Motivation
Environmental pollution, particularly the contamination of water resources, is a pressing global concern that has garnered significant attention from scientists The expansion of large-scale industries results in the ongoing discharge of toxic compounds into wastewater, which accumulate in the environment at high concentrations and are resistant to biodegradation Among these pollutants, derivatives of phenol, carboxylic acids, and pharmaceutical products pose direct and severe threats to living organisms.
Antibiotics commonly used in shrimp farming can negatively impact the environment and living organisms when released into wastewater Recent studies have highlighted the potential of advanced materials and nanomaterials, particularly titanium dioxide (TiO2), in various scientific and technological applications TiO2, recognized as a vital semiconductor, is extensively utilized in energy and healthcare sectors Its unique surface properties make it effective for photocatalysis, adsorption, and the decomposition of organic compounds, with these processes influenced by the substance's nature and concentration Furthermore, TiO2 plays a crucial role in the initial stages of catalysis, sensors, and drug delivery systems However, the detailed mechanisms and fundamental interactions of adsorptive processes on TiO2 surfaces remain underexplored.
Recent studies have concentrated on the removal of harmful substances detrimental to the environment through the use of nanomaterials and advanced technologies Various physical, chemical, and biological methods have been suggested to enhance efficiency in this process Additionally, new materials are currently being investigated for their effectiveness in addressing these environmental challenges.
The removal of organic pollutants, such as antibiotics in wastewater, can be effectively achieved through adsorption techniques using materials like activated carbon, filter membranes, and advanced oxidation processes These methods allow for the efficient capture of organic molecules on material surfaces, enhancing the treatment capacity for contaminated environments.
Recent studies have focused on identifying low-cost, environmentally friendly materials that effectively remove pollutants from the environment, as traditional methods tend to be expensive and overly complex.
Scientists have focused on clay minerals because of their excellent adsorption capacity, ease of fabrication, natural abundance, and environmental friendliness.
Clay mineral materials, known for their layered structures and extensive surface area, can enhance adsorption capacity and toxic substance removal through the addition or replacement of suitable cations Research into the adsorption of organic compounds and antibiotic residues using these materials holds both scientific and practical importance Vermiculite, in particular, shows promise as an effective treatment for persistent organic substances by effectively removing antibiotic residues from aquatic environments However, the intermolecular interactions and adsorption mechanisms occurring on mineral surfaces remain inadequately understood.
To effectively treat organic pollutants using TiO2 and clay minerals, it is crucial to understand the origin and role of surface interactions, as well as the stability of geometrical configurations during the adsorption process Insights into the adsorptive interactions between organic molecules and these materials are essential for comprehending the interactions between molecules and solid-state surfaces Recent advancements in molecular dynamics and quantum chemical modeling have enhanced the understanding of surface science The development of high-performance computing systems and efficient software has facilitated extensive theoretical studies, enabling scientists to investigate the structural and electronic properties, spectroscopy, and surface processes of TiO2 and clay minerals Consequently, theoretical investigations utilizing quantum chemical calculations to explore the adsorption and decomposition of organic molecules on material surfaces have emerged as a preferred method for understanding surface phenomena.
In summary, this theoretical study offers valuable insights that can be applied in future experimental research aimed at identifying effective materials for the treatment of organic pollutants The investigation, titled “Study on the Adsorption Ability of Organic Molecules,” highlights the significance of understanding adsorption processes to enhance environmental remediation efforts.
The study of TiO2 and clay mineral materials through computational chemistry methods holds significant scientific and practical value Our findings can guide future experimental observations and propose relevant experiments within Vietnam.
Research purpose
Our theoretical studies aim to determine stable structures formed during the adsorption of organic molecules on TiO2 and clay minerals, investigate the adsorption capabilities of various organic molecules and antibiotics on these surfaces, gain insights into surface interactions and their impact on the stability of complexes and adsorption processes, and evaluate the potential of TiO2 and clay minerals for future experimental studies focused on the adsorption and removal of antibiotics and organic pollutants in wastewater.
Object and scope of this study
The selected organic molecules and antibiotics include benzene and its derivatives, ampicillin, amoxicillin, benzylpenicillin, enrofloxacin, and tetracycline.
The material surfaces considered in this work include TiO 2 (rutile, anatase), kaolinite, and vermiculite.
This study focuses on the theoretical analysis of how organic compounds, particularly antibiotics, adsorb onto the surfaces of TiO2 (in both anatase and rutile forms) and various clay minerals, including kaolinite and vermiculite, utilizing computational chemistry techniques.
Research contents
This article begins with a review of prior research relevant to the topic, providing context for the current study It then outlines quantum chemical methods employed to address the Schrödinger equations, detailing their significance in theoretical chemistry Finally, the latter sections present computational findings for specific systems, enhancing the understanding of the applied methodologies.
Chapters 1 and 2 of Part 3 focus on the adsorption of organic molecules, particularly antibiotics, on the surfaces of TiO2 and clay minerals The research involves optimizing the structures of various organic molecules with functional groups such as -OH, -COOH, -NH2, -CHO, -NO2, and -SO3H, alongside different TiO2 surfaces (rutile-TiO2 (110) and anatase-TiO2 (101)) and clay minerals (vermiculite and kaolinite) Key tasks include designing stable adsorption structures for selected molecules, calculating energetic parameters related to their adsorption, and assessing the adsorption capabilities of these organic molecules on TiO2 and clay mineral surfaces, while also examining the role of intermolecular interactions in these systems.
In one of the crucial sections, conclusions and outlook, we summarize the significant results achieved in the present work and give some outlooks for further investigations.
Methodology
The density functional theory (DFT) methods with suitable and highly correlated functionals, such as the PBE, optPBE-vdW, vdW-DF-C09 [25], [72],
The optimization and calculation of key parameters, including the geometrical and electronic structures of organic molecules, antibiotics, and material surfaces, are essential for understanding their stable configurations Additionally, energy aspects such as adsorption, interaction, and deformation energies are calculated to assess the adsorption capabilities of molecules on material surfaces.
The VASP and GPAW packages, along with visualization software like Gaussview, VESTA, and Material Studio, are utilized to simulate the structures of TiO2 and clay minerals, as well as the configurations resulting from molecular adsorption on material surfaces These tools enable the calculation of energetic values and other essential parameters Furthermore, to explore intermolecular interactions, calculations involving DPE, PA, MEP, topological geometry, and EDT are conducted using Gaussian packages (versions 03 and later).
Details of calculations and analyses for the investigated systems are presented in the computational methods section.
Novelty, scientific and practical significance
Research on the adsorption of organic molecules with benzene rings onto TiO2 and clay minerals using computational chemistry has been insufficiently explored by scientists in Vietnam and globally This study provides valuable insights into the adsorption capabilities of these materials, which can help address environmental issues Understanding surface interactions is crucial for selecting effective materials to treat organic pollutants The findings offer a thorough assessment of the adsorption processes occurring on TiO2 and clay minerals, serving as a foundational investigation for future experimental studies aimed at pollutant removal and decomposition in the environment.
Our present work results give insights into the adsorption ability of organic compounds containing different functional groups such as -OH, -COOH, -CHO, >C=O,
Intermolecular interactions, including -NH2 and -SO3H groups on material surfaces, play a crucial role in the stability of complexes and the adsorption of molecules on TiO2 and clay minerals Utilizing quantum chemical methods can clarify these interactions, providing valuable insights for future research on the treatment of pollutants in wastewater.
This research has been validated through publications in esteemed peer-reviewed journals, including Surface Science, Chemical Physics Letters, Vietnam Journal of Chemistry, Vietnam Journal of Science and Technology, Vietnam Journal of Catalysis and Adsorption, and the Quy Nhon University Journal of Science.
DISSERTATION OVERVIEW
Organic pollutants and antibiotics residues in wastewaters
In recent decades, environmental pollution has become a significant global issue, drawing substantial attention from scientists and policymakers due to its lasting consequences The accumulation of benzene ring compounds in large quantities has been a notable aspect of human living conditions, highlighting the urgent need for effective solutions.
The increasing presence of antibiotics in wastewater has made it increasingly challenging to eliminate them from the environment, leading to significant negative impacts.
Antibiotics play a crucial role in the treatment of diseases in both humans and animals, and they are also widely utilized in the large-scale production of aquatic organisms Their applications extend across various fields, including medicine, biology, biochemistry, life sciences, and agriculture.
The uncontrolled release of antibiotic-containing waste is leading to significant environmental and health issues, particularly the pollution of aquatic resources, which adversely affects the growth of living organisms.
The shrimp farming industry in Vietnam plays a crucial role in the country's economy, contributing significantly to its development through high export values As the demand for both the quantity and quality of shrimp production rises, various advanced technologies and solutions have been implemented to enhance results However, the issue of water pollution from shrimp farming and processing remains a major concern, with wastewater containing harmful substances like antibiotic residues, nitrogen, and phosphorus compounds Notably, the use of antibiotics such as tetracycline, penicillins, and quinolones in shrimp farming has been prevalent and continues to lack stringent regulation, posing environmental risks.
For the well-being of society, it is imperative to safely remove pollutants, especially antibiotics, in wastewater discharged from shrimp farming.
2 TiO 2 nanomaterial and its applications
Nanotechnologies based on nanomaterials have been recently considered effective in solving wastewater problems [14] Furthermore, nanomaterials contribute to development of more efficient treatment processes among advanced water systems
Amorphous silica, calcium silicate, silica-based nanotubes, activated carbon, and graphene oxide have been identified as effective materials for the removal of antibiotics However, many of these options are costly and present challenges in regeneration after the adsorption process.
Titanium dioxide (TiO2) is a crucial semiconductor material used in photoreaction processes across energy, health, and food technology sectors Its unique surface properties make it effective in photocatalysis, adsorption, and the degradation of toxic compounds into simpler molecules Recent studies have highlighted the use of TiO2-based implants in biology and the adsorption of organic molecules on its surface The adsorption processes are influenced by the nanostructured surface of TiO2 films, which depend on factors such as substance nature, concentration, and environmental conditions Understanding the structure and properties of TiO2 surfaces is essential for designing efficient photocatalysts and solar cells Three stable phases of TiO2—rutile, anatase, and brookite—have been synthesized and utilized in various applications, including photocatalysis, sensors, and medical transmission.
The study of TiO2 phases reveals that rutile is the most stable form, with the (110) surface being the most stable plane, as confirmed by both experimental and theoretical research In contrast, anatase has garnered significant attention due to its superior photocatalytic activity compared to rutile, particularly the extensively studied (101) plane, which is the most prevalent in anatase.
Titanium dioxide (TiO2) is a key player in photocatalytic and photoelectrocatalytic processes due to its strong oxidation abilities, chemical stability, nontoxicity, and low cost Its effectiveness in removing pollutants is significantly influenced by factors such as particle size, specific surface area, crystalline phase, exposed plane surfaces, and the mass transfer rate of organic pollutants Additionally, adsorption is a crucial step in photocatalytic reactions, as it facilitates chemical reactions on the photocatalyst's surface and enhances sensor operation.
Noticeably, the adsorption of simple molecules has been examined in recent years [80], [81], [138] on different surfaces of TiO2 including rutile and anatase [102],
Recent studies have focused on the interactions between various organic molecules, including carboxylic acids, alcohols, ethers, and antibiotics, with TiO2 surfaces Research has utilized computational chemistry to analyze the geometrical structures and adsorption capabilities of amino acids and other organic compounds on TiO2 Notably, functionalized organic compounds such as >C=O, -COOH, -OH, -NH2, -CHO, and -CONH- demonstrate favorable adsorption on TiO2 However, previous investigations often lack detailed explanations of intramolecular interactions and do not adequately assess the stability of complexes or the adsorption processes involved, particularly concerning antibiotics.
10 molecules on TiO 2 surfaces are not analyzed in detail or received enough attention yet.
3 Clay minerals and their applications in the treatment of pollutants
Recent studies have focused on identifying effective materials for the removal of organic pollutants and antibiotic residues from wastewater Clay minerals, key components of various soil types, have emerged as promising adsorbents for wastewater treatment due to their remarkable properties, including high cation exchange capacity, excellent swelling ability, and large specific surface areas.
Clay minerals feature layered structures composed of different combinations of tetrahedral and octahedral sheets Notable examples include kaolinite, which has a 1:1 ratio of tetrahedral to octahedral sheets, and vermiculite, characterized by a 2:1 arrangement with two tetrahedral sheets surrounding a central octahedral sheet.
Kaolinite mineral is one of the potential materials used in the water purification industry to reduce soil pollution and catalysis for chemical reactions
Kaolinite features two distinct surfaces: the hydrogen-rich H-slab and the oxygen-rich O-slab Research by Harris and colleagues demonstrated that the H-slab exhibits superior adsorption capacity for organic compounds, including various dyes, compared to the O-slab and amorphous aluminum oxide The H-slab's high positive charge density enhances its ability to adsorb organic compounds with electrophilic functional groups, such as -OH and -COOH.
Johnson's research revealed that the H-slab of kaolinite exhibits superior adsorption capabilities compared to the O-slab for compounds like benzene, n-hexane, pyridine, and 2-propanol Chen and colleagues conducted both theoretical and experimental analyses on the adsorption of various amino/ammonium salts, including DDA, MDA, DMDA, and DTAC, finding that these cations are strongly adsorbed on both H-slab and O-slab surfaces through hydrogen bonding Similarly, Awad's study on the adsorption of 5-aminosalicylic acid indicated that amino/ammonium cations and derivatives have a stronger affinity for the H-slab compared to the O-slab This underscores the significance of investigating the adsorption of benzene derivatives with functional groups such as -CHO, -COOH, -OH, -NH2, and -SO3H on H-slab, as it is crucial for understanding the geometrical structure, stability of complexes, and intermolecular interactions on material surfaces.
Recent research indicates that clay minerals, particularly vermiculite, are promising adsorbents for effectively removing dyes, organic pollutants, and metal cations This effectiveness is attributed to their hydrophilic nature, high charge density surface, and unique layered crystalline structure.
Investigations on materials surfaces using computational chemistry
Quantum chemical computations are essential for understanding how molecules are adsorbed on clay minerals and TiO2 surfaces by determining the stability of various binding sites and the geometrical changes that occur during adsorption Theoretical studies utilizing density functional theory (DFT) have frequently explored the adsorption of organic molecules and antibiotics on TiO2, kaolinite, and vermiculite surfaces Previous research has examined the thermodynamic stabilities of different adsorbate-surface systems and their interaction roles However, a comprehensive understanding of surface interactions and their effects on the stability of configurations and the adsorption process remains underexplored.
Research on clay minerals and TiO2 materials in Vietnam, particularly their ability to adsorb organic pollutants, remains underexplored There is a notable scarcity of theoretical studies in this area, with existing investigations primarily focusing on materials like graphene, activated carbon, and zeolite Consequently, both theoretical and experimental research on clay minerals and TiO2 is still limited This highlights the potential for further exploration of surface phenomena, which could lead to valuable applications in environmental science.
THEORETICAL BACKGROUND AND COMPUTATIONAL METHODS
Quantum chemical approaches
In 1926, Erwin Schrödinger developed the time-dependent equation for a one-dimensional, one-particle system, integrating Planck's quantum theory with Louis De Broglie's concept of particle-wave duality.
(1.1) where h is Plank's constant and h
The potential field of the system, V(x,t), is influenced by the mass of the single particle, m, as well as the x-coordinate and time variable, t The function Ψ(x,t) is continuous and single-valued, and in a one-dimensional context, the probability of locating the particle within the interval from x to x + dx at time t is given by the square of the absolute value of Ψ(x,t), represented as |Ψ(x,t)|².
Equation (1.1) is quite complex, especially for many-body systems.
In chemistry, quantum systems are often analyzed in a stationary state, where the probability of locating a particle remains constant over time and is solely dependent on spatial coordinates This leads to the application of the time-independent Schrödinger equation, which serves as a simplified model for these systems.
This is simply rewritten as: ˆ where: H is the Hamiltonian operator, E is the energy of the system The (1.2a) and
(1.2b) are Schrửdinger equations that independent of time.
The Hamiltonian serves as the total energy operator in molecular systems, encompassing five key components: the kinetic energy of electrons, the kinetic energy of nuclei, the electrostatic attraction between nuclei and electrons, the repulsive forces among electrons, and the repulsive forces between nuclei.
U ee : the potential energy of interaction between electrons
It is fully represented by the following equation:
A 1 where: A, B: denote for the nuclei A and B
M A : mass ratio of nucleus A to one electron p, q: symbol for electrons in the system
Z A , Z B : number of units of nuclear charge A, B r pq : distance between the electrons p and q r pA : distance between the electron p and the nucleus A
R AB : distance between two nuclei A and B and nuclei
The fourth term in equation (1.4) remains indeterminate due to the indistinguishable nature of electrons, making it challenging to solve the Schrödinger equation for systems with multiple electrons Only systems with a single nucleus and a single electron, such as the hydrogen atom, can be solved explicitly For systems with two or more electrons, we can only obtain approximate solutions Ultimately, solving the Schrödinger equations provides the wave function Ψ and the total energy E of the system under investigation.
1.2 The Born - Oppenheimer approximation and Pauli’s exclusion principle 1.2.1 Born – Oppenheimer approximation
The Born-Oppenheimer approximation is a fundamental mathematical approach in molecular dynamics that facilitates the separation of nuclear and electronic motion within molecules When the nucleus is considered stationary relative to the electron, the electron's movement shows minimal dependence on nuclear motion Consequently, in the relevant equation, the second term is zero, while the last term remains constant, leading to the Hamiltonian operator for electrons that corresponds to the total electron energy.
For the movement of nuclei in the average field of the electrons, the nuclei operator has the form:
According to the Born − Oppenheimer approach, the complete wavefunction for the system containing N electrons, M nuclei can be rewritten: r i , R A el r i , R A nucl R A (1.7)
The Schrödinger equation cannot be precisely solved for multi-electron systems due to the complex interactions between electrons, making it impossible to determine their exact positions Instead, only the electron density can be calculated, representing the probability of finding a particle at a specific location.
The H el operator in equation (1.4) solely relies on the spatial coordinates of electrons, affecting only the spatial component of the wave function To comprehensively characterize electron properties, it is essential to incorporate the spin term into the wave function The spin functions (ω) and (ω), representing spin-up and spin-down states, can be selected to be orthogonal and normalized, ensuring they are orthonormal.
An electron is characterized by both spatial coordinates (r) and spin coordinates (ω), represented as x = {r, ω} Consequently, the wave function for a system of N electrons is expressed as ψ(x₁, x₂, , xN) and must exhibit antisymmetry when the coordinates (including both space and spin) of any two electrons, p and q, are exchanged.
The exclusion principle is the consequence that, if x p = x q for p ≠ q, then ψ(x 1 ,…, x p ,
…, x q ,…, x N ) = 0 (1.11) This means that none of the n particles may be in the same state (Pauli’s exclusion principle) [64], [73], [78].
Solving the Schrödinger equations accurately for systems with multiple nuclei and electrons is infeasible; however, approximate methods exist that simplify the problem significantly, often utilizing the variational principle Specifically, by examining a Hamiltonian H and a normalized function Ψ, we can determine the expectation value of the energy associated with that function.
H * H dr (1.12) where r represents all the integration coordinates.
The eigenfunctions Ψ of a Hamiltonian H are characterized by the condition of stationarity, meaning they do not change to the first order with small variations in Ψ This condition is equivalent to the Schrödinger equation, which relates the eigenfunctions Ψ_n to their corresponding eigenvalues E_n.
We label the ground state with n = 0 and the ground-state energy as E 0 The variational principle states, quite simply, for any different function Ψ,
This important finding shows that any function Ψ provides an upper estimate of the ground state's energy To approximate the unknown ground state, one can vary Ψ within a specified set of functions and identify the function that minimizes the Hamiltonian H.
The basis set comprises mathematical functions essential for constructing the wave function, playing a crucial role in solving Schrödinger equations To achieve accurate approximations, it is vital to enhance computational methods and select appropriate basis sets tailored to the specific systems under investigation A more extensive basis set provides a more realistic representation of electrons, leading to improved approximations Therefore, careful consideration of the chosen basis sets is necessary for each system to ensure optimal results.
There are two basic function types used in electronic structure calculations: Slater-type orbital (STO) and Gaussian-type orbital (GTO) with corresponding expressions in spherical coordinates:
In the context of Gaussian-type orbitals (GTO), the normalized factor is denoted as N, while r represents the distance calculated as the absolute value of the difference between the orbital coordinate vector and the nuclear coordinate A The spherical function is indicated by Y l,m, and ξ refers to the exponent associated with the respective Slater-type orbitals (STO) and GTO functions.
The use of Gaussian-type orbitals (GTO) in electronic structure calculations often emphasizes energy in the inner shell electron region, neglecting the chemically significant valence-shell electron region To accurately describe the outer valence shell, a sufficiently large basis set is essential, even though it may prolong computation times Therefore, combining a complete set of primitive Gaussian-type orbitals (PGTO) with a smaller basis set is necessary, resulting in a contracted basis set that simplifies the function into a contracted Gaussian-type orbital (CGTO).
In equation (1.17), a i is the reduction coefficients and k is the reduction order The Ψ CGTO is more similar to Ψ STO
1.4.2 Some popular basis sets i) Pople basis sets
Computational methods
The geometrical structures of organic molecules, antibiotics, and surfaces are optimized using the VASP program, based on experimental data Unit cells are selected with either four layers (two frozen and two relaxed) for the rutile-TiO2 (110) surface or two relaxed layers for the anatase-TiO2 (101) surface, allowing for an efficient investigation of molecular adsorption These surface structures feature double-bonded and neighboring bridging oxygen atoms, along with five-fold and six-fold coordinated titanium atoms, complemented by in-plane threefold coordinated oxygen atoms, as illustrated in Figure 4.
The simulated surfaces of r-TiO2 are designed with specific dimensions for the adsorption of various organic molecules and antibiotics, including formic acid, acetic acid, and benzene derivatives, as well as enrofloxacin (ER), ampicillin (AP), amoxicillin (AX), and tetracycline (TC) The dimensions for r-TiO2 are 13.24 Å x 9.10 Å x 25.00 Å, 17.82 Å x 13.15 Å x 35.00 Å, and 14.85 Å x 13.15 Å x 35.00 Å In contrast, the a-TiO2 is characterized by dimensions of 11.42 Å x 10.43 Å x 20.00 Å and 15.22 Å x 20.87 Å.
The adsorption of benzene derivatives and AP, AX antibiotics is evaluated using a Γ-center k-points grid for surface-related calculations The k-points in reciprocal space are strategically chosen at the Gamma center, utilizing ratios of 2 x 2 x 1 for r-TiO2 and 2 x 3 x 1 for enhanced accuracy in the analysis.
1 or 4 x 3 x 1 for a-TiO 2 based on cell size of investigated systems The converged plane-wave cut-off energy controlling the number of plane-wave basis
For our calculations at 500 eV, we selected 34 functions, utilizing the projected augmented wave (PAW) method as described by Kresse and Joubert Ionic position relaxations were conducted using the conjugate gradient algorithm, ensuring that the forces on all atoms were reduced to below 0.01 eV/Å.
In this study, we employed the Perdew-Burke-Ernzerhof (PBE) version of the generalized-gradient approximation (GGA) to account for exchange-correlation effects in all calculations, unless stated otherwise The adsorption energy (E_ads) and interaction energy (E_int) were calculated using specific expressions.
The equation E int = E comp – (E surf * + E mol * ) (1.65) represents the interaction energy of the surface-adsorbate system, where E comp, E surf, and E mol denote the energies of the optimized system, surface, and molecule, respectively The single-point energies, E surf * and E mol *, are fixed during the calculations The deformation energies of the adsorbate molecule (E def-mol) and rutile surface (E def-surf) are derived from the differences between the single-point and optimized energies Additionally, the optPBE-vdW functional is utilized in certain calculations to provide a comprehensive assessment of adsorption energies.
2.2.1 Adsorption of organic molecules on kaolinite surfaces
Kaolinite, with the chemical formula Al2Si2O5(OH)4, consists of uncharged layers linked by hydrogen bonds These bonds connect the hydrogen-rich (H-slab) and oxygen-rich facets of the kaolinite (001) surface.
The geometric structures of benzene derivatives and kaolinite surfaces were optimized using the VASP program The K⁺-supported kaolinite (001) surface, referred to as the K⁺-slab, was analyzed to evaluate the influence of cations on the adsorption capabilities of molecules on kaolinite, as illustrated in Figure 5.
Figure 5 The structure of kaolinite surfaces
The study examines H-slab systems of kaolinite with and without K+ cation, utilizing a cell size defined by the dimensions: a = 10.43 Å, b = 9.06 Å, and c = 25.00 Å, along with an approximate vacuum space of 15 Å A cutoff energy of 500 eV is selected for the analysis, employing the PBE functional in all calculations Additionally, the optPBE-vdW functional is applied in the K+-slab systems to comprehensively evaluate adsorption energies Key parameters such as adsorption energy (E_ads), interaction energy (E_int), and deformation energies are calculated using specified formulas.
2.2.2 Adsorption of antibiotics on vermiculite surface
The geometrical structures of the systems were optimized using the GPAW package, which employs the projector-augmented wave (PAW) method and represents wave functions on real-space orthorhombic grids through a finite-difference approach To model the vermiculite surface, a periodic slab was created based on crystallographic data from the American Mineralogist Crystal Structure Database This model, depicted in Figure 6, features Mg 2+ as the sole octahedral ion, while Si 4+ cations in the tetrahedral sheets are replaced by Al 3+ cations in a 1:3 ratio To accommodate the large size of the adsorbed molecule, the structure's single unit cell was expanded into a 2 x 1 super-cell with lattice parameters of a = 10.70 Å, b = 18.51 Å, c = 46.64 Å, and angles α = 90.00°, β = 97.12°, γ = 90.00°, corresponding to a monoclinic structure.
Figure 6 The model slab of vermiculite surface (red, yellow, grey, pink, and white colors displayed for O, Mg, Si, Al, and H atoms, respectively)
In this study, we simplified the model by removing water molecules that hydrate the Mg 2+ cations on the surface, allowing us to use the resulting surface for all DFT calculations We employed the PBE and vdW-DF-C09 functionals in our analysis.
The study involved the selection of 25 samples for geometry optimization and energy calculations, focusing on adsorption energy (E_ads) and interaction energy (E_int) using PBE and vdW-DF-C09 functionals Additionally, the deformation energies of both the adsorbate molecule (E_def-mol) and the vermiculite surface (E_def-surf) were computed in a manner similar to that used for TiO2 systems.
To enhance our understanding of the interactions among small organic molecules, antibiotics, TiO2 (both rutile and anatase), and clay minerals (such as kaolinite and vermiculite), we conducted calculations on proton affinity (PA) and deprotonation enthalpy (DPE) using the Gaussian 09 program at the B3LYP/6-31++G(d,p) level Additionally, we performed a topological analysis and examined the electron density (ρ(r)).
The Laplacian of electron density (∇²ρ(r)) at bond-critical points (BCPs) is determined through atoms-in-molecules theory using the AIM 2000 program at the B3LYP/6-31+G(d,p) level Additionally, the density potential energy (V(r)) and density kinetic energy (G(r)) at these BCPs are calculated using established expressions.
The molecular electrostatic potential (MEP) maps for antibiotic molecules were calculated at the B3LYP/6-31++G(d,p) level, revealing charge regions between -5.10 -5 e and 0.15 e, which are critical for assessing the stability of interactions with the vermiculite surface Additionally, Natural Bond Orbital (NBO) analysis was conducted using NBO 5.G software to evaluate the transfer of electron density between the interacting entities.
ADSORPTION OF ORGANIC MOLECULES ON MATERIALS SURFACES
Adsorption of organic molecules on rutile-TiO 2 (110) surface
We conducted calculations on the adsorption of formic, acetic, and benzoic acids, as well as phenol and nitrobenzene molecules on the rutile-TiO2 (110) surface (r-TiO2) Our study identified the most stable complexes for each adsorbate, designated as P1, P2, and P3.
P4, and P5 as shown in Figure 1.1.
P1 (adsorbate: HCOOH) P2 (adsorbate:CH 3 COOH) P3 (adsorbate:C 6 H 5 COOH)
Figure 1.1 Stable complexes for the adsorption of organic molecules on rutile-TiO 2
Table 1.1 Charge distribution in molecules at the B3LYP/6-31+G(d,p) level
(a),b) for O atoms in C(N)=O and O-H groups, (c) for H atoms in OH groups.
The NBO charge distribution at atomic levels within molecules was determined using the B3LYP/6-31+G(d,p) method, as detailed in Table 1.1 Additionally, Table 1.2 presents key parameters of the optimized structures following complexation, including interaction distances and bond angles.
Table 1.2 Some selected parameters of stable complexes at PBE functional (distance (r) in Å; angle (α) in degree) Ti∙∙∙O
* for C-H∙∙∙O; (1) for O atom in surface, (2) for O atom in formic acid
The interactions between organic molecules and r-TiO2 are characterized by Ti∙∙∙O distances of 1.95 to 2.35 Å, which closely align with the Ti-O bond lengths of r-TiO2 (1.80-2.20 Å), indicating strong adsorption interactions Additionally, O∙∙∙H hydrogen bonds formed during complexation range from 1.49 Å to 2.39 Å, suggesting their stability Notably, the hydrogen atom in the -COOH group of P1 is inclined to shift towards r-TiO2, resulting in the formation of a covalent O b-H bond measuring 1.05 Å The stability of these complexes is influenced by both Ti∙∙∙O interactions and O(C)-H∙∙∙O hydrogen bonds for P1 to P4, while only Ti∙∙∙O interactions are present for P5 Previous studies have also explored the significance of -COOH and -OH groups in enhancing the stability of these complexes.
The strong Ti∙∙∙O interactions arise from the Ti atoms on the surface bonding with the highly negatively charged oxygen atoms in C=O or O-H groups Additionally, O(C)-H∙∙∙O hydrogen bonds form between surface oxygen atoms and the positively charged hydrogen atoms in O-H and C-H bonds Notably, the distances between Ti and O increase in the sequence of P1 < P2 ≈ P3 < P4 < P5 complexes.
The α(TiOC(N)) angles for these interactions vary between 126.8° and 154.5°, with the α(TiOC) angles in P1, P2, P3, and P4 being larger than those of α(TiON) in P5 Additionally, the distance of the O∙∙∙H hydrogen bonds increases sequentially from P1 to P2, then to P3, and finally to P4.
P4 complexes exhibit varying interaction angles for H-O∙∙∙H hydrogen bonds, ranging from 159.0 to 178.0 degrees, with P1 showing the largest angle and P4 the smallest The angles in P1, P2, and P3 are comparable to classical hydrogen bonds, often referred to as "red-shifting" hydrogen bonds, indicating that these interactions are strong during complexation.
Table 1.2 presents the alterations in selected bonding lengths during adsorption phenomena, highlighting that the variations in O-H, C-O, N-O, and Ti-O bond lengths in isolated monomers and surfaces are minimal, ranging from 0.01 to 0.16 Å.
In contrast to P2, P3, P4, and P5, P1 exhibits significant changes in O-H and Ti-O distances, measuring 0.51 Å and 0.28 Å, respectively These substantial alterations result in the breaking of the O-H bond of the adsorbate, leading to the formation of a new O-H bond on the surface Consequently, the Ti atom on the surface shifts towards the molecular adsorbate, facilitating the creation of a new Ti-O bond.
The adsorption process of molecules on the TiO2 surface is assessed by calculating the adsorption, interaction, and deformation energies (E_ads, E_int, E_def-mol, and E_def-surf) using the PBE functional, as detailed in Table 1.3.
Table 1.3 Adsorption, interaction, and deformation energies (all in kcal.mol -1 ) for the adsorption processes on rutile-TiO 2 (110) surface
The adsorption energies of organic compounds on r-TiO2 are significantly negative, ranging from -18.5 to -28.8 kcal/mol, indicating strong adsorption The order of adsorption energies is benzoic acid < acetic acid < formic acid < phenol ≈ nitrobenzene, demonstrating a decrease in adsorption strength from acids to phenol and nitrobenzene This process is classified as chemical adsorption due to the large negative energies involved Additionally, the transfer of an H atom from the O-H bond in acid molecules to the r-TiO2 surface facilitates the formation of a complex through electrostatic interactions, leading to the dissociation of carboxylate anions on the surface These findings align with previous studies on the adsorption of formic, acetic, and benzoic acids on rutile-TiO2.
The interaction energies of stable complexes range from -22.6 to -114.3 kcal/mol, indicating strong Ti∙∙∙O and O-H∙∙∙O interactions that contribute to stable configurations Energy values decrease from nitrobenzene to phenol and acids, with formic acid exhibiting the most negative value due to higher negative charge densities at O atoms in C=O and C-O bonds, and positive charge densities at H atoms in O-H bonds The highest interaction energy for P1 arises from the formation of the O b-H covalent bond on r-TiO2 Consequently, the stability of Ti∙∙∙O and O-H∙∙∙O hydrogen bond interactions follows the order P1 > P3 > P2 > P4 > P5, showing that interactions between r-TiO2 and -COOH groups in P1, P2, and P3 are stronger than those with -OH and -NO2 groups in P4 and P5.
Deformation energy is a crucial factor in assessing the capability to detach the substrate from its stable surface configuration According to Table 1.3, the deformation energy values for the surface are marginally higher than those of the substrate.
In the case of the molecules, the energy required for the dissociation of formic acid is greater than that of the surface, particularly in the P1 scenario Consequently, the formation of stable structures on the surface is less favorable compared to the adsorbed molecules, with the exception of the separated monomers in P1.
1.1.3 The quantum chemical analysis for the interactions on the surface
To gain a better understanding of the interactions between the specified molecules and r-TiO2, we conduct a charge density and topological analysis, which are illustrated in Figures 1.2 and 1.3, and summarized in Table 1.4.
Figure 1.2 The charge density between adsorbent and adsorbates in stable complexes
Figure 1.3 The topological analysis for the first-layered structures
Charge distribution during interaction formation is notably concentrated in the first two-layered structures, as illustrated in Figure 1.2 Moreover, the orbital overlaps among the interacting atoms reinforce the establishment of these interactions The electron density overlaps in P1, P2, and P3 are more pronounced compared to P4 and P5, suggesting that P1, P2, and P3 exhibit greater stability in their interactions.
P4, P5 Hence, the interaction between –COOH group and rutile surface is more favorable than that of –OH and NO 2 groups following the adsorption process.
Table 1.4 The characteristic parameters for topological analysis (all in au)
*for C-H∙∙∙O hydrogen bonds; (1), (2) shown in Fig 1.3 (P1).
The presence of BCPs in Figure 1.3 and the values of electron density, its
Adsorption of benzene derivatives on rutile-TiO (110) and anatase-TiO 2 2 (101)
DFT calculations reveal the stable adsorption complexes of formic, acetic, and benzoic acids, as well as phenol and nitrobenzene, on the rutile-TiO2 (110) surface The study focuses on the adsorption energy of these organic molecules on r-TiO2, highlighting their interactions and stability on this specific substrate.
TiO 2 is estimated to be in the range of -18.5 to -28.8 kcal.mol -1 Both O∙∙∙Ti strong interaction and O-H∙∙∙O hydrogen bond significantly contributed to the stability of complexes Remarkably, the additional role of C-H∙∙∙O weak hydrogen bonds to the stability of complexes has been observed for the first time The adsorption on r-TiO 2 is more favorable for >COOH group than –OH and –NO 2 ones These processes are characterized as chemical adsorption The electron density overlap and AIM results provide a clearer understanding of the formation and role of O∙∙∙Ti and O(C)-H∙∙∙O intermolecular interactions following complexation.
1.2 Adsorption of benzene derivatives on rutile-TiO 2 (110) and anatase-TiO 2
Some parameters of geometrical structures of benzene derivatives, rutile-TiO 2
The study compares the calculated properties of rutile-TiO2 (110) and anatase-TiO2 (101) surfaces, referred to as r-TiO2 and a-TiO2, using the PBE functional via VASP with experimental data, as detailed in Table 1.5 The findings reveal minor discrepancies in bond length and angle, ranging from 0.00-0.08 Å and 0.5-2.4°, respectively, when compared to experimental results Thus, the PBE functional employed in this research proves to be a reliable choice for analyzing this system.
In addition, by using DFT calculations, we obtained 12 stable complexes for each system of interactions between the benzene derivatives, including C 6 H 5 CHO,
The optimized structures of the compounds C6H5COOH, C6H5NH2, C6H5OH, and C6H5SO3H, along with r-TiO2 and a-TiO2, are represented as r-PiX and a-PiX (where i = 1-4 and X = -CHO, -COOH, -NH2, -OH, -SO3H) These structures are illustrated in Figures 1.4 and 1.5, while the selected parameters for the complexes are compiled in Tables 1.6 and 1.7.
Table 1.5 Some selected parameters of molecules and TiO 2 surfaces
C 6 H 5 COOH a-TiO 2 r-TiO 2 r-P1CHO r-P2CHO r-P1COOH r-P2COOH r-P1NH 2 r-P2NH 2 r-P1OH r-P2OH r-P1SO 3 H r-P2SO 3 H r-P3SO 3 H r-P4SO 3 H
Figure 1.4 Stable complexes of adsorption of benzene derivatives on rutile-TiO 2
Table 1.6 Interaction distance (d, Å), bond angle (α, o ), and changes in the length of bonds ( r, Å) following the adsorption process for rutile systems
The complex interactions of titanium (Ti) with various functional groups, including d(Ti 5f ‧‧‧O/N) and d(O/C/N), are illustrated through multiple derivatives such as r-P1CHO, r-P2CHO, r-P1COOH, and others This study focuses on the bonding characteristics of Ti with oxygen atoms (O 1, O 2) in the -SO 3 H group, highlighting the involvement of two titanium atoms (Ti 1, Ti 2) Additionally, it examines the role of C-H O interactions and the hydrogen bonding between O b-H‧‧‧O at the TiO 2 surfaces, as depicted in Figure 4 of Part 2.
Table 1.7 Distance of intermolecular interactions (d, Å), bonding angle (α, o ), and changes of bond length (Δr, Å) upon adsorption process for anatase systems
Complex a-P1CHO a-P2CHO a-P1COOH a-P2COOH a-P1NH 2 a-P2NH 2 a-P1OH a-P2OH a-P1SO 3 H a-P2SO 3 H a-P3SO 3 H a-P4SO 3 H
X = C,N,O; * for C-HãããO; 1) 2) for TiãããO (Fig 1.5); (i) for O b -HãããO (Ti 5f and O b on TiO 2 surfaces as displayed in Fig 4 in Part 2)
47 a-P1CHO a-P2CHO a-P1COOH a-P2COOH a-P1NH 2 a-P2NH 2 a-P1OH a-P2OH a-P1SO 3 H a-P2SO 3 H a-P3SO 3 H a-P4SO 3 H Figure 1.5 Stable structures of adsorption of benzene derivatives on anatase-TiO 2
The distances between Ti 5f and O/N, as well as O/C/N-H, in the complexes range from 2.03-2.41 Å and 0.99-2.72 Å, respectively, which are smaller than the total van der Waals radii of the involved atoms (Ti, O, N, and H) This indicates the presence of Ti-O/N and O/C/N-H intermolecular contacts post-complexation Additionally, the hydrogen atom in the -SO3H group is likely to bond with O b in r-TiO2 and a-TiO2, forming O b -H covalent bonds The stable complexes arise from interactions between functional groups, five-coordination Ti atoms (Ti 5f), and O atoms at bridge sites (O b) on r-TiO2 and a-TiO2.
The interactive bonding angles α(Ti-O/N-C/S) range from 117.8° to 158.3°, with α(Ti-O-C/S) angles being larger than α(Ti-O-N) The α(O/C/N-H-O) bonding angles vary between 109.0° and 174.7°, with the extremes observed in r-P2SO3H and a-P4SO3H, as well as r-P1NH2 and a-P1OH These angles are comparable to those found in previous studies on O/C-HãããO hydrogen bonds Additionally, the lengths of Ti-O and O/C/N-H bonds increase by 0.02-0.17 Å and 0.01-0.80 Å, respectively Notably, the changes in O-H and Ti-O bonds in r-P1SO3H, a-P1SO3H, and a-P4SO3H are more pronounced than in other complexes, attributed to the high flexibility of the H atom in the -SO3H group, allowing it to shift towards r-TiO2 and a-TiO2 more readily than other groups This tendency aligns with prior reports on the flexibility of hydrogen atoms in acid functional groups.
[121] Hence, the obtained complexes are stabilized by Ti 5f ãããO interactions and O/N/C-HãããO hydrogen bonds.
1.2.2 Energetic aspects of the adsorption process
The energy aspects for stable configurations are listed in Tables 1.8 and 1.9
Table 1.8 Adsorption, interaction, and deformation energies of adsorption of benzene derivatives on rutile-TiO 2 (110) surface (all in kcal.mol -1 )
Complex r-P1CHO r-P2CHO r-P1COOH r-P2COOH r-P1NH 2 r-P2NH 2 r-P1OH r-P2OH r-P1SO 3 H r-P2SO 3 H r-P3SO 3 H r-P4SO 3 H
Table 1.9 Energetic aspects of adsorption of benzene derivatives on anatase-TiO 2
(101) surface (all in kcal.mol -1 )
Complex a-P1CHO a-P2CHO a-P1COOH a-P2COOH a-P1NH 2 a-P2NH 2 a-P1OH a-P2OH a-P1SO 3 H a-P2SO 3 H a-P3SO 3 H a-P4SO 3 H
The adsorption energy values for the complexes on rutile (r-TiO2) and anatase (a-TiO2) systems are significantly negative, ranging from -12.9 to -31.1 kcal/mol for rutile and -3.4 to -29.1 kcal/mol for anatase The adsorption energy values follow a specific order: -SO3H < -COOH < -NH2 < -CHO < -OH on both r-TiO2 and a-TiO2 Consequently, the adsorption ability of these molecules decreases in the order of -SO3H > -COOH > -NH2 > -CHO > -OH This indicates that organic molecules with acidic functional groups demonstrate stronger adsorption on titanium dioxide surfaces.
SO 3 H, - COOH) have stronger adsorption on TiO 2 surfaces than other ones (containing -OH, -CHO, -NH 2 groups) This is also similar to the previous investigation on adsorption of organic molecules on rutile-TiO 2 surface [85].
The interaction energy is a crucial factor in assessing molecular adhesion to material surfaces, with E int values ranging from -16.4 to -146.7 kcal/mol for rutile and -3.9 to -151.5 kcal/mol for other systems Notably, the interaction energy values follow the trend: -SO 3 H -NH2 > -COOH > -OH > -CHO Additionally, the positive charge density at H decreases in the same sequence, starting with -SO3H.
Adsorption of benzene derivatives on kaolinite (001) surface
The optimized structures of benzene derivatives adsorbed onto kaolinite
The stability of complexes on the H-slab surface is influenced by their configurations: P1 type exhibits a vertical structure with interactions between functional groups and the surface, while P2 type features a parallel structure with interactions concentrated at the benzene ring The stabilization of P1 complexes relies on O/N/C-H‧‧‧O contacts, whereas P2 complexes are primarily stabilized by H‧‧‧π and O/N/C-H‧‧‧O interactions Key structural parameters of these complexes are summarized in Table 1.14.
Figure 1.11 Stable structures of adsorption of derivatives on H-slab
Table 1.14 Distances of intermolecular contacts (d), changes in the bond lengths (Δr) involved in interactions in complexes (all in Å)
P3-SO 3 H a) for O-H‧‧‧O surf (surface), b) for H‧‧‧N, c) for O-H of surface, (i) for O surf -H‧‧‧O
Table 1.14 presents the distances for H‧‧‧O, H‧‧‧N, and H‧‧‧C/π (benzene ring) in various complexes, ranging from 1.46-2.58 Å, 2.25-2.81 Å, and 2.61-3.25 Å, respectively Notably, these measurements are generally less than the combined van der Waals radii of the interacting atoms, particularly highlighting the van der Waals radius of hydrogen.
C, N, O are 1.20 Å, 1.70 Å, 1.55 Å, and 1.50 Å, respectively) Therefore, it can be suggested that the hydrogen bonds of O/N/C-H‧‧‧O/π are formed in investigated complexes For P1-NH 2 and complexes of P2 type, the H‧‧‧N and H‧‧‧C(π) contacts exist although their distances are ca 2.81 Å and 2.61-3.25 Å, respectively, slightly larger than the total van der Waals radius of H and N/C atoms. Furthermore, the changes of C-H, C=O, S=O, S-O, N-H, O-H bonds length in molecules are pretty small, ca 0.06 Å Some C-O (in C-O-H) and S-O (in S-O-H) bonds are shortened in the range of 0.01-0.13 Å upon complexation In general, the formation of stable complexes leads to small changes in the geometrical structures of molecule and surface.
The H‧‧‧O surf distance in P1-SO₃H and P3-SO₃H is approximately 1.0 Å, which is comparable to the O-H bond length in organic molecules (around 0.98 Å) This suggests a significant shift of the H atom in the -SO₃H group towards the H-slab of kaolinite, facilitating the formation of a strong interaction.
O surf -H covalent bond following the adsorption process The hydrogen bonds of
The formation of surf-H‧‧‧O (-SO 3 -) occurs upon adsorption at a distance of 1.5 Å This study supports previous research indicating that the movement of hydrogen atoms in the strongly polarized bonds of acid groups, such as -COOH and -SO 3 H, towards material surfaces is a reasonable phenomenon.
1.3.2 Energetic aspects of the adsorption process
To assess the adsorption capability of molecules on the H-slab surface, we calculated key parameters such as adsorption energy (E_ads), interaction energy (E_int) of the complexes, and the deformation energies of both the molecules (E_def-mol) and the surface (E_def-surf), with the results presented in Table 1.15.
Table 1.15 Energetic parameters of complexes, molecules and surface upon adsorption processes (in kcal.mol -1 )
Complex P1-CHO P2-CHO P1-COOH P2-COOH P1-NH 2 P2-NH 2 P1-OH P2-OH P1-SO 3 H P2-SO 3 H P3-SO 3 H
The adsorption energy of the complexes, as detailed in Table 1.15, ranges from -3.0 to -24.8 kcal/mol, with P1 type complexes exhibiting more negative energy values than P2 type complexes, indicating that P1 complexes are more stable Furthermore, the interactive energy values for both P1 and P2 complexes vary from -6.8 to -99.2 kcal/mol.
60 and -3.6 to -7.6 kcal.mol -1 , respectively This confirms the higher strength of P1 complexes in comparison to P2.
In evaluating the stability of complexes formed by derivatives with various functional groups, the order of increasing adsorption energy (E ads) and interaction energy (E int) is identified as follows: -SO3H < -COOH < -OH < -CHO < -NH2 Consequently, the stability of these complexes decreases in the sequence: -SO3H > -COOH > -OH > -CHO.
The study reveals that the adsorption processes are primarily physical, with the exception of the -SO3H derivative, which exhibits chemisorption due to its significantly negative adsorption energy The progression of these processes leads to the formation of -COOH, -OH, -CHO, and ultimately -NH2.
The E ads and E int values for P1-SO3H and P3-SO3H are significantly more negative than those for P2-SO3H and other complexes, attributed to the H atom in the -SO3H group being readily transferred to the kaolinite surface, forming a covalent bond This transfer of the H atom in functional groups results in substantial alterations to both molecular and surface structures, as evidenced by the deformation energy (Edef-mol, Edef-surf) of the molecules and surfaces deviating from their stable configurations Specifically, the deformation energy values for P1-SO3H and P3-SO3H are approximately 60 kcal/mol for molecules and around 17 kcal/mol for surfaces.
The deformed energies of the complexes range from approximately 0.1 to 4.3 kcal/mol, indicating that the geometrical structures of the -SO3H derivative complexes are significantly distorted, while the geometric changes in the complexes of other derivatives are minimal.
1.3.3 Formation and role of intermolecular interactions
1.3.3.1 Deprotonation enthalpy of bonds and proton affinity at atoms in molecules
We conducted calculations on the dissociation energies (DPE) of O-H, C-H, and N-H bonds, along with the potential energy (PA) at O and N atoms involved in intermolecular interactions, utilizing the B3LYP/6-31+G(d,p) level of theory The results, including NBO charge analysis for atoms in functional groups, are detailed in Tables 1.10 and 1.11.
The negative charge densities, primarily located at the O/N atoms in functional groups, range from -0.507 to -0.936 e, allowing for strong interactions with the positively charged regions of the H-slab, particularly at H atoms, resulting in O/N‧‧‧H electrostatic interactions Conversely, positive charges, ranging from 0.152 to 0.538 e, are concentrated at H atoms in functional groups, facilitating the formation of H‧‧‧O electrostatic interactions during complexation Notably, in the C6H5SO3H molecule, the O atoms in the -SO3H group exhibit highly negative charge densities, leading to even stronger O‧‧‧ interactions.
H interactions with the H-slab in comparison to other derivatives Results imply that the ability to form O‧‧‧H interactions in complexes decreases in the ordering of -SO 3 H
The proton affinity (PA) at oxygen atoms plays a crucial role in understanding the formation of intermolecular contacts, particularly through the deprotonation of O-H and C-H bonds in functional groups As shown in Table 1.11, the PA values for O atoms vary from 178.8 to 203.2 kcal/mol, with an increasing trend observed in the order of -OH < -SO₃H.
The ability to form intermolecular contacts increases in the order of -COOH < -CHO, with DPE values for O-H/C-H bonds ranging from 341.1 to 404.3 kcal/mol, indicating that the strength of O/C-H‧‧‧O interactions decreases as follows: -SO3H > -COOH > -OH > -CHO Although the negative charge density and proton affinity at the nitrogen atom in C6H5NH2 are higher than those at oxygen atoms in other derivatives, the deprotonation of the N-H bond is less favorable than that of O-H bonds due to a higher DPE for N-H compared to O-H Consequently, the formation of H‧‧‧O contacts for the -NH2 group is slightly weaker than for -SO3H, -COOH, and -OH Additionally, DPE values for C-H bonds in the benzene ring, ranging from approximately 391.0 to 416.7 kcal/mol, are higher than those for O-H and N-H bonds in functional groups, suggesting that H‧‧‧C/π interactions at the benzene ring are less stable compared to O/N‧‧‧H interactions.
The topological geometries of the complexes are displayed in Figure 1.12 The
62 particular parameters for AIM analysis such as electron density (ρ(r)), Laplacian of electron density ( 2 ρ(r)), total electron density energy (H(r)) at the bond critical points (BCPs) are given in Table 1.16.
P1-CHO P1-COOH P1-NH 2 P1-OH P1-SO 3 H
Figure 1.12 Topological geometry of the most stable complexes for adsorption of organic molecules on H-slab
Table 1.16 Characteristics of topological geometries (ρ(r), 2 ρ(r), H(r), in au) and EDT (in e) at the B3LYP/6-31+G(d,p) level
Adsorption of benzene derivatives on a K + -supported kaolinite (001) surface 65 1 Stable complexes
MEP maps and NBO charges assess the electrostatic interactions between surface sites and molecules in complexes, highlighting the significance of K‧‧‧O/N and K‧‧‧π interactions in stabilizing P1 and P2 configurations Additionally, H‧‧‧O hydrogen bonds in P1COOH and P2NH2 contribute to this stabilization Notably, a slight movement of the K+ site on the surface occurs upon the adsorption of derivatives, particularly with C6H5COOH and C6H5NH2.
The adsorption energy values of configurations calculated by using the PBE and optPBE-vdW functionals are presented in Table 1.17.
Table 1.17 The adsorption energy of the stable complexes (in kcal.mol -1 )
P1CHO P2CHO P1COOH P2COOH P1NH 2 P2NH 2 P1OH P2OH
The formation energy (Ef) of the K+ -slab is calculated to be -6.2 kcal/mol using the PBE functional, determined by the equation Ef = EK+ -slab – EH -slab – EK+ This indicates that the K+ -slab exhibits slight stability, suggesting that the K+ ion can be favorably integrated into the structure.
67 shifted to other sites upon adsorption of molecules This is expressed in forming of stable configurations as displayed in geometrical structures.
The adsorption energy for various configurations ranges from -4.8 to -20.6 kcal/mol for the PBE functional and -8.7 to -22.8 kcal/mol for the optPBE-vdW functional, highlighting that incorporating van der Waals (vdW) forces enhances adsorption energy values by approximately 1.1 to 8.9 kcal/mol This variation in adsorption energy (E_ads) aligns with previous findings Notably, the E_ads values increase in the order of -CHO ≤ -COOH < -NH2 < -OH for both PBE and optPBE-vdW functionals, indicating that the stability and adsorption capacity of -CHO and -COOH derivatives on the K+-slab are superior compared to -NH2 and -OH derivatives.
Furthermore, the energy values for P1 type are more negative than that for
In -COOH systems, the stability of complexes is significantly influenced by K‧‧‧O electrostatic interactions, as evidenced by H-slab system results For -NH2 and -OH derivatives, P2 configurations exhibit more negative energy compared to P1, highlighting the substantial role of K‧‧‧π intermolecular interactions in stabilization, unlike K‧‧‧O/N contacts in P1 In the case of the -CHO derivative, PBE calculations indicate that the P1 configuration is more stable than P2, whereas vdW calculations suggest a slight stability advantage for P2 Thus, K‧‧‧O/N/π contacts and vdW forces are crucial for the stability of the configurations in these examined systems.
We conduct AIM and NBO calculations at the B3LYP/6-31+G(d,p) level to analyze the formation and strength of intermolecular contacts in complexes, with results presented in Figures 1.15, 1.16, and Table 1.18.
The presence of bond critical points (BCPs), represented as blue rings in topological geometries, signifies intermolecular interactions with K‧‧‧O/N/C contact ranges of 0.001-0.028 au and 0.002-0.150 au, respectively These measurements highlight the significance of molecular bonding in the studied systems.
68 the range of values for noncovalent interactions [9], [49], [50] With the small electron density, these contacts are evaluated as weak interactions.
Figure 1.15 The topological geometries of the stable complexes for K + -slab systems
Figure 1.16 The EDT maps of the stable complexes for K + -slab systems
Further, the ρ(r) values at K‧‧‧O BCPs decrease in the sequence of -CHO > -
COOH > -OH derivatives Similarly, the ρ(r) values at K‧‧‧C/π BCPs reduce in the ordering of -CHO > -COOH > -OH > -NH 2 Hence, the stability of complexes is
CHO > -COOH > -OH > -NH 2 derivatives Besides, H‧‧‧C/N/O contacts are regarded as additional terms in stabilization.
Table 1.18 The characteristics for topology analysis and total of electron density transfer (EDT) for K + -slab systems at the B3LYP/6-31+G(d,p) level
The examination of electron density transfers (EDT) in various configurations helps clarify the formation of intermolecular interactions Overlapping electron density between atoms indicates the establishment of these interactions.
70 transfers from molecules to surface as compared to the reverse transfers from surface to molecules.
DFT calculations reveal two primary trends in the adsorption of benzene derivatives on a K+-slab: interactions occur primarily at functional groups (P1 type) and at the benzene ring (P2 type) The adsorption of these organic molecules on kaolinite is characterized as weak chemical adsorption, with stability decreasing in the order of -COOH ≈ -CHO > -NH2 > OH The stability of the complexes is enhanced by K‧‧‧O/N/π electrostatic interactions between the K+ site on the surface and the negatively charged regions of the molecules Additionally, incorporating van der Waals (vdW) forces improves both the adsorption energy and the structural arrangement of the molecules on the K+-slab, while the presence of K+ cations on the kaolinite surface further enhances the adsorption capacity of these molecules.