INTERFACIAL AND CONFINED WATER Part 1 doc

Major part of water on the earth is the bulk phases: liquid phase in oceans, crystalline forms in polar ice caps and in glaciers, vapor phase in the air.. To understand specific propertie

INTERFACIAL AND CONFINED WATER

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INTERFACIAL AND CONFINED WATER

BY

Physical Chemistry Technical University of Dortmund Dortmund, Germany

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First edition 2008

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Printed and bound in Hungary

08 09 10 11 12 10 9 8 7 6 5 4 3 2 1

2.1 Surface transitions of fluids 172.2 Layering, prewetting, and wetting transitions of waternear hydrophilic surfaces 252.3 Drying transition of water near hydrophobic surfaces 502.4 Surface phase diagram of water 61

3.1 Surface critical behavior of fluids 683.2 Surface critical behavior of water 76

4.1 Effect of confinement on the phase transitions 914.2 Phase transitions of confined water 984.3 Capillary condensation and capillary evaporation 114

5.1 Percolation transition of hydration water 1215.2 Structure of water layers at hydrophilic surfaces 139

6 Role of interfacial water in biological function 151

v

vi Contents

7.1 Percolation transition of water in low-hydrated

biosystems 1657.2 Effect of hydration on the properties of biosystems 194

8 States of interfacial water in fully hydrated biosystems 215

Abundance of water on the earth and in space makes it involved inthe processes that are interesting for researches in various fields of sci-ence and technology For better understanding of these processes, it isnecessary to characterize water properties in a wide range of thermody-namic conditions Similar to other substances, water can exist in variousphase states with essentially different properties: vapor, liquid, crystallinephases, amorphous phases, glassy states Therefore, characterization ofwater properties should be based on the phase diagram, which showslocation of the phase transitions in thermodynamic space, i.e in temper-ature, pressure, density and other coordinates Major part of water on the

earth is the bulk phases: liquid phase in oceans, crystalline forms in polar

ice caps and in glaciers, vapor phase in the air Both on the earth and

in space, essential amount of water is affected by the proximity of ious boundaries A bulk three-dimensional phase may be terminated byanother phase of the same substance when two phases coexist For exam-ple, a liquid and vapor being at coexistence form liquid–vapor interface.Besides, a boundary may be formed by another substance being in crys-talline or amorphous phase, by extended surface of some macromolecularstructure, etc

var-The interfacial water, that is water near a boundary, plays an important

role in various biological, geological, technological, and other processes.For example, life is not possible without water, which exists mainly asinterfacial water in living organisms The presence of boundary breaksthe translational invariance present in a bulk system As a consequence,all system properties become local, i.e dependent on the position of thepart of fluid considered relative to the boundary The phase diagram ofany substance including water becomes much more complicated near

a boundary, in particular due to the appearance of the surface

tran-sitions Besides, the critical behavior of a fluid is strongly modified

vii

viii Preface

near a boundary, which strongly affects the fluid properties in a widetemperature range

Finite extension in one or more spacial directions makes the system

to be trapped in the pore geometry, which causes further complications

of its phase behavior The phase diagram of a system, confined betweentwo planar boundaries or within cylindrical boundary, differs from thebulk one In particular, bulk phase transitions and surface transitions aremodified due to the confinement in pores Structure of the real porousmaterials is often far from the simple slit or cylindrical geometry More-over, various porous media possess a highly disordered structure, andthis disorder further complicates the phase behavior of a confined sys-

tem On the earth, confined water may be found in various geological

materials, which possess a porous structure permeable for water siderable amount of water in living organisms is confined in cells andtheir counterparts Confined water can be often found in porous materialsused in technological processes Essential amount of confined water may

Con-be expected in comets, which presumably represent a mixture of dustand ice

To understand specific properties of interfacial and confined water atvarious thermodynamic conditions, we have to characterize the phasediagrams of water near surfaces and in various pores A wide variety

of such phase diagrams is expected, as they depend on the strength of thewater–wall interaction, heterogeneity, roughness and curvature of a wall,pore size, and shape, etc Knowledge of these phase diagrams opens a

way for the description of the water density distribution near the surfaces

and in pores, which is crucial for various structural and dynamic waterproperties Subsequent analysis of the properties of interfacial and con-fined water allows understanding of related phenomena Naturally, thephase behavior and properties of water show some regularities, whichare universal for a wide class of fluids or even lattice systems Knowledge

of these universal features allows to distinguish them from the peculiarfeatures, which are connected, first of all, with the strongly anisotropichydrogen-bonding interactions between water molecules

In Section 1 of this book, we give a brief description of the phasediagram of bulk water This includes analysis of the liquid–vapor coex-istence curve of water, a possibility to describe it in a universal way,

effect of the liquid–vapor critical point on the properties of supercritical

Preface ix

water, etc Besides, we consider some peculiar properties of a liquidbulk water, which appears at ambient and supercooled temperatures Therelation of these properties to the polyamorphism of water and to theliquid–liquid transitions of supercooled water is discussed Surface phasediagram of water is described in Section 2 Analysis of the surface tran-sitions of water starts with the brief overview of the theoretical, exper-imental, and simulation results obtained for lattices and simple fluids(Section 2.1) This is followed by the analysis of the surface transitions

of water near hydrophilic (Section 2.2) and hydrophobic (Section 2.3)surfaces Finally, the surface phase diagram of water is presented inSection 2.4 Section 3 is devoted to the surface critical behavior of water,which allows description of water density profiles near various surfaces.This analysis, presented in detail in Section 3.2, is based on the theory

of the surface critical behavior and its implementation in simple ids (Section 3.1) In Section 4, we consider the modifications of thephase diagram of water due to confinement in pores A brief overview

flu-of the general theoretical expectations and the results for simple fluids

is given in Section 4.1 Phase transitions of water in various pores arediscussed in Section 4.2 Phenomena of capillary evaporation and cap-illary condensation and characteristic properties of water in pores arebriefly described in Section 4.3 Upon adsorbing at hydrophilic surfaces,water may form mono- or bilayers (Section 5) In Section 5.1, we con-sider a percolation transition of water at hydrophilic surfaces, whichresults in the formation of water monolayer Main structural properties

of water layers at hydrophilic surfaces are described in Section 5.2 Role

of interfacial water in biology is analyzed in Section 6 In this analysis,

we show how various forms of biological activity depend on hydrationlevel, temperature, and pressure To clarify the role of interfacial water

in biological function, we consider separately low-hydrated (Section 7)and fully hydrated (Section 8) biosystems Experimental and simulationstudies of the percolation transition of hydration water in biosystemsare summarized in Section 7.1 The effect of this transition on variousproperties of biosystems is analyzed in Section 7.2 For fully hydratedbiosystems (Section 8), we analyze the effect of temperature and pres-sure on the various properties of hydration water, including connectivity

of the hydrogen bonds within the hydration shell The effect of the state

of hydration water on the properties of biosystems is discussed Finally,

x Preface

we summarize the current understanding of the properties of interfacialand confined water and formulate the open questions and controversialproblems in Section 9

In closing, we would like to express our deep gratitude to AlfonsGeiger for his hospitality, fruitful collaboration, support of our initia-tives in studies of water, and patience We have greatly profited fromcollaboration with Roland Winter, Nikolay Smolin, Aljaksei Krukau, andAlexey Mazur Our view on the properties of interfacial and confinedwater presented in this book is based on the fundamentals of the theory ofphase transitions and critical phenomena in presence of a boundary, and

we are greatly indebted to Kurt Binder, Michael Fisher, Gene Stanley,Robert Evans, Pablo Debenedetti, Gerhard Findenegg, and Josef Indekeufor elucidated and encouraging discussions, criticisms, and advices Thisbook has been made possible by financial support of our researches

by Deutsche Forschungsgemeinschaft through the Forschergruppe 436

“Polymorphism, dynamics and functions of water near molecular aries,” Schwerpunktprogramm 1155 “Molecular modeling and simula-tions in technology,” and Graduiertenkolleg 298 “Structure-DynamicsRelations in Microstructured Systems,” and by Bundesministerium furBildung und Forschung through the grant 01SF0303

bound-1 Phase diagram of bulk water

Properties of bulk fluid in distinct phase states differ so strongly thatgas, liquid, and solid states are studied in different fields of statisticalmechanics Phase state of a system may be identified based on the phasediagram Phase diagram of bulk water describes how the phase state ofwater changes with temperature and pressure It includes the liquid–vapor,liquid–solid, and solid–solid phase transitions and also hypothesized tran-sitions between amorphous (glassy or liquid) phases of supercooled water

In the solid state, water may form more than 14 crystalline forms [1].Among these ices, the hexagonal ice is the most abundant At atmosphericpressure, liquid water freezes into hexagonal ice at 273.15 K This liquid–solid transition is accompanied by the decrease in water density by about8% Other ices can be obtained by increasing the pressure above 2 kbar Inparticular, liquid water freezes into one of the high-density ices at ambienttemperature when the pressure exceeds 6 kbar

Major amount of water on the earth exists in the liquid bulk phase,which is close to the coexistence with a bulk water vapor present in theair Therefore, the liquid–vapor phase transition is of special importancefor understanding the water properties in the most of practically impor-tant situations Bulk liquid water coexists with saturated vapor in a widetemperature range of about 374 K from the freezing temperature up to theliquid–vapor critical point The liquid–vapor coexistence curve of bulk

water, i.e the temperature dependence of the densities of liquid (ρl) and

vapor (ρv) coexisting phases, is shown in Fig 1 by solid line A stablebulk vapor phase exists at the densities left to the lower density branch

of the coexistence curve Accordingly, a stable bulk liquid phase exists

at the densities right to the high-density branch of the coexistence curve.Water with a density inside two-phase region bounded by the coexistencecurve is not thermodynamically stable, and it decomposes into two coex-

isting phases with the densities ρland ρv By increasing the temperature,two coexisting liquid and vapor phases become more and more similar intheir properties until, at the critical point, all differences vanish Beyondthe critical point, only one homogeneous equilibrium water phase canexist, and all changes are continuous and smooth The coexistence pres-sure near the melting point is about 6· 10−3 of ambient pressure (1 bar),

1

2 Interfacial and confined water

Figure 1: Experimental liquid–vapor coexistence curve of water [3] (thick

solid line) Liquid–vapor coexistence curves of several water models: ST2

TIP 4P model to the extended scaling equation with leading asymptotic behaviordescribed by eq (1) is shown by thin solid line

and it increases by a factor of ∼36 000 when approaching the criticalpoint (Fig 2) The liquid–vapor critical point of water is located at the

critical temperature Tc = 647.096 K, critical pressure Pc = 22.064 MPa, and critical density ρc = 0.322 g/cm3[2, 3]

Although the liquid–vapor phase transition of bulk water is well studiedexperimentally, this is not the case for the phase transitions of interfacialand confined water, which we consider in the next sections Therefore,studies of the phase transitions of confined water by computer simula-tion gain a special importance For meaningful computer simulations, it

is necessary to have water model, which is able to describe satisfactorilythe liquid–vapor and other phase transitions of bulk water The coexistencecurves of some empirical water models, which represent a water molecule

as a set of three to five interacting sites, are shown in Fig 1 Some modeladequately reproduces the location of the liquid–vapor critical point and,

Phase diagram of bulk water 3

300

0.06 0.04 0.02

Figure 2: Left: liquid–vapor coexistence curve of water in the pressure–

temperature plane from freezing temperature to the critical point [3] Right:

curve [3, 20]

accordingly, the temperature dependence of the liquid water density athigh temperatures (SPCE model, see Fig 1) However, the same modelgives extremely low freezing temperature of water of about 214 K [4] Thecomparative analysis of various water models can be found in Refs [5–8].Generally there is no empirical water model, which adequately describesthe whole phase diagram of bulk water This is not surprising as most of thepopular water models were parameterized to fit some of the water prop-erties at some particular thermodynamic conditions Probably, the phasediagram of water in a wide thermodynamic range cannot be reproduced

by a water model with just a few sites [9] Therefore, there is an urgentneed in more adequate water models The available water models should

be used with caution, keeping in mind their limited abilities to reproducethe phase diagram and properties of water quantitatively

Upon heating, the densities of the coexisting vapor and liquid phasesapproach each other, and, asymptotically close to the critical temperature,their difference follows the universal power law:

Δρ = (ρl− ρv)/2 = B0τ β, (1)where Δρ is the order parameter of phase transition, τ = (Tc− T )/Tc

is a reduced deviation of temperature from Tc, β ≈ 0.326 [12] is a versal critical exponent, and B0 is a system-dependent amplitude Thebehavior of Δρ(τ) is shown in a double-logarithmic scale in Fig 3,

uni-where power law (1) is shown by straight dashed lines In the temperature

4 Interfacial and confined water

interval∼ 130◦below Tc(τ ≤ 0.2), Δρ(τ) closely follows the asymptotic

power law, whereas more complicate description is clearly necessary far

away from Tc In a wider temperature interval, order parameter may bedescribed by the extended scaling equation using Wegner expansion [13].The temperature dependence of the order parameterΔρ(τ) of water may

be described satisfactorily using several nonasymptotic corrections [11]:

ical density at T = Tc and changes mainly regulary with τ In the close

vicinity of the critical point, diameter of fluids shows a critical anomaly,which may behave as ∼ τ1−α [14] or ∼ τ 2β [15], or as superposition of

two contributions [16], where α ≈ 0.11 [12] Diameter of the coexistence

curve of bulk water may be described by the following equation [11]:

of the density fluctuations close to the Tc[17, 18] When approaching thecritical point, density fluctuations strongly increase and the correlation

length ξ, which describes their growth (extension), diverges as ξ = ξ0τ −ν,

where ν ≈ 0.63 [12] is a universal exponent and ξ0≈ 0.694 ˚A for water

along the coexistence curve [19] Under such circumstances, the scopic details of system structure are not important, and thermodynamicproperties depend mainly on the distance to the critical point expressed interms of temperature, pressure, or density The thermodynamic domain

micro-of universal behavior depends on the property considered, and it does notextend over more than several degrees for shear viscosity, for example.Strictly speaking, the true asymptotic range for the order parameterΔρ is

also rather narrow However, the corrections for nonasymptotic behaviornotably compensate each other, providing the behavior close to eq (1)

in the temperature range of dozens and hundreds degrees For instance,

Phase diagram of bulk water 5

theΔρ of model Lennard-Jones (LJ) fluids, which are used to describe

such simple fluids as noble gases, shows behavior close to the ∼ τ β in

the whole range of the existence of a liquid phase, i.e from Tcto freezingtemperature (see Fig 3) This “critical-like” behavior of the order param-eter in a wide temperature range is surprising because density fluctuationsseem to be negligible far away from the critical point

When approaching the critical point, all thermodynamic properties ofsystems behave in anomalous way [18, 21] In particular, isothermalcompressibility and heat capacity diverge, whereas diffusivity and otherdynamic properties show a critical slowing down An example of the

critical divergence is shown in Fig 2 for isobaric heat capacity CP ofliquid water along the coexistence curve The critical anomalies of var-

ious properties are similar when approaching Tc along the coexistencecurve or from supercritical temperatures along the critical isochore At

a given supercritical temperature, a property shows remnant of the ical anomaly, which is the largest at some pressure–density point As a

crit-result, there are a number of specific lines in T –ρ plane in supercritical

region, which emanate from the critical point and mark some specific

0.6

TIP4P water

LJ fluid experiment

Figure 3: A log–log plot of the order parameterΔρ vs reduced temperature τ

for real water [3] (thick solid line), TIP4P water model [28], and for LJ fluids