Colloid chemistry chapter 8 electrical property

Structure of Electrical Double Layer Helmholtz model... Structure of Electrical Double Layer... Structure of Electrical Double Layer... Structure of Electrical Double Layer... • The elec

Dr Ngo Thanh An

COLLOID CHEMISTRY Chapter 7 – Electrical properties of colloids

1 Structure of Electrical Double Layer

Helmholtz model

Gouy - Chapman model

1 Structure of Electrical Double Layer

Gouy - Chapman model

1 Structure of Electrical Double Layer

Stern model

1 Structure of Electrical Double Layer

Stern model, charge reversal

1 Structure of Electrical Double Layer

Stern model, overcharge

1 Structure of Electrical Double Layer

Stern model

1 Structure of Electrical Double Layer

Differentiate both sides of the above equation:

Differentiate

Therefore:

Finally, the charge density will be:

2 Potential of Electrical double layer

For a hyperbolic function:

C1 : constant

At at a very far distance, y = 0 and =0

Due to coshy = 1 when y=0, thus :

2 Potential of Electrical double layer

Multiply both numerator and denominator with

At x = 0, we have:

2 Potential of Electrical double layer

The above equation can be shown as follow:

Solve this equation:

In case, is pretty small (ze << kT) or  At 25oC,   25 mV:

2 Potential of Electrical double layer

Poisson – Boltzmann equation will be

Assume that

We will get:

Solve the above equation, we have

2 Potential of Electrical double layer

The boundary condition,

x = ,  C2 = 0

x = 0,  C1 = oFinally, we have:

2 Potential of Electrical double layer

3 Zeta potential

• The electric potential at the boundary of the double layer

is known as the Zeta potential of the particles and has values that typically range from +100 mV to -100 mV The magnitude of the zeta potential is predictive of the colloidal stability.

• Zeta Potential is an important tool for understanding the state of the nanoparticle surface and predicting the long term stability of the nanoparticle

3 Zeta potential

•The magnitude of the zeta potential gives an indication of the potential stability of the colloidal system

•If all the particles have a large negative or positive zeta potential they will repel each other and there is dispersion stability

•If the particles have low zeta potential values then there is no force to prevent the particles coming together and there is dispersion instability

3 Zeta potential

Zeta potential [mV] Stability

 < 30 unstable 30<  < 40 Moderately stable 40<  < 60 stable

 > 60 Very stable

3 Zeta potential

1 It does not matter whether the zeta potential is + or - its the magnitude which is important

2 the zeta potential is an indication of the surface potential, and so determines the magnitude of the electrical double layer repulsion The total interaction between particles is the sum of the electrical double layer and the van der Waals interaction

3 Zeta potential

• The word electrokinetic implies the combined effects of electrical and motion.

• Specifically, our interest centers on those phenomena in which a relative motion exists between two parts of the electrical double layer This may arise from the migration

of a particle relative to the continuous phase that surrounds it

4 Electrokinetic phenomena

4 Electrokinetic phenomena

Streaming potential

4 Electrokinetic phenomena

Electrophoresis: motion of particles

under influence of electric field

4 Electrokinetic phenomena

electro-osmosis: motion of liquid in porous body under influence of electric field;

4 Electrokinetic phenomena

5 Factors effect on Electrokinetic potential

6 Structure of micell

(potential-LỚP ION BAO QUANH : là các Ion đối (couterion) của ion tạo thế

NHÂN

MIXEN

Keo dương – Keo âm: do ion tạo thế quyết định

• Lớp ion tạo thế tạo thành bằng cách :

• Ion hóa bề mặt chất rắn

• Hấp phụ các ion trên bề mặt chất rắn: các ion phải giống ion trong

mạng tinh thể của chất rắn

• Hạt keo & Dung dịch keo trung hòa điện

Cấu tạo của hạt keo quyết định nhiều tính chất quan trọng của hệ keo: tính chất điện, khả năng hấp phụ,

tính bền

6 Structure of micell