Colloid chemistry chapter 2 update

For the molecules at the surface at the liquid/air interface  Only attractive cohesive forces with other liquid molecules which are situated below and adjacent to them..  They can d

Dr Ngo Thanh An

COLLOID CHEMISTRY Chapter 2 - Surface phenomena

1 Surface tension

Surface tension values

1 Surface tension

i i

A

G

dA dN

VdP SdT

dG

, ,

• The surface tension is the increase in the Gibbs free energy per increase in surface area at constant T, P and Ni

1 Surface tension

1 Surface tension

• In the liquid state, the cohesive forces between adjacent molecules are well developed.

For the molecules in the bulk of a liquid

• They are surrounded in all directions by other molecules for which they have an equal attraction

For the molecules at the surface (at the liquid/air interface)

 Only attractive cohesive forces with other liquid molecules which

are situated below and adjacent to them

 They can develop adhesive forces of attraction with the molecules

of the other phase in the interface

 The net effect is that the molecules at the surface of the liquid

experience an inward force towards the bulk of the liquid and pull the molecules and contract the surface with a force F

-Cohesive force is the force existing between like mole cules.

Adhesive force is the force existing between unlike molecules.

2 Surface and interfacial tension

• To keep the equilibrium, an equal force must be applied to oppose the inward tension in the surface.

• Thus SURFACE TENSION [ γ ] is the force per unit length that must be applied parallel to the surface so as to counterbalance the net inward pull and has the units of dyne/cm.

• INTERFACIAL TENSION is the force per unit length existing at the interface between two immiscible liquid phases and has the units of

dyne/cm.

2 Surface and interfacial tension

• The work W required to create a unit area of surface is known as

SURFACE FREE ENERGY/UNIT AREA (ergs/cm2) (1 erg =

dyne.cm)

• Its equivalent to the surface tension γ

• Thus the greater the area A of interfacial contact between the

phases, the greater the free energy

W = γ x ∆A

For equilibrium, the surface free

energy of a system must be at a

minimum

Thus liquid droplets tend to assume a

spherical shape since a sphere has

the smallest surface area per unit

volume

3 Surface free energy

For a single liquid (cohesion):

For two different liquids (adhesion):

AV AA

W  2 

AB BV

AV AB

) ,

( y x

f

f 

dy y

f dx

x

f df

x y

y

f

x

Review

i i

A G

dA dN

VdP SdT

dG

, ,

We have T S = Q > 0, because the area enlargement need

adsorbing heat This mean that  0





A Q

Finally,

Contact angles

Schematics of different wetting regimes: (a) Young’s model, (b) Wenzel model, and (c) Cassie model.

Wettability

Applications of

superhydrophobic surfaces

 Anti-adhesion and self-cleaning

- Self cleaning glasses

- Self cleaning textile

 Anti-biofouling applications

 Corrosion inhibition

 Drag reduction

 Surface roughness increases hydrophobicity

 Superhydrophobic if contact angle > 150°

 Superhydrophobicity leads to self-cleansing

Wettability

 Often forces that tend to spread a liquid

(interactions with solids or gas pressure

in a bubble) are balanced by surface

tension that tends to minimize interfacial

area, resulting in a curved liquid-gas

interface

 Particularly in porous media, the

liquid-gas interface shape reflects the “need” to

form a particular contact angle with solids

on the one hand, and a tendency to

minimize interfacial area within a pore

 A pressure difference forms across the

curved interface, where pressure at the

concave side of an interface is larger by

an amount determined by interfacial

curvature and surface tension

 These relationships between interfacial

curvature and pressure difference are

given by the Young-Laplace equation

DP = Pliq-Pgas When the interface curves into the gaseous phase (water droplet

in air)

DP = Pgas-Pliq When the interface curves into the liquid (air bubble in water, water in a small glass tube)

Curved Liquid-Vapor Interfaces

Curved interfaces and capillarity

For pendular rings between spherical particles (sand grains) the pressure difference is given as:

1

1

R R

Curved interfaces and capillarity

Thompson Kelvin equation

Thompson Kelvin equation

● When a small cylindrical capillary is dipped in a water reservoir a meniscus is formed in the capillary reflecting balance between contact angle and minimum surface energy.

● The smaller the tube the larger the degree of curvature, resulting in larger pressure differences across the air-water interface

● The pressure in the water is lower than atmospheric pressure (for wetting fluids) causing water to rise into the capillary until this upward capillary force is balanced by the weight of the hanging water column (equilibrium)

Capillary rise model

• Force balance can describe magnitude

of capillary rise.

Capillary rise:

r g

2

2

r h

r g

F W

h r g

Vg mg

Capillary Rise – Example 1

Problem Statement:

Calculate the height of capillary rise in a glass capillary tube having a

radius of 35 µm The surface tension of water is assumed to be 72.7 mN/m.

r g

3 1000 81

9

) 0 cos(

0727

1 s

m kg m

m

kg s

m

m

N

2 2

3 2

84

14 ]

m [

h





Capillarity (and Adsorption)

in Soils

Adsorption and Capillarity in Soils

The complex geometry of the soil pore space creates numerous

combinations of interfaces, capillaries, wedges, and corners around which water films are formed resulting in a variety of air water and solid water contact angles.

Water is held within this complex geometry due to capillary and adsorptive surface forces.

Due to practical limitations of present measurement methods no distinction is made between adsorptive and capillary forces All individual contributions are lumped into the matric potential

10 m

Bonus………… !!!!!!!!!

Bonus 1

Bonus 2

Bonus 4

Bonus 6

Bonus 6

Bonus 7

Bonus 8