1. Introduction to Surface and Colloid Chemistry

Size of colloidal molecule

1 nm to 1 μm

Size of macromolecule

0.1 nm to 10 nm

Colloidal dispersion

A colloidal dispersion is a mixture in which one phase is evenly distributed in the other phase. Two phases are mixed. Colloidal dispersions are meta-stable – stability is a kinetic phenomenon. They will sooner or later collapse. 

Type of dispersion: Liquid in a Gas

(Liquid) aerosol (e.g mist)

Type of dispersion: Solid in a Gas

(Solid) aerosol (e.g smoke)

Type of dispersion: Gas in a Liquid

Foam (whipped cream, Guinness)

Type of dispersion: Liquid in a Liquid

Emulsion (e.g mayonnaise)

Type of dispersion: Solid in a Liquid

Suspension (sol) (e.g paint)

Type of dispersion: Gas in a Solid

Solid foam (e.g styrofoam)

Type of dispersion: Liquid in a Solid

Solid emulsion (e.g butter)

Type of dispersion: Solid in a Solid

Solid suspension (e.g ruby glass)

Difference between a solution and a dispersion

Solutions

• Molecules that are dissolved through interaction with solvent molecules

• Solutions are controlled by inter-molecular interaction and can be thermodynamically stable

Dispersions

•  Dispersions are controlled by inter-molecular interaction (in the solution) as well as colloidal forces between interfaces

•  Dispersions are practically never thermodynamically stable

• Particles (and aggregates of molecules) are distributed (dispersed) in a continuous phase

Viscous force and gravitational force

Viscous drag force tries to counteract gravitational force

Stokes law

Add polymer →increased viscosity → reduced rate of sedimentation  

Reduced size of particle → reduced rate of sedimentation 

v_sed=rate of sedimentation [m/s]

g=gravitational field strength [m/s²]

r=radius of the spherical particle [m]

ρ_p=density of the particle [kg/m³]

ρ_f=density of the fluid [kg/m³]

η=dynamic viscosity of continuous phase [kg/(m•s)]

Assumptions for Stokes law

There are spherical particles and no interaction between particles.

Does sedimentation or kinetic energy affect velocity of small particles more?

For small particles for velocity from kinetic energy affects more movement: v_sed << v_kin

Brownian motion

Random movement of small particles due to temperature-induced density fluctuations in the surrounding medium. The rate depends on size, temperature and time. 

dx/dt=rate of random displacement [m/s]

x=random displacement  [m]

D= diffusion coefficient [m²/s]

t=time [s]

Stokes-Einstein

D= diffusion coefficient [m²/s]

k= Boltzmann constant (J/K)=1.38×10⁻²³ J/K

T= temperature (K)

η=dynamic viscosity of continuous phase [Pa•s]

d=hydrodynamic diameter of particle [m]

Monodisperse size distribution

only one size is present

Polydisperse size distribution

many different sizes are present. Polydispersion is more common than monodispersion

Surface tension

• The surface tension (surface energy) is a measure of the molecular interactions in a fluid.

• Surface tension (energy) γ is the work to generate the unit area of a new surface.

•  Surface tension is the work demanded for creation of a surface towards air

Surface tension of different liquids

Surface tension depends on the molecular interaction in and between the phases.

Why does mercury has extremly high surface tension?

Mercury has a very strong bond → extremely high surface tension (will not spread out) 

Does octane has higher surface tension than water?

No, water has higher

Laplace pressure

The Laplace pressure is the pressure difference between the inside and the outside of a curved surface that forms the boundary between two fluid region

Young-Laplace equation for a spherical droplet/bubble

The relation between pressure and surface tension is generalized in the Young-Laplace equation.

ΔP_L=Laplace pressure [Pa=N/m²]

γ=surface tension [N/m]

r=radius of droplet [m]

How does a change in chemsitry of surface affect surface tension?

Change in chemistry of surface (liquid or air for example) →  change in surface tension

Spreading

Spreading occurs if the surface energies of the 2 created surfaces γ_sl + γ_lv is < initial surface energy γ_sv

– S ≥ 0 : spreading occurs

– S < 0 :the liquid forms a finite lens

γ_vs is surface tension between solid and vapor, γ_vl is surface tension between liquid and vapor and γ_ls is surface tension between solid and liquid

Hydrophobic surface: γ (sl)+ γ (lv) > γ (sv)

Hydrophilic surface (spreading occurs): γ (sl)+ γ (lv) < γ (sv)