Colloidal Dispersions 2 Flashcards
Kinetic properties of Colloids
Kinetic properties – those related to the particle motion of colloidal systems Kinetic properties of colloids – Brownian motion Diffusion Viscosity Sedimentation Osmotic pressure
Brownian Motion
Brownian motion is the erratic motion of colloid particles (under microscope or ultramicroscopy)
Brownian motion is the result from the bombardment (collision) of the particles in the disperse medium
The velocity of the particles increases with decreasing particle size
Increasing the viscosity of the medium (e.g. by adding glycerin), decreases and finally stops the Brownianmovement
Diffusion
Colloidal particle diffusion obey Fick’s first law: from high concentration to low concentration until the concentration of the system is uniform throughout
Colloidal particle diffusion is the direct results of Brownian movement
three main rules of diffusion:
(a) the velocity of the molecules increases with decreasing particle size;
(b) the velocity of the molecules increases with increasing temperature;
(c) the velocity of the molecules decreases with increasing viscosity of the medium.
Osmotic Pressure
Use van Hoff equation
see slide 8
Sedimentation Rate
Sedimentation of colloidal particles is slow
Stokes’ sedimentation equation (the same as suspension)
When d is too small ( gravity; lower size limit of particles obeying Stokes’s equation
Ultracentrifuge
useful for determining the molecular weight of polymers (proteins) and degree of homogeneity of the sample
Viscosity
Viscosity is an expression of the resistance to flow of a system under an applied stress.
Viscosity is related to volume fraction
= 0(1 + 2.5 )
: apparent viscosity of the colloid
0: viscosity of dispersion media
: volume fraction (V internal / V total)
0 and η can be determined using a capillary viscometer
Applicable for diluted dispersion of spherical particle
Viscosity of colloids is also affected by shape of the particles (e.g. Spherocolloids relatively low viscosity, whereas linear particles are more viscous
Electrical properties of interfaces
Particles dispersed in liquid media may become charged;
Selective adsorption of a particular ionic species
Ionization of groups at the surface of the particles
Charges that arise from a difference in dielectric constants between particle and medium
Electrokinetic phenomena
The movement of a charged surface with respect to an adjacent liquid phase (provide method s to obtain the zeta potential
Electrophoresis: movement of a charged particle by f an applied potential through a liquid
Electro-osmosis; opposite of electrophoresis( liquid moves relative to the charged surface
Sedimentation potential, the reverse of electrophoresis; creation of a potential when particles undergo sedimentation.
Streaming potential differs from electro-osmosisin; create the potential by forcing a liquid to flow through a bed of particles
Stability of Colloid systems
Stabilization of colloids are accomplished by:
Provide the dispersed particles with an electric charge
Surrounding each particle with a protective solvent sheath that prevent mutual adhesion.
Stability of
Lyophobic colloids
Lyophilic colloids
Stability of Lyophobic Colloid
Thermodynamically unstable
Stability of lyophobic colloids is mainly controlled by particle surface charge
Addition of electrolyte affects the stability of the system significantly
Lowering zeta potential below its critical value, and consequently lowering primary maximum – DLVO theory
Too much electrolytes may lead to irreversible aggregation (coagulation) of lyophobic colloids (which is very sensitive)
DVLO theory of colloid stability
DLVO theory describes the stability of lyophobic colloids.
The forces on colloidal particles in dispersion are due to
Electrostatic repulsion (VR)
London-type van der Waal’s attraction (VA)
Therefore the total energy potential:
VT = VR + VA
Plot total potential (VT) vs. distance (H) –
DVLO theory
Primary maximum (Potential repulsion barrier)
Lowering primary maximum may lead to irreversible aggregation of Lyophobic colloids
Primary minimum (attraction)
If primary maximum is smaller than thermo energy, particles can reach the primary minimum region, and coagulation occurs
Secondary minimum (attraction)
Secondary minimum is very important in controlled flocculation of coarse dispersions
Stability of Lyophilic Colloid
Lyophilic colloids are usually thermodynamically stable
Stability of lyophilic colloids is related to particle charge, but not very sensitive to electrolyte changes
Addition of electrolyte in moderate amount– generally stable
Stability is mainly controlled by protective solvent sheath
Salt out: coagulate by high concentration of electrolytes: due to the desolvation by electrolytes ions
Organic solvents, such as alcohol and acetone, can also decrease the stability of hydrophilic colloids and cause coagulate. It destabilized the lyophilic nature of the colloidal interface.
Mixing oppositely charged hydrophilic colloids may leads to coacervation. e.g. gelatin(+) - acacia(-)
Protection vs Sensitization
Protection: adding large amount of hydrophilic colloids to a hydrophobic colloids (as protective colloids) to stabilize hydrophobic colloidal system
Sensitization: adding a small amount of oppositely charged colloid to a hydrophobic colloid to sensitize or coagulate the particles.
Reduction of zeta potential
Reduction of ionic layer
Aggregation
(a general term): collection of particles into groups
