PROPERTIES OF COLLOIDAL DISPERSIONS:
Brownian motion- Brownian motion describes the random zigzag movement of colloidal particles. Colloidal particles are subject to random collisions with the molecules of the dispersion medium; hence, each particle pursues an irregular and a complicated zigzag path. The motion of the molecules can be observed under an electron microscope. The velocity of the particles decreases with increasing particle size and by increasing the viscosity of the medium by adding glycerin or similar substances.
Diffusion- As a result of Brownian motion, colloidal particles spontaneously diffuse from a region of higher concentration to one of lower concentration until the concentration of the system is uniform throughout. The rate of diffusion is expressed by Fick’s first law as follows:
Dm/dt = -DAdc/dx
where dm is the mass of substance diffusing in time dt across an area A under the influence of a concentration gradient dC/dx (the minus sign denotes that diffusion takes place in the direction of decreasing concentration). D is the diffusion coefficient and has the dimensions of area per unit time. The above equation formulates the three main rules of diffusion:
- With decrease in particle size, the velocity of the molecules increases.
- With increase in temperature, the velocity of the molecules increases.
- With increase in viscosity of the medium, the velocity of the molecules decreases.
Sedimentation- The velocity v of sedimentation of a spherical particle of the diameter d and the density Ƿs, falling in a liquid of density Ƿl and viscosity ƞ is given by Stokes’ law:
v = d2(Ƿs – Ƿl)g/18 ƞ
where g is the acceleration due to gravity. If the particles are subjected only to the force of gravity, then because of Brownian motion, the lower size limit of particles obeying Stokes’ law is about 0.5 μm. Therefore, for sedimentation of colloidal particles of size less than 0.5μm, a stronger force than gravity is needed and a high-speed centrifuge (ultracentrifuge) is used, which can produce a force of about 106g.
Osmotic pressure- If a solution and a solvent are separated by a semipermeable membrane, the tendency to equalize chemical potentials (and hence concentrations) on either side of the membrane results in a net diffusion of solvent across the membrane. The pressure necessary to balance this osmotic flow is termed osmotic pressure. The osmotic pressure S of a dilute colloidal solution is described by the van’t Hoff equation:
π/c = RT/M+BC
where c is the concentration of the solution, M the molecular weight of the solute and B a constant depending on the degree of interaction between the solvent and solute molecules. Viscosity- Viscosity is an expression of the resistance to flow of a system under an applied stress. An equation of flow applicable to colloidal dispersions of spherical particles was developed by Einstein:
ƞ = ƞo(1+2.5Ø)
where ƞo is the viscosity of the dispersion medium and ƞ the viscosity of the dispersion when the volume fraction of colloidal particles present is Ø.
Light scattering- When a beam of light is passed through a colloidal solution, some of the light may be absorbed, some scattered and the remainder transmitted undisturbed through the sample. The colloidal solution appears turbid due to scattering of light by the colloidal particles and the effect is known as the Faraday–Tyndall effect. The turbidity of a sol is given by the following expression:
I = Io exp-ƞ
where Io is the intensity of the incident beam, I is the intensity of the transmitted light beam, l is the length of the sample and ƞ is the turbidity.
Electrical Properties- Colloidal particles carry electrical charge of either positive or negative type. Positively charged colloidal particles include dispersions containing metal oxides such as ferric oxide colloidal dispersion. Negatively charged colloidal particles include colloidal dispersions of sulphur and kaolin. In colloidal dispersion of proteins, the charge on particle may be positive, negative or neutral depending upon the pH of the medium.
Table 1 – Comparison of properties of lyophilic and lyophobic colloids
|Property||Lyophilic colloid||Lyophobic colloid|
|Dispersion will form spontaneously and is thermodynamically stable.
Rigid chains of a polymer held in the dry state increases entropy.
Dispersions usually of macromolecules and protein.
|Dispersion will not form spontaneously and is thermodynamically unstable. Particles remain dispersed due to electrical repulsion.
Dispersions usually of metals and inorganic crystals.
|Viscosity||Usually high, at sufłciently high disperse phase concentration a gel may be formed.
Dispersed particles are solvated and usually asymmetric.
Dispersed particles are unsolvated and usually symmetric.
|Stable at low electrolyte concentrations.
Salting out at high electrolyte concentrations.
|Very sensitive to added electrolyte, leading to aggregation in an irreversible manner.|
|Stability||Controlled by charge and solvation of particles.||Controlled by charge of particles.|
Multiple choice questions (MCQs)
1.In foams, the dispersed phase and dispersion medium, respectively, are
a)Clays and gels
b)Ointments and pastes
c)Solutions of soaps and proteins
d)Suspensions and emulsions
2.The characteristic of a lyophobic sol is that
a)A fine state of subdivision of dispersed phase
b)Dispersed particles are in the size range of 1nm to 1µm
c)Interface is very extensive
d)The presence of dispersed phase in a dispersion medium
3.Hydrosol refers to a system in which the dispersed phase and dispersion medium, respectively, are
4.Electrodialysis is used, when impurities in sol are
5.Under ultra-microscope, colloid particles appear as:
6.In the determination of gold number of a protective colloid, the endpoint is indicated by
7.The semi permeable membrane used in haemodialysis is
a)Insufficient to saturate the bulk phase
b)Insufficient to saturate the interface
c)Less than CMC
d)More than CMC
8.The stability of acacia in water is due to
9.The molecular weight of dispersed solids in a colloidal system can be determined using an instrument
a)Aid in dispersion
b)Decrease the zeta potential
c)Lower the interfacial tension
d)Offer a mechanical barrier
10.When the size of particle is less than 1nm then it is called
11.The movement of colloidal particles through a liquid under the influence of electric field is called
12.The potential difference developed when particles settle under the influence of gravity is called
13.According to Schulze-Hardy rule, the precipitating power increases rapidly with the
14.When a beam of light is passed through a colloidal solution, the path of light gets illuminated. This phenomenon is known as
15.The continuous collisions between the colloidal particles and molecules of dispersion medium produce zigzag movement of colloidal particles which is known as
a)20 to 50
b)50 to 100
c)Less than 20
d)More than 100
- c) Solutions of soaps and proteins
- b)Dispersed particles are in the size range of 1nm to 1µm
- d)More than CMC
- c)Lower the interfacial tension
- c)Salting out
- d)Surface area
- a)20 to 50
1. GAURAV KUMAR JAIN – THEORY & PRACTICE OF PHYSICAL PHARMACY, 1st edition 2012 Elsevier, page no. 195-198.
2. Martins Physical Pharmacy, 6th edition 2011, page no. 717-734.