Quality control of suspensions

A number of procedures have been suggested in the past for evaluating the physical stability of suspensions. Some of these are empirical in the sense that they have no mathematical base. Some methods currently being used are so drastic that they destroy the structure of the suspension. The methods used may be categorized as:

(a) Sedimentation methods:

 Since formation of the sediment and its redispersibility are two features related to the overall acceptability of suspensions, many evaluation procedures centre around sedimentation properties.

The simplest procedure for evaluation is to keep a measured volume of the suspension in a graduated cylinder in an undisturbed state for a certain period of time and note the volume of the sediment, which is expressed as ultimate height (hu). This, in relation to the initial volume of the suspension, (Ho) is expressed as sedimentation ratio. It should, however, be noted that sedimentation ratio (hu/Ho) is dependent on time and it is likely to vary at different periods of time. The sedimentation ratios at different periods of time can be plotted against time abscissa to give a curve that indicates the sedimentation pattern on storage. If the curve is horizontal to time axis it indicates a better suspension. However, if it steeps down it indicates a poor formulation. Sometimes it may be useful to dilute original suspension to known extents before determination of sedimentation ratios.

(b) Rheological methods:

 Evaluation of rheological behaviour of the suspension can help in predicting the settling pattern and can also provide clues to vehicle particle structure. In carrying out rheological study of a suspension generally low shear rates are employed and samples are evaluated undisturbed. By use of a Brookfield viscometer with T-spindle the rheological reatures at different depths in a sample can be studies. Data collected on samples stored for various periods of time can give useful information about the stability of the suspension.

(c) Electrokinetic methods:

 As has been discussed in the chapter on emulsions, the surface electric charge or zeta potential is instrumental in deciding the stability of disperse phase systems. Certain zeta potentials produce more stable suspension because of controlled flocculation. Hence, determination of zeta potential of a suspension can give valuable clues to its stability. The migration velocities of particles can be measured by electrophoretic methods and zeta potential calculated from it.

(d) Micromeritic methods: 

In the ultimate, the stability of a suspension is inter-related to the size of particles constituting its disperse phase. A growth in the particle size is a pointer towards its instability since such an occurrence can ultimately result in the formation of lumps or cake destroying the physical structure of a suspension and rendering it useless. Hence, an appreciation of change in particle size with passage of time can provide an insight into the stability aspect of a suspension. Changes in absolute particle size, particle size distribution, crystal habit etc. can be worked out by microscopy, coulter counter etc.