Quality control of emulsions

Evaluation is a necessary step in any formula development since it enables the development scientist to know whether his product possess the projected qualities or not. In biphasic products like emulsions the stability of physical structure is of paramount significance. The following evaluatory tests are usually done on the emulsions.

(a) Phase separation :

The rate and degree of phase separation in an emulsion can be easily determined by keeping a certain amount in a graduated cylinder and measuring the volume of separated phase after definite time intervals. The phase separation may result from creaming or coalescence of globules and these possibilities should be kept in mind.

Coalescence is a definite sign of instability and it generally becomes apparent within a month in instable formulations. The phase separation test can be accelerated by centrifugation at low/moderate speeds. One can at best expect a mixture of creamed and coalesced particles and in such a situation it may be difficult to make correct interpretations. High speed centrifugation (2,00,000 g) can be used to test the strength of interfacial films. At such speeds poor emulsions would crack up completely while only those with tough interfacial films may survive.

(b) Globule size :

Growth in the globule size after the preparation of an emulsion is an indication of its physical instability. Hence, size of globules and their size distribution is generally ascertained in an emulsion over a certain time span. In the beginning there generally occurs some change in the size but it may be due to inadequate surface coverage of globules by surfactants. Thereafter in a good formulation the globule size gets stabilized. If the globule size continues to grow it is indicative of a poor product which may get completely disorganized with time. The globule size is measured by microscopic methods or by electronic devices such as coulter counters. In either of these two techniques the original product has to be suitably diluted before estimations. This may introduce errors because of incomplete deflocculation or new patterns of flocculation in diluted emulsions. Hence, while making a judgement about the stability of the emulsion this aspect should be kept in mind.

(c) Flow properties: 

The rheological characteristics of an emulsion system depend upon globule size, emulsifier and its concentration, phase volume ratio etc., and hence determination of its flow characteristics could serve as an index of its stability. Therefore study of flow behavior of an emulsion is an integral part of its evaluation. Since most emulsions have non-Newtonion flow results may tend to differ if only one point viscosity measurements are made. Many emulsions exist in a flocculated state and disturbance caused due to measurements results in structure which may be contributing to their consistency. Use of a heliapath attachment with Brookfield viscometer helps in detection of creaming tendency and hence it is advisable to study rheological properties over extended periods of time can help in prediction of their long-term behaviors. Many emulsions show change in consistency with time which follows linear relationship when plotted on a log-log scale over a number of ten fold time intervals.

(d) Effect of thermal stresses: 

It is usual to evaluate the stability of an emulsion by subjecting it to high and low temperature in alternating cycles. The samples are first exposed to 60°C for a few hours and then to 0 to 4°C. Such exposures are repeated a number of times and emulsion stability assessed after each cycle. Such thermal stresses are in fact hard treatments and emulsions that survive them can be deemed to be really robust. It is to be noted that these thermal stresses cannot be directly correlated to actual shelf life situations. However, the extremes of temperatures may influence partitioning of the emulgent between the two phases or cause melting or crystallization of some fatty components and this gives an indirect clue to emulsion stability. The effect of thermal stresses on different emulsions do not follow any set pattern and vary from formulation to formulation. It has also not been possible to compute emulsion stability at room temperatures from its behavior at high temperatures. This evaluation has its significance in the fact that no formulator can possibly wait for years to evaluate shelf life of his product and hence he subjects the same to magnified artificial stresses in order to get some feel about structure stability. He may in this process accumulate valuable data which ultimately enables him to differentiate between “good” and “bad” products.