Influence of the temperature on the stability of an emulsion

Almost all emulsions are sensitive to heat and cold. Let us only discuss the influence exercised by elevated temperatures. A temperature increase will increase the Brownian movement of the droplets, which is promoted by the reduced viscosity of the dispersing phase: The number of collisions between the drop­lets, and the intensity of the collisions, will increase. In other words, there will be an increased tendency of the droplets to agglomerate.

Convection streams are formed in the liquid. This will also increase the number of collisions and hence the agglomeration tendency. The viscosity of the dispersing phase drops when the temperature rises. As a result, sedimentation or separation by flotation of the particles is facilitated, provided there is no significant change in the difference between the density of the dispersed and the dispersing phases. For example, if we have a water/perchloroethylene emulsion, the rate of separation of the water by flotation will increase approximately by a factor which amounts to 1.5, if the temperature goes up from 20°C to 90°C

If the temperature in the emulsion is increased, the rate at which chemical reactions take place will also be greatly accelerated. This means that emulsifying additives which tend to decompose are rapidly destroyed.

There is also the danger of vaporization of volatile components. But it must be pointed out that there are emulsions which can be brought to the boil with­out causing any trouble. Milk is a well known example.

But a temperature increase can also have positive effects. For example, the inter-facial tension is reduced when the temperature is raised.

Influence of additives on the stability of an emulsion

There is no hard-and-fast rule about the influence of additives such as dyestuffs or other auxiliaries required for dyeing, on the stability of an emulsion. This will mainly depend on the compatibility of these additives with the emulsifiers. Care must be exercised particularly in such cases where the additives have a bearing on the pH level.

Preparing of a water/perchloroethylene emulsion

First and foremost, we must of course select a suitable emulsifier. But several other factors must also be considered which can have a decisive influence on the performance of the emulsion. Among these are the sequence in which the mixture is prepared, especially the nature and time of the emulsifier addition, as well as duration and method of mixing, the temperature etc.

For w/p emulsions, it will be best to use emulsifiers soluble in perchloroethylene. These are added to the perchloroethylene at elevated temperature, and dis­solved. The temperature is then reduced, and water is slowly added while stirring, in such a manner that the water meets the emulsifier-containing perchloro­ethylene in the area of maximum turbulence.

Although there is no objection to preparing first a concentrated w/p emulsion and then to dilute it with perchloroethylene, the initial water concentration should not be too high, to prevent an inversion to the p/w type.

The emulsions we used in our trials, contained 8 g water and 5 g emulsifying additive in 1L perchloroethylene. The emulsifying additions we used were not sensitive to temperatures between 18°C and 120°C. The unbroken emulsions had a milky white appearance. To obtain data which should be as general as possible, independent of the individual dyestuff, we used emulsions free from dyestuff additions.

The distribution of droplet size in a water/perchloroethylene emulsion

The droplet size of the dispersed phase is one of the most important character­istics of an emulsion. But in almost all cases which occur in practice, there is a droplet size distribution. In the experimental determination of droplet size distribution, use is generally made of the fact that the settling velocity, or the rate at which the spherical particles rise in the liquid, depends on the radius of the particles. According to Stokes we have:

v = settling velocity or rate at which the particles rise [cm/sec]

r = radius of the droplets [cm]

Δρ - difference in density between the dispersing phase and the dispersed phase [g/cm³]

g = gravitational constant [cm/sec²]

η= viscosity of the dispersing phase [dyn sec/cm²]

The equation shows that large droplets will settle out or rise much quicker than small droplets. If the concentration or density of the dispersed phase in a plane of the measuring vessel is determined as a function of time, the droplet size distribution of the dispersed phase can be calculated. For droplets with a diameter below 1<m, the settling velocity, or the speed at which the particles rise, is very small. This has the disadvantage that the required observation time is long and that diffusion and convection phenomena exercise a disturbing influence. Sufficient droplet velocities can be achieved by effecting sedimentation or flotation in a centrifugal field of force exceeding gravitation by several orders of magnitude. The ultracentrifuge is suitable for this purpose.

For a quick survey we give a graphic representation of the droplet size dis­tribution. The share represented by the mass is plotted against the droplet size. For example, curve No. 13 in Fig. 7.4. should be read as follows: Very nearly 50% of the mass of the dispersed phase is present in the form of droplets with a diameter between 0 and 0.2 m. The greater the share (i.e. the number) of small droplets, and the more uniform they are, i.e. the steeper the curve shown in Fig. 2, the longer the life of the emulsion. We have not yet examined the influence exercised by the droplet size distribution of a w/p emulsion on the dyeing properties of the emulsion. But we can assume that in this case, too, small droplets, which should be as uniform as possible, will offer advantages. We determined the droplet size distribution at 20°C in different emulsions of the same composition, by means of the ultra-centrifuge. The results are shown in Fig. 7.4. (the measurements were carried out by Dr. [Mrs.] Lie). Although it appears that all six emulsions were prepared under identical conditions, the droplet size distributions are different. This proves that, after all, the droplet size distribution depends greatly on random changes in the conditions under which the emulsions have been prepared, e.g. on the speed at which water is added, or whether it is added to a more or less turbulent area.