7.7 Temperature

A change in temperature causes changes in the interfacial tension between the two phases，in the nature and viscosity of the interfacial film，in the relative solubility of the emulsifying agent in the two phases，in the vapor pressures and viscosities of the liquid phases，and in the thermal agitation of the dispersed particles. Therefore，temperature changes usually cause considerable changes in the stability of emulsions; they may invert the emulsion or cause it to break. Emulsifying agents are usually most effective when near the point of minimum solubility in the solvent in which they are dissolved since at that point they are most surface-active. Since the solubility of the emulsifying agent usually changes with temperature change，stability of the emulsion usually also changes because of this. Finally，anything that disturbs the interface decreases its stability，and the increased vapor pressure resulting from an increase in temperature causes an increased flow of molecules through the interface，with a resulting decrease in stability.