32 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 0,02 0.01 ¸ 0.00 0.0 [] 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Cornposi[ion Fraction Figure 6. With a higher initial emulsifier concentration ([•), the liquid crystalline film (Figure 5A) caused a strong and immediate evaporation retardation. Lower initial concentration (*) gave liquid crystal only peripherically (Figure 5B), and the retardation was modest. called the transport resistance, r, where 1/r is dimensionally equivalent to the diffusion coefficient divided by the path length (10,11). This equation is derived from Fick's first law, where the quantity of material diffusing through the area A, per unit of time, is given by: = -A. o.zX T where AC is the difference of concentration of the diffusing component, D is the diffusion coefficient, and Ax is the thickness of the section. In this case, the thickness of the section has to be replaced by the path length, since in a lameliar phase, the components can diffuse only via a tortuous path, 8 (12,13). We finally have: = -A ß ß AC where 1/r is a function of D/8.

EMULSION SYSTEM EVAPORATION 33 -2.0 -2.4 I I 0 5 10 15 20 lime / minu[es Figure 7. The increase of emulsifier concentration due to the evaporation of solvents was a logarithmic function of time. For pure water, r is of the order of 2 ß 10-3 s ß cm- • whereas with a diffusion , coefficient three orders of magnitude smaller, along with a tremendous increase in the path length, one can expect slow evaporation through a lameliar film (14). This is indeed what we observed. However, this simplistic model must be applied with caution, since the formation of the lameliar film has not been proven to be uniform, and hence the switch from an evaporation accompanied by convection to a diffusion- controlled one does not take place at the same time for all the surface exposed. SUMMARY The phase transition in a cosmetic emulsion system was followed during evaporation of water and the solvent. The result demonstrated the transition to more ordered structures with a reduced amount of water and organic solvents. REFERENCES (1) M. M. Breuer, in Encyclopedia of Emulsion Technology, Vol. 2, P. Becher, Ed. (Marcel Dekker, New York, 1983), p. 385.