Abstract: In this paper, a theoretical analysis on the relation between order parameters and activity coefficients of mixed model membranes is presented to overcome its empiricism in the previous studies- On the basis of statistical mechanics of order parameter, our discussion employs the mean-field approximation and leads, by means of classic thermodynamics, to a theoretical expression which demonstrates the concerned relation.Then a primary test that calculates the activity coefficients of POPCof system cholesterol/POPC (1-palmitoyl-2-oleoyl-sn-glycero-phosphatidylch by the proposed expression is carried out- Calculated values of activity coefficients for two concentrations at five temperatures are obtained.Com-pared to the.literature method, this work can better represent the behavior that activity coef-ficient changes with respect to temperature.In addition, the result shows that activity coeffi-cients are not sensitive to temperature within the given range of temperature.This indicates that the effect of temperature upon molecular interaction in the lipid bilayer of cholesterol/POPCis not s'ignificant.

Key words: Lipid bilayer, Mixed model membrane, Mean-field approximation, Order parameter, Activity coefficient