Two-component monolayers IV. The excess enthalpies and entropies of mixing in the octadecanol-docosyl sulfate system

Abstract

Surface pressure-area isotherms have been determined for the octadecanol-docosyl sulfate monolayer system on 0.1 M potassium chloride solution. Four different temperatures in the range 288 to 318°K and 11 different monolayer compositions were investigated. The Goodrich method was used to calculate the free energies, enthalpies, and entropies of mixing as excesses over the values of the corresponding properties at the lowest observable surface pressure. The small negative values of the excess free energy of mixing thus obtained were found to result from the near cancellation of relatively large contributions from the positive values of the excess enthalpy and the excess entropy. These results are attributed to changes in the hydration of the monolayers during mixing