Surface behaviour of C5, C6, C7 and C8 lecithins at the aqueous solution/air interface

Surface tensions of aqueous solutions of diacylphosphatidylcholines (diCnPC) with n=5, 6, 7 and 8 have been measured at 25 °C and pH 3.2, 7.4 and 10.0, respectively. From these data critical micelle concentrations (cmc), Gibbs energies of micellization (ΔG°m), of adsorption (ΔG°ads), of transfer from micelles to surface (ΔG°trans) and surface area minima per molecule have been obtained. The increase of the cmc with the pH was more significant for diC5PC, decreasing conform the carbon atoms number in the acyl chain raise. Surface areas decrease with increasing carbon number. The micellization is most favorable at low pH. The contributions to ΔG°m and ΔG°ads were of 1.8RT and 2RT per mol of CH2, respectively. The variation of ΔG°trans indicates that the processes of micellization and adsorption are most favorables conform the acyl chain increases.