2013 Volume 62 Issue 6 Pages 371-380
The ester-type asymmetrical tartaric gemini amphiphiles (Cm-Cn, where m and n are the number of carbon atoms of hydrophobic alkanoyl group, m+n=28) bearing two carboxyl groups and two different alkanoyl groups were prepared from L-tartaric acid, and the pressure-area (π-A) isotherms for a series of asymmetrical tartaric gemini amphiphiles were studied.
The π-A isotherms of asymmetrical Cm-Cn monolayers were classified into two groups. Group 1: The asymmetry was small (n/m <1.55), and a phase transition of the monolayer from the liquid-expanded to the liquid condensed state, and a subsequent transition to solid phases were observed. Group 2: The asymmetry was large (n/m >1.8), and only liquid-expanded state of the monolayer film was observed. Based on the subphase temperature (Tsub) dependence of monolayer static elasticity, es, the melting temperature (TL) of asymmetrical Cm-Cn monolayer was estimated to be TL = 31.7°C and 50.6°C for C13-C15 and C12-C16, respectively. Furthermore, assuming that asymmetrical C13-C15 can be viewed as an equimolar mixture of symmetrical 2C13 and 2C15, the temperature dependence of monolayers of 2C13 and 2C15 mixture at various ratios were also studied. As a result, all TL values of 2C14, C13-C15 and an equimolar mixture of 2C13 and 2C15 were almost the same. However, the variation of TL with the molar fraction of 2C15 (X2C15) was remarkably different from that of solid melting point Tm with X2C15.
