Monolayer behavior of asymmetrical ester-type tartaric gemini amphiphiles

The ester-type asymmetrical tartaric gemini amphiphiles (C(m)-C(n), 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 C(m)-C(n) 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 (T(sub)) dependence of monolayer static elasticity, es, the melting temperature (T(L)) of asymmetrical C(m)-C(n) monolayer was estimated to be T(L) = 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 T(L) with the molar fraction of 2C₁₅ (X(2C15)) was remarkably different from that of solid melting point T(m) with X(2C15).