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 (
Tsub) dependence of monolayer static elasticity, es, the melting temperature (
TL) of asymmetrical C
m-C
n monolayer was estimated to be
TL = 31.7°C and 50.6°C for C
13-C
15 and C
12-C
16, respectively. Furthermore, assuming that asymmetrical C
13-C
15 can be viewed as an equimolar mixture of symmetrical 2C
13 and 2C
15, the temperature dependence of monolayers of 2C
13 and 2C
15 mixture at various ratios were also studied. As a result, all
TL values of 2C
14, C
13-C
15 and an equimolar mixture of 2C
13 and 2C
15 were almost the same. However, the variation of
TL with the molar fraction of 2C
15 (
X2C15) was remarkably different from that of solid melting point
Tm with
X2C15.
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