Molecular area dependences of monolayers at the air/water interface

Abstract

The quantum chemical approach and a thermodynamic model for amphiphilic monolayers are used to find the temperature dependence of the area per amphiphile molecule in a monolayer at the onset of liquid-expanded – liquid-condensed phase transition (A_c). Quantum chemical calculations within the semiempiric PM3 method for clusterization thermodynamics and structure of surfactant dimers and trimers are used, as well as for assessment of their molecular area in an aggregate. Calculations are done for seven amphiphile classes: saturated and ethoxylated alcohols, saturated and unsaturated carboxylic acids, α-hydroxylic acids, N-acyl-substituted alanine and dialkyl-substituted melamine that are well experimentally explored. Calculations reflect the experimental data and show that a temperature increase leads to decrease of A_c for an amphiphile with a given alkyl chain length, and vice versa with a chain length increase at a fixed temperature the A_c value grows. It was shown that as the temperature increases the increment of A_c per methylene unit of the hydrocarbon chain becomes less significant. The average values of the slope reflecting the change of A_c per 1 ​°C for saturated and ethoxylated alcohols, saturated, cis- and trans-unsaturated carboxylic acids, α-hydroxylic acids, N-acyl-substituted alanine and dialkyl-substituted melamine are 0.57, 1.32, 1.14, 1.26, 1.15, 0.66, 1.07 and 0.67 ​Ų/°С, respectively. These data are quite similar to the experimental values of $-\frac{d{A}_c}{dT}$ for tetradecanoic acid, dipalmitoyl-phosphatidylcholine and dipalmitoyl phosphatididic acid. The proposed approach can be used as a predictive tool for amphiphiles lacking experimental data.