Thermodynamic and elastic fluctuation analysis of langmuir mixed monolayers composed by dehydrocholic acid (HDHC) and didodecyldimethylammonium bromide (DDAB)

Abstract

The physicochemical and elastic properties of Langmuir mixed monolayers composed by dehydrocholic acid (HDHC) and didodecyldimethylammonium bromide (DDAB) were evaluated. The experiments were performed with a constant surface pressure penetration Langmuir balance based on Axisymmetric Drop Shape Analysis (ADSA). The behavior of such amphiphiles in monolayer was clearly non-ideal and would be seriously influenced by the amount of HDHC molecules present. The presence of bile acid type molecules caused the monolayer be more condensed (Ac diminution) and the intermolecular attractive interactions be stronger (high epsilon0 values). This fact would be related to H-bond formation between water and carboxilate and carbonile groups in the cholesteric ring and agreed with the existence of laterally structured microdomains at the monolayer (determined by the analysis of the first virial coefficient, b0 < 1, of the state equation). The miscibility of both surfactants in the monolayer, their high bulk hydrophobicity (Pic > 35 mJ m-2) just with the obtained negative values of the free energy of mixing DeltaGmix, and the excess second virial coefficient (b1)E obtained allows us to infer that net attractive interaction existed between HDHC and DDAB molecules at the monolayer and that mixed systems would be able to be use in the formulation of supramolecular assemblies.