Monolayers (Langmuir films) behavior of multi-component systems composed of a bile acid with different sterols and with their 1:1 mixtures

Different physicochemical properties of Langmuir films (monolayers) composed of 10 mixed systems of a bile acid, deoxycholic acid (DC) with various plant sterols, such as stigmasterol (Stig), β-sitosterol (Sito) and campesterol (Camp) and a stanol, cholestanol (Chsta) in addition to an animal sterol, cholesterol (Ch) [these sterols and Chsta are abbreviated as St] and DC with 1:1 St mixtures; (Ch + Chsta), (Ch + Stig), (Stig + Chsta), (Ch + Sito) and (Ch + Camp) on the substrate of 5 M aqueous NaCl solution (pH 1.2) at 25 °C, were investigated in terms of mean surface area per molecule (Am), the partial molecular area (PMA), surface excess Gibbs energy (ΔG(ex)), interaction parameter (Ip) as well as activity coefficients (f1 and f2) in 2-D phase of each binary (or ternary) component system and elasticity (Cs−1) of formed films; these were analyzed on the basis of the respective surface pressure (π) versus Am isotherms as a function of mole fraction of Sts (Xst) in the DC/St(s) mixtures at discrete surface pressures. e findings are: (i) all the binary component systems did form patched film type monolayers consisting of (a) DC-dominant film solubilizing a trace amount of St molecules and (b) St dominant film dissolving a small amount of DC molecules, (ii) DC in 2-D phase exhibited a transition from LE film to LC film at a constant pressure ( π C 1 ) accompanied by compression and (iii) ΔG(ex) as well as Ip was found to be greatly dependent on (a) the combinations of DC with different St species and (b) to be markedly varied by a difference in mixing ratio of DC to Sts. Compressibility (or elasticity) analyses and fluorescence microscopy images could support the above findings as well as interpretation.