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 (A_m), the partial molecular area (PMA), surface excess Gibbs energy (ΔG_(ex)), interaction parameter (I_p) 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 A_m isotherms as a function of mole fraction of Sts (X_st) in the DC/St(s) mixtures at discrete surface pressures. Notable 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_C¹) accompanied by compression and (iii) ΔG_(ex) as well as I_p 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.