Spread mixed monolayers of deoxycholic and dehydrocholic acids at the air–water interface, effect of subphase pH. Characterization by axisymmetric drop shape analysis Original Research Article

Bile acids (deoxycholic and dehydrocholic acids) spread mixed monolayers behavior at the air/water interface were studied as a function of subphase pH using a constant surface pressure penetration Langmuir balance based on the Axisymmetric Drop Shape Analysis (ADSA). We examined the influence of electrostatic, hydrophobic and hydration forces on the interaction between amphiphilic molecules at the interface by the collapse area values, the thermodynamic parameters and equation of state virial coefficients analysis. The obtained results showed that at neutral (pH = 6.7) or basic (pH = 10) subphase conditions the collapse areas values are similar to that of cholanoic acid and consistent with the cross-sectional area of the steroid nucleus (≈ 40 Å2). The Gibbs energy of mixing values (ΔGmix < 0) and the first virial coefficients of the equation of state (b0 < 1) indicated that a miscible monolayer with laterally structured microdomains existed. The aggregation number (1 / b0) was estimated within the order of 6 (pH = 6.7) and 3 (pH = 10). At pH = 3.2, acidic subphase conditions, no phase separation occurs (ΔGmix < 0) but a high expanded effect of the monolayer could be noted. The mixed monolayer behavior was no ideal and no aggregates were formed (b0 ≥ 1). Such behavior indicates that the polar groups of the molecules interacts each other more strongly by repulsive electrostatic forces than with the more hydrophobic part of the molecule.