ABSTRACT
Binary mixed monolayers of octadecanoic acid and three related amphiphilic compounds (octadecanamide, octadecylamine, octadecylurea) have been investigated at the air/water interface by surface pressure-area (Pi-Â) isotherms and their resistances to water evaporation (r). In addition, the excess free energies of mixing (DeltaG(E)) were calculated using the Goodrich method. Both the ln r vs x and DeltaG(E) vs x plots exhibit marked deviations from linearity, indicating a high degree of miscibility and nonideal behavior of the components in the mixed films. For all of these binary systems the excess free energies of mixing have been found to be minimum for a certain composition corresponding almost to a maximum in evaporation resistances. Weak interactions were detected in octadecanoic acid/octadecanamide monolayers, whereas significant condensation effects were observed in 1 : 1 mixed films containing octadecanoic acid and octadecylamine. This is attributed to an acid-base equilibrium followed by the formation of a well-ordered arrangement of COO(-) and NH(3)(+) head groups bound to each other by electrostatic forces. The unusual polymorphism of octadecylurea monolayers could be influenced by adding small amounts of octadecanoic acid. The formation of the low-temperature phase (beta-phase) is completely suppressed, if the acid content exceeds 8 mol%. The octadecanoic acid seems to induce the formation of the high-temperature phase (alpha-phase), which is characterized by a vertical orientation of the hydrocarbon chains. Copyright 2000 Academic Press.
ABSTRACT
The interaction of dimyristoyl phosphatidylcholine (DMPC) with sodium cholate was studied by equilibrium penetration measurements. Surface pressure-area isotherms for DMPC on substrates containing various concentrations of sodium cholate are presented. From these isotherms, the saturation adsorptions of the bile salt for various surface concentrations of DMPC were calculated. It is shown that the equilibrium penetration of DMPC monolayers by sodium cholate can be satisfactorily described by the accessible-area theory of McGregor and Barnes. A plot of the surface excess concentration of cholate against the inverse of the area per DMPC molecule is linear. This plot yields a value for the adsorption into the accessible areas of the monolayer-covered surface that corresponds to the adsorption of cholate in a monolayer-free surface. The cholate molecules lie flat on the air-water interface. The effective cross-sectional area of the DMPC in the mixed monolayer is similar to the area occupied by a DMPC molecule in a close-packed liquid expanded monolayer. The composition of the penetrated monolayer at different areas per DMPC molecule was calculated. The penetration of cholate into DMPC monolayers is correlated with different results taken from the literature dealing with the incorporation of bile salts into lecithin bilayers. Copyright 1998 Academic Press.
