Molecular interactions in mixed monolayers of octadecanoic acid and three related amphiphiles

Binary mixed monolayers of octadecanoic acid and three related amphiphilic compounds (octadecanamide, octadecylamine, octadecylurea) have been investi gated at the air/water interface by surface pressure-area (Pi-(A) over cap) isotherms and their resistances to water evaporation (r). In addition, the excess free energies of mixing (DeltaG(E)) were calculated using the Goodr ich method. Both the In r vs x and DeltaG(E) vs x plots exhibit marked devi ations from linearity, indicating a high degree of miscibility and nonideal behavior of the components in the mixed films. For all of these binary sys tems the excess free energies of mixing have been found to be minimum for a certain composition corresponding almost to a maximum in evaporation resis tances. 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 attrib uted to an acid-base equilibrium followed by the formation of a well-ordere d arrangement of COO- and NH3+ head groups bound to each other by electrost atic forces. The unusual polymorphism of octadecylurea monolayers could be influenced by adding small amounts of octadecanoic acid. The formation of t he low-temperature phase (beta -phase) is completely suppressed, if the aci d content exceeds 8 mol%. The octadecanoic acid seems to induce the formati on of the high-temperature phase (alpha -phase), which is characterized by a vertical orientation of the hydrocarbon chains. (C) 2000 Academic Press.