Two-Dimensional Miscibility Behavior of Two Chemically Similar Amide Amphiphiles

• Published in: Journal of physical chemistry. C 2012-03, Vol.116 (10), p.6268-6274
• Main Authors: Stefaniu, C, Brezesinski, G, Vollhardt, D
• Format: Article
• Language: English
• Subjects: Condensed matter: structure, mechanical and thermal properties; Exact sciences and technology; Physics; Structure and morphology; thickness; Surfaces and interfaces; thin films and whiskers (structure and nonelectronic properties); Thermal properties of condensed matter; Thermal properties of crystalline solids; Thermodynamic properties; Thin film structure and morphology
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/jp211610k

The monolayer characteristics of two chemically similar amphiphiles 3-hydroxy-N-tridecyl propanoic acid amide (HTPA) and tetradecanoic acid-(2-hydroxyethyl)amide (TDAHA) and their selected mixtures are studied. Despite the slight structural difference (the position of the two substituents at the acid amide group), the pure components reveal large differences and peculiarities in the surface pressure–area (π-A) isotherms, the monolayer morphologies, and structures. Therefore, their miscibility behavior in monolayers is of special interest. At low temperatures (T ≤ 10 °C), the π-A isotherms of the pure components show a striking second critical point accompanied by an abrupt change of important 2D lattice parameters, indicating the existence of a second phase transition between two condensed phases. This second phase transition is strongly temperature-dependent for TDAHA, nearly temperature-independent for HTPA, and not occurring in the investigated mixtures. The results of Brewster angle microscopy and grazing incidence X-ray diffraction support ideal miscibility of HTPA-TDAHA monolayers already suggested by the linear relationship between the main phase transition pressure and the mole fraction. The lattice parameters of the mixed TDAHA-HTPA monolayers measured for three mole fractions (0.25, 0.5, and 0.75) at 5 °C are compared and discussed. The tilt angle in the mixed TDAHA-HTPA monolayers passes through a minimum, which is connected to the largest cross-sectional area and the smallest entropy change during the main LE/LC (liquid expanded/liquid condensed) transition. An HTPA-TDAHA phase diagram that illustrates the transitions between the different phases is proposed.