The adsorption properties of soluble, surface-chemically pure n-alkanoic acids at the air/water interface were investigated by evaluating equilibrium surface tension and surface potential versus concentration isotherms. There is no transition-like change in the adsorption isotherms of the n-alkanoic acids between n-pentanoic (C5) and n-undecanoic acid (C11). The isotherms are evaluated by the two-state approach to the adsorption equation and by the Gibbs equation. The nondissociated n-alkanoic acids' surface area demand per molecule adsorbed is not constant within the homologous series but decreases with increasing chain length until it approaches the (almost constant) value of the insoluble homologues. The limiting surface area demand per molecule adsorbed of the soluble n-alkanoic acids is compared with the corresponding data of the insoluble homologues obtained from surface pressure versus surface area isotherms as well as from crystal structure analyses. Standard free enthalpies of adsorption, limiting cross-sectional areas, and surface interaction parameters reveal a distinct effect of alternation within the homologous series. Interestingly, also the Henry constants are subject to the even/odd phenomenon. This is explained by the even and odd homologues' different surface arrangement of their terminal methyl groups with respect to the interface. The linear relationships describing the chain length dependences of the standard free enthalpy of adsorption and of the surface interaction parameter hold for chain lengths in the range 6 ≤ n C ≤ 11. n-Pentanoic acid has a somewhat different characteristic. Unlike the shorter chain homologues' adsorption, the adsorption of n-dodecanoic acid cannot be described by a monotonically proceeding process but seems to include also processes of association.

The main characteristics of Langmuir monolayers are radically changed by molecular recognition of hydrogen bond nonsurface-active species. The change in the thermodynamic, phase, and structural features by molecular recognition of dissolved uracil or barbituric acid by 2,4-di(n-undecylamino)-6-amino-1,3,5-triazine (2C11H23-melamine) monolayers is characterized by combination of surface pressure studies with Brewster angle microscopy (BAM) imaging and Grazing incidence X-ray diffraction (GIXD) measurements. Phase behavior of the 2C11H23-melamine monolayer and morphology of the condensed phase domains are changed drastically, but in a specific way, by molecular recognition of uracil or barbituric acid. The main characteristics of the interfacial system can be essentially affected by the kinetics of the recognition process. Pure 2C11H23-melamine monolayers show only small compact, but nontextured domains. The monolayers of 2C11H23-melamine-uracil assemblies develop well-shaped circular condensed-phase domains having an inner texture with alkyl chains essentially oriented parallel to the periphery and having a striking tendency to two-dimensional (2D) Ostwald ripening. The 2C11H23-melamine-barbituric acid monolayers form large homogeneous areas of condensed phase that transfer at smaller areas per molecule to a homogeneous condensed monolayer. BAM imaging of corresponding assemblies with ((CH3(CH2)11O(CH2)3)2-melamine having modified alkyl chains demonstrates the specific effect of the monolayer component. GIXD results reveal that molecular recognition of pyrimidine derivatives gives rise only to quantitative changes in the two-dimensional lattice structure. The striking differences in the main characteristics between the supramolecular species are related to their different chemical structures. Quantum chemical calculations using the semiempirical PM3 method provide information about the different nature of the hydrogen-bonding-based supramolecular structures.

Systems consisting of an amphiphilic melamine-type monolayer and a pyrimidine derivative dissolved in the aqueous subphase are good candidates for the formation of interfacial supramolecular assemblies by molecular recognition of hydrogen-bond nonsurface-active species. In the present work, the change in the thermodynamic, phase, and structural properties as a result of molecular recognition of dissolved thymine by 2,4-di(n-undecylamino)-6-amino-1,3,5-triazine (2 C11H23-melamine) monolayers is studied. The combination of surface pressure studies with Brewster angle microscopy (BAM) imaging and grazing incidence X-ray diffraction (GIXD) measurements is optimal for the characterization of the change in structure and phase behavior at the interfacial recognition process. The molecular recognition of the nonsurface-active thymine dissolved in aqueous subphase changes drastically the characteristic features (surface pressure-area isotherms, morphology of the condensed phase domains) of the 2 C11H23-melamine monolayer. It is demonstrated that the kinetics of the recognition process affect largely the main characteristics (phase behavior, morphology of the condensed phase domains) of the interfacial system. The monolayers of 2 C11H23-melamine-thymine assemblies form dumbbell-shaped condensed phase domains not yet observed in other Langmuir monolayers so far. GIXD results show that the molecular recognition of thymine causes only quantitative changes in the two-dimensional lattice structure. Complementary hydrogen bonding of two thymine molecules by one 2 C11H23-melamine molecule is concluded from the chemical structure of both components. Additional information about the nature of the hydrogen bonding on the basis of supramolecular assemblies is obtained by using the quantum chemical PM3 approximation. Energy and lengths of the hydrogen bonds of the optimized thymine-2 C11H23-melamine-thymine structure are calculated.