Adsorption Properties of Soluble, Surface-Chemically Pure n-Alkanoic Acids at the Air/Water Interface and the Relationship to Insoluble Monolayer and Crystal Structure Properties

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 areademandper molecule adsorbed of the solublen-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 ≤ 111. 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.