Novel results on the adsorption properties of n-alkyldimethylphosphine oxides at the air/water interface

The adsorption parameters of alkyldimethylphosphine oxides (from n-hexyl- t o n-tetradecyl) at the air/water interface were calculated from equilibrium surface tension (sigma(e)) versus concentration (c) isotherms at 295 K. To guarantee absence of measurable surface-active impurities in the adsorptio n layers, stock solutions of the surfactants were purified until the state of surface-chemical purity was achieved. The evaluation of the sigma(e) vs c isotherms was performed by applying a two-state approach to the surface e quation of state. All of them are described in the best way by ideal surfac e behavior. The critical micelle concentrations (cmc) and the calculated ad sorption parameters, i.e., standard free energy of adsorption (Delta G(ad)d egrees) and limiting surface area demand per molecule adsorbed (A(min),), r eveal distinct effects of alternation (even/odd phenomena). The A(min), val ues decrease with rising n-alkyl chain length. The results are discussed wi th respect to donor-acceptor interactions and surface conformational change s in the adsorption layer. As these surfactants' adsorption reveals ideal s urface behavior, the data may serve as a reliable basis for modeling adsorp tion.