Novel Results on the Adsorption Properties of n-Alkyldimethylphosphine Oxides at the Air/Water Interface

Abstract

The adsorption parameters of alkyldimethylphosphine oxides (from n-hexyl- to n-tetradecyl) at the air/water interface were calculated from equilibrium surface tension (σ e ) versus concentration (c) isotherms at 295 K. To guarantee absence of measurable surface-active impurities in the adsorption layers, stock solutions of the surfactants were purified until the state of surface-chemical purity was achieved. The evaluation of the σ e vs c isotherms was performed by applying a two-state approach to the surface equation of state. All of them are described in the best way by ideal surface behavior. The critical micelle concentrations (cmc) and the calculated adsorption parameters, i.e., standard free energy of adsorption (ΔG o ad ) and limiting surface area demand per molecule adsorbed (A min ), reveal distinct effects of alternation (even/odd phenomena). The A min values decrease with rising n-alkyl chain length. The results are discussed with respect to donor-acceptor interactions and surface conformational changes in the adsorption layer. As these surfactants' adsorption reveals ideal surface behavior, the data may serve as a reliable basis for modeling adsorption.