Permeability and thermodynamics study of quaternary ammonium surfactants-phosphocholine vesicle system

Quaternary ammonium compounds (QACs) are recognized as membrane active agents widely used as biocides. The main purpose of this work was to investigate the influence of the QAC head group and acyl chain length on their permeability-perturbing power and on their affinity for lipidic membranes. Permeability perturbations were assessed by measuring the release of calcein entrapped inside vesicles. The affinity of QACs for bilayers was investigated by isothermal titration calorimetry (ITC). QACs bearing C(16) chain were found to be more efficient to decrease the membrane permeability than their C(12) analogues. On the other hand, the chemical nature of the ammonium head group has practically no influence on the permeability perturbations caused by QACs bearing C(16) chains. It was difficult to assess the partitioning of the QACs between the aqueous and lipid phases since the ITC signals could also be associated to morphological changes such as vesicle aggregation. For the systems for which reliable thermodynamic parameters could be obtained, the Gibbs energy of transfer was similar to that for the micellization. The entropy variation represented the main contribution to the Gibbs energy, indicating that the insertion of QACs inside lipidic bilayers is driven by hydrophobic interactions.