Adsorption behavior of the ternary system of phospholipid 1-palmitoyl-2-oleoyl-sn-gylycero-3-phosphocholine in 3-hydroxypropionitrile with added tetrabutyl ammonium bromide

The adsorption behavior in a ternary system composed of the non-ionic surfactant POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine) and the ionic surfactant TBABr (tetrabutyl ammonium bromide) in the polar solvent HPN (3-hydroxypropionitrile) is investigated utilizing surface tension measurements and ARXPS (angular resolved X-ray photoelectron spectroscopy). The latter technique allows evaluating the surface excess of both surfactants. The combined experimental information (i.e. surface excess and surface tension) allows the determination of the chemical potential and thus the activities of both surfactants by virtue of the Gibbs equation. While the activity coefficient of TBABr is hardly affected by the presence of POPC, the activity coefficient of POPC strongly depends on both, the concentration of POPC and the concentration of TBABr. We observe the activity coefficient f_POPC to vary by more than 3 orders of magnitude in the 2-dimensional parameter space (c_POPC, c_TBABr). This finding reflects and explains the surprising behavior of the surface tension: adding TBABr to the binary system POPC/HPN may increase or decrease the surface tension, depending on the POPC concentration. Inspection of these results suggests that mixed POPC/TBABr aggregates are formed at low POPC concentrations while the typical features of micelle formation are absent in this ternary system even at very low TBABr concentration.