Adsorbed film of n-tetradecylphosphocholine at the tetradecane/water interface studied by interfacial tensiometry and X-ray reflection

The adsorbed film of n-tetradecylphosphocholine (C₁₄PC) at the tetradecane (C₁₄)/aqueous solution (W) interface was studied by interfacial tensiometry and X-ray reflection (XR). The interfacial tension γ was measured as a function of molality mC14PC and temperature T under atmospheric pressure. The interfacial density of C₁₄PC ΓC14PCH increases with increasing mC14PC and converges into 3μmolm^-2 (first saturation) at mC14PC≈0.08mmolkg-1 and into 4.2μmolm^-2 (second saturation) at the critical micelle concentration (CMC). The electron density profile determined by XR indicated that C₁₄PC molecules form monolayer with relatively loose packing at the first saturation. In the second saturation, on the other hand, the molecules form bilayer in which charge separated phosphocholine groups take upside-down arrangement to interact attractively between neighbors. The positive entropy changes associated with adsorption both from monomeric and from micellar states result from that the contribution of dehydration around C₁₄PC molecules, especially around PC groups, by the adsorption from aqueous solution overcomes that of a comparatively ordered molecular orientation at the interface. By comparing the results at the C₁₄/W interface with those at the Air (A)/W one, it was shown that in the monolayer and bilayer states, hydrophobic chains of C₁₄PC molecules are more ordered at the C₁₄/W than at the A/W interface because of attractive interaction between C₁₄PC and C₁₄ molecules. This leads to smaller value of the partial molar entropy and energy changes associated with the adsorption at the C₁₄/W than at the A/W interface. Furthermore the entropy of micelle formation changes from positive to negative with increasing T, suggesting that the entropy gain due to the dehydration exceeds the entropy loss by the aggregation at low temperatures, while the gain is less than the loss at high temperature.