Dynamic Surface Tensions and Micelle Structures of Dichained Phosphatidylcholine Surfactant Solutions

Surface tensions of aqueous solutions containing dialkyl phosphatidylcholines ((di-Cn)-PCs with n = 5, 6, 7, and 8) have been studied at the air−water interface, under equilibrium and dynamic conditions using du Nouy and maximum bubble pressure tensiometery, respectively. In the limit of short time (t → 0) the dynamic surface tension (DST or γ(t)) decays were consistent with a diffusion-controlled adsorption. However, at long times, closer to the equilibrium γ values, there was good evidence for a mixed diffusion−activation mechanism. Analysis of the limiting γ(t → ∞) data was consistent with an adsorption barrier of around 4 kJ mol-1. To check for the possible effects of micelles on the DST, approximate first-order micellar dissociation rate constants kmic were determined by stopped-flow spectrophotometry and a probe dye, Eosin Y. For example, with (di-C8)-PC, kmic was about 2 s-1. By use of measured values for kmic for these surfactants, it was shown that, just above the critical micelle concentration, the micelle breakdown kinetics have no discernible effect on the γ(t) decays. Small-angle neutron scattering (SANS) was also used to characterize the micelle structures, and analysis of the SANS data, using appropriate form factor expressions, indicated a micellar shape change spheres → ellipsoids → cylinders on increasing hydrophobic chain length from n = 5 to n = 8. These results show that the PCs have similar dynamic and equilibrium surface properties to structurally related dichained glucamides (Langmuir 1996, 12, 2701 and 1998, 14, 979; J. Colloid Interface Sci. 1997, 188, 423).