Dynamic Surface Tensions and Micelle Structures of Dichained Phosphatidylcholine Surfactant Solutions

Surface tensions of aqueous solutions containing dialkyl phosphatidylcholines ((di-C n )-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 k mic were determined by stopped-flow spectrophotometry and a probe dye, Eosin Y. For example, with (di-C8)-PC, k mic was about 2 s - 1 . By use of measured values for k mic 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).