Surface dilational viscoelasticity of aqueous surfactant solutions by surface quasi-elastic light scattering

We investigated the surface dilational viscoelasticities of aqueous solutions of dihexanoyl-phosphastidylcholine (DC₆PC) and dioctanoyl-phosphastidylcholine (DC₈PC), dodecyltrimethylammonium bromide (DTABr) and DTABr and sodium bromide (NaBr), dodecyltrimethylammonium tetrafluoroborate (DTABF₄) and bisdodecyltrimethylammonium sulfate (DTA₂SO₄), and nonionic surfactants with oxyethylene units (C₁₀E₅). We measured surface quasi-elastic light scattering (SQELS) spectra and analyzed the solutions by both surface rheological and phenomenological fitting approaches. For the DC₆PC-DC₈PC mixture, the electrocapillary wave (ECW) method was also applied. The molecular interactions between DC₈PC and DC₆PC in the adsorbed film clearly influenced the relaxation process on the order of several milliseconds (ECW). However, the effects were less influential on the order of several hundreds of microseconds (SQELS). The characteristic frequencies for relaxation owing to the staggered structure formation likely occurred in the range from 150 to 300 Hz. The dilational elasticity at 250–400 kHz of DTABr suggested that the main relaxation process influencing the dilational properties was neither diffusion of surfactants from the bulk phase to the subsurface nor an adsorption-desorption of surfactant ions between the adsorbed layer and subsurface. However, the diffusion of counterions between the Stern and diffuse layers occurred on a time scale of several microseconds. Studies of C₁₀E₅ suggested that the relaxation process captured by the SQELS studies was neither related to surfactant diffusion nor surfactant reorientation, but rather was likely to be a relaxation process where distortion of the hydration sphere and/or hydration-dehydration to/from the hydration sphere was caused by diffusion of water molecules.