| Summary: | 碩士 === 國立臺灣科技大學 === 化學工程系 === 103 === The dynamic/equilibrium surface tensions of three aqueous surfactant solutions (anionic surfactant 1-decanoic acid, nonionic surfactant 1-decanol, and cationic surfactant dodecylamine DDA) were measured by using a video-enhanced pendant bubble tensiometer. The theoretical ST profiles using the nonionic and ionic models (Langmuir, Frumkin and generalized Frumkin models) were compared with the experimental data of equilibrium and dynamic surface tensions in order to study the adsorption mechanism of these three surfactant molecules.
For 1-decanoic acid aqueous, a diffusivity of 5.50×10-6 cm2/s was obtained from the fit between the dynamic surface tension and the theoretical ST curves using ionic-gF model. The adsorption kinetics for 1-decanoic acid in aqueous 500 mM NaCl solution was found to be diffusion controlled with a diffusivity of 12.5×10-6 cm2/s.
For 1-decanol aqueous, a diffusivity of 6.60×10-6 cm2/s was obtained from the best-fit for aqueous solution without NaCl and a diffusivity of 6.14×10-6 cm2/s was obtained for aqueous solution without 100 mM NaCl.
There exists a plateau region at γ = 43.9 mN/m in the dynamic surface tension profiles of aqueous DDA solutions at 25oC. This plateau implies the existence of a phase transition for the DDA molecules adsorbed at air-water interface. A strange behavior was observed for DDA solution: γ(t) decreases from 72 mN/m (the surface tension of solvent) at t = 0, reaches a minimum γ, then increases with time and reach 72 mN/m. This special γ(t) relaxation implies that DDA molecules prefer to adsorb on solid surface instead of air-water interface. Therefore, two squirm pumps were used for exchanging DDA solution in the quartz cell before the ST measurement to maintain a constant bulk DDA concentration.
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