十二烷基硫酸钠水溶液的动态表面吸附性质

利用MPTC型气泡压力张仪研究了十二烷基硫酸钠(SDS)溶液在不同NaCl 浓度下的动态表面吸附性质, 分析了离子型表面活性剂在表面吸附层和胶束中形成双电层结构产生表面电荷对动态表面扩散过程和胶束性质的影响. 结果表明, SDS在表面吸附过程中, 表面电荷的存在会产生5.5 kJ·mol^-1的吸附势垒(E_a), 显著降低十二烷基硫酸根离子(DS^-)的有效扩散系数(D_eff). 十二烷基硫酸根离子的有效扩散系数与自扩散系数(D)的比值(D_eff/D)仅为0.013, 这表明SDS与非离子型表面活性剂不同, 在吸附初期为混合动力控制吸附机制. 加入NaCl可以降低吸附势垒. 当加入不小于80 mmol·L^-1 NaCl后, E_a小于0.3 kJ·mol^-1, D_eff/D在0.8-1.2之间, 表现出与非离子型表面活性剂相同的扩散控制吸附机制. 同时, 通过分析SDS胶束溶液的动态表面张力获得了表征胶束解体速度的常数(k₂). 发现随着NaCl 浓度的增大, k₂减小, 表明SDS胶束表面电荷的存在会增加十二烷基硫酸根离子间的排斥力, 促进胶束解体.

Dynamic Surface Adsorption Properties of Sodium Dodecyl Sulfate Aqueous Solution

The dynamic surface adsorption properties of aqueous sodium dodecyl sulfate (SDS) solutions were investigated at different concentrations of NaCl using bubble pressure tensiometry MPTC. In the case of ionic surfactants, the existence of a diffuse electric double layer on the surface adsorption layer and around the micelle produces a surface charge. Here, we discuss the influence of the surface charge on the dynamic surface diffusion processes and the micelle properties. It was found that the SDS adsorption process occurred in the presence of a 5.5 kJ·mol^-1 adsorption barrier (E_a) that was generated by the surface charge; this barrier significantly decreased the effective diffusion coefficient (D_eff) of the dodecyl sulfate ions (DS^-). The ratio of the effective diffusion coefficient to the monomer self-diffusion coefficient (D) (D_eff/D) was only 0.013. This indicated that at the beginning, the adsorption of SDS followed the mixed kinetic-diffusion controlled model; this is different from the behavior observed for nonionic surfactants. The adsorption barrier was reduced when NaCl was added. E_a was less than 0.3 kJ·mol^-1 after the addition of 80 mmol·L^-1 of NaCl. This resulted in values of between 0.8 and 1.2 for D_eff/D, which was consistent with the diffusion-controlled model that describes the behavior of nonionic surfactants. The characteristic constants for the micelle dissociation rate (k₂) were determined from the dynamic surface tension of the SDS micelle solutions. The calculated k values decreased as the NaCl concentration was increased, which demonstrated the existence of surface charge on the SDS micelles; this surface charge increased the repulsive forces between the dodecyl sulfate ions, and promoted the dispersion of the micelles.