The Adsorption Behavior Of Mixed System With Gemini On The Surface Of The Solution

Mixtures of cationic/anionic surfactants could usually exhibit strong synergistic effect, they showed not only high surface activity but also complex self-aggregation. Due to their important applications in many fields, the investigation of these mixtures attracted a lot of attentions. As a new series of surfactants, Geminis exhibited several peculiar properties because of novel molecular structures. however, few studies were focused on the mixture of cationic/anionic surfactants containing Gemini component until now. In this paper,we investigated the surface adsorption and the adsorption kinetics of the mixture of anionic Gemini surfactant C9pPHCNa and conventional cationic surfactant C₁₀TABr aqueous solutions. And we also investigated the adsorption kinetics of the mixture of C9pPHCNa and cationic Gemini surfactant 10-2-E₂-10 aqueous solutions. The main results are as follows: 1. The synergism occurred in the mixed systems and yielded maximum synergism at the mole fraction of C9pPHCNa in the bulk (α₁) equaling to 0.33, where the two components equal electricity mixing in solution. The corresponding minimum values of the critical micelle concentration (cmc_T), the surface tension (γ_cmc) and the total concentration required to reduce 20 mN·m^-1?1 surface tension of water (C_20,T) are respectively 0.6×10^-3mol·L^-1, 23.5 mN·m^-1?1 and 1.58×10^-5mol·L^-1. Over the range of α₁ studied, the molar compositions of C9pPHCNa and C₁₀TABr both in the micelle and on the surface are close each other, which reflects strong interaction between the molecules. As the differences of hydrophobicity and diffusion velocity between C9pPHCNa and C₁₀TABr, the maximum value of the surface adsorption at the concentration of the mixtures equaling to cmc (Γ_max) was observed at α₁=0.1. 2. The maximum bubble technique has been used to study the adsorption kinetics of single anionic Gemini surfactant C9pPHCNa and its mixtures with cationic surfactant C10TABr in aqueous solutions. The dynamic surface tension data was analyzed with the revised Ward and Tordai equations and the micelle dissociation kinetic model. The apparent diffusion coefficient Da below the cmc, the adsorption barrier εa and the micelle dissociation constant kmic were obtained. For single C9pPHCNa solution, it was proved that the initial adsorption was diffusion-only controlled. For the mixtures, the Das at short time were smaller than those at long time corresponding to the obviously adsorption barrier (εa) of 10²0 kJ·mol-1. The minimum εa appeared at the bulk mole fraction of C9pPHCNa (α1) being 0.33. The most stable mixed micelles were formed at α1=0.2 rather than at α1=0.33, owing to great discrepancy of hydrophobicity between the two components. These results indicated that the composition of mixed solution was an important factor affecting the adsorption kinetics and the micelle stability. 3. We also investigated the adsorption kinetics of the mixed solutions of C9pPHCNa /C10-2-E2-C10. They were different from what we had studied before, the dynamic surface tension of this mixture system was decreasing, until it reached a plateau, then it would continue to decline. We considered that it would attribute to the different adsorption status of the surfactants in the different adsorption processes, which is the adsorption of free C9pPHCNa and ion pairs of the mixture. And the dynamic surface tension of the C9pPHCNa/C10-2-E2-C10 mixtures was coincided with those of the free C9pPHCNa mono-molecules in the mixed solution at the first adsorption stage.