Hanzhen Yuan

Mechanism of Surfactant Micelle Formation

The mechanism of micelle formation of surfactants sodium dodecyl sulfate (SDS), n-hexyldecyltrimethylammonium bromide (CTAB) and Triton X-100 (TX-100) in heavy water solutions was studied by 1H NMR (chemical shift and line shape) and NMR self-diffusion experiments. 1H NMR and self-diffusion experiments of these three surfactants show that their chemical shifts (delta) begin to change and resonance peaks begins to broaden with the increase in concentration significantly below their critical micelle concentrations (cmcs). At the same time, self-diffusion coefficients ( D) of the surfactant molecules decrease simultaneously as their concentrations increase. These indicate that when the concentrations are near and lower than their cmcs, there are oligomers (premicelles) formed in these three surfactant systems. Carefully examining the dependence of chemical shift and self-diffusion coefficient on concentration in the region just slightly above their cmcs, one finds that the pseudophase transition model is not applicable to the variation of physical properties (chemical shift and self-diffusion coefficient) with concentration of these systems. This indicates that premicelles still exist in this concentration region along with the formation of micelles. The curved dependence of chemical shift and self-diffusion coefficient on the increase in concentration suggests that the premicelles grow as the concentration increases until a definite value when the size of the premicelle reaches that of the micelle, i.e., the system is likely dominated by the monomers and micelles. Additionally, the approximate values of premicelle coming forth concentration (pmc) and cmc were obtained by again fitting chemical shifts to reciprocals of concentrations at a different perspective, and are in good accordant with experimental results and literature values and prove the former conclusion.

Mechanism of the Mixed Surfactant Micelle Formation

This article provides a full description of the mixed micelle formation process at a molecular level. The mechanism of mixed micelle formation in binary surfactant aqueous solution systems, ionic/nonionic mixed systems (12-2-12/TX-100, 14-2-14/TX-100, and SDS/TX-100), and ionic/ionic mixed systems (12-2-12/TTAB, 14-2-14/TTAB, and SDS/CTAB), in heavy water solutions was studied by (1)H NMR spectroscopy. The critical micellization concentrations of each individual component in the mixed surfactant solutions were gained by analyzing changes in chemical shift and intensities of resonance peaks. The chemical shift changes indicated that in the 12-2-12/TX-100 and SDS/TX-100 systems, micelles of TX-100 formed first, and then 12-2-12 or SDS molecules were fused in the micelles, respectively, which has been proved by 2D NOESY experiments. In contrast, 14-2-14 was the first component to form the micelles in the 14-2-14/TX-100 system. Although 12-2-12 and 14-2-14 are analogs and differ only in the length of the hydrophobic chain by two methylene groups, they showed different behaviors in the micellization processes in the mixture with TX-100. The observation suggests that in the binary surfactant system under current study, the component with lower cmc in the mixed solution aggregates first; then, the other one fuses, resulting in the mixed micelles as the total concentration increases. The same results were obtained for the ionic/ionic solutions, 12-2-12/TTAB, 14-2-14/TTAB, and SDS/CTAB. The above results suggest that the two mixed surfactants do not aggregate synchronously. It obviously demonstrates that the so-called cmc of the mixed surfactant solution needs reconsideration.

NMR investigation of the exchange kinetics of quaternary ammonium dimeric surfactants C14-s-C14.2Br.

The exchange kinetics of cationic gemini surfactants of the alkanediyl-alpha-omega-bis(tetradecyldimethylammonium bromide) type, with alkanediyl being 1,2-ethylene, 1,3-propylene, and 1,4-butylene, were investigated by 1H NMR, 2D COSY, and 2D EXSY experiments. In contrast to the conventional surfactants, a second set of well-resolved resonance peaks appeared in the 1H NMR spectra of these surfactants when their concentrations reached their critical concentrations. These two sets of resonance peaks originate from their monomers and micelles, which are proved by the correlation in the 2D COSY experiments and the cross polarization in the 2D NOESY spectra. Therefore, exchanges between monomers in the bulk solution and in the micelles or other aggregates of this series of surfactants occur slowly on the NMR time scale. The exchange rate constants were obtained by both NMR line shape analysis and 2D EXSY experiments, which are very consistent with each other. The exchange rate constants for the gemini surfactants were found to be orders of magnitude less than those for the conventional single surfactants, and for geminis 14-s-14, the shorter the spacer, the slower the exchange dynamic. It still has been found that the fast exchange between monomers in the bulk solution and in the micelles for gemini surfactant 12-2-12 at 25 degrees C occurs slowly at 5 degrees C on the NMR time scale.

Aggregation Behavior of Quaternary Ammonium Dimeric Surfactant C14-s-C14•2Br Micelles

用^1H NMR弛豫、2D NOESY研究了双子表面活性剂14-s-14胶束在重水溶液中的聚集行为.^1H NMR自旋-晶格、自旋-自旋弛豫实验表明,联结基团质子及其邻近的亚甲基质子的运动受到严重限制,表明这些质子形成胶束疏水核的壳层;而远离极性头的疏水质子的运动则相对自由,说明其位于胶束的内部.14-s-14系列（s=2,3,4）的弛豫实验结果还表明,联结基团越短,分子的碳氢链在疏水核中的排列越紧密.与14-4-14的对应单链表面活性剂TTAB相比,它们的分子运动更受限制.14-4-14的2D NOESY谱给出了与16-4-16类似的交叉峰信息,这表明14-4-14同16-4-16一样,形成了表面不平滑的球形胶束.因此,烷烃链的长短对胶束中分子的排列方式没有影响.The aggregation behavior of gemini surfactants 14-s-14 micelles in heavy water solutions was studied using 1H NMR relaxation and two-dimensional nuclear Overhauser enhancement spectroscopy (2D NOESY). Spin-lattice and spin-spin relaxation measurements of the protons showed that the motion of the spacer protons and a part of the methylene protons next to the polar head group was seriously restricted. Therefore, these protons should be involved in the formation of the surface layer of the hydrophobic micellar core. The hydrophobic protons that were away from the polar head group were situated internal in the micellar core and moved relatively freely. Relaxation measurements of the 14-s-14 series(s=2, 3, 4) also showed that the molecules were more tightly packed as the length of the spacer decreased. Their motion was relatively more restricted than those of TTAB, the monomeric homologue of 14-4-14. 2D NOESY spectrum of 14-4-14 exhibited the same pattern as that of 16-4-16. This showed that the micelles of 14-4-14 were spherical, the surface of which exhibited a saw-toothed form, similar to that of 16-4-16. Consequently, the length of the alkyl chain did not have any effect on the relative arrangement of molecules in the micelles.

Self-aggregation of surfactants in water solution by NMR

Dynamic behavior of surfactant molecules, sodium dodecyl sulfonate (SDSN), cetyl trimethyl ammonium bromide (CTAB) and triton X-100, and their micelles in water solution at various concentrations were studied by chemical shift, self diffusion coefficient (D) and nuclear magnetic resonance relaxation (T1 and T2) measurements. Experimental results show that there exist turning points at surfactant concentration (C) nearby the critical micelle concentration (cmc) for all of the above-mentioned parameters, which approach the constant values when C equals 5 to 10 times cmc. T1 and T2 measurements show that when C<cmc, Triton X-100 molecules exhibit fast isotropic motion in water solution. After micellization, the motion of hydrophobic chains is far away from the extreme narrowing condition, however, the long hydrophilic oxyethylene chains move relatively freely, although their T1 and T2 become shorter in comparison with those of the molecules in mono-molecular state. The behavior of the hydrophobic chains within the micellar core of SDSN and CTAB is similar to that of Triton X-100 to some extent.Dynamic behavior of surfactant molecules, sodium dodecyl sulfonate (SDSN), cetyl trimethyl ammonium bromide (CTAB) and triton X-100, and their micelles in water solution at various concentrations were studied by chemical shift, self diffusion coefficient (D) and nuclear magnetic resonance relaxation (T1 and T2) measurements. Experimental results show that there exist turning points at surfactant concentration (C) nearby the critical micelle concentration (cmc) for all of the above-mentioned parameters, which approach the constant values when C equals 5 to 10 times cmc. T1 and T2 measurements show that when C<cmc, Triton X-100 molecules exhibit fast isotropic motion in water solution. After micellization, the motion of hydrophobic chains is far away from the extreme narrowing condition, however, the long hydrophilic oxyethylene chains move relatively freely, although their T1 and T2 become shorter in comparison with those of the molecules in mono-molecular state. The behavior of the hydrophobic chains within the micellar core of SDSN and CTAB is similar to that of Triton X-100 to some extent.

2,5-双取代烷基苯磺酸钠胶束微结构的 1 H NMR研究

Micellization of a series of newly synthesized dialkyl benzene sulfonates was studied using proton chemical shift changes, spin-lattice, and spin-spin relaxation NMR spectroscopy, and two-dimensional nuclear Overhauser enhancement spectroscopy (2D NOESY). The o-substituted chains are normal alkyl chains with varying lengths, and the m-substituted ones are branched alkyl chains. The results showed that the longer the o-substituted normal alkyl chain, protons of more methylene groups participated in the formation of the rigid surface layer of the hydrophobic micellar core. Consequently, the larger was the area per molecule adsorbed on the interface between oil and water at saturation. The branched m-substituted alkyl chains of the dialkyl benzene sulfonates were less tightly packed than the o-substituted normal alkyl chains in the hydrophobic micellar cores. The shorter the m-substituted branched alkyl chains, the looser they were packed in the hydrophobic micellar core. The relative arrangement of the surfactant molecules in the micelles was elucidated.利用核磁共振化学位移变化，自旋-自旋弛豫和2D NOESY（two-dimensional nuclear Overhauser enhancement spectroscopy）研究了一系列新合成的双取代烷基苯磺酸盐的胶束化．结果表明，邻位取代的是正烷烃链，间位取代的是支烷烃链．而且，邻位取代的烷烃链越长，参与形成胶束疏水核表面层的亚甲基个数越多．因此。每个分子在饱和吸附的油水界面上的面积越大．间位取代的分支链在胶束疏水核中堆积得没有邻位取代的正烷烃链紧密．分支链越短，堆积得越不紧密．描述了胶束中分子的相对排列。

NMR relaxation and micro-imaging study of polystyrene in concentrated cyclohexane solution

Abstract13C-relaxation times of polystyrene (PS) in its θ solvent, cyclohexane, are measured at different temperatures. A two-step model for the dissolution is proposed. Swelling of the polymer below the θ temperature is eventually the dispersion of the side group phenyl rings only. While above the θ temperature, complete dissolution is the dispersion of the main chain at a molecular level. The results of T1(C) are confirmed by1H-NMR imaging. NMR and its imaging are powerful twls to study the dynamic behavior of dissolution process of polymers in their θ solvents.