Aggregation behavior of mixed surfactants in non-polar solvents - Master's thesis - Dissertation

Aggregation Behavior of Mixed Surfactants in Non-Polar Solvents The aggregation behavior of surfactants in non-polar solvents is much more complicated than in aqueous solutions. Surfactants in aqueous solutions have a clear critical micelle concentration (CMC). There are still doubts about the CMC concept in non-polar solvents, but various methods such as dye solubilization, water solubilization, light scattering, fluorescence polarization, ultraviolet and nuclear magnetic resonance spectroscopy have confirmed and determined non-polar The presence of CMC in a solvent. The surfactant exists in a non-ionized state in a non-polar solvent, and its association mainly depends on the dipole-dipole and ion pair interaction between the amphiphilic molecules, then on a surface The addition of another surfactant, the surfactant, to the active agent solution will inevitably have a major impact on its aggregation behavior, but so far, no reports have been made on the aggregation behavior of mixed surfactants in non-polar solvents. In this paper, the CMC of the reverse micelles formed in n-heptane after mixing the anionic surfactant AOT and the nonionic surfactant Brij30 was determined by iodine spectroscopy and water solubilization method, in order to investigate the complex pairing of surfactants. The effect of its aggregation behavior. Diisooctylsulfonated sodium succinate (AOT, Sigma); Brij30 (C12E4, 12 oxyethylene-containing dodecyl alcohol, AcrosOrganics). Iodine is purified by sublimation, using water For the second re-distilled water, n-heptane is analytically pure reagent and then re-steamed by water. The absorption spectrum is determined by the American Instruments UV-1100 spectrophotometer, and the n-heptane solution of iodine is blank. The experiments were all carried out at a constant temperature of 25 °C. I2 can form an electron transfer compound with micelles or reverse micelles, and the absorption spectrum changes. This is the basis for the determination of CMC by iodine spectroscopy. The n-heptane solution of I2 has an absorption peak at 520 nm, after adding a certain concentration of surfactant. The absorption peak is lowered, and a new absorption peak appears at 390 nm. A n-heptane solution having a certain concentration of I2 (5×10-4 mol/L) is prepared, and the absorbance A is measured at 390 nm to A. The concentration of the surfactant is plotted. From the turning point of the curve, the CMC value can be obtained by taking 5 mL of the mixed surfactant n-heptane solution of different compositions and different concentrations. The water is gradually added to each solution by a micro-injector, and water is added each time. Immediately after shaking, until turbidity appeared immediately after standing, record the amount of water added, and correct it with a blank of n-heptane solvent to obtain a saturated solubilization amount S of water, solubilizing the water per liter of n-heptane solution. The number indicates (mol/L), as shown in Fig. 2.2. Results and discussion Iodine spectroscopy The iodine spectroscopy method was used to determine the CMC value of the reverse micelles formed in n-heptane when AOT and Brij30 were compounded in different ratios. Increase in the proportion of Brij30 in mixed surfactants, CMC first Lowering to a minimum value increases with the increase of Brij30 ratio. The reason may be: after adding a small amount of Brij30 in AOT, the repulsive force between AOT polar groups is reduced, and the surfactant is prone to aggregation. The CMC is reduced accordingly; further increasing the ratio of Brij30 will not significantly reduce the repulsive force between the AOT polar groups, but will increase the cross-sectional area of ​​the polar group, which hinders the aggregation of the surfactant from the geometrical considerations. CMC is also enlarged. Different ratios of AOT and Brij30 mixed surfactants in CMC water solubilization method in n-heptane. When the surfactant concentration is low, the solubilization amount of reverse micelles to water and its concentration are Linear relationship. The intersection of straight line and abscissa in Figure 2 can be regarded as CMC determined by water solubilization method. The results of CMC measurement for different systems are shown in Table 1. The results also show that CMC appears as the ratio of Brij30 increases. The trend of decreasing first and then increasing is consistent with the results determined by iodine spectroscopy. The slope of the straight line in Figure 2 reflects the average number of water molecules solubilized by each surfactant molecule in the reverse micelle, which can represent the ability to solubilize water. When the composition of the surfactant is different, the ability to determine the solubilization of water in each system is as follows. 2. With the increase of Brij30 ratio, the ability of mixing reverse micelles to solubilize water first increases and then decreases significantly, which is completely opposite to the change trend of CMC. According to the literature, the accumulation parameters of surfactant molecules forming anti-micelles v/a0lc has a direct influence on its solubilizing ability. The addition of Brij30 with smaller packing parameters reduces the stacking parameters of the mixed surfactant, so that the solubilizing ability of the mixed reverse micelles increases; but further increases the nonionic surface. The content of the active agent reduces the strength of the droplet interface film, increases the instability of the droplet, and thus has a tendency to reduce the solubilization ability of the reverse micelle, and the higher the content of the nonionic surfactant, the trend The more so, after observing the maximum solubilizing capacity, the solubilizing ability of the mixed reverse micelles decreases with the increase of the nonionic surfactant content. The solubilizing ability of the mixed reverse micelles composed of different surfactants is compared with the iodine spectrum. The CMC value measured by the method and the water solubilization method can be seen that when the content of the nonionic surfactant is low, the results measured by the two are substantially similar; and when the content of the nonionic surfactant is high, the water is solubilized. The measured CMC value is greater than the iodine spectrum The measured result. This may be because the CMC measured by iodine spectroscopy is the concentration at which the surfactant begins to aggregate, and the CMC measured by the water solubilization method is the surfactant when the formed aggregate has the ability to solubilize water. Concentration. With the increase of Brij30 content, the non-polar groups of AOT and Brij30 mixed surfactants gradually decrease relative to the polar groups, so when the Brij30 content is higher, only the surfactant concentration increases, and the aggregate radius increases. The curvature of the large and reverse micelle droplets is reduced, and the bulk density of the surfactant non-polar groups is increased to a certain value to effectively shield the oil-water interface with high surface tension caused by water solubilization. Solubility, so the CMC value measured by the water solubilization method is higher than that of the iodine spectrum; when the Brij30 content is low, the non-polar group of the surfactant is relatively large, and the formed small aggregates have solubilization Water capacity, the CMC values ​​measured by the two methods are similar. Keywords: mixed surfactant; non-polar solvent; US analytical instrument ; UV-1100; UV-1200