Ultramicroelectrode Voltammetric Investigation of Intermicellar Interaction and Micellar Growth of Sodium Dodecyl Sulfate in Aqueous NaCl Solutions

Effect of Catechol on the Size and Shape of CTAB Micelles in Aqueous Solutions

The effects of catechol on the change of cetyltrimethylammonium bromide (CTAB) micelles in aqueous solutions have been studied by Ultramicroelectrode (UME) cyclic voltammetry and Time-Resolved Fluorescence Quenching (TRFQ). It has been shown that the diffusion coefficient decreases and the micelle aggregation number increases with the addition of catechol. This indicates that catechol has influence on the size or shape of CTAB micelles. In order to analyze the change of the micelles, other micelle parameters like the effective radius (a), surface area of micelle (S), area of single CTAB molecule (A) and packing parameter (P) were calculated. The values of P which is relative to the shape of the micelles are consistent with the change of the diffusion coefficient. All the results show that catechol can accelerate the increase of the aggregate size and sphere-to-rod morphology change of CTAB micelles even with lower concentration of CTAB. This is because that hydrophobic force makes catechol insert into CTAB micelles between CTAB molecules, which leads to the increase of the radius of curvature. The increase of aggregate size and the sphere-to-rod morphology change are also promoted as a result.

Adsolubilization of dihydroxybenzenes into CTAB layers on silica particles

The adsolubilization of dihydroxybenzenes (catechol and hydroquinone) into cetyltrimethylammonium bromide (CTAB) layers on silica particles have been investigated by dye method and UV spectrum. It is found that the adsolubilization amount of catechol increases with increasing the concentration of CTAB, reaches a maximum value at the critical micelle concentration (CMC), and then decreases with further increment of CTAB concentration. For hydroquinone, different phenomenon is observed. The maximum adsolubilization amount reaches at critical surface aggregation concentration (CSAC) instead of CMC of the CTAB and then decreases to constant values. In order to analyze the adsolubilization difference between catechol and hydroquinone, we determined the interaction between them and CTAB in the bulk solution by measuring the diffusion coefficient of dihydroxybenzenes with ultramicroelectrode (UME). The individual contributions to the overall apparent diffusion coefficient of dihydroxybenzenes in CTAB solutions were also calculated. The results indicate that more hydroquinone than catechol could interact with CTAB molecules. However, rarely hydroquinone could interact with CTAB micelles compared with catechol. It is suggested that the substitution positions of hydroxyl lead to the difference of the interaction between dihydroxybenzenes and CTAB, which is responsible for the difference of the adsolubilization.

Study on the Structure Transition of CTAB/1-Butanol/Heptane/Water Microemulsions by Pt Ultramicroelectrode

Ultramicroelectrode voltammetric measurements have been used in the microemulsions of CTAB/1-butanol/heptane/water to calculate the diffusion coefficient which can reflect the structure transition of the microemulsions. It is found that with increasing water content of microemulsions, the apparent diffusion coefficient (D app) of the probe decreases monotonically with three transition points. The first two transition points indicate the change in the structure of microemulsions, that is from water in oil region to bicontinuous phase and then to oil in water region, while the third one differs from previous reports. The time resolved fluorescence quenching (TRFQ) technique has been carried out to obtain the aggregation number of the microemulsions in order to analyze the abnormal transition point. According to the TRFQ experiment, we find that the aggregation number of the microemulsion decrease with water content increasing between 50%–60%, and then tend to identical. These results suggest that the last transition point of the diffusion coefficient is because of the forces between oil droplets. The self-diffusion coefficient of oil droplet in the o/w region is calculated simultaneously. Based on these experimental results, we analyze the composition of the interface between oil and water.