Premicellar Aggregation Of Quarterary-ammonium Gemini Surfactants In Aqueous Solution

Surfactant can form micelles spontaneously in aqueous solution due to their amphiphilic character. Generally, amphiphilic molecules aggregate when the concentration of the solution reaches the critical micellar concentration (cmc). However, in some cases, so-called premicellization occurs, which means some small and incomplete aggregates are formed though the concentration of the solution is lower than cmc. It is important and necessary for people to understand the premicellization of surfactants because it can gives the microscopic information of the mechanic of surfactant self-organization, which may be helpful for us to understand the interaction between molecules. At present, however, few studies are focus on the premicellar aggregation. In this paper, we investigated the premicellization of C₁₂-s-C₁₂·2Br(s=2,3,4,5,6,8)and C₁₂-2-E_n-C₁₂·2Br(n=1,2,3) in aqueous solution by steady-state fluorescence method using 1-Pyrenebutyric acid (PBA) as probe. The main results are as follows: 1. The critical premicelle concentration (cpc) of C₁₂-s-C₁₂·2Br was determined by means of steady-state fluorescence spectroscopy technique. The value of cpc changes when the length of the spacer of C₁₂-s-C₁₂·2Br (s) increases from 2 to 8 and has a maximum at s=4, which is similar to the variation of cmc with s. On the other hand, the change of the ratio cmc/cpc with s goes in the reverse direction to the change of the cpc and has a minimum also at s=4. Compared with the conventional surfactant C₁₂TABr, C₁₂-s-C₁₂·2Br surfactants have a strong ability to form premicelles. 2. The cpc of C₁₂-2-E_n-C₁₂·2Br in aqueous solution was also determined. It increases with the increase of n, which is similar to the change of cmc. The ratio of cmc to cpc decrease with the increasing of n. 3. The premicellization of C12-2-E1-C12·2Br and C12-4-C12·2Br was promoted with addition of NaBr. And the effect of NaBr on C12-4-C12·2Br was more sensitive than on C12-2-E1-C12·2Br. 4. Aggregation number of pre-micelles was calculated from the aggregation number of micelles determined through the steady-state fluorescence technique and by analyzing the curve of IE/IM～logc of C12-s-C12·2Br using PBA as the probe. 5. We determined curves of γ～log c for C12-s-C12·2Br in aqueous solution and calculated Γcpc using the Gibbs equation. The Γcpc decrease with the increasing of s, which is similar to the change of Γcmc. The Γcpc/Γcmc<1 suggested that premicellization occurred in the bulk phase before the adsorption of the surfactant at the surface was saturated.