NMR Studies On Physicochemical Behaviours Of Geminic Surfactants In Aqueous Solution

The performance properties of surfactants depend on not only the surface/interface activity but also solution behaviors, such as aggregation and kinetic processes in the micellar solution. However, the microcosmic mechanism of the kinetics processes is still not fully understood. Moreover, it is highly desired to study the behaviors of the novel surfactants, for instance Gemini surfactants, as those surfactants display unique performance properties. NMR methods can provide structural and kinetics/dynamics information at molecular and atomic scale. Therefore, we applied NMR methods on studies of aggregation and micellar kinetics of Gemini, single-chained conventional, and mixed surfactant systems in aqueous solution. The results are summarized as follow:1. NMR lineshape analysis was used to determine the average residence time between the monomer state and micelle state of Gemini surfactants,14-s-14and12-s-12(s=2,3and4) and their conventional single-chained surfactants, TTAB and DTAB. Apparent activation energy of exchange between the two states were quantitatively measured by means of temperature changed experiment. Results indicate:the average residence time of14-s-14,12-s-12and TTAB (DTAB) are respectively on the time scale of s, ms and μs; the exchange energy of activation of surfactants plays a critical role on the average residence time, and is seriously affected by the length of side alkyl chain (m) as well as that of linked spacer group (s). Moreover, different effects of spacer length (s) on the apparent energy of activation and the micropolarity insides the micelle of dimeric surfactants hint that they might adopt two-step exchange mechanism in micellar solution.2. Both NMR lineshape analysis and2D EXSY experiments were used to investigate effects of different types of conventional surfactants on exchange dynamics of Gemini surfactants,14-s-14(s=2,3and4) in their binary mixed micellar solution. Results show that, TX-100and Brij-35(nonionic) accelarated, TTAB (cationic) barely affected and SDS (anionic) slowed the exchange rate of14-s-14. Results from DOSY experiment, relaxation time measurements, fluorescence probe experiments and chemical shift changes revealed that, TX-100and Brij-35loosened the molecule arrangement and decreased hydrophobic interactions in the micelle; TTAB little changed the molecule arrangement; and SDS introduced the electrostatic attraction effect, tightened the molecule arrangement and increased hydrophobic interactions in the micelle, which are the effect mechanisms of conventional surfactants on exchange rates of14-s-14. Besides, exchange rates of14-S-14with shorter spacer length (s) varied more obvious when mole fractions of conventional surfactant is increased, This implys that the14-s-14may adopt two-stepwise exchange mechanism in the mixed micellar solution.3. DOSY was applied to investigate the size change of micelle in the binary micellar solution of14-s-14/different types of conventional surfactant. Results from the diffusion coefficients of14-s-14in the bulk solution and that in the mixed micelle suggest that, the size of mixed micelle reduced and the premicelle of14-s-14growed when mole fractions of TX-100and Brij-35(conventional nonionics) are increased; the size of mixed micelle reduced and the premicelle of14-s-14changed little in presence of TTAB (conventional cationics); the size of mixed micelle growed and the premicelle of14-s-14reduced in presence mole fractions of SDS (conventional anionics).4. The self-assembly behavior of novel Gemini surfactants with pyrrolidone head groups, N, N’-dihexyl-N, N’-di(ethyl-2-pyrrolidone) ethylenediamine (Di-C6P) was studied by DOSY and Relaxation time measurements combined with UV-vis spectrum, fluorescence spectrum, dynamic light scattering (DLS) and cryo-transmission electronic microscopy (cryo-TEM) in detail. Results revealed that, pH change (from acidic to basic) induced spherical micelle to vesicle transitions (MVTs) of Di-C6P in diluted aqueous solutions. Changes in chemical shifts and relaxation time atio (T2/T1) suggested that higher pH reduces protonation of tertiary amine head group of Di-C6P, and may affact the packing parameter and induce MVTs of Di-C6P.