Umme Salma Siddiqui

Role of added counterions in the micellar growth of bisquaternary ammonium halide surfactant (14-s-14): 1H NMR and viscometric studies.

The change in the morphology of a series of dicationic gemini surfactants C(14)H(29)(CH(3))(2)N(+)-(CH(2))(s)-N(+)(CH(3))(2)C(14)H(29), 2Br(-) (14-s-14; s=4-6) on their interaction with inorganic (KBr, KNO(3), KSCN) and organic salts (NaBenz, NaSal) have been thoroughly investigated by means of (1)H NMR spectral analysis and the results are well supported by viscosity measurements. The presence of salt counterions results in structural transition (spherical to nonspherical) of gemini micelles in aqueous solution. With an increase in salt concentration all the three gemini surfactants showed changes in their aggregate morphology. This change is dependent on the nature and size of the added counterion. The effect of inorganic counterions on the micellar growth is observed to follow the Hofmeister series (Br(-) < NO(3)(-) < SCN(-)). The roles of organic counterions are discussed on the basis of probable solubilization sites of the substrate molecule in the gemini micelles, showing more growth in case of Sal(-) than Benz(-). The results are confirmed in terms of the obtained values of chemical shift (δ), line width at half height (lw), and relative viscosity (η(r)). Also, the growth of micelles was most pronounced for the gemini surfactant with the shortest spacer (s=4). This was attributed to the unique molecular structure of gemini surfactant micelles having flexible polymethylene spacer chain linking the twin polar headgroups.

Micellization Behavior of Butanediyl-1, 4-Bis(Dimethyldodecylammonium Bromide) Gemini Surfactant in Presence of Organic Additives

The surface and micellar properties of cationic gemini surfactant butanediyl-1, 4-bis(dimethyldodecylammonium bromide), referred to as C12-C 4 -C12, has been studied in pure aqueous solutions and in the presence of alcohols (butanol (C4OH), pentanol (C5OH), hexanol (C6OH)), and the corresponding amines (butylamine (C4NH2), pentylamine (C5NH2), hexylamine (C6NH2)) by surface tension measurements at 303 K. Parameters such as critical micelle concentration (CMC), C20 (the surfactant concentration required to reduce the surface tension of solvent by 20 mN m−1), the ratio of C20 and CMC, Γmax (maximum surface excess concentration at the air/solution interface), and Amin (minimum area per surfactant molecule) were evaluated. The CMC value of the gemini decreased with increasing concentration of additives. Mixed micelles were formed in each case and the CMC values of the mixtures were used to compute interaction parameters for mixed micelle formation in aqueous medium (βm), interaction parameters for mixed monolayer formation at the solution/air interface (βσ), and activity coefficients (f1 and f2). A synergistic interaction was observed both in the micelle as well as at the interface, as evident from the interaction parameters. Free energies of micellization ( ) and adsorption ( ) were also calculated.

Catalytic role of gemini surfactant micelles in the ninhydrin-L-isoleucine reaction

Effect of dicationic gemini surfactants C16H33(CH3)2N+-(CH2)s-N+(CH3)2C16H33, 2Br− (where s = 4, 5, 6) on the reaction of ninhydrin with L-isoleucine has been investigated spectrophotometrically as a function of [gemini], [L-isoleucine], [ninhydrin], and pH. The reaction follows first- and fractional-order kinetics, respectively, in [L-isoleucine] and [ninhydrin]. The gemini surfactant micellar media are found more effective for the reaction than their conventional monomeric counterpart CTAB. Furthermore, whereas typical rate constant (kψ) increase and leveling-off regions are observed with CTAB and geminis, the latter produce a third region of increasing kψ at concentrations ≥ 60 cmc’s. 1H NMR studies reveal that this unusual third-region effect of the geminis is due to changes in their micellar morphologies. Quantitative kinetic analysis has been performed on the basis of modified pseudo-phase model.

Growth of Gemini Surfactant Micelles Under the Influence of Additives: DLS Studies

Results are presented on the first extensive study on the influence of additives on the growth of gemini; alkanediyl α, ω - bis(dimethylcetylammonium bromide) surfactant micelles (16-s-16, with s = 5, 6); as measured by dynamic light scattering technique at 30°C. The effect of adding n-butanol, n-pentanol, n-hexanol, and n-hexylamine in the absence or presence of general ionic salt potassium bromide on 0.03 M gemini solutions were observed. The tendency for micelles to grow from spherical to rodlike structures is decisively influenced by the spacer length s. At 30°C, the micellar growth was more for s being 5, which has been interpreted in terms of short spacer having strong propensity for micellar growth. Addition of KBr plays a role in screening of the electrostatic interactions, thus promoting a change of morphology of the aggregates and giving rise to high hydrodynamic diameter (D h ) values. The micellar growth in presence of alcohols is interpreted in terms of the formation of the gemini–alcohol mixed micelles which followed the pattern C6OH>C5OH>C4OH. For equal chain length additives C6OH and C6NH2, the growth was more pronounced in case of alcohol. Also, in case of C6NH2, the value of D h reached to almost constancy or decreased to some extent, which is discussed in terms of its partitioning in aqueous phase. A combined presence of KBr and n-alcohols or n-hexylamine produced favorable conditions for micellar growth due to synergistic effect.

Synergism in cationic gemini – additive systems

A series of dicationic gemini surfactants with the general formula C16H33(CH3)2N+−(CH2)s−N+(CH3)2C16H33, 2Br− (where s = 4–6), designated as 16-s-16, were synthesised. Their interaction with organic additives: n-alcohols (C3H7OH, C7H15OH, C8H17OH) and the corresponding amines (C3H7NH2, C7H15NH2, C8H17NH2) in the absence and presence of KNO3 at 30°C was studied viscometrically to observe their effect on assembly formation and micellar transition. The simultaneous presence of KNO3 and organics induced rich aggregates morphologies in the gemini micellar systems by giving high viscosity values. On comparing the behaviour of the gemini surfactant series for a given alkyl chain length of the organic additive, the spacer is found to markedly influence the behaviour; shorter the spacer, earlier the sphere-to-rod transition. In the case of the conventional surfactant, CTAB, the concentration of KNO3 used with the geminis was insufficient to induce any transition.