The aggregation properties of three dicationic quaternary ammonium gemini surfactants with the same structure, except the spacer group, diethyl ether, six methylene, and p-xylyl, have been studied using electrical conductivity and fluorescence. The critical micelle concentration (cmc) and the micelle aggregation number (N) were determined, and the micropolarity and the microviscosity of the micelle were characterized. The micelle ionization degree (alpha) was obtained by a combination of the electrical conductivity data and the micelle aggregation number. Furthermore, the Gibbs free energy of micellization (deltaGmic) was studied. These results have shown that the nature of the spacer has an important effect on the aggregation properties of gemini surfactants in an aqueous solution. A hydrophilic, flexible spacer prompts micelle formation, which leads to a smaller cmc, smaller alpha, larger N, and more negative deltaGmic. Meanwhile, the microviscosity study indicates that the gemini surfactant with a hydrophilic, flexible spacer forms a more closely packed micelle structure than the one with a hydrophobic, rigid spacer.
The formation of complex monolayers between gemini surfactants (CsH2s-R,ω-(CmH2m+1N + (CH3)2Br -)2, abbreviated as C12-Cs-C12, s ) 3, 4, 6, 8, 10, 12) and DNA at the air-water interface was systematically investigated. The polyion-complex monolayers formed in situ through the electrostatic attraction between the ammonium groups of gemini surfactants and the phosphate groups of DNA. The effect of surfactant spacer length on the surface properties was investigated. A turning point of the surface properties (extrapolated molecular area and collapse pressure) of the gemini surfactant/DNA complex monolayers appears when the surfactant spacer is above a certain length (s ) 6). The gemini surfactant spacer taking a reverse U-shape conformation at the air-water interface is proposed to interpret the turning point. A quantitative kinetic analysis of the decay curves further confirms that the turning point appears at the surfactant spacer above its critical length, s ) 6. Moreover, the surface topographies of the gemini surfactant/ DNA complex monolayers were controlled by the spacer length of the gemini surfactants, which may be important in surface patterning and nanofabrication.
Microcalorimetric measurements have been made on the series of gemini surfactants [CMH2M+1(CH3)2N(CH2)SN(CH3)2CMH2M+1] Br2, designated CMCSCMBr2, where M and S indicate the numbers of carbons in the
side chains and spacer respectively, for M = 12 and S = 3, 4, 6, 8, 10, 12. For comparison, parallel
measurements have been made on the series of double-chain, singly charged surfactants dodecyldimethylalkylammonium bromides, designated as C12CNBr with N = 1, 2, 4, 6, 8, 10, 12, where N is the number of
carbons in the secondary alkyl chain. The choice of this second series was to compare each CMCSCMBr2 with
its nearest equivalent monomer C12CS/2Br. The values of the critical micelle concentrations (cmc) were found
to be in good agreement with other measurements, showing a maximum at S = 4−6 for the geminis and a
nonlinear variation with chain length for the C12CNBr series. For both series of surfactants, the enthalpies of
micellization, ΔH
mic, are all exothermic and show a marked minimum in magnitude at S = 4−6 or N = 4−6.
The variation of ΔH
mic and ΔS
mic across both series shows that the balance between enthalpic and entropic
contributions to the process of micellization changes substantially with S or N.
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