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.

A series of dissymmetric gemini surfactants, [C m H 2m+1 (CH 3 ) 2 N(CH 2 ) 6 N(CH 3 ) 2 C n H 2n+1 ]Br 2 (designated as C m C 6 C n Br 2 , with constant m + n ) 24 and m ) 12, 13, 14, 16, 18), have been investigated by electrical conductivity measurement, steady-state fluorescence measurement, and time-resolved fluorescence quenching. The critical micelle concentration (CMC) and the micelle aggregation number (N) were determined and the micropolarity of micelle was characterized. The micelle ionization degree (R) was obtained using a combination of electrical conductivity data and N. Furthermore, the Gibbs free energy of micellization (∆G mic ) and the entropy of micellization (∆S mic ) were studied. The results have shown that the degree of dissymmetry (m/n) has an important effect on the micellization in aqueous solution. As the m/n ratio increases, the CMC value decreases linearly, the N value at the CMC increases slightly (from 22 to 30), and R decreases slightly down the series. The value of ∆G mic becomes more negative for greater m/n ratios, which supports the belief that the micellization will be more spontaneous and the contribution, per CH 2 unit, to micellization increases as the m/n ratio increases. The calculated thermodynamic parameter |T∆S mic | is much larger than |∆H mic |, which indicates that the micellization of the C m C 6 C n Br 2 series is entropy-driven. However, the micropolarity of the micelle that is sensed by pyrene varies little, irrespective of the m/n ratio.

The complex formation between sodium carboxymethylcellulose (NaCMC) and dodecyltrimethylammonium bromide (DTAB) at various sodium bromide concentrations (C(NaBr)) has been studied by microcalorimetry, turbidimetric titration, steady-state fluorescence measurements, and the fluorescence polarization technique. The addition of salt is found to influence the formation of NaCMC/DTAB complexes markedly. At C(NaBr) = 0.00, 0.01, 0.02, 0.10, and 0.20 M, DTAB monomers form micelle-like aggregates on NaCMC chains to form NaCMC/DTAB complexes above the critical surfactant concentration (C1). At C(NaBr) = 0.23 M, DTAB molecules first form micelles above a 2.46 mM DTAB concentration prompted by the added salt, and then, above C1 = 4.40 mM, these micelles can aggregate with NaCMC chains to form NaCMC/DTAB complexes. However, at C(NaBr) = 0.25 M, there is no NaCMC/DTAB complex formation because of the complete salt screening of the electrostatic attraction between DTAB micelles and NaCMC chains. It is also surprisingly found that the addition of NaBr can bring out a decrease in C1 at C(NaBr) < 0.20 M. Moreover, the addition of NaBr to a mixture of 0.01 g/L NaCMC and 3.6 mM DTAB can directly induce the formation of NaCMC/DTAB complexes. This salt-enhancing effect on the complex formation is explained as the result of competition between the screening of interaction of polyelectrolyte with surfactant and the increasing of polyelectrolyte/surfactant interaction owing to the growth of micelles by added salt. When the increasing of polyelectrolyte/surfactant interaction exceeds the screening of interaction, the complex formation can be enhanced.

The micellization of six cationic gemini surfactants with various counterions, [C12H25(CH3)2N(CH2)6N(CH3)2C12H25]X2, designated as C12C6C12X2, with X = F-, Cl-, Br-, Ac-, NO3 -, and 1/2SO4 2- in aqueous solutions has been investigated by isothermal titration microcalorimetry (ITC) and conductivity measurements. The interaction of these surfactants with DNA in aqueous solutions has also been investigated by isothermal titration microcalorimetry. The critical micelle concentration (CMC) and the degree of micellar ionization (α), the critical aggregation concentration (CAC), the saturation concentration (C 2) of the aggregation, and the associated thermodynamic parameters were determined. The nature of the counterion significantly affects the processes of both micellization and aggregation. The trends for aggregation basically follow the Hofmeister (lyotropic) series, but the pattern of the variation of the enthalpy of aggregation often revealed a more complex behavior. Among the counterions examined, SO4 2- is the most effective anion for decreasing the CMC (or CAC). Both aggregation processes are mainly entropy-driven since the values of the entropy changes multiplied by temperature are much larger than the absolute values of the enthalpy changes. The binding of micelles to DNA is strongly dominated by the positive entropy gain on release of the small counterions from the micelles and from DNA. The interaction of all of the surfactants with DNA was dependent on the DNA concentration and may be associated with each micelle interacting with more than one DNA molecule.

We have used microcalorimetry to measure the critical micelle concentrations (CMC) and enthalpies of micellization (ΔH mic) of the series of dissymmetric cationic gemini surfactants, [C m H2 m +1(CH3)2N(CH2)6N(CH3)2C n H2 n +1]Br2, designated as C m C6C n Br2, with constant n + m = 24 and n = 6, 8, 10, 11, 12. There is a small decrease in CMC as the ratio m/n increases but this decrease is not much larger than the error. On the other hand there is a very large decrease in the enthalpy of micellization with m/n. This is consistent with a large increase in the hydrophobic contribution to micellization with m/n. To explain the relative invariance of the CMC, there must be a corresponding large increase in the entropy of micellization with m/n. The change in the hydrophobic contribution is explained in terms of the relative strengths of internal and external hydrophobic contacts down the series.

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