| Quaternary ammonium dimeric surfactants alkanediyl-a,o-bis(dimethyl dodecylammonium bromide) (Cm-s-Cm-2Br) are made up of two cationic surfactants connected at the lever of the head groups by a hydrocarbon spacer chain. The distance between the headgroups in Cm-s-Cm-2Br molecule can be controlled through designing and synthesizing different length of the spacer chain. This results in a change in the molecule shape and in the alkyl chain density of the surfactant molecule, and also in the charge density of the ionic headgroups and their hydration, which should directly affect self-association of surfactant molecules and structure of the aggregate.In this thesis, electrical conductivity experiments were carried out on aqueous solutions of Cm-s-Cm-2Br (/w=8,10,12,14; 5=2,6 and m=12; 5=3,4) as a function of surfactant concentration and temperature. The critical micelle concentration (cmc) was obtained from the break point in the curve of conductivity K vs concentration C. The Gibbs free energy, enthalpy and entropy changes in the process of micellization were evaluated based on mass action model. Using the steady fluorescence and viscosity measurements, self-assembly behavior of Ci2-5-Ci2-2Br (5=2,3,4,6) was examined in aqueous solution, including the aggregation number N, the shape and the structure of Ci2-5-Cn-2Br micelle. According to the stepwise association equilibrium model stated out by Moroi, it was investigated that solubilization of the polycylic aromatics (naphthalene, anthracene, pyrene) in aqueous micellar solution of Cm-s-Cm-2Br. The interaction between Cm-s-Cm-2Br micelles and the polycylic aromatics was also investigated by UV absorption spectrum technique. The results showed that:1 The cmc and the Gibbs free energy AG of the C s-C Br surfactants have much lower values compared with their monomer, alkyltrimethyl-ammonium bromide (CmTABr). The results show much stronger ability for the micelle formation of C|2-5-Ci2-2Br surfactants compared with that of CnTABr. A molecule of Cm-s-Cm-2Br rather than a single chain in the molecule determines the values of cmc and AG癿. The values of cmc increases first then decreases with increasing 5 and reaches a maximum at 5=4 for Ci2-s-Ci2-2Br, which is quite different from that of conventional surfactants.2 The Gibbs free energy, enthalpy and entropy changes show that the micellization of Cm-s-Cm-2Br surfactants is an entropy-driven process, which is similar to that of conventional surfactants. All the enthalpy-entropy compensation plots exhibits an excellent linearity. The intercept of compensation line of Cm-s-Cm-2Br surfactant decreases as 5 from 6 varying to 2 or m from 8 to 14. This implies that the shorter spacer or the longer alkyl chain favors to form more-packing and steadier micelle.3 The aggregation number N of Ci2-s-Ci2-2Br micelles increases with increasing concentration C. Specially for 5=2, the value of N rapidly increases at OT.Tmmol-L"1. The calculated result indicates asignificant change in the micellar shape. In fact, prolate ellipsoid micelles are formed at OT.Vmmol-L"1. The Viscosity measurement for Ci2-2-Ci2-2Br micellar solutions demonstrates the above results.4 The absorption spectra of the polycylic aromatics in the C]2-s-Ci2-2Br aqueous micellar solutions emerged a red shift. The red shift may be caused by cation-7i electron interaction between the polycylic aromatics and quaternary ionic head groups as the polycylic aromatics are generally located in the palisade layer of micelle. The values of AT increased with an order of naphthalene, anthracene to pyrene.5 In general, the lower aggregation number of micelle with the space chain length increasing, the stronger capacity of micellar solubilization. Therefore, the vacuum in the palisade layer of micelle is a key factor to determine the amount of polycylic aromatics incorporating into Cn-s-C12-2Br micelles. The stability in planar-ring of the molecule of polycylic aromatic hydrocarbon effected also its solubilization. It was shown by the solubilization isotherms that...
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