The Mixed Micellization Of Quaternary Ammonium Dimeric Surfactant C₁₂-s-C₁₂·2Br/Alcohol In Aqueous Solutions

Quaternary ammonium dimeric surfactants Ci2-s-Ci2-2Br (s=2,3,4,6) are made up of two identical amphiphilic moieties covalently connected at the level of the headgroups by a spacer group. By this way the separation between the polar headgroups within a molecule of dimeric surfactant is strongly decreased as s is rather small. Thus the surface activity is greatly enhanced due to close packing of these molecules at air/water interface. Alcohols added in the solution of dimeric surfactant affect significantly self-association behavior of the surfactant both in bulk solution and at interface. In this thesis, the effects of w-propanol (up to O^Smol-kg"1) , w-butanol (up to O^Smol-kg"1) , /j-pentanol (up to O.lOmol-kg'1) and n-hexanol (up to O.OlSmol-kg"1) on the micellization of Ci2-s-Ci2-2Br in aqueous solution have been investigated using conductivity and steady fluorescence measurements. The results showed that surfactant and alcohol form mixed micelles with addition of alcohol. In the mixed micelles, the alkyl chains of alcohol molecules penetrate into the micelle interior and the hydroxyl groups lie in the palisade layer of micelle, which weakens electrostatic repulsion between the ionic head groups of surfactants and results in a decrease of cmc and an increase of ionization degree of the micelle. The effects become much striking with increasing the alkyl chain length of alcohol molecule.The alcohol molecules intercalated between surfactant ions increase the average distance between ionic head groups for steric reasons. This leads to reduction of the headgroup density on the micelle surface and thus to increase of ionization degree of counterions. On the other hand, a part of the water in the palisade layer is probably replaced by alcohol and thus decrease the dielectric constant. This should also increase ionization degree of the mixed micelle.The micellar aggregation number, N, decreases with increasing the concentration of alcohol. However, the total aggregation number, NS+N&, increases with increase in the alcohol concentration except for propanol. The number of alcohol molecules in the mixed micelles increases with increasing the alkyl chain of alcohol.The molar fraction (x) of alcohol in the mixed micelle increases generally with increasing mA. The XA increases also with increasing the chain length of alcohol at given mA. At the same mA of ?hexanol, however, xf is almost the same for different 5.The different systems have been studied over a temperature range of 25 to 40 at mA=0.015 mol-kg. The standard free energy , enthalpy and entropy changes in the process of micellization were evaluated based on mass action model. The values of cmc and enthalpy of micellization increase with temperature. Both the entropy and the standard free energy of micellization are found to decrease with temperature or the chain length of alcohol molecule. The results showed that the micellization is an entropy-driven process, which is similar to that of conventional surfactants. But at the constant temperature increase with spacer length.