Abstract
Abstract Ultrasonic absorption coefficients in aqueous solutions of surfactants, octyltrimethylammonium bromide, dodecyltrimethylammonium bromide, and tetradecyltrimethylammonium bromide have been measured in the presence of 2-propanol or ethanol in the frequency range from 0.8 to 220 MHz at 25 °C. The frequency dependence of the observed absorption in solution of octyltrimethylammonium bromide has been characterized by a Debye-type relaxation equation with one single relaxation frequency which decreases with the surfactant concentration. On the other hand, the dependences in solutions of dodecyltrimethylammonium bromide and cetyltrimethylammonium bromide have been well fitted to the equation with two relaxation frequencies. One relaxation frequency decreases with the surfactant concentrations, and the other relaxation frequency shows concentration independence trend. The relaxation mechanisms have been considered on the basis of interaction between surfactant monomer and alcohol aggregate and of a conformational change of the surfactants with long hydrophobic groups to which plural alcohol molecules (dumpling cake) attract. The rate and thermodynamic parameters have been determined for aqueous solutions of octyltrimethylammonium bromide coexisting with 2.00 mol dm−3 2-propanol and 3.00 mol dm−3 ethanol. It has been speculated from the analysis that the dumpling cake of alcohols attaches and detaches on and from the alkyl chains of the surfactants. The number of alcohols in the aggregate is dependent on size of alcohol molecule although the forward and backward rate constants are not very dependent on coexisting alcohols. However, only the orders of the rate constants for the systems of dodecyltrimethylammonium bromide solution with 2.00 mol dm−3 2-propanol have been speculated because of too many parameters which should be determined. Following the proposed reaction model, the relation between the surfactant concentration dependence of adiabatic compressibility and the solution characteristics has been discussed.
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