Doctoral Dissertations

Date of Award

12-1973

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Chemistry

Major Professor

John Larsen

Abstract

Viscometry was used to investigate micelle sizes in hexadecyltrimethylammonium bromide solutions containing both inorganic salts and organic solutes. Increasing concentrations of sodium bromide produced rod-shaped micelles of increasing size. The electroviscous effect for hexadecyltrimethylammonium bromide micelles in aqueous solution was estimated to be 2.37. The solutes n-hexane and n-octane decreased micelle size in 0.1 M hexadecyltrimethylammonium bromide-0.1 M sodium bromide; the solutes benzene and 1-hexanol increased micelle size in the same system. The viscosity of 0.1 M hexadecyltrimethylammonium bromide was increased by electrolytes in the following order (a lyotropic series): sodium fluoride < sodium bromide < hydrogen bromide < sodium nitrate < potassium chlorate.

Counterion binding to micelles was investigated using ion-specific electrodes and calorimetry. At the critical micelle concentration the degree of dissociation of the micelles of decyltrimethylammonium bromide, dodecyltrimethylammonium bromide and hexadecyltrimethylammonium bromide was found to be independent of the concentration of added sodium bromide. The same behavior was observed for the first two surfactants at concentrations above the critical micelle concentration. For 0.005 M and 0.01 M hexadecyltrimethylammonium bromide the degree of dissociation of the micelles increased with increasing sodium bromide concentration; at 0.1 M surfactant the opposite behavior was observed.

A lyotropic series was found to exist for displacement of bromide ions from hexadecyltrimethylammonium bromide micelles by other ions; chloride - hydroxide < nitrate < tosylate. In the presence of increasing concentrations of hydroxide and nitrate ions the degree of dissociation (including all ions) of hexadecyltrimethylammonium bromide micelles decreased or stayed roughly constant. A lyotropic series for the enthalpy of binding of anions to these micelles was also observed: hydroxide < nitrate < tosylate (most exothermic).

Observed enthalpies of transfer for anions from water to 0.1 M hexadecyltrimethylammonium bromide or 0.075 M dodecyltrimethylammonium bromide could be arranged in a lyotropic series: carbonate < sulfate < acetate < formate < nitrate < bromide < tosylate (most exothermic). The observed enthalpies of transfer gave straight lines when plotted against the Stokes' Law hydrated radii for the anions. Lyotropic effects with cations were also observed for these surfactants. Observed enthalpies of transfer from water to 0.04 M sodium dodecylsulfate for cations also showed a lyotropic series: lithium < sodium < ammonium < potassium < tetramethylammonium < calcium.

Interactions of organic solutes with hexadecyltrimethylammonium bromide micelles were studied by calorimetry as well as viscometry. London dispersion interactions of solutes with surfactant N-methyl groups were found to be important. Compounds containing aromatic rings were found to have especially favorable enthalpies of transfer from water to 0.1 M hexadecyltrimethylammonium bromide. Observed enthalpies of solution for phenols in 0.1 M hexadecyltrimethylammonium bromide supported surface solubilization for these compounds. Ultraviolet spectroscopy also supported this type of solubilization.

Benzene and benzoic acid were found to displace bromide Ions from the micelles In 0.1 M hexadecyltrlmethylammonium bromlde-0.1 M sodium bromide. Benzene caused line-broadening in the nuclear magnetic resonance spectrum of the methylene and N-methyl protons of the surfactant.

Alkaline hydrolysis of alkyl salicylates was not catalyzed by hexadecyltrlmethylammonium bromide. This lack of catalysis was proposed to be due to a difficult proton transfer prior to decomposition of the tetrahedral Intermediate for the hydrolysis. Rate versus concentration profiles for bimolecular reactions on micellar surfaces when one reagent Is hydroxide Ion were rationalized in terms of decreasing hydroxide Ion concentration in the micellar phase. Hydroxide Ion binding data for hexadecyltrlmethylammonium bromide micelles supported this explanation.

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