Interfacial properties and aggregation of surfactants.
Item
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Title
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Interfacial properties and aggregation of surfactants.
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Identifier
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AAI9986359
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identifier
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9986359
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Creator
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Mathias, Joann Helena.
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Contributor
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Adviser: Milton J. Rosen
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Date
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2000
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Language
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English
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Publisher
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City University of New York.
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Subject
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Chemistry, Physical
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Abstract
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Bis(quaternary ammonium halide) surfactants (gemini surfactants) having, variously, diethylether, p-dimethylene phenylene, monohydroxypropyl and dihydroxybutyl spacer groups have been investigated by surface tension, interfacial tension and steady-state and dynamic fluorescence techniques. The critical micelle concentration (CMC) and area per molecule (Amin) are shown to deviate from the expected patterns of behavior as the number of carbon atoms in the alkyl chain (n) increases beyond a certain maximum. This aberrant behavior is observed at the hydrocarbon/water as well as the aqueous/air interface. The unexpected values of the physico-chemical parameters at long alkyl chain length have been interpreted on the basis of a concentration region in which sub-micellar or multi-layer structures are forming. Fluorescence measurements provide confirmation of CMC values by an alternative technique. Comparison of the fluorescence emission maxima profiles of the gemini surfactants with those of their mono-quaternary analogues demonstrates that there is a continuously changing shape with change in n for the geminis; whereas, the profiles for non-geminis are invariant. Long-chain geminis exhibit a gradually sloping sigmoidal profile indicating a variety of environments experienced by the probe between the totally aqueous environment at low surfactant concentration and the hydrophobic (entirety micellar) environment at high surfactant concentration. The large variation in the polarity of the probe environment between these two extremes may be attributed to the formation of sub-micellar structures.;Dynamic fluorescence measurements using the time-resolved single photon counting technique provided aggregation numbers for conventional and cationic gemini surfactants. Aggregation numbers of 2 (gemini molecules per micelle) were found in the surfactant concentration region between the expected and observed cmc values for long chain geminis, regardless of the chemical nature of the spacer, indicating dimer formation. Evidence of two observable cmc values was obtained by surface tension and steady state fluorescence measurements. Dimer formation below the first cmc is followed by normal aggregation growth until reaching the second cmc at which concentration dimers are again observed. The aggregation process for long chain geminis appears to be a dynamic process involving: dimer formation → growth → reformation.;In view of the considerable interest in these cost-effective and environmentally sound gemini surfactants, full understanding of their physico-chemical properties and aggregation behavior is highly desirable.
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Type
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dissertation
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Source
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PQT Legacy CUNY.xlsx
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degree
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Ph.D.
