INVESTIGATION OF THE SELF-ASSOCIATION OF FLUORINE-LABELED ALKYLAMMONIUM CARBOXYLATES IN NONAQUEOUS SOLUTIONS BY NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY (REVERSE MICELLES, DIPOLE-DIPOLE DISTANCE, THERMODYNAMICS, NMR)

BARBARA JOAN KONIECZKA, Purdue University

Abstract

The concentration dependence of ('19)F chemical shifts provides information about the self-association of N-alkylammonium-4,4,4-trifluorobutanoates in benzene and toluene. The aggregation is described by a single equilibrium model which yields small (less than four) noninteger average aggregation numbers (indicating that the systems are polydisperse) and equilibrium constants for the average aggregates. The equilibrium is better described by a sequential indefinite self-association model having K(,12) (NOT=) K(,13) = K(,14) = . . . The equilibrium constants obtained from this treatment are used to calculate population distributions for the species present in solution. In no case was there an appreciable concentration of any species larger than an eicosamer. The temperature dependence of the K(,i) is used to determine the enthalpies of formation of the individual i-mers formed from i ion-pairs. All values of the enthalpies are exothermic and they become more negative with increasing aggregation number and decreasing N-alkyl chain length. Dependence of the enthalpies on the chain length is exploited to separate the electrostatic and hydrocarbon contributions for the alkylammonium carboxylate-benzene systems. The electrostatic terms are all negative and become more exothermic with increasing aggregation number. The hydrocarbon contributions to the heat of reaction are positive. The entropies of association are also negative and become more negative as the chain lengthens. The electrostatic and hydrocarbon contributions to the entropy are also calculated. The electrostatic term is negative and becomes more negative as the degree of aggregation increases. The hydrocarbon contribution is positive and dependent on the chain length. The positive entropy contribution per methylene group is attributed to release of a solvent molecule upon association. The dipole approximation is used to calculate the separation between ion-pairs. The distance, 4.7 (ANGSTROM), may correspond to a lamellar structure of contact ion-pairs. The relative values of the contributions to the enthalpy and entropy terms indicate that the process is controlled by the electrostatic enthalpy of interaction of the ion-pairs.

Degree

Ph.D.

Subject Area

Chemistry

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