Abstract
Formation constants have been measured by a solvent distribution method for the ion pairing of an arene sulfonate, methyl orange dye, with two series of quaternary ammonium ions: R4N+(R=Et,n-Pr,n-Bu, andn-Pent) and C6H5CH2R3N+ (R=Me, Et,n-Pr,n-Bu,n-Pent, andn-Hex). Ion pairing increases dramatically as the length of the R group increases beyond butyl. Using a hard-sphere model for contact ion pairs, it is estimated that coulombic attraction contributes about −kT to the binding free energy and decreases slightly with increasing size of R4N+. Other factors related to solvation effects, of which cosphere overlap predominates, contribute from −2kT to −7kT of binding energy. Plots of logK for association as a function of cation size show an inflection with decreasing slope between R=propyl and R=butyl. Possible causes for the inflection are considered.
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Reeves, R.L., Cohen, H.L., Harkaway, S.A. et al. The effect of cation size on ion pairing of methyl orange dye with tetraalkylammonium and benzyltrialkylammonium ions in aqueous solution. J Solution Chem 5, 709–719 (1976). https://doi.org/10.1007/BF00643455
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DOI: https://doi.org/10.1007/BF00643455