School of Chemistry - Theses

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Author: Besford, Quinn Alexander × End date: 2015 × Start date: 2015 × Type: PhD thesis ×

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    Dipolar dispersion forces in liquids: mechanisms of hydrophobic and Hofmeister effects
    Besford, Quinn Alexander ( 2015)
    This thesis gives a study into dipolar dispersion interactions in liquids. A new mechanism for long-ranged dipolar dispersion forces, based on dipole pair correlation functions in solution, and the resulting free energy, is derived and shown to provide closer agreement with experiment than previous theories of dipolar dispersion forces.The new dipolar dispersion interaction is called φ-dispersion. The action of φ-dispersion between thermally rotating waters' produces a long-ranged attraction between hydrophobic surfaces immersed in water. Comparing to experiments for the long-ranged attraction, φ-dispersion is found to explain most of the observed force. Inclusion of φ-dispersion into the Derjaguin-Landau-Verwey-Overbeek theory improves the theory's accuracy for the prediction of attractive forces in aqueous solutions. The level of agreement with experiment cannot be found with Lifshitz theory. At high ionic strengths φ-dispersion is strongly screened, consistent with recent experiments which cannot be explained by Lifshitz theory. The surface tension of water primarily stems from lost φ-dispersion interactions, and accounts for almost all of the interfacial tension with n-alkanes. Simple models based on φ-dispersion are given for the prediction of contact angles and the hydrophobic collapse of polymers. The Hofmeister effect, whereby different ions make solutes more or less hydrophobic, is investigated by simulations. A new free energy resulting from ion-water correlations is found, the strength of which spans -17.6 kJ/mol for cesium to -175.2 kJ/mol for calcium. A Hofmeister series exists in the magnitude of this free energy, resulting from a change in water's structure due to each ion, and its impact on pair correlations. Most crucially, the calculations for the hydrophobic and Hofmeister effects are based on the same mechanism; a change in pair correlation functions due to a solutes presence. This thesis therefore demonstrates a mechanistic link between the hydrophobic and Hofmeister effects.