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Fundamental Questions About Entropy IV. The Hydrophobicity Of Alkanes: A Nearly Pure Entropic Effect

Pierre Huyskens, Günter Siegel
Published 2010 · Chemistry
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The low solubility of low-molecular weight alkanes in water is not due to the necessity of breaking H-bonds in water upon introducing the solute. It is also not due to the breaking of a special kind of cohesion forces “hydrophobic interactions” in the organic liquid. Experimental evidences are given demonstrating that the hydrophobic effect of water and alcohols with respect to liquid alkanes is nearly purely entropic. The introduction of the alkane has practically no influence on the fraction γ but extends the standardized domain of one molecule of the hydrogen bonded liquid. The hydrophobic effect is essentially due to this extension of the territory of the mobile order. To a good approximation the increase of the chemical potential of the alkane caused by this effect is given by μHB = -nAT(∂ΔSA mobile order/∂nB). On the other hand, contrary to the assumptions of multicomponent theories, the combinatorial entropy of the mixture is practically not affected by the presence of the H-bonds. From the value of the combinatorial entropy derived in part two, and neglecting all enthalpic effects one derives an equation predicting the solubilities of alkanes in water (in volume fractions) ϕB = exp (0.5(VB/18-1) + 0.5lnVB/18 - 2VB/18). This equation, that does not contain any adjustable parameter, predicts the solubility of all the alkanes within 50% error in a domain which varies from 10−4 to 10−7 volume fraction.

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