The hydrophobic interaction (HI) is the tendency of apolar species to aggregate in aqueous solutions, in order to minimize unfavourable interactions with water. HI is the principal force determining the structures of proteins and nucleic acids, and the binding of substrates to enzymes.¹

HI in aqueous solutions has been paid much attention recently. Reaction rates are frequently determined by HI between the reactants and water or with a cosolute. HI can stabilize the reactants thus diminishing the reaction rate. Accordingly, stabilization of encounter complexes of the reagents by HI brings about a rate enhancement.¹

We have shown that ultrasonication (US) can cause changes in the translational energy of species, thus leading to a solvent structure break or to a shift of solvation equilibria, HI included.² In aqueous ethanol solutions, at ethanol content 40 - 50 wt % hydrophobic reagents can be hidden in the clusters formed of ethanol molecules and thus made inaccessible for the reaction. Ethyl, n-propyl, and n-butyl acetates were used as probes of the inclusion of a reagent in the clusters. Indeed, the US effect on rates of the acid-catalyzed hydrolysis of esters correlated with the order of hydrophobicity of the esters. Butyl acetate should be the most powerfully held by clusters, and US was the least efficient in this case.

A logical inference was an unfavourable effect of US upon the reactions promoted by HI, e.g. Diels-Alder reaction, the benzoin condensation, etc.¹ Indeed, we observed slowing down of the benzoin condensation reaction of benzaldehyde by US in water and in water-ethanol solutions.³

In conclusion, if breaking down the stabilization of encounter complexes between the reagents, US suppresses the reaction rate, while perturbation of the solvent-stabilisation of the reagents by US accelerates the reaction. Thus, US is a useful tool for detection of hydrophobic interactions in solutions.

[1] Blokzijl W., Engberts J. B. F. N. (1993), Angew. Chem., Int. Ed. Engl., 32, 1545-1579 Otto S., Engberts J. B. F. N. (2003) Org. Biomol. Chem., 1, 2809-2820

[2] Tuulmets A., Salmar S., Hagu H. (2003) J. Phys. Chem., B., 107, 12891-12896 Salmar S., Cravotto G., Tuulmets A., Hagu H. (2006) J. Phys. Chem., B., 110, (in press)

[3] Tuulmets A., Hagu H., Salmar S. paper in preparation

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1. Effect of ultrasound on hydrolysis of 4-nitrophenyl acetate. A kinetic anomaly associated with the use of titanium horns.