Article Cite This: J. Phys. Chem. B XXXX, XXX, XXX−XXX
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Enthalpies of Solution of C‑Alkylresorcin[4]arenes in Water and Acetonitrile: Hydrophilic and Hydrophobic Effects Diana C. Riveros,† Glenn Hefter,*,‡ and Edgar F. Vargas† †
Laboratorio de Termodinámica de Soluciones, Departamento de Química, Universidad de los Andes, Bogotá D.C., Colombia Chemistry Department, Murdoch University, Murdoch, WA 6150, Australia
‡
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ABSTRACT: Enthalpies of solution, Δ sln H, of six Calkylresorcin[4]arene (C-ARA) solids, with alkyl substituents varying systematically from methyl (Me) to hexyl (Hx), have been measured in water, using solubility data, and in acetonitrile, by direct calorimetry. For all of the C-ARAs studied, the values of ΔslnH were highly favorable (exothermic) for dissolution into water but were strongly unfavorable (endothermic) for dissolution into acetonitrile (ACN). The differences between the two solvents varied systematically with increasing carbon chain length, from about 100 kJ·mol−1 (for Me) to 140 kJ·mol−1 (for Hx). These extraordinary variances can be attributed to the loss of hydrophilic hydration of the eight −OH groups on the rim of the C-ARAs and also by the loss of (enthalpically favorable) hydrophobic hydration of the alkyl chains upon transfer from the highly structured, strongly Hbonding water to the aprotic, relatively weak donor/acceptor ACN. Although ΔslnH values for the present C-ARAs in H2O are strongly favorable, they are more than counteracted by even larger negative changes in the entropy of dissolution, ΔslnS. This enthalpy/entropy compensation effect is consistent with the low aqueous solubilities (0.98
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>0.98
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>0.98
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>0.98
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>0.98 0.995 0.999
a
See the text for details.
synthesized by cyclic condensation under homogeneous conditions using literature procedures.3,20 A resorcinol solution in ethanol−water (1:1 v/v) was acidified with concentrated HCl(aq), and the appropriate aldehyde was added slowly. The stirred mixture was refluxed at 353 K for 24 h, precipitated by water addition, and cooled in an ice bath. The solid was collected by filtration, recrystallized from ethanol, washed with water, and dried at 10 kPa and 333 K for 48 h. All C-ARAs were kept in amber flasks inside a desiccator containing activated silica gel. Product purities were determined by HPLC to be ≥0.99 mass fraction. Solvents 1-propanol (1-PrOH) and acetonitrile (ACN) were commercial reagents, with mass fraction purities ≥0.995, used without further purification. High-purity water (Millipore Milli-Q system, USA) with a conductivity of 100 kJ·mol−1. With these estimates and noting that ΔslnH° is negative (i.e.,
