Thermodynamic evidence for nano-heterogeneity in solutions of the macrocycle C-butylresorcin[4]arene in non-aqueous solvents
Introduction
The occurrence of micelles (or, more generally, nano-heterogeneity) is usually considered to be a special feature of aqueous solutions, arising from the characteristic properties of water as a solvent, reflecting its unique structure and hydrogen-bonding capabilities. However, evidence exists for similar phenomena occurring in purely non-aqueous systems [1], [2], [3], [4]. The present paper provides evidence for the formation of aggregates of the C-butylresorcin[4]arene calixarene: 2,8,14,20-tetrabutyl-4,6,10,12,16,18,22,24-octahydroxo-resorcin[4]arene (CBRA, Fig. 1), using simple thermodynamic measurements, in a selection of protic and aprotic non-aqueous solvents of widely differing character.
Resorcin[4]arenes and their derivatives are versatile macrocycles with many actual or prospective applications in chemistry and biology [5], [6]. For example, they have been used for corrosion control [7] and in catalytic studies [8], [9]. In biological systems, the binding of a resorcin[4]arene with bovine serum albumin has been used to investigate antibody interactions [10]. Their uses in drug delivery and drug targeting [7], [11] have also been examined.
Because of their chemical diversity and structural variety the C-alkylresorcin[4]arenes are useful solutes for probing the behaviour of supramolecular systems in solution. Previous studies [12], [13] reported solution enthalpies of some C-alkylresorcin[4]arenes in alcohols. In this paper, apparent molar volumes and isentropic compressibilities of CBRA in five solvents have been determined from density and speed of sound measurements, respectively, at temperatures from 278.15 to 308.15 K.
Section snippets
Chemicals
CBRA was synthesized in acid media from resorcinol and pentanal using literature procedures [14], [15]. The compound so obtained was recrystallized from 1:1 (v/v) ethanol-water, dried under reduced pressure of ∼5 kPa at 360 K for 24 h, and then stored in dark flasks in a desiccator over freshly activated silica gel. The final purity of the compound was determined by HPLC to be >0.99 mass fraction. Thermogravimetric analysis, TGA, of the purified CBRA showed that the compound was not solvated (
Density of and speed of sound in the neat solvents
The present experimental densities and speeds of sound for the chosen solvents at 278.15 ≤ T/K ≤ 308.15 are listed in Table S1 in the Supporting Information. For DMSO the measurements were restricted to T ≥ 293.15 K, consistent with its melting point. Table S1 also lists for these solvents the corresponding literature values, which are in good agreement (typically better than ±0.1% relative) with the present results.
Apparent molar volumes and isentropic compressibilities
The densities of CBRA solutions in EtOH, 1-PrOH, ACN, DMF and DMSO at the
Discussion
The positive values of the slopes SV and Sκ (Eqs. (5), (6)) are consistent with increasing overlap of solvation shells with increasing solute concentration. Such overlap diminishes the degree of solvent structuring and thus increases both and Vϕ. It is also noted that SV(pre-agg) > SV(post-agg) for all systems studied (Tables 3 and S16), consistent with a decrease in the number of solute-solvent interactions in the presence of solute aggregation.
Conclusions
Apparent molar volumes and isentropic compressibilities of the macrocycle CBRA show two distinct regions of behaviour as a function of solute concentration in five non-aqueous solvents of widely differing character and over a range of temperatures. The observed behaviour is consistent with the existence of solute aggregation (nano-heterogeneity) in the solutions. Detailed consideration of the data suggests that the aggregates formed may contain significant quantities of solvent molecules rather
Acknowledgements
The authors thank the Chemistry Department, Faculty of Sciences, Universidad de los Andes, Bogotá Colombia for financial support.
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