Temperature effect on the solvation of two ionic resorcin[4]arenes from volumetric and acoustic properties in polar media
Introduction
Intermolecular interactions are the corner stone of supramolecular chemistry. For example, supramolecular receptors based on resorcin[4]arenes use hydrogen bonding or ionic interactions to form capsules and cation-π interactions to form inclusion complexes [1]. Such versatility arises from their structure (Fig. 1), more exactly from their internal cavity that allows inclusion of guest molecules. Additionally, eight hydroxyl groups are present for inter- and intra-molecular hydrogen bonding. Many of the interesting applications, which supramolecular receptors may have, take place in solution where processes like solvation compete against the formation of supramolecular ensembles [2]. Traditionally, this has been overcome with structural modifications at the expense of introducing new limitations, like reduced solubility. Consequently, if supramolecular structures are desired to be used in chemical applications, knowledge of the interactions with the surrounding media, namely solvation, is needed.
Solvation has been traditionally studied by analysis of bulk properties. Among them, partial or apparent molar volumes and isentropic compressibilities are two quantities, derived from the solution density and speed of sound, that are able to provide an overview of the interactions between solute and solvent. This information is obtained after appropriate analysis of the derived quantities as function of concentration. Thus, determination of these properties for solutions of supramolecular receptors is convenient. In this case, resorcin[4]arenes are suitable model solutes since their synthetic versatility [1] allows the preparation of different structures in large amounts with high purity.
In the present work, the standard molar volumes V2ο, and standard molar isentropic compressibilities κs,ϕο, of two ionic C-methylresorcin[4]arenes: (sodium (14,16,34 36,54,56,74,76–octahydroxy-2,4,6,8-tetramethyl-1,3,5,7(1,3) tetrabenzenacyclooctaphane 15,35,55,75 tetrayl) tetramethanesulfonate; Na4MeRA, Fig. 1, with R = -CH2SO3Na) and (1,1′,1″,1‴-(14,16,34,36,54,56,74,76-octahydroxy2,4,6,8-tetramethyl-1,3,5,7(1,3)- tetrabenzenacyclooctaphane-15,35,55,75-tetrayl)tetrakis(N,N-dimethylmethanaminium) chloride; TAM · (HCl)4, Fig. 1 with R = –CH2N(CH3)2·HCl) were studied in water and dimethylsulfoxide (DMSO). The compound Na4MeRA is known for its ability to interact with different guests in water [3] and to form with TAM·(HCl)4 an ionic ensemble with potential host-guest capabilities [4]. Unfortunately, the resulting capsule-resembling structure, based on ionic interactions, has a limited solubility [4]. The present study tries to address this limitation by elucidating the solvation of these macrocycles in solution. This was done by measuring density and speed of sound of the corresponding aqueous and DMSO solutions of each ionic resorcin[4]arene. A concentration range was chosen where solute-solute interactions are minimal, being <0.1 mol·kg−1 and 0.045 mol·kg−1 for aqueous and DMSO solutions, respectively. In the selected temperature range (273.15 to 308.15 K and 293.15 to 308.15 K for water and DMSO, respectively), changes in solvent-solvent interactions are appreciable due to its proximity to the melting points of the solvents.
Section snippets
Materials and chemicals
Synthesis of Na4MeRA and TAM·(HCl)4 was done by following already reported procedures [[3], [4], [5]]. After synthesis, these compounds were purified by recrystallization using water-ethanol mixtures. Characterization was done using 1H NMR, TG and Q-TOF LC/MS. The signal pattern of the 1H NMR in deuterated water (Fig. S1 of the Supporting Information) suggest a prevalence of the crown conformation. Analysis of the thermogravimetry (TG) suggests a mass loss coherent with the presence of the
Standard molar volumes
From the measured solution densities, ρ, Tables S1–S4 in the Supporting Information, the apparent molal volumes, Vϕ, of the solutions of TAM·(HCl)4 and Na4MeRA were calculated using the equation:where M2 is the molar mass of the C-methylresorcin[4]arene (Na4MeRA: 1008.9 g·mol−1, TAM·(HCl)4: 918.8 g·mol−1), m is the molal concentration and ρ0 is the density of the pure solvent.
Tables S1–S4 show the calculated Vϕ values as a function of m at different temperatures. For all
Conclusions
Standard molar volumes and compressibilities of two resorcin[4]arene salts were determined in water and DMSO as a function of temperature from measurements of solutions density and speed of sound. Analysis of the standard molar volumes revealed that they are dominated by the intrinsic volume of the resorcin[4]arene ions. Subtraction of the intrinsic contribution to yield the interaction volume Vinter, revealed that MeRA4− interacts more strongly with the solvents than TAM·H4+. These results
Acknowledgements
The authors gratefully acknowledge support from Facultad de Ciencias at Universidad de los Andes.
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Standard molar properties of ionic resorcinarenes in water and dimethysulfoxide
2023, Journal of Molecular Liquids