Vittorio Elia

Thermodynamics of Extremely Diluted Aqueous Solutions

An extensive thermodynamic study has been carried out on aqueous solutions obtained through successive dilutions and succussions of 1% wt/vol of some solutes up to extremely diluted solutions, (less than l × 10−5 mol kg−1 ) obtained via several 1/ 100 successive dilution processes. The interaction of acids or bases with the extremely diluted solutions has been studied calorimetrically at 25°C. Measurements have been performed of the heat produced by the mixing of acid or basic solutions of different concentrations, with bidistilled water or with the extremely diluted solutions. Despite the extreme dilution of the solutions, an exothermic heat of mixing in excess has been found in about the 92% of the cases, compared to the corresponding heat of mixing with the untreated solvent. Here, we show that successive dilutions and succussions may permanently alter the physical–chemical properties of the solvent water. The nature of the phenomenon here described still remains unexplained, but significant experimental results are obtained. A thermodynamic study on aqueous solutions gives interesting information about the behavior of solutes and their interactions with the solvent. The interaction of acids or bases with the extremely diluted solutions has been studied calorimetrically at 25°C. The extremely diluted solution is obtained starting from a solution at 1% wt/ vol. After succussion, that solution is named 1CH preceded by the name or formula of the solute. The succussion process consists of vertical shakings of the solution by means of a mechanical apparatus. In a simple succussion process, 100 vertical strokes in six seconds are given to the glass vessel containing the solution. To prepare the successive dilution, 1 g of this solution is added to 99 g of water that again gets succussed, obtaining the 2CH solution. The iteration of this process produces the extremely diluted solutions studied. Measurements have been performed of the heats of mixing of acid or basic solutions of different concentrations with bidistilled water or with solutions, at a concentration of 0.01 mol kg−1, used as reagent, whereas the concentrations of the extremely diluted solutions or with extremely diluted solutions. Procedures for the calorimetric determination of the heat of dilution or mixing are well developed.1 The experimental results are treated according to the MacMillan-Mayer approach,2 modified by Friedman and Krishnan.3 The enthalpies of mixing two solutions are given by the following equations:

Thermodynamics of the interaction of cyclodextrins with aromatic and α, ω-amino acids in aqueous solutions: a calorimetric study at 25°C

Abstract The interaction in water of α- and β-cyclodextrins with l -phenylalanine, l -tyrosine, l -tryptophan, and l -histidine has been studied calorimetrically at 25°C in pure water and in a phosphate buffer (pH 11.3). The interaction in water of α-cyclodextrin with some α, ω-amino acids was also studied. When a complex forms, calorimetry allows the calculation of both the enthalpy and the association constant, from which the free energy and the entropy of the process can be obtained. Aromatic amino acids form 1:1 inclusion complexes, characterized by low values of the association constants. The association occurs through the insertion of the guests aromatic ring into the hosts cavity, and is stronger at pH 11.3 than in pure water. For α, ω-amino acids the association constants increase with increasing lengths of the alkyl chains between the functional groups. For this class of substances the association is supposed to occur through an interaction mechanism different from inclusion, which involves mainly the exterior of cyclodextrin. The analysis of the signs and values of the thermodynamic parameters obtained permits hypotheses to be made about the forces involved in the association process.

Excess enthalpies of aqueous solutions of mono- and oligo-saccharide at 25°

Abstract The heats of dilution in water of d -xylose, d -fructose, d -galactose, d -mannose, lactose, and raffinose have been determined calorimetrically at 25°. The calorimetric data, expressed in terms of excess enthalpy, lead to an evaluation of pair- and triplet-interaction coefficients. Osmotic data, where known, permit the analogous coefficients of the excess free energy to be obtained and thence those of the excess entropy. Analysis of the excess functions and comparison with spectroscopic properties permits some qualitative hypotheses to be formulated on the molecular interactions occurring in these solutions.

Chiral recognition between enantiomeric α-aminoacids. A calorimetric study at 25°C

Homotactic and heterotactic enthalpic pair interaction coefficients were determined at 25°C from the enthalpies of dilution of binary and ternary aqueous solutions containing the L and D forms of α-aminoacids bearing alkyl chains of increasing length (alanine, α-aminobutyric acid, valine, norvaline, leucine, isoleucine and norleucine). Significant differences were found between the values of the homotactic (hDD or hLL) and heterotactic (hDL) pairwise coefficients, for the enantiomers of leucine and its isomers. The role of the zwitterionic interactions is discussed to explain the nature of the chiral recognition.

The hydrophobic effect in aqueous solutions of nonelectrolytes. I. Self-interactions of alkylureas

In order to clarify some aspects of the hydrophobic interactions, the enthalpies of dilution of monoethylurea, 1,3-dimethylurea, and 1,3-diethylurea have been determined calorimetrically at 25°C. The calorimetric data, expressed in terms of excess enthalpy, permit the evaluation of the pair and triplet interaction coefficients. The analyses of these and of the analogous coefficientsgxx andgxxx, derived from osmotic data, indicate a driving force favorable to the interactions among the hydrated solute molecules. Nevertheless, the positive values of thehxx andhxxx coefficients seem to suggest that the source of the effect is a rearrangement of the water molecules rather than a direct association of the solute molecules. There are evidences of a strict correlation between the enthalpic and the entropic effects.

Enthalpies of solvation for N-alkylamides in water and in carbon tetrachloride at 25°C

Molar enthalpies of solution at infinite dilution have been determined at 25°C for several N-alkyl and N,N-dialkylamides in water and in carbon tetrachloride, using a Calvet-type rotating calorimeter, and solution concentrations below 5×10−2 molal. Relevant enthalpies of transfer between the two solvents also have been derived. Molar enthalpies of solvation have been obtained by adding enthalpies of vaporization to solution values. Results are compared with those of other laboratories on other substituted amides, and their dependence on the number of carbon atoms in the chain is discussed. A possible computation of solvation enthalpies of functional groups is suggested and results for hydration of peptide or similar groups present in the compounds examined are discussed in terms of current models of their hydration and hydrogen bond formation.

Interactions in aqueous solutions of urea-like compounds. Heats of mixing of urea, monomethylurea and thiourea at 298.15 K

The heats of mixing of aqueous solutions of monomethylurea, urea and thiourea were determined at 298.15 K and the excess enthalpies obtained were expressed in terms of pair- and triplet-interaction coefficients.The validity of the empirical rule: hij2≈hij hjj, relating the self- and cross-interaction coefficients, was tested for the solutes under examination, for which both hii, hjj < 0.

Excess enthalpies of aqueous solutions of polyols at 25

The excess enthalpies of aqueous solutions of nine polyols were determined at 25°C and reported in the virial form. The most interesting and new feature of this family of solutes is that the sign of the enthalpic pair interaction coefficients hxx is positive for the first members of the series and negative for the higher homologues. Other points are the large differences found among the values of hxx for stereoisomers, whereas pairs of enantiomers show the same values within experimental errors. An application of the group additivity method is also discussed.

Inclusion compounds in water: Calorimetric and spectroscopic studies of the interaction of cyclomaltohexaose (α-cyclodextrin) with alkanols at 25°

Abstract A study of the interaction in water of some alkanols and alkanediols with cyclomaltohexaose (α-cyclodextrin, αCD), by calorimetry and 1 H-n.m.r. spectroscopy at 25°, indicates that the alkyl chains are included in the cavity of αCD, that 1:1 complexes are formed, and that the formation constants and enthalpies are markedly dependent on the number and position of the hydroxyl groups in the chain. For α,ω-alkanediols, the interaction is more complex than a simple inclusion.

Hydrophobic interactions of alkanols. A calorimetric study in water at 298.15 K

Enthalpies of dilution in water of binary aqueous solutions of n-diols of increasing alkyl chain length are performed using a flow microcalorimeter. The pairwise enthalpic coefficients of these hydrophobic solutes depend mainly on the mutual positions of the —OH groups in the molecule. A rationalization of the excess enthalpies of positional isomers is attempted.