Marcella Niccoli

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:

Experimental evidence of stable water nanostructures in extremely dilute solutions, at standard pressure and temperature.

This paper presents the results of several experimental methods (FT-IR spectroscopy, UV-vis spectroscopy, fluorescence microscopy (FM), Atomic Force Microscopy (AFM)) evidencing structural changes induced in extremely diluted solutions (EDS), which are prepared by an iterated process of centesimal (1:100) dilution and succussion (shaking). The iteration is repeated until an extremely high dilution is reached, so that the composition of the solution becomes identical to that of the solvent--in this case water--used to prepare it. The experimental observations reveal the presence of supramolecular aggregates hundreds of nanometres in size in EDS at ambient pressure and temperature, and in the solid state. These findings confirm the hypothesis--developed thanks to previous physico-chemical investigations--that formation of water aggregates occurs in EDS. The experimental data can be analyzed and interpreted with reference to the thermodynamics of far-from-equilibrium systems and irreversible processes.

Role of the functional group in the formation of the complexes between α-cyclodextrin and alkanols or monocarboxylic acids in aqueous solutions. A calorimetric study at 25 °C

Abstract The interaction of α -cyclodextrin with 1-alkanols, monocarboxylic acids and α , ω -diols has been studied calorimetrically at 25 °C in water, in phosphoric acid, pH 1.3, and in phosphate buffer, pH 11.3. When a complex is formed, calorimetry enables the calculation of both the enthalpy and the association constant, from which the free energy and the entropy of the process can be obtained. Inclusion complexes are formed by 1-alkanols and monocarboxylic acids. For alkanols, a model is proposed to explain the unusual trend of the association constants at increasing alkyl chain length. The association occurs through the insertion of the guests alkyl chain into the hosts cavity. However, for terms longer than C 6 , two forms of the guest can exist, each one associating to α CD with a different constant and enthalpy. α , ω -Diols associate through a mechanism which involves prevailingly the exterior of α -cyclodextrin. For terms longer than C 7 , another mechanism is proposed which provides the inclusion of the alkyl chain, with the hydroxyl groups both laying outside the cavity. The main role played by the different functional groups, and the forces involved in the association process are discussed in the light of the analysis of the signs and values of the thermodynamic parameters obtained.

Study of the Effects of Cosolvents on the Complexation of β-Cyclodextrin with Alkanols by Calorimetry at 298 K

The formation of complexes between β-cyclodextrin and 1-alkanols hasbeen studied calorimetrically at 298 K in water and in concentrated aqueoussolutions of ethanol or urea. When a complex is formed, calorimetry enables thecalculation of both the enthalpy and the association constant, from which thefree energy and the entropy of the process can be obtained. The effects ofethanol and urea on the hydration cospheres of the interacting substances havebeen investigated through the study of the binary solutions of the involvedsolutes in water and in the mixed solvents. The findings obtained are, then,related to the consequent changes in the association parameters.The forces involved in the association process are discussed in the light of the signsand values of the thermodynamic parameters obtained. The most important featurescoming out from this study are: (i) association in water is an entropy-driven process;(ii) in concentrated aqueous solutions of cosolvent, the enthalpic term contributessignificantly to the Gibbs energy, while the entropic contribution is smaller; (iii) forevery solvent medium employed, the invariance of the entropic contribution withincreasing alkyl chain length of the alkanol is an indication that the relaxation ofwater molecules from the cavity of the macrocycle mainly determines the associationprocess.

Preferential configurations in the aqueous solutions of dicarboxylic acids, diols and hydroxyacids: Calorimetric studies at 298 K

Enthalpies of dilution of binary and ternary aqueous solutions of malonic acid derivatives, α,ω-dicarboxylic acids, α,ω-diols and 2-hydroxyacids have been determined by flow microcalorimetry at 298 K. Enthalpic self- and cross-interaction coefficients of the virial expansion of the excess enthalpies were evaluated and interpreted using the “preferential configuration” model. This working model allows the rationalization of the coefficients in terms of nonstatistical interactions in solution. Information is obtained on the influence of the relative position of the functional groups on hydrophobic interactions. When the functional groups are separated, as in α,ω-substances, the hydroxy–carboxy interaction is a better promoter of hydrophobic interactions than hydroxy–hydroxy and carboxy–carboxy interactions. When the two functional groups are on the same carbon atom, as in 2-hydroxyacids, there is an increased cooperativity of hydrophobic interactions compared with malonic acid derivatives (α-dicarboxylic acids). Chiral recognition is not detected in the aqueous solutions of these substances. This failure has been explained in terms of an intramolecular interaction between the hydroxy and carboxy groups which would mask the influence of the chiral carbon atom on the overall interaction.

Calorimetric and conductometric titrations of nanostructures of water molecules in iteratively filtered water

This work continue the study of the physico-chemical properties of samples of pure, twice distilled water, when subject to a procedure of iterative filtrations through Pyrex glass filters (Büchner funnels). After the filtrations, electrical conductivity and heat of mixing with NaOH and HCl solutions increase. The hypothesis is that the iterative filtration procedure, that involves a flux of energy and material in an open system, is able to induce the formation of “dissipative structures” or nanostructures of water molecules (WNS). Water exhibits an extraordinary auto-organization potentiality triggered by several kinds of perturbations, including mechanical ones. We measured the heats of mixing of acid or basic solutions with such iterated filtered waters (IFW) and their electrical conductivity, comparing with the analogous heats of mixing, electrical conductivity of the solvent. We found some relevant exothermic excess heats of mixing and higher conductivity than those of the untreated solvent. The heats of mixing and electrical conductivity of IFW show a good correlation, underlining a single cause for the behavior of the samples.

Simultaneous determination of solubility, dissolution and dilution enthalpies of a substance from a single calorimetric experiment

Abstract The thermodynamics of dissolution in water of a set of substances has been studied calorimetrically. The examined substances were: potassium chloride, (glycyl-glycyl)diketopiperazine, (alanyl-alanyl)diketopiperazine, (leucyl-glycyl)diketopiperazine. They were chosen on the basis of their solubilities, going from a highly soluble electrolyte to the sparingly soluble diketopiperazines. It is shown that, using a commercially available calorimeter, it is possible to perform in a single calorimetric experiment the simultaneous determination of all thermodynamic parameters characterizing dissolution of a substance in a given solvent, i.e. solubility, dissolution enthalpy and dilution enthalpy. The solubility values in water obtained through the proposed method are in good agreement with those reported in the literature and obtained by other techniques.

The influence of cosolvents on hydrophilic and hydrophobic interactions. Calorimetric studies of parent and alkylated cyclomaltooligosaccharides in concentrated aqueous solutions of ethanol or urea

Heats of dilution in water and in aqueous 7 mol kg(-1) urea and 3 mol kg(-1) ethanol of binary solutions containing cyclomaltohexaose, cyclomaltoheptaose, cyclomaltooctaose, 2-hydroxypropyl-cyclomaltohexaose (HPαCD), 2-hydroxypropyl-cyclomaltoheptaose (HPβCD), methyl-cyclomaltohexaose (MeαCD), methyl-cyclomaltoheptaose (MeβCD) and 2-hydroxypropyl-cyclomaltooctaose (HPγCD) have been determined at 298.15K by flow microcalorimetry. The purpose of this study is to gain information about the influence of urea and ethanol, which have different effects on water structure, on hydrophilic and hydrophobic interactions. The pairwise interaction coefficients of the virial expansion of the excess enthalpies were evaluated and compared to those previously obtained for binary solutions of cyclomaltohexaose and cyclomaltoheptaose. The particular behaviour of cyclomaltooligosaccharides in water is put in evidence with respect to that shown by simple oligosaccharides. The values of the interaction coefficients greatly change in dependence of the solvent medium. They are negative in water for unsubstituted cyclomaltooligosaccharides, and positive for the alkyl-substituted ones, thus marking the major role of the hydrophobic interactions. In concentrated aqueous ethanol, coefficients are negative, while they are positive in concentrated aqueous urea. Urea solvates the hydroxyl group provoking the attenuation of hydrophilic and hydrophobic interactions. Instead, the presence of the cosolvent ethanol, which lowers the relative permittivity of the medium, enhances the strength of hydrophilic interactions.

Role of functional groups in determining interactions in ternary aqueous solutions of enantiomeric α-amino acids: a calorimetric study at 298 K

Cross-homochiral and cross-heterochiral enthalpic interaction coefficients were determined at 298 K by measuring the enthalpies of dilution in water of ternary solutions containing two different α-amino acids of the same or different chirality: phenylalanine, glutamine, leucine, asparagine, cysteine, methionine, citrulline, tryptophan. The interaction of these amino acids with small molecules such as glycine, formamide and urea was also studied in the attempt to separate the contribution of the functional groups to the overall interaction. The data are discussed in terms of the preferential configuration model. It is found that the interaction between the hydrophilic groups, the zwitterions, which is thermochemically attractive, mostly determines the values of the coefficients. In some cases, differences were found between the values of the cross-homochiral and cross-heterochiral coefficients: namely, the variation of the interactions between the side chains significantly influences the energetics of the interaction.

Complexation of modified cyclodextrins with hydroxylated substances in aqueous solutions. Calorimetric studies at 298 K

The formation of complexes between hydroxypropyl-β-cyclodextrin or methyl-β-cyclodextrin and 1-alkanols or cycloalkanols has been studied calorimetrically at 298 K in water and in concentrated aqueous solutions of ethanol with the aim of understanding the effects of the solvent medium on the association process. When a complex is formed, calorimetry enables the calculation of both the enthalpy and the association constant, from which the free energy and the entropy of the process can be obtained. The forces involved in the association process are discussed in the light of the signs and values of the thermodynamic parameters obtained. The most important conclusions from this study are: (i) for linear alkanols, hydrophobic interactions are largely the forces acting in the complexation. That is detected by the small enthalpies and by the high and always positive entropies. For cycloalkanols, entropies are positive or negative—an indication that other forces act in the complexation. (ii) In concentrated aqueous solutions of cosolvent, complexation is characterized by enthalpy and entropy changes which depend on the extent of alteration induced by the cosolvent on the structure of water and on the hydration shells of the interacting substances. (iii) A linear correlation exists between enthalpy and entropy of complexation, thus indicating that inclusion is a process dominated by aquation phenomena and ascribed to the modifications experienced by the solvent in the hydration shells of the interacting substances.