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Abstract  

The problem of interaction between organic and water moieties (neutral or ionized water molecular species) is of particular interest in chemistry in view of its implications to physico-chemical behavior of chemical and biological systems. Hydration patterns which result from interaction between hydrophilic and hydrophobic species are non trivial in chemistry. The key issue is that water molecules are able to aggregate in extremely large variety of structural modes. Tetrahedral geometry of intermolecular bonding around water molecule is analogous in geometrical terms to that of intramolecular geometry of carbon atom, known as a source of infinite number of organic structures. In general, space filling with hydrogen bonded water molecules is rather low. It may be illustrated in the following way: volume of neonium atoms is comparable to that of water molecules whilst having atomic mass just 10% higher than molecular mass of water. Thus, liquid neonium and liquid water would have similar densities if molecular packing is of comparable efficiency. The real values are much different, however. Liquid neonium at its boiling temperature has density of 1.20 g cm–3 , thus displaying significantly denser packing that that of water molecules. It certainly means that solid or liquid water has a ‘porous’ structure and may lead to molecular inclusion of foreign (guest) species in the intermolecular space of water framework. This property is not that simple, however, since inclusion of foreign (guest) species is, as a rule, associated with rearrangement of the host framework structure [1]. Anyway, inefficient packing of the mono-component host solid phases may be considered as a prerequisite for its pronounced clathration ability.

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Journal of Thermal Analysis and Calorimetry
Authors: J. Kučerík, H. Čechlovská, P. Bursáková, and M. Pekař

Abstract  

The thermodynamic stability of lignite humic acids (sodium salt) aggregates was studied by high resolution ultrasonic spectroscopy within the temperature interval from 5 to 90°C. The changes in differential ultrasonic velocity (U12) showed strong differences among humic solutions within the concentration range from 0.005 to 10 g L−1. Measurement revealed several transitions which were attributed to the weakening of humic secondary structure. Concentration around 1 g L−1 seemed to be a limit under which the change of the prevalence and importance of hydration occurred. Above this concentration the difference in U12 decreased following the temperature increase which was explained as a dominance of hydrophilic hydration. In contrast, below this concentration, the temperature dependence of U12 resulted in increasing tendency which was attributed to the prevalence of hydrophobic hydration, i.e. uncovering of apolar groups towards surrounding water. Additional experiments in which the humic sample was modified by hydrochloric acid resulted in a slight structural stabilization which lead to the conclusion that humic micelle-like subaggregates form an open-layer assemblies easily accessible for interaction with an extraneous molecule. That was partly verified by addition of propionic acid which brought about even larger reconformation of humic aggregates and exhibition of polar groups towards hydration water. The reversible changes in humate solutions induced by elevated temperatures provided the evidence about the existence of significant physical interactions among humic molecules resulting in formation of various kinds of aggregates. The nature of aggregates, mainly the stability and conformation, strongly depends on the concentration. Evidently, the changes observed in this work cannot be simply explained as expansions or conformational changes of macromolecular coils.

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Abstract  

In order to observe more directly the structural organization of water molecules around a non-polar molecule in an aqueous solution, heat capacity differences between two kinds of solutions (solution I and II) of quaternary ammonium salts were measured. In the solution I stable water structure was retained as much as possible and in the solution II water structure was destroyed either by heating to high temperatures or by irradiating with ultrasonic waves. It was found that the heat capacity differences ((Cp)II-(Cp)I) were slightly positive and its maximum values corresponded to 7-8 percent of the heat capacity of pure water itself.

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Abstract  

The heat capacities of binary aqueous solutions of 1,2-ethanediol, 1,2-propanediol and 1,2-butanediol were measured at temperatures ranging from 283.15 to 338.15 K by differential scanning calorimetry. The partial molar heat capacities at the infinite dilution were then calculated for the respective alkanediols. For 1,2-ethanediol or 1,2-propanediol, the partial molar heat capacities at the infinite dilution of increased with increasing temperature. In contrast, the partial molar heat capacities of 1,2-butanediol at the infinite dilution decreased with increasing temperature. Heat capacity changes by dissolution of the alkanediols were also determined. Heat capacity changes caused by the dissolution of 1,2-ethanediol or 1,2-propanediol were increase with increasing temperature. On the other hand, heat capacity changes caused by the dissolution of 1,2-butanediol are decrease with increasing temperature. Thus our results indicated that the structural changes of water caused by the dissolution of 1,2-butanediol differed from that of the two other alkanediols.

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Abstract  

Enthalpies of solution of 1,4-dioxane, 12-crown-4 ether (12C4), 15-crown-5 ether (15C5) and 18-crown-6 (18C6) have been analyzed from the point of view of preferential solvation of these cyclic ethers (crown ethers) by a molecule of acetone or dimethylsufoxide in the mixtures of water with acetone or dimethylsulfoxide. It has been observed that the carbonyl carbon atom replacement in acetone molecule by sulfur atom brings about completely different behavior of molecules of these solvents in relation to cyclic ethers dissolved in mixed solvents. Crown ethers are preferentially solvated by acetone (ACN) molecules, which is not observed in the case of dimethylsulfoxide (DMSO).

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Abstract  

Supercooling temperatures and enthalpies of mixing with some solvents have been examined for two kinds of solutions subjected to different thermal treatments (solutions I and II) of tetrahydrofuran (THF), isopropyl alcohol (2-PrOH), and ethyleneglycol butylether (BE), and ethyleneglycol isobutylether (i-BE) in order to observe more directly the structural organization of water molecules around a nonpolar molecule in an aqueous solution. For THF and 2-PrOH solutions, supercooling temperatures of solution I were found to be 2–3 degrees higher than those of solution II, and differences ΔHI-ΔHII were found to be about 3 kJ mol−1. It has been concluded that these results directly reflect the difference in the stability of hydrogen-bonded water networks in an aqueous solution.

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Abstract  

Enthalpies of solution of 15-crown-5 (15C5) in the mixtures of water with acetonitrile (AN) or propan-1-ol (PrOH) and benzo-15-crown-5 ether (B15C5) in the PrOH-water mixtures have been measured at 298.15 K. The values of standard enthalpies of solution of 15C5 are negative in the mixtures of water with AN within the whole range of mixture composition and in the mixtures water-PrOH for water content x w>0.1 and those of B15C5 are positive (except the standard enthalpy of B15C5 in pure water) in the system water-PrOH. The results of the calorimetric measurements together with the earlier data for B15C5 in water-acetonitrile mixtures are discussed with regard to the intermolecular interactions that occur in these systems.

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Abstract  

The structure of aqueous solutions of tetraethylammonium chloride was investigated using compressibility and density measurements and positron annihilation methods. The experimental results are different from those obtained earlier for systems where hydrophobic hydration dominates, although some evidences for formation of cage-like hydrates in liquid phase were observed. The results are interpreted, among others, in terms of competition among different hydrates of the tetraethylammonium cations, hydration of chloride anions, and formation of ionic pairs.

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Abstract  

The structure of aqueous solutions of 1,2-butanediol and 1,4-butanediol was investigated using adiabatic compressibility measurements and positron annihilation methods. In the case of 1,2-butanediol the experimental results are very similar to those obtained earlier for systems where hydrophobic hydration dominates. In both cases there are evidences for increased rigidity of the water network, which arises from the formation of hydrogen bonds between diols and water. The usefulness of both the methods applied in investigating the structure of liquid solutions was proved.

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Abstract  

Hydrophilic contributions to thermodynamic partition functions of some metal acetylacetonates and monothioacetylacetonates have been calculated by subtracting from the experimental quantities the contributions due to hydrophobic hydration of the chelate in the aqueous phase and to its interactions with the organic phase. These contributions have been evaluated with the use of a simple model describing solubility of hydrocarbons in water, and the theory of regular solutions, respectively. The results obtained are discussed in terms of different hydration of the chelates in their outer coordination sphere and—for coordinatively unsaturated chelates—also in the inner coordination sphere. Especially important are the effects due to replacing oxygen atoms in the acetylacetonate ligands by sulfur, strongly enhancing the extraction of metal chelates.

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