Thermodynamic parameters aqueous solutions

A striking feature of the partial molar volumes and heat capacities of aqueous electrolytes is their inverted-U shape as a function of temperature. Experimental data that cover a sufficiently large range of temperature invariably exhibit a maximum, generally somewhere between 50 and 100 °C. This was illustrated in Figures 10.6 and 10.12, which show data for the partial molar volume and heat capacity of NaCl. The existence of singular temperatures for water at -45 "C (228 K, Angell, 1982, 1983) and 374 °C (the critical temperature) makes it seem entirely reasonable that thermodynamic parameters of solutes in water should approach oo at these limits, and therefore reasonable that they should exhibit extrema (or inflection points) between these temperatures. ... 

A large programme utilizing temperature-jump relaxation methods for the study of tautomerism in aqueous solution has led the Dubois group to determine the kinetic and thermodynamic parameters of the equilibrium (130a) (130b) (78T2259). The tautomeric... 

Water plays a crucial role in the inclusion process. Although cyclodextrin does form inclusion complexes in such nonaqueous solvents as dimethyl sulfoxide, the binding is very weak compared with that in water 13 Recently, it has been shown that the thermodynamic stabilities of some inclusion complexes in aqueous solutions decrease markedly with the addition of dimethyl sulfoxide to the solutions 14,15>. Kinetic parameters determined for inclusion reactions also revealed that the rate-determining step of the reactions is the breakdown of the water structure around a substrate molecule and/or within the cyclodextrin cavity 16,17). 

Table 1. Thermodynamic parameters for the association of cyclodextrin with alcohol in an aqueous solution at 25 °C...

Papisov et al. (1974) performed calorimetric and potentiometric experiments to determine the thermodynamic parameters of the complex formation of PMAA and PAA with PEG. They investigated how temperature and the nature of the solvent affected the complex stability. They found that in aqueous media the enthalpy and entropy associated with the formation of the PMAA/PEG complex are positive while in an aqueous mixture of methanol both of the thermodynamic quantities become negative. The exact values are shown in Table II. The viscosities of aqueous solutions containing complexes of PMAA and PEG increase with decreasing temperature as a result of a breakdown of the complexes. 

Equilibrium constants calculated from the composition of saturated solutions are dependent on the accuracy of the thermodynamic model for the aqueous solution. The thermodynamics of single salt solutions of KC1 or KBr are very well known and have been modeled using the virial approach of Pitzer (13-15). The thermodynamics of aqueous mixtures of KC1 and KBr have also been well studied (16-17) and may be reliably modeled using the Pitzer equations. The Pitzer equations used here to calculate the solid phase equilibrium constants from the compositions of saturated aqueous solutions are given elsewhere (13-15, 18, 19). The Pitzer model parameters applicable to KCl-KBr-l O solutions are summarized in Table II. 

All three models may be fit to the thermodynamic data for aqueous solutions of NiClp, LiCl, and their mixtures by adjusting their Gurney parameters. Thus, it is difficult to tell from the thermodynamic data whether one model is more realistic than the others. However only model B is in satisfactory agreement with the spin relaxation data. (Ll)... 

The most important property of cyclodextrins is in their ability to accommodate guest molecules within their cavity, which has a volume of 262 per molecule or 157 mL per mol of [3-CD (cavity diameter 6.0-6.5 A). In aqueous solution, this cavity is filled with molecules of water the displacement of which by a less polar guest leads to an overall decrease in free energy. Stability constants and thermodynamic parameters for complexation of a vast number of guest molecules can be found in ref. [3]. 

The suitability of using solvent extraction for a given separation is determined by thermodynamic and kinetic considerations. The main thermodynamic parameter is the solute distribution ratio, Dyi, between the organic and the aqueous phase. This is given by [Eq. (4.3), Chapter 4] ... 

A thermodynamic parameter (dV/dnB)T,F,n g which describes how the volume of component S in a multicomponent system depends on the change in its amount expressed in mol. Hpiland recently summarized the partial molar volumes of numerous biochemical compounds in aqueous solution. See Dalton s Law of Partial Pressures Concentrations Molecular Crowding... 

The kinetic parameters of Zn(II)/Zn(Hg) electrode reaction in aqueous solution containing perchlorate, nitrate, chloride, and bromide ions were measured at different temperatures (5-50°C) [35]. The Arrhenius activation energy and thermodynamic parameters for the Zn(II)/Zn(Hg) system... 

The electrolyte is usually 20-28% aqueous solution of KOH. Solid-state compositions of KOH aqueous electrolyte obtained by addition of poly(ethylene oxide) [345] or polymer based electrolyte (based on polyacrylates) were also proposed [346]. For low temperature applications, higher concentrations of KOH were used, while for higher temperatures, sodium hydroxide was sometimes applied. The influence of the temperature from 0 to 200 °C, pressure and electrolyte concentration on the thermodynamic parameters of the cells, was studied in detail [347]. 

The amount of water in the reaction mixture can be quantified in different ways. The most common way is to nse the water concentration (in mol/1 or % by volume). However, the water concentration does not give much information on the key parameter enzyme hydration. In order to have a parameter which is better correlated with enzyme hydration, researchers have started to nse the water activity to quantify the amount of water in non-conventional reaction media (Hailing, 1984 Bell et al, 1995). For a detailed description of the term activity (thermodynamic activity), please look in a textbook in physical chemistiy. Activities are often very nselul when studying chemical equilibria and chemical reactions of all kinds, but since they are often difficult to measure they are not used as mnch as concentrations. Normally, the water activity is defined so that it is 1.0 in pure water and 0.0 in a completely dry system. Thus, dilute aqueous solutions have water activities close to 1 while non-conventional media are found in the whole range of water activities between 0 and 1. There is a good correlation between the water activity and enzyme hydration and thns enzyme activity. An advantage with the activity parameter is that the activity of a component is the same in all phases at eqnihbrium. The water activity is most conveniently measnred in the gas phase with a special sensor. The water activity in a liqnid phase can thns be measured in the gas phase above the liquid after equilibration. 

Although gibbsite and kaolinite are important in quantity in some soils and hydrothermal deposits, they have diminishing importance in argillaceous sediments and sedimentary rocks because of their peripheral chemical position. They form the limits of any chemical framework of a clay mineral assemblage and thus rarely become functionally involved in critical clay mineral reactions. This is especially true of systems where most chemical components are inert or extensive variables of the system. More important or characteristic relations will be observed in minerals with more chemical variability which respond readily to minor changes in the thermodynamic parameters of the system in which they are found. However, as the number of chemical components which are intensive variables (perfectly mobile components) increases the aluminous phases become more important because alumina is poorly soluble in aqueous solution, and becomes the inert component and the only extensive variable. 

Figure 6.9 Effect of CITREM concentration on the molecular and thermodynamic parameters of complex protein-surfactant nanoparticles in aqueous medium (phosphate buffer, pH = 7.2, ionic strength = 0.05 M 20 °C) (a) extent of protein association, k = Mwcomplex/Mwprotem (b) structure-sensitive parameter, p (c) second virial coefficient, A2 (rnolal scale) (d) effective charge, ZE (net number n of moles of negative charges per mole of original sodium caseinate nanoparticles existing at pH = 7.2 (Mw = 4xl06 Da)). The indicated cmc value refers to the pure CITREM solution. Reproduced from Semenova et al. (2007) with permission.

In addition to these kinetic studies, there has been a considerable amount of work on the thermodynamic parameters associated with this type of reaction. Thus the interaction of [M(edta)aq] with anions of 8-hydroxyquinoline-5-sulfonic acid (oxs ), ida2- and nitrilotriace-tic acid (nta3-) was thoroughly studied for M = Y, La-Lu (except Pm) by calorimetry and pH titration,419 and AG, AH, AS and K found. Some of these values are given in Table 5. It was, however, concluded from the variation of these parameters with atomic number that all M3+ in aqueous solution have the same coordination number (nine) but that the coordination number of the [M(edta)aq] species changes between Sm3+ and Tb. ... 

Table 104 Coordination Geometry and Thermodynamic Parameters for Nickel Complexes with Saturated Tetraaza Macrocydes in Aqueous Solution at 298 K...

The formation of 1 1 complexes between ethylenediamine-N, N1 -diacetic acid (edda) and Zn11 or Cd11 have been studied by calorimetric (AH) and potentiometric techniques.389 Earlier studies had omitted to allow for protonation reactions of the ethanoato groups in the ligand. Thermodynamic parameters for edda lie between those of nitrilotriacetate and triethylenetetra-mine. Edda appears to undergo a slow metal-ion-catalyzed hydrolysis in aqueous acid solution. 

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