Ionic strength, and dielectric constant

The kinetics of the reaction between bromopropionate and thiosulfate ions have been studied at 10-40 °C in various ethanol-water mixtures.107 Activation parameters were evaluated as a function of ionic strength and dielectric constant of the medium. The medium effect of mixed solvents on the rate constants of the Menshutkin reaction of triethylamine with ethyl iodide has been studied for binary mixtures of cyclohexane with benzene or ethyl acetate,108 and with chlorobenzene or dimethoxyethane.109 Rates were measured over the temperature range 293.1-353.1 K, and activation parameters were determined. 

These functions vary with pH, temperature, protein concentration, protein species or source, prior treatment, ionic strength, and dielectric constant of the medium. They also are affected by other treatments, macromolecules in the medium, processing treatments, and modification, etc. 

From the preceding discussion of the DLVO theory and equations 9.1-9.5 and 9.8, it is apparent that the stability of a lyophobic dispersion is a function of the particle radius and surface potential, the ionic strength and dielectric constant of the dispersing medium, the value of the Hamaker constant, and the temperature. Stability is increased by increase in the particle radius or surface potential or in the dielectric constant of the medium and by decrease in the effective Hamaker constant, the ionic strength of the dispersing liquid, or the temperature. 

The three variables of pH ionic strength and dielectric constant form an interrelated system for the fractionation of proteins by variation of solubilities, the classic example of separation achieved in this manner being the fractionation of plasma proteins carried out by Cohn and Edsall. 

The effect of ionic strength and dielectric constant of the medium on the rate explains qualitatively the reaction between two negatively charged ions, as seen in Scheme 1. The moderate AH and AS values are favorable for electron transfer reaction. The negative value of AS suggests that the intermediate complex is more ordered than the reactants [11]. The observed modest enthalpy of activation and a higher-rate constant for the slow step indicate that the oxidation presumably occurs via an innersphere mechanism. This conclusion is supported by earlier observations [12]. 

Autocatalytic reaction, complicated mechanism. Assumed rate law. J Data not tabulated dependence of k (or of itots ) on ionic strength and dielectric constant shown in graphs. Temperature dependence agrees with ref. 167. Data of ref. 167 recalculated in ref. 166. Q = 7-methyl-4,10-dithia-I,7,13-triazatridecane QS = l,ll-diamino-3,3,9-trithiaundecane see formulae (16) and (17). NSSN= l,8-diamino-3,6-dithiaoctane. Assumed. 

The acidic periodate oxidations of p-anisidine to 4-methoxy-l,2-benzoquinone, A,Al-diethyl-m-toluidine to methyl-1,4-benzoquinone, Mn(II)-catalysed oxidation of Al-methyl aniline to p-benzoquinone, and 3-chloro-2-methylaniline to 2-chloro-3-methyl-l,4-benzoquinone were studied in acetone/water medium. The reactions were first order in 104 and substrates, the rates increased with increasing ionic strength and dielectric constant, and the plot of 1/ 2 versus [H+] passed through a minimum. 

The functional dependence of the rate on system parameters is called the rate expression. This is frequently referred to as the rate law, but the term law ought to be reserved for fundamental principles, and not used to describe an empirical equation. Important parameters are the concentrations of the reactants and (sometimes) the products, and the temperature. Other quantities may influence the reaction rate. In gas phase reactions total pressure may play a key role (see Chapter 4), while for reactions in solution bulk properties such as ionic strength and dielectric constant may influence the rate. Rate expressions for stoichiometric reactions must be empirically determined, taking any mathematical form that fits the data. The rate will be expressed as a function of temperature, concentrations of species present, and any other parameters that are found to influence the rate... 

The reduction of toluidine blue by 8204 in HCI-H2O has a 1.39 order in dithionite ion. The reaction rate increases with increase in H+. The rate is affected by the ionic strength and dielectric constant of the medium and addition of 804 and N03 retard the rate. ... 

Summary, Gels of denatured protein can be formed only under very specific conditions of pH, ionic strength, and dielectric constant. Apparently just the right balance of attractive and repulsive forces between... 

With increases in both ionic strength and dielectric constant the rate of reaction (i) decreases while that of reaction (iv) increases, while reactions (ii) and (iii), in principle, are unaffected. 

The effect of ionic strength and dielectric constant on the aquation of the complex anion... 

Holba, V., Benko, J., Okalova, K. Effect of ionic strength and dielectric constant on base hydrolysis of monomethyl phthalate. Collect. Czech. Chem. Commun. 1978, 45(6), 1581-1587. 

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