Strength ionic

Ionic Strength Addition of neutral electrolyte, such as NaCl or KBr, causes a decrease in the adsorption of ionic surfactants onto an oppositely charged adsorbent and an increase in their adsorption onto a similarly charged adsorbent. 

These effects are presumably due to the decreased attraction between oppositely charged species and the decreased repulsion between similarly charged species at higher ionic strength. Both the efficiency and effectiveness of adsorption of ionic surfactants onto similarly charged substrates are increased by an increase in the ionic strength of the aqueous phase (Sexsmith, 1959 Groot, 1968 Connor, 1971). 

The presence of polyvalent cations, especially Ca2+, in the solution causes an increase in the adsorption of anionics. This may be due to the adsorption of Ca2+ onto the adsorbent, yielding -charged sites onto which negatively charged surfactant can adsorb (Van Senden, 1968). 

Increasing the ionic strength by adding salt, for instance, NaCl or Na2S04, decreases the solubility of analytes in the aqueous sample. This salting out effect, caused by the fact that water molecules are attracted to the salt ions and thus less available to solvate analytes, may increase the recovery of analytes. This is especially useful in trace determinations. However, increasing the ionic strength almost always increases the recovery and the effect of salt should therefore always be determined experimentally. 

Example 16.5 Estimate the ionic strength of a 100% ionized, 0.05 molar solution of Na2S04 in water. The two ions are Na and S04 . There must also be H and OH but these are normally in small enough concentrations to ignore. 

The gfon tion of Mg + is shown in ([14] p 1647) as -455.30 (kJ/mol) at 7=0 and 458.54 at 7=0.25, Chemical engineers would show that change as a change in activity coefficient, keeping onnation independent of concentrations and of pH. Biochemists take those changes into the values of and K. 

Chemical engineers who are comfortable with 7iL = exp ( Ag77 7) in which K is dimensionless and determined by 

the activity coefficient, f is an ion-specific correction factor describing how interactions among charged ions influence each other. Since the activity coefficient is a non-linear function of ionic strength, the activity is a non-linear function of the concentration, too. 

Only mean activity coefficients can be experimentally determined for salts, not activity coefficients for single ions. The Maclnnes Convention is one method for obtaining single ion activity coefficients and states that because of the similar size and mobility of the potassium and chloride ions  

The calculation of the ionic strength, the summation of the ionic forces, is one-half the sum of the product of the moles of the species involved, m and their charge numbers z,. 

Since an equilibrium is assumed between the transition state and the reactant(s), and because the corresponding equilibrium constant can be expressed in terms of activity coefficients and concentrations to account for the non-ideality of the medium, it follows that there should be an activity coefficient effect upon reaction rates. This is observed as a dependence of the rate constant upon ionic strength - the kinetic electrolyte effect [2]. Thus, for a bimolecular reaction, 

In a bimolecular process between ions of the same charge, an increase in the reaction rate will be observed as the ionic strength increases. Conversely, if the ions are of opposite charges, the reaction rate decreases. If one of the reactants is a neutral species (or if the reaction is unimolecular), the reaction rate becomes essentially independent of the ionic strength, according to this model, and this is approximately true in practice. These effects have been studied in detail and summarised graphically [22]. 

The choice of electrolyte for ionic strength control is not always straightforward, and the following points need to be considered. 

With these matters in mind, NaC104, NaCl and KC1 are usually the most appropriate electrolytes for maintenance of ionic strength. 

50-80 nm is observed. Most fibrils are composed of two thinner structures, 2-4 nm in width. Flocculates at 3.2 pD 6.8 and nematic fluids at 6.8 pD 7.2 also contain fibrils of the 5-10 nm in width and micrometers in length. 

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Often the van der Waals attraction is balanced by electric double-layer repulsion. An important example occurs in the flocculation of aqueous colloids. A suspension of charged particles experiences both the double-layer repulsion and dispersion attraction, and the balance between these determines the ease and hence the rate with which particles aggregate. Verwey and Overbeek [44, 45] considered the case of two colloidal spheres and calculated the net potential energy versus distance curves of the type illustrated in Fig. VI-5 for the case of 0 = 25.6 mV (i.e., 0 = k.T/e at 25°C). At low ionic strength, as measured by K (see Section V-2), the double-layer repulsion is overwhelming except at very small separations, but as k is increased, a net attraction at all distances... 

The presence of the large repulsive potential barrier between the secondary minimum and contact prevents flocculation. One can thus see why increasing ionic strength of a solution promotes flocculation. The net potential per unit area between two planar surfaces is given approximately by the combination of Eqs. V-31 and VI-22 ... 

The adhesion between two solid particles has been treated. In addition to van der Waals forces, there can be an important electrostatic contribution due to charging of the particles on separation [76]. The adhesion of hematite particles to stainless steel in aqueous media increased with increasing ionic strength, contrary to intuition for like-charged surfaces, but explainable in terms of electrical double-layer theory [77,78]. Hematite particles appear to form physical bonds with glass surfaces and chemical bonds when adhering to gelatin [79]. 

Fig. XrV-6. (a) The total interaction energy determined from DLVO theory for n-hexadecane drops for a constant ionic strength - 5.0 nm) at various emulsion pH (b) enlargement of the secondary minimum region of (a). (From Ref. 39.)...

For example, van den Tempel [35] reports the results shown in Fig. XIV-9 on the effect of electrolyte concentration on flocculation rates of an O/W emulsion. Note that d ln)ldt (equal to k in the simple theory) increases rapidly with ionic strength, presumably due to the decrease in double-layer half-thickness and perhaps also due to some Stem layer adsorption of positive ions. The preexponential factor in Eq. XIV-7, ko = (8kr/3 ), should have the value of about 10 " cm, but at low electrolyte concentration, the values in the figure are smaller by tenfold or a hundredfold. This reduction may be qualitatively ascribed to charged repulsion. 

Most of the Langmuir films we have discussed are made up of charged amphiphiles such as the fatty acids in Chapter IV and the lipids in Sections XV-4 and 5. Depending on the pH and ionic strength of the subphase, electrostatic effects can become quite important. Here we develop the theoretical foundation for charged films with the Donnan relationship. Then we mention the influence of subphase pH on film behavior. 

The Debye-Htickel limiting law predicts a square-root dependence on the ionic strength/= MTLcz of the logarithm of the mean activity coefficient (log y ), tire heat of dilution (E /VI) and the excess volume it is considered to be an exact expression for the behaviour of an electrolyte at infinite dilution. Some experimental results for the activity coefficients and heats of dilution are shown in figure A2.3.11 for aqueous solutions of NaCl and ZnSO at 25°C the results are typical of the observations for 1-1 (e.g.NaCl) and 2-2 (e.g. ZnSO ) aqueous electrolyte solutions at this temperature. 

One potentially powerfiil approach to chemical imaging of oxides is to capitalize on the tip-surface interactions caused by the surface charge induced under electrolyte solutions [189]. The sign and the amount of the charge induced on, for example, an oxide surface under an aqueous solution is detenuined by the pH and ionic strength of the solution, as well as by the isoelectric point (lEP) of the sample. At pH values above the lEP, the charge is negative below this value. 

Protems can be physisorbed or covalently attached to mica. Another method is to innnobilise and orient them by specific binding to receptor-fiinctionalized planar lipid bilayers supported on the mica sheets [15]. These surfaces are then brought into contact in an aqueous electrolyte solution, while the pH and the ionic strength are varied. Corresponding variations in the force-versus-distance curve allow conclusions about protein confomiation and interaction to be drawn [99]. The local electrostatic potential of protein-covered surfaces can hence be detemiined with an accuracy of 5 mV. 

In the second case, a thick double layer, Ka 1 (low ionic strength), is assumed. Wlren the surface potential is low, 1, a reasonable approximation is given by... 

This r dependence is also known as a Yukawa potential. This type of potential has been used to describe the behaviour of latex suspensions at low ionic strength. 

The solvent dielectric constant, ionic strength and temperature are chosen to fit the conditions of the experimental studies. The protein dielectric constant is assigned some small value, e.g. 4. The PB calculations are currently carried out with the atomic charges and radii of the PARSE parameter set, developed by Honig and coworkers [17] or that for CHARMM [12]. The PARSE parameter set... 

Table 2. Predicted intrinsic and apparent pKa values for the Cys403 residue in Yersinia phosphatase for different models of the structure the data refer to a temperature of 293 K and an ionic strength corresponding to 150 mM of monovalent salt. See the text for the detailed description of the conditions under which each pK estimation was made. The experimentally determined value is 4.67 [39]...

A cubic lattice is superimposed onto the solute(s) and the surrounding solvent. Values of the electrostatic potential, charge density, dielectric constant and ionic strength are assigned to each grid point. The atomic charges do not usually coincide with a grid point and so the... 

Klapper 1, R Hagstrom, RFine, K Sharp and B Honig 1986. Focusing of Electric Fields in tire Actir e Sit of CuZn Superoxide Dismutase Effects of Ionic Strength and Amino-Acid Substitution. Proteins Structure, Function and Genetics 1 47-59. 

Table 2.5. Apparent second-order rate constants for the catalysed Diels-Alder reaction between Ic and 2, equilibrinm constants for complexation of 2.4c to different Lewis-acids (Kj) and second-order rate constants for the reaction of these complexes with 2.5 (k at) in water at 2M ionic strength at 25°C.

In determining the values of Ka use is made of the pronounced shift of the UV-vis absorption spectrum of 2.4 upon coordination to the catalytically active ions as is illustrated in Figure 2.4 ". The occurrence of an isosbestic point can be regarded as an indication that there are only two species in solution that contribute to the absorption spectrum free and coordinated dienophile. The exact method of determination of the equilibrium constants is described extensively in reference 75 and is summarised in the experimental section. Since equilibrium constants and rate constants depend on the ionic strength, from this point onward, all measurements have been performed at constant ionic strength of 2.00 M usir potassium nitrate as background electrolyte . 

Table 2.7. Hammett p-values for complexation of 2.4a-e to different Lewis-adds and for rate constants (kcat) of the Diels-Alder reaction of 2.4a-e with 2.5 catalysed by different Lewis-acids in water at 2.00 M ionic strength at 25°C.

All measurements were performed at constant ionic strength (2.00 M using KNO3 as background electrolyte) and at pH 7-8. Ligand-catalyst ratio. 

The concentration of surfactant was 3.89 mM above the cmc in each case. Values taken from Chapter 2 and determined at a constant ionic strength of 2.0 M using KNOj as background electrolyte. 

Ionic strength 2.00 M (KNOj). Data taken from Chapter 2. 

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