The Effects of Added Electrolyte

The effects of added electrolytes on a micellar system were discussed in Chapter 4. For the case of ionic micelles, the effect of such addition is to decrease the cmc and increase the aggregation number. Such changes are predictable in micellar systems and might be expected to produce parallel effects on solubilization. In fact, however, the results are not always so easily analyzed. At surfactant concentrations near the cmc, it is usually found that the solubilizing power of a system will increase with the addition of electrolyte, as a result of the greater number of micelles available in the system. At surfactant concentrations well above the 

A study of the solubilization of decanol in solutions of sodium octanoate showed that at low surfactant concentrations the solubilization of the additive increased rapidly after the cmc was exceeded, and continued to do so for some time as the concentration of sodium chloride was increased. At higher surfactant concentrations, however, it was found that there was an initial increase in decanol incorporation, which reached a maximum and then began to decrease as the salt level continued to increase. When the octanoate concentration well exceeded the cmc, the addition of salt resulted in an immediate decrease in the ability of the system to incorporate the additive. Such complex interactions have been attributed to alterations in the thermodynamics of mixed micelle formation for the decanol and carboxylate salt. Similar results may be seen in systems where the increased electrolyte content produces a change in the character of the micellar system a sphere-to-rod micellar transformation or the development of a mesophase, for example. 

In the case of nonionic surfactants, the effects of added electrolytes seem to parallel their effects on the micellization process. When such addition produces an increase in micellar aggregation number, an increase in solubilizing capacity for hydrocarbon additives is also found. The results for the solubilization of polar materials is, again, less clear-cut. A similar trend is generally found for cationic and amphoteric surfactants. 

For ionic micelles, the effect of addition of electrolyte is to decrease the cmc and increase the aggregation number. Such changes are predictable in micellar 

FIGURE 16.2. The inclusion of a polar additive such as a medium to long-chain alcohol often results in an enhancement of the solubilization capacity of a surfactant system. The mechanism of such action probably stems from an increase in the size of the micelles and/or a change in the micellar environment for the solubilized material. 

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A number of investigations concerning the effect of added electrolytes and change in the solvent polarity have been reported (see ref. 25 pp. 17-19 for a more detailed account). The addition of neutral salts causes a large increase in the rate of reaction in both the one and two-proton mechanisms. In addition, the effect of increasing the water content in a dioxan-water solvent, provided that the concentration of water is above a certain threshold value, also produces a large increase in... 

Fig. 4. The effect of added electrolyte (Bu4N+ salts) on the reduction of approximately 20mM [Co(dmg)3(BF)2]BF4 by ferrocene at —20°C in CH2C12. The solid lines represent the parameters fit to all of the data, as described in the text. From top to bottom Br-, Cl-, BF, N03. In all cases, the rate constant in the absence of ion pairing, from the fitting, is 2.8 xlO5 M-1s-1. Reproduced from Ref. (5) by permission of Elsevier Science.

In this work, the critical micelle activity, cma, which is the activity of the surfactant at the cmc, is introduced and used Instead of the cmc to Investigate the free energy of micelle formation. It is found that upon the addition of an extra methylene group into the hydrocarbon chain, an approximately 3-fold reduction in cma is observed, irrespective of the hydrophilic head group. The effect of added electrolyte on cmc is also examined by the use of cma. 

The effect of added electrolyte and/or ionic strength on the rate constant(s) of a reaction in solution, exclusive of any role of that the electrolyte as a reactant or catalyst. 

Lagrange and co-workers (1994) have examined the effect of added electrolytes on the kinetics and find that the rate constant in the pH range from 2.5 to 3.5 increases with ionic strength in a manner consistent with the following ... 

The results in Table 13.1 have been collected for colloids bearing both positive and negative surface charges. One of the earliest (1900) generalizations about the effect of added electrolyte is a result known as the Schulze-Hardy rule. This rule states that it is the valence of the ion of opposite charge to the colloid that has the principal effect on the stability of the colloid. The CCC value for a particular electrolyte is essentially determined by the valence of the counterion regardless of the nature of the ion with the same charge as the surface. The numbers listed in parentheses in Table 13.1 are the CCC values in moles per liter for counterions of the... 

Aroyl esters of anthracene-9-methanol are photolysed in methanol to give products consistent with the anthracene-9-methyl cation as an intermediate.41 Rate constants for the solvolyses of secondary alkyl tosylates in fluorinated solvents were analysed in terms of the possible involvement of very short-lived carbocation-tosylate ion pair intermediates.42 The effect of added electrolytes on the rate of solvolysis of cumyl chloride and its -methyl derivative was studied in 90% aqueous acetone and 80% aqueous DMSO, with the results revealing a combination of a special salt effect and a mass law effect.43 Kinetic parameters obtained for the solvolysis of (8) (R1 = R2 = Me and R1 = Ar, R2 = H) show that there is substantial n, n participation in the transition state [e.g. (9). 44... 

Systematic studies have shown that the effect of added electrolyte on equilibria is independent of the chemical nature of the electrolyte but depends on a property of the solution called the ionic strength. This quantity is defined as... 

Ca ", were varied at a fixed concentrations of surfactant, ligand, and pH. For HDPA in CPC, the effect of added electrolyte (0.1 M NaCl) was evaluated. The rejection coefficient for Cu +, was calculated using the following equation. 

In summarizing the effects of added electrolyte and ion pairing, Wherland (ref. 7) concludes that, study of the effects of ion pairing would he greatly facilitated if the structures, concentrations and lifetimes of... ion paired species could he more directly evaluated. ... ... 

Effects of Ionic Strength. Figure 13 illustrates the effect of NaCl concentration on intrinsic viscosity for each terpolymer. Of course, this experiment should demonstrate only the effects of added electrolyte on individual chain contraction or expansion. The chains with sufficient monomer pairs exhibit increases in viscosity as expected with addition of NaCl. The best chain expansion is seen for the 5-5 sample, which is rapidly solvated with increasing ionic strength. The 5-10 sample shows some typical polyelectrolyte behavior because it has an excess of macroanions at pH 7. 

The effects of added electrolytes (salts) on protein solubility are complex. At low concentrations (typically with salt concentrations below about 0.5 mol dm ), electrostatic screening by the small ions in solution tends to reduce electrostatic interactions between the macromolecules, and the solubility of protein increases. Historically, this is known as the salting-in effect. However, at higher salt concentrations the solubility of protein tends to decrease due to the salting-out effect. 

There have been many studies of the effect of added electrolytes on ET rates [171, 234, 235], The main effect of ionic atmosphere is electrostatic screening, which is usually accounted for in terms of Debye-Huckel theory (mean-field, low-concentration approximation). At sufficiently low ionic strength, the corresponding component of the activation energy is simply proportional to the ratio of the Onsager radius (also referred to as the... 

S. S. Davis. L. Ilium, P. West and M. Galloway. Studies on the fate of fat emulsions following i.v. administration to rabbits and the effect of added electrolytes, Clin. Nutr., 6 13-197, 1987. 

Turning next to the effect of cosolutes on the CMC, this is an important and broad issue that we will come back to later. A most important matter is the effect of added electrolyte on the CMCs of ionics. This is illustrated in Figure 19.7 for the simplest and generally most important case of adding a 1 1 inert electrolyte to a solution of a monovalent surfactant. The following features are noted ... 

Figure 3 represents the effect of added electrolyte concentration on the [nl obtained from the modified Huggins plot for poly(4VMP/pSS) and poly(MPTMA/AMPS), and the usual Huggins plot for poly(METMA/MES). The intrinsic viscosity increases with increasing salt concentration for all three ampholytic systems. Similar results are also reported for other polyampholyte-salt systems (6,13,27,28). This behavior may be rationalized on the basis of chain expansion which results in increased solute-solvent interaction. The [ri] is related to the hydrodynamic volume of macromolecules in solution (29). An expansion of the chain results in the viscosity increase due to an increase in effective hydrodynamic volume of the solute in the given solvent. It is expected that the added electrolyte would disrupt the intramolecular and intermolecular interactions and allow the polymers to behave more freely. Thus, the increase in [n] may be related to extended chain conformations resulting from the increased polymer-solvent interactions. 

The effect of added electrolytes (sodium and potassium halides of progressively increasing molecular volume) in the concentration range 0.125—3 M on the viscosity behaviour of an aqueous sucrose solution (292 mM) between 25 and 40 °C has been investigated. Conductance data on the interaction of the sodium salts of several low-carbon aliphatic acids with sucrose in water and in formamide solutions have been reported and interpreted in terms of the effects that hydrocarbon chains have on hydrogen bonding in saturated solutions of sucrose. Conductance data have also been reported for the interaction of sucrose with symmetrical tetra-alkylammonium halides in formamide and in water in the temperature range 25—70°C. 

The addition of electrolyte to the diphenylmethane derivatives causes an increase in aggregation number as with typical surfactants. The effect of added electrolyte on the cmc of... 

An investigation on the effect of added electrolyte valence and species on polyelectrolytes studied using ACM was made in Reference [46], First-order differences in polyelectrolyte conformations, interactions, and hydrodynamics were reported by the authors when electrolytes of different valence and symmetry were added to the polyelectrolyte solution. Each relevant characteristic, however, followed the same scaling relationship to ionic strength but the prefactors were different. Electrostatic persistence length/electrostatic excluded volume calculations without adjustable parameters suggested that the effective linear charge density is considerably lower in the presence of divalent ions than monovalent ions. Consideration of the... 

Numerous studies have been reported of the effects of added electrolyte on the micellar properties of ionic surfactants. Values of CMC in the presence of... 

Figure 5.21 The effect of added electrolytes on the solubilization of heptane in 1 % aqueous solution 1-R9C6H4.O (CH2CH20)9.2H at 25° C. From Saito and Shinoda [168] with permission.

It would be wrong to assume that non-ionic stabilized emulsions are immune to the effect of added electrolytes. The addition of electrolytes to non-ionic stabilized emulsions can cause pronounced effects on stability. In solutions of non-ionic surfactants, the addition of electrolytes generally causes a dehydration of the ethylene oxide chains by disruption of hydrogen bonds. Selected salts have been shown to exhibit interaction with polyethylene oxide ethers, reducing their solvation and producing more compact molecular conformations [127,128]. 

In both parts of Figure 11.2, the effects of added electrolyte in reducing the extent of the double layer (thickness) and lowering the potential at the surface have been artificially separated. However, in practice both effects would occiu simultaneously and in a combined way reduce the double layer repulsion and consequently the stability of the dispersion. This is because the addition of electrolyte tends to lower the surface (zeta) potential and coagulation would then be expected at a lower ionic strength (see also Figure 10.13 with the zeta potential versus pH of TiOa at various salt concentrations). 

Additional anomalies can be found in the effect of added electrolyte on the erne s of these materials. It is usually found that the addition of electrolyte to... 

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