Critical salt concentration

Figure 4.11 The elFect of a on the precipitation of PAA by divalent ions. Q is the critical salt concentration. Based on Ikegami Imai (1962).

Fig. 19 and also 22 show the existence of a critical salt concentration for flocculation t high pH values (pH> 3.5). At low salt concentrations c s 10 mol/1 no flocculation exists... 

In the mucosal environment, effects of salt, pH, temperature, and lipids need to be taken into consideration for possible effects on viscosity and solubility. A pH range of 4-7 and a relatively constant temperature of 37°C can generally be expected. Observed solution properties as a function of salt and polymer concentration can be referred to as saline compatibility. Polyelectrolyte solution behavior [27] is generally dominated by ionic interactions, such as with other materials of like charge (repulsive), opposite charge (attractive), solvent ionic character (dielectric), and dissolved ions (i.e., salt). In general, at a constant polymer concentration, an increase in the salt concentration decreases the viscosity, due to decreasing the hydrodynamic volume of the polymer at a critical salt concentration precipitation may occur. 

In our model study reported in this contribution, we have chosen two double-chained C-13 alkylbenzenesulphonate surfactants (SLABS) of closely-related structure, which form micelles in aqueous solution in the absence of salt. However, when small amounts of electrolyte are added (e.g., —20mM NaCl), vesicles are spontaneously formed over a time period of seconds/minutes. These vesicle structures are then reasonably stable over a period of hours/days. The onset of vesicle formation can be readily characterised by the determination of the critical salt concentration (esc), needed to induce the formation of vesicles, from smaller aggregates or monomers. This parameter is easily determined experimentally from the increase in light scattering associated with self-assembly. It has now been determined for a number of electrolyte systems. 

Figure 9.12. Effect of electrolyte concentration (NaHCOs) on dispersibility and the determination of critical salt concentration (from Arora and Coleman, 1979, with permission).

Figure 9.15. Relationship between critical salt concentrations for reference clay minerals and soil clays in mixed-ion systems (raw data were taken from Arora and Coleman, 1979).

TABLE 9.4. Influence of pH on the Critical Salt Concentration (NaHCOj) of Reference Clay Minerals and Soil Clays... 

TABLE 9.5. Critical Salt Concentrations (CSC) of Mixtures of Kaolinite and Smectite Determined in NaHCQ4 Solutions... 

Constant capacitance model, 186 Coordination number, 118 Copper, 430, 433-434 Critical Salt Concentration (CCC), 381... 

Concentrated emulsions no longer form above a critical salt concentration, because at such high salt concentrations water and salt are organized to such an extent that the head group of the surfactant molecule no longer has more favorable interactions with the water molecules than with the oil molecules. For this reason the surfactant molecules are salted out from the water into the oil environment and the amount adsorbed on the water-oil interface becomes small. This also indicates that salt alone cannot stabilize the concentrated emulsion. 

This mathematical condition defines a critical separation in terms of a critical double layer thickness, k whidi can then be used to relate the critical salt concentration to the valence of the counterion. The result is the relation between the salt concentrations responsible for the CCC s being proportional to the counterion valences to the —6 power. 

This can be converted to molarity by dividing by Avogadro s number, 6 x 10", giving 0.08 M for the critical salt concentration to induce flocculation. 

The phenomenon of interest is about as simple to demonstrate as it is dramatic. If we imagine bubbles formed by passing a gas through a glass frit at the base of a column of water, the bubbles fuse on collision and grow in size as they ascend the column. However, on addition of an alkali halide, beyond a certain critical salt concentration the bubbles will not fuse and remain the same size. That phenomena has been explored by several authors. On the other hand, increasing concentrations of HCl do not have any effect on bubble coalescence (cf. Fig. 3.5)... 

There is no known mechanism that can account for these effects. Water structure has to be implicated. But there is clearly a remarkable correlation between the ions present in a salt and their effect on the coalescence phenomenon. A property a or P can be assigned to each anion or cation. The combination aa or PP results in inhibition of bubble coalescence at a critical salt concentration, whereas the combinations aP or Pa produces no effect at all. Different gases of widely different molecular size, from helium to sulphur hexafluoride, affect the transition concentration a little, but do not change the phenomenon. 

However, a very ln ortant question remains unanswered do PE/EMA and PE/EMA-salt blends exhibit one single PE-EMA or PE-EMA-salt phase or do they contain separate PE and EMA or EMA-salt phases In the melt With respect to the melt rheology data, if there are two separate phases in PE/EMA or PE/EMA-salt blends, non-superposibility of both G ai d G" would most likely occur. As previously mentioned, this Is not the case, which suggests that in the melt blends of PE and EMA or PE and EMA-salt form one phase with Ionic microphase separation occuring in blends containing EMA-salt, when the salt concentration is above the critical salt concentration. 

The shifts in lEP were observed for all salts, but only some Na and Li salts have an ability to reverse the sign of C to positive over the entire pH range (Figs. 3.102 and 3.103). The critical salt concentration inducing such an effect depends on the nature of the anion, and hard-soft acid-base interactions have been invoked to explain these effects. Small cations (Na, Li ) and large anions (T) show a... 

Actomyosin denatures also in situ under the influence of hypertonic salt solutions. When cod muscle is immersed in various concentrations of sodium chloride, there is a critical salt content in the fillet (8% to 10% NaCl) at which denaturation occurs together with a rapid loss of water and uptake of salt (Duerr and Dyer, 1952 Fougere, 1952). In herring, however, this critical salt concentration is much lower (3 % NaCl) (Nikkila and Linko, 1954b). The stability of the native configuration appears very variable, but we are unable to explain such differences. The need for a better knowledge of the protein itself is clearly stressed by these researches. Let us now consider the recent progress made in this direction. 

Influence of Added Salt on the Slow Mode. As with NaCl and CaCl2, the system NaPSS/LaCl3 presents a pseudo splitting phenomenon between the two modes at a critical salt concentration [32], The amplitude of the slow mode becomes very low and undetectable. Only the fast component of the autocorrelation function is present. These results are analogous to many observations made on a lot of polyelectrolyte solutions and recall the pseudo-transition from extraordinary phase to ordinary phase [31,32,34,37,64]. At last, in the upper one-phase at Cs 0.5 M (D-point on Figure 15), a large scattered intensity is observed with only one relaxation time. The value of the effective coefficient diffusion is about 10 7 cm2/s. 

Critical Salt Concentration (CSC). It was noted in several studies (13, 64-67) that the chemical fines migration occurs only when the... 

Figure 6 shows how the critical salt concentration is determined. A core sample is saturated with a suitable salt solution and mounted in a standard core holder and coreflood apparatus. Salt solution is then injected at a specified superficial velocity. Subsequently the salt concentration is reduced in small steps until a decrease in permeability is observed (36). 

The effect of ionic strength onto MB accumulation behavior was also studied by our group [60]. Chronocoulometric and voltammetric parameters for MB on binding to DNA at CPE were monitored. It was found that 10 mM ionic strength is the critical salt concentration. MB interacts to guanine electrostatically up to 10 mM NaCl, in the presence of higher concentrations of 10 mM of NaCI, MB intercalates to hydrogen bounds of dsDNA. 

Recent experiments show a critical salt concentration exists below which the clay particles are released from the pore wall. While this finding can be incorporated in the present model, preliminary results show that for most cases, minor corrections occur and agreement between model predictions and experimental data is quite good. 

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