Surface tension reduction electrolyte effect

By their nature, the experiments on gas bubble nucleation, whether electrolytic, chemically generated, or by pressure release, all have high concentrations of gas in solution during the bubble nucleation process, and thus show the most pronoimced effects of surface tension reduction. Two aspects of this observation are particularly noteworthy first, that He of all the gases so far tested shows the smallest ratio of theoryjmeasured—and is also the only gas with a b coefficient of 0. Second, that organic liquids with N2 gas dissolved in them (including the case of diethyl ether) show relatively modest ratios theoryjmeasured. In the case of diethyl ether, the liquid is relatively close... 

The characteristic effect of surfactants is their ability to adsorb onto surfaces and to modify the surface properties. Both at gas/liquid and at liquid/liquid interfaces, this leads to a reduction of the surface tension and the interfacial tension, respectively. Generally, nonionic surfactants have a lower surface tension than ionic surfactants for the same alkyl chain length and concentration. The reason for this is the repulsive interaction of ionic surfactants within the charged adsorption layer which leads to a lower surface coverage than for the non-ionic surfactants. In detergent formulations, this repulsive interaction can be reduced by the presence of electrolytes which compress the electrical double layer and therefore increase the adsorption density of the anionic surfactants. Beyond a certain concentration, termed the critical micelle concentration (cmc), the formation of thermodynamically stable micellar aggregates can be observed in the bulk phase. These micelles are thermodynamically stable and in equilibrium with the monomers in the solution. They are characteristic of the ability of surfactants to solubilise hydrophobic substances. 

Most detergents contain electrolytes, e.g. sulphate, bicarbonate, carbonate or citrate and the presence of these electrolytes increases the adsorption of anionic surfactants at the gas/liquid interface as already mentioned. This leads to a reduction of the surface tension at an equal solution concentration [7] and to a strong decrease of the cmc. The effect can be of several orders of magnitude. Similar to this are the effects of mixtures of surfactants with the same hydrophilic group and different alkyl chain length or mixtures of anionic and non-ionic surfactants as they are mostly used in detergency [8]. Mixtures of anionic and non-ionic surfactants follow the mixing rule (eqn. 3) in the ideal case ... 

The sugars sucrose, fructose and glucose have also been found to affect bubble coalescence. On addition to water these sugars raise the surface tension and are desorbed from the air-water interface. Thus their effect on bubble coalescence equally cannot be described in terms of surfactant-like behaviour and certainly no charge effects are involved. Hence, even if an "explanation" could be found within the confines of the primitive model of electrolytes, that explanation could not accommodate this observation. The reduction in bubble coalescence achieved with increasing concentration is shown in Fig. 3.7. 

The electrolyte content of the aqueous phase has a considerable effect on the wetting time of ionic surfactants, reflecting its effect on the reduction of surface tension by the surfactant, its solubility in water, and its CMC. Electrolytes that... 

In the case of ionised solutes, an increase in the salt concentration results in a reduction in the electrostatic repulsion between solute molecules as may be predicted by the modified Debye-Huckel theory of dilute electrolytes (Lietzke et al., 1968). Therefore, for an ionised solute, as the salt concentration is increased, the electrostatic component of the solvophobic equation is increased, resulting in an initial decrease of the In k term at low salt concentrations. As the salt concentration is further increased the surface tension effects increase and In k increases. 

The effect of the curvation of the micelle on solubilization capacity has been pointed out by Mukerjee (1979, 1980). The convex surface produces a considerable Laplace pressure (equation 7.1) inside the micelle. This may explain the lower solubilizing power of aqueous micellar solutions of hydrocarbon-chain surfactants for hydrocarbons, compared to that of bulk phase hydrocarbons, and the decrease in solubilization capacity with increase in molar volume of the solubilizate. On the other hand, reduction of the tension or the curvature at the micellar-aqueous solution interface should increase solubilization capacity through reduction in Laplace pressure. This may in part account for the increased solubilization of hydrocarbons by aqueous solutions of ionic surfactants upon the addition of polar solubilizates or upon the addition of electrolyte. The increase in the solubilization of hydrocarbons with decrease in interfacial tension has been pointed out by Bourrel (1983). 

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