Surfactant salting out

Initially, the vapor pressure measurements appear to be the most direct, but even here some assumptions are needed. The amount of alcohol in the micellar phase needs to be determined. To do this the difference in vapor pressure between a pure aqueous and a micellar solution is measured. If the ions of the surfactant salts out alcohol, the vapor pressure of pure water is not the correct comparison, and this could lead to lower partition coefficients. Thermodynamic data are well suited for model calculations, and both the models of DeLisi et al. ° ° and Hetu et al. fit the data well. Although in reasonable internal agreement, the partition coefficients calculated from partial molar volumes differ from those calculated from enthalpies the first is 927 or 944, the latter... 

This problem was resolved by Nakae et al. [7] using non-polar octadecylsilica as the stationary phase and a solution of 0.1 M of sodium perchlorate in methanol/water (80 20) as the mobile phase. The ternary system (water-alcohol-salt), previously used by Fudano and Konishi [8] as an eluent for the separation of ionic surfactants at higher concentrations, induced the so-called salting out effect . The addition of the organic solvent to the water modified the polarity of the eluent and produced a good separation within a short period of time [9]. It also has the function of dissociating the surfactant micelles in individual molecules that are dissolved in the eluent [8], The presence of the salt (NaC104) in the mobile phase has a considerable influence on... 

Washing and Cleaning Action. The properties of alkyl ether sulfates, due to the good solubility and the special hydrophilic/hydrophobic properties of the molecule, are of particular practical interest. From the investigations described in sections 2 and 3, it can be concluded that, in addition to the decrease in the Krafft Point, favorable properties for practical applications can be expected as a result of the inclusion of the oxyethylene groups into the hydrophobic part of the molecule. As is true for other anionic surfactants, the electrical double layer will be compressed by the addition of multivalent cations. By this means, the adsorption at the interface is increased, the surface activity is raised, and, furthermore, the critical micelle concentration decreased. In the case of the alkyl ether sulfates, however these effects can be obtained without encountering undesirable salting out effects. 

Schwarz et al. used ESI-MS detection to characterize the composition of binary (bile salt/phosphatidylcholine) and ternary (bile salt/phophatidy 1-choline/fatty acid) mixed micelles that were used in micellar affinity capillary electrophoresis (43,44). The detrimental effects of the surfactants turned out to be tolerable for short-time qualitative determinations. 

The addition of salts to the aqueous phase of concentrated emulsions can have profound effects on their stabilities. Water-in-oil HIPEs are generally stabilised by salt addition [10,12,13,21,80,90,112] however, the nature of the salt used was found to be important [13]. Salts which decrease the cloud point of the corresponding nonionic surfactant aqueous solutions, i.e. which have a salting-out effect, were more active. The interactions of the surfactant molecules at the oil/water interface were increased due to dehydration of the hydrophilic ethylene oxide groups on addition of salt. This was verified experimentally [113] by an ESR method, which demonstrated that the surfactant molecules at the oil/water interface become more ordered if the salt concentration is increased. 

Finally, some studies have been performed on the addition of salt to the aqueous phase of oil-in-water HIPEs [109]. For systems stabilised by ionic surfactants, increasing salt concentration reduces the double-layer repulsion between droplets however, stability is more or less maintained, probably due to steric and polarisation repulsions. Above a sufficiently high salt concentration, emulsions become unstable due to salting-out of the surfactant into the oil-phase. For nonionic surfactants, the situation is similar, except that there are no initial double-layer forces. In addition, Babak [115] found that increasing the electrolyte concentration reduced the barrier to coagulation between emulsion droplets, and therefore increased coalescence. Generally, therefore, stability of o/w HIPEs is not enhanced by salt addition. 

Composition of the subphase is of paramount importance for monolayer formation. Electrolytes, by virtue of their ability to salt-out organic molecules, often render surfactants insoluble and, thus, permit the formation of a well-behaved monolayer. A case in point is the previously cited example of optically active (jV-(a-methylbenzyl)stearamides [114]. Stable monolayers could not be formed with these molecules on water. Stable monolayers readily formed, however, on strong aqueous acid solutions [114],... 

With alkali halide-TBA-W or alkali halide-PD-W systems, the parameters Bne are negative for volumes and heat capacities (see Figures 1-5 and 10). This sign seems to be the one usually observed for the interaction of a hydrophobic with a hydrophilic solute (6). At intermediate cosolvent concentration, AYe°(W — W + TBA) and AYe°(W — W + PD) deviate in the direction we would expect for hydrophobic association the volume increases sharply, and the heat capacity decreases further. Inorganic electrolytes lower the critical micelle concentration of surfactants by salting out the monomers, thus favoring micellization (25) in a similar way, in the co-sphere of a hydrophilic ion, the hydrophobic bonding between the cosolvent molecules may be enhanced. 

DETAILED COMPARISON WITH PUBLISHED DATA IN THE PHYSICOCHEMICAL LITERATURE FOR SALTING OUT OF IDENTIFIED NONIONIC SURFACTANTS... 

The comparison of CMC data in distilled vs. hard river water shows that the decrease in CMC with hardness has the order anionics cationics nonionics (Rosen et al., 1996). Hardness increases the dependence of the CMC on alkyl carbon chain length of CnE0mS04, indicating that in hard water the influence of additional carbon atoms is the same for CnE0mS04 as for CnEOm surfactants (Rosen et al., 1996). The influence of ionic strength on micellization of nonionic surfactants is due to a salting out effect of the hydro-phobic moiety of the surfactant molecule (Carala et al., 1994). 

Figure 17 presents the results of an FTIR spectroscopic study of the effect of salt concentration on the cmt of 70 mM SDS. As Mantsch et al. (4) have shown in similar work with alkali hexadecylsulfates, the cmt can be related to the sudden increase in frequency of the v9 CH2 bands as a function of temperature. The large increase in the gauche conformer content of the methylene chains of the surfactant tails as they "melt" at the cmt is responsible for this frequency shift. The effect of added salt is to raise the cmt of SDS, which is the cause of the "salting out" of this ionic surfactant at any given temperature. The cmt values, taken as the midpoint of the discontinuities in the frequency-temperature plots, agree well with those obtained by other means (14,54). 

Surfactants Salting in is when the addition of electrolyte to a solution of non-ionic surfactant causes the critical micelle concentration to increase. Also, addition of electrolyte to an ionic surfactant solution in a multiphase system can drive surfactant from the oil phase into the aqueous phase. Salting out is when the addition of electrolyte causes the critical micelle concentration to decrease. Also, addition of electrolyte to an ionic surfactant solution in a multiphase system can drive surfactant from the aqueous phase into the oil phase. 

Generally speaking, for a stable emulsion a densely packed surfactant film is necessary at the interfaces of the water and the oil phase in order to reduce the interfacial tension to a minimum. To this end, the solubility of the surfactant must not be too high in both phases since, if it is increased, the interfacial activity is reduced and the stability of an emulsion breaks down. This process either can be undesirable or can be used specifically to separate an emulsion. The removal of surfactant from the interface can, for example, be achieved by raising the temperature. By this measure, the water solubility of ionic surfactants is increased, the water solubility of non-ionic emulsifiers is decreased whereas its solubility in oil increases. Thus, the packing density of the interfacial film is changed and this can result in a destabilisation of the emulsion. The same effect can happen in the presence of electrolyte which decreases the water solubility mainly of ionic surfactants due to the compression of the electric double layer the emulsion is salted out. Also, other processes can remove surfactant from the water-oil interface - for instance a precipitation of anionic surfactant by cationic surfactant or condensing counterions. 

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