Ionic and zwitterionic surfactants

MEKC is also performed using cationic, nonionic, and zwitterionic surfactants. Widely employed are cationic surfactant consisting of a long chain tetralkylammonium salt, such as cetyltrimeth-ylammonium bromide, which causes the reversal of the direction of the EOE, due to the adsorption of the organic cation on the capillary wall. Other interesting options include the use of mixed micelles resulting from the simultaneous incorporation into the BGE of ionic and nonionic or ionic and zwitterionic surfactants. Chiral surfactants, either natural as bile salts [207] or synthetic [208] are employed for enantiomer separations. 

Proteins often precipitate during isoelectric focussing because (1) they are concentrated into a sharp band and (2) they do not exhibit any net charge at their pi and can thus undergo extensive hydrophobic interaction. To improve solubility, a number of additives can be used. The denaturing agent urea may be used at concentrations of up to 6M. Non-ionic and zwitterionic surfactants are also commonly used. 

Notably, with ionic and zwitterionic surfactants, an additional entropy contribution, associated with the ionic head groups, must be considered. Upon partial neutralization of the ionic charge by the counter ions when aggregation occurs, water molecules are released. This will be associated with an entropy increase that should be added to the entropy increase resulting from the above hydrophobic effect. However, the relative contribution of the two effects is difficult to assess quantitatively. 

Rosen [35] has tabulated values of Tm for a wide variety of anionic, cationic, non-ionic and zwitterionic surfactants and has discussed the effect of surfactant structure on With hydrocarbon surfactants, the length of the hydrophobic group has little effect except when this exceeds 16 carbon atoms when a significant decrease in r jis noted, possibly due to coiling of the chain. Chain branching has only a small effect on T j, as has introduction of fluorine atoms into the hydrophobic chain. With polyoxyethylene non-ionic surfactants of fixed oxy-ethylene chain length the value of appears to be little influenced by the length of the hydrocarbon chain. 

K. Szczodrowski, B. Prdlot, S. Lantenois, J.-M. DouiUard, J. Zajac, Effect of heteroatom doping on surface acidity and hydrophilicity of Al, Ti, Zr-doped mesoporous SBA-15. Micro-porous Mesoporous Mater. 124(1-3), 84-93 (2009). doi 10.1016/j.micromeso.2009.04.035 J. Zajac, Mechanism of ionic and zwitterionic surfactant adsorption from dilute solutions onto charged non-porous and porous mineral oxides inferred from thermodynamic studies, in Recent Research Developments in Surface and Colloids, ed. by S.G. Pandalai (Research Signpost, Kerala, 2004), pp. 265-300... 

The binding constants between the anionic substrates and cationic micelles are large because of the combination of coulombic and hydrophobic effects so rate enhancements may be large even with dilute surfactant. There is binding with non-ionic and zwitterionic micelles despite the absence of coulombic attraction (Bunton et al., 1975). 

Table 17.3 shows that the slope of this type of plot ranges around 0.3 for all ionic surfactants, while it is approximately 0.5 for nonionic and zwitterionic surfactants. Since it is the logarithm of the CMC which is linearly related to the number of carbon atoms, the slopes... 

Few studies exist for ionic silicone surfactants. Several trisiloxane anionic, cationic and zwitterionic surfactants have been found to form micelles, vesicles and lamellar liquid crystals. As would be expected, salt shifts the aggregates toward smaller curvature structures [40]. 

However, surfactants incorporated into the electrolyte solution at concentrations below their critical micelle concentration (CMC) may act as hydrophobic selectors to modulate the electrophoretic selectivity of hydrophobic peptides and proteins. The binding of ionic or zwitterionic surfactant molecules to peptides and proteins alters both the hydrodynamic (Stokes) radius and the effective charges of these analytes. This causes a variation in the electrophoretic mobility, which is directly proportional to the effective charge and inversely proportional to the Stokes radius. Variations of the charge-to-hydrodynamic radius ratios are also induced by the binding of nonionic surfactants to peptide or protein molecules. The binding of the surfactant molecules to peptides and proteins may vary with the surfactant species and its concentration, and it is influenced by the experimental conditions such as pH, ionic strength, and temperature of the electrolyte solution. Surfactants may bind to samples, either to the... 

Submicroscopic, colloidal aggregates can influence chemical reactivity. Aqueous micelles are the most widely studied of these aggregates, and these micelles form spontaneously when the concentration of a surfactant (sometimes known as a detergent) exceeds the critical micelle concentration, cmc (1-3). Surfactants have apolar residues and ionic or polar head groups, and in water at surfactant concentrations not much greater than the cmc, micelles are approximately spherical and the polar or ionic head groups are at the surface in contact with water. The head groups may be cationic, (e.g., trimethylammonium), anionic, (e.g., sulfate), zwitterionic (as in carboxylate or sulfonate betaines), or nonionic. The present discussion covers the behavior of ionic and zwitterionic micelles and their effects on chemical reactivity. 

At or in the proximity of their pi values proteins exhibit a minimum total charge and reduced solubility in the electrolyte solution [350,370,385]. This increases the probability of aggregation, and is further enhanced by the low ionic strength of the ampholyte buffer. Under these conditions protein precipitation results from hydrophobic interactions. These interactions can be suppressed by addition of additives such as ethylene glycol (10-40 %), non-ionic or zwitterionic surfactants (1-4 %), or sorbitol to the ampholyte buffer. 

Other ionic surfactant systems were also studied, particularly isomerized a-olefin sulfonates [69] and other branched surfactants [70], polyallq l ones such as dioleyl phosphates [71], and zwitterionic surfactants [72]. 

Numerous micellar systems of both ionic and nonionic surfactants in water have been investigated [55,56], occasionally with an added electrolyte or another amphiphile as a third component. The electrical birefringence dynamics observed reflect the significant differences in the structure and thermodynamic behavior of ionic and nonionic systems, while zwitterionic amphiphiles [57] show their ambivalent nature. 

AFM imaging has been used to identify interfacial aggregate structure above the cmc for a variety of ionic, nonionic, and zwitterionic surfactants on both hydrophobic and hydrophilic surfaces. (Structures far below the cmc, corresponding to the low-density adsorption plateau, cannot be imaged readily because the tip-sample force is strongly hydrophobic and attractive in this regime.) The... 

As electrospray is a very soft ionisation technique, the spectra are dominated by molecular ion- or quasimolecular ion-related species. Ionic, nonionic and zwitterionic surfactants are all amenable to the technique. Unlike FAB or LSIMS, electrospray does not suffer from discrimination effects [6] and it is therefore applicable to the analysis of mixtures. Sensitivity is high 10 pg of cationic surfactants can be detected with a good signal-to-noise ratio. The spectrum contains peaks from the intact cation. It has been reported [7] that the limit of detection for positive-ion electrospray is of the order of several femtograms (1 fg = 10 g). 

The compounds triisobutyl (methyl) phosphonium tosylate (a) and trihexyl (tetradecyl) phosphonium bis 2,4,4-(trimethylpentyl)phosphinate (b) were synthesized (Fig. 4.14), and their surface-active properties studied.The polar compound (a) is water soluble and surface active, does not form micelles, but affects the micelliza-tion properties of ionic, nonionic, and zwitterionic surfactants more strongly than conventional electrolytes. The less polar compound (b) forms micelles and has very low aqueous solubility. Both compounds form mixed micelles with Triton X-100 nonionic surfactant in aqueous solution. Compound (a) replaces water to form microemulsions with isopropyl myristate as oil, stabilized by (b). Compound (a) showed a clear antitumor activity, for example, 5mg (a)mH in 0.9% NaCl solution caused 100% killing of Sarcoma-180 cell line in 1 h. More diluted solutions were still active 2.5 and 1 mg (a) mT caused 81 and 53% killing of the same cells, respectively. On the other hand, compound (b) was less active than (a) lOmg (b)mT in 0.9% NaCl solution caused 89% killing of Sarcoma-180 cell line in 2h. Note that the concentration of (b) employed was 33 times higher than its cmc (0.03 x 10" moll ). The efficiency of (a) with respect to (b) may be due to the fact that the former does not form micellar aggregates [89]. 

The linear part of the variation of Ihi with C, close to the cmc, has been used to obtain the values of the rate constants k+ and k for the association and dissociation of one surfactant to/from a micelle and of the distribution width 0 (see Equation 3.9), using known or estimated values of the mean micelle aggregation N. Many values of k" and k have been reported.2 2 2 > > > > ° Table 3.1 lists values for representative anionic, cationic (including dimeric), nonionic and zwitterionic surfactants. Note that the values listed in Table 3.1 have been obtained from an analysis of the 1/Xi vs. C data using Equation 3.9 or Equation 3.16, which is more correct for the ionic surfactants considered here. The two t5q)es of analysis 3ueld data than can differ by much more than the experimental error, as noted in several studies. Also the errors involved in the different methods used for measuring can be very different. The main conclusions are as follows ... 

---