Cationic surfactants versus anionic

Fig. 2.10. Equivalent conductance (A) and transport number of anion (T-) and cation (T+) for CTAB solutions at 35 °C plotted versus the square root of surfactant molarity. (From Ref.13))...

Figure 3.10 Mixing rules for surfactant mixtures, (a) u/k as a function of the average surfactant molecular weight (Msurf) for a mixture of two anionic surfactants, (b) Salinity (S) versus wt.% of non-ionic surfactant in a mixture of an anionic and a non-ionic surfactant, (c) Salinity (S) versus wt.% of cationic surfactant in a mixture of an anionic and a cationic surfactant.

For anionic surfactants colorimetric methods utilize the formation of an ion pair between the surfactant anion and a cationic dye. Similarly to two-phase titration, colorimetric determination is based on the fact that the ion pair is extractable into organic solvent, while the dye by itself is not. A characteristic example of the analysis of anionic surfactant is the determination of alkylsulfates and alkyl(aryl)sulfonates as their complex with methylene blue extracted into chloroform [31]. The absorbance of chloroform extract is measured at 625 nm versus chloroform background. This methods allows one to analyze alkylsulfates and alkyl(aryl)sulfonates separately. Alkylsulfates, in contrast to sulfonates, are easily hydrolyzed by hydrochloric acid. The products of hydrolysis do not interact with methylene blue and are not transferred into chloroform. Some other cationic dyes, such as dimidium bromide, can also be used. In fact, the use of the latter allows one to achieve much higher sensitivity than that obtained with methylene blue. 

Micellar Effects on Chemical Equflibria.—A few studies have been made of acid-base equilibria in micelles. Hydronium ion activity in anionic micelles has been measured conductimetrically using hydrophilic indicators, it being found that a plot of mn+ versus [H ]-t-[Na ] is linear with a slope of 0.82. The quantity mH+ is defined as the number of micellized hydrogen ions per surfactant head group, namely mH = [H ]tot—[H ]w/ [D]tot c.m.c., where [DJtot is the total catalyst concentration. The use of fluorescent indicators (21a) and (21b) in anionic, neutral, and cationic surfactantspermitted the evaluation of the electrical potential at the micellar surface as a function of added electrolytes. Indicator pK values for mixed micelles and pK values of weak... 

An increase in LS as surfactant concentration approaches CMC, followed by a decrease as the concentration is raised was observed for both cationic and anionic surfactants. The increase in the LS versus surfactant concentration, sharp enough to be considered an LS peak, occurs as the concentration of surfactant in aqueous solution approaches and passes the CMC. The angular dependence of LS showed the peaks are due to massive structures that only exist in the LS peak regime. Similar phenomena have been observed by others and different explanations were proposed, although the prevailing factor appears to be the release of hydrophobic impurities as the concentration of surfactant approaches and drops below the CMC [58, 59]. 

The cloud point temperature of the anionic-cationic surfactant mixtures is affected by the total surfactant concentration and by the relative concentration of the anionic and cationic surfactants. Solutions with excess anionic surfactant showed one minimum in their cloud point temperature versus total surfactant concentration graph (Fig. 10). The cloud point temperature of the minimum increased with an increase in the anionic surfactant mole... 

AP has been used to probe micellar media (Saroja et al., 1998). The probe is located at the micellar interface and is well suited to monitoring micellar aggregation. In fact, the sharp change in the fluorescence intensity versus surfactant concentration allows the critical micellar concentration (CMC) to be determined. Excellent agreement with the literature values was found for anionic, cationic and nonionic surfactants. The electroneutrality of 4-AP and its small size are distinct advantages over ionic probes like ANS or TNS. 

Anions and uncharged analytes tend to spend more time in the buffered solution and as a result their movement relates to this. While these are useful generalizations, various factors contribute to the migration order of the analytes. These include the anionic or cationic nature of the surfactant, the influence of electroendosmosis, the properties of the buffer, the contributions of electrostatic versus hydrophobic interactions and the electrophoretic mobility of the native analyte. In addition, organic modifiers, e.g. methanol, acetonitrile and tetrahydrofuran are used to enhance separations and these increase the affinity of the more hydrophobic analytes for the liquid rather than the micellar phase. The effect of chirality of the analyte on its interaction with the micelles is utilized to separate enantiomers that either are already present in a sample or have been chemically produced. Such pre-capillary derivatization has been used to produce chiral amino acids for capillary electrophoresis. An alternative approach to chiral separations is the incorporation of additives such as cyclodextrins in the buffer solution. 

The CMC of this new surfactant is several orders of magnitude lower than the CMC of its parent species. Figure 15 indicates a typical CMC plot versus the composition of the anionic-cationic (e.g., dodecyl sulfate-tetradecyl trimethyl ammomnium chloride) mixture in water. It can be seen that the CMCs of the anionic and cationic species are quite high, e.g., around 0.1 wt. %. As soon as a very small percentage of cationic is added to an anionic solution, the CMC falls several orders of magnitude. The same happens when a very small amount of anionic is added to a cationic solution. In both cases it seems that an equimolar catanionic species forms, and that its very low CMC dominates the mixing rule [84]. 

Although quantitative thin-film force measurements of polymer/surfactant-containing systems are relatively new, quite fascinating behaviour has already been found. In particular, the strongly interacting synergistic adsorption system (e.g. Case II) containing an anionic polyelectrolyte and a cationic surfactant has been extensively studied. This system has revealed both oscillatory force versus distance profiles, and interfacial gel formation within individual films. 

Cloud point phenomena is exhibited only by the complexes and not by their surfactant components. Figure 9 shows cloud point temperature versus anionic mole fraction for a system of anionic-cationic solutions where the additional hydrophilic group is carried by... 

Pc and Pz can be estimated from a plot of Pm [H + Xj versus [H The interaction between the tetracyclines and the ionic surfactants (Table 6.15) is of a different nature a relatively small Pm being observed for tetracycline in DTACand a large, presumably electrostatic interaction with NaLSat pH 2.1. The anionic-cation interaction would sufficiently alter transport properties so that dialysis rates would be altered it is perhaps wrong to ascribe the notation Km to the values obtained. 

Separation of two or more types of surfactants in cosmetics. To separate two or more types of surfactants in cosmetics involves using several detectors or columns as well as studying separation variables in particular pH. Kadono et al. (1987) proposed a reversed-phase LC method using a combination of the UV with RI as detector. A ratio of the UV/RI area versus a retention time that was specific to each surfactant. This method was applied to hair cleanser (shampoos and hair conditioners) containing anionic, cationic, amphoteric and nonionic surfactants. 

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