Sulfonated surfactants alkanesulfonates

Figure 7.2 illustrates the main routes for sulfonated or sulfated anionic surfactant manufacture [5]. It appears that paraffin sulfonates or alkanesulfonates (i.e., practically only SAS) are made from linear paraffins produced from kerosene or gas oil. 

TABLE 6 TLC Analysis of Alkanesulfonate and Petroleum Sulfonate Surfactants... 

Figure 20 shows the plot of the surface tension vs. the logarithm of the concentration (or-lg c-isotherms) of sodium alkanesulfonates C,0-C15 at 45°C. In accordance with the general behavior of surfactants, the interfacial activity increases with growing chain length. The critical micelle concentration (cM) is shifted to lower concentration values. The typical surface tension at cM is between 38 and 33 mN/m. The ammonium alkanesulfonates show similar behavior, though their solubility is much better. The impact of the counterions is twofold First, a more polarizable counterion lowers the cM value (Fig. 21), while the aggregation number of the micelles rises. Second, polarizable and hydrophobic counterions, such as n-propyl- or isopropylammonium and n-butylammonium ions, enhance the interfacial activity as well (Fig. 22). Hydrophilic counterions such as 2-hydroxyethylammonium have the opposite effect. Table 14 summarizes some data for the dodecane 1-sulfonates.

The HLB numbers decrease with increasing chain length, e.g., from 13.25 for sodium decane 1-sulfonate to 9.45 for the C18 homolog [72]. Typical HLB numbers for positional isomers range from 12.3 for sodium dodecane 1-sulfonate to 13.2 for the more hydrophilic 6 isomer [73]. The HLB numbers of alkanesulfonates are less influenced by the isomeric position of the functional group and by substituents than the cM values [68]. HLB numbers can be correlated with partition coefficients for the distribution of a surfactant between the aqueous and oily phases, which emphasizes that the partition coefficient is dependent on the carbon number [68]. 

These sulfonates are still valued as anionic surfactants for consumer products. The secondary alkanesulfonates are known to have high solubility, fast wetting properties, chemical stability to alkali, acids, and strong oxidants such as chlorine. 

Whilst the alkylbenzenesulfonates are equally used for powder and liquid detergents and cleaners, the field of use of the secondary alkanesulfonates (also called paraffin sulfonates) is rather in the liquid cleaners sector, because of their high solubility. This type of surfactant, produced by the conventional sulfochlorination process, became of some importance as early as the Second World War. 

Active agent content of anionics is usually determined by titration with a cationic surfactant such as benzethonium chloride. Two-phase titration is suitable for assay of alkylaryl sulfonates alkyl sulfates and hydroxysulfates, alkanesulfonates, fatty alcohol ethoxysul-fates and alkyIphenolethoxysulfates and dialkyIsulfosuccinates (4,5). The results on ma-... 

Aliphatic sulfates and sulfonates may be detected with a postcolumn extraction apparatus. As the anions elute from the column, they mix with a stream containing a fluorescent cation. The cation can only be extracted into an organic phase as an ion pair with the anionic surfactant. A fluorescence detector monitors the concentration of the cation extracted. This approach makes possible the analysis of alkylsulfates, alkanesulfonates, ester sulfonates, and ether sulfates by either normal phase or reversed-phase HPLC. Gradient analysis presents no difficulty (10,26,27). Such a system was also used with methylene blue in conjunction with a visible absorbance detector at 630 nm (9). Although methylene blue does not give the sensitivity of fluorescence detection, it is more selective for surfactants. Common fluorescent cations form extractible ion pairs with nonsurfactant anions, giving rise to interference and limiting the choices available for mobile phase modification. 

Ullner, H., I. Konig, C. Sander, U. Schwenk, LC separation of paraffin sulfonates, with special emphasis on primary paraffin sulfonates (in German), Tenside, 1980,17,169-170. Hauthal, H. G., Alkanesulfonates, in H. W. Stache, ed.. Anionic Surfactants Organic Chemistry, Marcel Dekker, New York, 1996. 

While the two-phase titration will not determine alkanedisulfonates if monosulfonates are absent, the presence of disulfonate impurities in commercial alkanesulfonate products will cause a variable interference (37). The response of the two-phase titration to molecules containing more than one sulfate or sulfonate group must be determined for each individual case. Lew reports stoichiometric reaction with each functional group for a C22 surfactant with two anionic sulfate groups separated by four carbon atoms (38). Wick-bold suggests addition of 10 mL 20% sodium sulfate solution to eliminate interference from disulfonates, and this was adopted in the ISO procedure (39). Alternatively, modification of the organic solvent to a 93 7 chloroform/l-hexanol mixture will allow complete recovery of disulfonates, regardless of pH (40). 

Hand dishwashing liquid detergents contain an anionic surfactant, perhaps an alcohol ether sulfate (or, in Europe, an alkanesulfonate), as major active agent, along with a lower level of an amide or amine oxide type nonionic to improve foaming. Betaines are sometimes used as foam boosters. A hydrotrope such as an alcohol or xylene sulfonate is generally also present. 

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