Anionic surfactants isolation

Nowadays, it is rare for infrared techniques to be used for qualitative or quantitative analysis of environmental materials. In either case, exhaustive separation of the surfactant from other materials must first be made. It is possible for inexperienced practitioners to go far wrong when identifying materials by IR, a technique best applied to pure compounds. Most environmental extracts, even after substantial cleanup, are mixtures which give complex spectra. It requires an experienced analyst to obtain useful information from the spectrum of a mixture containing unknown materials. As a general rule, a compound cannot be said to be present unless all of its characteristic absorbance bands are exhibited by the mixture. A once-common use of IR spectroscopy was confirmation of the identity of anionic surfactants isolated by the methylene blue spectrophotometric method. By proper choice of workup procedures and bands, this approach permitted exact determination of individual types of surfactants (78). 

The amount of residual sulfonate ester remaining after hydrolysis can be determined by a procedure proposed by Martinsson and Nilsson [129], similar to that used to determine total residual saponifiables in neutral oils. Neutrals, including alkanes, alkenes, secondary alcohols, and sultones, as well as the sulfonate esters in the AOS, are isolated by extraction from an aqueous alcoholic solution with petroleum ether. The sulfonate esters are separated from the sultones by chromatography on a silica gel column. Each eluent fraction is subjected to saponification and measured as active matter by MBAS determination measuring the extinction of the trichloromethane solution at 642 nra. (a) Sultones. Connor et al. [130] first reported, in 1975, a very small amount of skin sensitizer, l-unsaturated-l,3-sultone, and 2-chloroalkane-l,3-sultone in the anionic surfactant produced by the sulfation of ethoxylated fatty alcohol. These compounds can also be found in some AOS products consequently, methods of detection are essential. 

To synthesize new surfactants, having incorporated both structural elements, the known siloxanyl modified halogenated esters and ethers of dicyclopentadiene [5] were treated with different amines according to the reaction scheme. Triethylamine yielded quaternary ammonium salts directly. Alternatively, after reaction with diethylamine or morpholine, the isolated siloxanyl-modified tertiary amines were also converted to quaternary species. To obtain anionic surfactants, the halogenated precursors were initially reacted with n-propylamine. In subsequent reaction steps the secondary amines formed were converted with maleic anhydride into amides, and the remaining acid functions neutralized. Course and rate of each single reaction strongly depended on the structure of the initial ester or ether compound and the amine applied. The basicity of the latter played a less important role [6]. 

In 1998, UOP announced the development of a new Sorbex process called the MMP Sorbex process [15-19] that was capable of simultaneously separating both Cio i6 mono-branched paraffins and Cio i6 normal paraffins from a corresponding kerosene stream or n-paraffin-depleted Molex raffinate stream. Previously, no commercial process existed to isolate significant quantities of mono-methyl paraffin derived from either kerosene or n-paraffin depleted kerosene. Mono-methyl paraffins are desirable because they are needed for a new type of anionic surfactant. 

Recently we presented a new methodological approach for catalytic sulfoxidation which makes use of water as solvent in the presence of anionic surfactant, and hydrogen peroxide as terminal oxidant activated by an easily prepared chiral Pt(ll) complex, all under mild conditions (Figure 9.7). Moreover, the enantioenriched sulfoxides are isolated from the catalyst by means of simple diethyl ether extraction (Figure 9.8) which does not dissolve the catalyst. 

As mentioned earlier, cationic polymers can enhance the mildness of anionic surfactants toward skin. One factor is related to the ability of the polymer to reduce the activity of the surfactant monomer and, in turn, to lower its binding to the corneum. (See below.) It is not, however, possible to isolate this effect from other mechanisms involving the ability of the polymer to bind to the skin surface itself and influence its properties directly. 

Two implementation directives exist 82/243/EEC, which covers the class of anionic surfactants and 82/242/EEC, which covers the class of nonionic sm-factants. These implementation directives describe the precise methodology by which the surfactants must be extracted, isolated, and tested for their biodegradability [1]. 

For a series of anionic surfactants with the same ionic head group, the lifetime of a micelle decreases with decreasing alkyl chain length of the hydrophobic component. Branching of the alkyl chain could also play lui important role in the lifetime of a micelle. It is, therefore, important to carry out dynamic surface tension measurements when selecting a surfactant as an adjuvant as this may play an important role in spray retention. However, these above measurements should not be taken in isolation as other factors may also play an important role, e.g. solubilization which may require larger micelles. The selection of a surfactant as an adjuvant requires knowledge of the factors involved. 

Surfactant affinity for the skin was inversely proportional to the amount of dye staining the skin. The data from this study showed that lauryl sulfate and lauryl benzyl sulfate exhibited a greater affinity for the skin than ot-olefin sulfonate and lauryl(3EO)-sulfate. Later, Imokawa et al. [68] showed an apparent correlation between skin roughness and the adsorption of anionic surfactants to isolated stratum comeum. 

This is currently the only polyclinid ascidian in which such substances have been identified. These molecules may act as anionic surfactants and protect the colony from bacterial and/or fungal attack (natural antifouling agents). One of these derivatives is 1,19-phytanediol disulfate, previously isolated from Ascidia mentula by the same Italian team. Its carbon chains may be of symbiotic or dietary origin (see Chapter 26) (Aiello et al, 1997a, 2001). 

Many methods for isolation of surfactants are based upon the solubility of the materials. For example, the sodium salts of most anionic surfactants are soluble in acetone, while soap is not (26). It is difficult to make general statements in this field because of the many exceptions. For example, the sodium salt of the most common anionic surfactant, LAS, is only partially soluble in acetone. Surfactants which would be insoluble by themselves may become soluble in mixtures with other surfactants. Govindram and BGishnan report for a number of common surfactants that all are soluble in ethanol, but that only nonionics and free fatty acids are soluble in petroleum ether or ethyl acetate, with cocomonoethanola-mide being only partially soluble in petroleum ether (26). The solubility of anionics differs depending upon whether they are present as free acids or sodium salts. 

Most anionic surfactants are salts of moderately strong acids. As such, they can be titrated directly with base, provided that a suitable solvent system and visual or instrumental end point can be found. However, since anionic surfactants are generally found in the company of other ionic materials, some of them also acidic, direct acid-base titration is not used for most applications. For example, acid-base titration for assay of alkylarylsulfonate would risk high results because of titration of byproduct sulfate or other ions. Acid-base titration of LAS in a detergent formulation suffers from interference from such buffering compounds as sodium silicate and sodium tripolyphosphate. Titration with alkali is therefore limited to cases where the anionic surfactant can be isolated in pure form. 

In another version of this procedure the nonionic surfactant was first extracted batch-wise with sodium tetraphenylborate into 1,2-dichloroethane. The tetraphenylborate in the isolated organic phase was then titrated with a cationic surfactant, using Victoria Blue B as indicator (70). This titration can also be performed to an electrochemically detected end point. In this version, an excess of anionic surfactant is added to the cationic complex formed by the ethoxylated nonionic surfactant and potassium ion. The ion pair is extracted into dichloroethane, separated from the initial aqueous phase, then titrated with cationic surfactant in the presence of additional water. The ion pair of the anionic surfactant and Fe(II)(l,10-phenanthroline)3 is added as indicator. The end point of the titration is indicated when the last of the anionic surfactant is complexed by the cationic titrant, causing the iron-phenanthroline cation to migrate to the aqueous phase, where it is detected as a change in potential at a platinum electrode (71). 

A more efficient method of isolating anionic surfactants is extraction as part of an ion pair (33). An inorganic salt is added to decrease the solubility of the ion pair in the aqueous phase. Sometimes, the methylene blue spectrophotometric method described in Chapter 12 is used as the cleanup step. This permits the analyst to estimate the amount of surfactant isolated before proceeding with more definitive analytical techniques. Methylene blue may be removed from the surfactant extract by passage through a cation exchange column (56). If concentration is performed by liquid-liquid extraction of the ion pair with an alkyl quaternary compound, the UV spectrum of the ion pair is identical to that of LAS alone (55). 

When solvent sublation is used to isolate cationics, an anionic surfactant is sometimes added in excess to minimize adsorption of the cationic. This requires that the ion pair be separated later in the analysis. One method to minimize loss of cationics by adsorption to the apparatus is to pretreat the apparatus with a solution of the cationic, then rinse with water and solvent. Obviously, blank determinations must be run to confirm that this treatment is not a source of spurious high results (151). Dioctyldimethylammonium bromide has been used as an internal standard to correct for incomplete recovery of cationics (82). 

Hard surface household cleaners are available in various types. The concentrated products contain about 10% surfactant, often nonionics. The spray-on liquids, already diluted, may only contain 1% surfactant. Powdered hard surface cleaners are more likely to contain anionic surfactants, usually only 1 or 2%. Most household cleaners are made alkaline and usually contain a sequestering agent such as sodium gluconate. A water-soluble solvent is often added. Disinfectants, perfumes, and other components are present. Surfactants are isolated by extracting the dried solids with methanol or methylene chloride (24). 

A synthetic alternative to this is the chemical reduction of metal salts in the presence of extremely hydrophilic surfactants have yielded isolable nanometal colloids having at least 100 mg of metal per litre of water [105], The wide range of surfactants conveniently used to prepare hydrosols with very good redispersibility properties include amphiphilic betaines A1-A4, cationic, anionic, nonionic and even environmentally benign sugar soaps. Table 3.1 presents the list of hydrophilic stabilizers used for the preparation of nanostructured colloidal metal particles, and Table 3.2 shows the wide variety of transition metal mono- and bi-metallic hydrosols formed by this method [105,120],... 

The solvent sublation procedure of Wickbold [18] is another method that has been used for the analysis of LAS present in seawater [19,20], The solvent sublation technique (gaseous stripping into organic solvent, often ethyl acetate) has also been used to isolate and concentrate nonionic surfactants, e.g. AEs and APEO in aqueous samples [21,22], The co-extracted interferences can be eliminated by cation/anion ion-exchange and alumina chromatography [23,24]. 

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