Anionic surfactants assay

Electrodes suitable for the potentiometric determination of surfactants are either specially designed liquid or solid membrane electrodes or ion-selective electrodes that in addition to being selective to a particular ion, also quantitatively respond to surfactants. For example, a nitrate ion-selective electrode responds to anionic surfactants, a calcium ion-selective electrode is sensitive to quaternary ammonium salts, and a barium ion-selective electrode can be used for assaying polyethoxylates [43], In some cases it is possible for one to perform potentiometric determination of a counter-ion, e.g. one can titrate alkylpyridinium chloride or bromide salts with silver nitrate solution using silver wire as an indicator electrode [38]. 

FIGURE 6.13 (A) Viability of RAW 264.7 macrophages treated with different SLN formulations compared with cells treated with pure medium (100% viability). Viability is related to the total surfactant concentration in the cell culture medium and was quantified by an MTT assay. All SLN contained 10% Dynasan 114 and 1% of different surfactants (the anionic surfactants sodium cholate [NaCh] or lecithin [S75] or the cationic surfactant cetylpyridinium chloride [CPC]). (B) Viability of RAW 264.7 macrophages treated with different surfactant solutions (the anionic surfactants sodium cholate [NaCh] or lecithin [S75] or the cationic cetylpyridinium chloride [CPC]) compared to cells treated with pure medium (100% viability). Viability is related to the total surfactant concentration in the cell culture medium and was quantified by an MTT assay. 

Only the apphcation of LC-MS analysis can be conceived of as reliable surfactant analysis. To elaborate determination methods for the analysis of the anionic surfactant mixture of alkyl ethoxysuUates (AES) APCI and ESI-MS(-t/-) studies combined with in-source MS/MS examinations were performed and results were compared (cf. Pig. 15.4 and 15.5 and 15.3.3.2 ESI, surfactants). APCI fragment ion spectra revealed the aUcyl chain length and the number of ethoxylate moieties therefore APCI was found to be the method of choice [326]. To confirm determination methods apphed in surfactant analyses an inter-laboratory comparison study of LC-MS techniques and enzyme-hnked immunosorbent assay for the determination of surfactants in wastewaters was performed in seven laboratories. The quantitative determination of the non-ionic NPEOs, AEOs, coconut diethanol amides (CDEAs) and the anionic LAS, NPEO-sulfates and the secondary alkane sulfonates (SAS) was performed under APCI or ESI-interfacing conditions in positive or negative... 

Several surfactants of different types (nonionic, anionic, cationic, and zwitterionic) have been assayed, but most reported procedures use the anionic surfactant sodium dodecyl sulfate (SDS, CMC = 8.2xl0 moll-i at 25°C), which is readily available. Other common surfactants are the cationic cetyltrimethylammonium bromide (CTAB, CMC = 9 X 10 moll ), and the nonionic Brij-35 (polyoxyethylene (23) dodecyl ether, CMC = 1 X 10 moll ). Common solvents in RPLC (methanol, ethanol, propanol, acetonitrile, and tetrahydrofuran) are suitable modifiers. Other less polar solvents, such as butanol and pentanol, can... 

APE may be titrated in aqueous solution with tetraphenylborate ion, as described in Chapter 16. As with other ethoxylates, this titration is incomplete for products with less than about 10 moles of ethylene oxide. Chapter 16 describes how APE of lower degree of ethox-ylation may be converted to the corresponding sulfate and titrated as an anionic surfactant. APE is sometimes assayed simply by UV spectrophotometry. APE is easily determined by HPLC and indeed by all of the chromatographic methods described in this volume. 

Quaternary amines may be assayed by titration with an anionic surfactant by the potentio-metric or two-phase procedures described in Chapter 16. The ISO procedure specifies titration with sodium dodecyl sulfate, which is available in high purity (7). Sodium tetraphenylborate may also be used if potassium and ammonium ions are absent. Other approaches to assay are based on acid-base titration or quantification of the anion associated with the quaternary. 

Quaternary amine salts may be titrated with acid by conventional nonaqueous procedures. To be precise, the anion associated with the quaternary is titrated. In the case of halides, mercuric acetate is added to replace free halide ion with easily titratable acetate ion [see Section III] (8,9). Such methods are suitable for analysis of pure samples, which do not contain other salts. A more specific assay of cationic surfactants is the two-phase titration with an anionic surfactant, described in Chapter 16. 

The compendial method for assay of benzalkonium chloride is based upon reversal of the quaternization reaction. The compound is reacted with Nal, the benzyl iodide and tertiary amine are removed by extraction, and the iodide consumed is determined by titration with KIO3 (9). The alkyl chain length distribution is determined by HPLC. Other tests for characterization of benzalkonium salts are similar to those performed on alkyl quaternaries. These salts are also readily determined by two-phase or potentiometric titration with an anionic surfactant. 

Titrimetric methods were the first procedures to be widely applied to the analysis of anionic surfactants and they remain very useful for assay purposes. Their speed and low cost make them especially suitable for quality control. Cross has written a thorough and recent discussion of the titration of anionic surfactants (2)... 

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. 

The procedure is suitable for determination of anionic surfactants in concentrates and in detergent formulations and is used for assay of alkyl aryl sulfonates, alkyl sulfates, sulfated APE and AE, and sulfosuccinates. As described in Chapter 1, alkane monosulfonates require special precautions. Phosphate surfactants will generally not be determined. Since the analysis is conducted at pH 2, soap is not determined. Hydrotropes in low concentrations do not interfere, nor do most other detergent constituents. 

Procedure Assay of an Anionic Surfactant by Two-Phase Titration (6)... 

Procedure Potentiometric Titration for Assay of Anionic Surfactants (54)... 

Finally, MAbs and an immunoassay kit for LAS have been commercialized (see Table 3). The working range of the assay is between 20 and 500 pg L1. The antibodies are highly specific for LAS with alkyl chains between C8 and C12, whereas other anionic and nonionic surfactants tested showed no cross-reactivity. 

Ion pair extraction provides a standard method for estimating ionic surfactants either colorometrically or titrimetrically. For example a cationic surfactant such as cetrimide can be estimated by pairing it with a lipophilic anionic dye such as bromocresol purple. The ion pairing creates a coloured lipophilic ion pair, which can be extracted into an organic solvent such as chloroform and a quantitative measurement of the colour extracted can be made spectrophotometrically. This type of assay is described in the BP for Clonidine Injection and Benzhexol Tablets. 

Figure 8.1 Mice received between four and eight weekly i.t. doses of 100 jl of Alcalase in saline or detergent matrix. The detergent matrix consisted of anionic and non-ionic surfactants, silicate builders and perborate bleach. Blood samples were obtained from individual mice 5 days after the final i.t. dose. The blood samples were assayed for anti-Alcalase specific IgG 1 and IgE, respectively, by an ELISA method and by the RBL 2H3 cell 3H-serotonin release assay as previously described (Kawabata et al., 1996). The mice used for Figures A—C were BDF1 strain. (Data reprinted with permission from Kawabata et al., 1996.)...

The resistance of lipoplexes formed from compounds LI and L2 and plasmid DNA was studied by electrophoretic retardation assays. Initial complexation experiments showed that all compounds were able to form full lipid/DNA complexes despite the presence of fluorinated regions near the headgroup. Fluorinated lipoplexes were formed with an excess of pure compounds LI and L2 and their integrity was tested with anionic, cationic, or neutral surfactants. When lipoplexes were analyzed by agarose... 

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