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The use of the mixed aggregate method for the determination of amphiphilic substances is considered. The principles that support this method are presented, and the strategies devised so far for increasing sensitivity, selectivity and precision are discussed on the basis of both the theoretical background and the applications developed. The usefulness of this new approach in environmental and pharmaceutical applications is documented.

The determination of dialkyldimethylammonium surfactants (DDAS) based on the measurement of the critical micelle concentration (CMC) of mixed sodium dodecylsulphate (SDS)-DDAS aggregates in a basic medium ([NaOH]=4.8 × 10−3 M) is proposed. The dye Coomassie Brilliant Blue G (CBBG) was used as a photometric probe for the rapid determination of CMCs. Formation of CBBG-DDAS and DDAS-SDS premicellar aggregates of well-defined stoichiometrics at cationic and anionic surfactant concentrations far below their CMCs is demonstrated. Increased SDS concentration in the titration medium results in the formation of DDAS-SDS mixed micelles. The strong interaction between the opposite charged head group of DDAS and SDS permits these cationic surfactants to be determined at the ng ml−1 level with a nearly uniform response for all the DDAS tested (12–18 alkyl carbons). The relative standard deviation for 1.10 μg ml−1 ditetradecyldimethylammonium bromide (DTDAB) was 1.5%. The mixed-micelle based methodology was ap...

A new approach to the determination of polysorbates (PSs) in foods based on measurements of the critical micelle concentration of mixed micelles is proposed. PSs in a 0.25 M sulfuric acid medium are titrated photometrically with Triton X-100. The dye Coomassie brilliant blue G (CBBG) is used as a photometric probe for the rapid determination of critical micelle concentration values. The detection limits achieved for the PSs studied (viz. PS 20, PS 40, PS 60, PS 65 and PS 80) ranged from 0.05 to 0.08 μM. The relative standard deviation for 0.3 μM PS 60 was 2.8%. Food samples can be analyzed directly after dissolving PS from foodstuffs (baked bread, doughnuts, biscuits, butter, margarine, chocolate and noodle soup) with distilled water at room temperature. Average recoveries ranged from 98.0 to 99.3%.

Mixtures of anionic and cationic surfactants that form aggregates of pseudo-nonionic character and single-phase micellar solutions in the anionic- or cationic-rich range, were used to develop an effective approach to the determination of ionic amphiphilic substances based on the mixed-micelle methodology. Various alkyltrimethylammonium surfactants (ATAS) and sodium dodecyl sulphate (SDS) were tested as analytes and titrant, respectively. Nonionic–cationic aggregates made up of Triton X-100 and ATAS were comparatively studied. The linear concentration range achieved for ATAS based on the formation of pseudo-nonionic complexes was 0.1–0.6 μM, which permits these surfactants to be determined more sensitively than with both nonionic–cationic aggregates and monomer-based methods. Because of the opposite charge of the surfactants forming the pseudo-nonionic aggregates, strong synergism effects were observed for these systems. As a result, the critical micelle concentration for the mixture was significantly lower than those for the anionic and cationic surfactants alone, improving thus the detection limit for ATAS. The response was independent of the length of the hydrophobic chain in the cationic surfactant as the likely result of electrostatic interactions governing the incorporation of ATAS into SDS micelles. The relative standard deviation within- and between-assays for 0.3 μM of dodecyltrimethylammonium bromide was 3.3% and 3.9%, respectively.

The mixed aggregate (MA) method was evaluated for the routine determination of total ethoxylated nonionic surfactants in municipal raw and treated sewage. Nonionics in wastewater samples were enriched with a C8 solid-phase extraction cartridge, eluted with ethyl acetate–methanol, and separated from interferences with stacked strong anionic (quaternary ammonium form) and cationic (benzenesulfonic acid form) exchange columns. Sample extracts were evaporated to dryness and redissolved in distilled water. The total amount of ethoxylates, on a weight basis, was determined by measuring the critical micelle concentration of a mixed micelle made up of ethoxylates and the anionic surfactant sodium dodecyl sulfate, used as the reagent. The dye Coomassie Brilliant Blue G was used as a photometric probe. The minimum amount of ethoxylate required for a single determination was 1 μg. For raw sewage samples, mean recoveries and standard deviations were 98 ± 6 and 98 ± 4% at fortification levels of 0.3 and 0.6 μg/mL, respectively. For treated sewage samples, mean recoveries and standard deviations were 95 ± 2 and 100 ± 2% at fortification levels of 0.03 and 0.06 μg/mL, respectively. The MA method and the classical bismuth active substances and cobalt thiocyanate active substances procedures were compared. The former offers some significant advantages over the classical procedures in terms of sensitivity, selectivity, precision, simplicity, and rapidity; the type of response is independent of both the molecular weight and the chain length of the surfactant.