Nonionic surfactants detection

Since poly(oxyethylene)-type nonionic surfactants have a capability of facilitating the transfer of cations [51,52], the above interphase complexation may be seen as an example of precomplex formation before the bulk transfer of ions, which is seen when Aq (p is sufficiently positive. The presence of such precomplex formation at the interface, which is detectable voltammetrically [53], may have significance in the rate of complex formation and the selectivity in the bulk facilitated transfer. 

Direct determination of surfactants in complex matrices can also be carried out using ion-selective electrodes. Depending on the membranes and additives used, the detergent electrodes are optimized for the detection of anionic surfactants [81], cationic surfactants [82], and even nonionic surfactants [83]. The devices are sensitive to the respective group of surfactants but normally do not exhibit sufficient stability and reproducibility for their use in household appliances. With further optimization of membrane materials, plasticizers and measurement technology, surfactant-selective electrodes offer high potential for future applications. 

Positive ionisation is the method of choice for the detection of all nonionic surfactants generating molecular [M + H]+ or ammonium adduct ions ([M + NH4]+) in the presence of ammonium acetate. Often [M + Na]+ ions were also observed however, an excess of ammonium acetate will suppress their generation. While sensitivity in this mode is very high and can be improved by an excess of ammonium acetate to suppress sodium or potassium adduct ions, selectivity compared with negative ionisation for anionics is low. 

A simple, rapid, sensitive, and selective spectrofluorimetric method (2ex/ lem = 345/455nm) has been developed for the determination of zaleplon. Tang et al. have studied the influence of micellar medium on the absorption, fluorescent excitation, and emission spectra character of zaleplon The nonionic surfactant of Triton X-100 showed a strong sensitizing effect for the fluorescence of zaleplon in a pH 5.0 buffer. The possible enhancement mechanism was discussed. Based on the optimum conditions, the linear range was 1.32 x 10 8-1.00 x 10 mol/1. The detection limit was 4.0 x 10 mol/1 with a relative standard deviation (RSD) of 0.06%. The proposed method was successfully applied to the determination of zaleplon in tablets, serum, and urine. 

Leenheer, J. A., Wershaw, R. L., Brown, P. A., and Noyes,T. I. (1991). Detection of poly (ethylene glycol) residues from nonionic surfactants in surface-water by XH and BC nuclear-magnetic-resonance spectrometry. Environ. Sci. Technol. 25,161-168. 

Delgado, B., V. Pino, J.H. Ayala, V. Gonzalez, and A.M. Alfonso. 2004. Nonionic surfactant mixtures A new cloud-point extraction approach for the determination of PAHs in seawater using HPLC with fluori-metric detection. Anal. Chim. Acta 518 165-172. 

Future work in this area should focus on further development of novel extraction schemes that exploit one or more of the cited advantages of the nonionic cloud point method. It is worth noting that certain ionic, zwitterionic, microemulsion, and polymeric solutions also have critical consolution points (425,441). There appear to be no examples of the utilization of such media in extractions to date. Consequently, the use of some of these other systems could lead to additional useful concentration methods especially in view of the fact that electrostatic interactions with analyte molecules is possible in such media whereas they are not in the nonionic surfactant systems. The use of the cloud point event should also be useful in that it allows for enhanced thermal lensing methods of detection. 

Nonionic surfactants of the ethoxylate type are not so efficiently separated compared to ionic surfactants. The complexity of the surfactant mixtures and the lack of charge leads to insufficient peak resolution and high detection limits. 

Figure 29-9 illustrates a separation of the oligomers in a sample of the nonionic surfactant Triton X-100, Detection involved measuring the total ion current produced by chemical ioni/.aiion mass spectrometry. The mobile phase was carbon dioxide containing 1 % by volume of methanol. The column was a 30-m capillary column coated with a 1-pm lilm of 5% phenylpolysi-loxane. The column pressure was increased linearly at a rate of 2,5 bar/min. 

FtGURC 29-9 Chromaiograms for the nonionic surfactant Triton X-100 with lotal current mass spectrometric detection, i.d. - Inside diameter, (Reprinted with permission from R. D. Smith and H. R. Udseth. Anal. Chem., 1987. 

The addition of surfactants to culture media increases the yields of many enzymes. The results presented in Table 4 were obtained with secreted enzymes. No data are available for cell-bound enzymes. In some tests, no enzyme is detectable in the controls, but several units of activity per ml can be found in the presence of surfactants (i.e. infinite enhancement). The values shown in Table 4 are based on the surfactant Tween 80 at 0.1 %. For some systems, higher Tween concentrations give better yields, but there usually is a critical upper level. For some systems, other (nonionic) surfactants such as Triton are superior to Tween. In general, the enhancement factor is greatest for organisms which do not normally secrete much enzyme and least for those organisms already selected for their high yields. But even in the latter, an appreciable... 

EKC methodologies for aromatic amines are based on electrolytes containing SDS, " mixtures of SDS and nonionic surfactants, and bile salts as well as modifiers (tetraalkyl-ammonium salts, urea, " organic solvents, " - " - and CDs ). Combinations of anionic soluble polymers or crown ether and CDs have also been reported. Despite being chromophoric, alternative detectors for aromatic amines have been proposed (CE-ESI-MS, electrochemical, and fluorescence detection). 

NMR self-diffusion measurements indicated that all microemulsions consisted of closed water droplets and that the structure did not change much during the course of reaction. Hydrolysis was fast in microemulsions based on branched-chain anionic and nonionic surfactants but very slow when a branched cationic or a linear nonionic surfactant was employed (Fig. 11). The cationic surfactant was found to form aggregates with the enzyme. No such interactions were detected with the other surfactants. The straight-chain, but not the branched-chain, alcohol ethoxylate was a substrate for the enzyme. A slow rate of triglyceride hydrolysis for a Ci2E4-based microemulsion compared with formulations based on the anionic surfactant AOT [61,63] and the cationic surfactant cetyltrimethylammonium bromide (CTAB) [63] was observed in other cases also. Evidently, this type of lipase-catalyzed reaction should preferably be performed in a microemulsion based on an anionic or branched nonionic surfactant. Nonlipolytic enzymes such as cholesterol oxidase seem to function well in microemulsions based on straight-chain nonionic surfactants, however [64]. CTAB was reported to cause slow inactivation of different types of enzymes [62,64,65] and also, in the case of Chromobacterium viscosum lipase [66], to provide excellent stability. 

In the determination of nonionic surfactants according to the method described above (. 1.7.3), cationic surfactants are also detected. The... 

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