Surfactant-selective electrodes

A ring test proved that surfactant-selective electrodes are suitable for quantitative determination of anionic surfactants including alkanesulfonates [21]. The precision of this method, however, does not yet correspond to the state-of-the-art of the two-phase titration. Therefore, further development is needed to enhance the reproducibility and competitiveness of surfactant-sensitive titration. 

Surfactant concentration surfactant-selective electrode, potentiometry... 

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. 

Micelles of cationic surfactants have been found to form both in glycerol [44] and in ethylene glycol [18], The micelle formation of Ci6PyBr in ethylene glycol and glycerol was studied with surfactant-selective electrodes [45,46], The monomer concentration could in this way be measured at different total surfactant concentrations, and it was concluded that there is some premicellar aggregation... 

Potentiometric determination of surfactants is performed using various surfactant-selective electrodes. The procedure consists of dipping a surfactant-selective electrode into the test solution, measuring the electrode potential, and determining the concentration from voltage data via the Nemst equation. The response ofthe surfactant-selective electrodes varies day by day and is effected by different non-surfactant ions. For these reasons potentiometry is most often used as a technique for the determination of titration end-points rather than for the direct measurement of potential. One must also be aware that surfactant-selective electrodes can be used reliably only below the critical micellization concentration (CMC, see Chapter VI,3) of a surfactant. 

Yang L, Takisawa N, Kaikawa T, Shirahama K. 1996. Interaction of photosurfactants, [[[4 [(4 alkylphenyl)azo]phenyl]oxy]ethyl]trimethylammomium bromides, with alpha and beta cyclodextrins as measured by induced circular dichroism and a surfactant selective electrode. Langmuir 12(5) 1154 1158. 

Several NMR parameters are markedly different in the unimeric and self-assembled states and are therefore possible candidates for characterization of surfactant self-association processes. NMR can be a convenient alternative for the determination of critical micelle concentrations (erne s), but an analysis of variable-concentration NMR data generally also provides additional information. We here describe only how self-diffusion can be used to measure the free unimer concentration in simple and complex surfactant systems. In practice it is an alternative to surfactant-selective electrodes to measure surfactant activities. With a two-state assumption, we have, under the normal rapid exchange conditions [cf Eq. (7)],... 

The Royal Society of Chemistry for permission to reproduce the diagram of a surfactant-selective electrode (Figure 3.4) from Development of ion-selective electrodes for use in the titration of ionic surfactants in mixed solvent systems, Dowle, C. J., Cooksey, B. G. and Ottaway, J. M., Analyst, 112 (1987), 1299-1302. 

This method utilizes a surfactant selective electrode to monitor the interactions between the polymer and the surfactant. The EMF of the solution is dependent on the concentration of free surfactant in the solution. The binding of surfactant molecules to the polymer molecules results in a decrease in the free surfactant concentration and hence EMF. A known amount of concentrated surfactant solution is injected into a polymer solution and the EMF value is recorded when the equilibrium is reached. The process is repeated with further injections of surfactant solution. The EMF data are plotted against the surfactant concentration for the solutions with and without the polymer. 

Fig. 9.2 Titration of Zonyl FSA with 0.05 N Hyamine 1622. Metrohm model 670 titrator, Orion model 9342BN surfactant-selective electrode, and model 90-02 double-junction reference electrode. (From Ref. 10.)...

These compounds can be determined by either acid-base titration as carboxylic acids or by ion-pair titration. In the case of ion-pair titration, sarcosinates, like other carboxylic acids, can be titrated by two-phase titration at high pH with a cationic titrant (137). In addition, their titration has been demonstrated at low pH either by two-phase titration with an anionic surfactant (138) or one-phase titration with tetraphenylborate using a surfactant-selective electrode (139). These procedures are described in Chapter 16. Both acid-base titration and titration with a cationic surfactant will measure free fatty acid along with the acylsarcosine content. While titration at low pH with anionic titrants is presumably not subject to interference by free fatty acid, this has not been confirmed in the literature. 

Amines of low degree of ethoxylation can sometimes be titrated at low pH according to the methods for cationic surfactant titration described in Chapter 16. Alternatively, amine ethoxylates with alkoxy chain length less than about 10 can be converted to the corresponding ether sulfates and titrated as anionic surfactants, also discussed in Chapter 16 (101). Two-phase titration is required, since these compounds respond poorly to surfactant-selective electrodes. Higher ethoxylates are true nonionics and can be titrated as such with tetraphenylborate solution, also discussed in Chapter 16 (124). 

Schulz, R., Potentiometric titration of nonionic surfactants with the surfactant-selective electrode (in German), SOPW-Journal, 1996,122, 1022,1024-1028. 

Amphoglycinates can be determined by potentiometric titration with perchloric acid using a pH electrode or by titration with tetraphenylborate using a nonionic surfactant-selective electrode (18). 

Experimental surfactant-selective electrodes have been demonstrated as detectors for HPLC determination of alkylsulfates (28) and alkyl ether sulfates (29). The column effluents were diluted with water to prevent damage to the membrane from high methanol concentration. Such electrodes are subject to gradual deterioration of response and cannot be recommended for routine use. 

Masadome, T., T. Imato, N. Ishibashi, Surfactant-selective electrode ba.sed on plasticized polyfvinyl chloride) membrane and its application. Anal. ScL, 1987,3,121-124. 

Potentiometric titration of or Ca adducts of ethoxylates with tungstophosphoric acid has been demonstrated, using a tungsten wire electrode for end point detection. This is proposed for samples containing oil or solvents which would attack the PVC membrane of a surfactant-selective electrode (75). 

An electrode sensitive to both cationic and anionic surfactants can be made from a glass tube sealed at the bottom with a membrane of poly(vinylchloride) containing a high percentage of plasticizer, 40% or more. Dissolved in the plasticized PVC is a surfactant ion pair, such as hexadecyltrimethylammonium dodecylsulfate. The tube is filled with a dilute solution of anionic surfactant with a small amount of a chloride salt, into which a silver/ silver chloride electrode is inserted. For potential measurements in solutions, the circuit is completed with a reference electrode such as saturated calomel. Commercial surfactant-selective electrodes based on PVC membranes have been available since approximately 1990. 

An ion-selective field effect transistor device has been invented to serve as an anionic or cationic surfactant-selective electrode. In this case, a plasticized PVC membrane is made incorporating a surfactant salt. This membrane is cast over the gates area of a field effect transistor, the whole (except the sensing surface) is encapsulated in epoxy resin, and a suitable measuring circuit is connected. Application is similar to the polymer membrane electrodes described above (124-126). 

Surfactant-selective electrodes from different manufactures contain different additives in the membranes and necessarily differ from each other in their response to particular compounds. In particular, they differ in their response time and in the potential difference registered during a titration. This means that titration conditions must be optimized for each type of electrode, as well as for each surfactant (1). Care must be taken with other aspects of the titration also. The electrode should be rinsed with methanol and then water after each titration if it has a tendency to become coated with the ion pair. 

Since commercial surfactant electrodes are available, there is no reason for the analyst to prepare membranes and construct electrodes. Considerable know-how is involved in producing durable electrodes which behave reliably, so we should leave this to the instrument manufacturers. To illustrate this point, we may mention a study in which six electrodes were compared for use in end point detection of a titration of a cationic with an anionic. Two commercial surfactant-selective electrodes, two tetrafluoroborate selective electrodes, a nitrate selective electrode, and a homemade PVC-membrane electrode incorporating a tetraphenylborate salt were tested (135). Only one electrode, one of the commercial models, was traly suited for routine use, giving smooth potentiometric curves without reconditioning for over 100 titrations. The relative standard deviation of the end point was about 0.5%, while that for the other electrodes was 1.3-2.3%. The standard deviation of the end point potential was 3 mV for this electrode, compared to 8 mV or more for the other electrodes. Besides this, those electrodes not designed as surfactant electrodes required reconditioning (i.e., soaking in a dispersion of a surfactant ion pair) after 25 titrations in order to remain usable. 

This approach will not be practical for some time to come. The fundamental properties of surfactants (micelle formation, enrichment at interfaces) mean that the activity of a surfactant will usually differ from its absolute concentration (1). Just as serious is the technical problem that current surfactant-selective electrodes suffer from response which varies with their past and recent history they are also sensitive to the concentration of nonsurfactant ions. The result is that quantitative applications use electrodes not in direct measurements relating potential to concentration, but as indicators of the end point of a titration. In this latter application, it is not important that the electrode potential be exactly reproducible, but only that the potential change sharply as the surfactant concentration changes. For the titration of an anionic surfactant with a cationic surfactant, the electrode used for end point detection can be chosen to respond to either surfactant. Because of the drift in electrode potential, titrations must be conducted to an inflection in the titration curve rather than to a specific millivolt value. Details of the potentiometric titration methods can be found earlier in this chapter. The electrodes have also been demonstrated as detectors for flow injection analysis. 

The user should remain aware that surfactant-selective electrodes respond in nonlinear fashion in the range of the critical micelle concentration. The activity of individual surfactant molecules may actually decrease as the total concentration is raised to just over the CMC. If an organic solvent is used in place of water, the formation of micelles may not occur or may occur at higher concentration, but dimers and other aggregates may form. Even for titrations, electrodes are most reliably used below the critical micelle concentration of the surfactant. 

FIA of anionics without a phase separation may be based on the quenching effect of surfactants on the fluorescence intensity of 8-anilino-l-naphthalenesulfonic acid coupled with bovine serum albumin (83). Another method of avoiding a phase separation is to use a surfactant-selective electrode. Since interference is a problem, the surfactant-selective electrode approach is best coupled with online concentration with a reversed-phase column. Such a system has been demonstrated for trace analysis of sodium dodecylsulfate (84). 

A comparison of the results obtained with surfactant selective electrodes and the two-phase titration procedure reveals great accuracy for both techniques. However, the analysis of commercial cosmetic products and washing powders gave better results with the potentiometric titration method. 

A cyclic aza-oxa-cycloalkane, 7,13-bis( -octyl)-l,4,10-trioxa-7,13-diazacyclo-pentadecane (LI), was characterized and its interaction with anionic surfactants studied. Different PVC membrane anionic surfactant-selective electrodes were prepared using LI as ionophore and o-NPOE, bis(2-ethylhexyl) sebacate (BEHS) or DBP as plasticizers. The PVC-based membrane electrode containing o-NPOE as plasticizer showed a Nemstian response with a slope of 57.7 0.2 mV decade for lauryl sulfate (LS) in a concentration range from 3.3 x 10 to 6.7 x 10 mol L with a detection limit of 2.2 x 10 mol L The fabricated electrode was used for the determination of anionic surfactants in several mixtures, and the results obtained were compared to those found using a commercially available electrode. A similar ligand (7-methyl-7,13-di-octyl-l,4,10-trioxa-13-aza-7-azonia-cyclopentadecane)... 

In parallel to research considering surfactants sensors, some commercially surfactant selective electrodes have been launched into the market. Orion model 93-42, ASTEC model TSE 01 91, and Metrohm High Sense Tenside are examples with considerable long operational life time and is the most promising specimen on the market. It is stable over a wide pH range and its use permits differentiation between some detergents in mixtures simply by changing the pH of the analyzed sample. 

Birch BJ, Clarke DE, Lee RS, Oakes J (1974) Surfactant-selective electrodes Part III. Evaluation of a dodecyl sulphate electrode in surfactant solutions containing polymers and a protein. Anal Chim Acta 70 417 23... 

Anghel DF, Popescu G, Niculescu F (1980) Anal)4ical applications of surfactant selective electrodes-1. Cosmetic products. Tenside Surf Det 17 171-173... 

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