Analysis of Individual Surfactants

Besides the above procedures that are generally applicable to anionic surfactants, specialized methods have been developed to characterize individual surfactants. 

There is an ASTM standard, D1568, for analysis of LAS (6). It recommends determination of water by azeotropic distillation, unsulfonated material by extraction, and chloride by titration. pH is determined on a 0.1% aqueous solution and a procedure is given for 

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As the alkyl chain length of LAS increases, surface activity, sensitivity to water hardness, and solution viscosity increase while water solubility decreases. For most applications, the C] i-Ci3 range provides the best mix of properties. 

Analytical methods are available for very thorough characterization of the impurities in the alkylbenzene raw material used to produce LAS. These include trace levels of tetralins and of di- and polyaromatic compounds. Analysis is via preparative FIPLC separation into the major classes, followed by capillary GC/MS identification and quantification of individual components (52). In principle, this approach could also be applied to LAS, after desulfonation. 

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III. ANALYSIS OF INDIVIDUAL SURFACTANTS A. Fatty Quaternary Ammonium Salts... 

This chapter deals with the analysis of individual nonionic surfactants. Analysis of raw materials includes not only the determination of the active content, but also of impurities and by-products, and determination of oxyalkylene chain length. Any nonionic can be determined in formulated products by deionisation, which was described in section 4.6.2 and is not further discussed here. Nonionic fractions isolated in this way may have more than one principal active, and some of the methods given here are applicable to the analysis of such mixtures. There are also a few procedures for the determination of nonionics in formulated products. 

Impurities in anionic surfactants include sulfate salts from the sulfonation reaction and perhaps residual catalyst from previous steps, such as the KOH catalyst that may have been used to prepare the ethoxylated alcohol which was subsequently sulfated. Individual ions can be determined by dilution with water and application of the usual wet chemical tests or ion chromatography. Direct ion chromatographic analysis of a surfactant may require the addition of a low molecular weight alcohol to the aqueous buffer. 

TLC is used for qualitative analysis of mixtures of nonionics and determination of individual surfactants. It may also be used for determination of impurities, such as PEG in ethoxylates. 

Many amphoterics behave as cationic surfactants at acid pH, allowing them to be determined by the standard two-phase titration method. The mixed indicator method for the analysis of ionic surfactants, as described by Reid, Longman, and Heinerth, is adequate, but emulsions form in the vicinity of the end point so that precise analysis is difficult (115). Sometimes these problems can be minimized by reversing the titration. For example, a solution of sodium lauryl sulfate, buffer, and indicator is titrated with an aqueous sample containing a quaternary or amphoteric surfactant. The same end point is observed as described under the titration of anionics (94,95). Further improvement can be made by carefully adjusting the amount of ethanol added to the titration flask, depending on the individual surfactant being analyzed (116). 

Figure 13.22 shows the resolution of the surfactants Tween 80 and SPAN. The high resolution obtained will even allow the individual unreacted ethylene oxide oligomers to be monitored. Figure 13.23 details the resolution of many species in both new and aged cooking oil. Perhaps the most unique high resolution low molecular weight SEC separation we have been able to obtain is shown in Fig. 13.24. Using 1,2,4-trichlorobenzene as the mobile phase at 145°C with a six column 500-A set in series, we were able to resolve Cg, C, Cy, Cg, C9, Cio, and so on hydrocarbons, a separation by size of only a methylene group. Individual ethylene groups were at least partially resolved out to Cjg. This type of separation should be ideal for complex wax analysis.

The first two aspects entail relatively high concentrations of surfactants. In the last case, trace amounts are to be determined. When performing surfactant analysis, preconcentration and/or separation of the different surfactant classes are prerequisites for identifying and quantifying the compound in question. Furthermore, the trend is to analyze the individual components of any surfactant mixture. 

Earlier studies (ref. 440-442) with ordinary air microbubbles (without any synthetic surfactant coating) have already shown that echocardiographic contrast produced by microbubbles is useful in the qualitative analysis of blood flow and valvular regurgitation. In addition, quantitative studies (ref. 440) have shown a correlation between individual contrast trajectories on M-mode echocardiography and invasive velocity measurements in human beings. Meltzer et al. (ref. 441) have shown that velocities derived from the slopes of contrast trajectories seen on M-mode echocardiography correlate with simultaneous velocities obtained by Doppler techniques. (This correlation is expected because both measures represent the same projection of the microbubble velocity vector, that is, in the direction of the sound beam.) More detailed studies (ref. 442) confirmed that microbubble velocity obtained from either Doppler echocardiography or M-mode contrast trajectory slope analysis correlates well with actual (Doppler-measured) red blood cell velocity. Thus, these early studies have shown that microbubbles travel with intracardiac velocities similar to those of red blood cells. 

Adsorption isotherms represent a relationship between the adsorbed amount at an interface and the equilibrium activity of an adsorbed particle (also the concentration of a dissolved substance or partial gas pressure) at a constant temperature. The analysis of adsorption isotherms can yield thermodynamic data for the given adsorption system. Theoretical adsorption isotherms derived from statistical and kinetic data, and using the described assumptions (see 3.1), are known only for the gas-solid interface or for dilute solutions of surfactants (Gibbs). Those for the system gas-solid are of a few basic types that can be thermodynamically predicted81. From temperature relations it is possible to calculate adsorption and activation energies or rate constants for individual isotherms. Since there are no theoretically founded equations of adsorption isotherms for dissolved surfactants on solids, the adsorption of gases on solides can be used as a starting point for an interpretation. 

Capillary electrophoresis (CE) is an emerging analytical technique for determination of catechins. The majority of CE studies involve the analysis of catechins in tea infusion, extracts as well as supplements. The three variants of CE suitable for the analysis of catechins include capillary zone electrophoresis (CZE), micellar electro-kinetic chromatography (MEKC), and microemulsion electrokinetic chromatography (MEEKC) with UV detection. In general, the resolution of MEKC was found to be superior to CZE for separation of catechins. MEEKC is a relatively new technique, and the few reports available suggest that it offers a performance similar to MEKC. CE conditions are often quite complex, and many factors, such as buffer composition, pH, presence of surfactants, and column temperature, can all affect the quality of separation and should be optimized individually. On the other hand, CE offers several advantages over HPLC. The short analysis time (<20 minutes), low running costs, and reduced use of solvents make it an attractive alternative for routine analysis of catechins. 

It was shown by Rusanov et al. that the empirical HLB scale is supported by thermodynamics the analysis of work of transfer of surfactant molecules from aqueous phase to hydrocarbon phase revealed that group numbers, B are proportional to the work of transfer of individual groups present in the molecule, and that the work of transfer of the entire molecule is given by summation of works of transfer of individual groups. 

The ultimate goal in environmental analysis is the quantification of individual compounds separated from all their isomers and/or homologues. Chromatographic methods like HPLC, GC, or SFC are amongst the most powerful analytical instruments with regard to separation efficiency and sensitivity. Because of the low volatility of surfactants, HPLC is used far more often than GC. Since the launch of atmospheric pressure ionization (API) interfaces, LC-MS coupling is increasingly used for determination of surfactants (Table 30.5). 

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