Surfactants, isotopically labelled

Penfold et al. [62] have also used neutron reflectivity to study the adsorption (structure and composition) of the mixed anionic/nonionic surfactants of SDS and C12E6 at the hydrophilic silica-solution interface. This is rather different case to the cationic/nonionic mixtures, as the anionic SDS has no affinity for the anionic silica surface in the absence of the Ci2E6. The neutron reflectivity measurements, made by changing the isotopic labelling of the two surfactants and the solvent, show that SDS is coadsorbed at the interface in the presence of the Ci2E6 nonionic surfactant. The variations in the adsorbed amount, composition, and the structure of the adsorbed bilayer reflect the very different affinities of the two surfactants for the surface. This is shown in Fig. 7, where the adsorbed amount and composition is plotted as a function of the solution composition. 

Quantitative determinations of surfactants by FAB or FD-MS are rather difficult because of the need for isotopically labeled internal standards. 

While self-diffusion allows a model-free and straightforward extraction of structural information, the interpretation of relaxation data requires the use of models. Self-diffusion is experimentally less demanding than relaxation studies, and self-diffusion studies can often be performed more rapidly. Because of its excellent sensitivity, self-diffusion is generally based on H NMR. Under typical conditions, we can resolve different components of a microemulsion, and no isotopic labeling is required. Relaxation experiments require more care. Here, H NMR studies of selectively deuterated surfactants is often the method of choice. 

Measurement of self-diffusion coefficients by means of PGSE techniques has evolved to become one of the most important tools in the characterization of surfactant systems. In particular, this is true of those surfactant systems that are isotropic liquid solutions such as micellar systems and microemulsions. The technique has been described in a number of review articles [9,11-13]. An account of the most recent developments of the method can be found in Ref 9. We do not dwell on the technical aspects here but merely note that the technique requires no isotopic labeling (avoiding possible disturbances due to addition of probes) furthermore, it gives component-resolved diffusion coefficients with great precision in a minimum of measuring time. 

Even if a number of labeled stable isotopes (for instance, for PFCAs, PFASs, and FOSA) are commercially available, most PFCs of commercial standards are unavailable (including many PFAPs, fluorinated oxethanes, perfluoropolyethers, and cationic surfactants) and an MS approach is, therefore, needed to screen for target and nontarget PFCs [48]. 

Whereas all the neutron reflection studies of bilayer surfactant structures have relied on using the overall thickness of the layer to establish that it is a bilayer. Fragneto et al. were able to show this directly by using partially labelled CieTAB molecules [8]. The series of isotopic species 0 Ci6 dC ,dTAB with m = 4, 8 and 12, where 0 indicates... 

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