Surfactant structures, sample

Table 2.1 Schematics of molecular surfactant structures and sample surfactants Schematic of surfactant structure Sample surfactants...

Figure 12 Plot of the symmetric CH stretching frequency in C14AO/SDS mixed micelles at 20°C. a) the total surfactant concentration is 10 wt.% and all samples are from the Lt phase. Also plotted are the zero shear viscosities, q0. b) die total surfactant concentration is 18 wt.% and the surfactant structure varies dramatically with composition (Reproduced with permission from ref. 49. Copyright 1990 Steinkopff Verlag).

Figure 3. Sample surfactant structures belonging to each of the preference groups...

Briefly, AFM is a technique that maps the topography of a surface by plotting (on a color scale) the measured force between the surface and a small tip attached to a sensitive cantilever spring. In most applications, the tip is in direct contact with the surface, and the AFM performs as a sensitive contact profilometer. For imaging interfacial surfactant structures, however, contact forces disrupt the liquid crystalline aggregates. Therefore the repulsive colloidal stabilization forces between the surfactant layers adsorbed to the tip and sample are used as the contrast mechanism during imaging. 

Some differences between BCP/surfactant systems and surfactant/co-surfactant mixtures arise from the large difference in size and from the polymer-specific chain entropy contributions to the free energy of the systems. Moreover, in contrast to simple surfactant-based samples, the polydispersity of the polymer chains is also an important issue for structure formation. Polydispersity might lead to coexistence of different structures (e.g., spheres and worm-like structures) in the same polymer solution. This is due to the fact that small differences in the degree of polymerization of one of the blocks might lead to different packing parameters for a part of the size distribution. 

In general, the phase behavior of a POE nonionic surfactant (Figure 5.4) is more sensitive to surfactant structure than in the ionic case. Since the vast majority of such materials are, in fact, mixtures of POE chains of various lengths, phase diagrams lose a great deal of their theoretical utility, even though they may still be useful from a practical standpoint. While reproducible results can be obtained for a given sample of surfactant, another material of nominally the same structure may produce different results due to differences in POE chain distributions. As a result, it is not always a safe practice to extrapolate results for one sample to another of nominally the same material, even that provided by the same manufacturer. 

In reality, many proteins demonstrate mixed mode interactions (e.g., additional hydrophobic or silanol interactions) with a column, or multiple structural conformations that differentially interact with the sorbent. These nonideal interactions may distribute a component over multiple gradient steps, or over a wide elution range with a linear gradient. These behaviors may be mitigated by the addition of mobile phase modifiers (e.g., organic solvent, surfactants, and denaturants), and optimization (temperature, salt, pH, sample load) of separation conditions. 

In off-line coupling of LC and MS for the analysis of surfactants in water samples, the suitability of desorption techniques such as Fast Atom Bombardment (FAB) and Desorption Chemical Ionisation was well established early on. In rapid succession, new interfaces like Atmospheric Pressure Chemical Ionisation (APCI) and Electrospray Ionisation (ESI) were applied successfully to solve a large number of analytical problems with these substance classes. In order to perform structure analysis on the metabolites and to improve sensitivity for the detection of the various surfactants and their metabolites in the environment, the use of various MS-MS techniques has also proven very useful, if not necessary, and in some cases even high-resolution MS is required. 

Equidistant or clustered signals, characteristic of some anionic, nonionic or cationic surfactants (cf. Fig. 2.5.1(a) and (b). So the presence of non-ionic surfactants of alkylpolyglycolether (alcohol ethoxylate) type (AE) (structural formula C H2 i i-0-(CH2-CH2-0)x-H) could be confirmed in the formulation (Fig. 2.5.1(a)) applying APCI-FIA-MS in positive mode. AE compounds with high probability could also be assumed in the heavily loaded environmental sample because of the patterns of A m/z 44 equally spaced ammonium adduct ions ([M + NH4]+) shown in its FIA-MS spectrum in Fig. 2.5.1(b). 

Here, the mixture analytical FIA-MS-MS approach reached its limitation to identify compounds. Hence, LC separations prior to MS analysis are essential to separate compounds with the same m/z ratio but with different structures. The behaviour in the LC separation will be influenced by characteristic parameters of the surfactant such as linear or strongly branched alkyl chain, the type, the number and the mixture of glycolether groups—PEG and/or PPG—and the ethoxylate chains. The retardation on SPE materials applied for extraction and/or concentration also depends on these properties and can therefore be used for an appropriate pre-separation of non-ionic surfactants in complex environmental samples as well as in industrial blends and household detergent formulations. A sequential selective elution from SPE cartridges using solvents or their mixtures can improve this preseparation and saves time in the later LC separation [22],... 

In view of the inherent resistance of some surfactant metabolite isomers to complete mineralisation, efforts have to be mounted in order to obtain further insight into the reasons behind the persistence of these, such as the SPC and nonylphenol ethoxy carboxylates (NPECs). In order to achieve this, it would thus be indispensable to be able to fully elucidate the chemical structure of individual components, e.g. after isolation from environmental samples. Through the application of, for example, LC-ESI-MS-MS in combination with NMR analyses, this is now possible. 

Lundquist and the Stenhagens concentrated their efforts on the physical aspects of monolayer chemistry and did not elaborate then-work much in the direction of structural variation of the surfactant molecules. Their results show clearly, however, that the response of chiral monolayers to changes in surface pressure and temperature is sharply dependent on both the molecular structure of the surfactant and the optical purity of the sample. The Stenhagens were keenly aware of the possible application of the monolayer technique to stereochemical and other structural problems (72) however, they failed to exploit the full potential suggested by their initial results and, instead, pursued the field of mass spectrometry, to which they made substantial contributions. 

The force-area curves for racemic and (5 )-(+>2-tetracosanyl acetate were shown in Figures 17 and 18, respectively, while those of methyl esters of racemic and (5 )-(+)-2-methylhexacosanoic acid are found in Figs. 21 and 22, respectively. All these curves were obtained under identical experimental conditions at thevarious temperatures indicated in the figures. Simple inspection shows that the force-area curves of the two racemic samples are very similar, as are those for both optically pure samples. Lundquist suggested that this is merely a result of the very similar shapes and molecular structures of these chiral surfactants. Apart from the chain length, the only structural difference is limited to a reversal of the positions of the carbonyl group and ester oxygen. 

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