Surfactant spectroscopic probing

For many simple surfactant systems, the methods of electron and fluorescence spectroscopy are not directly applicable, but in quite a few cases either the surfactant or a solubilized molecule displays useful light absorbing and/or fluorescing properties. However, it is more frequently so that measurements are made on a spectroscopic probe added in small amounts to the system of interest. 

The spectroscopic probe pyridine-N-oxide was used to characterize polar microdomains in reverse micelles in supercritical ethane from 50 to 300 bar. For both anionic and nonionic surfactants, the polarities of these microdomains were adjusted continuously over a wide range using modest pressure changes. The solubilization of water in the micelles increases significantly with the addition of the cosolvent octane or the co-surfactant octanol. Quantitative solubilities are reported for the first time for hydrophiles in reverse micelles in supercritical fluids. The amino acid tryptophan has been solubilized in ethane at the 0.1 wt.% level with the use of an anionic surfactant, sodium di-2-ethylhexyl sulfosuccinate (AOT). The existence of polar microdomains in aggregates in supercritical fluids at relatively low pressures, along with the adjustability of these domains with pressure, presents new possibilities for separation and reaction processes involving hydrophilic substances. 

Supercritical fluids (SCFs) such as carbon dioxide have a "hydrocarbon-like solvent strength at typical conditions, so that they are appropriate solvents for lipophilic substances. The solvent strength may be raised significantly by the addition of small amounts of cosolvents such as ethanol to increase solubilities of moderately polar substances selectivelyQ), sometimes by several hundred percent(2,2 4). The solvent and cosolvent form clusters about solutes, in which the cosolvent concentrations are enhanced significantly( ,fi). The present objective is to explore the effects of considerably more powerful solvent additives, that is surfactants. Since very little is known about surfactants in SCFs, spectroscopic probes were used to measure polarities inside the reverse micelles. Polarity is a key indicator of the ability of a reverse micelle to solvate a hydrophile. Using the... 

Working with spectroscopic probes, one needs to know the solubilization site of the probe, which should be determined independently on the spectroscopic effect to be exploited. However, when micelles of a homologous series of surfactants are investigated, information on variation of solubilization sites may be obtained. Roelants et al. (177) concluded from activation energies of quenching processes that the quencher molecule iV-methyl-A7-decylaniline resides a little deeper in TTAC micelles than in CTAC micelles. 

De, S. and Girigoswami, A. 2004, Fluorescence resonance energy transfer-a spectroscopic probe for organized surfactant media. Journal of Colloidal Interface Science 271,485-495. 

According to various experimental information the hydrocarbon core of the "aqueous micelle has a liquid-like structure (3,4). This has been confirmed, in particular, by spectroscopic probing techniques (5,6). Hence the micelle in aqueous surfactant solutions presents itself to the surfactant monomer as an equivalent with respect to the (macroscopical) oil/water interface. It might be not unreasonable, therefore, to consider this type of micelle formation an "auto-solubilization" to stress the close resemblance between adsorption and homoassociation processes. The hydrocarbon core of a micelle in aqueous surfactant solutions is characterized by its excellent solvent power for crystalline non-polar compounds (7). This latter feature appears remarkable and could serve as a more fundamental distinction between "normal" and inverted micelles than the generally cited apparently more obvious differences. The free energy of micellization is customarily (8) referred to the standard free energy of a monomer in a micelle, i.e. AG° represents the free energy of transfer of a monomer from the aqueous solution to a micelle of size n. 

Grieser F, Dummond CJ (1988) The physicochemical properties of self-assembled surfactant aggregates as determined by some molecular spectroscopic probe techniques. J Phys Chem 92 5580-5593... 

The projects discussed in Section 20.2 have been centred around the spectroscopic probing of the structure and dynamics of surfactant systems and lyotropic mesophases, in particular hydrocarbon gels. Three related investigations not yet discussed in detail due to limitations in space shall be briefly mentioned. These are the study of the conformation of surfactant molecules by surface enhanced Raman spectroscopy, the development of a lipophilic dye probe for surfactant systems, and an outlook onto a different class of reaction gels created by hydrolysis of metal alkoxide precursors. 

Further studies have demonstrated that PFPE-based surfactants can form microemulsions (with water cores) in supercritical CO2 (21). At higher water loadings, the CO2 was saturated with water and micelles began to solubilize water, which demonstrated bulk-like properties using spectroscopic probes. Although the PFPE-ammonium carboxylate surfactant was able to aggregate in CO2 at low water concentrations, a double-tailed surfactant, Mn(PFPE)2, was not soluble in CO2 without water. However, in the presence of water, Mn(PFPE)2-based micelles formed and the water core was able to ionize the manganese. 

There is litde doubt that the surfactant headgroups and counterion species are excluded from the micelle core. There is, however, some debate concerning water penetration into the core. Evidence for water penetration into the micelle core generally comes from spectroscopic experiments employing probe molecules (22-23). The probe molecules have been found to lie in a partly hydrophilic environment, and this has been interpreted as indicating water penetration into the core. Recent NMR relaxation (24) and neutron scattering (25) data provide fairly unequivocal evidence for minimal water penetration into the micelle core, however. 

Some essential discoveries concerning the organization of the adsorbed layer derive from the various spectroscopic measurements [38-46]. Here considerable experimental evidence is consistent with the postulate that ionic surfactants form localized aggregates on the solid surface. Microscopic properties like polarity and viscosity as well as aggregation number of such adsorbate microstructures for different regions in the adsorption isotherm of the sodium dedecyl sulfate/water/alumina system were determined by fluorescence decay (FDS) and electron spin resonance (ESR) spectroscopic methods. Two types of molecular probes incorporated in the solid-liquid interface under in situ equilibrium conditions... 

Polymer surfactant interaction has been examined by using sodium 2-(N-dodecyIamino)naphthalene-6-sulphonate as a probe. Solute-solvent interaction of free base phthalocyanine has been examined in both polyethylene and polystyrene by the effect of pressure on spectroscopic hole burning s Fluorescence has been used to indicate the onset of aggregation in water soluble polymers s interaction of pyrenylmethyltributylphosphonium bromide with single strand polynucleotides , and the interaction of indole compounds with synthetic polyelectrolytes. ... 

Understanding of the structure of the adsorbed surfactant and polymer layers at a molecular level is helpful for improving various interfacial processes by manipulating the adsorbed layers for optimum configurational characteristics. Until recently, methods of surface characterization were limited to the measurement of macroscopic properties like adsorption density, zeta-potential and wettability. Such studies, while being helpful to provide an insight into the mechanisms, could not yield any direct information on the nanoscopic characteristics of the adsorbed species. Recently, a number of spectroscopic techniques such as fluorescence, electron spin resonance, infrared and Raman have been successfully applied to probe the microstructure of the adsorbed layers of surfactants and polymers at mineral-solution interfaces. 

Several spectroscopic techniques have been used to study different aspects of conventional W/O/S microemulsion structures and properties. The absorption and steady-state emission spectroscopy of probe molecules solubilized in a microemulsion system can find the polarity of the microemulsion at their solvation location [34]. Time-resolved emission spectroscopy also provides information on the dynamics and rotation relaxation of solvent in both classical W/O/S and IL microemulsions [34-36]. Water content, which is defined as the molar ratio of water to total surfactant ([water]/[surfactant]), is one of the key factors in a microemulsion [37]. The ionization degree of bioactivator in IL microemulsion was correlated with the water content (cOj,) using UV-Vis absorption spectra, as shown in Rgure 18.2. 

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