Fluorescence probing surfactant assemblies

Fluorescence Probing of the Surfactant Assemblies in Solutions and at Solid-Liquid Interfaces... 

These fluorescent probes have been successful in reporting the structural parameters of surfactant assemblies such as micelles [103], reverse micelles [104], ternary systems [105], swollen micelles [106], microemulsion [107], vesicles [108], liposomes [109], hemimicelles [110], monolayers [111] and bilayers [111]. 

The polarity within a surfactant assembly will be quite different from that of the bulk solution. It is useful to know the micropolarity of these assem-bhes for such applications where different substrates are compartmentalized inside these surfactants. The micropolarity of the surfactant assembhes can be determined using any fluorescence probe whose emission characteristics change with solvent polarity. The emissions of the probe are measured in solvents of known polarities and the polarity of the surfactant assembhes is determined by comparison. 

The fluidity (nanoviscosity) in an organized surfactant assembly on soUds can be substantially different from that in the bulk aqueous phase and hence, the diffusional resistance experienced by the probe in the micelle will be considerably different from that faced in the bulk solution [ 145]. Measurement of the viscosity or fluidity of the interior of a micelle is based on measurement of fluorescence properties that depend on the mobihty of the probe in the interior. A commonly used method for such studies involves the intramoleciflar... 

Another extremely useful method for cac determination, especially in the light of high sensitivity, is fluorescence emission spectroscopy [15]. Some aromatic water-insoluble dyes that are present in trace amounts in mixed polyelectrolyte-surfactant solutions have an ability to solubilize within the self-assembled surfactant aggregates and to change their photophysical properties because of the change of environmental polarity. Through this, they offer a very sensitive method for the determination of cac values. A typical and lately frequently used compound is pyrene, which is used as a fluorescence probe to assess various micellar properties. Pyrene exhibits a polarity dependent fluorescence spectrum with the ratio /,//3 (the ratio of the intensity... 

Information on microviscosity is obtained by studying the excimer forming capabilities of suitable fluorescent probes. The excimer, which is a complex of a ground state and excited state monomer, has a characteristic emission frequency. The intramolecular excimer formation for example, of 1,3-dinaphthyl propane (DNP), is a sensitive function of the microviscosity of its neighborhood. This property, expressed as the ratio of the excimer and monomer yield (/e//m) for DNP, has been determined for dodecyl sulfonate solutions and its adsorbed layer for the various regions of the adsorption isotherm (Fig. 4.18) (Somasundaran et al., 1986). Comparing the ratios thus obtained to the /e//m values of DNP in mixtures of ethanol and glycerol of known viscosities, a microviscosity value of 90 to 120 cPs is obtained for the adsorbed layer in contrast to a value of 8 cPs for micelles. The constancy of microviscosity as reported by DNP is indicative of the existence of a condensed surfactant assembly (solloids) that holds the probe. 

In the next part, we will focus our attention on nanosecond processes that occur in shells of self-assembled polymer micelle-like nanoparticles in aqueous media [56, 57]. Fluorescent probes that strongly bind to the nanoparticles have usually been employed to obtain information on the shell or on the immediate vicinity of nanoparticles. Suitable probes include amphiphilic fluorophores, i.e., fluorescent surfactants, such as prodan, laurdan, or patman (see chapter Huorescence Studies of Polymer Containing Systems , Fig. 2). They contain a fairly polar fluorescent head-group and a nonpolar aliphatic tail, which secures the favorable hydrophobic interaction and sorption on polymer nanoparticles. They bind to micelles [55, 56] and their localization depends on the polarity of the head-group and on the length of the tail. In the case of patman, the strongly polar head is usually located in the peripheric part of the solvated shell and the nonpolar tail is oriented towards the... 

Szajdzinska-Pietek E, Schlick S (2005) Self-assembling of ion-containing polymers and surfactants in aqueous solutions studied by ESR and fluorescence probes. J Mol Liq 117 (1-3) 153-164. doi 10.1016/j.molliq.2004.08.007... 

Ozawa, T. Asakawa, T. Ohta, A. Miyagishi, S. Halide-sensitive fluorescent probes for self-assembly of novel cationic fluoro surfactants. J. Oleo Sci. 2005, 54, 135-142. 

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