Surfactant microviscosity

Other solubilization and partitioning phenomena are important, both within the context of microemulsions and in the absence of added immiscible solvent. In regular micellar solutions, micelles promote the solubility of many compounds otherwise insoluble in water. The amount of chemical component solubilized in a micellar solution will, typically, be much smaller than can be accommodated in microemulsion fonnation, such as when only a few molecules per micelle are solubilized. Such limited solubilization is nevertheless quite useful. The incoriDoration of minor quantities of pyrene and related optical probes into micelles are a key to the use of fluorescence depolarization in quantifying micellar aggregation numbers and micellar microviscosities [48]. Micellar solubilization makes it possible to measure acid-base or electrochemical properties of compounds otherwise insoluble in aqueous solution. Micellar solubilization facilitates micellar catalysis (see section C2.3.10) and emulsion polymerization (see section C2.3.12). On the other hand, there are untoward effects of micellar solubilization in practical applications of surfactants. Wlren one has a multiphase... 

Lukac S (1984) Thermally induced variations in polarity and microviscosity of phospholipid and surfactant vesicles monitored with a probe forming an intramolecular charge-transfer complex. J Am Chem Soc 106 4386 -392... 

The structure of water droplets (often termed water pools) entrapped in spherical or near-spherical associations of amphiphilic surfactants is quite different from that of bulk water their polarity, microviscosity, and behavior as a function of temperature reflect the uniqueness of such media. 

Zana, R. 1999. Microviscosity of aqueous surfactant micelles Effect of various parardeKir fs. Chem. B. 103 9117-9125. 

Fluorescence polarization is a dimensionless ratio with values from 0.000 to 0.500 for dilute solutions containing fluorescing compounds (see Chapter 3). Polarization (P) measures the rotational diffusion of the fluorophore relative to its fluorescent half-life. If the half-life is short compared with the rate of rotational diffusion, P will be high. In contrast, if molecular rotation is faster than the excited state decay, then P wfll be low. Shinitzky proposed that for amni-otic fluid, lower polarization values reflected a decrease in the microviscosity of surfactant phospholipids. The fluidity of these phospholipids paralleled the change in lipid composition with maturation of the fetal lungs. This mechanism is incorrect for the NBD-PC method. NBD-PC binds to albumin and to surfactant thus the resulting polarization is a function of the surfactant/albumin ratio. ... 

Since photoisomerization involves motion of one-half of the probe molecule against the other, it remains unaffected by the overall motions of the organized assemblies, such as the motion of different segments of DNA, or of the peptide chains of a protein or the surfactants in a micelle. In contrast, in optical anisotropy studies, the bending and twisting motion of the macromolecular chains of DNA, proteins, or micelles become superimposed on the orientational motion of the probe. As a result, photoisomerization seems to be a better probe for the microviscosity of complex biological assemblies. 

Trimeric and oligomeric surfactants have also been prepared (Zana, 1995 Sumida, 1998 In, 2000 Onitsuka, 2001). Their CMC values are even smaller than those of the analogous geminis. As the number of hydrophobic groups per molecule increases for gemini quaternary C12 ammonium compounds with polymethylene -(CH2) j spacers, their surface layers become more dense, their micellar microviscosity increases, and their micellar shape changes from spherical to wormlike, to... 

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. 

The micropolarity and the microviscosity of the micelles of the [C H2m +1 N (CH3)2, Br ]2(CH2)j surfactants have been systematically investigated as a function of m and 5 using the fluorescent probes pyrene (micropolarity) and diphenylhexatriene or dipyrenylpropane (microviscosity) [108-110]. The results for the 12-5-12, 2Br" series have been compared to those for the series of the corresponding monomeric surfactants Ci2H25(Q/2H +i)N (CH3)2, Br". Figure 11 shows that the micropolarity is nearly the same for monomeric and gemini surfactant micelles at a given 5 value, except at 5 = 2. This result was explained on the basis of the similar compositions of the... 

FIG. 12 Microviscosity (product of the fluorescence lifetime by the intensity ratio monomer/ xcimer of dipyrenylpropane [110]) for micelles of the gemini surfactants 12-5-12, 2Br" ( ) and for their corresponding monomeric surfactants Ci2H25(Cj/2Hs+i) N (CH3)2 Br") ( ), relative to that in DTAB micelles, as a function of the carbon number 5. 

The forward reaction of Equation 3.4 is diffusion controlled and consequently its rate will vary inversely with the viscosity of the medium. The ratio /e/f m is commonly used as a measure of the ease of excimer formation, and /m being the excimer and monomer emission, respectively. Excimer formation in micellar systems requires at least two probe molecules per micelle for the reaction of Equation 3.4 to occur within the micelles. The ratio /e//m is thus dependent on the distribution of probe molecules among the micelles which is assumed to follow a Poisson distribution. At the commonly used probe/surfactant molar ratio of 0.01, Zachariasse [13] calculates from Poisson statistics that 27% of sodium dodecyl sulphate (NaDS) micelles are more than singly occupied. There is difficulty in the interpretation of the fluorescence data since excimer emission occurs alongside the partly quenched monomer fluorescence in doubly or higher occupied micelles, whereas singly occupied micelles show only unquenched monomer fluorescence. This situation leads to uncertainty in the calculated microviscosity and may explain the anomalous value of 150 cP proposed by Pownall and Smith [11] for the micro viscosity of the micellar core of hexadecyltrimethylammonium bromide. 

Many techniques have been used to characterize the state of water and the dynamics of water molecules in W/0 micro-emulsions. It is usual in that case to consider the molar concentration ratio Wo = [water]/[surfactant]. All the techniques used demonstrate that for low values of w , the water molecules are essentially bound to the counterions and to the surfactant polar heads. It is only when w exceeds a certain value (around 6-10 for AOT-based systems for instance) that free water can be detected. Properties such as the local microviscosity, or the local dielectric constant, are strongly affected by the state of water. 

The viscosity of the interior of micelles and the effects of surfactants on the structure and physical properties of the micelle can be determined by fluorescence polarization. Shinitzky et al. [9] studied the effects of introducing uncharged hydrophilic groups into the surface of a micelle on the fluidity of the micelle interior. Cetyl alcohol (CA) and cetyltrimethylammonium bromide (CTAB) were mixed in different ratios, and the microviscosity of the interior of the micelle was determined by measuring the polarization value of perylene embedded in... 

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