Partition micelle-water

Herein Pa and Pb are the micelle - water partition coefficients of A and B, respectively, defined as ratios of the concentrations in the micellar and aqueous phase [S] is the concentration of surfactant V. ai,s is fhe molar volume of the micellised surfactant and k and k , are the second-order rate constants for the reaction in the micellar pseudophase and in the aqueous phase, respectively. The appearance of the molar volume of the surfactant in this equation is somewhat alarming. It is difficult to identify the volume of the micellar pseudophase that can be regarded as the potential reaction volume. Moreover, the reactants are often not homogeneously distributed throughout the micelle and... 

Duller, W.J., Bakker, M.W.C., and Covers, H.A.J. Micellar solubility and micelle/water partitioning of polychlorinated biphenyls in solutions of sodium dodecyl sulfate. Environ. Sci. Technol, 29(4) 985-992, 1995. 

Effect of salt type and concentration The ionic strength of the aqueous solution in eontaet with a reverse micelle phase affects protein partitioning in a number of ways [18,23]. The first is through modification of electrostatic interactions between the protein surface and the surfaetant head groups by modifieation of the eleetrieal double layers adjacent to both the eharged inner mieelle wall and the protein surface. The second effect is to salt out the protein from the mieelle phase because of the inereased propensity of the ionie speeies to migrate to the micelle water pool, reduee the size of the reverse mieelles, and thus displace the protein. 

N Chen, Y Zhang, S Terabe, T Nakagawa. Effect of physicochemical properties and molecular structure on the micelle—water partition coefficient in micellar electrokinetic chromatography. J. Chromatogr. A 678 327-332 (1994). 

Fletcher PDl, Parrot D (1989) Protein-partitioning between water-in-oil microemulsions and conjugate aqueous phases. In Pileni MP (ed) Structure and reactivity in reverse micelles. Elsevier, Amsterdam, p 303... 

Caution should be exercised when considering temperature effects on solubilization by micelles, since the aqueous solubility of the solute and thus its micelle/water partition coefLcient can also change in response to temperature changes. For example, it has been reported that although tt solubility of benzoic acid in a series of polyoxyethylene nonionic surfactants increases with temperature, the micelle/water partition coefLci rt, shows a minimum at 2C, presumably due to the increase in the aqueous solubility of benzoic acid (Humphreys and Rhodes, 1968). The increasr in Km with increasing temperature was attributed to an increase in micellar size, as the cloud point temperature of the surfactant is approached (Humphreys and Rhodes, 1968). 

Nystatin Micelle-Water Partition Coefficients and Pluronic Micelle Core Polarities as a Function of Pluronic and Temperature... 

Valsaraj, K.T., Thibodeaux, L.J. (1989) Relationships between micelle-water and octanol-water partition constants for hydrophobic organics of environmental interest. Water Res. 23, 183-189. 

El Tayar et al., using micelle-water (mw) partition coefficients determined on a polyoxyethylene(23)-lauryl ether column [31], found a significant correlation between log Pmw and the activity of a series of antiplatelet agents, but no correlation when using log Poet. or calculated [10] log P-values (CLOGP) parameters. 

Quantitative approaches to describing reactions in micelles differ markedly from treatments of reactions in homogeneous solution primarily because discrete statistical distributions of reactants among the micelles must be used in place of conventional concentrations [74], Further, the kinetic approach for bimolecular reactions will depend on how the reactants partition between micelles and bulk solution, and where they are located within the microphase region. Distinct microphase environments have been sensed by NMR spectrometry for hydrophobic molecules such as pyrene, cyclohexane and isopropylbenzene, which are thought to lie within a hydrophobic core , and less hydrophobic molecules such as nitrobenzene and N,N-dimethylaniline, which are preferentially located at the micelle-water interface [75]. Despite these complexities, relatively simple kinetic equations for electron-transfer reactions can be derived for cases where both donors and acceptors are uniformly distributed inside the micelle or on its surface. 

The micelle/water partition coefficient for many solutes have been shown to correlate to the octanol/ water partition coefficientJ ° Data in Table 8, from Azaz and Donbrow, show that the micellar partition coefficients of the methylphenols increase with the number of methyl groups. Collett and Tobin showed that the micellar partition coefficients of several benzoic acid derivatives are proportional to their octanol-water partition coefficient for poloxamers (Table 9). Tomida et al. also illustrated that most of the 34 monosubstituted benzoic acids with Brij 35 have micellar partition coefficients that are inversely proportional to their aqueous solubilities and proportional to their octanol-water partition coefficients. The data of Tomida, Yotsuyanagi, and Ikeda[ °l for some steroid hormones (Table 10), further illustrate the parallelism between octanol-water and micelle-water partition coefficients. 

Other common partition coefficients are soil sorption partition coefficient, gas-solvent partition coefficient and micelle-water partition coefficient, together with leaching indices, which are partition indices thought of for environmental studies. 

Micelle-water partition coefficient, denoted as or in its logarithmic form as log Kmw or log is the partition of a solute between micellar and aqueous phases [Tanaka, Nakamura et al., 1994 Abraham, Chadha et al., 1995a]. 

Micelle-water partition coefficients are extracted by micelle chromatography (high performance liquid chromatography, HPLC) using micelle aqueous solution as mobile phase. For determination of retention times are measured using a usual HPLC system at various concentrations of micelle in the aqueous mobile phase and then estimated from the following equation ... 

As it was for the soil sorption partition coefficient, models for estimating micelle-water partition coefficients take advantage of the correlation with Kqw For example, K. values have been estimated from experimental octanol-water partition coefficients by... 

Tween 60) times of improvement compared with using water alone. Theoretically, the solubilization of hydrophobic compound by surfactant is initiated by surfactant monomer, but much greater and efficient solubilization is observed as the surfactant concentration increases beyond its critical micellar concentration, (CMC). The characteristics of surfactant and its ability to solubilize hydrophobic compounds beyond cmc can be quantified by an analogous micelle-water partition coefficient, Km ... 

In two-phase systems, however, where surfactant and water can partition between a fluid and a liquid phase, significant pressure effects occur. These effects were studied for AOT in ethane and propane by means of the absorption probe pyridine N-oxide and a fluorescence probe, ANS (8-anilino-l-naphthalenesulfonic acid) [20]. The UV absorbance of pyridine A-oxide is related to the interior polarity of reverse micelles, whereas the fluorescence behavior of ANS is an indicator of the freedom of motion of water molecules within reverse micelle water pools. In contrast to the blue-shift behavior of pyridine N-oxide, the emission maximum of ANS increases ( red shift ) as polarity and water motion around the molecule increase. At low pressures the interior polarity, degree of water motion, and absorbance intensity are all low for AOT reverse micelles in the fluid phase because only small amounts of surfactant and water are in solution. As pressure increases, polarity, intensity, and water motion all increase rapidly as large amounts of surfactant and water partition to the fluid phase. The data indicate that the surfactant partitions ahead of the water thus there is a constant increase in size and fluidity of the reverse micelle water pools up to the one-phase point. An example of such behavior is shown in Fig. 4 for AOT in propane with a total fVo of 40. The change in the ANS emission maximum suggests a continuous increase in water mobility, which is due to increasing fVo in the propane phase, up to the one-phase point at 200 bar. 

Figure 4 Partitioning of water and surfactant in the two-phase region, as indicated by the ANS emission maximum, for AOT (0.01 M) reverse micelles in liquid propane at 25°C with an overall Wq of 40. Arrows indicate emission maxima for one-phase AOT systems. (From Ref 20.)...

The kinetics of acid hydrolysis of the p-methoxybenzaldehyde-O-acyloximes (7) in SDS micelles modified by BuOH has also been fitted to the PIE model [86]. The substrates differ only in their hydrophobicities, and while the acetyl derivative partitions between water and micelles, the octanoyl derivative is wholly micelle-bound. The simple PIE model fits rate data in dilute HCl [Eq. (6)], but it underpredicts observed rate constants in more concentrated acid. This increased rate was analyzed in terms of a model that does not involve a constant value of a but allows concentrations of reactive and inert ions, and Na" ", in the micellar pseudophase to increase, following Langmuir isotherms [106]. This model was reasonably satisfactory except at high 1-butanol concentration. Alternatively, the rate data in more concentrated acid can be fitted in terms of Eq. (12). 

The use of micelles to model biomembranes in QSAR studies has however received little attention. Perhaps the difficulties associated with measuring micelle-water partition coefficients by conventional methods have been the major obstacles in conducting an extensive investigation of the use of micellar systems. The following contributions show that a combination of the unique characteristics of micelles and the capabilities of RPLC in physicochemical studies should be quite useful in QRAR research. 

Attempts are being made to determine non-aqueous phase liquid (NAPL) mole fraction and micelle-aqueous partition coefficients with rhamnolipids to understand equilibrium solubilization behaviour in surfactant-enhanced soil remediation situations [51]. A modification of Raoult s law was used for surfactant-enhanced solubilization but deviation from this ideal behaviour depended on the hydrophobicity of the compounds and the NAPL-phase mole fraction. Micelle-water partition coefficients were non-Unear in relation to the NAPL-phase mole fraction. Also enhancements by the surfactant were... 

Studies of water-in-scC02 reverse micelles derived from the surfactant ammonium carboxylate perfluoropolyether (PFPE) demonstrated that CO2 can partition into the PFPE reverse micelle [21], The important question related to the reaction rates and outcomes inside the nanoreactor is whether the pH within the reverse micelle water pool can be controlled by adjusting the CO2 pressure. In an attempt to answer this qnestion. Bright et al. quantitatively determined the pH within the PFPE water pools. Their results showed that the pH remains essentially constant as a fnnction of CO2 continnons phase pressure and micelle water loading [22]. 

Figure 6.7 Changes in micelle/water apparent partition coefficient (Pm) of a solubilizate after progressive addition of a second species. Polysorbate 80 concentrations range from 1 to 3% w/v. Each plot refers to a constant concentration of the surfactant, o-Hydroxybenzoic acid partition coefficients on addition of benzoic acid ((a) 1 % and (c) 3% w/v polysorbate 80). Benzoic acid partition coefficients on addition of o-hydroxybenzoic acid ((b) 1 % and (d) 3 % w/v polysorbate 80). Temperature, 25° 0.1°, pH = 2.0,and the initial solubilizate concentration, 3.2 x lO"" m. Redrawn from Alhaique et al [48].

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