Non-micellar solutions

Anionic micellar systems were found to increase the rate of the acid catalyzed hydrolysis of acetylsalicylic acid (Nogami et al., 1962), methantheline bromide (Nogami and Awazu, 1962), n-butyl acetate, t-butyl acetate, ethyl p-aminobenzoate, and ethyl o-aminobenzoate (Sakurada et al., 1967), but decreased that of methyl benzoate slightly (Sakurada et al., 1967). The acid catalyzed hydrolysis of anionic amphi-philes also generally tend to be accelerated by micellization (Table 5). The rates of the acid catalyzed hydrolyses of sodium sulfoethyl do-decanoate, sodium undecanoate, and sodium sulfobutyl caprylate are significantly greater in micellar than in non-micellar solutions while that of sodium dodecyl sulfoacetate is unaffected by micelle formation (Meguro and Hikota, 1968). 

The activation parameters for the acid-catalyzed hydrolysis of long chain alkyl sulfates compared to those for non-micellar ethyl sulfate calculated from potentiometric data indicate that the rate acceleration accompanying micellization is primarily a consequence of a decrease in the enthalpy of activation rather than an increase in the entropy (Kurz, 1962). However, the activation energies for the acid-catalyzed hydrolysis of sodium dodecyl sulfate calculated from spectrophotometric data have been reported to be identical (Table 8) for micellar and non-micellar solutions, but the entropy of activation for the hydrolysis of the micellar sulfate was found to be 6 9 e.u. greater than that for the non-micellar system (Motsavage and Kostenbauder, 1963). This apparent discrepancy may be due to the choice of the non-micellar state as the basis of comparison, i.e. ethyl sulfate and non-micellar dodecyl sulfate, to temperature dependent errors in the values of the acid catalyzed rate constant determined potentiometrically, or to deviations in the rate constants from the Arrhenius equation. 

All the definitions of the CMC discussed above reflect a general feature of surfactant solutions, namely, a qualitative change in the concentration dependencies of their properties at the CMC. This means that the thermodynamic state of such systems must also differ from the state below the CMC and cannot be described by conventional theories proposed for non-micellar solutions. A brief review of the thermodynamics of micellisation is presented in the following section. 

The third important property of ionic micellar solutions is the activity coefficient, which is closely related to the ionic strength. The mean activity coefficient of a non-micellar solution is usually defined as... 

It is well-known that the adsorption kinetics from non-micellar solutions can be described mathematically by a corresponding boundary problem for the diffusion equation [104, 105]. In the case of micellar solutions the diffusion equation for monomers must contain terms taking into account the influence of micelles. The single diffusion equation of monomers must be replaced by a system of two equations (for monomers and micelles). At last, it is necessary to introduce an additional boundary condition, which takes into account that micelles are not surface active. This are all alterations in the formulation of the mathematical problem. However, it will be shown below that the new problem is essentially more complex and can be solved analytically only for very particular situations and after introduction of additional simplifications. 

Obviously the influence of micelles must be taken into account in the first approximation only for diffusional and mixed adsorption mechanisms. If the rate-limiting step consists in crossing the adsorption barrier by the surfactant monomers, all the relations derived for non-micellar solutions hold for c > CMC too. 

Eq. (5.243) fully coincides with the Ward and Tordai equation. Hence it follows immediately that the presence of micelles in the solution leads to an acceleration of the adsorption and desorption processes if equilibrium between monomers and micelles can be assumed. All the relations obtained for non-micellar solutions can be used in this case too if the diffusion coefficient of monomers is replaced by the effective coefficient D. ... 

Expressions for the dilational visco-elasticity for non-micellar solutions E(ico) were obtained by Lucassen [57, 58] ... 

For calculations of the rheological dependences for CuEOg according to the theory of Lucassen [57, 58] for non-micellar solution we applied a combined reorientation model with two-dimensional compressibility in state with minimal molar area (O2 as... 

In the case of micellar solutions, studied in this work, the monomers interact via two-body potentials. The non-bonded particles interact via the repulsive part of a Lennard-Jones potential... 

The fractional ionization, a, of ionic micelles is increased by hydrophobic non-ionic solutes which decrease the charge density at the micellar surface and the binding of counterions (Larsen and Tepley, 1974 Zana, 1980 Bunton and de Buzzaccarini, 1982). Consistently, microemulsion droplets are less effective at binding counterions than otherwise similar micelles. 

The acceptor in the bulk solution causes a more efficient charge separation to give D+ and A- than non-micellar systems (Eq. (11)). 

Fig. 6 The Cole-Cole plot of the contribution from water to the frequency dependent dielectric function. The reduced real part [c (u )] is plotted against the reduced imaginary part (c (u>)]. Note the non-Debye character in the micellar solution.

For block copolymers with a polyacid or polybase block, the structure and properties of micellar solutions depend on the pH. For example, Morishima et al. (1982b) found that for a poly(9-vinylphenanthrene)-poly(methacrylic acid) (PVPT-PMA) diblock in water, the rate constant for the fluorescence quenching of phenanthrene groups by oxidative non-ionic quencher is pH dependent. These authors suggested that at low pH the polyacid units are not fully ionized and may participate in the formation of hydrophobic domains, cooperatively with PVPT. An alternative explanation is that the PM A chains are less solvated when... 

This chapter is concerned with experiments and theory for semidilute and concentrated block copolymer solutions.The focus is on the thermodynamics, i.e. the phase behaviour of both micellar solutions and non-micellar (e.g. swollen lamellar) phases. The chapter is organized very simply Section 4.2 contains a general account of gelation in block copolymer solutions. Section 4.3 is concerned with the solution phase behaviour of poly(oxyethylene)-containing diblocks and tri-blocks. The phase behaviour of styrenic block copolymers in selective solvents is discussed in Section 4.4. Section 4.5 is then concerned with theories for ordered block copolymer solutions, including both non-micellar phases in semidilute solutions and micellar gels. There has been little work on the dynamics of semidilute and concentrated block copolymer solutions, and this is reflected by the limited discussion of this subject in this chapter. 

In case of non-ionic surfactants in water, the behaviour of the water structure outlines three main concentration regions, which closely coincide with the three phases intersected by the experimental isotherms. In the micellar solution phase, no significant changes in the water structure are indicated, while, in the lamellar phase, rapid destruction of the tetrahedral hydrogen bond network occurs due to the confinement of the water between the hydrophilic surfaces of the lamellae. The dehydration of the surfactant head groups was found to start near the border between the lamellar and the reverse micellar solution phases. At higher concentrations, water demonstrates its trend to form clusters of tetrahedrally bonded molecules even at the very low content in the system. The results with surfactant solutions have been obtained by Raman spectroscopy (Marinov el al., 2001). 

The difference is ascribed to the smaller micelle cavity of succinimides relative to sulfonates. Mixed micelles of naphthalene-sulfonate-succinimide show weaker solubilization capacity than that of individual additives. The solubilization of water in a micellar system is closely related to the micelle core (Fontana, 1968). Addition of water to this non-polar solution, as engine lubricating oil is, produces a new set of phenomena. For small amounts of water, the micellar aggregates show swelling by uptake of water. The highly bounded water in reversed micelles makes surfactants less effective. 

Table III. Effect of Temperature on the Micellar Properties of Irradiated and Non-irradiated solutions...

Consequently, new investigations dealing with reactions in micellar solutions composed of functional surfactants, or mixtures of inert and functional surfactants, continue to appear in the literature. An interesting study of acid-catalyzed hydrolysis of 2-(p-tetradecyloxyphenyl)-l,3-dioxolane (p-TPD) in aqueous sodium dodecyl sulfate (SDS) solutions has been reported [13]. In this case,/ -TPD behaves as a non-ionic functional surfactant and apparently forms non-ideal mixed micelles with the anionic surfactant (SDS). Based on the observed kinetic data, the authors propose that, at elevated temperatures, the thermodynamic non-ideality results in the manifestation of two populations of micelles, one rich in SDS and the other rich in/>-TPD. 

Polarographic studies show, in addition to oxidation waves for [Os(bipy)3]2+, several reduction waves 130,158 this is also the case for [Os(4,4 -Me2bipy)3]2+, [Os(5,5 -Me2bipy)3]2+121 and [Os-(phen)3]2+. Recent cyclic voltammetry and coulombetry studies on [Os(bipy)3]2+ and [Os(phen)3]2+ in liquid S02 show successive one-electron oxidations to [Os(LL)3]3+ and [Os(LL)3]4+.lls There is a small but real difference in the Os111/u redox potential for [Os(phen)3]2+ in aqueous and in non-aqueous sodium lauryl sulfate micellar solutions.138 Correlations have been made between the oxidation potentials and charge-transfer transition frequencies in complexes [M(LL)3]2+ (Me = Fe, Ru, Os LL = bipy, 4,4 -Me2bipy, 5,5 -Me2bipy).159... 

The ability of micellar solutions and mlcroemulslons to dissolve and compartmentalize both polar and non-polar reactants has a significant effect on chemical reactivity. An Idealized representation of a typical micelle catalyzed reaction is depicted In Figure 2. Here the non-polar reactant is solubilized within the micelle while the ionic reactant is at the surface. The polar head groups of the surfactants generate a charge at the micelle surface which serves to attract an oppositely charged water soluble reactant increasing the concentration of that reactant near the micelle. The result Is an enhanced reaction rate. 

Because the MFEs of heavy atom-centered radicals are usually much smaller than those of light atom-centered radicals, micellar solutions and oil emulsions have been used for obtaining MP s of heavy atom-centered ones until quit recently. Recently, we have also observed MFEs of heavy atom-centered radicals even in usual non-viscous solvents. Some of such MFEs of heavy atom-centered radicals can be explained by the RM, but others by other mechanisms. 

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