Demicellization

An important kinetic aspect of the washing process is the rate of removal of oily stains by surfactant micelles. Several mechanisms have been suggested for the solubilization of oil into micelles. Chan [67] and Carroll [68] propose diffusion of micelles toward the oil-water interface, followed by demicellization,... 

To rate the wetting tendency of surfactants for hydrophobic surfaces. A graphitic powder, for example, with its low heat of wetting in water, yields much higher heat effects if immersed in solutions of surface active agents. Heats of dilution and of demicellization can be taken into account, if desired, to arrive directly at energies of interaction. 

P = 2RTM l is called the second virial coefficient it yields the same qualitative information about interaction as A[nq]2 in Eq. (4.6). Membrane osmometry seldom requires an accuracy to more than P cf. Doi and Edwards (1986) define a dilute solution as one in which P = 0—the ideal condition for accurately measuring Mn. Te is that temperature where P = 0 (Alberty and Silby, 1992). The fact that P provides information about solute-solute interactions, micellization and demicellization studies are made possible by the use of Eqs. (4.29) and (4.30). 

Strategy. The enthalpy effect measured in the studies of micellar solution structure is due to dilution and demicellization. This can be expressed by... 

It is possible to inclnde in a model a small bnt nonzero volume of the interfacial region in analyzing transport phenomena snch as mass transfer across or to an interface. Mathematical construction can now be performed to divide the domain into a small interfacial region, called the iimer region, where all surface effects apply. The bulk is the outer region where usual convective-diffusive mass transfer (and possibly reactions as in micellization-demicellization) dominate. This is exactly the method of matched asymptotic expansions... 

More complex situations such as solubilization of oils also exhibit nonequilibrium behavior near interfaces, but only at times. Carroll (1981) found that the rate of solubilization of a very insoluble oil drop by a micellar solution of nonionic surfactant (in moles per unit time per unit interfadal area) was iudepmdent of time and drop volume in a batch experiment. This behavior indicates that interfacial resistance, not mass transfer in the micellar solution, controls the rate of solubilization. His view was that first micelles completely demicellized to individual molecules in the immediate vicinity of the oil-water interface with the rate constant discussed in Chapter 4. These surfactant molecules were then... 

Although X2 could play a role in some systems, supporting data are sparse. Accordingly, we begin with a model that considers micelhzation-demicellization kinetics to be very fast, so that micelles are preserved as a whole. Solubilization takes place when a singly dispersed oil molecule colhdes with a micelle or when a micelle collides with the oil-water interface. 

FIGURE 6.26 Measured values of It (= KJc ) plotted against surfactant concentration (black). Also shown are the demicellization rate constant X2 from Lessner et al. (1981) (white). Reprinted from Shah and Neogi (2002) with permission from Elsevier. 

That is, the fluxes vary with the square root of a result they verified from experiments. Here, is the reaction rate constant where = Xf and experiments show thatXi or c. Here, x, is the first time constant in demicellization, which is smaller and represents the rate at which a single amphiphile is ejected from a micelle. In contrast, Xj, the second time constant, is used in Figure 6.26. It is also clear from Equation 6.13.iii that in the limit of large the reaction rate is infinite compared to the diffusion, and no reasonable expression results. Such a limiting case is quite relevant, and in such a situation a model such as Equation 6.101 may be the only recourse. 

In this chapter we examine some issues in mass transfer. The reader has already been introduced to some of the key aspects. In Chapter 3 (Section 7), flocculation kinetics of colloidal particles is considered. It shows the importance of diffusivity in the rate process, and in Equation 3.72, the Stokes-Einstein equation, the effect of particle size on diffusivity is observed, leading to the need to study sizes, shapes, and charges on colloidal particles, which is taken up in Chapter 3 (Section 4). Similarly some of the key studies in mass transfe in surfactant systems— dynamic surface tension, smface elasticity, contacting and solubilization kinetics—are considered in Chapter 6 (Sections 6, 7, 10, and 12 with some related issues considered in Sections 11 and 13). These emphasize the roles played by different phases, which are characterized by molecular aggregation of different kinds. In anticipation of this, the microstructures are discussed in detail in Chapter 4 (Sections 2,4, and 7). Section 2 also includes some discussion on micellization-demicellization kinetics. 

Although most polymers tend to accumulate at the fluid interface, reports involving the transfer of polymeric micelles (micellar shuttle) between two immiscible phases have been pubHshed. Poly(N-isopropylacrylamide) (PNIPAM), a thermally responsive polymer, is insoluble and can undergo a conformation change above its lower critical solution temperature of 32 ° C. The thermo reversible miceUization—demicellization process and micellar shuttle of PNIPAM-PEO diblock copolymer at a water-IL interface were investigated by dissipative particle dynamics (DPD) simulations (Soto-Figueroa et al, 2012). Simulation results confirm that the phase transfer behavior of polymeric micelles is controlled by the temperature effect that changes the diblock copolymer from hydrophilic to hydrophobic (as shown in Fig. 33). 

Soto-Figueroa C, del Rosario Rodriguez-Hidalgo M, Vicente L Dissipative particle dynamics simulation of the miceHization-demicellization process and micellar shuttle of a diblock copolymer in a biphasic system (water/ionic-liquid). Soft Matter 8 1871-1877, 2012. 

The enthalpy of micellization of many surfactants in aqueous solution has been determined in the past, using mostly cell type and flow microcalorimeters [6-8]. These determinations were based on measurements of the excess heat associated with dilution of a surfactant from a concentration above the cmc to a concentration below the cmc, which results in demicellization of the preexisting micelles. One diffleulty with these determinations relates to the dependence of the heat evolution (AQ) on the initial and final concentrations, probably due to secondary self-aggregations of the surfactants at high concentrations and/or pre-micellar dimer formation at low surfactant concentrations [6,9], These difficulties are at least partially responsible for the lack of consistent data on the thermodynamics of micelle formation [6]. 

In terms of molecular interactions, demicellization is analogous to dissolving hydrocarbons in water, which has been viewed [13] as being a two-step process ... 

Hydration of hydrophobic moieties and consequent formation of strong hydrogen bonds (A//cw), which, according to Gill et al. [13], should strongly favor transition of surfactant molecules into the water (i.e., favors demicellization)... 

The observed exothermic nature of demicellization of many surfactants at low temperatures (around 25°C) (see above) must mean that the heat gain associated with hydration of the hydrophobic moieties (possibly due to stronger hydrogen bonds in the water [14]) is greater than the sum of heat losses associated with reduction of the original hydrogen bonds in the water and disruption of the van der Waals interactions between hydrophobic moieties in the aggregates. 

Elevation of the temperature is likely to result in reduction of all the stabilizing energies. The observed endothermic nature of demicellization at high temperature... 

FIG. 2 Schematic explanation for the dependence of the heat of demicellization on chain length and temperature. The bold arrows represent the overall enthalpy of demicellization, the dotted arrows represent the excess enthalpy of hydration of the hydrophobic moieties of the amphiphiles, and the dashed arrows represent the sum of enthalpies resulting from van der Waals interactions between hydrophobic moieties in the aggregates and from hydrogen bonds between water molecules. 

When demicellization occurs, the heats of dilution of both monomers and micelles usually make only a minor contribution to the overall evolution of heat. In the following discussion we neglect these contributions. However, a more accurate evaluation of the heat of demicellization must be based on independent determination of the contribution of the heat of monomer dilution, which can be experimentally measured by dilution of a solution of a concentration below the cmc. This, of course, is possible only when the heat of dilution of monomers is... 

The infinite dilution protocol involves titration of a concentrated amphiphile solution with Dt > cmc into a cell containing no amphiphile, namely Do = 0. After titfation, the concentration in the cell is Di = DtV,/(Vc + vO, where Di < cmc. Since Vc v this protocol is denoted as infinite dilution and the heat evolution of the process reflects demicellization of aU the micelles that were present in the titrated volume. The major contribution to the heat of dilution (AQ) is the heat of demicellization of (Di cmc)Vt moles of surfactant. Hence,... 

FIG. 3 Schematic description of the expected dependence of the heat of demicellization on the total surfactant concentration in infinite dilution experiments. 

In another series of experiments, small volumes of aqueous solutions containing no surfactant are injected into the cell. When the initial concentration in the cell (Do) is lower than the cmc, the (relatively low) heat of dilution of monomers can be determined. When Dq cmc and Vc v the titration results in partial demicellization, the titrated volume becomes saturated with monomeric surfactant, and the total amount of surfactant that becomes dissociated equals Vt cmc so that... 

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