Rate of Solubilization

For the solubilization of highly insoluble hydrocarbons by POE nonionics into water, the rate of solubilization has been found (Carroll, 1981, 1982) to be directly proportional to the surfactant concentration above the CMC, and to increase with the polarity and decrease with the molecular weight of the oil. The rate is also strongly temperature dependent in the region of the cloud point (Section IIIB below), increasing rapidly as that temperature is approached. The mechanism suggested involves diffusion of the micelles to the hydrocarbon-water interface, where they dissociate and adsorb as monomers. This adsorption produces concerted desorption from the interface of an equivalent amount of monomeric surfactant, but in the form of micelles containing a quantity of solubilizate. 

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Fig. 9. Rate of solubilization and absorption of l44Ce from lung after inhalation of three forms of cerium.

A Proposed Theory. In earlier publications (1-3), a theory was proposed to correlate solubilization rate, interfacial tension and size of the surfactant aggregate (1) the interfacial tension lowering between the oil-surfactant solution interface is a function of the rate of solubilization of oil, and (2) the rate of solubilization (AS/At) is a function of the effective volume for solubilization ... 

During the solubilization experiment, oil continues to solubilize in the surfactant micelle and the M value continues to Increase. The change of M is reflected by the change of t. And dr/dt Ct = time) is taken as the rate of solubilization in our experiments. 

A "loose-complex hypothesis is highly speculative, but it is one that could fit the facts that have been established. It is compatible with the fact that the relative proportions of Cx to C i are important for maximum rate of solubilization (16). It could also explain the variation in the synergism between C i of one fungus and Cx of another, or the variation... 

Chen, B. FI., C. A. Miller, and P. R. Garrett. 1998. Rates of solubilization of triolein/fatty acid mixtures by nonionic surfactant solutionfeangmuir14 31—41. 

FIGURE 9.5 Schematic diagram of drop-on-fiber technique for measuring solubilization rates. Fiber radius and drop radius Z2, which varies with time, are needed to calculate the rate of solubilization. 

In many situations, for example, removal of oily soils during cleaning processes, both the amount and the rate of solubilization are important. However, considerably less is known about the rates than about the equilibrium conditions. 

For solutions of typical ionic surfactants with no added salts the studies of Carroll and Ward showed that solubihzation rates were much smaller than those for nonionic surfactants, presumably because the surfactant ions adsorbed at the oil-water interface repelled the micelles of like charge in the solution. Indeed, Bolsman et al. found no measurable solubilization of n-hexadecane into solutions of a pure benzene sulfonate and a commercial xylene sulfonate. They injected small oil drops into the surfactant solutions and observed whether the resulting turbidity disappeared over time due to solubilization. Similarly, Kabalnov found from Ostwald ripening experiments that the rate of solubilization of undecane into solutions of pure SDS was independent of surfactant concentration and about the same as the rate in the absence of surfactant. That is, the hydrocarbon presumably left the bulk oil phase in this system by dissolving in virtually miceUe-free water near the interface. In similar experiments TayloC and Soma and Papadopoulos observed a small increase in the solubilization rate of decane with increasing SDS concentration. De Smet et al., who used sodium dodecyl benzene sulfonate, which does not hydrolyze, found, like Kabalnov, a minimal effect of surfactant concentration. 

More recently, Pena and Miller investigated solubilzation rates of mixtures of n-decane and squalane into 2.5 wt% solutions of pure C,2Eg at 23°C using the oil drop method described above. They first measured the rate of solubilization of pure decane, confirming that the rate was controlled by interfacial phenomena as in Carroll s work, and demonstrated that pure squalane was not solubilized to any significant extent under these conditions. Next they measured solubilization rates of decane from various mixtures of the two hydrocarbons. Figure 9.7 shows results from one of these experiments together with predictions of a model based on assuming that the rate of decane... 

Ward, A.J.I., Kinetics of solubilization in surfactant-based systems, in Solubilization in Surfactant Aggregates, Christian, S.D. and Scamehorn, J.E., Eds., Marcel Dekker, New York, 1995, chap. 7. Chen, B.H., Miller, C.A., and Garrett, P.R., Rates of solubilization into nonionic surfactant solutions. Colloids Surf. A, 128, 129, 1997. 

In addition, as the wash temperature decreases, the chain length of the hydro-phobic group for optimum oily soil removal appears to decrease. Thus, for oily soil removal from polyester/cotton by blends of homogeneous 3EO and 8EO nonionics having similar cloud points, the order of maximum oil removal at 70° was Ci4 = Ci2>Cio, at 38°C it was Cio = Ci2>Ci4, and at 24°C it was Ci0> Cn > C 14. The difference has been ascribed to the rate of solubilization of the soil, since the rate for these surfactant blends decreases with increase in the length of the hydrophobe (Benson, 1982). 

There are various methods that can be used to measure the biological availability of magnesium supplements these can involve in vitro or laboratory studies such as the rate of solubilization in rumen fluid or weak acid solution. This method of study is less time consuming and less costly than pursuing animal studies, although in vivo methods are preferred. 

The rates of solubilization, however, and the apparent homogeneity of the solutions deserve comment. First, the extent of incorporation of the N-alkylacrylamide in the copolymer is not determinable from elemental analysis or NMR measurements because of the low concentration in the feed initially. Second, solubilization of purified, freeze-dried samples is often difficult because of inter- and intramolecular associations. Finally, even under dilute conditions, a time dependence on dissolution is observed with some solutions, which require weeks to reach constant values of apparent viscosity. Optical cloudiness with stringy texture is often observed with the longer alkyl chains at higher concentration. 

At temperatures significantly below the PIT (e.g., in the Winsor I phase region), the microemulsion can solubilize much less hydrocarbon (see Figure 4.25). The rate of solubilization is also slower (Miller and Raney, 1993) and interfacial tension is higher. As a result, soil removal is considerably lower, as may be seen from Figure 4.32. Well below the cloud point temperature, no... 

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... 

The available evidence suggests that, except for rather soluble oils, the rate of solubilization is often controlled by interfacial resistance. As diseussed previ-... 

Sailaja, D., Suhasini, K.L., Kumar, S., and Gandhi, K.S., Theory of rate of solubilization into surfactant solutions, Langmuir, 19, 4014, 2003. 

At equilibrium, the least soluble substance in a system that can form will precipitate. Much phosphate contained in sea water is precipitated as tricalcium orthophosphate or hydroxyl apatite, Caio(P04)6(OH)2, and fluorapatite, Caio(P04)6(F)2. Oceans floors are covered with these deposits and are referred to as marine pellets. There are many ways in which this problem may be approached, but it is obvious that if phosphates are to be leached from igneous rocks, large boulders will leach very slowly. Smaller particles of rock caused by grinding, weathering, and aging solubilize more rapidly than larger particles. As a first approximation, rates of solubilization are proportional to fresh surfaces of solubilized rocks. 

A study by stopped-flow techniques of the rate of solubilization of positively charged acridine (and related) dyes by the micelles of several sodium n-alkyl sulphates was reported by Robinson and co-workers [132]. The solubilization process was relatively slow k = 47.6 s for acridine orange in NaDS) due largely to charge effects and the rate was very dependent on the geometrical shape... 

A recent theory of the rate of solubilization into micellar solutions involves the dynamics of the solubilizate exchange between the micelles and the continuous phase. 

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