Solubilization, hydrocarbons aqueous micellar solutions

We begin with a relatively simple model, which was suggested some years ago by Mukeqee and which provides considerable insight on solubilization of small amounts of solutes in spherical micelles. Suppose that an aqueous micellar solution has reached its solubilization limit and is in equilibrium with an excess liquid phase of a pure hydrocarbon or some other compound of low polarity. Equating the chemical potentials j,g and of the solute in the bulk organic phase and in the micelles, we have... 

The effect of the curvation of the micelle on solubilization capacity has been pointed out by Mukerjee (1979, 1980). The convex surface produces a considerable Laplace pressure (equation 7.1) inside the micelle. This may explain the lower solubilizing power of aqueous micellar solutions of hydrocarbon-chain surfactants for hydrocarbons, compared to that of bulk phase hydrocarbons, and the decrease in solubilization capacity with increase in molar volume of the solubilizate. On the other hand, reduction of the tension or the curvature at the micellar-aqueous solution interface should increase solubilization capacity through reduction in Laplace pressure. This may in part account for the increased solubilization of hydrocarbons by aqueous solutions of ionic surfactants upon the addition of polar solubilizates or upon the addition of electrolyte. The increase in the solubilization of hydrocarbons with decrease in interfacial tension has been pointed out by Bourrel (1983). 

The presence of micelles can also result in the formation of different reaction products. A diazonium salt, in an aqueous micellar solution of sodium dodecyl sulfate, yielded the corresponding phenol from reaction with OH- in the bulk phase but the corresponding hydrocarbon from material solubilized in the micelles (Abe, 1983). 

It is well known that polycyclic aromatic hydrocarbons (PAHs) can be solubilized in aqueous solutions by micelle forming surfactants. As pointed out in our previous paper, these aqueous micellar conditions can be utilized in generating PAH-specific light emissions by electrochemical means at the rotating oxide-covered aluminum electrode. Because the aqueous micellar solutions are also well suited for chromatographic analysis of PAHs, the analytical feasibility experiments of the flow detector were carried out by utilizing the PAH-induced electroluminescence with Brij-35 and 9,10-diphenylanthracene (9,10-DPA) as the... 

Garrett et al. [108] and later Ashcroft et al. [109] have claimed that a rather novel application of the antifoam synergy found with hydrocarbon-calcium soap mixtures can be applied in this context. Consider then a cleaning liquid where both the hydrocarbon and a sodium soap (or fatty acid) are solubilized in a concentrated aqueous micellar solution of another surfactant. If use of the product involves dilution in hard water, then both the solubilized hydrocarbon and soap will be precipitated as the concentration of micelles decreases and the water hardness increases. It is claimed [108,109] that synergistic foam control is then observed as exemplified by the results shown in Table 8.2. Possibly precipitation of bulk phase hydrocarbon is nucleated on the calcium soap particles so that a hydrocarbon-calcium soap antifoam entity could be formed in situ giving rise to that synergy. 

Investigations of the solubilization of water and aqueous NaCl solutions in mixed reverse micellar systems formed with AOT and nonionic surfactants in hydrocarbons emphasized the presence of a maximum solubilization capacity of water, occurring at a certain concentration of NaCl, which is significantly influenced by the solvent used [132],... 

The second important characteristic of the micellar solution that relates to solubilization is the micelle size. Poor aqueous soluble compounds are solubilized either within the hydrocarbon core of the micelle or, very commonly, within the head group layer at the surface of the micelle or in the palisade portion of the micelle. Predictions of the micelle size have relied on the use of empirical relationships employed within a thermodynamic model, for instance the law of mass action where micellization is in equilibrium with the associated and unassociated (monomer) surfactant molecules (Attwood and Florence, 1983). 

At an air-water interface, a monolayer forms with heads lying down and tails up (toward air), whereas at an air-hydrocarbon interface the monolayer lies with tails down. By closing on the tail side, the sheetlike structure can be dispersed in aqueous solutions as spherical, rodlike, or disklike micelles (Fig. 3). Closure on the head side forms the corresponding inverted micelles in oil. Oil added to a micellar solution is incorporated into the interior of the micelle to form a swollen micellar solution. Thus, surfactant acts to solubilize substantial amounts of oil into aqueous solution. Similarly, a swollen inverted micellar solution enables significant solubilization of water in oil. 

In aqueous solutions the micellar assembly structure allows sparingly soluble or water-insoluble chemical species to be solubilized, because they can associate and bind to the micelles. The interaction between surfactant and analyte can be electrostatic, hydrophobic, or a combination of both [76]. The solubilization site varies with the nature of the solubilized species and surfactant [77]. Micelles of nonionic surfactants demonstrate the greatest ability for solubilization of a wide group of various compounds for example, it is possible to solubilize hydrocarbons or metal complexes in aqueous solutions or polar compounds in nonpolar organic solutions. As the temperature of an aqueous nonionic surfactant solution is increased, the solution turns cloudy and phase separation occurs to give a surfactant-rich phase (SRP) of small volume containing the analyte trapped in micelle structures and a bulk diluted aqueous phase. The temperature at which phase separation occurs is known as the cloud point. Both CMC and cloud point depend on the structure of the surfactant and the presence of additives. Table 6.10 gives the values of CMC and cloud point for the surfactants most frequently applied in the CPE process. 

Analogously, at point C the phase separation into a microemulsion of the composition D and a micellar solution of inverse micelles containing solubilized water takes place. This is a Winsor I (WI) type equlibrium. At the intermediate point E the system consists of a single microemulsion phase (ME). At even lower surfactant concentrations (line c), depending on the water/hydrocarbon ratio, the system will either separate into one of the two-phase systems (Winsor I or Winsor II), or a three-phase system may form at point F. This is a Winsor III (Will) equilibrium with an aqueous phase at the bottom, a microemulsion phase in the middle and a hydrocarbon phase at the top. 

Figure 25 shows the equilibria in sodium caprylate-hydrocarbon-water systems (the hydrocarbon can be octane or xylene). Here there is the one-phase system, Li, with homogeneous aqueous solutions and mesophases E and Ii. The hydrocarbon is solubilized in the micellar aqueous Li solution, but the sodium caprylate is not dissolved in the hydrocarbon. 

A popular representation of spherical micelles was devised by Hartley (26). As indicated in Fig. 1, the Hartley model of, e.g., an anionic micelle exhibits a spherical electric double layer composed of bulky, hydrated anionic heads of surfactant molecules and their counterions in the aqueous phase, while the hydrophobic tails, visualized as sticks, form a hydrocarbon-like micellar interior. Because of the high surface charge density of the micelle, there is only little electrolytic dissociation of counterions. The Hartley model explains the low conductivity of micellar solutions and the way surfactants work as detergents by solubilizing (i.e. incorporating) hydroi obic substrates. The model fails to explain certain NMR and fluorescence data that demonstrate some contact of... 

While various techniques, such as stopped flow, have been used to follow substrate kinetics, many kinetic measurements have involved the photophysical properties of solubilized probes. Because of the luminescent properties of their excited states, the aromatic hydrocarbons provide opportunities for monitoring movement of such probes across the micelle boundary. For example, long-lived phosphorescence of aromatic hydrocarbons has been monitored in micellar solutions containing ionic quenchers that themselves are repelled by the surfactant head groups. Since quenching must take place in the aqueous phase, phosphorescence lifetimes may be interpreted to provide rate constants for exit of the probe from the micelle. Some typical values obtained by this technique are given in Table III. Fluorescence data have also been used to obtain such information. 

One of the earliest generalizations concerning antifoams states that they must be present as undissolved particles (or drops) in the liquid to be defoamed [2-4]. Indeed the presence of antifoam materials at concentrations lower than the solubility limit can even enhance foamability [5, 6]. One weU-known example concerns the foamenhancing effect of dissolved polydimethylsiloxanes (PDMSs) on hydrocarbon lube oils [5]. Amaudov et al. [7] report a similar, but small, effect for solubilized 2-butyl octanol on the foamability of saline aqueous micellar sodium dodecylbenzene sulfonate solutions where the oil has a significant antifoam effect on the stability of foam when present at concentrations above the solubility limit. Another example concerns the effect of dodecanol on the foam of aqueous micellar anionic surfactant solutions. According to Amaudov et al. [7] drops of dodecanol in excess of the solubility limit function as weak antifoams—at least in the case of saline micellar solutions of sodium dodecylbenzene sulfonate. By contrast, Patist et al. [8] find that solubilized... 

With C12E5 as the nonionic surfactant at a 1 wt% level in water, quite different phenomena were observed below, above, and well above the cloud point when tetradecane or hexadecane was carefully layered on top of the aqueous solution. Below the cloud point temperature of 31 °C, very slow solubilization of oil into the one-phase micellar solution occurred. At 35 C, which is just above the cloud point, a much different behavior was observed. The surfactant-rich L phase separated to the top of the aqueous phase prior to the addition of hexadecane. Upon addition of the oil, the L, phase rapidly solubilizes the hydrocarbon to form an oil-in-water microemulsion containing an appreciable amount of the nonpolar oil. After depletion of the larger surfactant-containing drops, a front developed as smaller drops were incorporated into the microemulsion phase. This behavior is shown schematically in Figure 12.16. Unlike the experiments carried out below the cloud point temperature, appreciable solubilization of oil was observed in the time frame of the study, as indicated by upward movement of the oil-microemulsion interface. Similar phenomena were observed with both tetradecane and hexadecane as the oil phases. 

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