Micellar Solutions of Surfactants

It is well known that surfactants dissolved in aqueous solutions serve to enhance the solubility of ordinarily insoluble organic compounds, both solids and liquids. This phenomenon, commonly referred to as solubilization (ref. 501), has important commercial applications and as a consequence, has been the subject of considerable research (for reviews see ref. 450,502-504). Yet, as King and co-workers point out (ref. 501,505,506), less well known is the long-recognized fact that micellar solutions of surfactants are also capable of solubilizing gases (and vapors of low-molecular-weight compounds) in much the same manner as... 

The addition of solutes to micellar solutions of surfactants in water may give rise to different phenomena depending on the chemical nature of the additive. Ionic solutes carrying the same charge as the head groups of an ionic micelle... 

Stebe K. S., Lin S. Y, Maldarelly C., Remobilization surfactant-retarded particle interfaces. I. Stress-free conditions at the interfaces of micellar solutions of surfactants with fiist sorption kinetics, Phys. Fluids A, 1991, Vol. 3 (1), p. 3-20. 

In one of the reported works, TOPO-coated CdSe QDs were dispersed in a micellar solution of surfactant, which could be subsequently swollen by monomer [307], In detail, a toluene solution of CdSe was first added dropwise to a micellar solution of cetyltrimethyl ammonium bromide (CTAB). Then, a mixture of styrene, DVB, AA, and AIBN was added dropwise at 0°C. Finally, the system was heated to 70°C for 20 h. Submicrometer particles were prepared with an effective incorporation of hydrophobic TOPO-coated CdSe into carboxylic functionalized and crosslinked PS particles. The QD-tagged PS particles were then coated with a fluorescent silica shell through TEOS addition. Particle sizes ranging from 300 nm to 20pm were produced, depending on the recipe used for the synthesis. However, the solid content was quite low and there was no information about the amount of QDs incorporated. 

We have seen then that film rupture may occur because surface tension gradients are not sufficiently high to enable the film to withstand stress, because dllALA/d/t is always positive so that rupture is inevitable at a certain critical thickness, or because the Plateau border capillary pressure exceeds any maximum in the relevant disjoining pressure isotherm. However these phenomena are associated with low concentrations of surfactant (at least if we consider films formed slowly so that equilibrium between the air-liquid surface and the intralamellar liquid is maintained). Thus for example, we have CMC for AsqIAH < 0 and CMC. Elimination of both causes of rupture should therefore be readily achieved at sufficiently high concentrations of surfactant. The poor discrimination in foamability often found with relatively concentrated aqueous micellar solutions of surfactants may well be attributable to that cause. Interesting differences in foamability are, however, often revealed when films are either formed rapidly so that equilibrium adsorption is not obtained and conditions for stability are thereby violated or if antifoam is added to the solution. 

The generalization that antifoams must be present as undissolved entities has, however, occasionally been challenged [6,9,10]. A number of authors in fact report experimental results that purport to show antifoam effects due to additives that are solubilized in the foaming solution [11-13]. Thus, Ross and Haak [11], for example, identify two types of antifoam behavior associated with the effect of oils like tributyl phosphate and methyl isobutyl carbinol on the foam behavior of aqueous micellar solutions of surfactants such as sodium dodecylsulfate and sodium oleate. Wherever the oil concentration exceeds the solubility limit, emulsified drops of oil contribute to an effective antifoam action. However, it is claimed [11,14] that a weak antifoam effect is associated with the presence of such oils even when solubilized in micelles. The consequences of all this behavior are revealed if, for example, tributyl phosphate is added to micellar solutions of sodium oleate [11] at concentrations below the solubilization limit. A marked decrease in foamability is found immediately after dispersing the oil. As the oil becomes slowly solubilized, the foamability increases. However, even after the oil is completely solubilized, the foamability is still apparently less than that intrinsic to the uncontaminated surfactant solution [11]. By contrast, Arnaudov et al. [7] have more recently shown that the significant antifoam effect of n-heptanol on aqueous micellar solutions of sodium dodecylbenzene sulfonate (in the presence of NaCl) is almost completely eliminated after solubilization. 

There would appear then to be only limited evidence that oils which exhibit antifoam effects, when present as emulsified bulk phase, can also produce antifoam effects when present only as solubilizates in aqueous micellar solutions of surfactants. In many instances, alternative explanations for supposed observations of the latter are possible, which do invoke the presence of the oils as bulk phase. However some of the observations described here are difficult to dismiss. Of particular interest in this context are the findings of Koczo et al. [15], Lobo et al. [21], and Binks et al. [16] concerning the effect of solubilized alkanes on the foam stability of aqueous micellar solutions of various surfactants. Attempts to explain such effects by recourse to dynamic surface tension behavior after the manner of Ross and Haak [11] would appear to be unconvincing (see reference [22]). It is, however, possible that it may concern the effect of the solubilized oil on the relevant disjoining pressure isotherm. Wasan and coworkers [15,21] have suggested that the phenomenon is a consequence of the effect of solubilization of alkanes on intermicellar interactions. Lobo et al. [21] find that the instability of the foams formed from certain ethoxyl-ated alcohols in the presence of solubilized alkanes depends on the magnitude of the micellar second virial coefficient describing those interactions. Reduction of the... 

The solubilization of decanol and toluene in micellar solutions of surfactant [Q] with X = OH was investigated by small-angle neutron seattering (SANS). The micelles were modeled as spheres (for toluene solubilization) and cylinders (for decanol solubilization) in order to fit the SANS data. A growth of the micelle dimensions was thus evidenced upon addition of toluene and decanol [100]. 

Fluorescence kinetics of ArOH and ArO in micellar solutions of surfactants with various charges was studied by conventional single photon counting method using numerical deconvolution of decay curves. Concentrations of anionic surfactant - SDS ( 0.1 M ), cationic surfactant - DTAB (0.05 M ) and nonionic surfactant - Triton X-305 and Brij 35 (0.01 M ) were chosen by such a way, that more than 99% of hydro-xycompounds were solubilized in micellar phase and protoly-tic photoreaction take place nearly completely in this phase [11-14]. 

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