Diluted mixed surfactant solutions

Worm-Like Micelles in Diluted Mixed Surfactant Solutions Formation and Rheological Behavior... 

Small micelles in dilute solution close to the CMC are generally beheved to be spherical. Under other conditions, micellar materials can assume stmctures such as oblate and prolate spheroids, vesicles (double layers), rods, and lamellae (36,37). AH of these stmctures have been demonstrated under certain conditions, and a single surfactant can assume a number of stmctures, depending on surfactant, salt concentration, and temperature. In mixed surfactant solutions, micelles of each species may coexist, but usually mixed micelles are formed. Anionic-nonionic mixtures are of technical importance and their properties have been studied (38,39). 

Burchfield and Woolley [39,40] have described a one-step mass action model that includes only one micellar species, with the assumption that the surfactant is a strong 1 1 electrolyte at infinite dilution. A surfactant solution having a concentration above cmc is considered a mixed electrolyte. The mass action equilibrium is given by... 

Of the possible types of measurements, heats of micellar mixing obtained from the mixing of pure surfactant solutions are perhaps of the greatest interest. Also of interest is the titration (dilution) of mixed micellar solutions to obtain mixed erne s. While calorimetric measurements have been applied in studies of pure surfactants (6,7) and their interaction with polymers ( ), to our knowledge, applications of calorimetry to problems of nonideal mixed micellization have not been previously reported in the literature. 

Except for some anionic/cationic surfactant mixtures which form ion pairs, in a typical surfactant solution, the concentration of the surfactant components as monomeric species is so dilute that no significant interactions between surfactant monomers occur. Therefore, the monomer—mi celle equilibria is dictated by the tendency of the surfactant components to form micelles and the interaction between surfactants in the micelle. Prediction of monomer—micelle equilibria reduces to modeling of the thermodynamics of mixed micelle formation. 

KOH affects the size of the changed micelles (12.16-18). For mixed surfactant systems, this effect may be different for different values of surfactant ratios of the mixed surfactants. Furthermore, it was observed, during the earlier period of the present study, that the dilute solutions of sodium lauryl sulfate beccime turbid in the presence of KOH, presumably due to alkaline hydrolysis. Therefore, KOH was not used in this study. 

The microcalorimeter used (LKB, Sweden, bartch 2107) was described in detail in Ref. 11. The mixing procedure was the same as that described in Ref. 11, i.e., the heat of dilution of a surfactant solution (2 mL) was measured on mixing with 2 mL of solvent. In the reference cell the heat of mixing of 2 mL solvent with the... 

In all the measurements carried out in this study, for different ratios of NaDDS NaDOC, the dilution curves (Figure 1) of surfactant solution exhibited a clear break that corresponded with the c.m.c. as determined by other methods. This observation agrees with literature reports (1-11,18). The present data, however, show for the first time that mixed micelle systems also behave the same way as pure micellar systems, as measured by calorimetry. Further, because the aggregation number, N, of NaDDS Is much larger than that of NaDOC (16), Table I, the variation of enthalpy around the c.m.c. region is not related to the size of micelles. 

The CAD in the surfactant micelle was prepared by spreading 1 mL of 1 mmol/L dye solution (prepared in acetone) to each 50 mL Pyrex glass tube and air-dried so that the dye was evenly coated onto the tube wall. Various amounts of stock surfactant solution and additives (if any) were added to each tube and diluted by distilled and deionized water to a predetermined concentration with a final volume of 25 mL. Samples were then mixed with the solution, sealed with a Teflon-lined screw cap, and shaken in a mechanical rotary shaker at 20°C for 24 hrs. Different initial pH levels were adjusted by sulphuric acid or sodium hydroxide so that the consequence of initial pH to the overall reaction rates can be realised. 

The enthalpies of binding were determined for the binding of DP+ and CP+ cations to PSS anion [4], They were obtained in experiments in which a solution of NaPSS was mixed with a solution of DPC or CPC and the corresponding enthalpies of dilution of surfactants and NaPSS were accounted for. To keep the ionic strength of the initial and final solutions constant, in most cases both solutions contained an excess of the simple salt (NaCl). 

Another problem that had to be dealt widi was the preparation of the surfactant solution. In the first production scheme (down-hole emulsification), the water that was used to prepare the solution was warmed up so that surfactant dilution was facilitated. However, when switching to the second scheme (down-hole injec tion of diluent), the bitumen that came from the dis filiation tower was much hotter than in the previous process. The energy balance pointed to the fact that the water for the surfactant solution could not be warmed as much as before, otherwise the emulsification tern perature would be too high, and thus the representative point on the formulation-composition diagram would be too near SAD = 0. This new requisite would not allow for the total, in-line dilution of the nonionic surfactant, which was likely to produce a gel when mixed with water that was not hot enough (230). 

Hu et al. showed the preparation of highly porous nanorod-PANI-gra-phene nanocomposite films deposited onto the ITO substrate by in-situ electrochemical polymerization [135]. They used a reverse micelle technique by mixing oil and water in an aqueous solution of TritonX-100. The electro-migration of the ionic species is improved by the addition of dilute HNOj to the oil-water-surfactant solution followed by the polymerization of aniline monomer in presence of graphene dispersion. Figure 4.12 shows the schematic for the preparation of nanorod-PANI-graphene nanocomposites by reverse micelle in-situ electropolymerization technique. 

There is some disagreement within the surfactant literature as to the exact definition of solubilization, particularly as the ratio of surfactant to additive decreases, and one approaches the nebulous frontier between swollen micellar systems and the micro- and macroemulsion regions. For present purposes, solubilization will be defined as the preparation of a thermodynamically stable, isotropic solution of a substance (the additive ) normally insoluble or only slightly soluble in a given solvent by the addition of one or more amphiphilic compounds at or above their critical micelle concentration. By the use of such a definition, a broad area can be covered that includes both dilute and concentrated surfactant solutions, aqueous and nonaqueous solvents, all classes of surfactants and additives, and the effects of complex interactions such as mixed micelle formation and hydrotropes. It does not, however, limit the phenomenon to any single mechanism of action. 

Blankschtein and co-workers [65] have done pioneer work through theoretical modeling, aided by the computer, to predict the properties of mixed surfactant systems. Also, based on the necklace model proposed by Shirahama et al. [67,68], they have proposed a molecular thermodynamic theory of the com-plexation of nonionic polymers and surfactants in diluted aqueous solutions [66], Application of this method can help predict the interaction parameters for several nonionic polymer-surfactant mixtures. 

The quantity experimentally determined is the thermal effect accompanying the mixing in the calorimetric cell of a solubilizate-organic solvent solution with a water-surfactant-organic solvent microemulsion at a given surfactant concentration ([S]). This thermal effect, corrected for the enthalpy of dilution of both solutions and referred to 1 mole of the solubilizate, corresponds to the enthalpy of transfer (AH,) of the solubilizate from the organic to the micellar phase. In order to analyze the calorimetric data (AH, [S]), it is also necessary to develop... 

Adsorption enthalpies were measured in a Thermal Activator Monitor (TAM), an isothermal number 2277 microcalorimeter from LKB, Sweden. It contains a 25 ml stainless steel titration cell, fitted into a single detector measuring cylinder. The cell, with a reference ampoule, was especially designed for mixing liquids and adsorption from solution. For more details, and the execution of the measurements, see [6]. Basically, the heat evolved is measured by adding the surfactant solution to the kaolinite dispersion, where the heat of dilution is subtracted as the blank. In this way a plot q isiT) of the heat evolved as a function of the amount adsorbed F is obtained. 

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