Immersed in a surfactant solution

Gels that are usually employed in studies of surfactant uptake are previously saturated with the same solvent as that of surfactant solutions. When such gels are immersed in a surfactant solution, the binding should take place through the gel surface. The size and shape of the gel are then regarded as the primary factor directly affecting the uptake amount because it depends on the available surface area of the gel. 

Figure 23.12 depicts the change of XPS peaks as a consequence of water immersion. When similar experiments were carried out with a mixture of water/ethyl alcohol (50 50 volume ratio), the conspicuous appearance and increase with time were not observed as shown in Figure 23.13. The increase of O Is peak as function of water immersion time is depicted in Figure 23.14, which includes immersion in a dilute solution of a surfactant and 50 50 mixture of water and ethanol. The reason why the increase of O Is is influenced by the water mixtures is related to the driving force responsible for such changes. This aspect is discussed in Chapter 25.

Wen and Papadopoulos used videomicroscopy to measure the rate of shrinkage of a single drop of pure water immersed in a surfactant-containing oil phase, which itself was in contact with an aqueous solution of NaCl. They found that the solubilization rate of water was controlled by phenomena at the drop interface, suggesting that transport in multiple emulsions is limited by interfacial effects, not diffusion. 

Before the experiment, it was immersed in a dilute solution of the surfactant, n-dodecylpyridinium chloride (DPyCl) (Tokyo Kasei Co., Ltd.) containing 3x10 M sodium sulphate (Wako Pure Chemicals Industries, Ltd.). The structure of this surfactant is illustrated in Figure A. 1(c). 

Fig. 24-5). AFM results indicated the slow spontaneous formation of a silica-surfactant interfacial structure immersed in a dipping solution confirming a cooperative adsorption step even at low surfactant concentrations. 

Figure 5.20 Lateral immersion force versus center-to-center distance for two glass spheres of Ri = 0.6 mm radius measured with a torsion balance [601]. The two spheres are partially immersed in aqueous surfactant solution with Yl = 0.0368 Nm and Xc = 1.94 mm. Each particle is kept at the liquid surface by a support...

Similarly to quantitative determination of high surfactant concentrations, many alternative methods have been proposed for the quantitative determination of low surfactant concentrations. Tsuji et al. [270] developed a potentio-metric method for the microdetermination of anionic surfactants that was applied to the analysis of 5-100 ppm of sodium dodecyl sulfate and 1-10 ppm of sodium dodecyl ether (2.9 EO) sulfate. This method is based on the inhibitory effect of anionic surfactants on the enzyme system cholinesterase-butyryl-thiocholine iodide. A constant current is applied across two platinum plate electrodes immersed in a solution containing butyrylthiocholine and surfactant. Since cholinesterase produces enzymatic hydrolysis of the substrate, the decrease in the initial velocity of the hydrolysis caused by the surfactant corresponds to its concentration. Amounts up to 60 pg of alcohol sulfate can be spectrometrically determined with acridine orange by extraction of the ion pair with a mixture 3 1 (v/v) of benzene/methyl isobutyl ketone [271]. 

Bibette has used this method to study the effect of osmotic pressure on the stability of thin films in concentrated o/w emulsions [96], by means of an osmotic stress technique. The emulsion is contained in a dialysis bag, which is immersed in an aqueous solution of surfactant and dextran, a water-soluble polymer. The bag is permeable to water and surfactant, but impermeable to oil and polymer. The presence of the polymer causes water to be drawn out of the emulsion, increasing the phase volume ratio and the deformation of the dispersed droplets (Fig. 10). 

The semicontinuous polymerization was carried out in a 500-ml four-neck flask immersed in a constant temperature bath at 60° C, and equipped with a reflux condenser, a two-bladed stainless steel stirrer and a graduated dropping funnel. The water and surfactant were introduced into the flask, then nitrogen gas was bubbled with agitation for at least 20 minutes. The initiator solution was added, and after 5 minutes the mixture of monomer addition started and continued for a total of three hours. The polymerization was continued for at least one hour past this point. Under these conditions the rate of monomers addition was less than 1/10 of their maximum rate of polymerization (Rpmax). 

Stability of both foam and asymmetric aqueous films at the surface of organic liquids of different polarity has been studied as function of the surfactant concentration [551], Microscopic foam films were obtained in a glass cuvette (Tig. 3.120,c) by blowing an air bubble at the tip of a vertical capillary immersed into the surfactant solution. With a micrometric screw (not shown in the figure) the bubble was pressed carefully onto the solution/air interface, thus forming the film. 

In the ESCR test, samples of polymer are bent to introduce stress, and then they are immersed in a hot surfactant solution to determine how long it takes for the polymer chains to unravel and develop a crack. Surfactants and other chemicals often facilitate this unraveling process. ESCR increases (gets better) as the MW distribution of the polymer is broadened. 

WDD requires a non-aqueous solution of an enabling surfactant. The surfactant is retained in the non-polar fluid and the dissolved aqueous salts and colloids are removed with the water. The basis of the phenomenon is illustrated by the cartoon shown in Figure 3. A water-covered hydrophilic solid is immersed in a non-polar solution containing dissolved surfactant molecules (designated as V). The water initially is a film, but increasing adsorption of surfactant at the oil/water and oil/solid interfaces rapidly causes the water to bead up. Then the water rolls off as the contact angle approaches 180. This phenomenon is controlled by the venerable Young Equation ... 

Two theoretical force curves calculated by using Eqs (25) and (26) are shown in Fig. 18. In these calculations we used a Kelvin radius of 320 nm and an interfacial tension difference of 3.3 mN/m. The measured force curves fall in between the extreme cases of full equilibrium, where the volume of the condensate is changing with distance to minimize the free energy, and the case of no change in condensate volume with separation. Long-range forces due to capillary condensation have been observed previously by Petrov et al. who foimd that water condensed between two surfaces immersed in a microemulsion. (83). Capillary condensation of sparingly soluble surfactants between surfaces close to each other in surfactant solutions has also been reported (84). 

Cloud points were initially measured by heating 1% solutions of surfactant in distilled water at a rate of 1°C per minute in a test tube immersed in a water bath. When cloudiness was observed the heat source, was removed and the solution cooled until it was clear. The cloud point was taken as the midpoint between the observed temperature while heating and that during cooling. These were within l.O C of each other for pure surfactant. 

Prolonged exposure to water alone at elevated temperature causes a deterioration of barrier performance (21). If this elevated temperature includes exposure to soap, the deterioration is more serious. However, the latter is reduced if the exposed membrane is then immersed in Polymer JR solution before execution of the MVT test (at room temperature). A possible explanation is the formation of a polymer/surfactant complex in the membrane. 

The data in Table 13 for the vinylidene chloride-methyl acrylate copolymer bonded to polypropylene is quoted from a paper by Owens [75]. Owens coated a polypropylene sheet with an aqueous dispersion containing 80 parts vinylidene chloride, 20 parts methyl acrylate and 4 parts acrylic acid. The dispersion was surfactant free and the polypropylene surface had been flame treated. The resulting laminates were placed in some surfactant solutions, and to quote Owens In every case where Wa,l upon immersion in the liquid is negative, the coating... 

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