Surfactant drops

The area between phases A is the surface area of the drops. It will clearly be a strong function of the stirring characteristics (we assume that stirring is always fast enough to mix both phases). The presence of surfactants, drop size distributions, stirrer design, and circulation patterns. Interfacial area is frequently an unknown in emulsion reactors, but the above formulation should be applicable. Another complication in emulsion reactors is the fact that mass transfer coefficients depend strongly on drop size and stirring rate. The relevant parameter in an emulsion reactor is A km wilh neither factor known very well. 

The coupling property of various hydrotropes were evaluated using a special test procedure. In the procedure 100 mL of a mixture which is a two-layer blend of builders and organic surfactants (see Table 56) is stirred in a beaker while slowly adding test surfactants drop by drop to the beaker until the system gets clear and stays clear for at least 3 min. The effectiveness of the test surfactants were calculated as in Figure 11 and rated using Table 57. 

The topic of spreading rates is of importance in the technology of the use of mono-layers for evaporation control (see Section IV-6) it is also important, in the opposite sense, in the lubrication of fine bearings, as in watches, where it is necessary that the small drop of oil remain in place and not be dissipated by spreading. Zisman and coworkers have found that spreading rates can be enhanced or reduced by the presence of small amounts of impurities in particular, strongly adsorbed surfactants can form a film over which the oil will not spread [48]. 

A drop of surfactant solution will, under certain conditions, undergo a fingering instability as it spreads on a surface [27, 28]. This instability is attributed to the Marongoni effect (Section IV-2D) where the process is driven by surface tension gradients. Pesach and Marmur have shown that Marongoni flow is also responsible for enhanced spreading... 

The adsorption appears to be into the Stem layer, as was illustrated in Fig. V-3. That is, the adsorption itself reduces the f potential of such minerals in fact, at higher surface coverages of surfactant, the potential can be reversed, indicating that chemical forces are at least comparable to electrostatic ones. The rather sudden drop in potential beyond a certain concentration suggested to... 

Fig. XIV-16. A photomicrograph of a two-dimensional foam of a commercial ethox-ylated alcohol nonionic surfactant solution containing emulsified octane in which the oil drops have drained from the foam films into the Plateau borders. (From Ref. 234.)...

Dextrin Polyethylene glycol 400 Use 5 mL of 2% aqueous solution of chloride-free dextrin per 25 mL of 0. IM halide solution. Prepare a 50% (v/v) aqueous solution of the surfactant. Use 5 drops per 100 mL end-point volume. 

The surfactant is initially distributed through three different locations dissolved as individual molecules or ions in the aqueous phase, at the surface of the monomer drops, and as micelles. The latter category holds most of the surfactant. Likewise, the monomer is located in three places. Some monomer is present as individual molecules dissolved in the water. Some monomer diffuses into the oily interior of the micelle, where its concentration is much greater than in the aqueous phase. This process is called solubilization. The third site of monomer is in the dispersed droplets themselves. Most of the monomer is located in the latter, since these drops are much larger, although far less abundant, than the micelles. Figure 6.10 is a schematic illustration of this state of affairs during emulsion polymerization. 

Although the adsorption of surfactants tends to reduce mass-transfer coefficients by suppressing drop circulation, a sharp increase in mass transfer... 

SG sols were synthesized by hydrolysis of tetraethyloxysilane in the presence of polyelectrolyte and surfactant. Poly (vinylsulfonic acid) (PVSA) or poly (styrenesulfonic acid) (PSSA) were used as cation exchangers, Tween-20 or Triton X-100 were used as non- ionic surfactants. Obtained sol was dropped onto the surface of glass slide and dried over night. Template extraction from the composite film was performed in water- ethanol medium. The ion-exchange properties of the films were studied spectrophotometrically using adsorption of cationic dye Rhodamine 6G or Fe(Phen) and potentiometrically by sorption of protons. 

Additionally, the surfactant properties of filmers reduce the potential for stagnant, heat-transfer-resisting films, which typically develop in a filmwise condensation process, by promoting the formation of condensate drops (dropwise condensation process) that reach critical mass and fall away to leave a bare metal surface (see Figure 11.2). This function, together with the well-known scouring effect on unwanted deposits keeps internal surfaces clean and thus improves heat-transfer efficiencies (often by 5-10%). 

An evaluation of the retardation effects of surfactants on the steady velocity of a single drop (or bubble) under the influence of gravity has been made by Levich (L3) and extended recently by Newman (Nl). A further generalization to the domain of flow around an ensemble of many drops or bubbles in the presence of surfactants has been completed most recently by Waslo and Gal-Or (Wl). The terminal velocity of the ensemble is expressed in terms of the dispersed-phase holdup fraction and reduces to Levich s solution for a single particle when approaches zero. The basic theoretical principles governing these retardation effects will be demonstrated here for the case of a single drop or bubble. Thermodynamically, this is a case where coupling effects between the diffusion of surfactants (first-order tensorial transfer) and viscous flow (second-order tensorial transfer) takes place. Subject to the Curie principle, it demonstrates that this retardation effect occurs on a nonisotropic interface. Therefore, it is necessary to express the concentration of surfactants T, as it varies from point to point on the interface, in terms of the coordinates of the interface, i.e.,... 

The flux of surface-active agents from the surface into the bulk of the liquid may be controlled by the slower of the following processes 1) adsorption or desorption of surfactants at the surface or 2) diffusion of surfactants from the liquid bulk to the surface. Consequently, Levich evaluated the solution for a single drop... 

Eq. (207) can be derived also as a special case of a more general solution by Waslo and Gal-Or (Wl) which gives the terminal velocity of an ensemble of drops or bubbles in the presence of surfactants (see Sections IIF and 11IB for more details). 

Surfactants have a hydrophilic side of the molecule that attaches to water, and a hydrophobic side of the molecule that avoids water. In the absence of oils, the hydrophobic side sticks out of the surface of the water drop. There is no longer any water at the surface to form a strong surface tension, so the water no longer beads up, but spreads. The hydrophobic end of the molecule is also free to attach to grease, fat, or oil on the surface, which aids in the spreading. 

Electrochemical analytical techniques are a class of titration methods which in turn can be subdivided into potentiometric titrations using ion-selective electrodes and polarographic methods. Polarographic methods are based on the suppression of the overpotential associated with oxygen or other species in the polarographic cell caused by surfactants or on the effect of surfactants on the capacitance of the electrode. One example of this latter case is the method based on the interference of anionic surfactants with cationic surfactants, or vice versa, on the capacitance of a mercury drop electrode. This interference can be used in the one-phase titration of sulfates without indicator to determine the endpoint... 

The AOS and IOS surfactants discussed in this section are laboratory-prepared samples. The experiments were carried out by Borchardt [40] who used the spinning drop method throughout. Experimental details are given in the footnotes of the tables that follow. 

Tests were performed at 75°C using a University of Texas Model 500 spinning drop tensiometer. Active surfactant concentration in the aqueous phase prior to oil addition was 0.50% wt. The Kem River crude oil was from the Patricia Lease. The pH of the deionized water surfactant solutions was 8. The pH of the aqueous NaCl surfactant solutions was 9.5 unless otherwise noted. values represent the average deviation of two or three measurements at different times (0.75-1 h apart). D.I., deionized. 

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