Surfactant adsorption additive concentration

If an ionic surfactant is present, the potentials should vary as shown in Fig. XIV-5c, or similarly to the case with nonsurfactant electrolytes. In addition, however, surfactant adsorption decreases the interfacial tension and thus contributes to the stability of the emulsion. As discussed in connection with charged monolayers (see Section XV-6), the mutual repulsion of the charged polar groups tends to make such films expanded and hence of relatively low rr value. Added electrolyte reduces such repulsion by increasing the counterion concentration the film becomes more condensed and its film pressure increases. It thus is possible to explain qualitatively the role of added electrolyte in reducing the interfacial tension and thereby stabilizing emulsions. 

For instance, the time course of SPE demonstrates that the solvent phase surfactant concentration steadily decreases (Fig. 3) [58]. The w/o-ME solution s water content decreases at the same rate as the surfactant [58]. The protein concentration at first increases, presumably due to the occurrence of Steps 2 and 3 above, but then decreases due to the adsorption of filled w/o-MEs by the solid phase (Fig. 3) [58]. Additional evidence supporting the mechanism given above is the occurrence of a single Langmuir-type isotherm describing surfactant adsorption in the solid phase for several SPE experiments employing a given protein type (Fig. 4) [58]. Here, solid-phase protein molecules can be considered as surfactant adsorption sites. Similar adsorption isotherms occurred also for water adsorption [58]. 

For a series of lV-alkyl-2-pyrrolidinones that produce enhanced superspreading of the POE trisiloxane mentioned above on polyethylene film, it has been shown (Rosen, 2001) that the addition of the alkylpyrrolidinone to the trisiloxane surfactant produces little or no increase in the total surfactant at the hydrophobic solid-air or aqueous solution-air interfaces, but a considerable increase in the total surfactant adsorption at the hydrophobic solid-aqueous solution interface. This enhanced adsorption of surfactant at the aqueous solution-solid interface relative to that at the aqueous solution-air interface produces a decrease in the surfactant concentration at the air-solution interface in the thin precursor film at the wetting front (Figure 6-8). This results in a surface tension gradient in the precursor film promoting movement of the aqueous phase to the wetting front. 

Surfactants may not only stabilize system against coagulation, but may have an opposite effect, i.e. cause destabilization in cases when the surfactant adsorption proceeds against the polarity equalization rule (Chapter III,2), e.g., during chemisorption of surfactants from aqueous medium on a hydrophilic surface. For example, small additives of cationic surfactants cause coagulation of aqueous dispersions of clays and other silicates due to hydrophobization at T< rmax. Further increase in surfactant concentration results in the formation of a second (hydrophilizing) adsorption layer and leads to an increased... 

Eq. (10.63) can be hilfilled at low and high surface activity of surfactant. It is not recommended to use a surfactant of very high smface activity (cf. condition (8.72)) in the absence of a rear stagnant cap since the adsorption layer is carried away to the rear pole of the bubble. Addition of such surfactant does not enhance particle deposition on the leading surface of bubbles and does not prevent their detachment from the rear surface. The surfactant molecules are concentrated in the close neighbourhood of the rear pole of the bubble and detachment of particles can occur from any part of the surface in the vicinity of which the normal velocity components are directed to the liquid. 

When dealing with a foam, gas—liquid interfaces will be present in addition to solid—liquid and liquid—liquid interfaces. Surfactant adsorption at the gas—liquid interface is obviously required for foam formation and therefore cannot be considered a mechanism of surfactant loss. Because gas is always the nonwetting fluid, the presence of a gas phase is not expected to affect contact between the solid and the aqueous phase and is not likely to affect adsorption of a water-soluble surfactant at the solid—liquid interface. Limited data comparing surfactant adsorption from a foam with adsorption from a bulk liquid during flow through a sand pack have indicated that this is, indeed, the case (34). If surfactant adsorption at the gas—liquid interface were to affect adsorption at the solid—liquid interface, the effect would likely be a reduction in adsorption on the solid because of a reduced surfactant concentration in the bulk aqueous phase. 

In a solution containing a single pure surfactant, the monomer concentration and therefore the surfactant adsorption remain constant above the CMC because any additional surfactant is incorporated into micelles. In a surfactant mixture, the distribution of surface-active species in the monomer and micellar phases depends on their relative tendency to form micelles. The more hydrophobic components are incorporated into micelles preferentially, and the more hydrophilic components become en-... 

In addition to w/c microemulsions, o/c microemulsions may be formed for systems with strong surfactant adsorption. The area occupied by PFPE-C00 NH4 at the interface between 600 molecular weight polyethylene glycol (PEG) md CO2 is 440 per molecule based upon measurement of the interfacial tension versus surfactant concentration [21]. This surface coverage is sufficient for microemulsion formation as was verified with phase behavior measurements. Only 0.55 wt% of 600 molecular weight polyethylene glycol is soluble in CO2 at 45 °C and 300 bar. With the addition of 4wt% PFPE-C00 NH4 surfactant, up to 1.8 wt% is solubilized. The additional PEG resides in the core of the microemulsion droplets, consistent with the prediction from the adsorption measurement. 

Water-.soluble alcohols present a slightly po.sitive function, i.e.. their addition increases the hydrophilidiy of the surfaciantfalcoho amphiphile. but very liiilc indeed. As a consequence isopropanol effect in the correlation is negative, i.e, the same as decreasing salinity, 5CC-Butanol and /en-pentanol have practically no tormulation effect since their functions are essentially nil whatever the concentration. These arc the alcohols that are the more interface-seeking ones, and their miiin effect is to dilute the surfactant adsorption density without changing the formulation. 

Surfactants are used in a wide variety of applications such as ore flotation, cleaning, polymerization processes, and pharmaceuticals and agriculture. The usual role for the surfactant is to modify interfacial properties, whether they be liquid/liquid, solid/liquid, or gas/liquid interfaces. To be effective in any of these applications, however, the surfactant must adsorb strongly at the interface. In addition to its concentration at the interface, the conformation of the surfactant at the interface is also an important factor. While the influence of solution properties such as concentration, ionic strength, and pH on surfactant adsorption are well known, the properties of the other phase also exert a significant influence. 

Consider the complexity involved in modeling steric stabilization with a diblock copolymer. The reservoir bulk solution of copolymer is usually dilute (<1 wt % polymer) and the copolymer and solvent equilibrate between the bulk and surface regions. However, as solvent quality is decreased to the LCST phase boundary, the bulk solution will also separate into polymer-rich and polymer-lean phases. In addition, many diblock copolymers form self-assembled aggregates such as micelles and lamellae, if the concentration is above the critical micelle concentration. Thus, stabilizer can partition among up to four phases as solvent quality or polymer concentration is changed. The unique density dependence of supercritical fluids adds another dimension to the complex phase behavior possible. In the theoretical studies discussed below, surfactant adsorption energy, solubility, and concentration are chosen carefully to avoid micelle formation or bulk phase separation, in order to focus primarily on adsorption and colloid stability. 

There are two additional types of chemical flooding systems that involve surfactants which are briefly mentioned here. One of these systems utilizes surfactant-polymer mixtures. One such study was presented by Osterloh et al. [72] which examined anionic PO/EO surfactant microemulsions containing polyethylene glycol additives adsorbed onto clay. The second type of chemical flood involves the use of sodium bicarbonate. The aim of the research was to demonstrate that the effectiveness of sodium bicarbonate in oil recovery could be enhanced with the addition of surfactant. The surfactant adsorption was conducted in batch studies using kaolinite and Berea sandstone [73]. It was determined that the presence of a low concentration of surfactant was effective in maintaining the alkalinity even after long exposures to reservoir minerals. Also, the presence of the sodium bicarbonate is capable of reducing surfactant adsorption. 

In the conditioning process, under suitable alkaline conditions, both ionization of functional groups at the bitumen surface [33, 105] and adsorption of the natural anionic surfactant molecules at the bitumen/ aqueous interface [100,101,104] occur. Descriptions of the experimental techniques, including microelectrophoresis, employed to study the effects are given elsewhere [100,102,104,106]. Figure 14 shows how addition of NaOH in the process increases the concentrations of surfactant in the aqueous phase, which in turn increases the extents of surfactant adsorption at all of the aqueous phase interfaces present in the system gas/ aqueous, bitumen/aqueous, and solid/aqueous. The adsorption increases until monolayer coverage is achieved and thereafter either levels off or continues into multilayer adsorption. 

---