Dissolved surfactants

Ion Flotation and Foam Separation. Ions and dissolved surfactant molecules can be removed from solutions by the agency of foam. In this case ions are sandwiched in foam films. The scientific basis of these processes is weU understood and successes of metal ion recovery from solutions including U, Pt, Au, as weU as different surfactants (detergents) have been reported in the Hterature. 

On the other hand, if one starts from the oil phase with dissolved surfactant and add water, solubilisation of the latter takes place and hence solubilisation increases rapidly with reduction of temperature near the haze point of the surfactant. This is illustrated in Figure 2b which shows both the haze point and solubilisation curve. Between the two curves, an isotropic region of w/o solubilised system exists. At any given temperature, any increase in water weight fraction above the solubilisation limit results in water separation, i.e. w/o solubilised + water, whereas at a given surfactant concentration, any decrease in temperature below the haze point result in separation into water, oil and surfactant. 

The increase in conductivity is due to increase in dissolved surfactant, and this increase continues until all the crystallites dissolve. The peak in specific conductance is attained when the microemulsion is formed and the specific conductance levels off. The plateau of Figure 6 is often referred to as a "percolation threshold" (] ) and is reached when there is a disordered interspersion capable of bicontinuous structures ( ). Further addition of methanol results in a lowering of conductivity explained by the solution eventually approaching the conductivity of methanol. This is the region of molecular dispersion. These conductivity curves are similar to those observed by Lagues and Santerey (13) on a system of water, cyclohexane, sodium dodecylsulfate and 1-pentanol. 

Dissolve surfactants Igepal CO-630, Rhodapex CO-436 and Alkamide DC-212/S in water. 

In the phase separation model we take advantage of the fact that micellization has much in common with the formation of a separate liquid phase. At low concentration the chemical potential of the dissolved surfactants can be described by... 

The intermolecular attractions between the hydrocarbon chains in the interior of the micelle represent an energetically favourable situation but it is not one which is significantly more favourable than that which results from the alternative hydrocarbon-water attraction in the case of single dissolved surfactant molecules. Comparison of the surface tension of a typical hydrocarbon oil with the dispersion component of the surface tension of water (as discussed on page 67) illustrates this point. 

Adsorption isotherms represent a relationship between the adsorbed amount at an interface and the equilibrium activity of an adsorbed particle (also the concentration of a dissolved substance or partial gas pressure) at a constant temperature. The analysis of adsorption isotherms can yield thermodynamic data for the given adsorption system. Theoretical adsorption isotherms derived from statistical and kinetic data, and using the described assumptions (see 3.1), are known only for the gas-solid interface or for dilute solutions of surfactants (Gibbs). Those for the system gas-solid are of a few basic types that can be thermodynamically predicted81. From temperature relations it is possible to calculate adsorption and activation energies or rate constants for individual isotherms. Since there are no theoretically founded equations of adsorption isotherms for dissolved surfactants on solids, the adsorption of gases on solides can be used as a starting point for an interpretation. 

Dependence of the lifetime of foam bilayers on the concentration of dissolved surfactant. The stability of foam, emulsion and membrane bilayers can be characterised by their mean lifetime r which is the time elapsing form the moment of formation of a bilayer with a given radius until the moment of its rupture. Obviously, this is a kinetic characteristic of the bilayer stability and can only be applied to thermodynamically metastable bilayers. 

Dependence of the probability of observing a bilayer in a foam film on the concentration of dissolved surfactant. Experimental verification of the theory [399,402,403] of hole-mediated rupture of bilayers has also been conducted [382] by analysing data for the W(C) dependence with the help of Eq. (3.128). Studying this dependence is possible and particularly convenient at lower C values when the bilayer mean lifetime t is comparable with tr (see Eq. (3.122)). A characteristic feature of W, according to Eq. (3.128), is its sensitivity to changes of C only in a very narrow range. 

If it is assumed that at the moment of formation the film is not at equilibrium with the meniscus containing a dissolved surfactant, the vacancies concentration is changed, thus determining the initial nucleation rate Jq. So, the lateral diffusion movement of elementary vacancies affects the equilibrium establishment and film rupture, expressed by 7(f) [501]. 

As the thickness of a-films is determined by the layering of the liquid molecules adjacent to the solid, more than one thickness stratification] could in principle be expected. For pure water films or dilute aqueous solutions in the absence of dissolved surfactants or pol5miers, such stratification has never been observed. However, the phenomenon has been found for free aqueous films of concentrated surfactants and for wetting layers of large spherical organic molecules. ... 

A surface-active agent (or surfactant) is a substance that lowers the surface or interfacial tension of the medium in which it is dissolved. Surfactants have a characteristic molecular structure consisting of hydrophobic and hydrophilic groups. This is known as an amphipathic structure, and causes not only concentration of the surfactant at the surface and reduction of the surface tension of the solvent, but also orientation of the molecule at the surface with its hydrophilic group in the aqueous phase and its hydrophobic group oriented away... 

When nonionic surfactant is applied to a soil-aqueous system, the surfactant can exist as dissolved monomers, sorbed molecules on the soil, or aggregated groups of molecules called micelles. Molecules of HOCs in such a system can be solubilized in surfactant micelles, dissolved in the surrounding solution, sorbed directly on the soil, or sorbed in association with sorbed surfactant. The presence of nonionic surfactant micelles in the bulk solution of the system results in the partitioning of the HOC between two bulk solution compartments, commonly referred to as pseudophases. The micellar pseudophase consists of the hydrophobic interiors of surfactant micelles, whereas the aqueous pseudophase consists mainly of dissolved surfactant monomers and water. Micelles form when the bulk solution concentration exceeds the surfactant CMC. 

If surfactant is soluble in aqueous media, add surfactant directly to obtain a concentration of 0.75 to 10 g/1. If surfactant is poorly soluble, dissolve surfactant in a polar solvent (e.g., ethanol or acetone [ 1 to 5 mM]), and add ethanolic solution dropwise and slowly with stirring at refrigerator temperature. In a few cases, a hydrophilic organic solvent e.g., tetrahy-drofuran or ethyl alcohol, was added directly to the aqueous solution prior to adding the surfactant." ... 

The monomeric solubility of the surfactant in the water y cmon.a can be easily determined from surface tension measurements [37]. An interesting method to obtain ycmon.b is provided by the macroscopic phase behaviour through the determination of the mass fraction of surfactant y0 (see Fig. 1.3), i.e. the monomerically dissolved surfactant in both excess phases. Therefore, the volume fraction of the middle phase Vc/V has to be measured as a function of the mass fraction of surfactant y at a constant = 0.5 and the mean temperature f of the three-phase body [34, 38, 39]. Plotting Vc/V versus y yields yoat Vc/V = 0andy at Vc/V = 1.Then the monomeric solubility in the oil is calculated from... 

Figure 5.3 Film balances a. Langmuir trough having a movable barrier and a Wilhelmy tensiometer to measure the spreading pressure, n, for water insoluble monolayers, b. PLAWM (Pockels, Langmuir, Adam, Wilson and McBain) trough used for partially water-soluble monolayers, where a flexible membrane, which is fixed to the barrier, separates the surfactant solution and pure water departments to prevent the passage of dissolved surfactant molecules into the pure water department beneath the barrier.

The behavior of the surfactant molecules in an emulsion polymerization is complex. The adsorption of the surfactant on the rapidly and continually growing surface of the monomer-swollen latex particles reduces their concentration in the aqueous phase, and also upsets the balance in equilibrium between the dissolved surfactant and the surfactant present in the inactivated micelles (those in which polymerization is not occurring), as shown in Figure 5.11. The point is quickly reached at which the surfactant concentration in the solution falls below its critical micelle concentration, CMC. When this occurs, the inactive micelles become unstable and disintegrate to restore the balance. In time all of the micelles disappear and the monomer droplets shrink in size. After a conversion of 10-20%... 

Aside from the significance of the salting-out phenomenon itself, these observations are important for adsorption measurements in that it appears that the surfactant concentration actually in solution is less than 0.1 wt.% when appreciable concentrations of NaCl are present. Not only does the dissolved surfactant concentration appear to be less than about 0.1 wt.% but there is the effect on the apparent adsorption if the salted out surfactant partially or completely separates with the clay or other adsorbent being studied. Complete separation of the salted-out surfactant leads to large values of apparent adsorption and low equilibrium surfactant concentrations negligible separation of the salted-out surfactant leads to low adsorption and large apparent equilibrium surfactant concentrations but the actual dissolved surfactant con-... 

Thus Class I resonances correspond to either surfactant chains or to decane or to both. Spectrum 12 is from a biphasic 15.5 wt% dispersion at 37°C. No Class II peaks were observed with 1000 transients, nor did the situation change with 10,000 transients (nor did expanding the scale, as in Figure 10, Spectrum 15, make any difference). Thus the concentration of dissolved surfactant was too small to be distinguishable from the noise, in accord with the 0.04 wt% solubility measured at this temperature. 

The contribution of dissolved surfactant, whose concentration was only 0.001M, compared to 7M of decane, to the observed Class I peaks must have been negligible. Class II peaks were not observed in Spectrum 13 of the birefringent phase, and Class I peaks were broadened (linewidth about 30 Hz) compared to the peaks in Spectrum 12 (linewidth less than 5 Hz). Therefore it seems quite possible that the dispersed birefringent phase did give Class I peaks in Spectrum 12, but that these peaks, due to either the surfactant or to absorbed decane or to both, merged with those of the decane in the isotropic phase. 

Above 50°C, the solubility of surfactant in decane increased to about 9 wt% and Class II peaks were readily observed at 71°C and 15.5 wt% (Spectrum 14) and at 65°C and 7.7 wt% (Spectrum 16). It was noted that Class I peaks were much more intense than Class II peaks whenever the latter were observed (Spectra 14 and 16), evidently because they came from both surfactant and decane. The latter was present at 30-fold higher molar concentration than the dissolved surfactant. 

The role of adsorption kinetics and the diffusion of surfactants is especially important in controlling capillary impregnation. According to studies by N.N. Churaev, the solution impregnating the capillary quickly loses its dissolved surfactant due to adsorption of the latter on capillary walls, so the rate of impregnation may be limited by the diffusional transport of surfactant from the bulk of the solution to the menisci in the pores. 

The equilibrium between dispersed phase (i.e., micelles) and molecular solution of a surfactant (or the macroscopic phase, in case of saturation) exists in thermodynamically stable systems containing micelle-forming surfactants. One can, to a certain degree of approximation, describe the equilibrium between micelles consisting of m surfactant molecules and molecularly dissolved surfactant as a chemical reaction, namely [15,16]... 

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