Adsorption micellar solutions

Micellar flooding is a promising tertiary oil-recovery method, perhaps the only method that has been shown to be successful in the field for depleted light oil reservoirs. As a tertiary recovery method, the micellar flooding process has desirable features of several chemical methods (e.g., miscible-type displacement) and is less susceptible to some of the drawbacks of chemical methods, such as adsorption. It has been shown that a suitable preflush can considerably curtail the surfactant loss to the rock matrix. In addition, the use of multiple micellar solutions, selected on the basis of phase behavior, can increase oil recovery with respect to the amount of surfactant, in comparison with a single solution. Laboratory tests showed that oil recovery-to-slug volume ratios as high as 15 can be achieved [439]. 

Studies of the adsorption of surface active electrolytes at the oil-water interface provide a convenient method for testing electrical double layer theory and for determining the state of water and ions in the neighborhood of an interface. The change in the surface amount of the large ions modifies the surface charge density. For instance, the surface ionic area of 100 per ion corresponds to 16, /rC/cm. The measurement of the concentration dependence of the changes of surface potential were also applied to find the critical concentration of formation of the micellar solution [18]. 

There are a number of different factors which may affect the level of uptake and the energetics of adsorption from solution the chemistry and electrical properties of the solid surface and the molecular/micellar/polymeric structure of the solution must all be taken into account. Whenever possible, a study of both adsorption isotherms and enthalpies of displacement is worthwhile, but it is often necessary to complement these measurements with others including electrophoretic mobilities, FI7R spectra-and various types of microscopy. 

Ideally, the injected micellar solutions will be miscible with the fluids that they are in contact with in the reservoir and can thus miscibly displace those fluids. In turn, the micellar solutions may be miscibly displaced by water. Highest oil recovery will result if the injected micellar solution is miscible with the reservoir oil. If there are no interfaces, interfacial forces that trap oil will be absent. Injection of compositions lying above the multiphase boundary initially solubilizes both water and oil and displaces them in a misciblelike manner. However as injection of the micellar solution progresses, mixing occurs with the oil and brine at the flood front, and surfactant losses occur because of adsorption on the reservoir rock. These compositional changes move the system into the multiphase region. The ability of... 

Finally it should not be omitted to mention the fact that the rigid sphere fullerene, Ceo, can also be dissolved in vesicle membranes . When dissolved in hexane, chloroform or 1,2-dichloroethane, a narrow, concentration-independent absorption band at 334 nm (e 52000) was produced. In vesicles (lecithin, DODAB, DHP), the fullerene adsorption becomes concentration dependent whereby band-broadening, bathochromic shifts (343-360 nm) and loss of extinction (e 10000 4000) were observed in more concentrated solutions. 50 clearly aggregates within the vesicle membranes, a step not observed in micellar solutions. 

Bijsterbosch, H.D. Stuart, M.A.C. Fleer, G.J. Adsorption kinetics of diblock copolymers from a micellar solution on silica and titania. Macromolecules 1998, 31, 9281-9294. lijima, M. Nagasaki, Y. Okada, T. Kato, M. Kataoka, K. Core-polymerized reactive micellesm from heterotelechelic amphiphilic block copolymers. Macromolecules 1999, 32, 1140-1146. 

There is another phenomenon that is called polymer inaccessible pore volume (IPV). Laboratory data indicate that inaccessible pore volume is usually greater than the adsorption loss for polymers following a micellar solution (Trushenski et al., 1974). The competitive adsorption and IPV may make polymer penetrate the surfactant slug ahead of it. Therefore, surfactant-polymer interaction or incompatibility occurs not only in the surfactant-polymer process where the surfactant and the polymer are injected in the same slug, but also in the surfactant-polymer process where surfactant is injected before the polymer slug. 

Figure 18.12 Representation of the adsorption process from a micellar solution.

Fig. 4.11 Schematic of an adsorption process from micellar solutions...

The aggregation number also plays an important role in the total rate of the adsorption process from micellar solutions. The presence of dimers, which are assumed not to adsorb, increases the adsorption rate remarkably, although only 10% of the surfactant is aggregated in dimers (Fig. 4.14). 

In the discussion of the adsorption kinetics of micellar solutions, different micelle kinetics mechanisms are taken into account, such as formation/dissolution or stepwise aggregation/disaggregation (Dushkin Ivanov 1991). It is clear that the presence of micelles in the solution influences the adsorption rate remarkably. Under certain conditions, the aggregation number, micelle concentration, and the rate constant of micelle kinetics become the rate controlling parameters of the whole adsorption process. Models, which consider solubilisation effects in surfactant systems, do not yet exist. 

The aim of this chapter is to present the fundamentals of adsorption kinetics of surfactants at liquid interfaces. Theoretical models will be summarised to describe the process of adsorption of surfactants and surfactant mixtures. As analytical solutions are either scarcely available or very complex and difficult to apply, also approximate and asymptotic solutions are given and their ranges of application demonstrated. For particular experimental methods specific initial and boundary conditions have to be considered in these theories. In particular for relaxation theories the experimental conditions have to be met in order to quantitatively understand the obtained results. In respect to micellar solutions and the impact of micelles on the adsorption layer dynamics a detailed description on the theoretical basis as well as a selection of representative experiments will follow in Chapter 5. 

When an adsorption layer is pre-equilibrated with a micellar solution and expanded, then monomers will adsorb at the surface. As this decreases the monomer concentration locally, the monomers and micelles are out of equilibrium and micelles will disintegrate. Hence, locally the concentration of micelles decreases and micelles will diffuse too. As the result the presence of micelles in the solution bulk can be seen as extra source of matter, i.e. the micellar kinetics represents an additional relaxation mechanism to interfacia perturbations. 

Most of the traditional adsorption studies of surfactants correspond to dilute systems without aggregation in the bulk phase. At the same time micellar solutions are much more important from a practical point of view. To estimate the equilibrium adsorption, neglecting the effect of micelles can usually lead to reasonable results. However, the situation changes for nonequilibrium systems when the adsorption rate can increase by orders of magnitude when the of surfactant concentration is increased beyond the CMC. Current interest in the adsorption from micellar solutions is mainly caused by recent observations that the stability of foams and emulsions depends strongly on the concentration in the micellar region [1]. This effect can be explained by the influence of the micellisation rate on the adsorption kinetics. 

Recently it has been also shown that the surface tension of micellar solutions above the CMC can respond to the transition between spherical and rodlike micelles taking place in micellar solutions of certain surfactants in the presence of multivalent ions, such as Al [65], The qualitative explanation of this phenomenon is connected with the ability of ion to bind three surfactant headgroups, and, consequently, to lower the area per headgroup. According to Israelachvili et al., [11] this can induce a transition from spherical to rodlike micelles. On the other hand, the new micelles adsorb additional Al ions from the bulk. This leads also to a lower adsorption of Al at the solution-gas interface because the competitive adsorption of the counterions follows the same tendency as their bulk concentration. The desorption of Ap causes sharp increase of the surface tension with increasing molar ratio. 

The broad spectrum of recently developed experimental techniques together with traditional measurements of the surface tension has been applied during the last decade to surface layers of micellar solutions [66-76]. For example, Lehmann et al. studied the correlation between the results of a non-linear optieal technique (second-harmonic generation) and surface tension measurements [66], The concentration dependence of the second-order susceptibility component exhibits a kink point in the vicinity of the CMC with a subsequent levelling off (Fig. 5.5). Such behaviour can be explained by the approximate constancy of the adsorption above the CMC. 

It is well-established now that the concentration of surfactant ions in micellar solutions changes when the total surfactant concentration c is increased. This leads to changes in the adsorption value and, consequently, to changes in the surface tension. These alterations, however, are small, even for ionic surfactants. For relatively dilute solutions, i.e. c< 10 CMC, as a first approximation one can consider that the monomer concentration ci is constant (ci CMC). Actually, for c > CMC surface tension changes are usually low and in the range of accuracy of conventional methods. This fact evidences an approximate constancy of the adsorption. 

The situation changes for non-equilibrium systems. The dynamic surface properties of micellar solutions depend strongly on the concentration in a broad range of surface life time and/or of the frequency of surface compression and dilation. First of all this is related to the fact that the adsorption rate of surfactants increases with concentration for both sub-micellar and micellar solutions. As an example, dynamic surface tensions of SDS in 0.1 M NaCl measured by Fainerman and Lylyk [77] are shown in Fig. 7. As one can see entirely different values of the dynamic surface tension and of the adsorption can correspond to the same surface age at c > CMC. 

Numerous data on dynamic surface tension [77-93] and dynamic surface elasticity [94-103] of aqueous micellar solutions have been published until now. These data evidence the influence of micelles on the adsorption kinetics, although they are present only in the bulk phase. This effect can surprise on a first glance because it is well-known that the surface activity of micelles is negligible and hence their adsorption is almost zero. However, the influence of micelles can be easily explained if one takes into account that the adsorption kinetics of surfactants at fluid -fluid interface is determined by the diffusional exchange between the subsurface and the bulk phase [104, 105]. It is exactly the diffusion of monomers that changes in the presence of micelles. This point of view is widely accepted and difficulties arise only if one tries to obtain quantitative estimates of the observed effects. 

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