Surfactants adsorption at the solid/liquid interfac

Somasundaran, P., T. W. Healy, and D. W. Fuerstenau (1964), "Surfactant Adsorption at the Solid-Liquid Interface - Dependence", J. of Physical Chemistry 68, 3562-3566. 

Single Surfactant Systems. Surfactant adsorption at the solid/ liquid interface has been studied for several decades. Much of the early... 

A. Thermodynamics of Surfactant Adsorption at the Solid-Liquid Interface... 

In the various sections of this chapter, I will briefly describe the major characteristics of FT-IR, and then relate the importance of these characteristics to physiochemical studies of colloids and interfaces. This book is divided into two major areas studies of "bulk" colloidal aggregates such as micelles, surfactant gels and bilayers and studies of interfacial phenomena such as surfactant and polymer adsorption at the solid-liquid interface. This review will follow the same organization. A separate overview chapter addresses the details of the study of interfaces via the attenuated total reflection (ATR) and grazing angle reflection techniques. 

The adsorption of surfactants at the liquid/air interface, which results in surface tension reduction, is important for many applications in industry such as wetting, spraying, impaction, and adhesion of droplets. Adsorption at the liquid/liquid interface is important in emulsification and subsequent stabilization of the emulsion. Adsorption at the solid/liquid interface is important in wetting phenomena, preparation of solid/liquid dispersions, and stabilization of suspensions. Below a brief description of the various adsorption phenomena is given. 

Retention in Porous Media. Anionic surfactants can be lost in porous media in a number of ways adsorption at the solid—liquid interface, adsorption at the gas—liquid interface, precipitation or phase-separation due to incompatibility of the surfactant and the reservoir brine (especially divalent ions), partitioning or solubilization of the surfactant into the oil phase, and emulsification of the aqueous phase (containing surfactant) into the oil. The adsorption of surfactant on reservoir rock has a major effect on foam propagation and is described in detail in Chapter 7 by Mannhardt and Novosad. Fortunately, adsorption in porous media tends to be, in general, less important at elevated temperatures 10, 11). The presence of ionic materials, however, lowers the solubility of the surfactant in the aqueous phase and tends to increase adsorption. The ability of cosurfactants to reduce the adsorption on reservoir materials by lowering the critical micelle concentration (CMC), and thus the monomer concentration, has been demonstrated (72,13). 

A number of mechanisms can contribute to surfactant retention in a reservoir. The most important of these is, arguably, adsorption at the solid—liquid interface, because it cannot be eliminated completely. However, measures can be taken to minimize adsorption. 

Surfactant solubility and chemical stability are more easily assessed and controlled by proper surfactant selection than adsorption at the solid—liquid interface. In principle, proper foam-flood design should completely eliminate surfactant loss caused by the first two mechanisms. The... 

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. 

One of the factors that determines foam propagation and foam-flood economics is surfactant loss in the reservoir, most importantly adsorption at the solid—liquid interface. Adsorption levels of foaming surfactants, mostly those suitable for high salinity conditions, cover a wide range and lead to vastly different distances of foam propagation. Therefore, selection of a surfactant with minimal adsorption levels for the reservoir conditions of interest is crucial. 

Adsorption layers of the same kind as at fluid interfaces are also formed at low-energy solid -water surfaces, as it was established on PE, polystyrene, paraffin, carbon black, and other related materials. The classical Langmuir or Frumkin adsorption isotherm is often applicable to describe this behaviour. Studies on surfactant adsorption at various solid surfaces have been summarised in a great number of reviews [2, 7, 8, 54, 98, 101, 111, 121, 126, 141, 144, 145, 177, 186, 190, 194-198]. The adsorption at the solid/liquid interfaces is governed by a number of factors ... 

The relationship between adsorption and interfacial properties such as contact angle, zeta-potential and flotation recovery is illustrated in Figure 39.2 for cationic surfactant dodecylammonium acetate/quartz system (5). The increase in adsorption due to association of surfactants adsorbed at the solid-liquid interface into two dimensional aggregates called solloids (surface colloids) or hemi-micelles occurs at about 10 M DA A. This marked increase in adsorption density is accompanied by concomitant sharp changes in contact angle, zeta-potential and flotation recovery. Thus these interfacial phenomena depend primarily on the adsorption of the surfactant at the solid-liquid interface. The surface phenomena that reflect the conditions at the solid-liquid interface (adsorption density and zeta-potential) can in many cases be correlated directly with the phenomena that reflect the... 

Several different experimental techniques, such as fluorescence decay, electron spin resonance (ESR) spectroscopy, Raman spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, neutron reflectome-try, calorimetry, Fourier-transform infrared (FT-IR) adsorption spectroscopy, small-angle neutron scattering (SANS), ellipsometry and surface force measurements, have been used to study self-assembled surfactant structures at the solid-liquid interface (11). These measurements, although providing insight into the hemimicellization process, critical aggregation numbers... 

Success or failure of a surfactant flood may depend on the degree of retention of surfactants during the course of the flood and one of the possible mechanisms of surfactant retention is the adsorption at the solid-liquid interface. Several papers dealing with the adsorption of commercially available surfactants have been published (1,6,13,14) but a meaningful comparison of reported data is quite difficult since surfactants of various degrees of purity have been used and the concentration ranges, brine salinities, and temperatures have varied considerably. In addition, comparisons of the results are difficult because different experimental techniques have been employed. 

It is the objective of this paper to discuss fundamental aspects of the thermodynamics of adsorption at the solid-liquid interface, with emphasis on providing proper definitions of experimental variables such as the surface excess, selectivity, amount adsorbed, and the relationships among them. Types of surfactant adsorption isotherms for binary systems are discussed, and it is shown that an extreme caution must be taken when interpreting isotherms for surfactant mixtures. It is hoped that this discussion will facilitate a better understanding and interpretation of experimental results reported in the literature. 

Losses of surfactants in porous media may be affected more by retention of dispersed surfactants than by its adsorption at the solid-liquid interface. 

To conclude, we would like to stress that formation of hemimicelles, as a first step in surfactant adsorption onto a solid-liquid interface, has long been postulated in the literature and has been proven recently by neutron scattering [43] and AFM imaging [42] experiments. The significance of our results is that they show that the shape and size of the hemimicelles formed at the metal-solution interfaee may be conveniently controlled by the electrode potential. This control offers a new possibility to study the mechanism by which surfactants adsorb at the solid-liquid interface. 

It is known [5] that an integrated absorbance (A) obtained from FTIR-ATR spectra is proportional to the amount of surfactant adsorbed at the solid liquid interface hence, it can be used for qualitative analysis of surfactant adsorption. As far as adsorption to hydrophobic surfaces occurs by attaching —CH3 and —CH2 groups of surfactant molecules then A is calculated as the area under the peak corresponding to C—H str. mode (see the paragraph above) of FTIR-ATR spectra. 

An alternative (and perhaps more efficient) polymeric surfactant is the am-phipathic graft copolymer consisting of a polymeric backbone B (polystyrene or poly(methyl methacrylate)) and several A chains ( teeth ) such as poly(ethylene oxide). The graft copolymer is referred to as a comb stabiliser - the polymer forms a brush at the solid/liquid interface. The copolymer is usually prepared by grafting a macromonomer such as methoxy poly(ethylene oxide) methacrylate with poly(methyl methacrylate). In most cases, some poly(methacrylic acid) is incorporated with the poly(methyl methacrylate) backbone - this leads to reduction of the glass transition of the backbone, making the chain more flexible for adsorption at the solid/liquid interface. Typical commercially available graft copolymers are Atlox 4913 and Hypermer CG-6 supplied by ICI. 

Small-angle neutron scattering has been available for some twenty years now. By contrast, neutron reflectometry has only been actively pursued in the last five years. Like SANS, NR has been rapidly applied to many different materials, notably surfactants, and has not been confined to polymers alone. Essentially, NR can be used to measure the density profile and thickness of a surface layer provided that sufficient contrast is available. Applications of NR to polymers have included surface segregation, Langmuir-Blodgett films, interdiffusion and adsorption at the solid-liquid interface, and these will be mentioned here. 

Adsorption experiments conducted with PEO and alumina particles showed no adsorption of the former on the particles. Because it is known that SDS adsorbs on alumina and that PEO interacts with SDS, PEO adsorption tests were repeated with alumina pretreated with SDS. The authors observed that the presence of a surfactant on the alumina surface caused near-complete adsorption of the polymer as a consequence of its interaction with surfactant aggregates. This result showed that it was possible to force the adsorption of a polymer on a solid surface on which it spontaneously does not adsorb. Instead of drastically modifying the solid-surface properties to force the adsorption of PEO, it seems, by far, more convenient to form surfactant aggregates at the solid-liquid interface and then to allow the polymer to adsorb. 

Adsorption at the solid-liquid interface can proceed over long periods due to kinetic barriers in the SAR. The SAR spans concentrations that lead to a discrete thermodynamically stable aggregate morphology on the substrate, but where no aggregates are formally present in the bulk solution. Additionally, the fact that stepwise introduction of surfactant led to values of surface excess less than those seen at the same concentration for stepwise reductions in surfactant concentration (as shown in Figure 8.15) can be interpreted as evidence that equilibrium has not been reached in the former... 

Adsorption at the solid-liquid interface is associated with a decrease in free-surface energy. The adsorbed surfactant is held at the solid surface by physical,... 

For the solid-liquid system changes of the state of interface on formation of surfactant adsorption layers are of special importance with respect to application aspects. When a liquid is in contact with a solid and surfactant is added, the solid-liquid interface tension will be reduced by the formation of a new solid-liquid interface created by adsorption of surfactant. This influences the wetting as demonstrated by the change of the contact angle between the liquid and the solid surface. The equilibrium at the three-phase contact solid-liquid-air or oil is described by the Young equation ... 

Taking Simultaneous Micellizadon and Adsorption Phenomena into Consideration In the presence of an adsorbent in contact with the surfactant solution, monomers of each species will be adsorbed at the solid/ liquid interface until the dual monomer/micelle, monomer/adsorbed-phase equilibrium is reached. A simplified model for calculating these equilibria has been built for the pseudo-binary systems investigated, based on the RST theory and the following assumptions ... 

Surface wave, 17 422. See also S-wave Surfactant adsorption, 24 119, 133-144 at the air/liquid and liquid/liquid interfaces, 24 133-138 approaches for treating, 24 134 measurement of, 24 139 at the solid/liquid interface, 24 138-144 Surfactant blends, in oil displacement efficiency, 13 628-629 Surfactant-defoamers surface tension, <5 244t Surfactant-enhanced alkaline flooding,... 

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