Adsorption at the solid-liquid interface

The adsorption of surfactants at the solid-liquid interface is strongly influenced by a number of factors (1) the nature of the structural groups on the solid surface— whether the surface contains highly charged sites or essentially nonpolar groupings, 

FIGURE 2-5 Ion exchange. Reprinted with permission from M. J. Rosen, J. Am. Oil Chem. Soc. 52, 431 (1975). 

In the aqueous washing liquor the fabric surface and the pigment soil are charged negatively due to the adsorption of OH- ions and anionic surfactants and this leads to an electrostatic repulsion. In addition to this effect, a disjoining pressure occurs in the adsorbed 

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7rs = disjoining pressure in the adsorption layer of the substrate 7rp = disjoining pressure in the adsorption layer of the particle 

The non-specific adsorption of surfactants is based on the interaction of the hydrophilic headgroup and the hydrophobic alkyl chain with the pigment and substrate surfaces as well as the solvent. For the adsorption of surfactants, different models have been developed which take into account different types of interactions. A simple model which excludes lateral interactions of the adsorbed molecules is the Langmuir equation  

Qoo = equilibrium adsorbed amounts Qm = adsorbed amounts in a fully covered monolayer c = equilibrium concentration in solution b = constant 

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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. 

We extend our description to adsorption at the solid-liquid interface. For many systems we can use the same models as for gas adsorption on a solid surface, we only have to replace the pressure P by the concentration c. The adsorption of macromolecules to surfaces is briefly discussed in Section 10.3.2. For macromolecules desorption is often negligible and thermodynamic equilibrium is only reached after a very long time, if at all. 

Adsorption isotherms are used to quantitatively describe adsorption at the solid/ liquid interface (Hinz, 2001). They represent the distribution of the solute species between the liquid solvent phase and solid sorbent phase at a constant temperature under equilibrium conditions. While adsorbed amounts as a function of equilibrium solute concentration quantify the process, the shape of the isotherm can provide qualitative information on the nature of solute-surface interactions. Giles et al. (1974) distinguished four types of isotherms high affinity (H), Langmuir (L), constant partition (C), and sigmoidal-shaped (S) they are represented schematically in Figure 3.3. 

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. 

Several different forces may be involved in protein adsorption at the solid-liquid interface hydrogen bonding, electrostatic forces, and hydrophobic interactions. Entropic factors such as loss of water, structural deformation of the protein onto hydrophobic patches and dehydration of the protein may drive the adsorption process when there are non favourable electrostatic interactions. 

With a liquid-vapor interface, Gibbs [36] has developed a thermodynamic treatment of the variation of surface tension with composition. This derivation comes from the book Physical Chemistry of Surfaces by Adamson [2, p. 340]. This derivation sets the stage for adsorption at the solid—liquid interface, which will be discussed next. 

D.H. Everett, Adsorption at the Solid-Liquid Interface, in Specialist Periodical Report, Colloid Science. The Chemical Society (London), Vol. 1 (1973) 49 (ready access to the literature up to the year of publication non-aqueous systems). 

Adsorption at the solid/liquid interface plays a crucial role in preparative and analytical chromatography, and in heterogeneous catalysis, water purification and solvent recovery. These applications are, however, outside the scope of this book and we will be concerned with examples of the involvement of adsorption in more medical and pharmaceutical situations. 

The analytical forms of the adsorption isotherms and energy distribution functions given by Eqs.(3-6) were presented in our review [1]. By means of these equations there can be obtained the energy distribution function and parameter n which are important characterizations for the experimental adsorption systems. Consequently, by means of the functions F(Ei2) and the parameters n one can obtain quantitative characterization of the adsorbent heterogeneity, sorption properties of the solid, possibility to calculate the surface phase composition and potentiality for calculating the thermodynamic functions which characterize adsorption at the solid - liquid interface. 

R.W.O Brien and L.R.White, J.Chem.Soc., Faraday Trans.II, 74 (1978) 1607. A.J.Groszek (ed.). Proceedings of the BP Symposium on the Significance of the Heats of Adsorption at the Solid Liquid Interface, BP Research Centre, Sunbury-on-Thames, 1971. 

This flocculation is proposed to be due to strong adsorption at the solid-liquid interface (particle surface) due to the phase separation forming regions or patches rich in pyrrolidone. It is known... 

IV. AGGREGATE FRAGMENTATION INDUCED BY POLYMER ADSORPTION AT THE SOLID/LIQUID INTERFACE... 

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. 

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