Adsorption at solid surface

The experimental determination of the extent of adsorption usually involves shaking a known mass of adsorbent material with a solution of known 

The Langmuir equation (Equation 1.18) has been v ridely used in the interpretation of adsorption data of non-ionic surfactants 

Thus a plot of C2/n against C2 should give a straight line of slope 1/n and intercept l/n fc. 

The Langmuir equation is theoretically valid when the adsorbent is homogeneous, when there are no solute-solute or solute-solvent interactions either in solution or in surface layers and under conditions of monolayer adsorption [42]. 

The interpretation of adsorption isotherms of ionic surfactants is more complex due to a variety of factors mainly associated with the charge on the surfactant. 

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Adsorption at solid surfaces. In D. A. King, D. P. Woodruff, eds. The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Vol. 2. Amsterdam Elsevier, 1983. 

Szleifer I (1997) Protein adsorption on surfaces with grafted polymers a theoretical approach. Biophys J 72 595-612 Tanford C (1973) The hydrophobic effect. John Wiley Sons, Inc., Hoboken Van Dulm P, Norde W, Lyklema J (1981) Ion participation in protein adsorption at solid surfaces. J Colloid Interf Sci 82 77-82 Zoungrana T, Findenegg GH, Norde W (1997) Structure, stability and activity of adsorbed ensymes. J Colloid Interf Sci 190 437-448 Zoungrana T, Norde W (1997) Thermal stability and enzymatic activity of a-chymotrypsin adsorbed on polystyrene surfaces. Colloid Surf B 9 157-167... 

W. Norde, Driving forces for protein adsorption at solid surfaces, in Biopolymers at Interfaces, 2nd edn. M. Malmsten, Ed. New York CRC Press, 2003. 

Throughout most of this chapter we have been concerned with adsorption at mobile surfaces. In these systems the surface excess may be determined directly from the experimentally accessible surface tension. At solid surfaces this experimental advantage is missing. All we can obtain from the Gibbs equation in reference to adsorption at solid surfaces is a thermodynamic explanation for the driving force underlying adsorption. Whatever information we require about the surface excess must be obtained from other sources. 

Adsorption experiments are conducted at constant temperature, and an empirical or theoretical representation of the amount adsorbed as a function of the equilibrium gas pressure is called an adsorption isotherm. Adsorption isotherms are studied for a variety of reasons, some of which focus on the adsorbate while others are more concerned with the solid adsorbent. In Chapter 7 we saw that adsorbed molecules can be described as existing in an assortment of two-dimensional states. Although the discussion in that chapter was concerned with adsorption at liquid surfaces, there is no reason to doubt that similar two-dimensional states describe adsorption at solid surfaces also. Adsorption also provides some information about solid surfaces. The total area accessible to adsorption for a unit mass of solid —the specific area Asp — is the most widely encountered result determined from adsorption studies. The energy of adsorbate-adsorbent interaction is also of considerable interest, as we see below. 

D.A. King. D.P. Woodruff. The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Elsevier (1981,. ..). (A series of the "Advances" type, many experimental results, less emphasis on techniques. Volume 1 (1981) deals with clean solid surfaces. Vol. 2 (1983) with adsorption at solid surfaces. Volume 6 (1993) with co-(chemi-)-sorption, promoters and poisons.)... 

See for Instance, G.H. Findenegg, Principles of Adsorption at Solid Surfaces and Their Significance in Gos/Soiid and Liquxd/Solid Chromatography, in Theoretical Advancement in Chromatography and Related Separation Techniques, F. Dondi, G. Gulchon, Eds. NATO-ASI Series. Kluwer (1992) where other references can be found, and S.A. Busev, S.l. Zverev, O.G. Larionov and E.S. Yakubov, Zhur. Fiz. Khim. 56 (1982), 929 (transl. as Russ. J. Phys. Chem 56 (1982) 563). 

Besides process development and dissolution rate enhancement, the phenomenon of adsorption at solid surfaces is also useful in many other aspects of pharmaceutics. By adsorbing moisture onto its large surface area, colloidal silica has been frequently used as a... 

The basic approach regarding how adsorption at solid surfaces affects different aspects of pharmaceutical development, especially for solid dosage forms, is briefly reviewed. The three broad fields addressed are general pharmaceutical processing, dissolution rate enhancement for poorly water-soluble compounds, and some other applications using adsorption at solid surfaces. Case studies are introduced to aid in understanding the applications and/or principles involved. 

Adsorption at Solid Surfaces Pharmaceutical Applications Particle... 

OTHER APPLICATIONS USING ADSORPTION AT SOLID SURFACES... 

Let us now proceed with the discussion of the general trends of surfactant adsorption at solid surfaces. The most common method used to monitor adsorption phenomena in such systems is to study the concentration dependence of surfactant adsorption. In these studies solid surfaces with high specific surface areas, such as powders and highly porous adsorbents, are used. If the surface area of adsorbent, S), is not known, then the total amount of substance accumulated per unit weight of adsorbent, T, is determined. The latter can be established from the decrease in the concentration of adsorbing substance, Ac, in a solution of known volume, V, after adsorption equilibrium has been established, i.e. ... 

D.P. Woodruff, G.C. Wang, and T.M. Lu. Surface Structure and Order-Disorder Phenomena. In D.A. King and D.P. Woodruff, editors. Adsorption at Solid Surfaces. The Chemical Physics of Solid Surfaces and Heterogeneous Cataly.sis, Volume 2. Elsevier, New York, 1983. 

Competitive adsorption at solid surfaces is complex and it is therefore difficult to predict the outcome when new mixtures of amphiphiles or a new particle surface is introduced. The adsorption behaviour may, for instance, be very different at a pigment surface as compared to a latex surface. An anionic surfactant may adsorb more strongly than an associative thickener on the former but not on the latter. This type of competitive adsorption is difficult to predict a priori, the more so as most pigments are being surface modified by the pigment producer who usually does not reveal the exact surface composition. 

The notion monolayer usually refers to a layer of amphiphilic molecules at a fluid interface, being either a liquid/liquid or a liquid/gas interface. Even when the layer is incomplete or it is more than one molecular layer thick, it is still called a monolayer. The term monolayer may as well be used in case of adsorption at solid surfaces to distinguish it from a bilayer or multilayer. 

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