A molecule is adsorbed when it accumulates at a surface or an interface. Surfactants-being surface active-adsorb on almost every surface and at almost every interface. Surfactants adsorb at the interface between oil and water. Surfactants adsorb at the interface between liquids and gases (the water and air interface, for example). Still, their behavior at a solid-liquid interface is unique. Surfactant adsorption at the solid-liquid interface is especially interesting because adsorbed surfactants form aggregates on the surface of solids that can dramatically change the properties of the interface. This is an important factor in solid-liquid dispersions, in wetting of surfaces, and in cleaning.

Adsorption of a surfactant at a solid-liquid interface can be good or bad. When you want the surfactant to modify the properties of the interface, as you do in wetting applications, you want the surfactant to adsorb. In dispersions, surfactant adsorption is used to control the interaction between solid particles. But in cleaning operations, such as detergency in environmental applications of surfactants, and enhanced oil recovery most of the time surfactant adsorption represents a loss or even degradation in the cleaning efficiency. It is very important, therefore, to be able to choose a surfactant system so that the surfactant adsorption is low when adsorption represents a loss, and is high when it is needed to control the properties of the surface or interface.

Articles

• Minimizing Surfactant Losses Using Twin-Head Anionic Surfactants in Subsurface Remediation

  Illustrates the principal way we reduce surfactant adsorption: Choose a surfactant that is repelled by the charge on the surface where you don't want it to adsorb.

• Surfactant Adsolubilization and Modified Admicellar Sorption of Nonpolar, Polar, and Ionizable Organic Contaminants

  This paper illustrates another important property of adsorbed surfactants: They act a lot like micelles! This means that surfactants adsorbed on a surface can cause other molecules that wouldn't normally interact with the surface to also become adsorbed. When you want to clean the surface, this is to be avoided. When you want to modify the properties of the surface, this is an additional tool that can be used.

• Surfactant Adsorption on Oxides

  While surfactants will adsorb on anything, the adsorption of surfactants on metal oxides is especially important. Oxides are used as fillers and reinforcers in polymer composites, as pigments in inks, as opacifiers in paints, and as the base material in ceramics. Oxide layers also form spontaneously on all metal surfaces. Consequently, it is important to know the unique aspects of surfactant adsorption on oxides. One of the most important of these factors is the existence of an amphoteric (capable of acting as either an acid or a base) surface on all oxides. Thus, the surface of an oxide may have either a positive charge or a negative charge, depending on the pH. Each oxide surface also has a specific pH at which the net surface charge will be zero. This important property of the surface is called the Point of Zero Charge (PZC)

• Adsorption From Mixed Surfactant Systems

• Surfactant Adsorption in Porous Media

Recommended Reading

• Adsorption of Ionic Surfactants, , B. Hough and H. M. Rendall, in Adsorption from Solution at the Solid/Liquid Interface, G. D. Parfitt and C. H. Rochester, Editors, Academic Press, London, pages 247-320, 1983

• Adsorption of Nonionic Surfactants, , J. S. Clunie and B. T. Ingram, in Adsorption from Solution at the Solid/Liquid Interface, G. D. Parfitt and C. H. Rochester, Editors, Academic Press, London, pages 105-152, 1983

• Adsorption of Surfactants at the Solid/Liquid Interface, , R. Aveyard, Chapter 5, in Solid/Liquid Dispersions, Th. F. Tadros, Editor, Academic Press, London, 1987

• Evaluation of Ethoxylated Alkylsulfate Surfactants for use in Subsurface Remediation, , J. D. Rouse, D. A. Sabatini, R. E. Brown, and J. H. Harwell, Water Environment Research, 68, No. 2, 1996

• Adsolubilization, , J. H. O’Haver, L. L. Lobban, J. H. Harwell, and E. A. O’Rear III, in Solubilization in Surfactant Aggregates, S. D. Christian and J. F. Scamehorn, Editors, Marcel Dekker, New York, Chapter 8, pages 277-295, 1995

• Adsorption from Mixed Surfactant Systems, , J. F. Scamehorn and J. H. Harwell, in Mixed Surfactant Systems, K. Ogino and M. Abe, Editors, Chapter 9, pages 263-281, Marcel Dekker, New York, 1993

• Minimizing Surfactant Losses Using Twin-Head Anionic Surfactants in Subsurface Remediation, , J. D. Rouse, D. A. Sabatini, and J. H. Harwell, Envir. Sci. & Tech., 27, 2072-2078, 1993

• Adsorption of Nonionic Surfactants from Aqueous Solutions on Graphite: Adsorption Isotherms and Calorimetric Enthalpies of Displacement for C8E4 and Related Compounds, , G. H. Findenegg, B. Pasucha and H. Strunk, Colloids and Surfaces, 37, 223, 1989

• Surfactant Science and Technology, , Drew Myers, VCH Publishers, Weinheim, Federal Republic of Germany, Chapter 8, pages 273-325, 1988

• Fluorescence Probe Studies on the Structure of the Adsorbed Layer of Dodecyl Sulfate at the Alumina-Water Interface, , P. Chandar, P. Somasundaran, and N. J. Turro, J. Colloid Interface Sci., 117, 31, 1987

• Infrared Study of the Adsorption of Nonionic Surfactants on Silica [from Nonaqueous Solutions], , Y. Lijour, J.-V. Claves and P. Saumagne, J. Chem. Soc., Faraday Trans. 1, 83, 3283, 1987

• Effects of Counterions on Surfactant Aggregates at the Alumina/Aqueous Solution Interface, , D. Bitting and J. H. Harwell, Langmuir, 3, 500, 1987

• Adsorption of Nonyl Phenol Ethoxylates on Hydrophobic and Hydrophilic Surfaces, , J. R. Aston, D. N. Furlong, F. Grieser, P. J. Scales and G. G. Warr, in Adsorption at the Gas-Solid and Liquid-Solid Interface, J. Rouquerol and K.S.W. Sing, Editors, Elsevier Scientific Publishing Company, Amsterdam, page 97, 1982

• Relationship of Structure to Properties in Surfactants: III. Adsorption at the Solid-Liquid Interface from Aqueous Solution, , Rosen, M. J., J. Am. Oil Chem Soc., 52, 43, 1975