Langmuir 1992,8, 2649-2659
2649
Adsorption of Ionic Surfactants on Variable-Charge Surfaces. 1. Charge Effects and Structure of the Adsorbed Layer Marcel R. Biihmer and Luuk K.Koopal' Department of Physical and Colloid Chemistry, Wageningen Agricultural University, Dreijenplein 6, 6703 HB Wageningen, The Netherlands Received April 13, 1992. I n Final Form: July 31, 1992 The adsorption of an anionic surfactant on a variable-charge surface has been studied by comparing resulta obtained for sodium p-3-nonylbenzenesulfonate (SNBS)on rutile with theoretical predictions based on the self-consistent field lattice theory (SCFA) by Scheutjens,Fleer, and Leermakers. Both the measured and the calculated isotherms show strong cooperativeeffects at low coverages and have shapes which are characteristicfor the adsorption of anionic surfactanta on metal oxides. The experimentaland calculated adsorption isotherms at different salt concentrationshave a common intersection point (cip) corresponding to surface charge neutralization. Before the cip the surfactanta are adsorbed with their head groups in contact with the surface. After the cip bilayer formationstarta. Surfactant ions effectively screen the surface charge, so that this charge increases upon surfactant adsorption. From the surface charge the minimal amount of surfactant adsorbed Yhead-on"can be estimated. This amount depends mainly on the surface potential, which is fixed by the pH, and hardly on the concentrationof indifferent electrolyte. On the other hand, the amount of surfactant at the solution side of the bilayer mainly depends on the salt concentration and hardly on the surface potential. Due to the different charge screening mechanisms at the inner and the outer sides of the adsorbed layer, the layers are asymmetrical.
Introduction Adsorption isotherms of anionic surfactants from aqueous solution on metal oxide surfaces have been the subject of much research. The experimental work of the groups of F~erstenaul-~ and S~masundaran~*~ on alkanesulfonates adsorbed on alumina is often cited, and general agreement exists about the shape of the isotherm of an anionic surfactant on a metal oxide such as alumina; see, e.g., refs 1-9. Electrokinetic measurements have been frequently used to provide additional informati~n.'~*~ Typically, when plotted on a log-log scale, four regions can be distinguishedin the adsorption isotherms,as shown in Figure la. The isotherm extends over about three decades in concentration and adsorbed amount. In region I isolated surfactant moleculesadsorb. In region 11,which starts at very low coverages, the isotherm is steeper due to association of adsorbed molecules. The slope of the log-log isotherm decreases at the transition between regions I1and III. To interpret this change,the adsorption isotherm can be described with the relation r = ac exp(-ISGIRT), where r is the adsorbed amount, c is the concentration, a is a constant taking into account the correct dimensionality, and AG is the free energy of
* To whom correspondence should be addressed.
(1) Chander, 5.;Fuerstenau, D. W.; Stigter, D. In Adsorption from Solution;Ottewill, R. H., Rochester, C. H., Smith, A. L., Eds.; Academic Press: London, 1983; p 197. (2) Somaeundaran,P.; Fuerstenau, D. W. J. Phys. Chem. 1966,70,90. (3) Fuerstenau, D.W.; Wakamatsu, T.Faraday Discus. Chem. SOC. 1975. 59. 157. (4) Moudgil,B. M.; Somasundaran, P.;Soto, H. InReagents inMineral Technology;Somaeundaran,P., Moudgil, B. M., Us.;Surfactant Science Series; Dekker: New York, 1988, Vol. 27, p 79. (5) Chandar, P.;Somaeundaran,P.; T w o , N. J. J. Colloid Interface Sci. 1987, 117, 31. (6) Hough, D.B.; Rendall, H. M. In Adsorption from Solution at the SolidlLiquidInterface;Parfitt, G. D., Rochester, C. H., Eds.;Academic Prees: Lbndon, 1983; p 247. (7) Scamehorn,J.F.;Schechter,R. S.;Wade, W. H. J. Colloidlnterface Sci. 1982,85, 463. (8)Hywell, J. H.; Hoakins, J. C.; Schechter, R. S.; Wade, W. H. Langmuir 1985,1, 251. (9) Bitting, D.; Harwell, J. H. Langmuir 1987, 3, 500. , - - I
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Figure 1. Characteristic four-region isotherm for an anionic surfactanton a metal oxide surface on a log-log scale (a)and the electrophoreticmobility of the metaloxide particlea as a function of the concentration of anionic surfactant (b).
adsorption, including the configurational entropy. AG is a function of log c. If the slope of the log-log plot is unity, AG is constant. For a slope larger than unity, see region 11,a strong increase in AG is observed. For a slope smaller than unity AG decreases. In region I11 the slope of the log-log plot is lower then in region 11,indicating that the increase (slope >1)in AG is smaller than in region I11 or that AG decreases (slope
