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ABSTRACT: The equilibrium and kinetic aspects of the adsorption of alkyltrimethylammonium surfactants at the silica-aqueous solution interface have been investigated using optical reflectometry. The effect of added electrolyte, the length of the hydrocarbon chain, and of the counter- and co-ions has been elucidated. Increasing the length of the surfactant hydrocarbon chain results in the adsorption isotherm being displaced to lower concentrations. The adsorption kinetics indicate that above the cmc micelles are adsorbing directly to the surface and that as the chain length increases the hydrophobicity of the surfactant has a greater influence on the adsoption kinetics. While the addition of 10 mM KBr increases the CTAB maximal surface excess, there is no corresponding increase for the addition of 10 mM KCl to the CTAC system. This is attributed to the decreased binding efficiency of the chloride ion relative to the bromide ion. Variations in the co-ion species (Li, Na, K) have little effect on the adsorption rate and surface excess of CTAC up to a bulk electrolyte concentration of 10 mM. However, the rate of adsorption is increased in the presence of electrolyte. Slow secondary adsorption is seen over a range of concentrations for CTAC in the absence of electrolyte and importantly in the presence of LiCl; the origin of this slow adsorption is attributed to a structural barrier to adsorption.
Journal of Colloid and Interface Science 11/2003; 266(2):236-44. · 3.07 Impact Factor
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ABSTRACT: Until recently, the rapid time scales associated with the formation of an adsorbed surfactant layer at the solid-aqueous interface has prevented accurate investigation of adsorption kinetics. This has led to the mechanism of surfactant adsorption being inferred from thermodynamic data. These explanations have been further hampered by a poor knowledge of the equilibrium adsorbed surfactant morphology, with the structure often misinterpreted as simple monolayers or bilayers, rather than the discrete surface aggregates that are present in many surfactant-substrate systems. This review aims to link accepted equilibrium data with more recent kinetic and structural information in order to describe the adsorption process for ionic surfactants. Traditional equilibrium data, such as adsorption isotherms obtained from depletion approaches, and the most popular methods by which these data are interpreted are examined. This is followed by a description of the evidence for discrete aggregation on the substrate, and the morphology of these aggregates. Information gained using techniques such as atomic force microscopy, fluorescence quenching and neutron reflectivity is then reviewed. With this knowledge, the kinetic data obtained from relatively new techniques with high temporal resolution, such as ellipsometry and optical reflectometry, are examined. On this basis the likely mechanisms of adsorption are proposed.
Advances in Colloid and Interface Science 06/2003; 103(3):219-304. · 8.12 Impact Factor
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Langmuir 01/2001; 17(20):6155-6163. · 4.19 Impact Factor
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Langmuir 01/2000; 16(24):9374-9380. · 4.19 Impact Factor
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ABSTRACT: The equilibrium and kinetic aspects of the adsorption of alkyltrimethylammonium surfactants at the silica–aqueous solution interface have been investigated using optical reflectometry. The effect of added electrolyte, the length of the hydrocarbon chain, and of the counter- and co-ions has been elucidated. Increasing the length of the surfactant hydrocarbon chain results in the adsorption isotherm being displaced to lower concentrations. The adsorption kinetics indicate that above the cmc micelles are adsorbing directly to the surface and that as the chain length increases the hydrophobicity of the surfactant has a greater influence on the adsoption kinetics. While the addition of 10 mM KBr increases the CTAB maximal surface excess, there is no corresponding increase for the addition of 10 mM KCl to the CTAC system. This is attributed to the decreased binding efficiency of the chloride ion relative to the bromide ion. Variations in the co-ion species (Li, Na, K) have little effect on the adsorption rate and surface excess of CTAC up to a bulk electrolyte concentration of 10 mM. However, the rate of adsorption is increased in the presence of electrolyte. Slow secondary adsorption is seen over a range of concentrations for CTAC in the absence of electrolyte and importantly in the presence of LiCl; the origin of this slow adsorption is attributed to a structural barrier to adsorption.
Journal of Colloid and Interface Science.
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ABSTRACT: The adsorption of gemini surfactants of the form alkanediyl-R,ω-bis (dodecyldimethylammonium bromide) to a silica substrate is investigated using optical reflectometry (OR) and atomic force microscopy (AFM). The adsorption isotherms and kinetics of adsorption have been determined for spacer sizes of 2, 3, 4, 6, 8, 10, and 12. The maximum surface excess correlates strongly with the size of the spacer group. The smallest spacer size yields the largest surface excess. Soft-contact AFM imaging has shown that flattened ellipsoidal aggregates are present on the surface at the shortest spacer lengths. Images of the adsorbed layer cannot be obtained for spacer sizes greater than 3 because of a strongly attractive force regime. The origin of the attraction is attributed to proximal desorption induced by the approach of the tip toward the substrate. The linear increase in the area per adsorbed molecule with spacer size suggests that the aggregate structures become flattened with increasing spacer size but are otherwise similar.
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ABSTRACT: The adsorption of an anionic C12 surfactant and cationic C12 and C16 surfactants to a synthetic, anionic radio frequency glow discharge plasma polymer substrate has been investigated. This substrate has a similar charge density to that of amorphous silica but also possesses hydrophobic character, producing a water sessile contact angle of about 74° at pH values between 2 and 7. Both the cationic and anionic C 12 surfactants had the same saturation surface excess values on the plasma polymer substrate. Surprisingly, the addition of electrolyte did not affect the saturation surface excess, indicating that hydrophobic interactions between the surfactants and the substrate dominate the adsorption process. All surfactant systems show a change in slope in the isotherm at a surface excess of 0.3 mg‚m -2 . The likely conformation of the adsorbed surfactants at this surface excess is with the monomer lying flat on the substrate, to maximize hydrophobic interactions between the hydrocarbon chain and the surface. As the bulk concentration is increased further, adsorption is dominated by hydrophobic interactions. The structure of the adsorbed layer at saturation is discussed.
