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The effect of the anionic surfactant SDS (sodium dodecyl sulfate) on the adsorption behaviors of cationic hydroxyethyl celluloses (Polymer JR-125, JR-400, and JR-30M) and hydrophobically modified cationic cellulose (Quatrisoft LM-200) at silica surfaces in the presence of a 10 mM NaCl solution has been investigated by null ellipsometry. The adsorbed amount of LM-200 is found to be considerably larger than adsorbed amounts of other polymers. The rate of adsorption for the LM-200 is also lower than that for the Polymer JR series under comparable conditions. Electrostatic interaction is found to be the major driving force for the adsorption. The effect of SDS on adsorption was studied under two different conditions:  adsorption of polymer/SDS complexes from premixed solutions and additions of SDS to the preadsorbed polymer layers. In all cases, associative binding of the surfactant to the polymer seems to control interfacial behavior, which depends on the surfactant concentration. Maximum adsorption was obtained at the surfactant concentration below the phase separation region, and the complex desorbed from the surface at a high SDS concentration above the critical micelle concentration. The reversibility of the polymer/SDS complexes adsorption upon rinsing with NaCl solutions was also investigated. When the premixed polymer/SDS solutions at high SDS concentrations (>5 mM) were diluted by salt solutions, the adsorbed amount increased sharply with precipitation of the complex. A larger amount of the JR-400/SDS complex precipitated during the rinsing process seemed to adsorb to a larger extent to silica than the corresponding LM-200/SDS complex.

Adsorption of cationic high molecular weight polyacrylamides (CPAM) (M(w) is about 800 kDa) with different fractions of cationic units tau = 0.09 and tau = 0.018 onto silica surface was studied over a wide range of pH (4-9) and KCl concentration (c(s) = 10(-3)-10(-1) M) by in-situ null ellipsometry. We discuss how the adsorbed layer depends on the bulk conditions as well as kinetically responds to changes in solution conditions. The adsorbed amount Gamma of CPAM increases with pH for all studied electrolyte concentrations until a plateau Gamma is reached at pH > 6. At low pH we observed an increase in adsorbed amount with electrolyte concentration. At high pH there is no remarkable influence of added salt on the values of the adsorbed amount. The thickness of adsorbed polymer layers, obtained by ellipsometry, increases with electrolyte concentration and decreases with pH. At low c(s) and high pH the polyelectrolyte adsorbs in a flat conformation. An overcompensation of the surface charge (charge reversal) by the adsorbed polyelectrolyte is observed at high c(s) and low pH. To reveal the reversibility of the polyelectrolyte adsorption with respect to the adsorbed amount and layer thickness, parameters such as polyelectrolyte concentration (c(p)), c(s), and pH were changed during the experiment. Generally, similar adsorbed layer properties were obtained independent of whether adsorption was obtained directly to initially bare surface or by changing pH, c(s), or the concentration of polyelectrolyte solution in the presence of a preadsorbed layer, provided that the coverage of the preadsorbed layer was low. Once a steady state of the measured parameters (Gamma, d) was reached, experimental conditions were restored to the original values and corresponding changes in Gamma and adsorbed layer thickness were recorded. For initially low surface coverage it was impossible to restore the layer properties, and in this case we always ended up with higher coverage than the initial values. For initial high surface coverage it was usually possible to restore the initial layer properties. Thus, we concluded that polyelectrolyte appears only partially reversible to changes in the solution conditions due the slow rearrangement process within the adsorbed layer.

The effect of the anionic surfactant SDS (sodium dodecyl sulfate) on the adsorption behavior of cationic hydroxyethyl cellulose (Polymer JR-400) and hydrophobically modified cationic cellulose (Quatrisoft LM-200) at hydrophobized silica has been investigated by null ellipsometry and compared with the previous data for adsorption onto hydrophilic silica surfaces. The adsorbed amount of LM-200 is found to be considerably larger than the adsorbed amount of JR-400 at both surfaces. Both polymers had higher affinity toward hydrophobized silica than to silica. The effect of SDS on polymer adsorption was studied under two different conditions: adsorption of polymer/SDS complexes from premixed solutions and addition of SDS to preadsorbed polymer layers. Association of the surfactant to the polymer seems to control the interfacial behavior, which depends on the surfactant concentration. For the JR-400/SDS complex, the adsorbed amount on hydrophobized silica started to increase progressively from much lower SDS concentrations, while the adsorbed amount on silica increased sharply only slightly below the phase separation region. For the LM-200/SDS complex, the adsorbed amounts increased progressively from very low SDS concentrations at both surfaces, and no large difference in the adsorption behavior was observed between two surfaces below the phase separation region. The complex desorbed from the surface at high SDS concentrations above the critical micelle concentration. The reversibility of the adsorption of polymer/SDS complexes upon rinsing was also investigated. When the premixed polymer/SDS solutions at high SDS concentrations (>5 mM) were diluted by adding water, the adsorbed amount increased due to the precipitation of the complex. The effect of the rinsing process on the adsorbed layer was determined by the hydrophobicity of the polymer and the surface.

The adsorption of two cationic amphiphilic polyelectrolytes, which are copolymers of two charged monomers, triethyl(vinylbenzyl)ammonium chloride and dimethyldodecyl(vinylbenzyl)ammonium chloride (which is the amphiphilic one) with different contents of amphiphilic groups (40% (40DT) and 80% (80DT)), onto the hydrophilic silica-aqueous solution interface has been studied by in situ null ellipsometry and tapping mode atomic force microscopy (AFM). Adsorption isotherms for both polyelectrolytes were obtained at 25 degrees C and at different ionic strengths, and the adsorption kinetics was also investigated. At low ionic strength, thin adsorbed layers were observed for both polyelectrolytes. The adsorption increases with polymer concentration and reaches, in most cases, a plateau at a concentration below 50 ppm. For the 80DT polymer, at higher ionic strength, an association into aggregates occurs at concentrations at and above 50 ppm. The aggregates were observed directly by AFM at the surface, and by dynamic light scattering in the solution. The adsorption data for this case demonstrated multilayer formation, which correlates well with the increase in viscosity with the ionic strength observed for 80DT.

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The influence of sodium dodecyl sulfate (SDS) on the interfacial behavior of two amphiphilic polyelectrolytes, which are copolymers of the cationic monomers triethyl(vinylbenzyl)ammonium chloride and dimethyldodecyl(vinylbenzyl)ammonium chloride, at the silica-aqueous interface was studied. The fraction of amphiphilic monomers was varied, where 0DT, 40DT, and 80DT contained 0, 40, and 80 mol % monomers with dodecyl side chains, respectively. We used in situ ellipsometry to follow the kinetics of adsorption, in terms of adsorbed amount and adsorbed layer thickness, as well as the response of the adsorbed layers to changes in ionic strength and surfactant concentration. Different results were obtained when surfactant was added to the preadsorbed layers compared to the cases when complexes were preformed in the solution prior to the adsorption. In the whole range of concentrations studied, SDS interacts with 40DT and 80DT noncooperatively, whereas for 0DT cooperativity of binding is observed. The amount adsorbed increased significantly as the SDS concentration was close to the cmc. At high SDS concentrations, a lowering of the layer density was observed. For the amphiphilic polyelectrolytes, 40 DT and 80DT, no desorption from the interface was detected for the range of SDS concentrations studied, while 0DT features a maximum in adsorbed amount at concentrations close to the cmc of SDS. Adsorption of 40DT and 80DT from their mixtures with SDS is found to be path dependent with respect to the variation in SDS concentration, where the reversibility decreases with increasing SDS concentration above the expected charge neutralization point. The coadsorption of 80DT and SDS is highly irreversible with respect to changes in the ionic strength within the time scale of the experiment. In this study, we attempt to illustrate both general mechanisms and specific effects. With regard to the general behavior, it is important to note the charge regulation of both the silica surfaces and the polyion/surfactant complexes; an interplay between the two charge-regulating effects is the key to understanding our observations.

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