Ellipsometric Study of the Adsorption of Hydrophobically Modified Polyacrylates at Hydrophobic Surfaces

Abstract

The adsorption of hydrophobically modified polyelectrolytes (HMPEs) at hydrophobic surfaces was studied by ellipsometry. Both the adsorbed amount of polymer Γ and the layer thickness d were measured as a function of time. The HMPEs are polyacrylates carrying various fractions of grafted dodecyl chains. Hydrophobic interactions are responsible for the adsorption (unmodified polyacrylate does not adsorb at the surface). The surface anchoring of grafted dodecyl groups is coupled with the repulsion of charged acrylate groups. This results in the formation of large loops at the interface, separated by very short trains, since the probability to have two or more successive alkyl groups grafted at the polymer chain is small. As in the case of ordinary polyelectrolytes, the mean thickness of the adsorbed layer decreases with increasing solution ionic strength. Changes of the HMPE chemical composition (molar ratio, X, of alkyl units attached on the polyacrylate backbone) affect the adsorbed layer structure in a complex fashion. HMPEs with relatively few grafted chains form thick dilute adsorbed layers. Increasing the ratio of grafted chains to the number of polyelectrolyte monomers results in a monotonic decrease of the thickness and an increase of the HMPE density. However, the adsorbed amount exhibits a maximum at a relatively small ratio between 5% and 7%, above which the effect of the decrease of Γ caused by the decrease of the average loop size becomes larger than the opposing effect of the increasing surface density of anchors. We also discuss the shape of the adsorption isotherms, experimental evidence for the existence of a depletion layer located outside the adsorbed layer, and the time evolution of adsorption and desorption.

Ellipsometry Studies of Lipoprotein Adsorption

“…Since proteoglycans are highly charged polyelectrolytes, one would expect the adsorption to increase with the electrolyte concentration, since the electrolyte screens the repulsive electrostatic doublelayer interactions, opposing further adsorption (48). A similar behavior has previously been observed for (co)polyelectrolyte systems (49). However, in a number of previous investigations, electrolytes have been found to influence these systems in a significantly more complex way than this, both at surfaces (50) and in bulk solution (51)(52)(53), involving also cation specificity.…”

Formation of Adsorbed Protein Layers

“…As an example of this, Fig. 1c shows results obtained by Poncet et al on the adsorption at methylated silica of poly(acrylate) modified with grafted dodecyl chains (16). As can be seen, the adsorption initially increases strongly with the degree of hydrophobic modification, whereas at high substitution degrees the adsorbed amount decreases again, which is a consequence of the formation of thinner adsorbed layers.…”

“…Analogous results were obtained also for chitosan by Claesson and Ninham (51). Furthermore, Poncet et al found for hydrophobe-modified poly(acrylate) at methylated silica that increasing the concentration of NaCl resulted in a thinner adsorbed layer despite a higher adsorbed amount at the higher electrolyte concentration (16).…”

Low Hydrophobically Modified Poly(Acrylic Acid) Stabilizing Macroemulsions: Relationship between Copolymer Structure and Emulsions Properties

“…interfacial tension measurements at the n-dodecane/water Flocculation is likely to occur at low polymer concentrations interface are not available at the present time for this family as already discussed and deflocculation under shear (hystere-of amphiphilic polymers, their behavior at the solid (silica sis loop) is not a surprising feature. As mentioned earlier, surfaces)/liquid interface was studied by ellipsometry (46). more hydrophobic polymers (50 10C12Na) adsorb certainly Molecular weights in the range of 50,000 to 500,000 were more strongly at the interface (at least at low concentrations) shown to have little effect on the amount of adsorbed polythan less hydrophobic polymers (50 1C12Na), explaining mer at the interface.…”