The adsorption of hydrophobically modified carboxymethylcellulose on a hydrophobic solid: Effects of pH and ionic strength

Carboxymethylcellulose with 1.2% dodecyl groups (per glucose unit) was prepared by amidation with dodecylamine. This polymer behaves as a hydrophobically modified polyelectrolyte with the following thickening properties which are determined from viscosity data. It adsorbs from aqueous solution on spin-coated polystyrene films to various extents, depending on pH and ionic strength. The adsorbed amount has a surprising minimum at around pH 6 which coincides with a maximum in viscosity of a dilute solution, and with a very pronounced maximum in the hydrodynamic radius as determined from dynamic light scattering. To our knowledge, such behavior has not been reported before. It suggests that at low pH the polymer is present in the form of small aggregates which upon increasing the pH first swell and then break up into single molecules.