Polyelectrolyte brush layers studied by surface forces measurement: Dependence on pH and salt concentrations and scaling

Interactions between opposed brush layers of polyelectrolytes ionized poly(L-glutamic acid) (PLGA) or poly(L-tysine) (PLL) in water were directly investigated using surface forces measurement. The brush layers were prepared by the Langmuir-Blodgett deposition of amphiphiles bearing PLGA (degree of polymerization n = 21,44, and 48) or PLL (n = 41 and 52) as hydrophilic groups. The density of polyelectrolyte chains was around 0.4 chain/nm(2). Surface force profiles, consisting of a long-range electrical double layer repulsion and a short-range steric repulsion, were measured and analyzed with varying pH, salt concentration, and polyelectrolyte chain length. The surface potential obtained from the double layer repulsion indicated that nearly all of the ionized groups in the brush layers were neutralized by counterions. In the case of PLGA, the steric repulsion of the brush layer increased with increasing pH from 9 to 10, reflecting the increase in the ionization degree of the carboxylic groups, and then decreased above pH 10 due to salt effect. Similar behavior was also observed for PLL: the steric repulsion increased when the pH was changed from 4 to 3 and decreased at pH 2. In the cases of both PLGA and PLL, the steric repulsion decreased with increasing salt concentration (0.43-10 mM) at a fixed pH, which was likely due to a decrease in the osmotic pressure of the counterions. The distance at which the steric repulsion appeared (Do) corresponded to twice the length of an extended polyelectrolyte molecule. This distance Do remained practically the same under the pH and salt conditions studied for the brush of an identical polymerization degree. Twice the length of polyelectrolyte chains L-0 was defined as Do - 6 (nm), where 6 nm was twice the length of the long alkyl chains in polyelectrolyte amphiphiles, and was found to be proportional to the polymerization degree of polyelectrolyte n. The stress profiles, obtained by differentiating the force profiles, were scaled according to the distance to provide identical profiles for different polymer chain lengths. An equation describing the stress profiles was derived based on a model that attributed the steric force to the osmotic pressure of the counterions. The equation reproduced well the stress profiles of PLGA and PLL brushes, where the osmotic coefficient was a variable parameter. The osmotic coefficient was estimated to be 0.004 for both PLGA and PLL and much less than the value determined in solution (similar to0.2), indicating that the counterions bound more strongly to the polyelectrolytes in the brush layer than to isolated polyelectrolytes in solution. The advantage of characterizing polyelectrolytes by surface forces measurement was discussed.