Density-dependent jump in compressibility of polyelectrolyte brush layers revealed by surface forces measurement

Interactions between apposed brush layers of polyelectrolytes ionized poly(L-glutamic acid) (polymerization degree n = 21, 44, 48) were investigated in water at pH 10 by the direct surface forces measurement. Brush layers were prepared by the Langmuir-Blodgett (LB) deposition of amphiphiles bearing a poly(L-glutamic acid) chain and two octadecyl groups. The obtained surface force and stress profiles consisted of a long-range electrostatic repulsion and a short-range steric repulsion. The distance where the steric repulsion appeared was in good agreement with twice the length of the polyelectrolyte chains in the extended form. The stress profiles of the polyelectrolyte brushes at n = 21, 44, and 48 produced the identical curve when the distance was scaled by the length corresponding to twice the thickness of an undeformed polyelectrolyte layer. Interactions between the layers of a poly(L-glutamic acid) (n = 48) were also studied as a function of the polyelectrolyte chain density in the brush layers. The density was varied by mixing the poly(L-glutamic acid) amphiphile with dioctadecylphosphoric acid. The sudden increase in the short-range repulsion attributed to the steric component was found at the critical density of 0.20 +/- 0.07 chain/nm(2) with decreasing chain density in the brush layers, indicating the existence of the transition in the interaction mode of polyelectrolytes. The steric repulsion was quantitatively analyzed to provide the elastic compressibility modulus of the polyelectrolyte brushes. The obtained modulus of 0.6 +/- 0.1 pN/chain was in the high-density region, and 4.4 +/- 0.7 pN/chain was in the low-density region. The transition in the counterion binding to polyelectrolytes may account for the density-dependent change in the interactions of the polyelectrolyte chains.