Charge regulation in polyelectrolyte brushes studied by FTIR spectroscopy

Brush layers of a polyelectrolyte, ionized poly(glutamic acid) (polymerization degree n=48), were studied using FTIR spectroscopy as a function of the polyelectrolyte chain density. The Langmuir–Blodgett (LB) deposition of the mixed monolayers of a poly (glutamic acid) functionalized amphiphile and dioctadecylphospholic acid afforded the preparation of polyelectrolyte brushes at the desired density on various substrates. FTIR spectroscopy on the LB films demonstrated that the ionization degree of the carboxylic acid groups decreased with increasing polyelectrolyte density in order to maintain the charge density below the critical value of 0.8∼1 charge nm−3. The average distance between the charges at the maximum density was ca. 1.1±0.1 nm which is close to twice the Bjerrum length, the distance at which the Coulomb interaction between elementary charges is equal to the thermal energy.