Interactions between opposed brush layers of polyelectrolytes ionized poly(L-glutamic acid) (PLGA) or poly(L-lysine) (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 (D0) corresponded to twice the length of an extended polyelectrolyte molecule. This distance D0 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 L0 was defined as D0 -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 (∼0.2), 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.

A novel acridine derivative (1) connected with a disulfide bond through a long methylene spacer has been synthesized, and its interactions with DNA in solutions and at a self-assembled monolayer surface on gold have been studied. Spectroscopic and thermal data indicated that 1 intercalated successfully to DNA in buffer solutions. The mixed monolayers of 1 and 2, which has a similar molecular structure to that of 1 except for the lack of the acridine moiety, on gold substrates were then prepared. When DNA binding properties onto these monolayers were examined by means of a quartz crystal microbalance, there was a monolayer composition (mole fraction of 1 in the mixed monolayer, 0.04) at which the most effective binding of DNA took place. The interaction of this DNA-immobilized monolayer with polylysines (d, l) was examined by comparing circular dichroism spectra before and after binding of polylysines, and such a two-dimensionally immobilized DNA was found to have an ability to bind poly-l-lysine predominately.

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/nm2 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.