engleski

Protonation of the surfaces with physically adsorbed and chemically bound polyelectrolytes

The proton binding isotherms, measured in the potentiometric titrations at constant ionic strengths, give the insight into the protonation speciation of polyprotic systems. As well, their interpretation in terms of the effective pK gives a quantitative measure of the electrostatic contribution to the protonation free energy. In this light, the results of several proton binding studies are presented, for polyprotic systems consisitng of polyelectrolytes, either in dissolved state, or bound to charged or neutral surfaces. The weakly acidic nanoparticles (carboxyl latex, silica) with irreversibly adsorbed strongly cationic poly(diallyldimethylammonium chloride) (DADMAC) exhibit a charge reversal in a narrow pH-range, i.e. the point of zero charge, in which the system aggregates (1, 2). Compared with the bare nanoparticle surfaces, the adsorbed polyelectrolyte can induce both positive and negative excess surface charge, depending on the pH and ionic strength. This suggests that in these systems, the electrostatic attraction is the driving force for the adsorption. On the other hand, in the case of weakly basic chitosan adsorbed onto cellulose fibers, the adsorption reduces the charge of both the polyelectrolyte, and the surface (3). This is a consequence of a less polar environment for the amino groups compared with the polyelectrolite in its dissolved state, which suggests that the adsorption is not driven by electrostatic, but predominately hydrophobic interaction. Another interesting example is the spherical polyamine brush found at the surface of a "frozen" micelle, built in aqueous medium by an irreversible association of the amphiphilic block-copolymers. There, the proton binding isotherms indicate strong influence of the hydrophobic interactions at high ionic strength, while the electrostatic interactions are dominant at low ionic strength, and cause swelling upon charging.

Surface; charge; pK; pH; Electrical; Double; Layer; Potentiometric; Model; Polyelectrolyte; Brush; Titration

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