How Polyelectrolyte Adsorption Depends on History: A Combined Fourier Transform Infrared Spectroscopy in Attenuated Total Reflection and Surface Forces Study

Abstract

We present a systematic study of how adsorption history affects the thickness, surface forces, and interfacial rheology of a model cationic polymer. The polymer was quaternized poly-4-vinylpyridine, QPVP (weight-average degree of polymerization n w = 325 and 98% quaternized with ethyl bromide). The main comparisons concerned one-step adsorption from solution at a variable salt concentration up to 0.5 M NaCl, versus two-step adsorption (initial adsorption from buffer solution without added salt, then NaCl added later). The aqueous solutions were buffered at pH = 9.2 such that the surfaces (mica in the case of surfaces forces (SFA) experiments, oxidized silicon in the case of in situ infrared (FTIR-ATR) experiments) in each case carried a large negative charge. The SFA and FTIR-ATR experiments gave consistent estimates of the amount of polymer adsorbed, confirming the expectation that adsorption should be driven by electrostatic attraction to the surface of large opposite charge. The adsorbed amount showed little dependence on path, validating the common assumption of equilibration in this respect. However the layer thickness measured by surface forces, the shear nanorheology response at a given surface force, and the dichroism of pendant side groups of the polymer all showed a pronounced dependence on the path to reach the adsorbed state. We interpret the measurements to suggest that two-step adsorption produces an inhomogeneous layer comprised of a dense layer of segments closest to the solid surface and a sparse outer layer. In particular, two-step adsorption produced thicker layers and a greater tendency to decouple shear forces from those that resist compression in the normal direction, thereby lessening the shear forces at a given level of normal force.

Static and Dynamic Forces between Adsorbed Polyelectrolyte Layers (Quaternized Poly-4-vinylpyridine)

“…In this case the extension of the forces was significantly larger than obtained separately for the two adsorption conditions (note the different distance scale in Figure 4c), even though the adsorbed amount was only twice the one obtained in the first step (Table 1), and the magnitude of the force at smaller separations did not exceed the one in Figure 4a. This is consistent with FTIR experiments and previous force measurements 33 that suggest that the polymer chains adsorbing during a second adsorption step to the already partially covered surfaces have fewer contact points with the solid surface per chain and thus dangle far out into the solution to give a large increase in the range of the forces. The measured static force range suggests that some chains extend about 750 Å from each surface, which would be close to the contour length of a chain with n ) 325.…”

“…At very high concentration, 1 mg mL -1 , a larger adsorbed amount was obtained (cf. Table 1), 33 but in this case the observed decay length of the force, 30 Å, was shorter than the expected Debye length (taking into account the ionic strength of the polyelectrolyte in the adsorption solution kept between the surfaces in the experiments 33 ).…”

“…In FTIR experiments on silicon oxide surfaces, no desorption of 48% QPVP was found when rinsing with buffer solution with similar ionic strength as the adsorption solution, 19 nor did 98% QPVP desorb when the adsorption solution was replaced with 0.1 or 0.5 M NaCl solution. 33 It is thus unlikely that the amount of adsorbed polymer decreases significantly during the rinsing in the present experiments. This is supported by other studies where a 10% charged copolymer was found not to desorb from mica in pure water within 12 h, 12 and another, fully charged cationic polyelectrolyte was shown to desorb to a small extent only after immersion in dilute electrolyte solution for days.…”

“…An adsorbed amount similar to the one in our system was found in experiments where the 98% QPVP was allowed to adsorb onto mica from a 0.005 mg mL -1 solution in 1 mM borate buffer. 33 At this concentration, the long-range interactions were well described as purely electrostatic (decay length 60 Å), suggesting that the chains adsorbed in a flat conformation. At very high concentration, 1 mg mL -1 , a larger adsorbed amount was obtained (cf.…”

“…The adsorbed amount on each surface calculated from this thickness (assuming a density of 1.15 g/cm 3 as for uncharged PVP, 40 also for the values from ref 33) is given for each system in Table 1 together with values from other investigations. 18, 33 The compressed layers also contain solvent, so that the values given in Table 1 represent an upper bound for the adsorbed amount on mica at these conditions. The adsorbed amount was generally slightly larger on silicon oxide than on mica.…”