Polyelectrolyte adsorption: a subtle balance of forces

Abstract

The adsorption of polyelectrolytes on oppositely charged surfaces is investigated by numerical calculations based on a recent model for polyelectrolyte adsorption by Bóhmer et al.,1 which is an extension of the self-consistent-field theory of Scheutjens and Fleer for adsorption of uncharged homopolymers. First, the effects of salt concentration, segment charge, and surface charge density are described for the case of pure electrosorption. It appears that the adsorption always decreases with increasing salt concentration. We call this the screening-reduced adsorption regime. The conformation of adsorbed polyelectrolytes with moderate segment charge is rather extended, since they also gain adsorption energy for segments not attached to the surface, due to the long range character of the electrostatic interaction. The polyelectrolyte can be displaced from the surface at a certain critical salt concentration, cK, which depends on the segment charge t, and the surface charge density , according to cM = ( )10/11 following Muthukumar.2 Our numerical data agree rather well with this. The force balance is changed if the segments also have a specific (short-range, nonelectrostatic) interaction with the surface. Now, the screening-reduced adsorption regime occurs only for very low segment charge and sufficiently high surface charge density. The majority of the cases constitutes the screening-enhanced adsorption regime, where the adsorption increases with increasing ionic strength. A transition from the screening-reduced adsorption regime to the screening-enhanced adsorption regime, or vise versa, can take place if the strength of the nonelectrostatic attraction or the electrostatic interaction, by means of the segment charge or the surface charge density, is varied. In the screening-enhanced adsorption regime, a maximum in the adsorbed amount as a function of the salt concentration can occur if the counterions also have a specific interaction with the surface. The theoretical results agree qualitatively with reported experimental results.

Polyelectrolyte Adsorption on Solid Surfaces: Theoretical Predictions and Experimental Measurements

“…The practical relevance of polyion adsorption has generated substantial theoretical [6][7][8][9][10][11][12][13][14][15][16][17][18][19] and experimental 6,16,[20][21][22][23][24][25][26][27][28][29][30][31][32] efforts. As mentioned above, surface interactions in polyelectrolyte solutions can often be regulated by increasing or decreasing the addition of simple salt.…”

On the charge overcompensation of quenched polyelectrolyte stars electrostatically adsorbed onto a quenched oppositely charged planar surface

“…Under these conditions the classical one-gradient SCF theory predicts a slight undercompensation of the surface charge by adsorbed polyelectrolyte. 5 We found that in the framework of a two-gradient SCF model the equilibrium adsorbed amount near the plateau of the adsorption isotherm corresponds to noticeable overcompensation of the surface charge. To compare our results with predictions from Ref.…”

“…At high ionic strength, when the entropy gain of the small ions is less important, it is possible to displace the polymer from the interface. [1][2][3][4][5][6] At very low ionic strength the adsorption tends to saturate to a value for the adsorption that is very close to the exact surface charge compensation. Indeed, the exact surface charge compensation is expected at very low ionic strength.…”

In-situ analysis of polyelectrolyte complexes by flow cytometry

“…Adsorption experiments and modeling have been used to gain information about formed layers of polyelectrolytes (Van de Steeg et al 1992;Fleer et al 1993). Adsorption studies using QCM-D and SPR techniques have revealed that polyelectrolytes adsorb differently depending on their chemical properties, such as molar mass and charge density, onto model cellulose or silica surfaces (Tammelin et al 2004;Kontturi et al 2008;Strand et al 2017).…”