Adsorption of flexible polyelectrolytes : a theoretical and experimental study of polystyrene sulfonate adsorption on polyoxymethylene single crystals

The objective of the present work was to collect systematic adsorption data for a well-defined polyelectrolyte on an uncharged, homogeneous surface, and to compare these with the new theory that was recently developed by Van der Schee.In chapter 1 we shortly describe which factors determine the adsorption equilibrium. Understanding these in relation to their interplay is, ultimately, of great importance in numerous practical applications of adsorbing polyelectrolytes. Subsequently, we introduce the model system: polyoxymethylene (POM) single crystals as the adsorbent and polystyrene sulfonate (PSS) as the adsorbate.The preparation and characterization of the adsorbent is discussed in chapter 2. Single crystals of POX are flat, homogeneous and essentially uncharged, and offer a large specific surface area. Since they can be prepared reproducibly and in large amounts, they constitute a suitable model substrate in systematic adsorption studies. The crystallization procedure is discussed in some detail. The thickness of the crystals is obtained from EM and SAXS measurements, the results being in excellent mutual agreement. Combining this thickness with the crystal density the geometrical surface area is found to be 150 m ² /g. This is compared with the surface area obtained by BET analysis of nitrogen adsorption (30 m ² /g) and with the surface area that follows from adsorption of polyoxyethylated nonyl phenols from aqeous solution (60 m ² /g). The discrepancy in the results is explained in terms of different degrees of aggregation of POX crystals in the dry state and in suspension. Finally, some preliminary results of albumin and PSS adsorption are discussed.We characterized ten commercial samples of homodisperse PSS by elemental analysis, measurement of the extinction coefficient and viscometry. From the results, presented in chapter 3, we conclude that only five of them are sufficiently reliable. For these the degree of sulphonation is 97.5 %, the extinction coefficient at λ= 226 nm amounts to 11,600 M ^-1 cm ^-1 , and the intrinsic viscosities in 0.5 M NaCl conform to the Mark-Houwink relation with K = 1.087 g/g and a = 0.764.Systematic measurements of adsorption as a function of polyelectrolyte concentration, chain length and ionic strength are reported in chapter 4. Substantial adsorption occurs only at high ionic strength, as a consequence of the low affinity of the PSS for the POM surface. Adsorption isotherms are rounded. The adsorption increases more or less linearly with increasing ionic strength, and also we find a relatively strong dependence upon the molecular weight. From a comparison with adsorption results on hematite and with literature data on other systems, clear trends emerge as to the relation between adsorption characteristics and the affinity of the polyelectrolyte for the surface. This affinity may have both electrostatic and non- electrostatic components. With increasing affinity, isotherms become flatter, the molecular weight dependence weaker, and the effect of ionic strength smaller. Desorption experiments showed that the adsorption of PSS on POM is essentially reversible.Chapter 5 deals with polyelectrolyte adsorption theories. These may be derived from models for adsorption of uncharged polymers by incorporating the electrostatic free energy in the partition function of the system. We show in some detail how the lattice model of Roe for adsorption of uncharged polymers is extended to polyelectrolyte adsorption by Van der Schee. Systematic calculations based upon the new theory were performed. Major features are the high affinity character of adsorption isotherms, under all conditions, and the profound influence of the ionic strength upon the adsorption behaviour. At high salt concentration, the adsorption increases more strongly with ionic strength and molecular weight than at low salt concentration, and also the non-electrostatic segment-solvent interactions, expressed in the X-parameter, are more important. The non- electrostatic surface-polyelectrolyte affinity parameter Xs has a marked effect for any ionic strength. When either Xs or the salt concentration is low, the adsorption remains well below monolayer coverage. In that situation the presence of a surface charge of opposite sign to that of the polyelectrolyte gives rise to an additional adsorption of one (univalent) segment per elementary charge on the surface (charge compensation). For the thickness of the adsorbed layer we find 2 - 5 nm.Finally, in chapter 6 we compare experimental adsorption data for PSS on POM crystals with a recent lattice theory for polyelectrolyte adsorption. The dimensions of a lattice cell are estimated from the specific volume of the PSS monomer in combination with the persistence length of the chain, and the non-electrostatic segment-solvent interaction parameter χis assessed from literature data on phase separation. Qualitatively, the model reproduces the measured increase of adsorption with ionic strength and molecular weight. However, experimentally the adsorption increases more strongly than theoretically. For the ionic strength dependence we ascribe this to some variation of the solvent quality (χ) or the segment-surface interaction (χs) with the salt concentration. The theoretical increase of adsorption with molecular weight is probably too weak because the occurrence of free dangling tails at either end of an adsorbed chain is neglected in the Roe model. Experimental isotherms are rounded, whereas theoretically high-affinity isotherms are found. This may be a consequence of applying a lattice model for the bulk solution. The quantitative difference between experimental and theoretical adsorption is around 35 %, a satisfactory result in regard of the large number of model parameters and the uncertainties in their estimated values. Moreover, also the specific surface area of the adsorbent is liable to considerable error.