Uncharged Polymers Research Articles

Capillary electrophoretic separation of uncharged polymers using polyelectrolyte engines: Theoretical model

We recently demonstrated that the molecular mass distribution of an uncharged polymer sample can be analyzed using free-solution capillary electrophoresis of DNA–polymer conjugates. In these conjugates, the DNA is providing the electromotive force while the uncharged polydisperse polymer chains of the sample retard the DNA engine with different amounts of hydrodynamic drag. Here we present a theoretical model of this new analytical method. We show that for the most favourable, diffusion-limited electrophoresis conditions, there is actually an optimal DNA size to achieve the separation of a given polymer sample. Moreover, we demonstrate that the effective friction coefficient of the polymer chains is related to the stiffness of the two polymers of the conjugate, thus offering a method to estimate the persistence length of the uncharged polymer through mobility measurements. Finally, we compare some of our predictions with available experimental results.

Molar mass profiling of synthetic polymers by free-solution capillary electrophoresis of DNA-polymer conjugates.

The molar mass distribution of a polymer sample is a critical determinant of its material properties and is generally analyzed by gel permeation chromatography or more recently, by MALDI-TOF mass spectrometry. We describe here a novel method for the determination of the degree of polymerization of polydisperse, uncharged, water-soluble polymers (e.g., poly(ethylene glycol) (PEG)), based upon single-monomer resolution of DNA-polymer conjugates by free-solution capillary electrophoresis. This is accomplished by end-on covalent conjugation of a polydisperse, uncharged polymer sample (PEG) to a monodisperse, fluorescently labeled DNA oligomer, followed by electrophoretic analysis. The monodisperse, charged DNA "engine" confers to each conjugate an equal amount of electromotive force, while the varying contour lengths of the uncharged, polydisperse polymers engender different amounts of hydrodynamic drag. The balance of electromotive and hydrodynamic forces enables rapid, high-resolution separation of the DNA-polymer conjugates as a function of the size of the uncharged PEG tail. This provides a profile of the molar mass distribution of the original polymer sample that can be detected by laser-induced fluorescence through excitation of the dye-labeled DNA. We call this method free solution conjugate electrophoresis (FSCE). Theory-based analysis of the resulting electrophoresis data allows precise calculation of the degree of polymerization of the PEG portion of each conjugate molecule. Knowledge of the molecular mass of the uncharged polymer's repeat unit allows for direct calculation of the molar mass averages as well as sample polydispersity index. The results of these analyses are strikingly reminiscent of MALDI-TOF spectra taken of the same PEG samples. PEG samples of 3.4-, 5-, and 20-kDa nominal average molar mass were analyzed by FSCE and MALDI-TOF; the values of the molar mass averages, Mw and Mn, typically agree to within 5%. Measurements and molar mass calculations are performed without any internal standards or calibration. Moreover, when DNA-polymer conjugate analysis is performed in a chip-based electrophoresis system, separation is complete in less than 13 min. FSCE offers an alternative to MALDI-TOF for the characterization of uncharged, water-soluble polymers that can be uniquely conjugated to DNA.

Effect of a range of microbial polysaccharides on the diffusion of manganese ions using spatially resolved NMR relaxometry.

In accordance with the theory of contact exchange, it is hypothesized that the presence of negative charge in microbial exopolysaccharides increases the rate of cation transport. These typically acidic materials may provide a fast-track for the diffusion of nutrient cations through the polymer layer for uptake at the organism cell surface. We have measured the diffusion coefficient of a model cation, Mn(2+,) through xanthan, de-acetylated xanthan, scleroglucan and chitosan using spatially resolved NMR relaxometry. The concentration of Mn(2+) in solution was measured by recording the change in the spin-spin (T(2)) relaxation time of water (1)H over time in compartments either side of a polymer layer. This approach provides a sensitive, in situ, non-invasive method of measuring the rate of diffusion of paramagnetic cations through hydrophilic polysaccharides. The negatively-charged polysaccharides, xanthan and de-acetylated xanthan, permitted a significantly faster rate (2-2.5x) of cation transport compared to the uncharged polymer, scleroglucan. The positively-charged polysaccharide chitosan reduced the rate of Mn(2+) diffusion to around half the value obtained for scleroglucan. These results suggest that the presence and nature of fixed charges on the polysaccharide molecule affects the rate of cation transport in accordance with the theory of contact exchange. The presence of negative charge on microbial exopolysaccharides may thus improve the availability of nutrient cations at the organism cell surface.

Enhanced flocculation of colloidal dispersions by polymer mixtures

Bridging flocculation and electrolyte coagulation of negatively charged colloidal dispersions in the presence and absence, respectively, of uncharged polymers and polymer mixtures were studied. The relative coagulation and flocculation rates of particles in the presence of electrolyte and small polymer amounts were measured and the stability ratios have been calculated at various ionic strengths. Also, the structure of polymer layers formed in individual adsorption of polymers and in simultaneous competitive adsorption from binary polymer mixtures at particle/solution interfaces was investigated. The electrophoretic mobility and the diffusion coefficient of particles with and without adsorbed polymer were measured by laser Doppler-electrophoresis and photon correlation spectroscopy, respectively, and the electrophoretic and the hydrodynamic thickness of adsorbed polymer layers have been calculated. It was found that the adsorbed polymers may enhance or diminish the rate of successsful encounters between particles, even at low surface coverages, depending on the magnitude of the interparticle electrostatic repulsion. In addition, competitive adsorption of chemically different polymers for particle surfaces may result in considerable alteration in the conformation of macromolecules in the mixed adsorption layer. Close correlation was found between the effectiveness of polymers as flocculants and the thickness of adsorbed polymer layers formed at optimum polymer dosages on the particle surfaces. Binary mixtures of suitable polymers proved to be very efficient flocculants for the dispersions. The enhanced flocculating effect of some mixtures can be ascribed to extended polymer layers formed in competitive adsorption of chemically different macromolecules at particle/solution interfaces. These findings have relevance in many environmental technologies and offer a way of improving the effectiveness of solid–liquid separation processes.

Interaction between Poly(acrylic acid) and an Ethoxylated Nonionic Surfactant

The interaction between uncharged polymer and nonionic surfactants is usually weak or nonexistent, except for the association between polyacids and ethoxylated nonionic surfactants. In this paper we have studied the interaction between poly(acrylic acid) and an ethoxylated nonyl phenol ether. Below pH 3 an insoluble complex forms, which redissolves on addition of excess surfactant. While the amount of surfactant required to form the maximum amount of precipitate or redissolve the complex is linearly dependent on polymer concentration, that required for the onset of precipitation is not. Despite the polydisperse nature of the ethoxylate chain, little fractionation of the surfactant after precipitation was observed. The nonionic surfactant was preferentially partitioned into water rather than dodecane, except where the polymer/surfactant complex formed; in this case the complex dissolved mainly in the dodecane, forming a gel containing the oil. The origins of the effect of solution conditions and polymer ch...

Mixed Adsorption Layers of Uncharged Polymers at Particle/Solution Interfaces

Simultaneous competitive adsorption from binary mixtures of uncharged polymers on negatively charged colloidal dispersions and the structure of polymer layers formed at particle/solution interfaces were studied. From the adsorption isotherms determined in competitive adsorption at equal concentrations of two polymers in the bulk solution, preferential adsorption parameters for the polymers in pairs have been established, and used as a measure of the affinity of chemically different macromolecules for the solid surfaces. The diffusion coefficient of (monodisperse) particles with and without adsorbed polymer was measured by photon correlation spectroscopy, and the hydrodynamic thickness of adsorbed polymer layers has been calculated. The exchange of polymeric species adsorbed at the interfaces was detected by measuring the thicknesses of mixed layers after various contact times. The preferential adsorption parameters proved to be relevant markers for the various interfacial processes. Simultaneous adsorptio...

Direct analysis of experimental tie line data (two polymer–one solvent systems) using Flory–Huggins theory

A computational method is described which allows experimental tie line data for two polymer–one solvent mixtures to be analysed directly in terms of Flory–Huggins theory. Three Flory–Huggins parameters and their uncertainties are calculated. These are, one polymer molecular weight and two χ values (one polymer segment–solvent χ, i.e. χ12 or χ13, and χ23 the polymer–polymer segment interaction coefficient). Due to irreducible parameter correlation, the remaining two Flory–Huggins parameters, i.e. one polymer molecular weight and one polymer–solvent χ, must be fixed at experimentally determined values. The paper also describes application of the method to experimental (literature) tie line data for some typical ternary systems based on uncharged polymers (synthetic polymers and biopolymers). While the description of phase diagrams was generally good, discrepancies between calculated and experimental polymer molecular weights and second virial coefficients, together with a significant dependence of the χ values on molecular weight, warn that the Flory–Huggins approach is unlikely to be an accurate quantitative predictor of phase behaviour for systems of the type discussed. The results were encouraging enough, however, to suggest that semi-quantitative conclusions about the influence of molecular factors on the positions of binodals, tie lines and critical points, and on the slopes of tie lines, were likely to be valid for mixed uncharged ternary systems.

Electroosmotic mobility and aggregation phenomena of model polymers with permanent cationic groups

Capillary electrophoresis (CE) was applied to polyelectrolytes containing permanent cationic groups. The electrophoretic mobility (EM) of polycarboxybetaines at different pH strongly depends on the chemical environment of the cation. This promotes a pH dependent intra- and intermolecular aggregation. Using uncharged polymers as sieving media, the order of hydrodynamic radii superimposes the effect of number and location of the net charges on the overall mobility. EM of cationic polyelectrolytes is mainly determined by structural factors and substituent effects at the nitrogen.

Titration microcalorimetry of poly(vinylpyrrolidone) and sodium dodecylbenzenesulphonate in aqueous solutions

Microcalorimetric titrations are carried out on solutions containing the anionic surfactant sodium dodecylbenzenesulphonate (SDBS), and mixtures of SDBS and the uncharged polymer poly(vinylpyrrolidone) (PVP). Measurements are taken at different temperatures. Micellisation of SDBS is driven by hydrophobic bonding. The interaction enthalpy of mixed PVP/SDBS systems shows clearly a consecutive endothermic and exothermic region with increasing surfactant concentration. The endothermic part can be looked upon as an incremental binding isotherm and reflects the number of surfactant molecules involved in the association process. The exothermic region features inverse hydrophobic bonding behaviour. This is related to the flexible nature of the adsorbent, i.e. the polymer. Electrostatic repulsion between neighbouring surfactant molecules causes at increased surfactant concentrations structural rearrangements of the polymer-surfactant complexes. This is accompanied by losing inter- and intrachain linking and entropy gain since the expanded complexes can move more freely. Additional surfactants continue to adsorb on the vacant hydrophobic adsorption sites. The influence of the initial amount of polymer and the electrolyte concentration support our proposals.

Non-Equilibrium Adsorbed Polymer Layers via Hydrogen Bonding

The layer-by-layer or self-assembly (SA) technique, based on the electrostatic interaction between oppositely charged layers, provides a simple and elegant way of producing multilayer structures from a variety of materials. Multilayers are normally employed because one layer is usually too thin for any device application. We show that considerably thicker layers may be obtained if the adsorption process is non-self-limiting, unlike the case of mechanisms entirely based on electrostatic attraction. This was obtained in spontaneously adsorbed layers of polyaniline (PANi), poly(o-methoxyaniline) (POMA) and tetrahydrothiophenium polyphenylenevinylidene, a PPV precursor. The non-self-limiting adsorption is attributed to H-bonding which is more effective for uncharged polymers. It is also shown that the adsorption process is only non-self-limiting under appropriate experimental conditions, since adsorption must be performed under a non-equilibrium condition. Producing thicker layers via the non-self-limiting adsorption procedure may have important implications in device applications, but for that one needs to overcome the drawback caused by aggregation that affects film quality. For PANi and POMA, we have also demonstrated that multilayers can still be produced from uncharged polymers, simply by protonating them in an acid solution, and then alternating one of them with a polyanion.

Interactions between anionically modified hydroxyethyl cellulose and cationic surfactants

AbstractMacroscopic properties of aqueous solutions of several modified hydroxyethyl cellulose (HEC) samples and their interactions with cationic surfactants are studied by solubility, light scattering, electric birefringence, rheology, and surface tension measurements. Modified HEC samples carry anionic groups (an‐HEC D0) and anionic and hydrophobic groups in random distribution (HM‐an‐HEC D1–D4). The molar substitution of anionic (an) groups is about 0.07 in all samples while that of the hydrophobic (HM) groups ranges from 0 in an‐HEC D0 to 0.012 in HM‐an‐HEC D4. In a 1 wt% solution this corresponds to 2.7 mM anionic and 0 to 0.46 mM hydrophobic groups. In the dilute concentration range the polymers behave like typical polyelectrolytes whereas in the semi‐dilute range they resemble uncharged polymers. On addition of oppositely charged surfactants the phase behavior of all polyelectrolytes is similar. With increasing surfactant concentrations the transparent solutions become turbid and the phases separate. Finally, resolubilization takes place with excess surfactant concentrations. With the HM‐an‐HEC compounds viscoelastic solutions are formed with cationic surfactants. The intermolecular interaction between hydrophobic parts of the polymers and the surfactants and interactions of oppositely charged ionic groups of the two components lead to formation of a temporary network with gel‐like properties. With an‐HEC the interaction can only take place via charges. Viscosity enhancement with increasing surfactant concentration is therefore lower with an‐HEC than with HM‐an‐HEC compounds.

Viscometric study on the compatibility of polymer–polymer mixtures in solution

Abstract The viscosity behaviour of mixtures formed by two uncharged polymers in dilute solution has been studied at 25°C. The ternary systems assayed, and denoted solvent (1)/ polymer (2)/ polymer (3), have in common the poly(ether sulphone) (PES) as polymer 2, and poly(vinylidene fluoride) (PVDF), poly(methyl methacrylate) (PMMA) or poly(styrene) (PS) as polymer 3. The intrinsic viscosity and the viscometric interaction parameters have been experimentally measured for the binary (solvent/polymer) as well as for the ternary systems, and also theoretically evaluated for the latter. The estimation of the compatibility degree of the above polymer pairs have been done by means of three criteria: (i) the sign of Δbm, according to the traditional formalism developed by Krigbaum and Wall; (ii) the sign of a new defined Δb′m; and (iii) the sign of Δ[η]m. The two last criteria are proposed in this work and are based on treating the viscosity interaction parameter and the intrinsic viscosity as excess properties by similarity with those of real solutions. Both methods provide compatibility predictions in agreement with those made with the traditional one but they are much more simple in handling the calculations. Finally, the observed compatibility behaviour follows the order: (PES+PMMA)≫(PES+PVDF)>(PES+PS), in dimethylformamide as solvent, at least in the assayed composition range.

Titration Calorimetric Study of the Interaction between Ionic Surfactants and Uncharged Polymers in Aqueous Solution

The interaction between uncharged polymers (PEO, PPO, PVP, and EHEC) and ionic surfactants in dilute aqueous solution has been studied using isothermal titration microcalorimetry. The surfactants studied were lithium, sodium, and magnesium dodecyl sulfate, alkyltrimethylammonium halides (RTA+X- with R equal to C12, C14, and C16 and X either Br or Cl), and dodecylammonium chloride (DoAC). The critical aggregation concentration cac, the saturation concentration C2 and the amount of polymer-bound surfactant were derived from the calorimetric titration curves. The anionic surfactants interacted strongly with PEO while RTABr were indifferent. However, changing the counterion to chloride or modifying the headgroup to the unsubstituted ammonium ion gave observable interaction. The presence of extra salt lowered the cac and increased the C2 with a more pronounced effect for PPO and EHEC. An increase in temperature had no noticeable effect on cac and C2 although the shape of the calorimetric titration curves changed drastically. PPO 1000 showed both small molecule behavior and polymer character in the interaction with SDS. Due to the hydrophobicity of the backbone, PVP showed significant interaction with SDS well below the cac. PVP showed no affinity to the cationic RTAX surfactants.

Structure and Properties of Fluids Composed of Polyelectrolyte and Ionic Surfactant in the Organic Phase: Poly(allylamine) and Sodium Bis(2-ethylhexyl) Sulfosuccinate

The structure and properties of an oil-rich microemulsion containing the cationic polyelectrolyte poly(allylamine) hydrochloride, PAAN, were investigated. An anionic surfactant, sodium bis(2-ethylhexyl) sulfosuccinate AOT, was selected. The patterns of the small-angle X-ray scattering SAXS were drastically changed from those of the polymer-free AOT microemulsions. Electric conductivity and the SAXS patterns indicated the formation of the pearl-necklace structure in the polymer-containing microemulsion. Two kinds of the characteristic size, R1 and R2, were evaluated from the SAXS patterns. R1 was the droplets (pearl) size that was essentially controlled by water-to-surfactant mole ratio W0. R2 was the size of the polymer chain connecting the droplets. R2 of the salt-free system was near the end-to-end distance of an uncharged polymer chain whose degree of polymerization was the same as that of used PAAN. PAAN behaved like an uncharged polymer due to the strong electrostatic interaction with AOT. This was supported by the dynamic light scattering experiment. R2 was affected by the salt concentration in the aqueous solution, which was attributed to the change in the strength of the electrostatic interaction between AOT and PAAN. The AOT aggregates retained the original form even in the presence of the polymer strongly interacting with AOT.

A hydrophobic interaction site for lysozyme binding to polyethylene glycol and model contact lens polymers

Proton nuclear magnetic resonance ( 1H-NMR) spectroscopy is used to identify a preferred binding site for uncharged hydrophilic polymers on the surface of hen egg-white lysozyme. Chemical shift titrations show that exchangeable proton signals from amino acids Arg-61, Trp-62, Trp-63, Arg-73, Lys-96 and Asp-101 are selectively perturbed upon binding of poly(ethylene oxide), poly(ethylene glycol) and poly(ethylene-co-propylene oxide). The greatest binding-induced chemical shift changes are observed for Trp-62, Arg-61 and Arg-73 at the edge of the active site cleft of the protein, consistent with a predominantly hydrophobic interaction mode involving the polymer ethylene moieties. The more hydrophilic species poly(dihydroxypropyl methacrylate) causes similar but substantially smaller chemical shift effects than the other polymers, confirming the nature of the interaction. A dissociation constant of 76±5 m m is determined for the poly(ethylene glycol)–lysozyme complex. The relatively low affinity of the protein–polymer interactions compared to oligosaccharide substrate binding suggests that lysozyme activity is minimally affected by these materials.

Microviscosity in dilute aqueous solutions of SDS and non-ionic cellulose derivatives of different hydrophobicity: fluorescence probe investigations

The microviscosity in mixed micelles formed in dilute aqueous solutions of sodium dodecyl sulphate (SDS) and a set of non-ionic cellulose ethers of different hydrophobicity has been determined by means of steady-state fluorescence probe techniques. Two hydrophobic probes have been applied in this investigation: 1,3-di(1-pyrenyl)propane (P3P) and perylene. Reference measurements of microviscosity have also been performed on SDS solutions including the uncharged polymers poly(ethyleneoxide) (PEO) or poly(vinylpyrrolidone) (PVP). All compositions investigated showed qualitatively the same general behaviour with an abrupt increase in microviscosity at the critical surfactant concentration where the polymer-surfactant interaction starts ( c 1) followed by a maximum and an asymptotically declining region as the surfactant concentration was increased further. Comparison with a recent investigation of a specific ethyl(hydroxyethyl)cellulose (EHEC fraction CST-103)/ SDS/water system (Evertsson & Nilsson (1997) Macromolecules, 30, 2377) revealed that the maximum in microviscosity generally corresponds to a low degree of SDS adsorption (≈ 0.5 mmol of SDS per gram of polymer) and consequently to a high polymer content of the mixed micelles formed in the type of systems studied herein. The hydrophobicity of the cellulose derivatives was found to correlate to the amplitude of the overall microviscosity pattern for the mixed micelles, i.e. an increased polymer hydrophobicity gave an increased rigidity of the polymersurfactant aggregates. An approximately exponential relation was demonstrated between the maxima in microviscosity of the different mixed micelles and the surface activities of the corresponding cellulose derivatives. All polymer/surfactant combinations investigated gave aggregates with a higher rigidity than ordinary SDS micelles. The microviscosity of the mixed micelles of the cellulose derivatives and SDS formed close to c 1 increased as the temperature rose from 20 to 50 °C. This effect was attributed to an increased hydrophobicity of the cellulose ethers upon temperature elevation, hence giving rise to further close-packing of the aggregate structures.

Stabilization of small unilamellar DMPC-liposomes by uncharged polymers

Polymer-free and polymer-bearing small unilamellar (SUV) liposomes from dimyristoyl-phosphatidylcholine (DMPC) were prepared under standardized conditions. Polymer-bearing liposomes were formed by incorporating an uncharged polymer [hydrolyzed poly(vinyl alcohol) (PVA), poly(vinyl alcohol-co-vinylacetal) (PVA-Al), poly(vinyl alcohol-co-vinyl propional) (PVA-Prol) poly(vinyl alcohol-co-vinyl butiral) (PVA-Bul) copolymer or poly(vinyl pyrrolidone) (PVP)] into the membrane bilayer of vesicles. The kinetic (long-term) stability of the liposome dispersions stored in distilled water, in physiological NaCl solution and at various pH values, respectively, were studied. The physical stability of vesicles was tested by measuring the size and the zeta potential of liposomes by means of a Malvern Zetasizer 4 apparatus. It was shown that most of these polymers are effective steric stabilizers for the DMPC-liposomes. Among the polymers, the PVA-Bul and PVA-Prol copolymers and the PVP of high molecular mass exhibited the most efficient stabilizing effect at each pH studied, indicating that the formation of a relatively thick polymer layer around the lipid bilayers ensures an enhanced and prolonged physical stability of liposomes. Also, the butiral or propional side chain in the PVA-based copolymers presumably promotes the anchoring of macromolecules to the vesicles. Using these macromolecules, the colloidal interactions between vesicles can be modified and so the physical stability of liposomes and the kinetic stability of liposome dispersions can also be controlled.

Controls on polyacrylamide adsorption to quartz, kaolinite, and feldspar

Potentiometric titrations of quartz, kaolinite, feldspar, and partially hydrolysed polyacrylamide (HPAM), and sorption measurements of HPAM on the minerals, allows identification of the general mechanisms of polyacrylamide adsorption to aluminosilicates and quartz. Adsorption was monitored at the mineral solution interface by way of X-ray photoelectron spectroscopy (XPS). XPS spectra of the unreacted minerals show bands in the Ols, Si2p, Al2p, and Cls regions. Additional peaks are observed in the C1s and N1s regions after treatment with polyacrylamide and the latter is used in this study to monitor polymer adsorption. N1s peak intensities increase with polymer concentration to a maximum corresponding to surface site saturation. At a fixed polymer concentration, adsorption varies with pH-dependent surface charge. The adsorption mechanism changes with pH, reflecting variation in the pH-dependent concentrations of ionizable groups on polyacrylamide and at aluminosilicate surfaces, and the extent of hydrogen-bonding between uncharged mineral surface sites and polymer amide groups.

Adsorption of Polyelectrolyte Multilayers on Polymer Surfaces

Different from the conventional use of charged supports, the assembly of thin coatings by alternate adsorption of oppositely charged polyelectrolytes was realized on a variety of uncharged standard polymers, such as poly(propylene), poly(styrene), poly(methyl methacrylate), and poly(ethylene terephthalate). For the multilayer buildup, the polyelectrolyte pair poly(styrenesulfonate)/poly(choline methacrylate) was used. The quality of the coatings was investigated by UV/vis spectroscopy, time-of-flight secondary ion mass spectrometry (ToF-SIMS), and X-ray photoelectron spectroscopy (XPS). The multilayer deposition on poly(propylene) (PP) and poly(ethylene terephthalate) (PET) was investigated in detail. On polar poly(ethylene terephthalate) supports, the regular growth of multilayer assemblies is evidenced. In contrast, on less polar supports, in particular on PP, the quality of the poly(styrenesulfonate)/poly(choline methacrylate) multilayers is inferior. However, if a hydrophobically modified poly(choline methacrylate) is employed instead, good quality multilayers are obtained even on PP. Thus, by appropriate choice of the polyelectrolytes used, even very hydrophobic and polar substrates become useful for the alternate adsorption technique.

Adsorption of polyvinylpyrrolidone on kaolinite

The adsorption of the uncharged polymer polyvinylpyrrolidone (PVP) with molecular weights from 5000 to 600 000 g mol −1 on the non-swelling clay mineral kaolinite has been studied. The effects of the adsorbent concentration, solution pH and PVP molecular weight on the binding isotherms were investigated. The varying affinity character of the binding isotherms with the kaolinite concentration can be related to some molecular weight fractioning of the synthetic polymer samples. An adsorption decrease of about 20% in the pH range 3.5–10.3 indicates that the protonation of kaolinite edge surfaces may also be involved in the adsorption process. At pH 5.5, the maximum amount adsorbed varies with PVP molecular weight ⩽44 000 g mol −1. At higher molecular weights, a levelling-off of the adsorption is found at 1.2 mg m −2. The electrokinetic thickness, δ e , of PVP layers on the kaolinite surface was determined by microelectrophoresis. At ionic strengths ⩽10 −4 M, δ e tends to a limit which can be put equal to the hydrodynamic layer thickness, δ h . The molecular weight dependence of δ h in the presence of a fixed PVP concentration and the adsorbed amount dependence of δ h for various PVP molecular weights are discussed. At high PVP concentrations for surface kaolinite saturation, it can be shown that the molecular weight dependence of the polymer expansion in solution does not change greatly after adsorption. Thus, at high surface coverage, a relatively large increase of δ h up to 100 nm with a PVP molecular weight of 600 000 g mol −1 can be related to a high fraction of segments in loops. On the other hand, at low PVP surface coverages, ⩽0.6 mg m −2, a fairly constant δ h of about 10–14 nm regardless of the PVP molecular weight is found. The build-up of a condensed PVP layer which results from the flattening of the polymer coil structure can be suggested.