Influence of the ionic strength on the structure of polyelectrolyte films at the solid/liquid interface

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Abstract

The structure of polyelectrolyte multilayers built up by alternate adsorption of polyanions and polycations is investigated by X-ray reflectivity at the solid/air and neutron reflectivity at the solid/liquid interface. The experiments provide detailed information about the density gradient of polyelectrolyte chains across the film and show the influence of the water content of the film on the internal structure. The polyelectrolyte density is determined by the adsorption conditions (e.g. amount of NaCl) and cannot be changed by addition of salt after adsorption. After drying the film thickness is reduced by 30%.

Introduction

Ultrathin films are a new kind of material and promise applications in different fields like integrated optics and biosensors. With regard to transport mechanisms polymer films might reveal interesting properties as the polymer network may entrap large molecules while being penetrable by small ones. Hence, it is very interesting to achieve detailed information on the internal structure of these films.

The films investigated here are polyelectrolyte multilayers. They are built up by alternating adsorption of anionic and cationic polyelectrolytes from aqueous solutions [1], [2]. SAXS and UV-Vis spectroscopy experiments provided first proof of the layer structure of such self assembly films. Neutron reflectivity measurements showed that the polyelectrolytes are deposited as layers with an interdigitation smaller than a single layer thickness and indicate that there is no distinct layer-by-layer separation between polyelectrolytes of opposite charges [3], [4], [5]. The driving force for the formation of such multilayers seems to be the electrostatic attraction between the oppositely charged polyelectrolytes. Measurements of the surface potential resulted in a change of sign in surface charge after each single adsorption step, i.e. each additional single polyelectrolyte layer [6]. A special feature of molecular films is that the macroscopic properties can be controlled by the microscopic structure. By adding salt to the aqueous solution the thickness of the self assembly films prepared from these solutions can be increased. Hence, the total film thickness can be controlled with Å precision. [7]. The differences in film thickness are explained by different conformations of the chains: Without salt the polyelectrolyte chains are oriented flat and parallel to the substrate, with higher salt concentration of the aqueous solutions the chains form coils [8], [9] which are then adsorbed at the interface. Because of the screening of the charges along the polyelectrolyte chains the polymer is more entangled with larger thickness. TIRF (Total Internal Reflection Fluorescence) kinetic experiments where the fluorescence of a fluorescein labelled polyelectrolyte is measured indicate an increasing amount of adsorbate with increasing salt concentration [10]. Measurements of the rhodamine transport through the polyelectrolyte films revealed a higher diffusion coefficient of rhodamine for films prepared without salt. These findings lead to the conclusion that the polyelectrolyte density of films prepared without salt is lower than the polyelectrolyte density of films prepared from solutions with salt additive [11]. The adsorption measurements were carried out at the solid/liquid interface. So far, all scattering experiments on this system to determine film thickness and polyelectrolyte density were conducted at the solid/air interface. It is supposed that the polyelectrolyte layers are very sensitive to the water content of the environment. Hence, it is necessary to conduct structural investigations at the solid/liquid interface to clarify the internal structure in situ and under the same conditions, which were applied during the transport measurements. Here, and to our knowledge for the first time, we report on such a structural investigation of these thin polymer films under the initial film assembly conditions by combined neutron and X-ray reflectivity.

Section snippets

Materials and film preparation

Poly(styrene sulfonate) sodium salt (PSS) and Poly(allylamine hydrochloride) (PAM) were obtained from Aldrich (Steinheim, Germany). PSS was dialysed against Milli-Q water and freeze-dried. PEI and PAH were used without further purification. D2O was obtained from Aldrich. The Silicon wafers (80×50×15 mm) were purchased from Holm Siliciumbearbeitung, Tann, Germany and cleaned by the H2O/H2O2/NH3 (5:1:1) step of the so-called RCA method [12]. The wafer served as the top of a home made flow cell.

Results

Fig. 2 shows the neutron reflectivity spectra of the four wafers before and after adsorption. All spectra show Kiessig oscillations [16], due to the interference of beams reflected from the film/substrate interface and the film/water interface, respectively. The Kiessig oscillations yield direct information on the total thickness of the film. No Bragg-Peak is observed, i.e. there is no pronounced density variation within the repeat units of the films. For clarity, in the figure the

Discussion

The structure of the polyelectrolyte multilayer is influenced decisively by the ionic strength of the polyelectrolyte solution during preparation. The observed change in layer thickness is attributed to the increasing screening of charges along the polyelectrolyte chain with increasing counterion concentration which induces an increasing coiling of the polyelectrolyte chains. The structure of the polyelectrolyte layers is determined by the equilibrium with the environment during adsorption. The

Conclusions

Neutron reflectivity studies are conducted on polyelectrolyte multilayers at the solid/liquid interface. The conformation of the solvent swollen films is determined by the initial adsorption conditions. The thickness increases with the amount of salt in the polyelectroyte solution from which the films were adsorbed as CNaCl. No change in film thickness was observed upon rinsing with NaCl solution after completion of the adsorption process. After drying the multilayer thickness is reduced by 30%

Acknowledgements

R. St. wishes to thank the Sonderforschungsbereich 335, Anisotrope Fluide, and R. K. wishes to acknowledge the Deutsche Forschungsgemeinschaft for financial support. The authors are indebted to Arndt Steinhäuser for technical assistance.

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