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Adsorbed layers of polyelectrolyte are important in a number of energy-related applications. For example, polyelectrolyte films have been proposed as solid electrolytes in battery and fuel cell scenarios, where they offer excellent uniformity and low resistivity, and polyelectrolyte membranes are promising in certain gas phase separations of importance to energy production, such as oxygen/nitrogen (for coal burning) and hydrogen (for fuel cells). Substrate electric potential can strongly influence the adsorption process, and thus is an important control variable in engineering polyelectrolyte films of tailored properties. In recent experiments, we have uncovered conditions where polyelectrolyte adsorption to a conducting substrate at fixed electric potential may become continuous in the sense of scaling linearly with time over hours without any apparent saturation. Continuous polyelectrolyte adsorption under an applied potential offers great opportunities (single component polyelectrolyte films of controlled mass realized in a single step), but raises fundamental questions, such as how interfacial charge can accumulate without saturation. We seek here to answer these questions through molecular simulation of polyelectrolyte adsorption at a conducting interface.

The experimental results [1,2] suggest three important features to contribute to continuous polyelectrolyte adsorption: 1) dielectric discontinuity (i.e. the difference in dielectric constant between solution and adsorbing substrate), 2) attractive short range and ionic Coulombic interactions (as, e.g., from hydrogen bonding, van der Waals interactions, or ion correlations), and 3) charge regulation (i.e. polymer charge variation with local environment). We seek a model representation in order to probe the role of each of these factors in the adsorption process.

Model

The polyelectrolyte adsorption system consists of polyelectrolytes, the surface (and the corresponding dielectric discontinuity), and explicit ions. Polyelectrolytes are modeled as tangent hard spheres that may either be charged (q=+1) or neutral (q=0) with the charge state depending on system pH. The system contains small ions (q=-1 or q=+1) and a dielectric discontinuity corresponding to a metallic surface. Each polyelectrolyte unit (sphere) and the ions have a diameter corresponding to one half of the Bjerrum length (1/2 of 0.71 nm). The adsorption behavior of this polyelectrolyte system is governed by the electrostatic energy of the system. We have implemented a Monte Carlo simulation software to follow this the conformational behavior of this polyelectrolyte / counter-ion / salt system.

Results

We have examined the ion-free constant charge system numerically by calculating electrostatic energy as a function of polyelectrolyte separation. By comparing to the thermal energy, we obtain an initial understanding of the region where the dielectric discontinuity may play a role in promoting interfacial polymer-polymer binding. We have also investigated the presence of free ions and their associated complex charge correlations via Monte Carlo simulation. The results show that the presence of the dielectric discontinuity influences the adsorption process of charged polyelectrolytes significantly at short range for finite length polyelectrolytes. The calculations show that the presence of the surface dominates the monomer and short chain polymer behavior already very far from the surface, unless the oligomers are very close to one another (polymer-polymer distance less than the Bjerrum length). For longer polyelectrolytes, that is, more than 10 units long, the polyelectrolytes must be very close, within one Bjerrum length of the surface, for the dielectric discontinuity to influence the behavior at ambient temperature.

The inclusion of the more complex charge correlations due to ions and charge regulation in the polymers extends spatially the region at which the polyelectrolytes are adsorped to the surface layer. We have probed this using the Monte Carlo software implemented for the project. Explicit charge correlations are important in the system: when the rods are apart, the counterions condense around the polyelectrolytes independent of the other polymer rod in the system but at close range, the counterions act like glue condensing between the polymers and reducing the polymer-polymer repulsion to the degree that surface adsorption may take place at much larger range. The Monte Carlo simulation results indicate that charge screening of polymers may enhance adsorption and bind polymers together even in the absence of the dielectric discontinuity. The observed energy landscape, and correspondingly the parameter region at which spontaneous adsorption can occur depend on polymer charge, as well as, ionic strength of the solution.

We are currently systematically probing the charge dependence of the system behavior to fully evaluate the contributions of dielectric discontinuity, attractive short range and ionic Coulombic interactions, and charge regulation. The resulting knowledge of how these factors together enable continuous adsorption may usher in new processes for developing polyelectrolyte-based thin films for energy applications.

Bibliography

[1] A. P. Ngankam and P. R. Van Tassel, Proc. Nac. Acad. Sci. 104, 1140-1145 (2007).

[2] C. Olsen and P. R. Van Tassel, J. Colloid and Interface Science 329, 222-227 (2009).