The Zeta Potential Of PMMA Surfaces In Contact With Electrolytes Of Various Conditions: Theoretical and Experimental Investigation

The miniaturization of fluidic systems has developed into versatile technologies such as Lab-on-a-Chip which aims at the integration of all tasks that are performed in a (bio-)chemical laboratory on a single microfluidic chip. Typical tasks comprise unit operations such as sample enrichment, mixing, temperature cycling, and species separation with subsequent detection. A multitude of these unit operations can be conveniently realized by using electrokinetic phenomena since they can be controlled by an external electrical field induced in the micro-structures. Generally, electrokinetic phenomena are related to the presence of electrical surface charges of substrates -e.g. micro-channel walls or dispersed particles- in contact with a liquid. The origin of the surface charges can be manifold and their impact on the electrokinetic phenomena is also determined by the liquid parameter. Since surface charges cannot be easily measured, the zeta potential is considered as a relevant parameter for electrokinetic phenomena instead.

Poly(methyl methacrylate) (PMMA) is an attractive microfluidic substrate since micron size features can be manufactured by employing cost effective and rapid techniques such as hot embossing. However, the surface charge mechanisms of PMMA surfaces with respect to acidic and alkaline environments are not very well understood and the data found in literature shows significant discrepancies. In this work, the zeta potential of PMMA surfaces with respect to ionic strength, pH, temperature and the nature of co- and counter-ions of aqueous electrolytes are investigated both theoretically and experimentally. In terms of theoretical investigation, we numerically solve the Poisson-Boltzmann equation for a cylindrical coordinate system to obtain surface potential gradients for various electrolyte conditions. The assumption of a solely diffusive double layer configuration (i.e the Gouy-Chapman model) implies an absence of surface reactions and that the zeta potential is influenced by pure shielding.  In case of low to moderate ionic strengths, the computational results predict a linear correlation with the logarithm of the ionic strength and a minor influence of temperature and electrolyte composition. Experimentally, we engage electrophoretic light scattering to infer zeta potentials of PMMA particles dispersed in various aqueous electrolytes. In detail, negative zeta potential values are observed at all conditions which imply negatively charged surfaces. It is observed that the magnitude of the zeta potential values decreases linearly as the ionic strength of the aqueous electrolytes increases. If we normalize the zeta potential values with the logarithm of the ionic strength, a collapse of the data to a single curve is observed. The pH dependency of the PMMA zeta potential values can be explained as two distinct regions. We find that in an acidic to neutral pH range a negligible zeta potential variation while significant pH-dependency can be observed for an alkaline milieu. Further to our study on temperature dependency of zeta potentials, we find that parameter-corrected viscosities and permittivities play a vital role for the correct interpretation of the results. Finally, we observe that the electrolyte’s counter-ions have more contribution in the determination of zeta potential in comparison with the electrolyte’s co-ions. The data are used to infer an empirical correlation which can be used for modeling of electrokinetic phenomena and to derive design guidelines for electrokinetic unit operations for microfluidic applications.