Electrophoretic mobility of weakly-charged (dipolar) hydrogels in water: Contribution of hydrogen-bonding in the solvent dipole layer

• Uncharged and Weakly-charged nano-hydrogels have negative electrophoretic mobility.
• H-bonding in the dipole-layer is hypothesized to be responsible for these effects.
• Introduction of glycine to test this hypothesis leads to sharpened mobility distributions.
• The latter effect is related to Kohlrausch-boundary effects.
• Alternative theoretical treatments are presented.

Counterintuitive observations by dynamic light-scattering experiments of negative electrophoretic mobility in uncharged, lightly charged, and later, densely-charged hydrogel nanoparticles are presented. A tentative theory, emphasizing the roles of electric field energy density and induced dipole moments in the dipolar and hydrogen-bonding solvent layer surrounding the particle, is introduced to explain and rationalize these observations. Addition of co-solvent glycine seems to produce a Kohlrausch boundary regulating effect which again illustrates the importance of the dipole layer and hydrogen bonds within it. Further alternative theories involving electric field gradients are discussed which may be relevant to other uncharged systems (such as gold nanoparticles). A contribution to the dipolar solvent-induced mobility is derived in Appendix A. A proposal for a new treatment of traditional (i.e. charged colloid particle) electrokinetic phenomena is given in a second Appendix (Appendix B).

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