Diffusiophoresis in Zwitterionic Gradients

Diffusiophoresis (DP) has seen increasing attention for its ability to control and manipulate suspended objects using external solute gradients. While most work has focused on DP under electrolyte gradients, different types of solutes have seen much less attention.  Here we present theoretical and experimental studies of particle DP under gradients of zwitterions, i.e., electrical dipoles.  We develop a theory for zwitterion DP that predicts that colloids will always migrate up zwitterion gradients, but with velocities that scale linearly with the concentration gradient, in contrast to the logarithmic gradient scaling of conventional electrolyte DP.  Moreover, DP mobilities scale with the square of the zwitterion dipole moment – which depends on the distance between the charged ends of the molecule.

Experiments confirm the theory both qualitatively and semi-quantitatively. Using a microfluidic geometry we developed to impose truly steady state gradients, we can make direct and repeatable DP velocity measurements for hours at a time, thereby obtaining excellent statistics. We performed experiments under gradients of three distinct zwitterions with varying intercharge distances: Glycine, 4-aminobutyric acid (4-ABA) and 6-aminohexanoic acid (6-AHA). In all cases, colloidal particles migrated up the steady-state gradients with constant velocities, validating the linear dependence of DP on the imposed zwitterionic gradient. Also, DP velocities increased with increasing intercharge distance, being highest for 6-AHA, followed by 4-ABA and Glycine, as predicted by the theory. Furthermore, the intercharge distances of the zwitterionic molecules interpreted from experimentally-measured DP mobilities are in relatively good agreement with literature values, further corroborating our theory. Our results elucidate a previously unexplored phenomenon that opens new avenues for DP to be employed.