Zwitterion Gradients Drive Colloidal Migration

Diffusiophoresis (DP), a physical phenomenon wherein solute gradients drive the migration of suspended colloids, holds significant relevance in the microscale engineering of complex, nonequilibrium colloidal systems. While DP has been extensively studied under electrolyte and nonelectrolyte gradients, how gradients of other solute classes can potentially drive DP migration remains largely unexplored. In particular, zwitterions like amino acids are electrically neutral, like nonelectrolytes, yet carry charges like electrolytes. Here we demonstrate that zwitterion gradients drive DP migration, and develop a simple theory that successfully predicts DP velocities under a series of amino acids. This theory predicts colloidal migration to proceed up concentration gradients of zwitterions, in proportion to the dielectric increment of the zwitterion. Like for electrolyte gradients, the DP mobility depends on the particle zeta potential, yet DP velocity is proportional to the gradient (like nonelectrolytes), rather than the logarithmic gradient expected for electrolytes. Direct measurements of colloidal DP under gradients of three zwitterionic amino acids with decreasing dielectric increments (6-aminohexanoic acid, 4-aminobutyric acid, and glycine), agree quantitatively with the theory. This work establishes the unique characteristics of zwitterion-driven DP, opening avenues for its diverse applications, particularly in the domain of microscale colloidal transport.