Potential for Use of Liquid Crystals as Dynamically Tunable Electrophoretic Media

When two solutes have dissimilar structures, they move through an electrophoretic medium with different mobilities and, therefore, different speeds. This phenomenon is used to separate the charged components of a mixture. In a conventional electrophoretic separation, the velocities and diffusion coefficients of all solutes are constant because the transport properties of the medium are static, remaining unchanged over the length of the column and over the duration of separation. Here, we examine the question of how an electrophoretic separation might benefit from the use of a dynamic separation medium-one with the transport properties that can be tuned to values specified by the user. Liquid crystal (LC) polymers appear to be good candidates for tunable media, as their microstructure can be externally controlled. In particular, on application of a transverse external field, they undergo a transition from a configuration with randomly oriented side chains (‘‘random,” r ) to a configuration in which all side chains are aligned perpendicular to the axis of the column (‘‘aligned,” I ). There are then two principal values for the diffusion coefficient and migration velocity of a solute, viz., D,, ur, and D , , u , . D, is the diffusion coefficient in the random mode (m2/s), and D ~ is the diffusion coefficient in the aligned mode (m2/s). If the external field itself is made position-dependent, the diffusion coefficient of the solute can be modulated in space. Moreover, azobenzene-based LC polymers with response times as low as 200 ps have recently been synthesized (Ikeda and Tsutsumi, 19951, which suggests that it should also be feasible to modulate the solute velocities and diffusivities in time as well. This note summarizes a general theoretical study of electrophoretic separations in dynamic media. The emphasis is not a demonstration of the separation of specific solutes such as amino acid systems or DNA fragments, but rather a presentation of broad guidelines for solute separations in tunable media.