(397z) Interactive Forces Between SDS-Suspended Single-Wall Carbon Nanotubes and Agarose Gels

The separation of metallic and semiconducting fractions of as-prepared single-walled carbon nanotubes (SWCNT) is of great interest for their inclusion in a wide variety of devices. A commonly used method to produce large-scale separations of the two fractions is based on the flow of SWCNTs through a column packed with agarose gel. Prior studies have attributed the mechanism of separation to the selective adsorption of semiconducting SWCNTs onto the agarose gels. However, few details of the forces driving this selective adsorption are understood. Here we probe the driving forces that occur during the interaction of sodium dodecyl sulfate (SDS) suspended SWCNT with agarose gels using a combination of non-equilibrium and equilibrium studies. By altering the agarose backbone, the role of ionic, purely hydrophobic interaction, and predominantly π –π interactions on the selective adsorption could be examined. The results indicate that the surfactant orientation on the SWCNT surface is an important parameter during separation and that the primary mode of attraction is ion-dipole interactions between SDS-SWCNTs and hydroxyl groups on the agarose gel backbone. The selectivity of adsorption is then driven by the inherent differences in polarizability between the metallic and semiconducting species.  Furthermore, we report that simple Langmuir models for the adsorption behavior of SWCNTs onto agarose gels is not appropriate for all systems despite persistent use in the literature.  We recommend that further attempts to understand the entropic and enthalpic contributions to the free energy of these systems be measured with a series of calorimetric studies.