We study the interfacial behavior between the straight-chain alkyl surfactant sodium dodecyl sulfate (SDS) and single-walled carbon nanotubes (SWNTs) as a function of added electrolytes, including NaCl. We observe an increase in photoluminescence intensity and narrowing of spectral line widths with electrolyte addition, indicating a change in SDS aggregation number that leads to a pronounced volume change in the nanotube/SDS composite structure. By tuning the interfacial dynamics through NaCl addition and temperature change, we demonstrate that this volume change can be used to yield diameter-dependent separation of metallic and semiconducting SWNTs, without the use of any additional cosurfactant, through density gradient ultracentrifugation. The diameter-dependent fractionation follows the intrinsic relation expected for the density of unfunctionalized nanotubes, indicating a simple amplification of these inherent density differences as the mechanism for salt enhanced separations. Isolation of enriched metallic and semiconducting fractions further illustrates that the surface aggregation characteristics of SDS on metallic SWNTs are different from that on the semiconducting chiralities. These experiments illustrate the governing behavior of surface phenomena and interfacial forces on the diameter-dependent fractionation of SWNTs and point to new routes for enhancing existing separations strategies.