Dispersion of single-walled carbon nanotubes in water with the use of sodium dodecylsulfate allows the study of properties of individual nanotubes. Nanotube isolation is maintained in the dispersions by the electrostatic repulsion between the dodecylsulfate groups adsorbed on the nanotube surface. We present data showing changes in SWNT absorbance and emission spectra in response to titrating the dispersion with salt solutions. The results show an ability to manipulate surface charge density on the SWNTs through changes in surfactant equilibria and charge screening. The diameter dependence of these effects allows diameter-selective aggregation of SWNTs, with some specific cases of chirality-dependent stability being demonstrated.