The tunable solvent properties of gas expanded liquids (GXLs) have been previously used for the fractionation and separation of polydispersed ligand-stabilized metal nanoparticles into distinct monodispersed fractions. This work employs CO2 expanded hexane for silver nanoparticle synthesis within an AOT reverse micelle system where the tunable GXL solvent properties are used to control the nanoparticle size and polydispersity. The objective of this project is to answer two questions: (1) can nanoparticles with narrow and well-defined size distributions be synthesized in GXLs? and (2) how do the solvent properties impact the resulting nanoparticle size? In the reverse micelle synthesis, the AOT surfactant provides a nano-scale aqueous micelle core for nanoparticle nucleation, as well as, acts as a nanoparticle stabilizing ligand. Increasing the CO2 partial pressure in a GXL impacts the surfactant-solvent interaction and results in the synthesis of different sized nanoparticles. At ambient pressures, the mean particle diameter synthesized was 6.1 +/- 2.1 nm with W=40 and 5.4 +/- 2.0 nm with W=20, where W is the molar ratio of water to AOT. At CO2 partial pressures of 6.9 and 13.8 bar, there was no significant change in particle size, but decreases in the size distributions were observed. At CO2 partial pressures ranging from 20.7 to 41.4 bar, steady decreases in the mean particle diameter and size distribution were observed with values of 4.0 +/- 0.8 for W=40 and 4.1 +/- 1.0 for W=20 at 41.4 bar. This demonstrates some degree of nanoparticle size tunability within the GXL solvent, where smaller particle diameters and size distributions are achieved at higher CO2 compositions. (C) 2013 Elsevier B.V. All rights reserved.