Here we report on the temperature dependence of the density and refractive index of alkyl-sulfate room temperature ionic liquids (RTIL): 1-ethyl-3-methylimidazolium n-alkyl-sulfate, [C2mim][C nOSO3] (n = 2, 4, 6, 8). Moreover an X-ray scattering structural study on the salts' morphology is presented, highlighting the existence of nano-scale structural heterogeneities, whose size depends linearly on the anion alkyl chain length. This behaviour was observed in other RTILs and is related to nano-scale alkyl chains' segregation into a charged network. This study shows differences between alkyl-sulfate salts and other RTILs, providing new indication for further tuning the RTILs' properties by wise modification of chemical architecture. © 2010 Elsevier B.V. All rights reserved.