Selection of hydration medium depends on various factors. It is advised to use normal saline instead of DW, as the former will provide the desired osmolarity and will also prevent the disruption of vesicles during incorporation in any other secondary vehicle. However, buffers are generally employed either to stabilize the drug or to meet the non-irritancy criteria to the site of administration. The hydration medium also imparts substantial effect on vesicle size and drug loading of vesicles. It is advised to perform initial screening of the hydration medium to achieve the optimal vesicular dispersion.
There is no cut point difference in niosomes and liposomes on the basis of core material and, interestingly, both show great flexibility of core forming medium. You will find liposomes with aqueous core, alcohol, aqueous+alcohol and many other medias. You can just have a look at following review articles: