Liposome can be osmotically active or osmotically inactive. In osmotically active liposomes the lipid membrane is permeable and the osmosis will occur. This is not the case when liposomes are not osmotically active.The osmotic behavior of liposomes depend on their size, lamellarity and lipid composition.
Large unilamellar liposomes are osmotically active and therefore the osmolality of the solution that is encapsulated inside the large unilamellar liposomes should be equal to the buffer that is outside of the liposomes. If the liposomes are added to a medium for in vitro studies then the osmolality of the liposome solution should match the osmolality of the medium. If the liposomes are used for in vivo studies then the osmolality of liposomes should match the osmolality of blood.
Small unilamellar liposomes are NOT active osmotically and therefore there is not need to have iso-osmatic solutions inside and outside of liposomes.
Multilamellar phosphatidylcholine based liposomes that contain a small amount of charged lipids are osmotically active. Multilamellar phosphatidylcholine based liposomes that do not contain charged lipids are NOT osmotically active.
Very large oligolamellar liposomes with or without charged lipids are osmotically active.
Many liposome based fluorescent assays used 100 nm liposomes. Most of the liposome based injectable drugs are 100 nm in size. These are LUVs and therefore they are osmotically active.