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By using the former equation (8), we modified the equation which can show the dissimilar osmotic behavior of liposome with composition change. The slope of the new equation was presented as the ratio of osmotically active volume (Vact) to the total volume(Vtotal=Vact + Vdead; Vdead is osmotically inactive volume) of liposomes. We defined it as a Z-value, which can elucidate the dissimilarity of the osmotic activity of multilamellar liposomes with the change of phospholipid composition and the differences of physico-chemical properties of liposomes. Z-value was applied for studying the physico-chemical properties of liposomal membrane. The factor that affects on the Z-value was not the lipid concentration of liposome stock dispersion but the lipid composition of liposomal membrane. As the content of dicetylphosphate, the negative charged phospholipid, was increased, the osmotic activity, represented by Z-value, of multilamellar liposome was decreased. Under the hypertonic conditions (shrinking region), Z-value steadily increased and reached a maximum at 10mole percent cholesterol with increasing the cholesterol content.
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