pH 7.4 buffer. because pure water once release in blood may cause RBC hemolysis due to pH shock.

Osmotic pressure is the answer. 

Even though you can formulate them in water, as soon as you transfer them to any medium different from water the liposomes will experience an osmotic shock. This leads to different phenoma, but to make it simple the quality of the vesicles is lost.

in ph 7.4 the encapsulation of niosome is beter and for blood body ph in human .

to maintain the osmotic balance, it must be prepared in buffer (depending on end use) otherwise stability will be the issue, either it will burst out or shrink. 

this question has correctly been answered by colleagues above.

I agree with all this answers