I agree with Francois, even though I would call it lipid deposition rather than crystallisation. 

This may be due to a rapid evaporation (brutal will definitely lead to crystal-like structures as suggested by Francois) or a slightly-higher-than-normal temperature of the glass at the very beginning of the evaporation process. This would cause a local rapid evaporation of the solvent and a deposition of lipid onto the glass.

Last but not least, it may be cholesterol in excess. I found that using high %mol of Chol sometimes gives me this "weird" residue. Bear in mind that, even if you use the same formulation, the lipid film may differ greatly from batch to batch, so the ability to incorporate Chol.

Great thanks for your answers, what speed of rotation and vacuum do you suggest? About the molar ratio, it is 4/1 (pc/chol).

You have to find the best experimental setup by yourself, I am afraid. If you use Chloroform/Methanol as organic solvents, I would say you don't need to warm up the water bath and go deep vacuum. The other suggestion, even though I'm not a fan of rotary evaporation and I may be wrong, don't go full speed and get the flask/tube just touching the water surface if you really want to warm up the bath.

If you are working small scale, I always advise to make the film by hand using a nitrogen or argon stream. It may take a bit longer, but gives you full control and full feedback on what's going on in the tube.

Dear Favretto,

Your comments were really useful. Once again I thank you for providing valuable suggestions.

Hi. for your work you have to prepare 70/30(span,tween/cholestrol),60/40,50/50,20/80 solution (solvent=chlorform) and choise best of them. time 15-20 min and speed 180 rpm.

good luck

Totally agree with Marco;drying under nitrogen will give you a nice film..especially when handling low quantities.

Dear Mahmood and Anilda,

Great thanks for your useful suggestions.

What kind of phospholipids are you working with? Are they unsaturared, saturated, does the sample contain cholesterol? A fast evaporation is not convenient to obtain a correct film on the glass wall, particularly if you work with  phospholipids with high a temperature transition. Saturated phospholipid/cholesterol mixtures are specially difficult at a high cholestereol (CH) to phospholipid (Ph) ratios. In these cases we used to work with a non usual  mixture of chloroform and methanol. Instead of the classical 2:1 v/v ratio we increased the methanol content to 1:1 v/v ratio. With a CH/Ph 4:6 mol/mol ratio we had also to slightly (and carefully) warm the sample during evaporation to obtain a homogeneous film.

Dear Barnadas-Rodríguez,

Thanks for your suggestions. The utilized PC are unsaturated and  formulations contain cholesterol too.

Dear Shiva Emami,

Then, and depending on the cholesterol to phospholipid ratio, the white residue could be cholesterol. We also worked with unsaturated PC and CHOL but at the low ratio employed we did not find any problem.

Regards,

Ramon

Dear Ramon,

Great thanks for your help and suggestions.