I don't think you loose alochol during mixing. The problem is that e.g. 1 L water + 1 L alcohol gives you a final volume less than 2 L, because the alcohol (or water) molecules sit in between the water (or alcohol) molecules.

Just as Frank said, the volumes of ethanol and water are not additive. Their densities change when mixed. 

To prevent evaporation, simply cap the container you are storing your solutions in (or prepare them fresh each time).

Thank you all for your replies. Actually I want to do an in vitro study, in which i want to study the effect of alcohol on different cell lines. So Mr. Frank Van, when I mix the alcohol with DMEM media, alcohol will be there in the media? I asked because, on addition alcohol to the cells I expected cell death. But even at high concentration cells were alive. So I thought it would be due to the evaporation of alcohol.

Preparation and storage do  in cold environment so that you can avoid evaporation 

Furthermore, when I prepared 75% alcohol (final volume 2 mL) I added 1.5 mL and 0.5 DMEM media and added to the cells. After 24 hours of incubation, when I discarded the mixture, only approximately 0.5 mL were left. That's what I thought alcohol is getting evaporated. Because after adding alcohol I have to incubate them at 37 degree Celsius.

What is your condition? If you did your experiment in a open Petri dish, evaporation always happens not only for ethanol but also for water, and you'll lost your solvent. The question must be how much ethanol's evaporated, and this evaporation has any effect on your experiment!

Yes, I do the experiment in culture flask. Concentration of ethanol is very much is critical.

I would suggest you to design  blank experiments with desired concentration  of  ethanol and determine  ethanol concentration by standard methods over a period of  time.

I think you can change the solution in your flask after determined time!