why do lipids solve in 70% alcohol better than 95% alcohol?

Answer 1

Most lipids contain very long aliphatic hydrocarbons, which would really prefer the 95% alcohol, as well as very strongly charged phosphate groups, which would never dissolve in alcohol, 70% or 95%, by themselves. Since they're chemically linked 70% alcohol is a good "compromise" for them: the 70% alcohol dissolves the hydrocarbon part, and the 30% water dissolves the phosphate headgroups.

EDIT: In response to the two other answerers; Narkypoon: you're right, I was assuming he's talking about phopholipids, and as you said the same thing would be true for triglycerides, it would just be the polar head group, not a phosphate in that case. Bellorophon: while cholesterol would definitely prefer the 95% ethanol, I am not so sure that is true for triglycerides, and regardless the question asks "why" they dissolve better in 70% alcohol, not "if." Since the asker is asking to to explain why, not if, I answered the actual question.

Answer 2

Frankly, I am not sure that this is the case. I'd expect triglycerides and cholesterol to be more soluble in 95% . As for phospholipids, I am not sure -maybe they would be more soluble in 70% since having more water would favour the charged part of the phospholipid, but it is not so straightforward to predict the net effect on the solubility of the entire molecule.

If you are referring to using 70% alcohol for disinfection instead of 95% it is not because the lipids of the membrane are more soluble. The reason is that 95% ethanol causes too fast dehydration of the cell, blocking further entry of alcohol into the cell but also producing a dehydrated cell which under given conditions might be able to re-hydrate and revive.

Answer 3

Some Body: why have you assumed that asker is referring to phospholipids? These are predominantly found in the brain. Most body lipids are triglyceride type fats. That said your comparison of aliphatic chains being fat soluble & hydrophobic and the polar components (not just phosphate groups) being hydrophyllic is about right; 70% ethanol IS the compromise balance between the hydrophobic - hydrophyllic components.

Lipid BTW just means 'fat soluble'.