The external and internal environment of the cell is basically water, thus phospholipids organize themselves the way they do (bilayer). If the environment were to magically become mostly heptane, how would the phospholipid orientation change?
Phospholipids maintain their structure in the cell membrane because of the hydrophilic heads and the hydrophobic tails. Much unlike water, Heptane (CH3-CH2-CH2-CH2-CH2-CH2-CH3) is non polar and thus would not cause the heads to present themselves on the outer bilayer and the tails to tuck themselves away.
Because heptane is non polar, I wouldn't expect there to by any hydrogen-bonding going on, so I don't think that we would see an "inverted bilayer". I could be wrong though.
My thought - the cell membrane would cease to exist, I can't think of what would hold it together, but that seems a bit out there. Thoughts?