Absence of a vestigial vapor pressure paradox.

The enigmatic but much accepted vapor pressure paradox for oriented lipid bilayer samples was recently justified theoretically. Subsequently, recent experiments have shown that there is no vapor pressure paradox. The first result of this paper is to consider another degree of freedom that reverses the previous theoretical conclusion, so that theory and experiment are now in agreement that there is no vapor pressure paradox. However, this analysis also suggests the possibility of a vestigial vapor pressure paradox that would rationalize why the vapor pressure paradox was historically so persistent and that would have led to an improved protocol for obtaining bilayer structure. This vestigial vapor pressure paradox would involve a phase transition as a function of applied osmotic pressure. We test this possibility experimentally using combined neutron and x-ray scattering data. The conclusion from these experiments is that there is not even a vestigial vapor pressure paradox. However, this negative result validates an improved method for calibrating osmotic pressure in x-ray studies of oriented samples.