  We introduce a microscopic model of a lipid with a charged headgroup and
flexible hydrophobic tails, a neutral solvent, and counter ions. Short-ranged
interactions between hydrophilic and hydrophobic moieties are included as are
the Coulomb interactions between charges. Further, we include a short-ranged
interaction between charges and neutral solvent, which mimics the short-ranged,
thermally averaged interaction between charges and water dipoles. We show that
the model of the uncharged lipid displays the usual lyotropic phases as a
function of the relative volume fraction of the headgroup. Choosing model
parameters appropriate to dioleoylphosphatidylethanolamine in water, we obtain
phase behavior which agrees well with experiment. Finally we choose a solvent
concentration and temperature at which the uncharged lipid exhibits an inverted
hexagonal phase and turn on the headgroup charge. The lipid system makes a
transition from the inverted hexagonal to the lamellar phase which is related
to the increased waters of hydration correlated with the increased headgroup
charge via the charge-solvent interaction. The polymorphism displayed upon
variation of pH mimics that of the behavior of phosphatidylserine.
