Phase transformations in lipid A-diphosphate initiated by sodium hydroxide

The nature of the fluid phase transitions of charged-stabilized spherical lipid A-diphosphate clusters in aqueous dispersions was explored using a combination of small-angle X-ray scattering (SAXS) and electron microscopy. In contrast to previous studies, rather than removing NaCl, NaOH was added to the dispersions to promote crystallization. The fluid phase experiments were carried out employing titrations with mM·L(-1) NaOH (c(S)), along with variations in the particle-number density, n. When c(S) was increased, a new fluid disordered phase of self-assembled lipid A-diphosphate was encountered, followed by a crystalline bcc phase and then a new fluid phase containing 70 nm lipid A-diphosphate particles. The bcc crystal structure found in this regime had a lattice constant of 35.6 nm. By varying c(S) (mM L(-1)), it was possible to determine the effective charge, z(eff), for various n values and the screening parameter, k, for the excess electrolyte. For sufficiently large values of n, lipid A-diphosphate crystallized because of an increase in z(eff) at a constant c(S). When the c(S) was increased, the crystals melted with little change in z(eff). The existence of a bcc-fluid phase transition for different values of c(S) was supported by applying the Debye correlation function to the obtained data. An increase in c(S) enhanced interparticle interaction and attraction. The effective charge and k accounted for counterion condensation and many-body effects. If the effective charge determined from scattering measurements was used in the simulations, the equilibrium phase boundaries were consistent with predicted universal melting-line simulations. At any particle-number densities, n, when the melting line was reached, 70 nm clusters were formed.