Liquid-like ordered colloidal suspensions of lipid A: The influence of lipid A particle concentration

Abstract

Electrostatically stabilized aqueous dispersions of nm-sized free lipid A particles at low volume fractions (1.0×10−4⩽∅⩽3.5×10−4) in the presence of 1.0–10.0 mM NaCl (25 °C) have been characterized by static and quasielastic light scattering (QELS) techniques, electron microscopy (SEM and TEM), conductivity measurements, and acid–base titrations. QELS and electron microscopy (ρ̄TEM=8.0±0.6%) yield similar values for the particle size and particle size distribution (ρ̄QELS=10.9±0.75 %), whereas conductivity and acid–base titrations estimate surface chemical parameters (dissociation constant, ionizable sites, and Stern capacitance). Effective particle charges were determined by fits of the integral equation calculations of the polydisperse static structure factor, S̄(Q), to the light scattering data. Using the particle properties as determined from these experiments, the polydisperse structure factor, S̄(Q), was calculated as a function of volume fraction, ∅, which was found to be consistent with a S̄(Q) dependence on the number particle density. It can be concluded that, at low volume fractions and low ionic strength, the light scattering data are well represented by a Poisson–Boltzmann model (PBC) of fluid-like ordering of free lipid A in aqueous solution. We find that the light scattering data of this dispersion are best described by a model where only a small fraction of the ionizable phosphate groups is dissociated at neutral pH. Finally, light scattering studies of lipid A dispersions of volume fractions of 3.9×10−4⩽∅⩽4.9×10−4 indicate the presence of long-range order, resulting in distinct peaks which can be assigned either to a face-centered cubic (fcc) lattice (a=51.7 nm) or a body-centered cubic (bcc) lattice (a=41.5 nm), respectively.