  In concentrated macromolecular dispersions, far-from-ideal intermolecular
interactions determine the dispersion behaviors including phase transition,
crystallization, and liquid-liquid phase separation. Here, we present a novel
versatile capillary-cell design for analytical
ultracentrifugation-sedimentation equilibrium (AUC-SE), ideal for studying
samples at high concentrations. Current setups for such studies are difficult
and unreliable to handle, leading to a low experimental success rate. The
design presented here is easy to use, robust, and reusable for samples in both
aqueous and organic solvents while requiring no special tools or chemical
modification of AUC cells. The key and unique feature is the fabrication of
liquid reservoirs directly on the bottom window of AUC cells, which can be
easily realized by laser ablation or mechanical drilling. The channel length
and optical path length are therefore tunable. The success rate for assembling
this new cell is close to 100%. We demonstrate the practicality of this cell by
studying: 1) the equation of state and second virial coefficients of
concentrated gold nanoparticle dispersions in water and bovine serum albumin
(BSA) as well as lysozyme solution in aqueous buffers, 2) the gelation phase
transition of DNA and BSA solutions, and 3) liquid-liquid phase separation of
concentrated BSA/polyethylene glycol (PEG) droplets.
