Experimental Method to Distinguish between a Solution and a Suspension
Xufeng Xu, Quy K. Ong, Ting Mao, Paulo Jacob Silva, Seishi Shimizu, Luca Rebecchi, Ilka Kriegel, Francesco Stellacci
Abstract
Abstract Dispersion of objects in a fluid phase can be classified as solutions (Gibbs free energy of mixing, Δ G mix < 0) or suspensions (Δ G mix > 0) depending on their thermodynamic stability. Small objects tend to form solutions, larger ones suspensions, e.g., molecules versus micrometer‐sized colloids. Proteins and nanomaterials fall between these two size regimes. The long‐standing issue of whether proteins and nanoparticles are dissolved or suspended remains an important research question. Here, a simple, versatile, and experimentally robust method, based on sedimentation equilibrium analytical ultracentrifugation (SE‐AUC), which can determine whether proteins, nanoparticles, or polymers form solutions or suspensions, is presented. SE‐AUC determines the osmotic pressure profile for a dispersion. Such a profile for solutions (equilibrium one‐phase systems) is independent of the initial and the operating conditions. The opposite is true for suspensions that are nonequilibrium two‐phase systems. This study proves that bovine serum albumin and lysozyme form solutions while ferritin and apoferritin form suspensions.