Metastable Amorphous Dispersions of Hydrophobic Naphthalene Compounds Can Be Formed in Water without Stabilizing Agents via the "Ouzo Effect".

Hydrophobic molecules dissolved in water-miscible organic solvents are used in vitro for biological membrane studies and for testing of potential pharmaceuticals in high-throughput screenings. When these solutions are introduced into an aqueous environment, it is possible that metastable "ouzo-like" dispersions form from liquid-liquid phase separation. It is therefore hypothesized that when solutions of naphthalene compounds in water-miscible solvents are added to water, metastable dispersions will form. Millimolar solutions of naphthalene, -phenyl-1-naphthylamine (NPN), 1-aminonaphthalene, 1-iodonaphthalene (INAP), 1,4-dimethoxynaphthalene, and 1-naphthol were prepared in either dimethyl sulfoxide, ethanol, or acetone at concentrations similar to those used in biological membrane studies. Each solution was diluted 10-fold in water. Particle formation was characterized by qualitative observations, dynamic light-scattering, nephelometry, and optical microscopy. It was discovered that two of the compounds tested made metastable dispersions: INAP and NPN. The initial particle sizes were ∼400 nm (radius), with turbidity ranging from 1,000 to 20,000 NTU, depending on the initial concentrations used. Fluorescence microscopy imaging showed spherical particles that do not aggregate while under observation. Slow-nucleating crystallization occurs over days, presumably from a heterogeneous nucleation process. The formation of these dispersions has implications for in vitro delivery of hydrophobic molecules to biological membranes.