Constructing compartments to divide space controllably and spontaneously into subunits at the submicron scale is a significant challenge facing nanotechnology. We have developed a simple method of creating nanocompartments in vitro via the "interdigitated" bilayer phase formed by adding ethanol to a variety of saturated phospholipids. At temperatures below the gel-liquid crystalline transition, Tm, the interdigitated lipid-ethanol sheets are rigid and flat; when the temperature is raised above Tm, the sheets become flexible and close on themselves and the surrounding solution to form closed compartments. During this closure, the sheets can entrap other vesicles, biological macromolecules, or colloidal particles. The result is efficient and spontaneous encapsulation without disruption of even fragile materials to form biomimetic nanoenvironments for possible use in drug delivery, in colloidal stabilization, or as microreactors.