Sandwich crystals of butyl paraben†
Butyl paraben crystals having a porous layer in between two solid non-porous layers have been produced by cooling crystallization in mixtures of ethanol and water. The outer layers are transparent and fully crystalline, while the middle layer appears to be polycrystalline and is full of pores of various dimensions, down to below 0.1 μm diameters. The thickness of the porous layer reaches about 40% of the whole crystal. The crystals contain one polymorph only and appear to be essentially fully crystalline. They are stable for more than a year when stored on the shelf at room temperature. When the crystals dissolve, the porous layer dissolves faster, leaving the outer layers for slower dissolution. The sandwich crystals are easily cleaved through the middle layer parallel to the (100) plane. This type of sandwich crystals may provide new useful properties to pharmaceutical solids, e.g. larger specific surface area, higher dissolution rates and improved compaction properties.