Effects of pH and surface pressure on morphology of glycine crystals formed beneath the phospholipid Langmuir monolayers

Ordered molecular monolayers of dipalmitoyl phosphatidylcholine (DPPC) were used as templates to induce the nucleation and growth of glycine crystals. It was found that α-glycine crystals were formed under the DPPC monolayers regardless of pH values of the aqueous supersaturated glycine solutions. The morphologies and orientations of the glycine crystals varied with pH of the solutions and surface pressures of the monolayers. When acidic and neutral aqueous supersaturated glycine solutions were used as subphases, the glycine crystals are plate-like habit with an elongated (0 1 0) crystal face preferentially oriented parallel to the plane of the monolayers; when basic solutions were used, the crystals are pyramidal habit. At surface pressures below 25 mN/m at the beginning of crystallization, plate-like α-glycine crystals were formed from the neutral solution; while at higher surface pressures, such as 35 and 40 mN/m, prismatic crystals were formed. The morphology of the glycine crystals can be tuned by changing the experimental conditions.