An in situ dissolution study of aspirin crystal planes (100) and (001) by atomic force microscopy

Purpose. To observe in situ and on individual aspirin crystal faces the comparative rates and processes of dissolution of the dominant faces. Methods. The kinetics of the dissolution rate of two aspirin crystal planes (001) and (100) under 0.05M HCl are studied in situ at room temperature using Atomic Force Microscopy. The dissolution process of each crystal plane was followed by observed changes in topographic features. Results. The results revealed that crystal plane (001) dissolves by receding step edges, and has a dissolution rate of 0.45 nm s(-1). Conversely, plane (100) displays crystal terrace sinking at an average rate of 2.93 nm s(-1). Calculated intrinsic dissolution values (g s(-1) cm(-2)) for planes (001) and (100) are 1.37 x 10(-7) g s(-1) cm(-2) and 8.36 x 10(-7) g s(-1) cm(-2), respectively. Conclusions. These values indicate that the rate of flux of material from plane (100) is approximately six times greater than that from plane (001), under 0.05M HCl. Interpretation of the data, based upon intrinsic dissolution rates and dissolution rate velocities, correlate with reported variations in the dissolution behavior of commercial aspirin products. These observations illustrate the suitability of the technique for characterizing the dissolution behavior of crystalline drugs.