Dali M V, Carstensen J T
School of Pharmacy, Madison, Wisconsin 53706-1515, USA.
Pharm Res. 1996 Jan;13(1):155-62. doi: 10.1023/a:1016010207729.
To study the effect of change in the shape factor of real crystals on their dissolution behavior using a potassium dichromate crystal as a model for particulates in general.
A model geometry (parallelepiped) has been suggested for a dissolving particle. Single crystals of potassium dichromate which are monoclinic prisms were grown individually from supersaturated solutions at 40 degrees C. Dissolution studies were carried out on five such crystals in 0.1N H2SO4 at 25 degrees C and a stirrer speed of 50 +/- 1 rpm. The five crystals had different degrees of non-isometricity. Initial dimensions of the crystals were measured using image analysis techniques. The shape factor of the dissolving crystal as a function of time was obtained indirectly from the dissolution data.
The shape factor of a single crystal changed significantly after about 50% dissolution. The nature of this change depended on the degree of non-isometricity of the crystal. The change in shape factor of the dissolving crystal was accounted for in the Hixson-Crowell cube root law, and a modified form of the cube root equation was developed. This equation for dissolution explained the observed upward curvature in the cube root law plot.
The shape factor for any non-isometric particle cannot be considered to be constant over the dissolution event, as is commonly assumed. This change has an appreciable effect on the dissolution behavior of crystals. This study is particularly of significance for elongated shapes like needles and platelets. By the methodology described here, it was possible to determine the initial shape factor of the crystal and the intrinsic dissolution rate constant.
以重铬酸钾晶体作为一般颗粒的模型，研究真实晶体形状因子的变化对其溶解行为的影响。
针对溶解颗粒提出了一种模型几何形状（平行六面体）。在40℃下从过饱和溶液中分别生长出单斜棱柱形的重铬酸钾单晶。在25℃、搅拌速度为50±1转/分钟的条件下，在0.1N硫酸中对五颗这样的晶体进行溶解研究。这五颗晶体具有不同程度的非等轴性。使用图像分析技术测量晶体的初始尺寸。溶解晶体的形状因子随时间的变化是从溶解数据中间接获得的。
单晶在溶解约50%后，其形状因子发生了显著变化。这种变化的性质取决于晶体的非等轴性程度。溶解晶体形状因子的变化在希克森-克劳威尔立方根定律中得到了解释，并推导出了立方根方程的修正形式。这个溶解方程解释了立方根定律图中观察到的向上曲率。
任何非等轴颗粒的形状因子在溶解过程中不能像通常所假设的那样被认为是恒定的。这种变化对晶体的溶解行为有显著影响。这项研究对于针状和片状等细长形状尤为重要。通过这里描述的方法，可以确定晶体的初始形状因子和固有溶解速率常数。
