J. Phys. Chem. B 2001, 105, 7021-7026
7021
Identification of Crystalline and Amorphous Regions in Low Molecular Weight Materials Using Microthermal Analysis P. G. Royall,† V. L. Kett, C. S. Andrews, and D. Q. M. Craig* The School of Pharmacy, The Queen’s UniVersity of Belfast, 97 Lisburn Road, Belfast BT9 7BL, U.K. ReceiVed: February 5, 2001; In Final Form: May 9, 2001
The use of microthermal analysis as a means of differentiating between amorphous and crystalline forms of a model low molecular weight solid has been investigated, with a view to establishing the strengths and limitations of the technique as a means of identifying different physical forms of the same substance within a single sample. Samples of the drug indometacin were prepared in amorphous, crystalline, and partially crystalline forms and the samples studied using microthermal analysis (MTA) and modulated temperature differential scanning calorimetry (MTDSC). MTDSC studies revealed a simple melting process for the crystalline material corresponding to the γ form of the drug, while a glass transition, recrystallization and subsequent melting was seen for the amorphous indometacin; in addition, the sample demonstrated a shift in the phase angle at approximately 65 °C which has been previously associated with flow of the sample in the DSC pan. MTA studies indicated that the thermal conductivity is dominated by the surface topology and no clear differentiation between amorphous and crystalline regions was obtained in this mode. A discontinuity was seen in the localized thermal analysis (LTA) profile for the crystalline sample at 152 °C corresponding to the melting of the material. Similarly, the amorphous material showed a shift in probe position at 64 °C, these responses being ascribed to sample softening. Topographical examination of the sample following LTA studies indicated the presence of a “crater” approximately 10-20 µm in diameter, giving an indication of the scale of scrutiny of the experiment. Studies using different scanning rates (2 to 20 °C/s) did not demonstrate significant changes to the temperatures of transition and crater size. Studies on partially crystalline samples clearly showed that the technique was able to differentiate between the two physical regions in LTA mode. The technique may therefore have considerable potential as a means of identifying distinct physical regions in a single sample. However, consideration needs to be given to the experimental and interpretative issues raised in this investigation.
Introduction There has been considerable recent interest in the preparation and behavior of amorphous low molecular weight materials such as pharmaceutical and food constituents.1,2 This interest has arisen for two principal reasons. In the first instance, there are many circumstances whereby the preparation of a glassy dosage form has advantages over the equivalent crystalline system, examples including the preparation of amorphous drugs for improved dissolution and bioavailability3,4 and the manufacture of freeze and spray dried dried dosage forms for peptide and proteinaceous drugs.5,6 In addition, however, it is now recognized that the accidental generation of amorphous material on particle surfaces during processing (e.g., mixing, milling) may result in significant changes to the behavior of the system in question.7 For example, amorphous surface regions at a level of only 2 wt % of the crystalline bulk may cause agglomeration of micronized particles formulated for delivery to the lung.8 It is therefore recognized that there is a need for a more profound understanding of the nature and behavior of the amorphous state within a pharmaceutical context. To this effect attention has focused on studying the behavior of wholly amorphous materials with a view to understanding * To whom correspondence should be addressed. E-mail: duncan.craig@ qub.ac.uk. † Present address; Department of Pharmacy, School of Health and Life Sciences, King’s College London, Franklin-Wilkins Building, 150 Stanford Street, London SE1 8WA, U.K.
the glass transitional, relaxation and recrystallization behavior, while to date partially amorphous systems have been investigated more from the viewpoint of detecting rather than characterizing the amorphous fraction. This strategy has arisen as a simple result of the paucity of available techniques capable of directly studying the glassy material in semicrystalline pharmaceutical systems. Consequently, there is a strong argument for the development of new approaches with which the amorphous and crystalline fractions may be studied in isolation within a single system. A recent development within the thermal analysis field has been the introduction of microthermal analysis (MTA). This term refers to what is becoming a family of techniques whereby a thermal probe is combined with a microscopic technique, thereby allowing heating or thermal imaging of specific regions of a sample, usually to micron or submicron resolution. The first attempts at achieving such an analysis are attributed to Williams and Wickramasinghe9 who obtained thermal conductivity images by measuring the potential difference between an atomic force microscope (AFM) tip and the sample surface as a function of temperature. Since then the technique has developed such that the AFM tip may be replaced by a V-shaped Wollaston wire, whereby the silver sheath is etched away to expose the platinum core at the apex of the tip10 (Figure 1). This tip may then act as a resistive heat source as well as a resistance thermometer and may apply both linear and modu-
10.1021/jp010441k CCC: $20.00 © 2001 American Chemical Society Published on Web 06/29/2001
7022 J. Phys. Chem. B, Vol. 105, No. 29, 2001
Royall et al. and interpret the data from the MTA studies. The objective of the present study was therefore to examine the use of the technique for the study of a model drug, indometacin, for which there is already a substantial body of information available and which is also known to recrystallize over a time period suitable for the present studies. In this manner the suitability of the technique for this application may be established and the utility of the technique for the study of low molecular weight samples ascertained. Materials and Methods
Figure 1. SEM image of the microthermal analysis tip.
lated heating signals to a sample.11,12 The method has now been commercialised and has attracted considerable interest within the polymer sciences13,14 and more recently within the pharmaceutical arena.15-17 There are now a number of texts available that outline the principles of the technique14,15 but in essence the method allows measurement in the following modes. First, the tip may act as a (somewhat crude) AFM tip in the conventional sense, with resolution generally in the 100 nm-1 µm range. Second, the method may be used as a means of thermal conductivity scanning whereby the tip forms part of a Wheatstone bridge circuit. On rastering over regions of sample with differing thermal conductivities, the heat flow from the tip to the sample will vary which will in turn be reflected by differences in the applied current required to maintain a constant probe temperature. In addition, a quasi-isothermal alternating heating signal may be applied to the tip to improve baseline sensitivity and to allow AC imaging in order to control the depth sensitivity and to facilitate quantitative measurement of thermal diffusivity. The method may also be used as a means of nonisothermal thermal analysis (localized thermal analysis, LTA) by moving the tip to a chosen site on the surface and heating at a specified rate. By use of an identical but remote reference probe the temperature differential or heat flow into the sample may be measured, either in DC or AC mode. In addition, the method may be used as a means of localized thermomechanical analysis, whereby a controlled force is applied to the probe and the position of the tip measured as a function of temperature, hence the method is analogous to macroscopic thermomechanical analysis. Further innovations such as the use of a dynamic mechanical load,18 the use of pulsed force measurements19 and the combination of the method with other techniques such as gas chromatographymass spectroscopy15 and FT-IR20 have also been described. Given the above, the technique clearly offers numerous possibilities for the study of multicomponent systems in general and, in the present context, semicrystalline systems in particular. However, the use of the technique for the study of low molecular weight materials is very limited to date. Such studies are of interest for two principal reasons. First, if proof of concept could be provided that microthermal analysis can differentiate between crystalline and amorphous regions in such a sample then the implications for the study of a wide range of pharmaceutical systems is profound. Second, low molecular weight pharmaceuticals provide excellent samples for testing the strengths and limitations of the technique itself due to their high purity and, in many cases, the substantial body of available data on the physical characteristics of the systems with which to validate
Indometacin (1-(p-chlorobenzoyl)-5-methoxy-2-methylindole3-acetic acid) was supplied by Sigma (stated purity of greater than 99%, γ polymorph) and used as received. Compacts of crystalline material were prepared in an infrared sample press by placing 400 mg of powder in an 8 mm diameter die. A mass equivalent of 3 tons was applied for 2 min and the resulting compact was removed and used immediately. Amorphous samples of indometacin were prepared by heating the crystalline material to the molten state and then cooling to room temperature under ambient conditions in a small crystallization dish. Fragments of the glassy material were removed and placed in a sealed sample jar and stored at 20 °C for 1 month. Thermogravimetric analysis studies indicated a water content of
