Powder Studies

The advantages of the synchrotron radiation already mentioned permit excellent work on crystalline powders, especially accurate determination of unit cell. Moreover, 

Anyhow we believe that structural studies on small crystals, when available, should be preferred as indicated previously. 

Any kind of phase transition or transformation under the influence of physical (T, P,.. .) or chemical factors can be accurately studied with synchrotron radiation. 

Another example is provided by Tolocko et al., who took advantage of the high speed of data collection to study the recrystallization process in deformed silver by time-resolved X-ray diffraction The sample deformation was obtained by cutting and the first diffraction curve has been obtained 400 m sec after the deformation. Another 59 diffraction curves were stored with 50 m sec time interval. These curves allow to follow the structural changes during recrystallization. 

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Because of the large number of powders studied and their generally refractory character, it appears that it can safely be concluded that substantial particle comminution does not occur in shock-compressed powders. Rather,... 

In Section 6.11. a discussion of the measurement of the spectrum will be given. Here, however, it should be mentioned that a complete explanation of the spectrum of powdered samples with respect to the total vibrational assignment of all the observed bands is usually impossible. The number of single crystal studies is much smaller than the number of powder studies. 

On the other hand, Davies and Cox (1998) have reported a dependence of the reaction probability on the gas-phase concentration of HN03 as well as water vapor. The reaction probabilities they report are also one to two orders of magnitude smaller than those of the other powder studies. They interpret their data in terms of a modified version of the Beichert and Finlayson-Pitts (1996) mechanism, in which HN03 first physisorbs to the surface and then diffuses to a reactive site that holds water. A modified Langmuir reaction scheme has been proposed by Ghosal and Flemminger (1999), which explains the observed dependence on... 

It has long been known that nitrocotton may be precipitated from solution in the form of a gel on the addition of a metallic powder, e.g. copper, aluminium. Jenkins [32] examined the activity of different metals as coagulating agents, and found lead to be the strongest, and zinc the weakest coagulating agent. The metallic powders studied have been ranged with respect to this property to form a series as follows ... 

Silver(i).—Group VII Donors. A study of the phase equilibria of the AgI-NaI-HzO system has previously led to the isolation of the compound AgI,NaI,3H20 194 u.v-visible spectra and X-ray powder studies indicate the formulation Na[AgI2],3H20.195 The phase transition for the system AgI-I -(KNa)N03 at 280 °C has been studied at high iodide concentrations.196 The results indicate the presence of [Agl4]3 and polynuclear species. Thermodynamic calculations on the distribution of Agl between solid or liquid (K2Ag)I and a nitrate melt have been reported.197 It was concluded... 

Carbon-Doped Anatase Ti02 Powders as a Visible-Light Sensitive Photocatalyst Oxidative annealing of TiC used to afford yellow doped powders. Study focus as in Entry 5. 310... 

Unless otherwise stated, this chapter relates to ordinary Portland cements hydrated in pastes at 15-25°C and w/c ratios of 0.45-0.65. XRD powder studies on such pastes have been reported by many investigators (e.g. C38,M67). The rates of disappearance of the phases present in the unreacted cement are considered more fully in Section 7.2.1. Gypsum and other calcium sulphate phases are no longer detectable after, at most, 24 h, and tbe clinker phases are consumed at differing rates, alite and aluminate phase reacting more quickly than belite and ferrite. The ratio of belite to alite thus increases steadily, and after about 90 days at most, little or no alite or aluminate phase is normally detectable. 

V. Pecharsky and P. Zavahj, Fimdamentals of Powder Diffraction and Structural Characterization of Materials , Springer, Berlin, 2005, Comprehensive treatment of all aspects of powder diffractometry and determination of crystal structures from powder data. Contains up to date information on software for X-ray powder studies. Also contains an extensive hst of additional texts and key journal articles. 

This section discusses a selection of NMR results with an emphasis on powder studies, on experiments that describe the dynamics of water at the protein surface, and on lysozyme as a model protein. Methods and theory are not discussed. For review discussions see Kuntz and Kauz-mann (1974), Bryant (1978), Koenig (1980), and Fung (1986). A recent review by Bryant (1988) is an elegant summary of the theory and results for NMR measurements of protein hydration, in powders and in solution. 

Hoofman RJOM, van der Laan GP, de Haas MP, Tanigaki K. (1997) Charge migration in pulse-irradiated undoped C60 powder studied with the time-resolved microwave conductivity technique. Synth Met 86 2355-2356. 

In powder studies, the surface area of the powder is an important parameter. It can be measured directly by means of gas adsorption studies, in which the amount of gas or another molecular item, such as dye molecules adsorbed onto the powder to form a monolayer, is determined. Several books have been written describing the theory and procedures for gas adsorption studies. Before 1977, it was believed that one of the basic problems with surface area estimates by gas... 

All the experimental data can now be approximated by a very simple equation (equation 8) containing a single parameter, ao, and this is shown as a continuous line in Figure 5 from which it can be seen that with a small number of exceptions the data points are well approximated. The reason for the deviation of the results for the sand bed shown in Figure 5 is believed to be due to differences in the particle size distribution of the material. Size analysis showed that the sand contained a higher proportion of both large and small particles than the other powders studied and thus it... 

Although BF provides a direct SSA measurement, the techniques of PSD, MIP and gas adsorption provide complementary information on texture characteristics to better understand and interpret results from process and formulation development studies. However, in some cases, BF can be considered as a complete and adequate quality control method which can easily be set to work within the frame work of production control provided that the method has been validated and applies well to the powder studied... 

For the calculation of SSA from PSD results, a spherical shape of particle is assumed. As a consequence, the surface of a particle is 4 tR R beeing the mean radius of particles. No assumption on the particles shape is made when using the gas adsorption technique which allows for measurement of the overall surface developed by the powder. Since for the micronized DS powder studied here no intraparticular porosity is observed, the powder surface measured by gas adsorption corresponds to the sum of each particle surface. 

Drug Substance (DS) Specific Surface Area (SSA) has been estimated by permeabilimetry, gas adsorption, laser light scattering and Mercury Intrusion Porosimetry (MIP). Because of the simplifying and different assumptions made, none of these experimental methods can provide the absolute SSA value and a perfect agreement between the values obtained by each technique is not found However, differences in theoretical assumptions made for each technique and observed results have been useful for understanding and interpreting the texture of the powder studied. 

Cech, F. and Slansky, E., 1965. X-ray powder study and thermal investigation of AIPO4 2H2O minerals. Acta Univ. Carol. Geol., 1965, No. 1 1—30. 

Jones R, Pollock HM, Geldart D, et al. Inter-particle forces in cohesive powders studied by AFM effects of relative humidity, particle size and wall adhesion. Powder Technol 2003 132(2-3) 196-210. 

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