Surface area dissolution experiments

If the spreading is into a limited surface area, as in a laboratory experiment, the film front rather quickly reaches the boundaries of the trough. The film pressure at this stage is low, and the now essentially uniform film more slowly increases in v to the final equilibrium value. The rate of this second-stage process is mainly determined by the rate of release of material from the source, for example a crystal, and the surface concentration F [46]. Franses and co-workers [47] found that the rate of dissolution of hexadecanol particles sprinkled at the water surface controlled the increase in surface pressure here the slight solubility of hexadecanol in the bulk plays a role. 

It is advantageous with a new drug substance to be able to estimate what its solubility will be prior to carrying out dissolution experiments. There are several systems of solubility prediction, most notably those published by Amidon and Yalkowsky [14-16] in the 1970s. Their equation for solubility of p-aminobenzo-ates in polar and mixed solvents is a simplified two-dimensional analog of the Scatchard-Hildebrand equation and is based on the product of the interfacial tension and the molecular surface area of the hydrocarbon portion of a molecule. 

Another important consequence of the constant rate of release diffusion model is that it mimics many of the features that have commonly been attributed to surface reaction (matrix dissolution) control. If one were to account for changes in surface area over time, the predicted long-term dissolution rate due to surface reaction control would also yield constant element release. In surface reaction controlled models, the invariant release rate with respect to time is considered to be the natural consequence of the system achieving steady-state conditions. Other features of experiments commonly cited as evidence for surface reaction control, such as relatively high experimental activation energies (60-70 kJ/ mol), could be explained as easily by the diffusion-control model. These findings show how similar the observations are between proponents of the two models it is only the interpretation of the mechanism that differs. 

Dissolution of a drug substance is controlled by several physicochemical properties, including solubility, surface area, and wetting properties. For insoluble compounds, dissolution is often the rate-limiting step in the absorption process. Knowledge ofthe dissolution rate of a drug substance is therefore very useful for formulation development. The appropriate dissolution experiments can help to identify factors that contribute to bioavailability problems, and also assist in the selection of the appropriate crystal form and/or salt form. Dissolution tests are also used for other purposes such as quality control and assisting with the determination of bioequivalence (Dressman et al., 1998). 

The dissolution rate at about 25 C depends upon acid concentration and surface area of the metal. Typically, initial batches of solution from the dissolver average 50 +5 g Pu/L the concentration increases to 60 +10 g Pu/L when using a cycle of 1 hour dissolving time followed by displacement of two- thirds of the solution. A more complete treatment of both ambient temperature and elevated temperature dissolving experiments is given elsewhere 2, 3). 

The rate of dissolution of synthetic and deep sea biogenic (pteropods) aragonite in seawater have also been determined in the laboratory by the pH-stat method (57). The results of the experiments to determine the change in the rate of dissolution as a function of undersaturation are presented in Figure 16. The pteropods were found to dissolve at. only about 3% the rate, per unit surface area, of the synthetic aragonite. The results also indicate a change in the empirical reaction order from 2.92 to 7.37 at = 0,44. The rate equations for pteropod dissolution are ... 

Stone et al. (S29) developed by a mathematical analysis the functional relationship between the rate of extraction of silica from pure quartz in sodium hydroxide solution and time, temperature, sodium hydroxide concentration, and particle size. With the use of response surface methodology, a comprehensive picture of this dissolution process was obtained from a few well-chosen experiments. The fractional extraction of silica can be expressed by a second-order equation. The effect of quartz particle size and temperature are predicted to be about equal and greater than the influence of sodium hydroxide concentration and reaction time. The reaction rate is controlled by the surface area of the quartz. An increase in sodium hydroxide concentration increases the activation energy for the reactions and is found to be independent of quartz size. 

During dissolution, mass of the mineral decreases and specihc surface area generally increases. Most researchers use the initial mineral mass and surface area to normalize reaction rate, but for experiments where the extent of reaction is large, the hnal surface area may be used to normalize the rate (Stillings and Brantley, 1995). Reactors are run until outlet concentration reaches a constant steady-state value. Dissolution rates are then reported with respect to solution chemistry as measured in the effluent. For example, measured rate is reported with respect to the outlet rather than inlet pH. 

Weathering rates in the field are as much as one to two orders of magnitude slower than dissolution rates measured in the laboratory (Benedetti et al., 1994 see Chapter 5.05). The difference is due to a number of factors (i) there are differences in surface area between laboratory minerals and natural minerals (ii) secondary precipitates may protect primary mineral surfaces in the field (iii) in soils, most flow is through macropores and not all mineral surfaces are continually exposed to flowing solutions as they are in laboratory experiments and (iv) most... 

Note that all these data concern experiments in the absence of reaction and in conditions where it can be assumed that all surfaces of the packing are wetted (dissolution of solid particles). However, when a reaction is occurring, mass transfer may be constrained to a region close to the pore openings on the outer surface of the particles and hence the effective surface area for mass transfer could be considerably less than the external surface of the catalyst, leading to lower values k a. More studies are needed on porous catalysts at conditions where external transport is important. 

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