Partition coefficient temperature control

Figure 15. Variation in Dxh for garnet versns reciprocal temperature for experimental data sources listed in Table lb at a variety of pressures n = 33). A distinction is made between mantle solidus partition coefficients (Salters and Longhi 1999 Salters et al. 2002 McDade et al. 2003a,b) and the rest. Note the strong temperatnre dependence, which is qnalitatively similar to that incorporated in Equation (25b). The scatter is due to additional compositional controls.

Important factors influencing the analyte volatilisation process are related to diffusion, porosity, and surface area (for solids). To obtain reproducible results it is necessary to control storage temperature and time strictly. The temperature of the sample is very important because of the specific boiling points of the various analytes. The partition coefficient, K, at equilibrium, is... 

Gas-liquid relationships, in the geochemical sense, should be considered liquid-solid-gas interactions in the subsurface. The subsurface gas phase is composed of a mixture of gases with various properties, usually found in the free pore spaces of the solid phase. Processes involved in the gas-liquid and gas-solid interface interactions are controlled by factors such as vapor pressure-volatilization, adsorption, solubility, pressure, and temperature. The solubility of a pure gas in a closed system containing water reaches an equilibrium concentration at a constant pressure and temperature. A gas-liquid equilibrium may be described by a partition coefficient, relative volatilization and Henry s law. 

Fig. 19. Experimental partition coefficients and those predicted by size exclusion theory (Eq. (4), shown as a solid line) for vitamin B12 (a relatively large, hydrophilic solute) in 10 x 4 PNIPAAm gel. Although size exclusion theory consistently underestimates the value of K, the sharp drop predicted at the transition temperature is observed. Error bars (standard deviation of three samples) not shown are smaller than the symbol. The dotted line is to guide the eye. Reprinted from the Journal of Controlled Release (1992) 18 1, by permission of the publishers, Elsevier Science Publishers BV [70]...

Solid phase micro-extraction (SPME) allows isolation and concentration of volatile components rapidly and easily without the use of a solvent. These techniques are independent of the form of the matrix liquids, solids and gases can be sampled quite readily. SPME is an equilibrium technique and accurate quantification requires that the extraction conditions be controlled carefully. Each chemical component will behave differently depending on its polarity, volatility, organic/water partition coefficient, volume of the sample and headspace, speed of agitation, pH of the solution and temperature of the sample (Harmon, 2002). The techniques involve the use of an inert fiber coated with an absorbant, which govern its properties. Volatile components are adsorbed onto a suitable SPME fiber (which are usually discriminative for a range of volatile components), desorbed in the injection chamber and separated by a suitable GC column. To use this method effectively, it is important to be familiar with the factors that influence recovery of the volatiles (Reineccius, 2002). 

Temperature control may also be required to control the sensitivity of the coating to the analyte. Apart from the aforementioned temperature sensitivity of coating physical parameters, the partition coefficient between analyte in the ambient gas or liquid phase and that sorbed by the coating is typically exponentially dependent on absolute temperature. For simple physical interactions, such as an organic solvent being sorbed by a polymer film, the largest contribution to this effect is the strong temperature dependence of the solvent s vapor pressure. The sensitivity of the coated device is thus temperature dependent [34]. 

The classical measurement of LogP is the shake flask method [17]. A known amount of drug is dissolved in a flask containing both octanol phase and aqueous buffer at controlled pH to ensure the existence of only nonionic form (at least two units from the drug pA) ). The flask is shaken to equilibrate the sample between two phases. There must be no undissolved substance present in both phases. After the system reaches its equilibrium, which is time- and temperature-dependent, the concentration of drug is analyzed by HPLC in both phases. Partitioning coefficient is calculated as... 

DYNAMICS OF DISTRIBUTION The natural aqueous system is a complex multiphase system which contains dissolved chemicals as well as suspended solids. The metals present in such a system are likely to distribute themselves between the various components of the solid phase and the liquid phase. Such a distribution may attain (a) a true equilibrium or (b) follow a steady state condition. If an element in a system has attained a true equilibrium, the ratio of element concentrations in two phases (solid/liquid), in principle, must remain unchanged at any given temperature. The mathematical relation of metal concentrations in these two phases is governed by the Nernst distribution law (41) commonly called the partition coefficient (1 ) and is defined as = s) /a(l) where a(s) is the activity of metal ions associated with the solid phase and a( ) is the activity of metal ions associated with the liquid phase (dissolved). This behavior of element is a direct consequence of the dynamics of ionic distribution in a multiphase system. For dilute solution, which generally obeys Raoult s law (41) activity (a) of a metal ion can be substituted by its concentration, (c) moles L l or moles Kg i. This ratio (Kd) serves as a comparison for relative affinity of metal ions for various components-exchangeable, carbonate, oxide, organic-of the solid phase. Chemical potential which is a function of several variables controls the numerical values of Kd (41). 

Partition coefficients of solutes are influenced by aqueous-solution properties (pH, ionic strength) and the solvent (hydrophobicity, polarity). Partition coefficients are relatively insensitive to temperature or solute concentrations over the ranges normally used in fermentation-broth processing. Among the aqueous properties, the pH is one of the more important variables (particularly for weak bases and acids). Several primary and secondary metabolites, such as penicillin, are weak acids or bases, and the pH can be used to control and even reverse the distribution coefficient. A list of pharmaceutical products that are weak acids or bases and are extracted with solvent is given in Table 4. 

Sometimes unravelling the separate effects of temperature and composition can be difficult, especially where the liquidus temperature of a melt is a function of composition. Such is the problem with Ni partitioning between olivine and a basaltic melt. Two experimental studies, published at the same time, seem to show conflicting results. Leeman and Lindstrom (1978) showed that the prime control on the olivine partition coefficient for Ni in a natural basaltic melt was temperature whilst Hart and Davis (1978) showed that there is clear inverse correlation between the melt composition and partition coefficient. To resolve the apparent conflict... 

The chemical structure of an encapsulated molecule is an important parameter that can influence the partition coefficient and then the controlled release into a food. Alcohols and short-chained esters had higher partition coefficients in the oil/polymer system, than in the water/polymer system. Several studies have attempted to model the relationship between the encapsulated molecule, the composition of the food, and the partition coefficient (Arab Tehrany and Desobry 2004). It is also known that matrix crystallinity and glass transition of the matrix are key factors for an efficient controlled release of an active compound. A controlled transition from glassy to rubbery state (temperature, water activity) leads to the best system for good food preservation. A lot of work stiU has to be done to allow perfect control of an active compound release. 

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