Dissociation, Partitioning, and Solubility

Generally, to produce a biological response, a drug molecule must first cross at least one biological membrane. The biological membrane acts as a lipid barrier to most drugs and permits the absorption of lipid-soluble substances by passive diffusion while lipid-insoluble substances can diffuse if at all across the barrier only with considerable difficulty. The interrelationship of the dissociation constant, lipid solubility, and pH at the absorption site and absorption characteristics of various drugs are the basis of the pH-partition theory. 

The interrelationship between the dissociation constant and lipid solubility of a drag, as well as the pH at the absorption site, is known as the pH-partition theory of drag absorption. Accordingly, rapid transcellular passive diffusion of a drag molecule may be due to ... 

The stabilitycompatibility, and toxicity in mice, the absorption in dogs, and the effect of antacids on the availability of vasi-cinone (185) have likewise been studied. Its solubility, intrinsic dissolution rate, dissociation constant, and partition coefficient have been determined. ... 

Physico-chemical properties and spectroscopic signals constitute the most important class of experimental chemical measurements, also playing a fundamental role as molecular descriptors both for their availability as well as their interpretability. Examples of physicochemical measurable quantities are refractive indices, molar refractivities, parachors, densities, solubilities, partition coefficients, dipole moments, chemical shifts, retention times, spectroscopic signals, rate constants, equilibrium constants, vapour pressures, boiling and melting points, acid dissociation constants, and so on [Baum, 1997 Horvath, 1992 Lyman, Reehl et al., 1982 Reid, Prausnitz et al., 1988]. 

Employing Equation 9.41 and selecting drugs that represent a wide range of absorption characteristics, from poorly absorbed compounds to those that are virtually completely absorbed, the utility of the AP parameter as a predictor of the fraction absorbed was assessed (47). The physicochemical properties, such as the partition coefficient, solubility, fraction available in unionized form, dissociation constant, fraction of dose absorbed, and calculated dimensionless parameter AP for drugs selected in this study (47), are reported in Table 9.6. The observed... 

Blanchard J, Boyle JO and Van Wagenen S, Determination of the partition coefficients, acid dissociation constants and intrinsic solubility of carbenoxolone, /. Pharm. ScL, Tl, 548-552 (1988). 

Mode of action and effectiveness of a microbicide are determined by the interplay of the chemical and physical properties of the active ingredient molecule, which in turn depend on the molecular structure. The properties involved include water- and lipid-solubility, polarity, ionogenicity, degree of dissociation, partition factor, reactivity and stability. The differences in modes of action should always be considered as they may result in different consequences for the application of microbicides. 

The following physico-chemical properties of the analyte(s) are important in method development considerations vapor pressure, ultraviolet (UV) absorption spectrum, solubility in water and in solvents, dissociation constant(s), n-octanol/water partition coefficient, stability vs hydrolysis and possible thermal, photo- or chemical degradation. These valuable data enable the analytical chemist to develop the most promising analytical approach, drawing from the literature and from his or her experience with related analytical problems, as exemplified below. Gas chromatography (GC) methods, for example, require a measurable vapor pressure and a certain thermal stability as the analytes move as vaporized molecules within the mobile phase. On the other hand, compounds that have a high vapor pressure will require careful extract concentration by evaporation of volatile solvents. 

Water solubility, dissociation constant(s) and n-octanol/water partition coefficients allow one to predict how an analyte may behave on normal-phase (NP), reversed-phase (RP), or ion-exchange solid-phase extraction (SPE) for sample enrichment and cleanup. 

The major differences between behavior profiles of organic chemicals in the environment are attributable to their physical-chemical properties. The key properties are recognized as solubility in water, vapor pressure, the three partition coefficients between air, water and octanol, dissociation constant in water (when relevant) and susceptibility to degradation or transformation reactions. Other essential molecular descriptors are molar mass and molar volume, with properties such as critical temperature and pressure and molecular area being occasionally useful for specific purposes. A useful source of information and estimation methods on these properties is the handbook by Boethling and Mackay (2000). 

The calculation is illustrated in Table 1.5.5 for pentachlorophenol. The experimental aqueous solubility is 14.0 g/m3 at a pH of 5.1. The environmental pH is 7. Higher environmental pH increases the extent of dissociation, thus increasing the Z value in water, increasing the apparent solubility, decreasing the apparent KqW and Henry s law constant and the air-water partition coefficient, and decreasing the soil-water partition coefficient. 

An alternative way of expressing the partition constant of a sparingly soluble salt is to define its solubility product Rsp (also called the solubility constant Rs). Ks is defined as the product of the ion activities of an ionic solute in its saturated solution, each raised to its stoichiometric number v . Ks is expressed with due reference to the dissociation equilibria involved and the ions present. 

It must be noted that the partition coefficient is not the ratio of the pollutant solubilities in the two pure liquids. This change can result in significant differences, particularly with compounds of low aqueous solubility. The measurement of partition coefficients may be complicated by the involvement of other equilibrium processes such as pKa and pH values. For example, the following reaction shows the dissociation of a monoprotic organic acid ... 

PhC properties most investigated by scientists to date are their water solubility (s, mg/mL), volatility (correlated to the Henry constant H) (pg m atr/pg m wastewater), biodegradability (correlated to pseudo-first-order degradation constant bioi L gSS d ), acid dissociation constant K, distribution and sorption (through the sludge-water distribution coefficient K, expressed in L gSS or the octanol-water partition coefficient Kg ). The main focus has been to find any correlations between these parameters and to determine PhC removal rates during the different treatment steps. Thus, different properties have been quantified for many compounds, and software, such as EPl Suite 4.00 [54], consenting their estimation, is available. 

As indicated earlier (Section 3.1.1) the sorption of organic compounds onto dissolved matter can significantly increase the solubility of the compound. This can in turn affect the fate of these chemicals in the environment. We can use physicochemical parameters such as distribution coefficients (log D), aqueous acid dissociation constants (pAia), and octanol-water partition coefficients (p/to )-These attributes are also linked to the acidity and alkalinity of the environment as well as lipohilicity of the compound. The mathematical relationships between these attributes are outlined below to explore how each of these impacts the fate of PPCPs in the environment. 

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