Solubility trends

Table 4.18 shows the solubility trend for some common hydrocarbons. 

Another way to look at this solubility trend is to note that n-propanol,... 

The chlorides, bromides, nitrates, bromates, and perchlorate salts are soluble in water and, when the aqueous solutions evaporate, precipitate as hydrated crystalline salts. The acetates, iodates, and iodides are somewhat less soluble. The sulfates are sparingly soluble and are unique in that they have a negative solubility trend with increasing temperature. The oxides, sulfides, fluorides, carbonates, oxalates, and phosphates are insoluble in water. The oxalate, which is important in the recovery of lanthanides from solutions, can be calcined directly to the oxide. This procedure is used both in analytical and industrial applications. 

Karathanasis AD, Thompson YL, Evangelou VP. 1990. Temporal solubility trends of aluminum and iron leached from coal spoils and contaminated soil materials. J Environ Qual 19 389-395. 

Figure 13.4 The log (blood-brain barrier permeability-surface area (PS) product) is plotted versus the log (partition coefficient) 1 octanol/saline for different molecules of varying molecular weight. The line in bold font denotes the lipid solubility trend line.

Small molecules with molecular weights of approximately 200 Da, such as AZT or 21-132, have permeability values below the lipid solubility trend line, and are presumed to be actively effluxed from the BBB (see below). Molecules such as D-glucose and L-DOPA have permeability values 3-4 log orders of magnitude above the lipid solubility trend line, owing to carrier-mediated transport through the BBB in vivo which results in enhanced transport. 

The proposal that these solubility trends result from the effects of cations on water-structure has been criticised (Feillolay and Lucas, 1972). Instead, dispersion forces between salt and solute are used to explain why added Bu4N+Br raises the solubility of helium and methane (Lucas and de Trobriand, 1971). 

The abundances of trace elements in rivers depends both on their abundances in the continental crust and their mobility during weathering and transport. In order to depict a global solubility trend of trace elements, dissolved concentrations (Cw) are normalized to those of the upper continental crust (Cc) (Figure 2). Data from the continental crust are from Li (2000). In this figure, major elements in river waters are also shown and all normalized concentrations are compared to the value for sodium. It is important to note that the Cw/Cc ratio is a global mobility index rather than a solubility index because, as will be shown below, a number of very different processes contribute to the occurrence of trace elements in river dissolved load. In addition, for a... 

In reality, solvation involves donor-acceptor interactions, which may not be purely electrostatic in nature (see below), so that neutral molecules may also be strongly solvated. Solvent molecules are ordered round the solute, not only in the primary solvation sphere but (especially with ions) affecting more distant molecules. Solvation therefore produces a decrease in entropy, which can be substantial with small highly charged ions, and contributes to acid-base strength, complex formation and solubility trends (see Topics E2-E4Y... 

Proper understanding of solute behavior in sub-H20 systems requires evaluation of both solute-sub H2O phase behavior (particularly in engineering scale systems where solute concentrations are finite), solute solubility trends in hot pressurized water, the diffusion coefficients of solutes in water as a function of temperature and their role in facilitating mass transport, and the potential effect of pressure - often trivialized as a major factor in SWE 16) - in affecting analyte extractions fi om sample matrices via sub-H20. All of these factors ultimately impact the resultant SWE, SWF, and SWR process, reinforcing one another fortuitously as temperature is increased, leading to an increase in solute flux into the sub-H20 medium. 

The degree of solubility, how much solute can dissolve in a given volume of solvent, is a quantitative measure of solubility. It is difficult to predict the solubility of each and every compound. However, general solubility trends are based on the following considerations ... 

Although the above, and similar, arguments are qualitative, they provide a helpful means of assessing the pattern in solubilities for series of ionic salts we stress ionic because equations 6.55 and 6.56 assume an electrostatic model. Our discussions in Section 5.15 and earlier in this section indicated how partial covalent character in silver halides affects solubility trends. 

Ionic compounds have very peculiar solubility trends. Some are highly soluble, whereas some others have very little solubility. The solubility of ionic compounds can be explained in terms of the interactions between the ions and the water molecules. Let s take sodium chloride as an example. Sodium chloride has a solubility of 360 g per liter or 36 g per 100 ml at room temperature. 

The identification, structural and thermal characterization of new polymorphs is an important topic in solid-state chemistry and requires a battery of techniques that includes X-ray diffraction and spectroscopic methods, in addition to thermal analysis methods and dissolution techniques to determine solubility trends. Such studies are described by Caira in Chapter 16, as well as more recent theoretical techniques aimed at the prediction of the crystal structures of new polymorphs. Crystal polymorphism is particularly important in pharmaceutical products, so there is an emphasis on this area. Systems displaying solvatomorphism (the ability of a substance to exist in two or more crystalline phases arising from differences in their solvation states) molecular inclusion and isostructurality (the inverse of polymorphism) are also given due attention in this chapter. 

The solubility trends for another ubiquitous substance in natural products, water, is illustrated in Figure 3 [14]. Here the mole percent of water dissolved in SC-CO2 as a function of pressure and temperature is shown. The solubility trends as a function of pressure are relatively monotonic and the amount of dissolved water in SC-CO2 is a slight function of temperature. Contrasting the weight percent solubility of water versus that exhibited by triglycerides in SC-CO2, one certainly sees the effect of solute polarity on the resultant solubility. This slight solubility of water in SC-CO2 can have some profound effects in SFE and SFR as will be shown later. 

One of the more profound effects of solute solubility trends in supercritical fluids is exhibited by the course of an extraction as a function of time or volume of fluid passed... 

This problem-solving tip is based on the simple fundamental concept that opposite charges attract. Higher and more concentrated charges have stronger attractive forces that can resist the solution process that pulls the ions apart. Further, transition metal and heavier main group metalloid cations form stronger bonds to most anions. You can use these concepts to explain most of the solubility trends. 

Our discussions in Section 6.15 and earlier in this section indicated how partial covalent character in silver halides affects solubility trends. 

The explanation for the observed solubility trend in the aUrahne earth hydroxides is that the lattice enCTgy decreases more rapidly in the series Mg(OH)2, Ca(OH)2, Sr(OH)2, and Ba(OH)2 than does the energy of hydration in the series of ions Mg Ca ... 

Note in each case trends in the ease of precipitation reflecting solubility trends. 

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