Molecular solutes in water

Some compounds that consist of molecules (covalent compounds) are soluble in water and some are not. Some of those that are soluble form ions when they dissolve, but most do not. Let us now take a look at what happens at the molecular level in these situations. 

A few polar covalent solutes, namely acids, dissolve in water and also form ions in the process. The strength of the interaction with the water molecule is sufficient to break a covalent bond in these solutes and to form ions. Inorganic acids, such as HCl, H2SO4, and HNO3, completely ionize and are called strong acids. This ionization can be written as follows for hydrochloric acid (HCl), for example  

FIGURE 10.3 An illustration of a poiar covalent compound, such as sugar, dissolving in water. The water molecules, by virtue of their polar nature, pull the polar sugar molecules from the crystal array, and the compound dissolves. 

FIGURE 10.4 An illustration of a strong acid dissolving in water. The acid is a polar covalent compound, like sugar, but when it dissolves in water, it ionizes—that is, the hydrogen atoms break away from the molecule, forming hydrogen ions (+ ions) and - ions. It is a total, complete ionization. 

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In a very extensive test of the SM5 method (a type of GBM), Hawkins et al. found the average absolute deviation in AGsoivation to be 0.38 kcal/mole for 260 molecular solutes in water and in 90 organic solvents 131 for ions in water, it was 3.8 kcal/mole, for experimental AGsolvation between -58 and -110 kcal/mole. Semiempirical quantum-mechanical procedures were used. 

D[2t A is calculated with Eq. (6-31) using MrB = 212. In Table 6-7 diffusion coefficients of high molecular solutes in water calculated with Eq. (6-32) are compared with experimental values (Tanford, 1961). 

A surfactant molecule is an amphiphile, which means it has a hydrophilic (water-soluble) moiety and a hydrophobic (water-insoluble) moiety separable by a mathematical surface. The hydrophobic tails of the most common surfactants are hydrocarbons. Fluorocarbon and perfluorocarbon tails are, however, not unusual. Because of the hydrophobic tail, a surfactant resists forming a molecular solution in water. The molecules will tend to migrate to any water-vapor interface available or, at sufficiently high concentration, the surfactant molecules will spontaneously aggregate into association colloids, i.e., into micelles or liquid crystals. Because of the hydrophilic head, a surfactant (with a hydrocarbon tail) will behave similarly when placed in oil or when put in solution with oil and water mixtures. Some common surfactants are sodium or potassium salts of long-chained fatty acids (soaps), sodium ethyl sulfates and sulfonates (detergents), alkyl polyethoxy alcohols, alkyl ammonium halides, and lecithins or phospholipids. 

Pluronics, also known as poloxamers, are a class of synthetic block copolymers which consist of hydrophilic poly(ethylene oxide) (PEO) and hydrophobic poly(propylene oxide) (PPO), arranged in an A-B-A triblock structure, thus giving PEO-PPO-PEO (Fig. 11.7) (Batrakova and Kabanov 2008). They can be found either as liquids, pastes or solids (Ruel-Gariepy and Leroux 2004). Due to their amphiphilic characteristics (presence of hydrophobic and hydrophilic components), pluronics possess surfactant properties which allow them to interact with hydrophobic surfaces and biological membranes (Batrakova and Kabanov 2008). Being amphiphilic also results in the ability of the individual block copolymers, known as unimers, to combine and form micelles in aqueous solutions. When the concentration of the block copolymers is below that of the critical micelle concentration (CMC), the unimers remain as molecular solutions in water. However, as the block copolymer concentration is increased above the CMC, the unimers will self-assemble and form micelles, which can take on spherical, rod-shaped or lamellar geometries. Their shapes depend on the length and concentration of the block copolymers (i.e. EO and PO), and the temperature (Kabanov et al. 2002). Micelles usually have a hydrophobie eore, in this case the PO chains, and a hydrophilic shell, the EO ehains. 

Enzymes are proteins of high molecular weight, several of which have been isolated in a pure State consequently their precise nature is in some instances still obscure. They form solutions in water and in dilute salt solutions, and are precipitated when such solutions are saturated with ammonium sulphate. 

MethylceUulose with a methyl DS less than about 0.6 is alkali-soluble. Erom about 1.6 to 2.4, it is water-soluble (most commercial grades) above 2.4, it is soluble in a wide variety of organic solvents. MethylceUulose solutions in water start to gel at 55° C, independent of molecular weight. The gelation is a function of the DS, rate of heating, and type and amounts of additives such as salts. As the temperature increases, the viscosity initially decreases (typical behavior). When the gelling temperature is reached, the viscosity sharply rises until the flocculation temperature is reached. Above this temperature, the viscosity coUapses. This process is reversible with temperature (75). 

Hydrophobicity ( water-hate ) can dominate the behavior of nonpolar solutes in water. The key observations are (1) that very nonpolar solutes (such as saturated hydrocarbons) are nearly insoluble in water and (2) that nonpolar solutes in water tend to form molecular aggregates. Some authors refer to item 1 as the hydrophobic effect and to item 2 as the hydrophobic interaction. Two extreme points of view have been taken to account for these observations. 

Valence and oxidation state are directly related to the valence-shell electron configuration of a group. Binary hydrides are classified as saline, metallic, or molecular. Oxides of metals tend to be ionic and to form basic solutions in water. Oxides of nonmetals are molecular and many are the anhydrides of acids. 

Melo, T. B. lonescu, M. A. Haggquist, G. W. Naqvi, K. R. (1999). Hydrogen abstraction by triplet flavins. I time-resolved multi-chaimel absorption spectra of flash-irradiated riboflavin solutions in water Spectrochimica Acta, Part A Molecular and Biomolecular Spectroscopy, Vol.55, No.ll, (September 1999), pp. 2299-2307, ISSN 1386-1425. 

Three hundred precipitations of the aqueous solution with alcohol were necessary before a constant-rotating product was obtained. [a]D20 = — 51.4° (water). (Alcohol was removed from the sample by repeated evaporation with water followed by drying for two hours at 80° in a high vacuum.) Cryoscopic determination of the molecular weight in water solution gave an average of 2600 for the original and 2825 for the deacetylated product. 

A second type of ternary electrolyte systems is solvent -supercritical molecular solute - salt systems. The concentration of supercritical molecular solutes in these systems is generally very low. Therefore, the salting out effects are essentially effects of the presence of salts on the unsymmetric activity coefficient of molecular solutes at infinite dilution. The interaction parameters for NaCl-C02 binary pair and KCI-CO2 binary pair are shown in Table 8. Water-electrolyte binary parameters were obtained from Table 1. Water-carbon dioxide binary parameters were correlated assuming dissociation of carbon dioxide in water is negligible. It is interesting to note that the Setschenow equation fits only approximately these two systems (Yasunishi and Yoshida, (24)). 

A quantitative analysis of the structure-retention relationship can be derived by using the relative solubility of solutes in water. One parameter is the partition coefficient, log P, of the analyte measured as the octanol-water partition distribution. In early work, reversed-phase liquid chromatography was used to measure log P values for drug design. Log P values were later used to predict the retention times in reversed-phase liquid chromatography.The calculation of the molecular properties can be performed with the aid of computational chemical calculations. In this chapter, examples of these quantitative structure-retention relationships are described. 

To estimate how many dye molecules fit into the dendritic micelles, UV-titra-tion experiments have been employed. In comparison with the spectra of a pure pinacyanol chloride solution in water, the peaks of the absorption maxima of the dye in the presence of the dendrimer are shifted bathochromically due to solvatochromic effects, which indicates the incorporation of the dye within the branches of the dendrimer. At dye-to-dendrimer molar ratios higher than 4 1, in addition to the bathochromic shifts, hypsochromically shifted peaks start to appear, indicating that the dendrimer is not incorporating further dyes. We interpret this as an incorporation of up to four dyes within the branches of the dendrimer. This observation correlates with the calculated available space within the dendrimer, obtained from the molecular simulations. Further studies of the interactions of the dyes within the dendritic micelle are in progress. 

Since all the derivatives studied, except (III), had a very poor solubility in water (<2 g/1), they were inoculated as suspensions in 4% hydroxypropylcellulose (Klucel J. F., Hercules Co) solutions in water. Such Klucel solutions were shown to be nontoxic after i.p. (intra-peritoneal) inoculation. Moreover, the size of the molecular aggregates in such Klucel solution was much less than 1 pm, as demonstrated by electron microscopy. 

Polyethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) (Pluronic) block copolymer is a very efficient reducing agent and nanoparticle stabilizer. Au NPs of about 10 nm can be stabilized with PEO-PPO-PEO block copolymer solutions in water and at room temperature and using HAuC14 as precursor. The formation of gold nanoparticles is controlled by the overall molecular weight and relative block length of the block copolymer [118]. 

The soluble Kollidon products form reversible complexes with many hydrophobic active substances, and clear solutions in water are thus obtained. This may be affected by the molecular weight. The longer the chains or the higher the -value of the Kollidon type are, the stronger the solubility effect is, and thus the greater the solubility that can be obtained by the active substance. In practice, this effect was mostly exploited for the solubilization of antibiotics in human and veterinary medicine. There are also restrictions on the use of this substance in human parenter-als. In many countries the -value must not exceed 18, and there is also a restriction on the amount to be used for each dose administered in intramuscular application. 

Many oxides of nonmetals form acidic solutions in water and hence are called acid anhydrides. The familiar laboratory acids HN03 and H2S04, for instance, are derived from acidic binary oxides. Even oxides that do not react with water can be regarded as the formal anhydrides of acids. A formal anhydride of an acid is the molecule obtained by striking out the elements of water (H, H, and O) from the molecular formula of the acid. Carbon monoxide, for instance, is the formal anhydride of formic acid, HCOOH, although CO does not react with cold water to form the acid. 

A solution of an unknown molecular substance in water at 300 K gives rise to an osmotic pressure of 4.85 atm. What is the molarity of the solution ... 

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