Aggregation of Organics

Much of the essential physical chemistry of the hydrophobic effect has emphasized the transfer of small organics from the gas phase to water. As we have said, hydrocarbons have very low solubilities in water. While this is the characteristic feature of the hydrophobic effect, other thermodynamic effects are seen, including unusual entropy effects and often large heat capacity effects. To a very good approximation, AG° of transfer scales with surface area of the hydrocarbon that is exposed to water on dissolution. The exact scaling factor is debated and appears to depend on context. Values as low as 15 cal/ mol in AG° for every A of exposed aliphatic or aromatic hydrocarbon and as high as 75 cal / mol A are reported, but a more typical range is 30-50 cal/mol A. If we settle on 40 cal/mol A, and assume a surface area of 29 A for a CH2 in an alkane, then every additional CH2 adds 1.2 kcal/mol of destabilization in a hydrophobic effect. 

Given the 30-50 cal / mol A value, one would expect that once they are in water, hydrocarbons should minimize their exposed surface area. They can do this in two ways shape changes and aggregation. As an example of the first, consider n-butane in water. Not surprisingly, gauche butane is a more compact structure than anti butane. We would expect a 

Some Values of it and the Incremental Gibbs Free Energy of Transfer from n-OctanoI to Water  

Gauche butane reduces exposed surface area 

In nature, the more common amphiphiles are phospholipids. These are derivatives of glycerol (1,2,3-trihydroxypropane), in which two alcohols form esters with long chain carboxylic acids. The third alcohol forms a phosphate ester, and the phosphate then makes another ester with a simpler alcohol. This creates structures such as phosphatidyl choline, phosphatidyl serine, and phosphatidyl ethanolamine (see next page). The polar group can 

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This aggregation of organic molecules in water actually leads to accelerated rates for some organic reactions in water. Diels-Alder and other sigmatropic reactions work very well in water, despite not being soluble or miscible in that solvent. This effect is discussed further in Chapter 7. 

Micelles Electrically charged colloidal particles that are usually composed of aggregates of organic... 

S.D. Cox, T.E. Gier, and G.D. Stucky, Second Harmonic Generation by the Self-aggregation of Organic Guests in Molecular Sieve Hosts. Chem. Mater., 1990, 2, 609-619. 

Micelles, discrete aggregates of organic molecules or ions that can incorporate catalytic groups, act as individual microreactors, their performance being influenced by the distribution of reactants between the micelle and the surrounding liquid. 

Trivalent and some bivalent cations can precipitate HSs under the right pH conditions from dilute solutions. Precipitation or aggregation of organics with cations is facilitated at low temperatures, most likely due to the copredpitation effect with carbonates (Steinberg and Muenster (1985)). However carbonates ate more soluble at low temperatures. 

For HA, at low pH (3.5), particulates were measured in the absence and presence of calcium. Calcium increased the measured particulate size, indicating aggregation of organic colloids. At pH 8 particles formed in the absence of calcium, which shows that there may be some undissolved HA. At pH 10, no particulates were measured and the HA was fully dissolved. Once calcium was added at pH 10, particulates were measured, possibly confirming the co-precipitation of organics with calcite. In the presence of 25 mM calcium, particles are smallest at pH 8 and largest at pH 3.5. Organic concentration did not have a measurable effect on the size of the colloids, and at 25 mM CaCl 2 particles formed at 25, 50, and 75 mgL" HA as DOC. 

Nonpolar substances are in this case liquid, more rarely solid aggregations of organic matter whose composition is dominated by carbon (80-90% by weight) and hydrogen (10-15%), and are also present O, S and N. A typical feature of these substances is almost total absence in their composition of polar compoxmds and of a clearly dominating component, which would play a role of the solvent. They include a huge number of various compounds with a broad range of physical properties. 

R 655 X. Jiang and G. Ji, Two Important Frontiers of Physical Organic Chemistry Aggregation of Organic Molecules and Radical Chemistry , Huaxue Tongbao, 2003, 66,147... 

The most important function of carbohydrates is their role as a mediator in cellular communication [190]. They participate in processes such as insemination, differentiation and aggregation of organs as well as in bacterial and viral cell infection [191]. It is obvious that new therapeutic possibilities are opening up in the fight against disease. Free ohgosaccharides found in human milk are... 

Pastore M, De Angelis F (2010) Aggregation of organic dyes on Ti02 in dye-sensitized solar cells models an ab initio investigation. ACS Nano 4(l) 556-562... 

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