Acids molecular structure affecting strength

So far, we have focused on how differences in molecular structure affect the solubilities and activity coefficients of organic compounds in pure water at 25°C. The next step is to evaluate the influence of some important environmental factors on these properties. In the following we consider three such factors temperature, ionic strength (i.e., dissolved salts), and organic cosolutes. The influence of pH of the aqueous solution, which is most important for acids and bases, will be discussed in Chapter 8. 

The strength of acids and bases is affected by molecular structure, steric effects, resonance, inductive effects, hydrogen bonding, and solvation. 

The entire network structure is divided into several components, so-called molecular budding-blocks or tectons, which consist of core parts and interaction parts. The geometrical shape of the core part controls the direction of the interaction. The interaction parts are substituents such as carboxylic acid groups for hydrogen bonds and carboxylates for coordination bonds, which control the interaction strength and affect the equilibrium of the crystallization process. By choosing appropriate molecular building blocks, we can construct the desired molecular networks and frameworks. 

Some polymers such as poly(acrylic acid) or polyacrylamide precipitate from aqueous solutions when cooled (normal solubility behavior) whereas others such as poly(ethylene oxide), poly(propylene oxide), or poly(methacrylic acid) phase separate when heated (inverse solubility behavior). Solution turbidimetiy is often used to obtain plots of phase-separation temperatures termed cloud point vs concentration for fixed solvent conditions. Changes in ionic strength, molecular weight, and addition of co-solvents or structure breakers affect the shapes of phase behavior curves. The important conclusion of such studies is that the total free energy of the polymer and water must be considered to predict phase behavior. The structin-e and dynamics of water surroimding polynucleotides, proteins, polysaccharides, and lipids are also major determinants of biological activity (8-10). 

Structural elements of unsaturated polyester resins that affect the properties of the cured products include molecular weight and molecular weight distribution, distribution of various repeating units, type of unsaturation, and ratio of acid to glycol components. An overall concept regarding the structural features of polyester resin and the performance properties of cured product is not available. However, it is known that the structural features determine the values of hardness, impact strength, heat-distortion temperature, water absorption, chemical resistance, and heat resistance. 

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