Formaldehyde aqueous-phase equilibrium

Concentrations of S(IV) in fogwater were far in excess of those expected to be in equilibrium with ambient S02(g). Elevated formaldehyde concentrations suggest the formation of a formaldehyde-S(IV) complex kinetic and model studies have shown that this complex is very stable and that its formation leads to high aqueous-phase S(IV) concentrations. 

FIGURE 7.13 Equilibrium fraction of total formaldehyde in the aqueous phase as a function of cloud liquid water content at 298 K. 

While Chapter 5 deals with models which are applicable to a wide variety of non-electrolyte systems, separate chapters have been composed where systems are described which require specialized models. These are electrolytes (Chapter 7), polymers (Chapter 10) and systems where chemical reactions and phase equilibrium calculations are closely linked, for example, aqueous formaldehyde solutions and substances showing vapor phase association (Chapter 13). Special phase equilibria like solid-liquid equilibria and osmosis are discussed in Chapters 8 and 9. respectively. 

Formaldehyde is a low-boiling substance with a normal boiling point of approx. 254 K. It is not stable in its pure form, so it usually occurs in aqueous or methanolic solutions. Mixtures of formaldehyde and water or alcohols are not binary solutions in the usual sense, as formaldehyde reacts with both of them to a wide variety of species which are not stable as pure compounds themselves. Therefore, the standard procedure for building up a thermodynamic model by setting up the pure component properties and the binary interaction parameters fails in this case. The formaldehyde-water-methanol system is a good example freactive phase equilibrium, where a special model has to be developed. This has been done by the group of Maurer [2-6]. 

As with resoles, we can use a three-phase model to discuss formation of a novolac. Whereas the resole is activated through the phenol, activation in novolacs occurs with protonation of the aldehyde as depicted in Scheme 12. The reader will note that the starting material for the methylolation has been depicted in hydrated form. The equilibrium level of dissolved formaldehyde gas in a 50% aqueous solution is on the order of one part in 10,000. Thus, the hydrated form is prevalent. Whereas protonation of the hydrate would be expected to promote dehydration, we do not mean to imply that the dehydrated cation is the primary reacting species, though it seems possible. 

The kinetics of the formation and condensation of mono- and dimethylolureas and of simple UF condensation products has been studied extensively. The formation of mono-methylolurea in weak acid or alkaline aqueous solutions is characterized by an initial fast phase followed by a slow bimolecular reaction [4,5]. The first reaction is reversible and is an equilibrium which proceeds to products due to the uptake of the products, the methylolureas, by the second reaction. The rate of reaction varies according to the pH with a minimum rate of reaction in the pH range 5 to 8 for a urea/formaldehyde molar ratio of 1 1 and a pH of 6.5 for a 1 2 molar ratio [6] (Fig. 1). The 1 2 urea/formaldehyde reaction has been proved to be three times slower than the 1 1 molar ratio reaction [7]. 

The Sj 2 reaction, X + RY XR + Y", has been simulated with MC equilibrium calculations by Jorgensen and coworkers [81, 82]. The procedure used by these authors involves three steps i) the lowest energy reaction path is determined for the in vacuo system by using ab initio molecular orbital calculations ii) inter-molecular potential functions are obtained to describe the interactions between the substrate and a solvent molecule these potentials depend on the internal structure of the substrate iii) MC simulations are carried out to determine the free energy profile for the reaction in solution. This is a difficult computational task since importance sampling methods are required to explore all the values of the reaction coordinate. A similar technique was used by Madura and Jorgensen [83] in simulating the nucleophilic addition of hydroxide ion to formaldehyde in the gas phase and in aqueous solution. 

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