Polyacrylic acid (PAA)

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Polyacyrlic Acid (PAA) and Molecular Weights

Question: Maybe this is a silly question and I'm missing something obvious, but "Why is Mat Code 481 a poly acrylic acid 2000 molecule but the associated inflow PAA is only 1/3 of a molecule ?" What happens is that the PAA inflow uses the molecular weight for 1/3 of the Mat Code 481, but the equilibrium uses the full molecule.

Analysis: When the PAA work was done in 2005, OLI explored the literature data for PAA of all possible molecular weights and found that when PAA concentrations (C_PAA) are in monomolar of PAA (number of monomer units per unit volume or solvent mass), the C_PAA vs pH plots are independent of the molecular weights of PAA. In fact, most literature data for PAA concentrations were reported as monomolar (or monomolal). OLI therefore developed only the PAA with molecular weight of 2000 for a specific data service project, with the intention of using this PAA for all PAA with different molecular weight, as long as the input concentrations for PAA are in monomolal. The model thus developed reproduces well the titration curves for PAA with different molecular weights. The user just use the name "PAA" as their inflow component to introduce PAA, and use monomolal for their PAA concentrations.

For the PAA species that show in the mod file, there are indeed 4 species: PAA2000ION, HPAA2000ION, H2PAA2000ION, and H3PAA2000ION, but the last one is actually a neutral species of H3PAA2000 with a charge of 0. The reason being using -ION as a suffix in the name is to avoid letting the user to use a more complex name "H3PAA2000" to introduce PAA in their inflow, which would also cause confusion about the molecular weights (of having to be 2000). Instead, a species called "PAA" with only general information was created to be used to introduce PAA easily as its name shows. The charge balance is done in the normal way for all other systems.

As the actual species (PAA2000ION, HPAA2000ION, H2PAA2000ION, and H3PAA2000ION) for the equilibrium calculations are defined for one mole of PAA(2000), and the "PAA" as defined in the MSEPUB is only used as input name (it has only general information), the definition used in the material code (1/3 of 481) does not affect the calculations.

Editor: James W Berthold Author: Peiming Wang