Apparent inhibition of thermal decomposition of hydrogencarbonate ion by poly(acrylic acid). The effect of molar mass and end-group functionality

• Alkaline scale formation is a thermodynamically controlled process.
• Polyacrylic acid appears to prevent heterogeneous decomposition of hydrogencarbonate.
• Hydrophobic end-group functionalization improves this behavior by 8 to 12% (at 10 ppm).

The thermal decomposition of the hydrogencarbonate ion has been previously described by a bimolecular mechanism or a unimolecular mechanism. In this work the Gibbs free energy of the competing reactions for both the unimolecular and bimolecular mechanisms was calculated for typical concentrations found in thermal desalination plants. Activity coefficients were estimated using the Pitzer equations. At low temperature the bimolecular mechanism is thermodynamically favored, while above 80 °C the unimolecular mechanism is favored, consistent with observations of alkaline scale formation in thermal desalination plants. The rate coefficient of thermal decomposition of HCO3− at 97.2 °C in the absence and presence of 10 ppm of poly(acrylic acid) (PAA) with different end groups and molar mass was determined. PAA was found to retard the rate of decomposition by up to 49% and for all end groups of PAA the rate coefficient of thermal decomposition of 40 ppm HCO3− increased with increasing molar mass. The results are consistent with PAA preventing heterogeneous decomposition of HCO3− on interfaces. The rate of partitioning of PAA to these interfaces should increase with decreasing molar mass and resulting mobility of PAA, and may also be affected by self-assembly behavior.