Extending the diffuse layer model of surface acidity behavior: I. Model development

• Published in: Chemical speciation and bioavailability Vol. 18; no. 3; pp. 105 - 116
• Main Author: Loux, Nicholas T.
• Format: Journal Article
• Language: English
• Published: Taylor & Francis 01-01-2006
• Subjects: acidity constant; charging energy; diffuse layer model; excess free energy; hydration energy
• ISSN: 0954-2299; 2047-6523
• DOI: 10.3184/095422906782146221

Considerable disenchantment exists within the environmental research community concerning our current ability to accurately model surface-complexation-mediated low-porewater-concentration ionic contaminant partitioning with natural surfaces. Several authors attribute this unacceptable variation to uncertainties in our understanding of surface acidity behavior. In support of this contention, many authors are not satisfied with our ability to model surface acidity behavior in controlled, pure phase laboratory systems.
The present work was designed to more closely examine the theoretical aspects of those variable energies likely to be contributing to the excess free energies associated with surface acidity behavior. Conclusions from the work include: (1) the excess free energy term associated with charge/potential relationships is given by: ΔG
excess,sigma/psi
= ΔG
electrostatic
+ ΔG
hydration
+ ΔG
dipole
+ ΔG
Debye_Huckel
, (2) the excess free energy terms included in effective acidity constant behavior includes a charging energy term: ΔG
excess,pK
= ΔG
excess,sigma/psi
+ ΔG
charging
, (3) under conditions where -ΔG
excess,sigma/psi
»RT, ΔG
excess,sigma/psi
≈ ΔG
excess,pK
≈ ΔG
electrostatic
+ ΔG
hydration
(thereby supporting historical methodologies), (4) under conditions where -ΔG
excesssigma/psi
≈ RT, ΔG
excess
is better approximated by: ΔG
excess,sigma/psi
≈ ΔG
electrostatic
ΔG
hydration
and ΔG
excess,pK
≈ ΔG
excess,sigma/psi
+ ΔG
charging
, (5) ΔG
charging
becomes increasingly significant with decreased counterion condensation, and (6) asymmetric behavior in the charge dependency of effective acidity constant behavior is predicted-particularly when charging energies become significant.