Ion-size parameter in newly defined species

[mstec@ichpw.pl]

Hi,

I'm trying to add N-methyldiethanolamine species to my simulation. Basic definition (association reaction log K) was straight forward:

SOLUTION_MASTER_SPECIES
Mdea    Mdea    1.0   119.16 119.16
SOLUTION_SPECIES
Mdea = Mdea
    log_k     0

H+ + Mdea = MdeaH+
    log_k     8.567
    -a_e 49.585588 0.004349 -7.934690 -17.090111 5.393322

And now I'm trying to add ion-size -gamma parameter.  Literature sources state that either MDEA or MDEAH+ have 4.5 ionic diameter.   As far as I found in phreeqc.dat database I have to add -gamma 4.5 0.0 line to primary master species definition. But what about proton association reaction? In second reaction I have 2 ionic species and 1 molecular, having 9.0 (H+), 4.5 (MDEA) and 4.5 (MDEAH+) diameters. What is the interpretation of -gamma parameter in such case?

Best regards,
Marcin Stec

[dlparkhurst]

For uncharged species, the ion size parameter has no effect because the Debye-Huckel term is multiplied by the charge of the species.

The ion size parameter is an extension of the original D-H theory, which assumed species were point charges. While it may have some physical significance, the ion size is often a fitting parameter to match mean-activity coefficient experimental data. To be honest, individual-ion activity coefficients have enough uncertainties, that I would not worry too much about the value. Many species in the most of the ion-association databases use the Davies equation, which effectively assigns the same ion-size parameter to each species.

[mstec@ichpw.pl]

Thank You for the explanation. I'll try to use the ion size as a fitting parameter and compare the model and experimental results.