What is a "BINTER"?

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What the heck is a BINTER?

The BINTER parameter in the database - more correctly formatted Binter - is a type of interaction parameter between an ion and a neutral molecule in the aqueous phase. This parameter only appears in the AQ thermodynamic framework (not MSE) and is generally replaced by the Pitzer activity model.

So what is it?

Before Pitzer

In the days before the Pitzer activity model was developed, J. Setchenow (Z.Physik.Chem., 4, 117 (1889)) created an activity relationship to model the effect of salts on the solubility of gasses.

Neutral molecules in water are affected by other species in solution. The salting in and out of a gas is a typical example. When Oxygen is dissolved into pure water, it has a typical solubility. When salt is added, the solubility decreases. This is most-likely due to an interaction between the sodium ions and the neutral oxygen molecule and the interaction between the chloride ions and the neutral oxygen molecule

The solubility of oxygen in NaCl solutions at 25 oC, 1 atmosphere

This characterizes a phenomenon known as salting in/out. The formulation is in terms of the ratio of solubilities in pure water to an aqueous salt solution at a constant temperature.

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Solubility of gas in pure water
Solubility of gas in a salt solution
Setchenow cofficiient
Concentration of salt

In this case, the K is approximately equal to -0.0002. Unfortunately, this approach is limited to a single temperature and a single salt.

The Binter parameter was introduced into the OLI code in the 1970's to model the Setchenow effect in multi-component systems. A coefficient was regressed for an ion-molecule interaction. The equation regressed against was:

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The Activity Coefficient for molecule m
Binter for molecule m
Ionic Strength
for all ions in solution


molality of ion i
Binter for molecule m

Binter has largely been replaced in the OLI code by either Pitzer or the MSE framework. You will still see Binter parameters in AQ framework PUBLIC databank.

Author: Jim Berthold