A change in ionic strength at ethanol addition

[MichaelZ20]

Hi!
Can somebody help to explain the strange behavior of ionic strength at ethanol addition to a brine.

#DATABASE c:\phreeqc\database\pitzer.dat
SOLUTION_MASTER_SPECIES
Ethanol Ethanol 0.0 46.7 46.7

SOLUTION_SPECIES
Ethanol = Ethanol
   log_k 0
   Vm 58.39
END
   
SOLUTION_SPREAD
units ppm                                    
Number   pH   Alkalinity           Cl           S(6)           N(5)    Na           K   Ca   Mg   B      
              as HCO3                              
1         7.34      854           2994   4539   475    2619   53   844   526   6.6
END

USE SOLUTION 1

EQUILIBRIUM_PHASES
Gypsum 0 0

REACTION 1
Ethanol 1           
10 moles in 200 steps  # 467 g/kgw         

USER_GRAPH 1
-headings RXN Ionic_strength SI_Gypsum
-axis_titles "Ethanol addition, g/kgw" "Ionic strength" "SI_Gypsum"
-chart_title ""
-axis_scale x_axis 0 500 50
10 GRAPH_X RXN*GFW("C2H5OH")
20 GRAPH_Y MU
30 GRAPH_SY SI("Gypsum")
END

[dlparkhurst]

The addition of ethanol does not affect the ionic strength directly because it is uncharged. However, the addition does affect the activity of water. The activity of water is basically the mole fraction of water, which decreases as the concentration of ethanol increases.

So, the activity of water decreases, and the following mass action equation applies:

K = [H+][OH-]/[H2O]

where [] indicates activity. So the activity product of H+ and OH- decreases as activity of water decreases. In this system, the ionic strength does decrease, but it is near zero, so the activity coefficients remain close to 1.0. That means the activity product decrease must be caused by concentration decreases in H+ and OH-, which are the only contributions to ionic strength. Thus, ionic strength has to decrease.

[MichaelZ20]

David, thank you for your reply, but when I run the file I get the more complicated change in ionic strength.
In the initial brine (before ethanol addition) it is 0.2251.
After adding the 1-st portion of ethanol (0.05 mol) it jumps to 0.2137, and then gradually increases to the initial value (0.2251 at adding 8.75 moles of ethanol). After this, the ionic strength stays constant.

[dlparkhurst]

The tabs of SOLUTION_SPREAD do not transfer through the forum, and in my previous post, I neglected to fix them, so I was using pure water instead of the brine. My bad.

When the brine is used, the effects are still related to the activity of water. Incremental addition of ethanol decreases the activity of water. The mass action equation:

K = [Ca+2][SO4-2][H2O]^2

indicates that as activity of water decreases, concentration of Ca+2 and SO4-2 must increase if equilibrium obtains. Initially, the solution is supersaturated with gypsum, and gypsum precipitates. As more ethanol is added, gypsum becomes more soluble, and less and less gypsum precipitates, so the concentrations of Ca+2 and SO4-2 progressively increase; larger concentrations cause ionic strength to increase. However, at a point, the gypsum is undersaturated and no longer precipitates, at which point, Ca+2, SO4-2, and ionic strength are more or less constant.

In detail, continued addition of ethanol does decrease the activity of water, but the effect is much less than the changes due to gypsum precipitation.

[MichaelZ20]

David, thank you very much. Now it is clear.
The data have been taken from V. Gomis, M.D. Saquete, J. Garcia-Cano (2013). CaSO4 solubility in water–ethanol mixtures...