Stable phases were obtained even with 0 ppm of ions concentration

[rkramamoorthy]

Hi all,

I have been playing with PHREEQC to understand the possible NaYF4 phases that emerge in aqueous solutions for different initial concentrations of starting species. Surprisingly, the same phase was identified as the stable phase for the entire simulation region. So, i put 0 ppm for Na and Y species. Still, i am getting the phase that was stable for the earlier conditions. Now my questions are:

1. How can the thermodynamic condition for the different concentrations of ion species be controlled in PHREEQC?
2. Is it possible with the PHREEQC to simulate a thermodynamically stable phase with different sizes of crystals, in particular, at nanometer size?

For your reference, the code is attached below:

Code: [Select]

TITLE NaYF4-Test

SOLUTION 1 Pure water
        pH      7.0
        temp    25.0
  units ppm
  F 32800
  Na 0
  Y 0
SOLUTION_MASTER_SPECIES
Y Y+3 0 Y 88.906

SOLUTION_SPECIES
Y+3 =  Y+3
log_k 0

PHASES
Alpha-NaYF4

NaYF4 = Na+ + Y+3 + 4F-
-log_k -23.26

Beta-NaYF4

NaYF4 = Na+ + Y+3 + 4F-
-log_k -25.69
 

EQUILIBRIUM_PHASES 1
        Alpha-NaYF4          0.0     1.0
        Beta-NaYF4           0.0     1.0
REACTION_TEMPERATURE 1
        25.0 75.0 in 51 steps
USER_GRAPH 1 Example 2
        -headings Temperature Alpha-NaYF4 Beta-NaYF4 Y
        #-axis_scale x_axis 25 75 5 0
        #-axis_scale y_axis auto 0.05 0.1
        -axis_titles "Temperature, in degrees celsius" "Saturation index" "Molality"
        -initial_solutions false
  -start
  10 graph_x TC
  20 graph_y SI("Alpha-NaYF4") SI("Beta-NaYF4")
  30 graph_sy TOT("Y")
  -end
END

[dlparkhurst]

No surprise. The two phases have the same reaction, but Beta has the smaller log K. It will be more stable under all conditions. Kinetically, Alpha could form, but thermodynamically, it should convert to Beta.

Different crystal sizes would have different free energies, which translates to different log Ks. Alpha and Beta would be an example. You could form small crystals of Alpha through KINETICS if the precipitation rate were faster than Beta. But again, thermodynamically, Alpha should convert to Beta, or smaller crystals should convert to larger crystals.

Code: [Select]

Alpha-NaYF4
NaYF4 = Na+ + Y+3 + 4F-
-log_k -23.26

Beta-NaYF4
NaYF4 = Na+ + Y+3 + 4F-
-log_k -25.69



[rkramamoorthy]

Is it possible, using PHREEQC, to identify the size-induced stable phase? If yes, how can I program that? If not, please suggest other alternatives.

What i understood is that if the concentration of species is zero, then the possibility of forming any targeted phase should be zero. But PHREEQC predicts Beta is the thermodynamically stable phase even with zero ions. How is that possible?

[dlparkhurst]

You provided 1 mole of Alpha and 1 mole of Beta in the EQUILIBRIUM_PHASES definition. If you look at the output, you will see that all of the Alpha converted to Beta.

I don't know how to deal with size distributions.