Incorrect activity coefficients for molality-based phases

[speth]

Problem description

The molar activity coefficients (returned by getActivityCoefficients) for phases that use molalities internally (those derived from the MolalityVPSSTP class) are not correct.

Steps to reproduce

For the Debye-Huckel model:

>>> p = ct.Solution('test/data/debye-huckel-all.yaml', 'debye-huckel-dilute')
>>> p()

  debye-huckel-dilute:

       temperature             300  K
          pressure          101325  Pa
           density         55.9229  kg/m^3
  mean mol. weight         21.3322  amu
         potential               0  V
                pH           8.861

                          1 kg            1 kmol
                       -----------      ------------
          enthalpy    -1.22199e+07       -2.607e+08     J
   internal energy    -1.22217e+07       -2.607e+08     J
           entropy         4194.41        8.948e+04     J/K
    Gibbs function    -1.34782e+07       -2.875e+08     J
 heat capacity c_p          158171        3.374e+06     J/K
 heat capacity c_v    <not implemented>

                           X           Molalities         Chem.Pot.    ChemPotSS    ActCoeffMolal
                                                          (J/kmol)      (J/kmol)
                     -------------    ------------     ------------  ------------    ------------
            H2O(L)      0.738115          55.5093     -3.07518e+08   -3.06815e+08        1.02227
               Na+      0.124393           9.3549     -2.57445e+08   -2.57956e+08       0.131209
               Cl-      0.124393           9.3549     -1.83605e+08   -1.84117e+08       0.131209
                H+   1.39607e-10       1.0499e-08     -5.08919e+07              0       0.131209
               OH-   1.83035e-08       1.3765e-06     -2.65584e+08   -2.26855e+08       0.131209
          NaCl(aq)     0.0130966          0.98492     -4.32958e+08    -4.3292e+08              1
          NaOH(aq)   5.16408e-08       3.8836e-06     -5.14827e+08   -4.83751e+08              1
          SiO2(aq)   4.01294e-07       3.0179e-05     -9.25875e+08   -8.99913e+08              1
      NaH3SiO4(aq)   9.14817e-07       6.8798e-05     -1.74563e+09   -1.72172e+09              1
           H3SiO4-   1.36043e-08       1.0231e-06     -1.50517e+09    -1.4657e+09       0.131209

>>> p.activities
array([7.54555306e-01, 1.22745026e+00, 1.22744995e+00, 1.37756687e-09,
       1.80609658e-07, 9.84920000e-01, 3.88360000e-06, 3.01790000e-05,
       6.87980000e-05, 1.34240277e-07])

>>> 0.131209 * 9.3549  # molal activity coefficient times molality for Na+
1.2274470741  # matches reported activity

>>> p.activity_coefficients * p.X  # should equal the activities, but doesn't
array([7.54555306e-01, 2.21125165e-02, 2.21125109e-02, 2.48168672e-11,
       3.25368299e-09, 1.77433338e-02, 6.99630540e-08, 5.43674685e-07,
       1.23939597e-06, 2.41833859e-09])

Likewise for IdealMolalSoln:

>>> p = ct.Solution('test/data/thermo-models.yaml', 'ideal-molal-aqueous')
>>> p()

       ideal-molal-aqueous:
     
            temperature          298.15  K
               pressure          101325  Pa
                density         12.0577  kg/m^3
       mean mol. weight           18.07  amu
              potential               0  V
     
                               1 kg            1 kmol
                            -----------      ------------
               enthalpy     0.000735017          0.01328     J
        internal energy        -8403.38       -1.518e+05     J
                entropy         5.18177            93.63     J/K
         Gibbs function    -2.15747e+06       -3.899e+07     J
      heat capacity c_p          1595.8        2.884e+04     J/K
      heat capacity c_v    <not implemented>
     
                                X           Molalities         Chem.Pot.    ChemPotSS    ActCoeffMolal
                                                               (J/kmol)      (J/kmol)
                          -------------    ------------     ------------  ------------    ------------
                 H2O(l)      0.997484          55.5093     -3.89686e+07   -3.89623e+07       0.999997
                CO2(aq)    0.00179697              0.1     -4.46703e+07   -3.89623e+07              1
                H2S(aq)    0.00053909             0.03     -4.76549e+07   -3.89623e+07              1
                CH4(aq)   0.000179697             0.01     -5.03783e+07   -3.89623e+07              1

>>> p.activities
array([0.99748108, 0.1       , 0.03      , 0.01      ])  # equals molal activity coeff times molality, except for solvent

>>> p.activity_coefficients
array([0.99999682, 1.0025221 , 1.0025221 , 1.0025221 ])  # not just returning molality based coefficients

>>> p.activity_coefficients * p.X
array([9.97481078e-01, 1.80150000e-03, 5.40450000e-04, 1.80150000e-04])  # should equal activities, but don't

System information

• Cantera version: 2.6.0 or main at 1ab81ac

Additional context

This was discovered as part of #1299, which implements Cantera/enhancements#114.