SeaFreeze 1.1.1

Thermodynamic properties of the phases of H2O and NaCl (aq)

SeaFreeze

V1.1.1

The SeaFreeze package allows to compute the thermodynamic and elastic properties of water and ice polymorphs (Ih, II, III, V, VI and ice VII/ice X) in the 0-100 GPa and 220-10000 K range, with the study of icy worlds and their ocean in mind. It is based on the evaluation of Gibbs Local Basis Functions parametrization (https://github.com/jmichaelb/LocalBasisFunction) for each phase. The formalism is described in more details in Brown (2018), Journaux et al. (2019), and in the liquid water Gibbs parametrization by Bollengier, Brown, and Shaw (2019).

Installation

This package will install SeaFreeze, LBFTD, and MLBspline and their dependencies.

Requires Python ≥ 3.11.

Run the following command to install:

pip install SeaFreeze

To upgrade to the latest version:

pip install --upgrade SeaFreeze

getProp

Calculates thermodynamic and elastic properties of a phase of water or solution.

Usage

The main function of SeaFreeze is getProp, which has the following parameters:

• PT: the pressure (MPa) and temperature (K) conditions at which the thermodynamic quantities should be calculated -- note that these are required units, as conversions are built into several calculations This parameter can have one of the following formats:
  • a 1-dimensional numpy array of tuples with one or more scattered (P,T) tuples
  • a numpy array with 2 nested numpy arrays, the first with pressures and the second with temperatures -- each inner array must be sorted from low to high values a grid will be constructed from the P and T arrays such that each row of the output will correspond to a pressure and each column to a temperature
• phase: indicates the phase of H₂O. Supported phases are
  • 'Ih' — ice Ih; Feistel & Wagner 2006
  • 'II' — ice II; Journaux et al. 2020
  • 'III' — ice III; Journaux et al. 2020
  • 'V' — ice V; Journaux et al. 2020
  • 'VI' — ice VI; Journaux et al. 2020
  • 'VII_X_French' — ice VII / ice X; French & Redmer 2015
  • 'water1' — liquid water ≤ 500 K, ≤ 2300 MPa; Bollengier et al. 2019 (recommended for 200–355 K)
  • 'water2' — liquid water up to 100 GPa; Brown 2018
  • 'water_IAPWS95' — IAPWS-95; Wagner & Pruss 2002
  • 'NaClaq' — stitched LP+HP NaCl(aq), P=[0, 10001] MPa, T=[229, 2001] K (recommended for NaCl)
  • 'NaClaq_LP' — 2026 low-P NaCl(aq) spline only, P=[0, 1001] MPa
  • 'NaClaq_HP' — 2026 high-P NaCl(aq) spline only, P=[500, 10001] MPa
  • 'NaClaq_5GPa_2024' — legacy Brown 2024 NaCl(aq) spline, P=[0, 5000] MPa

The output of getProp is a SimpleNamespace object whose attributes match those of the Matlab SF_getprop function exactly.

Pass verbose=True to print lbftd diagnostic warnings (e.g. extrapolation outside the spline domain); silent by default.

Deprecation note: seafreeze.seafreeze() (the old function name) still works but emits a DeprecationWarning and will be removed after 2026-06-21. Use getProp instead.

All phases (pure water/ice and NaClaq):

Quantity	Symbol	Unit
Gibbs Energy	G	J/kg
Entropy	S	J/K/kg
Internal Energy	U	J/kg
Enthalpy	H	J/kg
Helmholtz free energy	A	J/kg
Density	rho	kg/m³
Isobaric heat capacity	Cp	J/kg/K
Isochoric heat capacity	Cv	J/kg/K
Isothermal bulk modulus	Kt	MPa
Pressure derivative of Kt	Kp	−
Isentropic bulk modulus	Ks	MPa
Thermal expansivity	alpha	1/K
Bulk sound speed	vel	m/s
Adiabatic temperature gradient	Js	K/MPa
Grüneisen parameter	gamma_Gruneisen	−
Pressure echo	P	MPa
Temperature echo	T	K

Solid ice phases additionally (Ih, II, III, V, VI, VII_X_French):

Quantity	Symbol	Unit
Shear modulus	shear	MPa
P-wave velocity	Vp	m/s
S-wave velocity	Vs	m/s

NaClaq additionally (NaClaq, NaClaq_LP, NaClaq_HP, NaClaq_5GPa_2024):

Quantity	Symbol	Unit
Solute chemical potential	mus	J/mol
Solvent (water) chemical potential	muw	J/mol
Partial molar volume of solute	Va	cm³/mol
Apparent molar heat capacity	Cpa	J/mol/K
Partial molar volume	Vm	cm³/mol
Partial molar volume of water	Vw	cm³/mol
Partial molar heat capacity	Cpm	J/mol/K
Osmotic coefficient	phi	−
Excess volume	Vex	cm³/mol
Water activity	aw	−
Molality echo	m	mol/kg
Solute mole fraction	xs	−
Solvent mole fraction	xw	−
Mass fraction factor	f	kg-soln/kg-H₂O

NaN values are returned for conditions outside the parametrization boundaries.

Example

import numpy as np
from seafreeze import seafreeze as sf

# list supported phases
sf.phases.keys()

# evaluate thermodynamics for ice VI at 900 MPa and 255 K
PT = np.empty((1,), dtype='object')
PT[0] = (900, 255)
out = sf.getProp(PT, 'VI')
# view a couple of the calculated thermodynamic quantities at this P and T
out.rho     # density
out.Vp      # compressional wave velocity

# evaluate thermodynamics for water at three separate PT conditions
PT = np.empty((3,), dtype='object')
PT[0] = (441.0858, 313.95)
PT[1] = (478.7415, 313.96)
PT[2] = (444.8285, 313.78)
out = sf.getProp(PT, 'water1')
# values for output fields correspond positionally to (P,T) tuples 
out.H       # enthalpy

# evaluate ice V thermodynamics at pressures 400-500 MPa and temperatures 240-250 K
P = np.arange(400, 501, 2)
T = np.arange(240, 250.1, 0.5)
PT = np.array([P, T], dtype='object')
out = sf.getProp(PT, 'V')
# rows in output correspond to pressures; columns to temperatures
out.A       # Helmholtz energy
out.shear   # shear modulus

seafreeze.whichphase: determining the stable phase of water

Usage

SeaFreeze includes a function to determine which of the supported phases is stable under the given pressure and temperature conditions.

whichphase(PTm, solute='water1', path=defpath)

• PTm — same format as getProp (PT for pure water, PTm for NaCl solutions)
• solute — optional; set to 'NaCl' to use NaClaq as the liquid phase, enabling freezing-point-depression phase maps; PTm then requires a molality axis [P, T, m]

The output is a NumPy array of integers: 0 = liquid, 1 = ice Ih, 2 = II, 3 = III, 5 = V, 6 = VI; numpy.nan outside all parametrizations.

• Scattered (P,T): each value corresponds to the same index in the input
• Grid: each row corresponds to a pressure and each column to a temperature

phasenum2phase(phaseInt) converts an integer phase number back to a material string.

Example

import numpy as np
from seafreeze import seafreeze as sf

# determine the phase of water at 900 MPa and 255 K
PT = np.empty((1,), dtype=object)
PT[0] = (900, 255)
out = sf.whichphase(PT)
# map to a phase using phasenum2phase
sf.phasenum2phase(out[0])

# determine phase for three separate (P,T) conditions
PT = np.empty((3,), dtype=object)
PT[0] = (100, 200)
PT[1] = (400, 250)
PT[2] = (1000, 300)
out = sf.whichphase(PT)
# show phase for each (P,T)
[(pt, sf.phasenum2phase(pn)) for (pt, pn) in zip(PT, out)]

# find the likely phases at pressures 0-5 MPa and temperatures 240-300 K
P = np.arange(0, 5, 0.1)
T = np.arange(240, 300)
PT = np.array([P, T], dtype=object)
out = sf.whichphase(PT)

# phase map for a 2 mol/kg NaCl solution (freezing-point depression)
PTm = np.array([np.arange(0, 500, 10), np.arange(240, 300, 0.6),
                np.full(50, 2.0)], dtype=object)
out = sf.whichphase(PTm, solute='NaCl')

---

Phase boundaries: seafreeze.phaselines

SeaFreeze 1.1.0 adds a dedicated module for computing and plotting phase boundary curves — the equilibrium (P, T) loci between any two supported phases. It is the Python equivalent of the Matlab SF_PhaseLines / SF_WPD stack.

Public API

Function	Returns	Description
phase_range(material)	PhaseRange(P, T, m)	Knot-domain bounds of the Gibbs spline for one material
phase_lines(matA, matB, …)	PhaseLineResult (or list)	Equilibrium (P, T) curve between two phases
wpd(…)	matplotlib.figure.Figure	Full water phase diagram plot

phase_lines parameters

Parameter	Default	Description
matA, matB	—	Phase names (same as getProp; order does not matter)
m	None	Molality (mol/kg) — required when one phase is 'NaClaq'; accepts a scalar or list; m=0 gives the pure-water limit via the NaClaq EoS
T	auto	1-D array of temperatures (K) to use as the evaluation grid
segment	'all'	'all', 'stable', or 'meta' — which part of the curve to return

The PhaseLineResult object has attributes matA, matB, P (MPa), T (K), stable (bool mask), segment, and m.

wpd parameters

Parameter	Default	Description
ax	None (new figure)	Matplotlib Axes to plot onto; creates a new figure if omitted
solute	'none'	'NaCl' to overlay NaClaq melting curves
m	None	Molality list for the NaCl overlay
show_meta	True	Show metastable extensions as dashed gray lines
phase_labels	False	Annotate phase fields (Ih, II, III, V, VI, Liquid)

Example — Ice Ih melting curves with NaCl

m=0 uses the NaClaq EoS at the pure-water limit. Higher concentration depresses the melting temperature across the entire pressure range.

import matplotlib.pyplot as plt
import matplotlib.cm as cm
import numpy as np
from seafreeze.phaselines import phase_lines

m_vals   = [0.0, 0.5, 1.0, 2.0, 4.0]
m_labels = ['0 (pure water)', '0.5', '1.0', '2.0', '4.0']
colors   = cm.viridis(np.linspace(0.0, 0.85, len(m_vals)))

fig, ax = plt.subplots(figsize=(7, 5))
for m, lbl, c in zip(m_vals, m_labels, colors):
    r = phase_lines('Ih', 'NaClaq', m=m, segment='stable')
    ax.plot(r.P, r.T, '-', color=c, lw=2, label=f'm = {lbl} mol/kg')
ax.set_xlabel('Pressure (MPa)')
ax.set_ylabel('Temperature (K)')
ax.set_title('Ice Ih melting curves (NaClaq EoS)')
ax.legend(fontsize=9)
ax.grid(True, alpha=0.3)
plt.tight_layout()
plt.show()

Example — Full pure-water phase diagram

Use show_meta=False to hide metastable extensions and phase_labels=True to annotate each stability field.

from seafreeze.phaselines import wpd

with warnings.catch_warnings():
    warnings.simplefilter('ignore')
    fig = wpd(show_meta=False, phase_labels=True)
plt.show()

wpd also accepts a solute='NaCl' keyword together with a list of molalities to overlay NaCl melting curves on the diagram:

fig = wpd(show_meta=False, phase_labels=True, solute='NaCl', m=[0.5, 2.0, 4.0])

---

Important remarks

Water representation

The ice Gibbs parametrizations are optimized to be used with water1 (Bollengier et al. 2019), particularly for phase-equilibrium calculations. Using other water parametrizations will lead to incorrect melting curves. water2 (Brown 2018) and water_IAPWS95 (IAPWS-95) are provided for high-pressure extension (up to 100 GPa) and comparison only. The authors recommend water1 for any application in the 200–355 K range and up to 2300 MPa.

Range of validity

SeaFreeze stability prediction is currently considered valid down to 130K, which correspond to the ice VI - ice XV transition. The ice Ih - II transition is potentially valid down to 73.4 K (ice Ih - ice XI transition). The ice VII and ice X representation extend to 1TPa (1e6 MPa) and 2000K.

References

• Bollengier, Brown and Shaw (2019) J. Chem. Phys. 151, 054501; doi: 10.1063/1.5097179
• Brown (2018) Fluid Phase Equilibria 463, pp. 18-31
• Feistel and Wagner (2006), J. Phys. Chem. Ref. Data 35, pp. 1021-1047
• Journaux et al. (2020) JGR: Planets 125, e2019JE006176
• Wagner and Pruss (2002), J. Phys. Chem. Ref. Data 31, pp. 387-535
• French and Redmer (2015), Physical Review B 91, 014308

Authors

• Baptiste Journaux - University of Washington, Earth and Space Sciences Department, Seattle, USA
• J. Michael Brown - University of Washington, Earth and Space Sciences Department, Seattle, USA
• Penny Espinoza - University of Washington, Earth and Space Sciences Department, Seattle, USA
• Erica Clinton - University of Washington, Earth and Space Sciences Department, Seattle, USA
• Tyler Gordon - University of Washington, Department of Astronomy, Seattle, USA
• Ula Jones - University of Washington, Earth and Space Sciences Department, Seattle, USA

Change log

Changes since 0.9.0

• 1.1.1: Added matplotlib to Python dependencies; removed numpy<2 upper bound for NumPy 2.x compatibility.
• 1.1.0: added seafreeze.phaselines module — phase boundary computation (phase_lines, phase_range) and the full water phase diagram plotter (wpd); NaClaq melting curves for Ih, II, III, V, and VI; cross-validated against the Matlab SF_PhaseLines implementation to < 0.01 K. getProp output now matches Matlab SF_getprop exactly: added Js, gamma_Gruneisen, P/T echoes, NaClaq mixing properties (m, xs, xw, f, Vw); removed Python-only V, gam, Gex from default output; individual per-spline .mat files replace the monolithic spline archive.
• 1.0: added NaCl aqueous solution EOS and concentration dependent thermodynamic variables.
• 0.9.4: Adjusted python readme syntax and package authorship info
• 0.9.3: add ice VII and ice X from French and Redmer (2015). LocalBasisFunction spline interpretation software integrated into SeaFreeze Python package. Adjusted packaging to work better with pip
• 0.9.2.post2: whichphase returns numpy.nan if PT is outside the regime of all phases
• 0.9.2: add ice II to the representation.
• 0.9.1: add whichphase function

Changes from 0.8

• rename function get_phase_thermodynamics to seafreeze
• reverse order of PT and phase in function signature
• remove a layer of nesting (seafreeze.seafreeze rather than seafreeze.seafreeze.seafreeze)

License

SeaFreeze is licensed under the GPL-3 License :

Copyright (c) 2019, B. Journaux

This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, version 3.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program. If not, see https://www.gnu.org/licenses/.

THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW. EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM "AS IS" WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU. SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING, REPAIR OR CORRECTION.

Acknowledgments

This work was produced with the financial support provided by the NASA Postdoctoral Program fellowship, by the NASA Solar System Workings Grant 80NSSC17K0775 and by the Icy Worlds node of NASA's Astrobiology Institute (08-NAI5-0021).

Illustration montage uses pictures from NASA Galileo and Cassini spacecrafts (from top to bottom: Enceladus, Europa and Ganymede). Terrestrial sea ice picture use with the authorization of the author Rowan Romeyn.