Fluid Properties

A Fluid has four properties: a Reference Diffusivity, a Diffusive Reduction Formula, a set of Relative Diffusivities, and a set of Partition Coefficients (or Solubilities).

The Relative Diffusivities, Partition Coefficients and Solubilities must be specified for every species. Note, however, that they are actually elemental properties, as opposed to species properties. That is, if you have defined species that represent isotopes of the same chemical element, these properties must be identical for all isotopic species of the element. (If you are not simulating isotopes, you need not be concerned with this, as there is then a one-to-one correspondence between each species and each element).

Reference Diffusivity: The reference diffusivity is the diffusivity for a "standard" or "reference" species in the fluid. It must have dimensions of area / time. As will be seen below, this "reference" value can be modified (multiplied by a specified factor) for each individual species.

Diff. Reduction Formula: The Diffusivity Reduction Formula is a dimensionless value that has the impact of decreasing the effective diffusivity based on the the fluid’s saturation level within any Cell pathways in which it is present. This allows you to more accurately simulate diffusion through partially saturated porous media. If diffusion through partially saturated media is not of interest, simply leave this at its default value (1).

When used, this formula is intended to directly reference the fluid’s saturation level using a special local available property called “Saturation”. It must be referenced as “~Saturation” in the equation entered into the field. Note that the ~Saturation local available property can only be referenced directly in this field; it cannot be used in a separate element that is then linked into this field.

When the fluid is used within a Cell pathway, GoldSim internally computes the saturation of the fluid in the pathway, and uses this value to compute the Diffusive Reduction Formula (and hence the diffusive mass flux) into and out of the Cell.

Relative Diffusivities: A Relative Diffusivity is unitless, and within GoldSim, is multiplied by the Reference Diffusivity to determine the actual diffusivity of each species in the fluid. This input requires that you enter a vector by element (a vector with an item for each chemical element) or a vector by species (a vector with an item for each species).

By default, the vector is defined locally as a vector by (chemical) element, which means that you can edit it directly from this dialog (by pressing the Edit… button to the right of the input field).

You can enter constants, expressions, or links into the fields for each element. Note that the default value for each element is 1 (i.e., the diffusivity for the element is identical to the Reference Diffusivity).

If you wish to enter a link to define the entire vector, rather than defining the vector locally, you must first press the Clr button. This will clear the local vector, and make the edit field available to you for entering a link. When entering a link, you can link to either to a vector by element (a vector with an item for each chemical species) or a vector by species (a vector with an item for each species). The vector must be dimensionless.

Note: If you are simulating isotopes of the same element, regardless of how the data is entered (as species or elements), GoldSim will automatically ensure that isotopic species use the same values for the Relative Diffusivity (if data is entered as species, the value for the first species in the list will be used for other isotopes).

Note: If you link to a vector by species, and then subsequently decide to specify the vector locally (by pressing the Edit… button), the dialog for entering the data locally will be presented in terms of species. To change this back to a local vector by chemical elements, you must first link to a dimensionless vector of elements, and then press the Edit… button again.

Partition Coefficients: The partition coefficient is defined as the ratio, at thermodynamic equilibrium, of the concentration in the Fluid to that in the Reference Fluid. The partition coefficient for a Fluid is dimensionless. This input requires that you enter a dimensionless vector by element (a vector with an item for each chemical element) or a vector by species (a vector with an item for each species).

You can enter constants, expressions, or links into the fields for each element. Note that the default value for partition coefficients is zero (i.e., species do not partition into the fluid).

Note: If you link to a vector by species, and then subsequently decide to specify the vector locally (by pressing the Edit… button), the dialog for entering the data locally will be presented in terms of species. To change this back to a local vector by elements, you must first link to a dimensionless vector of elements, and then press the Edit… button again.

Note: If you are simulating isotopes of the same element, regardless of how the data is entered, GoldSim will automatically ensure that isotopic species use the same values for the (molar) Solubility. If data is entered as species, the molar solubility for the first species in the list will be applied to all isotopes. If the data is entered as elements, and solubilities are entered on a mass basis, a molar solubility is computed based on the Atomic Weight of the first species in the list.

Note: For a gas, the partition coefficient with respect to water is sometimes referred to as the Henry’s Law constant.

Solubilities: Rather than entering the partition coefficients for a Fluid, you can alternatively enter the solubility.

The solubility represents the maximum concentration that the chemical element can have in the Reference Fluid within a Cell pathway (with any additional mass being precipitated out of solution). This input requires that you enter a vector by element (a vector with an item for each chemical element) or a vector by species (a vector with an item for each species).

In this case, GoldSim automatically computes the partition coefficients for the Fluid with respect to the Reference Fluid. (By definition, the partition coefficient for a species is simply the ratio of the solubility in the Fluid to the solubility in the Reference Fluid).

A negative solubility indicates that you do not wish to apply any solubility constraints for this element (i.e., the element can never become limited by solubility).

You can enter constants, expressions, or links into the fields for each element. Note that the default value for each element is -1 mol/l. GoldSim interprets a negative solubility as indicating that the chemical element is infinitely soluble (i.e., no solubility constraint will be applied).

By default, the vector must have dimensions of moles/volume. However, by changing the Display Units immediately to the right of the Solubilities field, you can also choose to specify the Solubilities in terms of mass/volume.

Note: If you link to a vector by species, and then subsequently decide to specify the vector locally (by pressing the Edit… button), the dialog for entering the data locally will be presented in terms of species. To change this back to a local vector by elements, you must first link to a dimensionless vector of elements, and then press the Edit… button again.

Warning: If you choose to specify solubilities (rather than partition coefficients) for a Fluid, and the Reference Fluid has unlimited (negative) solubilities for some of the elements, you must also specify unlimited (negative) solubilities for the same elements (and only those elements) in the Fluid. If there is an inconsistency in specifying the solubilities (i.e., unlimited solubilities in one fluid but defined solubilities in another), GoldSim will provide a fatal error message when you try to run the model.

Note: Solubilities are only applied within Cell pathways (and are not applicable to any other type of pathway).

Note: You can paste data from a spreadsheet, a Word table or a comma-delimited text file into any of the vector editing dialogs for a Fluid by copying the data to the clipboard, single-clicking in the cell representing the top of the target range, and pressing Ctrl+V.