Added a :chargeNeutralityNecessary() function and boolean.

If charge neutrality is necessary for the proper specification
of thermo functions, this is true. If not it's set to false.
Currently, it's true for molality based formulations, and false
otherwise. It's false for Ideal gases.

Cantera/src/thermo/MolalityVPSSTP.cpp

@@ -45,6 +45,12 @@ namespace Cantera {
     m_xmolSolventMIN(0.01),
     m_Mnaught(18.01528E-3)
   {
+    /*
+     * Change the default to be that charge neutrality in the
+     * phase is necessary condition for the proper specification
+     * of thermodynamic functions within the phase
+     */
+    m_chargeNeutralityNecessary = true;
   }
 
   /*

Cantera/src/thermo/ThermoPhase.cpp

@@ -25,6 +25,24 @@
 using namespace std;
 
 namespace Cantera {
+
+    //! Constructor. Note that ThermoPhase is meant to be used as
+    //! a base class, so this constructor should not be called
+    //! explicitly.
+    ThermoPhase::ThermoPhase() :
+         Phase(), 
+         m_spthermo(0), m_speciesData(0),
+         m_index(-1), 
+         m_phi(0.0), 
+         m_hasElementPotentials(false),
+         m_chargeNeutralityNecessary(false)
+    {
+    }
+
+   ThermoPhase::~ThermoPhase() 
+   {
+     delete m_spthermo;
+   }
     /**
      * Copy Constructor for the ThermoPhase object. 
      *
@@ -37,7 +55,8 @@ namespace Cantera {
 	m_speciesData(0),
 	m_index(-1),
 	m_phi(0.0), 
-	m_hasElementPotentials(false)
+	m_hasElementPotentials(false),
+        m_chargeNeutralityNecessary(false)
     {
 	/*
 	 * Call the assignment operator

Cantera/src/thermo/ThermoPhase.h

@@ -75,14 +75,67 @@ namespace Cantera {
    * and 5 functions multiplied together makes 25 possible functions. That's 
    * why %ThermoPhase is such a large class.
    *
-   *  
+   * <H3>
    * Mechanical properties
+   * </H3>
    *
-   * Treatment of the electrochemical potential 
+   * <H3>
+   * Treatment of the %Phase Potential and the electrochemical potential of a species
+   * </H3>
    *
-   * Treatment of other potential energy contributions.
+   *  The electrochemical potential of species k in a phase p, \f$ \zeta_k \f$,
+   *  is related to the chemical potential via
+   *  the following equation, 
+   * 
+   *       \f[
+   *            \zeta_{k}(T,P) = \mu_{k}(T,P) + z_k \phi_p 
+   *       \f]
    *
+   *   where  \f$ \nu_k \f$ is the charge of species k, and \f$ \phi_p \f$ is
+   *   the electric potential of phase p.
+   *      
+   *  The potential  \f$ \phi_p \f$ is tracked and internally storred within
+   *  the base %ThermoPhase object. It constitutes a specification of the
+   *  internal state of the phase; it's the third state variable, the first 
+   *  two being temperature and density (or, pressure, for incompressible
+   *  equations of state). It may be set with the function,
+   *  ThermoPhase::setElectricPotential(),
+   *  and may be queried with the function ThermoPhase::electricPotential().
+   *  
+   *  Note, the overall electrochemical potential of a phase may not be
+   *  changed by the potential because many phases enforce charge
+   *  neutrality:
+   *
+   *       \f[
+   *            0 = \sum_k z_k X_k
+   *       \f]
+   *
+   *  Whether charge neutrality is necessary for a phase is also specified
+   *  within the ThermoPhase object, by the function call
+   *  ThermoPhase::chargeNeutralityNecessary(). Note, that it is not
+   *  necessary for the IdealGas phase, currently. However, it is
+   *  necessary for liquid phases such as Cantera::DebyeHuckel and
+   *  Cantera::HMWSoln for the proper specification of the chemical potentials.
+   *
+   *
+   *  This equation, when applied to the \f$ \zeta_k \f$ equation described
+   *  above, results in a zero net change in the effective Gibbs free
+   *  energy of the phase. However, specific charged species in the phase
+   *  may increase or decrease their electochemical potentials, which will
+   *  have an effect on interfacial reactions involving charged species,
+   *  when there is a potential drop between phases. This effect is used
+   *  within the Cantera::InterfaceKinetics and Cantera::EdgeKinetics kinetics 
+   *  objects classes.
+   *  
+   *
+   *  Other internal state variables, that track the treatment of other
+   *  potential energy contributions, by adding contributions to the
+   *  chemical potential to create an effective chemical potential, 
+   *  may be added at a later time.
+   *
+   * <H3>
    * Setting the %State of the phase
+   * </H3>
    *
    * Instantiation of %ThermoPhase properties occurs via the following path.
    *
@@ -190,14 +243,10 @@ namespace Cantera {
     //! Constructor. Note that ThermoPhase is meant to be used as
     //! a base class, so this constructor should not be called
     //! explicitly.
-    ThermoPhase() : Phase(), m_spthermo(0), m_speciesData(0),
-		    m_index(-1), m_phi(0.0), m_hasElementPotentials(false) {}
+    ThermoPhase();
 
     //! Destructor. Deletes the species thermo manager.
-    virtual ~ThermoPhase() {
-      delete m_spthermo;
-    }
-
+    virtual ~ThermoPhase();
    
     //!Copy Constructor for the %ThermoPhase object. 
     /*!
@@ -408,7 +457,7 @@ namespace Cantera {
 
     //! Returns the electric potential of this phase (V).
     /*!
-     *  Units are Volts
+     *  Units are Volts (which are Joules/coulomb)
      */ 
     doublereal electricPotential() const { return m_phi; }
 
@@ -589,8 +638,15 @@ namespace Cantera {
     
     //!  Get the species electrochemical potentials. 
     /*!
-     * These are partial molar quantities.  This method adds a term \f$ Fz_k
-     * \phi_k \f$ to each chemical potential.
+     *  These are partial molar quantities.  This method adds a term \f$ F z_k
+     *  \phi_p \f$ to each chemical potential.  
+     *  The electrochemical potential of species k in a phase p, \f$ \zeta_k \f$,
+     *  is related to the chemical potential via
+     *  the following equation, 
+     *
+     *       \f[
+     *            \zeta_{k}(T,P) = \mu_{k}(T,P) + F z_k \phi_p 
+     *       \f]
      *
      * @param mu  Output vector of species electrochemical
      *            potentials. Length: m_kk. Units: J/kmol
@@ -1405,6 +1461,21 @@ namespace Cantera {
 
     //@}
 
+    //! Returns the chargeNeutralityNecessity boolean
+    /*!
+     * Some phases must have zero net charge in order for
+     * their thermodynamics functions to be valid.
+     * If this is so, then the value returned from this
+     * function is true.
+     * If this is not the case, then this is false.
+     * Now, ideal gases have this parameter set to false,
+     * while solution with  molality-based activity
+     * coefficients have this parameter set to true. 
+     */
+    bool chargeNeutralityNecessary() const {
+      return m_chargeNeutralityNecessary;
+    }
+
             
   protected:
 
@@ -1439,6 +1510,16 @@ namespace Cantera {
     //! Boolean indicating whether there is a valid set of saved element potentials for this phase
     bool m_hasElementPotentials;
 
+    //! Boolean indicating whether a charge neutrality condition is a necessity
+    /*!
+     * Note, the charge neutrality condition is not a necessity for ideal gas phases. There may
+     * be a net charge in those phases, because the NASA polynomials for ionized species 
+     * in Ideal gases take this condition into account. 
+     * However, liquid phases usually require charge neutrality in order for their derived
+     * thermodynamics to be valid.
+     */ 
+    bool m_chargeNeutralityNecessary;
+
   private:
 
     //! Error function that gets called for unhandled cases