1
Physico-chemistry of the Soil-Water System

The Earth is at a distance from the Sun that allows water to be stable in the three states: solid, liquid and gas. The properties of water have been carefully measured (Table 1.1), first using thermodynamics methods, then with spectroscopic methods (UV, IR, Raman), while physicists and chemists were trying to establish the link between macroscopic data and atomic and molecular data by means of statistical physics and physical chemistry. This link is not yet completely established and remains semiempirical. Water still remains both a "unique chemical constituent" [Fra79] and a "forgotten biological compound", which is too banal and ubiquitous to be looked at carefully.

Table 1.1. Coordinates of two triple points and the critical point of water

1.1. The "abnormal" properties of water

Water has "abnormal" properties compared to other liquids [Car92]:

• - H2O is liquid under standard conditions (STP: 25°C, 1 bar), whereas H2S is gaseous, although O is lighter than S;
• - ice Ih is less dense than liquid. Otherwise, during the glaciations, a continuous layer of ice would have settled at the bottom of the ocean and would have never thawed. The specific gravity of H2O(l.) passes through a maximum at 3.984°C;
• - the compressibility coefficient passes through a minimum at 46.5°C;
• - heat capacity passes through a minimum at 37.5°C;
• - sound velocity passes through a maximum around 70°C;
• - viscosity is very high and passes through a minimum when the pressure rises;
• - the surface tension and the dielectric constant ? are both very high and decrease with the temperature;
• - the phase diagram of water is very complex, with at least 15 varieties of ice, including two amorphous ones.

Many properties (73!) thus show non-monotonic, nonlinear variations with T and P, but extremums are observed for different values of P and T depending on the quantity under consideration. Some properties of water are presented in Table 1.2. The enthalpy of fusion of ice I at 0°C is close to 6 kJ mol-1, and the enthalpy of vaporization of liquid water at 100°C is close to 41 kJ mol-1.

Table 1.2. Values of the specific volume, the enthalpy and the dielectric constant of water at 0°C, 100°C and 1 bar and at the critical point (see Chaplin, Martin "Water structure and science", www1.lsbu.ac.uk/water and the several references cited)

1.1.1. The thermodynamic properties of pure water

Water is the most studied pure substance and has been for a long time - in the liquid, solid or vapor form - consider the importance of the steam engine1, but still, there is no simple state equation for water. In addition to hexagonal ice Ih and cubic ice Ic, there are 13 other varieties of solid water, including two amorphous phases, according to pressure and temperature.

The chemical potential of free pure liquid water, namely which is not bound to the soil (see section 1.6), depends only on P and T:

It is identical to the Gibbs free energy of formation of liquid water from pure oxygen and hydrogen. In standard conditions of temperature and pressure (P0 = 105 Pa, T0 = 298.15 K), its value is - 237.140 kJ mol-1. The thermodynamic properties of water under STP conditions are given in Table 1.3.

Table 1.3. Thermodynamic properties of pure liquid water and OH- under standard conditions [Bra89]

The high value of heats of changes in state of water derives directly from the "abnormal" properties of water and from the strong cohesion of liquid water. As a result, there is a strong influence of evaporation and condensation on climate regulation.

1.1.2. The stability field of water according to the pH and pe

Pure water can dissociate into H+ and OH- or H2 and O2. The expression of the equilibrium reactions and the material balance constraint makes it possible to define the conditions for water stability. The pH is defined by:

where {H+} is the activity of the hydrated proton H+(aq).

The pe is defined by:

where {e-} is the electron activity. The pe is related to the redox potential E through the Nernst relation:

where F is the Faraday constant, R is the ideal gases constant2 and T is the absolute temperature (K), such that T = t + 273.15, where t is the temperature (°C), and E is the redox potential (V). The zero of the potential scale is defined by convention as the couple H+/H2(g), which is known as a normal hydrogen electrode. The pe and pH diagrams are equivalent to the Eh and pH diagrams [Pou63], with the advantage that pe and pH are dimensionless. These are master variables for the study of stability conditions of chemical species and solid phases in solution [Sil67]. The standard deviation of measurements under thermostated conditions in stable environments are typically of the order of 0.03 for the pH and 1.7 for the pe.

1.1.2.1. Graphical representation

The equations:

delineate the domain of existence of water according to the pe and pH conditions in standard conditions of temperature and pressure (P = 1 bar and T = 298.15 K) (see the demonstration in the Appendix, section 1.10). This domain is very wide: 14 orders of magnitude of the activity of H+ and 21 orders of magnitude of the electron activity (Figure 1.1). Water thus allows numerous acid-base and redox reactions to proceed.

Figure 1.1. Water stability domain in the pe and pH diagram at 25°C, 1 bar

The equations:

define the water neutrality lines. Their intersection point defines the neutrality point of pure water.

1.2. Properties of the water molecule

1.2.1. Geometry of the isolated water molecule

The water molecule is a dipole (Figure 1.2). In a first simplified representation, each covalent bond is depicted with a pair of electrons shared between O and H, and therefore two pairs of free electrons remain.

Figure 1.2. Structure of the isolated water molecule. The angle is equal to a = 104.52°, and the distance O-H is 0.9572 ± 0.0003Å

The isolated water molecule exhibits a large dipole moment

EXERCISE 1.1.- Based on the dipole moment of the water molecule and the geometry of the water molecule...