The molar absorptivity of interparticle water in clay-water systems

Infared spectroscopy was used to study the molar absorptivity, ε{lunate}, for OD stretching of HDO mixed in different proportions with various clay minerals. The value of ε{lunate} was found to decrease with increasing m_c m_w, the mass ratio of clay to water, according to the empirical equation ε{lunate} = ε{lunate}⁰ exp( β_ε{lunate}m_c m_w), where ε{lunate}⁰ is the value of ε{lunate} for bulk HDO and β_ε{lunate} is a constant that is proportional to the specific surface area of the clay. Theoretical equations were used to relate the infrared absorption intensity to the arrangement and interaction of the interparticle water molecules. It was found that both ( ∂u ∂Q₁)², the square of the partial derivative of the dipole moment with respect to the normal coordinate of vibration, and τ, the transition moment relaxation time, obeyed empirical equations of the same form as the one which describes the variation in ε{lunate}. The changes in ( ∂u ∂Q₁)² and τ were interpreted to mean that water next to clay surfaces has a different charge distribution and a slower rate of rotation than bulk water. Also, since ε{lunate} was independent of the surface charge density and, hence, of the concentration of the counterions, it was concluded that the particle surfaces are responsible for these differences. As shown by an analysis of the data, the effect of the particle surfaces on the adjacent water extends to at least 40 Å.