Effect of clay aggregation on water diffusivity using low field NMR.

Area of Science:

• Colloid and Surface Science
• Materials Science
• Geochemistry

Background:

• Understanding water diffusion in clay suspensions is crucial for various applications, including soil science and materials engineering.
• Smectite clays exhibit complex behaviors in aqueous solutions due to their layered structure and surface charge.

Purpose of the Study:

• To investigate the effects of smectite concentration and salinity on water diffusivity (D) in Na-smectite suspensions.
• To explore the relationship between macroscopic organization, particle interactions, and water diffusion dynamics.
• To establish a unifying framework for water diffusivity based on electrochemical properties.

Main Methods:

• Nuclear Magnetic Resonance (NMR) techniques, specifically measuring water diffusivity (D) and T(2) transverse relaxation times.
• Preparation of Na-smectite suspensions with varying clay fractions and salinities (NaCl, CaCl(2)).
• Analysis of T(2) relaxation distributions to infer suspension microstructure (dispersed vs. flocculated).

Main Results:

• Water diffusivity (D) decreases with increasing smectite fraction and increases with salinity.
• Higher salinity, particularly with CaCl(2), promotes clay particle aggregation.
• T(2) relaxation times reveal differences between dispersed (mono-modal) and flocculated (bi-modal) systems, indicating particle spacing.
• A universal curve for water diffusivity is achieved using Debye length and particle spacing, showing decreased diffusivity when these are comparable.

Conclusions:

• Water diffusivity in smectite suspensions is strongly dependent on clay concentration, salinity, and particle organization.
• Salinity-induced aggregation significantly impacts water diffusion pathways.
• The interplay between the electro-diffuse layer thickness (Debye length) and particle spacing governs water diffusivity, highlighting the role of electrochemical properties.