New approaches to the study of hydrophobicity and wetting of soils: methods and theories

Abstract

New approaches to the study of hydrophobicity and wetting of soils were investigated with the following research objectives: 1) to evaluate the use of fluorescence probes, nile red and pyrene, as tools to assess the polarity and viscosity of organics adsorbed to soils; 2) to assess, using a variety of kinetic methods, what steps are involved in wetting soils; and 3) to assess the validity of current theories for the anomalously high contact angles often measured for water on soils, and investigate an alternative explanation based on the geometry of a water drop sitting on hydrophobic particles. Whilst it was possible to image nile red emission after adsorption to soils, issues of emission intensity and soil auto-luminescence suggest that nile red is not a useful probe for soil studies. Fluorescence measurements were made using pyrene as an in-situ polarity and viscosity/mobility probe. Using pyrene co-deposited with organics on acid-washed sand, excimer kinetics showed a decrease in environment mobility as the organic layer was changed from a liquid to a hard wax. Spectra from natural soils indicated varying environmental polarity and heterogeneity within the soil samples studied. A theoretical model for soil wetting, involving adhesional-immersional wetting followed by branching capillary wetting, is proposed, and a series of experiments to assess the validity of this model described. Methods used include: water drop penetration time (WDPT) test, mass of soil grains wetted over time; time taken for penetration of a water drop into different soil thicknesses; optical microscopy; WDPT measurements with salt solutions of different densities. An alternative interpretation of the anomalously high contact angles measured on soils is proposed based on a correction factor for water on particles. To assess this, measurements were made using regularly arranged ballpoint needles and metal spheres, and acid-washed sand and natural soil, coated in paraffin wax. .

Pyrene and nile red fluorescence probes for in‐situ study of polarity and viscosity of soil organic coatings implicated in soil water repellency

“…Samples of sandy soils (0-10 cm depth) were taken from six naturally water-repellent soils (Balshaw, 2019;Doerr et al, 2005). Four were from Gower, South Wales: two dune soils from Nicholaston, NIC1 and NIC2 (51 34 0 N 04 07 0 W), and two dune soils under pine forest from Llanmadoc, LLAN1 and LLAN2 (51 37 0 N 04 15 0 W).…”