Nano-scale investigation of soil particles and model material surfaces with biofilms
and contrasting hydrophobicity using atomic force microscopy (AFM)
R. Bryant1, S. Cheng1,2, S.H. Doerr2, C.J. Wright1, J.V. Bayer1 , P.R. Williams1
1School of Engineering, Swansea University, Singleton Park, Swansea SA2 8PP, UK.
2 School of the Environment and Society, Swansea University, Singleton Park, Swansea SA2 8PP, UK.
r.bryant@swan.ac.uk
Organic coatings (biofilms) can alter the physic-chemical properties of mineral soil
particles. In this study, surface images and force measurements were obtained using
atomic force microscopy (AFM) to investigate surface properties of (i) model ‘soil’
material of varying hydrophobicity (smooth glass surfaces and acid-washed sand (AWS)
coated with soil-derived humic acid (HA) or lecithin (LE)), and (ii) individual soil
particles derived from hydrophobic and wettable sandy soils. Height and phase images
and phase distributions (obtained from the latter) provide insight into the complex surface
morphology and heterogeneous distributions of organic matter on model material and soil
particle surfaces. Adhesion force measurements taken in air and water allow
quantification of capillary forces (reflecting surface hydrophobicity at very high
resolution) and the distribution of organic material upon underlying inorganic surfaces
respectively. Higher adhesion forces in water indicate a local strong hydrophobic
interaction whereas lower adhesion forces in humid air reflects weak hydrophilic
interactions. These measurements allow direct evaluation of the contributions of particle
surface properties, determined at micro and nanometer scales, to the water repellency of
corresponding bulk soils.
Adhesion forces on glass/AWS surfaces, determined in air, decreased in the order
LE, glass/AWS and HA, and, in water, in the order HA, LE and glass/AWS. Forces at
model surfaces indicate that, in air, a reduction in adhesion corresponded with increased
hydrophobicity, but in water, corresponded with a decrease. Adhesion force
measurements on hydrophobic soil particles in water were found to be larger than those
for wettable ones, but the high roughness and complexity of surfaces obscure an opposite
trend for measurements in air. The combination of force measurements, applied for the
first time to soil particles, together with those on model surfaces, and independent
assessments of water repellency of corresponding bulk systems, indicate good qualitative
agreement of the manifestation of hydrophobic behavior at both bulk and nano-scales.