Environ. Sci. Technol. 2009, 43, 6500–6506
Investigation of Surface Properties of Soil Particles and Model Materials with Contrasting Hydrophobicity Using Atomic Force Microscopy S H U Y I N G C H E N G , †,‡ R O B E R T B R Y A N T , * ,‡ STEFAN H. DOERR,† CHRISTOPHER J. WRIGHT,‡ AND P. RHODRI WILLIAMS‡ Institute of Environmental Sustainability, School of the Environment and Society, and Centre for Complex Fluids Processing, Multidisciplinary Nanotechnology Centre, School of Engineering., Swansea University, Singleton Park, Swansea, SA2 8PP, U.K.
Received February 18, 2009. Revised manuscript received June 19, 2009. Accepted July 21, 2009.
Surface images and force measurements obtained using atomic force microscopy (AFM) were used to assess the hydrophobicity of particles from soils and model soil material (smooth glass and acid-washed sand (AWS) exposed to soilderived humic acid (HA) or lecithin (LE)). Height and phase images, and phase distributions (from soil particles) show complex morphology and heterogeneously distributed organic matter. Forces at model surfaces indicate that, in air, reduction in adhesion corresponded with increased hydrophobicity, but in water, corresponded with a decrease (and serve to guide interpretation of data from natural particles). Adhesion forces on hydrophobic soil particles in water were larger than those for hydrophilic ones, but surface roughness and complexity may obscure any opposite trend for measurements in air. Combination of force measurements, applied for the first time to soil particles, together with those on model surfaces, and independent assessments of hydrophobicity of corresponding single particle layers, indicate good, but not consistent, qualitative agreement between hydophobicity at bulk and nanoscales. AFM is likely to facilitate detailed evaluation of soil particle surface hydrophobicity, which contributes to bulk wetting behavior of soils and other porous systems, including assessments of the potential for contributions to superhydrophobicity from surfaces at the microand nanoscales.
Introduction Soil comprises a heterogeneous mixture of various mineral particles and organic materials that physically and chemically bind to form complex aggregates (1). The structure and chemistry of soil constituents ultimately determine the properties and behavior of bulk soil material. For example, content and composition of soil organic matter (SOM) influence the physicochemical nature of the soil interface * Corresponding author phone: +44 1792 295201; fax: +44 17925756; e-mail:
[email protected]. † Institute of Environmental Sustainability. ‡ Centre for Complex Fluids Processing. 6500
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including its wettability (i.e., water repellency, usually defined in terms of water droplet penetration) (2-4). In both moistening and drying soils, it is likely that molecules rearrange, insofar as possible, to organize into hydrophobic and hydrophilic regions (5). Adsorption of SOM on minerals may alter their reactivity and the affinity of resulting composites to water. This affects soil hydrology, soil erosion, and mobility of nutrients and contaminants in soils (1, 6, 7). Present understanding of the influence of adsorbed SOM is based on investigations on bulk soil material, since direct and quantitative investigation of constituent particle surfaces at the size scale of adsorbed organic macromolecules was not previously possible. Recent developments in atomic force microscopy (AFM) have facilitated such investigations (8). AFM has been applied to many research areas for imaging surface structure, at the nanometer scale, and estimating adhesion forces at selected contact points (9-12). Differences in surface properties are detected from phase shifts in cantilever oscillation during tapping mode (TM) scanning (13-15). In TMAFM, data for topographical and phase images are captured simultaneously. Since data provide no information as to how surface domains correspond with the various materials present, the determination of phase attributes remains a consuming task. Studies of block polymers poly(styrene-ethylene/butylenes-styrene) (SEBS) using TMAFM (14) concluded that higher spots in AFM height images and brighter domains in phase images correspond with hard polystyrene (PS). Others (16) suggest that such topographic high spots should correspond with PS, whereas others (17, 18) considered it to be the opposite. It is evident such phase images serve to identify different components in composite materials and differentiate regions of high and low surface hardness or adhesion (19). Phase images reflect variations in composition, friction, adhesion, and/or any other surface property that dissipates energy provided, by the piezo drive, to the cantilever thereby affecting its oscillation, so consistency of methods is required. Forces between AFM probes and sample surfaces can be measured in both air and liquid. In humid air a water capillary forms between the tip and specimen introducing a meniscus force (or capillary force), which dominates the adhesion force in air (11, 20, 21). Immersion of specimen and tip in liquid eliminates this contribution allowing estimation of van der Waals and other small forces (22, 23). Such force measurements show that hydrophobic surfaces exhibit mutual longrange and strong attractive forces in aqueous solutions (24-26). AFM has been used to provide direct and quantitative information on morphological features of soil particles and colloidal surfaces (27-30), and of humic substances deposited on smooth surfaces (16, 31-35). The interaction between AFM tips and soil particle surfaces has rarely been studied due the high surface roughness, which influences adhesion forces by reducing contact areas between tip and specimen. Previously (8) we demonstrated that phase distributions represent the distribution of soil particle SOM and that the magnitude and distribution of adhesion forces (mainly the capillary forces) between AFM tips and soil particles were affected by both local surface roughness and the presence of adsorbed organics. This paper presents outcomes of AFM morphological studies combined with adhesion force measurements, made in air and water, for characterization of individual particles (i) derived from water repellent and nonrepellent soil samples including those examined previously (8), additional sandy soils, and from (ii) model specimens of known composition. 10.1021/es900158y CCC: $40.75
2009 American Chemical Society
Published on Web 08/05/2009
TABLE 1. Model Materials and Soil Samples and Their Masses of Extractable Organic Material, Surface Roughness (RRMS), Average Contact Angles (CA), and Adhesion Forces (Fad) Obtained in Air and Water sample code
mass extracteda (g/kg soil)
Rrms (nm)
CA (°)
Fadin air nN
Fadin water nN
9.76 ( 1.50 0.55 ( 0.23 3.3 ( 0.4 1.30 ( 0.44 3.7 ( 1.3 0.9 ( 0.6
0.58 ( 0.20 5.62 ( 3.16 1.38 ( 1.12 74.0 ( 38.2 103.6 ( 79.5 88.6 ( 41.6 53.0 ( 25.7 106.4 ( 49.9 86.1 ( 52.1 71.2 ( 27.4 78.6 ( 42.8 65.5 ( 35.7
43.3 62.4 12.9 51.9 79.2 23.5 111.7 82.2 121.3 74.5 104.3 68.8
19.9 ( 1.3 6.6 ( 2.8 24.8 ( 1.8 7.3 ( 3.3 5.7 ( 3.6 9.7 ( 5 8.0 ( 5.4 11.2 ( 8.8 7.9 ( 3.8 7.41 ( 4.16 11.1 ( 7.3 12.0 ( 7.0
0.24 ( 0.10 2.4 ( 1.6 0.88 ( 0.20 0.38 ( 0.20 1.9 ( 1.6 1.25 ( 1.14 0.85 ( 0.88 0.30 ( 0.26 0.85 ( 0.64 0.27 ( 0.42 0.71 ( 0.81 0.69 ( 0.52
glass HA on glass LE on glass AWS HA-AWS LE-AWS NL1 NLC PT2 PTC AU2 AUC a
The mass (of organic material) extracted from soil are taken from ref (31).
The results provide information about nanoscale distributions of adsorbed SOM. Adhesion force data from model surfaces serve to guide interpretation of those from natural soils, such as distinctions between adsorbed hydrophobic and hydrophilic material, and facilitate, for the first time, a comparison between independent assessments of water repellency made using larger regions of model surfaces and bulk soils, and the manifestation of this phenomenon at nanoscales.
Materials and Methods Model Materials and Surfaces. Model materials used were acid-washed quartz sand, (AWS); particle diameter 0.27 ( 0.07 mm, and flat glass slide covers; 11 mm diameter), obtained from Riedel-de Hae¨n and Fisher Scientific, UK respectively. Humic acid, sodium salt (HA) was obtained from Sigma-Aldrich, UK and lecithin (LE) from Fisher Scientific, UK. HA (∼120 kDa) was used as it is a main constituent of soil organic matter implicated in soil-water repellency (3) and LE (∼5.5 kDa) because it is a small, uniform surface active compound of plant origin. Water, with a pH of 5.8 ( 0.2 and specific conductivity glass > LE). The lower Fad and higher standard error of AWS most probably results from factors associated with local surface roughness. This may reduce the area, and affect the geometry, of contact between AFM tips and the surface leading to formation of weak and easily disrupted capillaries on a hydrophilic surface similar to those formed on a smooth hydrophobic surface. The force distribution of HA-AWS possessed a significant frequency at very low force (∼0 nN), suggesting the presence of regions where capillary formation is extremely weak and others similar in magnitude to those of AWS. The distribution of forces of LE-AWS particles exhibits a significant frequency at high force (>10 nN). These distributions possess common regions in the force range 3-10 nN where the surfaces are essentially indistinguishable from each other. These may simply reflect AWS surfaces that are predominantly free from adsorbed material. Previously (8), the force distribution (in air) of particles drawn from NL samples was found to be a poor indicator of the distribution of organic material around particle surfaces. The broad and rather random force distributions of natural soil particle surfaces reflect contributions arising from the inherent roughness of the underlying mineral phases, augmented, or ameliorated, by that of naturally adsorbed organic material, and variations in surface chemistry of this material (as previously described). The Fad obtained from such distributions may not be associated with the dominant mode and, therefore, may not serve as the most appropriate statistic to represent the property. Force Measurement in Water. Typical forces of interaction, as a function of the separation distance between an AFM tip, glass, and HA and LE exposed on the glass surfaces show increasingly repulsive interactions (as the tip approaches the surface) in that sequence (Figure 4). Although
FIGURE 4. Typical force-separation distance curves measured in water between AFM tip and glass and glass coated with HA and LE, (a) approach curves and (b) retraction curves. the AFM tip is likely to possess negative surface charge due to oxidation of the surface (37), the approach curve to glass (Figure 4a) shows a negligible repulsion suggesting minimal surface charge on the glass which consistently returned a water CA of 43°. The repulsive forces for HA and LE may indicate that they presented a negatively charged surface to the tip providing a Coulombic interaction of electrical double layers. Force curves for retraction of the tip from the surfaces (Figure 4b) show a small region of negative (i.e., attractive) force at a separation of LE > smooth glass and show much larger relative variation than similar repetitions made in air (Table 1). This variation may result from dissolution of organic material from the glass, observed through the (in situ) optical microscope, and/or adsorption of organic molecules on the AFM tip. Typical maps of adhesion force distributions of AWS, HA-, and LE-AWS particles between AFM tip and particle surfaces, made in water, over regions of 1 µm2, available in the SI, show a featureless distribution of adhesion force for AWS falling in a narrow range, consisting of a dominant mode at LE-AWS > AWS is the same as that found for the forces between corresponding smooth glass surfaces in water, but differs from the trend for forces in air. Distributions of adhesion force (in water) between natural soil particles and AFM tips, over regions of 1 µm2, show differences depending on the SOM content, its quality and properties on the surface (Figure 5). The force distribution for particles drawn from (water repellent) NL1 was much broader than that for (wettable soil) NLC (Figure 5a). Although both contain outlying data points at high forces, these were significantly more numerous for NL1 suggesting that NLC had a surface more sparsely populated with organic material (consistent with, and complementing previous findings 7, 8). Similar results were found for maps and force distributions of soil particles from PT2 and PTC (Figure 5b) and AU2 and AUC (Figure 5c). The adhesion force distribution for PT2 is much broader than that for PTC, but both show a very low amplitude and scatter of data extending to below 5.0 nN. This may arise from the organic material present on particle surfaces from both sample types. Particles from PTC show significant proportions of adhesion forces below ∼0.6 nN 6504
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FIGURE 5. Force distribution from 900 measurements in water by AFM for (a) NL1 and NLC; (b) PT2 and PTC; (c) AU2 and AUC. and the force distributions for PT2 particles show reduced amplitudes in this range, with a broad peak extending to 2.5 nN. AU2 and AUC both show the predominant peaks at low force, and AU2 exhibits a broad peak extending to 3.7 nN. This is consistent with the quantity and heterogeneity of SOM present on soil particle surfaces reported previously (8, 39). The fact that adhesion forces exhibited by particles from water repellent soils NL1 and PT2 in water were higher than those from their wettable counterparts NLC and PTC may be significant (Table 1). It suggests that correlation between nanoscale particle surface and bulk soil-water repellencies may be more than simply fortuitous. For both NL and PT pairs, these results reflect the higher levels of extractable organic material in the bulk samples. Although some SOM may exist in free particulate form, and adsorbed forms may be nonuniformly distributed across or within various mineral particle size fractions, the existence of scattered submicrometer-scale hydrophobic surface domains is likely to cause, or at least contribute to, bulk soil-water repellency. These domains provide scope for retention of various additional adsorbents including microbes and viruses (40) with affinities for hydrophobic
surfaces, which may further influence evolution of local soil structure and properties. Implications. The higher adhesion forces (Fad) in water, exhibited by soil particles that form hydrophobic single particle layers (CA > 90°) NL1 and PT2 in comparison with their hydrophilic counterparts (NLC and PTC) (CA < 90°) and the opposite case for Fad in air (Table 1) may have important implications. It suggests that correlation between nanoscale particle surface properties and bulk soil hydrophobicity may be more than simply fortuitous. For both these sample pairs, results reflect the higher levels of extractable organic material in bulk samples of NL1 and PT2. Data for the pair AU2 and AUC are ambiguous. Topographical, phase and adhesion force data and images from precisely the same areas of specimen surface are likely to provide critical additional information in relation to the detailed distribution of surface properties with topography. Ideally this involves scanning with various probes and relocation of a reference point on a rough surface following necessary disruption of the geometry between probe and specimen. This presents a demanding experimental challenge. Recently (41) the potential for development of superhydrophobicity in soils, where roughness and surface chemistry act in concert, has been identified. AFM is likely to allow (i) a more detailed evaluation of soil particle surface hydrophobicity, which in turn determines bulk wetting behavior of soils and other porous particulate systems, and (ii) to assess the potential for contributions to superhydrophobicity from surfaces at micro- and nanoscales.
(11) (12) (13) (14) (15)
(16)
(17) (18) (19)
(20)
Acknowledgments This work was supported by UK Natural Environment Research Council grant NE/C003985/1 and Advanced Fellowship NER/J/S/2002/00662. L.W. Dekker and J.J. Keizer kindly provided soil samples. We are grateful to the UK Engineering and Physics Science Research Council for supporting the facilities used in this study.
(23)
Supporting Information Available
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Details of soil samples, specimen preparation, AFM methods and maps of adhesion force on AWS particles. This material is available free of charge via the Internet at http://pubs. acs.org.
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