Nanoscale Forces between Basal Mica Surfaces in Dicarboxylic Acid Solutions: Implications for Clay Aggregation in the Presence of Soluble Organic Acids

The stability of organomineral aggregates in soils has a key influence on nutrient cycling, erosion, and soil productivity. Both clay minerals with distinct basal and edge surfaces and organic molecules with reactive functional groups offer rich bonding environments. While clay edges often promote strong inner-sphere bonding of −COOH-laden organics, we explore typically weaker, outer-sphere bonding of such molecules onto basal planes and its significance in organomineral interactions. In this surface force apparatus study, we probed face-specific interactions of negatively charged mica basal surfaces in solutions containing carboxyl-bearing, low-molecular-weight dicarboxylic acids (DAs). Our experiments provide distance-resolved, nanometer-range measurements of forces acting between two (001) mica surfaces and simultaneously probe DA adsorption. We show that background inorganic ions display crucial importance in nanoscale forces acting between basal mica surfaces and in DA adsorption: Na+ contributes to strong repulsion and little binding of dicarboxylic anions, while small amounts of Ca2+ are sufficient to screen the basal surface charge of mica, facilitate strong adhesion, and enhance dicarboxylic anion adsorption by acting as cationic bridges. Despite reversible and weak adsorption of DAs, we resolve their multilayer binding via assembly of hydrophobic chains in the presence of Ca2+, pointing the importance of abundant, less reactive basal clay surfaces in organomineral interactions.

S2 TABLE S1. Chosen previous works studying adsorption of low molecular weight, soluble organic acids onto clay minerals and clay aggregation behavior in the presence of these organic acids.
year authors title mineral organic molecules methods adsorption characteristics face-specific information reference
Interactions of a marine humic acid with clay minerals and a natural sediment.
chlorite, illite, kaolinite, natural sediments humic acid batch adsorption, XRD, thermogravimetric analysis, infrared spectroscopy, chemical modelling Sorption of humic acid is higher in the presence of NaCl. Humic acid adsorption is reversible. Humic acid does not intercalate into interlayer spaces of clays. The work indicates that humic acid bonds via electrostatic and van der Waals forces, with a small proportion of humic acid chemically bonded with clays via -COOH groups. The authors suggest that the sorption is the highest in regions with highest proportion of broken bonds.
The formation of humic coatings on mineral particles under simulated estuarine conditions-a mechanistic study kaolinite, vermiculite humic and fulvic acids batch adsorption, specific surface area measurements Humic acids are more likely to adsorb by chemisorption, whereas more hydrophilic organic molecules are physisorbed. There is no face specific information, although the authors suggest that sorption is larger for smaller clay particles due to higher amount of broken edges. The authors report pH-dependent sorption of citric acid onto clays. Kaolinite adsorbs the highest amount of citric acid between pH 4.5 and 7, while illite between pH 5 and 7. Sorption is smaller both at lower and higher pHs. Authors suggest only outer-sphere adsorption of citrate on illite and kaolinite edges and no evidence of adsorption on negatively charged silica faces of both kaolinite and illite.   (Figures 2, 3, 4 Figure 5, with pH partially adjusted with varying concentrations of Ca(OH)2.       Daniele, P. G., et al. (1985). Journal of the Chemical Society, Dalton Transactions, (11), 2353-2361; c) Nakajima, D., et al. (2014). Applied surface science, 321, 364-370.

Dissociation Constants
Oxalic 2    Electrical double layer forces and surface potentials 0 were estimated using the linear superposition approximation (LSA) method at constant potential (CP-LSA), adapted from Israelachvili (Intermolecular and surface forces. −1 is Debye length (m -1 ), is bulk concentration of each ion species in the solution (M), is ion valency, 0 is electrical permittivity of vacuum (F/m), is the water dielectric constant, k is the Boltzmann constant, is temperature (K), is the radius of the SFA cylindrical samples (m), and is the distance between the surfaces (m). As the amount of divalent species in dicarboxylic acid solutions is negligible at low pH, we assumed monovalent electrolytes composed of − and + species, where 2− is a fully dissociated dicarboxylic anion. Neutral 2 species do not contribute to EDL repulsion. S16 TABLE S8: Theoretical and fitted Debye lengths for high pH dicarboxylic acid solutions and inorganic control for forces shown in Figure 4A. Theoretical Debye lengths were calculated based on the solution ionic strength ( ), including complexation if the association constants were available. complexation has been modelled in PhreeqC software using association constants summarized in Table  S5. 2− is a fully dissociated dicarboxylic anion. 0.9 ± 0.1 C7 pimelic 0.9 ± 0.1 S17 FIGURE S1: Representative forces (F) normalized with the radius of contact curvature (R) measured as a function of surface separation (D) in high pH (8.3) NaCl control experiment, with ionic strength (IS) of 150 mM. The measured experimental Debye length was = 0.9 ± 0.1 nm. The inset plot shows variation of adhesion measured in a representative experiment.