Ca²⁺ Ions Decrease Adhesion between Two (104) Calcite Surfaces as Probed by Atomic Force Microscopy

Abstract

Solution composition-sensitive disjoining pressure acting between the mineral surfaces in fluid-filled granular rocks and materials controls their cohesion, facilitates the transport of dissolved species, and may sustain volume-expanding reactions leading to fracturing or pore sealing. Although calcite is one of the most abundant minerals in the Earth’s crust, there is still no complete understanding of how the most common inorganic ions affect the disjoining pressure (and thus the attractive or repulsive forces) operating between calcite surfaces. In this atomic force microscopy study, we measured adhesion acting between two cleaved (104) calcite surfaces in solutions containing low and high concentrations of Ca 2+ ions. We detected only low adhesion between calcite surfaces, which was weakly modulated by the varying Ca 2+ concentration. Our results show that the more hydrated calcium ions decrease the adhesion between calcite surfaces with respect to monovalent Na + at a given ionic strength, and thus Ca 2+ can sustain relatively thick water films between contacting calcite grains even at high overburden pressures. These findings suggest a possible loss of cohesion and continued progress of reaction-induced fracturing for weakly charged minerals in the presence of strongly hydrated ionic species.

Detecting Early-Stage Cohesion Due to Calcium Silicate Hydration with Rheology and Surface Force Apparatus

“…We have previously shown with the SFA that the viscous, amorphous calcium carbonate gel-like phase, which reprecipitated on two confined calcite surfaces, significantly increased the repulsion between these confining surfaces . In general, low-surface-charge calcium carbonate surfaces repel each other in water due to their strong surface hydration, ,, and Ca 2+ ions do not enhance attraction between calcite surfaces . In contrast, reprecipitation of C 3 S particles increases the local pH and produces highly charged C–S–H nanoplatelets, which attract each other mainly via ion-correlation forces in the presence of Ca 2+ (at pH above 11, most of the surface silanol groups are deprotonated and the surfaces undergo charge reversal due to Ca 2+ adsorption).…”

“… 68 In general, low-surface-charge calcium carbonate surfaces repel each other in water due to their strong surface hydration, 69 , 78 , 79 and Ca 2+ ions do not enhance attraction between calcite surfaces. 80 In contrast, reprecipitation of C 3 S particles increases the local pH and produces highly charged C–S–H nanoplatelets, which attract each other mainly via ion-correlation forces in the presence of Ca 2+ 27 (at pH above 11, most of the surface silanol groups are deprotonated 18 and the surfaces undergo charge reversal due to Ca 2+ adsorption). Here, in our SFA experiments, the C–S–H product remains on the surfaces despite repeated pull-off events and shows that the C–S–H layers are strongly linked to the unreacted C 3 S surfaces.…”

Hygroscopic Growth of Adsorbed Water Films on Smectite Clay Particles

“…The water film thickness h on mineral surfaces at subsaturation conditions ranges from a few Å to ∼100 nm. ,− In atmospheric modeling of mineral dust transport, where the long-lifetime particles are thought to carry a limited amount of adsorbed water under subsaturation conditions, , the relationship between Π and h is of particular importance because it can provide a parameterization for the hygroscopic growth and surface reactions of mineral dust particles. , The relationship remains poorly constrained, as illustrated by the use of a range of formulas. Experimental studies have generally used an exponential relation, Π ∝ exp­(− h /δ), with an empirical decay length δ ranging from ∼3 to 300 Å. ,, Meanwhile, theoretical studies suggest an inverse power law relation, Π ∝ h – b , with significant disagreement regarding the value of the inverse power-law exponent ( b = 1, 2, or 3). , Furthermore, empirical relations established in different experimental studies vary significantly with underlying physical and chemical features including mineral type, surface active site, surface charge density, surface roughness, and aqueous chemistry. − …”

“… 32 , 33 Furthermore, empirical relations established in different experimental studies vary significantly with underlying physical and chemical features including mineral type, surface active site, surface charge density, surface roughness, and aqueous chemistry. 34 − 36 …”