Surface-Modified and Unmodified Calcite: Effects of Water and Saturated Aqueous Octanoic Acid Droplets on Stability and Saturated Fatty Acid Layer Organization.

Natalia A. Wojas,Patrick A. C. Gane,Per M. Claesson,Joachim Schoelkopf,Agne Swerin,Mikael Järn,Viveca Wallqvist

doi:10.1021/acs.langmuir.1c02387

Abstract

A profound understanding of the properties of unmodified and saturated fatty acid-modified calcite surfaces is essential for elucidating their resistance and stability in the presence of water droplets. Additional insights can be obtained by also studying the effects of carboxylic acid-saturated aqueous solutions. We elucidate surface wettability, structure, and nanomechanical properties beneath and at the edge of a deposited droplet after its evaporation. When calcite was coated by a highly packed monolayer of stearic acid, a hydrophilic region was found at the three-phase contact line. In atomic force microscopy mapping, this region is characterized by low adhesion and a topographical hillock. The surface that previously was covered by the droplet demonstrated a patchy structure of about 6 nm height, implying stearic acid reorganization into a patchy bilayer-like structure. Our data suggest that during droplet reverse dispensing and droplet evaporation, pinning of the three-phase contact line leads to the transport of dissolved fatty carboxylic acid and possibly calcium bicarbonate Ca(HCO3)2 molecules to the contact line boundary. Compared to the surface of intrinsically hydrophobic materials, such as polystyrene, the changes in contact angle and base diameter during droplet evaporation on stearic acid-modified calcite are strikingly different. This difference is due to stearic acid reorganization on the surface and transport to the water–air interface of the droplet. An effect of the evaporating droplet is also observed on unmodified calcite due to dissolution and recrystallization of the calcite surface in the presence of water. In the case where a water droplet saturated with octanoic acid is used instead of water, the stearic acid-coated calcite remains considerably more stable. Our findings are discussed in terms of the coffee-ring effect.

Highlights

Calcium carbonate (CaCO3) is the most abundant inorganic biomineral in nature, which is due to the predominance of limestone over other carbonate rocks.[1]
AND DISCUSSION we first consider morphological and nanomechanical changes that occur on calcite and carboxylic acidmodified calcite surfaces, as a result of exposure to droplets of pure water and water saturated with octanoic acid
The calcite surface area that was located under the water droplet and at the edge of the water droplet is different from the area only exposed to air

Summary

INTRODUCTION

Calcium carbonate (CaCO3) is the most abundant inorganic biomineral in nature, which is due to the predominance of limestone over other carbonate rocks.[1]. As calcium carbonate has a high surface energy (about 500− 600 mJ/m2 for the {101̅4} plane), it readily adsorbs organic molecules.[34,35] for fundamental studies, a clean, freshly fractured CaCO3 surface should be used for effective surface modification, after which the modified surface should be stored in the dry pure air (or preferably vacuum) environment. This is, not realistically achievable in industrial processes.[36] The calcite surface itself is highly reactive and readily undergoes recrystallization with time in air and at high relative humidity. Experiments with octanoic acid-saturated aqueous droplets show that the CRE can be suppressed to achieve higher stability of the initially homogeneous fatty acid layer

EXPERIMENTAL SECTION

RESULTS AND DISCUSSION

MORPHOLOGICAL AND NANOMECHANICAL CHANGES

CONCLUSIONS

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