Surface-Modified and Unmodified Calcite: Effects of
Water and Saturated Aqueous Octanoic Acid Droplets on Stability and
Saturated Fatty Acid Layer Organization
posted on 2021-11-18, 22:13authored byNatalia A. Wojas, Agne Swerin, Viveca Wallqvist, Mikael Järn, Joachim Schoelkopf, Patrick A. C. Gane, Per M. Claesson
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.