Abstract. Divergent ice nucleation (IN) efficiencies of quartz, an important component
of atmospheric mineral dust, have been reported in previous studies. We show
here that quartz particles obtain their IN activity from milling and that
quartz aged in water loses most of its IN efficiency relative to freshly
milled quartz. Since most studies so far reported IN activities of commercial
quartz dusts that were milled already by the manufacturer, IN active samples
prevailed. Also, the quartz surface – much in contrast to that of feldspars
– is not prone to ammonia-induced IN enhancement. In detail we investigate
the influence of solutes on the IN efficiency of various silica
(SiO2) particles (crystalline and amorphous) with special focus on
quartz. We performed immersion freezing experiments and relate the observed
variability in IN activity to the influence of milling, the aging time and to
the exposure conditions since milling. Immersion freezing with silica
particles suspended in pure water or aqueous solutions of NH3,
(NH4)2SO4, NH4HSO4, Na2SO4 and
NaOH, with solute concentrations corresponding to water activities
aw=0.9–1.0, were investigated in emulsified droplets by means
of differential scanning calorimetry (DSC) and analyzed in terms of the onset
temperature of the heterogeneous freezing signal Thet and the
heterogeneously frozen water volume fraction Fhet. Quartz
particles, which originate from milling coarse samples, show a strong
heterogeneous freezing peak in pure water with Thet equal to
247–251 K. This IN
activity disappears almost completely after aging for 7 months in pure water
in a glass vial. During this time quartz slowly grew by incorporating silicic
acid leached from the glass vial. Conversely, the synthesized amorphous
silica samples show no discernable heterogeneous freezing signal unless they
were milled. This implies that defects provide IN activity to silica
surfaces, whereas the IN activity of a natural quartz surface is negligible,
when it grew under near-equilibrium conditions. For suspensions containing
milled quartz and the solutes (NH4)2SO4, NH4HSO4
or Na2SO4, Thet approximately follows
ThetΔawhet(aw), the
heterogeneous freezing onset temperatures that obey Δawhet criterion, i.e., ThetΔawhet(aw)=Tmelt(aw+Δawhet) with Δawhet being
a constant offset with respect to the ice melting point curve, similar to
homogeneous IN. This water-activity-based description is expected to hold
when the mineral surface is not altered by the presence of the solutes. On
the other hand, we observe a slight enhancement in Fhet in the
presence of these solutes, implying that the compliance with the Δawhet criterion does not necessarily imply constant
Fhet. In contrast to the sulfates, dilute solutions of
NH3 or NaOH (molality ≥5×10-4 mol kg−1) reveal
Thet by 3–8 K lower than ThetΔawhet(aw), indicating a significant impact
on the mineral surface. The lowering of Thet of quartz suspended
in dilute NH3 solutions is opposite to the distinct increase in
Thet that we found in emulsion freezing experiments with
aluminosilicates, namely feldspars, kaolinite, gibbsite and micas. We ascribe
this decrease in IN activity to the increased dissolution of quartz under
alkaline conditions. The defects that constitute the active sites appear to
be more susceptible to dissolution and therefore disappear first on a
dissolving surface.
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