Abstract. 2-Methylbutane-1,2,3,4-tetraol (hereafter named tetraol) is an important
oxidation product of isoprene and can be considered as a marker compound for
isoprene-derived secondary organic aerosols (SOAs). Little is known about
this compound's physical phase state, although some field observations
indicate that isoprene-derived secondary organic aerosols in the tropics tend
to be in a liquid rather than a solid state. To gain more knowledge about the
possible phase states of tetraol and of tetraol-containing SOA particles, we
synthesized tetraol as racemates as well as enantiomerically enriched
materials. Subsequently the obtained highly viscous dry liquids were
investigated calorimetrically by differential scanning calorimetry revealing
subambient glass transition temperatures Tg. We also show that
only the diastereomeric isomers differ in their Tg values, albeit
only by a few kelvin. We derive the phase diagram of water–tetraol mixtures
over the whole tropospheric temperature and humidity range from determining
glass transition temperatures and ice melting temperatures of aqueous tetraol
mixtures. We also investigated how water diffuses into a sample of dry
tetraol. We show that upon water uptake two homogeneous liquid domains form
that are separated by a sharp, locally constrained concentration gradient.
Finally, we measured the glass transition temperatures of mixtures of tetraol
and an important oxidation product of α-pinene-derived SOA:
3-methylbutane-1,2,3-tricarboxylic acid (3-MBTCA). Overall, our results imply
a liquid-like state of isoprene-derived SOA particles in the lower troposphere
at moderate to high relative humidity (RH), but presumably a semisolid or even
glassy state at upper tropospheric conditions, particularly at low relative
humidity, thus providing experimental support for recent modeling calculations.
Physical state of 2-methylbutane-1,2,3,4-tetraol in pure and internally mixed aerosols