Fullerene hydration is an interesting but lesser known fact concerning
the C60 and C70 fullerenes. Contrary to expectations due to their apparently hydrophobic character, they may be dissolved in water. (C60-Ih)[5,6]
fullerene C70 can be dispersed in water (> 2 mM
) by transferring
from an organic solvent using sonication without the need
of stabilizers (for example, γ-cyclodextrin ) or chemical
modification  (normally its solubility direct from solid is eleven orders lower at about 20 fM).
It has also been solubilized by ultrasonication in strong
aqueous inorganic acids  as has C70 .
The result is a kinetically-stable molecular colloidal solution
also containing a variety of negatively charged clusters.d It forms a transparent orange solution with a broad absorption
band (400-500 nm). This fullerene is an electronegative molecule
showing some aromatic behavior in the twenty six-membered (but
not the twelve five-membered) rings with the pi-orbitals
electron density biased outwards .c C60 has been shown to have a high free energy of hydration and hence a high affinity for water .
The solubility of single C60 molecules may be explained
if the fullerene may sit (ideally) in an icosahedral water
missing its inner water dodecahedron. The size of the C60 molecule and the central aqueous dodecahedron are similar (see opposite).e
The inner twenty remaining water molecules are
ideally situated to form -OH···pi hydrogen bondsa to each
of the twenty 6-membered rings in the fullerene, by positioning
directly over these rings; the optimum such positioning for
the hydrogen bond to a benzene molecule. These 20 water
molecules can then be linked through the 60 fully hydrogen
bonded water molecules from the next shell of the icosahedral
cluster. This arrangement would present a negatively charged
surface to the environment, as found experimentally. In such
a structure the carbon atoms would be centers of electron-deficiency
and capable of interacting with lone pair electrons donated
by extra water molecules. Such water molecules have room to
sit under the outer shell water molecules to which they can
hydrogen bond if some outer shell hydrogen bonds are broken
or distorted. The strong hydration around the monomeric C60 molecules has been proposed to prevent the occurrence of toxic
reactions .b The surrounding water in this complex can be partially replaced by disaccharides such as lactose .
An increased tendency to ionize by these carbon-linked water
molecules would increase the negative charge on the C60 molecules, make the C60 solution acidic as found
 and give rise to the orange
color of solutions. The resultant symmetrical positioning
of six hydroxide ions (see below right)
increases the electron density so strengthening the -OH···pi-electrons hydrogen bonding. The corresponding hydrogen ions may be associated
with the water in the immediately surrounding shell or the
In the structures given above only the innermost
80 water molecules in the icosahedral cluster are shown to
clarify the structuring. Recent spectroscopic studies are
consistent with this structure even though the authors propose
Interactive structures are given (Jmol). Without water hydrogen bonding to the outside of these clusters, they will collapse.
A potential arrangement
of six electron-donating hydroxide ions (as above
right), shown blue, and the 20 hydrogen atom-donating
hydrogen bonds, shown red, is given on the connectivity
map of C60; also showing the most important
Kekulé structure [946a].f These 20 positions are also the most prominent positions given by molecular dynamics simulations using a state-of-the-art quantum mechanical polarizable force field .
C60 molecules in aqueous solution form colloidal
clusters based on 3.4 nm-sized icosahedral arrangements
of 13 C60 molecules ,
where water separates the C60 molecules .
Such an arrangement is shown opposite within an expanded
(but now strain-free) cluster of water icosahedral clusters.
The water network is formed by fully tessellated tetrahedral tricyclo decamer (H2O)10 structures (see below for one). The diameter of the cluster (carbon atoms)
opposite is slightly larger at 3.5 nm. Ions that destroy the expanded
water network also coagulate such C60 hydrosols
(see the Hofmeister series) .
The structure is also compatible with recent
findings by Grigoriy Andrievsky using piezogravimetry 
(20 - 24 H2O per C60). This result agrees
with low temperature differential calorimetry 
where two types of water were evident, fully hydrogen bonded
water melting at 0 °C (~60 H2O per C60)
showing required hydrogen-bonding to hydrogen-bond deficient
water, melting at -2.3 °C (19 ± 1 H2O
per C60) with 30% less enthalpy change. The ratio
of inner sphere to outer (second) sphere water molecules varies
between 1:3 for single molecules and 2:3 for infinite sized
aqueous C60 clusters.
Interactive structures are given (Jmol).
Theoretical calculations show that a water molecule
may be placed inside the (C60-Ih)[5,6]
fullerene cage with stabilization (relative to the isolated
molecule) due to stronger -OH···π hydrogen bonding than found in the hydration sphere surrounding
the fullerene .
Although there is room for more water molecules inside the
cage, positioning them there is energetically unfavorable
due to poor hydrogen bonding caused by the cramped environment
(C70-D5h(6))[5,6] fullerene can also be dissolved in water . Solution is achieved using a mixture of sulfuric and nitric acids with or without ultrasonic irradiation; the soluble product proven to be unchanged C70 by mass spectrometry. The hydration of C70 may be similar to that of C60 as above, involving aqueous icosahedral clusters, and utilizing -OH···pi-electrons hydrogen bondsa. In this case the icosahedral water
again missing its inner water dodecahedron, is split into two halves hydrating the top and bottom parts of the C70 (see below). Around the middle of the molecule an incomplete hydration shell may form which naturally involves five water molecules (asterisked below) that are ideally placed to hydrogen bond to the five extra π-centers in the extra five six-membered rings that C70 possesses over the 20 in C60.
Inner aqueous sphere only, C70(H2O)90; C70 shown as blue-green, -OH···pi-electrons hydrogen bonds shown as violet-dashed with hydrogen bonds between the water molecules shown as red links, hydrogen atoms are not shown.
The connectivity map for (C70-D5h(6))[5,6] fullerene is shown opposite indicating the electron rich centers that may hydrogen bond to water in the surrounding aqueous cluster. The blue dots show the positions of the additional sites (compared with (C60-Ih)[5,6]) for hydrogen bonding. It is clear that the -OH···π hydrogen bonds are not equivalent in C70 whereas they were in C60 as there are now three different environments for the electron-rich six-membered rings (in the ratio of 10:10:5).
An interactive structure of C70(H2O)90 is given (Jmol).
a Such -OH···pi-electrons hydrogen bonds have been found to possess about half the binding
energy of -OH···O hydrogen bonds with,
optimally, the -OH atoms centrally and vertically placed and
the distances from the oxygen atom to the aromatic centroid
of about 3.1-3.7Å (compare 3.2Å for all 20
such bonds/fullerene in the above model) .
They have been found as the optimum structure of C60(H2O) . Water-benzene -OH···π hydrogen bonds are evident below 340 °C  and are responsible for the higher than expected solubility of benzene in water.
b Although disputed
from some quarters, the toxicity
widely reported for some C60 preparations may
be solely due to the presence of significant amounts of solvent
or impurities . Tests for
physiological harm should be experimental as modeling studies
may ignore this hydration effect (for example, ). Other work has shown that hydrated fullerene may have some health benefit .
c Although C60 has been modeled (by some) as a purely hydrophobic structure possessing no electrostatic interactions, it is important to recognize that there are effective charge separations between the (positive) carbon atoms and the (negative) centers of the six membered rings and also that the
C60 molecule is polarizable. Without allowing for these factors, such modeling is likely to be unreliable. [Back]
d Other work only produces colloidal clusters . [Back]
e A derivative of the C60 fullerene, C60(OH)24 (see right) dissolves and hydrates well in water. In this molecule the hydroxyl groups are closely arranged and encourage high density water forming around the fullerene, with limited (but strong) hydrogen bonding to the hydroxyl groups (<2 per OH group compared with the preferred hydration of 3 per OH group) and consequentially there are many broken hydrogen bonds in the surrounding water . [Back]
f The 30 double bonds actually have bond orders of 1.44 and the 60 single bonds (all 12 pentagons) have bond order of 1.28, so using the 'spare' Pz electrons donated from each of the 60 carbon atoms [946b]. [Back]