6 research outputs found
A Study of the Hydration of the Alkali Metal Ions in Aqueous Solution
The hydration of the alkali metal ions in aqueous solution
has been studied by large angle X-ray scattering (LAXS) and double
difference infrared spectroscopy (DDIR). The structures of the dimethyl
sulfoxide solvated alkali metal ions in solution have been determined
to support the studies in aqueous solution. The results of the LAXS
and DDIR measurements show that the sodium, potassium, rubidium and
cesium ions all are weakly hydrated with only a single shell of water
molecules. The smaller lithium ion is more strongly hydrated, most
probably with a second hydration shell present. The influence of the
rubidium and cesium ions on the water structure was found to be very
weak, and it was not possible to quantify this effect in a reliable
way due to insufficient separation of the OâD stretching bands
of partially deuterated water bound to these metal ions and the OâD
stretching bands of the bulk water. Aqueous solutions of sodium, potassium
and cesium iodide and cesium and lithium hydroxide have been studied
by LAXS and MâO bond distances have been determined fairly
accurately except for lithium. However, the number of water molecules
binding to the alkali metal ions is very difficult to determine from
the LAXS measurements as the number of distances and the temperature
factor are strongly correlated. A thorough analysis of MâO
bond distances in solid alkali metal compounds with ligands binding
through oxygen has been made from available structure databases. There
is relatively strong correlation between MâO bond distances
and coordination numbers also for the alkali metal ions even though
the MâO interactions are weak and the number of complexes of
potassium, rubidium and cesium with well-defined coordination geometry
is very small. The mean MâO bond distance in the hydrated sodium,
potassium, rubidium and cesium ions in aqueous solution have been
determined to be 2.43(2), 2.81(1), 2.98(1) and 3.07(1) Ă
, which
corresponds to six-, seven-, eight- and eight-coordination. These
coordination numbers are supported by the linear relationship of the
hydration enthalpies and the MâO bond distances. This correlation
indicates that the hydrated lithium ion is four-coordinate in aqueous
solution. New ionic radii are proposed for four- and six-coordinate
lithiumÂ(I), 0.60 and 0.79 Ă
, respectively, as well as for five-
and six-coordinate sodiumÂ(I), 1.02 and 1.07 Ă
, respectively.
The ionic radii for six- and seven-coordinate K<sup>+</sup>, 1.38
and 1.46 Ă
, respectively, and eight-coordinate Rb<sup>+</sup> and Cs<sup>+</sup>, 1.64 and 1.73 Ă
, respectively, are confirmed
from previous studies. The MâO bond distances in dimethyl sulfoxide
solvated sodium, potassium, rubidium and cesium ions in solution are
very similar to those observed in aqueous solution
A Coordination Chemistry Study of Hydrated and Solvated Cationic Vanadium Ions in Oxidation States +III, +IV, and +V in Solution and Solid State
The coordination chemistry of hydrated and solvated vanadiumÂ(III),
oxovanadiumÂ(IV), and dioxovanadiumÂ(V) ions in the oxygen-donor solvents
water, dimethyl sulfoxide (DMSO), and <i>N</i>,<i>N</i>â˛-dimethylpropyleneurea (DMPU) has been studied in solution
by extended X-ray absorption fine structure (EXAFS) and large-angle
X-ray scattering (LAXS) and in the solid state by single-crystal X-ray
diffraction and EXAFS. The hydrated vanadiumÂ(III) ion has a regular
octahedral configuration with a mean VâO bond distance of 1.99
Ă
.
In the hydrated and DMSO-solvated oxovanadiumÂ(IV) ions, vanadium
binds strongly to an oxo group at ca. 1.6 Ă
. The solvent molecule
trans to the oxo group is very weakly bound,
at ca. 2.2 Ă
, while the remaining four solvent molecules, with
a mean VâO bond distance of 2.0 Ă
, form a plane slightly
below the vanadium atom; the mean OîťVî¸O<sub>perp</sub> bond angle is ca. 98°. In the DMPU-solvated
oxovanadiumÂ(IV) ion, the space-demanding properties of the DMPU molecule
leave no solvent molecule in the trans position to the oxo group,
which reduces the coordination number to 5. The OîťVî¸O
bond angle is consequently much larger, 107°, and the mean
VîťO and VâO bond distances decrease to 1.58 and 1.97
Ă
,
respectively. The hydrated and DMSO-solvated dioxovanadiumÂ(V) ions
display a very distorted octahedral configuration with the oxo groups
in the cis position with a mean VîťO bond distance of 1.6 Ă
and a OîťVîťO bond angle of ca. 105°. The solvent
molecules trans to the oxo groups are weakly bound, at ca. 2.2 Ă
,
while the remaining two have bond distances of 2.02 Ă
. The experimental
studies of the coordination chemistry of hydrated
and solvated vanadiumÂ(III,IV,V) ions are complemented by summarizing
previously reported crystal structures to yield a comprehensive description
of the coordination chemistry of vanadium with oxygen-donor ligands
On the Structure and Volumetric Properties of Solvated Lanthanoid(III) Ions in Amide Solutions
The
coordination chemistry and the volumetric properties of three
representative lanthanoidÂ(III) ionsî¸lanthanumÂ(III), gadoliniumÂ(III),
and lutetiumÂ(III)î¸have been studied in three amide solvents
with gradually increasing spatial demand upon coordination: <i>N</i>,<i>N</i>-dimethylformamide (dmf) < <i>N</i>,<i>N</i>-dimethylacetÂamide (dma) < <i>N</i>,<i>N</i>-dimethylpropionamide (dmp). Large angle
X-ray scattering (LAXS) and EXAFS have been used to determine the
structure of the solvated lanthanoidÂ(III) ions in solution, further
supplemented with a crystallographic study on octakisÂ(<i>N</i>,<i>N</i>-dimethylacetamide)ÂlanthanumÂ(III) triflate, [LaÂ(dma)<sub>8</sub>]Â(CF<sub>3</sub>SO<sub>3</sub>)<sub>3</sub>. The selection
of ions and solvents allows an estimate of the steric congestion effects
on the resulting coordination number, CN, ranging from nine for lanthanumÂ(III)
ions in dmf to seven for the smaller lutetiumÂ(III) ion in space-demanding
dma. The standard partial molar volumes of the solvated lanthanoidÂ(III)
ions in water and dmf are reflected in the CNs, as these solvent molecules
are small enough to not interfere with each other upon coordination.
However, the larger and more space-demanding dma displays a different
pattern with an almost constant standard partial molar volume and
a decreasing CN, counterbalancing the difference in ionic radius of
the lanthanoidÂ(III) ion
On the Structure and Volumetric Properties of Solvated Lanthanoid(III) Ions in Amide Solutions
The
coordination chemistry and the volumetric properties of three
representative lanthanoidÂ(III) ionsî¸lanthanumÂ(III), gadoliniumÂ(III),
and lutetiumÂ(III)î¸have been studied in three amide solvents
with gradually increasing spatial demand upon coordination: <i>N</i>,<i>N</i>-dimethylformamide (dmf) < <i>N</i>,<i>N</i>-dimethylacetÂamide (dma) < <i>N</i>,<i>N</i>-dimethylpropionamide (dmp). Large angle
X-ray scattering (LAXS) and EXAFS have been used to determine the
structure of the solvated lanthanoidÂ(III) ions in solution, further
supplemented with a crystallographic study on octakisÂ(<i>N</i>,<i>N</i>-dimethylacetamide)ÂlanthanumÂ(III) triflate, [LaÂ(dma)<sub>8</sub>]Â(CF<sub>3</sub>SO<sub>3</sub>)<sub>3</sub>. The selection
of ions and solvents allows an estimate of the steric congestion effects
on the resulting coordination number, CN, ranging from nine for lanthanumÂ(III)
ions in dmf to seven for the smaller lutetiumÂ(III) ion in space-demanding
dma. The standard partial molar volumes of the solvated lanthanoidÂ(III)
ions in water and dmf are reflected in the CNs, as these solvent molecules
are small enough to not interfere with each other upon coordination.
However, the larger and more space-demanding dma displays a different
pattern with an almost constant standard partial molar volume and
a decreasing CN, counterbalancing the difference in ionic radius of
the lanthanoidÂ(III) ion
Chromium(III) Complexation to Natural Organic Matter: Mechanisms and Modeling
Chromium is a common soil contaminant,
and it often exists as chromiumÂ(III).
However, limited information exists on the coordination chemistry
and stability of chromiumÂ(III) complexes with natural organic matter
(NOM). Here, the complexation of chromiumÂ(III) to mor layer material
and to Suwannee River Fulvic Acid (SRFA) was investigated using EXAFS
spectroscopy and batch experiments. The EXAFS results showed a predominance
of monomeric chromiumÂ(III)-NOM complexes at low pH (<5), in which
only Cr¡¡¡C and CrâOâC interactions were
observed in the second coordination shell. At pH > 5 there were
polynuclear
chromiumÂ(III)-NOM complexes with Cr¡¡¡Cr interactions
at 2.98 Ă
and for SRFA also at 3.57 Ă
, indicating the presence
of dimers (soil) and tetramers (SRFA). The complexation of chromiumÂ(III)
to NOM was intermediate between that of ironÂ(III) and aluminumÂ(III).
ChromiumÂ(III) complexation was slow at pH < 4: three months or
longer were required to reach equilibrium. The results were used to
constrain chromium-NOM complexation in the Stockholm Humic Model (SHM):
a monomeric complex dominated at pH < 5, whereas a dimeric complex
dominated at higher pH. The optimized constant for the monomeric chromiumÂ(III)
complex was in between those of the ironÂ(III) and aluminumÂ(III) NOM
complexes. Our study suggests that chromiumÂ(III)-NOM complexes are
important for chromium speciation in many environments
Amorphous Calcium Carbonate Constructed from Nanoparticle Aggregates with Unprecedented Surface Area and Mesoporosity
Amorphous
calcium carbonate (ACC), with the highest reported specific
surface area of all current forms of calcium carbonate (over 350 m<sup>2</sup> g<sup>â1</sup>), was synthesized using a surfactant-free,
one-pot method. Electron microscopy, helium pycnometry, and nitrogen
sorption analysis revealed that this highly mesoporous ACC, with a
pore volume of âź0.86 cm<sup>3</sup> g<sup>â1</sup> and
a pore-size distribution centered at 8â9 nm, is constructed
from aggregated ACC nanoparticles with an estimated average diameter
of 7.3 nm. The porous ACC remained amorphous and retained its high
porosity for over 3 weeks under semi-air-tight storage conditions.
Powder X-ray diffraction, large-angle X-ray scattering, infrared spectroscopy,
and electron diffraction exposed that the porous ACC did not resemble
any of the known CaCO<sub>3</sub> structures. The atomic order of
porous ACC diminished at interatomic distances over 8 Ă
. Porous
ACC was evaluated as a potential drug carrier of poorly soluble substances
in vitro. Itraconazole and celecoxib remained stable in their amorphous
forms within the pores of the material. Drug release rates were significantly
enhanced for both drugs (up to 65 times the dissolution rates for
the crystalline forms), and supersaturation release of celecoxib was
also demonstrated. Citric acid was used to enhance the stability of
the ACC nanoparticles within the aggregates, which increased the surface
area of the material to over 600 m<sup>2</sup> g<sup>â1</sup>. This porous ACC has potential for use in various applications where
surface area is important, including adsorption, catalysis, medication,
and bone regeneration