A
fundamental understanding of the relationship between interfacial properties
and surfactant molecular architecture is very important in design of
surfactants suitable for the formation of stable micelles, microemulsions,
emulsions, latexes and solid nanoparticle dispersions in CO2. In the last nine months much progress has
been made in designing, constructing and testing new laboratories involving
NMR, tensiometry, sum frequency generation, dynamic light scattering and Raman
spectroscopy.
Dynamics of
Surfactants in CO2 by Pulsed Field Gradient (PFG) NMR (1) Hardware and software were implemented on the Bruker Advance 500
NMR spectrometer for diffusion measurements. Representatives from Bruker and
Nalorac came to the NMR laboratory to get the spectrometer and probes up to
specifications and to train users.
Special NMR pulse sequences were put into operation on the Bruker
spectrometer. Pressure cells were
evaluated for NMR diffusion measurements with the following conclusions:
Toroid cavity detector: An extensive study of these detectors shows that
TCDs provide poor NMR resolution, poor temperature control, and no provision
for 2H lock. With current
technology TCDs are not a practical choice for CO2 diffusion
studies.
Folded microcapillary tubes (150 microns): These cells offer a high
pressure rating for safety and stability to convection currents. They suffer from low filling factors, i.e. low S/N ratios.
1-mm ID tubes: A cell was designed with a PEEK body, Swagelok union and ferrules,
and filling (at the bottom) by means of a microcapillary tube. Pyrex tubes are satisfactory for resolution,
but are limited to fairly low pressures (to 3000 psig). Accurate bore PEEK tubes cannot be prepared
in 1-mm size and zirconium oxide tubes are too expensive. Sapphire 1-mm tubes are still being sought.
NMR diffusion studies were
initiated on the PtBMA-b-PFOMA diblock copolymer system. Exchange between micelles and unimers in the
bulk CO2 phase was demonstrated.
Diffusion curves were obtained for several diffusion times, but the data
do not yet permit definitive analysis.
Interfacial
Properties in CO2-Based Systems (2) Formation of emulsions and microemulsions in CO2 has
been achieved in recent years with the use of anionic surfactants. In the past nine months, Johnston’s group
has investigated polydimethylsiloxane and perfluoropolyether based nonionic and
cationic surfactants with molecular weights varying between 500 and 10,000
g/mol. We have designed, constructed
and tested a new user-friendly high-pressure pendant drop tensiometer, fully
automated, capable of measuring static and dynamic surface (air/water) and
interfacial (liquid/supercritical fluid) tension up to 400 bar. Stability of
water-in-CO2 (W/C) emulsions has exceeded 24h. Interfacial tension measurements have been
used to determine surfactant adsorption and to identify the balanced state
where the surfactant prefers water and carbon dioxide equally. The balanced state is the key point in
controlling emulsion stability.
Nanoparticle
Colloids in CO2 (3)
Nanocrystals, 20–100 Ĺ in diameter, exhibit unique size-dependent
optical, catalytic, magnetic, and electronic properties compared to their bulk
counterparts. In this program, a
single-phase arrested growth method was developed to prepare 55 Ĺ diameter
silver nanocrystals capped with perfluoro-decanethiol ligands. The presence of the fluorinated ligand was
verified with FTIR spectroscopy. The
fluorocarbon-coated nanocrystals disperse readily in carbon dioxide at
pressures as low as 62 bar and 25oC and polar solvents such as
acetone. This is the first example of
nanocrystals sterically stabilized in pure CO2. Compared to hydrocarbon stabilized
nanocrystals, nanocrystals coated with the fluorinated ligands exhibit a
reduced electron mean free path as indicated by peak broadening in the
absorbance spectra. As a second
approach, palladium nanoparticles have been encapsulated in dendrimers in CO2
in collaboration with Crooks at Texas A&M.
The encapsulated nanoparticles were about 1 nm, and were used to
catalyze the industrially attractive Heck reaction of iodobenzene with methyl
acrylate. High selectivities indicate
that nanoparticle in dendrimers in CO2 represent a unique reaction
environment.
Structure and
Dynamics of Surface Bound/Attached Species in CO2 (4) Progress has been made on several aspects of our proposed
research. A high performance polymer-based NMR sample cell has been built and
operated. We have successfully demonstrated the use of this cell to record
high-resolution multinuclear NMR spectra at pressures ranging from ambient to
400 bar to follow phase behavior. We
have successfully adapted reverse microemulsion synthetic procedures to prepare
well-defined silica and metallic core-silica shell nanoparticles for use as
model surfaces for our NMR studies and as surface enhanced Raman spectroscopy
(SERS) substrates in Raman scattering studies, respectively. In the latter class of nanoparticles we
have, to the best of our knowledge, prepared the first silica coated alloy
nanoparticles from SERS active metals.
We have validated the use of these materials for studies of surface
dynamics, but the ability to obtain information regarding surface viscosity is
still unresolved.
In
the area of Raman spectroscopy we have developed a serpentine folded,
high-pressure Raman cell for use in high-pressure fluids which allows a greater
detection volume than a single pass capillary cell. The preliminary results indicate that these cells are not
susceptible to the stress-induced birefringence effects observed in larger cell
windows under high-pressure conditions, a key advancement. We have also recently demonstrated the
concept of an evanescent sensor for detecting the removal of material by CO2. We anticipate that this approach will have
utility in studying deposition and removal kinetics of materials on solid
interfaces in CO2 environments.
We
have discovered a new class of compounds that interact strongly with CO2.
Melting points are reduced dramatically in contact with gaseous CO2. Several derivatives of this class completely
dissolve in liquid and supercritical CO2 at up to 35 % (w/w).
Dynamics and
Structure of Colloids in CO2 (5)
The
effect of stabilizers on the particle formation stage in dispersion
polymerization of methyl methacrylate in supercritical CO2 has been
studied by in-situ turbidimetry by Webber and Johnston. The average particle diameter and number
density at the end of this formation stage were compared for
poly(dimethylsiloxane), (PDMS), poly(1,1-dihydroperfluoroctylacrylate) and
perfluoropolyether based stabilizers.
The diameters are 250 nm for the first two stabilizers. The fundamental understanding of the
mechanism of particle formation in dispersion polymerization gained from this
study is of great practical interest for the development of stabilizers to
control the final product morphology of polymer latexes. Another goal of this program has been to
design a novel, high-pressure, low-angle dynamic light scattering (DLS)
apparatus for the study of water-in-carbon dioxide microemulsions and other
colloids from 1 to 10 nm. It is capable
of studying continuous angles from 10 - 30° in a high-pressure cell
using two sapphire windows. The
high-pressure cell has been constructed and tested. It will complement the
laboratory at UNC for larger angles starting at 45 degrees.
Polarization
Microscopy (32) Samulski has initiated
studies to corroborate the existence of specific interactions between CO2 and
fluorohexane reported by Dardin and Samulski.
He has examined chemical shifts in solvent mixtures (e.g., hexane + carbon
disulfide; hexane + carbon tetrachloride) to see if the fluorine-19 shifts
scale with mixed-solvent susceptibilities.
They do. But there is evidence
for specific interactions between the fluorohexane “probe” and CS2. We will pursue similar studies with
supercritical ethane and CO2 in order to refine the nature of the
fluoropolymer-CO2 interactions.
He
has started design and fabrication of a high-pressure microscopy cell for work
on liquid crystal formation of surfactants in CO2. Quotations for the microscope, video recorder, and associated computational
equipment are now in hand. The arrival
in August of a new post-doctoral associate with experience in liquid crystal
microscopy will enable us to finalize the equipment purchase.
Non-Homogeneous
Aggregates (34) We have recruited a post-doctoral
fellow who has focused on the design and construction of spectroscopic cells
that can also be used as reaction vessels.
We have sought to contrast the surface properties of synthetic
nanoparticulate TiO2 clusters prepared respectively in hydroxylic and
aprotic solvents and in supercritical CO2. We have constructed cells that simultaneously allow monitoring of
absorption and fluorescence spectral changes accompanying these nanosyntheses.
Studies
of the mechanism of formation and stability of water-in-CO2
microemulsions and emulsions will be expanded.
Interfacial tensions will be measured at the CO2-water
interfaces with various surfactant structures as a function of formulation
variables such as temperature, salinity, pH and CO2 density. The properties of microemulsions and
emulsions and the dynamics of surfactants at the water-CO2 interface
will be investigated by a series of measurements: interfacial tension, NMR
relaxation, diffusion by NMR and dynamic light scattering (DLS), and sum
frequency generation (SFG). We will
calibrate the new Varian 600 MHz spectrometer for diffusion measurements and
will switch to 1-mm ID capillary high-pressure cells. This instrumentation will be used to study water in CO2
microemulsion systems to obtain trapping efficiencies, particle sizes, and
exchange rates.
A
new SFG system will allow the characterization of the structure at an interface
and changes that occur upon exposure to CO2 as a function of
thermodynamic conditions. This
technique represents a surface active, vibrational spectroscopy, which we use
to investigate practically any CO2-based interfacial system. In NMR and Raman experiments, we will fully
characterize the surface structure and interactions of CO2 solvent
systems in contact with C8 and C18 silica samples,
including fluorinated versions.
We
plan to complete NMR measurements on PtBMA-b-PFOMA diblock copolymer to obtain
diffusion curves using microcapillary
cells. The data sets will be analyzed
simultaneously to obtain sizes of unimers and micelles, rate constants for
exchange, and dissociation constants.
The
low angle high-pressure DLS cell will be calibrated, and drop sizes of W/C
microemulsions and solid nanoparticle dispersions will be studied. The thickness of PDMS and other polymer
layers on colloids will also be studied and contrasted with the SFG
measurements.
Nanoparticles will be synthesized in organized molecular assemblies in carbon dioxide with various surfactants. The surfactants will restrict the growth of the particles to dimensions from 1 to 100 nm. Inorganic nanoparticles will include metals and metal oxides whereas organic nanoparticles will be composed of polymers. We will continue our efforts to design and test the pressure-holding spectroscopic cells as reaction vessels for nanoscale preparations of clusters and particulate aggregates. We plan to expand the synthetic work within the next year to include the preparation of shell-core semiconductor clusters, whose photocatalytic activity will be studied.
Water-in-carbon
dioxide emulsions have been formed with cationic surfactants, and the stability
of the emulsions has been related to the properties of the surfactants at the
interface, as measured by tensiometry.
High-resolution multinuclear NMR spectra at pressures ranging from
ambient to 400 bar have been recorded with a polymer-based sample cell. The exchange rates for unimers in
PtBMA-b-PFOMA-diblock copolymer micelles have been determined by NMR to
understand the micelle dynamics. A
serpentine folded, high-pressure Raman cell has been developed which is not
susceptible to the stress-induced birefringence effects observed in larger cell
windows under high-pressure conditions. Palladium nanoparticles have been
encapsulated in dendrimers in CO2 and used to control the
selectivity in the Heck reaction of iodobenzene and methylacrylate. A fundamental understanding of the mechanism
of particle formation and stabilization in dispersion polymerization has been
developed by in-situ turbidimetry
measurements for a series of stabilizers.