| 1. |
- Halthur, Tobias, et al.
(författare)
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Stability of Polypeptide Multilayers As Studied by in Situ Ellipsometry : Effects of Drying and Post-Buildup Changes in Temperature and pH
- 2004
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 126, s. 17009-17015
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Tidskriftsartikel (refereegranskat)abstract
- Polyelectrolyte multilayers (PEM) of poly(L-glutamic acid) (PGA) and poly(L-lysine) (PLL) with an initial layer of polyethyleneimine (PEI) were built on silica and titanium surfaces using the layer-by-layer (LbL) technique. The stability of the film during drying/rewetting, temperature cycles, and pH shifts was studied in situ by means of ellipsometry. The film thickness was found to decrease significantly (approximately 70%) upon drying, but the original film thickness was regained upon rewetting, and the buildup could be continued. The thickness in the dry state was found to be extremely sensitive to ambient humidity, needing several hours to equilibrate. Changes in temperature and pH were also found to influence the multilayer thickness, leading to swelling and deswelling of as much as 8% and 10-20% respectively. The film does not necessarily regain its original thickness as the pH is shifted back, but instead shows clear signs of hysteresis.
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| 2. |
- Tyrode, Eric, et al.
(författare)
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Adsorption of CTAB on hydrophilic silica studied by linear and nonlinear optical spectroscopy
- 2008
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Ingår i: Journal of The American Ceramic Society. - : American Chemical Society. - 0002-7820 .- 1551-2916 .- 0002-7863 .- 1520-5126. ; 130:51, s. 17434–17445-
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Tidskriftsartikel (refereegranskat)abstract
- Vibrational sum-frequency spectroscopy (SFS) and total internal reflection Raman scattering (TIR Raman) have been used to study the adsorption of hexadecyltrimethylammonium bromide (CTAB) to hydrophilic silica. These two complementary techniques permit the determination of the adsorbed amount with a sensitivity of 1% of the maximum surface coverage, changes in the average tilt of the adsorbed molecules, the presence of asymmetric aggregates in the adsorbed film, and the structure and orientation of the water molecules in the interfacial region. The TIR Raman spectra show a monotonic increase with CTAB concentration with no measurable changes in the relative intensities of the different polarization combinations probed, implying that no significant changes occur in the conformational order of the hydrocarbon chain. In the sum-frequency (SF) spectra, no detectable peaks from the surfactant headgroup and hydrophobic chain were observed at any surface coverage. Major changes are observed in the water bands of the SF spectra, as the originally negatively charged silica surface becomes positively charged with an increase in the adsorbed amount, inducing a change in the polar orientation of the water molecules near the surface. The detection limits for hydrocarbons chains in the SF spectra were estimated by comparison with the SF spectrum of a disordered octadecyltrichlorosilane monolayer. The simulations demonstrate that the asymmetry in the adsorbed CTAB layer at any concentration is less than 5% of a monolayer. The results obtained pose severe constraints on the possible structural models, in particular at concentrations below the critical micellar concentration where information is scarce. The formation of hemimicelles, monolayers and other asymmetric aggregates is ruled out, with centrosymmetric aggregates forming from early on in the adsorption process.
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| 3. |
- Tyrode, Eric, et al.
(författare)
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Hydration State of Non-ionic Surfactant Monolayers at the Liquid/Vapor Interface : Structure Determination by Vibrational Sum Frequency Spectroscopy
- 2005
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 127, s. 16848-16859
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Tidskriftsartikel (refereegranskat)abstract
- The OH stretching region of water molecules in the vicinity of nonionic surfactant monolayers has been investigated using vibrational sum frequency spectroscopy (VSFS) under the polarization combinations ssp, ppp, and sps. The surface sensitivity of the VSFS technique has allowed targeting the few water molecules present at the surface with a net orientation and, in particular, the hydration shell around alcohol, sugar, and poly(ethylene oxide) headgroups. Dramatic differences in the hydration shell of the uncharged headgroups were observed, both in comparison to each another and in comparison to the pure water surface. The water molecules around the rigid glucoside and maltoside sugar rings were found to form strong hydrogen bonds, similar to those observed in tetrahedrally coordinated water in ice. In the case of the poly(ethylene oxide) surfactant monolayer a significant ordering of both strongly and weakly hydrogen bonded water was observed. Moreover, a band common to all the surfactants studied, clearly detected at relatively high frequencies in the polarization combinations ppp and sps, was assigned to water species located in proximity to the surfactant hydrocarbon tail phase, with both hydrogen atoms free from hydrogen bonds. An orientational analysis provided additional information on the water species responsible for this band.
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| 4. |
- Ahlquist, Mårten, et al.
(författare)
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Product Protection, the Key to Developing High Performance Methane Selective Oxidation Catalysts
- 2009
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 131:47, s. 17110-17115
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Tidskriftsartikel (refereegranskat)abstract
- Selective, direct conversion of methane to methanol might seem an impossible task since the C-H bond energy of methane is 105 kcal mol(-1) compared to the C-H bond energy for methanol of 94. We show here that the Catalytica catalyst is successful because the methanol is protected as methyl bisulfate, which is substantially less reactive than methanol toward the catalyst. This analysis suggests a limiting performance for systems that operate by this type of protection that is well above the Catalytica system.
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| 5. |
- Ahlquist, Mårten, et al.
(författare)
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The mechanism of the phosphine-free palladium-catalyzed hydroarylation of alkynes
- 2006
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 128:39, s. 12785-12793
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Tidskriftsartikel (refereegranskat)abstract
- The mechanism of the Pd-catalyzed hydroarylation and hydrovinylation reaction of alkynes has been studied by a combination of experimental and theoretical methods (B3LYP), with an emphasis on the phosphine-free version. The regioselectivity of the hydroarylation and hydrovinylation shows unexpected differences, which could be attributed mainly to the higher steric demand of the cyclohexenyl group as compared to the phenyl group. Hydroarylation of alpha,beta-acetylenic carbonyl substrates yields a very unusual anti-Michael selectivity, which is shown to result from reaction of the nonconjugated double bond, leaving the conjugation intact. In all cases were the regioselectivities reproduced by the calculations.
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| 6. |
- Andrén, Oliver C. J., et al.
(författare)
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Heterogeneous Rupturing Dendrimers
- 2017
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126.
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Tidskriftsartikel (refereegranskat)abstract
- Utilizing macromolecular scaffolds as templates for the production of small molecules that are distinctively different from the original monomer feedstock has many potential applications. Herein, as a proof-of-concept, a family of dendrimers displaying internally queued disulfide bridges were synthesized and exploited as flawless macromolecular templates that selectively rupture into a set of monomeric mercaptans. Disassembly was accomplished in a reducing environment, using DTT as an external stimulus, and the thiol constituents were successfully isolated. Their composition was dictated by three dendritic regions, i.e., (i) the symmetrical trithiol of the core (C3), (ii) the interior-asymmetric trithiols (CD2), and (iii) the periphery-asymmetric monothiols (DB2), in which B functionality is of an orthogonal nature. Taking into account the steady state between disulfides and thiols in all living cells, the collapse of the dendrimers to a multitude of smaller thiols was intracellularly assessed as a means to disrupt the balance of reactive oxygen species (ROS) often elevated in cancer cells. Indeed, the fragmentation induced a significant increase of ROS in human lung carcinoma A549 cells. These findings can potentially alter the perception of dendrimers being limited to carriers to being prodrugs for intracellular delivery of ROS with the potential to fight cancer.
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| 7. |
- Azam, M. S., et al.
(författare)
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Silica Surface Charge Enhancement at Elevated Temperatures Revealed by Interfacial Water Signals
- 2020
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 142:2, s. 669-673
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Tidskriftsartikel (refereegranskat)abstract
- The structure of water adjacent to silica is sensitive to the degree of deprotonation of surface silanol groups. As a result, close inspection of signals originating from these water molecules can be used to reveal the surface charge density. We have used nonlinear vibrational spectroscopy of the water O-H stretching band over a temperature range of 10-75 °C to account for the increase in surface potential from deprotonation. We demonstrate that the behavior at the silica surface is a balance between increasing surface charge and a decreasing contribution of water molecules aligned by the surface charge. Together with a model that accounts for two different types of silanol sites, we use our data to report the changes in enthalpy and entropy for deprotonation at each site. This is the first experimental determination of these thermodynamic parameters for hydrated silanol groups at the silica surface, critical to a wide range of geochemical and technological applications.
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| 8. |
- Baber, Ashleigh E., et al.
(författare)
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In Situ Imaging of Cu2O under Reducing Conditions : Formation of Metallic Fronts by Mass Transfer
- 2013
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 135:45, s. 16781-16784
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Tidskriftsartikel (refereegranskat)abstract
- Active catalytic sites have traditionally been analyzed based on static representations of surface structures and characterization of materials before or after reactions. We show here by a combination of in situ microscopy and spectroscopy techniques that, in the presence of reactants, an oxide catalyst's chemical state and morphology are dynamically modified. The reduction of Cu2O films is studied under ambient pressures (AP) of CO. The use of complementary techniques allows us to identify intermediate surface oxide phases and determine how reaction fronts propagate across the surface by massive mass transfer of Cu atoms released during the reduction of the oxide phase in the presence of CO. High resolution in situ imaging by AP scanning tunneling microscopy (AP-STM) shows that the reduction of the oxide films is initiated at defects both on step edges and the center of oxide terraces.
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| 9. |
- Baias, Maria, et al.
(författare)
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De Novo Determination of the Crystal Structure of a Large Drug Molecule by Crystal Structure Prediction-Based Powder NMR Crystallography
- 2013
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 135:46, s. 17501-17507
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Tidskriftsartikel (refereegranskat)abstract
- The crystal structure of form 4 of the drug 4-[4-(2-adamantylcarbamoyl)-5-tert-butyl-pyrazol-1-yl]benzoic acid is determined using a protocol for NMR powder crystallography at natural isotopic abundance combining solid-state H-1 NMR spectroscopy, crystal structure prediction, and density functional theory chemical shift calculations. This is the first example of NMR crystal structure determination for a molecular compound of previously unknown structure, and at 422 g/mol this is the largest compound to which this method has been applied so far.
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| 10. |
- Baranov, A. I., et al.
(författare)
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Unique (infinity Ni8Bi8S)-Ni-1 metallic wires in a novel quasi-1D compound. Synthesis, crystal and electronic structure, and properties of Ni8Bi8SI
- 2001
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 123:49, s. 12375-12379
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Tidskriftsartikel (refereegranskat)abstract
- A new quasi-one-dimensional compound Ni8Bi8SI has been synthesized and its crystal structure determined from single-crystal X-ray diffraction data. The structure of Ni8Bi8SI consists of [(infinityNi8Bi8S)-Ni-1] columns separated by iodine atoms. Conductivity and magnetic susceptibility measurements (down to 4.2 K) show that Ni8Bi8SI is a one-dimensional metal and exhibits Pauli paramagnetic properties. These observations are in good agreement with the results from electronic structure calculations. An analysis of the chemical bonding employing difference electron charge density maps reveals strong multicenter Ni-Bi bonds and pair Ni-S interactions within the [(infinityNi8Bi8S)-Ni-1] columns. Only electrostatic interactions are inferred between the columns and iodine atoms.
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