WorldCat Identities

HARVARD UNIV CAMBRIDGE MA Dept. of CHEMISTRY

Overview
Works: 36 works in 38 publications in 1 language and 38 library holdings
Publication Timeline
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Most widely held works by HARVARD UNIV CAMBRIDGE MA Dept. of CHEMISTRY
The Structure and Reactivity of Alkylsiloxane Monolayers Formed by Reaction of Alkyltrichlorosilanes on Silicon Substrates( Book )

1 edition published in 1989 in English and held by 1 WorldCat member library worldwide

Long-chain alkyltrichlorosilanes, C13Si (CH2)nX, adsorb from solution onto silicon-silicon dioxide (Si/SiO2) substrates and form ordered alkylsiloxane monolayer films. These films were characterized by wettability, ellipsometry, and XPS. Except for very short chains (n = 0,1,2), the wetting of these monolayers was approximately independent of chain length. The presence of small amounts of water was necessary for the formation of these films. The alkylsiloxane monolayers were stable in common organic solvents, water, and acid, but were destroyed by prolonged exposure to base. Simple reactions on vinyl-terminated monolayers generated alcohol-, carboxylic acid-, and bromine- terminated films whose contact angles were lower than the starting monolayers, but whose lengths were largely unchanged. Measurements of the contact angle of acid-terminated interfaces as a function of pH indicated that ionization of soluble carboxylic acids. Monolayers containing mixtures of methyl- and carboxyl-functionalities exhibited wetting properties that mirrored the composition of the interface
Comparisons of Self-Assembled Monolayers on Silver and Gold: Mixed Monolayers Derived from HS(CH2)21X and HS(CH2)10Y (X, Y - CH3, CH2OH) Have Similar Properties( Book )

1 edition published in 1991 in English and held by 1 WorldCat member library worldwide

This paper describes the preparation of self-assembled monolayers (SAMs) by adsorption of alkanethiols on evaporated silver, and compares these SAMs with analogous SAMs prepared on gold. The work concentrated on SAMs derived from mixtures of alkanethiols having long and short chains and terminating in hydrophobic and hydrophilic tails: HS(CH2) 10CH3, HS(CH2) 21CH3, HS(CH2)10CH2OH, and HS(CH2)21CH2OH. The thickness and the compositions of these SAMs were established by X ray photoelectron spectroscopy (XPS). As is observed with self- assembled alkanethiolate monolayers on gold, the composition of the monolayers parallels but does not equal the solution composition. Less pronounced differences are observed between compositions in solution and on the surface on silver than have previously been reported on gold. Rates of exchange of surface thiolates with thiols in solution are similar on silver and gold
Supersonic Pyrolysis Jets for Diamond Film Deposition( Book )

2 editions published between 1991 and 1992 in English and held by 1 WorldCat member library worldwide

Recent progress in the production of high-intensity supersonic free jets of hydrocarbon radicals (e.g. CH2Hn, C3Hn), has made it possible to investigate the growth mechanism of diamond and diamond-like carbon by bombardment of a substrate with a well-characterized supersonic flow of defined radicals. This report details the first phase of the project, encompassing source design, and the engineering of specific radical precursors. We have constructed and tested new higher-temperature ceramic Zirconium Oxide and Silicon Carbide pyrolysis nozzles that can operate continuously at 1700 C for several hours. The two successful designs were used to generate intense beams of hydrocarbon radicals and carbenes by thermal decomposition of appropriate precursors that were inaccessible with our previous Aluminum Oxide designs. The new nozzle design makes possible quantitative sequential homolytic cleavage of a Carbon-Bromine bond to generate a radical, or two C-Br bonds to generate a carbene. We generated a molecular beam of propadienylidene (1-C3H2) from 1,3- dibromopropyne in this fashion
Modelling Organic Surfaces with Self-Assembled Monolayers( Book )

1 edition published in 1989 in English and held by 1 WorldCat member library worldwide

The interfacial properties of organic materials are of critical importance in many applications, especially the control of wettability, adhesion, tribology, and corrosion. The relationships between the microscopic structure of an organic surface and its macroscopic physical properties are, however, only poorly understood. This short review presents a model system that has the ease of preparation and the structural definition required to provide a firm understanding of interfacial phenomena. Long-chain thiols, HS(CH2)nX, adsorb from solution onto gold and form densely packed, oriented monolayers. By varying the terminal functional group, X, of the thiol, organic surfaces can be created having a wide range of structures and properties. Keywords: Self assembly, Monolayers, Thiols, Gold, Organic surfaces
Molecular Self-Assembly and Nanochemistry: A Chemical Strategy for the Synthesis of Nanostructures( Book )

1 edition published in 1991 in English and held by 1 WorldCat member library worldwide

Molecular self assembly is the spontaneous association of molecules under equilibrium conditions into stable, structurally well-defined aggregates joined by non-covalent bonds. Molecular self-assembly is ubiquitous in biological systems, and underlies the formation of a wide variety of complex biological structures. Understanding self-assembly and the associated non- covalent interactions that connect complementary interacting molecular surfaces in biological aggregates is a central concern in structural biochemistry. Self- assembly is also emerging as a new strategy in chemical synthesis, with the potential of generating non-biological structures having dimensions of 1-10 2 nanometers. Structures in the upper part of this range of sizes are presently inaccessible through chemical synthesis, and the ability to prepare them would open a route to structures comparable in size (and perhaps complementary in function) to those that can be prepared by microlithography and other techniques of microfabrication
A Study by Contact Angle of the Acid-Base Behavior of Monolayers Containing W-Mercaptocarboxylic Acids Absorbed on Gold: An Example of Reactive Spreading( Book )

1 edition published in 1989 in English and held by 1 WorldCat member library worldwide

Monolayers formed by the spontaneous assembly of organic thiols from solution onto gold provide a high degree of control over the chemistry and structure of an organic surface. In this paper we exploit the flexibility of these self-assembled monolayers to study the influence of the polarity of an interface on the acidity of carboxylic acids. Methyl-terminated thiols, such as HS(CH2)10CH3, adsorb from solution onto gold and form non-wettable, hydrophobic surfaces composed predominantly of nonpolar methyl groups. Thiols terminated by carboxylic acid groups form hydrophilic monolayers that are wetted by water. By coadsorbing mixtures of HS(CH2)10CH3 and HS(CH2)10CO2H in various mole fractions, we can synthesize surfaces in which the carboxylic acid groups are surrounded largely by nonpolar methyl groups, by other polar acid groups, or by mixtures of the two. This system allows us to examine the effect on the advancing contact angle of a chemical reaction occurring at the interface between a drop and a surface- in this case, neutralization of a carboxylic acid by aqueous base. Reactive spreading appears to be complex and may lead to different wetting behavior for the same chemical reaction on different substrates. A knowledge of the acid-base properties of carboxylic acids and other organic acids and bases at solid-liquid interfaces is important for understanding phenomena as diverse as the stability of colloids, the folding of proteins and the mechanisms of catalysis by enzymes. Keywords: Ethanols; Polarity; Polyethylene carboxylic acids; Contact angles; Reactive spreading; Titration
Omega-Terminated Alkanethiolate Monolayers on Surfaces of Copper, Silver and Gold Have Similar Wettabilities( Book )

1 edition published in 1991 in English and held by 1 WorldCat member library worldwide

Long chain alkanethiols (HS(CH2)nX) adsorb from solution onto the surfaces of freshly evaporated copper, silver and gold films and form oriented monolayers. Both polar and non-polar tail groups (X) can be accommodated in these adsorptions. Adsorption on all three metals generates self-assembled monolayers (SAMs) exhibiting similar wetting properties, and thus, by inference, having related oriented structures. XPS data suggest that the omega-terminated eta-alkanethiolate monolayers, like those derived from simple alkanethiols, are composed of trans-extended chains having orientations on copper and silver that are closer to the perpendicular to the surface than are those on gold. We have prepared and characterized monolayers (mixed monolayers) by exposure of all three metals to mixtures of HS (CH2)11OH and HS9CH2)11CH3
The Reactivity of Carboxylic Acid and Ester Groups in the Functionalized Interfacial Region of 'Polyethylene Carboxylic Acid' (PE-CO2H) and Derivatives: Differentiation of the Functional Groups into Shallow and Deep Subsets Based on a Comparison of Contact Angle and ATR-IR Measurements( Book )

1 edition published in 1986 in English and held by 1 WorldCat member library worldwide

The carboxylic acid groups present in the functionalized interface of polyethylene carboxylic acid (PE-CO2H, a material prepared from low-density polyethylene film have been differentiated by reaction with aqueous chromic acid) into two subsets: those sufficiently close to the surface of the polymer to influence its wettability by water and those too deep to do so. This differentiation was accomplished by taking advantage of differences in rates of esterification of carboxylic acid groups in different regions of the functionalized interface, and of differences in rates of hydrolysis of ester groups derived from them. The subset of functional groups influencing wettability comprises <30% of the total groups present in the functionalized interface and appears to be homogeneous in its chemical reactivity. The remaining groups (approx. 70% of the total) do not directly influence wettability and appear to become less reactive with increasing depth in the polymer. The surface and subsurface carboxylic acid and ester moieties are both less reactive in hydrolysis and formation reactions than are these groups in organic molecules in solution. Reactivities of the interfacial functional groups depend on structure in ways having no analogy in reactions in solution
Formation of Monolayers by the Coadsorption of Thiols on Gold: Variation in the Length of the Alkyl Chain( Book )

1 edition published in 1989 in English and held by 1 WorldCat member library worldwide

Mixtures of two long-chain alkanethiols, HS(CH2)nX and HS(CH2)mY (X, Y = CH3, OH; n> m), in which the alkyl chains have different lengths, adsorb from solution onto gold and form monolayers comprising a densely packed inner region adjacent to the gold surface and a disordered outer region in contact with the solution. When X = Y = CH3 (n = m), this disordered phase makes the mixed monolayer more oleophilic than the ordered, pure (i.e., single-component) monolayers. When X = Y = OH, the pure monolayers are wet by water, but the mixed monolayers are less hydrophilic because nonpolar polymethylene chains are exposed at the surface. When X = Ch3, Y = OH (n = 21, m = 11), a very sharp transition occurs from a monolayer composed largely of the longer, methyl-terminated component to the shorter, hydroxyl-terminated component as the mole fraction of HS(CH2)11OH in the adsorption solution is increased. From solutions containing two thiols, adsorption of the thiol with the longer chain is preferred. This preference is greater when the monolayers are adsorbed from ethanol than from isooctane. The mixed monolayers do not act as ideal two-dimensional solutions. The adsorption isotherms suggest a positive excess free energy of mixing of the two components in the monolayer. The compositions of the monolayers appear to be determined largely by thermodynamics, although in some cases there is also a kinetic contribution to the composition
Acid-Base Behavior of Carboxylic Acid Groups Covalently Attached at the Surface of Polyethylene: The Usefulness of Contact Angle in Following the Ionization of Surface Functionality( )

1 edition published in 1985 in English and held by 0 WorldCat member libraries worldwide

Oxidation of polyethylene with chromic acid/sulfuric acid generates a material (PE-CO2H) having a high density of carboxylic acid and ketone functionalities in a thin surface layer on the polymer. This paper determines the extent of ionization of the surface and near-surface carboxylic acid groups of these materials in contact with water as a function of pH using three experimental techniques: measurement of attenuated total reflectance infrared (ATR-IR) spectra, measurement of contact angles, and direct potentiometric titration. On the basis of correlations between results obtained using these three techniques, we propose an equation relating the contact angles an aqueous solution having a given value of pH and the extent of ionization (a subscript i) of those carboxylic acid groups that are directly exposed to the solution. These carboxylic acid groups have broad titration curves, and have CO2H groups that are less acidic than soluble carboxylic acids. The initial ionization of these carboxylic acid groups occurs when the solution is approximately pH = 6. The detailed structures of these oxidized polymer surface layers and the nature of the interactions between the carboxylic acid and carboxylate ions in them are still not completely defined. Salt effects on the extent of ionization a subscript i at a particular value of pH are unexpectedly small, and suggest that charge-charge interactions between carboxylate ions may not dominate the titration curves. This work demonstrates the usefulness of contact angle in following chemical changes occurring in organic functional groups of surfaces
Manipulation of the Wettability of Surfaces on the 0.1 to 1 Micrometer Scale Through Micromachining and Molecular Self-Assembly( )

1 edition published in 1992 in English and held by 0 WorldCat member libraries worldwide

Micromachining allows the formation of micrometer-sized regions of bare gold on the surface of a gold film supporting a self-assembled monolayer (SAM) of alkanethiolate. A second SAM forms on the micromachined surfaces on exposing the entire system--the remaining undisturbed gold-supported SAM and the micromachined features of bare gold--to a solution of dialkyldisulfide. By preparing an initial hydrophilic SAM from HS(CH2)15COOH, micromachining features into this SAM, and covering these features with a hydrophobic SAM formed from [CH(3)(CH(2))(11)S]2, it is possible to construct micrometer-scale hydrophobic lines in a hydrophilic surface. These lines provide new structures with which to manipulate the shapes of liquid drops
From Minerals to Materials: A Facile Synthetic Route to Preceramic Polymers for Aluminum Oxide( )

1 edition published in 1992 in English and held by 0 WorldCat member libraries worldwide

Reaction of boehmite, (Al(O)(OH)n, with an excess of carboxylic acid (HO2CR) results in the formation of the carboxy substituted alumoxanes, (Al(O) x(OH)y(O2CR)z)n where 2x + y + z = 3 and R = alkyl substituents. The alumoxanes have been fully characterized by SEM, elemental analysis, IR and multinuclear NMR spectroscopy. The physical properties of the alumoxanes are highly dependent on the identity of R, and range from insoluble crystalline powders, e.g. R = CH3, to powders which readily form solutions or gels in hydrocarbon solvents, e. g. R = C5H11. All of the alumoxanes decompose under mild thermolysis to yield gamma-alumina
Organized Nanorod-Superconductor Composites (Publications/Patents/Presentations/Honors/Students Report)( )

1 edition published in 1997 in English and held by 0 WorldCat member libraries worldwide

The overall goal of this project is to develop rational approaches for controlling the structure and interfaces of high-temperature superconductors (HTSs) and other complex solids at the nanometer scale. Our emphasis on nanometer scale is motivated by the recognition that control of structure in this size regime leads in general to materials with enhanced and/or novel electrical, mechanical, optical and magnetic properties. In this regard, our main objective has been to control the nanometer scale defect structure in HTSs since this will enable the intrinsic problem of thermally-activated flux flow, which limits critical currents in all HTS materials, to be significantly reduced. Specific objectives that have been pursued during the past year include (1) the design and synthesis of one-dimensional nanostructures (nanorods) that are chemically and structurally compatible with HTS materials, (2) the synthesis and structural characterization of nanorod/HTS nanocomposites, and (3) the elucidation of critical current behavior of nanorod/FiTS materials as a function of temperature and magnetic field
Correlation between Surface Free Energy and Surface Constitution( )

1 edition published in 1992 in English and held by 0 WorldCat member libraries worldwide

Self-assembled monolayers (SAMs) of alkylsiloxanes on elastomeric PDMS (polydimethylsiloxane) were used as model systems to study interactions between surfaces. Surface free energies of these chemically modified surfaces were estimated by measuring the deformations that resulted from the contact between small semispherical lenses and flat sheets of the elastomer under controlled loads. The measured surface free energies correlated with the surface chemical compositions of the SAMs and were commensurate with the values estimated from the measurements of contact angles. This study provides, for the first time, direct experimental evidence for the validity of estimates of the surface free energies of low-energy solids obtained from contact angles
The Physical-Organic Chemistry of Surfaces, and Its Relevance to Molecular Recognition( )

1 edition published in 1989 in English and held by 0 WorldCat member libraries worldwide

Understanding the interactions between liquids and the surfaces of organic solids is important for the study of molecular recognition. One method for studying these interactions is based on a physical organic approach: formation of well characterized organic surfaces and comparison of the contact angles of liquids on these surfaces. The technique of self-assembly provides an excellent route for the syntheses of organic surfaces; alkanethiols, for example, self-assemble from solutions to form monolayers on gold. Properties of these monolayers, including their thickness and their wettability by water, can be controlled by manipulating the structure and functionality of the molecules used to form the monolayer. The wettability of monolayers on gold formed from HS(CH2)(n)CH3 or HS(CH2)110(CH2)(n)CH3, where n is the number of methylene groups, depends only on the composition the outermost 5-10 A angstroms of the monolayer. The wettability of monolayers formed from mixtures of HS(CH2)11CH3 and HS(CH2)21CH3 depends both on the structure of the outermost part of the monolayer and the structure of the liquid. These and other results suggest that this physical-organic approach has and will continue to provide valuable insight into molecular recognition. Keywords: Biocompatibility, Proteins, Ligands, Biochemistry. (AW)
Molecular Recognition in Gels, Monolayers, and Solids( )

1 edition published in 1991 in English and held by 0 WorldCat member libraries worldwide

This paper describes work in four area:affinity electrophoresis of carbonic anhydrase in cross-linked polyacrylamide-derived gels containing immobilized derivatives of aryl sulfonamides; inhibition of the hemagglutination of erythrocytes induced by influenza virus using water-soluble polyacrylamides bearing sialic acid groups; the application of self-assembled monolayers (SAMs) of alkyl thiolates on gold to the study of protein adsorption on organic surfaces; and the use of networks of hydrogen bonds to generate new classes of non-covalently assembled organic materials, both in solution and in crystals. This paper summarizes research in two areas of molecular recognition:affinity polymers and molecular self assembly. We illustrate these areas by examples drawn from affinity gel electrophoresis, soluble synthetic macromolecular inhibitors of binding of influenza virus to erythrocytes protein adsorption on self assembled monolayers and self assembling hydrogen bonded molecular aggregates
How to Make Water Run Uphill( )

1 edition published in 1992 in English and held by 0 WorldCat member libraries worldwide

A surface having a spacial gradient in its surface free energy was capable of causing drops of water placed on it to move uphill. This motion was the result of an imbalance in the forces due to surface tension acting on the liquid-solid contact line on the two opposite sides ('uphill' or 'downhill') of the drop. The required gradient in surface free energy was generated on the surface of a polished silicon wafer by exposing it to the diffusing front of a vapor of decyltrichlorosilane. The resulting surface displayed a gradient of hydrophobicity (with the contact angle of water changing from 97 deg. to 25 deg.) over a distance of 1 centimeter. When the wafer was tilted from the horizontal plane by 15 deg., with the hydrophobic and lower than the hydrophilic, and a drop of water (1 to 2 microliters) was placed at the hydrophobic end, the drop moved toward the hydrophilic end with an average velocity of about 1 to 2 mm/sec. In order for the drop to move, the hysteresis in contact angle on the surface had to be low (<or = 10 deg.)
Tert-butylalumoxanes: Synthetic Analogs for Methylalumoxane (MAO) and New Catalytic Routes to Polyolefins and Polyketones( )

2 editions published in 1994 in English and held by 0 WorldCat member libraries worldwide

Alkyl alumoxanes, the products from the partial hydrolysis of aluminum trialkyls, have been shown to be active catalysts and co-catalysts for a variety of polymerization processes. Despite industrial useage, no one has been able to isolate individual compounds and demonstrate a structure activity correlation. The purpose of this study is to: (a) isolate single alumoxane species, (b) determine the function of alumoxanes in Ziegler Natta Catalysis, and (c) develop new polymers, employing these catalysts
Investigations of Metal Adhesion and Reactions on Solid Lubricant Surfaces Using Scanned Probe Microscopies( )

1 edition published in 1997 in English and held by 0 WorldCat member libraries worldwide

Scanning tunneling and atomic force microscopy studies of metal dichalcogenide materials demonstrate that wear proceeds via surface oxidation, and that wear rates are directly related to the oxidative stability of the surfaces. Surface oxidation was also shown to produce oriented MoO3 nanocrystals on MoS2 surfaces. This system was shown to be uniquely suited for nanotribology studies because the interface structure and contact area are atomically deformed and the MoO3 nanocrystals can be moved controllably with lateral force microscope tip. Highly anisotropic friction has been observed whereby MoO3 nanocrystals moved along only specific directions of the MoS2 surface lattice. An atomic model of the interface was developed to explain these In addition, chemical force microscopy has been used to measure adhesion and friction forces between probe tips and substrates covalently modified with self-assembled monolayers (SAMs) that terminate in distinct functional groups such as COOH, CH3,and NH2 in ethanol and water solvents. The measured adhesive forces were found to agree well with predictions of the Johnson, Kendall, and Roberts (JKR) theory of adhesive contact, and thus show that the observed adhesion forces correlate with the surface free energy. Electrostatic contributions to adhesive forces have also been characterized using COOH/NH2 functionalized tip/surface that exists as COO( - )/NH3(+) in aqueous solution. The dependence of friction forces on the tip and sample functionality has also been shown to be the basis for chemical force microscopy in which lateral force images are interpreted in terms of the strength of adhesive interactions between functional groups. Chemically sensitive imaging of photopatterned monolayers using probe tips modified with different functional groups has been demonstrated
Formation of Monolayer Films by the Spontaneous Assembly of Organic Thiols from Solution onto Gold( )

1 edition published in 1988 in English and held by 0 WorldCat member libraries worldwide

Long-chain alkanethiols, HS(CH2)nX, adsorb from solution onto gold surfaces and form ordered, oriented monolayer films. The properties of the interfaces between the films and liquids are largely independent of chain length when n> 10; in particular, wetting is not directly influenced by the proximity of the underlying gold substrate. The specific interaction of gold with sulfur and other 'soft' nucleophiles and its low reactivity toward most 'hard' acids and bases make it possible to vary the structure of the terminal group, X, widely and thus permit the introduction of a great range of functional groups into a surface. Studies of wettability of these monolayers, and of their composition using X-ray photoelectron spectroscopy (XPS), indicate that the monolayers are oriented with the tail group, X, exposed at the monolayer-air or monolayer-liquid interface. The adsorption of simple n-alkanethiols generates hydrophobic surfaces whose free energy (19 mJ sq m) is the lowest of any hydrocarbon surface studied to date. (TTL)
 
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