Showing papers by "George M. Whitesides published in 1989"
TL;DR: In this paper, the authors used contact angles and optical ellipsometry to study the kinetics of adsorption of monolayer films and to examine the experimental conditions necessary for the formation of high-quality films.
Abstract: : 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. Measurement of contact angles is a useful tool for studying the structure and chemistry of the outermost few angstroms of a surface. This work used contact angles and optical ellipsometry to study the kinetics of adsorption of monolayer films and to examine the experimental conditions necessary for the formation of high-quality films.
3,193 citations
TL;DR: In this article, the authors characterized the properties of ordered alkylsiloxane monolayer films and showed that they were stable in common organic solvents, water, and acid, but were destroyed by prolonged exposure to base.
Abstract: : 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.
1,189 citations
TL;DR: In particular, the ratio of the concentration of the t\r'o components in a mixed monolayer is not the same as in solution but reflects the relative solubilities of the components in solution and interactions between the tail groups, X, in the monolayers as discussed by the authors.
Abstract: Long-chain alkanethiols, HS(CH2)nX, adsorb from solution onto gold and form oriented. ordered monolayers. Monolayers exposing more than one functional group at the surface can be generated by coadsorption of t*'o or more thiols from solution. In general, the ratio of the concentration of the t\r'o components in a mixed monolayer is not the same as in solution but reflects the relative solubilities of the components in solution and interactions between the tail groups, X, in the monolayers. Multicomponent monolayers do not phase-segregate into single-component domains large enough to influence the contact angle (a few tens of angstroms across) and also do not act as ideal two.dimensional solutions. In the two-component system HS(CH2)J/HS(CH2)"CH3 in ethanol, where X is a polar tail group such as CH2OH or CN, adsorption of the polar component is particularly disfavored at low concentrations of the polar component in the monolayer. These isotherms may arise from poor solvation of the polar tail groups in the quasi-two-dimensional alkane solution provided by the methyl tail groups. From dilute solutions in alkanes, adsorption of HS(CH2)r9CH2OH is strongly preferred over HS(CHz)roCHr, probably due to the stabilization afforded by intramonolayer hydrogen bonds between the hydroxyl tail groups. The wettability of mixed monolayers is not linear in the composition of the surface. (n a surface comprised of a polar and a nonpolar component, the polar component is more hydrophilic when its concentration in the monolayer is low than when the monolayer is composed largely of the polar component.
900 citations
TL;DR: In this paper, it was shown that the hysteresis in the contact angle on these monolayers cannot be explained by theories of wetting based on macroscopic heterogeneity, and the wettability of mixed, methylterminatea can be partially rationalized by the geometric m€an approximition, but a full description probably requires inclusion of the entropy of mixing at the monolayer-liquid interface.
Abstract: when X = Y = oH' the pure monolayers a." *eti"d by water, but the mixed monolayers are less hydrophilic because nonpolar polymethylenecha insareexposedatt hesurface. whinX =cHr, Y = oH (n=2L,m= ll),averysharptr ansitionoccurs from a monolayer composed largely of the longer, methyl-terminated component to the shorter, hydroxyl-terminated component as the mole fraction of HS(CHz) r roH in-the adsorption solution is increased. From solutions .ontuining two thiols, adsorption of the thiol with the longer chain is preferred. Tiris preference is greater when the monolayers are adsorbed from ethanol than from isooctane' The mixed monolayers do not uit ", ideal two-dimensional solutions. T-he 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. The two components in the mixed monolayers do not phase-segregate into macroscopic islands (greater than a few tens of angstroms across) but are probably not randomly dispersed within the monolayer, ThL wettability of mixed, methyl-terminatea can be partially rationalized by the geometric m€an approximition, but a full description probably requires inclusion of the entropy of mixing at the monolayer-liquid interface. The hysteresis in the contact angle on these monolayers cannot be explained by theories of wetting based on macroscopic heterogeneity. Contact angles ur? -o.. sensitive than optical ellipsometry or X-ray photoelectron spectroscopy to certain types of changes in the coriposition and structure of these monolayers.
648 citations
TL;DR: In this paper, organic monolayers were formed on gold slides by adsorption from ethanol of HS(CH2)10CH2OH, HS( CH2) 10CH3, S(CH 2)10 CH2OH2, S((CH 2 )10CH 2OH2), S( CH 2) 10Ch32), and of binary mixtures of these molecules in which one component was terminated by a hydrophobic methyl group and one by hydrophilic alcohol group.
Abstract: : Ordered, organic monolayers were formed on gold slides by adsorption from ethanol of HS(CH2)10CH2OH, HS(CH2)10CH3, S(CH2)10CH2OH2, S(CH2)10Ch32, and of binary mixtures of these molecules in which one component was terminated by a hydrophobic methyl group and one by a hydrophilic alcohol group. The compositions of the monolayers were determined by X-ray photoelectron spectroscopy (XPS). Wettability was used as a probe of the chemical composition and structure of the surface of the monolayer. When monolayers were formed in solutions containing mixtures of a thiol and a disulfide, adsorption of the thiol was strongly preferred (approx. 75:1). Monolayer, Gold, Surface, Contact angles, Thiols, Disulfides.
460 citations
TL;DR: In this article, a model system that has the ease of preparation and the structural definition required to provide a firm understanding of interfacial phenomena is presented, where long-chain thiols, HS(CH2)nX, adsorb from solution onto gold and form densely packed, oriented monolayers.
Abstract: 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. More complex systems can be constructed by coadsorbing two or more thiols with different terminal functional groups or with different chain lengths onto a common gold substrate. By these techniques, controlled degrees of disorder can be introduced into model surfaces. We have used these systems to explore the relationships between the microscopic structure of the monolayers on a molecular and supramolecutar scale and their macroscopic properties. Wettability is a macroscopic interfacial property that has proven of particular interest.
447 citations
TL;DR: In this paper, the thickness of C10-C18 alkylsiloxane monolayers on silicon-silicon dioxide substrates has been measured with ellipsometry and low-angle X-ray reflection.
Abstract: The thicknesses of C10-C18 alkylsiloxane monolayers on silicon-silicon dioxide substrates have been measured with ellipsometry and low-angle X-ray reflection. Although, for any given sample, thicknesses measured by the two methods agree to within experimental error, ellipsometric measurements are systematically larger by approximately 2 {angstrom}. This difference may result from variations in the sensitivity of the two techniques to the structure of the interface between silicon dioxide and the alkylsiloxane monolayer. The x-ray reflectivity measurements provide evidence that these organic monolayers do not build up as island structures and demonstrate that the approximate area projected by each alkyl group in the plane of the monolayer is {approximately} 21 {plus minus}3 {angstrom}{sup 2}. Preliminary studies indicate that this technique can be used to follow the changes in the structure of a monolayer which result from chemical transformations. The influence of damage that is induced by x-ray radiation on these measurements is discussed.
400 citations
TL;DR: This procedure provides a method for modifying the surface characteristics of microlithographically generated patterns and offers a versatile technique for controlling solid-vapor and solid-liquid interfacial properties in systems having patterns with dimensions of the order of 1 micrometer.
Abstract: This work demonstrates the practicality of forming two self-assembled monolayers (SAMs), independently but simultaneously, by adsorption of two different adsorbates from a common solution onto a substrate exposing two different materials at its surface. The experimental procedure and the degree of independence achieved in the resulting SAMs are illustrated by examination of monolayers obtained by adsorption of alkanethiols on gold and alkane carboxylic acids on alumina. This procedure provides a method for modifying the surface characteristics of microlithographically generated patterns and offers a versatile technique for controlling solid-vapor and solid-liquid interfacial properties in systems having patterns with dimensions of the order of 1 micrometer.
329 citations
327 citations
TL;DR: Experiments with sialylated oligosaccharides confirm literature reports that mutations at amino acid 226 change the specificity of hemagglutinin for alpha( 2,6) and alpha(2,3) glycosidic linkages and suggest that sialic acid is the only component that contacts the protein.
Abstract: The equilibrium binding of influenza virus hemagglutinin to derivatives of its cell-surface ligand, sialic acid, was measured by nuclear magnetic resonance (NMR) spectroscopy. Binding was quantified by observing perturbations of sialic acid resonances in the presence of protein. The major perturbation observed was a chemical shift of the N-acetyl methyl resonance, presumably due to the proximity of the methyl group to tryptophan 153. X-31 hemagglutinin binds to the methyl alpha-glycoside of sialic acid with a dissociation constant of 2.8 mM and does not bind to the methyl beta-glycoside. Replacing the 4-hydroxyl group of sialic acid with an acetyl group has little effect, while replacing the 7-hydroxyl group with an acetyl prevents binding. Experiments with sialylated oligosaccharides confirm literature reports that mutations at amino acid 226 change the specificity of hemagglutinin for alpha(2,6) and alpha(2,3) glycosidic linkages. The NMR line broadening of sialyloligosaccharides suggests that sialic acid is the only component that contacts the protein. Saccharides containing two sialic acid residues appear to have two separate binding modes. Hemagglutinin that has undergone a low pH induced conformational change retains the ability to bind sialic acid.
324 citations
TL;DR: In this paper, the authors exploit the flexibility of self-assembled monolayers to study the influence of the polarity of an interface on the acidity of carboxylic acids.
Abstract: : 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.
TL;DR: A review about the potential of enzymes as catalysts for the synthesis of carbohydrates: advances in the enzyme-catalysed construction of natural and unnatural monosaccharides; methods for enzyme-Catalysed formation of glycosyl bonds by Lenoir and non-Lenoir pathways.
TL;DR: In this paper, the attenuation length of electrons with energies in the range 940-1400 eV in self- assembled monolayers of n-alkanethiols adsorbed on gold was determined.
Abstract: : Quantitative analysis of data obtained by X-ray photoelectron spectroscopy (XPS) requires a knowledge of the escape depths of electrons from the surface of a sample. In order to derive the composition of a homogeneous material from the intensities of the photoelectrons originating from different elements, one needs to know not only the relative atomic cross-sections, but also the variation of the attenuation length, gamma, with the energy of the photoelectrons. The ability to derive an elemental depth profile of a layered material from the variation in the photoelectron intensity with the angle of emission requires a knowledge of the absolute value of gamma. The recent growth of interest in thin organic films has generated an immediate need for accurate, reliable values of gamma in organic materials in general, and in thin, densely packed hydrocarbon films in particular. In this paper we have determined the attenuation length of electrons with energies in the range 940-1400 eV in self- assembled monolayers of n-alkanethiols adsorbed on gold. (JS)
TL;DR: Etude par voltammetrie cyclique et spectrometrie Auger de l'adsorption d'un complexe de fer sur une microelectrode de platine dans l'acetonitrile
Abstract: Etude par voltammetrie cyclique et spectrometrie Auger de l'adsorption d'un complexe de fer sur une microelectrode de platine dans l'acetonitrile
TL;DR: In this article, the use of sulfur-containing organic monolayer films improves adhesion between gold and silicon dioxide, and the structures of these monolayers were analyzed using contact angle, ellipsometry, and XPS.
Abstract: The use of sulfur-containing organic monolayer films improves adhesion between gold and silicon dioxide. The structures of these monolayers were analyzed using contact angle, ellipsometry, and XPS. The zone of adhesive failure was at or near the gold-monolayer interface.
TL;DR: Les supports utilises sont la silice, le carbone, l'alumine, and le alumine as discussed by the authors, i.e., silice and carbone.
Abstract: Les supports utilises sont la silice, le carbone, l'alumine. La cinetique de la reaction est etudiee
TL;DR: A variety of ketones were examined in order to determine the substrate specificity, regioselectivities, and enantioselectivity of cyclohexanone oxygenase.
TL;DR: In this article, the authors present a model system that has the ease of preparation and the structural definition required to provide a firm understanding of interfacial phenomena of organic surfaces. But the relationship between the microscopic structure of an organic surface and its macroscopic physical properties are poorly understood.
Abstract: : 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.
TL;DR: L'oxydation electrochimique du propanol-2 est realisee sur une electrode de platine dans une solution d'acide sulfurique as mentioned in this paper.
Abstract: L'oxydation electrochimique du propanol-2 est realisee sur une electrode de platine dans une solution d'acide sulfurique
TL;DR: In this paper, the cyclic bis(disulfide) diners derived from trans -2-butene-1,4-dithiol and cis -1,2-cyclohexane dithiol were tested with a catalytic amount of sodium methanethiolate.
TL;DR: In the presence du piegeur de radicaux du titre, le bromure de cycloheptyle reagit avec le magnesium pour former la (cyclohepticoxy-1 tetramethyl-2,2,6,6) piperidine comme produit principal as mentioned in this paper.
Abstract: En presence du piegeur de radicaux du titre, le bromure de cycloheptyle reagit avec le magnesium pour former la (cycloheptyloxy-1 tetramethyl-2,2,6,6) piperidine comme produit principal. Les radicaux cycloheptyles apparaissent etre les intermediaires dans la formation du bromure de cycloheptylmagnesium
TL;DR: In this paper, the substrate specifity of D-lactate dehydrogenase (D-LDH, EC 1.1.28) was compared to that of L-Lactate deacetylase (L-LDA), and three procedures that use DLDH in synthesis and two methods for recycling NADH.
Abstract: This note compares the substrate specifity of D-lactate dehydrogenase (D-LDH, EC 1.1.1.28) to that of L-lactate dehydrogenase (L-LDH, EC 1.1.1.27), illustrates three procedures that use D-LDH in synthesis and two methods for recycling NADH, and provides experimental details illustrating the use of D-LDH in organic synthesis.
TL;DR: The use of LDH has advantages of convenience, stability of the regenerating system, and economy as discussed by the authors, and Pyruvate analogs, monomethyl oxalate and S -methyl thiooxalate, were examined as possible oxidants but were not substrates of lactate dehydrogenase.
TL;DR: In this paper, the use of an inverted RAMA catalyst for the reduction of the ketone in this class of carbohydrates to an alcohol has been discussed, where the structure of this aldose and the location of the vicinal diol formed in the aldol reaction can be control led through the structure in R'. The ketone group derived from the DHAP offers control of the chemistry at the end of the sugar distal to the aldehyde.
Abstract: t'\"0:'\":\" group at C2 rather than an aldehyde group at C I . Conversion of a ketose to an aldose is not straightforward.T Here we descr ibe a new st ra tegy for us ing RAMA ( the \" inver ted\" s t ra teg) . Scheme I ) that increases the usefu lness of th is enzyme as a cata lys t in the synthes is o f sugars . We a lso demonst ra te the va lue of r id i to l dehydrogenase ( lDH) as a catalyst for the diastereospecif ic reduction of the ketone in this class of carbohydrates to an alcohol.8'e RAMA-catalyzed aldol condensation between DHAP and a half-protected dialdehyde, OCHR'(CHg;n, generates a protected aldose having a ketone (that derived from DHAP) at Cn-,. Dephosphorylat ion. reduction. or other transformation of the ketone and deprotect ion o f the a ldehyde prov ide the a ldose. Both the structure of this aldose and the location of the vicinal diol formed in the aldol reaction can be control led through the structure of R'. The ketone group derived from the DHAP offers control of the chemistry at the end of the sugar distal to the aldehyde. Scheme II i l lustrates this * inverted\" approach to the synthesis of sugars us ing RAMA wi th syntheses of t -xy lose (4) and 2deoxy-o-ar abi no-hexose (9). RAMA-cata lyzed (50 un i ts ) condensat ion o f d ie thoxyacet a ldehyde ( l ) r0 ( l mmol . added in f ive por t ions over 5 days) and of ructose 1,6-d iphosphate (1 mmol) in the presence of t r iosephosphate isomerase (EC 5.3.1 .1 , ca. 200 un i ts ) , fo l lowed by treatment in situ with acid phosphatase (AP, 20 units), afforded 2 in 60Vo overal l yield. 'r Conversion of ketone 2 (1 mmol) to alcohol 3 with L stereochemistry was accomplished in 69Vo yield, using IDH (from Condida utilis,l0 units),e coupled with formate dehydrogenase (FDH, 10 units) and sodium formate (3 mmol)
TL;DR: The enzymes phosphotransacetylase or carnitine acetyltransferase and their respective substrates provide the basis for efficient recycling systems for acetyl-CoA and the syntheses and substrate activities of analogs of these two substances are surveyed.
TL;DR: Description du protocole experimental permettant la synthese de phosphoenolpyruvate a partir d'acide phosphoglycerique.
Abstract: Description du protocole experimental permettant la synthese de phosphoenolpyruvate a partir d'acide phosphoglycerique
01 Nov 1989
TL;DR: The influence of polar ester or amide functionality on Theta was measured for a series of surface-functionalized polyethylene films bearing ester and amide groups having different sizes of alkyl group as mentioned in this paper.
Abstract: : Advancing contact angles Theta (a) were measured for a series of surface-functionalized polyethylene films bearing ester (-CO2C(n)H(2n+1))and amide (-CONH(n)H(2n+1)) groups having different sizes of alkyl group. The influence of the polar ester or amide functionality on Theta (a) is no longer detectable for alkyl groups having n >or= 6. The size of the alkyl group required to shield a polar functional group at the surface of the polymer from the contacting water inferred from these experiments is remarkable similar to that observed previously with the more highly ordered, self-assembled monolayers of omega-mercapto ethers (HS(CH2)16OC(n)H(2n+1)) on gold. (aw)