Showing papers in "Langmuir in 1995"

Catalytic Engineering of Carbon Nanostructures

Abstract: Catalytically grown carbon nanofibers are novel materials that are the product of the decomposition of carbon-containing gases over certain metal surfaces. Studies in our laboratory have demonstrated that the structure and properties of the fibers can be tailored by careful control of a number of parameters including the nature of the metal surface, the composition of the gas-phase reactant, the temperature, and the incorporation of either gas-phase or solid additives. High-resolution transmission electron microscopy studies have revealed that the nanofibers consist of well-ordered graphite platelet structures, the arrangement of which can be engineered to desired geometries by choice of the correct catalyst system. When the data from these examinations are combined with the information of the associated catalyst particle morphology, it is possible to develop models that describe many of the structural characteristics as well as some previously unknown mechanistic features of the various carbon nanofiber conformations. 26 refs., 3 figs.

Characterization of the Surface of a Citrate-Reduced Colloid Optimized for Use as a Substrate for Surface-Enhanced Resonance Raman Scattering

Abstract: Citrate-reduced colloids can be used to give reproducible, sensitive, and selective analysis by surface-enhanced (resonance) Raman scattering (SE(R)RS). Control of the chemistry at the colloid surface is essential to realize the potential ofthis method. This study is aimed at understanding the surface chemistry in aqueous solutions, characterizing the nature of the final surface, and developing a robust method for SE(R)RS analysis at the surface. An optimized procedure for the reduction of silver nitrate with trisodium citrate is described. Visible absorption and photon correlation spectroscopies of colloid formation indicate that the initial reduction ofAg 1 to Ag 0 occurs within 2 min of citrate addition, and the initial particles formed are large (60-80 nm) and polydisperse. Subsequent heating initially provides a less polydisperse mixture of 20-30 and 40-50 nm particles and finally an approximately monodisperse distribution of smaller particles (∼27 nm). Solution NMR studies of the colloidal suspension indicate the presence of citrate and its decomposition products, acetoacetic acid and formate in solution throughout colloid formation. Raman scattering from aggregated aliquots of colloid indicates two forms of citrate depending on the stage of preparation, but neither acetoacetic acid nor formate is detected as being adsorbed at the silver surface. The final, approximately monodisperse particles are believed to be stabilized by a surface layer of silver citrate, with pendant negative groups. The colloids are stable for over 2 months. The SE(R)RS effect requires controlled aggregation of the colloid. The aggregation process is generally induced by the addition of acid or activating ions, for example, Cl - or I - . Aggregation with acid (HNO 3 ) and with poly(L-lysine) and ascorbic acid are compared. The poly(L-lysine) method is more effective, enhancing the monodispersity of colloidal aggregates. The reproducibility of SERRS (relative standard deviation (RSD) <5%) is acceptable for analytical purposes, whereas that from aggregation with acid (HNO 3 )(RSD = 18.5%) is not. Futhermore, at low analyte concentrations, SE(R)RS from both the analyte and the citrate layer are observed on aggregation with nitric acid. However, SE(R)RS is only observed from the analyte on aggregation with poly(L-lysine) and ascorbic acid. The advantages for trace analysis of anionic, neutral, and cationic species of using reagents which alter surface charge and dielectric constant are illustrated.

Drainage of a Thin Liquid Film Confined between Hydrophobic Surfaces

Abstract: We investigate theoretically the drainage of a thin liquid film between two undeformed hydrophobic spheres. The role ofhydrophobicity is revealed in the apparent slippage of liquid over the solid. The origin of the slippage effect is probably linked with a decrease in viscosity in the very thin near-to-wall layer. The solution is obtained for arbitrary values of slip lengths (from zero to infinity) as well as for arbitrary radii of curvature of approaching surfaces. The main result consists in that the pressure and the drag force yield the product of corresponding expressions for similar hydrophilic spheres and some corrections for slippage. These corrections depend only on the relationships between the gap and the slip lengths. As a result, at distances that are much greater than both slip lengths of approaching surfaces, the liquid flow is the same as that for hydrophilic surfaces. If the gap width exceeds considerably only one of the slip lengths then the pressure and the resistance will be equal to those experienced by hydrophilic sphere moving toward the free bubble surface. If the gap is much smaller than both slip lengths, the flow will be like that which arises when two bubbles approach each other. In the latter case, the hydrodynamic drag is not inversely dependent on the gap but is inversely proportional to the slip lengths and only logarithmically dependent on the gap. The correction for slippage plays a dramatic role in the coagulation processes. The main result for coagulation consists in the possibility for collision to occur at a finite time. Also, this correction needs to be taken into account when the various properties of confined liquids (first of all the hydrophobic attractive force) are investigated with the drainage technique.

Activated Carbon Surface Modifications by Nitric Acid, Hydrogen Peroxide, and Ammonium Peroxydisulfate Treatments

Abstract: A series of activated carbons with different degrees of activation was treated with HNO 3 , H 2 O 2 , and (NH 4 ) 2 S 2 O 8 in order to introduce oxygen surface complexes. The effects of the oxidizing treatments on the surface area, the pore texture, and the surface chemical nature were analyzed by means of N 2 and CO 2 adsorption, mercury porosimetry, FTIR, TPD, electrophoretic, and mass titration measurements. Results obtained show that the HNO 3 treatment affects the surface area and the porosity of the samples to a greater extent than the other treatments. Carboxyl groups were essentially fixed after the three treatments, although ketone and ether groups, as detected by FTIR, were also fixed after the treatments with peroxides. The most important conclusion was that the stronger acid groups were fixed after the (NH4) 2 S 2 O 5 treatment rather than after the HNO 3 treatment, in spite of the fact that this latter treatment fixed the largest number of oxygen complexes that evolved as CO 2 .

Influence of Evaporation on Contact Angle

Abstract: The evaporation of sessile drops of water and n-decane placed on various substrates has been studied using a projection method. Drop dimensions and contact angle have been measured as a function of time. The first stage of the experiment corresponds to a saturated atmosphere ; then, when evaporation is allowed to occur, two or three different stages appear (depending on the surface roughness). For the first of these, a model is proposed which allows us to calculate the coefficient of diffusion of the liquid vapor in air.

Alkanediyl-.alpha.,.omega.-Bis(Dimethylalkylammonium Bromide) Surfactants (Dimeric Surfactants). 5. Aggregation and Microstructure in Aqueous Solutions

Abstract: A series of alkanediyl-α,ω-bis(dimethylalkylammonium bromide) surfactants, referred to as dimeric m-s-m surfactants (where m and s are the carbon numbers of the side alkyl chains and of the alkanediyl spacer), have been investigated by time-resolved fluorescence quenching (determination of micelle aggregation number of the 12-s-12 series) and transmission electron microscopy at cryogenic temperature (determination of the nature and shape of the aggregates present in solutions of 12-s-12 and 16-s-16). These m-s-m surfactants can be considered as the dimers of the quarternary ammonium surfactants having two unequal alkyl chains with carbon numbers m and s/2. The 12-s-12 surfactants were found to form spherical micelles at concentration close to the critical micelle concentration. The homologues with s=6, 8, and 10 showed only a small to moderate micellar growth upon increasing concentration, suggesting that the micelles remained spheroidal in the concentration range investigated. However, the homologues with s=3, 4, and 5 showed a significant micellar growth, which increased as s decreased. The solutions of 12-2-12 showed shear-thickening at the concentration of 1.4% and became viscoelastic above 2%. CryoTEM showed the presence of giant entangled threadlike micelles in the aqueous solutions of this surfactant. Elongated micelles were also seen in solutions of 12-3-12. Only spherical or spheroidal micelles were present in solutions of 12-4-12, 12-8-12, and 12-12-12, at the concentration investigated, in agreement with the fluorescence quenching results. Vesicles, often double-lamellar were observed in solutions of 12-16-12 and 12-20-12. Vesicles were also present in the solutions of the corresponding «monomeric» surfactants. In the case of the 16-s-16 series, vesicles, bilayer membrane fragments and threadlike micelles were observed in 16-3-16 solutions. Only threadlike micelles were seen in 16-4-16 solutions, whereas 16-6-16 solutions showed slightly elongated micelles and 16-8-16 solutions spherical micelles. The above results clearly indicate that the dimeric surfactants investigated tend to form in aqueous solutions aggregates of lower curvature than the corresponding «monomeric» surfactants

Organization of Au Colloids as Monolayer Films onto ITO Glass Surfaces: Application of the Metal Colloid Films as Base Interfaces To Construct Redox-Active Monolayers

Abstract: Au colloid films are organized on indium tin oxide (ITO) surfaces using (aminopropyl)siloxane or (mercaptopropyl)siloxane as base monolayer for the deposition of the metal colloid. Different Au colloids, ranging in particles of diameters 25, 30, 35, and 120 nm, were deposited on the monolayer-modified ITO surfaces. For the small particles, 25 nm, an almost continuous Au colloid film is formed with interparticle spacing of 10-25 nm. The surface coverage of the Au colloid on the (aminopropyl)siloxane monolayer is higher than that for the (mercaptopropyl)siloxane-modified ITO. The Au colloid films provide active surfaces for the self-assembly of redox-active thiolate monolayers. 8-(N-Methyl-4,4'-bipyridinyl)octanoic acid was covalently linked to a cystamine monolayer assembled onto the Au colloids. For the 25-nm Au colloid the surface coverage by the redox active unit, 6.8×10 -10 mol-cm -2 , is ca. 12-fold higher than that of the (aminopropyl)siloxane monolayer-modified ITO, lacking the Au film. The surface coverages of the Au colloid films by the bipyridinium monolayers increase as the colloid particle sizes decrease

Spectroscopic Probe Analysis of Protein-Surfactant Interactions: The BSA/SDS System

Abstract: The interactions and structure of complexes formed between bovine serum albumin (BSA) and the anionic surfactant sodium dodecyl sulfate has been investigated by spectroscopic probe techniques. Steady state and time-resolved fluorescence, electron spin resonance, and deuterium NMR spectroscopy have been employed as a multitechnique approach to investigate the structures which occur along the proteinsurfactant isotherm. Three models of the protein-surfactant complex are considered. The results of the multitechnique approach are consistent with a structure which is predominantly of the "necklace and bead" type in which the unfolded protein wraps around surfactant micelles.

Capillary Hysteresis in Nanopores: Theoretical and Experimental Studies of Nitrogen Adsorption on MCM-41

Abstract: Capillary hysteresis in cylindrical nanopores has been studied using MCM-41 as the prime example of a mesoporous material. These materials, due to their regular pore structure, can be considered to be candidates for reference adsorbents for standardizing adsorption measurements and methods for characterization of porous solids. They provide a unique opportunity for verification of theoretical models employed for predicting phase equilibrium in confined geometry. Three samples with monodisperse pore channels have been synthesized and examined using X-ray diffraction (XRD). Nitrogen adsorption isotherms were modeled using nonlocal density functional theory (NLDFT) in a wide range of pore sizes (18-80 A). Theoretical isotherms for pore channels with sizes corresponding to those identified by XRD were compared with experimental isotherms at different temperatures between 70 and 82 K. The latter have been measured independently on two different adsorption setups. The theoretical thermal dependence ofthe thermodynamic adsorption-desorption hysteresis predicted by NLDFT is confirmed by the experimental measurements. It is shown that at 77.4 K NLDFT quantitatively predicts equilibrium phase transitions in cylindrical channels of MCM-41. Theoretical and experimental results prove that the nitrogen hysteresis observed at temperatures below 77.4 K is associated with metastability of the adsorption branch of the isotherm. The absence of experimental hysteresis on samples with pore size of about 40 A at temperatures above 77.4 K cannot be explained by the capillary critical temperature for a given pore size being achieved as was assumed previously.

A Simple Method for the Production of a Two-Dimensional, Ordered Array of Small Latex Particles

Abstract: We present here a simple technique for depositing wide monolayers of submicron-sized latex particles. The method is straightforward and does not require any special apparatus. We also studied the quality of the arrays formed, giving information on either the large scale structure or the small scale one. With this technique we succeeded to deposit arrays of 42 nm sized latex spheres, as far as we know, the smallest regular monolayer array ever deposited.

Phase diagram of colloidal dispersions of anisotropic charged particles : equilibrium properties, structure, and rheology of laponite suspensions

Abstract: We discuss the phase diagram of aqueous dispersions of colloidal platelike charged particles (300 A x 10 A). Particle concentration and ionic strength are the two parameters controlling the system. The suspensions undergo a sol/gel transition without macroscopic phase separation. Shear rheology is used to monitor this transition and to locate the appearance of the mechanical gel phase. Increasing the ionic strength shifts the sol/gel transition to lower volume fraction. Direct inspection of this gel phase by cryofracture, TEM and SAXS shows correlated but well-separated particle populations. In order to check the reversibility and the equilibrium properties of this transition, the equation of state was determined by osmotic stress. At fixed ionic strength, the osmotic pressure first increases at low particle concentration, then reaches a pseudoplateau, and increases again for higher concentrations. The location of such a singularity in the equation of state of the suspension defines a thermodynamical transition coinciding with the mechanical phase transition. In order to analyze the origin of this gel or glassy phase, the role of particle anisotropy, coupled with diffuse layer repulsion, is discussed.

Stripe patterns formed on a glass surface during droplet evaporation

Abstract: When a suspension droplet dries on a glass surface, particles collect near the edge of the droplet boundary (contact line) and often leave a striped pattern as the droplet evaporates. During drying, the motion of the droplet contact line resembles stick-slip motion and it shrinks toward the center with an oscillatory motion. To explain the oscillatory motion and the mechanism of the stripe formation, we formulated a mathematical model that includes a friction force which the contact line feels when particles flow from the inside of the droplet to the droplet boundary. As a result of competition between this friction force and surface tensions at the contact line, the droplet oscillates as it dries and generates a striped film composed of particles

Structure and Dynamics of Block Copolymer Colloids

Abstract: Polymer colloids offer great potential in creating tailor-made supramolecules because of wider variations in the molecular architecture when compared with small detergent molecules. The delicate balance between inter- and intramolecular interactions together with interface/surface and geometrical contraints challenges both theory and experiment. Many experiments and applications have been made. A short-chain triblock copolymer is used as an example to discuss the methodology developed to investigate the structure and dynamics of such polymer colloids and to see the type of information which can be made available by using a combination of physical techniques. 65 refs., 12 figs., 3 tabs.

Self-Assembled Monolayers of Long-Chain Hydroxamic Acids on the Native Oxides of Metals

Abstract: Departments of Materials Science and Engineering and Chemistry, Uniuersity of lllinois,Urbana-Champaign, (Irbana, Illinois 61801Receiued September 79, 1994eLong-chain alkanehydroxamic acids adsorb on the native oxides of metals and formed oriented self-assembled monolayers (SAMs). This study examined SAMs of hydroxamic acids on the native oxides ofcopper, silver, titanium, aluminum, zirconium, and iron. These SAMs were characterized usingwettability,X-ray photoelectron spectroscopy (XPS), and polarized infrared external reflectance spectroscopy (PIERS).Alkanehydroxamic acids give better monolayers than the corresponding alkanecarboxylic acids on certainbasic metal oxides (especially copper(Il) oxide). On the native oxide of copper (which has an isoelectricpoint greater than the pK" of the hydroxamic acid), the ligand is bound to the surface predominantly asthe hydroxamate. The strength of the interaction between copper oxide and the hydroxamate allowsincorporation of polar tail groups into the monolayer. On acidic or neutral metal oxides (e.g., TiO2), thepredominant species bound to the surface is the hydroxamic acid. Alkanehvdroxamic acids on titaniumdioxide bind relatively weakly but. nonetheless. form SAlts that are more stable than those from carboxylicacids (although not as stable as those from alkanephosphonic acids r.

Dynamics of Colloid Deposition in Porous Media: Blocking Based on Random Sequential Adsorption

Abstract: An improved theoretical model is presented for quantifying the dynamics of colloid deposition in granular porous media. The model characterizes the transient aspects ofirreversible particle deposition onto spherical collector surfaces where repulsive electrostatic forces between colloidal particles limit deposition to monolayer coverage. The transient deposition rate normally associated with particle deposition is depicted in the model by a dynamic blocking function derived from random sequential adsorption (RSA) mechanics. The RSA blocking function has a nonlinear power law dependence on surface coverage, in contrast to the linear Langmuirian blocking function used in previous dynamic deposition models for porous media. A technique involving the calculation of the jamming limit from experimental particle breakthrough curves is utilized for determining the excluded area parameter, an integral component of the dynamic blocking function. Parameter estimation and curve fitting techniques are not required by the model as all parameter values are calculated a priori using available theoretical principles. Parameter values are incorporated into the theoretical model to produce theoretical particle breakthrough curves based on both RSA and Langmuirian dynamic blocking functions. A comparison of theoretical results with experimental particle breakthrough curves demonstrates the utility of RSA mechanics as a means of describing particle blocking dynamics associated with colloid deposition in granular porous media.

Surface Plasmon Resonance Permits in Situ Measurement of Protein Adsorption on Self-Assembled Monolayers of Alkanethiolates on Gold

Abstract: This paper demonstrates that surface plasmon resonance (SPR) spectroscopy can be used to measure the nonspecific adsorption of proteins to self-assembled monolayers (SAMs) of alkanethiolates on gold in situ and in real time. Mixed SAMs comprising hexa(ethylene glycol) and methyl groups that have values of XMe 0.5 : the amount of adsorbed protein correlated with XMe. The initial rate for adsorption of fibrinogen to a methyl-terminated SAM (XMe = 1.0) followed first-order kinetics. The combination of SAMs and SPR described here is particularly well suited for investigations of the interactions of proteins with structurally well-defined organic surfaces.

Young-Dupre Revisited

Abstract: The Young-Dupre equation for the work of adhesion of a liquid drop to a solid surface, where the solid surface is in equilibrium with the vapor of the liquid, is given as W = γ L (1 + cos θ), where γ L is the surface tension of the liquid and θ the contact angle. This work (W) has generally been identified with the free energy of adhesion. It is shown here that it constitutes the total work of adhesion only under the artificial condition that the sessile drop retains its shape after detaching from the solid surface. Under real conditions, W represents only one component of the total free-energy change taking place when a drop is separated from, or attached to, a vapor-equilibrated smooth solid surface. In the present work, a Net Free Energy of Adhesion, ΔF N , is derived which gives the total free energy necessary to separate a sessile drop from a smooth solid surface to form a free sphere (its negative, of course, is the free energy of attachment of the sphere). It is given by ΔF N = πr 2 γ L [(2α/sin θ) 2/3 - α], where r is the radius of the solid-liquid interface and α, called the effective area, is [2/(1 + cos θ)] - cos θ. The Net Free Energy of Adhesion and Young-Dupre work of adhesion are compared as functions of the contact angle. This is done for systems of constant solid-liquid interfacial area and for systems of constant drop volume.

Preparation of rodlike gold particles by UV irradiation using cationic micelles as a template

Abstract: Colloidal gold was prepared by irradiation of HAuCl 4 solutions with 253.7 nm light in the presence of rodlike micelles of hexadecyltrimethylammonium chloride. Rodlike colloidal gold was obtained at above some concentrations of HAuCl 4 with increasing irradiation time. This result suggests that the rodlike micelle operates as a template for the preparation of anisotropic gold particles.

Temperature Effects on Structural Properties of Pluronic P104 and F108 PEO-PPO-PEO Block Copolymer Solutions

Abstract: The effects of temperature on the micellization properties and the structure of the micelles for two poly(ethylene-oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO-PPO-PEO) copolymers, having similar size hydrophobic (PPO) blocks and different size hydrophilic (PEO) blocks, are reported. Critical micellization concentration (CMC) and temperature (CMT) values for aqueous copolymer solutions were obtained from a dye solubilization method and corroborated with differential scanning calorimetry, surface tension, density, light scattering intensity, and fluorescence spectroscopy experiments; very good agreement among the different techniques is observed. The extraction of CMC and CMT values from the characteristic features of the data obtained from the various techniques is discussed. The hydrodynamic radii of the copolymer micelles, determined using dynamic light scattering, remained constant over the temperature range investigated; the polydispersity of the micelle size decreased with temperature. The micropolarity in aqueous copolymer solutions was probed as a function of temperature using the I{sub 1}/I{sub 3} intensity ratio of the pyrene vibrational fine structure recorded in fluorescence emission spectra. The decreasing values for the intensity ratio, following the formation of micelles, were considered to be a linear combination of the temperature effects on polarity observed in bulk PEO and PPO homopolymers, and in water. 59 refs., 10 figs., 2 tabs.

Phenomenology and kinetics of lipid bilayer spreading on hydrophilic surfaces

Abstract: We studied the spreading of phospholipid bilayer membranes and the conditions for the formation of continuous bilayers on rough (glass, glass-MgF 2 , glass-MgF 2 -SiO 2 ) and smooth (mica) solids using reflection interference contrast microscopy as an analytical tool. We show that two fundamentally different spreading mechanisms are possible : (i) The sliding of a single bilayer on a thin lubricating water film and (ii) the rolling of thin lobes of two juxtaposed bilayers in a tank tread type motion. In the first mechanism the spreading velocity of straight interface exhibits a square root behavior, υ ∼ t 1/2 , allowing an estimate of the frictional coupling of the membrane to the substrate. On smooth surfaces (e.g., freshly cleaved mica) the dissipation is dominated by shear flow in the ultrathin water film separating the bilayer from the substrate. On rough surfaces in contrast (e.g., glass) friction is caused by two-dimensional flow of pinning centers through the spreading membrane. In the latter case the advancing front exhibits a self-similar interface roughness which grows with time. The growth of the roughness is analyzed, and a static roughness exponent ζ = 0.61 ± 0.04 is found. The rolling of membranes occurs on dehydrated solid-bilayerinterfaces with the substrate adjacent bilayer being immobilized. In this case a viscous fingering type spreading pattern is observed. From a practical point of view the rolling motion results in separated lipid patches with intermediate uncovered spots, while spreading by membrane sliding leads to continuous substrate-supported bilayers.

Computer Simulation of Interlayer Molecular Structure in Sodium Montmorillonite Hydrates

Abstract: Molecular structure in the interlayers of sodium Wyoming montmorillonite with one, two, or three water monolayers was investigated by Monte Carlo and molecular dynamics simulation, based on tested water-water, water-counterion, clay-water, and clay-counterion potential unctions. Calculated layer spacings and thermodynamic properties, as well as interlayer water configurations and interlayer-species self-diffusion coefficients, were in good agreement with available experimental data. Inner-sphere surface complexes of Na+ with tetrahedral charge sites persisted in all hydrates, whereas the outer-sphere surface complexes with octahedral charge sites found in the one-layer hydrate dissociated partially into an incipient diffuse layer in the two- and three-layer hydrates. Even in the three-layer hydrate, interlayer water molecules did not replicate their structure or mobility in bulk aqueous solution. Improved diffraction and spectroscopic data on interlayer water are needed to guide future simulation studies.

Evidence for a Unique Chain Organization in Long Chain Silane Monolayers Deposited on Two Widely Different Solid Substrates

Abstract: We address the longstanding issue of substrate effects in alkylsiloxane monolayer self-assembly. With proper substrate prehydration, we observe formation of identical quality, compact, quasi-crystalline monolayers on oxidized silicon and gold substrates with widely different chemical character, the former capable of covalent bonding to the adsorbed molecules via silanol groups and the latter devoid of reactive surface sites. Infrared spectroscopy, ellipsometry, and wetting measurements show identical average film structures consisting of highly extended chains tilted at 10 (±2)° with significant end-gauche defect content. This observed substrate independence is consistent with our previous hypothesis that substrate-bound water promotes the decoupling of the organic film from the underlying solid surface.

Formation and in Situ Modification of Monolayers Chemisorbed on Ultraflat Template-Stripped Gold Surfaces

Abstract: We have previously described the preparation of ultraflat Au(111) surfaces as substrates for scanning probe microscopy. We report here alternative ways to produce polycrystalline Au(111)thin films of similar, high quality (i.e., with mean roughness smaller than 5 A over 25 μm 2 ). All of them are based on the same principle, i.e., that of exposing the very first layer of gold atoms which had deposited onto mica. One alternative route leads to substrates which are transparent enough for optical microscopy. Two other routes make use of ceramic glues, providing substrates which can be handled with most organic solvents without disruption of the Au(111) layer. The substrates prepared by the latter procedures can thus be used to produce gold-directed self-assembled monolayers (SAMs) from nearly all ω-functionalized alkanethiols or dialkyl disulfides ; they can also be used for the in situ chemical modification of SAMs on gold. For this purpose, appropriate reaction chambers have been developed. As an example of the use of the new techniques described here, we report the preparation ofa SAM ofN-palmitoylcysteamine, either from ex situ synthesis of the precursor or from in situ acylation of the amino head groups of cysteamine SAM.

Adsorption of DNA to mica, silylated mica, and minerals : characterization by atomic force microscopy

Abstract: The nature of DNA adsorption to various substrates has been investigated by atomic force microscopy. The substrates used were mica, silylated mica, calcite, fluorite, and barite. The presence of a divalent cation, either in solution or accessible to the DNA on the substrate, greatly improves DNA adsorption. A comparison of DNA adsorption from different buffers onto mica and silylated mica shows that buffer composition is much less important for DNA adsorption to silylated mica and also suggests that for good adsorption DNA must be electrostatically adsorbed to the surface.

A near edge X-ray absorption fine structure spectroscopy and X-ray photoelectron spectroscopy study of the film properties of self-assembled monolayers of organosilanes on oxidized Si(100)

Abstract: n-Alkyltrichlorosilane films with different alkyl chain length (n-propyltrichlorosilane [PTS, CH 3 (CH 2 ) 2 -SiCl 3 ] ; n-octadecyltrichlorosilane [OTS, CH 3 (CH 2 ) 17 SiCl 3 ] ; n-triacontyltrichlorosilane [TCTS, CH 3 (CH 2 ) 29 -SiCl 3 ]) and trimethoxysilane films with amino- and methyl-terminated alkyl chains (n-octadecyltri-methoxysilane [OTMS, CH 3 (CH 2 ) 17 Si(OCH 3 ) 3 ] ; (3-aminopropyl)trimethoxysilane [APTMS, NH 2 (CH 2 ) 3 Si-(OCH 3 ) 3 ] ; (17-aminoheptadecyl)trimethoxysilane [AHTMS, NH 2 (CH 2 ) 17 Si(OCH 3 ) 3 ]) ofdifferent chain length have been characterized using near edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS) to probe the molecular order and orientation in self-assembled films. Furthermore, the influence of the different headgroups (trichlorosilanes vs trimethoxysilanes), chain lengths, and chain terminal functions (methyl vs amino) on the resulting film structures was investigated. It was found that methyl-terminated silanes with an alkyl chain of intermediate length like OTS and OTMS tend to form self-assembled monolayers which are oriented almost perpendicular to the surface. The different chemistries of these two compounds during adsorption seems to result in slightly different film structures. Aminosilane (AHTMS) films of the same chain length as OTS and OTMS, however, are completely disordered. XPS indicates, that this is probably due to the interaction of the amino group with the surface. Short alkyl chain silanes with a terminating methyl or amino group (APTMS, PTS) result in films with no defined orientation of the molecules. Also long chain methyl-terminated alkylsilane films (TCTS) are less well oriented than OTS films, which gives evidence that there is an optimal chain length for ordering in self-assembled organosilane films.

Steric and Bridging Forces between Surfaces Bearing Adsorbed Polymer: An Atomic Force Microscopy Study

Abstract: Direct measurement of the forces of interaction between a 10 μm radius sphere of zirconia with a flat polished zirconia ceramic plate in aqueous electrolyte solution has been performed. The interaction profiles of these surfaces after the addition of a high molecular weight (M w =750 000) polyacrylic acid were also recorded. The surface coverage of the polymer adsorbed at the solid/aqueous interface was controlled by varying the added polymer concentration and by varying the equilibration time of the polymer with the two surfaces at large separations. At a long equilibration times (24 h) and higher polymer concentration (30 μg/L) a repulsive interaction force was observed between the surfaces upon their approach, this repulsive force beginning at twice the radius of gyration and increasing monotonically as the separation is decreased. At a lower polymer concentration (10 μg/L) and short equilibration time (<1 h) the polymer was seen to bridge between the sphere and the flat plate. At the scan rates employed here, for the approach and separation of the surfaces, the bridging polymer remained adsorbed to both surfaces tethering them together

Scanning Force Microscopies Can Image Patterned Self-Assembled Monolayers

Abstract: This paper demonstrates that scanning probe microscopies (SPMs) can image patterned self-assembled monolayers (SAMs) formed by microcontact printing. Lateral force microscopy (LFM) and force modulation microscopy (FMM) showed contrast between regions of patterned SAMs terminated by different chemical functionalities. Normal force microscopy (NFM) typically showed less contrast than LFM or FMM but provided information about the topography of the surface. Chemical functionality (at the interface between the SAM and air) dominated imaging in these experiments. Changes in the morphology of the surface or changes in the humidity of the environment for imaging did not influence significantly the contrast in our experiments. The sharpness of the contrast suggests the use of LFM and SAMs in studying tribology on the submicrometer scale.

Fractal Morphology Analysis of Combustion-Generated Aggregates Using Angular Light Scattering and Electron Microscope Images

Abstract: Experimental studies of the fractal morphology of flame-generated aggregates are described here, considering not only the fractal dimension, D f , but also the fractal prefactor (lacunarity), k g , both of which are shown to be needed to fully characterize aggregates. Measurements were made using angular light scattering (ALS) and thermophoretic sampling followed by transmission electron microscopy (TEM) for soot aerosols found in laminar and turbulent flame environments. D f and the prefactor k g were simultaneously inferred from ALS measurements using the optical properties of aggregates composed of small primary particles. TEM-based inferences of these fractal properties involve analysis of aggregate-projected images from which the actual morphologies are obtained by correlating the radius of gyration to the outer radius (half of the maximum length) of an aggregate. Both of our procedures for determining the detailed morphology of aggregates yield D f = 1.7 ± 0.15 and k g = 2.4 ± 0.4 for carbonaceous soot, in good agreement with earlier TEM measurements involving multiple angle images. Furthermore, we show that these values also seem to be valid for other materials such as alumina aggregates, suggesting that not only the fractal dimension but also the fractal prefactor are universal properties of aggregates found in combustion environments due to similar mechanisms of aggregation. This universality for D f and k g is observed at various positions in four different flame types with nine various gaseous and liquid fuels for aggregates' with mean primary particle radii and number of primary particles in aggregates in the range 12-26 nm and 2-10 4 , respectively.

Cationic Surfactant Adsorption by Swelling and Nonswelling Layer Silicates

Abstract: The adsorption of hexadecyltrimethylammonium (HDTMA) in both swelling (montmorillonite) and nonswelling (kaolinite) layer silicates was studied by adsorption isotherms, X-ray diffraction and electrophoretic mobility. The objectives were to determine the effects of clay type and solution composition on the adsorption of cationic surfactants and to reveal the relation between the structure of the adsorbed surfactant layer and the stability of surfactant-clay complexes. We found that HDTMA adsorption by montmorillonites was more complex than by a nonswelling clay such as kaolinite. The major complexity arose from the fact that the structure of adsorbed HDTMA layer in the interlayers of montmorillonites depended strongly on the initial degree of clay dispersion. Highly dispersed clays (e.g., Na-montmorillonite in dilute NaCl solutions) lead to a random HDTMA distribution on the surfaces, larger distances between silicate layers, and hence reduced lateral interactions between the alkyl chains of adsorbed HDTMA. Accordingly, cation selectivity coefficients were low at low HDTMA loadings and increased dramatically as HDTMA loading increased, resulting in a unique S-shaped adsorption isotherm, which has not been observed for nonswelling clays. Initially flocculated clays (e.g., Na-montmorillonite in concentrated NaCl solutions or Ca- or Cs-montmorillonite) lead to HDTMA segregation and more compact adsorption layers, which enhanced the lateral interactions between alkyl groups. As a result, the cation selectivity coefficients were high even at low HDTMA loadings and changed little as loading increased, similar to that for nonswelling clays. Both cation exchange and hydrophobic bonding were involved in HDTMA adsorption at high loadings (e.g., >0.75 CEC). Charge density of clays, ionic strength of the bulk solution, and the type of companion anions all affect the adsorption of HDTMA via hydrophobic bonding.