Response surface modelling and kinetic studies for the experimental estimation of measurement uncertainty in derivatisation

Response surface modelling is proposed as an approach to the estimation of uncertainties associated with derivatisation, and is compared with a kinetic study. Fatty acid methyl ester formation is used to illustrate the approach, and kinetic data for acid-catalysed methylation and base-catalysed transesterification are presented. Kinetic effects did not lead to significant uncertainty contributions under normal conditions for base-catalysed transesterification of triglycerides. Uncertainties for acid-catalysed methylation with BF3 approach significance, but could be reduced by extending reaction times from 3 to 5 min. Non-linearity is a common feature of response surface models for derivatisation and compromised first-order estimates of uncertainty; it was necessary to include higher order differential terms in the uncertainty estimate. Simulations were used to examine the general applicability of the approach and to study the effects of poor precision and of change of response surface model. It is concluded that reliable uncertainty estimates are available only when the model is statistically significant, robust, representative of the underlying behaviour of the system, and forms a good fit to the data; arbitrary models are not generally suitable for uncertainty estimation. Where statistically insignificant effects were included in models, they gave negligible uncertainty contributions.     

Headgroup percolation and collapse of condensed langmuir monolayers

We present a study of Langmuir isotherms and 2D bulk moduli of binary lipid mixtures, where changes in monolayer collapse pressure (Pic) are followed while varying the relative amounts of the two components. For monolayers containing dipalmitoylphosphocholine (DPPC) with either hexadecanol (HD) or palmitic acid (PA), a distinctly non-monotonic change in Pic is observed with varying composition. At low mole fractions, there is a slight decrease in Pic as films get richer in DPPC, while a sharp increase to pure DPPC-like values is observed when the mole fraction exceeds approximately 0.7. The sudden transition in collapse pressure is explained using the principles of rigidity percolation, and important ramifications of this phenomenon for biological surfactant are discussed.     

Spectroscopy and potential energy surface of the H2-CO2 van der Waals complex: experimental and theoretical studies

A 4-D ab initio potential energy surface is calculated for the intermolecular interaction of hydrogen and carbon dioxide, using the CCSD(T) method with a large basis set. The surface has a global minimum with a well depth of 212 cm(-1) and an intermolecular distance of 2.98 A for a planar configuration with both the O-C-O and H-H axes perpendicular to the intermolecular axis. Bound state calculations are performed for the H(2)-CO(2) van der Waals complex with H(2) in both the para and ortho spin states, and the binding energy of paraH(2)-CO(2)(50.4 cm(-1)) is found to be significantly less than that of orthoH(2)-CO(2)(71.7 cm(-1)). The surface supports 7 bound intermolecular vibrational states for paraH(2)-CO(2) and 19 for orthoH(2)-CO(2), and the lower rotational levels with J< or = 4 follow an asymmetric rotor pattern. The calculated infrared spectrum of paraH(2)-CO(2) agrees well with experiment. For orthoH(2)-CO(2), the ground state rotational levels allowed by symmetry are found to have (K(a), K(c))=(even, odd) or (odd, even). This somewhat unexpected fact enables the previously observed experimental spectrum to be assigned for the first time, in good agreement with theory, and indicates that the orientation of hydrogen is perpendicular to the intermolecular axis in the ground state of the orthoH(2)-CO(2) complex.     

Negative surface potential produced by self-assembled monolayers of helix peptides oriented vertically to a surface

Self-assembled monolayers (SAMs) of helix peptides oriented vertically to a gold surface were prepared and the surface potential measured using the Kelvin technique up to 140°C. Negative surface potentials of a few hundred millivolts were observed for the helix peptide SAMs, indicating the occurrence of the large dipole moment of the helices directing toward the surface. The longer the helix peptide, the larger was the negative surface potential obtained. The absolute value of the surface potential decreased with increase in temperature due to thermal perturbation in the helical structure. However, Fourier transform infrared reflection–absorption spectroscopy revealed that perturbation is not significant and the α-helical conformation is stable even at 140°C.     

Effect of surface chemical composition on the surface potential and iso-electric point of silicon substrates modified with self-assembled monolayers

Self-assembled monolayer (SAM)-modified nano-materials are a new technology to deliver drug molecules. While the majority of these depend on covalently immobilizing molecules on the surface, it is proposed that electrostatic interactions may be used to deliver drugs. By tuning the surface potential of solid substrates with SAMs, drug molecules could be either absorbed on or desorbed from substrates through the difference in electrostatic interactions around the selected iso-electric point (IEP). In this work, the surface of silicon substrates was tailored with various ratios of 3-aminopropyltrimethoxysilane (APTMS) and 3-mercaptopropyltrimethoxysilane (MPTMS), which form amine- and thiol-bearing SAMs, respectively. The ratio of the functional groups on the silicon surface was quantified by X-ray photoelectron spectrometry (XPS); in general, the deposition kinetics of APTMS were found to be faster than those of MPTMS. Furthermore, for solutions with high MPTMS concentrations, the relative deposition rate of APTMS increased dramatically due to the acid-base reaction in the solution and subsequent electrostatic interactions between the molecules and the substrate. The zeta potential in aqueous electrolytes was determined with an electro-kinetic analyzer. By depositing SAMs of binary functional groups in varied ratios, the surface potential and IEP of silicon substrates could be fine-tuned. For <50% amine concentration in SAMs, the IEP changed linearly with the chemical composition from <2 to 7.18. For higher amine concentrations, the IEP slowly increased with concentration to 7.94 because the formation of hydrogen-bonding suppressed the subsequent protonation of amines.     

Calculation of the surface potential and surface charge density by measurement of the three-phase contact angle

The silica/silicon wafer is widely used in the semiconductor industry in the manufacture of electronic devices, so it is essential to understand its physical chemistry and determine the surface potential at the silica wafer/water interface. However, it is difficult to measure the surface potential of a silica/silicon wafer directly due to its high electric resistance. In the present study, the three-phase contact angle (TPCA) on silica is measured as a function of the pH. The surface potential and surface charge density at the silica/water surface are calculated by a model based on the Young-Lippmann equation in conjunction with the Gouy-Chapman model for the electric double layer. In measurements of the TPCA on silica, two distinct regions were identified with a boundary at pH 9.5-showing a dominance of the surface ionization of silanol groups below pH 9.5 and a dominance of the dissolution of silica into the aqueous solution above pH 9.5. Since the surface chemistry changes above pH 9.5, the model is applied to solutions below pH 9.5 (ionization dominant) for the calculation of the surface potential and surface charge density at the silica/aqueous interface. In order to evaluate the model, a galvanic mica cell was made of a mica sheet and the surface potential was measured directly at the mica/water interface. The model results are also validated by experimental data from the literature, as well as the results obtained by the potentiometric titration method and the electro-kinetic measurements.     

Side-by-side characterization of electron tunneling through monolayers of isomeric molecules: a combined experimental and theoretical study

We report a new experimental method for measuring relatively small differences in electron tunneling through two distinct monolayers. We place them side by side using scanning probe nanolithography and compare the tunneling currents by conductive probe atomic force microscopy under identical force, voltage, and tip contamination conditions. We demonstrate the validity of our approach by applying it to two isomeric molecules with similar length and functional groups, with only the position of two functional groups, one aromatic and the other aliphatic, being inverted with respect to each other. The relative values of the two tunneling currents, calculated using density functional theory and the Tersoff-Hamann approach, compare very well with the experimental data, providing us with an example of theory vs experiment agreement that is rather uncommon in this field.     

Vinylcyclobutane-cyclohexene rearrangement: theoretical exploration of mechanism and relationship to the Diels-Alder potential surface

[graph: see text] The vinylcyclobutane-cyclohexene rearrangement has been studied computationally with density functional theory and complete active space SCF calculations. The rearrangement proceeds through a diradical that exists on a very flat potential energy surface. Transition structures for conformational processes, only slightly higher in energy than the minimum energy reaction path, account for the stereochemistries of products observed in the thermal rearrangements of vinylcyclobutane derivatives. The connection of this rearrangement to the Diels-Alder reaction of butadiene with ethylene is discussed.     

502—Temperature dependence of membrane potential and surface pressure of bilayers and monolayers of dipalmitoylphosphatidylcholine

The temperature effects on the membrane potential of bilayers made from synthetic dl-β, γ-dipalmitoyl-α-phosphatidylcholine in NaCl and CaCl₂ solutions are measured and the membrane selectivity variation is discussed. An evaluation of the activation energies of the membrane-potential-determining process is performed and the calculated values are compared with data obtained from surface pressure measurements, at different temperatures, of monolayers of the same phospholipid formed on NaCl and CaCl₂ solution of variable concentrations (10^?3-2 M).     

Effect of microstructure on molecular oxygen permeation through condensed phospholipid monolayers

A method is presented that allows novel measurement of the effect of microstructure on the oxygen permeability of highly condensed, polycrystalline phospholipid monolayers. Oxygen permeability of the polycrystalline shell coating a stationary microbubble is measured directly using an apposing microelectrode in the induced transfer mode and modeling oxygen flux through the shell and intervening aqueous medium. Varying cooling rate through the phospholipid main phase transition permits control of shell microstructure by manipulation of crystalline domain size and shape. Domain boundary density, defined as the ratio of the mean domain perimeter to the mean domain area, of the microbubble shell is determined by fluorescence microscopy. Oxygen permeability was shown to increase linearly with domain boundary density at a constant phospholipid acyl chain length and, accordingly, was shown to decrease exponentially with increasing chain length at a constant domain boundary density. Modification of the energy barrier theory to account for microstructural effects, in terms of the domain boundary density, provides a general equation to model passive transport through polycrystalline monolayer films. Results from this method show promise in determining the gas transport kinetics of medical microbubbles and the gas exchange characteristics of biological monolayers.     

The surface potential of a spherical colloid particle: functional theoretical approach

By using the iterative method in functional analysis, the potential of the electrical double layer of a spherical colloid particle, which is represented by the so-called Poisson-Boltzmann (PB) equation, has been solved analytically under general potential conditions. With the help of the diagram method in mathematics, the surface potential of the particle has been defined from the second iterative solution. The influence of the parameters included in the solutions on the surface potential has been studied. The results show that the surface potential of the particle increases as the temperature of the system, the aggregation number, and the concentration of ions increase, but decreases with an increase in the dielectric constant and the valence of the ions. The corresponding space charge density also has been illustrated in this work.     

Surface energy and surface tension of condensed matter and the principle of minimum potential energy of systems (revised)

We consider the essence and relation of the surface energy and surface tension of condensed matter: which is which … and (most important question here)—when? For the first time, this consideration is based not on reversible thermodynamics but, as an approximation, on the Principle of Minimum Potential Energy, given two factors: (1) the time-dependent dynamic transformation of the potential energy of the system into the surface energy and into the surface tension (stress); (2) elasticity of structured surface layers of the liquids.     

Protein-directed assembly of binary monolayers at the interface and surface patterns of protein on the monolayers

Ferritin-directed assembly of binary monolayers of zwitterionic dipalmitoylphosphatidylcholine and cationic dioctadecyldimethylammonium bromide (DOMA) at the interface and surface patterns of ferritin on the monolayers have been investigated using a combination of infrared reflection absorption spectroscopy, surface plasmon resonance, and atomic force microscopy. Ferritin binding to the binary monolayers at the air-water interface at the surface pressure 30 mN/m, primarily driven by the electrostatic interaction, gives rise to a change in tilt angle of hydrocarbon chains from 15 degrees +/- 1 degrees to 10 degrees +/- 1 degrees with respect to the normal of the monolayer at the mole fraction of DOMA (XDOMA) of 0.1. The chains at XDOMA = 0.3 are oriented vertical to the water surface before and after protein binding. A new mechanism for protein binding to the binary monolayers is proposed. The secondary structures of the adsorbed ferritin are prevented from changing to some extent due to the existence of the monolayers. The amounts of the bound protein on the monolayers at the air-water interface are increased in comparison with those on the pre-immobilized monolayers at low XDOMA. The increased amounts and different patterns of the adsorbed protein at the monolayers are mostly attributed to the formation of multiple binding sites available for ferritin, which is due to the lateral reorganization of the lipid components in the monolayers induced by the protein in the subphase. The created multiple binding sites on the monolayer surfaces through the protein-directed assembly can be preserved for subsequent protein binding.     

Combined theoretical and experimental study on the adsorption of methanol on the ZnO(1010) surface

The structure, dynamics, and energetics of methanol adlayers on the nonpolar ZnO(1010) surface have been studied by He-atom diffraction (HAS), high-resolution electron energy loss spectroscopy (HREELS), thermal desorption spectroscopy (TDS), and density functional calculations. The experimental and theoretical data consistently show that at temperatures below 357 K methanol forms an ordered adlayer with a (2 × 1) periodicity and a coverage of one monolayer in which half of the methanol molecules are dissociated. The ordering of the methanol molecules is governed by repulsive interactions between the methyl groups of the methanol molecules. This repulsive interaction is also responsible for the formation of a second, low-density phase at higher temperatures with half monolayer coverage of undissociated methanol which is stable up to 440 K.     

A theoretical study of the ground-state potential surface of guanine and its binding with oxygen and water

A molecular orbital geometry optimization study of the potential surface of guanine, guanine? O₂, and guanine? O₂? water reaction products in the ground state has been carried out. The origin of the asymmetric double-well potential surface of guanine suggested earlier on the basis of experimental observations has been explained. The most stable binding of O₂ with guanine (G) is found to occur at the C4C5 double bond of the latter molecule. In the case of G? O₂? water reaction product (HOO? G? OH), the groups ? OOH and ? OH bind at C4 and C8, respectively. The possiblity of a polymeric reaction product of the type R1? (G? O? O? G)n? R2 (R1, R2 = H, OH) has also been suggested. These results are broadly supported by experimental observations. The mechanism of spectral oscillations observed in UV-irradiated guanine solutions has been discussed.