Reversible activation of diblock copolymer monolayers at the interface by pH modulation, 1: Lateral chain density and conformation

This study focuses on the design of chemically regulated surfaces that allow for reversible control of the interactions between biological matter (cells and proteins) and planar substrates. As a tunable interlayer, we use a monolayer of a near-monodisperse poly[2-(dimethylamino)ethyl methacrylate-block-methyl methacrylate] (PDMAEMA-PMMA) diblock copolymer. Owing to the relatively large fraction (50%) of the hydrophobic PMMA block, this copolymer forms a stable Langmuir monolayer at the air/water interface. Both in situ and ex situ film balance experiments suggest that the hydrophilic PDMAEMA block adsorbs to the air/water interface in its uncharged state (pH 8.5), but stretches into the subphase in its charged state (pH 5.5). Optimization of the preparation protocols enables us to fabricate stable, homogeneous diblock copolymer films on hydrophobized substrates via Langmuir-Schaefer transfer at well-defined lateral chain densities. Ellipsometry and X-ray reflectivity studies of the transferred films confirm that the film thickness can be systematically regulated by the lateral chain densities. The transferred copolymer films remain stable in water for about a week, suggesting that they are promising materials for the creation of pH-controlled solid substrates for the support of biological matter such as proteins and cells.     

Alkane/Alcohol mixed monolayers at the solid/liquid interface

In this work, we present the behavior of solid monolayers of binary mixtures of alkanes and alcohols adsorbed on the surface of graphite from their liquid mixtures. We demonstrate that solid monolayers form for all the combinations investigated here. Differential scanning calorimetry (DSC) is used to identify the surface phase behavior of these mixtures, and elastic neutron incoherent scattering has been used to determine the composition of the mixed monolayers inferred by the calorimetry. The mixing behavior of the alcohol/alkane monolayer mixtures is compared quantitatively with alkane/alkane and alcohol/alcohol mixtures using a regular solution approach to model the incomplete mixing in the solid monolayer with preferential adsorption determining the surface composition. This analysis indicates the preferential adsorption of alcohols over alkanes of comparable alkyl chain length and even preferential adsorption of shorter alcohols over longer alkanes, which contrasts strongly with mixtures of alkane/alkane and alcohol/alcohol of different alkyl chain lengths where the longer homologue is always found to preferentially adsorb over the shorter. The alcohol/alkane mixtures are all found to phase separate to a significant extent in the adsorbed layer mixtures even when molecules are of a similar size. Again, this contrasts strongly with alkane/alkane and alcohol/alcohol mixtures where, although phase separation is found for molecules of significantly different size, good mixing is found for similar size species.     

Little exchange at the liquid/solid interface: defect-mediated equilibration of physisorbed porphyrin monolayers

The transition from low to high density 2D surface structures of copper porphyrins at a liquid/solid interface requires specific defects at which nearly all exchange of physisorbed molecules with those dissolved in the supernatant occurs.     

Vermiculite-aliphatic amine interactions at the solid/liquid interface: a thermodynamic approach

Polar n-alkylmonoamines of general formula H₃C(CH₂) _n NH₂ (n = 1, 3, 5) interacted with layered silicate vermiculite at the solid/liquid interface. The maximum amount of amine intercalated (N _f ) inside the interlamellar space were 0.62, 0.46, and 0.38 mmol g⁻¹, to give the following order of intercalation ethyl → butyl → hexylamines. The layered vermiculite solid was suspended in deionized water and calorimetrically titrated with this series of amines, to give favorable thermodynamic data, such as exothermic enthalpy, negative Gibbs free energy and positive entropy data.     

Self-assembly at the liquid/solid interface: STM reveals

The liquid/solid interface provides an ideal environment to investigate self-assembly phenomena, and scanning tunneling microscopy (STM) is the preferred methodology to probe the structure and the properties of physisorbed monolayers on the nanoscale. Physisorbed monolayers are of relevance in areas such as lubrication, patterning of surfaces on the nanoscale, and thin film based organic electronic devices, to name a few. It's important to gain insight in the factors which control the ordering of molecules at the liquid/solid interface in view of the targeted properties. STM provides detailed insight into the importance of molecule-substrate (epitaxy) and molecule-molecule interactions (hydrogen bonding, metal complexation, and fluorophobic/fluorophilic interactions) to direct the ordering of both achiral and chiral molecules on the atomically flat surface. By controlling the location and orientation of functional groups, chemical reactions can be induced at the liquid/solid interface, via external stimuli, such as light, or by controlled manipulation with the STM tip. The electronic properties of the self-assembled physisorbed molecules can be probed by taking advantage of the operation principle of STM, revealing spatially resolved intramolecular differences within these physisorbed molecules.     

Molecular weight dependence of diblock copolymer segregation at a polymer/polymer interface

Utilizing forward recoil spectrometry (FRES), we have determined the segregation isotherm which describes the interfacial excess z_i^* of diblock copolymers of poly (d₈-styrene-b-2-vinylpyridine) (dPS-PVP) at the interface between the homopolymers PS and PVP as a function of ?_∞, the volume fraction of diblock copolymer remaining in the host homopolymer. All the samples were analyzed after annealing at temperatures and times sufficient to achieve equilibrium segregation. The effect of the degree of polymerization of both the diblock copolymers and the host homopolymers on the segregation isotherm is investigated. When the degree of polymerization of the homopolymer is much larger than that of the diblock copolymer, the normalized interfacial excess (z_i^*/R_g), where R_g is the radius of gyration of an isolated block copolymer chain, is a universal function of that portion of the block copolymer chemical potential due to chain stretching. The existence of such a universal function is predicted by theory and its form is in good agreement with self-consistent mean field calculations. Using these results, one can predict important aspects of the block copolymer segregation (e.g., the saturation interfacial excess) without recourse to the time-consuming numerical calculations. © 1994 John Wiley & Sons, Inc.     

Structural analysis of PEO-PBO copolymer monolayers at the air-water interface

X-ray reflectivity (XR) and grazing incidence X-ray diffraction (GIXD) have been used to examine an oxyethylene-b-oxybutylene (E(23)B(8)) copolymer film at the air-water interface. The XR data were fitted using both a one- and a two-layer model that outputted the film thickness, roughness, and electron density. The best fit to the experimental data was obtained using a two-layer model (representing the oxyethylene and oxybutylene blocks, respectively), which showed a rapid thickening of the copolymer film at pressures above 7 mN/m. The large roughness values found indicate a significant degree of intermixing between the blocks and back up the GIXD data, which showed no long range lateral ordering within the layer. It was found from the electron density model results that there is a large film densification at 7 mN/m, possibly suggesting conformational changes within the film, even though no such change occurs on the pressure-area isotherm at the same surface pressure.     

Dynamics in physisorbed monolayers of 5-alkoxy-isophthalic acid derivatives at the liquid/solid interface investigated by scanning tunneling microscopy

Monolayers of isophthalic acid derivatives at the liquid/solid interface have been studied with scanning tunneling microscopy (STM). We have investigated the dynamics related to the phenomenon of solvent co-deposition, which was previously observed by our research group when using octan-1-ol or undecan-1-ol as solvents for 5-alkoxy-isophthalic acid derivatives. This solvent co-deposition has now been visualized in real-time (two frames per second) for the first time. Dynamics of individual molecules were investigated in mixtures of semi-fluorinated molecules with video-STM. The specific contrast arising from fluorine atoms in STM images allows us to use this functionality as a probe to analyze the data obtained for the mixtures under investigation. Upon imaging the same region of a monolayer for a period of time we observed that non-fluorinated molecules progressively substitute the fluorinated molecules. These findings illustrate the metastable equilibrium that exists at the liquid/solid interface, between the physisorbed molecules and the supernatant solution.     

Analysis of the gas states at a liquid/solid interface based on interactions at the microscopic level

The states of gas accumulated at the liquid/solid interface are analyzed on the basis of the continuum theory, in which the Hamaker constant is used to describe the long-range interaction at the microscopic scale. The Hamaker constant is always negative, whereas the "gas" spreading coefficient can be either negative or positive. Despite the complexity of gas, including that the density profile may not be uniform due to absorption on both solid and liquid surfaces, we predict three possible gas states at the liquid/solid interface, that is, complete "wetting", partial "wetting", and pseudopartial "wetting". These possible gas states correspond, respectively, to a gas pancake (or film) surrounded by a wet solid, a gas bubble with a finite contact angle, and a gas bubble(s) coexisting with a gas pancake. The typical thickness of the gas pancakes is at the nano scale within the force range of the long-range interaction, whereas the radius of the gas bubbles can be large. The state of a gas bubble(s) coexisting with a gas film is predicted theoretically for the first time. Our theoretical results can contribute to the development of a unified picture of gas nucleation at the liquid/solid interface.     

pH-responsive diblock copolymer micelles at the silica/aqueous solution interface: Adsorption kinetics and equilibrium studies

The adsorption behavior of two examples of a weakly basic diblock copolymer, poly(2-(dimethylamino)ethyl methacrylate)-block-poly(2-(diethylamino)ethyl methacrylate) (PDMA-PDEA), at the silica/aqueous solution interface has been investigated using a quartz crystal microbalance with dissipation monitoring and an optical reflectometer. Dynamic and static light scattering measurements have also been carried out to assess aqueous solution properties of such pH-responsive copolymers. In alkaline solution, core-shell micelles are formed above the critical micelle concentration (cmc) by both copolymers, whereas the chains are molecularly dissolved (as unimers) at all concentrations in acidic solution. As a result, the adsorption behavior of PDMA-PDEA diblock copolymers on silica is strongly dependent on both the copolymer concentration and the solution pH. Below the cmc at pH 9, the cationic PDMA-PDEA copolymers adsorb as unimers and the conformation of the adsorbed polymer is essentially flat. At concentrations just above the cmc, the initial adsorption of copolymer onto the silica is dominated by the unimers due to their faster diffusion compared to the much larger micelles. Rearrangement of the adsorbed unimers and/or their subsequent displacement by micelles from solution is then observed during an equilibration period, and the final adsorbed mass is greater than that observed below the cmc. At concentrations well above the cmc, the much higher proportion of micelles in solution facilitates more effective competition for the surface at all stages of the adsorption process and no replacement of initially adsorbed unimers by micelles is evident. However, the adsorbed layer undergoes gradual rearrangement after initial adsorption. This relaxation is believed to result from a combination of further copolymer adsorption and swelling of the adsorbed layer.     

Supramolecular nanopatterns self-assembled by adenine-thymine quartets at the liquid/solid interface

By means of scanning tunneling microscopy (STM), we have observed for the first time well-ordered supramolecular nanopatterns formed by mixing two complementary DNA bases: adenine (A) and thymine (T), respectively, at the liquid/solid interface. By mixing A and T at a specific mixing molar ratio, cyclic structures that were distinctly different from the structures observed by the individual base molecules separately were formed. From an interplay between the STM findings and self-consistent charge density-functional based tight-binding (SCC-DFTB) calculation method, we suggest formation of A-T-A-T quartets constructed on the basis of A-T base pairing. The formation of the A-T-A-T quartets opens new avenues to use DNA base pairing as a way to form nanoscale surface architecture and biocompatible patterned surfaces particularly via host-guest complexation that might be suitable for drug design, where the target can be trapped inside the cavities of the molecular containers.     

Nonlinear viscoelasticity of sorbitan tristearate monolayers at liquid/gas interface

The interfacial rheology of sorbitan tristearate monolayers formed at the liquid/air interface reveal a distinct nonlinear viscoelastic behavior under oscillatory shear usually observed in many 3D metastable complex fluids with large structural relaxation times. At large strain amplitudes (gamma), the storage modulus (G') decreases monotonically whereas the loss modulus (G') exhibits a peak above a critical strain amplitude before it decreases at higher strain amplitudes. The power law decay exponents of G' and G' are in the ratio 2:1. The peak in G' is absent at high temperatures and low concentration of sorbitan tristearate. Strain-rate frequency sweep measurements on the monolayers do indicate a strain-rate dependence on the structural relaxation time. The present study on sorbitan tristearate monolayers clearly indicates that the nonlinear viscoelastic behavior in 2D Langmuir monolayers is more general and exhibits many of the features observed in 3D complex fluids.     

Molecular patterning at a liquid/solid interface: the foldamer approach

Molecular patterning has received a lot of attention in the past decade; however, the functionalization of these surface-confined 2D patterns on the nanoscale level remains a challenge. Assembling 2D patterns from oligomeric foldamers turns out to be an interesting approach to accomplishing the controlled positioning of functional elements. We designed a family of peptidomimetic foldamers bearing a 2D turn element folding at the liquid/solid interface. The turning element was developed while studying derivatives with one turning unit. Furthermore, folding was found to be induced by the confinement of the surface. This achievement paves the way for the design of foldamers with multiple turns, providing a higher versatility in the functionalization of nanopatterns.     

Ellipsometric characterization of ethylene oxide-butylene oxide diblock copolymer adsorption at the air-water interface

Ellipsometry was used to determine the adsorbed layer thickness (d) and the surface excess (adsorbed amount, Gamma) of a nonionic diblock copolymer, E(106)B(16), of poly(ethylene oxide) (E) and poly(butylene oxide) (B) at the air-water interface. The results were obtained (i) by the conventional ellipsometric evaluation procedure using the change of both ellipsometric angles Psi and Delta and (ii) by using the change of Delta only and assuming values of the layer thickness. It was demonstrated that the calculated surface excesses from the different methods were in close agreement, independent of the evaluation procedure, with a plateau adsorption of about 2.5 mg/m(2) (400 A(2)/molecule). Furthermore, the amount of E(106)B(16) adsorbed at the air-water interface was found to be almost identical to that adsorbed from aqueous solution onto a hydrophobic solid surface. In addition, the possibility to use combined measurements with H(2)O or D(2)O as substrates to calculate values of d and Gamma was investigated and discussed. We also briefly discuss within which limits the Gibbs equation can be used to determine the surface excess of polydisperse block copolymers.     

Switchable self-assembly of a bioinspired alkyl catechol at a solid/liquid interface: competitive interfacial, noncovalent, and solvent interactions

The large tendency of catechol rings to adsorb on surfaces has been studied by STM experiments with molecular resolution combined with molecular-dynamics simulations. The strong adhesion is due to interactions with the surface and solvent effects. Moreover, the thermodynamic control over the differential adsorption of 1 and the nonanoic solvent molecules has been used to induce a new temperature-induced switchable interconversion. Two different phases that differ in their crystal packing and the presence of solvent molecules coexist upon an increase or decrease in the temperature. These results open new insight into the behavior of catechol molecules on surfaces and 2D molecular suprastructures.     

Characterizing the pH-responsive behavior of thin films of diblock copolymer micelles at the silica/aqueous solution interface

The pH-responsive behavior of cationic diblock poly(2-(dimethylamino)ethyl methacrylate)-block-poly(2-(diethylamino)ethyl methacrylate) copolymer micelles adsorbed at the silica/aqueous solution interface has been characterized. The micellar morphology of this copolymer, initially adsorbed at pH 9, can be dramatically altered by lowering the solution pH. The original micelle-like morphology of the adsorbed copolymer chains at pH 9 completely disappears as the pH is decreased to 4, and a brush-like layer structure is produced. This change results from protonation of the copolymer chains: the subsequent electrostatic repulsions within the film drive the copolymer chains to expand into the aqueous phase. Returning the solution pH from 4 to 9 causes this brush-like layer to collapse, with atomic force microscopy images suggesting degradation of the film. Hence, the pH-responsive behavior of the copolymer film exhibits irreversible morphological changes. Measurements of the adsorbed/desorbed amounts of the copolymer film were conducted using both a quartz crystal microbalance with dissipation monitoring (QCM-D) and optical reflectometry (OR). After an initial rinse at both pH values, the OR adsorbed mass becomes almost constant during subsequent pH cycling, whereas the corresponding QCM-D adsorbed mass changes significantly but reversibly in response to the solution pH. Since the QCM-D measures a bound mass that moves in tandem with the surface, the discrepancy with the OR data is due to changes in the amount of bound water in the copolymer film as a result of the pH-induced changes in surface morphology. The larger effective mass observed at pH 4 suggests that the brush-like layer contains much more entrapped water than the micellar films at pH 9. The pH dependence of the contact angle of the adsorbed film is consistent with the changes observed using the other techniques, regardless of whether the solution pH is altered in situ or the aqueous solution is completely replaced. In fact, comparison of these two approaches provides direct evidence of the exposure of adsorbed micelle core blocks to the solution during pH cycling and the concomitant impact upon all the other measurements.     

Tuning molecular orientation with STM at the solid/liquid interface

Triptycene molecular orientation has been tuned with a STM tip at a Cu(111) surface in solution from flat, to tilt, to vertical. The tuning is completely bias dependent and reversible. The study is important in the fields of nanoscience and technology.     

Molecular interactions at the solid liquid interface with special reference to flotation and solid particle stabilized emulsions

Colloid and Polymer Science - The experiments carried out with a variety of pure surface active agents, employed in flotation of ores as either collectors or frothers, show that (a) long chain...     

Adsorption and thermodynamics of biosurfactant,surfactin, monolayers at the air-buffered liquid interface

Adsorption of surfactin, a powerful lipopeptide biosurfactant, at the air-liquid interface has been investigated in this article. The adsorption took place from buffered solutions containing relatively high concentrations of surfactin co- and counterions. Dynamic surface tension measurements were used to follow the self-assembly of surfactin at the interface until equilibrium surface pressure Π ^e is reached at a given surfactin concentration (C _s). Gibbs adsorption equation in conjunction with the Langmuir adsorption isotherm was used to predict surfactin surface excess as a function of the biosurfactant concentration up to the critical micelle concentration (CMC). The predicted surface excess at saturation (Γ _∞ ) is 1.05?±?0.05 μmol m^?2, corresponding to an area per molecule (A _∞ ) of 159?±?8 Å². The adsorption equilibrium constant (K?=?(1.5?±?0.6)?×?10⁶ M^‐?1) was also estimated from the nonlinear regression of Π ^e???C _s data in region B of the Π ^e???ln?C _s plot. The value of K suggests that surfactin has strong affinity for the interface, which is in line with its known high surface activity. Gibbs elasticity (E _G) of the interfacial surfactin monolayers, which is an important thermodynamic property, was also predicted at different surfactin concentrations. The limiting value (at the CMC) of E _G was found to be 183 mN m^?1, which is comparable to those reported in the literature for similar systems. The findings reported in this work reveal that the surface tension measurements coupled with appropriate theoretical analysis could provide useful information comparable to those obtained using highly sophisticated techniques.