The effect of solvent quality on the slip flow penetration of polymer films was evaluated by monitoring small-molecule mass transport under varying laminar flow rates using Förster resonance energy transfer in combination with total internal reflectance fluorescence microscopy (FRET-TIRFM). For thin films of poly(N-isopropylacrylamide) (pNIPAM), solvents with solvent quality ranging from good to poor were studied. The solvents used were composed of varying mole ratios of methanol and water in order to take advantage of the unique cononsolvency phenomenon of pNIPAM such that differences in the physicochemical properties of these solvents were insignificant for fluorescence detection. FRET quenching of a donor fluorophore covalently tethered on the substrate surface at the bottom of the pNIPAM film by a solution-confined acceptor was monitored as a function of time. Quenching curves were fit to a combined Taylor–Aris–Fickian mass transport model for the acceptor, rhodamine B (RhB) or 2-nitrobenzylaclohol (2-NBA), allowing apparent diffusion coefficients to be determined and used to assess slip flow penetration into the polymer film. An increase in the apparent diffusion coefficient of tracer molecules was observed with increasing laminar flow rate for all solvents, indicating that mass transport processes in the pNIPAM film are significantly perturbed by laminar slip flow penetration. In going from poor solvents, 31 mol % MeOH/H2O and 20 mol % MeOH/H2O, to the theta solvent, 13 mol % MeOH/H2O, and finally to a good solvent, 100% methanol, the slip length increases from 25 to 37 to 70 to 128 nm, with the corresponding percentage of the film penetrated by slip flow increasing from 19 to 27 to 42 to 57%, respectively. The apparent diffusion coefficients of the two acceptors, RhB and 2-NBA, which differ substantially in size and charge, in pNIPAM films under identical conditions were found to be of the same order of magnitude, albeit with a small difference (∼10%) due to inherently different diffusive properties. Therefore, the dominant mechanism for the mass transport of small molecules in densely grafted thin pNIPAM brush films is suggested to be linear Fickian diffusion under the chosen laminar flow conditions with linear flow velocities ranging from 192 to 2952 μm/s. High-quality fits to a Taylor–Aris–Fickian diffusion model of the experimental breakthrough curves obtained with both acceptor molecules further substantiate the proper use of this model and the validity of the FRET-TIRFM method.

The evolution of solution aggregates of the anionic form of the native monorhamnolipid (mRL) mixture produced by Pseudomonas aeruginosa ATCC 9027 is explored at pH 8.0 using both experimental and computational approaches. Experiments utilizing surface tension measurements, dynamic light scattering, and both steady-state and time-resolved fluorescence spectroscopy reveal solution aggregation properties. All-atom molecular dynamics simulations on self-assemblies of the most abundant monorhamnolipid molecule, l-rhamnosyl-β-hydroxydecanoyl-β-hydroxydecanoate (Rha-C10-C10), in its anionic state explore the formation of aggregates and the role of hydrogen bonding, substantiating the experimental results. At pH 8.0, at concentrations above the critical aggregation concentration of 201 μM but below ∼7.5 mM, small premicelles exist in solution; above ∼7.5 mM, micelles with hydrodynamic radii of ∼2.5 nm dominate, although two discrete populations of larger lamellar aggregates (hydrodynamic radii of ∼10 and 90 nm) are also present in solution in much smaller number densities. The critical aggregation number for the micelles is determined to be ∼26 monomers/micelle using fluorescence quenching measurements, with micelles gradually increasing in size with monorhamnolipid concentration. Molecular dynamics simulations on systems with between 10 and 100 molecules of Rha-C10-C10 indicate the presence of stable premicelles of seven monomers with the most prevalent micelle being ∼25 monomers and relatively spherical. A range of slightly larger micelles of comparable stability can also exist that become increasing elliptical with increasing monomer number. Intermolecular hydrogen bonding is shown to play a significant role in stabilization of these aggregates. In total, the computational results are in excellent agreement with the experimental results.

The potential of glycolipid surfactants, composed of a sugar headgroup and lipid tail, as highly biodegradable and less toxic alternatives to commonly used surfactants motivates the systematic study of structure–function relationships of various glycolipid surfactants. Advances in the efficient synthesis of high purity amphipathic glycolipid surfactants enable the analysis of a diverse set of glycolipids. The solution phase properties of two suites of glycolipid surfactants, n-alkyl-O-melibiosides and n-alkyl-O-cellobiosides, with varying straight-chain alkyl tails of 8, 10 and 12 carbons are investigated in this work. This study substantiates their efficient surfactant performance and reveals their aggregate structure and microenvironment as it relates to molecular structure. Surface tensiometry demonstrates critical micelle concentrations of 0.2–40 mM with minimum surface tension values of 36–40 mN m−1. Time resolved fluorescence quenching (TRFQ) spectroscopy is used to calculate micelle aggregation number (12–70 molecules per micelle) and assess relative micropolarity and microfluidity within the aggregates. The average aggregation numbers determined by TRFQ and apparent hydrodynamic radii determined by dynamic light scattering vary with alkyl chain length and headgroup size, but these properties are relatively insensitive to changes in concentration. The aggregates form nearly spherical, tightly-packed micelles with a small population of vesicles and polydispersity present at low concentrations or with short alkyl chain length.

Polymer–fluid interfaces are used widely in a variety of applications, including separations, which require exposure of the polymer to dynamic flow conditions. Despite the ubiquity of such interfaces, the importance of convective mass transport within the near-interface region of a polymer is a fundamental process that is still poorly defined. As a step toward better defining mass transport behavior within the near-interface portion of a polymer, in this work, a new application of a spectroscopic method based on the combination of Förster resonance energy transfer (FRET) and total internal reflectance fluorescence microscopy (TIRFM) is reported that allows quantification of the penetration depth of a laminar flow field (i.e., the slip length) in a densely grafted, thin poly(N-isopropylacrylamide) (pNIPAM) film as a model polymer system. Specifically, decay curves from FRET of an acceptor with a donor attached at the substrate surface are fit to a combined Taylor–Aris–Fickian mass transport model to extract apparent linear diffusion coefficients of acceptor molecules for different flow rates. Apparent diffusion coefficients range from 1.9 × 10–12 to 9.1 × 10–12 cm2/s for near-surface flow linear velocities ranging from 192 to 2952 μm/s. This increase in apparent diffusion coefficient with fluid flow rate suggests increasing contributions from convective mass transport that are indicative of flow field penetration into the polymer film. The depth of penetration of the flow field is estimated to range from ∼6% of the polymer film thickness in a good solvent at ∼192 μm/s to ∼60% of the film thickness at ∼2952 μm/s. Thus, flow field penetration into polymer thin films, with its concomitant contributions from convective mass transport within the near-interface region of the polymer, is demonstrated and quantified experimentally.

Self-assembled monolayers (SAMs) of dipolar phosphonic acids can tailor the interface between organic semiconductors and transparent conductive oxides. When used in optoelectronic devices such as organic light emitting diodes and solar cells, these SAMs can increase current density and photovoltaic performance. The molecular ordering and conformation adopted by the SAMs determine properties such as work function and wettability at these critical interfaces. We combine angle-dependent near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) to determine the molecular orientations of a model phenylphosphonic acid on indium zinc oxide, and correlate the resulting values with density functional theory (DFT). We find that the SAMs are surprisingly well-oriented, with the phenyl ring adopting a well-defined tilt angle of 12–16° from the surface normal. We find quantitative agreement between the two experimental techniques and density functional theory calculations. These results not only provide a detailed picture of the molecular structure of a technologically important class of SAMs, but also resolve a long-standing ambiguity regarding the vibrational-mode assignments for phosphonic acids on oxide surfaces, thus improving the utility of PM-IRRAS for future studies.