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Surface nanostructures are increasingly more employed for controlled protein assembly on functional nanodevices, in nanobiotechnology, and in nanobiomaterials. However, the mechanism and dynamics of how nanostructures induce order in the adsorbed protein assemblies are still enigmatic. Here, we use single-molecule mapping by accumulated probe trajectories and complementary atomic force microscopy to shed light on the dynamic of in situ assembly of human plasma fibrinogen (HPF) adsorbed on nanostructured polybutene-1 (PB-1) and nanostructured polyethylene (PE) surfaces. We found a distinct lateral heterogeneity of HPF-polymer nanostructure interface (surface occupancy, residence time, and diffusion coefficient) that allow identifying the interplay between protein topographical nanoconfinement, protein diffusion mechanism, and ordered protein self-assembly. The protein diffusion analysis revealed high-diffusion polarization without correlation to the anisotropic friction characteristic of the polymer surfaces. This suggests that HPF molecules confined on the nanosized PB-1 needle crystals and PE shish-kebab crystals, respectively, undergo partial detachment and diffuse via a Sansetsukon-like nanocrawling mechanism. This mechanism is based on the intrinsic flexibility of HPF in the coiled-coil regions. We conclude that nanostructured surfaces that encourage this characteristic surface mobility are more likely to lead to the formation of ordered protein assemblies and may be useful for advanced nanobiomaterials.

Single-molecule Förster Resonance Energy Transfer (FRET) was used to study the dynamic association of mobile donor-labeled ssDNA oligonucleotides ("target") with covalently immobilized complementary acceptor-labeled ssDNA oligonucleotides ("probe"). While probe-target association events were resolved for all experiments, such FRET events were far more likely to occur in systems with complementarity and on hydrophobic, as compared to hydrophilic, surfaces. The distribution of donor-acceptor association-time intervals did not exhibit simple first-order kinetics, and when decomposed into a superposition of first-order processes, only a small fraction of events corresponded to a long-lived state that was presumed to represent true DNA hybridization, while the majority of association events were transient, representing nonspecific associations or partial hybridization. The structure of the DNA target and probe affected both the stability of the hybridized state, as well as the likelihood that an association between the two led to hybridization. In particular, the likelihood of hybridization decreased for longer target strands and for targets with stem-loop secondary structure. The presence of oligonucleotide secondary structure reduced the stability of hybridization, while greater complementarity increased stability of the hybridized state. Interestingly, increased ionic strength (i.e., greater electrostatic screening) increased the probability of hybridization but did not influence the lifetime of the hybridized state. Combined, these observations provide a nuanced view of surface-mediated DNA hybridization, where various factors independently influence the probability and stability of hybridization.

Using high throughput single-molecule total internal reflection fluorescence microscopy (TIRFM), we have acquired molecular trajectories of bovine serum albumin (BSA) and hen egg white lysozyme during protein layer formation at the silicone oil-water interface. These trajectories were analyzed to determine the distribution of molecular diffusion coefficients, and for signatures of molecular crowding/caging, including subdiffusive motion and temporal anticorrelation of the instantaneous velocity vector. The evolution of these properties with aging time of the interface was compared with dynamic interfacial tension measurements. For both lysozyme and BSA, we observed an overall slowing of protein objects, the onset of both subdiffusive and anticorrelated motion (associated with crowding), and a decrease in the interfacial tension with aging time. For lysozyme, all of these phenomena occurred virtually simultaneously, consistent with a homogeneous model of layer formation that involves gradual crowding of weakly interacting proteins. For BSA, however, the slowing occurred first, followed by the signatures of crowding/caging, followed by a decrease in interfacial tension, consistent with a heterogeneous model of layer formation involving the formation of protein clusters. The application of microrheological methods to single molecule trajectories described here provides an unprecedented level of mechanistic interpretation of interfacial events that occurred over a wide range of interfacial protein coverage.

Self-assembled monolayers (SAMs) were prepared from a novel two-tailed partially fluorinated thiol (F8C11/C16), possessing one hydrocarbon chain and one chain with an extended fluorinated segment, and from mixtures of F8C11/C16 and hexadecanethiol (C16) on gold, with the expectation that the internal chemical dissimilarity and wedge-like shape of F8C11/C16 would lead to unique self-organizational motifs. The SAMs were systematically characterized using ellipsometry, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), contact angle goniometry, and polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS). Based on this characterization, the one-component F8C11/C16 SAMs exhibited relatively poor molecular organization compared to traditional alkanethiols, forming low coverage monolayers with significant molecular disorder. However, the series of mixed SAMs formed from F8C11 and F8C11/C16 were anomalously well ordered as indicated by film thickness, surface coverage, and the frequencies of characteristic vibrational modes. AFM images of these mixed SAMs exhibited nanoscale fibrillar structures in a birds-nest morphology, suggesting that in the presence of a C16 matrix, the F8C11/C16 component organized into the two-dimensional analogue of discrete bilayers. Control experiments involving mixed SAMs comprised of F8C11/C16 and a single-tailed partially fluorinated thiol (F8C11) or C16 and F8C11 exhibited no appreciable indication of interesting self-organization beyond an evenly dispersed mixing of the thiolates or phase separation, respectively.

Custom-designed surfactants, known as "linactants", have the ability to reduce the line tension between coexisting phases within mixed monolayers of chemically dissimilar compounds at the air-water interface. Thus far, linactants have been successfully identified for only one type of chemical dissimilarity, involving mixed monolayers of hydrocarbon and fluorocarbon surfactants. In the present work, we have pursued a more general interpretation of linactant compounds by extending the concept to a new system that is comprised of a mixture of aliphatic (pentadecanoic acid) and aromatic (p-terphenyl carboxylic acid) compounds. We found that the "bare" line tension between phases of this mixed monolayer was ~4 pN, and within the same order of magnitude as our previous measurement in mixed monolayers containing hydrocarbons and fluorocarbons. Furthermore, we examined a homologous series of potential linactant compounds possessing an aliphatic tail of variable length and a p-terphenyl block. We determined that linactants with longer tails were able to reduce the line tension more efficiently and effectively. In particular, the addition of only 0.14% of a linactant with an 11-carbon chain reduced the line tension by more than a factor of 2. We hypothesize that the efficiency of this particular linactant is associated with its long tail; this creates strong van der Waals interactions with the aliphatic chains and enables the tail to adopt conformations that facilitate π-stacking interactions with the aromatic compounds within the monolayer.

The interactions between adsorbate molecules and hydrophobic surfaces are of significant interest due to their importance in a variety of biological and separation processes. However, it is challenging to extrapolate macroscopic ensemble-averaged force measurements to molecular-level phenomena. Using total internal reflection fluorescence microscopy to image individual molecules at hydrophobic solid-aqueous interfaces, we directly observed dynamic behavior associated with the interactions between fluorescently labeled dodecanoic acid (our probe molecules) and self-assembled monolayers (SAM) comprising n-alkyltriethoxysilanes with systematically increasing chain length (from n = 4-18). In all cases, we observed at least two characteristic surface residence times and two diffusive modes, suggesting the presence of multiple distinct adsorbed populations. In general, the mean surface residence time increased and the mobility decreased with increasing SAM chain length, consistent with stronger probe-surface interactions. However, these trends were not primarily due to changes in characteristic residence times or diffusion coefficients associated with the individual populations but rather to a dramatic increase in the fraction associated with the long-lived slow-moving population(s) on long-chain SAMs. In particular, on longer (16-18 carbon) alkylsilane monolayers, the probe molecule exhibited far fewer desorption-mediated "flights" than on short (4-6 carbon) monolayers. Additionally, probes on the longer chain surfaces were much more likely to exhibit extended surface residence times as opposed to short transient surface visits.

Recent work shows that coating a supported palladium catalyst with a self-assembled monolayer (SAM) of alkanethiols can dramatically improve selectivity in the hydrogenation of 1-epoxy-3-butene (EpB) to 1-epoxybutane. Here, we present the results of surface-level investigations of the adsorption of EpB and related molecules on SAM-coated Pd(111), with an aim of identifying mechanistic explanations for the observed catalytic behavior. Alkanethiol SAM-covered Pd(111) surfaces were prepared by conventional techniques and transferred to ultrahigh vacuum, where they were characterized using Auger electron spectroscopy (AES) and temperature-programmed desorption (TPD) of EpB and other probe molecules. Whereas previous studies have shown that EpB undergoes rapid decomposition via epoxide ring opening on uncoated Pd(111), TPD studies show that EpB does not undergo substantial ring opening on SAM-covered surfaces but rather desorbs intact at temperatures less than 300 K. Systematic comparisons of EpB desorption spectra to spectra for other C(4) oxygenates suggest that the SAM creates a kinetic barrier to epoxide ring-opening reactions that does not exist on the uncoated surface. The EpB desorption spectra as a function of exposure show behavior similar to the desorption of olefins from Pd(111), indicating that the binding of the olefin functionality, in contrast to that of the epoxide ring, is not significantly perturbed. EpB desorption spectra from surfaces with less well-ordered SAMs show the presence of weakly bound states not observed on well-ordered SAM surfaces. The lower activity observed on catalysts covered with less well-ordered SAMs is hypothesized to occur due to partial confinement of adsorbates into these weakly bound, less active states.

Single-molecule total internal reflection fluorescence microscopy was used to obtain real-time images of fluorescently labeled hexadecanoic (palmitic) acid molecules as they adsorbed at the interface between fused silica and three different solvents: hexadecane (HD), tetrahydrofuran (THF), and water. These solvents were chosen to explore the effect of solvent polarity on the activation energy associated with the attachment rate, i.e., the rate at which molecules were transferred to the surface from the near-surface layer. Direct counting of single-molecule events, made under steady-state conditions at extremely low coverage, provided direct, model-independent measurements of this attachment rate, in contrast with conventional ensemble-averaged methods, which are influenced by bulk transport and competing detachment processes. We found that the attachment rate increased with increasing temperature for all solvents. Arrhenius analyses gave activation energies of 5+/-2 kJ/mol for adsorption from HD, 10+/-2 kJ/mol for adsorption from THF, and 19+/-2 kJ/mol for adsorption from water. These energies increased systematically with the solvent polarity and, therefore, with the expected strength of the solvent-substrate interaction. We hypothesize that the adsorption of amphiphilic solute molecules from solution can be regarded as a competitive exchange between solute molecules and surface-bound solvent. In this scenario, adsorption is an activated process, and the activation energy for attachment is associated with the solvent-substrate interaction energy.

The structure and conformation of self-assembled monolayers (SAMs) derived from the adsorption of a specifically designed double-chained partially fluorinated thiol having the formula 12,12,13,13,14,14,15,15,16,16,17,17,18,18,19,19,19-heptadecafluoro-2-tetradecylnona-decane-1-thiol (2) onto the surface of evaporated gold were examined by ellipsometry, contact angle goniometry, polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS), and X-ray photoelectron spectroscopy (XPS). The results were compared to those of SAMs generated from normal hexadecanethiol (1) and a structurally related single-chained partially fluorinated thiol having the formula 12,12,13,13,14,14,15,15,16,16,17,17,18,18,19,19,19-heptadecafluorononadecane-1-thiol ( 3). Collectively, the studies demonstrate that the double-chained adsorbate 2 forms SAMs on gold in which the alkyl chains are less densely packed and less conformationally ordered than those in the SAMs derived from each of the single-chained adsorbates. Furthermore, the fluorocarbon moieties in the SAMs derived from 2 are more tilted from the surface normal than those in the SAMs derived from 3. The low values of contact angle hysteresis suggest, however, that the double-chained adsorbate 2 generates homogeneous monolayer films on the surface of gold.

Alkylsilane self-assembled monolayers (SAMs) on oxide substrates are commonly used as liquid crystal (LC) alignment layers. We have studied the effects of alkyl chain length, photolytic degradation, and mechanical rubbing on polar and azimuthal LC anchoring. Both gradient surfaces (fabricated using photolytic degradation of C18 SAMs) and unirradiated SAMs composed of short alkyl chains show abrupt transitions from homeotropic to tilted alignment as a function of degradation or chain length. In both cases, the transition from homeotropic to tilted anchoring corresponds to increasing wettability of the SAM surfaces. However, there is an offset in the critical contact angle for the transition on gradient vs unirradiated SAMs, suggesting that layer thickness is more relevant than wettability for LC alignment. Mechanical rubbing can induce azimuthal alignment along the rubbing direction for alignment layers sufficiently near the homeotropic-to-planar transition. Notably, mechanical rubbing causes a small but significant shift in the homeotropic-to-tilted transition, e.g., unrubbed C5 SAMs induce homeotropic anchoring, but the same surface after rubbing induces LC pretilt.

Individual fatty acid molecules adsorbed at the interface between hexadecane and fused silica have been tracked using total internal reflection fluorescence microscopy. Two cooperative diffusive mechanisms are observed: continuous small-scale Brownian motion and occasional large "jumps." The continuous diffusion exhibits evidence of confinement. The effective interfacial diffusion coefficients for each mechanism increase systematically with temperature; an Arrhenius analysis gives an activation barrier of approximately 50 kJ/mol for "jumping" and an upper limit of approximately 10 kJ/mol for confined diffusion.

An amphiphilic photoisomerizable macrocycle has been prepared that forms stable Langmuir monolayers at the air-water interface. The hydrophilic core of the molecule switches between closed and open isomers upon irradiation by the appropriate wavelengths of light. Isotherm measurements, Brewster angle microscope images, and atomic force micrographs (of transferred Langmuir-Blodgett films) suggest a phase transition between a face-on to a tilted edge-on molecular orientation as a function of surface concentration. In the face-on phase, in situ photoisomerization results in a reversible increase in surface pressure due to greater molecular crowding in the open configuration.

A method is presented for the measurement of contact angles of particles in the size range 100-1000 microm, using an optical microscope. This method is used to characterize the wettability of polyethylene particles produced in a gas-phase polymerization process. The as-polymerized particles are shown to be significantly more hydrophobic than melt-processed polyethylene materials. The surface structure of the particles is examined with scanning electron microscopy, and the increased hydrophobicity of the as-polymerized particles is attributed to the nanoscale texture of the particle surfaces.

We have studied the anchoring of the nematic liquid crystal 5CB (4'-n-pentyl-4-cyanobiphenyl) as a function of the surface wettability, thickness of the liquid crystal layer, and temperature by measuring the birefringence of a hybrid aligned nematic cell where the nematic material was confined between octadecyltriethoxysilane-treated glass surfaces, with one surface linearly varying in its hydrophobicity. A homeotropic-to-tilted anchoring transition was observed as a function of the lateral distance along the hydrophobicity gradient, typically in a region corresponding to a water contact angle of approximately 64 degrees. The effect of the nematic layer thickness was measured simultaneously by preparing a wedge cell where the thickness varied along the direction perpendicular to the wettability. The detailed behavior of the onset of birefringence was found to be consistent with a dual-easy-axis model that predicts a discontinuous anchoring transition from homeotropic to planar. The anchoring was independent of temperature, except within 1 degrees C of the nematic-to-isotropic transition temperature (T(NI)). As the temperature approached T(NI), the tendency for planar anchoring gradually increased relative to that for homeotropic anchoring.

We report the nanoscale structural changes associated with the interfacial gelation of adsorbed beta-casein layers as a function of aging time. Adsorbed layers were transferred to solid supports and imaged by atomic force microscopy. The aging of the layer was accompanied by the formation of distinct disk-shaped protein nanoparticles ( approximately 20 nm in diameter). Under conditions where a gelled layer was expected (from previous interfacial rheology experiments), we observed ordering of the particles and the formation of elongated aggregates or linear rows. Brewster angle microscopy images were also obtained during the adsorption and gelation processes and during the degradation of the protein layer following addition of the surfactant sodium dodecyl sulfate (SDS). If SDS was added prior to interfacial protein gelation, the layer developed a foamlike morphology consistent with a fluid interfacial protein layer. However, if SDS was added after gelation, the protein layer was observed to fracture, consistent with the behavior of a solid phase.

Self-assembled monolayer growth of octadecanoic acid on single-crystal C-plane (0001) and R-plane (1102) sapphire (α-Al2O3) has been investigated by ex situ tapping mode atomic force microscopy, contact angle measurements, and Fourier transform infrared spectroscopy. Partial monolayers, interrupted during growth, contain islands of a densely packed well-ordered phase within a lower-lying disordered molecular matrix. The areal fraction of the ordered phase increases as a function of immersion time. The growth kinetics are sensitive to the history of the sapphire substrate; e.g., complete monolayers were formed from 1.5 mM hexadecane solution in ∼1 min on freshly annealed (i.e., dehydrated) substrates in comparison with ∼1 h on substrates aged for 1 day under ambient conditions. The growth is slightly faster on C-sapphire than R-sapphire. Although weakly bound, the complete monolayers are very well-ordered, conformal, and essentially defect-free.

A magnetic rod interfacial shear rheometer was used to measure the properties of β-casein adsorbed at the air/solution interface as a function of aging time. Over a wide range of concentrations (1 × 10-6 to 2 × 10-2 wt % β-casein) the initial rheology of the adsorbed surface layer is dominated by a viscous response of the interface. For solutions in the range 5 × 10-3 to 2 × 10-2 wt %, interfacial gelation is observed after ∼15 h of aging, long after the surface tension has stabilized. In particular, although both components of the complex interfacial shear modulus (i.e., the storage and loss moduli) gradually increase with aging time, the ratio of the loss to the storage modulusthe loss tangentdecreases and drops below unity. The frequency dependence of the shear modulus is consistent with sol−gel transitions observed in bulk systems and described within the context of percolation theory.

Grazing incidence X-ray diffraction (GIXD) and Brewster angle microscopy (BAM) were used to study the miscibility and phase behavior of Langmuir monolayers composed of a mixture of a saturated (stearic) and a trans-monounsaturated (elaidic) fatty acid. In contrast with suggestions from previous thermodynamic measurements, these compounds were poorly miscible in monolayers, and phase separation was always observed between domains of a liquid crystalline stearic acid-rich phase and a disordered elaidic acid-rich phase. The molecular packing density inside the ordered domains of mixed monolayers was within 2% of the density of pure stearic acid monolayers at the same surface pressure, suggesting that this phase contained at most a very small fraction of the larger elaidic acid molecules. However, the presence of this small concentration of unsaturated chains depressed the L2 to Ov phase transition by ∼7 mN/m.

We have built a magnetic needle viscometer capable of measuring the surface shear viscosity of Langmuir monolayers at constant surface pressure or molecular area. A Langmuir trough with dual symmetrical compression/expansion barriers is positioned between two electromagnetic coils that create a homogeneous magnetic field gradient. The gradient drives a Teflon-encapsulated magnetic needle floating on the monolayer along the surface to a terminal velocity. A channel created by two glass plates centers the needle and minimizes drift. The ratio of monolayer surface shear viscosity at a given composition, temperature, surface pressure, and so forth to a reference surface shear viscosity is determined by taking the appropriate ratio of the limiting velocities of the magnetic needle for a given applied force. The monolayer viscometer was calibrated using thin films of silicone oil of known viscosity. We demonstrate the utility of the viscometer by showing that the shear viscosity of saturated dipalmitoylphosphat...

Brewster angle microscopy was used to study the microrheological response of fatty acid Langmuir monolayers subjected to shear flow. Slippage of domains past each other was observed during shear, and elastic recovery of domain shape was monitored following the cessation of shear. Experiments were performed as a function of shear rate, total strain, and temperature in two tilted hexatic monolayer phases, L2 and Ov. Three regimes were found as a function of temperature. At low temperature (T = 17 °C), the degree of slippage and recovery was shear rate independent. At high temperature (T ≥ 25 °C), the recovery decreased and the slippage increased systematically with increasing shear rate. At an intermediate temperature (T = 21 °C), a distinct transition from low-temperature to high-temperature behavior was observed at a shear rate of 0.35 s-1. These regimes correlated precisely to previous observations of parabolic or triangular velocity profiles in monolayer channel flow. Taking all temperatures and shear r...

We have observed the surface pressure-driven flow of Langmuir monolayers through a narrow channel by means of Brewster angle microscopy (BAM). The monolayers, composed of long chain carboxylic acids, were studied in a “hexatic” liquid crystalline mesophase (L2 phase). The velocity profile across the channel demonstrated transient behavior; i.e., it evolved from the expected parabolic shape to triangular as a function of flow rate, location along the channel, and time elapsed after start-up of flow. These transient effects appear to be related to the total strain in the system and suggest the importance of elasticity in the monolayer flow response. For situations in which the velocity profile was triangular, the distribution of domain widths across the channel indicated that the degree of domain stretching was systematically less than expected from the amount of strain the monolayer had experienced. This implied that elasticity of domain boundaries and/or slippage between domains resists domain stretching ...

We have observed the dissolution of a self-assembled monolayer of octadecylphosphonic acid from a mica surface into organic solvent in real time using atomic force microscopy. Holes in the monolayer are observed to nucleate, grow, and percolate across the sample, leaving isolated monolayer islands that gradually decrease in size. The relative rates of hole growth and hole nucleation suggest that removing a molecule from the monolayer/hole boundary is about 5 × 104 times more likely than removing a molecule from within a continuous region of monolayer. The rate of dissolution is increased by flowing solvent through the cell compared to stagnant solvent and increased even further by rapid stirring using the AFM tip. The coverage kinetics can be quantitatively described by a model that incorporates desorption from “hole” regions and diffusive solution-phase transport through a stagnant layer of finite thickness. The coverage kinetics and hole nucleation and growth rates are consistent with a picture of monol...

Dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylglycerol (DMPG) mixed monolayers at the air-water interface, with and without 5 mM calcium in the subphases, were investigated by pressure-area measurements and simultaneous fluorescence microscopy. Condensed-phase domains were observed over both types of subphases and quantified by analyzing the microscope images. The domains' shape changed and area increased with the mole fraction of DMPG, and hysteresis effects in the compression-decompression cycles were more prevalent on the PG-rich side of the phase diagram. The origin of hysteresis and other nonequilibrium effects in monolayers is discussed, and the equilibrium pressure-composition phase diagram for this binary system is presented which shows a measured liquidus curve and calculated solidus curve. The overall effect of calcium is to facilitate formation of condensed-phase domains.

The time-dependent growth of octadecyltrimethylammonium bromide (C18TAB) monolayers at mica surfaces from aqueous solution, at a temperature below the Krafft point, is investigated with transmission IR spectroscopy, wetting measurements, and atomic force microscopy (AFM). Under these deposition conditions, C18TAB adsorbs predominately as the monomer rather than as an aggregated solution species resulting in monolayer films. Formation of the C18TAB monolayer is characterized by two distinct growth regimes. A rapid initial adsorption is observed which results in a 2-D liquid phase comprised of randomly oriented alkyl chains. When the films are allowed to assemble for times longer than 24 h, distinct, densely packed, monolayer high islands of C18TA+ molecules are observed (2-D solid). As the assembly time is increased further, the relative surface coverage of the semicrystalline islands increases substantially. In this growth regime, the mica surface is covered by a two-phase film comprised of the 2-D liquid...

We have studied the thermal melting of two disk-shaped molecules at the air−water interface. Brewster angle microscopy images of both compounds indicated first order phase transitions from a low-temperature solid phase to a high-temperature fluid phase. The high-temperature phase could be either isotropic or nematic. Grazing-incidence diffraction measurements provided corroborating evidence of reversible melting, in the form of Bragg peaks that vanished at high temperature and reappeared upon cooling. Furthermore, we observed that the diffraction peaks obtained upon recooling were substantially sharper than those obtained initially, and the domain sizes seen in Brewster angle microscopy were substantially larger, indicating an enhancement of crystalline domain size by thermal annealing.

The surface coverage of an adsorbed monolayer of octadecylphosphonic acid on mica was varied via solution concentration and immersion time, thereby controlling the surface free energy. For low surface coverage, a stable wetting layer of deposition solution was formed. For slightly greater coverage, the substrate was initially wetted by a thin liquid layer which quickly ruptured, leaving the sample dry. For large coverage, the liquid layer receded from the sample edge. Atomic force microscopy and video microscopy showed that the intermediate regime corresponds to dewetting via nucleation and growth of holes (0.25−5 μm in diameter) in the liquid layer while the dewetting at large coverage proceeds from the sample edge. The dependence of the wetting mode upon surface coverage and solution concentration was explained in terms of a competition between two rates: the nucleation rate of holes in the liquid layer and the speed at which the three-phase line advances.

Langmuir−Blodgett monolayers, deposited from a Langmuir monolayer (on the water surface) in a two-dimensional liquid (LE) state, were examined using atomic force microscopy (AFM), transmission infr...

In 1937 Katherine Blodgett demonstrated that immersingcertainLangmuir-Blodgett (LB) films inbenzene for short periods of time “skeletonized” the films.1 She determined, using optical interference methods on optically-thick multilayers, that the refractive index of the skeletonized films was reduced but the film thickness remained approximately equal to that of the original film. She hypothesized that the skeletonization removed some molecules, resulting in gaps throughout the filmsbut that the overall film retained its integrity. Since the films successfully skeletonized are prepared at a pH lower than necessary for full conversion of free acid to soap, it is reasonable to assume that themolecules removed by dissolution in benzene correspond to the free acid and that the molecules converted to soap remain in the film. The kinetics of this skeletonization process and the optical constants of the resulting films have been previously studied.1-5 However, the details of themixing of acid and soap molecules and, hence, the structure of the skeletonized film have remained a mystery.

Self-assembled monolayers are formed by exposing freshly cleaved mica to a solution of octadecylphosphonic acid in tetrahydrofuran. Atomic force microscope images of samples immersed in solution for varying exposure times show that prior to forming a complete monolayer the molecules aggregate into dense islands (1.8 ± 0.2 nm high) on the surface. The islands have a compact, rounded morphology. The cosine of the contact angle between hexadecane and the partial monolayers has an approximately linear dependence on coverage. However, the cosine of the contact angle of water decreases linearly to about 50% coverage (the percolation threshold of the phosphonate islands) and then appears to saturate.