Session N18: Surfaces, Interfaces, and Colloids I

Interfacial interactions of alkane and ether molecules tethered to mesoporous MCM-41 using quasielastic neutron scattering

The motion of 1,3-diphenylpropane (DPP, $\equiv $Si-O-C$_{6}$H$_{4}$(CH$_{2})_{3}$C$_{6}$H$_{5})$ and phenethyl phenyl ether (PPE, $\equiv $Si-O-C$_{6}$H$_{4}$(CH$_{2})_{2}$OC$_{6}$H$_{5}$ and $\equiv $Si-O-C$_{6}$H$_{4}$O(CH$_{2})_{2}$C$_{6}$H$_{5})$ tethered to the interior pore surface of MCM-41 silica was investigated using quasielastic neutron scattering. Measurements of the elastic intensity were carried out in the temperature range 50-380 K to probe the changes in dynamics between DPP and PPE as well as investigate the role of ether oxygen location in the PPE isomers. Full quasielastic scans over an energy range of $\pm $ 17 $\mu $eV were performed at temperatures of 240 K, 280 K, and 320 K. Analysis of the quasielastic data elicits the role that temperature has on the motion of these molecules tethered within the pores. The two PPE isomers exhibit much different dynamics illustrating the importance of hydrogen bonding to the silica surface.   

Thermodynamic Investigation of thin n-Heptane films adsorbed on Magnesium Oxide (100) surfaces

The thermodynamic properties of thin films of \textit{n-Heptane} on MgO (100) were measured using high resolution adsorption isotherms between 205 K and 275 K. Heats of adsorption were derived for first and second layer adsorbed on substrate and yield 42.38 $\pm $ 0.98 kJ/mol and 40.58 $\pm $ 0.46 kJ/mol, respectively. The isothermal compressibility was determined as a function of temperature and used to identify two possible phase transitions at 246.2 $\pm $ 1.5 K for the first and at 250.7 $\pm $ 0.8 K for the second layer respectively. These results will be compared to previous thermodynamic and neutron diffraction measurements of other shorter chained alkanes.   

Structure of Iron Oxide Water Interface: a combined CTR and DFT study

Chemical reactions at the mineral and water interface play an important role in many natural and technological processes, from controlling the fate and transport of environmental contaminants, and biological availability and geochemical cycling of iron to the electronic device fabrications. The interface structure and composition of a mineral, dictates the chemical interactions that take place between the mineral and its environment, and also play a pivotal role in nanoparticle growth process. Therefore, we have conducted a systematic investigation of the solid-solution interface structure of two of the most common and stable phases of iron oxides systems: hematite ($\alpha $-Fe$_{2}$O$_{3})$ (1-102) and goethite ($\alpha $-FeOOH) (100) -- using the crystal truncation rod (CTR) diffraction technique. The distribution and form of surface hydroxyl groups at the interface are described using combined CTR and \textit{ab initio} density functional theory (DFT) calculations. Goethite (100) interface structure is determined to be a relaxed double hydroxyl termination with the presence of two semi-ordered water layers exposing two types of hydroxyl groups at the surface and the hematite (1-102) interface structures show vacancies in the near surface metal occupancies and different distributions of surface hydroxyl groups.   

Structure and dynamics of fluorinated alkanes on silicon dioxide surfaces

Despite their great promise in various applications, the structure and dynamics of fluorinated alkanes at interfaces is still an open question. In particular, the knowledge from both theoretical and experimental perspectives is very limited when it comes to understanding the interface between these systems and a solid substrate. Molecular dynamics simulations based on the All Atom OPLS model are used to predict the equilibrium structure and dynamics of short fluorinated alkanes on both amorphous and crystalline silicon dioxide surfaces. In order to understand the effect of layer-layer interaction on the ordering of chains in a given layer, the thickness of the liquid film is increased layer-by-layer from monolayer to multilayers. Results for structural and dynamics of the liquid films near the silicon dioxide surfaces will be presented.   

Delayering of Intermediate-Length Alkanes Adsorbed on Solid Surfaces

We have recently discovered that a film of the intermediate-length alkane, dotriacontane ($n$-C$_{32}$H$_{66}$ or C32) does not completely wet SiO$_{2}$ and highly oriented graphite surfaces on a nanometer length scale.\footnote{M. Bai \textit{et al}., cond-mat/0611497.} In a narrow temperature range near the bulk melting point $T_{b}$, we observe a single layer of C32 molecules oriented with their long axis perpendicular to the surface. On heating just above $T_{b}$, these molecules undergo a delayering transition to three-dimensional droplets that remain present up to their evaporation point. Here we report noncontact Atomic Force Microscopy and synchrotron x-ray measurements indicating that a similar delayering transition occurs for films of other intermediate-length alkanes: C24, C25, C30, and C36 deposited from solution onto a SiO$_{2}$ surface. These results raise a number of interesting questions including whether the delayering transition is driven by conformational changes in the molecules and what implications the nonwetting behavior may have for lubricating nanoscale devices.   

Linear surfactant-mediated spreading of nanodroplets: molecular dynamics simulation

We utilized molecular dynamics simulations to probe surfactant-mediated spreading of nanodroplets on a solid surface. We find that the spreading speed is strongly influenced by the attraction of the hydrophobic surfactant tail to the solid surface. When this attraction is sufficiently strong, surfactant molecules partition to the liquid-solid interface and can lead to an inhomogeneous distribution of surfactant over the liquid-vapor interface, which could drive the Marangoni convection. The result also shows that the surfactant molecules can assemble into micelles. The repulsion between micelles leads to break-off and migration of the micelles from the liquid-solid to the gas-solid interface and spreading is facilitated in this way. Our model system contains features that have been connected with superspreading in experimental studies and provides insight into the workings of a successful surfactant. REF: J. Chem. Phys. 125, 174708 (2006).   

State-resolved inelastic and reactive scattering dynamics of gases with liquid surfaces

Energy transfer dynamics between gas and liquid surfaces are investigated by colliding a molecular beam of CO$_{2}$ with low vapor pressure liquids in vacuum. Nascent quantum states of CO$_{2}$ are probed via direct infrared absorption of the $\nu_{3}$ asymmetric stretch with a Pb-salt diode laser. The high spectral resolution ($\sim $20 MHz) of the laser provides the means to characterize the translational, rotational, vibrational, and angular distributions of the scattered CO$_{2}$. Experiments have probed an array of collision energies, incident and final scattering angles, liquids, and surface temperatures. In each case, multi-channel dynamics have been observed and characterized as trapping-desorption (TD) and impulsive scattering (IS). Rotational and translational distributions show considerable excitation above the surface temperature (T$_{S})$, while the vibrational distributions remain colder than T$_{S}$. Similar experiments have probed the HF(v,J) product from reactive scattering of fluorine atoms with a hydrocarbon surface. Both the inelastic and reactive scattering distributions are well-characterized by a two-temperature model where T$_{TD} \quad \sim $ T$_{S}$ and T$_{IS} \quad >$ T$_{S}$.   

Dielectric Response of Thin Surface Water Layer

Recently, we have reported a negative dielectric constant ($\varepsilon $') for various nano-particle assemblies (urea- coated Ba$_{0.8}$Rb$_{0.4}$TiO(C$_{2}$O$_{4}$)$_{2}$ and Al$_2 $O$_3$ {\it et al}). There is a close correlation between the zero-frequency electrical conductivity and the occurrence of the negative $\varepsilon $'. The large surface area of the nano- assemblies and moisture level play an important role. To determine the surface water effect on the negative $\varepsilon $', we carried out different dielectric measurements (time and frequency domain with different bias voltages) for differently prepared surfaces with controlled humidities. We will present our data and analysis for surface water ranging from a few layers to ~100 $\mu$m. The contribution of water decomposition by electrolysis to $\varepsilon $' will also be evaluated.   

Lateral Adhesion Balance (LAB) -- a Novel Surface Characterization Technique

Drop lateral adhesion to a surface and the condition for drop sliding along a surface are key issues in many disciplines including biophysics, environmental science, fluid dynamics and agriculture. Yet, to date, except for the tilt stage method, which is extremely limited in range of forces, there is no systematic experimental instrumentation to measure the forces required for drop sliding. We present a new instrument that uses centrifugal forces to slide any drop along a surface. Beyond extending the range of measurable drop-surface interaction, the instrument enables decoupling of some parameters that are bound to be coupled with the simple tilt stage method. Specifically the tilt stage method has two variables varying at the same time: the lateral and normal forces. This violates a fundamental principle of experimental science which leads to obscure understanding of surface characteristics. The LAB avoids this problem.   

How Water Meets a Hydrophobic Surface: Reluctantly and with Fluctuations

By definition hydrophobic substances hate water. Water placed on a hydrophobic surface will form a drop in order to minimize its contact area. What happens when water is forced into contact with a hydrophobic surface? One theory is that an ultra-thin low- density region forms near the surface. To investigate the existence of this layer, we have employed three surface sensitive techniques, time-resolved phase-modulated ellipsometry, surface plasmon resonance, and X-ray reflectivity. Both ellipsometry and X-ray reflectivity provide strong evidence for the low-density layer and illuminate unexpected temporal behavior.   

Studies of dynamical layering in adsorbed alkane films by molecular dynamics simulations and quasielastic neutron scattering.

From experiments using a surface apparatus it is known that alkane fluids confined between two surfaces exhibit a layered structure at the molecular level. This static layering has motivated us to consider the possibility that the individual molecular layers in fluid alkane films adsorbed on a solid surface also exhibit different dynamical properties. Here we report molecular dynamics (MD) simulations of the diffusive motion in layers of tetracosane molecules ($\rm C_{24}H_{50}$) (C24) adsorbed on graphite in time scales from 1 to 100 ps and from 1 to 4 ns and compare the results with high--energy-- resolution quasielastic neutron scattering spectra that probe motions on these time scales. The MD simulations are set up to answer the questions: a) is interlayer diffusion of C24 molecules significant on these time scales? b) are the diffusive motions in the layers different? and c) what is the nature of the diffusive motions observed in the high--energy--resolution quasielastic neutron scattering experiments using the Disk Chopper Spectrometer (1-100 ps) and the High Flux Backscattering Spectrometer (1-4 ns) at NIST?   

Contact Angle Measurements by AFM on Droplets of Intermediate-Length Alkanes Adsorbed on SiO$_{2}$ Surfaces

We have recently discovered that films of intermediate-length alkanes ($n$-C$_{n}$H$_{2n+2}$; 24 $<$ n $<$ 40) do not completely wet a SiO$_{2}$ surface on a nanometer length scale [2]. In a narrow temperature range near the bulk melting point $T_{b}$, we observe a single layer of molecules oriented with their long axis perpendicular to the surface. On heating just above $T_{b}$, these molecules undergo a delayering transition to three-dimensional droplets that remain present up to their evaporation point. Here we report measurements by noncontact Atomic Force Microscopy of the contact angle of these droplets for a film of hexatriacontane ($n$-C$_{36}$H$_{74}$ or C36). Our preliminary measurements indicate that there is a weak maximum in the contact angle at $\sim \quad T_{b}$ + 3 \r{ }C. Further measurements are planned to investigate whether the weak maximum in the contact angle is consistent with the droplets supporting a surface freezing effect as at the bulk fluid/air interface. $^{2}$M. Bai, K. Knorr, M. J. Simpson, S. Trogisch, H. Taub, S. N. Ehrlich, H. Mo, U. G. Volkmann, F. Y. Hansen, cond-mat/0611497.   

Adsorption and Growth Studies of CF$_{4}$ on CF$_{3}$Cl - Covered HOPG

We have studied the adsorption and growth of CF$_{4}$ on a CF$_{3}$Cl-covered graphite surface from 60 K to 105 K, using infrared reflection absorption spectroscopy (IRAS) supplemented by ellipsometry. For the monolayer liquid phase of CF$_{3}$Cl, the CF$_{4}$ initially mixes/dissolves in the CF$_{3}$Cl layer and then continuously replaces the CF$_{3}$Cl on the surface. However, there remains a trace of CF$_{3}$Cl even after a number of layers of CF$_{4}$ are deposited on the surface. The orientation of the residual CF$_{3}$Cl is different from the orientation of the original CF$_{3}$Cl monolayer. For the monolayer solid phases of CF$_{3}$Cl, the CF$_{4}$ adsorbs on top of the CF$_{3}$Cl layer, with little or no solubility. With two layers of CF$_{3}$Cl on the graphite, CF$_{4}$ displaces one of the layers and adsorbs on top of the remaining layer.