Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session Y19: Surfaces, Interfaces, and Colloids III |
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Sponsoring Units: DCP Chair: Nancy Levinger, Colorado State University Room: Colorado Convention Center 104 |
Friday, March 9, 2007 11:15AM - 11:27AM |
Y19.00001: Surfactant Adsorption at the Air-Liquid and Hydrophobic Solid-Liquid Interfaces: Unraveling the Mechanism for Superspreading Makonnen Payne, Charles Maldarelli, Alexander Couzis In this paper we report our findings with regard to the synergistic interactions between polyethylene oxide surfactants of the general structure CiEj and compare the behavior to a known super wetting surfactant. Pendant drop tensiometry experiments and sessile drop contact angle measurements on hydrophobic surfaces were conducted on combinations of CiEj surfactants with 1-dodecanol. We found that a number of combinations were capable of reducing significantly the air-liquid tension, however only systems that exhibited the propensity to form extended liquid crystalline phases, as shown by the combination of cross-polarized microscopy, cryo-TEM, and light scattering experiments, were able to improve on the wetting performance of the these systems. We have also conducted the parallel experiment focused on the surfactant adsorption at the hydrophobic solid-liquid interface. Using in-situ infrared internal reflection spectroscopy and complimentary sum-frequency generation spectroscopy, we are able to dynamically interrogate the surfactant adsorption kinetics and interfacial water structure evolution at the hydrophobic solid-liquid interface. We will relate these findings to gain insight into the molecular requirements for superspreading. [Preview Abstract] |
Friday, March 9, 2007 11:27AM - 11:39AM |
Y19.00002: Interrogating the strength of the bond between salt and water: a combined DFT and MP2 study Bo Li, Angelos Michaelides, Matthias Scheffler The interaction of water with salt (NaCl) is of widespread importance and of considerable general interest. Although numerous theoretical studies have been reported [1], none has provided a convincingly reliable estimate of the strength of the bond between water and a flat NaCl surface, such as NaCl(001). Moreover, the computed adsorption energies predicted by DFT vary from 0.2-0.7 eV/H$_{2}$O depending on the choice of exchange-correlation functional [2]. Here, we address this issue through an extensive series of periodic Hartree-Fock and post-Hartree Fock [M{\o}ller-Plesset perturbation (MP2) and coupled cluster (CCSD(T))] calculations. Periodic Hartree-Fock calculations have been performed for H$_{2}$O molecules adsorbed on NaCl slabs, and by evaluating the local dependence of the correlation contribution to the adsorption energy with respect to cluster size, we obtain accurate MP2 and CCSD(T) estimates of the H$_{2}$O adsorption energy on NaCl(001). Our computed adsorption energy, which is around 0.6 eV/H$_{2}$O, comes close to the experimental value [3] and is at the upper end of the range predicted by DFT. [1] A. Verdaguer, G. M. Sacha, H. Bluhm, and M. Salmeron, Chem. Rev. 106, 1478 (2006). [2] B. Li, A. Michaelides, and M. Scheffler, in preparation. [3] L. W. Bruch, A. Glebov, J. P. Toennies, and H. Weiss, J. Chem. Phys. 103, 5109 (1995). [Preview Abstract] |
Friday, March 9, 2007 11:39AM - 11:51AM |
Y19.00003: Cluster Morphology and Aggregation Kinetics in Dense Aerosols Rajan Dhaubhadel, Amitabha Chakrabarti, Christopher Sorensen We studied the cluster morphology and kinetics of an aggregating aerosol system using the small angle light scattering technique. Exploding a mixture of a hydrocarbon gas and oxygen in closed chamber generated a system of aggregating soot particles. The soot particles started as individual monomers, ca. 38 nm radius, grew to bigger clusters with time and finally stopped evolving after spanning a network across the whole system volume. The gelled clusters showed a hybrid morphology with a lower fractal dimension at length scales of a micron or smaller and a higher fractal dimension at length scales greater than a micron. The study of the kinetics of the aggregating system showed that the system gelled when the homogeneity \textit{$\lambda $} attained a value 0.4 or higher. The aggregation kernel $K$ appearing in SE was also determined using the light scattering data.. The observed data indicated a slight increase in $K$ value when the system was denser. [Preview Abstract] |
Friday, March 9, 2007 11:51AM - 12:03PM |
Y19.00004: Adsorption of Water, Methanol and Toluene on the Surface of Soot Particulate Matter Mohsen Yeganeh, Shawn Dougal, Bernard Silbernagel, El-Mekki El-Malki Soot particles are byproducts of incomplete hydrocarbon combustion. The adsorption of water and organic molecules on the soot surfaces is of technological and environmental importance. We have applied a newly developed technique of SFG spectroscopy for high surface area materials [1] to investigate the adsorption of water, methanol, and toluene on the surface of standard soot particles from the National Institute of Standards and Technology (NIST). Adaptations of standard SFG procedures are required to compensate for the high refractive index of these carbonaceous materials. We find that adsorption of water, methanol, and toluene on the soot surface is reversible at room temperature. We also find that UV radiation modifies the surface composition of these soot particles. The effect of UV radiation on the adsorption, as well as the kinetics and thermodynamics of methanol adsorption/desorption will also be discussed. [1] Mohsen S. Yeganeh, Shawn M. Dougal, and Bernard G. Silbernagel, Langmuir \textbf{22}, 637 (2006) [Preview Abstract] |
Friday, March 9, 2007 12:03PM - 12:15PM |
Y19.00005: Drops Bouncing on a Superhydrophobic Surface Frank van Swol Low solid interfacial energy and fractally rough surface topography confer to Lotus plants superhydrophobic (SH) properties like high contact angles, rolling and bouncing of liquid droplets, and self-cleaning of particle contaminants. By exploiting the porous fractal structure of a novel, synthetic SH surface it is possible to produce a synthetic lotus effect. This is relevant to realizing self-cleaning properties for particle concentration, and the slippery nature of the coating can be used to enhance the performance of fluidic devices. Here we report on molecular dynamics (MD) and some classical density functional theory calculations that provide valuable insight into the conditions needed to cause liquid droplets to form and bounce on a surface. The MD results report on the details of the droplets impacting surfaces of varying wettability, ranging from complete wetting to complete drying. For the SH surfaces we present results for the contact time, between droplet and surface, as a function of impact velocity, droplet size and surface friction. [Preview Abstract] |
Friday, March 9, 2007 12:15PM - 12:27PM |
Y19.00006: Pair Interaction of microparticles at oil-water interfaces Chuan Zeng, Anthony Dinsmore We confined microparticles at fluid-fluid interfaces and measured the interaction between them. Aggregates of colloidal particles were observed, suggesting an attractive capillary force at long range, which cannot be explained by gravity. We report measurements of the interaction between carboxylate-modified polystyrene spheres (radius $\sim $ 1 micron) at the oil-water interface using image analysis and particle tracking. The interaction between two isolated spheres was measured from particle trajectories and analyzed through the Markovian Dynamics Extrapolation method developed by J.C. Crocker and D.G. Grier. Different choices of oil (1,1,1-trifluoroheptan, silicone oil, decahydronapthalene, etc.) and various sample geometries were explored. The role of surfactants will be discussed. We acknowledge support from NASA through the Fluid Physics program (NRA 02-OBPR-03-C). [Preview Abstract] |
Friday, March 9, 2007 12:27PM - 12:39PM |
Y19.00007: Selective crystal nucleation at a soft organic template. Sumit Kewalramani, Kyungil Kim, Guennadi Evmenenko, Pulak Dutta Langmuir monolayers floating on supersaturated aqueous solutions can guide the growth of oriented inorganic crystals.$^{1}$ Although, oriented crystal growth under such monolayers has been observed for a variety of inorganic species, barium fluoride and strontium fluoride are the only systems where an epitaxial match between the organic and inorganic lattices is directly observed.$^{2}$ We have studied different growth stages of this model system by varying the subphase conditions. We find that, in the earliest nucleation stages, strained epitaxial thin films of barium fluoride and barium fluoride chloride can be grown under the same organic structures. We will also discuss late crystal growth stages, specifically orientation/misorientation effects in thick films, which are governed by reorganization of preformed crystals. 1. S. Mann, Biomineralization Principles and Concepts in Bioinorganic Materials Chemistry, Oxford University Press, Oxford, 2001. 2. J. Kmetko et al. Phys. Rev. Lett. \textbf{89}, 186102-1 (2002). [Preview Abstract] |
Friday, March 9, 2007 12:39PM - 12:51PM |
Y19.00008: A Light Scattering Study of Solvent Solidification in Colloidal Suspensions Melissa Spannuth, Simon Mochrie, John Wettlaufer Intermolecular forces between a solid and a foreign substrate can lead to the formation of a thin film of liquid between the two surfaces at temperatures below the solid's bulk melting temperature. These forces also result in fluid flow towards colder regions of the interface when a temperature gradient is present. When the fluid component of a colloidal suspension solidifies, the particles become encased within the ice both individually and in clusters. Near the melting temperature, a premelted layer bathes the particles, providing a restricted, but finite volume of liquid that facilitates Brownian motion through the background solid. We will present the results of our X-ray Photon Correlation Spectroscopy experiments by which we can characterize the dynamics of the particles in a solidifying colloidal suspension and thereby extract information about the amount of premelted liquid. [Preview Abstract] |
Friday, March 9, 2007 12:51PM - 1:03PM |
Y19.00009: Ion-size effect at the surface of silica hydrosol Aleksey Tikhonov The forces of electrical imaging strongly polarize the surface of colloidal silica. I used X-ray scattering to study the ion-size effect in the adsorbed 2-nm-thick compact layer of alkali ions at the surface of concentrated solutions of 5-nm, 7-nm, and 22-nm particles, stabilized either by NaOH or a mixture of NaOH and CsOH, with the total bulk concentration of alkali ions ranging from 0.1- to 0.7-mol/L. The observed structure of the compact layer is almost independent of the size of the particles and concentration of alkali base in the sol; it can be described by a two-layer model, i.e., a $\sim $ 6 - 8-Angstrom-thick layer of directly adsorbed hydrated alkali ions with a surface concentration $\sim 3\times 10^{18}$ m$^{-2}$, and a $\sim $ 13-Angstrom-thick layer with a surface concentration of sodium ions $\sim 8\times 10^{18}$ m$^{-2}$. In cesium-enriched sols, Cs$^{+}$ ions preferentially adsorb in the first layer replacing Na$^{+}$; their density in the second layer does not depend on the presence of cesium in the sol. The difference in the adsorption of Cs$^{+}$ and Na$^{+}$ ions can be explained by the ion-size-dependent term in the electrostatic Gibbs energy equation derived earlier by others. Brookhaven National Laboratory is supported by U.S.D.O.E., Contract No. DEAC0298CH10886. X19C is partially supported through funding from the ChemMatCARS National Synchrotron Resource and the University of Chicago. [Preview Abstract] |
Friday, March 9, 2007 1:03PM - 1:15PM |
Y19.00010: Steric effects on the dynamics of electrolytes Mustafa Sabri Kilic, Martin Bazant, Armand Ajdari The classical Poisson-Boltzmann (PB) theory of electrolytes assumes a dilute solution of point charges with mean-field electrostatics. Even for very dilute solutions, however, it predicts absurdly large ion concentrations (exceeding close packing) for surface potentials of only a few tenths of a volt. In this talk, we adopt a simple model for steric effects with only one parameter (the effective ion size), and we analyze the charging of a thin double layer, which must form a condensed layer of close-packed ions near the surface at high voltage. A surprising prediction is that the differential capacitance varies non-monotonically with the applied voltage, and thus so does the response time of an electrolytic system. This effect is able to predict the reversal of AC electro-osmotic flow at high voltage and high frequency, as well as the increase of capacitance with temperature in molten salts. [Preview Abstract] |
Friday, March 9, 2007 1:15PM - 1:27PM |
Y19.00011: Transport of colloids in porous medium. Hsiang-Ku Lin, Roya Zandi, Leonid P. Pryadko Pathogenic microorganisms such as bacteria and viruses in groundwater cause over one million illnesses per year in the United States. Despite the considerable research, the transport of microorganisms (colloids) in porous media is not well understood. In the reported work, we present a phenomenological filtration model that describes transport of colloids and the dynamics of colloid deposition and release at the attachment sites. The model has a soliton-like solution for the filtering front separating ``clean'' anterior and ``dirty'' posterior regions. The computed breakthrough curves and time-dependent deposition curves are in good agreement with experimental measurements. [Preview Abstract] |
Friday, March 9, 2007 1:27PM - 1:39PM |
Y19.00012: Charge Inversion by Electrostatic Complexation: Molecular Dynamics Simulations Jordi Faraudo, Alex Travesset Ions near interfaces play an important role in many biological and physico-chemical processes and exhibit a fascinating diverse range of phenomena. A relevant example is charge inversion, where interfacial charges attract counterions in excess of their own nominal charge, thus leading to an inversion of the sign of the interfacial charge. In this work, we argue that in the case of amphiphilic interfaces, charge inversion can be generated by {\em complexation}, that is, electrostatic complexes containing several counterions bound to amphiphilic molecules. The formation of these complexes require the presence at the interface of groups with conformational degrees of freedom with many electronegative atoms. We illustrate this mechanism by analyzing all atomic molecular dynamics simulations of a DMPA (Dimirystoil-Phosphatidic acid) phospholipid monolayer in contact with divalent counterions. The results are found to be in agreement with recent experimental results on Langmuir monolayers. We also discuss the implications for biological systems, as Phosphatidic acid is emerging as a key signaling phospholipid. [Preview Abstract] |
Friday, March 9, 2007 1:39PM - 1:51PM |
Y19.00013: Microscale Curveballs: Translational and Rotational Coupling of Colloids Stephen Anthony, Minsu Kim, Steve Granick Optically anisotropic MOON particles (modulated optical nanoparticles) allow the simultaneous measurement of translation and rotation for individual particles. Through chemical modification, these particles can be made hydrophilic, amphiphilic, or hydrophobic. Among those three, the boundary condition exhibits varying amounts of slip, which registers in the value of the rotational diffusion constant. Additionally, for the amphiphilic case, the translational and rotational motions are no longer independent of each other, exhibiting coupling due to the asymmetric hydrodynamic drag. Understanding these processes is fundamental to particle dynamics, with implications to kinetically limited processes such as the self-assembly of multi-unit proteins. Addressing the influence of these varied boundary conditions, this study presents single-particle tracking of micron-sized spherical colloids suspended in deionized water, tracked and quantified using home-written algorithms. [Preview Abstract] |
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