Hydrophobic-induced surface reorganization: molecular dynamics simulations of water nanodroplets on perfluorocarbon self-assembled monolayers (original) (raw)
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Structure and Depletion at Fluorocarbon and Hydrocarbon/Water Liquid/Liquid Interfaces
Physical Review Letters, 2008
The results of x-ray reflectivity studies of two oil/water (liquid/liquid) interfaces are inconsistent with recent predictions of the presence of a vapor-like depletion region at hydrophobic/aqueous interfaces. One of the oils, perfluorohexane, is a fluorocarbon whose super-hydrophobic interface with water provides a stringent test for the presence of a depletion layer. The other oil, heptane, is a hydrocarbon and, therefore, is more relevant to the study of biomolecular hydrophobicity. These results are consistent with the sub-angstrom proximity of water to soft hydrophobic materials. PACS Numbers: 82.70.Uv, 68.05.-n, 61.05.cm Kaoru et al., "Structure and Depletion…" 2
Langmuir, 2005
The local structure of water near hydrophobic surfaces of different surface topographies has been analyzed by molecular dynamics simulation. An alkane crystal has been taken as the parent model for a hydrophobic surface. Surface structures were created by placing pits into it, which were half a nanometer deep and several nanometers wide. Around all structures, the water has a lower density, less orientational ordering, fewer water-water hydrogen bonds, and fewer surface contacts than for a flat unstructured surface. This indicates that the structured surfaces are more hydrophobic than the flat surface. Of the structures investigated, pits with a diameter of approximately 2.5 nm were effective in increasing the hydrophobic character of the surface.
Characterization and Direct Force Measurements of Fluorocarbon monolayer Surfaces
We have prepared hydrophobic surfaces by silylating fused silica in three different ways. Heptadecafluoro1,1,2,2,tetrahydrodecyltriethoxysilane (FTE) was either (i) spread on the airwater interface, allowed to polymerise and then deposited as an LB film (at surface pressures of 10, 20 and 35 mN/m designated LB10, LB20 and LB35) or (ii) allowed to react with the silica surface in a CHCl 3 solution (FTE/CHCl 3 ), or (iii) allowed to react with the silica in the undiluted liquid state (FTE/Neat). The surfaces thus prepared were scanned by atomic force microscopy; their chemical composition analysed by Xray photoelectron spectroscopy; wettability studies with water were performed; and adhesion or pulloff forces between two such surfaces in humid air and water were determined. The FTE/Neat surface was significantly less stable and less hydrophobic than the other surfaces, although AFM scans indicated comparable smoothness. Considerable amounts of excess material could be removed from this surface by rinsing with ethanol or water. The FTE/CHCl 3 surfaces and the LB10 surfaces were the smoothest, with a mean roughness of 0.14 nm, whereas LB20 ≈ and LB35 were rougher and showed randomly distributed bulges protruding 2.5 3 nm above the surfaces. All surfaces appeared amorphous and the coverage was similar (90100%) for all LB surfaces, but lower for FTE/CHCl 3 (80%), which also showed some loss on rinsing.
Three-phase contact force equilibrium of liquid drops at hydrophilic and superhydrophobic surfaces
Journal of Colloid and Interface Science, 2013
Hydrophilic and superhydrophobic surfaces were obtained by modifying a dendritic Au surface with carboxyl-or methyl-terminated self-assembled monolayers, respectively. The hydrophilic surface generates capillary forces which induce water flow through the grooves defined by the dendritic structure, resulting in a halo structure; the superhydrophobic surface on the other hand completely repels water drops. The contrasting behaviors exhibited by the two surfaces with nominally identical morphology but different surface chemistry are explained semi-quantitatively in terms of the equilibrium of surface forces developing at the three-phase (air-solid-water) contact lines.
A liquid interaction with ultrahydrophobic surfaces
EPJ Web of Conferences, 2016
The interaction of the liquid with ultra-hydrophobic surfaces was so far studied through estimation of static contact angles. It appears now that this interaction is more complex, and cannot be described only with static methods. Effect of ultra-hydrophobic surfaces and their advantages are also particularly in dynamic interaction with liquids. One of the parameters that determine the character of the dynamic interaction is presence of air film close to the surface. The thickness of air film can be measured with long distance microscopy and the interaction with the flow using micro PIV method. Here we present the results of measurements of the air film that is created close to ultra-hydrophobic surfaces and the dependence of its thickness on the Re number.
The state of fluorocarbon surfactant monolayers at the air-water interface and on mica surfaces
Journal of Colloid and Interface Science, 1990
Monolayers of a cationic, double-chained fluorocarbon surfactant (N-( a-trimethylammonioacetyl)-O-O'-bis( IH, 1H, 2H, 2H-perfluorodecyl)-L-glutamate chloride) were studied at the air-water interface and adsorbed on the muscovite mica basal plane. The stability of the monolayer at the air-water interface was considerably higher on a 10-2 M KBr subphase than on pure water. Fluorocarbon surfactant-coated mica surfaces were obtained either by Langmuir-Blodgett (LB) deposition or by adsorption from a polar solvent. The adsorbed layer was characterized by means of ESCA, transfer ratio and contact angle measurements. A more densely packed layer could be obtained by LB deposition than by adsorption from solution. For this surfactant the preferential conformation of the molecule in layers adsorbed from solution is different from that in LB layers. At low adsorption densities the advancing contact angle increases rapidly with the adsorbed amount. The receding angle is hardly affected until the area per molecule is less than about 140 ~2. At higher adsorption densities, the receding contact angle is more sensitive to the adsorbed amount than the advancing angle. The reason for the low salt stability, the low adhesion force in water, and the numerous cavities forming around the contact region when surfaces coated by this fluorocarbon surfactant are brought together in water is proposed to be due to (molecular size) heterogeneities in the surfactant layer,
Langmuir, 2011
Semifluorinated alkanes (C n F 2n+1 C m H 2m+1 , FnHm for short) are molecules that associate two antagonist moieties in the same chain: a hydrocarbon block and a fluorocarbon block. 1À4 The former block exhibits a hydrophobic character and substantial conformational freedom because of the low-energy barrier of trans/gauche interchanges (typically 1.2 k B T). 5 The latter block adopts a helical and more rigid configuration with a higher energy barrier to the rotation of carbonÀcarbon bonds (typically 1.8 k B T) 6 because of the bulkiness of the fluorine atom. Moreover, fluorinated chains are both hydrophobic and lipophobic and consequently tend to segregate when mixed with hydrocarbon chains. 7 Also, the cross sections of the two blocks are different: 0.18 nm 2 for the hydrocarbon chain and 0.28 nm 2 for the fluorocarbon chain. Because of this combination of antagonisms, semifluorinated alkanes exhibit self-assembling properties, alone 4,9 or in mixtures with other compounds, in two or three dimensions. The bulk crystalline phases of FnHm seem to depend strongly on the chain length (both n and m) leading to layered structures for which parallel, antiparallel, interdigited, and other structures have been proposed. 12 At higher temperatures, some FnHm exhibit mesophases with smectic ordering. 4,13À16 The primitive surfactant nature of FnHm diblocks is revealed by their capacity to form micelles in both fluorocarbons and hydrocarbons 17,18 and to induce gelification. 3 In aqueous solutions, they stabilize phospholipid vesicles, 19 fluorocarbon-in-water emulsions, 20 and hydrocarbon-in-fluorocarbon emulsions. 2 The synthesis and behavior of FnHm diblocks has recently been reviewed. 4 Pure FnHm can also exhibit surface freezing as observed for F12Hm (m = 8, 14, and 19). 21 FnHm monolayers have been the subject of many studies both at liquid and solid surfaces using different preparation and characterization techniques. Indeed, it is known since Gaines' pioneering work that, despite the absence of a polar headgroup, FnHm can form stable Langmuir monolayers. 22 Following these first experiments, many papers have dealt with their interfacial properties. Various authors have studied by surface pressure measurement the behavior on the surface of water of several FnHm (n = 3À12 and m = 6À20) and have shown that most of these molecules form monolayers at the airÀwater interface and have uncovered more complex behavior. 10,19,23À25 Because of their hydrophobic character, FnHm Langmuir monolayers segregate vertically, spontaneously or upon compression, when mixed with other amphiphilic compounds, such ABSTRACT: We have determined the structure formed at the airÀwater interface by semifluorinated alkanes (C 8 F 17 C m H 2m+1 diblocks, F8Hm for short) for different lengths of the molecule (m = 14, 16, 18, 20) by using surface pressure versus area per molecule isotherms, Brewster angle microscopy (BAM), and grazing incidence x-ray experiments (GISAXS and GIXD). The behavior of the monolayers of diblocks under compression is mainly characterized by a phase transition from a low-density phase to a condensed phase. The nonzero surface pressure phase is crystalline and exhibits two hexagonal lattices at two different scales: a long-range-order lattice of a few tens of nanometers lateral parameter and a molecular array of about 0.6 nm parameter. The extent of this organization is sufficiently large to impact larger scale behavior. Analysis of the various compressibilities evidences the presence of non organized molecules in the monolayer for all 2D pressures. At room temperature, the self-assembled structure appears generic for all the F8Hm investigated.
Multiscale Effect of Hierarchical Self-Assembled Nanostructures on Superhydrophobic Surface
Langmuir, 2014
In this work, we describe self-assembled surfaces with a peculiar multiscale organization, from the nanoscale to the microscale, exhibiting the Cassie−Baxter wetting regime with extremely low water adhesion: floating drops regime with roll-off angles < 5°. These surfaces comprise bundles of hierarchical, quasi-one-dimensional (1D) TiO 2 nanostructures functionalized with a fluorinated molecule (PFNA). While the hierarchical nanostructures are the result of a gas-phase self-assembly process, their bundles are the result of the capillary forces acting between them when the PFNA solvent evaporates. Nanometric features are found to influence the hydrophobic behavior of the surface, which is enhanced by the micrometric structures up to the achievement of the superhydrophobic Cassie−Baxter state (contact angle (CA) ≫ 150°). Thanks to their high total and diffuse transmittance and their self-cleaning properties, these surfaces could be interesting for several applications such as smart windows and photovoltaics where light management and surface cleanliness play a crucial role. Moreover, the multiscale analysis performed in this work contributes to the understanding of the basic mechanisms behind extreme wetting behaviors.
Molecular dynamics simulation of water condensation on surfaces with tunable wettability
2020
Water condensation plays a major role in a wide range of industrial applications. Over the past few years, many studies have shown interest in designing surfaces with enhanced water condensation and removal properties. It is well known that heterogeneous nucleation outperforms homogeneous nucleation in the condensation process. Because heterogeneous nucleation initiates on a surface at a small scale, it is highly desirable to characterize water-surface interactions at the molecular level. Molecular dynamics (MD) simulations can provide direct insight into heterogeneous nucleation and advance surface designs. Existing MD simulations of water condensation on surfaces were conducted by tuning the solid-water van der Waals interaction energy as a substitute for modeling surfaces with different wettabilities. However, this approach cannot reflect the real intermolecular interactions between the surface and water molecules. Here, we report MD simulations of water condensation on realistic surfaces of alkanethiol self-assembled monolayers with different head group chemistries. We show that decreasing surface hydrophobicity significantly increases the electrostatic forces between water molecules and the surface, thus increasing the water condensation rate. We observe a strong correlation between our rate of condensation results and the results from other surface characterization metrics, such as the interfacial thermal conductance, contact angle, and the molecular-scale wettability metric of Garde and co-workers. This work provides insight into the water condensation process at the molecular scale on surfaces with tunable wettability.