Properties of a water layer on hydrophilic and hydrophobic self-assembled monolayer surfaces: A molecular dynamics study

The microscopic behaviors of a water layer on different hydrophilic and hydrophobic surfaces of well ordered self-assembled monolayers (SAMs) are studied by molecular dynamics simulations. The SAMs consist of 18-carbon alkyl chains bound to a silicon(111) substrate, and the characteristic of its surface is tuned from hydrophobic to hydrophilic by using different terminal functional groups ( CH 3 , COOH). In the simulation, the properties of water membranes adjacent to the surfaces of SAMs were reported by comparing pure water in mobility, structure, and orientational ordering of water molecules. The results suggest that the mobility of water molecules adjacent to hydrophilic surface becomes weaker and the molecules have a better ordering. The distribution of hydrogen bonds indicates that the number of water-water hydrogen bonds per water molecule tends to be lower. However, the mobility of water molecules and distribution of hydrogen bonds of a water membrane in hydropho- bic system are nearly the same as those in pure water system. In addition, hydrogen bonds are mainly formed between the hydroxyl of the COOH group and water molecules in a hydrophilic system, which is helpful in understanding the structure of interfacial water.     

Atomic force microscopy measurement of heterogeneity in bacterial surface hydrophobicity

The structure and physicochemical properties of microbial surfaces at the molecular level determine their adhesion to surfaces and interfaces. Here, we report the use of atomic force microscopy (AFM) to explore the morphology of soft, living cells in aqueous buffer, to map bacterial surface heterogeneities, and to directly correlate the results in the AFM force-distance curves to the macroscopic properties of the microbial surfaces. The surfaces of two bacterial species, Acinetobacter venetianus RAG-1 and Rhodococcus erythropolis 20S-E1-c, showing different macroscopic surface hydrophobicity were probed with chemically functionalized AFM tips, terminating in hydrophobic and hydrophilic groups. All force measurements were obtained in contact mode and made on a location of the bacterium selected from the alternating current mode image. AFM imaging revealed morphological details of the microbial-surface ultrastructures with about 20 nm resolution. The heterogeneous surface morphology was directly correlated with differences in adhesion forces as revealed by retraction force curves and also with the presence of external structures, either pili or capsules, as confirmed by transmission electron microscopy. The AFM force curves for both bacterial species showed differences in the interactions of extracellular structures with hydrophilic and hydrophobic tips. A. venetianus RAG-1 showed an irregular pattern with multiple adhesion peaks suggesting the presence of biopolymers with different lengths on its surface. R. erythropolis 20S-E1-c exhibited long-range attraction forces and single rupture events suggesting a more hydrophobic and smoother surface. The adhesion force measurements indicated a patchy surface distribution of interaction forces for both bacterial species, with the highest forces grouped at one pole of the cell for R. erythropolis 20S-E1-c and a random distribution of adhesion forces in the case of A. venetianus RAG-1. The magnitude of the adhesion forces was proportional to the three-phase contact angle between hexadecane and water on the bacterial surfaces.     

Water Structure and Phase Transition Near a Surface

This article concerns the density and orientational structure of water near uncharged (hydrophobic) and charged (hydrophilic) surfaces and the surface-induced phase transition accompanying a powerful, attractive surface force. Previously reported studies on the water structure using the integral equation theories and computer simulations are critically reviewed, and some controversial aspects are reexamined by additional calculations. Effects of the surface curvature and surface charge density on the structure are analyzed in detail. A new analysis is performed on the phase transition for water containing a trace amount of a hydrophobic component at surfaces. It is shown that the transition occurs even at charged surfaces and the attraction induced between surfaces predominates over the strong Coulombic repulsion at sufficiently small separations. At highly charged surfaces, however, the transition does not occur.     

Friction and adhesion forces of Bacillus thuringiensis spores on planar surfaces in atmospheric systems

The kinetic friction force and the adhesion force of Bacillus thuringiensis spores on planar surfaces in atmospheric systems were studied using atomic force microscopy. The influence of relative humidity (RH) on these forces varied for different surface properties including hydrophobicity, roughness, and surface charge. The friction force of the spore was greater on a rougher surface than on mica, which is atomically flat. As RH increases, the friction force of the spores decreases on mica whereas it increases on rough surfaces. The influence of RH on the interaction forces between hydrophobic surfaces is not as strong as for hydrophilic surfaces. The friction force of the spore is linear to the sum of the adhesion force and normal load on the hydrophobic surface. The poorly defined surface structure of the spore and the adsorption of contaminants from the surrounding atmosphere are believed to cause a discrepancy between the calculated and measured adhesion forces.     

The hydrophobic effect: molecular dynamics simulations of water confined between extended hydrophobic and hydrophilic surfaces

Structural and dynamic properties of water confined between two parallel, extended, either hydrophobic or hydrophilic crystalline surfaces of n-alkane C(36)H(74) or n-alcohol C(35)H(71)OH, are studied by molecular dynamics simulations. Electron density profiles, directly compared with corresponding experimental data from x-ray reflectivity measurements, reveal a uniform weak de-wetting characteristic for the extended hydrophobic surface, while the hydrophilic surface is weakly wetted. These microscopic data are consistent with macroscopic contact angle measurements. Specific water orientation is present at both surfaces. The ordering is characteristically different between the surfaces and of longer range at the hydrophilic surface. Furthermore, the dynamic properties of water are different at the two surfaces and different from the bulk behavior. In particular, at the hydrophobic surface, time-correlation functions reveal that water molecules have characteristic diffusive behavior and orientational ordering due to the lack of hydrogen bonding interactions with the surface. These observations suggest that the altered dynamical properties of water in contact with extended hydrophobic surfaces together with a partial drying of the surfaces are more indicative of the hydrophobic effect than structural ordering, which we suggest to be independent of surface topology.     

Fluorocarbon Surfactant Polymers: Effect of Perfluorocarbon Branch Density on Surface Active Properties

We describe a series of fluorocarbon surfactant polymers designed for modifying fluorocarbon surfaces such as poly(tetrafluoroethylene). Novel fluorocarbon surfactant polymers poly(N-vinyldextranaldonamide-co-N-vinylperfluoroundecanamide), in which hydrophilic dextran oligosaccharides and hydrophobic perfluoroundecanoyl groups were incorporated sequentially onto a poly(vinylamine) backbone, were synthesized and characterized by FT-IR, NMR, and XPS spectroscopy. By adjusting the feed ratio of dextran to fluorocarbon branches, surfactant polymers with different hydrophilic/hydrophobic balances were prepared. The surface activity of the surfactants at the air/water interface was demonstrated by significant reductions in water surface tension. Surfactant adsorption and adhesion at the solid PTFE/aqueous interface were examined under well-defined dynamic flow conditions, using a rotating disk system. The surface activity at the air/water interface and adhesion stability on PTFE under an applied shear stress both increase with increasing density of fluorocarbon branches on the polymer backbone. The results show that stable surfactant adhesion on PTFE can be achieved by adjusting the hydrophilic dextran to hydrophobic fluorocarbon branch ratio.     

特殊浸润性表面的液滴凝结

以铝片为基底, 经电化学腐蚀和沸水处理制备了多级微纳米结构; 通过气相沉积和涂油分别制备了超疏水表面、 疏水超润滑(slippery)表面和亲水slippery表面; 探究了表面不同的特殊浸润性(超亲水、 超疏水、 疏水slippery和亲水slippery)对液滴凝结的影响. 结果表明, 超亲水表面的液滴凝结属于膜状冷凝, 超疏水表面和slippery表面的液滴凝结均属于滴状冷凝. 超疏水表面液滴合并时, 合并的液滴会不定向弹离表面. 疏水slippery表面和亲水slippery表面由于表面浸润性的不同导致液滴成核密度和液滴合并的差异, 亲水slippery表面凝结液滴的最大体积远大于疏水slippery表面凝结液滴的最大体积. 4种表面的雾气收集效率由大到小依次为亲水slippery表面>疏水slippery表面>超亲水表面>超疏水表面.     

Ultrathin poly(N-isopropylacrylamide) grafted layer on polystyrene surfaces for cell adhesion/detachment control

We investigated physicochemical properties of two types of poly(N-isopropylacrylamide) (PIPAAm)-grafted tissue culture polystyrene (TCPS) surfaces, to elucidate the influential factors for thermally regulated cell adhesion and detachment to PIPAAm-grafted surfaces. The two types of PIPAAm-grafted surfaces were prepared by the electron beam polymerization method. Attenuated total reflection Fourier transform infrared spectroscopy revealed that amounts of the grafted polymers were 1.4 +/- 0.1 microg/cm2 for PIPAAm-1.4 and 2.9 +/- 0.1 microg/cm2 for PIPAAm-2.9. Both PIPAAm-grafted surfaces showed hydrophobic/hydrophilic property alterations in response to temperature. However, PIPAAm-1.4 surfaces were more hydrophobic (cos theta = 0.21 at 37 degrees C and cos theta = 0.35 at 20 degrees C) than PIPAAm-2.9 (cos theta = 0.42 at 37 degrees C and cos theta = 0.50 at 20 degrees C) both above and below the PIPAAm's transition temperature. Thicknesses of the grafted PIPAAm layers were estimated to be 15.5 +/- 7.2 nm for PIPAAm-1.4 and 29.5 +/- 8.4 nm for PIPAAm-2.9, by the use of UV excimer laser and atomic force microscope. Bovine carotid artery endothelial cells (ECs) adhere to the surfaces of PIPAAm-1.4 and proliferate to form confluent cell monolayers. The cell monolayers were harvested as single cell sheets by temperature decrease from 37 to 20 degrees C. On the contrary, ECs did not adhere to the surfaces of PIPAAm-2.9. This phenomenon was correlated with an adsorption of cell adhesion protein, fibronectin, onto surfaces ofPIPAAm-1.4 and -2.9. In the case of nano-ordered thin grafted surfaces, the surface chain mobility is strongly influenced by the thickness of PIPAAm grafted layers because dehydration of PIPAAm chains should be enhanced by the hydrophobic TCPS surfaces. PIPAAm graft amounts, that is, thickness of the PIPAAm grafted layers, play a crucial role in temperature-induced hydrophilic/hydrophobic property alterations and cell adhesion/detachment behavior.     

分子动力学模拟多巴在自组装膜上的黏附性

为了更好地理解贻贝在表面的黏附机理,实现水下胶黏,采用分子动力学方法研究了多巴在自组装膜上的黏附性:采用伞形取样和加权柱状图分析方法计算了多巴在不同自组装膜表面的黏附自由能,使用拉伸分子动力学模拟研究了多巴在不同自组装膜表面上黏附后的脱附力.结果表明,多巴在带负电的羧基自组装膜上的黏附能比在带正电的氨基自组装膜上的大,多巴更容易黏附到带负电表面;多巴在带电表面的黏附能比未带电表面的黏附能更强,表明在带电表面黏附更稳定.进一步分析了多巴在不同表面的取向分布,发现多巴与不同表面相互作用的方式不同:与疏水表面主要通过苯环相互作用;与亲水表面主要通过羟基相互作用;与负电表面主要通过氨基相互作用;与正电表面主要通过羧基相互作用.通过模拟比较了多巴在不同自组装膜上的脱附力,发现多巴在带电表面的脱附力比在未带电表面的大,与黏附能的趋势一致.对比4种非带电表面的脱附力,发现多巴在疏水性甲基自组装膜表面的脱附力最大,黏附更稳定,随着表面疏水性的增加,脱附力增大,黏附稳定性增强.本工作可为研发新型水下胶黏剂提供理论指导.     

Assembly of nanoparticles at the contact line of a drying droplet under the influence of a dipped tip

The manipulation of colloidal nanoparticles (NPs) in a drying droplet has critical importance not only for several industrial applications but also their assembly into patterns on surfaces. The influence of a tip with hydrophilic or hydrophobic surfaces dipped into a drying droplet on hydrophilic or hydrophobic surfaces on the behavior of 98 nm latex NPs was investigated. The formation of concentric rings on hydrophilic glass surfaces regardless of the surface chemistry of the dipped tip was observed. On the other hand, no pattern formation on hydrophobic surfaces was observed with the insertion of the tip. With a hydrophilic tip, the concentric rings were formed due to stick-slip motion of the solvent contact line resulting from competition between pinning and capillary forces while the capillary effect was not effective until the surface of the tip was changed by adherent NPs making the tip surface available for water adherence with a hydrophobic tip, which results in the pulling of droplet towards the tip. It is also found that the tip thickness and suspension concentration significantly influences the formation of concentric rings on surfaces. This simple procedure can be used to influence the distribution or assembly of NPs in the droplet area.     

Non-DLVO silica interaction forces in NMP-water mixtures. II. An asymmetric system

The interaction between energetically asymmetric hydrophilic and hydrophobic surfaces has fundamental and practical importance in both industrial and natural colloidal systems. The interaction forces between a hydrophilic silica sphere and a silanated, hydrophobic glass plate in N-methyl-2-pyrrolidone (NMP)-water binary mixtures were measured using atomic force microscopy (AFM). A strong and long-range attractive force was observed in pure water and was attributed to the formation of capillary bridges associated with nanoscale bubbles initially present on the hydrophobic surface. When NMP was added, the capillary force and corresponding pull-off force became less attractive, which was explained readily in terms of the surface wettability by the binary solvent mixture. Similar to the case of symmetric (two hydrophilic) surfaces, the range of attraction between the asymmetric surfaces was maximized at around 30 vol % NMP, which is consistent with the formation of a thick adsorbed macrocluster layer on the hydrophilic silica surface.     

Bond-strengthening in staphylococcal adhesion to hydrophilic and hydrophobic surfaces using atomic force microscopy

Time-dependent bacterial adhesion forces of four strains of Staphylococcus epidermidis to hydrophobic and hydrophilic surfaces were investigated. Initial adhesion forces differed significantly between the two surfaces and hovered around -0.4 nN. No unambiguous effect of substratum surface hydrophobicity on initial adhesion forces for the four different S. epidermidis strains was observed. Over time, strengthening of the adhesion forces was virtually absent on hydrophobic dimethyldichlorosilane (DDS)-coated glass, although in a few cases multiple adhesion peaks developed in the retract curves. Bond-strengthening on hydrophilic glass occurred within 5-35 s to maximum adhesion forces of -1.9 +/- 0.7 nN and was concurrent with the development of multiple adhesion peaks upon retract. Poisson analysis of the multiple adhesion peaks allowed separation of contributions of hydrogen bonding from other nonspecific interaction forces and revealed a force contribution of -0.8 nN for hydrogen bonding and +0.3 nN for other nonspecific interaction forces. Time-dependent bacterial adhesion forces were comparable for all four staphylococcal strains. It is concluded that, on DDS-coated glass, the hydrophobic effect causes instantaneous adhesion, while strengthening of the bonds on hydrophilic glass is dominated by noninstantaneous hydrogen bond formation.     

Liquid–paper interactions during liquid drop impact and recoil on paper surfaces

The spreading and recoiling of water drops on several flat and macroscopically smooth model surfaces and on sized paper surfaces were studied over a range of drop impaction velocities using a high-speed CCD camera. The water drop spreading and recoiling results on several model hydrophobic and hydrophilic surfaces were found to be in agreement with observations reported in the literature. The maximum drop spreading diameter for those model surfaces at impact was found to be dependent upon the initial drop kinetic energy and the degree of hydrophobicity/hydrophilicity of the surface. The extent of the maximum drop recoiling was found to be much weaker for hydrophilic substrates than for hydrophobic substrates. Sized papers, however, showed an interesting switch of behaviour in the process of water drop impaction. They behave like a hydrophobic substrate when a water drop impacts on it, but like a hydrophilic substrate when water drop recoils. Although the contact angle between water and hydrophilic or hydrophobic non-porous surfaces changes from advancing to receding as reported in literature, the change of contact angle during water impact on paper surface is unique in that the level of sizing was found to have a smaller than expected influence on the degree of recoil. Atomic force microscopy (AFM) was used to probe fibres on a sized filter paper surface under water. The AFM data showed that water interacted strongly with the fibre even though the paper was heavily sized. Implications of this phenomenon were discussed in the context of inkjet print quality and of the surface conditions of sized papers. Results of this study are very useful in the understanding of inkjet ink droplet impaction on paper surfaces which sets the initial condition for ink penetration into paper after impaction.     

Detachment of Particles from Surfaces: An AFM Study

We have measured interactions between hydrophilic and hydrophobic surfaces in an aqueous medium at various pH and ionic strengths as well as in some organic solvents using atomic force microscopy and analyzed them in terms of particle adhesion and detachment from surfaces. In hydrophilic systems the forces observed were found to be well described by DLVO theory at large separation distances. Very long range hydrophobic forces were not observed in hydrophilic-hydrophobic systems. Nevertheless, the jump into contact was found to occur at distances greater that those predicted by just van der Waals attraction. The interaction between two hydrophobic surfaces was dominated by the long-range attraction due to hydrophobic forces. This interaction was found to be sensitive to the type of substrate as well as to the pH and electrolyte concentration. Measured pull-off forces showed poor reproducibility. However, average values showed clear trends and were used to estimate interfacial energies or work of adhesion for all systems studied by means of the Derjaguin approximation. These values were compared to those calculated by the surface tension component theory using the acid-base approach. Good qualitative agreement was obtained, giving support for the usefulness of this approach in estimating interfacial energies between surfaces in liquid media. A comparison of the measured adhesion force with hydrodynamic detachment experiments showed good qualitative agreement. Copyright 2001 Academic Press.     

Development of a classical force field for the oxidized Si surface: application to hydrophilic wafer bonding

We have developed a classical two- and three-body interaction potential to simulate the hydroxylated, natively oxidized Si surface in contact with water solutions, based on the combination and extension of the Stillinger-Weber potential and of a potential originally developed to simulate SiO(2) polymorphs. The potential parameters are chosen to reproduce the structure, charge distribution, tensile surface stress, and interactions with single water molecules of a natively oxidized Si surface model previously obtained by means of accurate density functional theory simulations. We have applied the potential to the case of hydrophilic silicon wafer bonding at room temperature, revealing maximum room temperature work of adhesion values for natively oxidized and amorphous silica surfaces of 97 and 90 mJm(2), respectively, at a water adsorption coverage of approximately 1 ML. The difference arises from the stronger interaction of the natively oxidized surface with liquid water, resulting in a higher heat of immersion (203 vs 166 mJm(2)), and may be explained in terms of the more pronounced water structuring close to the surface in alternating layers of larger and smaller densities with respect to the liquid bulk. The computed force-displacement bonding curves may be a useful input for cohesive zone models where both the topographic details of the surfaces and the dependence of the attractive force on the initial surface separation and wetting can be taken into account.     

Cleaning and hydrophilization of atomic force microscopy silicon probes

The silicon surface of commercial atomic force microscopy (AFM) probes loses its hydrophilicity by adsorption of airborne and package-released hydrophobic organic contaminants. Cleaning of the probes by acid piranha solution or discharge plasma removes the contaminants and renders very hydrophilic probe surfaces. Time-of-flight secondary-ion mass spectroscopy and X-ray photoelectron spectroscopy investigations showed that the native silicon oxide films on the AFM probe surfaces are completely covered by organic contaminants for the as-received AFM probes, while the cleaning methods effectively remove much of the hydrocarbons and silicon oils to reveal the underlying oxidized silicon of the probes. Cleaning procedures drastically affect the results of adhesive force measurements in water and air. Thus, cleaning of silicon surfaces of the AFM probe and sample cancelled the adhesive force in deionized water. The significant adhesive force values observed before cleaning can be attributed to formation of a bridge of hydrophobic material at the AFM tip-sample contact in water. On the other hand, cleaning of the AFM tip and sample surfaces results in a significant increase of the adhesive force in air. The presence of water soluble contaminants at the tip-sample contact lowers the capillary pressure in the water bridge formed by capillary condensation at the AFM tip-sample contact, and this consequently lowers the adhesive force.     

Molecular explanation for why talc surfaces can be both hydrophilic and hydrophobic

While individual water molecules adsorb strongly on a talc surface (hydrophilic behavior), a droplet of water beads up on the same surface (hydrophobic behavior). To rationalize this dichotomy, we investigated the influence of the microscopic structure of the surface and the strength of adhesive (surface-water) interactions on surface hydrophobicity. We have shown that at low relative humidity, the competition between adhesion and the favorable entropy of being in the vapor phase determines the surface coverage. However, at saturation, it is the competition between adhesion and cohesion (water-water interactions) that determines the surface hydrophobicity. The adhesive interactions in talc are strong enough to overcome the unfavorable entropy, and water adsorbs strongly on talc surfaces. However, they are too weak to overcome the cohesive interactions, and water thus beads up on talc surfaces. Surprisingly, even talc-like surfaces that are highly adhesive do not fully wet at saturation. Instead, a water droplet forms on top of a strongly adsorbed monolayer of water. Our results imply that the interior of hydrophobic zeolites suspended in water may contain adsorbed water molecules at pressures much lower than the intrusion pressure.     

Molecular dynamics simulations of polyelectrolyte adsorption

We have performed molecular dynamics simulations of polyelectrolyte adsorption at oppositely charged surfaces from dilute polyelectrolyte solutions. In our simulations, polyelectrolytes were modeled by chains of charged Lennard-Jones particles with explicit counterions. We have studied the effects of the surface charge density, surface charge distribution, solvent quality for the polymer backbone, strength of the short-range interactions between polymers and substrates on the polymer surface coverage, and the thickness of the adsorbed layer. The polymer surface coverage monotonically increases with increasing surface charge density for almost all studied systems except for the system of hydrophilic polyelectrolytes adsorbing at hydrophilic surfaces. In this case the polymer surface coverage saturates at high surface charge densities. This is due to additional monomer-monomer repulsion between adsorbed polymer chains, which becomes important in dense polymeric layers. These interactions also preclude surface overcharging by hydrophilic polyelectrolytes at high surface charge densities. The thickness of the adsorbed layer shows monotonic dependence on the surface charge density for the systems of hydrophobic polyelectrolytes for both hydrophobic and hydrophilic surfaces. Thickness is a decreasing function of the surface charge density in the case of hydrophilic surfaces while it increases with the surface charge density for hydrophobic substrates. Qualitatively different behavior is observed for the thickness of the adsorbed layer of hydrophilic polyelectrolytes at hydrophilic surfaces. In this case, thickness first decreases with increasing surface charge density, then it begins to increase.     

Adsorption of the fusogenic peptide B18 onto solid surfaces: insights into the mechanism of peptide assembly

The adsorption and assembly of B18 peptide on various solid surfaces were studied by reflectometry techniques and atomic force microscopy. B18 is the minimal membrane binding and fusogenic motif of the sea urchin protein bindin, which mediates the fertilization process. Silicon substrates were modified to obtain hydrophilic charged surfaces (oxide layer and polyelectrolyte multilayers) and hydrophobic surfaces (octadecyltrichlorosilane). B18 does not adsorb on hydrophilic positively charged surfaces, which was attributed to electrostatic repulsion since the peptide is positively charged. In contrast, the peptide irreversibly adsorbs on negatively charged hydrophilic as well as on hydrophobic surfaces. B18 showed higher affinity for hydrophobic surfaces than for hydrophilic negatively charged surfaces, which must be due to the presence of hydrophobic side chains at both ends of the molecule. Atomic force microscopy provided the indication that lateral diffusion on the surface affects the adsorption process of B18 on hydrophobic surfaces. The adsorption of the peptide on negatively charged surfaces was characterized by the formation of globular clusters.