Wetting study of imidazolium ionic liquids

In this work, we present a systematic contact angles study of a series of 1-alkyl, 3-methyl-imidazolium ionic liquids (ILs) on well-defined polar and nonpolar monolayer surfaces supported on Si wafers. The advancing and receding contact angles of ILs were used to determine the surface energy of the monolayer surfaces using Neumann's equation-of-state and Zisman's critical surface tension approaches. In parallel, the contact angles of conventional probe fluids (molecular liquids) including water, formamide, methylene iodide, ethylene glycol, and hexadecane were determined on the same surfaces. The results obtained showed a great deal of similarity in wetting behavior of ionic vs molecular probe fluids: the contact angles of both sets of liquids followed the same patterns in accord with the surface tension of the fluid. A good agreement was found between the surface energy determined by different sets of liquids.     

The wettability of fluoropolymer surfaces: influence of surface dipoles

The wettabilities of fluorinated polymers were evaluated using a series of contacting probe liquids ranging in nature from nonpolar aprotic to polar aprotic to polar protic. Fully fluorinated polymers were wet less than partially fluorinated polymers, highlighting the weak dispersive interactions of fluorocarbons. For partially fluorinated polymers, the interactions between the distributed dipoles along the polymer backbone and the dipoles of the contacting liquids were evaluated using both polar and nonpolar probe liquids. The results demonstrate that the surface dipoles of the fluoropolymers generated by substituting fluorine atoms with hydrogen or chlorine atoms can strongly interact with polar contacting liquids. The wettabilities of the partially fluorinated polymers were enhanced by increasing the density of dipoles across the surfaces and by introducing differentially distributed dipoles.     

Coulombic dragging of molecules on surfaces induced by separately flowing liquids

We demonstrate that ions or polar molecules can be driven by fluctuating Coulombic forces induced by flowing polar liquids at nanometer separations. We simulate this intriguing phenomenon on small ions and polar molecules driven on the surfaces of carbon nanotubes through which a flow of water is passing. Our simulations show that the average velocities of the driven molecules are close to those of the passing liquid. These transport phenomena open the door for many potential applications.     

Formation of ZnO hexagonal micro-pyramids: a successful control of the exposed polar surfaces with the assistance of an ionic liquid

Wurtzite ZnO hexagonal micro-pyramids, with all exposed surfaces being polar +/- (0001) and {1011} planes, have been successfully synthesized using ionic liquids as solvents.     

Partial Air Wetting on Solvophobic Surfaces in Polar Liquids

The properties of solvophobic surfaces in polar liquids are studied by sedimentation experiments as well as by force measurements using a scanning force microscope (SFM). Depending on whether the polar liquid contacts the solvophobic surface under normal air pressure or under vacuum the experimental results are different. Sedimentation velocities of vacuum-contacted solvophobic surfaces are similar to those of solvophilic vacuum- or air-contacted ones. However, for the air-contacted solvophobic surfaces there is a slip boundary condition of the hydrodynamic flow causing a change of the sedimentation velocity of about 20%, and a long-range attraction varying with the polarity of the liquid molecule is observed between them. These effects can be explained by an incomplete air dewetting of the solvophobic surface when brought into contact with the polar liquid at normal air pressure. Copyright 1999 Academic Press.     

On Neglecting the Polar Nature of Halogenated Hydrocarbons in the Surface Energy Determination of Polar Solids from Contact Angle Measurements

The generally accepted strategy of neglecting the polar nature of halogenated liquids in the surface energy determination using the Lifshitz-van der Waals/Lewis acid-base (LW/AB) approach may lead to erroneous and inconsistent results for polar solids. This was demonstrated in a simulation study carried out on monopolar basic surfaces using water, glycerol, and hypothetical liquids whose surface energy characteristics (gamma(L)(LW)=50 mJ/m(2), gamma(L)(-)=0, and gamma(L)(+)=0-1 mJ/m(2)) were chosen to approximate halogenated hydrocarbons. Neglect of the liquid polarity overestimates the LW component and underestimates the basic parameter of the solid surface energy. This effect increases rapidly with an increase in the actual (nonzero) gamma(L)(+) value of supposedly apolar liquid. Consequently, results with an appropriate level of precision can be obtained only with liquids having gamma(L)(+)<0.02 mJ/m(2). For liquids having gamma(L)(+) approximately 0.5 mJ/m(2) (diiodomethane, s-tetrabromoethane, and, probably, other halogenated hydrocarbons), neglect of the liquid polarity causes errors up to 15% in the LW component and up to 100% in the basic parameter of the solid surface energy. The quantitative trends established in the simulation study were indeed observed in an experimental study performed on the surfaces of poly(methyl methacrylate) and polystyrene using water, glycerol, and diiodomethane or s-tetrabromoethane as the test liquids. Copyright 2000 Academic Press.     

Mixed alkanethiol monolayers on gold surfaces: Wetting and stability studies

Studies of wetting and stability of mixed monolayers containing hydrophobie and hydrophilic components are discussed. We are reporting the observation of an apparent concentration-driven transition in the cosine of the contact angles of liquids on mixed monolayers. It is suggested that this phenomenon is due to a possible (true or rounded) surface phase transition, resulting in the formation of a prewetting water layer. This formation is triggered by variations in the quenched distribution of random surface fields. The variation of the surface free-energy, both polar and dispersive parts, has been determined as a function of surface OH-concentration. The surface free-energy of the 100% OH surface is close to that found for water, as might be expected for a surface coated with several monolayers of water. Zisman plots obtained for several of the surfaces using polar and nonpolar liquids give γ_c values which follow the observed dispersive contribution to the total surface free energy, and thus do not present a good approximation to the surface free energy (i.e., γ_c < γ_sv).Contact angle variation was studied on self-assembled alkanethiol monolayers containing mixtures of OH and CH₃ groups at their air-monolayer interface. It was found that these high free energy organic surfaces yielded contact angles which were not stable over long periods of time. The extent of the variation was found to be related to the surface free energy (%OH). The effect of different storage environments and temperature on the changing contact angles are discussed. We propose that monolayer surfaces containing high concentrations of OH groups on mobile organic chains are not stable. Such monolayer surfaces may stabilize over time, depending on the chain length, by surface reorganization and the adsorption of contaminants.     

Characterization of biomaterials polar interactions in physiological conditions using liquid–liquid contact angle measurements: Relation to fibronectin adsorption

Wettability of biomaterials surfaces and protein-coated substrates is generally characterized with the sessile drop technique using polar and apolar liquids. This procedure is often performed in air, which does not reflect the physiological conditions. In this study, liquid/liquid contact angle measurements were carried out to be closer to cell culture conditions. This technique allowed us to evaluate the polar contribution to the work of adhesion between an aqueous medium and four selected biomaterials widely used in tissue culture applications: bacteriological grade polystyrene (PS), tissue culture polystyrene (tPS), poly(2-hydroxyethyl methacrylate) film (PolyHEMA), and hydroxypropylmethylcellulose-carboxymethylcellulose bi-layered Petri dish (CEL). The contributions of polar interactions were also estimated on the same biomaterials after fibronectin (Fn) adsorption. The quantity of Fn adsorbed on PS, tPS, PolyHEMA and CEL surfaces was evaluated by using the fluorescein-labeled protein. PolyHEMA and CEL were found to be hydrophilic, tPS was moderately hydrophilic and PS was highly hydrophobic. After Fn adsorption on PS and tPS, a significant increase of the surface polar interaction was observed. On PolyHEMA and CEL, no significant adsorption of Fn was detected and the polar interactions remained unchanged. Finally, an inverse correlation between the polarity of the surfaces and the quantity of adsorbed Fn was established.     

Direct Measurement of Repulsive van der Waals Interactions Using an Atomic Force Microscope

The forces of interaction between a flat poly(tetrafluoroethylene) (PTFE) surface and gold spheres (of radii 3–8 μm) were measured as a function of apparent surface separation for different intervening media. For air, fluorinated alkanes, and polar liquids the interaction between the surfaces was found to be attractive. With intervening liquids of low-polarity the interaction was found to be repulsive. This repulsion is attributed to a negative composite Hamaker coefficient leading to van der Waals repulsion.     

Importance of surface tension for physical paint properties

Surface tension is a parameter of decisive importance for characterizing painted and unpainted surfaces related to wetting and adhesion phenomena. This article presents measurements of the surface tension of solids by means of an automatic contact angle measurement device. The evaluation of the surface tension is based on a separation into polar and disperse components. In addition, this paper briefly touches on other, more far-reaching approaches (acid/base) and discusses a method for the determination of dynamic surface tension of liquids.     

Triphilic Ionic‐Liquid Mixtures: Fluorinated and Non‐fluorinated Aprotic Ionic‐Liquid Mixtures

We present here the possibility of forming triphilic mixtures from alkyl‐ and fluoroalkylimidazolium ionic liquids, thus, macroscopically homogeneous mixtures for which instead of the often observed two domains—polar and nonpolar—three stable microphases are present: polar, lipophilic, and fluorous ones. The fluorinated side chains of the cations indeed self‐associate and form domains that are segregated from those of the polar and alkyl domains. To enable miscibility, despite the generally preferred macroscopic separation between fluorous and alkyl moieties, the importance of strong hydrogen bonding is shown. As the long‐range structure in the alkyl and fluoroalkyl domains is dependent on the composition of the liquid, we propose that the heterogeneous, triphilic structure can be easily tuned by the molar ratio of the components. We believe that further development may allow the design of switchable, smart liquids that change their properties in a predictable way according to their composition or even their environment.     

Glass capillary gas chromatography of highly polar solutes like diols,acids and amines deactivation of fused silica and pretreated alkali glass support surfaces

Summary The method of support surface deactivation by PSD (alkylpolysiloxane degradation) at temperature between 300 and 450°C previously described was used to deactivate both fused silica and alkali glass surfaces of capillary columns. The latter surfaces had to be pretreated before deactivation with aqueous HCl leaching or by the dealkalisation method using flowing HCl gas at 450°C and subsequent rinsing with water for alkali removal. Excellent alkylpolysiloxane columns with regard to tailing and irreversible adsorption of highly polar solutes have been obtained on both fused silica and the pretreated alkali glass. Fused silica does not require pretreatment before deactivation by the PSD-method, however. Good polyethyleneglycol (Carbowax 20 M) columns can also be obtained by coating the two types of surfaces when no deactivation is necessary. Deactivation by the PSD method cannot be applied in this case because polar stationary liquids do not adhere to alkylpolysiloxane deactivated surfaces. Sample capacity problems arising when separating highly polar solutes with non-polar stationary phases have also been investigated.     

Conformations and vibrational spectra of 2-chloro-3-fluoro-1-propene and 2-bromo-3-fluoro-1-propene

The infrared spectra of 2-chloro-3-fluoro- and 2-bromo-3-fluoro-1-propene as vapours and liquids were recorded in the region 4000–4050 cm^?1. Additional spectra of the amorphous and crystalline solids at ?170 °C and of the liquids in polar and non-polar solvents were recorded between 4000 and 200 cm^?1.Raman spectra, including semi-quantitative polarization measurements of the liquids were obtained. Spectra were also recorded with the samples dissolved in polar and non-polar solvents, and unannealed as well as annealed crystalline solids were studied at ?180 °C.Approximately 14 vibrational bands present in the spectra of the liquids, solutions and the glassy solids vanished in the infrared and Raman spectra of the crystals. From various criteria it can be concluded with certainty that the more polar (gauche) and less polar (cis) conformers were present in the crystalline chloro- and bromo- compounds, respectively. From infrared and Raman band intensities it was estimated that the conformational equilibrium in chlorofluoro-propene was highly displaced towards cis in the vapour, with both conformers approximately equally abundant in the liquid state (30 °C). For bromofluoro- propene the equilibrium was still further displaced towards the cis conformer.A striking similarity between the spectra of the two compounds was ob- served. The fundamental frequencies have been tentatively assigned and checked by force constant calculations. Dipole moments and relative stabilities of the conformers were estimated by a CNDO calculation.     

Monopolar surfaces

Following the development of a methodology for determining the apolar components as well as the electron donor and the electron acceptor parameters of the surface tension of polar surfaces, surfaces of a number of quite common materials were found to manifest virtually only electron donor properties and no, or hardly, any electron acceptor properties. Such materials may be called monopolar; they can strongly interact with bipolar materials (e.g., with polar liquids such as water); but one single polar parameter of a monopolar material cannot contribute to its energy of cohesion. Monopolar materials manifesting only electron acceptor properties also may exist, but they do not appear to occur in as great an abundance. Among the electron donor monopolar materials are: polymethylmethacrylate, polyvinylalcohol, polyethyleneglycol, proteins, many polysaccharides, phospholipids, nonionic surfactants, cellulose esters, etc. Strongly monopolar materials of the same sign repel each other when immersed or dissolved in water or other polar liquids. The interfacial tension between strongly monopolar surfaces and water has a negative value. This leads to a tendency for water to penetrate between facing surfaces of a monopolar substance and hence, to repulsion between the molecules or particles of such a monopolar material, when immersed in water, and thus to pronounced solubility or dispersibility. Monopolar repulsion energies can far outweigh Lifshitz-van der Waals attractions as well as electrostatic and "steric" repulsions. In aqueous systems the commonly observed stabilization effects, which usually are ascribed to "steric" stabilization, may in many instances be attributed to monopolar repulsion between nonionic stabilizing molecules. The repulsion between monopolar molecules of the same sign can also lead to phase separation in aqueous solutions (or suspensions), where not only two, but multiple phases are possible. Negative interfacial tensions between monopolar surfactants and the brine phase can be the driving force for the formation of microemulsions; such negative interfacial tensions ultimately decay and stabilize at a value very close to zero. Strongly monopolar macromolecules or particles surrounded by oriented water molecules of hydration can still repel each other, albeit to an attenuated degree. This repulsion was earlier perceived as caused by "hydration pressure". A few of the relevant colloid and surface phenomena are reviewed and re-examined in the light of the influence of surface monopolarity on these phenomena.     

Conformations and vibrational spectra of 2,3-dichloro and 2,3-dibromo-1-propene

The infrared spectra of 2,3-dichloro- and 2,3-dibromo-1-propene in the region 4000-200 cm^?1 were recorded of the liquids and of the crystalline solids at ?180 °C. Raman spectra, including semiquantitative polarization measurements were obtained of the liquids. Additional spectra were recorded of the samples dissolved in polar and non-polar solvents and of unannealed as well as of annealed crystalline solids at ?180 °C.Approximately 13 vibrational bands present in the spectra of the liquids and solutions as well as of the amorphous solid vanished in the crystal spectra. From the intensity variations with changing solvent polarity it was concluded that the conformer present in the crystal was more polar (gauche) than the other, simultaneously present in the liquid (cis). A striking similarity between the spectra of the two compounds was observed. All the fundamentals for the gauche conformers have tentatively been assigned.     

A physical approach to specifically improve the mobility of alkane liquid drops

Seamless control of resistance to liquid drop movement for polar (water) and nonpolar alkane (n-hexadecane, n-dodecane, and n-decane) probe liquids on substrate surfaces was successfully demonstrated using molten linear poly(dimethylsiloxane) (PDMS) brush films with a range of different molecular weights (MWs). The ease of movement of liquid drops critically depended on polymer chain mobility as it relates to both polymer MW and solvent swelling on these chemically- and topographically identical surfaces. Our brush films therefore displayed lower resistances to liquid drop movement with decreasing polymer MW and surface tension of probe liquid as measured by contact angle (CA) hysteresis and tilt angle measurements. Subsequently, while mobility of water drops was inferior and became worse at higher MWs, n-decane drops were found to experience little resistance to movement on these polymer brush films. Calculating CA hysteresis as Δθ(cos) = cos θ(R) - cos θ(A) (θ(A) and θ(R) are the advancing and receding CAs, respectively) rather than the standard Δθ = θ(A) - θ(R) was found to be advantageous for estimation of the actual dynamic dewetting behavior of various probe liquids on an inclined substrate.     

Self-assembly of alkanols on Au(111) surfaces

Self-assembled monolayers (SAMs) of alkanols (1-C(N)H(2N+1)OH) with varying carbon-chain lengths (N = 10-30) have been systematically studied by means of scanning tunneling microscopy (STM) at the interfaces between alkanol solutions (or liquids) and Au(111) surfaces. The carbon skeletons were found to lie flat on the surfaces. This orientation is consistent with SAMs of alkanols on highly oriented pyrolytic graphite (HOPG) and MoS2 surfaces, and also with alkanes on reconstructed Au(111) surfaces. This result differs from a prior report, which claimed that 1-decanol molecules (N = 10) stood on their ends with the OH polar groups facing the gold substrate. Compared to alkanes, the replacement of one terminal CH3 group with an OH group introduces new bonding features for alkanols owing to the feasibility of forming hydrogen bonds. While SAMs of long-chain alkanols (N > 18) resemble those of alkanes, in which the aliphatic chains make a greater contribution, hydrogen bonding plays a more important role in the formation of SAMs of short-chain alkanols. Thus, in addition to the titled lamellar structure, a herringbone-like structure, seldom seen in SAMs of alkanes, is dominant in alkanol SAMs for values of N < 18. The odd-even effect present in alkane SAMs is also present in alkanol SAMs. Thus, the odd N alkanols (alkanols with an odd number of carbon atoms) adopt perpendicular lamellar structures owing to the favorable interactions of the CH3 terminal groups, similar to the result observed for odd alkanes. In contrast to alkanes on Au(111) surfaces, for which no SAMs on an unreconstructed gold substrate were observed, alkanols are capable of forming SAMs on either the reconstructed or the unreconstructed gold surfaces. Structural models for the packing of alkanol molecules on Au(111) surfaces have been proposed, which successfully explain these experimental observations.     

Nanostructure self-organization of ionic liquids

The theory of integral equations was applied to investigate the formation of structures in ionic liquids (ILs). The effect of temperature and the length of the cation tails on the structural properties of a system was studied. An intermediate type of ordering in ILs as compared with common liquids was observed. The formation of polar and nonpolar domains was revealed, with the distribution of the polar domains having the shape of a three-dimensional net coexisting with nonpolar domains. The characteristic scale of intermediate ordering was shown to increase as a power function without disturbing the shape of the distribution of polar domains as the length of the cation tails grew.     

Selection of Ionic Liquids for Free Radical Polymerization Processes

Summary: Ionic liquids are efficient solvents for free radical homo- and copolymerization. Important parameters for selection of ionic liquids are their liquidus range, their viscosity, and their polarity. Viscosity of ionic liquids strongly influences the degree of polymerization of homopolymers. Micropolarity of ionic liquids can be used to explain differences in the composition of copolymers made on the basis of a relatively nonpolar methacrylate and a highly polar zwitterionic methacrylate.     

Surface patterning and its application in wetting/dewetting studies

Recent advances in microfabrication have allowed one to pattern the surface of a solid substrate with patches of different wettabilities on the micrometer-sized scale. These textured surfaces provide a well-characterized model system for studying the wetting and dewetting behaviors of liquids on heterogeneous surfaces. They also present a well-defined template to direct the self-organization of liquids on the surfaces of solid substrates, and to form patterned microstructures of various materials without using expensive, clean-room facilities. As demonstrated in a number of studies, the three-dimensional morphologies of the liquid microstructures could be easily controlled by changing the two-dimensional features patterned on the surface of a solid substrate. These demonstrations suggest that microfabrication based on surface patterning and selective wetting or dewetting will offer immediate advantages in applications such as fabrication of microreactor arrays and microfluidic devices, where a liquid (or solution) is the primary material to be patterned.