Wettability and corrosion of [NTf2] anion-based ionic liquids on steel and PVD (TiN, CrN, ZrN) coatings

Thewetting and corrosion behavior of three bis(trifluoromethylsulfonyl)imide-based ionic liquids: 1-Dodecyl-3- methylimidazolium bis(trifluoromethylsulfonyl)imide [C12MIM][NTf2], tributylmethylammonium bis(trifluoromethylsulfonyl)imide [N4441][NTf2] and methyltrioctylammonium bis(trifluoromethylsulfonyl)imide [N1888][NTf2] are tested in this research. The surface tension was measured for temperatures of 293–353 K resulting in the expected linearly decreasing behavior with temperature increase. In addition, contact angle measurements were made on AISI 52100 steel and three coatings (TiN, CrN and ZrN) obtained by PVD technique, finding the regular behavior in hydrophobic (non-polar) systems: high contact angles led to high surface tensions. Complementary parameters like spreading parameter and polarity fraction were calculated to enhance the wetting evaluation of these ionic liquids. [N1888][NTf2]/TiN resulted as the best IL-surface combination for a good wettability, due to the higher dispersion of the charge on the large size cation in this IL and the higher values of total and polar component of the surface free energy for this coating. Finally, SEM-EDS analysis determined that [N1888][NTf2]/ZrN was the best option in order to avoid corrosion problems. The evaporation of water, present as impurity in the ionic liquids, was found the main reason because of corrosion did not occur in the tests carried out at 100 °C

Molecular level studies of polymer behaviors at the water interface using sum frequency generation vibrational spectroscopy

Industrial plastics, biomedical polymers and numerous other polymeric systems are contacted with water for everyday functions and after disposal. Probing the interfacial molecular interactions between widely used polymers and water yields valuable information that can be extrapolated to macroscopic polymer/water interfacial behaviors so scientists can better understand polymer bio‐compatibility, hygroscopic tendencies and improve upon beneficial polymer behavior in water. There is an ongoing concerted effort to elucidate the molecular level behaviors of polymers in water by using sum frequency generation vibrational spectroscopy (SFG). SFG stands out for its utility in probing buried interfaces in situ and in real time without disrupting interfacial chemistry. Included in this review are SFG water interfacial studies performed on poly(methacrylate) and (acrylate)s, poly(dimethyl siloxane)s, poly(ethylene glycol)s, poly(electrolyte)s and other polymer types. The driving forces behind common water/polymer interfacial molecular features will be discussed as well as unique molecular reorientation phenomena and resulting macroscopic behaviors from microscopic polymer rearrangement. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013 Macroscopic polymer surface properties such as hydrophilicity, biocompatibility, and structural stability in water may be extrapolated from microscopic behaviors of polymer functional groups. Sum frequency generation vibrational spectroscopy (SFG) has been used to directly probe polymer molecular vibrational modes at the water interface, revealing functional group orientation changes, movement trends, and ordering at polymer surfaces. This review covers a variety of SFG studies of polymer water interactions performed in the preceding decade to the present.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/96247/1/23221_ftp.pd

Surface tension, interfacial tension and contact angles of ionic liquids

Ionic liquids combine the properties of molten salts (they are liquids composed predominantly of ions) and organic liquids (a variety of chemical bonds and interactions are relevant). Their unique properties have attracted significant attention over the last decade. Their interfacial properties are now coming under scrutiny because (i) they are important for their performance in specific applications and also because (ii) they provide an opportunity to study diverse chemical systems under vacuum. Recent examples from the literature are used to illustrate what is currently known. The surface tension, interfacial tension and wettability of ionic liquids resemble these of polar molecular organic liquids. It is physically clear there is a relation between surface tension and molecular structure but only general trends have been identified so far. Structural studies of the free surface of ionic liquids (e.g. with surface spectroscopy) are possible and reveal unique information about the interfacial molecular orientation. The amount of empirical data available is growing rapidly and elements of systematisation are beginning to appear. © 2011 Elsevier Ltd

The molecular-kinetic approach to wetting dynamics: Achievements and limitations

The molecular-kinetic theory (MKT) of dynamic wetting was formulated almost 50 years ago. It explains the dependence of the dynamic contact angle on the speed of a moving meniscus by estimating the non-hydrodynamic dissipation in the contact line. Over the years it has been refined to account explicitly for the influence of (bulk) fluid viscosity and it has been applied successfully to both solid-liquid-vapour and solid-liquid-liquid systems. The free energy barrier for surface diffusion has been related to the energy of adhesion. The MKT provides a qualitative explanation for most effects in dynamic wetting. The theory is simple, flexible, and it is widely used to rationalize the physics of wetting dynamics and fit experimental data (dynamic contact angle versus contact line speed). The MKT predicts an intermediate wettability as optimal for high-speed coating as well as the maximum speeds of wetting and dewetting. Nevertheless, the values of the molecular parameters derived from experimental data tend to be scattered and not particularly reliable. This review outlines the main achievements and limitations of the MKT and highlights some common cases of misinterpretation

Electrowetting: Electrocapillarity, saturation, and dynamics

Electrowetting is an electrocapillary phenomenon, i.e. the surface charge generated at the solid-liquid interface through an external voltage improves the wettability in the system. The Young-Lippmann equation provides the simplest thermodynamic framework and describes electrowetting adequately. Saturation, i.e. the reduced or nullified effectiveness of the external voltage below a threshold contact angle value, was and remains the most controversial issue in the physics of electrowetting. A simple estimation of the limits of validity of the Young model is obtained by setting the solid-liquid interfacial tension to zero. This approach predicts acceptably the change in electrowetting mechanism but not the minimal value of the contact angle achievable during electrowetting. The mechanism of saturation is, in all probability, related to charge injection into the dielectric layer insulating the working electrode but physical details are scarce. Surface force and spectroscopic techniques should be deployed in order to improve our understanding of the surface charging of insulators immersed in conductive liquids. Electrowetting in solid-liquid-liquid systems is generally more effective and robust. Electrowetting offers new ways of studying the dynamics of liquid movement as it allows selective changes in the wettability of the system

Spontaneous liquid marble formation on packed porous beds

The encapsulation of aqueous and organic solvents with particles used to form liquid marbles implies there are attractive interactions between the particles and those different liquids. This is often masked, however, by the impact of the droplet kinetic energy on marble formation. We investigated droplet wetting and evaporation when drops were gently placed (without rolling or shaking) on beds of silanised glass beads. Particle coating of the drop surface occurred within seconds of liquid contact with the particle bed. This ruled out evaporation from causing the particles to appear to rise up the surface of the drop as it was reduced in volume. Particles attach to the fresh liquid surface created during the droplet oscillations immediately after contact. The further ordered advance of the particles over the drop surface and the close-packed arrangement of the particles revealed the role of capillary forces in the coating process. By minimising the kinetic energy of the droplet contact with the particles, we found that maximum particle coating occurs at liquid surface tensions just above the critical wetting tension of the beads

Free running droplets on packed powder beds

We observed that water drops placed on horizontal beds of fine molybdenite particles move freely over the bed surface for about 1 second. The drops collect an irregular coating of unevenly distributed particles as they bounce and roll. We manipulated the distance that the drops travel, and hence the area of the droplet surface coated with particles, by varying the water surface tension and the kinetic energy of the initial droplet impact on the bed surface. Our results highlight the role of contact angle hysteresis in particle encapsulation of liquid drops