Surface tension of water and C 10 EO 8 solutions at the interface to hexane vapor saturated air

Response to Comment on "Vapor Lubrication for Reducing Water and Ice Adhesion on Poly(dimethylsiloxane) Brushes": Organic Vapors Influence Water Contact Angles on Hydrophobic Surfaces

Fast removal of water drops from solid surfaces is important in many applications such as on solar panels in rain, in heat transfer, and for water collection. Recently, a reduction in lateral adhesion of water drops on poly(dimethylsiloxane) (PDMS) brush surfaces after exposure to various organic vapors was reported. It was attributed to the physisorption of vapor and swelling of the PDMS brushes. However, it was later pointed out that a change in the interfacial energies by vapor adsorption could also have caused low drop adhesion. To find out how strongly each effect contributes, contact angles of water drops on three hydrophobic surfaces in different vapors are measured. In water-soluble vapors, a substantial decrease is observed in contact angles. This decrease can indeed be explained by a vapor-induced change in the interfacial tensions. The very low contact angle hysteresis on PDMS surfaces in saturated n-hexane and toluene vapor cannot be explained by a change in interfacial tensions. The observation supports the hypothesis that these vapors adsorb into the PDMS and form a lubricating layer. It is hoped that these findings help to solve fundamental problems and contribute to applications, such as anti-icing, heat transfer, and water collection.

A Multistate Adsorption Model for the Adsorption of C14EO4 and C14EO8 at the Solution/Air Interface

The dynamic and equilibrium properties of adsorption layers of poly (oxyethylene) alkyl ether (CnEOm) can be well described by the reorientation model. In its classical version, it assumes two adsorption states; however, there are obviously surfactants that can adsorb in more than two possible conformations. The experimental data for C14EO4 and C14EO8 (dynamic and equilibrium surface tensions and surface dilational visco-elasticity as measured by bubble profile analysis tensiometry) are used to verify if a reorientation model with more than two possible adsorption states can better describe the complete set data of CnEOm adsorption layers at the water/air interface. The proposed refined theoretical model allows s different states of the adsorbing molecules at the interface. The comparison between the model and experiment demonstrates that, for C14EO4, the assumption of s = 5 adsorption states provides a much better agreement than for s = 2, while for C14EO8, a number of s = 10 adsorption states allows an optimum data description.

Thermodynamics, Kinetics and Dilational Visco-Elasticity of Adsorbed CnEOm Layers at the Aqueous Solution/Air Interface

The adsorption behaviour of linear poly(oxyethylene) alkyl ether (CnEOm) is best described by a reorientation model. Based on a complete set of experimental data, including the adsorption kinetics, the equilibrium surface tension isotherm and the surface dilational visco-elasticity, the thermodynamic and kinetic adsorption parameters for some CnEOm at the water/air interface were determined. For the study, six CnEOm surfactants were selected (n = 10, 12 and 14 and m = 4, 5 and 8) and were studied by bubble profile analysis and maximum bubble pressure tensiometry. A refined theoretical model based on a reorientation-adsorption model combined with a diffusion-controlled adsorption kinetics and exchange of matter allowed us to calculate the surface layer composition by adsorbing molecules in different orientations. It turns out that at larger surface coverage, the adsorption rate decreases, i.e., the apparent diffusion coefficients are smaller. This deceleration can be explained by the transition of molecules adsorbed in a state of larger molar surface area into a state with smaller molar surface area.

Interactions of Oil Drops Induced by the Lateral Capillary Force and Surface Tension Gradients

The interacting forces on the objects at the air-liquid interface are important for researching self-assembly of objects. Due to the interacting forces among objects, the objects self-assemble at the air-liquid interface and organize into ordered structures. In general, for the interacting of oil drops, they can attract coalescence or repel dispersion. However, according to our former research, we find that interactions of oil drops can be motion like interactions of gas atoms. The oil drops cannot coalesce to form a big oil drop or separate to form ordered structures. They attract at a long distance but repel when they almost contact. In this study, according to our simulation, we demonstrate that the atomic-like motion of oil drops is mainly caused by the surface tension gradient and the lateral capillary force. Based on the results, our research will facilitate the fundamental understanding about interactions among oil drops. Apart from giving detailed demonstration about the interacting forces among oil drops, our research may also put forward research about self-assembly, oil emulsification, and microfluidics.

Adsorption of C14EO8 at the interface between its aqueous solution drop and air saturated by different alkanes vapor

The dynamic and equilibrium surface tension for drops of aqueous C₁₄EO₈ solutions at the interface to pure air or pentane, hexane, heptane and toluene saturated air, and the dynamic surface tension of pure water at these interfaces are presented. Two theoretical models were employed: both assuming a diffusion controlled adsorption of the surfactant, and either a diffusion or kinetic barrier governed adsorption of the alkanes. The experimental results are best described by the model which implies a diffusion control for the C₁₄EO₈ molecules and the existence of a kinetic barrier for the alkane molecules. The desorption of alkanes from the surface layer after equilibration and their subsequent removal from the measuring cell was studied as well. The desorption process was shown to be slow for heptane and hexane. However, for the pentane vapor the desorption is quite rapid, and after the desorption commences the surface tension becomes equal to that at the interface with pure air.