CO 2-wettability of sandstones is a key variable which determines structural and residual trapping capacities and strongly influences multi-phase fluid dynamics in the rock. An increasing number of researchers has now estimated this wettability by conducting contact angle measurements on quartz, however, there is a large uncertainty associated with the reported data. We demonstrate clearly that the main factor which leads to this broad data spread is due to surface contamination. It is clear that typically inappropriate cleaning methods were used which resulted in artificially high contact angle measurements. We used surface cleaning methods typically prescribed in the surface chemistry community and found that the water contact angle θ on a clean quartz substrate is low, 0-30°, and that θ increases with pressure. We conclude that quartz is strongly water-wet at high pressure conditions.

Aqueous-based synthesis is one of the most popular methods to prepare nanoparticles. In these procedures, surfactants are needed to regulate the growth and final particle size. While there are numerous evidence on the decisive role of surfactants, a quantitative description remains elusive. This study develops a theoretical model to correlate the surfactant activities to particle growth. In the model, the "penetrability" of ions within surfactant layer is used to combine surface reaction and adsorption/desorption processes. The penetrability was then directly correlated to surfactant size. The theory was verified by synthesis of iron oxide nanoparticles with series of cationic surfactants. Eight surfactants, with same headgroup and increasing hydrocarbon tail, were employed. The experimental data showed a deterministic correlation between surfactant tails and particle size. The experimental correlation between surfactant length and particle size was predicted by the model. The modeling results verify the role of surfactant as capping agent during particle growth. More importantly, it provides a theoretical framework to control particle size in wet synthesis.

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The synergistic adsorption of a binary surfactant mixture was investigated by tensiometry and neutron reflectometry. The results directly contradicted the conventional Gibbs adsorption equation. The accompanied molecular simulation demonstrated a multilayer arrangement at the synergic conditions, with three distinctively oriented water layers. The positive synergism can be explained by considering the relationship between water orientation and surface tension, in a similar manner to Langmuir's proposal in 1920s. In spite of the supporting evidence, the relationship has not been quantified in literature. The molecular orientation and arrangement are not included in the current theoretical framework, which simplifies the adsorbed zone into a single monolayer. A new theoretical framework is needed to properly quantify the interfacial adsorption for the mixed surfactant systems.

A high-speed camera was used to observe the motion of the three-phase contact (TPC) line for a small rising bubble ruptured by a submerged horizontal glass plate. The experimental data for the radial position of the TPC line as a function of time were used to examine both hydrodynamic and molecular-kinetic models previously developed for wetting/dewetting processes. It was found that both models were not able to describe the experimental data using the physically consistent values of parameters. A better fit could be obtained if the equilibrium contact angle, obtained from the TPC versus time measurements, was used in place of the thermodynamic contact angle determined by the Wilhelmy plate technique; however, the parameters obtained by this fitting could not be physically justified. Importantly, the equilibrium contact angle was found to be a function of the bubble radius and was significantly different from the Young contact angle. This radius dependence of contact angles and other curvature-dependent effects, which are not considered in the hydrodynamic and molecular-kinetic models, may be the cause of the deviations from the experimental results.

Alcohols have an amphiphilic characteristic and are employed in industrial processes to enhance interfacial properties. In this study, the change in surface potential (ΔV) and surface tension of 1-hexanol were measured on the subsurface of electrolyte solutions (NaCl at 0.02, 0.2, and 2 M). The results were fitted by a newly proposed model, which includes the influence of electrolytes and surface concentration of surfactant at the air-water interface. The findings were compared to those of a previous study on methyl isobutyl carbinol (MIBC). Most significantly, the modeling results showed opposite behaviors between the two systems: adsorbed MIBC enhances the presence of cations, whereas adsorbed 1-hexanol enhances the presence of anions. The difference highlights the significance of the molecular structure on the arrangement at the air/water interface.

The Gibbs adsorption equation has been indispensable in predicting the surfactant adsorption at the interfaces, with many applications in industrial and natural processes. This study uses a new theoretical framework to model surfactant adsorption at the air/water interface without the Gibbs equation. The model was applied to two surfactants, C14TAB and C16TAB, to determine the maximum surface excesses. The obtained values demonstrated a fundamental change, which was verified by simulations, in the molecular arrangement at the interface. The new insights, in combination with recent discoveries in the field, expose the limitations of applying the Gibbs adsorption equation to cationic surfactants at the air/water interface.

This article presents a new configuration of a water droplet floating on oil surface. The configuration is characterized by an acute contact angle (i.e., θ2 < π/2). In contrast, the previously identified droplet had an obtuse contact angle, which was easily sunk by a small disturbance. By employing a common surfactant, the new configuration was experimentally verified in a mineral oil with a density similar to that of crude oils. The new droplet is kinetically more stable than the previous configuration and can sustain strong disturbances. The results also highlight the significance of dynamic interfacial adsorption on the stability of the floating droplet.