Hofmeister Effect on the Interfacial Free Energy of Aliphatic and Aromatic Surfaces Studied by Chemical Force Microscopy

Recent Advances in the Quantification and Modulation of Hydrophobic Interactions for Interfacial Applications

Hydrophobic interaction is responsible for a variety of colloidal phenomena, which also plays a key role in achieving the desired characteristics and functionalities for a wide range of interfacial applications. In this feature article, our recent advances in the quantification and modulation of hydrophobic interactions at both solid/water and air/water interfaces in different material systems have been reviewed. On the basis of surface forces apparatus (SFA) measurements of hydrophobic polymers (e.g., polystyrene), a three-regime hydrophobic interaction model that could satisfactorily encompass the hydrophobic interaction with different ranges was proposed. In addition, the atomic force microscope (AFM) coupled with various techniques such as the colloidal probe, the electrochemical process, and force mapping were employed to quantify the hydrophobic interaction from different perspectives. For the hydrophobic interactions involving deformable bubbles, the bubble probe AFM combined with reflection interference contrast microscopy (RICM) was used to simultaneously measure the interaction force and spatiotemporal evolution of the thin film drainage process between air bubbles and hydrophobized mica surfaces in an aqueous medium. The studies on the interactions of air bubbles with self-assembled monolayers (SAMs) demonstrated that the range of hydrophobic interactions does not always increase monotonically with the hydrophobicity of interacting surfaces as characterized by the static water contact angle; viz., surfaces with similar hydrophobicity can exhibit different ranges of hydrophobic interaction, while surfaces with different hydrophobicities can exhibit a similar range of hydrophobic interactions. It is found that the hydrophobic interaction can be modulated by tuning the surface nanoscale structure and chemistry. Moreover, the long-range "hydrophilic" attraction that resembles the hydrophobic interaction was discovered between water droplets and polyelectrolyte surfaces in an oil medium, on the basis of which polyelectrolyte coating materials were designed for oil cleaning, oil/water separation, and demulsification. The interfacial applications, remaining challenges, and future perspectives of hydrophobic interactions are discussed.

Hofmeister Series: Insights of Ion Specificity from Amphiphilic Assembly and Interface Property

Hofmeister series (HS), ion specific effect, or lyotropic sequence acts as a pivotal part in a number of biological and physicochemical phenomena, e.g., changing the solubility of hydrophobic solutes, the cloud points of polymers and nonionic surfactants, the activities of various enzymes, the action of ions on an ion-channel, and the surface tension of electrolyte solutions, etc. This review focused on how ion specificity influences the critical micelle concentration (CMC) and how the thermoresponsive behavior of surfactants, and the dynamic transition of the aggregate, controls the aggregate transition and gel formation and tunes the properties of air/water interfaces (Langmuir monolayer and interfacial free energy). Recent progress of the ion specific effect in bulk phase and at interfaces in amphiphilic systems and gels is summarized. Applications and a molecular level theoretical explanation of HS are discussed comprehensively. This review is aimed to supply a fresh and comprehensive understanding of Hofmiester phenomena in surfactants, polymers, colloids, and interface science and to provide a guideline to design the microstructures and templates for preparation of nanomaterials.

Calcium-Mediated Adhesion of Nanomaterials in Reservoir Fluids

Globally, a small percentage of oil is recovered from reservoirs using primary and secondary recovery mechanisms, and thus a major focus of the oil industry is toward developing new technologies to increase recovery. Many new technologies utilize surfactants, macromolecules, and even nanoparticles, which are difficult to deploy in harsh reservoir conditions and where failures cause material aggregation and sticking to rock surfaces. To combat these issues, typically material properties are adjusted, but recent studies show that adjusting the dispersing fluid chemistry could have significant impact on material survivability. Herein, the effect of injection fluid salinity and composition on nanomaterial fate is explored using atomic force microscopy (AFM). The results show that the calcium content in reservoir fluids affects the interactions of an AFM tip with a calcite surface, as surrogates for nanomaterials interacting with carbonate reservoir rock. The extreme force sensitivity of AFM provides the ability to elucidate small differences in adhesion at the pico-Newton (pN) level and provides direct information about material survivability. Increasing the calcium content mitigates adhesion at the pN-scale, a possible means to increase nanomaterial survivability in oil reservoirs or to control nanomaterial fate in other aqueous environments.

Diffusion of ionic fluorescent probes atop polyelectrolyte brushes

The lateral diffusion of ionic fluorescent molecules atop polyelectrolyte brushes was adopted to probe the distribution of counterions of the polyelectrolyte brushes. With a combination of single molecule fluorescence techniques, fluorescence correlation spectroscopy and single molecule fluorescence imaging, the lateral diffusion of the ionic probes (sulforhodamine B, rhodamine 6G) at the top of the model polyelectrolyte brushes with the opposite charges, poly([2-(methylacryloyloxyl)ethyl] trimethylammonium chloride) (PMETAC) and polystyrene sulfonate (PSS), was studied with different external salt concentrations. A huge decrease of the diffusion rate of the probes was observed at salt concentrations 2-3 orders of magnitude lower than that for any detectable change of brushes thickness could be observed. The results reflect the early collapse of the top portion of the polyelectrolyte brushes and also the penetration of the probes into the brushes due to the increase of osmotic pressure by the salt level in the solution. The diffusion of the fluorescent counterion can serve as a very sensitive probe of the structure atop the polyelectrolyte brushes.

Hofmeister effect on the interfacial dynamics of single polymer molecules

The Hofmeister effect on interfacial dynamics has been discovered for single charged polymer molecules (sodium polystyrene sulfonate) adsorbed on a hydrophobic surface from an aqueous solution. The presence of ions in the aqueous solution affects the surface diffusivity, and its amplitudes and the surface friction follow the Hofmeister series-the kosmotropic ions slowed down the surface diffusivity and the chaotropic ions speeded it up. The amplitude of the surface friction exhibits a good correlation with the surface tension increment, indicating the interfacial feature of the Hofmeister effect.