Atomic Force Microscopy Force Mapping Analysis of an Adsorbed Surfactant above and below the Critical Micelle Concentration

Self-Assembled Sodium Dodecyl Sulfate Structures on Mineral Surfaces Following Rapid Solvent Removal

Sodium dodecyl sulfate (SDS) is a widely used surfactant with applications ranging from detergents to cell lysis and nanomaterial exfoliation. Additionally, SDS can form self-assembled structures on different substrates under specific conditions. While extensive research has explored SDS self-assembly at the liquid-solid interface, less is known about the structures formed at the solid-air interface following solvent removal. In this study, we investigated SDS self-assembled structures on HOPG (highly oriented pyrolytic graphite), talc, and mica substrates using spin-coating and spread-coating methods. Scanning probe microscopy revealed a range of morphologies, including hemicylindrical micelles, lamellar bilayers, and quasi-1D structures, shaped by the interaction between SDS and the substrate surface. On HOPG, hemicylindrical micelles were observed in dilute solutions, whereas lamellar 2D structures, likely bilayer stacks, formed in both dilute and concentrated samples. On talc, lamellar bilayers demonstrated temporal evolution and thermal stability up to 160 °C. Mica samples exhibited quasi-1D structures, 2D bilayers, and thinner lamellar 2D structures, with evidence suggesting the presence of a methyl-terminated monolayer. Thermal annealing tests indicated that quasi-1D structures lost organization at 60 °C, whereas bilayers remained stable up to 150 °C, at least. The results highlight the complexity of SDS self-assembly at the solid-air interface, emphasizing the critical role of local environmental factors. These findings provide insights into surfactant behavior during solvent removal and establish a foundation for further exploration of self-assembled systems under ambient conditions.

Boundary Lubrication with Adsorbed Anionic Surfactant Bilayers in Hard Water

The adsorption behavior of an anionic surfactant, hydroxy alkane sulfonate with an alkyl chain length of 18 (C18HAS), from its hard water solution onto a mica surface and resulting lubrication properties were investigated. Because of the double chain-like chemical structure and aggregation behavior, C18HAS formed vesicles in hard water, which adsorbed onto a negatively charged mica surface via cation (Ca2+) bridging and then transformed into a bilayer film. The number of bilayers formed on the surface was evaluated by force curve measurements using an atomic force microscope (AFM), and the results showed a time-dependent increase of the number of adsorbed bilayers. Friction and lubrication properties were evaluated for the confined film of the C18HAS hard water solution between mica surfaces using the surface forces apparatus (SFA). When the two surfaces were brought into contact under load and sheared against each other, the lubricating film consisted of two adsorbed C18HAS bilayers whose friction coefficient μ was of the order of 10-3 or below. The detailed analysis of the friction features revealed that the slipping in the boundary film does not occur at the interface between the opposed headgroup region of the two adsorbed bilayers, which is the typical mechanism for the low friction of adsorbed phospholipid bilayers extensively studied in the literature. Instead, slipping occurs at the interface between opposed liquid-like alkyl chain tails within the adsorbed bilayers; the low friction coefficient comes from the existence of two slip planes in the boundary film.

Wettability of rock minerals and the underlying surface forces: A review of the implications for oil recovery and geological storage of CO2

The wettability of subsurface minerals is a critical factor influencing the pore-scale displacement of fluids in underground reservoirs. As such, it plays a key role in hydrocarbon production and greenhouse gas geo-sequestration. We present a comprehensive and critical review of the current state of knowledge on the intermolecular forces governing wettability of rock minerals most relevant to subsurface fluid storage and recovery. In this review we first provide a detailed summary of the available data, both experimental and theoretical, from the perspective of the fundamental intermolecular and surface forces, specifically considering the roles played by the surface chemistry, fluid properties, as well as other significant factors. We subsequently offer an analysis of the effects of chemical additives such as surfactants and nanoparticles that have emerged as viable means for manipulating wettability. In each example, we highlight the practical implications for hydrocarbon production and CO2 geo-storage as two of the most important current applications. As the physico-chemical mechanisms governing the wetting phenomena are the main focus, special emphasis is placed on nano-scale experimental approaches along with atomic-scale modeling that specifically probe the underlying intermolecular and surface forces. Lastly, we discuss the gaps in the current state of knowledge and outline future research directions to further our fundamental understanding of the interactions and their impact on the wetting characteristics of Earth's minerals.

Nanomechanics of self-assembled surfactants revealed by frequency-modulation atomic force microscopy

Surfactants play a critical role in bottom-up nanotechnologies due to their peculiar nature of controlling the interfacial energy. Since their operational mechanism originates from the molecular-scale formation and disruption processes of molecular assemblies (i.e., micelles), conventional static-mode atomic force microscopy has made a significant contribution to unravel the detailed molecular pictures. Recently, we have successfully developed a local solvation measurement technique based on three-dimensional frequency-modulation atomic force microscopy, whose spatial resolution is not limited by jump-to-contact. Here, using this novel technique, we investigate molecular nanomechanics in the formation and disruption processes of micelles formed on a hydrophobic surface. Furthermore, an experiment employing a hetero-nanostructure reveals that the nanomechanics depends on the form of the molecular assembly. Namely, the hemifusion and disruption processes are peculiar to the micellar surface and cause a higher energy dissipation than the monolayer surface. The technique established in this study will be used as a generic technology for further development of bottom-up nanotechnologies.

Real-Time Monitoring the Staged Interactions between Cationic Surfactants and a Phospholipid Bilayer Membrane

The cationic surfactant-lipid interaction directs the development of novel types of nanodrugs or nanocarriers. The membrane action of cationic surfactants also has a wide range of applications. In this work,...

Comprehensive review on surfactant adsorption on mineral surfaces in chemical enhanced oil recovery

With the increasing demand for efficient extraction of residual oil, enhanced oil recovery (EOR) offers prospects for producing more reservoirs' original oil in place. As one of the most promising methods, chemical EOR (cEOR) is the process of injecting chemicals (polymers, alkalis, and surfactants) into reservoirs. However, the main issue that influences the recovery efficiency in surfactant flooding of cEOR is surfactant losses through adsorption to the reservoir rocks. This review focuses on the key issue of surfactant adsorption in cEOR and addresses major concerns regarding surfactant adsorption processes. We first describe the adsorption behavior of surfactants with particular emphasis on adsorption mechanisms, isotherms, kinetics, thermodynamics, and adsorption structures. Factors that affect surfactant adsorption such as surfactant characteristics, solution chemistry, rock mineralogy, and temperature were discussed systematically. To minimize surfactant adsorption, the chemical additives of alkalis, polymers, nanoparticles, co-solvents, and ionic liquids are highlighted as well as implementing with salinity gradient and low salinity water flooding strategies. Finally, current trends and future challenges related to the harsh conditions in surfactant based EOR are outlined. It is expected to provide solid knowledge to understand surfactant adsorption involved in cEOR and contribute to improved flooding strategies with reduced surfactant loss.

Experimental and Molecular Dynamics Simulation Study on the Effects of the Carbon Chain Length of Gemini Surfactants on the Inhibition of the Acid-Rock Reaction Rate

A series of gemini surfactants were synthesized to examine their adsorption properties. The properties of gemini surfactants, including critical micelle concentration, electrostatic potential distributions, charge, occupied volume, lowest unoccupied molecular orbital (LUMO), and highest occupied molecular orbital (HOMO), were evaluated using conductivity and density functional theory (DFT) calculations. The calculation results indicated that the electrostatic potential distributions were similar among the four gemini surfactants. Moreover, surfactants with longer carbon chains are more likely to be oblique on the rock surface according to the energy gap between the HOMO of the surfactants and the LUMO of the calcite surface. Experimental tests and molecular dynamics (MD) simulations were conducted to analyze the calcite-surfactant interactions. Combined with the free energy (ΔG) based on the contact angle and adsorption energy (E) based on MD simulation, the adsorption ability increases as the carbon chain length decreases. MD simulation is used to understand the form of surfactant molecules on the calcite at an atomic scale at different times. An obvious aggregation of gemini surfactants was found with an increase in the carbon chain length, which reduces the adsorption density and covered area of the surfactant. The adsorption behavior of the gemini surfactant is beneficial for isolating H⁺ transfer and retarding the acid reaction with the rock. The retarding ability and etching morphology were studied by acid etching. The acid-rock reaction rate showed that 12-4-12 had the best inhibition performance. Meanwhile, the uneven surface pattern following 12-4-12 etching is beneficial for maintaining the acid fracturing conductivity.

Directly probing surfactant adsorption on nanoscopic trenches and pillars

HYPOTHESIS

Confinement causes a change in the amount of surfactant adsorbed and in the adsorption morphology.

EXPERIMENTS

Two cationic surfactants, tetradecyltrimethylammonium bromide (TTAB) and cetylpyridinium chloride (CPC) were adsorbed at the silica-water interface. Atomic force microscopy (AFM) force curves were measured on 50 nm and 80 nm wide trenches. Force curves were also measured on silica pillars, and the results were quantified based on distance from the edge.

FINDINGS

Trenches: Adsorbed surfactants films in 50 nm and 80 nm trenches showed the same break-through values. However, compared to unconfined values, TTAB in trenches had decreased break-through and adhesion forces while CPC in trenches had increased break-through and adhesion forces, indicating that surfactant identity varies the confinement effect. Pillars: Near the edge, few surfactants adsorb, and those that do extend in the direction normal to the surface. While the experimental data agree qualitatively with previous coarse-grained molecular dynamic simulations, the length scales at which the phenomena are detected differ by ~ half-order of magnitude. Specifically, experimental data show measurable effects on adsorbed surfactant morphology at a distance from the edge 10-20 times the length of a surfactant molecule after accounting for the ~8 nm size of the probe.

The Role of Chemical Heterogeneity in Surfactant Adsorption at Solid-Liquid Interfaces

Chemical heterogeneity of solid surfaces disrupts the adsorption of surfactants from the bulk liquid. While its presence can hinder the performance of some formulations, bespoke chemical patterning could potentially facilitate controlled adsorption for nanolithography applications. Although some computational studies have investigated the impact of regularly patterned surfaces on surfactant adsorption, in reality, many interesting surfaces are expected to be stochastically disordered and this is an area unexplored via simulations. In this paper, we describe a new algorithm for the generation of randomly disordered chemically heterogeneous surfaces and use it to explore the adsorption behavior of four model nonionic surfactants. Using novel analysis methods, we interrogate both the global surface coverage (adsorption isotherm) and behavior in localized regions. We observe that trends in adsorption characteristics as surfactant size, head/tail ratio, and surface topology are varied and connect these to underlying physical mechanisms. We believe that our methods and approach will prove useful to researchers seeking to tailor surface patterns to calibrate nonionic surfactant adsorption.

Solid-Liquid-Liquid Wettability of Surfactant-Oil-Water Systems and Its Prediction around the Phase Inversion Point

Surfactant-oil-water (SOW) systems are important for numerous applications, including hard surface cleaning, detergency, and enhanced oil-recovery applications. There is limited literature on the wettability of solid-liquid-liquid (SLL) systems around the surfactant phase inversion point (PIP), and the few references that exist point to wettability inversion accompanying the microemulsion (μE) phase inversion. Despite the significance of this phenomenon and the extreme changes in contact angles, there are no models to predict SLL wettability as a function of proximity to the PIP. Recent works on SLL wettability in surfactant-free systems suggest that SLL contact angles can be predicted with an extension of Neumann's equation of state (e-EQS) if the interfacial tension (IFT or γ_o-w) is known and if there is a good estimate for the interfacial energy between the wetting phase and the surface (γ_{S-wetting liquid}). In this work, IFT predictions for SOW systems around the PIP were obtained via the combined hydrophilic-lipophilic difference (HLD) and net-average-curvature (NAC) framework. To test the hypothesis that the combined HLD-NAC + e-EQS can predict wettability inversion around the PIP, with a given γ_S-μE, the contact angles (measured through the light oil phase, θ_O) for the μE of sodium dihexyl sulfosuccinate-toluene-saline water system were measured on high surface free energy (SFE) materials (glass, stainless steel, and mica) and on polytetrafluoroethylene (low SFE) around the PIP. Considering that at the PIP, most systems have a contact angle of 90°, an estimated γ_S-μE = 1/4γ_o-w@PIP was found to be suitable for the systems considered in this work and for systems presented in the literature. The largest deviations between the predictions and the experimental values were found in the positive HLD range (surfactant in the light oil phase). Although there is room for improvement, this framework can estimate the wetting behavior of SOW systems starting solely from formulation parameters.