Oil-soluble Surfactant Research Articles

Mechanisms of CO2-rich industrial waste gas enhanced shale oil recovery in kerogen slit based on adsorption behavior, gas flooding and surfactant synergy study

Enhanced oil recovery (EOR) by injection of CO2-rich industrial waste gas has been proposed as an energy-efficient and environmentally friendly method for shale oil reservoirs, in which the waste gas can be utilized and sequestered to reduce greenhouse gas emissions. In this work, the adsorption behaviors of shale oil and waste gas components in kerogen slits and the effects of gas flooding on EOR were investigated using molecular dynamics (MD) method, which can provide technical guide for shale oil development. In terms of the adsorption behaviors, an intensity decrease of oil adsorption is observed in kerogen slits after the injection of the waste gas components. Desorption of shale oil during injection of SO2 and CO2 is more pronounced in shallow reservoirs, whereas it is more evident for NO and N2 in deep reservoirs. The desorption efficiency can be 10.91 % for pure CO2 injection and 9.09 % for NO injection at a depth of 1000 m, and the relative stability ratios of CO2 and NO sequestration are 77.09 % and 76.06 %, respectively. For gas flooding, the existence of pressure difference provides favorable conditions for gas molecules to pass through the oil zone, and waste gas can be more effective than pure CO2 as driving gas. For the waste gas injection, the percentage of CO2 retained in the kerogen nanopores is much higher than that of pure CO2 injection, suggesting that more CO2 molecules can enter into kerogen matrix to take part in the displacement process. Furthermore, the effect of surfactants on oil desorption and displacement was also investigated and the interaction energy of the surfactant/oil/gas system was calculated and analyzed, finding that addition of the oil-soluble surfactants can be beneficial to shale oil displacement. This work can provide a comprehensive evaluation of the feasibility of CO2-rich industrial waste gas application for EOR, which is of great significance in the effective production of shale oil and the sequestration of waste gas in shale reservoirs.

Extreme Li-Mg selectivity via precise ion size differentiation of polyamide membrane

Achieving high selectivity of Li+ and Mg2+ is of paramount importance for effective lithium extraction from brines, and nanofiltration (NF) membrane plays a critical role in this process. The key to achieving high selectivity lies in the on-demand design of NF membrane pores in accordance with the size difference between Li+ and Mg2+ ions, but this poses a huge challenge for traditional NF membranes and difficult to be realized. In this work, we report the fabrication of polyamide (PA) NF membranes with ultra-high Li+/Mg2+ selectivity by modifying the interfacial polymerization (IP) process between piperazine (PIP) and trimesoyl chloride (TMC) with an oil-soluble surfactant that forms a monolayer at oil/water interface, referred to as OSARIP. The OSARIP benefits to regulate the membrane pores so that all of them are smaller than Mg2+ ions. Under the solely size sieving effect, an exceptional Mg2+ rejection rate of over 99.9% is achieved. This results in an exceptionally high Li+/Mg2+ selectivity, which is one to two orders of magnitude higher than all the currently reported pressure-driven membranes, and even higher than the microporous framework materials, including COFs, MOFs, and POPs. The large enhancement of ion separation performance of NF membranes may innovate the current lithium extraction process and greatly improve the lithium extraction efficiency.

Interfacial Adsorption of Oil-Soluble Kraft Lignin and Stabilization of Water-in-Oil Emulsions.

In this paper, the potential of esterified Kraft lignin as a novel oil-soluble surfactant was examined. The lignin was chemically modified by esterification with lauric or stearic acid, making it soluble in solvents such as toluene or n-decane. Adsorption at the oil-water interface was then studied by the Du Noüy ring-method. The oil-soluble lignin behaved similar to water-soluble lignin surfactants, both the qualitative and quantitative progression of interfacial tension. Modeling revealed a surface excess of 7.5-9.0 × 10-7 mol/m2, area per molecule of 185-222 Å2, and a diffusion coefficient within the range 10-10 to 10-14 m2/s; all of which are in line with existing literature on water-soluble lignosulfonates. The data further suggested that the pendant alkyl chains were extended well into the paraffinic solvent. At last, bottle tests showed that the oil-soluble lignin was able to stabilize oil-in-water emulsions. The emulsion stability was affected by the concentration of lignin or NaCl as well as the oil phase composition. Aromatic oils exhibited lower emulsion stability in comparison to the aliphatic oil. In conclusion, a new type of surfactant was synthesized and studied, which may contribute to developing green surfactants and novel approaches to valorize technical lignin.

High-efficiency mechanism of enhancement of spreading performance of oil involving surfactant-laden oil droplet spreading over water surface

This study clarified the high-efficiency mechanism of enhancement of the spreading performance of oil by allowing a droplet of an organic solvent to spread over the surface of water with the addition of oil-soluble surfactants. Firstly, the effect of Span 85, Span 80, Span 20, octylphenol polyoxyethylene (4) ether (OP-4), and sodium dioctyl sulfosuccinate (AOT) on the interfacial parameters of an n-decane droplet was evaluated by measuring the surface tension and interfacial tension. Then the spreading coefficient was calculated to determine whether the surfactant-laden oil droplets could spread at the air–water interface. Moreover, the law governing the spreading of the surfactant-laden oil droplets over water was systematically studied. Furthermore, the characteristics of the dynamic behavior of the surfactants were investigated by molecular dynamics simulations to elucidate the spreading mechanism. The results of the experiments and simulations explained how the oil-soluble surfactants Span 85, Span 80, Span 20, OP-4, and AOT significantly improved the spreading performance of oil droplets over the water surface.

Comparison of interfacial interactions between bovine serum albumin with water- and oil-soluble surfactants: An interfacial rheological study

The interfacial interactions between bovine serum albumin (BSA) with a water-soluble surfactant polyoxyethylene sorbitan monooleate (Tween 80) and an oil-soluble surfactant sorbitan monooleate (Span 80), respectively, were studied using the interfacial dilatational rheological experiments. The BSA molecules were irreversibly adsorbed at interface and formed a network with high dilatational modulus. The increasing interfacial pressure induced the transformation of BSA molecules from a compact globular to an unfolded flexible conformation, which enhanced the relaxation process. Tween 80 formed hydrophilic complexes with BSA in the aqueous phase, significantly slowing down the dynamic adsorption. In addition, Tween 80 played a dominant role at the interface due to its higher interfacial activity, thus inhibiting the interfacial performance of BSA. As BSA and Span 80 diffused towards the interface from different phases, they adsorbed rapidly at the interface and formed a mixed adsorption layer, thus exhibiting synergistic effects in decreasing the interfacial tension. However, the alternate arrangement of BSA and Span 80 at the interface disrupted the inter-protein interactions, resulting in a significant decrease in the dilatational modulus of the interfacial film. The obtained results showed that Span 80 from the oil phase and Tween 80 from the aqueous phase interacted with the protein in different ways, and thus presented distinct dynamic adsorption processes and interfacial performance. The present work provided a theoretical foundation for their practical application in food, cosmetic, and pharmaceutical industries.

Directions for Improving the Compositions of Reverse Emulsions for Well Plugging

Well killing is the most common technological process in oil production. Inverse emulsions are used as an effective fluid in the combined killing method. At present, some potential possibilities of inverse emulsions have not been analyzed both in terms of improving their compositions and technological methods of practical implementation. The paper considers a number of promising research areas for obtaining, improving the compositions of inverted emulsions and the method of their application with injection into the bottomhole formation zone before filling the wellbore with mineralized water. Natural oils of co-emulsifiers - ethoxylated oil-soluble surfactants, and filtration reducers - technical lignosulfonates, starch, latex were proposed as constituent components of hydrocarbon media. Attention was focused on the need for experimental evaluation of the rate of settling of reverse emulsions along the wellbore in formation water and oil, obtaining and studying stable reverse emulsions using high-density salts of CaBr2, ZnBr2, ZnCl2, polyhydric alcohols.

Triglyceride mixtures: Cold-bursting and double emulsion formation

It was shown recently that a solid-to-solid phase transitions (α-to-β, gel-to-crystal), typical for many lipid substances, can lead to formation of nanoporous network inside lipid micro-particles dispersed in aqueous surfactant solutions (Cholakova et al. ACS Nano2020, 14, 8594). These nanopores are spontaneously infused by the aqueous phase when appropriate combination of water-soluble and oil-soluble surfactants is applied. As a result, the initial lipid micro-particles can spontaneously burst into much smaller nanoparticles, just by cooling and heating of the initial dispersion. Under certain conditions, the infused aqueous phase is entrapped in the moment of lipid particle melting and double emulsion of type water-in-oil-in-water (W/O/W) is formed. The current study aims to clarify how the composition of the lipid micro-drops and surfactants affect the observed phenomena. Selected mixtures of monoacid triglycerides are studied systematically. The results show that the bursting efficiency usually decreases when the complexity of the lipid mixture increases, due to the expanded temperature interval for lipid melting. Nevertheless, complete particle bursting and lipid nanoparticles with diameters down to 20 nm are formed even for the most complex lipid compositions under appropriate conditions. The key mechanisms leading to efficient fragmentation and double emulsion formation are clarified, and the main governing factors are explored. On this basis, we reveal that the system behavior can be switched between complete particle bursting and W/O/W emulsion formation by: (1) Change in the cooling and heating rates without any changes in the chemical composition, (2) Change in the concentration of oil-soluble surfactant, and/or (3) Change in the phase in which the oil-soluble surfactant is introduced initially. Thus, we have formulated guiding rules to control the formation of lipid nanoparticles and W/O/W emulsions with triglyceride mixtures promising multiple potential applications.

Effect of Oil-Soluble/Water-Soluble Surfactants on the Stability of Water-in-Oil Systems, an Atomic Force Microscopy Study.

The stabilization mechanism of water-in-oil (W/O) emulsions has been studied by measuring the interactions between two water droplets in n-tetradecane using atomic force microscopy. The effects of water-soluble surfactants (SDS/CTAB/Tween 80), an oil-soluble surfactant (Span 20), and the coexistence of the water and oil-soluble surfactants on the stability of water droplets in oil were investigated separately. It is found that the addition of oil-soluble surfactants (Span 20) prevents the coalescence of water droplets in oil. To discuss the role of an oil-soluble surfactant, we analyzed the force curve by applying the theoretical model. The results demonstrate that the oil-soluble surfactant (Span 20) stabilizes dispersed droplets by adsorbing onto the interface and forming a relatively tighter layer with the increase in surfactant concentration, which hinders film rupture. This behavior of the surfactant could also be properly characterized by steric hindrance. A further step was taken by introducing another water-soluble surfactant. It is found that the addition of either SDS or CTAB into the water phase is futile in inducing droplet coalescence in the presence of Span 20. In contrast, Tween 80 was found to be effective in destabilizing water droplets, which could be due to the competitive adsorption between Tween 80 and Span 20 at the interface. By characterizing the interfacial adsorption of Tween 80 and Span 20 with a theoretical adsorption isotherm model, the result indicates that interface replacement would result in a loose adsorption layer that is insufficient to hinder droplet coalescence. Our study provides an intriguing understanding of the role of surfactants in the stabilization and destabilization of water-in-oil emulsions.

The influence of vegetable additives on the flocculation of asphaltenes in oil dispersed systems

The study examines the existing asphaltene flocculation inhibitors in oil dispersed systems — oil-soluble surfactants of various nature. It is proposed to use cheap surfactants of vegetable origin as flocculation inhibitors — phosphatide concentrate and sodium sulfosuccinate. The process of flocculation of asphaltenes on model mixtures of asphaltenes of different concentrations with toluene and precipitants, as well as the influence of the proposed inhibitors on the aggregative stability of the mixtures, was studied. The flocculation starting point (onset point) of each mixture was determined by the photocolorimetric method.

Temperature-Triggered Release of Chromium Chloride from Nanocapsules for Controlled Burst Release and Gelation of Hydrolyzed Polyacrylamide to Plug High-Permeability Channels

Summary Chromium(III) (Cr3+)-hydrolyzed polyacrylamide (HPAM) gels have been applied extensively as blocking agents for sweep efficiency improvement. Previous studies focused on delaying the gelation time and ignored the diffusion of the crosslinkers during the transportation process. The gelation time of Cr3+-HPAM was too long to be controlled. This study systematically describes a novel approach of using thermoresponsive nanocapsules to precisely control the release of Cr3+. The nanocapsules are successfully prepared by a controlled nanoprecipitation of hydrophobic polymers [poly (methyl methacrylate)] (PMMA) and Pluronic® P-123 onto stable aqueous Cr3+ solution nanodroplets. The stable aqueous nanodroplets are obtained by double inverse miniemulsions with oil-soluble surfactant Span® 80. The nanoprecipitation occurs when heating the mixture at 50℃, which leads to the evaporation of solvent and precipitation of the PMMA into the interface of the aqueous droplets to form the shells. Pluronic P-123 is introduced to stabilize the double miniemulsion and enhance the precipitation efficiency of the shell polymer during the fabrication process. The fabricated nanocapsules show a size range from 211.9 to 297.2 nm depending on the feed contents of the Cr3+. The thermoresponsive function of Pluronic P-123 is studied and applied as a temperature-trigger on the shell. Gelation results show that the thermoresponsivity of Pluronic P-123 dominates the release rate rather than the diffusion rate through PMMA, which could be used to shorten the gelation interval time.

Physical mechanisms for droplet size and effective viscosity asymmetries in turbulent emulsions

By varying the oil volume fraction, the microscopic droplet size and the macroscopic rheology of emulsions are investigated in a Taylor–Couette turbulent shear flow. Although here oil and water in the emulsions have almost the same physical properties (density and viscosity), unexpectedly, we find that oil-in-water (O/W) and water-in-oil (W/O) emulsions have very distinct hydrodynamic behaviours, i.e. the system is clearly asymmetric. By looking at the micro-scales, the average droplet diameter hardly changes with the oil volume fraction for O/W or for W/O. However, for W/O it is about $50\,\%$ larger than that of O/W. At the macro-scales, the effective viscosity of O/W is higher when compared to that of W/O. These asymmetric behaviours are expected to be caused by the presence of surface-active contaminants from the walls of the system. By introducing an oil-soluble surfactant at high concentration, remarkably, we recover the symmetry (droplet size and effective viscosity) between O/W and W/O emulsions. Based on this, we suggest a possible mechanism responsible for the initial asymmetry and reach conclusions on emulsions where interfaces are fully covered by the surfactant. Next, we discuss what sets the droplet size in turbulent emulsions. We uncover a $-6/5$ scaling dependence of the droplet size on the Reynolds number of the flow. Combining the scaling dependence and the droplet Weber number, we conclude that the droplet fragmentation, which determines the droplet size, occurs within the boundary layer and is controlled by the dynamic pressure caused by the gradient of the mean flow, as proposed by Levich (Physicochemical Hydrodynamics, Prentice-Hall, 1962), instead of the dynamic pressure due to turbulent fluctuations, as proposed by Kolmogorov (Dokl. Akad. Nauk. SSSR, vol. 66, 1949, pp. 825–828). The present findings provide an understanding of both the microscopic droplet formation and the macroscopic rheological behaviours in dynamic emulsification, and connects them.

Phase inversion emulsification of paraffin oil/polyethylene wax blend in water: A comparison between mixed monomeric and monomeric/gemini surfactant systems

• Natural-based gemini surfactant (GS) reduced water surface tension more than Tween80. • Using GS reduced water phase volume fraction at phase inversion point (f w,inv ). • Change in f w,inv with polyethylene was alike change in phases interfacial tension. • Rheological properties of emulsions highly depended on crystallinity of oil phase. • Higher stability of emulsions was achieved by GS due to their higher zeta potential. Despite the environmental attractiveness of phase inversion emulsification, polyethylene wax-based emulsions are prepared by high-energy emulsification methods. In this research, phase inversion emulsification of paraffin oil/high density polyethylene wax (PEW) blend in water was conducted using mixed surfactants systems. Sorbitan monooleate (Span 80) was used as the oil-soluble surfactant, while either polysorbate 80 (Tween 80) or a synthesized sunflower oil-based gemini surfactant (SF6) was utilized as the water-soluble surfactant. In order to conduct phase inversion emulsification, the 85 °C water phase containing 5 wt% Tween 80 or SF6 was added to the melted oil phase containing 30 to 60 wt% PEW in the blend and 5 wt% Span 80 at 120 °C with the addition rate of 5 mL/min. Phase inversion point was detected by monitoring the dissolution of the emulsion in water during emulsification and proved by electrical conductivity measurements. A lower effectiveness was found for the mixed monomeric surfactant system compared to the gemini surfactant-containing system. For each surfactant system, the trend of change in water phase volume fraction at phase inversion point upon increase in the PEW content in the oil phase was in accordance with the trend of change in the interfacial tension between the water phase and the oil phase. Using the mixed monomeric/gemini surfactant system resulted in emulsions with larger values of absolute zeta potential and drop sizes comparable to their monomeric surfactant-containing counterparts. The absolute zeta potential of the emulsions containing 30, 50, and 60 wt% PEW in the blend was about 70, 54, and 13 mV higher in case of using the mixed monomeric/gemini surfactant system. The gemini surfactant-containing emulsions demonstrated less shear-thinning behavior and the general trend of change in their viscosity and storage modulus with formulation was in agreement with that in the enthalpy of melting of the dried emulsions. Usage of the mixed monomeric/gemini surfactant system led to higher long-term stability of the emulsions, revealed by lower rate of water evaporation from the emulsions. The results of the current study can be applied for preparation of non-polar polyethylene wax-based emulsions using phase inversion emulsification technique. This investigation also opens up the opportunity to use efficient gemini surfactants in low-energy phase inversion emulsification.

Analysis of influence of surfactant on the properties of diluted asphalt mixtures

In order to reduce the dosage of diluent in diluted asphalt mixture, this paper introduces a surfactant to improve the performance of asphalt mixture, the oil-soluble surfactant was added to the diluted asphalt mixture, and the effect of oil-soluble surfactant on the performance of diluted asphalt mixture during construction was investigated by mixing adhesion test, penetration test, cohesive test and rotary compaction test. At the same time, combined with rutting test, low temperature bending test and Marshall test, the effect of oil soluble surfactant on the performance of diluted asphalt mixture in service period was analyzed.The test results show that 1% oil-soluble surfactant can reduce diesel diluent at least 4%, and the addition of surfactant can significantly improve the construction performance of diluted asphalt mixture. At the same diluent dosage, the strength of asphalt mixture diluted by surfactant increases significantly, the low temperature performance and water stability are improved, and the high temperature performance is slightly insufficient and slightly reduced. The other road performance can meet the standard of hot mix.

Probiotic encapsulation in water-in-oil high internal phase emulsions: Enhancement of viability under food and gastrointestinal conditions

Polyglycerol polyricinoleate (PGPR) was used as the oil-soluble surfactant and beeswax was used as the oil phase to formulate a water-in-oil (W/O) high internal phase emulsions (HIPEs) for the encapsulation and protection of probiotics. The physicochemical properties of the W/O HIPEs and the survival of the encapsulated probiotics when exposed to acidic conditions and in vitro digestion were investigated. The viability of the probiotics decreased slightly when exposed to high-speed shearing. The rheological analysis, microstructural images, physicochemical stability showed that the W/O HIPEs remained relatively stable. The survival of the probiotics loaded in the SK-HIPEs (prepared with skim milk) was much higher than in the NS-HIPEs (prepared with normal saline) during storage at 4 °C. An in vitro gastrointestinal model showed that encapsulation of the probiotics enhanced their survival. This study provides useful insights into the utilization of W/O HIPEs to improve the efficacy of probiotics in the food industry.

UCST-Type Polymer Capsules Formed by Interfacial Complexation.

Formation of aqueous-core polymer capsules exhibiting an upper critical solution temperature (UCST) was achieved using surfactant-polymer interfacial complexation in water-in-oil inverse emulsions. In fluorinated oil, Coulombic interactions between Krytox, an anionic oil-soluble surfactant, and a cationic poly(lysine) grafted with poly(acrylamide-co-acrylonitrile) enabled the formation of an adsorbed polymer shell at the surface of water droplets. The thermoresponsiveness of the polymer shell was assessed by fluorescence microscopy with and without the presence of nanoparticles, including gold particles. We show that, above the cloud point, polymers with a balanced fraction of UCST grafts form flat adlayers that (i) spontaneously entrap nanoparticles upon cooling and (ii) switch from fluid-like dynamics at high temperature to solid-like dynamics below the cloud point. This system offers a straightforward mean to prepare temperature-sensitive capsules in mild, biocompatible conditions and to concentrate nanoparticles (including nanoheaters) in their shell.

Determent of oil-soluble surfactants on aggregation of model asphaltene compound and synergistic effect of their mixtures on foaming property

Oil-based foam stabilized by dispersed asphaltene is potential in the oil recovery process of water-sensitive formations. But asphaltenes are prone to aggregate, which jeopardizes the stability of the foam. This paper investigated the effect of different VO79 (model asphaltene compound violanthrone-79) systems on the performance of oil-based foams through surface tension and foaming tests. And simulations were performed to investigate the microscopic mechanism of ODA (octadecyl amine) and Span20 (sorbitan monolaurate) on the dispersion effect of VO79 and the effect of their mixtures on the interfacial film formation. The experimental results showed that the mixed system of ODA, Span20, and VO79 significantly enhances the performance of the oil-based foam. According to simulative results, ODA and Span20 blending are able to form ODA: Span20 molecular pairs, inhibiting the aggregation of model asphaltenes. The blend of ODA, Span20, and VO79 tends to form a ternary supramolecular complex, assembling into a dense and stretchable interfacial film. The synergistic effect generated by the ternary blends is capable of optimizing the performance of oil-based foams.

Microfluidic emulsification with a surfactant and a particulate emulsifier: Dripping-to-jetting transitions and drop size scaling

We report an experimental investigation on the effect of emulsifiers on the dripping-to-jetting transition and droplet size scaling in a co-flow microcapillary device. The emulsifiers studied include an oil-soluble surfactant Span 80, a water-soluble nanosheet, and their combination. We characterize the dispersed phase behaviors in the state space comprising the Weber number of the inner phase Wein and the capillary number of the outer phase Caout. Results show that the dripping-to-jetting (D-J) transition occurs at Wein ≈ O(1) for the no-emulsifier case and Wein ≈ 65 for the Span 80 system. In the case of the nanosheet, the D-J transition is dependent on concentration with Wein ≈ 0.08–5.6. For the combination system, depending on the nanosheet concentration, the critical Wein varies from 0.3 to 21. We explain that these emulsifier-dependent critical Wein result in terms of additional interfacial stresses arising due to surfactant transport and adsorbed nanosheets. Combining droplet size data from this study and literature, we find that in the dripping regime, the droplet diameter decreases with increasing Caout, yielding a scaling exponent of ≈−0.33 and a pre-factor that could be dependent on the type of emulsifier. In the jetting regime, the droplet diameter increases with the flow rate ratio of the two phases, with a scaling exponent of ≈ 0.37 that is independent of the type of emulsifier. The results from this work help to better understand the influence of surfactants and particulate emulsifiers in microfluidic emulsification.

Oil soluble surfactants as efficient foam stabilizers

The surface and foam properties of two oil-soluble surfactants, Span 60 and Brij 72, and two water-soluble surfactants, Tween 60 and Brij S20, are compared. Aqueous surfactant solutions containing sugar at high concentration (63 wt%) are also studied in the context of sweet food-related foams. The experimental results show that foams with very small bubbles of ca. 5 µm are formed and remain stable for more than 10 days at room temperature when oil-soluble surfactants are dispersed in the sugar-rich solutions. The excellent stability of these foams is related to (1) The multilamellar vesicles present in the respective surfactant dispersions which increase the viscosity and decrease the rate of water drainage from the foam; (2) The very slow exchange of surfactant molecules between the interface and the adjacent aqueous solution that leads to very significant difference in the surface tension of shrinking and expanding bubbles, which in turn decreases the rate of bubble Ostwald ripening. The foams formed from Span 60 are more stable as compared to the foams formed from Brij 72, due to the higher viscosity and the slower interface-solution exchange. The foams formed from Brij S20 solutions are least stable - they are completely destroyed after 1 day at room temperature, even in the presence of sugar. The main conclusion of this study is that oil soluble surfactants can form condense adsorption layers on the bubble surfaces after adsorption from aqueous solutions with and without sugar in it. The formed layers, which present a better molecular packing, reduce the gas permeability and decelerate the Ostwald ripening disproportionation of the foam This is obtained by a relatively low interfacial tension imparted by continuous shrinkage and expansion of bubbles during foam generation, ultimately ensuring long-term stabilization of formed foams.

PH-Dependent Interfacial Tension and Dilatational Modulus Synergism of Oil-Soluble Fatty Acid and Water-Soluble Cationic Surfactants at the Oil/Water Interface.

While the concept of interfacial tension synergism in surfactant mixtures is well established, little attention has been paid to the possibility of synergistic effects on the interfacial rheology of mixed surfactant systems. Furthermore, interfacial tension synergism is most often investigated for mixtures of surfactants residing in a single phase. Here, we define dilatational modulus synergism and report a study of interfacial tension isotherms and complex dilatational moduli for a binary surfactant system with the two surfactants accessing the oil/water interface from opposite sides. Using an oil-soluble fatty acid surfactant (palmitic acid, PA) that may be ionized at the oil/water interface and a quaternary ammonium water-soluble cationic surfactant (tetradecyltrimethylammonium bromide, TTAB), the binary interfacial interaction was tuned by the aqueous phase pH. Interfacial tensions and dilatational moduli were measured by the pendant drop method for the binary surfactant system as well as the corresponding single-surfactant systems to identify synergistic effects. The possible occurrence of dilatational modulus synergism was probed from two perspectives: one for a fixed total surfactant concentration and the other for a fixed interfacial tension. The aqueous pH was found to have a controlling effect on both interfacial tension synergism and the dilatational modulus synergism. The conditions for interfacial tension synergism coincided with those for the storage modulus synergism: both tension and storage modulus synergisms were observed under all conditions tested at pH 7 where PA was mostly deprotonated, for both perspectives examined, but not for any conditions tested at pH 3 where PA is mostly protonated. The loss modulus synergism exhibited more complex behaviors, such as frequency and interfacial tension dependences, but again was only observed at pH 7. The tension and modulus synergism at pH 7 were attributed to the increased attraction between ionized PA and cationic TTAB and the formation of catanionic complexes at the oil/water interface.

Synergistic and Competitive Adsorption of Hydrophilic Nanoparticles and Oil-Soluble Surfactants at the Oil-Water Interface.

Fundamental insights into the interplay and self-assembly of nanoparticles and surface-active agents at the liquid-liquid interface play a pivotal role in understanding the ubiquitous colloidal systems present in our natural surroundings, including foods and aquatic life, and in the industry for emulsion stabilization, drug delivery, or enhanced oil recovery. Moreover, well-controlled model systems for mixed interfacial adsorption of nanoparticles and surfactants allow unprecedented insights into nonideal or contaminated particle-stabilized emulsions. Here, we investigate such a model system composed of hydrophilic, negatively, and positively charged silica nanoparticles and the oil-soluble cationic lipid octadecyl amine with in situ synchrotron-based X-ray reflectometry, which is analyzed and discussed jointly with dynamic interfacial tensiometry. Our results indicate that negatively charged silica nanoparticles only adsorb if the oil-water interface is covered with the positively charged lipid, indicating synergistic adsorption. Conversely, the positively charged nanoparticles readily adsorb on their own, but compete with octadecyl amine and reversibly desorb with increasing concentrations of the lipid. These results further indicate that with competitive adsorption, an electrostatic exclusion zone exists around the adsorbed particles. This prevents the adsorption of lipid molecules in this area, leading to a decreased surface excess concentration of surfactants and unexpectedly high interfacial tension.