The Interfacial Dilational Rheology of Surfactant Solutions with Low Interfacial Tension

In this paper, the spinning drop method was used to measure the oil-water interfacial dilational modulus of four different types of surfactants with low interfacial tension (IFT), including the anionic surfactant sodium dodecyl sulfate (SDS), the nonionic surfactant Triton X-100 (TX100), the zwitterionic surfactant alkyl sulfobetaine (ASB), and the extended surfactant alkyl polyoxypropyl ether sodium sulfate (S-C13PO13S). Based on the experimental results, we found that the spinning drop method is an effective means of measuring the interfacial dilational modulus of the oil-water interface with an IFT value of lower than 10 mN/m. For common surfactants SDS and TX100, the interfacial dilational modulus decreases rapidly to near zero with an increase in concentration when the IFT is lower than 1 mN/m. On the other hand, ASB has the highest interfacial dilatation modulus of 50 mN/m, which comes from the flatness of its unique hydrophilic group structure. The interfacial dilational modulus of S-C13PO13S showed a moderate plateau value of 30 mN/m with a broader concentration change. This is due to the fact that the main relaxation process dominating the interfacial film properties comes from the long helical polyoxypropyl chain. Through the large-size hydrophilic groups in betaine molecules and the long PO chains in the extended surfactant molecules, an interfacial film with controllable strength can be formed in a low IFT system to obtain a higher interfacial dilational modulus. This is of great significance in improving the emulsification and oil displacement of chemical flooding in reservoir pores.

pH-Switchable Surfactant-Based Microemulsions: Reversible Transition between Microemulsification and Demulsification Triggered by Suitable Acids and Bases

pH-switchable surfactant-based microemulsions (SBMEs) are those that can switch reversibly between a monophasic state and a fully phase-separated state under the alternation of acids and bases, which is rarely reported. By using an equimolar mixture of sodium dodecyl sulfate and N,N-dimethyldodecylamine (SDS-C12A) as a pH-switchable surfactant, a pH-switchable SDS-C12A-based microemulsion (SDS-C12A-ME) has been fabricated for the first time. The main principles of the reversible switching are the reversible destruction/formation of the emulsifier, SDS-C12A-n-butanol, film at the oil-water interface due to the alternating protonation/deprotonation of C12A caused by acids and bases. The byproducts, H2O and salt, had an adverse effect on the reversibility of SDS-C12A-ME, with salt having a greater adverse effect than H2O. However, the reversibility of SDS-C12A-ME could be enhanced by suitable acids and bases. For example, for the same oil-in-water (O/W) SDS-C12A-ME, the number of switching cycles with HCl-choline hydroxide (ChOH) as a stimulus can be as large as 11, but only 3 with HCl-NaOH as a stimulus. By using the methyl methacrylate photochemical polymerization as a model, such a pH-switchable SBME can function as a recyclable reaction medium, while the resultant poly(methyl methacrylate) has a considerably reproducible molecular weight and narrow molecular weight distribution (polydispersity index is around 1.2) over three cycles. It is anticipated that the results presented in this work will serve as a reference for the design and fabrication of pH-switched SBMEs and also that such pH-switched SBMEs may have potential applications in practical technological areas such as industrial reaction media, drug delivery, microreactors, etc.

Viscoelasticity of a carbon nanotube-laden air-water interface

The viscoelasticity of a carbon nanotube (CNT)-laden air-water interface was characterized using two different experimental methods. The first experimental method used a Langmuir-Pockels (LP) trough coupled with a pair of oscillating barriers. The second method is termed the Bicone-Trough (BT) method, where a LP trough was custom-built and fit onto a rheometer equipped with a bicone fixture to standardize the preparation and conditioning of a particle-laden interface especially at high particle coverages. The performance of both methods was evaluated by performing Fast Fourier Transform (FFT) analysis to calculate the signal-to-noise ratios (SNR). Overall, the rheometer-based BT method offered better strain control and considerably higher SNRs compared to the Oscillatory Barriers (OB) method that oscillated barriers with relatively limited positional and speed control. For a CNT surface coverage of 165 mg/m2 and a frequency of 100 mHz, the interfacial shear modulus obtained from the OB method increased from 39 to 57 mN/m as the normal strain amplitude increased from 1 to 3%. No linear viscoelastic regime was experimentally observed for a normal strain as small as 0.5%. In the BT method, a linear regime was observed below a shear strain of 0.1%. The interfacial shear modulus decreased significantly from 96 to 2 mN/m as the shear strain amplitude increased from 0.025 to 10%.

Synergistic Effect of Salt and Anionic Surfactants on Interfacial Tension Reduction: Insights from Molecular Dynamics Simulations

Surfactants are commonly utilized in chemical flooding processes alongside salt to effectively decrease interfacial tension (IFT). However, the underlying microscopic mechanism for the synergistic effect of salt and surfactants on oil displacement remains ambiguous. Herein, the structure and properties of the interface between water and n-dodecane are studied by means of molecular dynamics simulations, considering three types of anionic surfactants and two types of salts. As the salt concentration (ρsalt) increases, the IFT first decreases to a minimum value, followed by a subsequent increase to higher values. The salt ions reduce the IFT only at low ρsalt due to the salt screening effect and ion bridging effect, both of which contribute to a decrease in the nearest head-to-head distance of surfactants. By incorporating salt doping, the IFTs can be reduced by at most 5%. Notably, the IFTs of different surfactants are mainly determined by the hydrogen bond interactions between oxygen atoms in the headgroup and water molecules. The presence of a greater number of oxygen atoms corresponds to lower IFT values. Specifically, for alkyl ethoxylate sulfate, the ethoxy groups play a crucial role in reducing the IFTs. This study provides valuable insights into formulating anionic surfactants that are applicable to oil recovery processes in petroleum reservoirs using saline water.

An in-depth investigation of the impact of salt nature on the formulation of microemulsion systems

Electrolytes have a wide range of technological applications. Despite the recent improvements in characterizing and predicting the phase behavior of microemulsion systems by hydrophilic-lipophilic deviation (HLD) and net-average curvature (NAC) frameworks, they are ineffective in the presence of different salts. This work seeks to bridge this gap by investigating the influence of salt nature on the microemulsion phase formulation. First, a one-dimensional salinity scan on different microemulsion systems consisting of sodium dodecyl benzene sulfonate as a surfactant, hexane as an oil and, several brines was carried out, and the effect of each salt on the phase behavior were precisely evaluated. The results for optimum salinity and solubilization parameter of different salts were consistent with the Hofmeister series. In addition, multiple linear regression model is presented to accurately predicting the optimum salinity of different salts using this research data and all the available experimental data. The results revealed that the values estimated by this model is in significant consistency with the experimental data by correlation coefficient of 0.92. Finally, the effect of salt type on the NAC parameters (length parameter, and characteristic length[Formula: see text] were evaluated to improve the predicting ability of this equation of state in the presence of various salts. We found that salt nature has a significant impact on both these parameters. It was found that the length parameter is linearly dependent on the optimum ionic strength of salts while the salting-out capacity of each salt was predominant factor affecting the characteristic length.

Binding of Ca2+ Ions to Alkylbenzene Sulfonates: Micelle Formation, Second Critical Concentration and Precipitation

Anionic surfactants, such as sodium linear alkylbenzene sulfonates (NaLAS), are utilized in various fields, including industry, household, and agriculture. The efficiency of their use in aqueous environments is significantly affected by the presence of cations, Ca2+ and Mg2+ in particular, as they can decrease the concentration of the surfactant due to precipitation. To understand cation-sulfonate interactions better, we study both NaLAS colloidal solutions in the presence of CaCl2 and precipitates forming at higher salt concentrations. Upon addition of CaCl2, we find the surface tension and critical micelle concentration of NaLAS to decrease significantly, in line with earlier findings for alkylbenzylsulfonates in the presence of divalent cations. Strikingly, an increase in the surface tension is discernible above 0.6 g L-1 NaLAS, accompanied by the decrease of apparent micelle sizes, which in turn gives rise to transparent systems. Thus, there appears to be a second critical concentration indicating another micellar equilibrium. Furthermore, the maximum salt tolerance of the surfactant is 0.1 g L-1 Ca2+, above which rapid precipitation occurs yielding sparingly soluble CaLAS2∙2H2O.

Effect of Divalent and Monovalent Salts on Interfacial Dilational Rheology of Sodium Dodecylbenzene Sulfonate Solutions

This study presents the equilibrium surface tension (ST), critical micelle concentration (CMC) and the dilational viscoelasticity of sodium dodecylbenzene sulfonate (SDBS)-adsorbed layers in the presence of NaCl, KCl, LiCl, CaCl2 and MgCl2 at 0.001–0.1 M salt concentration. The ST and surface dilational viscoelasticity were determined using bubble-shape analysis technique. To capture the complete profile of dilational viscoelastic properties of SDBS-adsorbed layers, experiments were conducted within a wide range of SDBS concentrations at a fixed oscillating frequency of 0.01 Hz. Salts were found to lower the ST and induce micellar formation at all concentrations. However, the addition of salts increased dilational viscoelastic modulus only at a certain range of SDBS concentration (below 0.01–0.02 mM SDBS). Above this concentration range, salts decreased dilational viscoelasticity due to the domination of the induced molecular exchange dampening the ST gradient. The dilational viscoelasticity of the salts of interest were in the order CaCl2 > MgCl2 > KCl > NaCl > LiCl. The charge density of ions was found as the corresponding factor for the higher impact of divalent ions compared to monovalent ions, while the impact of monovalent ions was assigned to the degree of matching in water affinities, and thereby the tendency for ion-pairing between SDBS head groups and monovalent ions.

Salt-induced stability and modified interfacial energetics in self-faceting emulsion droplets

HYPOTHESIS

The counterintuitive temperature-controlled self-faceting of water-suspended, surfactant-stabilized, liquid oil droplets provides new opportunities in engineering of smart liquids, the properties of which are controllable by external stimuli. However, many emulsions exhibiting self-faceting phenomena have limited stability due to surfactant precipitation. The emulsions' stability may be enhanced, and their inter-droplet electrostatic repulsion tuned, through controlled charge screening driven by varying-concentration added salts. Moreover, in many technologically-relevant situations, salts may already exist in the emulsion's aqueous phase. Yet, salts' impact on self-faceting effects has never been explored. We hypothesize that the self-faceting transitions' temperatures, and stability against surfactant precipitation, of ionic-surfactants-stabilized emulsions are significantly modified by salt introduction.

EXPERIMENTS

We explore the temperature-dependent impact of NaCl and CsCl salt concentration on the emulsions' phase diagrams, employing optical microscopy of emulsion droplet shapes and interfacial tension measurements, both sensitive to interfacial phase transitions.

FINDINGS

A salt concentration dependent increase in the self-faceting transition temperatures is found, and its mechanism elucidated. Our findings allow for a significant enhancement of the emulsions' stability, and provide the physical understanding necessary for future progress in research and applications of self-faceting phenomena in salt-containing emulsions.

The Oscillatory Spinning Drop Technique. An Innovative Method to Measure Dilational Interfacial Rheological Properties of Brine-Crude Oil Systems in the Presence of Asphaltenes

The oscillatory spinning drop method has been proven recently to be an accurate technique to measure dilational interfacial rheological properties. It is the only available equipment for measuring dilational moduli in low interfacial tension systems, as it is the case in applications dealing with surfactant-oil-water three-phase behavior like enhanced oil recovery, crude oil dehydration, or extreme microemulsion solubilization. Different systems can be studied, bubble-in-liquid, oil-in-water, microemulsion-in-water, oil-in-microemulsion, and systems with the presence of complex natural surfactants like asphaltene aggregates or particles. The technique allows studying the characteristics and properties of water/oil interfaces, particularly when the oil contains asphaltenes and when surfactants are present. In this work, we present a review of the measurements of crude oil-brine interfaces with the oscillating spinning drop technique. The review is divided into four sections. First, an introduction on the oscillating spinning drop technique, fundamental and applied concepts are presented. The three sections that follow are divided according to the complexity of the systems measured with the oscillating spinning drop, starting with simple surfactant-oil-water systems. Then the complexity increases, presenting interfacial rheology properties of crude oil-brine systems, and finally, more complex surfactant-crude oil-brine systems are reviewed. We have found that using the oscillating spinning drop method to measure interfacial rheology properties can help make precise measurements in a reasonable amount of time. This is of significance when systems with long equilibration times, e.g., asphaltene or high molecular weight surfactant-containing systems are measured, or with systems formulated with a demulsifier which is generally associated with low interfacial tension.

How to Use the Normalized Hydrophilic-Lipophilic Deviation (HLDN) Concept for the Formulation of Equilibrated and Emulsified Surfactant-Oil-Water Systems for Cosmetics and Pharmaceutical Products

The effects of surfactant molecules involved in macro-, mini-, nano-, and microemulsions used in cosmetics and pharmaceuticals are related to their amphiphilic interactions with oil and water phases. Basic ideas on their behavior when they are put together in a system have resulted in the energy balance concept labeled the hydrophilic-lipophilic deviation (HLD) from optimum formulation. This semiempirical equation integrates in a simple linear relationship the effects of six to eight variables including surfactant head and tail, sometimes a cosurfactant, oil-phase nature, aqueous-phase salinity, temperature, and pressure. This is undoubtedly much more efficient than the hydrophilic-lipophilic balance (HLB) which has been used since 1950. The new HLD is quite important because it allows researchers to model and somehow predict the phase behavior, the interfacial tension between oil and water phases, their solubilization in single-phase microemulsion, as well as the corresponding properties for various kinds of macroemulsions. However, the HLD correlation, which has been developed and used in petroleum applications, is sometimes difficult to apply accurately in real cases involving ionic–nonionic surfactant mixtures and natural polar oils, as it is the case in cosmetics and pharmaceuticals. This review shows the confusion resulting from the multiple definitions of HLD and of the surfactant parameter, and proposes a “normalized” Hydrophilic-Lipophilic Deviation (HLDN) equation with a surfactant contribution parameter (SCP), to handle more exactly the effects of formulation variables on the phase behavior and the micro/macroemulsion properties.