Hydrotrope-induced inversion of salt effects on the cloud point of an extended surfactant

Methods of changing low molecular weight gel properties through gelation kinetics

Low molecular weight gels continue to attract notable interest, with many potential applications. However, there are still significant gaps in our understanding of these systems and the correlation between the pre-gel and final gel states. The kinetics of the gelation process plays a crucial role in the bulk properties of the hydrogel and presents an opportunity to fine-tune these systems to meet the requirements of the chosen application. Therefore, it is possible to use a single gelator for multiple applications. This review discusses four ways to modify the pre-gelled structures before triggering gelation. Such modifications can enhance the material's intended performance, which may result in significant advancements in high-tech areas, such as drug delivery, cell culturing, electronics, and tissue engineering.

Study on the Behavior of Saturated Cardanol-Based Surfactants at the Crude Oil/Water Interface through Molecular Dynamics Simulations

Cardanol is a green biosurfactant with broad application prospects, which is expected to be used to enhance oil recovery (EOR). This paper designed two types of surfactants (extended and nonextended), including six kinds of nonionic and anion-nonionic surfactants. The position changes of PO and EO chains and the effects of different hydrophilic groups on the interface properties were studied with molecular dynamics simulations by constructing a model of crude oil (containing four components) and water molecules. The results of interfacial tension and solvent-accessible surface area showed that the interfacial properties of sulfate were better than those of sulfonates and nonionic surfactants. Meanwhile, the interface properties of nonextended surfactants were better than those of extended surfactants. The gyration radius (Rg) and tilt angle data demonstrated that when EO chains were located between hydrophobic groups and PO chains (nonextended surfactants), the adsorption capacity of surfactants at crude oil and water interfaces could be effectively improved. The radial distribution function of the hydrophilic group and hydrophobic group of surfactants with water molecules and four components of the crude oil molecule, respectively, explained that surfactants (8EO8POSO4) had better emulsification performance when the intermolecular interactions between crude oil and water two phases were relatively balanced. This study provides a theoretical reference for the design of oil-displacement surfactants and the mechanism analysis of emulsification properties.

Dilational Rheology of Extended Surfactants at the Air/Water and Oil/Water Interfaces: Effect of Propylene Oxide Group Numbers

In this paper, the interfacial properties of extended surfactants with different oxypropylene (PO) groups were comprehensively investigated by using interfacial dilational rheology. The differences in molecular orientation, spatial configuration, and relaxation process were compared at the gas-water interface and oil-water interface. The influences of the PO groups on the interface viscoelasticity were analyzed, providing important theoretical support for the wide application of extended surfactants. Experimental results show that the lower number of PO groups in extended surfactants does not cause differences in their presence states on the interface; however, once it increases, the longer PO segment will spiral up in the direction perpendicular to the interface, forming a spatial configuration like a thin cylinder. Compared with air, the PO group has better solubility in the oil phase. The chain segment can still maintain a helical extension from the beginning to the end as a result. However, the upper layer of the thin cylinder will collapse to a certain extent at the surface. Moreover, the orientation of the hydrophobic side has a dynamic process of "tilting to upright" with the increase of adsorption amount or in response to interfacial dilation and compression. The increase of PO number or the insertion of oil molecules has little effect on dilational modulus, and the interfacial film strength is generally relatively low. That is to say, the better emulsifying and solubilizing ability of PO-containing extended surfactants may be more attributed to the matching steric effect at interface or better packing action in bulk phase than to higher film strength.

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.

Synthesis and Study of a New Type of Nonanionic Demulsifier for Chemical Flooding Emulsion Demulsification

The application of chemical flooding improves the stability of the produced emulsion, which reduces the demulsification efficiency of conventional demulsifiers. To improve the demulsification effect, in this paper, a new multibranched nonanionic polyether demulsifier, FYJP, was prepared by grafting carboxylate based on a nonionic demulsifier. The FYJP demulsifier could generate an initiator through p-tert-butylphenol, triethylenetetramine, and methanol, which was polymerized with ethylene oxide (EO) and propylene oxide (PO) to produce a nonionic polyether demulsifier. Sodium chloroacetate was used to modify the polyether demulsifier to obtain a new type of nonanionic polyether demulsifier. The FYJP polyether demulsifier was characterized by the hydrophilic-lipophilic balance (HLB) value, relative solubility (RSN), and surface activity of the demulsifier, and the demulsification mechanism was analyzed by a microscopic demulsification process test, and the effect of demulsifier dosage on the demulsification effect was discussed. Meanwhile, a dehydration test was carried out. The experimental results showed that the highest dehydration rate of the demulsifier was 94.7% at 85 °C, 100 ppm demulsifier dosage, 50 mL of a W/O emulsion, and 120 min demulsification time. The abovementioned studies show that FYJP is an effective demulsifier for chemical flooding emulsions, and this work promises to provide a reference for future demulsifier research.

Phase diagrams and microstructures of aqueous short alkyl chain polyethylene glycol ether carboxylate and carboxylic acid triblock surfactant solutions

HYPOTHESIS

The surfactant C₈EO₈CH₂COOH (Akypo LF2) and its salts have a small hydrophobic and a significantly longer hydrophilic part. As a consequence, there must be a significant steric constraint, once these surfactant molecules form micelles. In addition, the partially charged headgroups should bring some additional fine-tuning via electrostatic interactions to this "essentially non-ionic" surfactant.

EXPERIMENTS

Phase diagrams of binary mixtures of water and C₈EO₈CH₂COOH are established over large concentration and temperature ranges, also at different pHs and in the presence of sodium and calcium ions. Surface tensions and osmotic pressures are measured to understand the systems. To evaluate the microstructures, also Dynamic Light Scattering and Small-Angle X-ray Scattering are performed.

FINDINGS

Apart from the formation of coacervates at very low surfactant concentrations, spherical micelles persist over the whole concentration and temperature range and do not change in size and shape. At very high surfactant concentrations, above 60% by weight, where the headgroups are no longer fully hydrated, the standard core-shell structure of micelles vanishes and highly stabilized aggregates of 8-26 octyl chains are suspended in interdigitated polyoxyethylene layers and form an "osmotic brush". When the acid is partially transformed to a sodium salt, the repulsion between the micelles increases, whereas bridging between micelles prevails, when the counterions are calcium cations. Remarkably, the negative charges of the headgroups are randomly distributed in the hydrophilic ethylene oxide shell. Altogether, a phase diagram without lyotropic liquid crystalline phases and an extreme shift of the cloud-point in temperature and composition is found, similar to the phase diagram of C₈EO₈OH already known in literature. The phase properties can be explained by the curvature and packing constraints together with the Lindemann rule applied to short hydrocarbon chains.

Salt-tolerance of alkyl-glyceryl ether carboxylates hydrotropes and surfactants. Dramatic effect of the methylation of the glyceryl spacer

HYPOTHESIS

The insertion of polyether spacers between the anionic head and the alkyl chain of ionic surfactants significantly improves their salt-tolerance. The aim of this work is to study whether the petro-based polyethoxy spacer can be replaced by a glyceryl ether group for high salinity applications.

EXPERIMENTS

A series of amphiphilic sodium salts of alkyl glyceryl ether carboxylates are synthesized with different alkyl chain lengths from 4 to 12 and various spacers between the glyceryl and the carboxylate groups. Their aggregation behavior is studied by tensiometry and their amphiphilicities are assessed by the PIT-slope method. The dramatic effect of the methylation of the glyceryl spacer on the salt-tolerance is highlighted, and rationalized by DFT calculations and molecular dynamics.

FINDINGS

In contrast to the corresponding sodium soap, n-C₆H₁₃-CO₂Na, and to the non-methylated counterpart, the sodium salt of 1-pentyl-3-methyl glyceryl ether methylene carboxylate ([5.0.1]-CH₂CO₂Na) exhibits an excellent salt-tolerance since it remains water-soluble with NaCl or CaCl₂ concentrations greater than 20 wt% at 25 °C. Amphiphiles with short alkyl chains (<C₈) act as hydrotropes whereas longer compounds behave as surfactants whose CMC are lower than their corresponding isomers with ethoxy spacers n-C_i(EO)₂CH₂CO₂Na.

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.

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.

What dominates the interfacial properties of extended surfactants: Amphipathicity or surfactant shape?

HYPOTHESIS

The properties of conventional surfactants (c-surfactants) are generally accepted to be amphipathicity-dominated, but extended surfactants (e-surfactants) are additionally polypropylene oxide (PPO)-dependent; this additional property makes us wonder how an intramolecular PPO spacer would be "extended" at various interfaces and what is responsible for the excellent all-round properties of e-surfactants.

EXPERIMENTS

A series of novel sodium medium alkyl chain PPO-b-PEO sulfates (2-ethylhexyl polypropylene oxide-block-polyethylene oxide sulfates, C₈P_pE_eS) were designed, synthesized and structurally identified. Tensiometry was applied to estimate the surfactant shape at the air/water surface. Surface tension, interfacial tension, emulsifying power, electrolyte tolerance, adsorption onto oil sands and thermal hydrolysis stability were measured to evaluate the effect of the PPO coil on the interfacial and micellar properties of the e-surfactants.

FINDINGS

On the basis of obtaining greater values for e-surfactants than c-surfactants for both surface area (a_m) per surfactant molecule and the corresponding shape factor (S), we were surprised to find that e-surfactants form a rugby ball shape not only at the air/water surface but also at the oil/water interface; this result is potentially explained by the PPO spacer coiling and collapsing to produce dense packing at the monolayer adsorption, which is rationally borrowed by other interfaces. Many positive or negative correlations were observed between the interfacial/micellar properties of C₈P_pE_eS and a_m values, which seems that the surfactant shape dominants the properties of the e-surfactants. In fact, the properties of C₈P_pE_eS are dominated by the dynamic amphipathicity and assisted by the rugby ball shape of the molecules because of both being driven by the dynamic biphasic affinity of the PPO coil in response to the external environment; these findings provide soft interfacial materials specially adapted for surfactant flooding.

Comparative Study of Conventional/Ethoxylated/Extended n-Alkylsulfate Surfactants

A series of novel anionic e-surfactants n-C _cP _pS was molecular designed and synthesized from long-chain fatty alcohols by polypropoxylation and sulfation followed by neutralization. Excellent all-round performance of extended surfactants (e-surfactants) interests us how a simple polypropylene oxide (PPO) spacer has great effects on properties. By a comparative study of conventional/ethoxylated/extended n-alkylsulfate surfactants, we were surprised to find that e-surfactants are in an obvious rugby shape at the air/water surface according to molecular surface area ( a_m), and it comes down to the intramolecular PPO spacer coiling and surface-induced collapse. On the basis of the interfacial properties of the e-surfactants, it is found that the PPO spacer can provide both hydrophilic and lipophilic contributions to an e-surfactant molecule. The synergism between PPO spacers and alkyl chains indicates that a certain PPO spacer can adjust the contributions in view of different alkyl chain lengths. Therefore, it is both the rugby-shaped molecular geometry of e-surfactants and the dynamic amphipathicity of a PPO spacer that makes e-surfactants behave with excellent interfacial and solution properties for household cleaning. Therefore, this work gives us a hint that the molecular geometry of surfactants plays a vital role in interfacial and solution properties similar to molecular amphipathicity.

Foam and Rheological Properties of a Kind of Extended Surfactants

The foam and rheological properties of four extended surfactants C12–14PmE2S (m = 0, 3, 5, and 8) of four different concentrations were investigated in this paper. The foam properties, including, foamability, foam stability, liquid carrying ability, foam drainage and foam morphology were characterized. The rheological measurements were carried out through frequency sweep in the oscillation mode. The foamability and foam stability decreased with increasing numbers of PO groups embedded in those surfactants. At the same concentration, the longer the PO chain of the surfactant molecule, the larger the liquid carrying capacity of the foam. The foaming number decreases as the time increases, and longer PO chains were associated with faster foam number decline. The rheological measurements show that those surfactant solutions are “liquid-like”. With increasing surfactant concentration, the viscous modulus of C12–14P8E2S increases gradually, whereas that of C12–14E2S decreases.

Clouding phenomenon in amphiphilic systems: A review of five decades

Phase separation in amphiphilic systems is an important phenomenon. The temperature at which an amphiphilic solution phase separates is known as Cloud Point (CP). This article reviews in detail the process of phase separation in various amphiphiles (surfactants, polymers and drugs) and effect of different classes of additives on the CP of these amphiphilic systems. Ions affect the CP of drugs in a different way: kosmotropes and hard bases decrease while chaotropes and soft bases increase the CP of nonionic and cationic surfactants. Anionic surfactants show CP in presence of quaternary salts only. Thus, depending upon the nature and concentration of additive, the CP of an amphiphilic system gets increased or decreased and, hence, properties of the system may be tuned as per the need and use. A system with CP at high concentration can be made to phase separate at lower concentration by simply introducing an appropriate additive in it. This makes the system cost effective. On the other hand, if not required, a low CP can be enhanced with the help of another type of a suitable additive.

Cloud point phenomena in aqueous solutions of an anionic gemini surfactant with a dibenzene spacer in the presence of tetra-n-propyl-ammonium bromide

The cloud point behaviour of a carboxylate gemini surfactant,O,O′-bis(sodium 2-dodecylcarboxylate)-p-dibenzenediol was observed in the presence of tetra-propyl-ammonium bromide.

Interactions of univalent counterions with headgroups of monomers and dimers of an anionic surfactant

Specific ion effects in solution are related to the hydrated ion size and ion hydration, electrostatic interactions, dispersion forces, ion effects on water structure, and ion modification of surface tension. In this study, we tried to identify which factor determines the ion specificity observed. The preference and energy of metal cations binding with the headgroups of dodecylsulfate (DS) monomers and dimers were determined by mass spectrometry. In the gas phase, cation binding to DS dimer headgroups depends strongly on the cation radius. On the other hand, the interactions between DS monomer headgroups and chaotropic ions depend on the cation polarizability, and the binding of kosmotropic cations to DS monomer headgroups strongly depends on the Gibbs free energies of ion hydration. DS dimers are related to surfactants having doubly charged headgroups, and DS monomers are related to surfactants with singly charged headgroups. Our spectrometric study of the strength of counterion binding to free monomers of a surfactant provides insight into surfactant-counterion interactions at micellar interfaces in bulk solution.

Self-assembly of fatty acids in the presence of amines and cationic components

Fatty acids can self-assemble under various shapes in the presence of amines or cationic components. We assemble and compare these types of self-assembly leading toward a catanionic system either with a cationic surfactant or with an amine component playing the role of counter-ion. First, we focus on the molar ratio as a key driving parameter. Known and yet un-known values from other quantities governing the colloidal properties of these systems such as structural surface charge, osmotic pressure, molecular segregation, rigidity, in plane colloidal interactions and melting transition are discussed. We include also recent results obtained on the interfacial and foaming properties of these systems. We will highlight the specificity of these self-assemblies leading to unusual macroscopic properties rich of robust applications.

Interaction, solubilization and location of p-hydroxybenzoic acid and its sodium salt in micelles of moderately hydrophilic PEO-PPO-PEO triblock copolymers

Micelles of ABA type triblock copolymers (where A is polyethylene oxide PEO and B is polypropylene oxide PPO) viz. Pluronic® P103, P104 and P105 (each containing almost the same PPO mol wt. ~ 3250 g/mol and 30, 40 and 50 wt.% of PEO, respectively) in the presence of p -hydroxybenzoic acid (PHBA) and its sodium salt (Na-PHBA) were examined by viscosity, dynamic light scattering (DLS), small angle neutron scattering (SANS) and NMR. Spherical polymeric micelles (apparent hydrodynamic diameter ~ 20 nm) in water at 30 °C grow in the presence of PHBA and transform into prolate-ellipsoidal shape with an increased aggregation number. The micellar transition was favored at higher PHBA concentration, temperature and for copolymers with more hydrophobicity. The PHBA salt, however, increased cloud point and showed only a marginal decrease in aggregation number even at much higher concentrations. The location of PHBA in micelle was elucidated by nuclear Overhauser enhancement spectroscopy (NOESY).

Effect of salts on the phase behavior and the stability of nanoemulsions with rapeseed oil and an extended surfactant

For many decades, the solubilization of long-chain triglycerides in water has been a challenge. A new class of amphiphiles has been created to overcome this solubilization problem. The so-called "extended" surfactants contain a hydrophilic-lipophilic linker to reduce the contrast between the surfactant-water and surfactant-oil interfaces. In the present contribution, the effects of different anions and cations on the phase behavior of a mixture containing an extended surfactant (X-AES), a hydrotrope (sodium xylene sulfonate, SXS), water, and rapeseed oil were determined as a function of temperature. Nanoemulsions were obtained and characterized by conductivity measurements, light scattering, and optical microscopy. All salting-out salts show a transition from a clear region (O/W nanoemulsion), to a lamellar liquid crystalline phase region, a clear phase (bicontinuous L(3)), and again to a lamellar liquid crystalline phase region with increasing temperature. For the phase diagrams with NaSCN and Na(2)SO(4), only one clear region (O/W nanoemulsion) was observed, which turns into a lamellar phase region at elevated temperatures. Furthermore, the stability of the nanoemulsions was investigated by time-dependent measurements: the visual observation of phase separation, droplet size by dynamic light scattering (DLS), and optical microscopy. The mechanism of the different phase transitions is also discussed.