Role of salts in performance of foam stabilized with sodium dodecyl sulfate

Classification of surfactants and admixtures for producing stable aqueous foam

Surfactants and foam have captured the interest of researchers worldwide due to their unique behavior of surface activity, the dynamic nature of foam formation, and simultaneous destruction. The present review focuses on the surfactants' classification, surfactant-solvent interaction, foam formation, characteristics, and a range of admixtures to enhance the foam performance. Although surfactants have been researched and developed for decades, recently, their sustainability has been given special attention. One such aspect is the development of green foaming agents from natural and renewable sources and assessing their suitability for different applications. Further, widely researched parameters are the type of surfactant, surfactant concentration, surfactant-solvent interaction, and foam production method on the foamability of a surfactant solution and related foam characteristics, including stability and texture. However, still, there is no rule to predict the best foam. Another vital concern is the non-standardization of foam assessment methods across industries and regions. Recently, research has progressed in identifying suitable admixtures for foam performance enhancement and utilizing them to produce stable foams for application in enhanced oil recovery, drug delivery, and manufacturing of aerated food products and foamed concrete. Although foam stabilization using various admixtures has been recognized well in the literature, the underlying mechanism requires further research. The interaction of surfactant and admixtures in solution is complicated and requires more research.

Unusual Structural Insights Revealed by Rheo-SAXS Studies of Nonaqueous Crystalline Gels

Glycerol is a nonaqueous polar solvent and is of interest in many industrial areas due to its beneficial properties, such as green production and biocompatibility. Our previous works have shown the presence of a fibrillar phase on the microscale that consists of lamellar sodium dodecyl sulfate (SDS) crystals containing interstitial glycerol on the nanoscale. The phase is gel-like at room temperature and demonstrates shear-thinning behavior upon application of a shear. Initially, small-angle X-ray scattering coupled with rheology (rheo-SAXS) measurements were performed to elucidate the structural transition of the gel phase under an applied shear, but it became clear that the aging process of the gel has a profound impact on both the gel nanostructure and also the mechanical properties. For younger gels, both the dissolution of SDS crystallites and the alignment of the fibrillar phase were seen. However, in the aged gels, an unexpected foam was formed at shear rates γ ˙ > 700 s-1. The microscopic structure of the foam phase was imaged using polarizing light microscopy and brightfield and darkfield optical microscopy. The nanostructure of the foam phase was investigated using rheo-SAXS. The foam phase was shown to be stabilized by the presence of SDS crystallites at the air-liquid interface, and the stability of the foam is high with foam persisting even t = 3 months after formation. These results highlight the importance of investigating green nonaqueous media and the gel aging process, both of which are interesting not only on a fundamental level but also for a range of industrial applications, from personal care products and cosmetics to food science.

Interface-Templated Crystal Growth in Sodium Dodecyl Sulfate Solutions with NaCl

Many ionic surfactants, such as sodium dodecyl sulfate (SDS) crystallize out of solution if the temperature falls below the crystallization boundary. The crystallization temperature is impacted by solution properties and can be decreased with the addition of salt. We studied SDS crystallization at liquid/vapor interfaces from solutions at high ionic strength (sodium chloride). We show that the surfactant crystals at the surface grow from adsorbed SDS molecules, as evidenced by the preferential orientation of the crystals identified by using grazing incidence X-ray diffraction. We find a unique time scale for the crystal growth from the evolution of structure, surface tension, and visual inspection, which can be controlled through varying the SDS or NaCl concentrations.

Impact of Pressure and Temperature on Foam Behavior for Enhanced Underbalanced Drilling Operations

Foam, a versatile underbalanced drilling fluid, shows potential for improving the drilling efficiency and reducing formation damage. However, the existing literature lacks insight into foam behavior under high-pH drilling conditions. This study introduces a novel approach using synthesized seawater, replacing the conventional use of freshwater on-site for the foaming system's liquid base. This approach is in line with sustainability objectives and offers novel perspectives on foam stability under high-pH conditions. Experiments, conducted with a high-pressure, high-temperature (HPHT) foam analyzer, investigate how pressure and temperature affect foam properties. The biodegradable foaming agent ammonium alcohol ether sulfate (AAES) is employed. Results demonstrate that the pressure significantly impacts foam stability. Increasing pressure enhances stability, reducing decay rates and promoting uniform bubble sizes, especially at lower temperatures. This highlights foam's capacity to withstand high-pressure conditions. Conversely, the temperature plays a substantial role in foam decay, particularly at elevated temperatures (75 and 90 °C). Decreased liquid viscosity accelerates the liquid drainage and foam decay. While pressure mainly influences the AAES foam stability at temperatures up to 50 °C, temperature becomes the dominant factor at higher temperatures. Temperature's impact on foamability is minimal under constant pressure, maintaining consistent gas volume for maximum foam height. However, foam stability is sensitive to temperature variations, with increasing temperature leading to a more significant bubble size increase gradient. These findings stress the importance of considering temperature effects in foam drilling, particularly in deep and high-temperature environments. AAES foam exhibits stability at lower temperatures, making it suitable for surface and intermediate drilling. Understanding temperature-induced changes in foam structure and bubble size is essential for optimizing performance in high-temperature and deep drilling scenarios.

Comparative Study on Interfacial Properties, Foam Stability, and Firefighting Performance of C6 Fluorocarbon Surfactants with Different Hydrophilic Groups

Liquid fuel is flammable and hazardous, and a pool fire is one of the most serious disasters. Therefore, it is important to develop high-performance firefighting agents. To synthesize aqueous film-forming foam (AFFF) formulations, two C6 short-chain fluorocarbon surfactants Capstone 1157 (FC1157) and sodium perfluorohexylethyl sulfonate (SF852) with different hydrophilic groups were introduced, and three hydrocarbon surfactants sodium dodecyl sulfate (SDS), decyl glucoside (APG0810), and coco glucoside (APG0814) were chosen. The AFFF formulations based on the short-chain fluorocarbon-hydrocarbon compounding system were developed, and the firefighting performance of the formulations was assessed according to the standard pool fire extinction test. The results indicated that amphoteric FC1157 was slightly more effective than anionic SF852 in extinguishing small-scale pool fires and could reduce heat flux more effectively than SF852. Fluorocarbon surfactant FC1157 has been shown to suppress large pool fires much better than SF852, possibly due to its higher foam stability, higher foaming property, lower dynamic surface tension, and lower bubble coarsening rate. Both formulations we studied were more effective than commercial AFFF formulations. A concentration of 0.1-0.3% of FC1157 in an AFFF solution was optimal for extinguishing high-boiling-point oil fires.

Experimental Study of the Influence of Inorganic Salts on Foam Stability

The aqueous film-forming foam (AFFF) extinguishing agent is suitable for fighting various hydrocarbon fuel fires due to its dual fire-fighting effect of foam and liquid film. Because the action law and microscopic mechanism of inorganic salts on the stabilization process of surfactant-generated AFFF are not perfect, this paper employs an experimental approach to investigate the effects of inorganic salt types and concentrations on sodium dodecyl sulfate-containing foam systems (SDS). Prior to critical micelle concentration (CMC), increasing inorganic salt concentration decreased solution surface tension, but the opposite was true after CMC. The CMC value of an SDS solution decreases as inorganic salt concentration increases, and the "salting effect" of inorganic salt cations also has an effect on the CMC value. Based on the resistance of the liquid film at the gas-liquid interface affecting gas transport, the foam evolution was divided into three stages: foam generation, liquid drainage, and gas transfer. The effect of inorganic salts on these three stages was studied at the molecular level, and it was discovered that the addition of NH₄Cl and MgCl₂ could improve the saturation adsorption at the gas-liquid interface, reduce the surface tension of the surfactant solution, and improve foam stability. Meanwhile, inorganic salts can change the force of gas molecules, so the equilibrium force of gas across the liquid membrane increases as inorganic salt concentration increases. Additionally, the addition of inorganic salts raises the diffusive drainage coefficient, but the gravity drainage coefficient still reigns supreme in the predecay period.

Thermodynamics of complex chemical equilibria in surfactant mixtures

A thermodynamic approach was developed to predict the precipitation conditions of surfactants using the solubility product relationship between surfactant monomer concentrations, in order to calculate the monomer-precipitate equilibrium. This approach provides an explicit equation which predicts the amount of solid phase which forms in any surfactant mixture. All calculations of the total change in Gibbs energy (ΔG) were performed for concentrations of both surfactants that were below their CMC values. The elaborated ΔG-pH diagrams offer the possibility to determine the areas of thermodynamic stability of the solid phases depending on the chemical composition and acidity of the studied system. It was shown that with increasing concentration of the surfactant and the metal ion, the range of precipitate formation, either as slightly soluble salt or as slightly soluble acid, was extended by a few pH units in all cases.

Fast-Curing Geopolymer Foams with an Enhanced Pore Homogeneity Derived by Hydrogen Peroxide and Sodium Dodecyl Sulfate Surfactant

The properties of porous and lightweight ceramic foam that can be cured at room temperature using metakaolin-based geopolymers were studied. A geopolymer slurry was prepared using metakaolin and a potassium-based alkaline medium at room temperature, and the obtained viscous paste was expanded via gaseous methods, by means of the decomposition of peroxide at room temperature. Therefore, geopolymer (GP) foam developed in this study through multivariate geopolymer, foaming agents, and surfactants can be cured at room temperature (within 5 days) without a separate heat treatment process. The homogeneous micropores were obtained through the stabilization of the interface between geopolymer slurry and oxygen gas bubbles generated through the base-catalyzed decomposition of hydrogen peroxide. The porosity was confirmed to be 29% and 54% before and after using sodium dodecyl sulfate (SDS). The compressive strengths and densities were 1.57 MPa and 0.75 g/cm3 for GP foam without SDS, and 3.63 MPa and 0.48 g/cm3 for GP foam with SDS. Through the mercury intrusion porosimetry analysis, the pores were further refined from 100 µm to 30 µm when SDS was used, and at the same time, the variation of pore size was minimized, so that a relatively uniform pore size was maintained. In addition, the thermal conductivity is 0.0803 W/m·K and the pore size is 33.2 μm, which is smaller in pore diameter than the geopolymer containing only hydrogen peroxide. As a result, although the hydrogen peroxide alone sample has excellent thermal conductivity, the use of a surfactant is recommended for fine micropore size control. While reducing the non-uniform distribution of pores and the size of micropores generated through the direct foaming method as an inorganic binder, the possibility of an insulation finish was also confirmed by reducing the weight.

A pulp foam with highly improved physical strength, fire-resistance and antibiosis by incorporation of chitosan and CPAM

Bio-inspired borate cross-linked pulp foam (PF) with high porosity and low density can be widely used in many fields. However, PF is flammable, and lack of mechanical strength and antibacterial activity. To solve these issues, an ultra-strong PF was prepared by incorporation of chitosan and cationic polyacrylamide (CPAM). Results showed that the obtained PF exhibited highly improved mechanical properties (the compressive strength (485 kPa at a strain of 50%) was over 6 times higher compared with the borate cross-linked PF without chitosan and CPAM, and it was even higher than most of the reported cellulose-based porous materials). Also, the prepared PF has good performance on fire-retardance (hard to light), thermal insulation, antibiosis and sound absorption, due to the synergistic actions of borate, chitosan and CPAM. Additionally, spent liquor in preparing PF could be fully recycled, and thus this sustainable approach has potential for large-scale production of high-performance PF.

Thermal Stability of Gel Foams Stabilized by Xanthan Gum, Silica Nanoparticles and Surfactants

The foams stabilized by nanoparticles (NPs), water-soluble polymers, and surfactants have potential application prospects in the development of new, environmentally friendly firefighting foams. In the present study, a gel foam containing a water-soluble polymer (xanthan gum, XG), hydrophilic silica NPs, hydrocarbon surfactant (SDS), and fluorocarbon surfactant (FS-50) were prepared. The surface activity, conductivity, viscosity, and foaming ability of foam dispersions were characterized. The gel foam stability under a radiation heat source and temperature distribution in the vertical foam layer were evaluated systematically. The results show that the addition of NPs and XG has a significant effect on the foaming ability, viscosity and foam thermal stability, but has a very subtle effect on the conductivity and surface activity. The foaming ability of the FS-50/SDS solution was enhanced by the addition of NPs, but decreased with increasing the XG concentration. The thermal stability of the foams stabilized by SDS/FS-50/NPs/XG increased with the addition of NPs and increasing XG concentration. Foam drainage and coarsening were significantly decelerated by the addition of NPs and XG. The slower foam drainage and coarsening are the main reason for the intensified foam thermal stability. The results obtained from this study can provide guidance for developing new firefighting foams.

Effect of selected monovalent salts on surfactant stabilized foams

Surfactant-stabilized foams have been at the centre of scientific research for over a century due to their ubiquitous applications in different industries. Many of these applications involve inorganic salts either due to their natural presence (e.g. use of seawater in froth floatation) or their addition (e.g. in cosmetics) to manipulate foam characteristics for the best outcomes. This paper provides a clear understanding of the effect of salts on surfactant-stabilized foams through a critical literature survey of this topic. Available literature shows a double effect of salts (LiCl, NaCl and KCl) on foam characteristics in the presence of surfactants. To elucidate the underlying mechanisms of the stabilizing effect of salts on foams, the effect of salts on surfactant-free thin liquid films is first discussed, followed by a discussion on the effect of salts on surfactant-stabilized foams with the focus on anionic surfactants. We discuss two distinctive salt concentrations, salt transition concentration in surfactant-free solutions and salt critical concentration in surfactant-laden systems to explain their effects. Using the available data in literature supported by dedicated experiments, we demonstrate the destabilizing effect of salts on foams at and above their critical concentrations in the presence of anionic surfactants. This effect is attributed to retarding the adsorption of the surfactant molecules at the interface due to the formation of nano and micro-scale aggregates.