Aqueous Foam Stabilized by Hydrophobic SiO2 Nanoparticles using Mixed Anionic Surfactant Systems under High-Salinity Brine Condition

A primary concern of surfactant-assisted foams in enhanced oil recovery (EOR) is the stability of the foams. In recent studies, foam stability has been successfully improved by the use of nanoparticles (NP). The adhesion energy of the NP is larger than the adsorbed surfactant molecules at the air–water interface, leading to a steric barrier to mitigate foam-film ruptures and liquid-foam coalescence. In this study, the partially hydrophobic SiO₂ nanoparticles (SiO₂-NP) were introduced to anionic mixed-surfactant systems to investigate their potential for improving the foamability and stability. An appropriate ratio of internal olefin sulfonate (C_15-18 IOS) and sodium polyethylene glycol monohexadecyl ether sulfate (C₃₂H₆₆Na₂O₅S) was selected to avoid the formation of undesirable effects such as precipitation and phase separation under high-salt conditions. The effects of the NP-stabilized foams were investigated through a static foam column experiment. The surface tension, zeta potential, bubble size, and bubble size distribution were observed. The stability of the static foam in a column test was evaluated by co-injecting the NP-surfactant mixture with air gas. The results indicate that the foam stability depends on the dispersion of NP in the bulk phase and at the water–air interface. A correlation was observed in the NP-stabilized foam that stability increased with increasing negative zeta potential values (−54.2 mv). This result also corresponds to the smallest bubble size (214 μm in diameter) and uniform size distribution pattern. The findings from this study provide insights into the viability of creating NP-surfactant interactions in surfactant-stabilized foams for oil field applications.     

Mixtures of anionic and cationic surfactants with single and twin head groups: Adsorption and precipitation studies

This research reports on the adsorption and precipitation of mixtures of anionic and cationic surfactants having single and twin head groups. The surfactant mixtures investigated were: (i) a single-head anionic surfactant, sodium dodecyl sulfate (SDS), in a mixture with the twin-head cationic surfactant pentamethyl-octadecyl-1,3-propane diammonium dichloride (PODD)—adsorption was studied on negatively charged silica; and (ii) a twin-head anionic surfactant, sodium hexadecyl-diphenyloxide disulfonate (SHDPDS), and the single-head cationic surfactant dodecylpyridinium chloride (DPCI)—adsorption was studied on positively charged alumina. Whereas the mixed surfactant system of SHDPDS/DPCI showed adsorption on alumina that was comparable to the of SHDPDS alone, the mixed surfactant system of SDS/PODD showed increased adsorption on silica as compared with PODD alone. The adsorption of the SDS/PODD mixture increased as the anionic and cationic system approached an equimolar ratio. Precipitation diagrams for mixtures of single- and twin-head surfactant systems showed smaller precipitation areas than for single-head-only surfactant mixtures. Thus, the combination of single- and double-head surfactants helps reduce the precipitation region and can increase the adsorption levels, although the magnitude of the effect is a function of the specific surfactants used.     

Mixtures of anionic and cationic surfactants with single and twin head groups: Solubilization and adsolubilization of styrene and ethylcyclohexane

Mixtures of anionic and cationic surfactants with single and twin head groups were used to solubilized styrene and ethylcyclohexane into mixed micelles and adsolubilize them into mixed admicelles on silica and alumina surfaces. Two combinations of anionic and cationic surfactants were studied: (i) a single-head anionic surfactant, sodium dodecyl sulfate (SDS), with a twin-head cationic surfactant, pentamethyl-octadecyl-1,3-propane diammonium dichloride (PODD), and (ii) a twin-head anionic surfactant, sodium hexadecyl-diphenyloxide disulfonate (SHDPDS), with a single-head cationic surfactant, dodecylpyridinium chloride (DPCl). Mixtures of SDS/PODD showed solubilization synergism (increased oil solubilization capacity) when mixed at a molar ratio of 1∶3; however, the SHD-PDS/DPCl mixture at a ratio of 3∶1 did not show solubilization enhancement over SHDPDS alone. Adsolubilization studies of SDS/PODD (enriched in PODD) adsorbed on negatively charged silica and SHDPDS/DPCl adsorbed on positively charged alumina showed that while mixtures of anionic and cationic surfactants had little effect on the adsolubilization of styrene, the adsolubilization of ethylcyclohexane was greater in mixed SHPDS/DPCl systems than for SHDPDS alone. Finally, it was concluded that whereas mixing anionic and cationic surfactants with single and double head groups can improve the solubilization capacity of micelles or admicelles, the magnitude of the solubilization enhancement depends on the molecular structure of the surfactant and the ratio of anionic surfactant to cationic surfactant in the micelle or admicelle.     

Krafft Point Prediction of Anionic Surfactants Using Group Contribution Method: First-Order and Higher-Order Groups

Krafft point is one of the key properties of anionic surfactants to indicate their solubility in an aqueous phase. Below the Krafft point, the surfactant is a useless solid. We have developed a prediction model for the Krafft point based on the Marrero and Gani Group Contribution (GC) concept. Literature data were analyzed and a new third-order level was proposed to improve the model accuracy. The Krafft points of 53 anionic surfactants were collected including alkyl sulfonate, alkyl sulfate, alkyl benzene sulfonate, alkyl ethoxy sulfate, alkyl ester sulfonate, alkyl ester sulfate, alkyl disulfate, alkyldiphenylether disulfonates, alkyl diester sulfonate, alkyl naphthalene sulfonate, and fluorocarbon surfactants. The coefficients of the fractional groups in a surfactant molecule reveal the dependence of the Krafft point on the internal structure. Compared to the more complicated models such as those based on advanced statistical concept, the GC model for the Krafft point developed in this work gives a higher accuracy with a simpler calculation.     

Ethoxy Carboxylate Extended Surfactant: Micellar,Adsorption and Adsolubilization Properties

The micellar, adsorption, and adsolubilization properties of a novel ethoxy carboxylate extended surfactant are measured and compared to an extended sulfate surfactant. The critical micelle concentration (CMC) of the ethoxy carboxylate extended surfactant is measured to be 0.02 mM while it is 0.07 mM for the extended sulfate surfactant. Adsorption and adsolubilization studies are carried out on alumina oxide surfaces. The extended sulfate surfactant has a higher maximum adsorption capacity onto the aluminum than the ethoxy carboxylate extended surfactant (0.47 vs. 0.14 mmol/g, respectively). For adsolubilization, the extended sulfate surfactant shows a slightly higher phenanthrene adsolubilization compared to the ethoxy carboxylate extended surfactant (log K_adm of 6.15 vs. 5.71, respectively). In contrast, for solubilization, the ethoxy carboxylate extended surfactant exhibits higher phenanthrene solubilization capacities than the extended sulfate surfactant (log K_mic of 5.61 vs. 5.42, respectively). Relative to surfactant loss from the solid surface, the ethoxy carboxylate extended surfactant shows a higher desorption capacity as compared to the extended sulfate surfactant. From these measurements, the ethoxy carboxylate extended surfactant has better properties for micellar applications (lower CMC, higher K_mic), while the extended sulfate surfactant has better properties for admicellar applications (higher q_max and K_adm values, and less desorption).     

Ethoxy Carboxylate Extended Surfactant: Surface Charge of Surfactant-Modified Alumina, Adsolubilization and Solubilization of Phenylethanol and Styrene

Adsolubilization of contaminants by surfactant-modified material is an important phenomenon for surfactant-based environmental technologies. Recently, extended surfactants have been shown to enhance the adsolubilization capacity of organic solutes. In this study, two extended surfactants (ethoxy propoxylated carboxylate extended surfactant—C16PO4EO5C and propoxylate extended sulfate surfactant—C16PO4S) were selected for modifying positively charged alumina surfaces with the aim of enhancing adsolubilization of organic solutes with varying degrees of polarity (phenanthrene, styrene, and phenylethanol). The nature of the charged surface as a function of extended surfactant adsorption was evaluated through the zeta potential measurements. The results showed that at maximum bilayer coverage, the zeta potential of the alumina surface remained constant and was oppositely charged (negative) to the unmodified alumina (positive). Zeta potential measurements showed that the adsorbed bilayer of carboxylate-based extended surfactant produced more negatively charged surface. Surfactant desorption results showed that the surfactant-modified surface retained their negatively charge, albeit reduced, indicating that partial desorption occurred but not to the point that the positively charged alumina surface was realized. The adsolubilization results suggest a benefit of the ethoxy groups in adsolubilizing the polar phenylethanol in the palisade layer.     

Effect of Ethylene Oxide Group in the Anionic–Nonionic Mixed Surfactant System on Microemulsion Phase Behavior

The phase behavior of microemulsions stabilized by a binary anionic–nonionic surfactant mixture of sodium dihexyl sulfosuccinate (SDHS) and C12-14 alcohol ethoxylate (C_{12 − 14}E_j) that contains an ethylene oxide (E_j) group number, j, of either 1, 5, or 9 was investigated for oil remediation. The oil–water interfacial tension (IFT) and optimal salinity of the microemulsion systems with different equivalent alkane carbon numbers (EACN) were examined. The anionic–nonionic surfactant ratio was found to play a pivotal role in the phase transition, IFT, and optimal salinity. The minimum IFT of mixed SDHS − C_{12 − 14}E_j systems were about three times lower than those of neat SDHS systems. A hydrophilic–lipophilic deviation (HLD) empirical model for the mixed anionic–nonionic surfactant system with the characteristic parameter was proposed, as represented in the excess free energy term . The results suggested that the mixed system of SDHS − C_{12 − 14}E₁ was more lipophilic, while SDHS − C_{12 − 14}E₉ was more hydrophilic than the ideal mixture (no excess free energy during the microemulsion formation), and the SDHS − C_{12 − 14}E₅ system was close to the ideal mixture. The findings from this work provide an understanding of how to formulate mixed anionic–nonionic microemulsion systems using the HLD model for oils that possess a wide range of EACN.     

Styrene Solubilization and Adsolubilization on an Aluminum Oxide Surface Using Linker Molecules and Extended Surfactants

The impact of lipophilic linker and extended surfactant properties on admicelle formation and styrene adsolubilization were evaluated through adsorption and adsolubilization studies on aluminum oxide. While linker-based systems achieved a higher maximum adsorption than extended surfactants, the extended surfactants reached maximum adsorption at a lower aqueous surfactant concentration. Results of solubilization and adsolubilization studies are summarized by the extent of solute solubilization into micelles and admicelles, as captured through the micellar partition coefficient, K _mic, and the admicellar partition coefficient, K _adm. The extended-surfactant-based micelles showed greater solubilization capacity than linker-based micelles. Relative to the effect of the number of propoxy groups for extended surfactants with the same alkyl chain length, the results show that the solubilization capacity increases when the PO number increases for both C12,13- and C14,15-based surfactant series. Thus, adsolubilization using extended-surfactant-based admicelles showed adsolubilization enhancement but required lower amounts of surfactants to form admicelles. These results thus provide insights into external and internal linker-based and extended-surfactant-based admicellar systems and highlight the differences observed between them and admicelles based on conventional surfactant systems.

Solubilization and Adsolubilization of Polar and Nonpolar Organic Solutes by Linker Molecules and Extended Surfactants

Adsolubilization reaches its maximum when a surfactant adsorbed onto the solid–liquid interface achieves complete bilayer or maximum adsorption. The attempt to enhance the adsolubilization of organic solute is accomplished by increasing interaction between the hydrophobic core of adsorbed admicelles and the organic solute. Solubilization and adsolubilization were studied with linker-based and extended-surfactant-based systems. Extended surfactants have propylene oxide (PO) groups of intermediate polarity inserted between hydrophobic and lipophilic moieties in the surfactant molecule. This study evaluated the adsolubilization of polar (phenylethanol) and nonpolar (ethylcyclohexane) solutes into conventional linker-based and extended-surfactant-based admicelles. The results demonstrated that the extended-surfactant-based systems showed higher solubilization capacity than the conventional sodium dodecyl sulfate alone or with linker. For the polar solute, the presence of PO group has a greater effect than the number of PO groups or the tail length, while for the nonpolar solute as the number of POs groups and the tail length increased, the adsolubilization capacity also increased. Preliminary explanations for these observations are provided.

Phase stability,fuel properties,and diesel engine performance of palm-oil-based microemulsion biofuels

Microemulsification and blending are two viscosity-modifying techniques of vegetable oils for direct use with diesel engine. In this study, alcohol blends are mixtures of ethanol, diesel, and palm-oil biodiesel while microemulsion biofuels are thermodynamically stable, clear, and single-phase mixtures of diesel, palm oil, and ethanol stabilized by surfactants and cosurfactants. Although there are many studies on biofuels lately, there is limited research on using biodiesel as a surfactant in microemulsion formulations and applied on engine performance at different engine loads. Therefore, the objectives are to investigate phase stability and fuel properties of formulated biofuels (various blends and microemulsions), to determine the engine performance at different engine loads (no load, and from 0.5 to 2.0 kW), and to estimate laboratory-scale cost of the selected biofuels compared to diesel and biodiesel. The results showed that phase stability and fuel properties of selected microemulsion biofuels are comparable to diesel and biodiesel. These microemulsion biofuels can be applied to the diesel engine at different loads while diesel-ethanol blends and palm-oil-biodiesel-ethanol blends cannot be. It was found that the energy efficiencies of the system using microemulsion biofuels were slightly lower than the average energy efficiency of diesel engine. From this study, it can be summarized that microemulsion biofuels can be formulated using palm-oil biodiesel (palm-oil methyl ester) as a bio-based surfactant and they can be considered as environmentally-friendly alternatives to diesel and biodiesel. However, cost considerations showed that the raw materials should be locally available to reduce additional costs of microemulsion biofuels.