Ultralow Interfacial Tension Values Research Articles

Experimental Investigation on Spontaneous Imbibition of Surfactant Mixtures in Low Permeability Reservoirs.

Spontaneous imbibition of surfactants could efficiently enhance oil recovery in low permeability sandstone reservoirs. The majority of studies have considered the application of individual surfactants to alter wettability and reduce interfacial tension (IFT). However, a significant synergistic effect has been reported between different types of surfactants and between salts and surfactants. Therefore, this study systematically studied the capability of a binary surfactant mixture (anionic/nonionic) and a ternary surfactant mixture (anionic/nonionic/strong base-weak acid salt) in imbibition enhanced oil recovery (IEOR). The interfacial properties and the cores' wettability were explored by IFT and contact angle measurements, respectively. Subsequently, the imbibition performances of different types of surfactant solutions were discussed. The results suggested that the surfactants' potential to enhance oil recovery followed the order of ternary surfactant mixture > binary surfactant mixture > anionic > nonionic > amphoteric > polymer. The ternary surfactant mixture exhibited strong capacity to reverse the rock surface from oil-wet (125°) to strongly water-wet (3°), which was more significant than both binary surfactant mixtures and individual surfactants. In addition, the ternary surfactant mixture led to an ultralow IFT value of 0.0015 mN/m, achieving the highest imbibition efficiency (45% OOIP). This research puts forward some new ideas on the application of the synergistic effects of surfactants in IEOR from low-permeability sandstone reservoirs.

Study on the Synergistic Effects between Petroleum Sulfonate and a Nonionic–Anionic Surfactant for Enhanced Oil Recovery

Nonionic–anionic surfactants are expected to be applied in chemical flooding due to their important properties such as ultralow IFT values, good salt tolerance, and no chromatographic separation in porous media. In this study, a new type of nonionic–anionic–hydrophobic group structure surfactant N, N-dihydroxyethylalkylamide carboxylate (EAMC) was synthesized. The synergistic effects between petroleum sulfonate (KPS) and EAMC in reducing interfacial tension (IFT) and emulsification properties were studied. The influences of salt, alkali and Ca2+ on the IFTs of surfactant solutions were also investigated. One-dimensional core flooding experiments were used to characterize the enhanced oil recovery capability of the KPS and EAMC mixed system. The experimental results show that both EAMC and KPS have high interfacial activity and can reduce IFTs to about 0.01 mN/m order of magnitude against decane at optimized concentrations. The area occupied by the hydrophilic group of EAMC on the interface is smaller than that of its own hydrophobic group. The interfacial film formed by EAMC alone is relatively loose. The IFTs of KPS containing different structure petroleum sulfonates is affected by the difference in the adsorption rate of petroleum sulfonates on the interface, which shows that both the dynamic and equilibrium interfacial tensions can have the lowest values. However, the IFTs of the EAMC solutions against crude oil can be reduced to ultralow values because the mixed tight adsorption film is formed by EAMC and crude oil fraction molecules with synergistic effect. On the other hand, the KPS molecule has a hydrophobic part with large size and no synergism with crude oil fractions can be observed in the solutions containing only KPS. The combination of EAMC and KPS shows synergistic effect, namely ultralow IFT values, good emulsification properties, high alkali tolerance, and good salt and Ca2+ tolerance during a wide percentage range of EAMC. The best formula of EAMC and KPS system can be applied for EOR after polymer flooding. The studies in this paper are helpful for the design and application of a chemical flooding formula with nonionic–anionic–hydrophobic group structure surfactants.

Effect of bivalent cations on the interfacial tensions of extended anionic surfactant solutions

Extended surfactants have shown great potential in enhanced oil recovery (EOR). In this paper, the effects of extended surfactant concentration, polypropylene oxide (PO) number, and various counter ions (Na+, Ca2+, and Mg2+) on the oil–water interfacial tension (IFT) of extended surfactant solutions were investigated in detail. The selected extended surfactants were sodium dodecyl polyoxypropylene sulfate (L-C12POmS, m = 5, 10, 15). Based on our experimental results, under NaCl conditions, with an increase in the PO number, the space occupied by the surfactant molecules on the side of the oil phase increases, which matches the size of the anionic head, and the surfactant film becomes tight. An ultralow IFT value appears for L-C12PO15S at 0.5% NaCl against decane. The addition of bivalent cations can significantly reduce the IFT. As the Ca2+ concentration increases, IFTs of APS solutions against alkanes can achieve ultralow values at lower PO numbers and against alkanes with a wide ACN range. For the MgCl2 system, the smaller hydration radius of Mg2+ improves the tightness of the anionic head, resulting in a crowded arrangement of the PO chains on the oil phase side. Therefore, the ability of Mg2+ to reduce the IFT against alkanes is weaker than that of Ca2+. Moreover, the surface-active substances in the crude oil adsorb competitively on the interface, resulting in an increase in IFT. By adding Ca2+or Mg2+, a tight adsorption film of surfactant molecules will be formed, and the IFT value reaches ultralow against crude oil. The studies in this paper have important implications for the application of extended surfactant solutions to enhance oil recovery under high salt reservoir conditions.

The mechanism responsible for lowering interfacial tension between heavy crude oil and betaine solutions

To clarify the mechanism responsible for the reduction of interfacial tension (IFT), the dynamic and equilibrium IFTs of different betaine solutions against n-alkanes, kerosene, heavy oil fractions model oil and crude oil have been investigated by spinning-drop interfacial tensiometer. The experimental results show that the synergism for lowering IFT between heavy crude oil and betaine solutions are controlled by two main factors. One is “size compatibility” between surfactant molecules and surface-active substances in crude oil. The other is the optimized ratio of crude oil fraction to betaine. The betaine molecules with large size of hydrophilic group can form compact interfacial film with surface-active fractions in heavy crude oil by mixed adsorption. Therefore, betaines with large hydrophilic groups show an apparent synergism with heavy crude oil in lowering IFT and can produce ultralow IFT value at optimized concentration.

Improved As-Synthesized Oleic Amido Propyl Betaine Surfactant Mixture for Stable Ultra-Low Interfacial Tension: Effect of Mixed Co-Solvents

As-synthesized oleic amido propyl betaine surfactant mixture, that was produced through a “direct formulation through synthesis” process, exhibited ultra-low oil/water interfacial tension (IFT) values as low as 3.5 × 10−4 mN/m when dissolved in seawater at a reservoir temperature of 96 °C. The as-synthesized surfactant, which was left untreated, had a slightly cloudy appearance when mixed with seawater. Polar solvents were introduced to this surfactant to improve its aqueous solubility by changing its overall hydrophilicity, particularly on the oil/water interface. In this study, two types of glycol ether co-solvent, i.e., ethylene glycol monobutyl ether and diethylene glycol monobutyl ether, were used at different concentrations in a single application and as a mixture of co-solvents at a certain ratio. The behavior of the as-synthesized surfactant with the presence of these co-solvents was investigated. As a result, it showed that the co-solvent helps in solubility improvement and alters the interfacial tension behavior of the surfactant. Ethylene glycol monobutyl ether was found to be efficient in maintaining an ultra-low IFT value of the surfactant. However, the aqueous solubility of the surfactant was not significantly improved. In contrast, the addition of diethylene glycol monobutyl ether showed improvement of the aqueous solubility of the surfactant, but it tends to increase the IFT above ultra-low value. Based on this understanding, a set of co-solvent mixing ratios were tested, and the results showed further improvement in both the oil/water interfacial tension behavior and surfactant aqueous solubility. The most stable oil/water IFT of 3.36 × 10−3 mN/m and clearer surfactant solutions were obtained for ratio C at 35 wt.% presence of co-solvent.

Characterization of petroleum sulfonate synthesized via gas-phase SO3 sulfonation in rotating packed bed and its application in enhanced oil recovery

In this work, petroleum sulfonate (PS), which can be used for enhanced oil recovery (EOR), was synthesized by sulfonation of distillate oil using gaseous SO3 in a rotating packed bed. The prepared PS was characterized by FT-IR, NMR, negative electrospray ionization fourier transform ion cyclotron resonance mass spectrometry [(−) ESI FT-ICR MS] and thermal gravimetric analysis (TGA). Results showed that the PS contains N1, O1, O2, O3, N1O2, O3S, O4S, and N1O3S classes, among which O3S, O4S, and N1O3S are the three main classes. And the chemical composition of the PS was obtained by the combined analysis of the double-bond equivalent (DBE) and carbon number (CN). TGA results showed that PS is thermally stable at the conventional reservoir temperature (less than 200 °C). In addition, the EOR performance of PS was studied by measuring surface tension, interfacial tension (IFT), wettability alteration and core-flooding experiments. Critical micelle concentration (CMC) of the prepared PS with a value of 0.2% was also determined by surface tension method. An ultra-low oil-water IFT value 1.327 × 10−3 mN/m was obtained at the CMC of PS solution. It was also found that adding PS into brine can decrease the contact angle below 90°, indicating that it can alter the rock wettability from oil-wet to water-wet surface which contributes to enhancing oil recovery. Finally, core-flooding experiments were carried out with different PS concentrations. The results showed that an additional recovery of about 30% after conventional water flooding can be obtained at a PS concentration of 0.3%. This study indicated that the PS synthesized by gas-phase SO3 sulfonation in RPB has good EOR performance, and the work is helpful for the learning of the relationship between PS composition and EOR performance and give the guidance for PS’s synthesis.

Cationic/Nonionic Mixed Surfactants as Enhanced Oil Recovery Fluids: Influence of Mixed Micellization and Polymer Association on Interfacial, Rheological, and Rock-Wetting Characteristics

In this study, the efficacy of ionic/nonionic mixed surfactant systems as a promising chemical route toward enhanced oil recovery applications is investigated. The critical micelle concentration of the (CTAB + Tween 60) surfactant system was confirmed using conductivity studies and surface tensiometry. Thermodynamic analyses revealed that both adsorption and micellization processes in mixed surfactant compositions are more pronounced/effective as compared to pure surfactant solutions. Addition of polymer resulted in improved micellar stability in mixed surfactant systems by steric interactions. Ultralow interfacial tension values were obtained for mixed surfactant systems using a spinning drop technique. In the presence of carboxymethylcellulose (polymer), the viscosity of surfactant slugs are improved, leading to sweep efficiency during oil displacement process. Viscoelasticity investigations reveal that elastic modulus (G′) dominate over viscous modulus (G″) at an angular frequency of >1 rad/s, showing ...

Synthesis, characterization and enhanced oil recovery potential analysis through simulation of a natural anionic surfactant

Enhanced oil recovery (EOR) especially surfactant slug injection has received a great attention as an oil recovery technique. Surfactants are capable in reducing the interfacial tension (IFT) and altering the wettability of rock from subsurface reservoir. Here, a new anionic surfactant was synthesized from Madhuca longifolia (Mahua) oil. The surfactant was characterized using FTIR, GC–MS, FE-SEM and EDX analysis and tested rigorously using IFT, wettability and flooding experiments for its application in EOR. The surfactant showed an ultralow IFT value around 1 × 10−2 at optimal salinity. Thermodynamic parameters were calculated using surface tension as a function of temperature and the studies showed that adsorption of molecules at the interface is favorable over micellization. The surfactant was found to be thermally stable by TGA analysis for its application at high temperature reservoir conditions. It showed the ability to alter the wettability of oil-wet sandstone rock surface to water-wet. The core-flooding experiments were conducted using individual surfactant slug and surfactant-polymer slug injection, which showed an additional recovery of almost 20% over convention water flooding. The experimental results were simulated using computational fluid dynamics (CFD) analysis. The results obtained after simulation study and core-flooding experiment results are very much comparable. The comprehensive evaluation of thermal, interfacial and oil recovery studies provides grounds for potential application in EOR.

Bio-based surfactant for enhanced oil recovery: Interfacial properties, emulsification and rock-fluid interactions

Anionic surfactants are very effective for application in enhanced oil recovery (EOR) in sandstone reservoirs because of their promising surface-active properties. Considering the non-biodegradability of some of the commercial surfactants, the present study deals with an anionic surfactant synthesized from naturally occurring soap-nut oil with excellent interfacial properties for application in EOR. The soap-nut surfactant showed the promising properties of interfacial tension (IFT) reduction, emulsification, rock wettability alteration, which are desirable for its application in EOR. An ultra-low IFT value of 2.123 × 10−2 mN/m between crude oil and surfactant solution was obtained at critical micelle concentration (CMC), which was further reduced to 2.037 × 10−3 mN/m at optimum salinity. Phase behaviour studies showed the formation of Winsor III emulsion phase which is beneficial for recovery of the trapped oil from the fine pores of the reservoir by miscibility mechanism. The surfactant exhibited favourable rock-wetting ability from oil-wet rock to water-wet condition. The loss of surfactant by adsorption on the rock surface was studied, and the adsorption data were analysed by Langmuir and Freundlich models. Injection of a small pore volume of surfactant-slug augmented with polymer slug as mobility buffer recovered more than 30% of original oil in place (OOIP) after the conventional water-flooding.

Thermodynamic and physicochemical properties evaluation for formation and characterization of oil-in-water nanoemulsion

Kinetically stable nanoemulsion with droplet size <500 nm has emerged as an advanced material in wide industrial applications because of its unique structural and physicochemical properties. Nanoemulsions are stabilized by surfactants or emulsifiers by lowering the interfacial tension (IFT) between the dispersed and continuous phases to an ultra-low value. In the present study, surfactants are screened based on their ability to lower the IFT values for proper formulation of stable nanoemulsion. The critical micelle concentration (CMC) of surfactants obtained from the interfacial tension-concentration isotherm curve (IFCV) was used to evaluate surface excess concentration (Гmax), minimum surface area per molecule (Amin), adsorption efficiency (pC20), surface tension at CMC (γCMC), effectiveness of adsorption (Πcmc) along with thermodynamic parameters (ΔGmic and ΔGads). The experimental results of thermodynamic parameters reveal that adsorption phenomenon predominates over micellization. A meticulous evaluation of derived parameters suggests the suitability of non-ionic surfactant (Tween 80) over ionic surfactants (CTAB and SDS) for formation of stable oil-in-water (O/W) emulsion. In solubilization study, the ultra-low IFT value (10−3 to 10−5 mN/m) between oil-middle phase emulsion with 0.5 wt% Tween 80 at varying salinity calculated by Chun-Huh equation indicates the increased solubility of oil in emulsion. Nanoemulsions prepared by high energy method using Tween 80 and n-heptane with droplet size in the range 91.05 to 40.16 nm manifest kinetic stability due to continuous Brownian motion of droplets. The dissolving capacity of crude oil in nanoemulsion was determined by miscibility test. The synergetic effect of silica nanoparticles (NPs) in presence of Tween 80 shows enhancement in stability of nanoemulsion with drop in droplets size from 36.15 to 21.37 nm and surface charge of −59.05 mV. The formulated nanoemulsions show Pseudo-plastic behavior with viscosity in the range of 9–12.5 mPa-s, which is further improved in presence of nanoparticles.

Effects of Polyether Surfactants on Dynamic Interfacial Tension of Betaine Solutions against Crude Oil

AbstractThe dynamic interfacial tension (IFT) of betaine and betaine/polyether‐nonionic surfactant‐mixed systems against hydrocarbons, kerosene, and crude oil–water was studied using a spinning‐drop tensiometer. The influence of average molecular weight of polyether‐nonionic surfactants on IFT of mixed solutions was investigated. On the basis of the experimental results, one can find that it is difficult to reach the ultralow IFT value for betaine solution against hydrocarbon and kerosene because of the mismatch between the hydrophobic and hydrophilic groups. After purification, kerosene still contains a small amount of carboxyl groups, which can exert a synergistic effect on surfactants resulting in a lower IFT. The IFT of betaine and mixtures against Daqing crude oil can reach an ultralow value because of the mixed adsorption of surfactant and petroleum soap molecules. For mixed solutions, with the increasing concentration of added polyether, the decrease of petroleum soaps at the oil–water interface results in the destruction of synergistic effects.

Characterizations of surfactant synthesized from palm oil and its application in enhanced oil recovery

The present work deals with synthesis of an anionic biodegradable surfactant for application in chemical enhanced oil recovery (EOR) process. Alpha sulfonated ethyl ester (α-SEE) was synthesized from palm oil via trans-esterification process. The surfactant was characterized by FTIR, GC, TGA, FE-SEM and EDX analyses. Critical micelle concentration (CMC) was determined by surface tension measurement at air–aqueous interface. The efficiency of α-SEE was studied by calculating the interfacial tension (IFT) between crude oil and surfactant solution, and by investigating the ability of the surfactant to alter the wettability nature of carbonate and quartz surfaces. Salt effect was studied at CMC of surfactant to obtain an ultra-low IFT value of the order of 10−3 mN/m at optimal salinity. Addition of organic alkali also showed synergistic effect on IFT between crude oil and surfactant solution. The surfactant favorably altered the wettability of oil-wet carbonate and quartz surfaces to water-wet, which is desirable for oil recovery. The surfactant showed potential application in EOR owing to its enhanced interfacial properties and rock-wetting characteristics. Flooding experiments were conducted with surfactant slugs at different α-SEE concentrations to achieve about 25%–27% additional oil recoveries after conventional flooding.

Dynamic interfacial tensions of binary nonionic–anionic and nonionic surfactant mixtures at water–alkane interfaces

The influences of nonionic surfactant on interfacial properties of anionic–nonionic surfactant against alkanes are studied by measuring the dynamic interfacial tension (IFT). The surfactants chosen are fatty alcohol polyoxyethylene carboxylate (C12EO3C) and fatty alcohol polyoxyethylene (C12E3) with various counterions, Na+, Mg2+, and Ca2+. On the basis of our experimental results, one can find that the addition of nonionic surfactants C12E3 could achieve an ultralow IFT value at some specific alkane carbon numbers under NaCl condition and the obvious synergistic effect on reducing IFT can be observed during all experimental alkane carbon numbers. However, for MgCl2 systems, the ultralow IFT values can be achieved by pure C12EO3C solutions and one can observe synergistic effect and antagonistic effect on IFTs at higher and lower alkane carbon numbers respectively by adding nonionic surfactant. On the other hand, for CaCl2 systems, the addition of 0.05% C12E3 shows the same influence on the IFTs as that of NaCl systems, while the similar effect as that of MgCl2 systems will be observed when adding 0.1% C12E3. An interfacial model combined two mechanisms, controlling hydrophilic–lipophilic balance and forming mixed adsorption film, responsible for varying IFT has been provided based on the difference of ion radius. By adding nonionic surfactant, the mechanisms of forming mixed adsorption film and controlling hydrophilic–lipophilic balance play crucial roles in effecting IFTs for NaCl systems and MgCl2 systems, respectively. Moreover, for CaCl2 systems, the responsible mechanism of forming mixed adsorption film will change into controlling hydrophilic–lipophilic balance with the increase of nonionic surfactant concentration.

Estimation of the Capability of Surfactant in Reducing the Interfacial Tension in Surfactant/Alkali/Crude Oil Systems via Quantification

The dynamic interfacial tension in surfactant/alkali/acidic oil systems was investigated in a mode of three arylalkyl surfactant flooding. The results indicate that there exists an optimum range of alkali concentrations when the minimum interfacial tension value is below 10−2 mN/m (the ultra-low interfacial tension value) at certain surfactant concentrations. The minimum interfacial tension presents a similar parabolic relationship with alkali concentrations and also a similar hyperbolic relationship with surfactant concentrations as well. Thus, a convexity can be built by the results of three variables, i.e., the minimum interfacial tension, surfactant concentration, and alkali concentration. The capability of synthesized arylalkyl surfactants in reducing the interfacial tension can be expressed by the values of the volumes formed by the three variables. The higher value indicates the stronger capability of the surfactant in reducing the interfacial tension. This method for the capability of surfactant being quantified would offer a new way to choose the surfactants in enhanced oil recovery.

A family of alkyl sulfate gemini surfactants. 2. Water–oil interfacial tension reduction

A family of sulfate gemini surfactants that has great potential for various industrial applications was previously prepared and characterized in our lab. The unique and versatile structure of these surfactants has endowed them with properties that are attractive for enhanced oil recovery (EOR). A detailed experimental study was carried out and is presented here on the water–oil interfacial properties of these novel molecules. This series of seven sulfate gemini surfactants of different hydrophobic tail and spacer group lengths shows systematic trends in interfacial tension (IFT) reduction with changing solution conditions. These molecules exhibit extraordinary aqueous stability even in high salinity and hard brines. Ultra-low interfacial tension values were measured with low surfactant concentrations. The results from this study showed the potential of utilizing these surfactants at low concentrations and in harsh reservoir environments.

A New Family of Anionic Surfactants for Enhanced-Oil-Recovery Applications

Summary A new family of anionic surfactants that has great potential for enhanced-oil-recovery (EOR) applications was synthesized and characterized in our laboratory. The unique and versatile structure of these surfactants has endowed them with properties that are attractive for EOR. A detailed experimental study was carried out and is presented here on the oil/water and solid/water interfacial properties of seven novel molecules. The interfacial properties of this series of seven anionic surfactants with different lengths of hydrophobic tail and linking spacer group show systematic trends in interfacial tension (IFT) and static adsorption density with changes in solution conditions. These molecules showed excellent aqueous stability even in high-salinity and hard brines. Ultralow IFT (ULIFT) values were measured at low surfactant concentrations. The synthesized Gemini surfactants also showed lower maximal adsorption densities than the conventional single-chain surfactants. The results from this study showed the potential of using these surfactants at low concentrations and in harsh reservoir conditions.

Formulating Middle Phase Microemulsions Using Extended Anionic Surfactant Combined with Cationic Hydrotrope

The phase behavior of systems containing sodium alkyl polypropylene oxide sulfate with equimolar ratio of polypropylene oxide ammonium chloride was determined as a function of salt concentration and alkane carbon number at ambient temperature. Visual inspection as well as cross polarizers were used to detect anisotropy. Solubilization ratios for oil and brine in the middle phases were measured and used to calculate the interfacial tension (IFT) between the microemulsion and oil or brine. The fish diagram is presented showing a minimum amount of 0.20 wt% surfactant needed to form the middle phase. Swelling increases with decreasing the alkane carbon number of the oil. Ultra low IFT values were predicted for the systems investigated.

Microemulsion Phase Behavior of Aerosol-OT Combined with a Cationic Hydrotrope in the Dilute Region

The phase behavior of systems containing minimum amounts of sodium bis (2-ethylhexyl) sulfosuccinate with equimolar ratio of tetra ethyl ammonium chloride were studied as a function of salt concentration and alkane carbon number at ambient temperature. Visual inspection as well as cross polarizers were used to detect anisotropy. Solubilization ratios for oil and brine in the middle phases were measured and used to calculate the interfacial tension. Ultra-low interfacial tension values were predicted for the systems containing heptane, octane and nonane as model oil. Different phase behavior was observed for systems with higher alkane number.

Mathematical modelling of enhanced oil recovery by alkali solutions in the presence of cosurfactant and polymer

A mathematical model describing oil recovery by alkali solutions in the presence of cosurfactants and polymers is presented. According to previously published experimental results, oil recovery is sensitive to transient interfacial tension ( IFT) particularly near ultra low IFT values. Unlike previously published models, the present work accounts for the transient interfacial tension and nonequilibrium mass transfer phenomena. Adsorption at the solid/liquid interface has been described by a surface excess model. Over thirty simulation runs have been performed to investigate the effects of alkali, cosurfactant and polymer concentrations, chemical slug size and dynamic IFT on oil recovery. The model was found to be sensitive to initial alkali, cosurfactant and polymer concentrations. For the cases studied, a 0.1% alkali, 0.1% polymer and 0.01% cosurfactant solution yielded the highest oil recovery. The presence of optimal concentration was recognized for cosurfactant. This optimum behaviour was attributed to dynamic interfacial tensions.

Physical and chemical properties of a biosurfactant synthesized by Rhodococcus species H13-A

The chemical and physical properties of a biosurfactant synthesized by hexadecane-grown Rhodococcus species H13-A are described. The biosurfactant is an anionic glycolipid consisting of 1 major and 10 minor components. The hydrophilic portion of the molecule is trehalose, which is acylated with normal C(10) to C(22) saturated and unsaturated fatty acids, C(35) to C(40) mycolic acids, hexanedioic and dodecanedioic acids, and 10-methyl hexadecanoic and 10-methyl octadecanoic acids. The major glycolipid species was identified as 2,3,4,6,2',3',4',6'-octaacyltrehalose, plus minor glycolipid species of di-, tetra- and hexa-acyltrehalose derivatives. The glycolipid exhibited a critical micelle concentration of 1.5 mg/mL and minimum interfacial tension value of 2 × 10(-2) mN/m against decane, with a further reduction in interfacial tension to 6 × 10(-5) mN/m in the presence of the cosurfactant pentanol. The phase behavior of the glycolipid indicates the formation of a surfactant-rich, "middle-phase" microemulsion containing liquid crystals, both of which are associated with surfactant systems having ultralow interfacial tension values. Key words: trehalose lipids, glycolipids, biosurfactants.