Bulk Micelles Research Articles

Interface Adsorption versus Bulk Micellization of Surfactants: Insights from Molecular Simulations.

Surfactants play essential roles in many commonplace applications and industrial processes. Although significant progress has been made over the past decades with regard to model-based predictions of the behavior of surfactants, important challenges have remained. Notably, the characteristic time scales of surfactant exchange among micelles, interfaces, and the bulk solution typically exceed the time scales currently accessible with atomistic molecular dynamics (MD) simulations. Here, we circumvent this problem by introducing a framework that combines the general thermodynamic principles of self-assembly and interfacial adsorption with atomistic MD simulations. This approach provides a full thermodynamic description based on equal chemical potentials and connects the surfactant bulk concentration, the experimental control parameter, with the surfactant surface density, the suitable control parameter in MD simulations. Self-consistency is demonstrated for the nonionic surfactant C12EO6 (hexaethylene glycol monododecyl ether) at an alkane/water interface, for which the adsorption and pressure isotherms are computed. The agreement between the simulation results and experiments is semiquantitative. A detailed analysis reveals that the used atomistic model captures well the interactions between surfactants at the interface but less so their adsorption affinities to the interface and incorporation into micelles. Based on a comparison with other recent studies that pursued similar modeling challenges, we conclude that the current atomistic models systematically overestimate the surfactant affinities to aggregates, which calls for improved models in the future.

Renaturation of Myoglobin Denatured by Sodium Dodecyl Sulfate by Removal of Dodecyl Sulfate Ions Bound to the Protein Using Sodium Cholate.

The secondary and tertiary structures of myoglobin were disrupted by sodium dodecyl sulfate (SDS) but were hardly affected by the bile salt, sodium cholate (NaCho). This disruption was induced by the binding of dodecyl sulfate (DS) ions to the protein. In this study, the removal of DS ions bound to the protein was attempted using NaCho. The extent of removal of DS ions was estimated by the restoration of the secondary and tertiary structures of the protein disrupted by SDS. The secondary structural change was followed by monitoring mean residue ellipticity at 222 nm, [θ]222, which was frequently used as a measure of α-helical content. The tertiary structural change was followed by examining the Soret band absorbance of the protein. Evidently, the magnitude of [θ]222 of myoglobin in the SDS solution initially decreased and then increased back to almost its original value as the NaCho concentration increased. The initial decrease in [θ]222 indicated the cooperation of NaCho and SDS in disrupting the secondary structure at low concentrations of both surfactants. This cooperation was also observed in the tertiary structural change as a shift of the Soret band maximum wavelength, λmax, and a decrease in the molar absorption coefficient, εmax, at λmax. Above a certain NaCho concentration, the position of λmax and the magnitude of εmax were also restored to their original states. The secondary and tertiary structures were simultaneously restored by adding NaCho. These recoveries were attributed to removal of the DS ions bound to the protein. This removal might be due to the ability of cholate anions to strip DS ions bound to the protein. The stripped DS ions are more likely to form SDS-NaCho mixed micelles in bulk than SDS-NaCho mixed aggregates on the protein.

Interplay of temperature and calcium content in beta-casein solutions: From controlled self-aggregation of micelles in bulk to the design of stable foams

We describe in this study the aggregation behaviour of β-casein micelles from milk in bulk aqueous solution as function of both temperature and calcium content, and its influence on the foaming properties, in order to test if the different aggregation states of β-casein makes possible the design of proteins-based thermoresponsive foams. First, we characterized the morphology of the self-assembled β-casein molecules in solution by coupling turbidity measurements, Dynamic Light Scattering and Small Angle Neutron Scattering. They self-organize into individual micelles at low temperature (20°C) whatever the calcium content, and transit in a reversible way into aggregates of micelles at large temperature in presence of calcium, with a threshold transition that depend both on temperature and calcium content. The micelles aggregation is driven by the calcium through association with serine phosphate groups localized on the hydrophilic part of the β-casein. In the micelles regime, we demonstrated that the addition of calcium tunes the aggregation number of unimers per micelle in the same way than an increase of temperature through a change of hydrophobic interactions. The hydrophilic chains of the corona are however in a good solvent and interact through excluded volume interactions, even when the β-casein micelles aggregates themselves. The internal molecular structure of the micelles is thus not modified by calcium bridges, which explains the complete reversibility of the aggregation process over temperature cycling. Second, we studied the foam stability versus time as a function of the temperature and calcium content by measuring the kinetic evolution of both the foam volume and the liquid fraction. Foams produced by solutions containing only β-casein micelles were stable in terms of foam volume on a timescale of 1 h at 20°C but drained quickly. However, foams become unstable when the temperature was increased above 20°C. In presence of calcium, the aggregation of β-casein micelles inside the foam liquid channels enabled to increase foam stability at larger temperature by acting as a cork, which slows down the drainage. The increase of foam stability by such aggregates is however not sufficient on the long term to allow the design of thermoresponsive foams.

Promising aqueous dispersions of carbon black for semisolid flow battery application

In this systematic study, we attempt to thoroughly understand the aggregation mechanism of conductive carbon black in aqueous dispersing medium and optimize the formulation of suspension electrodes for aqueous sodium–ion semisolid flow battery applications. For the first time, we present branched micelle solution as an electrolyte that introduces the widest electrochemical stability window of 3.5 V, from –3 to + 0.5 V vs. Ag/AgCl, with relatively low viscosity, rendering it as promising aqueous dispersing medium for suspension electrodes (anolytes). The simultaneous rheo-electrical measurements revealed that the aggregation of conductive ketjenblack (KB) particles was controlled by the electrostatic repulsion forces that doubly originate from the inter-aggregate soft interaction and bulk micelle interactions. This remarkable interaction rendered the dispersions electrically percolated at low carbon content (0.9 wt%) and importantly able to conserve the electrical conductivity under extremely high shear rates. Under the quiescent conditions, the percolated KB dispersion could sustain higher load of active material (up to 30 wt% Na2Ti3O7 as an active material) without severe influence on the rigidity (rheological plateau modulus G0) and electrical conductivity (Σ) of the suspensions. These promising findings propound further investigation for the electrochemical performance of these aqueous sodium–ion suspension electrodes under flow conditions.

Microfluidic Engineering of RGD[1]‐Terminated Nanocarriers Micellization and In‐Situ Docetaxel Encapsulation: An Atomistic Insight

AbstractThe interactions between ligands and polymers are of great interest to drug delivery researches. In this work for the first time, Poly (lactic‐co‐glycolic acid) based polymers with arginine‐glycine‐aspartate (RGD) peptide ligands which are immediately self‐assembled into micellar nanoparticles (NPs) in contact with water have been investigated in the atomistic level. In this regard, the effect of polymer and RGD concentrations on the stability, size, interaction energies and other molecular properties of nanocarriers has been investigated using molecular dynamics simulation. In the next step, the microfluidic micellization and docetaxel (DTX) encapsulation has been simulated in comparison to bulk micellization. Results indicated that by employing only 20 wt % RGD‐conjugated polymer, the particles reach the stable properties and compactness without DTX. But, when the DTX was added, 30 wt.% of RGD‐conjugated polymers that assembled in microfluidic system is the best state nanocarrier for the smart target drug delivery for transfer of DTX drug into the Cancer cell. Comparisons of the results acquired in the current paper confirmed that engineering and tuning the self‐assembly in the interface of phases in microchennel resulted in a precise control on the particle size and resulted in the preparation of smaller and more stable NPs that can be exploited as ultra‐small, smart drug carriers.

Modulation of micellization behavior of imidazolium based surface active ionic liquids by aromatic anions in aqueous medium

Surface active ionic liquids (SAILs) represent a class of ionic surfactants that many times display better surface-active properties as compared to conventional ionic surfactants. The micellization of SAILs can be further tailored by the addition of salts, and aromatic salts comprising hydrotropic anions have never been explored for altering the micellization of SAILs in low concentration range. Considering this, we have investigated the effect of addition of three aromatic salts i.e. Sodium 4-hydroxy benzenesulfonate, sodium benzenesulfonate, and sodium p-toluenesulfonate on the micellization of 1-dodecyl-3-methylimidazolium chloride, [C12mim][Cl] and its amide functionalized counterpart, 3-(2-(dodecylamino)−2-oxoethyl)−1-methyl-1H-imidazol-3-ium chloride, [C12Amim][Cl]. The surface-active behavior of SAILs has been investigated by tensiometry, whereas micellization in bulk has been probed through conductivity, steady-state fluorescence, dynamic light scattering (DLS) and zeta-potential measurements. The possible location of aromatic anions adsorbed on the micellar surface has been probed using 1H NMR measurements. Isothermal titration calorimetry has been utilized to gain insights into the thermodynamic aspects of micellization. The inferences gained from different techniques have been correlated and found to be in good agreement with each other. It has been observed that the functionalization of alkyl chain length of SAIL along with relative hydrophobicity and bulkiness of the aromatic anions control the characteristic properties of micellization.

Foam film stratification studies probe intermicellar interactions

Ultrathin foam films containing supramolecular structures like micelles in bulk and adsorbed surfactant at the liquid-air interface undergo drainage via stratification. At a fixed surfactant concentration, the stepwise decrease in the average film thickness of a stratifying micellar film yields a characteristic step size that also describes the quantized thickness difference between coexisting thick-thin flat regions. Even though many published studies claim that step size equals intermicellar distance obtained using scattering from bulk solutions, we found no reports of a direct comparison between the two length scales. It is well established that step size is inversely proportional to the cubic root of surfactant concentration but cannot be estimated by adding micelle size to Debye length, as the latter is inversely proportional to the square root of surfactant concentration. In this contribution, we contrast the step size obtained from analysis of nanoscopic thickness variations and transitions in stratifying foam films using Interferometry Digital Imaging Optical Microscopy (IDIOM) protocols, that we developed, with the intermicellar distance obtained using small-angle X-ray scattering. We find that stratification driven by the confinement-induced layering of micelles within the liquid-air interfaces of a foam film provides a sensitive probe of non-DLVO (Derjaguin-Landau-Verwey-Overbeek) supramolecular oscillatory structural forces and micellar interactions.

Electrochemical Stimulation of Water-Oil Interfaces by Nonionic-Cationic Block Copolymer Systems.

Variable interfacial tension could be desirable for many applications. Beyond classical stimuli like temperature, we introduce an electrochemical approach employing polymers. Hence, aqueous solutions of the nonionic-cationic block copolymer poly(ethylene oxide)114-b-poly{[2-(methacryloyloxy)ethyl]diisopropylmethylammonium chloride}171 (i.e., PEO114-b-PDPAEMA171 with a quaternized poly(diisopropylaminoethyl methacrylate) block) were investigated by emerging drop measurements and dynamic light scattering, analyzing the PEO114-b-qPDPAEMA171 impact on the interfacial tension between water and n-decane and its micellar formation in the aqueous bulk phase. Potassium hexacyanoferrates (HCFs) were used as electroactive complexants for the charged block, which convert the bishydrophilic copolymer into amphiphilic species. Interestingly, ferricyanides ([Fe(CN)6]3-) act as stronger complexants than ferrocyanides ([Fe(CN)6]4-), leading to an insoluble qPDPAEMA block in the presence of ferricyanides. Hence, bulk micellization was demonstrated by light scattering. Due to their addressability, in situ redox experiments were performed to trace the interfacial tension under electrochemical control, directly utilizing a drop shape analyzer. Here, the open-circuit potential (OCP) was changed by electrolysis to vary the ratio between ferricyanides and ferrocyanides in the aqueous solution. While a chemical oxidation/reduction is feasible, also an electrochemical oxidation leads to a significant change in the interfacial tension properties. In contrast, a corresponding electrochemical reduction showed only a slight response after converting ferricyanides to ferrocyanides. Atomic force microscopy (AFM) images of the liquid/liquid interface transferred to a solid substrate showed particles that are in accordance with the diameter from light scattering experiments of the bulk phase. In conclusion, the present results could be an important step toward economic switching of interfaces suitable, e.g., for emulsion breakage.

Interplay between surfactant self-assembly and adsorption at hydrophobic surfaces: insights from dissipative particle dynamics

ABSTRACT We study the self-organisation of aqueous surfactants in bulk phase, and the adsorption and self-organisation of the aqueous surfactants at a planar hydrophobic surface by dissipative particle dynamics. Nonionic surfactants, n-alkyl poly(ethylene oxide) CE, and water are coarse-grained into mesoscopic beads comprising 1–3 heavy atoms and two water molecules, respectively. The size of the mesoscopic beads is related to the molar volume of the underlying molecular fragments while the bead–bead interaction parameters are calibrated against the water-octanol partition coefficients. We focus on the CE, CE, CE, and CE surfactants in water that form spherical micelles in the bulk. The bulk micellization is primarily affected by the alkyl tail length which is demonstrated by an order of magnitude decrease in the critical micelle concentration when going from the aqueous CE to aqueous CE solutions. Surfactants strongly adsorb on the hydrophobic surface, adopting lying-down configurations and forming hemispheres which are in equilibrium with the spherical micelles in the bulk. In contrast to the bulk phase, the surfactant adsorption behaviour is influenced by both the alkyl tail and head chain lengths.

Exploring relationship of the state of N-dodecyl betaine in the solution monomer, at the interface and in the micelle via configurational entropy

Interfacial adsorption and bulk micellization of surfactant are common in natural world, and have prominent applications both in production and life. Surfactant molecules circularly flux among solution bulk, interface, and micelle in surfactant solution. Molecular dynamics (MD) simulations are carried out to compare the state of surfactant among solution monomer, interfacial monolayer, and micelle. Surface coverages from extremely dilute adsorption to saturated adsorption are all considered. Using the quasi harmonic approximation method, configurational entropies of N-dodecyl betaine (NDB) in different environments are obtained. The NDB monomers possess the largest freedom owing to spatial and non-aggregated superiorities. Entropy decomposition, including translational, rotational and vibrational entropies, is calculated to give further comparison and analysis. A new Born-Haber cycle is proposed to depict the state of NDB in different environments and to present entropy change of NDB itself in different mass transport processes. In particular, entropy changes of NDB in physical adsorption process below and above critical micelle concentration are obtained. The NDB molecule exhibits entropy decrease in interfacial adsorption, bulk micellization, and the transport of surfactant from micelle to surface layer.

Removal of Dodecyl Sulfate Ions Bound to Human and Bovine Serum Albumins Using Sodium Cholate.

The secondary structures of human serum albumin (HSA) and bovine serum albumin (BSA) were disrupted in the solution of sodium dodecyl sulfate (SDS), while being hardly damaged in the solution of the bile salt, sodium cholate (NaCho). In the present work, the removal of dodecyl sulfate (DS) ions bound to these proteins was attempted by adding various amounts of NaCho. The extent of removal was estimated by the restoration of α-helical structure of each protein disrupted by SDS. Increases and decreases in α-helical structure were examined using the mean residue ellipticity at 222 nm, [θ]222, which was frequently used as a measure of α-helical structure content. The magnitudes of [θ]222 of HSA and BSA, weakened by SDS, were restrengthened upon the addition of NaCho. This indicated that the α-helical structures of HSA and BSA that were disrupted by the binding of DS ions were nearly reformed by the addition of NaCho. The NaCho concentration at which the maximum restoration of [θ]222 of each protein was attained increased nearly linearly with SDS concentration. These results indicated that most of the bound DS ions were removed from the proteins but the removal was incomplete. The removal of DS ions, examined by means of the equilibrium dialysis, was also incomplete. The α-helical structure restoration and the DS ion removal by NaCho were considered to be due to the ability of cholate anions to strip the surfactant ions bound to HSA and BSA. These stripped DS ions appeared to be more likely to form SDS-NaCho mixed micelles in bulk rather than SDS-NaCho mixed aggregates on the proteins.

Micelle and Surface Tension of Double-Chain Cationic Surfactants.

Since the early 20th century, the slightly disparate measurements of a surfactant’s critical micelle concentration, via either surface tension or electrical conductivity, have been assumed one and the same. As a consequence, the possibility that micelles can adsorb at the air/water surface has been disregarded and has led to some abnormalities in the literature that remain as yet unresolved. In this paper, we closely examined the two critical concentrations for a double-chain cationic surfactant. We confirmed that the two concentrations represent two different physical phenomena. Furthermore, the results verified the existence of surface micelles, which are different from the bulk micelles. The formation of the surface micelles can be explained by the structural changes of the adsorption layer, which was also corroborated by molecular simulations. The findings open new challenges to examine the surface adsorption, which offers new insights into the molecular levels.

Synthesis and Surface Properties of Anionic Vinylguaiacol Based Surfactants

AbstractBy changing amino acid head group four biobased anionic surfactants were synthesized using 4-vinylguaiacol and methylbromo undecanoate. The synthesized surfactants were characterized by NMR and mass spectrometry analysis. The surface active properties such as surface tension, wetting power, foaming characteristic, emulsion stability, calcium tolerance were studied and compared with those of sodium lauryl sulfate (SLS). Excellent emulsion stabilities and calcium tolerances were observed for the synthesized surfactants. These surfactants show a lower critical micelle concentration as compared to the conventional surfactants. The bulk micellization properties were studied using dynamic light scattering and fluorescence anisotropy technique. The antimicrobial and cytotoxicity studies were also carried out.

Synthesis, Surface Properties and Effect of an Amino Acid Head Group of 11‐(2‐Methoxy‐4‐vinylphenoxy)undecanoicacid‐Based Anionic Surfactants

AbstractOur present research describes the surface properties of three biobased anionic surfactant synthesized from vinylguaiacol and 11‐bromo undecanoic acid. To further improve its hydrophobicity and bioavailability, amino acid head group incorporation was carried out. All these synthesized compounds were thoroughly characterized using NMR and mass spectroscopy. The performance properties such as foaming, wetting, emulsification value and calcium tolerance were evaluated. The studied surfactants possess excellent emulsion stability and moderate calcium tolerance as compared to commercially available surfactant sodium lauryl sulfate (SLS). The micelle formation and the thermodynamics involved at the air–water interface were estimated from surface tension measurements. These surfactants showed a higher tendency towards adsorption at the air–water interface than micellization. Dynamic light scattering and steady state fluorescence anisotropy study were carried out to shed light on the bulk micellization properties of the synthesized surfactant. Along with spherical micelles of <5 nm size, larger aggregates (35–84 nm) were observed with higher anisotropy values. FESEM images further confirmed the larger spherical micelles formed by these surfactants. The surfactants formed chiral aggregates above the critical micelle concentration as indicated by circular dichroism spectra. These surfactants may be suitable candidates for additives to detergents to improve their calcium tolerance especially in the case of hard water. Furthermore, a low foaming ability along with high emulsion stability may find these surfactants to be better replacement of the conventional surfactant used as emulsifiers in many industrial applications.

Charging Mechanism for Polymer Particles in Nonpolar Surfactant Solutions: Influence of Polymer Type and Surface Functionality.

Surface charging phenomena in nonpolar dispersions are exploited in a wide range of industrial applications, but their mechanistic understanding lags far behind. We investigate the surface charging of a variety of polymer particles with different surface functionality in alkane solutions of a custom-synthesized and purified polyisobutylene succinimide (PIBS) polyamine surfactant and a related commercial surfactant mixture commonly used to control particle charge. We find that the observed electrophoretic particle mobility cannot be explained exclusively by donor-acceptor interactions between surface functional groups and surfactant polar moieties. Our results instead suggest an interplay of multiple charging pathways, which likely include the competitive adsorption of ions generated among inverse micelles in the solution bulk. We discuss possible factors affecting the competitive adsorption of micellar ions, such as the chemical nature of the particle bulk material and the size asymmetry between inverse micelles of opposite charge.

Removal of Reactive Blue 21 from Aqueous Solution by Sorption and Solubilization in Micellar Media

This manuscript reports comparative study for the removal of health risk organic dye reactive blue 21 from aqueous solution using sorption and solubilization. Activated carbon was used as sorbent to remove dye. The bath sorption experiments were carried out at different pH, sorbent dosage, contact time, initial concentration, and agitation speed. The applicability of the Langmuir and Freundlich isotherm models was examined. The influence of cationic surfactant, cetyltrimethyl ammonium bromide (CTAB), and an anionic surfactant, sodium dodecyl sulfate (SDS) on spectral properties of selected dye in aqueous solutions was investigated by means of UV–Visible spectroscopy in sub-micellar and micellar concentration range. Absorption spectra of this dye in the presence of surfactants may confirm which type of interaction is possible between molecules of dye and surfactants. It is expected that initially molecules of dye may attach with monomeric molecules of surfactant in premicellar region while in post-micellar region they are accommodated within micelle. The values of micelle–water partition coefficient, Kx, free energy of partition, ΔGx, of RB21 between bulk water and micelles, drug-surfactant binding constant, Kb, and free energy of binding, ΔGb, were calculated from differential spectroscopic data.

The anionic biosurfactant rhamnolipid does not denature industrial enzymes.

Biosurfactants (BS) are surface-active molecules produced by microorganisms. Their combination of useful properties and sustainable production make them promising industrial alternatives to petrochemical and oleochemical surfactants. Here we compare the impact of the anionic BS rhamnolipid (RL) and the conventional/synthetic anionic surfactant sodium dodecyl sulfate (SDS) on the structure and stability of three different commercially used enzymes, namely the cellulase Carezyme® (CZ), the phospholipase Lecitase Ultra® (LT) and the α-amylase Stainzyme® (SZ). Our data reveal a fundamental difference in their mode of interaction. SDS shows great diversity of interaction toward the different enzymes. It efficiently unfolds both LT and CZ, but LT is unfolded by SDS through formation of SDS clusters on the enzyme well below the cmc, while CZ is only unfolded by bulk micelles and on average binds significantly less SDS than LT. SDS binds with even lower stoichiometry to SZ and leads to an increase in thermal stability. In contrast, RL does not affect the tertiary or secondary structure of any enzyme at room temperature, has little impact on thermal stability and only binds detectably (but at low stoichiometries) to SZ. Furthermore, all enzymes maintain activity at both monomeric and micellar concentrations of RL. We conclude that RL, despite its anionic charge, is a surfactant that does not compromise the structural integrity of industrially relevant enzymes. This makes RL a promising alternative to current synthetic anionic surfactants in a wide range of commercial applications.

Multiblock copolymer solutions in contact with a surface: self-assembly, adsorption, and percolation.

Amphiphilic copolymers are often used as compatibilizing or stabilizing agents, either in solution or at surfaces. In the special case of multiblock copolymers the connectivity of the blocks combines with the antagonistic behavior of the different types of blocks. Here we report on the behavior of solutions of amphiphilic multiblock copolymers with a large number of blocks and a low fraction of solvophobic monomers in contact with an attractive surface. Using lattice Monte Carlo simulations, the influence on the structures of the solvent quality and the type of surface from noninteracting to strongly attractive to the solvophobic monomers can be assessed. In the presence of a surface bulk micelles are formed that are not different in size and shape from the micelles observed in the absence of a surface. When increasing the surface attraction, solvophobic monomers tend to adsorb either as isolated blocks or forming surface micelles. Evidence is given of a surface concentration threshold above which surface micelles can form due to microphase separation. These surface micelles are in equilibrium with bulk micelles, some of which are connected to the surface through a path of either hydrophobic and/or hydrophilic blocks or hydrophobic cross-links, or both. The size distributions of bulk and connected micelles are similar. With increasing surface concentration surface micelles get organized due to the steric repulsion between core-shell surface micelles. Moreover, these organized surface micelles percolate. The connected micelles form a concentrated layer parallel to the attractive surface. In addition, these systems are governed by two very different time scales: The fast one leads to micellar self-assembly in the bulk and at the surface while the slow one prevents the system from reaching equilibrium in the course of the simulations and corresponds to the transfer of copolymers from the bulk to the attractive surface.

The Effect of NaCl on the Krafft Temperature and Related Behavior of Cetyltrimethylammonium Bromide in Aqueous Solution

AbstractThis paper presents the effect of NaCl on the Krafft temperature (TK), surface adsorption and bulk micellization of cetyltrimethylammonium bromide (CTAB) in aqueous solution. The critical micelle concentration (CMC) of CTAB in the presence of NaCl increased and then decreased with increasing temperature. Thus, the CMC–temperature data can be represented by a bell‐shaped curve. The micellar dissociation (fraction of counterion binding) and energetic parameters (free energy, enthalpy and entropy changes) of both adsorption and micellization were calculated. The processes were found to be both enthalpy and entropy controlled and appeared to be more and more enthalpy driven with increasing temperature. An enthalpy–entropy compensation rule was observed for both adsorption and micelle formation. The TK of the surfactant decreased significantly in the presence of NaCl, which is a sharp contrast to the usual behavior of the effect of electrolytes on the TK of classical ionic surfactants. The surface excess concentrations decreased with increasing temperature. However, the values were much higher in the presence of NaCl compared to the corresponding values in pure water. The solubilization behavior of a water‐insoluble dye, Sudan red B (SRB), in the micellar system was studied by the UV–visible spectrophotometric technique. The molar solubilization ratio in the presence of NaCl was found to be about three times higher than that in pure water, indicating that the solubilization of SRB in the CTAB micelles significantly increases in the presence of NaCl.

Micellization and interfacial behavior of a synthetic surfactant–biosurfactant mixture

The interfacial behavior and micellization of the mixture of surfactin (a powerful biosurfactant) and a commonly used synthetic surfactant, sodium dodecylbenzenesulphonate (SDOBS), have been investigated. The adsorption of the two surfactants from the liquid phase into the air–liquid interface resulted in the formation of a mixed interfacial monolayer with a mole fraction of the biosurfactant as 0.55 despite that its mole fraction in the liquid bulk was much lower (0.076). The interaction between the adsorbed molecules of surfactin and SDOBS at the interface was found to be attractive (βI=−1.15). However, the mixed micelles formed from the two surfactants with a mole fraction of the biosurfactant as 0.50 have demonstrated a repulsive interaction (βM=1.35). The formation of a mixed interfacial monolayer comprising SDOBS and surfactin was found to be a spontaneous process (ΔGI=−2.42kJmol−1). Furthermore, the formation of the mixed micelles in the liquid bulk was also thermodynamically feasible (ΔGM=−0.88kJmol−1). The findings reported in this study could form the basis for utilizing mixed biosurfactant–synthetic surfactant systems in many industrial applications with the ultimate aim of producing surface active agents formulations that are green and sustainable.