Effect of surfactants on peroxidase activity. III. Effect of nonionic surfactants

The effect of a series of nonionic surfactants on the initial rate of the peroxide oxidation of 5-aminosalicylic acid in solution catalyzed by horseradish peroxidase was studied. As the surfactant concentration increases, the peroxidation rate first increases, then decreases, and the increase/decrease cycle is repeated. The primary increase may be induced by a change in properties of the medium under the action of surfactants, and the following decrease, by the enzyme inhibition. The secondary increase may be explained by to a change in the enzyme conformation and an increase in the accessibility of its active site for the substrate due to the immobilization of the protein in the surfactant aggregates, whereas the secondary decrease, by a shielding of the protein with these aggregates. For communication II, see [1].     

Effect of surface-active agents on peroxidase activity. IV. Effect of lipids and fatty acids from milk

The effect of surfactants, lipids and fatty acid salts isolated from cow milk on the activity of heme-containing horseradish peroxidase in solution was studied. As the surfactant concentration increases, the rate of the enzymic reaction successively decreases, increases, and again decreases, down to zero in the case of the fatty acid salts. The initial deceleration of the reaction rate results from the enzyme inhibition. The subsequent increase is caused by an improved accessibility for the substrate and the enhanced activity of the catalytic site of the enzyme due to its immobilization in the surfactant aggregates. A shielding of the protein by these aggregates can explain the secondary deceleration of the enzymic reaction rate. The general character of the dependence is similar and does not depend on the surfactant structure for a series of fatty acid salts and phospholipids; however, it is quite different in the case of cholesterol and sphingomyelin.     

Effect of surfactants on peroxidase activity: IV. Effect of milk lipids and fatty acid salts

The effect of surfactants, lipids and fatty acid salts isolated from cow milk on the activity of hemecontaining horseradish peroxidase in solution was studied. As the surfactant concentration increases, the rate of the enzymic reaction successively decreases, increases, and again decreases, down to zero in the case of the fatty acid salts. The initial deceleration of the reaction rate results from the enzyme inhibition. The subsequent increase is caused by an improved accessibility for the substrate and the enhanced activity of the catalytic site of the enzyme due to its immobilization in the surfactant aggregates. A shielding of the protein by these aggregates can explain the secondary deceleration of the enzymic reaction rate. The general character of the dependence is similar and does not depend on the surfactant structure for a series of fatty acid salts and phospholipids; however, it is quite different in the case of cholesterol and sphingomyelin. For communication III, see [1].     

A freeze-fracture transmission electron microscopy and small angle x-ray diffraction study of the effects of albumin, serum, and polymers on clinical lung surfactant microstructure

Freeze-fracture transmission electron microscopy shows significant differences in the bilayer organization and fraction of water within the bilayer aggregates of clinical lung surfactants, which increases from Survanta to Curosurf to Infasurf. Albumin and serum inactivate all three clinical surfactants in vitro; addition of the nonionic polymers polyethylene glycol, dextran, or hyaluronic acid also reduces inactivation in all three. Freeze-fracture transmission electron microscopy shows that polyethylene glycol, hyaluronic acid, and albumin do not adsorb to the surfactant aggregates, nor do these macromolecules penetrate the interior water compartments of the surfactant aggregates. This results in an osmotic pressure difference that dehydrates the bilayer aggregates, causing a decrease in the bilayer spacing as shown by small angle x-ray scattering and an increase in the ordering of the bilayers as shown by freeze-fracture electron microscopy. Small angle x-ray diffraction shows that the relationship between the bilayer spacing and the imposed osmotic pressure for Curosurf is a screened electrostatic interaction with a Debye length consistent with the ionic strength of the solution. The variation in surface tension due to surfactant adsorption measured by the pulsating bubble method shows that the extent of surfactant aggregate reorganization does not correlate with the maximum or minimum surface tension achieved with or without serum in the subphase. Albumin, polymers, and their mixtures alter the surfactant aggregate microstructure in the same manner; hence, neither inhibition reversal due to added polymer nor inactivation due to albumin is caused by alterations in surfactant microstructure.     

Self-assembly of cationic surfactants on the carbon nanotube surface: insights from molecular dynamics simulations

The insolubility of carbon nanotubes (CNTs) in aqueous media has been a limitation for the practical application of this unique material. Recent studies have demonstrated that the suspend ability of CNT can be substantially improved by employing appropriate surfactants. Although various surfactants have been tested, the exact mechanism by which carbon nanotubes and the different surfactants interact is not fully understood. To deepen the understanding of molecular interaction between CNT and surfactants, as well as to investigate the influence of the surfactant tail length on the adsorption process, we report here the first detailed large-scale all-atomistic molecular dynamics simulation study of the adsorption and morphology of aggregates of the cationic surfactants containing trimethylammonium headgroups (C₁₂TAB and C₁₆TAB) on single-walled carbon nanotube (SWNT) surfaces. We find that the aggregation morphology of both C₁₂TAB and C₁₆TAB on the SWNT is dependent upon the number of the surfactants in the simulation box. As the number of the surfactants increases the random monolayer structure gradually changes to the cylinder-like monolayer structure. Moreover, we make a comparison between the C₁₂TAB and C₁₆TAB adsorption onto SWNTs to clarify the role of the surfactant tail length on the adsorption process. This comparison indicates that by increasing the number of surfactant molecules, the larger number of the C₁₆TAB molecules tend to adsorb onto SWNTs. Further, our results show that a longer chain yields the higher packed aggregates in which the surfactant heads are extended far into the aqueous phase, which in turn may increase the SWNTs stabilization in aqueous suspensions.     

Role of surfactant on the proteolysis of aqueous bovine serum albumin

Nonionic and ionic surfactants diminish the initial rate of proteolysis of aqueous bovine serum albumin (BSA) by subtilisin Carlsberg. Surfactants studied include: nonionic tetraethylene glycol monododecyl ether (C12E4); anionic sodium dodecyl sulfate (SDS), anionic sodium dodecylbenzenesulfonate (SDBS), and cationic dodecyltrimethylamonium bromide (DTAB). Kinetic data are obtained using fluorescence emission. Special attention is given to enzyme kinetic specificity determined by fitting initial-rate data to the Michaelis-Menten model. All surfactants reduce the rate of proteolysis, most strongly at concentrations near and above the critical micelle concentration (CMC). Circular dichroism (CD), tryptophan/tyrosine fluorescence spectra, and tryptophan fluorescence thermograms indicate that BSA partially unfolds at ionic surfactant concentrations near and above the CMC. Changes in BSA conformation are less apparent at ionic surfactant concentrations below the CMC and for the nonionic surfactant C12E4. Subtilisin Carlsberg activity against the polypeptide, succinyl-Ala-Ala-Pro-Phe-p-nitroanilide, decreased due to enzyme-surfactant interaction. At the concentrations and time frames studied, there was no enzyme autolysis. Importantly, aqueous proteolysis rates are significantly reduced at high surfactant concentrations where protein-micellar-surfactant aggregates occur. To explain the negative effect of surfactant on subtilisin Carlsberg proteolytic activity against BSA, we propose that micelle/protein complexes hinder enzyme access.     

Influence of sulfobetaines on the stability of the Citrobacterdiversus ULA-27 beta-lactamase.

The activity and stability of beta-lactamase from Citrobacter diversus ULA-27 have been investigated in the presence of different ionic and zwitterionic surfactants. All the sulfobetaine surfactants tested allow the enzyme to retain its full activity, but the best stabilizing effect is greatly dependent on their structure. Very little variations on the monomer headgroup can significantly reduce enzyme deactivation or speed up the loss of activity with respect to buffer alone. The whole hydrophobic/hydrophilic balance on the headgroup seems to have a determining role in preserving beta-lactamase activity and structure. The presence of zwitterionic surfactants stabilizes the protein conformation toward denaturation by urea and low-temperature inactivation. Similar experiments were performed in the presence of other two zwitterionic surfactants, an amine oxide, dimethylmyristylamine oxide (DMMAO) and a carboxybetaine, cetyldimethylammonium methanecarboxylate (CB1-16). The former stabilizes the enzyme even better than the sulfobetaines, the latter quickly deactivates it. Therefore, the factors responsible for beta-lactamase stabilization are dependent not only on the zwitterionic nature of the surfactant headgroup but also specific interactions between the surfactant and the protein may be important.     

The interaction between Beta-lactoglobulin and sodium N-dodecyl sulphate.

1. The binding of sodium n-dodecyl sulphate to beta-lactoglobulin was studied in the pH range 3.5-7.0 by equilibrium dialysis, ultracentrifugation and microcalorimetry. 2. At low binding concentrations (less than 30 bound surfactants anions per protein molecule) the complexes formed aggregates in solution. 3. At higher binding concentrations aggregation does not occur at low ionic strength (0.01 mol/litre), but continues at high ionic strength (0.1 mol/litre). 4. At 25 degrees C the enthalpy of interaction of sodium n-dodecyl sulphate with beta-lactoglobulin can be interpreted as the sum of the enthalpies of formation of a complex with 2 bound surfactant anions, with an enthalpy change of -9.5 kJ-mol-1 of bound surfactant, and complexes containing at least 22 bound surfactant anions, with limiting enthalpies per bound surfactant anion of -12.4 kJ-mol-1 at pH 3.5 and -3.25 kJ-mol-1 at pH 5.5. 5. The binding of surfactant and the enthalpy of interaction at pH 3.5 ARE NOT SIGNIFICANTLY AFFECTED BY THE ADDITION Of 8 M-urea. 6. The data indicate that at low binding concentrations the interaction is of an ionic nature, and is accompanied by a conformational change in the protein.     

Effect of ethyleneoxide groups of anionic surfactants on lipase activity

The use of enzymes in laundry and dish detergent products is growing. Such tendency implies dedicated studies to understand surfactant‐enzyme interactions. The interactions between surfactants and enzymes and their impact on the catalytic efficiency represent a central problem and were here evaluated using circular dichroism, dynamic light scattering, and enzyme activity determinations. This work focuses on this key issue by evaluating the role of the ethyleneoxide (EO) groups of anionic surfactants on the structure and activity of a commercial lipase, and by focusing on the protein/surfactant interactions at a molecular level. The conformational changes and enzymatic activity of the protein were evaluated in the presence of sodium dodecyl sulfate (SDS also denoted as SLE₀S) and of sodium lauryl ether sulfate with two EO units (SLE₂S). The results strongly suggest that the presence of EO units in the surfactant polar headgroup determines the stability and the activity of the enzyme. While SDS promotes enzyme denaturation and consequent loss of activity, SLE₂S preserves the enzyme structure and activity. The data further highlights that the electrostatic interactions among the protein groups are changed by the presence of the adsorbed anionic surfactants being such absorption mainly driven by hydrophobic interactions. © 2016 American Institute of Chemical Engineers Biotechnol. Prog., 32:1276–1282, 2016     

Studies on surfactant-biopolymer interaction. I. Microcalorimetric investigation on the interaction of cetyltrimethylammonium bromide (CTAB) and sodium dodecylsulfate (SDS) with gelatin (Gn), lysozyme (Lz) and deoxyribonucleic acid (DNA)

The interaction of the surfactants cetyltrimethyl ammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) with the biopolymers gelatin (Gn), lysozyme (Lz) and deoxyribonucleic acid (DNA) was studied by isothermal titration microcalorimetry at varied biopolymer concentration, pH and temperature. The nature of interaction of the surfactants with the biopolymers was assessed from the observed enthalpy-[surfactant] profiles. The biopolymer-induced aggregation of the surfactants was observed. The enthalpies of aggregation of amphiphiles, binding of aggregates with macromolecules, organisational change of bound aggregates, and threshold concentrations for micelle formation of surfactants in the presence of biopolymers were estimated. The results collected on the three biopolymers were analysed and compared.     

Correlation between surfactant/micelle structure and the stability of bacteriorhodopsin in solution.

The rate of solubilization and isothermal bleaching of bacteriorhodopsin (bR) in a series of nine alkylammonium surfactants is studied by using time-resolved optical spectroscopy. The surfactant series RN(+)R'(3) covers a range in tail length (R = C(12)H(25), C(14)H(29), or C(16)H(33)) and headgroup size and hydrophobicity (R' = CH(3); C(2)H(5), or C(3)H(7)). The rate of bleaching increases initially with increasing surfactant concentration but decreases at higher concentrations. Possible explanations for this behavior are discussed. The kinetic data are consistent with the penetration of the surfactant into the protein interior. Interaction of the surfactants with the protein is a complicated, multistep process, and the rate curves are a function of at least four variables: 1) the micellar environment, 2) the length of the surfactant tail, 3) the size of the headgroup, and 4) the hydrophobicity of the headgroup. Our data provide new insights into the molecular characteristics that help define the performance of surfactants in the solubilization and denaturation of membrane-bound proteins.     

Effect of cationic and non-ionic surfactants on the hydrolysis of N-glutaryl- -phenylalanine catalysed by chymotrypsin iso-enzymes

The hydrolysis of N-glutaryl- -phenylalanine p-nitroanilide catalysed by various chymotrypsin (CT) iso-enzymes (α-CT, β-CT, δ-CT, and γ-CT) has been studied in the presence of cationic and non-ionic surfactants at concentration higher than the critical micellar concentration. The enzyme activity was tested in the presence of the following surfactants: cetyltrimethylammonium bromide (CTABr), cetyldimethylethylammonium bromide (CDMEABr), cetyltripropylammonium bromide (CTPABr), Triton X100 (TX100) and polyoxyethylene 9 lauryl ether (PO9). The activity of the iso-enzymes depends on the surfactant concentration and it varies with the surfactant head group dimensions (CTPABr>CDMEABr>CTABr). For all the iso-enzymes, superactivity has been detected only in the presence of CTPABr and CDMEABr. The extent of superactivity depends on the enzyme used (δ-CT>β-CT>γ-CT>α-CT). The observed reaction rate has been compared with the prediction of a theoretical model for enzymatic activity in the presence of surfactant aggregates in aqueous media developed in a previous paper. The results can be explained by introducing an equilibrium relation between the enzyme confined in the free bulk water and in the bound water pseudo-phase, and by allowing for different catalytic behaviours of the two forms of enzyme.The theoretical model enables the initial reaction rate to be related to the substrate concentration with an overall Michaelis–Menten equation. Good agreement has been found between experimental and model predicted values of the kinetic parameters.     

Kinematic viscosity of therapeutic pulmonary surfactants with added polymers

The addition of various polymers to pulmonary surfactants improves surface activity in experiments both in vitro and in vivo. Although the viscosity of surfactants has been investigated, the viscosity of surfactant polymer mixtures has not. In this study, we have measured the viscosities of Survanta and Infasurf with and without the addition of polyethylene glycol, dextran or hyaluronan. The measurements were carried out over a range of surfactant concentrations using two concentrations of polymers at two temperatures. Our results indicate that at lower surfactant concentrations, the addition of any polymers increased the viscosity. However, the addition of polyethylene glycol and dextran to surfactants at clinically used concentrations can substantially lower viscosity. Addition of hyaluronan at clinical surfactant concentrations slightly increased Infasurf viscosity and produced little change in Survanta viscosity. Effects of polymers on viscosity correlate with changes in size and distribution of surfactant aggregates and the apparent free volume of liquid as estimated by light microscopy. Aggregation of surfactant vesicles caused by polymers may therefore not only improve surface activity as previously shown, but may also affect viscosity in ways that could improve surfactant distribution in vivo.     

Inactivation of pulmonary surfactant due to serum-inhibited adsorption and reversal by hydrophilic polymers: experimental

The rate of change of surface pressure, pi, in a Langmuir trough following the deposition of surfactant suspensions on subphases containing serum, with or without polymers, is used to model a likely cause of surfactant inactivation in vivo: inhibition of surfactant adsorption due to competitive adsorption of surface active serum proteins. Aqueous suspensions of native porcine surfactant, organic extracts of native surfactant, and the clinical surfactants Curosurf, Infasurf, and Survanta spread on buffered subphases increase the surface pressure, pi, to approximately 40 mN/m within 2 min. The variation with concentration, temperature, and mode of spreading confirmed Brewster angle microscopy observations that subphase to surface adsorption of surfactant is the dominant form of surfactant transport to the interface. However (with the exception of native porcine surfactant), similar rapid increases in pi did not occur when surfactants were applied to subphases containing serum. Components of serum are surface active and adsorb reversibly to the interface increasing pi up to a concentration-dependent saturation value, pi(max). When surfactants were applied to subphases containing serum, the increase in pi was significantly slowed or eliminated. Therefore, serum at the interface presents a barrier to surfactant adsorption. Addition of either hyaluronan (normally found in alveolar fluid) or polyethylene glycol to subphases containing serum reversed inhibition by restoring the rate of surfactant adsorption to that of the clean interface, thereby allowing surfactant to overcome the serum-induced barrier to adsorption.     

Effect of surfactant type on surfactant--protein interactions at the air-water interface

The displacement of the proteins (beta-lactoglobulin and beta-casein) from an air-water interface by the nonionic (Tween 20 and Tween 60) and ionic (sodium dodecyl sulfate, cetyltrimethylammonium bromide, and lyso-phosphatidylcholine-lauroyl) surfactants has been visualized by atomic force microscopy (AFM). The surface structure has been sampled by the use of Langmuir-Blodgett deposition onto mica substrates to allow imaging in the AFM. In all cases, the displacement process was found to occur through the recently proposed orogenic mechanism (Mackie et al. J. Colloid Interface Sci. 1999, 210, 157-166). In the case of the nonionic surfactants, the displacement involved nucleation and growth of surfactant domains leading to failure of the protein network and subsequent loss of protein into the bulk phase. The surface pressure dependence of the growth of surfactant domains and the failure of the network were found to be the same for both Tween 20 and Tween 60, demonstrating that the breakdown of the protein film was dominated by the mechanical properties of the network. The displacement of protein by ionic surfactants was found to be characterized by nucleation of surfactant domains with little domain growth prior to failure of the network. The size of the domains formed by ionic surfactants was found to be limited by the strong intersurfactant repulsive forces between the charged headgroups. Screening of these charges led to an increase in the size of the domains. The surface pressure at which the network continuity was lost was found to be dependent on the type of surfactant and, in all cases, to occur at higher surface pressures than that required for nonionic surfactants. This has been attributed to surfactant-protein binding that initially strengthens the protein network at low surfactant concentrations. Evidence obtained from surface shear rheology supports this assertion.     

Surfactant Assemblies and their Various Possible Roles for the Origin(S) of Life

A large number of surfactants (surface active molecules) are chemically simple compounds that can be obtained by simple chemical reactions, in some cases even under presumably prebiotic conditions. Surfactant assemblies are self-organized polymolecular aggregates of surfactants, in the simplest case micelles, vesicles, hexagonal and cubic phases. It may be that these different types of surfactant assemblies have played various, so-far underestimated important roles in the processes that led to the formation of the first living systems.Although nucleic acids are key players in the formation of cells as we know them today (RNA world hypothesis), it is still unclear how RNA could have been formed under prebiotic conditions. Surfactants with their self-organizing properties may have assisted, controlled and compartimentalized some of the chemical reactions that eventually led to the formation of molecules like RNA. Therefore, surfactants were possibly very important in prebiotic times in the sense that they may have been involved in different physical and chemical processes that finally led to a transformation of non-living matter to the first cellular form(s) of life. This hypothesis is based on four main experimental observations: (i) Surfactant aggregation can lead to cell-like compartimentation (vesicles). (ii) Surfactant assemblies can provide local reaction conditions that are very different from the bulk medium, which may lead to a dramatic change in the rate of chemical reactions and to a change in reaction product distributions. (iii) The surface properties of surfactant assemblies that may be liquid- or solid-like, charged or neutral, and the elasticity and packing density of surfactant assemblies depend on the chemical structure of the surfactants, on the presence of other molecules, and on the overall environmental conditions (e. g. temperature). This wide range of surface characteristics of surfactant assemblies may allow a control of surface-bound chemical reactions not only by the charge or hydrophobicity of the surface but also by its “softness”. (iv) Chiral polymolecular assemblies (helices) may form from chiral surfactants.There are many examples that illustrate the different roles and potential roles of surfactant assemblies in different research areas outside of the field of the origin(s) of life, most importantly in investigations of contemporary living systems, in nanotechnology applications, and in the development of drug delivery systems. Concepts and ideas behind many of these applications may have relevance also in connection to the different unsolved problems in understanding the origin(s) of life.     

Effect of surface-active substances on bioluminescence intensity of bacteria

The study of sensitivity of luminous bacteria isolated from the Black and Azov seas to surfactants from various classes was carried out. It was shown that cationic surfactants had a strong inhibition effect on bacterial luminescence in contrast to anionic and in particular nonionic surfactants. To increase the luminous bacteria sensitivity to the action of OP-10 (nonionic surfactant) and ABS (anionic surfactant), which are widely used in industry, several approaches have been developed. They include modulation of bacterial sensitivity by the additives of cationic substances, use of luminous bacteria at a logarithmic stage of growth, realization of biotesting at low pH = 5.5. The use of these approaches allows to lower effective concentrations of OP-10 and ABS, which caused a decrease of bioluminescence by 50%, 3-200 times and opens perspectives for the use of the bioluminescent method to study these surfactants toxicity on the principle of biosensorics.     

Quantifying the biodegradation of phenanthrene by Pseudomonas stutzeri P16 in the presence of a nonionic surfactant.

The low water solubility of polycyclic aromatic hydrocarbons is believed to limit their availability to microorganisms, which is a potential problem for bioremediation of polycyclic aromatic hydrocarbon-contaminated sites. Surfactants have been suggested to enhance the bioavailability of hydrophobic compounds, but both negative and positive effects of surfactants on biodegradation have been reported in the literature. Earlier, we presented mechanistic models of the effects of surfactants on phenanthrene dissolution and on the biodegradation kinetics of phenanthrene solubilized in surfactant micelles. In this study, we combined the biodegradation and dissolution models to quantify the influence of the surfactant Tergitol NP-10 on biodegradation of solid-phase phenanthrene by Pseudomonas stutzeri P16. Although micellized phenanthrene does not appear to be available directly to the bacterium, the ability of the surfactant to increase the phenanthrene dissolution rate resulted in an overall increase in bacterial growth rate in the presence of the surfactant. Experimental observations could be predicted well by the derived model with measured biokinetic and dissolution parameters. The proposed model therefore can serve as a base case for understanding the physical-chemical effects of surfactants on nonaqueous hydrocarbon bioavailability.     

Effect of selected non-ionic surfactants on the flow behavior of aqueous veegum suspensions

The aim of this work was to investigate the influence of some non-ionic surfactants, Tween 80 and Brij 98, on the viscosity and flow behavior of a commercial montmorillonite clay, Veegum Granules. The effect of different concentrations of the surfactants on the shear stress-shear rate rheograms of hydrated concentrated clay suspensions was determined by shear viscometry. The addition of either surfactant increased the plastic viscosity and the yield stress of the suspensions. Furthermore both surfactants altered the thixotropy of the suspensions to an extent that depended on both the surfactant concentration and the time of equilibration of the surfactant and Veegum. Brij 98 had a greater and more rapid effect. It is proposed that the surfactant polar head-groups anchor at the tetrahedral sheet surface, leaving the alkyl chains extending away from the edges and faces. Consequently, the alkyl chains undergo hydrophobic interactions that facilitate the association between the platelets and increase the physical structure within the suspension. Stereochemical differences between the polar groups may lead to differences in the way the surfactants associate with the tetrahedral sheet and hence their ultimate effect on the rheological behavior. There is a significant interaction between these surfactants and montmorillonite clays, and the rheological changes that occur could have a major impact on any pharmaceutical formulation that uses these ingredients. Published: March 30, 2007     

Influence of coprecipitation and interactions with anionic surfactants on the activity of plant phospholipase D.

The effects of two surfactants (sodium dodecyl sulfate and sodium dodecyl dioxyethylene sulfate) of similar structure but differing water solubility (of their calcium salts) on the enzymatic activity of cabbage phospholipase D have been studied. The solubility difference is insignificant because the two surfactants activate phospholipase D similarly. To elucidate the mechanism of their influence on the enzyme, the phase behavior in the reaction media and the interactions of the surfactants with the enzyme were investigated by potentiometry and by light scattering and UV spectroscopy. Calcium dodecyl dioxyethylene sulfate (which is more soluble in water than calcium dodecyl sulfate) precipitates in the presence of phosphatidylcholine, the substrate of the enzymatic reaction. In the reaction media phospholipase D was involved into a precipitate consisting of calcium salts of the surfactants and phosphatidylcholine that might be interpreted as its immobilization. In addition, the surfactants were adsorbed on the enzyme, unfolding the globular enzyme molecule due to electrostatic repulsion between adsorbed surfactant anions. The observed increase in the functional activity of phospholipase D is accounted for by transfer to an optimal tertiary structure for the enzyme molecule in the course of consecutive conformational transitions induced by the surfactants.