Showing papers on "Critical micelle concentration published in 2000"

Rhamnolipid-Induced Removal of Lipopolysaccharide from Pseudomonas aeruginosa: Effect on Cell Surface Properties and Interaction with Hydrophobic Substrates

Abstract: Little is known about the interaction of biosurfactants with bacterial cells. Recent work in the area of biodegradation suggests that there are two mechanisms by which biosurfactants enhance the biodegradation of slightly soluble organic compounds. First, biosurfactants can solubilize hydrophobic compounds within micelle structures, effectively increasing the apparent aqueous solubility of the organic compound and its availability for uptake by a cell. Second, biosurfactants can cause the cell surface to become more hydrophobic, thereby increasing the association of the cell with the slightly soluble substrate. Since the second mechanism requires very low levels of added biosurfactant, it is the more intriguing of the two mechanisms from the perspective of enhancing the biodegradation process. This is because, in practical terms, addition of low levels of biosurfactants will be more cost-effective for bioremediation. To successfully optimize the use of biosurfactants in the bioremediation process, their effect on cell surfaces must be understood. We report here that rhamnolipid biosurfactant causes the cell surface of Pseudomonas spp. to become hydrophobic through release of lipopolysaccharide (LPS). In this study, two Pseudomonas aeruginosa strains were grown on glucose and hexadecane to investigate the chemical and structural changes that occur in the presence of a rhamnolipid biosurfactant. Results showed that rhamnolipids caused an overall loss in cellular fatty acid content. Loss of fatty acids was due to release of LPS from the outer membrane, as demonstrated by 2-keto-3-deoxyoctonic acid and sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis and further confirmed by scanning electron microscopy. The amount of LPS loss was found to be dependent on rhamnolipid concentration, but significant loss occurred even at concentrations less than the critical micelle concentration. We conclude that rhamnolipid-induced LPS release is the probable mechanism of enhanced cell surface hydrophobicity.

On the measurement of critical micelle concentrations of pure and technical-grade nonionic surfactants

Abstract: The critical micelle concentrations (CMC) of nine commercial nonionic surfactants (Tween 20, 22, 40, 60, and 80; Triton X-100; Brij 35, 58, and 78) and two pure nonionics [C12(EO)5 and C12(EO)8] were determined by surface tension and dye micellization methods. Commercially available nonionic surfactants (technical grade) usually contain impurities and have a broad molecular weight distribution owing to the degree of ethoxylation. It was shown that the surface tension method (Wilhelmy plate) is very sensitive to the presence of impurities. Much lower CMC values were obtained with the surface tension method than with the dye micellization method (up to 6.5 times for Tween 22). In the presence of highly surfaceactive impurities, the air/liquid interface is already saturated at concentrations well below the true CMC, leading to a wrong interpretation of the break in the curve of surface tension (γ) vs. concentration of nonionic surfactant (log C). The actual onset of micellization happens at higher concentrations, as measured by the dye micellization method. Furthermore, it was shown that when a commercial surfactant sample (Tween 20) is subjected to foam fractionation, thereby removing species with higher surface activity, the sample yields almost the same CMC values as measured by surface tension and dye micellization methods. It was found that for monodisperse pure nonionic surfactants, both CMC determination methods yield the same results. Therefore, this study indicates that precaution should be taken when determining the CMC of commercial nonionic surfactants by the surface tension method, as it indicates the surface concentration of all surface-active species at the surface only, whereas the dye method indicates the presence of micelles in the bulk solution.

Molecular dynamics simulation of the kinetics of spontaneous micelle formation

Abstract: Using an atom based force field, molecular dynamics (MD) simulations of 54 dodecylphosphocholine (DPC) surfactant molecules in water at two different concentrations above the critical micelle concentration have been performed. Starting from a random distribution of surfactants, we observed the spontaneous aggregation of the surfactants into a single micelle. At the higher DPC concentration (0.46 M) the surfactants aggregated into a worm-like micelle within 1 ns, whereas at lower concentration (0.12 M) they aggregated on a slower time scale (∼12 ns) into a spherical micelle. The difference in the final aggregate is a direct consequence of the system achieving the lowest free energy configuration for a given quantity of surfactant within the periodic boundary conditions. The simulation at low surfactant concentration was repeated three times in order to obtain statistics on the rate of aggregation. It was found that the aggregation occurs at a (virtually) constant rate with a rate constant of k = 1 × 10-4 p...

Micellar liquid chromatography

Abstract: Micellar liquid chromatography (MLC) is a reversed-phase liquid chromatographic mode with a solution containing a surfactant above the critical micellar concentration, as mobile phase. An organic solvent is added to improve the elution strength and peak shape. The interaction of solutes with the surfactant monomers adsorbed on the stationary phase, in combination with the solubilization capability of micelles, has profound implications on retention, selectivity, and efficiency. However, the most relevant features of MLC are the elution of compounds in a wide range of polarities (neutral and ionic) using isocratic elution, and the feasibility of direct injection of physiological fluids.

Counterion effects on hexadecyltrimethylammonium surfactant adsorption and self-assembly on silica

Abstract: Combining optical reflectometry and atomic force microscopy (AFM), we have studied the effects of the surfactant counterion on the adsorption isotherms, kinetics, and layer structure for cationic hexadecyltrimethylammonium (C16TA+) surfactants on negatively charged silica surfaces. The adsorption kinetics suggest that the adsorption mechanism changes at the critical micelle concentration (cmc). A change in mechanism is also suggested by differences observed in the state of interfacial self-assembly on either side of the cmc. Above the cmc, increasing the binding affinity of the counterion (from chloride to bromide) increased the surface excess concentration by approximately 60% and changed the structure of the adsorbed surfactant layer from aggregates with circular projections to wormlike micelles. The addition of 10 mM KCl or KBr increased the surfactant surface excess concentration for both counterions. Below the cmc, the counterion has only a small effect on the structure of the adsorbed layer, and the...

Theory of Surfactant Aggregation in Water/Ethylene Glycol Mixed Solvents

Abstract: The aggregation behavior of surfactants in mixed solvents composed of water and ethylene glycol is predicted using an extension to our theory of aqueous surfactant solutions. The extension accounts for the dependence of (i) the surfactant tail transfer free energy, (ii) the aggregate core-solvent interfacial free energy, and (iii) the headgroup interaction free energy on the composition of the mixed solvent. As the proportion of ethylene glycol in the mixed solvent increases, the model predicts an increase in the critical micelle concentration (cmc), a decrease in the average aggregate size, an increase in the aggregate polydispersity, and a stronger dependence of the average aggregation number on the total surfactant concentration. The model reveals that (a) large values for cmc originate primarily from the smaller magnitude of the surfactant tail transfer free energy in the mixed solvent, (b) the aggregation numbers are small mainly because of the smaller magnitude of the hydrocarbon-mixed solvent inter...

Solution behavior of poly(styrene)-block-poly(2-vinylpyridine micelles containing gold nanoparticles

Abstract: Formation and structural transformation of inverse poly(styrene)-block-poly(2-vinylpyridine) micelles whose polyvinylpyridine core was loaded with HAuCl4 or with elementary gold nanoclusters was studied by combined static and dynamic light scattering. A transformation in the morphology from spherical particles (small Rg/Rh ratio) to large anisomeric objects (large Rg/Rh ratio) was observed by decreasing the concentration of the block copolymer below the critical micelle concentration. At this point, the polymer chains are molecularly dispersed and no longer able to prevent uncontrolled growth of the gold nanoclusters.

Thermogelling Biodegradable Polymers with Hydrophilic Backbones: PEG-g-PLGA

Abstract: The aqueous solutions of poly(ethylene glycol) grafted with poly(lactic acid-co-glycolic acid) flow freely at room temperature but form gels at higher temperature. The existence of micelles in water at low polymer concentration was confirmed by cryo-transmission electron microscopy and dye solubilization studies. The micellar diameter is about 9 nm, and the critical micelle concentration is in a range of 0.01−0.05 wt %. The critical gel concentration, above which a gel phase appears, was 16 wt %, and the sol-to-gel transition temperature was slightly affected by the concentration between 16 and 25 wt %. At sol-to-gel transition, viscosity and modulus increased abruptly, and 13C NMR showed molecular motion of hydrophilic poly(ethylene glycol) backbones decreased while that of hydrophobic poly(lactic acid-co-glycolic acid) side chains increased. The hydrogel of PEG-g-PLGA with hydrophilic backbones was transparent during degradation and remained a gel for 1 week, suggesting a promising material for short-te...

Redox Catalytic Properties of Palladium Nanoparticles: Surfactant and Electron Donor−Acceptor Effects

Abstract: Dye reduction catalyzed by palladium nanoparticles has been investigated in water and surfactant media. The initial rate of dye reduction strongly depends on the nature of the reducing agent and dye. In most cases the rate becomes surface controlled and depends on the E1/2 values of the dye and the reducing agent. However, stronger reducing agents make the reduction process a diffusion controlled one. A detailed study has been carried out to understand the effect of surfactant on the reaction rate. A variation of the reaction rates has been observed in surfactants above and below the critical micelle concentration (cmc). Below the cmc, surfactants are adsorbed onto the Pd surface and act as a binding site for both the dye and the reducing agent. Above the cmc, surfactants form micelles which provide additional binding sites. Catalytic selectivity was also achieved by proper selection of reducing agent and surfactant.

Adsorption Kinetics and Structural Arrangements of Cationic Surfactants on Silica Surfaces

Abstract: The adsorption of cetyltrimethylammonium bromide (CTAB) to the silica−aqueous interface has been studied using optical reflectometry. The effect of pH, salt, and surface preparation on the surface excess and adsorption kinetics has been studied. The adsorption kinetics have been measured and compared to the theoretical diffusion limited flux of surfactant, the quotient being the “sticking ratio”. Analysis of the sticking ratio as a function of CTAB concentration reveals that the adsorption process is cooperative above the critical micelle concentration (CMC). At the critical surface aggregation concentration (CSAC), adsorption proceeds slowly in the absence of salt and takes hours to reach an equilibrium value. At all other concentrations and even at the CSAC when salt is present, the adsorption is complete within minutes. These results indicate that, above the CMC, micelles adsorb directly to the silica surface, and this is reflected in the structure of the surface layer. At the CSAC the equilibrium surf...

What Is So Special about Aerosol-OT? 1. Aqueous Systems†

Abstract: To identify why Aerosol-OT (sodium bis(2-ethylhexyl) sulfosuccinate) is such a versatile surfactant, adsorption and aggregation in aqueous solutions have been investigated with 11 different AOT-related compounds. The first set, denoted di-CnSS with n = 4, 5, 6, 7, and 8, contain linear chains. In addition six different branched chain systems have been studied, including Aerosol-OT itself. These surfactants span extremes of branching, from 3,5,5-trimethyl-1-hexyl, through 2-ethyl-1-hexyl (AOT), to 1-ethyl-2-methyl-1-pentyl (see Figure 1), and therefore it is possible to delineate structure−performance relationships. For one model compound, di-C6SS, adsorption studies were made with two complementary techniques, drop volume tensiometry (DVT) and neutron reflection (NR). The tensiometrically derived area per molecule at the critical micelle concentration Acmc was 60 ± 3 A2, which is in excellent agreement with that obtained directly by neutrons, 59 ± 2 A2. The procedures for obtaining a sufficiently high sur...

Interaction, Stability, and Microenvironmental Properties of Mixed Micelles of Triton X100 and n-Alkyltrimethylammonium Bromides: Influence of Alkyl Chain Length

Abstract: Micellar properties of binary surfactant systems of Triton X100 (TX100) with three different n-alkyltrimethylammonium bromides (n = 12 (DTAB), 14 (TTAB), and 16 (CTAB)) were investigated by the fluorescence probe technique. The critical micelle concentration (cmc) values of the corresponding mixtures in the whole range of composition were obtained by the pyrene 1:3 ratio method. In order to estimate the interaction between the surfactants in the mixed micelles, the cmc data were treated by using the conventional regular solution approach for mixed micelles. It was found that whereas the interaction parameter values (β12) remained fairly constant for the TX100−TTAB and TX100−CTAB systems, the TX100−DTAB system was not well modeled by using that theory. However, in all cases the results showed deviation from ideal behavior. The stability of the mixed micelles was also discussed in the light of Maeda's treatment (J. Colloid Interface Sci. 1995, 172, 98), and the observed differences between the three systems...

SANS Investigation of Polyether Block Copolymer Micelle Structure in Mixed Solvents of Water and Formamide, Ethanol, or Glycerol

Abstract: The solution structure of a representative polyether block copolymer (Pluronic P105: EO37PO58EO37, where EO and PO denote ethylene oxide and propylene oxide segments, respectively) has been invest...

A Theoretical Analysis of Synergistic Effects in Mixed Surfactant Systems

Abstract: Synergistic effects in mixed binary surfactant systems have been investigated by analyzing the main contributions to the free energy of forming a mixed surfactant aggregate. We show that a nonlinear behavior of the critical micelle concentration (cmc) with respect to the surfactant composition of the aggregates is determined by a nonlinear behavior of the free energy per aggregated surfactant. It appears that synergistic effects are due mainly to entropic free energy contributions related with the surfactant headgroups. For a mixture of a monovalent ionic and a nonionic surfactant in the absence of added salt we obtain, entirely because of electrostatic reasons, a negative deviation from ideal behavior of the cmc vs the aggregate composition corresponding to an interaction parameter β ≈ −1, whereas β values on the order of −5 or even less can arise for mixtures of two ionic surfactants with the same charge number but with different hydrocarbon moieties. Moreover, we introduce a novel expression for the fr...

Aggregation Properties and Mixing Behavior of Hydrocarbon, Fluorocarbon, and Hybrid Hydrocarbon−Fluorocarbon Cationic Dimeric Surfactants

Abstract: The synthesis, phase behavior, and mixing properties of cationic dimeric (gemini) surfactants having one or two perfluoroalkyl chains is described and compared to that of analogue hydrocarbon surfactants. The combination of the dimeric character of the amphiphiles with the high hydrophobicity of the fluorocarbon chains leads to very low critical micelle concentration values and unusually low characteristic times of exchange between surfactants in the bulk solution and surfactants in the aggregate. This results in slow exchange on the NMR time scale, which allows a practical study of micellization and comicellization by NMR. With respect to the morphology of the aggregates, the fluorocarbon amphiphiles do not simply behave as more hydrophobic amphiphiles. Instead, they assemble into various structures including very stable unilamellar and multilamellar vesicles not seen in their hydrocarbon analogues. The hybrid hydrocarbon−fluorocarbon dimeric surfactant exhibits features of both families. In particular i...

Solution Properties of Micelle Networks Formed by Nonionic Surfactant Moieties Covalently Bound to a Polyelectrolyte: Salt Effects on Rheological Behavior

Abstract: Rheological properties of aqueous solutions of micelle networks formed by a copolymer of sodium 2-(acrylamido)-2-methylpropanesulfonate (AMPS) and an associative macromonomer, methacrylate substituted with HO(CH2CH2O)25C12H25 (C12E25) (20 mol % content in the copolymer), were investigated as a function of the polymer concentration (Cp), added salt concentration ([NaCl]), and shear stress. The polymer-bound C12E25 surfactant moieties form micelles via intra- and interpolymer association. These polymer-bound micelles are bridged by polymer chains and hence form a network structure. The solution viscosity of this micelle network increases markedly with increasing Cp near 10 g/L (at [NaCl] = 0.10 M) and beyond this Cp the number of micelle bridges greatly increases with increasing Cp. The extent of the micelle bridging depends on [NaCl]. When Cp is sufficiently high, where interpolymer associations are favorable, steady-state viscosity measured at a low-shear rate increases with increasing [NaCl], exhibiting ...

Study of Interaction of Poly(ethylene imine) with Sodium Dodecyl Sulfate in Aqueous Solution by Light Scattering, Conductometry, NMR, and Microcalorimetry

Abstract: Light scattering studies show that in aqueous solution poly(ethylene imine) (PEI) exists largely in the form of individual macromolecules plus a small fraction of aggregates. The aggregates make a large contribution to the scattering signal but only a very small contribution to the solution viscosity. Addition of sodium dodecyl sulfate (SDS) to the solution has a number of interesting consequences. Microcalorimetry experiments show that well below the critical micelle concentration of SDS, individual SDS molecules add to the PEI through an exothermic process. At higher SDS concentrations, there is a noncooperative adsorption, which is endothermic in nature, of SDS micelles onto the polymer chains. The surfactant−polymer complex likely contains several polymer molecules. These solutions are characterized by a higher specific conductivity than can be explained by the sum of the conductivities of all the individual ions in solution, even if the Na+ and DS- ions were free in solution and not bound to the poly...

Degradation of Alkylphenol Ethoxylate Surfactants in Water with Ultrasonic Irradiation

Abstract: During the last years, many efforts have been devoted to the elimination of alkylphenol ethoxylate surfactants from aqueous systems. In this paper, the sonochemical degradation of aqueous solutions of Triton X-100 was performed at an ultrasonic frequency of 358 kHz and an applied power of 50 W. Analysis of the reaction products by HPLC-ES−MS suggests that the hydrophobic alkyl chain is the preferential site for oxidation. Alkylphenol, or short-chain ethoxylated phenols, were not generated as byproducts. To verify this hypothesis, the sonochemical degradation of the corresponding alkylphenols (e.g., tert-octylphenol) was performed under the same conditions; in these cases, similar rate constants and products were observed. These results differ from those reported for the biodegradation of alkylphenol ethoxylates. A substantial increase in the rate constant was observed for the degradation of Triton X-100 below its critical micelle concentration. This observation indicates that micelle formation serves to e...

Effects of Sodium Salicylate on the Microstructure of an Aqueous Micellar Solution and Its Rheological Responses.

Abstract: In this article, we consider the effects of sodium salicylate on the microstructure evolution and rheological responses of an aqueous cetyltrimethylammonium bromide (CTAB) solution. The experimental runs covered CTAB solutions ranging from dilute to semidilute, which were far above its critical micelle concentration. Sodium salicylate (NaSal) was used as a structure-forming agent with the molar ratio of NaSal to CTAB ranging from 0.1 to 10.0. The experimental results showed that the rheological responses of the surfactant solution were influenced strongly by both the CTAB concentration and the molar ratio. At low molar ratios, below 0.3, the surfactant solutions behaved like a Newtonian fluid. However, as the molar ratio increased, the deviation from Newtonian behavior became pronounced. Specifically, for 0.05 M CTAB solutions with molar ratios ranging from 1.0 to 5.0, an apparent yield stress developed at low shear rates and a stress plateau was displayed at intermediate shear rates. When the shear rate exceeded a certain threshold value, the shear stress increased, again passing over the plateau value. In addition, viscoelastic response and relaxation behavior were observed. The relaxation behavior after the cessation of flow was strongly dependent on the molar ratio, which was also confirmed by rheo-optical observations. The optical anisotropy measured by rheo-optical methods was closely related to flow-induced stretching and alignment of the wormy micelles and was consistent with the rheological responses.

The Critical Micelle Concentration of Asphaltenes As Measured by Calorimetry

Abstract: Micellization of asphaltenes in solution has been investigated using a micro calorimetric titration procedure (Andersen, S. I.; Birdi, K. S. J. Colloid Interface Sci. 1991, 142, 497). The method uses the analysis of heat of dissociation and dilution of asphaltene micelles when a pure solvent (or solvent mixture) is titrated with a solution of asphaltene in the same solvent. The asphaltene concentration of the injected solution is at a level above the critical micelle concentration (CMC). In the present paper the procedure is applied in investigation of asphaltenes as well as subfractions of asphaltenes obtained by solvent extraction with toluene/heptane mixtures. These subfractions get more complex in structure as the toluene content of the extraction solvent increases and at the same time the CMC of the asphaltenes in pure toluene decreases. There is an indication that the presence of “lower” asphaltene species leads to less association in solution due to the interaction between these and the heavier asp...

Methods, devices and systems for characterizing proteins

Abstract: A method of characterizing a polypeptide, comprising providing a first capillary channel (104) having a separation buffer disposed within, wherein the separation buffer comprises a non-crosslinked polymer solution, a buffering agent, a detergent, and a lipophilic dye. The separation buffer is provided such that, at the time of detection, the detergent concentration in the buffer is not above the critical micelle concentration. The polypeptide is introduced into one end of the capillary channel. An electric field is applied across the polymer solution at different rates. The polypeptide is then detected as it passes a point (176) along the length of the capillary channel.

Salt-Induced Contraction of Polyelectrolyte Diblock Copolymer Micelles

Abstract: -PA(85)] solutions, the partial structure factors pertaining to PS-PS andPA-PA density correlations, as well as the composition structure factor were obtained with small-angleneutronscatteringandcontrastmatchinginthewater.Thecopolymersself-assemblewithanaggregationnumber ˘100 into spherical micelles made of a PS-block core, surrounded by a coronal layer formed bythe PA blocks. The addition of salt has no effect on the size of the core and the aggregation number. Atfullcoronachargeandminimalscreeningconditions,thePAchainsarealmostfullystretchedintheradialdirection away from the core. With increasing salt concentration, the micelle contracts and the coronachain statistics can be described with a two-region density-scaling model. In the inner coronal region thestatistics is unaffected by the salt, whereas for larger radial distances the scaling is similar to neutralpolymer stars. Both the micelle radii and the crossover distance between the two different density-scalingregimes comply with theory for osmotic star-branched polyelectrolytes in the salt-dominated regime. Forlow fractional corona charge, the density scaling is determined by charge annealing effects. Here, theaddition of salt does not affect the density scaling, but the micelle nevertheless contracts and eventuallyprecipitates at high ionic strength. Despite the high salt concentrations required to compete with thesalinityinthecoronallayergeneratedbythecounterionscomingfromthepolyelectrolyteblocks,therangein micelle dimension is similar to the one that can be covered by variation in pH.