Study of Fuchsin Acid Fading in Micellar Media

The rate constant of alkaline fading of fuchsin acid (FA^2?) was measured in the presence of nonionic (TX‐100), cationic (dodecltrimethylammonium bromide, DTAB), and anionic (sodium dodecyl sulfate, SDS) surfactants. FA^2? has three negatively charged substituents and one positive charge, and this makes the behavior of FA^2– different from dyes such as bromophenol blue. It was observed that the reaction rate constant decreased in the presence of TX‐100, DTAB, and SDS. Binding constants of FA^2? to TX‐100, DTAB, and SDS and the related thermodynamic parameters were calculated by the stoichiometric (classical) model. The results show that the binding of FA^2? to SDS is endothermic in both regions, and the binding of FA^2? to DTAB and TX‐100 is exothermic in one region and endothermic in another region of the used concentration range of these surfactants. Also, the binding constants of FA^2? to surfactant molecules of SDS/TX‐100 and DTAB/TX‐100 mixed micelles were obtained.     

Effect of ionic micellar medium on kinetics and mechanism of oxidation of bovine serum albumin: A pulse radiolysis study

Effect of protein–micelle interaction on bovine serum albumin (BSA) oxidation by trichloromethyl peroxyl radical (CCl₃O₂^·) in anionic sodium dodecyl sulfate (SDS) and cationic cetyltrimethyl ammonium bromide (CTAB) micellar media has been studied using nanosecond pulse radiolysis technique. Viscosity measurement and light scattering studies have suggested that SDS and CTAB micelles produce BSA–micelle aggregates of different sizes and polydispersity. Oxidation kinetics and transients have been affected both by anionic SDS and cationic CTAB micelles but in a different manner. Tryptophanyl-CCl₃O₂^· adduct radical to tyrosyl radical transformation in BSA has been observed in anionic SDS micelles but not in cationic CTAB micelles. Similar studies have also been done with tryptophan and tyrosine amino acids, which undergo oxidation in BSA. The study suggests that Coulombic and hydrophobic interactions between micelles and protein affect the structure of the protein to shield its functional amino acids, like tryptophan and tyrosine, to neutral oxidizing radical.     

Toluene solubilization induces different modes of mixed micelle growth

The effect of toluene solubilization on the size and mobility of Triton X100 (TX100) micelles and TX100/sodium dodecyl sulfate (SDS) mixed micelles was studied by turbidimetry, dynamic light scattering, and capillary electrophoresis. Micelle growth due to toluene solubilization was observed for both surfactant systems; however, two different modes of growth were seen. Mixed micelles in 0.1 M NaCl are spherical (apparent diameter d(app) = 8 nm) and remain so while taking up 3 mM toluene, with a volume increase per micelle of deltaV(m) = 50 nm3. In 0.5 M NaCl, the large d(app) of both nonionic and mixed micelles (14 and 24 nm, respectively) indicate ellipsoidal or rodlike shapes, and their large increases in d(app) upon addition of 3 mM toluene thus correspond to elongational growth, with the same deltaV(m) = 50 nm3. Further addition of toluene to TX100/SDS in 0.5 M NaCl results in a dramatic increase in micelle size followed by an unexpected bimodal size distribution. The addition of excess toluene leads to the formation of ca. 140 nm toluene droplets, stabilized mainly by monomers of the high critical micelle concentration surfactant, SDS. These microemulsions coexist with the smaller (20 nm) swollen mixed micelles.     

Role of manganese(II), micelles,and inorganic salts on the kinetics of the redox reaction of L‐sorbose and chromium(VI)

The kinetics of the reduction of chromium(VI) to chromium(III) by L ‐sorbose in HClO₄ was studied between 30 and 80°C at various concentrations of reactants and acidities in both aqueous and micellar sodium dodecyl sulfate (SDS)/TritonX‐100(TX‐100) solutions. Under pseudo‐first‐order conditions the reaction rate is fractional‐order in [L ‐sorbose] and [H⁺], and first‐order in [Cr^VI] both in the absence and in the presence of surfactant micelles. The reaction is accelerated by addition of manganese(II) and is routed through the same mechanism as shown by the kinetic studies in the absence and presence of surfactants. The rate enhancement in presence of SDS/TX‐100 micelles indicates that essentially all the reactive species are bound to micelles under the experimental conditions. The observed catalyses are explained with the modified Menger and Portnoy model. Inorganic salts (NaBr, LiBr, NH₄Br) inhibit the reaction in presence of SDS micelles, which confirms exclusion of the reactive species of chromium(VI) from the reaction site. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 543–554, 2003     

Kinetics of interaction of Histidine and Histidine Methyl Ester with Ninhydrin in micellar media

Kinetics of the interaction of histidine and histidine methyl ester with ninhydrin under varying concentrations of reactants, anionic (sodium dodecyl sulphate, SDS), cationic (cetyltrimethylammonium bromide, CTAB) and non‐ionic (Triton X‐100, TX‐100) micelles have been carried out. Rate of the reaction was found to be independent of the initial concentration of histidine (and histidine methyl ester) but was dependent on [Ninhydrin]. The SDS micelles had no effect on the rate of the reaction. In the presence of the CTAB micelles a small enhancement in the rate was observed. The rate − [CTAB] profile showed that the increase in [CTAB] increased the rate up to a maximum value and a further increase had a decreasing effect on the rate. The rate was enhanced by TX‐100 also but, unlike CTAB micelles, TX‐100 possessed a curve without peak for the rate − [TX‐100] profile. The following rate equation was obeyed by the reaction in CTAB and TX‐100 micelles: Values of k_w, k_m, and K_S were evaluated and are reported herein. ©1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 103–111, 1999     

Distribution of the methyl radical adduct of N‐benzylidene‐1‐diethoxyphosphoryl‐1‐methylethylamine N‐oxide‐type nitrones in water–sodium dodecyl sulfate micelles: an electron paramagnetic resonance spin trapping study

The behavior of the methyl radical adduct of six β‐phosphorylated nitrones in the N‐benzylidene‐1‐diethoxyphosphoryl‐1‐methylethylamine N‐oxide series in the presence of sodium dodecyl sulfate (SDS) micelles was followed by electron paramagnetic resonance spectroscopy. Except when the highly hydrophilic trap 4‐PyOPN (2) was used, all the adducts were found to partition significantly between micelles and the bulk aqueous phase. The average correlation time τ of the exchange of spin adducts between SDS micelles and water was found to be in the range 5 × 10^?8—4 × 10^?7 s, which is in the region of the life time of an SDS monomer in the micelle structure. In each case, the adduct affinity for the micelles has been quantified by evaluating its micelle–water distribution coefficient K_d. Copyright © 2002 John Wiley & Sons, Ltd.     

Salt effects on solute exchange in sodium dodecyl sulfate micelles

We describe the influence of sodium chloride on the rate of solute exchange in aqueous SDS micelles for a water-insoluble solute, a pyrene-containing triglyceride 1. The initially prepared solutions contained a small fraction of micelles containing two molecules of 1 and a large excess of empty micelles. These solutions showed a measurable excimer emission (of intensity I(E)) that was stable for days to weeks in the absence of added salt. Following additions of salt, I(E) decayed exponentially (rate constant, k(obs)) accompanied by an increase in pyrene monomer emission. Values of k(obs) increased strongly with ionic strength (k(obs) similar [Na(+)](4)). There was no contribution of the empty micelle concentration beyond its contribution to the sodium ion concentration. We conclude that the solute exchange involves spontaneous fragmentation of the SDS micelles into two submicelles, each bearing a molecule of 1, which then grow back to normal micelles through condensation of SDS monomers. We propose a model for the fragmentation process in which large amplitude surface fluctuations "pinch off" a subunit that becomes a submicelle. These fluctuations bring sulfate headgroups into close proximity. Fluctuations leading to fission become important only in the presence of sufficient counterion concentration to reduce the electrostatic repulsion between neighboring headgroups.     

Micelle and Solvent Relaxation in Aqueous Sodium Dodecylsulfate Solutions

Dielectric spectra have been measured for aqueous sodium dodecylsulfate (SDS) solutions up to 0.1 mol L^?1 at 25 °C over the frequency range 0.005≤ν GHz^?1≤89. The spectra exhibit two relaxation processes at approximately 0.03 GHz and 0.2 GHz associated with the presence of micelles in addition to the dominant solvent relaxation process at approximately 18 GHz and a small contribution at approximately 1.8 GHz due to H₂O molecules hydrating the micelles. Detailed analysis reveals that the micelles bind 20 water molecules per SDS unit, but not as strongly as trimethylalkylammonium halide surfactants do. The relaxation times and amplitudes of both micelle relaxation processes can be simultaneously analysed with the theory of Grosse, yielding the effective volume of a SDS unit in the micelle and the lateral diffusion coefficient of the bound counterions. The findings of this investigation fully corroborate recent molecular dynamics simulations on structure and dynamics of SDS micelles.     

Novel Enzymatic Potentiometric Approaches for Surfactant Analysis

The present study described a novel application of simple potentiometric enzymatic method for analysis of surfactants based on their inhibitory effect on acetylcholinesterase enzyme (AChE). The enzymatic activity was measured through monitoring hydrolysis of acetylcholine (ACh) with a disposable acetylcholine potentiometric sensor. Comprehensive investigations were carried out including the effect of incubation time, cholinesterase enzyme and the working calibration ranges. Based on inhibition of AChE, different cationic, anionic and nonionic surfactants were determined in the concentration range from 0 to 40 μg mL⁻¹ with detection limits reaching 0.07 μg mL⁻¹ depending on the nature of surfactants. The degree of AChE inhibition caused by different tested surfactants were as follows: cetylpyridinium chloride (CPC) > benzyldimethylhexadecyl ammonium chloride (BDHAC) > Hyamine (Hy)>cetyltrimethylammonium bromide (CTAB) > Triton X‐100 (TX‐100) > sodium dodecyl sulphate (SDS). The proposed method was applied for determination of surfactants in pharmaceutical formulation, detergents products and environmental samples with acceptable sensitivity and reproducibility.     

Kinetics of micellization: its significance to technological processes

The association of many classes of surface active molecules into micellar aggregates is a well-known phenomenon. Micelles are often drawn as static structures of spherical aggregates of oriented molecules. However, micelles are in dynamic equilibrium with surfactant monomers in the bulk solution constantly being exchanged with the surfactant molecules in the micelles. Additionally, the micelles themselves are continuously disintegrating and reforming. The first process is a fast relaxation process typically referred to as τ₁. The latter is a slow relaxation process with relaxation time τ₂. Thus, τ₂ represents the entire process of the formation or disintegration of a micelle. The slow relaxation time is directly correlated with the average lifetime of a micelle, and hence the molecular packing in the micelle, which in turn relates to the stability of a micelle. It was shown earlier by Shah and coworkers that the stability of sodium dodecyl sulfate (SDS) micelles plays an important role in various technological processes involving an increase in interfacial area, such as foaming, wetting, emulsification, solubilization and detergency. The slow relaxation time of SDS micelles, as measured by pressure-jump and temperature-jump techniques was in the range of 10⁻⁴–10¹ s depending on the surfactant concentration. A maximum relaxation time and thus a maximum micellar stability was found at 200 mM SDS, corresponding to the least foaming, largest bubble size, longest wetting time of textile, largest emulsion droplet size and the most rapid solubilization of oil. These results are explained in terms of the flux of surfactant monomers from the bulk to the interface, which determines the dynamic surface tension. The more stable micelles lead to less monomer flux and hence to a higher dynamic surface tension. As the SDS concentration increases, the micelles become more rigid and stable as a result of the decrease in intermicellar distance. The smaller the intermicellar distance, the larger the Coulombic repulsive forces between the micelles leading to enhanced stability of micelles (presumably by increased counterion binding to the micelles). The Center for Surface Science & Engineering at the University of Florida has developed methods using stopped-flow and pressure-jump with optical detection to determine the slow relaxation time of micelles of nonionic surfactants. The results show relaxation times τ₂ in the range of seconds for Triton X-100 to minutes for polyoxyethylene alkyl ethers. The slow relaxation times are much longer for nonionic surfactants than for ionic surfactants, because of the absence of ionic repulsion between the head groups. The observed relaxation time τ₂ was related to dynamic surface tension and foaming experiments. A slow break-up of micelles, (i.e. a long relaxation time τ₂) corresponds to a high dynamic surface tension and low foamability, whereas a fast break-up of micelles, leads to a lower dynamic surface tension and higher foamability. In conclusion, micellar stability and thus the micellar break-up time is a key factor in controlling technological processes involving a rapid increase in interfacial area, such as foaming, wetting, emulsification and oil solubilization. First, the available monomers adsorb onto the freshly created interface. Then, additional monomers must be provided by the break-up of micelles. Especially when the free monomer concentration is low, as indicated by a low CMC, the micellar break-up time is a rate limiting step in the supply of monomers, which is the case for many nonionic surfactant solutions. Therefore, relaxation time data of surfactant solutions enables us to predict the performance of a given surfactant solution. Moreover, the results suggest that one can design appropriate micelles with specific stability or τ₂ by controlling the surfactant structure, concentration and physico-chemical conditions, as well as by mixing anionic/cationic or ionic/nonionic surfactants for a desired technological application.     

PHOTOLUMINESCENT PROPERTIES OF OCTADECYLRHODAMINE B IN MICELLES OF LOW-MOLECULAR-WEIGHT DETERGENTS AND WATER-SOLUBLE TRIBLOCK COPOLYMERS

The fluorescence intensity, lifetime and degree of polarization of octadecylrhodamine B (ORB) have been measured in order to examine the usefulness of this molecule as a probe of micelle properties for low-molecular-weight detergents and water-soluble triblock copolymers. The surfactants examined are hexadecyltrimethylammonium chloride (HTAC), Triton X-100 (TX-100), sodium dode-cylsulfate (SDS), sodium tetradecylsulfate (STS), and Pluronic L64 (ethylene oxide [EO]₁₃ propylene oxide₃₀ EO₁₃, L64). The fluorescence intensity and degree of polarization of ORB show drastic increases at the critical micelle concentrations (CMC) of HTAC, TX-100 and L64, indicating that ORB is cooperatively incorporated into the micelles upon micellization. This feature demonstrates the validity of ORB as a probe for detecting micelle formation of these surfactants. However, in the case of SDS and STS, the fluorescence intensity starts to rise at concentrations far below the CMC, and the degree of polarization does not show significant changes at the CMC. The details of the interactions between ORB and the anionic surfactants have been unclear. These facts imply that some caution is needed for the applications of ORB to the systems containing anionic surfactants. The local viscosity of L64 micelles has been determined by polarization and lifetime measurements. The structure of the block copolymer micelles and the locations of the probe in the micelles are discussed in terms of the viscosity data.     

十二烷基硫酸钠水溶液的动态表面吸附性质

利用MPTC型气泡压力张仪研究了十二烷基硫酸钠(SDS)溶液在不同NaCl 浓度下的动态表面吸附性质, 分析了离子型表面活性剂在表面吸附层和胶束中形成双电层结构产生表面电荷对动态表面扩散过程和胶束性质的影响. 结果表明, SDS在表面吸附过程中, 表面电荷的存在会产生5.5 kJ·mol^-1的吸附势垒(E_a), 显著降低十二烷基硫酸根离子(DS^-)的有效扩散系数(D_eff). 十二烷基硫酸根离子的有效扩散系数与自扩散系数(D)的比值(D_eff/D)仅为0.013, 这表明SDS与非离子型表面活性剂不同, 在吸附初期为混合动力控制吸附机制. 加入NaCl可以降低吸附势垒. 当加入不小于80 mmol·L^-1 NaCl后, E_a小于0.3 kJ·mol^-1, D_eff/D在0.8-1.2之间, 表现出与非离子型表面活性剂相同的扩散控制吸附机制. 同时, 通过分析SDS胶束溶液的动态表面张力获得了表征胶束解体速度的常数(k₂). 发现随着NaCl 浓度的增大, k₂减小, 表明SDS胶束表面电荷的存在会增加十二烷基硫酸根离子间的排斥力, 促进胶束解体.     

Use of Cr(phen)33+ as luminescence probe to study the binding constants of phenols to micelles of SDS

Steady-state and time resolved luminescence quenching measurements of (²T/²E)Cr(phen)₃³⁺ were used to investigate the association of phenols to sodium dodecyl sulfate (SDS) micelles. Steady-state results show the quenching process occurs in the micellar pseudo phase. Scatchard plots indicate that the process is a partition between aqueous and micelles. The k₊ and k₋ rate constant have been evaluated from time resolved data and the binding constants were obtained. The trend found in the K's were 4-H-Ph < 2,6-diMe-Ph < 4-Br-Ph. We concluded that it is possible to use *Cr(phen)₃³⁺ as a luminescent probe to determine association parameters for quenchers to micelles of SDS.     

The size of sodium dodecyl sulfate micelles in the presence ofn-alcohols as determined by fluorescence quenching measurements

The steady-state fluorescence quenching technique was used to investigate the effect of the presence of a series of alcohol homologues of mid-sized straight chain on the size of mixed micelles of sodium dodecyl sulfate (SDS). We used pyrene at concentration of ca. 10^–6M, where only its monomer exhibits any fluorescence, as fluorescent probe, and cetylpiridinium chloride at concentrations in the range (1–9)×10^–5 M as quencher. This technique allows one to determine the micellar aggregation number. The number of alcohol molecules per micelle was calculated from reported values for the micelle-water partition coefficient. On the assumption of spherical micelles, their hydrophobic radii was then calculated. The hypothesis that micelle size is determined by the available surface area per charged headgroup is discussed in the light of the results obtained.     

Interaction of slightly cross-linked gels of poly(diallyldimethylammonium bromide) with sodium dodecyl sulfate. Diffusion of surfactant ions in gel

The dynamics of the changing microenvironment of the fluorescent probe pyrene in slightly cross-linked networks of poly(diallyldimethylmmonium bromide) during diffusion of sodium dodecyl sulfate (SDS) in the gel phase has been investigated by fluorescence spectroscopy. Values of the spectral ratio I₃/I₁ for pyrene monomer included in SDS micelles in the swollen networks fall between the corresponding values for pyrene in water and for pyrene dissolved in SDS micelles in aqueous solution. In the narrow interval of the surfactant concentrations in the gel phase, the formation of pyrene excimers is observed. The values of the critical micelle concentration in the gel phase (ca. 5 × 10^?4 to 8 × 10^?4 mol/L) are tenfold lower than in aqueous solutions of the surfactant. The effective micellar diffusion coeffecient D in the gel phase increases with increasing swelling of the network. © 1993 John Wiley & Sons, Inc.     

Micellar effects on absorption spectra and protolytic equilibria of phenyl-pyridyl-ketones

The absorption spectra of the three isomeric phenyl-pyridyl-ketones were studied in anionic (SLS), cationic (CTAB) and non-ionic (Triton X-100) micellar solutions. The spectral changes of the n, π* transition in micelles compared to net water provided information on the location of the ketone molecules in the micelle and the partitioning of ketones between micelles and water. The apparent pK^a values of the three pyridyl ketones were determined in anionic, cationic and non-ionic micelles and compared to those in aqueous solution. The results indicate that the ketones reside within the interfacial head group region of the micelle. This location affects the protolytic equilibria: a decrease in nitrogen basicity in Triton X-100 (ΔK^a∼−0.6) and CTAB (ΔpK^a−0.5) and an increase in SLS (ΔpK^a+1.5) are attributed to a reduced polarity at the micelle surface and to an electrostatic surface potential.     

Molecular dynamics study of solubilization of cyclohexane,benzene, and phenol into mixed micelles composed of sodium dodecyl sulfate and octaethylene glycol monododecyl ether

Molecular dynamics calculations of a mixed micelle composed of sodium dodecyl sulfate (SDS) and octaethylene glycol monododecyl ether (C₁₂E₈) were performed for six compositions (SDS/C₁₂E₈ = 100/0, 80/20, 60/40, 40/60, 20/80, and 0/100) to investigate the composition dependence of the mixed micelle structure and solubilization of cyclohexane, benzene, and phenol molecules by the micelle. The radial density distribution of the hydrophilic polyoxyethylene (POE) group of C₁₂E₈ as a function of distance from the micelle center is very sharp for micelles with high SDS content because the POE group captures a Na⁺ ion in solution and wraps around it to form a compact crown-ether-like complex. The hydrophobic dodecyl groups of SDS and C₁₂E₈ were separately distributed in the mixed micelle core. ΔG(r) evaluated for each solute showed that despite the structural changes of the micelle the binding strength of the solute molecules to the micelle did not change significantly. © 2019 Wiley Periodicals, Inc.     

Effect of sphere to rod transition on the probe microenvironment in sodium dodecyl sulphate micelles: A time resolved fluorescence anisotropy study

The effect of different hydrotropic salts on the microenvironment at the anionic head group region of sodium dodecyl sulphate (SDS) micelle has been studied through time-resolved fluorescence anisotropy measurements of a solubilized probe, coumarin-153 (C153). The organic cations of the hydrotropic salts used in this study, i.e. aniline hydrochloride (AHC) and o-, m- and p-toluidine hydrochlorides (OTHC, MTHC and PTHC, respectively), differ in their charge to size ratio and hydrophobicity. Present study utilizes the sensitivity of the fluorescence technique to understand the changes in the micropolarity and microviscosity experienced by the fluorescent probe, C153, solubilized in the micellar Stern layer, on addition of different hydrotropic salts. Significant changes are observed in the rotational relaxation dynamics of the probe with increasing concentration of the salts. The changes in the rotational relaxation dynamics clearly reflect the sphere to rod transition in the SDS micelles and correspond nicely with the reported results from dynamic light scattering measurements. The growth behavior of SDS micelles is found to be sensitive to the hydrophobicity of the organic cations. The charge to size ratio of the organic cations also indicated to play a role in inducing the sphere to rod transition in the SDS micelles. The interesting observation made from this study is that the sphere to rod transition of SDS micelles is largely facilitated by the presence of the hydrotropic salts and such a transition is successfully indicated by the simple fluorescence anisotropy measurements of a probe in the micelle carried out in the presence of different hydrotropic salts.     

Interaction of Sulfonated Ruthenium(II) Polypyridine Complexes with Surfactants Probed by Luminescence Spectroscopy

Novel anionic [RuL₂L′]²⁻ complexes, where L stands for (1,10‐phenanthroline‐4,7‐diyl)bis(benzenesulfonate) (pbbs; 3a ) or (2,2′‐bipyridine)‐4,4′‐disulfonate (bpds; 3b ), and L′ is N‐(1,10‐phenanthrolin‐5‐yl)tetradecanamide (pta; 2a ) or N‐(1,10‐phenanthrolin‐5‐yl)acetamide (paa; 2b ), were synthesized, and their interaction with the prototypical surfactants sodium dodecylsulfate (SDS), cetyl trimethyl ammonium bromide (CTAB), and Triton X‐100 (TX‐100) was investigated by electronic absorption, luminescence spectroscopy, emission‐lifetime determinations, and O₂‐quenching measurements. [Ru(pbbs)₂(pta)]²⁻ ( 5a ) displayed cooperative self‐aggregation in aqueous medium at concentrations above 1.3 μM ; the observed association was enhanced in the presence of either β‐cyclodextrin or NaCl. This amphiphilic Ru^II compound showed the strongest interaction with all the detergents tested: nucleation of surfactant molecules around the luminescent probe was observed below their respective critical micellar concentrations. As much as a 12‐fold increase of the emission intensity and a 3‐fold rise in the lifetime were measured for 5a bound to TX‐100 micelles; the other complexes showed smaller variations. The O₂‐quenching rate constants decreased up to 1/8 of their original value in H₂O (e.g., for [Ru(bpds)₂(pta)]²⁻ ( 6a ) bound to CTAB micelles). Luminescence‐lifetime experiments in H₂O/D₂O allowed the determination of the metal‐complex fraction exposed to solvent after binding to surfactant micelles. For instance, such exposure was as low as 25% for pta complexes⋅CTAB aggregates. The different behaviors observed were rationalized in terms of the Ru^II complex structure, the electrostatic/hydrophobic interactions, and the probe environment.