Flow-injection chemiluminescence method for determination of critical micelle concentration of surfactants

In this study, chemiluminescence (CL) behaviour of Luminol-H₂O₂ in the presence of the different concentrations of four surfactants, cetyltrimethylammonium bromide (CTAB), cetylpyridinium bromide (CPB), sodium dodecyl sulphate (SDS) and polyoxyethylene dodecyl ether (Brij-35), was investigated. A novel method for the direct determination of critical micelle concentration (CMC) of the surfactants using flow-injection CL is described. Under the optimum conditions, the luminescence intensity of the Luminol-H₂O₂ system increased gradually with increasing concentration of the surfactants before the CMC, but rapidly reached to the emission maximum at the CMC, followed by a decrease after the CMC. The concentrations of the surfactants corresponding to the luminescence maximum are in agreement with the literature CMC values. The main factors affecting the determination of CMC are discussed. The mechanistic studies show that the luminescence peaks observed in the experiment were mainly because of the protective effect of the micelle against the transition of the excited species and the retarding effect of the micelle structures on the CL reaction rate.     

Determination of the critical premicelle concentration, first critical micelle concentration and second critical micelle concentration of surfactants by resonance Rayleigh scattering method without any probe

The purpose of this work is to determine the values of critical premicelle concentration (CPMC), first critical micelle concentration (FCMC) and second critical micelle concentration (SCMC) of surfactants using a common spectrofluorophotometer by recording resonance Rayleigh scattering (RRS) signal without any probe. The plot of the RRS intensities at the maximum scattering wavelength (I(RRS)(max)) versus surfactant concentrations (c) was constructed to obtain the I(RRS)(max)-c curve. From the inflexions in I(RRS)(max)-c curve, the CPMC, FCMC and SCMC values of a surfactant can be obtained sensitively. The FCMC of some anionic, cationic and nonionic surfactants such as sodium dodecyl sulfate (SDS), sodium dodecyl benzene sulfonate (SDBS), cetyltrimethylammonium bromide (CTAB), cetylpyridinium chloride (CPC), Tween-20, and Tween-80 were determined by RRS method and the values are in good agreement with those obtained from conductivity and surface tension measurements and literature values. The CPMC and SCMC of SDS and CTAB were also determined by RRS method respectively and the values conform to literature values too. Furthermore, RRS method can also be used to determine the FCMC of an amphiphilic macromolecule-hemoglobin, whose structure resembles a surfactant. From the experimental results, it is concluded that RRS method can be applied to the simultaneous determination of the CPMC, FCMC and SCMC values in a sensitive, accurate and no probe way.     

Effect of compressed CO2 on the critical micelle concentration and aggregation number of AOT reverse micelles in isooctane

The effect of compressed CO2 on the critical micelle concentration (cmc) and aggregation number of sodium bis-2-ethylhexylsulfosuccinate (AOT) reverse micelles in isooctane solution was studied by UV/Vis and fluorescence spectroscopy methods in the temperature range of 303.2-318.2 K and at different pressures or mole fractions of CO2 (X(CO2)). The capacity of the reverse micelles to solubilize water was also determined by direct observation. The standard Gibbs free energy (DeltaGo(m)), standard enthalpy (DeltaHo(m)), and standard entropy (DeltaSo(m)) for the formation of the reverse micelles were calculated by using the cmc data determined. It was discovered that the cmc versus X(CO2) curve and the DeltaGo(m) versus X(CO2) curve for a fixed temperature have a minimum, and the aggregation number and water-solubilization capacity of the reverse micelles reach a maximum at the X(CO2) value corresponding to that minimum. These results indicate that CO2 at a suitable concentration favors the formation of and can stabilize AOT reverse micelles. A detailed thermodynamic study showed that the driving force for the formation of the reverse micelles is entropy.     

表面活性剂在溶液中聚集形态的动力学模拟

用耗散颗粒动力学模拟方法(DPD)展示了表面活性剂分子在溶液中的聚集形态，用扩散程度表征了表面活性剂溶液中的自组装情况。结果发现：这种分子动力学模拟方法能够直观地得到表面活性剂的聚集形态；随着表面活性剂的浓度增加，聚集形态依次从球状胶束、棒状或虫状胶束，六角状相，向层状相变化。     

Detection of the critical micelle concentration of cationic and anionic surfactants based on aggregation-induced emission property of hexaphenylsilole derivatives

We report a fluorescence “turn-on” method to detect the critical micelle concentration (CMC) of surfactants. This method works well for both cationic and anionic surfactants. It employs an unprecedented mechanism (aggregation-induced emission, or AIE) to determine the CMC values, and the results are consistent with the data obtained by the classical techniques. In addition, this method renders the convenient detection of the CMC values. Any large and professional instruments are unnecessary, instead, a portable UV lamp and an ultrasonic generator are enough to carry out the detection in an ordinary laboratory. Considering that micelles are interesting entities and have found applications in many important fields such as emulsion polymerization, template of nanosized materials synthesis, controllable drug delivery and macromolecular self-assembling. Our experimental results may offer a facile, sensitive and promising method to detect the formation of micelles constructed by the new amphiphilic molecules and macromolecules. Supported by the National Natural Science Foundation of China (Grant Nos. 50573065 & 50873086) and the Natural Science Foundation of Zhejiang Province (Grant No. Z406018)     

The aggregation of nonionic surfactants in the presence of poly(methacrylic acid)

The solution properties of homogeneous hexaethylene and octaethylene glycol mono(n-dodecyl) ethers, C₁₂E₆ and C₁₂E₈, respectively, and octaethylene glycol mono(n-decyl) ether, C₁₀E₈, with poly(methacrylic acid) (PMA) were investigated by dye solubilization, surface tension, fluorescence, viscosity, and pH measurements. The data were discussed regarding non-cooperative and cooperative binding of surfactant to polymer. Whereas in the interaction with poly(acrylic acid) (PAA), the critical aggregation concentrations (cac or T ₁) of these surfactants were lower than the respective critical micelle concentration (cmc), in that with the more hydrophobic PMA, T ₁’s of C₁₂E₆ and C₁₂E₈ were higher than the respective cmc, but that of C₁₀E₈ was lower than its cmc. These may be ascribed to the hydrophobic microdomains (HMD) of the PMA coil in water, probably in its inside. It is considered that some surfactants are bound first to the HMD non-cooperatively and then they are abruptly bound cooperatively at T ₁. This raises T ₁ higher than cmc when the cmc is low, and the amount bound by the HMD is relatively large and vice versa. T ₁ of C₁₂E₆ or C₁₂E₈ is the former case, and that of C₁₀E₈ is the latter. Thus, different from PAA, T ₁ for PMA + nonionic surfactant system consists of the amount of non-cooperative binding and the cac of the cooperative binding in equilibrium. Therefore, this T ₁ has a different meaning from that for PAA and should be called apparent T ₁. As the binding to the HMD is dependent on PMA concentration and cac is not, which is like in the PAA system, separation of apparent T ₁ from the HMD binding was achieved by extrapolating T ₁’s to zero PMA concentration (denoted intrinsic T ₁). This value for C₁₂E₈ was found to be lower than the respective cmc and also lower than the respective T ₁ for PAA. With increase in surfactant concentration, the pH of PMA solution rose and demonstrated a peak. This pH rise and fall may be induced by loosening of the HMD coil due to binding increase and by rearrangement of PMA + surfactant complex in high surfactant concentrations region. By raising the initial pH, the HMD were loosened; consequently, T ₁ rose a little, and at higher pH, no surfactant binding took place.     

The aggregation of aqueous dodecylphosphonic acid in dodecyltrimethylammonium hydroxide mixtures

The aggregation of aqueous dodecylphosphonic acid (DPA) and dodecyltrimethylammonium hydroxide (DTAOH) mixtures was studied by several methods. The behavior of DPA-rich mixtures is close to that of pure DPA. This is probably due to the preservation of the hydrogen-bonded structure of the micellar headgroup layer. The behavior is almost ideal. Betweeny _DPA =0.5 and 0.33 (y _DPA being the mole fraction of DPA in the surfactant mixture), the hydrogen-bonded structure of the micellar headgroup layer is destroyed. A sort of micellar azeotrope is formed, and the maximum of non-ideal interaction between the two surfactants is attained aty _DPA =0.4. Fory _DPA <0.33 the system behaves as a common mixture of a cationic surfactant and a non-ionic one (DPA.2LTA). There is a phenomenon of counterion condensation on aggregates at concentrations over the CMC.     

The effect of NaOH on dodecyltrimethylammonium hydroxide aggregation in aqueous solutions

The aggregation of dodecyltrimethylammonium hydroxide (DTAOH) in aqueous NaOH solutions was studied as a function of NaOH concentration. As in NaOH-free DTAOH aqueous solutions, the surfactant underwent a stepwise aggregation mechanism. Changes in the structure of aggregates produced an increase of the concentration at which premicellar aggregates could solubilize hydrophobic dyes and also in the concentration at which hydroxide inons join the aggregates.     

A proton NMR study on aggregation of cationic surfactants in water: effects of the structure of the headgroup

 ¹H NMR chemical shifts of solutions of the following cationic surfactants in D₂O were determined as a function of their concentrations: cetyltrimethylammonium chloride, CTACl, a 1 : 1 molar mixture of CTACl and toluene, cetylpyridinium chloride, CPyCl, cetyldimethylphenylam-monium chloride, CDPhACl, cetyldimethylbenzylammonium chloride, CDBzACl, cetyldimethyl-2-phenylethylammonium chloride, CDPhEtACl, and cetyldimethyl-3-phenylpropylammonium chloride, CDPhPrACl. Plots of observed chemical shifts versus [surfactant] are sigmoidal, and were fitted to a model based on the mass-action law. Satisfactory fitting was obtained for the discrete protons of all surfactants. From these fits, we calculated the equilibrium constant for micelle formation, K, the critical micelle concentration, CMC and the chemical shifts of the monomer, δ_mon and the micelle δ_mic. ¹H NMR-based CMC values are in excellent agreement with those which we determined by surface tension measurements of surfactant solutions in H₂O, allowing for the difference in structure between D₂O and H₂O. Values of K increase as a function of increasing the size of the hydrophilic group, but the free energy of transfer per CH₂ group of the phenylalkyl moiety from bulk water to the micellar interface is approximately constant, 1.9±0.1 kJ mol^-1. Values of (δ_mic–δ_mon) for the surfactant groups at the interface, e.g., CH₃–(CH₂)₁₅–N+(CH₃)₂ and within the micellar core, e.g., CH₃–(CH₂)₁₅–N⁺ were used to probe the (average) conformation of the phenyl group in the interfacial region. The picture that emerges is that the aromatic ring is perpendicular to the interface in CDPhACl and is more or less parallel to it in CDBzACl, CDPhEtACl, and CDPhPrACl. Received: 23 February 1996 Accepted: 29 August 1996     

用氯离子电极同时测定阳离子表面活性剂的CMC及反离子缔合度

临界胶束浓度(CMC)是研究表面活性剂的一个重要参数,胶束的反离子缔合度(K)是重要的特性参数。本文以阳离子表面活性剂十烷基三甲基氯化铵(DTMAC)、十二烷基三甲基氯化铵(DTAC)、十四烷基三甲基氯化铵(TTAC)、十六烷基三甲基氯化铵(CTAC)和十八烷基三甲基氯化铵(OTAC)水溶液体系为研究对象,用氯离子选择电极分别测定了其水溶液体系胶束的CMC和K。     

系列离子液体型Gemini咪唑表面活性剂在水溶液中的分子动力学模拟

利用分子动力学模拟方法研究了系列离子液体型Gemini咪唑表面活性剂在水溶液中的表面活性和胶束化能力. 模拟结果表明,压力张量法得到的表面张力模拟值偏小,需乘以修正系数矫正;分子动力学模拟得到的临界胶束浓度变化规律与实验相符,可以定性比较不同结构的离子液体型Gemini咪唑分子间的胶束化能力;温度的升高会加剧分子的热运动,不利于离子液体型Gemini咪唑表面活性剂在水溶液中形成胶束;此外,研究还发现联接基不同的离子液体型Gemini咪唑表面活性剂可能遵循不同的胶束化机理.S≤6时,单个分子自组装成胶球后发生聚合形成大胶团.随着咪唑上长烷烃链碳数的增加,[C_n-4-C_nim]胶束化能力提高;而随着联接链长度增加,[C₁₀-S-C₁₀im]胶束化能力降低;当S ＞6时,分子联接基弯曲并伸入其它分子烷烃链内部以减小头基分离力,从而形成稳定的胶束或胶团.随着联接基团亚甲基数的增加,头基斥力减小,附加疏水相互作用增强,[C₁₀-S-C₁₀im]胶束化能力提高.     

Estimating the temperature dependence of peptide folding entropies and free enthalpies from total energies in molecular dynamics simulations

The temperature dependence of thermodynamic quantities, such as heat capacity, entropy and free enthalpy, may be obtained by using well-known equations that relate these quantities to the enthalpy of the molecular system of interest at a range of temperatures. In turn, the enthalpy of a molecular system can be estimated from molecular dynamics simulations of an appropriate model. To demonstrate this, we have investigated the temperature dependence of the enthalpy, heat capacity, entropy and free enthalpy of a system that consists of a beta-heptapeptide in methanol and have used the statistical mechanics relationships to describe the thermodynamics of the folding/unfolding equilibrium of the peptide. The results illustrate the power of current molecular simulation force fields and techniques in establishing the link between thermodynamic quantities and conformational distributions.     

Absolute free energies of binding of peptide analogs to the HIV-1 protease from molecular dynamics simulations

The constants of binding of five peptide analogs to the active site of the HIV-1 aspartic-protease are calculated based on a novel sampling scheme that is efficient and does not introduce any approximations in addition to the energy function used to describe the system. The results agree with experiments. The squared correlation coefficient of the calculated vs. the measured values is 0.79. The sampling scheme consists of a series of molecular dynamics integrations with biases. The biases are selected based on an estimate of the probability density function of the system in a way to explore the conformational space and to reduce the statistical error in the calculated binding constants. The molecular dynamics integrations are done with a vacuum potential using a short cutoff scheme. To estimate the probability density of the simulated system, the results of the molecular dynamics integrations are combined using an extension of the weighted histogram analysis method (C. Bartels, Chem. Phys. Letters 331 (2000) 446-454). The probability density of the solvated ligand-protein system is obtained by applying a correction for the use of the short cutoffs in the simulations and by taking into account solvation with an electrostatic term and a hydrophobic term. The electrostatic part of the solvation is determined by finite difference Poisson-Boltzmann calculations; the hydrophobic part of the solvation is set proportional to the solvent accessible surface area. Setting the hydrophobic surface tension parameter equal to 8 mol(-1) K(-1) A(-2), absolute binding constants are in the muM to nM range. This is in agreement with experiments. The standard errors determined from eight repeated binding constant determinations are a factor of 14 to 411. A single determination of a binding constant is done with 499700 steps of molecular dynamics integration and 4500 finite difference Poisson-Boltzmann calculations. The simulations can be analyzed with respect to conformational changes of the active site of the HIV-1 protease or the ligands upon binding and provide information that complements experiments and can be used in the drug development process.     

The influence of shear rate, temperature and chain conformation on the critical concentration c

 When viscometry is used, a crossover phenomenon is observed separating the dilute solutions into extremely dilute solutions and dilute solutions. The critical concentration c ^**, determined from this crossover phenomenon, strongly depends on the shear rate in the solution. At very high values of shear rate the critical concentration c ^** becomes very low and depends only on the contour length of the elongated chains of different polymers. An increase of the temperature induces an increase of c ^** because the relaxation time of the chains decreases. If a polymer adopts a rodlike conformation (in a given solvent at a given temperature) the excluded volume of its chains increases and its critical concentration c ^** decreases. Received: 14 October 1996 Accepted: 3 March 1997     

The mechanism of formamide hydrolysis in water from ab initio calculations and simulations

The neutral hydrolysis of formamide in water is a suitable reference to quantify the efficiency of proteolytic enzymes. However, experimental data for this reaction has only very recently been obtained and the kinetic constant determined experimentally is significantly higher than that predicted by previous theoretical estimations. In this work, we have investigated in detail the possible mechanisms of this reaction. Several solvent models have been considered that represent a considerable improvement on those used in previous studies. Density functional and ab initio calculations have been carried out on a system which explicitly includes the first solvation shell of the formamide molecule. Its interaction with the bulk has been treated with the aid of a dielectric continuum model. Molecular dynamics simulations at the combined density functional/molecular mechanics level have been carried out in parallel to better understand the structure of the reaction intermediates in aqueous solution. Overall, the most favored mechanism predicted by our study involves two reaction steps. In the first step, the carbonyl group of the formamide molecule is hydrated to form a diol intermediate. The corresponding transition structure involves two water molecules. From this intermediate, a water-assisted proton transfer occurs from one of the hydroxy groups to the amino group. This reaction step may lead either to the formation of a new reaction intermediate with a marked zwitterionic character or to dissociation of the system into ammonia and formic acid. The zwitterionic intermediate dissociates quite easily but its lifetime is not negligible and it could play a role in the hydrolysis of substituted amides or peptides. The predicted pseudo-first-order kinetic constant for the rate-limiting step (the first step) of the hydrolysis reaction at 25 degrees C (3.9x10(-10) s(-1)) is in excellent agreement with experimental data (1.1x10(-10) s(-1)).     

The Concentration of Hydrogen Peroxide Generated during Aggregation of α-Synuclein in vitro Is Lower than 5 nmol／L

Using a fluorometric method with a detection limit of 5 nmol/L, here it is reported that albeit positive results were got from bovine serum albumin (BSA) and chicken ovalbumin (OVA) as published in literature, no detectable amount of hydrogen peroxide (H2O2) was generated during α-synuclein (α-Syn) aggregation in vitro even in the presence of transition metal ions Cu(Ⅱ) or Fe(Ⅲ). The results suggest that the concentration of H2O2 generated during aggregation of α-Syn in vitro be lower than 5 nmol/L beyond the detection limit of the adopted method and it is far too poor to be responsible for the cytotoxicity of α-Syn aggregates, thus allowing people to extensively elucidate the mechanism underlying neurotoxicifies of the aggregates formed by some amyloidogenic proteins.     

Dynamics of liposomes in the fluid phase

We review the currently available material on the morphology and dynamics of phospholipids assembled into liposomes. Key information obtained from neutron scattering, nuclear magnetic resonance (NMR), and other techniques plays a crucial role in understanding the vital role of lipids in sustaining life in living organisms. We concentrate on the dynamics in the biologically important fluid phase in the time range from picoseconds to seconds, which includes a discussion of the center of mass diffusion of liposomes, membrane fluctuations; and lateral, rotational, and flip-flop motions of the lipids. We emphasize on the sensitivity of the dynamics on interactions with a variety of biologically relevant molecules such as cholesterol. By a comparison of data from literature, we witness a good agreement of the results from different techniques and studies.