Cholesterol Monohydrate Dissolution in Bile Salt‐Lecithin Solutions

A modified dissolution rate equation was used to quantitatively investigate the contribution of simple bile salt (BS) micelles and mixed BS‐lecithin (L) micelles to a cholesterol monohydrate (ChM) dissolution. Using a least‐squares technique to assess the relationship between the ChM dissolution rate and BS concentration at a constant L concentration, good curve‐fittings were obtained when the BS monomer concentration was set to equal the critical micellar concentration (CMC). For taurochenodeoxycholate (TCDC), a dihydroxy BS, the resulting values of parameters show that the simple TCDC micelle rate constant (ks) increases, but the mixed TCDC‐L micelle rate constant (kM ) decreases with increasing L concentrations. As for taurocholate (TC), a tri‐hydroxy BS, a ChM dissolution study was conducted over the initial 2 hour period in different TC‐L solutions. A similar curve‐fitting analysis revealed that the simple TC micelle ks is independent of L concentration and is much higher than the kM of mixed TC‐L micelles. Moreover, the outcome of the analysis supports previously reported equilibrium dialysis study results concerning the BS to L ratio of mixed BS‐L micelles. According to the collision theory, the resulting ks and kM values are interpreted.     

稳态荧光探针研究Guerbet醇聚氧乙烯醚羧酸钠的聚集行为

以芘为荧光探针、二苯甲酮为猝灭剂,用稳态荧光探针法测定了合成的4种具有支链结构的Guerbet醇聚氧乙烯醚羧酸钠的临界胶束浓度和胶团的聚集数.结果表明,用稳态荧光探针法得到的临界胶束浓度(CMC)数值与表面张力法相差不大.结合临界胶束浓度、胶束微环境的极性和胶束聚集数的变化规律,推测了这类表面活性剂形成的胶束聚集体的结构形态.     

Implicit solvent simulations of DPC micelle formation

The formation of micelles by dodecylphosphocholine (DPC) is modeled by treating the surfactants in atomic detail and the solvent implicitly, in the spirit of the EEF1 solvation model for proteins. The solvation parameters of the DPC atoms are carried over from those of similar atoms in proteins. A slight adjustment of the parameters for the headgroup was found necessary for obtaining an aggregation number consistent with experiment. Molecular dynamics simulations of 960 DPC molecules at different concentrations are used to obtain the aggregation number, the micelle size distribution, and the CMC. At 20 mM concentration we obtain an aggregation number of 53-56 and a CMC of 1.25 mM, values close to the experimental ones. At 100 mM the aggregation number increases to 90. Simulations of individual micelles of varying size show that the effective energy per surfactant molecule is initially a decreasing function of aggregation number but stabilizes at about 60 molecules. The van der Waals term and the desolvation of nonpolar groups contribute to micellization, whereas the desolvation of polar groups opposes it. From the difference between the effective energy and the free energy (calculated from the CMC), the translational and rotational entropy contributions to the free energy are estimated at about 7 kcal/mol per monomer. The micelles obtained here are more irregular than those obtained in explicit water simulations. This modeling approach allows the study of larger surfactant aggregates for longer times and the extraction of thermodynamic in addition to structural information.     

壬基酚聚氧乙烯醚型Gemini季铵盐表面活性剂的合成 及其表面性质的研究

合成了系列壬基酚聚氧乙烯醚型Gemini季铵盐表面活性剂(GNPQA), 用核磁、红外和元素分析对它们的结构进行了表征, 考察了反应条件对转化率的影响, 并用表面张力法和稳态荧光探针法对GNPQA的表面性能及胶束聚集数(N)进行了研究. 结果表明, 较优的反应条件: 反应时间为12 h, 反应温度为70 ℃, 反应原料摩尔比为n(双聚壬基酚聚氧乙烯醚)∶n(三乙胺)∶n(环氧氯丙烷)＝1∶1∶1; GNPQA的临界胶束浓度(CMC)值较相应的单体壬基酚聚氧乙烯醚型季铵盐表面活性剂(NPQA)降低了1～2个数量级, 显示了较高的表面活性; 当GNPQA溶液浓度为5～9倍CMC时, N值随浓度增大而线性增大; 随着氧乙烯(EO)单元数的增长, GNPQA的CMC和N值均逐渐减小; 结合GNPQA的表面性能参数和N值的变化规律, 探讨了这类表面活性剂表面及胶束聚集体的结构形态.     

Molecular dynamics simulation and thermodynamic modeling of the self-assembly of the triterpenoids asiatic acid and madecassic acid in aqueous solution

The self-assembly behavior of the triterpenoids asiatic acid (AA) and madecassic acid (MA), both widely studied bioactive phytochemicals that are similar in structure to bile salts, were investigated in aqueous solution through atomistic-level molecular dynamics (MD) simulation. AA and MA molecules initially distributed randomly in solution were observed to aggregate into micelles during 75 ns of MD simulation. A "hydrophobic contact criterion" was developed to identify micellar aggregates from the computer simulation results. From the computer simulation data, the aggregation number of AA and MA micelles, the monomer concentration, the principal moments of the micelle radius of gyration tensor, the one-dimensional growth exhibited by AA and MA micelles as the aggregation number increases, the level of internal ordering within AA and MA micelles (quantified using two different orientational order parameters), the local environment of atoms within AA and MA in the micellar environment, and the total, hydrophilic, and hydrophobic solvent accessible surface areas of the AA and MA micelles were each evaluated. The MD simulations conducted provide insights into the self-assembly behavior of structurally complex, nontraditional surfactants in aqueous solution. Motivated by the high computational cost required to obtain an accurate estimate of the critical micelle concentrations (CMCs) of AA and MA from evaluation of the average monomer concentration present in the AA and MA simulation cells, a modified computer simulation/molecular-thermodynamic model (referred to as the MCS-MT model) was formulated to quantify the free-energy change associated with optimal AA and MA micelle formation in order to predict the CMCs of AA and MA. The predicted CMC of AA was found to be 59 microM, compared with the experimentally measured CMC of 17 microM, and the predicted CMC of MA was found to be 96 microM, compared with the experimentally measured CMC of 62 microM. The AA and MA CMCs predicted using the MCS-MT model are much more accurate than the CMCs inferred from the monomer concentrations of AA and MA present in the simulation cells after micelle self-assembly (2390 microM and 11,300 microM, respectively). The theoretical modeling results obtained for AA and MA indicate that, by combining computer simulation inputs with molecular-thermodynamic models of surfactant self-assembly, reasonably accurate estimates of surfactant CMCs can be obtained with a fraction of the computational expense that would be required by using computer simulations alone.     

Concentrations of Monomers and Cylindrical Micelles above the Second CMC

Based on thermodynamically substantiated linear dependence of the work of cylindrical micelle formation on the aggregation number within a wide range of aggregation numbers where the cylindrical micelles are accumulated in a surfactant solution, the second critical micellization concentration (CMC) is introduced as an overall surfactant concentration at which the ratio of the total amount of substance in cylindrical micelles to the amount of substance in monomers is equal to 0.1, i.e., it is already noticeable. It is shown that this ratio increases rather rapidly with a monomer concentration. The coefficient of the linear dependence of the work of cylindrical micelle formation on the aggregation number in the important practical situation where the ratios of the total concentration of cylindrical micelles and total amount of substance in these micelles to the monomer concentration are equal by the order of magnitude to 1 and 10⁵, respectively, while disc micelles and extended bilayers are still not appeared. In the same situation, the ratios of the total concentration of spherical micelles and total amount of substance in these micelles to the monomer concentration are equal by the order of magnitude to 1 and 10², respectively. The relationship between the overall surfactant concentration and monomer concentration is found. It is shown that the second CMC exceeds by two orders of magnitude the first CMC corresponding to the onset of the noticeable accumulation of surfactant in spherical micelles. The distribution of cylindrical micelles over the aggregation numbers is analyzed. It is demonstrated that, in agreement with the experiment, the distribution is almost uniform in the considerable part of the wide range of aggregation numbers and drops exponentially in the remaining (right-hand) part of this range. Experimental result is confirmed that the total concentration of cylindrical micelles, the mean value, and the mean statistical scatter of aggregation numbers in a cylindrical micelle is proportional to the square root of the overall surfactant concentration. The balance equation of surfactant amount in the vicinity of the final equilibrium state of a materially isolated solution is linearized. This linearization makes it possible to express the deviations of monomer and aggregate concentrations from their equilibrium values at the lower boundary of the region of the linear dependence of the work of cylindrical micelle formation on the aggregation numbers via the deviations of experimentally observed total concentrations of spherical and cylindrical micelles from their equilibrium values. The case of the solutions of such surfactants, for which spherical shape appeared to be unrealizable due to their molecular structure and packing conditions, is considered separately.     

Molecular dynamics simulation of self-assembly of n-decyltrimethylammonium bromide micelles

In this paper, a molecular dynamics simulation of surfactant self-assembly using realistic atomistic models is presented. The simulations are long enough to enable the observation of several processes leading to equilibrium, such as monomer addition and detachment, micelle dissolution, and micelle fusion. The self-assembly of DeTAB surfactants takes place in three stages: fast aggregation of monomers to form small disordered oligomers; ripening process by which larger aggregates grow at the expense of smaller ones; slower stage involving collisions between large micelles. The first two stages were described well by a simple kinetic model with a size-independent rate constant estimated from the self-diffusion coefficient and collision radius of an isolated monomer. The average cluster size, area per headgroup, degree of counterion dissociation, and critical micelle concentration estimated from the simulation are in reasonable agreement with experimental values. An all-atom and united-atom surfactant model were compared, and the results were seen to be almost independent of the choice of model. DeTAB micelles are spheroidal, with a hydrophobic core composed of tail atoms surrounded by a hydrophilic corona of head atoms. A Stern layer composed of bromide counterions was also identified. Water molecules solvate the counterions and the head atoms, penetrating into the micelle up to the location of the atom connecting the head to the aliphatic tail, in agreement with recent experimental observations.     

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.     

Cholesterol Binding to Simple Micelles in Aqueous Bile-Salt–Cholesterol Solutions

The true thermodynamic activity (A_T) of cholesterol (Ch) in aqueous solutions containing taurocholate (TC)–Ch was determined by employing a direct assay of a 1 × 2-cm silicone polymer film with 0.025 cm thickness. Using theA_Tdata, information on the nature of micellar species present in the TC–Ch system, and employing a binding-site model previously developed for tauroursodeoxycholate (TUDC)–Ch and taurochenodeoxycholate (TCDC)–Ch systems, it appeared that the Ch-binding affinity for simple bile-salt micelles corresponds precisely with the order of hydrophobicity, TUDC < TC < TCDC. Further, although simple TC micelles and simple TCDC micelles have similar binding capacities, the first Ch binding to a simple TC micelle may not significantly facilitate the second Ch binding, as occurs in simple TCDC micelles. For TUDC–Ch, TC–Ch, and TCDC–Ch systems, the concentration of bound simple micelles increased with increasingA_Tvalues, whereas the unbound simple micelle concentration decreased proportionally. These results provide insights into the possible influence of bile-salt species on Ch-binding to simple micelles in bile-salt–Ch solutions.     

Electrophoretic properties of dodecyltrimethylammonium bromide micelles in KBr solution

Charge in ionic micelles determines the trends of their stability and their practical applications. Charge can be calculated from zeta potential (zeta) measurements, which, in turn, can be obtained by Doppler microelectrophoresis. In this study, the electrophoretic properties of dodecyltrimethylammonium bromide (DTAB) in KBr aqueous solution (0-6 mM) were determined by Doppler microelectrophoresis. At very low surfactant concentrations (up to 6 mM), zeta potential was quite constant and due to the ionized monomers (DTA+). Above 6 mM, zeta potential increased to a maximum at surfactant concentrations still below the critical micellar concentration (CMC). This increase could be explained by a formation of nonmicellar aggregates of DTAB. Then, above the CMC, zeta potential underwent an abrupt reduction, which was dependent qualitatively and quantitatively on KBr concentration, and which could be due to an increase of the number of counterions adsorbed on the micelle surface. Calculation of effective micellar charge from zeta potential gave the surface charge density. Comparing this value with the theoretical, obtained from geometrical considerations, a fraction of 0.29 of charged micellar headgroups was obtained when DTAB was in aqueous solution, which is consistent with the value obtained by conductivity measurements.     

阳离子表面活性单体DMOHAC的合成及其胶束化行为

以环氧氯丙烷、丙烯酰胺及N,N-二甲基十八烷基叔胺为原料,经开环和季铵化两步反应合成了新型阳离子表面活性单体,N,N-二甲基十八烷基(2-羟基-3-丙烯酰胺丙基)氯化铵（DMOHAC, 1）,其结构经FT-IR表征。用TG和摩尔电导率法研究了1的热稳定性和1、十八烷基三甲基氯化铵(OTAC)、1/OTAC混合胶束的CMC值。结果表明：1的热稳定性能良好,1和OTAC的CMC值分别为0.10 mmol·L^-1和0.33 mmol·L^-1。当n（1）: n（OTAC）=2 : 1时,1/OTAC混合胶束的CMC值为0.12 mmol·L^-1;当n（1）: n（OTAC）=1 : 2时,1/OTAC混合胶束的CMC值为0.21 mmol·L^-1。1/OTAC混合胶束的CMC值介于1和OTAC之间。     

Amphiphilic polymeric micelle as pseudostationary phase in electrokinetic chromatography for analysis of eight corticosteroids in cosmetics

Amphiphilic polymeric micelle, as a novel pseudostationary phase in EKC was used to determine eight kinds of corticosteroids namely hydrocortisone, prednisolone, hydrocortisone acetate, prednisone, cortisone acetate, prednisolone acetate, dexamethasone, and triamcinolone acetonide in cosmetics. Amphiphilic random copolymer poly(methyl methacrylate‐co‐methacrylic acid) (P(MMA‐co‐MAA)) was micellizated via neutralization in alkaline aqueous solution. The influences of the molar ratio of monomer MMA to MAA, the concentration of polymer and pH on the polymeric micelle microstructure and EKC performances were investigated. As molar ratio of MMA to MAA in P(MMA‐co‐MAA) increased, both CMC and environmental polarity of the inner core in polymeric micelle decreased dramatically. With increasing monomer ratio, the size of polymeric micelles increased firstly, and then decreased, finally increased again. ζ potential of the micelle had a slight decline trend. As increment of polymer concentration, the size of the polymeric micelle increased steadily. By optimizing the monomer ratio, the polymer concentration, and pH of the running buffer, as well as operation conditions such as separation voltage and temperature, the eight analytes could be separated within 16.5 min using 7.5 mg/mL polymer with the monomer ratio of 7:3 dissolved in pH 9.2 borax buffer as the running buffer. The method has been used for analysis of corticosteroids in cosmetic samples with simple extraction; the recoveries for eight analytes were between 85.9 and 106%. This method was of accuracy, repeatability, pretreatment simplicity, and could be applied to the quality control of cosmetics.     

Evidence of the existence of micelles in the fibrillogenesis of beta-amyloid peptide

Alzheimer's disease (AD) is characterized by the deposition of fibrillar deposits formed by the amyloid beta (Abeta) peptide. The most widely accepted model of fibrillogenesis of Abeta affirms that fibrillogenesis occurs in two distinct stages, nucleation and elongation. A modification of the model includes the formation of micelles. We have demonstrated with accurate experimental determinations the existence of aggregates with micellar properties (namely, the critical micellar concentration, CMC, and aggregation number). Values of the CMC were obtained by analysis of surface tension (17.5 microM) and changes in the fluorescence of pyrene (17.6 microM), respectively. The average aggregation number determined by fluorescence quenching was 25, and it was independent of peptide concentration. The presence of micelles implies that above the CMC all excess peptide is incorporated into micelles, and consequently, the monomer concentration is kept almost constant. Thus, micelles act as a peptide reservoir. Micelles are located on-pathway, since they serves as nucleation centers. Experimental data support the model, since above 17.7 microM the time of half-aggregation is independent of peptide concentration, and the overall reaction of the conversion of monomer peptide into fibril can be treated as an apparent first-order reaction.     

石油磺酸盐水溶液及其油/水平衡体系

应用ＥＳＲ法研究了石油磺酸盐水溶液及其油／水平衡 水溶液胶束的形成及其微观性质,结果表明,石油磺酸盐／３％正丁醇／０．３％,氯化十二烷平衡体系的有效临界胶束浓度远高于石油磺酸盐在３％正丁醇,０．３％氯内中的临界胶束浓度（ＣＭＣ）,发产生正比结果的原因。     

表面活性剂胶束化物理化学性质研究

通过电导法考查温度和盐浓度对十二烷基硫酸钠(SDS)临界胶束浓度(CMC)的影响,研究表面活性剂形成胶束过程的物理化学性质。根据拟相分离模型求得胶束化热力学函数,并讨论体系电导活化能随温度和SDS浓度变化关系。结果表明:SDS的CMC随温度升高而增加,随氯化钠浓度增大而减小。在热力学上SDS在水溶液中形成胶束是一个自发、放热、熵增的过程;在动力学上,SDS溶液电导率与温度关系符合Arrhenius公式,通过电导活化能信息可揭示离子型表面活性剂形成胶束的机理特征。     

Boltzmann distributions and slow relaxation in systems with spherical and cylindrical micelles

Equilibrium and nonequilibrium distributions of molecular aggregates in a solution of a nonionic surfactant are investigated at the total surfactant concentration above the second critical micelle concentration (CMC2). The investigation is not limited by the choice of a specific micellar model. Expressions for the direct and reverse fluxes of molecular aggregates over the potential humps of the aggregation work are derived. These aggregation work humps set up activation barriers for the formation of spherical and cylindrical micelles. With the aid of the expressions for molecular aggregate fluxes, a set of two kinetic equations of micellization is derived. This set, along with the material balance equation, describes the molecular mechanism of the slow relaxation of micellar solution above the CMC2. A realistic situation has been analyzed when the CMC2 exceeds the first critical micelle concentration, CMC1, by an order of magnitude, and the total surfactant concentration varies within the range lying markedly above the CMC2 but not by more than 2 orders of magnitude. For such conditions, an equation relating the parameters of the aggregation work of a cylindrical micelle to the observable ratio of the total surfactant concentration and the monomer concentration is found for an equilibrium solution. For the same conditions, but in the nonequilibrium state of materially isolated surfactant solution, a closed set of linearized relaxation equations for total concentrations of spherical and cylindrical micelles is derived. These equations determine the time development of two modes of slow relaxation in micellar solutions markedly above the CMC2. Solving the set of equations yields two rates and two times of slow relaxation.     

Polysorbate20 adsorption layers below and above the critical micelle concentration over aluminum; cloud point and inhibitory role investigations at the solid/liquid interface

Non‐ionic polysorbate20 surfactant was used to produce adsorption protective layers below and above its critical micelle concentration (CMC) at the liquid/solid interface. The well‐ordered accumulation of surfactant molecules on the metal surface below the CMC led to the formation of oriented surfactant monolayers. On the other hand, as the surfactant concentration increased above the CMC, the monodisperse micelles, free surfactant molecules and oriented surfactant monolayers undergo aggregate formation and produce a turbid solution. The gradual increase in the number and size of aggregates leads to phase separation and hence disassembled protective layers that allow easier penetration of corrosive HCl at a metal surface. This was demonstrated by inhibition efficiency, activation energy, enthalpy and entropy of activation values. Two‐dimensional irregular crystalline sheets accumulated at the surface of aluminum, as shown by scanning electron micrographs. Adsorption of polysorbate20 at the aluminum surface exhibited a Temkin isotherm fit. Larger desorption processes at the cloud point demonstrate aggregate formation and phase separation, and hence poorer adsorption layers at the metal surface. Copyright © 2012 John Wiley & Sons, Ltd.     

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。     

Micellization of bile salts in aqueous medium: a fluorescence study

Owing to the physiological importance of the micellization process of bile salts, the critical micelle concentration (CMC) becomes a fundamental parameter in the evaluation of their biological activities. The present study suggests fluorescence probing, using 1,6-diphenylhexatriene (DPH), as a simple, convenient, sensitive and economic method for monitoring the micellization process of bile salts in aqueous medium. Three independent parameters: fluorescence intensity, anisotropy and lifetime of DPH have been employed successfully for determining the CMC of two bile salts, sodium deoxycholate (NaDC) and sodium cholate (NaC), in aqueous medium. The CMC values reported by all the above three parameters of DPH are found to be same and it is 16 mM for NaC and 6 mM for NaDC at 25 degrees C in unbuffered solution. The effect of temperature and ionic strength on the micellization process has also been investigated employing DPH as a fluorescent probe. Increasing temperature leads to the formation of fluffier micelles with less rigid interior for both NaC and NaDC. The micelle core of NaC is less perturbed by the presence of NaCl whereas in case of NaDC, the aggregates provide DPH a more nonpolar and rigid environment in presence of NaCl than that in absence of salt.     

Mixed micelles of tetradecyltrimethylammonium bromide and chlorhexidine digluconate in aqueous solutions of isopropyl alcohol

The influence of isopropyl alcohol (IPA) on the size and composition of the mixed micelles in mixtures of tetradecyltrimethylammonium bromide (TTAB) and chlorhexidine digluconate (CG) has been determined as a function of the composition of the systems. The addition of 0.5 M and 1.0 M IPA had little significant effect on the composition of the mixed micelles as determined both by analysis of critical micelle concentration (CMC) data using a theoretical treatment based on excess thermodynamic quantities and by an empirical treatment of conductivity data. Static and quasielastic light scattering measurements showed a progressive decrease of the aggregation number and hydrodynamic radius of TTAB micelles on addition of IPA, but minimal changes in the properties of the small CG aggregates. The results show that the micellar weight in the TTAB/CG/IPA solutions is determined by the ratio of the surfactants in the system and for each TTAB/CG ratio decreases on addition of IPA.