Solubilization of a water-insoluble dye in aqueous NaBr solutions of alkylpyridinium bromides and its relation to micellar size and shape

In order to investigate the effect of added salt on micelle size, shape, and structure the solubilization of Orange OT in aqueous NaBr solutions of decylpyridinium bromide (DePB), dodecylpyridinium bromide (DPB), tetradecylpyridinium bromide (TPB), and hexadecylpyridinium bromide (CPB) has been examined. The solubilization powers of DePB and DPB micelles increase with increasing NaBr concentration up to 2.86 and 3.07 mol dm^–3, respectively, but above these concentrations remain unaltered. This suggests that spherical micelles of DePB and DPB can have a maximum and constant size at NaBr concentrations higher than these threshold concentrations. On the other hand, the solubilization powers of TPB and CPB micelles increase in the whole range of NaBr concentration studied. The dependencies of the solubilization powers of their micelles on the counterion concentration change at 0.10 and 0.03 mol dm^–3 NaBr, respectively, as suggests that TPB and CPB micelles undergo the sphere–rod transition at those concentrations. Orange OT is a more suitable probe for detecting the presence of the maximum- and constant-size spherical micelle than Sudan Red B.     

The sphere-rod transition of micelles of dodecyldimethylammonium bromide in aqueous NaBr solutions,and the effects of counterion binding on the micelle size,shape and structure

Micelle size and shape of dodecyldimethylammonium bromide have been determined by measurement of light scattering from its aqueous NaBr solutions. In water and in the presence of NaBr up to 0.07 M, the Debye plots give straight lines with positive slopes, and spherical micelles having molecular weight less than 30 000 are formed. At higher NaBr concentrations, the Debye plots decrease with increasing micelle concentration, indicating the aggregation of the primary spherical micelles into larger secondary micelles. The molecular weight and the radius of gyration of the secondary micelles increase with increasing NaBr concentration, and the relation between molecular weight and radius of gyration suggests that they are rodlike and flexible. Linear logarithmic relations between micelle molecular weight and ionic strength hold for spherical and rodlike micelles, respectively, and the threshold concentration of NaBr for the sphere-rod transition is located at 0.07 M. The spherical micelle of dodecyldimethylammonium ions has a size more than 20 surfactant ions larger in NaBr solutions than in NaCl solutions, and their rodlike micelle has a shorter length in NaBr solutions than in NaCl solutions, when compared at an identical aggregation number, indicating 2 more surfactant ions in its cross-section.     

Surface and bulk properties of aqueous decyltrimethylammonium bromide-hexadecyltrimethylammonium bromide mixed system

The aqueous mixed system decyltrimethylammonium bromide (C(10)TAB)-hexadecyltrimethylammonium bromide (C(16)TAB) was studied by conductivity, ion-selective electrodes, surface tension, and fluorescence spectroscopy techniques. The mixture critical micelle concentration, cmc(*), aggregation number, N( *), and micelle molar conductivity, Lambda(M)(cmc), showed that the system aggregation is strongly nonideal. Both cmc(*) and N( *) results were analyzed with two different procedures: (i) the regular solution theory on mixed micelles or Rubingh's theory, and (ii) by the determination of the partial critical micelle concentration of the amphiphile component i in the presence of a constant concentration of the other amphiphile component, cmc(i)( *). The Rubingh procedure gives micelles richer in C(16)TAB than the overall mixtures, while procedure (ii) gives micelles having the same composition as in the complete surfactant mixture (alpha(C(10)TAB). Mixed micelles are larger than pure surfactant ones, with nonspherical shape. Using a literature model, the cause of the synergistic effect seems to be a reduction of the hydrocarbon/water contact at the micelle surface when mixed micelles form. Conductivity and ion-selective electrodes indicate that highly ionized premicelles form immediately before the cmc(*). The air/solution interface is strongly nonideal and much richer in C(16)TAB than the composition in the bulk. When micelles form there is a strong desorption from the air/solution interface because micelles are energetically favored when compared with the monolayer.     

Morphology and mechanical properties of surfactant aggregates at water-silica interfaces: molecular dynamics simulations

Dilute and concentrated surfactant systems at the solid-liquid interface are examined using classical molecular dynamics simulations. Particular emphasis is placed on understanding how surfactants aggregate and form the micellar structure, how micelles change shape at high concentrations in aqueous media and in the presence of hydrophilic surfaces, and at what force this micellar structure breaks apart during indentation of micelle-covered surfaces with a proximal probe microscope tip. The specific system of interest is C12TAB (n-dodecyltrimethylammonium bromide) surfactant in an aqueous medium that is modeled with empirical potentials. The simulations predict that the micelle structure in water is compact and either spherical or elliptical in shape. In the presence of a hydrophilic surface of silica, the structure evolves into a flat elliptical shape, in agreement with experimental findings. The simulated indentation of the micelle/silica system causes the micelle to break apart at an indentation force of about 1 nN and form a surfactant monolayer. The predicted force curve is in excellent agreement with experimental measurements.     

Properties of two polymerizable surfactants aqueous solutions: dodecylethylmethacrylatedimethylammonium bromide and hexadecylethylmethacrylatedimethylammonium bromide. I. Critical micelle concentration

The aggregation of two polymerisable surfactants dodecylethylmethacrylatedimethylammonium bromide (C₁₂PS) and hexadecylethylmethacrylatedimethylammonium bromide (C₁₆PS) was studied with a battery of methods. Both surfactants form premicelles at low concentration, and show a critical micelle concentration and a transition between spherical and rod-like micelles. The micelle ionization degree and the adsorption at the air/solution interface were also studied. Results are interpreted on the basis of the conformation of the polar head group.     

表面活性剂胶束形状随浓度转变的核磁共振研究

运用核磁共振一维氢谱和自扩散实验方法研究了聚乙烯乙二醇异辛酚醚(TX-100)、十二烷基苯磺酸钠(SDBS)和十四烷基三甲基溴化铵(TTAB)三种不同类型的表面活性剂在重水溶液中的胶束形状转变, 发现它们在临界胶束浓度以上的各自相应浓度都有胶束形状的变化(由球状转变为椭球状或棒状). 在常温常压和没有其他添加剂的情况下, 表面活性剂溶液浓度高于其临界胶束浓度时, 球状胶束开始形成. 核磁共振一维氢谱和自扩散实验的结果显示, 当溶液浓度继续增加到一定程度时, 溶液中表面活性剂分子的化学位移和自扩散系数的变化速率都有明显的转折, 这说明溶液中球状胶束开始发生转变. 进一步通过仔细分析对比核磁共振一维氢谱中各基团谱峰, 发现表面活性剂胶束亲水表面上的质子的化学位移变化速率要远高于其疏水内核中的质子, 据此推测胶束形状很可能由球状转变为椭球状或棒状.     

Polymorphism of Micelles in Aqueous Cetyltrimethylammonium Bromide Solutions

A viscometric study was made of the polymorphous transformation of micelles in an aqueous solution of cetyltrimethylammonium bromide in the presence of NaBr and sodium methylbenzenesulfonate. The effects of temperature and nature and concentration of additives on the second critical micelle concentration were studied. The thermodynamic characteristics of the micellar transition are estimated.     

A calorimetry and light scattering study of the formation and shape transition of mixed micelles of EO20PO68EO20 triblock copolymer (P123) and nonionic surfactant (C12EO6)

The interaction between the nonionic surfactant C12EO6 and the poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer EO20PO68EO20 (P123) has been investigated by means of isothermal titration and differential scanning calorimetry (DSC) as well as static and dynamic light scattering (SLS and DLS). P123 self-assembles in water into spherical micelles at ambient temperatures. At raised temperatures, the DSC data revealed a sphere-to-rod transition of the P123 micelles around 60 degrees C. C12EO6 interacts strongly with P123 micelles in aqueous solution to give mixed micelles with a critical micelle concentration (cmc) well below the cmc for pure C12EO6. The presence of C12EO6 also lowers the critical micelle temperature of P123 so aggregation starts at significantly lower temperatures. A new phenomenon was observed in the P123-C12EO6 system, namely, a well-defined sphere-to-rod transition of the mixed micelles. A visual phase study of mixtures containing 1.00 wt % P123 showed that in a narrow concentration range of C12EO6 both the sphere-to-rod transition and the liquid-liquid phase separation temperature are strongly depressed compared to the pure P123-water system. The hydrodynamic radius of spherical mixed micelles at a C12EO6/P123 molar ratio of 2.2 was estimated from DLS to be 9.1 nm, whereas it is 24.1 nm for the rodlike micelles. Furthermore, the hydrodynamic length of the rods at a molar ratio of 2.2 is in the range of 100 nm. The retarded kinetics of the shape transition was detected in titration calorimetric experiments at 40 degrees C and further studied by using time-resolved DLS and SLS. The rate of growth, which was slow (>2000 s), was found to increase with the total concentration.     

Mechanism of surfactant micelle formation

The mechanism of micelle formation of surfactants sodium dodecyl sulfate (SDS), n-hexyldecyltrimethylammonium bromide (CTAB) and Triton X-100 (TX-100) in heavy water solutions was studied by 1H NMR (chemical shift and line shape) and NMR self-diffusion experiments. 1H NMR and self-diffusion experiments of these three surfactants show that their chemical shifts (delta) begin to change and resonance peaks begins to broaden with the increase in concentration significantly below their critical micelle concentrations (cmc's). At the same time, self-diffusion coefficients ( D) of the surfactant molecules decrease simultaneously as their concentrations increase. These indicate that when the concentrations are near and lower than their cmc's, there are oligomers (premicelles) formed in these three surfactant systems. Carefully examining the dependence of chemical shift and self-diffusion coefficient on concentration in the region just slightly above their cmc's, one finds that the pseudophase transition model is not applicable to the variation of physical properties (chemical shift and self-diffusion coefficient) with concentration of these systems. This indicates that premicelles still exist in this concentration region along with the formation of micelles. The curved dependence of chemical shift and self-diffusion coefficient on the increase in concentration suggests that the premicelles grow as the concentration increases until a definite value when the size of the premicelle reaches that of the micelle, i.e., the system is likely dominated by the monomers and micelles. Additionally, the approximate values of premicelle coming forth concentration (pmc) and cmc were obtained by again fitting chemical shifts to reciprocals of concentrations at a different perspective, and are in good accordant with experimental results and literature values and prove the former conclusion.     

Endogenous growth of the population of reverse micelles

The coupling reaction between cetylbromide (CB) and trimethylamine (TMA) to yield the surfactant cetyltrimethylammonium bromide (CTAB) is studied in the system chloroform/isooctane (2/1,v/v)/water in which CTAB forms reverse micelles. This system affords an endogenous micelle population growth, i.e., an increase of the concentration of the micelles due to appearance of the surfactant in situ. The reaction is studied in the presence of preexisting CTAB reverse micelles. The rate of CTAB formation is measured by NMR spectroscopy, and the endogenous micelle population growth is directly monitored by time-resolved fluorescence quenching analysis. Under our experimental conditions, a 100% yield of the chemical reaction brings about a fourfold increase in the population of the reverse micelles. Since the water concentration is constant during chemical reaction, the newly formed water pools are formed at the expense of the initial ones, which brings about a decrease of the average water pool radius during micellar growth. The implication of the endogenous micelle population growth as a model for biological systems is briefly discussed.     

Spontaneous vesicle formation of single chain and double chain cationic surfactant mixtures

The concentration vs composition diagram of aggregate formation of the dodecyltrimethylammonium bromide (DTAB) and didodecyldimethylammonium bromide (DDAB) mixture in aqueous solution at rather dilute region was constructed by analyzing the surface tension, turbidity, and electrical conductivity data and inspected by cryo-TEM images and dynamic light scattering data. Although the aqueous solution of DTAB forms only micelles, the transition from monomer to small aggregates and then to vesicle was found at 0.1 < X2 相似文献   

Spectrophotometric study of the interaction of some triphenylmethane dyes and 1-carbethoxypentadecyltrimethylammonium bromide

The effect of the cationic surfactant 1-carbethoxypentadecyltrimethylammonium bromide at above and below its critical micelle concentration on the absorption spectra and the values of the apparent dissociation constant pK(2) of the dyes Phenol Red, Bromophenol Blue and Bromocresol Green has been studied, along with the influence of a strong electrolyte (NaCl) on the effects produced by the presence of surfactant micelles in the dye solution.     

Self-consistent field modeling of linear nonionic micelles

A self-consistent field theory is used to predict structural, mechanical, and thermodynamical properties of linear micelles of selected nonionic surfactants of the type C(n)()E(m). Upon increase in surfactant concentration the sudden micelle shape transition from spherical to cylindrical (second critical micelle concentration (cmc)) is analyzed. The cylindrical micelles consist of a body (with radius R(c) and length L) and two slightly swollen endcaps. For small L, the shape resembles a dumbbell. With increase in the length of the body, an oscillatory behavior in the grand potential of the micelle is found. The wavelength of the oscillation (lambda(d)) is proportional to the surfactant tail length n. The amplitude of these oscillations decreases exponentially with a decay length xi. In the limit of very long micelles, the grand potential converges to the endcap (free) energy E(c). This endcap energy increases approximately quadratic with the tail length and diminishes by increasing the headgroup size m. The micelle size distribution is generated showing non-monotonic features due to the presence of short dumbbells and becomes exponential when L > 8R(c). It is also shown that the endcap energy can be estimated in first order by the grand potential of the spherical micelle that coexists with infinitely long cylindrical micelles. The persistence length l(p) of these linear micelles is evaluated to estimate the relative importance of conformational entropy for these micelles.     

Miscibility of dodecyltrimethylammonium bromide and dodecyltrimethylammonium chloride in surface-adsorbed film and micelle

Surface tension of aqueous solutions of mixtures of dodecyltrimethylammonium bromide (DTAB) and dodecyltrimethylammonium chloride (DTAC) has been measured and analyzed by using thermodynamic relations. The adsorbed film has been found to contain more DTAB molecules than the solution. The shape formed by the curves of the total molality at constant surface tension against the solution and surface compositions indicates the ideal mixing of the DTAB and DTAC molecules in the adsorbed film. Micellar composition has been estimated at the critical micelle concentration (CMC). The micelles have been found to be richer in DTAB than the solution, but poorer in DTAB than the adsorbed film at the CMC. The DTAB and DTAC molecules have been shown to mix ideally in the micelles. From the comparison with the results on the system of decylammonium bromide and decylammonium chloride, it has been concluded that, on the mixing of surfactants differing only in counter ions, the adsorbed film is influenced more significantly by the ionic head group of the surfactant than the micelle.     

Sphere-to-rod transition of non-surface-active amphiphilic diblock copolymer micelles: a small-angle neutron scattering study

Micellization behavior of amphiphilic diblock copolymers with strong acid groups, poly(hydrogenated isoprene)-block-poly(styrenesulfonate), was investigated by small-angle neutron scattering (SANS). We have reported previously (Kaewsaiha, P.; Matsumoto, K.; Matsuoka, H. Langmuir 2005, 21, 9938) that this strongly ionic amphiphilic diblock copolymer shows almost no surface activity but forms micelles in water. In this study, the size, shape, and internal structures of the micelles formed by these unique copolymers in aqueous solution were duly investigated. The SANS data were well described by the theoretical form factor of a core-shell model and the Pedersen core-corona model. The micellar shape strongly depends on the hydrophobic chain length of the block copolymer. The polymer with the shortest hydrophobic chain was suggested to form spherical micelles, whereas the scattering curves of the longer hydrophobic chain polymers showed a q-1 dependence, reflecting the formation of rodlike micelles. Furthermore, the addition of salt at high concentration also induced the sphere-to-rod transition in micellar shape as a result of the shielding effect of electrostatic repulsion. The corona thickness was almost constant up to the critical salt concentration (around 0.2 M) and then decreased with further increases in salt concentration, which is in qualitatively agreement with existing theories. The spherical/rodlike micelle ratio was also constant up to the critical salt concentration and then decreased. The micelle size and shape of this unique polymer could be described by the common concept of the packing parameter, but the anomalously stable nature of the micelle (up to 1 M NaCl) is a special characteristic.     

Second CMC in surfactant micellar systems

Experimental reports of surfactant systems displaying a second critical micelle concentration (second CMC) have been surveyed. It turns out that surfactant micelles usually show a growth behavior with some typical features. (i) Micelles grow weakly at low surfactant concentrations but may switch to a much stronger growth behavior at higher concentrations. The second CMC is defined as the point of transition from weakly to strongly growing micelles. (ii) Micelles are found to be non-spherically shaped below the second CMC. (iii) At the second CMC micelles are found to be much smaller, with aggregation numbers typically 100–200, than expected for flexible micelles. (iv) Micelles of intermediate size are present in a narrow concentration regime close to the second CMC. (v) Micelles grow much stronger above the second CMC than expected from a sphere-to-rod transition. The conventional spherocylindrical micelle model predicts a smooth growth behavior that contradicts the appearance of a second CMC. Modifying the model by means of including swollen end caps neither account for the presence of micelles with intermediate size, nor the strong growth behavior above the second CMC. Taking into account micelle flexibility is not consistent with the rather low micelle aggregation numbers observed at the second CMC. On the other hand, a recently proposed alternative theoretical approach, the general micelle model, have been demonstrated to take into account basically all features that are typical of experimentally observed micellar growth behaviors.     

Formation and microstructure transition of F127/TX-100 complex

Formation and structure transition of the complex composed of triblock copolymer F127 and nonionic surfactant TX-100 have been investigated by 1H NMR spectroscopy, dynamic light scattering (DLS), and isothermal titration calorimetry (ITC). Three TX-100 concentration regions are identified, within which TX-100/20 mg/mL F127 complex undergoes different temperature-induced structure transitions. In low concentration region (< 9.42 mM), F127 single molecular species (unimers) wrap around TX-100 micelles forming F127/TX-100 complex with TX-100 micelle as the skeleton at a lower temperature (5 degrees C), and the skeleton transfers to F127 micelle at higher temperature (40 degrees C); in intermediate TX-100 concentration region (9.42-94.85 mM), the skeleton of F127/TX-100 complex transfers from TX-100 micelle successively into F127 micelle and TX-100 micelle again upon heating. The interaction of F127 with TX-100 is saturated in high TX-100 concentration region (> 157.57 mM), and free TX-100 micelles coexist with larger clusters of F127/TX-100 complexes. In addition, TX-100-induced F127/TX-100 complex formation and structure transition are also investigated at constant temperatures. The results show that within 5-10 degrees C, F127 unimers mainly adsorb on the surface of TX-100 micelles just like normal water soluble polymers; in the temperature region of 15-25 degrees C, TX-100 micelles prompts F127 micelle formation. Within 30-40 degrees C, TX-100 inserts into F127 micelles leading to the breakdown of F127 aggregates at higher TX-100 concentrations, and the obtained unimers thread through TX-100 micelles forming complex with TX-100 micelle as skeleton.     

Investigation on the mixed micellar systems of cationic surfactants with propylene glycol and its oligomers

The effects of the addition of several propylene glycol oligomer additives on the micellar properties of the cationic mixed micelle system of cetyltrimethylammonium bromide (CTAB) and tetradecyltrimethylammonium bromide (TTAB) were investigated. Through conductivity studies, the critical micelle concentration (cmc) and counter-ion binding values were observed in the presence of the additives. A regular solution approach was employed to determine the effects of the additives on the interactions between the two surfactants to form the mixed micelles. The effects of the additives on the interaction parameter and the composition of the mixed micelles were evaluated. Excess Gibbs energy of mixing and the values of activity coefficients were also calculated and described in terms of non-ideality of the system.     

Investigation of SDS, DTAB and CTAB micelle microviscosities by electron spin resonance

Electron spin resonance spectroscopy (ESR) of the nitroxide labelled fatty acid probes (5-, 16-doxyl stearic acid) was used to monitor the micelle microviscosity of three surfactants at various concentrations in aqueous solution: sodium dodecyl sulphate (SDS), dodecyltrimethylammonium bromide (DTAB) and cetyltrimethylammonium bromide (CTAB). At low surfactant concentration, there is no micelle, the ESR probe is dissolved in water/surfactant homogeneous phase and gives his microviscosity. At higher surfactant concentration, an abrupt increase in microviscosity indicates the apparition of micelles and, the solubilization of the probes in micelles. The microviscosity of the three surfactants, in a large surfactant range, was obtained as well as the critical micelle concentration (CMC). The microviscosity increased slightly with the increase in surfactant concentration. Phosphate buffer lowered the CMC value and generally increased the microviscosity.     

Quenching and dequenching of pyrene fluorescence by nucleotide monophosphates in cationic micelles

The fluorescence behavior of pyrene solubilized in the hexadecyltrimethylammonium bromide aqueous micellar solution in the presence of adenosine 5'-monophosphate (AMP) and uridine 5'-monophosphate (UMP) was investigated. AMP and UMP were found to influence oppositely the fluorescence of micellized pyrene. UMP acts as quencher, while AMP acts as dequencher. Both effects saturate at high nucleotide concentration (about 40 mM). Dequenching of micellized pyrene fluorescence is induced also by addition of disodium hydrogen orthophosphate (Na 2HPO 4), while loading with sodium bromide (NaBr) quenches the fluorescence. Furthermore, in absence of micelles, pyrene fluorescence depends on the UMP, according to the Stern-Volmer relation, but is unaffected by AMP. Dynamic light scattering experiments showed that the size and shape of aggregates is not affected by different types of nucleotide loaded into the solution; thus, we conclude that the opposite photophysical effect exploited by AMP and UMP are uncorrelated to any change in micellar microstructure. The whole fluorescence data set was successfully accounted for by assuming that the anionic nucleotides compete with the surfactant counterion (bromide) for the surface of the micelle. Accordingly, substitution of bromide with the more effective quencher UMP results in a strong decrease of the pyrene fluorescence, while the substitution of bromide with the nonquencher AMP results in an increase in the pyrene fluorescence.