Effect of alkyl chain length and size of the headgroups of the surfactant on solvent and rotational relaxation of Coumarin 480 in micelles and mixed micelles

The effect of alkyl chain length and size of the headgroups of the surfactant on the solvation dynamics and rotational relaxation of Coumarin 480 (C-480) has been investigated using dynamic Stokes' shift of C-480 in different types of alkyltrimethylammonium bromide micelles and mixed micelles. The rotational relaxation time increases with increase in alkyl chain length of the surfactant. The increase in the number of alkyl chains of the surfactant leads to the more close packed micelles, hence the microviscosity of the micelles increases and consequently rotational relaxation time increases. Solvation time also increases due to the increase in number of alkyl chains of the surfactant. The change in solvation and rotational relaxation time is more prominent in micelles compared to mixed micelles. The solvation and rotational relaxation time also increase with the increase in size of the headgroup of the surfactant.     

Research on the vesicle-micelle transition by 1H NMR relaxation measurement

We have investigated how the dynamics of surfactant molecules changes with the vesicle-micelle transition by (1)H NMR relaxation studies on the sodium decyl sulfate (SDeS)-decyltrimethylammonium bromide (DeTAB)-deuterium oxide system. The study has been planned with reference to the phase diagram of the SDeS-DeTAB-water system deduced from thermodynamic analysis of the surface tension data. The spin-lattice relaxation time (T(1)) and the spin-spin relaxation time (T(2)) are measured at 90 and 400 MHz at various total molalities, m, and compositions, X(2), of the surfactants. The data were analyzed according to the "two-step" model developed by Wennerstr?m et al. and molecular dynamics of the surfactant is discussed from the viewpoint of correlation time tau(f) associated with the local fast motion of the surfactant molecule, correlation time tau(s) associated with the slow overall motions of the aggregate and surfactant molecules within it, and local order parameter S. We find tau(s) of vesicles is an order of magnitude larger than that of micelles signifying that the tumbling of vesicle particles and surfactant diffusion over the vesicle are much slower than those for micelle. Tau(f) and S for vesicles are also larger than those for micelles. Molecular environments of the surfactant are also discussed from the dependence of the chemical shifts on m at constant X(2) or from that on X(2) at constant m. When the chemical shifts in vesicle and micelle are compared at constant m, the chemical shifts in vesicle are displaced to a lower magnetic field than those in micelle, which implies that the surfactant molecules are arranged more closely to each other in the vesicle than in the micelle.     

Characteristic behavior of short-term dynamics in reorientation for Gay-Berne particles near the nematic-isotropic phase transition temperature

A specific transition behavior was found in the tumbling motion near the nematic-isotropic phase boundary using molecular dynamics simulations of the Gay-Berne mesogenic model under isobaric conditions at a reduced pressure P* of 2.0. The relaxation time for the motion obtained from the second-rank orientational time correlation function and the rotational diffusion coefficient showed a clear jump at the nematic-isotropic phase transition temperature. Regardless of the temperature dependence of the relaxation time, the change in the rotational diffusion coefficient evaluated from the orientational order parameters and the relaxation time agreed qualitatively with that of real mesogens. The rotational viscosity coefficients gamma(1) and gamma(2) were obtained from the simulation data for the relaxation time for the short-term dynamics and for the rotational diffusion coefficients. gamma(1) was proportional to (2), where is the second-rank orientational parameter. Furthermore, the rotational behavior of the model was compared with that of the Debye approximation in the isotropic phase.     

Angular and frequency dependent spin relaxation study of liquid crystalline cyanobiphenyls

NMR field-cycling measurements of the Larmor frequency (v) and angular (Δ) dependences of the longitudinal proton spin relaxation time T₁ for the nematic liquid crystals 5CB and 8CB allow a more detailed analysis of the underlying molecular motions than data available previously. All T₁ (v, Δ) dispersion profiles essentially distinguish three frequency ranges where T₁ is governed by either local field effects, collective motions (director order fluctuations), or rotational and translational diffusion of individual molecules or molecular groups, respectively. The angular dependence supports and extends previous conclusions about the significance of the order fluctuation term at low (kHz) and high (MHz) Larmor frequencies; in addition it is the basis for the disentanglement of local field effects, which involve Jeener's dipolar relaxation, and of the sophisticated rotational relaxation models suggested in the literature by Dong, Nordio and Vold. It is found that Vold's third rate concept gives the best explanation of the measurements. The results on the rotational diffusion processes essentially agree with deuteron studies from the literature, but also reveal clear distinctions with regard to the anisotropy parameter σ, essentially due to the improved separation from the order fluctuation contribution.     

Generalized gaussian model for uniaxial rotational motion: application to the calculation of spectroscopic responses

An exact model aimed at describing uniaxial rotational motions, based on a rotational adapted Gaussian statistics, is presented. In its simplest form, it depends on only two parameters, an order parameter which can vary from 1 (perfect order) to 0 (isotropic diffusion) and a time-dependent correlation parameter rho which varies from 1 to 0 between initial and infinite times. This model yields closed form expressions for the correlation functions relevant to the main spectroscopic techniques (dielectric absorption, light and neutron scattering, NMR line shape, spin-lattice relaxation, etc.) for all values of the two parameters. According to the functional form postulated for rho(t), in particular forms decaying as power laws at long times, one obtains shapes for the spectroscopic correlation functions and spectra that are similar to those experimentally observed in a large variety of complex systems (liquid crystals, polymers, gels, and amorphous and glassy materials), especially in confined geometries, which often resemble "stretched" exponentials. A simple way to introduce time coherent effects through a modification of rho(t) is proposed. Examples of theoretical correlation functions and spectra are presented. Important remarks concerning the application of this model to the analysis of real data are made. This model is the rotational analogue of the Gaussian translational model developed recently (Volino et al. J. Phys. Chem B 2006, 110, 11217).     

丙三醇水溶液玻璃结构松弛现象学研究

为了考察水含量对丙三醇水溶液玻璃体结构松弛行为的影响，利用差示扫描量热法(DSC)测量了五种高浓度丙三醇水溶液的玻璃化转变温度和玻璃化转变区域的比热容，利用TNM模型进行了结构松弛的现象学分析. 松弛时间的分析结果表明，水溶液玻璃中水含量越高，则松弛过程越快. TNM模型的计算结果表明，随着水含量的增加，玻璃体系的结构松弛活化能和非指数参数都有逐渐降低的趋势，而非线性参数和指前因子逐渐增加.     

Interaction of ionic liquid with water with variation of water content in 1-butyl-3-methyl-imidazolium hexafluorophosphate ([bmim][PF6])/TX-100/water ternary microemulsions monitored by solvent and rotational relaxation of coumarin 153 and coumarin 490

The interaction of water with room temperature ionic liquid (RTIL) [bmim][PF6] has been studied in [bmim][PF6]/TX-100/water ternary microemulsions by solvent and rotational relaxation of coumarin 153 (C-153) and coumarin 490 (C-490). The rotational relaxation and average solvation time of C-153 and C-490 gradually decrease with increase in water content of the microemulsions. The gradual increase in the size of the microemulsion with increase in w0 (w0=[water]/[surfactant]) is evident from dynamic light scattering measurements. Consequently the mobility of the water molecules also increases. In comparison to pure water the retardation of solvation time in the RTIL containing ternary microemulsions is very less. The authors have also reported the solvation time of C-490 in neat [bmim][PF6]. The solvation time of C-490 in neat [bmim][PF6] is bimodal with time constants of 400 ps and 1.10 ns.     

Unusual behavior of PEG/PPG/Pluronic interfaces studied by a spinning drop tensiometer

The effects of surfactants on the interfacial tension driven retraction of elongated drops were studied in a spinning drop tensiometer. Experiments were conducted on polypropylene glycol (PPG) drops suspended in polyethylene glycol (PEG), with Pluronic block copolymers as surfactants. Two unusual observations are reported here. In the first, initially-elongated drops generated at high rotational speed were allowed to retract by reducing the rotational speed. Pluronic-laden drops would not retract completely, but would instead maintain strongly nonspherical shapes indefinitely. We attribute such "nonretraction" to an interfacial yield stress induced by the Pluronic surfactant. In the second, drops being heated while spinning at a constant speed would elongate sharply at some temperature, and subsequently breakup. Such "autoextension" and breakup indicate complex nonmonotonic changes in interfacial tension with time during heating. We propose that autoextension occurs because at low temperature, interfacially-adsorbed surfactant is crystallized and hence trapped at the interface at a concentration far above equilibrium.     

Nuclear magnetic resonance investigation of the micellar properties of two-headed surfactant systems: the disodium 4-alkyl-3-sulfonatosuccinates. 2. The dynamics of the chains comprising the interior of two-headed surfactant micelles

The dynamics of the carbons comprising the micelles of two members of the family of two-headed surfactants, the disodium 4-alkyl-3-sulfonatosuccinates, has been determined via the application of the two-step model to the 13C relaxation rates and the nuclear Overhauser enhancements (nOe's) at 200 MHz. The NMR relaxation times, determined from the inversion recovery method, increase steadily as we descend the chain from the headgroup region. The relaxation rate profiles and the order parameters have been calculated from the two-step model for the micellar sulfosuccinate aggregates. We note that the order parameter profile and the fast motion correlation time profile for these two-headed surfactants are distinctly different from those of a typical single-headed, single-tailed surfactant such as dodecyltrimethylammonium bromide, particularly in the headgroup region of the micelle. All these results are interpreted in terms of the effect of adding a second headgroup to a single-headed, single-tailed surfactant.     

Shear and extensional rheology of solutions of mixtures of poly(ethylene oxide) and anionic surfactants in ionic environments

Interactions between a high molecular weight poly(ethylene oxide) (PEO) and the anionic surfactant sodium dodecyl benzene sulfonate (SDBS) in aqueous solutions were investigated by shear and extensional rheometry. Results for mixtures between PEO and sodium dodecyl sulfate (SDS) are also presented for comparison purposes. Addition of anionic surfactants to PEO solutions above the critical aggregation concentration (CAC), at which micellar aggregates attach to the polymer chain, results in an increase in shear viscosity due to PEO coil expansion, and a strengthening of interchain interactions. In extensional flows, these interactions result in a decrease of the critical shear rate for the onset of the characteristic extension thickening of the PEO solutions that is due to transient entanglements of polymer molecules. The relaxation times associated with these transient entanglements are not directly proportional to the shear viscosity of the solutions, but rather vary more rapidly with surfactant concentration. In the presence of an electrolyte, coil contraction results in lower shear viscosities and a decrease in the extension thickening effects at surfactant concentrations just beyond the CAC. The relaxation times associated with transient entanglement reach a minimum at the same surfactant concentration as the shear viscosity, which indicates that coil contraction is responsible for the observed effects in both types of flow. However, the increase in extensional-flow entanglement relaxation times is much more abrupt than the decrease in shear viscosity. All these results point to a greater sensitivity of extensional flows on the molecular conformation of PEO/surfactant complexes.     

Relaxation time, diffusion, and viscosity analysis of model asphalt systems using molecular simulation

Molecular dynamics simulation was used to calculate rotational relaxation time, diffusion coefficient, and zero-shear viscosity for a pure aromatic compound (naphthalene) and for aromatic and aliphatic components in model asphalt systems over a temperature range of 298-443 K. The model asphalt systems were chosen previously to represent real asphalt. Green-Kubo and Einstein methods were used to estimate viscosity at high temperature (443.15 K). Rotational relaxation times were calculated by nonlinear regression of orientation correlation functions to a modified Kohlrausch-Williams-Watts function. The Vogel-Fulcher-Tammann equation was used to analyze the temperature dependences of relaxation time, viscosity, and diffusion coefficient. The temperature dependences of viscosity and relaxation time were related using the Debye-Stokes-Einstein equation, enabling viscosity at low temperatures of two model asphalt systems to be estimated from high temperature (443.15 K) viscosity and temperature-dependent relaxation time results. Semiquantitative accuracy of such an equivalent temperature dependence was found for naphthalene. Diffusion coefficient showed a much smaller temperature dependence for all components in the model asphalt systems. Dimethylnaphthalene diffused the fastest while asphaltene molecules diffused the slowest. Neat naphthalene diffused faster than any component in model asphalts.     

Dynamics of solvent and rotational relaxation of Coumarin 153 in a room temperature ionic liquid, 1-butyl-3-methylimidazolium octyl sulfate, forming micellar structure

The solvent and rotational relaxation of Coumarin 153 (C-153) was investigated by picosecond time-resolved fluorescence spectroscopy in a room temperature ionic liquid (RTIL), 1-butyl-3-methylimidazolium octyl sulfate ([C4mim][C8SO4]). This is a typical RTIL, which form micellar structure above certain concentration of the RTIL (0.031 M). Dynamic light scattering (DLS) measurements show that the average hydrodynamic diameter ( Dh) of a [C4mim][C8SO4]-water micelle is 2.8 (+/-0.2) nm. Both the solvent and rotational relaxation of C-153 are retarded in this micelle compared to the solvation time of a similar type of dye in neat water. However, the solvent relaxation in this ionic liquid surfactant is different from that of a conventional ionic surfactant. The slow component of the solvation dynamics in C8H17SO4Na or TX-100 micelle is on the nanoseconds time scale, whereas in [C4mim][C8SO4] micelle the same component is on the subnanoseconds time scale. The different molecular motions with different time scale is the main reason behind this difference in the solvation time in micelles composed of RTIL with other conventional micelles.     

Mass transport in micellar surfactant solutions: 2. Theoretical modeling of adsorption at a quiescent interface

Here, we apply the detailed theoretical model of micellar kinetics from part 1 of this study to the case of surfactant adsorption at a quiescent interface, i.e., to the relaxation of surface tension and adsorption after a small initial perturbation. Our goal is to understand why for some surfactant solutions the surface tension relaxes as inverse-square-root of time, 1/t(1/2), but two different expressions for the characteristic relaxation time are applicable to different cases. In addition, our aim is to clarify why for other surfactant solutions the surface tension relaxes exponentially. For this goal, we carried out a computer modeling of the adsorption process, based on the general system of equations derived in part 1. This analysis reveals the existence of four different consecutive relaxation regimes (stages) for a given micellar solution: two exponential regimes and two inverse-square-root regimes, following one after another in alternating order. Experimentally, depending on the specific surfactant and method, one usually registers only one of these regimes. Therefore, to interpret properly the data, one has to identify which of these four kinetic regimes is observed in the given experiment. Our numerical results for the relaxation of the surface tension, micelle concentration and aggregation number are presented in the form of kinetic diagrams, which reveal the stages of the relaxation process. At low micelle concentrations, "rudimentary" kinetic diagrams could be observed, which are characterized by merging of some stages. Thus, the theoretical modeling reveals a general and physically rich picture of the adsorption process. To facilitate the interpretation of experimental data, we have derived convenient theoretical expressions for the time dependence of surface tension and adsorption in each of the four regimes.     

Inertial and bias effects in the rotational brownian motion of rodlike molecules in a uniaxial potential

Inertial effects in the rotational brownian motion in space of a rigid dipolar rotator (needle) in a uniaxial potential biased by an external field giving rise to asymmetry are treated via the infinite hierarchy of differential-recurrence relations for the statistical moments (orientational correlation functions) obtained by averaging the Euler-Langevin equation over its realizations in phase space. The solutions of this infinite hierarchy for the dipole correlation function and its characteristic times are obtained using matrix continued fractions showing that the model simultaneously predicts both slow overbarrier (or interwell) relaxation at low frequencies accompanied by intermediate frequency Debye relaxation due to fast near-degenerate motion in the wells of the potential (intrawell relaxation) as well as the high frequency resonance (Poley) absorption due to librations of the dipole moments. It is further shown that the escape rate of a brownian particle from a potential well as extended to the Kramers turnover problem via the depopulation factor yields a close approximation to the longest (overbarrier) relaxation time of the system. For zero and small values of the bias field parameter h, both the dipole moment correlation time and the longest relaxation time have Arrhenius behavior (exponential increase with increasing barrier height). While at values of h in excess of a critical value however far less than that required to achieve nucleation, the Arrhenius behavior of the correlation time disappears.     

Effect of water, methanol, and acetonitrile on solvent relaxation and rotational relaxation of coumarin 153 in neat 1-hexyl-3-methylimidazolium hexafluorophosphate

The dynamics of solvent relaxation in ionic liquid (IL)-water, IL-methanol, and IL-acetonitrile mixtures have been investigated using steady state and picosecond time-resolved fluorescence spectroscopy. We have used Coumarin 153 (C-153) and 1-hexyl-3-methylimidazolium hexafluorophosphate ([hmim][PF(6)]) as fluorescence probe and IL, respectively. The steady-state emission spectra showed that the gradual addition of cosolvents increases the polarity of the mixtures. In neat [hmim][PF(6)] and all IL-cosolvent mixtures, solvation occurs in two well-separated time regimes within the time resolution of our instrument. A substantial portion of the solvation has been missed due to the limited time resolution of our instrument. The gradual addition of cosolvents decreases the viscosity of the medium and consequently solvation time also decreases. The decrease in solvation time is more pronounced on addition of acetonitrile compared to water and methanol. The rotational relaxation time of the probe is also decreasing with gradual addition of the cosolvents. The decrease in viscosity of the solution is responsible for the decrease in the rotational relaxation time of the probe molecule.     

Local dynamics of micelles of new long-chain surfactants in aqueous media

The molecular dynamics, organization, and phase state of aqueous solutions of new long-chain cationic surfactants with saturated hydrocarbon radicals (from C₁₆ to C₂₂) containing one or two hydroxyl groups in their polar heads are studied by the spin-probe EPR spectroscopy. In the region of micellar solutions, local mobility of surfactant molecules slightly changes with an increase in the length of hydrocarbon radical, whereas the order parameter of micelles increases notably. The addition of two hydroxyl groups to the polar part of long-chain (C ₂₂) surfactant molecule considerably decreases local mobility and increases the ordering of micellar system compared to the micelles of analogous surfactant with one hydroxyl group. Phase transition from micellar to a solid state is observed in this system with a decrease in temperature. The addition of KCl to aqueous surfactant solution lowers the local mobility, increases the order parameter of micelles, and can cause changes in the phase state of a system. In the presence of salt, the correlation time of probe rotation and its order parameter depend on surfactant concentration. Apparently, this is explained by changes in the shape of micelles upon variations in surfactant concentration.     

Dynamics of solvent and rotational relaxation of glycerol in the nanocavity of reverse micelles

The dynamics of solvent and rotational relaxation of Coumarin 480 and Coumarin 490 in glycerol containing bis-2-ethyl hexyl sulfosuccinate sodium salt (AOT) reverse micelles have been investigated with steady-state and time-resolved fluorescence spectroscopy. We observed slower solvent relaxation of glycerol confined in the nanocavity of AOT reverse micelles compared to that in pure glycerol. However, the slowing down in the solvation time on going from neat glycerol to glycerol confined reverse micelles is not comparable to that on going from pure water or acetonitrile to water or acetonitrile confined AOT reverse micellar aggregates. While solvent relaxation times were found to decrease with increasing glycerol content in the reverse micellar pool, rotational relaxation times were found to increase with increase in glycerol content.     

Sedimentation of a cylindrical particle in a Carreau fluid

The drag coefficient of an isolated, rigid cylindrical particle in a Carreau fluid is evaluated. The result of numerical simulation reveals that, in general, the shear-thinning nature of a Carreau fluid yields a drag coefficient smaller than that for the corresponding Newtonian fluid. Also, the smaller the Reynolds number, the more appreciable the decrease of the drag coefficient as the relaxation time constant of the Carreau fluid increases. The influence of the index parameter of a Carreau fluid on the drag coefficient depends largely on the magnitude of the relaxation time constant and is insensitive to the Reynolds number. Only if the relaxation time constant is sufficiently large is the influence of the index parameter on the drag coefficient significant. If the Reynolds number and/or the relaxation time constant is sufficiently large, the flow field upstream of a particle becomes asymmetric to that downstream. In general, the influence of the index parameter, the relaxation time constant, and the Reynolds number on the flow field follows the order index parameter相似文献   

Rheology of viscoelastic solutions of cationic surfactant. Effect of added associating polymer

Rheological studies were performed with aqueous salt solutions of viscoelastic cationic surfactant erucyl bis(hydroxyethyl)methylammonium chloride (EHAC) and its mixtures with hydrophobically modified polyacrylamide. The solutions of surfactant itself above the concentration of crossover of wormlike micelles exhibit two regions of rheological response. In the first region, they behave like polymer solutions in semidilute regime characterized by viscoelastic behavior with a spectrum of relaxation times. In the second region, unlike polymer solutions their relaxation after shear is dominated by a single relaxation time. Being composed of "living" micelles, the EHAC solutions easily lose their viscosity at the variation of the external conditions. For instance, heating from 20 to 60 degrees C reduces viscosity by up to 2 orders of magnitude, while added hydrocarbons induce a sudden drop of viscosity by 3-6 orders of magnitude. Polymer profoundly affects the rheological properties of EHAC solutions. The polymer/surfactant system demonstrates a 10,000-fold increase in viscosity as compared to pure-component solutions, the effect being more pronounced for polymer with less blocky distribution of hydrophobic units. A synergistic enhancement of viscosity was attributed to the formation of common network, in which some subchains are made up of elongated surfactant micelles, while others are composed of polymer. At cross-links the hydrophobic side groups of polymer anchor EHAC micelles. In contrast to surfactant itself, the polymer/surfactant system retains high viscosity at elevated temperature; at the same time it keeps a high responsiveness to hydrocarbon medium inherent to EHAC.