Investigations of Water Morphology in Reverse Micelles: Mesoscopic Simulation and IR Spectral Analysis

Abstract

A simple model, i.e., sodium bis(2‐ethylhexyl) sulfosuccinate [Aerosol OT (AOT)] represented by one‐head and two‐tail beads tied together by a harmonic spring and water or isooctane by one bead, was put forward via dissipative particle dynamics (DPD) simulation method. According to the experimental AOT/water/isooctane system, the aggregates of simulated reverse micelle can be obtained in the three‐dimensional cell. Three types of water morphology, such as bound water, trapped water, and bulky water, were distinguished using the water isodensity slice in DPD simulation. The IR spectra experiment also showed three types of water in the same system. One conclusion is that DPD simulation can be considered as an adjunct to experiments and provide other valuable information for the experiment.     

MesoDyn Simulation Study on Phase Diagram of Aerosol OT/isooctane/water System

There is a growing interest in the study of surfactant self-assemble in oil/water/surfactant system because of their applications not only in traditional colloid chemistry but in analytical, synthetic, and medicinal chemistry as well1,2. In these systems, one of the most commonly studied surfactants which can form reverse micelles is sodium bis(2-ethylhexyl) sulfosuccinate, i.e. Aerosol OT (AOT)3. The properties of the AOT reverse micelles have been discussed by some experimental methods4…     

Conserving the linear momentum in stochastic dynamics: Dissipative particle dynamics as a general strategy to achieve local thermostatization in molecular dynamics simulations

Stochastic dynamics is a widely employed strategy to achieve local thermostatization in molecular dynamics simulation studies; however, it suffers from an inherent violation of momentum conservation. Although this short‐coming has little impact on structural and short‐time dynamic properties, it can be shown that dynamics in the long‐time limit such as diffusion is strongly dependent on the respective thermostat setting. Application of the methodically similar dissipative particle dynamics (DPD) provides a simple, effective strategy to ensure the advantages of local, stochastic thermostatization while at the same time the linear momentum of the system remains conserved. In this work, the key parameters to employ the DPD thermostats in the framework of periodic boundary conditions are investigated, in particular the dependence of the system properties on the size of the DPD‐region as well as the treatment of forces near the cutoff. Structural and dynamical data for light and heavy water as well as a Lennard–Jones fluid have been compared to simulations executed via stochastic dynamics as well as via use of the widely employed Nose–Hoover chain and Berendsen thermostats. It is demonstrated that a small size of the DPD region is sufficient to achieve local thermalization, while at the same time artifacts in the self‐diffusion characteristic for stochastic dynamics are eliminated. © 2016 Wiley Periodicals, Inc.     

Phase Behavior of Amphiphilic Molecules in the Presence of One Solvent: A Dissipative Particle Dynamics Study

We simulate the phase behavior of amphiphilic molecules in the presence of one solvent by DPD. In general, DPD has successfully captured most of the effects of composition of amphiphilic molecules, solvent selectivity, and solvent amount, on the phase transition behavior obtained by both SCMF calculations and experiments. When a neutral good solvent is added, the solutions undergo a lyotropic transition analogous to the thermotropic transition in the melts. Furthermore, the order‐disorder transition results obtained via DPD are in good agreement with theoretical predictions by including the fluctuation effects, as well as with experiments. In the selective solvents, various transitions from the “normal” phases (i.e., the minority blocks form the minor domains) to even the “inverted” phases (formed by the majority blocks) have been observed by varying solvent selectivity and solvent amount. Since the packing order of the spheres is greatly affected by the finite size of the simulation box, it becomes difficult to examine the most stable packing array of spheres via DPD as has been predicted by SCMF theory. However, DPD reveals a possible spherical order of A15, which has been ignored in current SCMF work but observed in some amphiphilic molecule systems.

     

Phase behavior of lyotropic rigid-chain polymer liquid crystal studied by dissipative particle dynamics

The phase behavior of lyotropic rigid-chain liquid crystal polymer was studied by dissipative particle dynamics (DPD) with variations of the solution concentration and temperature. A chain of fused DPD particles was used to represent each mesogenic polymer backbone surrounded with the strongly interacted solvent molecules. The free solvent molecules were modeled as independent DPD particles, where each particle includes a lump of solvent molecules with the volume roughly equal to the solvated polymer segment. The simulation shows that smectic-B (S(B)), smectic-A (S(A)), nematic (N), and isotropic (I) phases exist within certain regions in the temperature and concentration parameter space. The temperature-dependent S(B)∕S(A), S(A)∕N, and N∕I phase transitions occur in the high concentration range. In the intermediate concentration range, the simulation shows coexistence of the anisotropic phases and isotropic phase, where the anisotropic phases can be the S(B), S(A), or N phases. Mole fraction and compositions of the coexisted phases are determined from the simulation, which indicates that concentration of rigid rods in isotropic phase increases as the temperature increases. By fitting the orientational distribution function of the systems, the biphasic coexistence is further confirmed. From the parameter α obtained for the simulation, the distribution of the rigid rods in the two coexistence phases is quantitatively evaluated. By using model and simulation methods developed in this work, the phase diagrams of the lyotropic rigid-chain polymer liquid crystal are obtained. Incorporating the solvent particles in the DPD simulation is critical to predict the phase coexistence and obtain the phase diagrams.     

Effect of the environment on the stability of surfactant–polypeptide self‐assembled complexes: Molecular dynamics simulations of stoichiometric complexes formed by N‐dodecyltrimethylammonium and poly(α,L‐glutamate) in chloroform,methanol, and water

A series of molecular dynamics simulations have been performed to study the supramolecular structure of self‐assembled complexes formed by N‐dodecyltrimethylammonium cations and the synthetic polypeptide poly(α,L ‐glutamate). The influence of the type of solvent has been investigated, considering explicit environments of chloroform, water, and methanol on a stoichiometric complex containing 15 residues. In chloroform, the complex stabilizes in a regular structure: the polypeptide adopts an α‐helix conformation that is regularly surrounded by surfactant molecules to form electrostatic interactions through a multiple interaction pattern. However, this structure destabilizes in methanol and water: (a) the α‐helix unfolds in the two solvents and (b) the electrostatic links between the surfactant molecules and the polyanion are disrupted in aqueous solution, although these interactions are still preserved in methanol. The role of the solvent environment in stabilizing or destabilizing the polypeptide secondary structure, the organization of the surfactant molecules, and predominantly the surfactant–polypeptide supramolecular organization is discussed in detail. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1122–1133, 2006     

Dissipative particle dynamics simulation of gold nanoparticles stabilization by PEO–PPO–PEO block copolymer micelles

Dissipative particle dynamics (DPD) was used to simulate the formation and stabilization of gold nanoparticles in poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO–PPO–PEO) block copolymer micelles. Primary gold clusters that were experimentally observed in the early stage of gold nanoparticle formation were modeled as gold bead in DPD simulation. It showed that gold beads were wrapped by the block copolymer and aggregated into spherical particles inside the micelles and forming stable Pluronic–gold colloids with two-layer structures. Increasing Pluronic concentration, molecular weight, and PPO block length led to the formation of more uniform and more stable gold nanoparticles. Density profiles of water beads suggested that the micelles, especially the hydrophobicity of the micellar cores, played an important role in stabilizing gold nanoparticles. Dynamic process indicated that the formation of gold nanoparticles was controlled by the competition between aggregation of primary gold clusters and the stabilization by micelles of block copolymers.. The DPD simulation results of gold–copolymer–water system agree well with previous experiments, while more structure information on microscopic level could be provided.     

Pressure‐induced phase separation of polymer‐solvent systems with dissipative particle dynamics

Dissipative Particle Dynamics (DPD) can be used to simulate the liquid‐liquid phase separation behaviour of polymer‐solvent systems. However, the DPD model fails to predict the lower critical solution temperature (LCST) behaviour common to many of these polymer‐solvent systems. In this article, a method analogous to the extended Flory‐Huggins theory is presented to overcome this shortcoming.     

Temperature effects on spreading of water nano‐droplet on poly(methyl methacrylate): A molecular dynamics simulation study

In this article, the effect of temperature on the spreading behavior of a water nano‐droplet on poly(methyl methacrylate) substrate is investigated. The contact angle analysis illustrates that the spreading process occurs in a stage‐like manner and the increase in temperature causes a regime change from partial to total wetting. The interaction energy distributions show that there exist sites on the surface which could trap water molecules and provide a better path for other molecules to overcome the asperities. Estimations of the coefficients of self‐diffusivity suggest that temperature has a major effect in the reorientation stage, which results in the formation of the interfacial layer. In the second stage of spreading, temperature affects the process by providing sufficient energy for water molecules to overcome the interactions with the substrate. Therefore, this stage is controlled by the movement of water molecules on the surface and is highly influenced by their interaction with the surface asperities, strong interaction sites, and the carbonyl groups. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1532–1541     

Anti‐freezing,Conductive Self‐healing Organohydrogels with Stable Strain‐Sensitivity at Subzero Temperatures

Conductive hydrogels are a class of stretchable conductive materials that are important for various applications. However, water‐based conductive hydrogels inevitably lose elasticity and conductivity at subzero temperatures, which severely limits their applications at low temperatures. Herein we report anti‐freezing conductive organohydrogels by using an H₂O/ethylene glycol binary solvent as dispersion medium. Owing to the freezing tolerance of the binary solvent, our organohydrogels exhibit stable flexibility and strain‐sensitivity in the temperature range from −55.0 to 44.6 °C. Meanwhile, the solvent molecules could form hydrogen bonds with polyvinyl alcohol (PVA) chains and induce the crystallization of PVA, greatly improving the mechanical strength of the organohydrogels. Furthermore, the non‐covalent crosslinks endow the conductive organohydrogels with intriguing remoldability and self‐healing capability, which are important for practical applications.     

Anti‐freezing,Conductive Self‐healing Organohydrogels with Stable Strain‐Sensitivity at Subzero Temperatures

Conductive hydrogels are a class of stretchable conductive materials that are important for various applications. However, water‐based conductive hydrogels inevitably lose elasticity and conductivity at subzero temperatures, which severely limits their applications at low temperatures. Herein we report anti‐freezing conductive organohydrogels by using an H₂O/ethylene glycol binary solvent as dispersion medium. Owing to the freezing tolerance of the binary solvent, our organohydrogels exhibit stable flexibility and strain‐sensitivity in the temperature range from −55.0 to 44.6 °C. Meanwhile, the solvent molecules could form hydrogen bonds with polyvinyl alcohol (PVA) chains and induce the crystallization of PVA, greatly improving the mechanical strength of the organohydrogels. Furthermore, the non‐covalent crosslinks endow the conductive organohydrogels with intriguing remoldability and self‐healing capability, which are important for practical applications.     

Polysoaps in Backbone‐Selective Solvents: Effects of Side‐Chain Length on Collapse Dynamics

Abstract

Molecular dynamics (MD) simulations of model comb‐graft heteropolymers were performed to understand general mechanistic features of coil‐to‐micelle relaxation after instantaneous quench from a nonselective solvent to solvent conditions selective for the backbone monomers and poor for the side‐chain monomers. The systems considered were single bead‐spring molecules with backbones of 30 monomers and 10 equally spaced side chains of 1, 5, 10, or 20 monomers each, immersed in dense liquids of 20,000 simple solvent particles. We find that the coil‐to‐micelle relaxation time, τ _r , averaged over 50 independent trajectories for each set of topological parameters considered, decreases with increasing side‐chain length. A two‐stage relaxation mechanism is observed: (1) a fast collapse and aggregation of neighboring side chains to form a chain of “protomicelles,” followed by (2) a slow intramolecular aggregation of protomicelles. Fast collapse dominates for molecules with relatively longer side chains due to relatively higher probabilities of initial contacts between side‐chain monomers in different side chains, while slow intramolecular aggregation dominates for molecules with relatively shorter side chains.     

An ultrafast and clean method to manufacture poly(vinyl alcohol) bead foam products

Poly (vinyl alcohol) (PVA) foam is a promising environment‐friendly packaging material due to the good biodegradability and excellent mechanical properties. Besides, PVA can be produced on a large scale viathe non‐petroleum routes. However, the preparation of complex‐shaped PVA foam products has not been realized, because PVA is a water‐soluble and semi‐crystalline polymer with a high melting temperature (226°C), which cannot be welded through the conventional bead foaming technology. In this article, a clean and efficient strategy based on microwave foaming and sintering was innovatively developed to manufacture the PVA bead foam products. First, the expandable PVA beads were prepared through polar solvent‐plasticization, followed by supercritical carbon dioxide (scCO₂)‐impregnation in solid‐state. The impregnated beads were then surface plasticized with polar solvent by simple coating. Thus, the incorporated polar solvent in the internal and superficial regions of PVA beads was rapidly heated upon exposure to the microwave irradiation, which simultaneously induced the CO₂ foaming and interfacial melting, respectively. In this way, the expansion and welding of PVA beads were completed in a one‐step procedure. Meanwhile, the complex‐shaped PVA bead foam products with excellent elasticity and intra‐bead adhesive strength were prepared within a short period of 30 seconds. Therefore, the microwave heating can be considered as an efficient strategy for preparing the high‐performance polymer bead foam products, especially for these high‐melting temperature or glass‐transition temperature polymers.     

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。     

Preparation of non‐spherical particles from amphiphilic block copolymers

Simple self‐assembly techniques to fabricate non‐spherical polymer particles, where surface composition and shape can be tuned through temperature and the choice of non‐solvents was developed. A series of amphiphilic polystyrene‐b‐poly(2‐ethyl‐2‐oxazoline) block copolymers were prepared and through solvent exchange techniques using varying non‐solvent composition a range of non‐spherical particles were formed. Faceted phase separated particles approximately 300 nm in diameter were obtained when self‐assembled from tetrahydrofuran (THF) into water compared with unique large multivesicular particles of 1200 nm size being obtained when assembled from THF into ethanol (EtOH). A range of intermediate structures were also prepared from a three part solvent system THF/water/EtOH. These techniques present new tools to engineer the self‐assembly of non‐spherical polymer particles. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 750–757     

Effects of the Bead‐Solvent Interaction on the Dynamics of Macromolecules, 1

Summary: Hamiltonian dynamics and a chain model are used to study the dynamics of macromolecules immersed in a solution. From the Hamiltonian of the overall system, “macromolecule + solvent,” a master and a Fokker‐Planck equation are then derived for the phase‐space distribution of the macromolecule. In the Fokker‐Planck equation, all the information about the interaction among the beads of the macromolecule as well as the effects of the surrounding solvent is described by friction tensors, which are expressed in terms of the bead‐solvent interaction and the dynamic structure factor of the solvent. To explore the influence of the bead‐solvent potential on the dynamics of macromolecules, the friction tensors are calculated for a dumbbell molecule and for three choices of the interaction (Yukawa, Born‐Mayer, and Lennard‐Jones). Expressions are derived, in particular, for the friction tensor coefficients of the center‐of‐mass and the relative coordinates of the dumbbell. For the long‐term behaviour of the internal momentum autocorrelation function, moreover, an “algebraic decay” is found, in contrast to the (unphysical) exponential decay as known from phenomenological theory.

Yukawa, Born‐Mayer and Lennard‐Jones bead‐solvent interaction potentials.     

Switchable Self‐Assembly of a Bioinspired Alkyl Catechol at a Solid/Liquid Interface: Competitive Interfacial,Noncovalent, and Solvent Interactions

The large tendency of catechol rings to adsorb on surfaces has been studied by STM experiments with molecular resolution combined with molecular‐dynamics simulations. The strong adhesion is due to interactions with the surface and solvent effects. Moreover, the thermodynamic control over the differential adsorption of 1 and the nonanoic solvent molecules has been used to induce a new temperature‐induced switchable interconversion. Two different phases that differ in their crystal packing and the presence of solvent molecules coexist upon an increase or decrease in the temperature. These results open new insight into the behavior of catechol molecules on surfaces and 2D molecular suprastructures.     

Water‐Binding‐Mediated Gelation/Crystallization and Thermosensitive Superchirality

Determination of molecular structural parameters of hydrophobic cholesterol–naphthalimide conjugates for water binding capabilities as well as their moisture‐sensitive supramolecular self‐assembly were revealed. Water binding was a key factor in leading trace water‐induced crystallization against gelation in apolar solvent. Ordered water molecules entrapped in self‐assembly arrays revealed by crystal structures behave as hydrogen‐bonding linkers to facilitate three‐dimensional growth into crystals rather than one‐dimensional gel nanofibers. Water binding was also reflected on the supramolecular chirality inversion of vesicle self‐assembly in aqueous media via heating‐induced dehydration. Structural parameters that favor water binding were evaluated in detail, which could help rationally design organic building units for advancing soft materials, crystal engineering, and chiral recognition.     

The Solvent‐Dependent Shift of the Amide I Band of a Fully Solvated Peptide as a Local Probe for the Solvent Composition in the Peptide/Solvent Interface

We determine the shift and line shape of the amide I band of a model AK peptide from molecular dynamics (MD) simulations of the peptide dissolved in methanol/water mixtures with varying composition. The IR spectra are determined from a transition dipole coupling exciton model. A simplified empirical model Hamiltonian is employed, which takes into account both the effect of hydrogen bonding and the intramolecular vibrational coupling. We consider a single isolated AK peptide in a mostly helical conformation, while the solvent is represented by 2600 methanol or water molecules, simulated for a pressure of 1 bar and a temperature of 300 K. Over the course of the simulations, minor reversible conformational changes at the termini are observed, which are found to only slightly affect the calculated spectral properties. Over the entire composition range, which varies from pure water to the pure methanol solvent, a monotonous shift towards higher frequency of the IR amide I band of about 8 wavenumbers is observed. This shift towards higher frequency is comparable to the shift found in preliminary experimental data also presented here on the amide I′ band. The shift is found to be caused by two counter‐compensating effects. An intramolecular red shift of about 1.2 wavenumbers occurs, due to stronger intramolecular hydrogen bonding in a methanol‐rich environment. Dominating, however, is the intermolecular solvent‐dependent shift towards higher frequency of about 10 wavenumbers, which is attributed to the less effective hydrogen‐bond‐donor capabilities of methanol compared to water. The importance of the solvent contribution to the IR shift, as well as the significantly different hydrogen formation capabilities of water and methanol, makes the amide I band sensitive to composition changes in the local environment close to the peptide/solvent interface. This allows, in principle, an experimental determination of the composition of the solvent in close proximity to the peptide surface. For the AK peptide case, we observe at low methanol concentrations a significantly enhanced methanol concentration at the peptide/solvent interface, supposedly promoted by the partially hydrophobic character of the AK peptide’s solvent‐accessible surface.