Long-range hydration effect of lipid membrane studied by terahertz time-domain spectroscopy

The hydration state of biomolecules is believed to affect their self-assembly. The hydration state of phospholipid bilayers is studied precisely by terahertz spectroscopy, by which water perturbed by a lipid membrane is detected sensitively from the observation of the relaxation dynamics of water molecules in the subpicosecond time scale. Combined with x-ray observation of the lamellar structure of the lipid, a long-range hydration effect on up to 4-5 layers of water is confirmed. Most water molecules in the lamellae fall into the hydration water, and condensation of them is also indicated.     

Dissipative particle dynamics simulation for the effect of interaction on the self-assembly behaviours of heterogemini surfactant in aqueous solution

Self-assembly behaviours of heterogemini surfactant in aqueous solution have been explored by means of dissipative particle dynamics simulation. Five kinds of heterogemini molecules are involved and a variety of novel morphologies have been obtained. Results based on detailed analyses show that head–tail and tail–water interactions play different roles in the formation of six kinds of self-assembly morphologies. The key factors are tail–water and head–tail repulsions separately for morphology with small (sphere) and relatively large scales (rod, planar grid, lamella and tunnels). Besides, the appearance of network in only one system can be regarded as a particular type of intermediate state. Coexistence of several sphere micelles in an amplified dissipative particle dynamics system gives us a better understanding of interactions inside the soft matter. Our simulation results can provide a theoretical guide to further research towards self-assembly behaviours of heterogemini surfactants and practical applications of these matters.     

Anomalous temperature dependence of surfactant self-assembly from aqueous solution

The self-assembly of surfactant molecules from aqueous solutions exhibits anomalous and as yet unexplained temperature dependence. These effects include (a) a decrease in surfactant solubility, (b) an increase of the critical micelle concentration, and (c) increasing self-assembly on solid surfaces with increasing temperature near ambient temperature. These are the opposite of what one intuitively expects based on increasing molecular motion with temperature, and of what is observed for simpler fluids. Using a lattice gas theory that allows for inhomogeneity, hydrogen bonding, and micelle formation, we show conclusively that all of these phenomena arise from hydrogen bonding between water and the surfactant headgroups. While theory and experiment show very good qualitative agreement when such H bonding is included, its omission leads to complete reversal of the temperature dependence.     

硫杂杯[4]芳烃基胶束自组装荧光探针的结构性质及检测性能的研究

水体中重金属污染因威胁生态环境和人类健康而被受广泛关注。荧光探针由于具有快速高效检测重金属的特性,一直是该领域的研究热点。通常,荧光探针在结构上包括对待测物质起识别作用的受体和能产生信号响应的荧光体,并逐步形成了内在型、共轭型、系综型和模板辅助自组装型等四种结构类型。近年来,基于受体和荧光体在表面活性剂胶束内自组装而形成的胶束自组装型荧光探针因结构简单、易于制备、能直接应用于水环境等特点逐渐受到重视。以对铜离子具有优异结合性能的对叔丁基硫杂杯[4]芳烃（TCA）为受体,以芘、荧蒽、蒽、菲、苝等分子为荧光体,通过表面活性剂胶束自组装制备针对Cu2+检测的胶束自组装型荧光探针,采用参比法测定了胶束自组装荧光探针的荧光量子产率,采用稳态荧光法测定了胶束聚集数,同时通过计算荧光猝灭率分别考察了荧光体种类、复配表面活性剂等因素对该探针的Cu2+检测性能的影响情况。实验结果显示,采用十二烷基硫酸钠（SDS）、曲拉通100（TX-100）、聚氧乙烯月桂醚（Brij35）等三种不同的表面活性剂对探针荧光体的荧光量子产率产生了明显影响,测得的荧光探针荧光量子产率介于0.25~0.47,且三者逐渐增大,说明表面活性剂改变了胶束内荧光分子芘所处微环境的极性,且不同类型表面活性剂对微环境极性的影响程度有所差异,微环境极性的增强对极性更大的激发态芘具有更强的稳定作用。而受体TCA的加入对荧光体所处微环境极性影响较小,未对荧光量子产率产生较大影响。但TCA的加入使探针的胶束聚集数明显减少,这归因于具有两亲性的受体TCA分子通过胶束自组装进入并分散在表面活性剂分子层中,形成共胶束结构,从而改变了表面活性剂分子的聚集状态。荧光体变更对荧光探针的Cu2+检测性能有显著影响,在同样条件下,以荧蒽、蒽、菲作为荧光体的探针检测Cu2+所得到的荧光猝灭率远高于芘、苝,这主要是因为不同荧光体在从激发态返回基态时辐射跃迁所释放能量不同,其能量与受体TCA识别Cu2+所需能量之间的匹配度越高,荧光猝灭率越大。不同类型的表面活性剂之间的复配能明显提升荧光探针检测性能,当非离子/阴离子、非离子/阳离子型复配表面活性剂之间的复配比例分别为7∶3和1∶1时荧光猝灭率达到最大值,且均高于单一表面活性剂时的荧光猝灭率。这说明不同类型表面活性剂复配的最佳比例存在较大差异,但均有效地增强了受体与荧光体的分散性及自组装性能,提高了对Cu2+的检测性能。研究结果将为新型胶束自组装荧光探针的设计和应用提供数据参考。     

Origin of the short-range, strong repulsive force between ionic surfactant layers

We study the electrostatic interaction between two ionic surfactant layers by performing molecular dynamic simulations of salt-free thin water films coated by surfactants (Newton black films). We find a strong exponentially decaying short-range repulsion not explained by classical Poisson-Boltzmann theory. This electrostatic force is shown to be mainly due to the anomalous dielectric response of water near charged surfactant layers. This result clarifies the much debated physical mechanism underlying the controversial "hydration forces" observed in experiments. In the case of ionic thin films, the "hydration forces" can be identified with the electrostatic forces induced by the layers of highly polarized water originated at the interfaces.     

Potential controlled surface aggregation of surfactants at electrode surfaces – A molecular view

By describing studies of three prototypical surfactants with similar hydrophobic tails but differently charged headgroups, this review provides a summary of the rich phase behavior of soluble surfactant molecules at electrified interfaces. With the use of electrochemical, scanning probe microscopy, and neutron scattering techniques we have been able to fully explore the adsorption and surface aggregation of these molecular systems. Furthermore, we have been able to provide compelling evidence of electric field-driven phase transitions in these surfactant films and their aggregated structures. Cumulatively, our results demonstrate that the electrical state of a surface (namely surface charge or applied potential) plays an integral role in determining the morphology of surfactants at solid interfaces. Unlike other aggregate shape determining factors such as the surfactant packing parameter, the electrical parameter can readily be adjusted in situ, providing a tunable means to control films of soft condensed matter.     

Glass transition in biomolecules and the liquid-liquid critical point of water

Using molecular dynamics simulations, we investigate the relation between the dynamic transitions of biomolecules (lysozyme and DNA) and the dynamic and thermodynamic properties of hydration water. We find that the dynamic transition of the macromolecules, sometimes called a "protein glass transition," occurs at the temperature of dynamic crossover in the diffusivity of hydration water and also coincides with the maxima of the isobaric specific heat C_{P} and the temperature derivative of the orientational order parameter. We relate these findings to the hypothesis of a liquid-liquid critical point in water. Our simulations are consistent with the possibility that the protein glass transition results from crossing the Widom line, which is defined as the locus of correlation length maxima emanating from the hypothesized second critical point of water.     

Theory of interfacial phase transitions in surfactant systems

The spin-1 Ising model, which is equivalent to the three-component lattice gas model, is used to study wetting transitions in three-component surfactant systems consisting of an oil, water, and a nonionic surfactant. Phase equilibria, interfacial profiles, and interfacial tensions for three-phase equilibrium are determined in mean field approximation, for a wide range of temperature and interaction parameters. Surfactant interaction parameters are found to strongly influence interfacial tensions, reducing them in some cases to ultralow values. Interfacial tensions are used to determine whether the middle phase, rich in surfactant, wets or does not wet the interface between the oil-rich and water-rich phases. By varying temperature and interaction parameters, a wetting transition is located and found to be of the first order. Comparison is made with recent experimental results on wetting transitions in ternary surfactant systems.This paper is dedicated to J. K. Percus in honor of his 65th birthday.     

Surface triple points and multiple-layer transitions observed by tuning the surface field at smectic liquid-crystal-water interfaces

We present an ellipsometric study of the interface between a smectic liquid crystal and water in the presence of a nonionic surfactant. The surfactant concentration serves as a handle to tune the surface field. For sufficiently large surfactant concentrations, a smectic phase is present at the interface in the temperature range above the smectic-A-isotropic bulk transition; when the bulk transition is approached, the thickness of this surface phase grows via a series of layer-by-layer transitions at which single smectic layers are formed. At lower surfactant concentrations, transitions appear at which the thickness of the surface phase jumps by multiple smectic layers, thereby implying the existence of triple points at which surface phases with different smectic layer numbers coexist. This is the first experimental demonstration of such surface triple points which are predicted by theoretical models.     

Structural transitions of solvent-free oligomer-grafted nanoparticles

Novel structural transitions of solvent-free oligomer-grafted nanoparticles are investigated by using molecular dynamics simulations of a coarse-grained bead-spring model. Variations in core size and grafting density lead to self-assembly of the nanoparticles into a variety of distinct structures. At the boundaries between different structures, the nanoparticle systems undergo thermoreversible transitions. This structural behavior, which has not been previously reported, deviates significantly from that of simple liquids. The reversible nature of these transitions in solvent-free conditions offers new ways to control self-assembly of nanoparticles at experimentally accessible conditions.     

局限空间中水的相变与动态学研究

在本篇论文中,主要讨论水分子在局限空间中的运动及相变行为,所使用的方法为双量子过滤核磁共振光谱及T₁反转回覆光谱,体系为利用MCM-41吸附不同量的重水. 本研究中所使用的这两种光谱方法各有其独特之处,其中双量子过滤核磁共振光谱是利用被吸附的重水分子中氘核残余四极作用力所产生双量子讯号进行侦测,因此是特别针对表面吸附的水分子进行观测, 而T₁反转回覆光谱则是侦测整体孔洞內水分子的行为,借由两种光谱的谱线分析相互比对,得到在MCM-41內各层水分子对温度变化的完整动态学行为. 在描述表面水分子运动上,采用的是修改过的锥体模型,主要将水分子分成相对其对称轴的摇摆运动及旋转运动,在研究中发现,表面水分子是被MCM-41表面单独的SiOH所吸附,比例上为一个水分子对一个SiOH,而表面分子的运动会受到第二层水分子的形成与否所影响,一但第二层水分子 的量够多时,其平移扩散运动会借由碰撞影响表面水分子的摇摆运动,而且在孔洞中属于非表面吸附的水分子,在随温度变化至240~250 K之间时会有相变发生,相变的温度则会随水量充填在MCM-41內的多寡而改变,当水分子越多时,因为彼此间空间有限,使得氢键网路结构与一般正常在大量水分子体系的结构有些相异,因此使得相变温度会随之下降. 除此之外,本研究中亦提出不同的双量子过滤核磁共振脉冲序列以及不同的模型来讨论.     

Ultralow elastic stability of salt ice at low temperatures

A model is developed to describe the elastic stability of saline ice with a homogeneous distribution of the components in the volume at high pressures (P > 0.1 GPa) and low temperatures (T < 220 K). This model is based on the basic principles of the theory of elastic percolation and the theory of ion hydration (formation of spherical clusters with water molecule dipoles oriented toward ions) in a liquid solution. This model can be used to find the dependence of the elastic stability of an ice solution on the concentration and temperature for any salts decomposing into ions with various valences and effective radii. Good agreement between the calculated and experimental dependences indicates the existence of hydration spheres in an ice solution, which is characterized by a narrower size distribution of spheres as compared to the liquid state and by a steeper decrease in the sphere size when the salt concentration increases in the range x ?? 0.001?C0.01. The stability of a solid ice solution depends on the temperature and the salt concentration in a complicated manner, having specific features in the form of maxima near phase transitions in a water matrix as a result of competing effects that enhance or weaken the elastic contributions of the frozen spheres. As follows from the model calculations, solid ice solutions can have an ultralow elastic stability (which is lower than that of pure water ice by a factor of 5?C30), which was experimentally detected even at low weight fractions of salts at a level of x ?? 0.0001?C0.01 for T < 220 K.     

The influence of macromolecular polymerization of spin-lattice relaxation of aqueous solutions

The docking or polymerization of globular proteins is demonstrated to cause changes in proton NMR spin-lattice (T1) relaxation times. Studies on solutions of lysozyme, bovine serum albumin, actin, and tubulin are used to demonstrate that two mechanisms account for the observed changes in T1. Polymerization displaces the hydration water sheath surrounding globular proteins in solution that causes an increase in T1. Polymerization also slows the average tumbling rate of the proteins, which typically causes a contrary decrease in T1. The crystallization reaction of lysozyme in sodium chloride solution further demonstrates that the "effective" molecular weight can either decrease or increase T1 depending on how much the protein is slowed. The displacement of hydration water increases T1 because it speeds up the mean motional state of water in the solution. Macromolecular docking typically decreases T1 because it slows the mean motional state of the solute molecules. Cross-relaxation between the proteins and bound water provides the mechanism that allows macromolecular motion to influence the relaxation rate of the solvent. Fast chemical exchange between bound, structured, and bulk water accounts for monoexponential spin-lattice relaxation. Thus the spin-lattice relaxation rate of water in protein solutions is a complex reflection of the motional properties of all the molecules present containing proton magnetic dipoles. It is expected, as a result, that the characteristic relaxation times of tissues will reflect the influence of polymerization changes related to cellular activities.     

Dependence of thermodynamic properties of model systems on some dissipative particle dynamics parameters

The influence of systematic perturbation of input interaction parameters on thermodynamic equilibrium properties is studied employing dissipative particle dynamics (DPD) simulations. The values of both the excess pressure and the surface tension are found to be very sensitive to the values of the soft repulsion parameter between unlike DPD particles for high values of the coarse-graining level (number of water molecules per DPD particle). For the case in which a molecular surfactant is present at the interface we have determined the dependence of these properties on the values of the parameters that characterize the bonding force between polymer beads. No significant differences were found between linear and branched surfactants.     

Solvent-induced DNA conformational transition

Modified water models with scaled charges are used to investigate solvent polarity effects on DNA structure. Several intensive molecular dynamics simulations of the DNA EcoRI dodecamer d(CGCGAATTCGCG) in different model solvents are performed. When the polarity of the solvent molecule decreases, from overpolarized to less polarized, DNA experiences the conformational transitions of constrained-->B form-->(A-B)mix-->A form. We demonstrate that one important cause of these structure changes is the competition between hydration and direct cation coupling to the free oxygen atoms in the phosphate groups on DNA backbones.     

Anharmonic transitions in nearly dry L-cysteine I

Two special dynamical transitions of universal character have recently been observed in macromolecules (lysozyme, myoglobin, bacteriorhodopsin, DNA and RNA) at T* ~100-150 K and T(D) ~180-220 K. The underlying mechanisms governing these transitions have been the subject of debate. In the present work, a survey is reported on the temperature dependence of structural, vibrational and thermodynamical properties of a nearly anhydrous amino acid (orthorhombic polymorph of the amino acid l-cysteine at a hydration level of 3.5%). The temperature dependence of x-ray powder diffraction patterns, Raman spectra and specific heat revealed these two transitions at T* = 70 K and T(D) = 230 K for this sample. The data were analyzed considering amino acid-amino acid, amino acid-water, water-water phonon-phonon interactions and molecular rotor activation. Our results indicated that the two referred temperatures define the triggering of very simple and particular events that govern all the interactions of the biomolecular: activation of CH(2) rigid rotors (T < T* ), phonon-phonon interactions between specific amino acid and water dimer vibrational modes (T* < T < T(D)), and water rotational barriers surpassing (T > T(D)).     

Microphase separation in polymer solutions containing surfactants

A microphase separation in solutions containing a polymer and a mixture of two solvents, one of which consists of amphiphilic molecules (surfactant), is considered theoretically in the weak-segregation regime. A surfactant molecule is described as a dimer consisting of hydrophobic and polar parts. The energy gain due to the orientation of surfactant molecules can lead to the appearance of non-homogeneities in the solution, where density fluctuations cause the orientational ordering of surfactant molecules. The difference in the interaction energies of hydrophobic and polar groups of a surfactant with solvent is considered as a main reason for orienting surfactant molecules. The free energy is calculated for various morphologies (lamellar, cylindrical hexagonal, spherical particles arranged at different cubic lattices). The phase diagrams are presented. With worsening the solvent quality, the transitions from disordered to a macro-separated state at low polymer and surfactant concentrations or to a body-centered-cubic, then hexagonal, and then lamellar structure at high polymer and surfactant concentrations are predicted. The amphiphilicity degree of surfactant molecules should exceed a certain critical value to make a microstructure formation possible. The period of the lamellar microstructure decreases with increasing the surfactant and polymer concentrations.     

Light scattering and fluorescence probe studies on micellar properties of Triton X-100 in KCl solutions

The effect of KCl on micelle formation and structure of Triton X-100 (TX-100) was investigated by using combined static and dynamic light scattering measurements, together with the fluorescence probe technique. An analysis of the light scattering data, including hydrodynamic radius and micellar aggregation number, accounted for both micelle growth and hydration. Fluorescence studies using pyrene as a probe were carried out to determine the critical micelle concentration (CMC) as a function of solution composition. In addition, with the aim of gaining information on the possible changes in the micro-environmental properties of TX-100 micelles, fluorescence probe studies, including intermolecular pyrene excimer formation and fluorescence polarization of coumarin 6 associated with micelles, were carried out. It was found that the addition of electrolyte induces a decrease in the CMC and an increase in both aggregation number and hydration. However, complementary data of partial specific volume and cloud point of the surfactant suggested that the main contribution to micellar hydration is due to water mechanically trapped in the micelle. Fluorescence measurements do not indicate changes in the micellar micropolarity, probably due to modifications of the solubilization site of the probe caused by the micellar growth. Both pyrene excimer formation and fluorescence polarization of coumarin 6 revealed an increase in microviscosity with electrolyte addition, which is consistent with increased micellar hydration.     

Effect of electric and magnetic fields on the orientation structure of a ferronematic liquid crystal

We analyze uniform orientation phases in soft ferronematics (suspensions of magnetic nanoparticles in nematic liquid crystals) induced by electric and magnetic fields. It is shown that the competition between the electric and magnetic fields can lead to various sequences of orientation transitions in a ferronematic depending on the energy of coupling between the director and magnetization. We obtain and analyze phase diagrams of these transitions. A sequence of re-entrant transitions in the orientation structure (angular phase-homeotropic phase-angular phase-planar phase) is predicted for a certain range of the coupling energies and electric field strengths.