Collective membrane motions of high and low amplitude, studied by dynamic light scattering and micro-interferometry

Undulations of lipid bilayers were experimentally studied for the two limiting cases of high and weak lateral tension using two well established model systems: freely suspended planar lipid bilayers, so-called black lipid membranes (BLM) for high-tension studies and large unilamellar vesicles (LUV) for measurements at weak tension. This variation in tension results in changes of undulation amplitudes from several hundred nm (LUV) down to 1 nm (BLM), thus requiring different physical methods for their detection. We have employed microinterferometric techniques (RICM) for studying the regime of weak tension and dynamic light scattering (DLS) for that of high tension. The dedicated DLS set-up allowed the measurements of undulations over a wide wave vector range of 250 < q/cm-1 < 35,000 cm-1. This enabled the observation of collective membrane modes in two regimes, the oscillating one at low q and the overdamped regime at high q. The transition between both regimes at the bifurcation point is rather abrupt and depends on the lateral tension of the bilayer, as is demonstrated by comparing the dispersion curves of pure lipid and of lipid-cholestrol BLMs over the same q-range. The DLS measurements allowed a critical test of a hydrodynamic theory of the dispersion behaviour of membrane collective modes under tension. The DLS measurements are compared with RICM results of undulatory excitations of giant vesicles weakly adhering to substrates in the 10(-6)-2.5 x 10(-7) m wavelength regime and at low frequencies (0.1-25 Hz). Experimental evidence for the strong decrease in the relaxation rate by the hydrodynamic coupling of the membrane with the wall is established.     

Renormalized one-loop theory of correlations in disordered diblock copolymers

A renormalized one-loop (ROL) theory developed in previous work [P. Grzywacz, J. Qin, and D. C. Morse, Phys. Rev E. 76, 061802 (2007)] is used to calculate corrections to the random phase approximation (RPA) for the structure factor S(q) in disordered diblock copolymer melts. Predictions are given for the peak intensity S(q?), peak position q?, and single-chain statistics for symmetric and asymmetric copolymers as functions of χ(e)N, where χ(e) is an effective Flory-Huggins interaction parameter and N is the degree of polymerization. The ROL and Fredrickson-Helfand (FH) theories are found to yield asymptotically equivalent results for the dependence of the peak intensity S(q?) upon χ(e)N for symmetric diblock copolymers in the limit of strong scattering, or large χ(e)N, but to yield qualitatively different predictions for symmetric copolymers far from the ODT and for asymmetric copolymers. The ROL theory predicts a suppression of S(q?) and a decrease of q? for large values of χ(e)N, relative to the RPA predictions, but an enhancement of S(q?) and an increase in q? for small χ(e)N. The decrease in q? near the ODT is shown to be unrelated to any change in single-chain statistics, and to be a result of inter-molecular correlations. Conversely, the predicted increase in q? at small values of χ(e)N is a direct result of non-Gaussian single-chain statistics.     

Magnetically aligned phospholipid bilayers with large q ratios stabilize magnetic alignment with high order in the gel and L(alpha) phases

The magnetic alignment behavior ofbicelles (magnetically alignable phospholipid bilayered membranes) as a function of the q ratio (1,2-dihexanoyl-sn-glycerol phosphatidylcholine/1,2-dimyristoyl-sn-glycerol phosphatidylcholine mole ratio) and temperature was studied by spin-labeled X-band electron paramagnetic resonance (EPR) spectroscopy and solid-state 2H and 31P NMR spectroscopy. Well-aligned bicelle samples were obtained at 45 degrees C for q ratios between 2.5 and 9.5 in both the EPR and NMR spectroscopic studies. The molecular order of the system, S(mol), increased as the q ratio increased and as the temperature decreased. For higher q ratios (> or = 5.5), bicelles maintained magnetic alignment when cooled below the main phase transition temperature (approximately 30 degrees C when in the presence of lanthanide cations), which is the first time, to our knowledge, that bicelles were magnetically aligned in the gel phase. For the 9.5 q ratio sample at 25 degrees C, S(mol) was calculated to be 0.83 (from 2H NMR spectra, utilizing the isotopic label perdeuterated 1,2-dimyristoyl-sn-glycerol phosphatidylcholine) and 0.911 (from EPR spectra utilizing the spin probe 3beta-doxyl-5alpha-cholestane). The molecular ordering of the high q ratio bicelles is comparable to literature values of S(mol) for both multilamellar vesicles and macroscopically aligned phospholipid bilayers on glass plates. The order parameter S(bicelle) revealed that the greatest degree of bicelle alignment was found at higher temperatures and larger q ratios (S(bicelle) = -0.92 for q ratio 8.5 at 50 degrees C).     

Concentration fluctuations in polymer gel investigated by neutron scattering: static inhomogeneity in swollen gel

By using small-angle neutron scattering (SANS) and neutron spin echo (NSE), we have quantitatively investigated the static inhomogeneity in poly (N-isopropyl acrylamide) gel (PNIPA) in microscopic length scales of 0.015相似文献   

表面活性剂在低能固体表面上的吸附Ⅰ. γ-θ法研究辛基硫酸钠、溴化辛基三甲胺及其等比混合物在石蜡/水溶液界面上的吸附

(1) 用γ-θ法研究C_8H_(17)SO_44Na、C_8H_(17)N(CH_3)_3Br及其1:1混合物在石蜡/水溶液界面上的吸附, 得到Langmuir型吸附等温线。 (2) 首次计算两表面活性剂在固/液界面吸附层中的分子相互作用参数, 并与它们在气/液、液/液界面吸附层中的数值相比较。 (3) 结果表明γ-θ法不仅简单、方便、样品用量少, 而且有获得信息多的特点, 用几毫升样品可得到有关体系表面性质的十多种数据。     

Adsorption of anionic and cationic surfactants on anionic colloids: supercharging and destabilization

We present herein a study on the adsorption of anionic (SDS), cationic (CTAB), and nonionic (C(12)E(5)) surfactants onto anionic silica nanoparticles. The effects of this adsorption are studied by means of the static structure factor, S(q), and the collective diffusion coefficient, D(c), obtained from small-angle X-ray scattering and dynamic light scattering measurements, respectively. The effective charge on the particles was determined also from classical electrophoresis and electroacoustic sonic-amplitude measurements. The surface tension of the sample was also investigated. Of particular note is the adsorption of SDS onto the silica nanoparticles, which leads to supercharging of the interface. This has interesting repercussions for structures obtained by the layer-by-layer (LbL) technique, because emulsions stabilized with supercharged and hydrophobized silica are perfect candidates for use in a multilayer system.     

Evaluation of surface tension and Tolman length as a function of droplet radius from experimental nucleation rate and supersaturation ratio: metal vapor homogeneous nucleation

Zinc and silver vapor homogeneous nucleations are studied experimentally at the temperature from 600 to 725 and 870 K, respectively, in a laminar flow diffusion chamber with Ar as a carrier gas at atmospheric pressure. The size, shape, and concentration of aerosol particles outcoming the diffusion chamber are analyzed by a transmission electron microscope and an automatic diffusion battery. The wall deposit is studied by a scanning electron microscope (SEM). Using SEM data the nucleation rate for both Zn and Ag is estimated as 10(10) cm(-3) s(-1). The dependence of critical supersaturation on temperature for Zn and Ag measured in this paper as well as Li, Na, Cs, Ag, Mg, and Hg measured elsewhere is analyzed. To this aim the classical nucleation theory is extended by the dependence of surface tension on the nucleus radius. The preexponent in the formula for the vapor nucleation rate is derived using the formula for the work of formation of noncritical embryo [obtained by Nishioka and Kusaka [J. Chem. Phys. 96, 5370 (1992)] and later by Debenedetti and Reiss [J. Chem. Phys. 108, 5498 (1998)]] and Reiss replacement factor. Using this preexponent and the Gibbs formula for the work of formation of critical nucleus the dependence of surface tension on the radius R(S) of the surface of tension is evaluated from the nucleation data for above-mentioned metals. For the alkali metals and Ag the surface tension was determined to be a strong function of R(S). For the bivalent metals (Zn, Hg, and Mg) the surface tension was independent of radius in the experimental range. A new formula for the Tolman length delta as a function of surface tension and radius R(S) is derived by integration of Gibbs-Tolman-Koenig equation assuming that delta is a monotonic function of radius. The formula derived is more correct than the Tolman formula and convenient for the elaboration of experimental data. Using this formula the values of delta are determined as a function of R(S) from the experimental nucleation data. It is determined that all the metals considered are characterized by strong dependence of delta on radius; for the bivalent metals delta changes sign.     

Universal growth of microdomains and gelation transition in agar hydrogels

Investigations were carried out on aqueous sols and gels of agar (extracted from red seaweed Gelidiella acerosa) to explore the growth of microdomains en route to gelation. Isothermal frequency sweep studies on gel samples revealed master plots showing power-law dependence of gel elastic modulus, |G*|, on oscillation frequency, omega as |G*| approximately omegan, independent of temperature, with 0.5Tg). The S(q,t) behavior close to the gel transition point (Tg approximately (38+/-3 degrees C determined from rheology) followed a stretched exponential function: S(t)=A exp(-t/ts)beta. The beta factor increased from 0.25 to 1 as the gel temperature approached 25 degrees C from Tg, and relaxation time, ts, showed a peak at T approximately 30 degrees C. The SLS data (in the sol state) suggested the scaling of scattered intensity, Is(q) approximately epsilon(-gamma) (epsilon=(T/Tg-1), T>Tg) with gamma=0.13+/-0.03, and the presence of two distinct domains characterized by a Guinier regime (low q) and a power-law regime (high q). Close to and above Tg (+2 degrees C), IS(q) scaled with q as Is(q) approximately q(-alpha) with alpha=2.2+/-0.2, which decreased to 1.4+/-1 just below Tg (-2 degrees C), implying a coil-helix transition for 0.2% (w/v) and 0.3% (w/v) samples. For a 0.01% sample, alpha=3.5+/-0.5 which indicated the presence of spherical microgels.     

Investigation of structure factors in dense colloidal suspensions using frequency domain photon migration: polydisperse systems

Frequency domain photon migration (FDPM) technique was employed to investigate the structure factors of dense, polydisperse colloidal suspensions. The angle-integrated structure factors, [S(q)], extracted from FDPM measurements of scattering properties at volume fractions ranging from 0.05 to 0.4, were compared with the values predicted from the polydisperse hard sphere Percus-Yevick (HSPY) model, as well as decoupling approximation (DA) and local monodisperse approximation (LMA) models that incorporated independently measured particle size information. Results show that the polydisperse HSPY model is the most suitable for accounting for particle interactions which predominantly arise from volume exclusion effects. Furthermore, the influence of size polydispersity upon [S(q)] is most significant at high volume fractions. The static structure factors at small wave vector q, S(0), were also assessed from dual wavelength FDPM measurements by using the small wave number approximation as well as the local monodisperse approximation. The measured S(0) agrees well with the values predicted by the polydisperse HSPY model.     

Atomic Force Microscopy Study of the Adhesion of Saccharomyces cerevisiae

The influence of the liquid properties and the operating variables on the intrinsic volumetric flow rate, q(0), of the droplets at the liquid-atmosphere interface in nonfoaming adsorptive bubble separation and that, W(f0), of the liquid in foam at the liquid-foam interface in foam separation was studied to estimate the enrichment ratio of surface-active substance in the two techniques. Each intrinsic flow rate was determined by the extrapolation method, which the authors proposed previously, and was mainly influenced by superficial gas velocity, liquid viscosity, and surface tension. Although the changes in the surface tension and the liquid viscosity were small, they greatly affected the intrinsic flow rates for both the droplets and the liquid in foam. The experimental results were applied to a dimensional analysis. Dimensionless volumetric flow rates, q(0)/Q and W(f0)/Q, were successfully correlated with a dimensionless number (Ohnesorge number, Z=&mgr;(L)/(rho(L)sigmad(b))(0.5)). q(0)/Q=6.66 Z(1.46) and W(f0)/Q=2.53Z(0.533) were obtained for the nonfoaming and the foaming regions within errors of 30 and 35%, respectively. The enrichment ratio of surface-active substance in the droplets and the liquid in foam to the bulk liquid was derived by using these correlations for the nonfoaming and the foaming regions, respectively. A good agreement of the experimental enrichment ratio and the calculated ones was obtained by using the correlations for nonfoaming and foaming. Copyright 2001 Academic Press.     

Dynamical Structure Factor of Fluid Ar36

Abstract

The continued fraction representation of the correlation function and a secant hyperbolic form of the memory function have been used to calculate the dynamical structure factor S(q, ω) of fluid Ar³⁶ for the densities ranging from n? = 0.668 to 0.763 and a temperature to 120 K. The parameters of the memory function have been determined from the fourth and sixth frequency sum rules of S(q, ω). The predicted results for S(q, ω) have been compared with recent neutron scattering data. A good agreement has been achieved. Further, it is also found that the position of first minima of full width at half maxima of S(q, ω) shifts towards smaller wave number side with decrease in density while, the position of first maxima is independent of density.     

丙烯酰胺型阴离子表面活性单体的化学结构与其胶束化行为

对两种丙烯酰胺型阴离子表面活性单体(2-丙烯酰胺基十四烷磺酸钠, NaAMC₁₄S; 2-丙烯酰胺基十二烷磺酸钠, NaAMC₁₂S)的化学结构与胶束化行为的关系进行了较深入的研究. 使用紫外分光光度法测定了NaAMC₁₄S, NaAMC₁₂S及十二烷基磺酸钠(SDS)在水中的溶解度, 同时采用表面张力法(环法)测定了它们在不同温度下的临界胶束浓度CMC; 采用稳态荧光探针法测定了不同浓度的胶束聚集数与本征胶束聚集数. 实验结果表明, 与普通表面活性剂相比, 由于丙烯酰胺型阴离子表面活性单体分子中具有两个亲水头基, 在水中的溶解性能较强, 故具有较低的Krafft温度; 在溶液表面的饱和吸附量低, 故降低水表面张力的能力较差, 即表面活性差; 疏水缔合的胶团较为疏松, 故聚集数很小; 胶束内分子间的疏水相互作用较弱, 故临界胶束浓度CMC较高.     

Exploring the thermodynamic derivatives of the structure factor of dense protein solutions

Using small-angle X-ray scattering data of concentrated solutions of the protein lysozyme taken at different pressures and temperatures, the isothermal pressure derivative and the isobaric temperature derivative of the structure factor S(q) were determined. The pressure derivative of S(q) allows us to test various models for the triplet correlation function g(3). Significant differences were found in comparison to simple liquids reflecting the more complex interaction potential in dense protein solutions.     

Size dependence, stability, and the transition to buckling in model reverse bilayers

Molecular dynamics simulations of a model bilayer made of surfactant dimers in a Lennard-Jones solvent are reported for three sizes of the systems up to an area of 100sigma x 100sigma and for a large interval of the specific areas: from hole formation under tension deep into the floppy state of a buckling compressed bilayer. The transition to the floppy state appears quite abrupt and discontinuous; in the floppy state the lateral tension is negative and scales with size while vanishing from below. The structure factor was also determined for all three sizes and all areas; for most part the apparent tension is larger than the lateral tension whereas the apparent rigidity constant--always positive--is low in the floppy state and increasing in the tensioned state. Both do not scale visibly with size. The replacement of the 1q(2) capillary-wave divergence by another pole is accounted for and explained.     

A model for transits in dynamic response theory

The first goal of vibration-transit (V-T) theory was to construct a tractable approximate Hamiltonian from which the equilibrium thermodynamic properties of monatomic liquids can be calculated. The Hamiltonian for vibrations in an infinitely extended harmonic random valley, together with the universal multiplicity of such valleys, gives an accurate first-principles account of the measured thermodynamic properties of the elemental liquids at melt. In the present paper, V-T theory is extended to nonequilibrium properties, through an application to the dynamic structure factor S(q,omega). It was previously shown that the vibrational contribution alone accurately accounts for the Brillouin peak dispersion curve for liquid sodium, as compared both with molecular-dynamics (MD) calculations and inelastic x-ray scattering data. Here it is argued that the major effects of transits will be to disrupt correlations within the normal-mode vibrational motion and to provide an additional source of inelastic scattering. We construct a parametrized model for these effects and show that it is capable of fitting MD results for S(q,omega) in liquid sodium. A small discrepancy between model and MD at large q is attributed to multimode vibrational scattering. In comparison, mode coupling theory formulates S(q,omega) in terms of processes through which density fluctuations decay. While mode coupling theory is also capable of modeling S(q,omega) very well, V-T theory is the more universal since it expresses all statistical averages, thermodynamic functions, and time correlation functions alike, in terms of the same motional constituents, vibrations and transits.     

Heterogeneous Nucleation of n-Butanol Vapor on Submicrometer Charged and Neutral Particles of Lactose and Monosodium Glutamate

Condensation of a supersaturated vapor of n-butanol on monodisperse submicrometer particles of lactose and monosodium glutamate is investigated in a flow cloud chamber (FCC). The dependence of critical supersaturation S(cr) on the particle size in the range 30 to 90 nm is experimentally examined. The results show that the size dependence of S(cr) qualitatively agrees with that predicted by the Fletcher version of the Volmer theory of heterogeneous nucleation, but to a lesser degree. The experimental S(cr) is smaller than the theoretical prediction even with the line tension and surface diffusion taken into account, and they induce heterogeneous nucleation better than perfectly wetted particles. The discrepancy can not be fully accounted for by the effects of line tension and surface diffusion and the existing theory concerning the curvature-dependent surface tension. The condensation on single positive-charged particles of diameter 30, 60, and 90 nm is also examined. A lowering of S(cr) at an efficiency much larger than the prediction by Volmer's theory for ion-induced nucleation is observed, and the charge effect fades away as particle size increases. Copyright 2000 Academic Press.     

Charge-induced facial-selectivity in the formation of new cationic planar chiral iridacycles derived from aniline

The reaction of chiral chlorido-iridacyclic 2-(4-N,N-dimethylaminophenyl)pyridines with solvato-type [Cp*M(S)(3)](q+) (M = Ru, S = MeCN, q = 1; M = Ir, S = MeC(O)Me, q = 2) complexes produces new cationic racemic planar chiral iridacycles in an efficient and diastereospecific way.     

Thin films of nematic liquid crystal on liquid substrate. Bulk and surface structures,surface and line tensions

Summary The structure of a liquid crystal (L.C) thin film on water has been studied optically and the molecular orientation at its interfaces has been deduced. The surface tension shift corresponding to the orientations of the molecules either normal or parallel to the free surface has been deduced. From the study of the formation and of the stability of holes in the thin film we deduce the order of magnitude of the line tensionx. This tension originates in the elastic deformation of LC in the neighbourhood of the hole.