Experimental determination of the mj distribution in inelastic scattering of Na2 by He

The orientational distribution of Na₂ molecules scatterd by He has been determined in a molecules beam experiment. At large angle scattering where inelastic collisions are dominant, a high degree of alignment has been observed. This alignment depends strongly on the rotational quantum number J after scattering. The J dependence can be explained by assuming that during collisions m_j is conserved when the quantization axis is chosen parallel with the geometric apse.     

Monitoring coalescence behavior of soft colloidal particles in water by small-angle light scattering

The fractal dimension (D _f) of the clusters formed during the aggregation of colloidal systems reflects correctly the coalescence extent among the particles (Gauer et al., Macromolecules 42:9103, 2009). In this work, we propose to use the fast small-angle light scattering (SALS) technique to determine the D _f value during the aggregation. It is found that in the diffusion-limited aggregation regime, the D _f value can be correctly determined from both the power law regime of the average structure factor of the clusters and the scaling of the zero angle intensity versus the average radius of gyration. The obtained D _f value is equal to that estimated from the technique proposed in the above work, based on dynamic light scattering (DLS). In the reaction-limited aggregation (RLCA) regime, due to contamination of small clusters and primary particles, the power law regime of the average structure factor cannot be properly defined for the D _f estimation. However, the scaling of the zero angle intensity versus the average radius of gyration is still well defined, thus allowing one to estimate the D _f value, i.e., the coalescence extent. Therefore, when the DLS-based technique cannot be applied in the RLCA regime, one can apply the SALS technique to monitor the coalescence extent. Applicability and reliability of the technique have been assessed by applying it to an acrylate copolymer colloid.     

Density dependence of electron mobility in dense gases

The density (N) dependence of electron mobility (μ) in various dense gases (H₂, N₂, O₂, CO₂ and rare gases) has been calculated using a multiple-scattering approach. Deviation of the high density gas from its perfect gas behaviour has been taken into account through the temperature-dependent second virial coefficient. Multiple scattering of electrons leads to shifts in their kinetic energy and it also changes their distribution functions. This unified approach predicts both positive and negative effects. The positive (negative) effect entails on increase (decrease) of μ withN. We have assessed the available data on momentum transfer cross-sections by comparing the mobility at very low densities (Nμ)₀ with experimental results. The density dependence is studied by comparing the calculated ratioNμ)/Nμ)₀ with the observed values and other theoretical work. The Legler model which assumes a constant cross-section is inadequate for predicting the observed density dependence. We obtain good agreement with available experimental work for all the atomic and molecular species studied here.     

Atom desorption energies for sodium clusters

The desorption energies of supported sodium clusters have been determined as a function of cluster size. Na _n clusters were formed by surface diffusion of sodium atoms adsorbed from a thermal atomic beam on a LiF(100) single crystal. Measurements have been performed by temperature programmed thermal desorption. The signals reflect fractional order desorption kinetics. The average cluster size could be controlled by varying the total number of sodium atoms on the surface. It was determined from scattering experiments. We find that the binding energies vary between 0.55 and 0.8 eV and only approach a constant value for clusters with diameters as large as 1,000 Å.     

Characterization of lignin using multi-angle laser light scattering and atomic force microscopy

Small differences in the isolation techniques of lignin can result in significant changes in its molecular structure and configuration. Light scattering (evaluated at 18 different angles in a plane), Atomic Force Microscopy (AFM) and Near Infrared Spectroscopy (NIR) proved very effective for evaluating the characteristics of lignin. Zimm plots were generated using Zimm, Debye and Berry formalisms to evaluate the weight average molecular weight (MW), radius of gyration (r_g), hydrodynamic radius (r_h) and second virial coefficient (A₂). Two types of lignin and nine different solvents were used for the study, to analyze the conformation of lignin molecules in different solvents expected to be used in lignin degradation and subsequent analysis. Absolute MW and r_g decreased and the dn/dc increased when the solvent used for lignin was changed from water to sodium hydroxide. The two types of lignin also exhibited different values for all the above estimated parameters. This study also highlighted the differences between the unlyophilized and lyophilized lignin in terms of aggregation, pH dependence and stability over time. This aggregation has never been seen on a ultraviolet (UV) or refractive index (RI) detector that has been used so far for liquid chromatography (LC) reducing the reliability of lignin depolymerization data obtained without light scattering.     

Light scattering behaviour of low-molecular-weight polymers in mixed solvents with very high selective sorption

Some low-molecular-weight polymers (poly(butyl methacrylate), nylon 6, nylon 12) in the solvent system m-cresol/n-heptane, with effective refractive index increments (?n/?c)_μ in the range 0.34?0.81 ml/g, have been studied by light scattering. These extremely large values, measured by differential refractometry of solutions in the state of dialysis equilibrium, are mainly due to very great selective sorption of one solvent component (m-cresol) on the polymers studied. The effect inherent to these systems, viz. that of the free solvent composition change, has been discussed and exemplified by the light scattering behaviour of solutions of oligomers of butyl methacrylate. The light scattering anomalies observed with solutions of some nylons in m-cresol/n-heptane have been interpreted as due to interaction of polymer end-groups. It has been demonstrated that the determination of $\text{M}{}_{\mathrm{w}}$ of the order of 10³ by light scattering in systems with large (?n/?c)_μ values, i.e. with large scattering power, is possible with good accuracy.     

Effects of Hofmeister anions on the flocculation behavior of temperature-responsive poly(N-isopropylacrylamide) microgels

Effects of some sodium salts (NaCl, NaClO₃, and NaSCN) in the Hofmeister series on deswelling and temperature-induced aggregation behavior of microgels of poly(N-isopropylacrylamide) (PNIPAAM) and PNIPAAM-co-PAA with attached poly(acrylic acid) moieties were investigated with the aid of turbidimetry and dynamic light scattering. Addition of salt in the concentration range 0.1–0.5?M generated aggregation of the PNIPAAM microgel particles at elevated temperatures, but it was no distinct difference between chaotropic and kosmotropic anions. In contrast, the flocculation behavior at high temperatures for PNIPAAM-co-PAA revealed a prominent influence of salinity and type of anion on the formation of aggregates. The aggregation transition was shifted to the highest temperature for the most chaotropic anion (SCN^?), and the aggregation transition at the same salt concentration is consistent with the typical Hofmeister series. The turbidity results from the PNIPAAM-co-PAA microgels disclosed a two-step transition for the considered anions, and both a low and high temperature change in the turbidity data was observed. The high-temperature transition followed the Hofmeister series.     

Synthesis and application of head-to-head-type styrene dimers bearing two fluoroalkyl end-groups

Head-to-head-type styrene and substituted styrene dimers bearing two fluoroalkyl end-groups have been efficiently synthesized by a simple reaction of perfluoroalkyl iodide with styrene under radical conditions as a mixture of meso and racemic forms. The meso form obtained from the mixture by recrystallization gave a crystal suitable for X-ray diffraction study and the crystal structure was found to be based on π-stacking of benzene rings and aggregation of fluoroalkyl chains. Dynamic light scattering measurements showed that meso-styrene dimers bearing two fluoroalkyl end-groups can form the nanometer size-controlled self-assemblies through the intermolecular π-stacking of benzene rings and aggregation of end-capped fluoroalkyl groups in methanol.

The density dependence of the depolarized light scattering at high frequencies in rare gas fluids

Measurements of the depolarized light scattering spectra of venon at densities between 0.3 and 2.65 times the critical density are presented. The second moments are compared with the second moments of the scattering functions S(k, ω) of argon at corresponding densities. A conclusion is drawn about the wavevector of the modes that contribute to the high-frequency part of the spectra that are observed at high, liquid densities.     

Measurements of the hyperfine dimer spectrum for H2-Ne,H2-Ar,H2-Kr by the magnetic beam renosance technique

Measurements are performed on ortho H₂(I = 1, j = 1, vibrational ground state)-X dimers with X = Ne, Ar, Kr; the dimers H₂-X occur in their van der Waals stretch ground state. The dimer end-over-end rotational states (quantum number L) are investigated for L ?2. The rf hyperfine-transition frequencies are measured, 30 ? v ? 600 kHz. Combining our results with scattering data from the Göttingen group, for H₂Ne the full potential could be described with high accuracy. For the other systems, a simple molecular parameter is derived pertaining to the dimer system. As our spectroscopy mainly permits a sensitive probing of the well region, new scattering data are needed to determine accurate model potentials over the full range.     

Adsorption and aggregation properties of novel star-shaped gluconamide-type cationic surfactants in aqueous solution

The adsorption and aggregation behavior of novel star-shaped gluconamide-type cationic surfactants N-dodecyl-N,N-bis[(3-D-gluconylamido)propyl]-N-alkylammonium bromide (C_nDBGB, where n represents hydrocarbon chain lengths of 10, 12, and 14) has been studied on the basis of static/dynamic surface tension, conductivity, dynamic light scattering (DLS), and transmission electron microscopy (TEM) data. The static surface tension of the C_nDBGB aqueous solution measured at the critical micelle concentration (CMC) is observed to be significantly lower than that of the corresponding monomeric surfactants. The dynamic surface tension results indicate that adsorption process of above CMC is a mixed diffusion–kinetic adsorption mechanism. From the results of temperature dependent conductivity measurements, we could obtain the degree of counterion binding (β) and the thermodynamic parameters such as standard free energy (ΔG _mic ⁰ ), enthalpy (ΔH _mic ⁰ ), and entropy (ΔS _mic ⁰ ) of aggregation. With a combination of the DLS and TEM data, a size transformation of the micelles is suggested to occur with an increase in the concentration.     

Determination of the ionization constants of natural cyclodextrins by high-resolution 1H-NMR and photon correlation spectroscopy

The purpose of this investigation has been to establish reference pK _a values in D₂O for the three natural CDs by high-resolution ¹H-NMR, according to the recent guidelines provided by the IUPAC for the determination of extreme pK _a values. The most alkaline conditions achieved in this study than in previous pH potentiometric assays have made possible to deduce the pK _a for the three acidic groups of each CD. In addition, we have studied the effects of the ionization of ??-CD on the aggregation properties of this macrocycle in H₂O by dynamic light scattering (DLS) as a function of pH. This procedure provides an indirect way of measuring the pK _a of ??-CD either by tracking the percentage of scattered light or the hydrodynamic radii of the species involved and reveals that the aggregates of ??-CD break and reduce their size progressively upon ionization of the OH^? groups in positions 2 and 3.     

Aggregation Behavior of Pluronic-L64 in Presence of Sodium Dodecyl Sulphate in Water

The effect of sodium dodecyl sulfate (SDS) on the micellization and aggregation behavior of a poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO-PPO-PEO) amphiphilic copolymer (Pluronic L64: EO₁₃ PO₃₀ EO₁₃) have been investigated by various techniques like, cloud point, viscosity, isothermal titration calorimetry (ITC), differential scanning calorimetry (DSC), fluorescence spectroscopy, room temperature phosphorescence (RTP), and small angle neutron scattering (SANS). Addition of SDS in L64 solutions shows mark alteration of different properties. We observed synergistic interaction between SDS and Pluronic L64. The changes in the critical micelle concentration (CMC), critical micelle temperature (CMT), cloud point (CP), micelle size, and shape has been correlated and reported in terms of structure dynamics and mechanics. The ITC titrations have been used to explore the different stages of binding and interactions of SDS with L64. The enthalpies of aggregation for copolymer-SDS aggregates binding, organizational change of bound aggregates, and the threshold concentrations of SDS in the presence of copolymer were estimated directly from ITC titration curves. The effect of temperature on enthalpy values has been reported in terms of different aggregation state. Fluorescence and RTP for L64 were used to investigate the change in micellar environment on the addition of SDS at different temperature. Appearance and shifting of SANS peaks have been used to monitor the size and inter micellar interaction on addition of SDS in L64 solution. Cloud point and viscosity elaborate the penetration of SDS molecule in L64 micelle and hence changing the micellar architect.     

On the interaction of copper with defects in gallium arsenide

The behavior in gallium arsenide of copper introduced through the melt has been investigated. Measurements have been made on the electrical and photoluminescent properties of single crystals that had either been quenched from T_cryst, slowly cooled after crystallization or thermally annealed at 1100°C after completion of the growth. On quenching from T_cryst, formation of very shallow acceptors is observed. On slow cooling or annealing at 1100°C, acceptor levels are formed of the order of 0.12 and 0.02 eV. The concentration of carriers for both levels diminishes with lower temperatures of annealing and with concentration of copper added. The 0.12 eV level is assigned to the center (Cu_GaV_Ga)⁻.     

Interfacial behaviour of block polyelectrolytes,III. Formation of surface micelles

The aggregation of (poly)styrene/quaternized (poly)-4-vinylpyridine diblocks into regular circular structures has been observed in Langmuir-Blodgett films removed from a pure water surface. These novel aggregates, termed surface micelles, form at low surface pressures (< 2 mN/m) and exhibit rich isotherm behaviour as Langmuir films. A distinct first order phase transition is observed for the decylated derivatives of a series of diblocks (i.e. (PS)₂₆₀(C₁₀PVP⁺I⁻)_n; n = 70, 120, 240) which has been interpreted as a transition from an entirely surface-adsorbed micelle to a surface micelle whose polyelectrolyte chains have been forced into the aqueous subphase. Transmission electron and atomic force micrographs of LB films provide for direct visualization of these surface micelles as well as the means to estimate their aggregation numbers, N.     

Thermodynamics of micellization of alkylimidazolium surfactants in aqueous solution

Alkylimidazolium salts are a very important class of compounds. So far, calorimetry has hardly been used to characterize their solution behaviour. The enthalpies obtained from indirect methods have an intrinsic large uncertainty, and nowadays it is clear that calorimetry is the most sensitive technique for directly measuring the thermodynamic properties of aggregation.In this work, isothermal titration calorimetry (ITC) was used along with conductivity to determine the thermodynamics of aggregation of 1-alkyl-3-methylimidazolium chlorides ([C_nmim]Cl, n = 8, 10, 12, and 14) in aqueous solution. The critical micelle concentrations, cmc, were obtained from conductivity and calorimetry, and the enthalpies of micelle formation, ΔH_mic, were derived from the calorimetric titrations. From conductivity, we could also derive the values for the degree of ionisation of the micelles (α), the molar conductivity (Λ^M) of the [C_nmim]Cl micellar species and the molar conductivity at infinite dilution (Λ^∞) for the [C_nmim]⁺ cations.Values are therefore reported for the enthalpy (ΔH_mic), the Gibbs free energy (ΔG_mic) and entropy (ΔS_mic) changes for micelle formation. Further, the aggregate sizes and aggregation numbers were obtained by light scattering (LS) measurements.The observed variation of the thermodynamic properties with the alkyl chain length is discussed in detail and compared with the traditional cationic surfactants 1-alkyl-trimethylammonium chlorides, [C_nTA]Cl. The difference in the values of the thermodynamic parameters for both types of surfactants is here related to the structure of their head groups.     

Small-angle X-ray scattering in sodium dodecyl sulfate solutions and micelle clustering

The small-angle X-ray scattering (SAXS) in micellar sodium dodecyl sulfate solutions has been studied in the range of overall concentrations c from 8 mM (CMC₁) to 300 mM and the absolute values of scattering vector q from 0.07 to 3.0 nm^–1. The total intensity of isotropic scattering has been revealed to increase with solution concentration. At c > 27 mM, the SAXS spectra have been found to exhibit an interference peak, which testifies a correlation in the arrangement of micelles in the bulk solution. This peak corresponds to the magnitude of q close to 1.55 nm^–1. Using the position of this maximum, average distance r₀ between the centers of micelles has been determined, which is equal to 4.1 nm and remains almost unchanged upon an increase in the overall concentration of sodium dodecyl sulfate. The observed regularities have been explained in terms of the DLVO theory taking into account the electrostatic and molecular intermicellar interaction.     

Measurement of the Compton profiles of CH4 and C2H4 by high energy electron impact spectroscopy

We present a new experimental determination of the Compton profiles of CH₄ and C₂H₄ molecules using high energy electron impact. The observable q range has been extended up to 10 au (6 au for C₂H₄). Good agreement is found with an earlier X-ray scattering experiment and with new theoretical calculations.     

Micellization behavior of the ionic liquid lauryl isoquinolinium bromide in aqueous solution

The micellization behavior of the ionic liquid lauryl isoquinolinium bromide ([C₁₂iQuin]Br) in aqueous solution has been assessed using surface tension, electrical conductivity, and ¹H nuclear magnetic resonance (NMR) measurements. The results reveal that the critical micelle concentration (CMC) and constant surfactant tension (γ _cac) are lower than that of butyl isoquinolinium bromide ([C₄iQuin]Br), octyl isoquinolinium bromide ([C₈iQuin]Br, and lauryl pyridinium bromide ([C₁₂Pyr]Br). ¹H NMR spectra show the evidence of paralleled π-stacking of adjacent isoquinoline rings. To elucidate the effect of the π–π interactions on the aggregation process, thermodynamic parameters such as the standard free energy, enthalpy, and entropy of aggregation have been discussed. These parameters are evaluated from the CMC with temperature by fitting these values to expressions derived from a micellization thermodynamic model. The enthalpy–entropy compensation phenomenon has been observed in the micellization process of [C₁₂iQuin]Br in water, and the presence of isoquinoline cations is responsible for the decrease in the ΔH _mic ^? , compared with [C₁₂Pyr]Br which has the same alkyl chain and counter-ion.     

Mobility and aggregation of free clusters soft landed on amorphous and crystalline carbon substrates

Neutral antimony clusters produced by a gas aggregation source have been deposited at room temperature on thin films of amorphous carbon and cleavage surfaces (0001) of graphite. Antimony islands generated from different mean size distributions of preformed clusters Sb_n(n = 4, 90, 150, 250, 600, 2000) have been investigated by transmission electron microscopy. Only compact islands have been observed on amorphous carbon, whereas an evolution from compact to dendritic shapes occurs on graphite substrate as the mean size of the deposited clusters increases. For clusters containing more than 150 atoms the dendritic islands exhibit a fractal character whose dimensional analysis yields a fractal dimension of 1.63 ± 0.07. The different models for island growth are discussed in the light of these results.