Small-angle neutron scattering studies of different mixed micelles composed of dimeric and monomeric cationic surfactants

Measurements of small-angle neutron scattering (SANS) cross sections from different mixed micelles composed of CTAB and Br-, n-C16H33N+Me2-(CH2)(m)N+Me2-n-C16H33, Br- (16-m-16, 2Br(-), where m = 3, 5, and 10), in aqueous media (D2O) are reported. The data have been analyzed using the Hayter and Penfold model for macroion solution to compute the interparticle structure factor S(Q) taking into account the screened Coulomb interactions between the micelles. The aggregate composition matches with that predicted from an ideal mixing model. The SANS analysis further indicates that the extent of aggregate growth and the Variations of shapes of the mixed micelles could be modulated by the amount of dimeric surfactant present in these mixtures. With the spacer chain length m less than or equal to 4 in the dimeric surfactant, the propensity of micellar growth is particularly pronounced. The effect of the variation of the temperature for the mixed micellar system (23.1 mol % of 16-3-16, 2Br(-)) was also examined. The systemic microviscosities that the mixed micellar aggregates offer to a solubilized, extrinsic fluorescence probe, 1,6-diphenyl-1,3,5-hexatriene, were determined. The variation of the microviscosities of the mixed micelles as a function of percentages of the dimeric surfactants could be explained in terms of conformational variations and progressive looping of the spacer chain of dimeric surfactants in mixed micellar aggregates with increasing m values.

The algebra and geometry of SU(3) matrices

We give an elementary treatment of the defining representation and Lie algebra of the three-dimensional unitary unimodular group SU(3). The geometrical properties of the Lie algebra, which is an eight dimensional real Linear vector space, are developed in an SU(3) covariant manner. The f and d symbols of SU(3) lead to two ways of 'multiplying' two vectors to produce a third, and several useful geometric and algebraic identities are derived. The axis-angle parametrization of SU(3) is developed as a generalization of that for SU(2), and the specifically new features are brought out. Application to the dynamics of three-level systems is outlined.

Local environmental structures of Mo in the superionic conducting glass (AgI)(0.6)(Ag2MoO4)(0.4) by anomalous X-ray scattering

The anomalous X-ray scattering (AXS) method using Mo K absorption edges has been employed for obtaining the local structural information of superionic conducting glass having the composition (AgI)(0.6)(Ag2MoO4)(0.4). The possible atomic arrangements in the near-neighbor region of this glass were estimated by coupling the results with the least-squares variational analysis so as to reproduce the differential intensity profile for Mo as well as the ordinary scattering profile. The coordination number of oxygen around Mo is found to be about 4 at the distance of 0.180 mn. This implies that the most probable structural entity in the glass is the MoO4 tetrahedral unit which has been proposed based on infrared spectroscopy. The value of the coordination number of I- around Ag+ is estimated as 4.4 at 0.287 nm, suggesting an arrangement similar to that of crystalline or molten AgI.

Micellar structures of dimeric surfactants with phosphate head groups and wettable spacers: A small-angle neutron scattering study

Dimeric or gemini surfactants consist of two hydrophobic chains and two hydrophilic head groups covalently connected by a hydrophobic or hydrophilic spacer. This paper reports the small-angle neutron scattering (SANS) measurements from aqueous micellar solutions of two different recently developed types of dimeric surfactants: (i) bis-anionic C16H33PO4--(CH2)(m)-PO4-C16H33,2Na(+) dimeric surfactants composed of phosphate head groups and a hydrophobic polymethylene spacer, referred to as 16-m-16,2Na(+), for spacer lengths m = 2, 4, 6, and 10, (ii) bis-cationic C16H33N+(CH3)(2)-CH2-(CH2-O-CH2)(p)-CH2-N+ (CH3)(2)C16H33,2Br(-) dimeric surfactants composed of dimethylammonium head groups and a wettable polyethylene oxide spacer, referred to as 16-CH2-p-CH2-16,2Br(-), for spacer lengths p = 1 - 3. The micellar structures of these surfactants are compared with the earlier studied bis-cationic C16H33N+ (CH3)(2)-(CH2)(m)-N+ (CH3)(2)C16H33,2Br(-) dimeric surfactants composed of dimethylammonium head groups and a hydrophobic polymethylene spacer, referred to as 16-m-16,2Br(-). It is found that 16-m-16,2Na(+), similar to 16-m-16,2Br(-), form various micellar structures depending on the spacer length. Micelles an disklike for rn = 2, rodlike for m = 4, and prolate ellipsoidal fur m = 6 and 10. The micelles of 16-CH2-p-CH2-16,2Br(-) are prolate ellipsoidal for all the values of p = 1 - 3. It is also found that micelles of 16-m-16,2Na(+) and 16-CH2-p-CH2-16,2Br(-) are large in comparison to those of 16-in-16,2Br(-) for similar spacer lengths. This is connected with the fact that both in 16-m-16,2Na(+) and 16-CH2-p-CH2-16,2Br(-), the head group or the spacer is more hydrated as compared to that in the 16-m-16,2Br(-). An increase in the hydration of the spacer or the head group increases the screening of the Coulomb repulsion between the charged head groups. This effect has been found to be more pronounced in the dimeric surfactants having wettable spacers. [S1063-651X(99)00303-7].

Helical structures: The geometry of protein helices and nanotubes

In nature, helical structures arise when identical structural subunits combine sequentially, the orientational and translational relation between each unit and its predecessor remaining constant. A helical structure is thus generated by the repeated action of a screw transformation acting on a subunit. A plane hexagonal lattice wrapped round a cylinder provides a useful starting point for describing the helical conformations of protein molecules, for investigating the geometrical properties of carbon nanotubes, and for certain types of dense packings of equal spheres.

Roy equation analysis of ππ scattering

We analyse the Roy equations for the lowest partial waves of elastic ππ scattering. In the first part of the paper, we review the mathematical properties of these equations as well as their phenomenological applications. In particular, the experimental situation concerning the contributions from intermediate energies and the evaluation of the driving terms are discussed in detail. We then demonstrate that the two S-wave scattering lengths a00 and a02 are the essential parameters in the low energy region: Once these are known, the available experimental information determines the behaviour near threshold to within remarkably small uncertainties. An explicit numerical representation for the energy dependence of the S- and P-waves is given and it is shown that the threshold parameters of the D- and F-waves are also fixed very sharply in terms of a00 and a20. In agreement with earlier work, which is reviewed in some detail, we find that the Roy equations admit physically acceptable solutions only within a band of the (a00,a02) plane. We show that the data on the reactions e+e−→ππ and τ→ππν reduce the width of this band quite significantly. Furthermore, we discuss the relevance of the decay K→ππeν in restricting the allowed range of a00, preparing the grounds for an analysis of the forthcoming precision data on this decay and on pionic atoms. We expect these to reduce the uncertainties in the two basic low energy parameters very substantially, so that a meaningful test of the chiral perturbation theory predictions will become possible.

Characterization of vesicles from ion-paired gemini surfactants by small angle neutron scattering

Detailed small angle neutron scattering ( SANS) studies were carried out with the aqueous vesicular (unilamellar) suspension of dimeric ion-paired lipids (2a-2c) for spacer lengths corresponding to n-values of 2, 6 and 10 and monomeric ion-paired lipid (3) below and above the phase transition temperature of each amphiphile. The vesicular structure strongly depends on the spacer chain length. The mean vesicle size is smallest for the lipid with a short spacer, n = 3 and it increases with the increase in the spacer chain length. The bilayer thickness also decreases with the increase in the spacer chain length. The size polydispersity increases with the increase in the spacer chain length (n-value).

Determination of critical micelle concentration by hyper-Rayleigh scattering

The critical micelle concentration (CMC) of several surfactants that contain an NLO chromophore, either at the hydrocarbon tail, or at the hydrophilic headgroup, or even as a counterion, was determined by hyper-Rayleigh scattering (HRS). In all cases, the HRS signal exhibited a similar variation with surfactant concentration, wherein the CMC is inferred from a rather unprecedented drop in the signal intensity. This drop is attributed to the formation of small pre-micellar aggregates, whose concentrations become negligible above CMC. In addition, a probe molecule, which upon protonation yielded a species with significantly enhanced HRS intensity, was developed and its utility for the determination of the CIVIC of simple fatty acids was demonstrated.

On the solution of the problem of scattering of surface water waves by a sharp discontinuity in the surface boundary conditions

Closed-form analytical expressions are derived for the reflection and transmission coefficients for the problem of scattering of surface water waves by a sharp discontinuity in the surface-boundary-conditions, for the case of deep water. The method involves the use of the Havelock-type expansion of the velocity potential along with an analysis to solve a Carleman-type singular integral equation over a semi-infinite range. This method of solution is an alternative to the Wiener-Hopf technique used previously.

Encapsulation of cobalt phthalocyanine in zeolite-Y: Evidence for nonplanar geometry

Cobalt (11) phthalocyanine (CoPc) molecules have been encapsulated within the supercage of zeolite-Y. The square-planar complex, being larger than the almost spherical cage, is forced to adopt a distorted geometry on encapsulation. A comparative spectroscopic and magnetic investigation of CoPc encapsulated in zeolite-Y and in the unencapsulated state is reported. These results supported by molecular modeling have been used to understand the nature and extent of the loss of planarity of CoPc on encapsulation. The encapsulated molecule is shown to be the trans-diprotonated species in which the center of inversion is lost due to distortions required to accommodate the square complex within the zeolite. Encapsulation also leads to an enhancement of the magnetic moment of the CoPc. This is shown to be a consequence of the nonplanar geometry of the encapsulated molecule resulting in an excited high-spin state being thermally accessible.

Structural characterization of DC magnetron-sputtered TiO2 thin films using XRD and Raman scattering studies

Titanium dioxide films have been deposited using DC magnetron sputtering technique onto well-cleaned p-silicon substrates at an oxygen partial pressure of 7 x 10(-5) mbar and at a sputtering pressure (Ar + O-2) Of I X 10(-3) mbar. The deposited films were calcinated at 673 and 773 K. The composition of the films as analyzed using Auger electron spectroscopy reveals the stoichiometry with an 0 and Ti ratio 2.08. The influence of post-deposition annealing at 673 and 773 K on the structural properties of the titanium dioxide thin films have been studied using XRD and Raman scattering. The structure of the films deposited at the ambient was found to be amorphous and the films annealed at temperature 673 K and above were crystalline with anatase structure. The lattice constants, grain size, microstrain and the dislocation density of the film are calculated and correlated with annealing temperature. The Raman scattering study was performed on the as-deposited and annealed samples and the existence of Raman active modes A(1g), B-1g and E-g corresponding to the Raman shifts are studied and reported. The improvement of crystallinity of the TiO2 films was also studied using Raman scattering studies. (C) 2003 Elsevier Ltd. All rights reserved.

Brillouin Scattering Investigation of Solvation Dynamics in Succinonitrile-Lithium Salt Plastic Crystalline Electrolytes

Temperature dependent Brillouin scattering studies have been performed to ascertain the influence of solvent dynamics on ion-transport in succinonitrile-lithium salt plastic crystalline electrolytes. Though very rarely employed, we observe that Brillouin spectroscopy is an invaluable tool for investigation of solvent dynamics. Analysis of various acoustic (long wavelength) phonon modes observed in the Brillouin scattering spectra reveal the influence of trans-gauche isomerism and as well as ion-association effects on ion transport. Although pristine SN and dilute SN-LiClO(4) samples show only the bulk longitudinal-acoustic (LA) mode, concentrated SN-LiClO(4) (similar to 0.3-1 M) electrolytes display both the bulk LA mode as well as salt induced brillouin modes at ambient temperature. The appearance of more than one brillouin mode is attributed to the scattering of light from regions with different compressibilities (''compactness''). Correspondingly, these modes show a large decrease in the full width at half-maximum (abbreviated as nu(f)) as the temperature decreases. Anomalous temperature dependent behavior of nu(f) with addition of salt could be attributed to the presence of disorder or strong coupling with a neighbor. The shape of the spectrum was evaluated using a Lorentzian and Fano line shape function depending on the nature and behavior of the Brillouin modes.