Resonant effects in evanescent wave scattering of polydisperse colloids

Measurements and predictions are reported to understand large variations in evanescent wave (EW) scattering intensities between different particles from the same batch of single mode, polydisperse colloids. Measured EW scattering intensity distributions are obtained for three different micrometer sized latex particles irreversibly deposited onto glass surfaces. Predicted EW scattering intensity distributions are obtained using measured particle size distributions as input in a Mie theory for the three-dimensional scattering of a sphere under EW illumination. Good agreement is observed between measured and predicted EW scattering intensity distributions using no adjustable parameters. Our results indicate how finite polydispersity together with resonant effects produce large, nonlinear intensity variations between particles that appear to be physically and chemically uniform. Our findings allow such resonant effects to be understood and exploited in EW based particle-surface characterization techniques (e.g., using total internal reflections, surface plasmons) and chemical and biomolecular sensing applications (e.g., using whispering gallery modes).     

Photoswitchable orientational patterns of confined domains in monolayers

We report here a photoswitch process that involves collective molecular reorientation in a monolayer of an azobenzene derivative. Using polarized light we force the transition between two clearly distinguishable orientational mesoscopic configurations that can be monitored by reflection optical microscopy. A model that combines thermodynamic and kinetic arguments is proposed, and it is able to reproduce both the two states and the mechanism involved in the transition. We conclude that the phenomenon reported here is essentially different from the usual electric-field-induced molecular alignment often found in liquid crystalline materials and devices. Instead, it involves a photoexcitation concomitant with an H-aggregation process.     

Numerical methods for inverse problems in electrooptics of polydisperse colloids

In this paper we propose a new method for the determination of the distribution of electrical and geometrical particle parameters based on electrooptical experimental data. The electrooptical method leads to the solution of inverse ill-posed problems. The main equations for the determination of the distribution of particles on these parameters are presented. To find out the distribution functions from the electrooptical experimental data one has to solve the first-kind Fredholm integral equation corresponding to the problem under study. The proposed method of its solution is based on the penalty functions method. The results of modelling that let us compare the various numerical methods are presented.     

Calculation of long-range orientational order parameters in impure nematic liquid crystals using an extended van der Waals model

For the atended van der Waals model of a binary nematic system consisting of rigid biaxial molecules, we consider the following properties: the relation between the orientational order parameters of a nematic liquid crystal (NLC) S⁽¹⁾ and a nonmesomckphic impurity S⁽²⁾, the relation between S⁽²⁾ and the biaxiality parameter D⁽²⁾ of the impurity, and the lowered clearing temperature of NLC upon dissolution of the nonmesogen. The dependence of these characteristics on the fom and size of component molecules and the intermolecular attraction potential is analyzed. The model reproduces the experimental dependence of S⁽²⁾ and D⁽²⁾ on temperature and structure of the nematic matrix.Scientific Research Institute of Chemistry, St. Petersburg State University. Translated fromZhurnal Strukturnoi Khimii, Vol. 34, No. 5, pp. 106–115, September–October, 1993.Translated by L. Smolina     

Short-range orientational order in polymer-solvent systems

This review presents the results on studying short-range order in polymer solutions by polarization optical, electrooptical, and spectroscopic methods. The effect of short-range orientational order on dynamic birefringence, photoelasticity, electric-field-induced birefringence, IR absorption spectra, and fluorescence polarization in polymer-solvent systems has been studied. The problem concerning the account for internal field in optical and electrooptical experiments is considered. We present the results on studying the effect of characteristics of both a polymer and a solvent on the parameters characterizing short-range orientational order in polymer solutions.     

Raman study of orientational order in polymers

In this paper, we present some analytical results in the development of the classical Bower theory of Raman scattering in the partially oriented polymers. We consider more general case of arbitrary molecular orientation instead of Bower's assumption about sample having three mutually perpendicular two-fold axes of rotational symmetry. We also present here the experimental data on the study of structure of one-way-drawn polyethylene tapes and polysulfone hollow fiber membranes.     

Photoinduced orientational order of dichloride anion radicals

The photo-orientation of dichloride anion radicals (Cl₂^−·) in a glassy solution of 5 M LiCl is discussed. The quantitative characteristics of orientation of paramagnetic molecules were determined using the anisotropy of optical absorption and the angular dependence of the EPR spectrum. The orientational distribution function of ordered anion radicals was determined by joint computer modeling of the EPR spectrum recorded at different directions of the symmetry axis of a sample relative to the magnetic field of a spectrometer. It was found that the value of the order parameter (−0.1 ± 0.01), calculated from the orientational distribution function coincides with the value obtained under the measurements of the linear dichroism in the range of the detection error (−0.12 ± 0.01).     

Quantitative characterization of orientational order in liquid carbon tetrachloride

A simple geometrical construct is proposed for a clear-cut classification of the relative orientation between two tetrahedral molecules in terms of six orientational classes. When applied to sort out configurations from condensed phase simulations, it leads to a quantitative characterization of orientational order: A definite percentage for each class is obtained as a function of the distance between molecular centers. The basic picture that emerges, for liquid carbon tetrachloride, is that the dominant configuration for each distance is such that the number of chlorines in between both carbons diminishes with increasing separation, with a configuration here termed edge-to-face being the dominant one at contact. Regarding the range of orientational order, remnants are still noticeable at approximately 20 A, i.e., up to the fourth solvation shell. Beyond this distance the distributions are hardly distinguishable from the analytical predictions for random orientation. The analysis of the small fluctuations at such long distances shows that there are no significant differences between the ranges of positional and orientational order.     

Mechanisms of solute orientational order in nematic liquid crystals

The order parameters of a number of different solutes dissolved in a number of different nematic liquid-crystal solvents are measured and compared. It is shown that the order parameters can all be rationalized with the ansatz that there exist two independent ordering mechanisms operating in the liquid crystals employed. With this ansatz it is possible to fit the experimental order parameters to better than 5%. This opens the possibility of the accurate prediction of order parameters in ordered liquids.     

Tip-induced orientational order of surfactant micelles on gold

Using liquid-cell atomic force microscopy, we investigate aqueous solutions of alkyltrimethylammonium halide surfactants at the Au111 surface. The long, micellar surfactant surface aggregates cover the gold surface completely and exhibit two types of orientational order for chloride and bromide counterions, respectively. We observe lateral forces perpendicular to the scanning direction, which we explain by anisotropic friction between the probe and the oriented micelles. Conversely, we show that these friction forces can be employed to modify the spatial conformation of the micellar adlayer. Where previous methods have failed to provide control over the orientation down to the level of individual micelles, we use this technique to achieve a very high degree of order over more than 100 micelle diameters.     

Dynamics of photoinduced orientational order of azo-dyes in polymer films

Dynamical and spectral behavior of photoassisted electrical poling and photoinduced anisotropy are studied in films of DR1-doped PMMA, DR1-PMMA copolymer and new DR1-Polyimide copolymer. The orientational mobility of DR1 is characterized both in ordering processes (angularly selective trans-cis photoisomerization) and in disordering relaxation processes. Mobility is increased by the photoisomerization and decreases slowly with time, after the end of optical pumping. The transient dichroism recorded simultaneously on six probe wavelengths shows a spectral inhomogeneity of the absorption band of DR1 and of the photoinduced anisotropy (particularly in polyimide films).     

Effect of hydrogen bonds on orientational order in liquid-crystalline polymers

The orientational behavior of nematic LC copolymers (4-{[6-(acryloyloxy)hexanoyl]oxy}phenyl-4-propoxybenzoate)-co-(4-(6-acryloyloxypropyl-1-oxy)benzoic acid) in a magnetic field has been studied by ²H NMR spectroscopy. An increase in the content of carboxyl groups in the copolymer leads to an appreciable growth of the orientational order parameter S _zz .     

Structure of colloids and macroionic solutions with soft,long-range interactions

The structure of strongly interacting, charged dispersions is examined using the Ornstein-Zernike integral-equation formalism with thermodynamic consistency conditions (i.e., with the so-called HMSA closure conditions). It is shown that for highly charged dispersions interacting through screened Coulombic (Yukawa) interactions this integral-equation approach predicts the static structure factorS(q) in excellent agreement with Monte Carlo results and that it is better than the rescaled mean-spherical approximation. The Sogaimi potential (which predicts Coulombic attraction under identical physicochemical conditions) is also considered here as a model potential for representing soft, long-ranged interactions with an attractive component. None of the integralequation formalisms, including the HMSA theory and the reference hypernetted-chain (RHNC) theory, leads to a sufficient level of thermodynamic consistency for the latter potential, and the results deviate noticeably from the Monte Carlo results. We further demonstrate that fitting the experimentally observed structure factors in the neighborhood and beyond the primary peak inS(q) could lead to inaccurate conclusions concerning the nature of interparticle forces, particularly in the case of soft, long-range interactions.     

Short-range orientational order and thermodynamic properties of polymer solutions

The concept of short-range orientational order was applied to the calculation of thermodynamic characteristics of polymer solutions in terms of the lattice model. It was shown that allowance for short-range order makes it possible to explain negative values of the entropy of mixing and the existence of a lower critical solution temperature in both polymer solutions and solutions of low-molecular-mass compounds. It was found that systems with lower critical solution temperature can exist even when the degree of orientational order in solutions slightly increases as compared with the corresponding values of this parameter in their components.     

Shear-induced long-range uniaxial assembly of polyaromatic monolayers at molecular resolution

The control of spatial arrangements of molecular building blocks on surfaces opens the foundational step of the bottom-up approach toward future nanotechnologies. Contemporarily, the domain size of monolayers exhibiting crystallinity falls in the submicrometer scale. Developed herein is a method that allows the alignment of polyaromatics with one-single domain for as long as 7 mm. Even more exciting is the fact that the method is applicable to every laboratory and costs practically nothing. The monolayers are prepared simply by placing a piece of folded lens paper against the substrate and the deposition solution containing the compound of interest. The preparation scheme is similar to the Couette flow where the laminar flow takes place between two concentric walls, one of which rotates and creates viscous drag proven useful to align macromolecules. The method can induce an edge-on orientation for 3,6,11,14-tetradodecyloxydibenzo[g,p]chrysene (DBC-OC12), 3,6,12,15-tetrakis(dodecyloxy)tetrabenz[a,c,h,j]anthracene (TBA-OC12), and hexakis(4-dodecyl)-peri-hexabenzocoronene (HBC-C12) and unsubstituted coronene which would otherwise adopt the face-on arrangement on graphite. This finding will be useful to the research and industry that demands high quality alignment of polyaromatics such as OTFTs, optical polarizers, and nanodevices associated with molecular self-assembly.     

In situ determination of orientational distributions in Langmuir monolayers by total internal reflection fluorescence

A new application of the polarized total internal reflection fluorescence (PTIRF) technique to study the orientation distribution of a fluorophore within a Langmuir monolayer in situ on an aqueous subphase is described. The technique utilizes the measurement of polarized fluorescence, excited by the evanescent field appearing upon total internal reflection. The excitation by the evanescent field is achieved by launching the beam into a prism that is brought into contact with the monolayer from above. We also show here that a combination of PTIRF of monolayers on water and those freshly deposited onto the prism by horizontal lift in the same experiment provide enough data to determine the dielectric constant of the actual local environment of the fluorophore in the monolayer to eliminate the ambiguity of the orientation determination, arising from uncertainty in the normal component of excitation field. The new technique was applied to several model systems: fatty acid monolayers containing amphiphilic dyes DiI or BODIPY and also a monolayer of a synthetic amphiphilic porphyrin-binding peptide AP0. This technique is more accurate than polarized epifluorescence (PEF) in determining the fluorophore orientation distribution due to the much higher normal component of the excitation, achievable in the evanescent field, and to the lack of surface vibrations caused by capillary waves. Comparison of the new PTIRF approach with PEF shows that the monolayer structure is not disturbed by weak van der Waals attachment to the hydrophobic substrate.     

Relationship of orientational order parameters of impurity molecules in a nematic matrix

Institute of Physics, Siberian Division, Academy of Sciences of the USSR. Translated from Zhurnal Strukturnoi Khimii, Vol. 33, pp. 168–170, January–February, 1992.     

Considerations in the determination of orientational order parameters from X-ray scattering experiments

An assessment of the data processing and analysis methods used to obtain the second- and fourth-rank orientational order parameters of liquid crystals from X-ray scattering experiments has been carried out, using experimental data from four extensively studied alkyl-cyanobiphenyls and calculated data generated from two general types of theoretical orientational distribution function. The application of a background subtraction and two different baseline correction methods to the scattering profiles is assessed, along with three different methods to analyse the processed data. The choice of baseline correction method is shown to have a significant effect: an offset to zero overestimates the order parameters from the experimental and calculated data sets, particularly for lower order parameters arising from broad distributions, whereas an offset to a value estimated from regions of low scattering intensity provides experimental values close to those reported from other experimental techniques. By contrast, the three different analysis methods are shown generally to result in relatively small absolute differences between the order parameters. We outline a straightforward general approach to experimental X-ray scattering data processing and analysis for uniaxial phases that results in order parameters that match well with those reported using other experimental techniques.     

Dependence of molecular polarizability on the orientational order in nematic liquid crystals

The dispersion dependences of refractive indices in the visible range were used to obtain experimental values of the Lorentz tensor components L _j and the mean molecular polarizability $ \bar \gamma $ \bar \gamma for five nematic liquid crystals belonging to two homological series. The dependence of L _j components on the homologue number, mesophase temperature, birefringence value, and the orientational order of molecules in the nematic phase and upon a nematic-smectic A phase transition was revealed. The effect of the isotropization of the Lorentz tensors and the local field tensor with decreasing birefringence and molecular polarizability anisotropy Δγ was confirmed. The quadratic dependence $ \bar \gamma $ \bar \gamma (S) on the molecular orientational order parameter S in the nematic phase was found. It was invariant with respect to the nematic-smectic A transition. The dependences $ \bar \gamma $ \bar \gamma (S) and Δγ(S) are explained within molecular statistical theory as consequences of the correlation between orientational and conformational degrees of freedom of molecules. These conformational degrees of freedom are related to the internal rotation of molecular fragments, which affects the electronic conjugation of the fragments and the oscillator strengths of molecular transitions.