Brownian dynamics in strongly scattering porous media - dynamic light scattering with single-mode matching

Brownian motions in porous media represent a challenging problem not only from a theoretical but also from an experimental point of view. A very powerful technique for the measurement of Brownian motions is Dynamic Light Scattering (DLS). For our problem, however, the classic version of DLS is useless because the porous matrix scatters very strongly and the particles to be measured are completely masked. Sometimes it is possible to suppress the background scattering by matching the refractive indices of the matrix and the confined liquid, but this restricts the applicability to a handful of suitable model systems. We have introduced a new technique which overcomes this difficulty: The keyword is single-mode matching and the idea is to select from the complicated random light field generated by the strongly scattering medium only a single mode so that the contribution from the matrix to the selected mode vanishes because of destructive interference. The first experimental results on the dynamics of latex particles in aqueous suspensions confined in a packing of glass beads are very promising. Their quantitative analysis demonstrates clearly the feasibility of measuring the diffusion coefficient within an opaque strongly scattering porous medium.     

Static and dynamic light scattering in turbid suspensions

The application of static and dynamic light scattering to many colloidal systems of practical interest has often been considered too complicated due to strong multiple scattering. There are two new approaches to overcome this problem. One of them aims at suppressing contributions from multiple scattering using novel cross‐correlation schemes. While this relies on the suppression of multiple scattering, the so‐called diffusing wave spectroscopy (DWS) works in the limit of very strong multiple scattering. DWS can be used for the characterization of dynamic and static properties of colloidal systems on a large range of time and length scales ranging from a few Ångstroms to hundreds of nanometers. We demonstrate that a wealth of information can be obtained from these methods on the structure, dynamics, interaction effects, stability, aggregation, and sol‐gel transition in colloidal dispersions.     

Incoherent subharmonic light scattering in isotropic media

Incoherent subharmonic light scattering in isotropic media is a new kind of nonlinear light scattering, which involves single input photon and multiple output photons of equal frequency. We investigate theoretically the dependence of the subharmonic scattering intensity on the hyperpolarizability of molecules and the incident intensity using nonlinear optics theory similar to that used for Hyper-Rayleigh scattering and degenerate optical parametric oscillators. It is derived that the subharmonic scattering intensities grow exponentially or superexponentially with the hyperpolarizability of molecules and the incident intensity.     

Resonance light scattering properties of Eu3+ in gold colloid

Gold colloidal containing rare-earth ions Eu3+ were prepared at room temperature. Fluorescence spectra and resonance light scattering (RLS) spectra of Eu3+ ions and gold colloid containing Eu3+ were measured. For solution containing Eu3+, RLS features show two peaks at the edges of the visible light wavelength region. The short wavelength peak takes place at about 400 nm and the longer wavelength peak is the corresponding 1/2 fraction frequency RLS peak, which takes place at about 780 nm. When gold colloids were added to the solution containing Eu3+, both these two RLS peaks were enhanced. We believe that the energies, which are absorbed by the surface plasmon resonance in the gold nanoparticles, are efficiently transferred into the Eu3+ ions to cause the increased scattering.     

Dynamic light scattering by non-ergodic media

The theory of dynamic light scattering (DLS) by non-ergodic media, systems such as polymeric gels whose density fluctuations are partially “frozen”, is discussed. We derive the relationship between the time-averaged time correlation function of the scattered intensity, the quantity measured in a single DLS experiment, and the ensemble-averaged correlation function of the sample's density fluctuations. The connection between DLS, speckle and the reciprocal-space structure of the medium is also discussed.     

Size dispersion and colloid mediated radionuclide transport in a synthetic porous media

Size dispersion effects during the migration of natural submicron bentonite colloids (<200 nm) through a ceramic column are observed for the first time by laser-induced breakdown detection (LIBD) at ppm (parts per million) mass concentration. Larger size fractions ( approximately 200 nm) arrive prior to smaller size fractions (<100 nm) at the column outlet in agreement with model predictions and earlier findings with carboxylated polystyrene spheres. By addition of trace amounts of americium(III) and plutonium(IV), colloid mediated transport of these radionuclides is studied. The peak arrival times of Pu-244 and Am-241, as measured by ICP-MS, match the bentonite colloid breakthrough and occur significantly prior to the conservative tracer (HTO) indicating the colloid-borne migration of tri- and tetravalent radionuclides.     

Static and dynamic light scattering study of fluorinated polymer colloids with a crystalline internal structure

The light scattering properties of polymer colloids made of polytetrafluoroethylene with added copolymers are described in some detail. The particles possess a partially crystalline internal structure which makes them optically anisotropic, and consequently gives rise to a strong depolarized component in the scattered light intensity. We present a complete theory of static and dynamic light scattering which includes the effects of polycrystallinity, of optical polydispersity, and of interparticle interactions. We also discuss applications to surfactant adsorption studies, to sedimentation experiments and to fractal aggregation processes     

Static light scattering from poly(ethylene oxide) in methanol

Static light scattering measurements were performed on dilute solutions of monodisperse poly(ethylene oxide) (PEO) in methanol at 25°C. PEOs of five different molecular weights ranging from nominal M_w = 8.6 × 10⁴ to 9.13 × 10⁵ were used. Linear Zimm plots were obtained for all the PEO samples: no downturn was observed at small angles, indicating that no large aggregates of PEO molecules exist in the solution. From the plots, values of the weight-average molecular weight, M_w, the radius gyration, R_G, and the second virial coefficient, A₂, were successfully determined for respective PEOs. Observed relationship between R_G and M_w indicates that methanol is certainly a good solvent for the polymer. © 1996 John Wiley & Sons, Inc.     

A light scattering study of concentrated dispersions in nonaqueous media

Interparticle interactions between colloidal poly(methylmethacrylate) particles stabilised by poly(12-hydroxystearic acid) in non-aqueous media have been investigated using time-average light scattering. The problem of multiple scattering was avoided by using a binary mixture of solvents such that the colloidal particles were optically matched. This enabled the static structure factor to be measured and from the small scattering vector expansion the osmotic pressure to be determined. The softness of the pairwise interaction potential has been exposed using the Chandler-Weeks-Anderson perturbation theory. However, it is concluded that dispersions of the type studied can be reasonably well approximated by a hard sphere fluid model.     

Influence of structure in heterodyne electrophoretic light scattering

Heterodyne light scattering is employed to probe the electrophoretic flow behavior of a concentrated, deionized model colloidal suspension. Both for the fluid and crystalline regime, the power spectra are well described as velocity distributions, which let assume that the power spectra are dominated by the self-dynamic structure factor. Furthermore, all measured power spectra show an increase of the frequency integrated intensity with the electric field. Since the number of particles should remain constant this finding is unexpected. These effects are rationalized on the basis of a new theoretical model including the influence of the structure factor and of the mobility polydispersity on the power spectra.     

Resolving the coupled effects of hydrodynamics and DLVO forces on colloid attachment in porous media

Transport of colloidal particles in porous media is governed by the rate at which the colloids strike and stick to collector surfaces. Classic filtration theory has considered the influence of system hydrodynamics on determining the rate at which colloids strike collector surfaces, but has neglected the influence of hydrodynamic forces in the calculation of the collision efficiency. Computational simulations based on the sphere-in-cell model were conducted that considered the influence of hydrodynamic and Derjaguin-Landau-Verwey-Overbeek (DLVO) forces on colloid attachment to collectors of various shape and size. Our analysis indicated that hydrodynamic and DLVO forces and collector shape and size significantly influenced the colloid collision efficiency. Colloid attachment was only possible on regions of the collector where the torque from hydrodynamic shear acting on colloids adjacent to collector surfaces was less than the adhesive (DLVO) torque that resists detachment. The fraction of the collector surface area on which attachment was possible increased with solution ionic strength, collector size, and decreasing flow velocity. Simulations demonstrated that quantitative evaluation of colloid transport through porous media will require nontraditional approaches that account for hydrodynamic and DLVO forces as well as collector shape and size.     

Three dimensional cross-correlation dynamic light scattering by non-ergodic turbid media

We investigate dynamic light scattering by non-ergodic turbid media with an adapted version of the method proposed by Pusey and van Megen [Physica A 157, 705 (1989)]. Our formulation follows the derivation of the original method by extending it to the three dimensional cross-correlation scheme (3DDLS). The main finding is an expression to obtain the dynamic structure factor from light scattering that takes into account the system turbidity and the peculiarities of the 3D geometry. From 3DDLS measurements in well-controlled solid-like systems of different turbidity, we confirm that our results can be interpreted reasonably well by the theoretical approach described here. Good agreement is found with earlier reported results on similar systems.     

Combined effects of Ca2+ and humic acid on colloid transport through porous media

We report the separate and combined effects of humic acid and Ca²⁺ ions on the transport of colloidal particles through a sand-packed column. Polystyrene latex particles with a sulfate functional group were used as model colloids. The concentrations of both the inlet solution and the effluent solutions were measured during each experimental run. Breakthrough curves were obtained by taking the ratios of each effluent sample concentration to the inlet solution concentration. In the absence of humic acid, the results indicate that increasing the concentration of Ca²⁺ increases particle attachment to the sand, thus causing decreased transport rates of latex particles through the porous bed matrix. Once 4 mg/l humic acid was added to the system, changes were observed in the effect that Ca²⁺ has on latex particle breakthrough. In a system containing calcium, increasing the humic acid concentration was shown to reduce particle attachment and increase transport rates. In the absence of calcium, the ratios for the outlet-to-inlet concentrations were similar for each concentration of humic acid. The electrophoretic mobility was also measured in order to determine the role of electrostatic repulsion in the latex particle transport. The electrophoretic mobility of the latex particles was found to be dependent on humic acid concentration in the absence of Ca²⁺ but not in its presence. Received: 2 February 2001 Accepted: 6 2001     

Static light scattering from microgel particles: model of variable dielectric permittivity

We perform static light scattering experiments on a dilute suspension of microgel particles and model the resultant form factors Pq by assuming an exponentially decaying dielectric permittivity. The result is that Pq is a Lorentzian function of the scattering wavevector q for length scales greater than the particle size; the width approximately corresponding to twice the particle radius. This simple model reasonably accounts for scattered light from both swollen and shrunken microgel phases.     

Classical light scattering studies on structure formation in polymer solutions

The scattered light is very sensitive to the presence of supermolecular structures in polymer solutions, offering the possibility to study structure formation in situ by classical light scattering and to characterize the generated particles. An improved analysis of light scattering data yields information on the structure type, polydispersity of the systems, particle mass and size as well as the degree of swelling. The flocculation process of ampholytic modified poly(acrylonitrile) was investigated in dependence on the pH-value and the content of different salts in the solvent. Results obtained give a deeper insight into the flocculation mechanism.     

Static and dynamic light scattering of a random coil polymer in rodlike polymer solution

We have studied ternary solutions of rodlike and random coil polymers in a common solvent. In these solutions, the rods scatter weakly, and almost all scattering comes from the coil component. From light scattering measurements, we gained information about the thermodynamic and hydrodynamic properties of the coil in rodlike polymer solutions.     

Scattering structure factor of colloidal gels characterized by static light scattering, small-angle light scattering, and small-angle neutron scattering measurements

The scattering structure factor of a colloidal gel in a q range of 5 orders of magnitude has been determined by combining static light scattering, small-angle light scattering, and neutron scattering measurements. It exhibits simultaneously two types of structure information: a mass fractal scaling within the clusters that constitute the gel and a surface fractal scaling for length scales larger than that of the clusters. Such scattering behavior can be well interpreted by the pair-correlation function proposed in the literature to model an ideal structure constituted of mass fractal objects inside surface fractal objects.     

Static light scattering study of complex formation between protein and neutral water-soluble polymer

Complex formation of poly(N-isopropylacrylamide) (PNIPA) having a weight-average molecular weight of 1,720,000 g/mol with human serum albumin (HSA), ovalbumin (OVA) and lysozyme (LYZ) was studied in an aqueous medium containing 0.01 M NaCl and adjusted to pH 3. The polymer–protein mixtures at different molar ratios (r_m) were examined by static light scattering (SLS). The analysis of SLS data using our own approach [Kokufuta et al., Langmuir 15 (1999) 940; Biomacromolecules 4 (2003) 728] showed that the molecular weight of each resulting complex is smaller than that of the interpolymer complex composed of two polymer chains plus one protein. This indicates the formation of an intrapolymer complex in all the polymer–protein systems studied. Thus, at each r_m we calculated the number of bound proteins per polymer, the value of which was OVA > HSA > LYZ in order. These results were compared with the hydropathy profiles of each protein which are a good tool for obtaining an information about distribution of hydrophobic and hydrophilic segments in a protein. It has become apparent that the hydrophobic interaction between polymer and protein plays an important role in the intrapolymer complex formation.