Light scattering - diagnostic methods for colloidal dispersions

A Hitchhiker's Guide to Particle Sizing Techniques

Accurate characterization of particle size and particle size distributions is mandatory in nanotechnology and a broad range of colloidal sciences. The size of colloidal particles can be determined using various techniques in direct and reciprocal space, including electron microscopy and static and dynamic light scattering. Differential dynamic microscopy was introduced recently and offers a new alternative. In this paper we present a systematic study of particle size determination using various techniques. We compare the results and highlight advantages and disadvantages. Unexpectedly we find that differential dynamic microscopy offers the unique possibility to determine the particle size in highly turbid samples.

Sizing of reverse micelles in microemulsions using NMR measurements of diffusion

This paper reports the size of reverse micelles (RMs) in AOT/octane/H(2)O and CTAB/hexanol/H(2)O microemulsions using magnetic resonance (MR) pulsed field gradient (PFG) measurements of diffusion. Diffusion data were measured using the pulsed gradient stimulated echo (PGSTE) experiment for surfactant molecules residing in the RM interface. Inverse Laplace transformation of these data generated diffusion coefficients for the RMs, which were converted into hydrodynamic radii using the Stokes-Einstein relation. This technique is complementary to those previously used to size RMs, such as dynamic light scattering (DLS) and small-angle X-ray scattering (SAXS), but also offers several advantages, which are discussed. RM sizes, determined using the PGSTE method, in the AOT (sodium bis(2-ethylhexyl) sulfosuccinate) and CTAB (cetyltrimethylammonium bromide) microemulsions were compared with previous DLS and SAXS data, showing good agreement. Methods for determining number distributions from the PGSTE data, through the use of scaling factors, were investigated.

Investigating the microstructure of a yield-stress fluid by light scattering

We report the results of an experimental study of the microstructure of dispersions of Carbopol ETD 2050, a model yield-stress fluid. Using two different light scattering instruments, measurements were made over three decades in scattering wave vector, from 0.02 to 25 μm⁻¹. These measurements reveal microstructure characterized by two length scales: a longer length scale, 6 μm and larger, that depends on Carbopol concentration and the pH of the dispersion and a shorter length scale of approximately 400 nm that is independent of both sample concentration and pH. We relate these results to shear rheology measurement of the yield stress of these materials.

Fast microscopic method for large scale determination of structure, morphology, and quality of thin colloidal crystals

We present a novel fast microscopic method to analyze the crystal structures of air-dried or suspended colloidal multilayer systems. Once typical lattice spacings of such films are in the range of visible light, characteristic Bragg scattering patterns are observed. If in microscopic observations these are excluded from image construction, a unique color coding for regions of different structures, morphologies, and layer numbers results. Incoherently scattering defect structures, however, may not be excluded from image construction and thus remain visible with high resolution.

Light scattering investigations on dilute nonionic oil-in-water microemulsions

Dilute 3-component nonionic oil-in-water microemulsions formulated with either a polyoxyethylene surfactant (C18:1E10 or C12E10) or the alkylamine-N-oxide surfactant, DDAO (C12AO), and containing either a triglyceride or an ethyl ester oil have been examined using dynamic and static light-scattering techniques. Analysis of the results showed distinct differences in the tested oil's mode of incorporation into the microemulsion droplets, with both the molecular volume of the oil and the hydrophobic chain length of the surfactant being important. For example, microemulsions formulated by C18:1E10 and containing one of the larger molecular volume oils (that is, either a triglyceride, Miglyol 812, or soybean oil) or the ethyl ester of fatty acid oil, ethyl oleate, exhibited first a decrease and then an increase in hydrodynamic size and surfactant aggregation number, suggesting that the asymmetric C18:1E10 micelles became spherical upon the addition of a small amount of oil and grew thereafter because of further oil being incorporated into the core of the spherical microemulsion droplet. A similar conclusion of sphericity could not be drawn for microemulsions stabilized by C18:1E10 and containing one of the oils smaller in molecular volume (namely tributyrin, ethyl butyrate, or ethyl caprylate) where neither the aggregation number nor the hydrodynamic radius changed much upon the addition of oil. This result suggested that these oils were preferentially located in the interfacial surfactant monolayer, behaving in much the same way as a cosurfactant. A different trend of results, however, was seen for microemulsions prepared using C12E10 and C12AO, most likely because these surfactants produced approximately spherical micelles. In this case, the microemulsions containing the oils larger in molecular volume tended to exhibit an increase in surfactant aggregation number and hydrodynamic size, suggesting the growth of spherical micelles, while the smaller oils (in particular ethyl butyrate) caused a significant decrease in surfactant aggregation number incompatible with their being incorporated into the centre of the droplet, suggesting that the oils were being located in the interfacial surfactant monolayer. These results suggest that the various oils are incorporated into the microemulsions in very different ways.

Circular-array optical-fiber probe for backscattering photon correlation spectroscopy measurements

A miniaturized probe comprising a circular array of optical fibers coupled to a graded-index microlens has been designed for analyzing colloidal solutions by photon correlation spectroscopy (PCS). The system's suitability for PCS measurements was validated by experimental testing performed in bulk solutions and small drops of monodisperse and bimodal test colloidal solutions.