P. Somasundaran

Two- and one-dimensional flocculation of silica spheres on substrates

Abstract A new method for assessing polymer adsorption onto a macroscopic surface and the variation of the adsorption density along the surface is proposed. The presence of the adsorbed polymer is inferred from the adherence of colloidal particles to the surface; conditions are selected such that the particles would not bond if the adsorbed polymer were not present. The method is used to monitor increases in polymer adsorption density and to reveal nonuniform distribution of the adsorbed polymer in patterns having a scale larger than the size of the decorating particles. The technique has been demonstrated to study the effect of chemical pretreatments of surfaces on polymer-induced flocculation of silica to glass. Also, interesting one- and two-dimensional decorations have been obtained, possibly as a result of the surface heterogeneity.

Monitoring nanoparticles in the presence of larger particles in liquids using acoustics and electron microscopy

Monitoring the presence of nanoparticles in dispersions having broad particle size distributions can be a problem for many measurement techniques because large particles or even aggregates of the smaller particles can mask the presence of the sought after nanoparticles. The ability of many existing techniques to detect the nanoparticles when present in broad polydisperse systems is largely unknown, yet it is critical for proper selection of the measuring technique for characterizing a particular nanodispersion. Acoustic spectroscopy is already a known and proven tool for studying nanoparticles in systems with a narrow size distribution. The purpose of this paper is to evaluate the sensitivity of acoustic spectroscopy for determining the nanoparticle content of very polydisperse systems. We used eight different ZnO powders from different manufacturers to prepare 5wt.% dispersions, each dispersed in water. The stability of each dispersion was optimized by pH adjustment and addition of sodium hexametaphosphate as determined by maximizing the measured zeta-potential. According to the acoustic measurement, the median size of these different ZnO dispersions varied from 200nm to 700nm. Independent TEM photographs in general confirmed the size variation between the samples. Independent DLS measurements failed to provide particle size data correlating with TEM. The acoustic measurements further showed that each dispersion contained a different relative content in the nanoparticle fraction. The precision with which the nanoparticle fraction could be determined was better than 2% of the total solid loading for all samples. In order to verify consistency of this measurement we performed a mixing study by adding dispersion with the largest nanoparticle content to the dispersion with the smallest nanoparticle content, in small increments. This test confirms that the acoustic sensitivity threshold is about 2% of nanoparticles in the broad polydisperse dispersions of dense metal oxide particles.

Adsorption of sodium dodecyl sulfate onto clathrate hydrates in the presence of salt

This work presents the effect of NaCl on the adsorption of sodium dodecyl sulfate (SDS) at the cyclopentane (CP) hydrate-water interface. The adsorption isotherms and the SDS solubility in NaCl solutions are obtained using liquid-liquid titrations. The solubility data are determined at typical hydrate forming temperatures (274-287 K) to ensure that the adsorption isotherms are obtained within SDS solubility limits in NaCl solutions. The isotherms show L-S (Langmuir-Step) type behaviors with 1mM and 10mM NaCl solutions while L type isotherm is determined for 25 mM NaCl solutions due to the low SDS solubility in this salt concentration. Zeta potentials of CP hydrate particles in the aqueous solutions support the shape of the adsorption isotherm with the 1mM NaCl solution. The 1mM NaCl case shows the highest SDS adsorption amount among the cases with 0 mM, 10 mM, and 25 mM NaCl solutions. In this case, the competition for adsorption between Cl(-) and DS(-) is not as strong compared to the 10 and 25 mM NaCl cases and the presence of Na(+) ions may reduce the repulsion between DS(-) ions, which results in a higher adsorption of DS(-) ions and enhanced enclathration.

Investigation of colloidal properties of modified silicone polymers emulsified by non-ionic surfactants.

Functionalized silicones are a unique class of hybrid materials due to their simultaneous hydrophobic-oleophobic properties, which results in applications in a variety of surface modification techniques. Prior research has shown that changes in surface charge and turbidity of modified silicone emulsions as a function of pH have a marked effect on their performance in coating applications. The emulsion droplet size is also believed to play significant role in such coating applications. In this work, modified silicone polymer emulsions stabilized by non-ionic surfactants were studied using dynamic light scattering (dilute) and electroacoustic (concentrated) spectroscopy to monitor the emulsion droplet size. The dilute and concentrated regime studies showed the emulsion droplet to be in nanometer range with no appreciable change in size as a function of pH. Electroacoustic studies showed a small fraction of droplets to be present in the micron size range. The emulsions were examined using Cryo-TEM technique, and the effect of pH and dilution on hydrophobicity of nanodomains was studied by employing fluorescence spectroscopy. It is shown from pyrene excimer behavior that both the dilution and pH have an effect on emulsion stability with a presence of critical surfactant concentration after which the emulsion was destabilized. It is proposed that the emulsion stability characteristics and the particle size distribution both play a significant role in their ability to impart desired macro and nano surface properties to treated substrates through electrostatic interactions and selective binding.

Modification of surface properties of cellulosic substrates by quaternized silicone emulsions

The present work describes the effect of quaternization of silicones as well as the relevant treatment parameter pH on the frictional, morphological and relaxation properties of fabric substrates. Due to their unique surface properties, silicone polymers are extensively used to modify surface properties of various materials, although the effects of functionalization of silicones and relevant process conditions on modification of substrates are not well understood. Specifically we show a considerable reduction in fabric friction, roughness and waviness upon treatment with quaternized silicones. The treatment at acidic pH results in better deposition of silicone polymers onto the fabric as confirmed through streaming potential measurements which show charge reversal of the fabric. Interestingly, Raman spectroscopy studies show the band of C-O ring stretching mode at ∼1095 cm(-1) shift towards higher wavenumber indicating lowering of stress in fibers upon appropriate silicone treatment. Thus along with the morphological and frictional properties being altered, silicone treatment can lead to a reduction in fabric strain. It is concluded that the electrostatic interactions play an initial role in modification of the fiber substrate followed by multilayer deposition of polymer. This multi-technique approach to study fiber properties upon treatment by combining macro to molecular level methods has helped in understanding of new functional coating materials.

In Situ kinetics measurements of surfactant adsorption on colloidal alumina using ESR spectroscopy

Abstract An electron spin resonance spectroscopy technique is employed to investigate in situ the kinetics of surfactant adsorption on colloidal particles. Using this technique, it was found that 40% of the adsorption of Aerosol OT at the alumina/cyclohexane interface takes place within 5 s after addition of the surfactant to the suspension.