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Advances in Colloid and Interface Science (v.127, #1)
Probing fast kinetics in complex fluids by combined rapid mixing and small-angle X-ray scattering by P. Panine; S. Finet; T.M. Weiss; T. Narayanan (pp. 9-18).
Stopped-flow mixing coupled to small-angle X-ray scattering (SAXS) is an established technique for investigating structural kinetics in solution down to the millisecond range. More recently, the emphasis has shifted to the sub-millisecond range using continuous flow microfluidic mixing devices. The aim of this article is to review the present status and limitations when applying mixing techniques to a wide range of soft matter and biological systems. In the case of SAXS, special consideration of the mixing quality is necessary for a quantitative description of the scattered intensity. This is demonstrated through two representative examples involving protein refolding and micellar self-assembly.
Keywords: Fast mixing devices; Small angle X-ray scattering; Stopped-flow method; Time resolved scattering
Surfaces and interfacial water: Evidence that hydrophilic surfaces have long-range impact by Jian-ming Zheng; Wei-Chun Chin; Eugene Khijniak; Eugene Khijniak Jr.; Gerald H. Pollack (pp. 19-27).
It is generally thought that the impact of surfaces on the contiguous aqueous phase extends to a distance of no more than a few water-molecule layers. Older studies, on the other hand, suggest a more extensive impact. We report here that colloidal and molecular solutes suspended in aqueous solution are profoundly and extensively excluded from the vicinity of various hydrophilic surfaces. The width of the solute-free zone is typically several hundred microns. Such large exclusion zones were observed in the vicinity of many types of surface including artificial and natural hydrogels, biological tissues, hydrophilic polymers, monolayers, and ion-exchange beads, as well as with a variety of solutes. Using microscopic observations, as well as measurements of electrical potential and UV–Vis absorption-spectra, infrared imaging, and NMR imaging, we find that the solute-free zone is a physically distinct and less mobile phase of water that can co-exist indefinitely with the contiguous solute-containing phase. The extensiveness of this modified zone is impressive, and carries broad implication for surface–molecule interactions in many realms, including cellular recognition, biomaterial–surface antifouling, bioseparation technologies, and other areas of biology, physics and chemistry.
Keywords: Water; Gel; Colloid; Hydrophilic surface; Interface
Polarization model for poorly-organized interfacial water: Hydration forces between silica surfaces by Marian Manciu; Oscar Calvo; Eli Ruckenstein (pp. 29-42).
The main goal of this paper is to review the theoretical models which can be used to describe the interactions between silica surfaces and to show that a model proposed earlier by the authors (the polarization model), which accounts concomitantly for double layer and hydration forces, can be adapted to explain recent experiments in this direction. When the water molecules near the interface were considered to have an ice-like structure, a strong coupling between the double layer and hydration forces (described by the correlation length between neighboring dipoles, λ m) generates long range interactions, larger than the experimentally determined interactions between silica surfaces. Arguments are brought that a gel layer is likely to be formed on the surface of silica, which, by generating disorder in the interfacial water layers, can decrease strongly the value of λ m. Since the prediction of λ m involves a choice for the microscopic structure of water, which is often unknown, the polarization model is also presented here as a phenomenological theory, in which λ m is used as a fitting parameter. Two extreme cases are considered. In one of them, the water molecules near the interface are considered to have an ice-like structure, whereas in the other they are considered randomly distributed. In the first case, the dipole correlation length λ m=14.9 Å. In the second limiting case, λ m can be of the order of 1 Å. It is shown that, for λ m=4 Å, a more than qualitative agreement with the experiment could be obtained, for reasonable values of the parameters involved (e.g. surface dipole strength and density, dipole location, surface charge).
Keywords: Hydration force; Colloidal interactions; Silica
Photocatalytic paper from colloidal TiO2—fact or fantasy by Robert Pelton; Xinglian Geng; Michael Brook (pp. 43-53).
Photocatalytic paper encompasses a range of materials based on paper and nonwoven fabrics which performs a function based on the light-activated catalytic activity of colloidal TiO2. The literature describing photocatalytic paper is surveyed, including mechanisms, applications, limitations and future opportunities. The technology is in its infancy with less than 10 patents and as many scientific publications appearing over the last decade. The main applications described are the destruction of organic molecules (mineralization) and photo-disinfection (sterilization). These disclosures build upon a much larger literature describing photochemical properties of TiO2 both supported on non-cellulose substrates or simply as suspended particles in water or air.Current photocatalytic paper developments include methods to fix TiO2 to cellulose substrates to minimize photochemical damage to the paper. Another theme is the use of multiple approaches, such as zeolites, for enhanced mineralization, and metals, such as silver and copper, for enhanced photocatalytic disinfection.
Keywords: Titanium dioxide; Photocatalysis; Mineralization; Photoactive paper; Disinfection; Sterilization