Advances in Colloid and Interface Science (v.173, #C)

Review of the dielectric properties of nanofiltration membranes and verification of the single oriented layer approximation by Darren L. Oatley; Laia Llenas; Ramon Pérez; Paul M. Williams; Xavier Martínez-Lladó; Miquel Rovira (1-11).
The structuring of water at soft solid surfaces remains an area of great interest to colloid science as a whole and has many applications in relation to colloid stability, foams, and wetting films as well as being central to membrane separations. Quantitatively calculating the structural components of thin layers of water and the interaction forces of hydrated molecules with the surface of pores through a layer of water having modified structure is one of the most important challenges in the physics of surface phenomenon. In this paper these effects are reviewed and discussed in relation to the confines of a capillary pore. Membrane nanofiltration is extremely complex and is dependent on the micro-hydrodynamics and interfacial events occurring at the membrane surface and within the membrane nanopores. There is significant debate as to the exact nature of these complex phenomena and rejection is typically attributed to a combination of steric and electrical effects. The electrical effects are less well understood and in particular the contribution of dielectric exclusion. A review of the two competing descriptions of dielectric exclusion is presented along with the theories currently used in modelling this phenomena. A series of rejection experiments of 0.01 M salt solutions at the membrane isoelectric point has been performed for the NF270 and NF99HF membranes. The dielectric constants inside the nanopore are calculated and these values were consistent for three of the salts studied, indicating that a simplistic model based on Born theory is accurate enough for engineering calculations and that ion solvation is most likely to be the more appropriate dielectric exclusion mechanism for true nanofiltration membranes.Display Omitted► Dielectric exclusion from nanofiltration membranes is reviewed. ► Two major mechanisms are accepted, that of image forces and ion solvation. ► A detailed account of both mechanisms is provided with discussion. ► New evidence supporting the single oriented solvent layer approximation is provided. ► Ion solvation appears to be the dominant mechanism for true nanofiltration membranes.
Keywords: Nanofiltration; Dielectric exclusion; Ion solvation; Modelling;

Revisiting the Brewster Angle Microscopy: The relevance of the polar headgroup by Cristina Roldán-Carmona; Juan J. Giner-Casares; Marta Pérez-Morales; María T. Martín-Romero; Luis Camacho (12-22).
The Brewster Angle Microscopy (BAM) is a powerful microscopy technique allowing the in situ visualization of the morphology of Langmuir monolayers at the air/water interface. The use of the BAM for attaining structural insights in the molecular arrangement of the Langmuir monolayers is widespread. In this review, we examine the reflection of a Langmuir monolayer under a rather different perspective than classical: the influence of the polar headgroup of the amphiphiles in the BAM images is taken into account. The relevance of the polar headgroup as the main cause of the BAM features has been the focus of a reduced number of BAM studies. An emerging experimental and theoretical framework from recent bibliography is discussed. Different theoretical scenarios are considered, concerning the size and absorption of radiation of the polar headgroup. Two qualitative examples showing physical phenomena regarding the reflectivity changes in a BAM experiments are discussed. The anisotropy in the BAM images as inner textures is of special interest. Quantitative structural information of the molecular arrangement of the monolayer is obtained by simulating the textures of the domains observed. The quantitative assessment of the detailed molecular arrangement of the polar headgroup by BAM is highly valuable, as this information can hardly be obtained from other experimental techniques. The procedure for extracting quantitative structural data from the experimental BAM pictures is revised in detail from the recent bibliography for further application of this model to different Langmuir monolayers.Display Omitted► The influence of the polar headgroup in the BAM pictures is reviewed. ► An emerging framework for the interpretation of BAM pictures is presented. ► The anisotropy as textures within the domains is quantitatively analyzed. ► A multiple wavelength BAM instrument is proposed.
Keywords: Brewster Angle Microscopy; Langmuir monolayer; 2D domains; Anisotropy; Interfacial aggregation;

Suspension flow in microfluidic devices — A review of experimental techniques focussing on concentration and velocity gradients by A.M.C. van Dinther; C.G.P.H. Schroën; F.J. Vergeldt; R.G.M. van der Sman; R.M. Boom (23-34).
Microfluidic devices are an emerging technology for processing suspensions in e.g. medical applications, pharmaceutics and food. Compared to larger scales, particles will be more influenced by migration in microfluidic devices, and this may even be used to facilitate segregation and separation. In order to get most out of these completely new technologies, methods to experimentally measure (or compute) particle migration are needed to gain sufficient insights for rational design. However, the currently available methods only allow limited access to particle behaviour.In this review we compare experimental methods to investigate migration phenomena that can occur in microfluidic systems when operated with natural suspensions, having typical particle diameters of 0.1 to 10 μm. The methods are used to monitor concentration and velocity profiles of bidisperse and polydisperse suspensions, which are notoriously difficult to measure due to the small dimensions of channels and particles. Various methods have been proposed in literature: tomography, ultrasound, and optical analysis, and here we review and evaluate them on general dimensionless numbers related to process conditions and channel dimensions. Besides, eleven practical criteria chosen such that they can also be used for various applications, are used to evaluate the performance of the methods.We found that NMR and CSLM, although expensive, are the most promising techniques to investigate flowing suspensions in microfluidic devices, where one may be preferred over the other depending on the size, concentration and nature of the suspension, the dimensions of the channel, and the information that has to be obtained. The paper concludes with an outlook on future developments of measurement techniques.Display Omitted► Various experimental techniques can visualise concentration and velocity profiles. ► The techniques are compared based on dimensionless numbers and practical criteria. ► NMR and CSLM can be used to measure micron sized particles in confined geometries. ► Improved resolution and combination of techniques are expected to give more insights.
Keywords: Microfluidic devices; Experimental techniques; Suspension; Particle migration;

Fluorinated semiconductor photocatalysts: Tunable synthesis and unique properties by Shengwei Liu; Jiaguo Yu; Bei Cheng; Mietek Jaroniec (35-53).
Semiconductor photocatalysts are of great significance in solar energy conversion and environmental remediation. To overcome serious drawbacks of these materials with respect to narrow light-response range and low quantum efficiency, a variety of strategies have been developed in the past decades to enhance the light harvesting and excitation as well as the charge transfer against recombination. In particular, fluorination of semiconductor photocatalysts can be employed to modify their surface and bulk properties, and consequently, to enhance their photocatalytic performance. This review presents a comprehensive description of the F-mediated synthesis and unique properties of fluorinated semiconductor photocatalysts, in particular titanium dioxide (TiO2). The available strategies for the synthesis of fluorinated photocatalysts include post-synthesis fluorination and in-situ fluorination. Depending on the synthesis route and conditions, it is possible to control the chemical nature of incorporated fluorine (such as adsorbed fluoride and lattice-doped fluorine) and the fluoride-mediated crystal modification and organization, which often results in exceptional surface and bulk physicochemical properties, giving rise to unique photocatalytic properties. Significantly, the surface fluorination induces unusual adsorption behavior and interfacial charge transfer dynamics, directly affecting photocatalytic redox properties of the surface-fluorinated photocatalysts. The lattice fluorine-doping, sole or cooperative with other complementary co-dopants, introduces special localized electronic structures and surface defect states, accounting for the exceptional visible-light photoactivity of the fluorine-doped photocatalysts. Finally, recent advances in the synthesis and properties of fluorinated photocatalysts are summarized along with perspectives on further developments in this area of research.Display Omitted► A brief overview of fluorinated photocatalysts is presented. ► Post-synthesis and in-situ fluorination methods are reviewed. ► Surface fluorination effects are discussed. ► Doping of photocatalysts with fluorine is reviewed. ► Future perspectives in the area of fluorinated photocatalysts are outlined.
Keywords: Photocatalyst; TiO2; Fluorine; Surface modification; Crystal engineering;