Foreword

Forewor

Reversible coagulation of colloidal suspension in shallow potential wells: direct numerical simulation

Brownian dynamics computer simulations of aggregation in 2D colloidal suspensions are discussed. The simulations are based on the Langevin equations, pairwise interaction between colloidal particles and take into account Brownian, hydrodynamic and colloidal forces. The chosen mathematical model enables to predict the correct values of diffusion coefficient of freely moving particle, the mean value of kinetic energy for each particle in ensemble of interacting colloidal particles and residence times of colloidal particles inside the potential wells of different depths. The simulations allow monitoring formation and breakage of clusters in a suspension as well as time dependence of the mean cluster size

Formation of stable clusters in colloidal suspensions

The experimental evidences and available theoretical explanations on formation of stable clusters in colloidal suspensions are reviewed. The clusters form in the parameters range intermediate between that corresponds to a stable suspension built up by singlets and that causing the irreversible coagulation of the suspension. The stable clusters can appear as a result of a competition between a short range attraction and a long range repulsion between colloidal particles or due to reversible flocculation in the shallow secondary potential well

Colloidal dynamics: influence of diffusion, inertia and colloidal forces on cluster formation

Computer simulations of colloidal suspensions are discussed. The simulations are based on the Langevin equations, pairwise interaction between colloidal particles and take into account Brownian, hydrodynamic and colloidal forces. Comparison of two models, one taking into accout inertial term in Langevin equation and other based on diffusional approximation proposed in Ermak D.L., and McCammon J.A. J. Chem. Phys., 1978, 69, 1352 have shown that both models the prediction of the correct values of the diffusion coefficient and residence time of particle in a doublet and ere therefore suitable to study the dynamics of formation and breakage of clusters in colloidal suspensions. It is shown that the appropriate selection of the time step and taking into account inertia of particles provides also the correct value of the average kinetic energy of each particle during the simulations, what allows to use the model based on full Langevin equations as a reference model to verify the validity of the numerical scheme for simulation using diffusion approximation

Spreading behaviour of aqueous trisiloxane solutions over hydrophobic polymer substrates

The kinetics of spreading of aqueous trisiloxane solutions over different solid hydrophobic substrates has been investigated experimentally. Two pure trisiloxane surfactants with 6 and 8 polyoxyethylene groups at concentrations closed to the critical aggregation concentration (CAC) and the critical wetting concentration (CWC) were used in the spreading experiments. The following three hydrophobic substrates (Teflon AF, Parafilm, and Polystyrene) having different surface properties were used. It was found that the spreading behaviour depends on the hydrophobic/roughness properties of substrates. The rapid spreading and complete wetting were observed for both trisiloxane surfactant solutions at the CWC on a substrate with a moderate hydrophobicity. For both highly hydrophobic Teflon AF and Parafilm substrates only partial wetting was found. The experiments have shown that the spreading behaviour over all substrates proceeds at two stages. At the CAC for both trisiloxanes on all substrates the time lag of the spreading was detected

Interaction forces between colloidal particles in liquid: Theory and experiment

The interaction forces acting between colloidal particles in suspensions play an important part in determining the properties of a variety of materials, the behaviour of a range of industrial and environmental processes. Below we briefly review the theories of the colloidal forces between particles and surfaces including London–van der Waals forces, electrical double layer forces, solvation forces, hydrophobic forces and steric forces. In the framework of Derjaguin–Landau–Verwey–Overbeek (DLVO) theory, theoretical predictions of total interparticle interaction forces are discussed. A survey of direct measurements of the interaction forces between colloidal particles as a function of the surface separation is presented. Most of the measurements have been carried out mainly using the atomic force microscopy (AFM) as well as the surface force apparatus (SFA) in the liquid phase. With the highly sophisticated and versatile techniques that are employed by far, the existing interaction theories between surfaces have been validated and advanced. In addition, the direct force measurements by AFM have also been useful in the explaining or understanding of more complex phenomena and in engineering the products and processes occurring in many industrial applications

Spreading of aqueous trisiloxane surfactant solutions and conventional surfactant solutions over PTFE AF coated silicone wafers

Kinetics of spreading of aqueous trisiloxane solutions Tn (with n = 4, 6 and 8 ethoxy groups) and conventional aqueous surfactant solutions (Tween 20, C12E4, SDS) over silicon wafers coated with PTFE AF is experimentally investigated. It has been found that trisiloxane solutions spread on highly hydrophobic PTFE AF coated silicone wafers; however, they do not show superspreading behavior on these highly hydrophobic substrates. Solutions of conventional non-ionic surfactants investigated show kinetics of spreading similar to trisiloxanes. Three regimes of spreading have been found: (i) at low concentrations complete non-wetting both at an initial and at final stages of spreading, (ii) a transition from initial non-wetting to partial wetting at the end of the spreading process at intermediate concentrations and (iii) partial wetting both at the beginning and the end of the spreading process at higher concentrations. Transition from the first regime (i) to the second regime (ii) takes place at the critical aggregation concentration (CAC) or critical micelles concentration (CMC), transition from regime (ii) to regime (iii) happens at the critical wetting concentration (CWC). In the case of regime (i) the spreading of non-ionic surfactants solutions investigated on PTFE AF coated silicone wafers is slow and follows a theoretically predicted law. In the case of regimes (ii) and (iii) the spreading of the non-ionic surfactant solutions investigated proceeds in two stages: the fast short first stage, which is followed by a much slower second stage. The latter stage is similar to the case of C < CAC/CMC and follows the similar theoretical law. It is shown that the slow stages in both cases develop according to a previously described theoretical model (Starov et al. J. Colloid Interface Sci., (2000), 227(1), 185). According to this theory the surfactant molecules adsorb in front of the moving three phase contact line (autophilic phenomenon), which results in a partial hydrophilisation of an initially hydrophobic substrate and a spreading as a consequence. We assume that the first stage of the spreading is related to the disintegration of surfactant aggregates in the vicinity of the moving three phase contact line

Effect of lithium chloride additive on forward osmosis membranes performance

The research efforts on the development of ideal forward osmosis membranes with high water flux and low reverse salt flux have been devoted in the recent years. In this study, thin film composite polyamide forward osmosis membranes were prepared. The porous polysulfone (PSU), polyphenylsulfone (PPSU), and polyethersulfone (PESU) substrates used in this study were prepared by the phase inversion process, and the active rejection layer was prepared by interfacial polymerization. All the membranes showed highly asymmetric porous structures with a top dense upper layers and finger-like porous substrates with macro voids in the bottom layer. The addition of 3% lithium chloride (LiCl) to the membrane substrates resulted in an increase in both the water flux and reverse salt flux. PSU and PESU showed the highest water flux when the active layer faced the feed solution (AL-FS), while the largest water flux was obtained when the active layer faced the draw solution (AL-DS). For all the membranes, the water flux under the AL-DS orientation was higher than that under the AL-FS orientation