Advances in Colloid and Interface Science (v.146, #1-2)

Quantitative imaging of colloidal flows by Rut Besseling; Lucio Isa; Eric R. Weeks; Wilson C.K. Poon (1-17).
We present recent advances in the instrumentation and analysis methods for quantitative imaging of concentrated colloidal suspensions under flow. After a brief review of colloidal imaging, we describe various flow geometries for two and three-dimensional (3D) imaging, including a ‘confocal rheoscope’. This latter combination of a confocal microscope and a rheometer permits simultaneous characterization of rheological response and 3D microstructural imaging. The main part of the paper discusses in detail how to identify and track particles from confocal images taken during flow. After analyzing the performance of the most commonly used colloid tracking algorithm by Crocker and Grier extended to flowing systems, we propose two new algorithms for reliable particle tracking in non-uniform flows to the level of accuracy already available for quiescent systems. We illustrate the methods by applying it to data collected from colloidal flows in three different geometries (channel flow, parallel plate shear and cone plate rheometry).
Keywords: Colloids; Suspensions; Confocal microscopy; Particle tracking; Flow; Rheology;

A review of the kinetic models, recently developed by the authors for the nucleation mechanism of protein folding and for the barrierless thermal denaturation, is presented. Both models are based on the mean first passage time analysis. A protein is treated as a random heteropolymer consisting of hydrophobic, hydrophilic, or neutral beads. As a crucial idea of the model, an overall potential around the cluster of native residues wherein a residue performs a chaotic motion is considered as the combination of the average dihedral, effective pairwise, and confining potentials. The overall potential as a function of the distance from the cluster center has a double well shape which allows one to determine its emission and absorption rates by the first passage time analysis. One can thus develop a theory for the nucleation mechanism of protein folding and calculate the temperature dependence of the folding time. A kinetic model for protein denaturation occurring in a barrierless way has been also developed by using the same approach. The numerical calculations for two model proteins (one consisting of 124 amino acids and the other of 2500 amino acids) demonstrate that the models can predict folding and unfolding times consistent with experimental data.
Keywords: Protein folding; Protein denaturation; Mean first passage time; Nucleation pathway; Optimal folding temperature; Spinodal decomposition;

In order to focus on salient interactions it is customary to design a parameter network representing the reference (ideal) molecular behavior. Such reference properties are subtracted from the experimental data and the difference is analyzed. Each network is based on generally agreed thermodynamic dependent and independent variables defining unambiguously the state of the system. Unfortunately, more correction terms have been introduced making the relationship with traditional thermodynamic networks dependent on each system investigated. A critical comparison is made on the conceptual constraints introduced when developing the two most utilized reference thermodynamic networks. The first is represented by the standard mixing and excess functions of Gibbs free energy. The second represents tailored models involving van der Waals liquids and polymer solutions. Although the mixing and excess functions are formally same, their definition varies dramatically. Unfortunately this influences the analysis of divergence from the reference function, resulting in serious consequences as shown by the entropy and enthalpy obtained from temperature dependency. Moreover, entirely new functions relating to multiple solubility parameters in excess of standard van der Waals behavior are developed since reference data for these models are easily accessible.
Keywords: Solubility parameter; Exchange energy density parameter; Interchange cohesive pressure parameter; Work of adhesive pressure interaction; Interaction parameter; Energy of mixing; Excess energy; Combinatory energy; Residual energy;

Hofmeister series and specific interactions of charged headgroups with aqueous ions by Nina Vlachy; Barbara Jagoda-Cwiklik; Robert Vácha; Didier Touraud; Pavel Jungwirth; Werner Kunz (42-47).
In this paper, we propose a Hofmeister-like ordering of charged headgroups. To this purpose we review various literature data and complete them with some new experimental and computational results on interactions of ions with alkyl sulfates and carboxylates. We further combine the proposed headgroup ordering with the law of matching water affinities in order to obtain a general description and predictions of ion-headgroup interactions. Examples from colloidal chemistry and from biological systems are provided to illustrate the power of this approach.
Keywords: Hofmeister series; Specific-ion effects; Surfactant headgroups; Molecular dynamics;

Normal capillary forces by Hans-Jürgen Butt; Michael Kappl (48-60).
A liquid meniscus between two lyophilic solid surfaces causes an attractive force, the capillary force. The meniscus can form by capillary condensation or by accumulation of adsorbed liquid. Under ambient conditions and between hydrophilic surfaces, capillary forces usually dominate over other surface forces. They are relevant in many processes occurring in nature and technical applications, for example the flow of granular materials and friction between surfaces. Here we review normal capillary forces, focusing on a quantitative description with continuum theory. After introducing the capillary force between spherical surfaces, we extend the discussion to other regular and irregular surfaces. The influence of surface roughness is considered. In addition to capillary forces at equilibrium, we also describe the process of meniscus formation. Assumptions, limits, and perspectives for future work are discussed.
Keywords: Capillary condensation; Meniscus force; Granular matter; Condensation;