Advances in Colloid and Interface Science (v.171-172, #C)

This paper reviews the shear rheology of suspensions of microscopic particles. The nature of interparticle forces determines the microstructure, and hence the deformation and flow behavior of suspensions. Consequently, suspensions were classified according to the resulting microstructure: hard-spheres, stabilized, or aggregated particles. This study begins with the most simple case: flowing suspensions of inert, rigid, monomodal spherical particles (called hard-spheres), at low shear rates. Even for inert particles, we reviewed the effect of several factors that produce deviations from this ideal case, namely: shear rate, particle shape, particle size distribution, and particle deformability. Then we moved to suspensions of colloidal particles, where interparticle forces play a significant role. First we studied the case of dispersed or stabilized suspensions (colloidal dispersions), where long range repulsive forces keep particles separated, leading to a crystalline order. Second we studied the more common case of aggregated or flocculated suspensions, where net attractive forces lead to the formation of fractal clusters. Above the gelation concentration (which depends on the magnitude of the attractive forces), clusters are interconnected into a network, forming a gel. We differentiate between weak and strong aggregation, which may lead to weak or strong gels, respectively. Finally, we reviewed the case of filler/matrix composite suspensions or gels, where rigid or viscoelastic particles (fillers) are dispersed in a continuous viscoelastic material (matrix), usually a gel. For each type of suspension, predictive curves of fundamental rheological properties (viscosity, yield stress, elastic and complex moduli) vs. particle volume fraction and shear rate were obtained from theoretical or empirical models and sound experimental data, covering ranges of practical interest.Display Omitted► We reviewed the shear rheology of suspensions of microscopic particles. ► We classified them as hard-sphere, stabilized, and aggregated suspensions. ► We also studied filler-matrix composite suspensions. ► We discussed the main rheological models of each type of suspension. ► For each case, we plotted predictive curves of fundamental rheological parameters.
Keywords: Shear rheology; Hard-sphere suspensions; Stabilized suspensions; Aggregated suspensions; Colloidal gels; Filler-matrix composites;

State of art in porphyrin Langmuir–Blodgett films as chemical sensors by Gabriele Giancane; Ludovico Valli (17-35).
Porphyrins are tetrapyrrolic macrocycles with a fascinating and multifarious variegation of properties of essential significance in up-to-date and leading technologies. From a different point of view, the Langmuir–Blodgett technique allows the immobilisation of films with an accurate regulation of molecular organisation and thickness. As a logical upshot, this manuscript concerns a substantial object of consideration in contemporary research, the utilisation of Langmuir–Blodgett multilayers of porphyrins in sensing elements for the detection of analytes in different matrices. Investigations on the morphological, optical, structural and surface characteristics of these films are remarkably related to the significant properties of sensors with the ultimate goal of rationalising the innermost intercourses between the sensing behaviour and the peculiarities and molecular organisation brought about by the deposition method. The integration of the typical electrical and optical characteristics of porphyrins with the potentialities of the Langmuir–Blodgett multilayer has originated not only encouraging projects but has afforded also certainties on the accomplishment of operative chemical sensors.Display Omitted► We report the use of LB film of porphyrin in chemical sensors. ► The proposed topic is of paramount importance to the present. ► Several transduction mechanisms have been reported.
Keywords: Porphyrins; Langmuir-Blodgett films; Sensors;

Casein micelles and their internal structure by Cornelis G. de Kruif; Thom Huppertz; Volker S. Urban; Andrei V. Petukhov (36-52).
The internal structure of casein micelles was studied by calculating the small-angle neutron and X-ray scattering and static light scattering spectrum (SANS, SAXS, SLS) as a function of the scattering contrast and composition. We predicted experimental SANS, SAXS, SLS spectra self consistently using independently determined parameters for composition size, polydispersity, density and voluminosity. The internal structure of the casein micelles, i.e. how the various components are distributed within the casein micelle, was modeled according to three different models advocated in the literature; i.e. the classical sub-micelle model, the nanocluster model and the dual binding model. In this paper we present the essential features of these models and combine new and old experimental SANS, SAXS, SLS and DLS scattering data with new calculations that predict the spectra. Further evidence on micellar substructure was obtained by internally cross linking the casein micelles using transglutaminase, which led to casein nanogel particles. In contrast to native casein micelles, the nanogel particles were stable in 6 M urea and after sequestering the calcium using trisodium citrate. The changed scattering properties were again predicted self consistently.An important result is that the radius of gyration is independent of contrast, indicating that the mass distribution within a casein micelle is homogeneous. Experimental contrast is predicted quite well leading to a match point at a D2O volume fraction of 0.41 ratio in SANS. Using SANS and SAXS model calculations it is concluded that only the nanocluster model is capable of accounting for the experimental scattering contrast variation data. All features and trends are predicted self consistently, among which the ‘famous’ shoulder at a wave vector value Q = 0.35 nm-1 In the nanocluster model, the casein micelle is considered as a (homogeneous) matrix of caseins in which the colloidal calcium phosphate (CCP) nanoclusters are dispersed as very small (about 2 nm) “cherry stones” at an average distance of 18.6 nm. Attached to the surface of the nanoclusters are the centers of phosphorylation (3-5 nearby phosphorylated amino acid residues) of the caseins. The tails of the caseins, much larger than the CCP clusters, then associate to form a protein matrix, which can be viewed as polymer mesh with density fluctuations at the 2 nm scale. The association of the tails is driven by a collection of weak interactions. We explicitly use weak interactions as a collective term for hydrophobic interactions, hydrogen bonding, ion bonding, weak electrostatic Van der Waals attraction and other factors (but not the strong calcium phosphate interaction) leading to self association. The association is highly cooperative and originates in the weak interactions. It is the cooperativety that leads to a stable casein micelle. Invariably, κ-casein is thought to limit the process of self association leading to stabilization of the native casein micelle.Display Omitted► The internal structure of a casein micelle consists of a protein matrix in which calcium phosphate nanoclusters of about 2 nm radius are dispersed. ► The protein matrix has density fluctuations on a scale of a about two nm radius. ► A casein micelle contains about 285 nanoclusters and five times more protein clusters. ► In pooled milk a casein micelle has a volume average radius of 61 nm and a polydispersity of 0.35. ► A new method for calculating scattering spectra of composite particles is presented.
Keywords: Casein micelle; Structure; Nanocluster model; SANS; Model calculations;

Adhesion of organic films to substrates is important in applications that involve solid surfaces in sliding contact. Although the thickness of self-assembled monolayers (SAMs) is only a few nanometers, they can drastically modify the frictional properties of the underlying substrate, and thus have great potential for serving as boundary lubricants on micro- and nano-scales. This review focuses on the relationship between the structural and compositional properties of SAMs and their frictional response. Adhesion of SAMs to the substrate surface usually occurs through chemisorption of the head groups on the constituent molecules, with molecular interactions such as van der Waals interactions playing important roles in organizing the molecules into surface films, and in controlling their tribological behavior. The durability and wear resistance of SAMs depend on the nature and strength of the binding forces between the head groups and the substrate surfaces, while the adhesion and friction forces are strongly influenced by the interactions of the terminal groups with the counterfaces. Results from both experimental measurements and molecular dynamics simulations consistently indicate that structural ordering of alkyl chains in SAMs reduces their frictional response, and that SAMs formed by molecules with alkyl chains longer than 8 to 10 methylene units are well organized, exhibiting low levels of friction. Less densely packed or more disordered monolayers inherently possess greater numbers of conformational defects at room temperature and present lower barriers to defect creation under the action of a contacting surface, and thus exhibit higher friction. Cross-linking of the spacer chains can reduce the frictional response of disordered films by increasing the chain ordering, but has little impact on SAMs that are already well ordered. On the other hand, introduction of sterically demanding terminal groups and dissimilar molecules reduces molecular ordering in SAMs and increases their frictional response. Significant growth in the application of SAMs to control the adhesion, friction, and wear of materials is expected with better understanding of the frictional properties of SAMs controlled by their structures and compositions and with technological progresses.Display Omitted► Micro-/nanoelectromechanical systems demand robust ultrathin films for lubrication. ► SAMs can drastically modify the frictional properties of the contacting surfaces. ► Structural ordering of the alkyl chains in SAMs controls their frictional properties. ► SAMs are promising for serving as boundary lubricants on micro- and nano-scales. ► Development of experimental and theoretical tools can gain more understanding on SAMs.
Keywords: Self-assembled monolayer (SAM); Friction; Adhesion; Wear; Chain ordering; Nano-lubrication;

Similar to hydrogenated surfactant mixtures, the ones of hydrocarbon and fluorocarbon surfactants can also self-assemble into various aggregates, including mixed micelles and vesicles, however, completely different phase behavior and self-assembly of CH/CF surfactant mixtures in solution can be observed and exhibit novel features because of the repellence between the two hydrophobic chains. These systems of hydrocarbon and fluorocarbon surfactant mixtures can provide important considerations for both theoretical and applied interest. Several advantaged techniques, including small-angle neutron scattering (SANS), small-angle X-ray scattering (SAXS), 19F- and 1H-NMR, cryo-TEM, and Freeze-fracture TEM (FF-TEM) have been widely employed to characterize these mixture systems. In this review, the aggregation behavior, self-assembly aggregation, and interaction of hydrocarbon and fluorocarbon surfactant mixtures in solution are described and focused three aspects, (i) immiscibility and nonideal mixing in hydrocarbon and fluorocarbon surfactant mixed micelles systems; (ii) spontaneous vesicles of hydrocarbon and fluorocarbon surfactant mixtures in aqueous solution; and (iii) self-assembled aggregates of hybrid fluorocarbon/hydrocarbon surfactants in aqueous solutions.Unil-, multi-lamellar vesicles, large vesicles enclosing small unilamellar vesicles formed by hydrocarbon and fluorocarbon surfactant mixtures in water at 25 °C can be obtained by means of FF-TEM image, and the small-angle X-ray scattering (SAXS) measurements show clearly the lamellar structures.Display Omitted► New results of hydrocarbon and fluorocarbon surfactant mixtures in solution were summarized. ► The aggregation behavior, self-assembly aggregation, and interaction are described in detail. ► Three aspects are focused.
Keywords: Hydro- and fluoro-carbon surfactants; Phase behavior; Self-assembly; Interaction; Vesicles;

The numerical values of points of zero charge (PZC, obtained by potentiometric titration) and of isoelectric points (IEP) of various materials reported in the literature have been analyzed. In sets of results reported for the same chemical compound (corresponding to certain chemical formula and crystallographic structure), the IEP are relatively consistent. In contrast, in materials other than metal oxides, the sets of PZC are inconsistent. In view of the inconsistence in the sets of PZC and of the discrepancies between PZC and IEP reported for the same material, it seems that IEP is more suitable than PZC as the unique number characterizing the pH-dependent surface charging of materials other than metal oxides. The present approach is opposite to the usual approach, in which the PZC and IEP are considered as two equally important parameters characterizing the pH-dependent surface charging of materials other than metal oxides.Display Omitted► Well-organized, up-to-date and critical summary of IEP of various materials. ► Erroneous results have been filtered out. ► Complements a previous review in Advances CIS devoted to metal oxides.
Keywords: Point of zero charge; Isoelectric point; Zeta potential; Electrokinetic potential; Mass titration;

by Rob van Daalen (87).