Advances in Colloid and Interface Science (v.179-182, #C)

Honorary Note: John Ralston, AO by Terry Blake; Daniel Fornasiero; Tom Healy; Brian Vincent (1-4).

Some basic elements of the adsorption of proteins on solid surfaces are briefly reviewed, emphasizing immunoglobulins. The paper focuses on the physicochemical interactions and considers the precautions that have to be taken to let the protein adsorb in a way in which it is biologically active. Contributing factors include surface pretreatment, composition of the solution, (pH, nature and concentration of electrolytes, etc.), extent of reversibility, and lateral interactions in the adsorbed state. Particular attention is paid to the option of partially pre-coating the adsorbent by irreversibly adsorbed polymers to induce the later adsorbing immune globulin molecules to assume a biologically preferred orientation and conformation.Display Omitted► The adsorption of polymers and proteins depends on the kinetics of attachment. ► The degree of occupancy and the rates of relaxation are coupled parameters. ► Conditions for optimal specific biological activity can be defined.
Keywords: Proteins; Immunoglobulins; IgG; Interface; Adsorption; Orientation; Conformation; Biological activity;

Although polymerization of reactive surfactants (surfmers) in micelles and other self-assembled phases has been studied for at least 30 years, the last decade or so has seen substantial advances in understanding both the structure and dynamics of these systems. In this review we highlight the new insights yielded primarily by small-angle neutron scattering (SANS) using high-flux sources, the perspective this provides for realizing topochemical polymerization in micellar systems, and the prospects and new developments for further exploiting SANS in this field. We present some new neutron contrast variation results exemplifying these elements.Display Omitted► Recent studies on polymerizable micelles are reviewed. ► Tail and head-type polymerizable micelles are compared. ► Thermodynamic versus kinetic trapping of micelle morphology is discussed. ► Advances in understanding morphology due to SANS and cryo-TEM are presented.
Keywords: Polymerizable micelle; Surfmer; Polysoap; Self-assembly; Small-angle neutron scattering (SANS);

Forced wetting of a reactive surface by T.D. Blake (22-28).
The dynamic wetting of water on gelatin-coated poly(ethylene terephthalate) (GC-PET) has been investigated by forced wetting over a wide speed range and compared with earlier data obtained with unmodified PET. The results were analysed according to the molecular-kinetic theory of dynamic wetting (MKT). Both substrates show complex behaviour, with separate low- and high-speed modes. For the GC-PET, this is attributed to a rapid change in the wettability of the substrate on contact with water, specifically a surface molecular transformation from hydrophobic to hydrophilic. This results in a smooth wetting transition from one mode to the other. For the PET, the bimodal behaviour is attributed to surface heterogeneity, with the low-speed dynamics dominated by interactions with polar sites on the substrate that become masked at higher speeds. In this case, the transition is discontinuous. The study has general ramifications for the investigation of any wetting processes in which a physicochemical transformation takes place at the solid surface on contact with the liquid. In particular, it shows how forced wetting, combined with the MKT, can reveal subtle details of the processes involved. It is unlikely that similar insight could be gained from spontaneous wetting studies, such as spreading drops.Display Omitted► Complex interaction of water with gelatin-coated PET revealed by forced wetting. ► Hydrophobic to hydrophilic transition modelled by zeroth order kinetics. ► Superficially similar results with uncoated PET are due to surface heterogeneity. ► Molecular-kinetic theory of wetting can interpret the results for both surfaces. ► The work has general implications for studies of reactive wetting.
Keywords: Dynamic wetting; Coating; Molecular-kinetic theory; Gelatin; Hydrophobic to hydrophilic transition; Heterogeneous surfaces;

Wetting is important when solids are brought in contact with a liquid and for movement of micro-drops on solids. The Young equation that describes the wetting of a solid and the relations between adhesion and wetting and vapor adsorption and wetting are discussed first. The characterization of low-energy surfaces is discussed as these surfaces are important for electrowetting on a dielectric (EWOD) and the characterization methods reveal that both dispersion and acid–base interactions are important for wetting. Surfactant solutions are very suitable for modifying the wetting behavior; the distinct differences between low- and high-energy surfaces in relation to surfactant adsorption are discussed. The effects of surface charges on the wetting behavior are considered for both adsorbed charges and charges due to an applied electric potential (electrowetting). The physical interpretation of Young-Lippmann equation, which describes EWOD, is critically examined and contact angle saturation is briefly discussed.Display Omitted► Solid–liquid adhesion is governed by van der Waals and acid–base interactions. ► Surfactant adsorption modifies wetting of low- and high-energy surfaces differently. ► The “effective” SL-interfacial tension is the excess Gibbs energy of the SL-interface. ► Contact angle saturation occurs when the interfacial water structure becomes unstable. ► An unstable water structure prevents a further increase of the acid–base interactions.
Keywords: Wetting; Work of adhesion; Critical surface tension; Surfactant adsorption; Protonic surface charge; Electrowetting; Electronic surface charge; Contact angle saturation;

Surface charge and wetting characteristics of layered silicate minerals by Xihui Yin; Vishal Gupta; Hao Du; Xuming Wang; Jan D. Miller (43-50).
The surface characteristics, including surface charge and wettability, of layered silicates are reviewed based on experimental results and molecular dynamics simulation (MDS) results. The surface charge features of important layered silicates including mica, talc, and kaolinite are described from atomic force microscopy (AFM) measurements, electrophoresis measurements, and/or results from potentiometric titration. In addition, the wetting characteristics of the silica tetrahedral surface which is common to all layered silicates are examined with different experimental techniques and results are discussed. The wettability of trilayer silicates and bilayer silicates is discussed, particularly the wettability of the silica tetrahedral face and alumina octahedral face of kaolinite based on MDS results as well as recent AFM results.Display Omitted► Recent results on the surface characteristics of layered silicates are reviewed. ► The charging behavior of the talc and muscovite edge surfaces is discussed. ► Surface potentials of the kaolinite silica face and the alumina face are presented. ► Hydrophobicity for the kaolinite silica face is demonstrated from AFM results.
Keywords: Layered silicates; Surface charge; Wettability; AFM; MDS;

Charging of particles in aqueous suspensions is primarily related to potential determining ions, such as silver and iodide ions at silver halide particle surfaces. Proton is considered as a (secondary) potential determining ion at hydrated metal oxide surfaces. Indifferent electrolytes neutralize at increased concentration the surface charge but do not reverse it. However, in the presence of a non-Coulombic interaction the surface charge may be enhanced or reversed at increased ionic strength. Such interaction is denoted specific which may be due to enhanced van der Waals dipolar, Lewis acid–base, solvation (Hofmeister) and/or Born solvation effects. Alternatively, these interactions have been characterized in terms of (semi) empirical ion and surface properties, such as hard–soft acid–base (HSAB) interaction. Within the Stern layer closest to the particle surface truly specific effects are related to the inner Helmholtz plane (IHP) in order to distinguish them from the charge and solvation related effects occurring within the outer Helmholtz plane (OHP). We review some recent observations on the particular influence of ions on the charging of titania particles in aqueous 1:1, 2:1, 1:2 and mixed electrolyte suspensions.The stability of titania suspensions can be conveniently controlled with 1:1, 2:1, 1:2 and mixed electrolytes. Their influence on the colloidal stability which is not explained by the DLVO theory is highlighted.Display Omitted► The charging and stability of titania suspensions can be conveniently controlled with 1:1, 2:1, 1:2 and mixed electrolytes. ► The concentration dependent differences between alkali and earth alkali cations result from solvation (hydration) effects. ► The concentration dependent influences of divalent anions may be related to their complexation behavior with surface sites. ► Combination of 1:1 and 2:1 or 1:2 (mixed) electrolytes leads partly to non-additive effects as a function of concentration. ► The difference between anatase and rutile depends on their manufacture plus properties and solids fraction of the suspension.
Keywords: Titania suspensions; Partial charge model; Multisite complexation model; DLVO model; Stern layer adsorption; Schultze–Hardy model; 1:1, 2:1, 1:2 and mixed electrolytes; ζ-Potential; Isoelectric points; Common intersection points; Settling; Viscosimetry; Yield stress;

Nanofluids mediating surface forces by Georgia A. Pilkington; Wuge H. Briscoe (68-84).
Fluids containing nanostructures, known as nanofluids, are increasingly found in a wide array of applications due to their unique physical properties as compared with their base fluids and larger colloidal suspensions. With several tuneable parameters such as the size, shape and surface chemistry of nanostructures, as well as numerous base fluids available, nanofluids also offer a new paradigm for mediating surface forces. Other properties such as local surface plasmon resonance and size dependent magnetism of nanostructures also present novel mechanisms for imparting tuneable surface interactions. However, our fundamental understanding, experimentally and theoretically, of how these parameters might affect surface forces remains incomplete. Here we review recent results on equilibrium and dynamic surface forces between macroscopic surfaces in nanofluids, highlighting the overriding trends in the correlation between the physical parameters that characterise nanofluids and the surface forces they mediate. We also discuss the challenges that confront existing surface force knowledge as a result of this new paradigm.Display Omitted► Nanofluids as a new paradigm for mediating surface forces. ► Forces tuneable via nanostructure size, shape, surface chemistry and concentration. ► Many outstanding questions, challenges and opportunities.
Keywords: Surface forces; Nanofluids; Nanoparticles; Complex fluids; Colloidal interactions;

Investigating forces between charged particles in the presence of oppositely charged polyelectrolytes with the multi-particle colloidal probe technique by Michal Borkovec; Istvan Szilagyi; Ionel Popa; Marco Finessi; Prashant Sinha; Plinio Maroni; Georg Papastavrou (85-98).
Direct force measurements are used to obtain a comprehensive picture of interaction forces acting between charged colloidal particles in the presence of oppositely charged polyelectrolytes. These measurements are achieved by the multi-particle colloidal probe technique based on the atomic force microscope (AFM). This novel extension of the classical colloidal probe technique offers three main advantages. First, the technique works in a colloidal suspension with a huge internal surface area of several square meters, which simplifies the precise dosing of the small amounts of the polyelectrolytes needed and makes this approach less sensitive to impurities. Second, the particles are attached in-situ within the fluid cell, which avoids the formation of nanobubbles on the latex particles used. Third, forces between two similar particles from the same batch are being measured, which allows an unambiguous determination of the surface potential due to the symmetry of the system. Based on such direct force measurements involving positively and negatively charged latex particles and different polyelectrolytes, we find the following forces to be relevant. Repulsive electrostatic double-layer forces and attractive van der Waals forces as described by the theory of Derjaguin, Landau, Verwey, and Overbeek (DLVO) are both important in these systems, whereby the electrostatic forces dominate away from the isoelectric point (IEP), while at this point they vanish. Additional non-DLVO attractive forces are operational, and they have been identified to originate from the electrostatic interactions between the patch-charge heterogeneities of the adsorbed polyelectrolyte films. Highly charged polyelectrolytes induce strong patch-charge attractions, which become especially important at low ionic strengths and high molecular mass. More weakly charged polyelectrolytes seem to form more homogeneous films, whereby patch-charge attractions may become negligible. Individual bridging events could be only rarely identified from the retraction part of the force profiles, and therefore we conclude that bridging forces are unimportant in these systems.Display Omitted► Novel multi-particle colloidal probe technique based on the atomic force microscope is discussed. ► Forces between charged particles in the presence of polyelectrolytes are studied. ► Repulsive electrostatic double-layer forces are dominant away from the isoelectric point (IEP). ► Close to the IEP attractive van der Waals and patch-charge forces are important.
Keywords: Surface forces; Polyelectrolytes; Dendrimers; Water–solid interface; Latex particles; Atomic force microscopy; Direct force measurements;

The forces acting in colloidal suspensions and affecting their stability and aggregation kinetics are considered. The approximations used for these forces in numerical simulations and the importance of the balanced account for both colloidal forces and hydrodynamic interactions are discussed. As an example the results of direct numerical simulations of kinetics of aggregation either with account for hydrodynamic interaction between particles or without it are compared by varying the parameters of the interaction potential between particles and fraction of solid. Simulations are based on the Langevin equations with pairwise interaction between particles and take into account Brownian, hydrodynamic and colloidal forces. It is confirmed that the neglecting of hydrodynamic interaction results in an accelerated growth of aggregates. The results of numerical simulations of aggregation kinetics are compared with well known analytical solutions.Aggregates formed at the fraction of solid 0.32; time of aggregation 20 s; depths of the potential well U min  = 6 kT.Display Omitted► Forces affecting stability and aggregation kinetics of colloidal suspensions. > Brownian dynamic simulations of aggregation kinetics. ► Reversible aggregation with constant average size of aggregates at 4-8 kT. ► Aggregates growth rate is overestimated by neglecting of hydrodynamic interactions.
Keywords: Colloidal suspensions; Colloidal forces; Hydrodynamic interactions; Aggregation; Langevin equations;

Is adhesion superficial? Silicon wafers as a model system to study van der Waals interactions by Peter Loskill; Hendrik Hähl; Thomas Faidt; Samuel Grandthyll; Frank Müller; Karin Jacobs (107-113).
Adhesion is a key issue for researchers of various fields, it is therefore of uppermost importance to understand the parameters that are involved. Commonly, only surface parameters are employed to determine the adhesive forces between materials. Yet, van der Waals forces act not only between atoms in the vicinity of the surface, but also between atoms in the bulk material. In this review, we describe the principles of van der Waals interactions and outline experimental and theoretical studies investigating the influence of the subsurface material on adhesion. In addition, we present a collection of data indicating that silicon wafers with native oxide layers are a good model substrate to study van der Waals interactions with coated materials.Display Omitted► Adhesion on coated materials is an interplay of surface and bulk interactions. ► Subsurface materials contribute to adhesion via van der Waals interactions. ► Si wafers with native oxide layers are a good model system for coated materials.
Keywords: Adhesion; Multilayer; van der Waals-interactions; Silicon oxide layer;

The analytical model of nanoparticle recovery by microflotation by N. Mishchuk; J. Ralston; D. Fornasiero (114-122).
A model of collision and collection of Brownian submicron particles based on the creation of a convective–diffusion layer near a bubble surface and overcoming the energy barrier created by particle/bubble interaction is developed. Simple analytical expressions describing the rate of collision and collection efficiency are obtained. The collision and collection minimums and the limits of theory applicability are analysed.The collection efficiency of submicron particles at their different surface characteristics.Display Omitted► Brownian particles create a convective–diffusion layer around a rising bubble. ► Interaction of submicron particles with a bubble should be described by an energy model. ► Concentration of particles may be analysed using “population balance equations”. ► Simple analytical expressions for collision and collection efficiency are obtained.
Keywords: Collection; Collision; Convective–diffusion layer; Energy barrier; Microflotation; Population balance equations;

Generation and characterization of submicron size bubbles by Chendi Wu; Kirsten Nesset; Jacob Masliyah; Zhenghe Xu (123-132).
A baffled high intensity agitation (BHIA) cell was used to generate submicron size bubbles of an average diameter around 500 nm by hydrodynamic cavitation. The generation of submicron size bubbles by BHIA cell was found to be largely dependent on the agitation speed of impellers. The duration of agitation and temperature showed only a marginal effect on generation of submicron size bubbles. Surface properties such as zeta-potential and stability of submicron size bubbles were found to be highly dependent on the chemistry of solutions in which the bubbles are generated. The presence of surfactant and frother in water was found to be beneficial for generating a larger number of submicron size bubbles that are more stable, having a life time of up to 24 h.Characteristics of submicron size bubbles generated by hydrodynamic cavitation in SDS (a) and DF250 (b) solutions at pH 8.5.Display Omitted► Review of current methods for nanobubble generation. ► Successful generation of stable submicron size bubbles by hydrodynamic cavitation. ► Submicron size bubbles stable for at least 24 h in frother or surfactant solutions. ► Fine tune of surface charge of submicron size bubbles by solution chemistry. ► Specific adsorption of calcium on CO2 bubbles, but not on air/water vapour bubbles.
Keywords: Submicron size bubble; Hydrodynamic cavitation; Bubble generation; Bubble stability; Zeta-potential;

In most fields of technological application of superhydrophobic materials, such as protection against corrosion, icing, and capillary condensation, or micro fluidics applications, a superhydrophobic surface has to operate in contact with aggressive aqueous media. Therefore, the peculiarities of behaviour of hydrophobic and superhydrophobic surfaces on prolonged contact with water and the mechanisms of possible degradation of superhydrophobicity need to be discussed. In this study, a consideration of the physicochemical processes accompanying the contact of hydrophobic and superhydrophobic materials with water, acid, alkaline and saline aqueous solutions is presented on the basis of experimental data on three-phase equilibrium obtained by the sessile drop method. It is shown that simultaneous analysis of the contact angle and contact diameter of the sessile drop and liquid/vapour surface tension allows one to attribute degradation of the superhydrophobic and hydrophobic state to reversible and irreversible processes such as hydrolysis of hydrophobic molecules, growth of an oxide layer and so on. A method for estimating both the portion of wetted area and the intrinsic wettability state (hydrophobic versus hydrophilic) of texture elements for a heterogeneous wetting regime is proposed and discussed.Display Omitted► Hydrophobic agent from coating undergoes hydrolysis and desorption in contact with water. ► Hydrophobicity degrades due to formation of water adsorption layers on the coating. ► Hydrophobicity degrades due to desorption of hydrophobic agent from the interface. ► Portion of wetted area in heterogeneous wetting regime can be defined in sessile drop method.
Keywords: Three phase equilibrium parameters; Superhydrophobicity deterioration; Contact angle; Hydrophobic agent hydrolysis; Water adsorption layers;

Surface topographical factors influencing bacterial attachment by Russell J. Crawford; Hayden K. Webb; Vi Khanh Truong; Jafar Hasan; Elena P. Ivanova (142-149).
Substratum surface roughness is known to be one of the key factors in determining the extent of bacterial colonization. Understanding the way by which the substratum topography, especially at the nanoscale, mediates bacterial attachment remains ambiguous at best, despite the volume of work available on the topic. This is because the vast majority of bacterial attachment studies do not perform comprehensive topographical characterization analyses, and typically consider roughness parameters that describe only one aspect of the surface topography. The most commonly reported surface roughness parameters are average and root mean square (RMS) roughness (R a and R q respectively), which are both measures of the typical height variation of the surface. They offer no insights into the spatial distribution or shape of the surface features. Here, a brief overview of the current state of research on topography-mediated bacterial adhesion is presented, as well as an outline of the suite of roughness characterization parameters that are available for the comprehensive description of the surface architecture of a substratum. Finally, a set of topographical parameters is proposed as a new standard for surface roughness characterization in bacterial adhesion studies to improve the likelihood of identifying direct relationships between substratum topography and the extent of bacterial adhesion.Display Omitted► A set of standard topographical parameters for the characterization of surface roughness has been proposed. ► Adoption of these parameters for nanoscale roughness characterisation would assist a more accurate prediction of bacterial adhesion behaviour. ► A greater understanding of bacterial adhesion would be beneficial in many industrial applications.
Keywords: Surface characterization; Surface topography; Surface roughness parameters;

Inorganic materials using ‘unusual’ microorganisms by Vipul Bansal; Atul Bharde; Rajesh Ramanathan; Suresh K. Bhargava (150-168).
A promising avenue of research in materials science is to follow the strategies used by Mother Nature to fabricate ornate hierarchical structures as exemplified by organisms such as diatoms, sponges and magnetotactic bacteria. Some of the strategies used in the biological world to create functional inorganic materials may well have practical implications in the world of nanomaterials. Therefore, the strive towards exploring nature's ingenious work for designing strategies to create inorganic nanomaterials in our laboratories has led to development of biological and biomimetic synthesis routes over the past decade or so. A large proportion of these relentless efforts have explored the use of those microorganisms, which are typically not known to encounter these inorganic materials in their natural environment. Therefore, one can consider these microorganisms as ‘unusual’ for the purpose for which they have been utilized — it is in this context that this review has been penned down. In this extensive review, we discuss the use of these ‘unusual’ microorganisms for deliberate biosynthesis of various nanomaterials including biominerals, metals, sulfides and oxides nanoparticles. In addition to biosynthesis approach, we have also discussed a bioleaching approach, which can provide a noble platform for room-temperature synthesis of inorganic nanomaterials using naturally available raw materials. Moreover, the unique properties and functionalities displayed by these biogenic inorganic materials have been discussed, wherever such properties have been investigated previously. Finally, towards the end of this review, we have made efforts to summarize the common outcomes of the biosynthesis process and draw conclusions, which provide a perspective on the current status of the biosynthesis research field and highlights areas where future research in this field should be directed to realize the full potential of biological routes towards nanomaterials synthesis. Furthermore, the review clearly demonstrates that the biological route to inorganic materials synthesis is not merely an addition to the existing list of synthesis routes; biological routes using ‘unusual’ microorganisms might in fact provide an edge over other nanomaterials synthesis routes in terms of their eco-friendliness, low energy intensiveness, and economically-viable synthesis. This review has significant importance for colloids and interface science since it underpins the synthesis of colloidal materials using ‘unusual’ microorganism, wherein the role of biological interfaces for controlled synthesis of technologically important nanomaterials is clearly evident.Display Omitted► We review biosynthesis of inorganic nanomaterials using ‘unusual’ microorganisms. ► The inorganic nanomaterials include biominerals, metals, sulfides and oxides. ► The ‘unusual’ microorganisms predominantly include bacteria, fungi, and algae. ► The current status and future perspective on biosynthesis is discussed.
Keywords: Nanomaterials; Metal; Oxide; Sulfide; Biosynthesis; Bioleaching;