Packing in multimodal crystals and particle assemblies as models for dynamic packing during spontaneous and enforced particle movement: Settling, viscosity and elasticity.

Colloid particles (CP, 10-8-10-6 m = 10-1000 nm) are used as models for atom scale processes, such as crystallization since the process is experimentally observable. Packing of atoms in crystals resemble mono-, bi-, and trimodal packing of noncharged hard spheres (particles). When the size of one particle exceeds the two others an excluded volume consisting of small particles is created around large particles. This is also the case when colloid particles are dispersed in water. The formation of an excluded volume does not require attraction forces, but it is enforced by the presence of dissolved primary (cations) and secondary (protons of surface hydroxyls) potential determining ions. The outcome is an interfacial solid-liquid charge. This excluded volume, denoted Stern layer is characterized by the surface potential and charge density. Charge neutrality is identified by point of zero charge (pHpzc and pcpzc). Outside Stern layer another excluded volume is formed of loosely bound counterions which interact with Stern layer. The extent of this diffuse layer is given by inverse Debye length and effective ζ-potential. The overall balance between attractive and repulsive energies is provided by Derjaguin-Landau-Veerwey-Overbeek (DLVO) model. Charge neutrality is identified at isoelectric point (pHiep and pciep). The dependence of viscosity and yield stress on shear rate may be modeled by von Smoluchowski's volumetric collision frequency multiplied by some total interaction energy given by DLVO model. Equilibrium and dynamic models for settling and enforced particle movement (viscosity) are presented. Both compressive yield stress (sedimentation) and cohesive energy (viscoelasticity) are characterized by power law exponents of volume fraction. The transition of disperse suspensions (sols) to spanning clusters (gels) is identified by oscillatory rheology. The slope of linear plots of logarithmic storage (G´) and loss (G") moduli against logarithm of frequency or logarithm of volume fraction provide power law exponents from the slopes. These exponents relate to percolation and fractal dimensions characterizing the particle network. Moreover, it identifies the structure formation process either as diffusion limited cluster-cluster (DLCCA) or as reaction limited cluster-cluster (RLCCA) aggregation.

Sizing and packing of particles - Characterization of mono-, di- and trimodal particle assemblies.

The influence of particle size and shape on the properties of mono-, di- and trimodal particle assemblies is evaluated. The relative increase of surface area over bulk when particle size is reduced renders particles in the colloid (10-100 nm) and nano (1-10 nm) ranges extraordinary properties. Asymmetric particle shapes are characterized by sphericity and represented by equivalent spheres. The average diameter of particle size classes (size ranges) of powders are dependent on two experimentally determined properties. Average particle sizes (median, mean and mode) for each size class are extracted from size distributions of powders. Mono-, di- and trimodal particle packing efficiency is expressed as volume fractions and inverted volume fractions of close-packed hard spheres and related to standard cubic, orthoromic, tetragonal-sphenoidal and rombohedral-hexagonal packing properties. Simple models are presented to reveal the relative influence of fine, medium, and coarse particles and their ratios on powder properties. Experimental challenges relate to the influence of test compartment size and shape on particle layering and of particle shape on packing density. Particle asymmetry induces preferential aggregation through bond and site percolation resulting in dense closed or loose open cluster structures relating to particle segregation. Clusters may be characterized by structural fractals while textural fractals identify the particles involved. A modified Flory-Huggins lattice model for macromolecular solutions enables determination of combinatory entropy for cluster formation. A model is presented which relates time dependent volume fraction to logarithmic time dependence of compaction. This review concerns mixing of dry particles which corresponds to molecular processes at the gaseous (continuum, vacuum) reference state.

Characterization of molecule clustering and liquid transport at nearly ideal solid surfaces.

Vapor adsorption, mobility, two-dimensional (monolayer formation) and three-dimensional (multilayer formation) clustering is evaluated. Two-dimensional vapor diffusion is compared to results obtained from molecular kinetic (MK) model fits. Three-dimensional clustering results in condensation of multimolecular vapor layers to thin films. Thin films are characte-rized by line tension and liquid spreading by hydrodynamic (HD) models. Although it is experimentally shown that steady-state wetting ranges are intersected by a chaotic slip-stick range, MK and HD models are combined to molecular hydrodynamic (MH) models with the aim to cover this slip-stick range. The results of MK, HD and MH model fits are, however rather poor (unphysical results). Thin film (α-phase) models are compared to thick film (ß-phase) models. In order to improve model designs, established phenomenological relation-ships known from irreversible thermodynamics are presented. Forced wetting, expressed as generalized fluxes can be made dependent on multiple generalized conjugate forces which enables identification of dominant interactions to be introduced in future improved transport models.

Quantitative evaluation of key properties of dry and wet metal oxides and metal hydroxides as well as of their potential determining cations in aqueous solutions.

When potential determining cations are dissolved from solids, it is a reversed process to precipitation of solids from their electrolyte solutions. In both cases there is a transport of substance and charges across the solid-liquid interface. It is obvious that a comprehensive understanding of the process involves characterization of the (dry) solid, of the solid-liquid interface and of the potential determining cation electrolyte. The aim of this review is to quantitatively evaluate the most important properties of metal oxides and metal hydroxides, of their constituent cation electrolytes and of their interactions at the solid-liquid interface. In this way the relations between commonly used key parameters frequently reported in text-books and listed in tables can be established. No external additive, other than protons/ hydroxyls (pH) are introduced to the system. Moreover, the most successful semi-quantitative models for solids cohesion and dissolution, for cation release from their native solids and for cation interaction with water are reviewed. In order to secure credibility 148 samples (1 < zM < 8) were selected for this quantitative evaluation. The key properties are listed in 22 Tables, 8 extensive Appendices and mutually correlated in 37 Figs. For mutual comparison energies are scaled as kJ/mol.

A rational approach to basic equilibrium thermodynamics.

The elegance and general applicability of classical thermodynamics made a great impression on Albert Einstein as quoted: A theory is the more impressive the greater the simplicity of its premises, the more different kinds of things it relates and the more extended its area of applicability. Therefore the deep impression that classical thermodynamics made upon me. It is the only physical theory of universal content, which I am convinced will never be overthrown, within the framework of applicability of its basic concepts. In this review, basic relationships between partial derivatives of internal energy, enthalpy, Helmholtz and Gibbs (free) energies are presented in a condensed and self-consistent "Thermodynamic Wheel of Connections" (TWC). As a support for experimentalists a complete set of first- and second-order partial derivatives of basic state functions (U, F, H, G) derived with respect to state variables (P, T, V, S) under isothermal, isobaric, isochoric and isentropic conditions are presented as a Table. The basic TWC-network remains unchanged when expanded by additional conjugative state parameter pairs, such as chemical potential - amount of substance and surface/interfacial tension - contact area. The extension enables characterization of first- and second-order phase transitions of bulk phases and interphases in terms of first-, second- and third-order partial derivatives of Gibbs energy as well as by first- and second-order partial derivatives of chemical potential and Derjaguin's disjoining pressure. Semi-three-dimensional interface (Guggenheim convention) state functions are derived by subtracting corresponding parameters from total state functions. Then properties become dependent on the location and extension of the interface. For truly two-dimensional mono-molecular Langmuir films (Gibbs convention), first- and second-order partial derivatives of basic interfacial state functions (Us, Fs, Hs, Gs) may be derived with respect to state variables (π, T, A, Ss) under isothermal, isobaric, isoareal and isentropic conditions. They are assembled as interfacial "Thermodynamic Family Three" (TFT) s. Replacing π by P, A by V and omitting upper index s it is converted to previously published TFT for bulk phases.

Critical evaluation of models for self-assembly of short and medium chain-length surfactants in aqueous solutions.

During numerous visits to our Laboratory professor Johannes (Hans) Lyklema emphasized the importance of a holistic view on thermodynamics. In order to fulfill this aim he assembled the monumental Fundamentals of Interface and Colloid Science series. The basic state functions (internal energy, enthalpy and free energies) are interrelated by Gibbs and Helmholtz relationships. First-order phase transitions are characterized by first-order state variables (temperature, pressure, entropy, volume). Interactions are, however best expressed by second-order partial derivatives (compressibility, heat capacity and expansivity). They are related to the first-order state variables by relaxation contributions quantifying the degree of cooperativity of self-assembly processes leading to phase separation. In particular they exhibit the limit when phase transitions are changed to second-order processes. This was the focus of my first review dedicated to the memory of professor Lyklema, "Characterization of van der Waals type bimodal,- lambda,- meta- and spinodal phase transitions in liquid mixtures, solid suspensions and thin films" (ACIS 253 (2018) 66). In the present review the attention is placed on short and medium chain-length surfactant self-assembly in aqueous solutions without additives (salts or solubilizates). The dependence of state functions described above on concentration, temperature and pressure is compared to corresponding dynamic molecular processes occurring on different time, frequency and length scales including structure analysis. It is convincingly shown that Hartley-Tanford space filled spherical anhydrous micelle core - polar shell model designed for long chain-length surfactants (cmc < 0.01 mol/dm3, N > 50) cannot be enforced on short and medium chain-length surfactant non-sperical micelles (cmc close to unity, N < 20). Moreover, it is shown that a proper validity evaluation of proposed models for micelle formation is seriously undermined by their application to only a narrow concentration range near critical micelle concentration (cmc). When successful each model should characterize all self-assembly processes occurring (also at limiting association concentration, lac, at second critical concentration, 2cc and at third critical concen-tration, 3cc) within the entire concentration range of thermodynamically stable surfactant solutions. All other self-assembly processes except micelle formation are rarely considered. The pre-micelle formation at lac is, for example omitted as deviations from presented models. The reviewed reports are therefore selected on the basis of maximum investigated concentration range and of largest possible number of homologues.

Evaluation of particle charging in non-aqueous suspensions.

Factors influencing the sign and size of effective surface (zeta) potential in suspensions of very low dielectric constants are evaluated. For non-aqueous suspensions it was found that Gutmann's donor number (DN = negative Lewis type molar acid-base adduct formation enthalpy) was successfully related to zeta potential changes, similarly as pH is optimal for aqueous suspensions. Negative molar proton dissociation enthalpy (Brϕnsted type HD number), negative hydrogen bond enthalpy (HB number), logarithmic hydrogen bond equilibrium constant (molar Gibbs free energy), standard reduction potential of solvated protons (Eo(HL+/H2)), electrolytic dissociation potential of water (Eo(H2O/H2,O2)) and electron exchange Fermi potentials could equally well be related to zeta potential changes. All these properties were linearly dependent on each other. Correlations to products of Gutmann's DN and AN numbers and other relevant properties such as polar, hydrogen bond and acid-base contributions to solubility parameters and to surface tensions were found to be less successful particularly when very polar liquids were encountered. Commonly used DLVO models for repulsive interaction energy between pair of particles in aqueous electrolyte suspensions have been simplified when dealing with low-polar, non-polar and apolar suspensions. When evaluating factors contributing to attractive and repulsive interaction energies, it is found that in order for the models to be relevant the extension of diffuse charging has to be much larger than the distance to repulsive barrier ensuring suspension stability. At this limit and at high surface potentials, the repulsive energy grows exceptionally large being in the range of lattice energy of each solid. The models fail when surface potential is low and the extension of diffuse charging is much smaller than the distance to repulsive barrier. Then interaction energies are reasonable. The investigated (Au, SiO2, Glass, TiO2, Al2O3, CaCO3, MgO) suspensions fall between these limits. The attractive energy is small but significant as compared to repulsive energy. All energies were larger than the estimated lower limit for stable suspensions.

Characterization of van der Waals type bimodal,- lambda,- meta- and spinodal phase transitions in liquid mixtures, solid suspensions and thin films.

The perfect gas law is used as a reference when selecting state variables (P, V, T, n) needed to characterize ideal gases (vapors), liquids and solids. Van der Waals equation of state is used as a reference for models characterizing interactions in liquids, solids and their mixtures. Van der Waals loop introduces meta- and unstable states between the observed gas (vapor)-liquid P-V transitions at low T. These intermediate states are shown to appear also between liquid-liquid, liquid-solid and solid-solid phase transitions. First-order phase transitions are characterized by a sharp discontinuity of first-order partial derivatives (P, S, V) of Helmholtz and Gibbs free energies. Second-order partial derivatives (KT, B, CV, CP, E) consist of a static contribution relating to second-order phase transitions and a relaxation contribution representing the degree of first-order phase transitions. Bimodal (first-order) and spinodal (second-order) phase boundaries are used to separate stable phases from metastable and unstable phases. The boundaries are identified and quantified by partial derivatives of molar Gibbs free energy or chemical potentials with respect to P, S, V and composition (mole fractions). Molecules confined to spread Langmuir monolayers or adsorbed Gibbs monolayers are characterized by equation of state and adsorption isotherms relating to a two-dimensional van der Waals equation of state. The basic work of two-dimensional wetting (cohesion, adsorption, spreading, immersion), have to be adjusted by a horizontal surface pressure in the presence of adsorbed vapor layers. If the adsorption is extended to liquid films a vertical surface pressure (Π) may be added to account for the lateral interaction, thus restoring PV = ΠAh dependence of thin films. Van der Waals attraction, Coulomb repulsion and structural hydration forces contribute to the vertical surface pressure. A van der Waals type coexistence of ordered (dispersed) and disordered (aggregated) phases is shown to exist when liquid vapor is confined in capillaries (condensation-liquefaction-evaporation and flux). This pheno-menon can be experimentally illustrated with suspended nano-sized particles (flocculation-coagulation-peptisation of colloidal sols) being confined in sample holders of varying size. The self-assembled aggregates represent critical self-similar equilibrium structures corres-ponding to rate determining complexes in kinetics. Overall, a self-consistent thermodynamic framework is established for the characterization of two- and three-dimensional phase separations in one-, two- and three-component systems.

Critical evaluation of dipolar, acid-base and charge interactions I. Electron displacement within and between molecules, liquids and semiconductors.

Specific dipolar, acid-base and charge interactions involve electron displacements. For atoms, single bonds and molecules electron displacement is characterized by electronic potential, absolute hardness, electronegativity and electron gap. In addition, dissociation, bonding, atomization, formation, ionization, affinity and lattice enthalpies are required to quantify the electron displacement in solids. Semiconductors are characterized by valence and conduction band energies, electron gaps and average Fermi energies which in turn determine Galvani potentials of the bulk, space charge layer and surface states. Electron displacement due to interaction between (probe) molecules, liquids and solids are characterized by parameters such as Hamaker constant, solubility parameter, exchange energy density, surface tension, work of adhesion and immersion. They are determined from permittivity, refractive index, enthalpy of vaporization, molar volume, surface pressure and contact angle. Moreover, acidic and basic probes may form adducts which are adsorbed on target substrates in order to establish an indirect measure of polarity, acidity, basicity or hydrogen bonding. Acidic acceptor numbers (AN), basic donor numbers (DN), acidic and basic "electrostatic" (E) and "covalent" (C) parameters determined by enthalpy of adduct formation are considered as general acid-base scales. However, the formal grounds for assignments as dispersive, Lifshitz-van der Waals, polar, acid, base and hydrogen bond interactions are inconsistent. Although correlations are found no of the parameters are mutually fully compatible and moreover the enthalpies of acid-base interaction do not correspond to free energies. In this review the foundations of different acid-base parameters relating to electron displacement within and between (probe) molecules, liquids and (semiconducting) solids are thoroughly investigated and their mutual relationships are evaluated.

Critical evaluation of dipolar, acid-base and charge interactions II. Charge exchange within electrolytes and electron exchange with semiconductors.

Electron displacements may be considered as a general measure of semiconductor activity as well as of dipolar, acid-base and charge interactions. Electron transfers during reduction and oxidation reactions between dissolved cations and anions correspond to an extreme Lewis acid-base electron displacement. Brϕnsted proton release (protolysis) represents an extremely weakened hydrogen bond. The most common electrostatic (Born, PCM) and chemical (pKa matching) models for electron and proton exchange between dissolved species are reviewed using aluminium species as examples. Dissolution of ions from solids (salts) may be considered as a reversed precipitation reaction. For partly covalent solids dissociation is dependent on electron or vacancy (hole) transfers to the solid which connects oxidation and reduction reactions to electron displacements in semiconductors. The electron exchange is characterized by Femi energy of semiconductors and of electrolytes. The standard reduction potential may thus be converted to Fermi energy of connected electrochemical cells. In disconnected particle suspensions (sols) the electron activity is a more appropriate parameter which may be converted both to standard reduction potential of ions and to Fermi energy of semiconductors. Dissolution of potential determining cations and anions and hydrolysis of surface sites determines the charging (electron transfer to/from surface) of solids. Both electrostatic (MUSIC) and chemical equilibrium constant models are available for Brϕnsted equilibrium of surface hydroxyls. Point of zero charge is a result of positive and negative charge matching and it represents the optimal condition for condensation of polynuclear species by olation and oxolation. The capability of partial charge (PCM) model to predict condensation is evaluated. Acidity (pH), composition and temperature dependence of aluminium species is illustrated by solubility limits of contributing species and by phase diagrams. Influence of ions on macroscopic suspension properties, such as wetting and electrophoretic mobility is evaluated with reference to point of zero charge and to isoelectric point. Restrictions to the use of zeta-potentials are related to the surface potential and particle size - Debye length ratios. Macroscopic settling (particle precipitation) and viscosity of suspensions (sols) are discussed with reference to Deryagin-Landau-Verwey-Overbeek (DLVO) model. The primary dependence on counterion valence is evaluated according to Schulz-Hardy approach. The secondary dependence on counterion hydration (Hofmeister or lyotropic effect) and ion association (Debye-Hückel limiting model) are discussed.

On the incompatibilities of interaction scales and processes with focus on the work of adhesion.

The mutual compatibility of Hamaker constants, solubility parameters or cohesive energy densities (CED) and surface/interface tensions are evaluated. It is shown that the partial contributions (dispersive, Lifshitz-van der Waals, dipolar induction, dipolar orientation, polar, acid, base and hydrogen bond) to Hamaker constants, solubility parameters or cohesive energy densities and surface/interface tensions are mutually inconsistent. The published reference data for a single set of liquids is moreover shown to be exceedingly scattered; making the parallel use of these scales challenging. Reference processes designed for bringing two and three phases into mutual contact are conflicting. The two-phase processes within Hamaker and exchange energy density (EED) frameworks agree, but the three-phase models differ. As a free-standing parameter the EED is however comparable. The two-phase adhesion process is shown to be incompatible with the other contact processes and the three-phase adhesion process is opposite to them. One reason for this controversy is the different averaging of interfacial properties. While interfacial Hamaker constants and solubility parameters or cohesive energy densities are geometric averages of corresponding intervening phase properties, this practice is replaced by the work of adhesion being geometrically averaged as works of cohesion. As a result, there exist three conflicting models for the adhesion process: the Dupré work of adhesion, the Girifalco-Good geometric averaged works of cohesion and Fowkes reduced interfacial or interphasial tension process. None of these agree with the commonly accepted standard Hamaker contact processes and they should be replaced with the compatible extended work of adhesion process originally suggested by Dupré. The models offered for the conversion of Hamaker constants and solubility parameters or cohesive energy densities to surface tensions involve conversion factors and equilibrium distances between planes of molecules in liquids. The equilibrium distance for different close packings derived from molar liquid volumes are about 2-5 times larger than the cutoff distances obtained from simulations. Using volumetric equilibrium distances, the conversion factors for dispersive, polar and total Hamaker constant and solubility parameter or cohesive energy densities to surface tensions become nearly equal but they are different for each liquid.

Influence of charge exchange in acidic aqueous and alcoholic titania dispersions on viscosity.

Charging effects resulting from adsorption of acid, acid anions, and protons on titania (anatase) surfaces in anhydrous or mixed alcohol-water dispersions is summarized. The suddenly enhanced conductivity as compared to titania-free solutions has previously been modeled and explained as surface-induced electrolytic dissociation (SIED) of weak acids. This model and recently published results identifying concurrent surface-induced liquid (solvent) dissociation (SILD) are evaluated with experimentally determined conductivity and pH of solutions, zeta-potential of particles, and viscosity of dispersions. Titania (0-25wt%)-alcohol (methanol, ethanol, and propanol) dispersions mixed with (0-100wt%) water were acidified with oxalic, phosphoric, and sulfuric acids. It was found that the experimental results could in many cases be condensed to master curves representing extensive experimental results. These curves reveal that major properties of the systems appear within three concentration regions were different mechanisms (SILD, surface-induced liquid dissociation; SIAD, surface-induced acid dissociation) and charge rearrangement were found to be simultaneously active. In particular, zeta-potential - pH and viscosity - pH curves are in acidified non-polar solvents mirror images to those dependencies observed in aqueous dispersions to which hydroxyl is added. The results suggest that multiple dispersion and adsorption equilibria should be considered in order to characterize the presented exceptionally extensive and complex experimental results.

Liquid spreading on solid surfaces and penetration into porous matrices: Coated and uncoated papers.

Liquid spreading on solid surfaces and penetration into porous matrices (powders and coated papers) are investigated. The influence of chemical and structural heterogeneity on equilibrium and dynamic surface wetting is evaluated both experimentally and theoretically. Single capillary systems are used to identify the predominating mechanisms for acceleration, momentum, inertial and viscous liquid penetration. Different stages of vertical and horizontal penetration of liquids from non-limited, restricted (sessile drop) and cut-off sources into powders and papers are evaluated with reference to a range of frequently used models. For all types of liquid transport power-law exponents are used to relate all observations. The applicability of models from which the exponents are derived is discussed. Results are compared to theoretical predictions for liquid penetration. Models are of general validity, but the focus is placed on probe liquid spreading on and penetrating into coated and uncoated papers. This sets a particular challenge, since papers are heterogeneous layered composites of powder compacts on fibrous network. For the evaluation of models published results are supported by extended original results.

Phase equilibriums, self-assembly and interactions in two-, three- and four medium-chain length component systems.

The Scandinavian surface (surfactant) and colloid science owes much of its success to Per Ekwall and Björn Lindman. In this review the main topics shared by their research groups at Åbo Akademi University in Finland and at Lund University in Sweden are described. The nature of surface active substances (cosolvents, co-surfactants and surfactants) and microemulsions are evaluated. It is shown that the properties of medium-chain length surfactants differ dramatically from long-chain surfactants. The phase equilibriums of binary systems are related to the phase equilibriums of ternary and quaternary systems referred to as microemulsions or more recently also as nanoemulsions. A distinction is made between hydrotrope liquids, detergentless microemulsions, surfactant mixture systems and microemulsions. Three component systems are assembled to "true" quaternary microemulsions. An exceptionally comprehensive network of thermodynamic parameters describing molecular site exchange and micelle formation are derived and related mutually. Gibbs free energy, enthalpy, entropy, volume, heat capacity, expansivity and compressibility can be used to illustrate the degree of aggregation cooperativity and to evaluate whether micelle formation is of a first-, second- or intermediate order phase transition. Theoretical simulations and experimental results show that the associate structures of medium-chain length surfactants are quite open and may be deformed due to small aggregation numbers. The self-assembly occurs over a number of distinct steps at a series of experimentally detectable critical concentrations. Despite the low aggregation tendency their phase behavior equals those of long-chain homologs in surfactant mixture and microemulsion systems. A number of models describing the self-assembly are reviewed. Nuclear magnetic resonance (shift, relaxation rate and diffusion), Laser Raman and infrared spectroscopies were chosen as key instruments for molecular interaction characterization since they were used in the collaboration between the research groups in Åbo and in Lund. A new method is introduced in order to evaluate the traditional procedure for extracting limiting parameters which also enables an illustration of the degree of cooperativity. The focus is laid mainly on aqueous, alcoholic, saline and, to a limited extent oil phases of one-, two-, three- and four component systems of water-sodium carboxylates-alcohol-oil. The extensive thermodynamic characterization of these liquid phases and liquid crystalline phases is left out due to space restrictions.

Interaction between oxalic acid and titania in aqueous ethanol dispersions.

The charging effects resulting from adsorption of oxalic acid and oxalate anions on titania (anatase) surfaces in anhydrous or mixed water-ethanol suspensions is summarized. The suddenly enhanced electrical conductance with respect to titania free solutions has previously been explained in terms of surface-induced electrolytic dissociation (SIED) of weak acids. A recently published model has previously been found to successfully characterize the complex SIED effect. The model is evaluated experimentally by recording the conductance and pH of the dispersion and the zeta potential of the particles. The experimental results can be condensed to master curves, which reveal the major properties of the systems and facilitate further modeling of extensive experimental results. The equilibrium and transport properties of solutions and particles were related, but different mechanisms was found to be active in each case. The results suggest that at least three adsorption equilibria should be considered in order to improve the model.

Peculiar charging effects on titania in aqueous 1:1, 2:1, 1:2 and mixed electrolyte suspensions.

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.

Rheological characterization of the influence of PVOH on calcite suspensions.

Flow properties of the calcite/poly(vinyl alcohol) (PVOH) system were studied and related to the microstructure of the suspension. Adsorption of PVOH on calcite was confirmed, and it results in a shift of the slipping plane out from the surface. The charge density at the surface is assumed to remain unchanged. Since the PVOH used is only partially hydrolyzed, the most likely adsorption conformation consists of residual acetate groups adsorbed to the surface and vinylalcohol groups extending outward from the surface as loops and tails. The microstructure and flow properties of the calcite/PVOH system was found to go through several different stages as a function of PVOH concentration. At low PVOH concentrations a gradual weakening of the initially formed floc network is observed as a function of PVOH concentration. Further addition of PVOH eventually leads to breakdown of the flocs which results in a sterically stabilized suspension with a very low viscosity. This state persists for a narrow concentration range of PVOH, and increasing the PVOH concentration over a certain limit leads to a second gradual increase in viscosity. The system is believed not to undergo reflocculation at high PVOH concentrations as judged from the nonelastic nature of the suspensions. Instead, the polymers form a viscous matrix in the solution while the particles remain well-dispersed. At high enough PVOH concentration, the free volume available for the particles is greatly reduced, and the viscosity increases sharply.

Critical comparison of molecular mixing and interaction models for liquids, solutions and mixtures.

Surface properties of condensed matter, in particular solids are frequently characterized with probe liquids. The liquids are assigned physico-chemical parameters, such as solubility parameters, surface/interfacial tensions and Hamaker constants. Each parameter has been subdivided into two-to-five van der Waals (London, Debye and Keesom) and Lewis contributions. A critical comparison reveals that each contribution varies considerably distorting the balance between them. Despite this scatter each set of parameters representing a particular molecular interaction shows similar trends. Experimental verification of these multi-parameter contributions in multi-components systems remain, however uncertain. Three models involving solubility parameters, surface/interfacial tensions and Hamaker constants were compared for internal and mutual conceptual consistency. It is shown that Fowkes definition of work of adhesion as interfacial tension contradicts Dupre's definition as work process of adhesion. The exchange energy density (EED) process differs from the work of adhesion process by a factor two for the interfacial average term and for three-component systems the models differ substantially. The processes which are represented by Hamaker constants are in accord with the EED process for two-component systems, but assumed equal to work process of adhesion for three-component systems. Although the process representation is common for all models, it is shown that they represent only a fraction of the total energy balance.

Solvents, in which ionic surfactants do not affect the zeta potential.

The influence of solvent polarity on adsorption behavior of DOSS (anionic surfactant) has been studied by means of electroacoustic method. DOSS substantially affected the zeta potential of titania and alumina dispersed in methanol and in hydrocarbons. In 1-propanol, 2-propanol, and 1-butanol, which have intermediate polarity between methanol and hydrocarbons, the effect of the ionic surfactant on the zeta potential of solid particles was less significant.

Surface-induced electrolytic dissociation of oxalic acid in polar organic solvents.

The presence of titania powder (chiefly anatase) enhanced electrolytic dissociation of oxalic acid in lower aliphatic alcohols (but not in water). The surface-induced dissociation was manifested in enhanced electric conductance of a dispersion containing solvent, oxalic acid, and titania, which was substantially higher than the conductance of dispersion containing only solvent and titania and of solution of oxalic acid in that solvent. This phenomenon can be applied to control the electrokinetic potential of particles in polar organic solvents.