P. M. Costanzo

DLVO and non-DLVO interactions in hectorite

The Derjaguin-Landau-Verwey-Overbeek (DLVO) theory sums the attractive van der Waals and repulsive electrostatic forces as a function of separation distance to predict the interaction between charged particles immersed in a liquid. In aqueous media, however, non-electrostatic polar (electron acceptor/electron donor or Lewis acid/base) forces between particles with high energy surfaces often are comparable to, or greater than, the components of DLVO theory. By means of contact angle measurements on smooth self-supporting clay films, the values of the polar surface forces (AB) and the van der Waals forces (LW) of hectorite were measured. Determinations of ζ were used to derive the electrostatic forces (EL). Calculations based on the values obtained for the EL, LW, and AB forces show that for smooth spheres with a radius of 1 µm in a ≥ 0.1 M NaCl solution a net attraction exists leading to flocculation. At NaCl concentrations of ≤ 0.01 M, a repulsion energy of about +500 to +1300 kT exists at separation distances ≤ 50 Å, preventing contact between particles, thus ensuring stability of the colloidal suspension. At these concentrations, theory predicts that small clay particles or edges of clay crystals having an effective radius of curvature ≤ 10 Å should be energetic enough to overcome the repulsion barrier which prevents flocculation. Experimentally, for NaCl solution concentrations of ≥ 0.1 M, suspensions of hectorite particles flocculated, whereas at concentrations of ≥ 0.01 M, the suspensions remained stable. These experimental results agree with the predictions made by summing all three forces, but contradict the calculations based on classical DLVO theory.

The surface free energies of talc and pyrophyllite

The components of the interfacial surface tension of talc and pyrophyllite were determined by measuring the rate of the capillary rise of a number of liquids through thin, sedimented deposits of the powdered minerals. The rate of capillary rise of a liquid in a powder is related to the contact angle between the liquid and the solid by the Washburn equation. The contact angles thus derived were used to determine the apolar (Lifshitzvan der Waals) component, γLW, and the polar, electronacceptor and electron-donor parameters, γ⊕ and γ⊖ respectively, of the Lewis acid/base component of the total interfacial surface energy using the Young equation. The values of γLW for talc and pyrophyllite (31.5 and 34.4 mJ/m2) are slightly smaller than for smectite clay minerals (e.g., the value for hectorite is 39.9 mJ/m2), the electron donor parameter values are roughly comparable for talc and pyrophyllite (γ⊕ = 2.4 and 1.7 mJ/m2) as are the values of the electron acceptor parameter (γ⊖ = 2.7 and 3.2 mJ/m2). The well-known hydrophobicity of these two minerals is due to the remarkably small value (for silicate minerals) of γ⊖(γ⊕ is normally small or zero for silicates and many other oxides). The small values of both γ⊕ and γ⊖ mean that the Lewis acid/base interactions between talc or pyrophyllite and highly polar water molecules are very weak. In contrast, low-charge smectites, the minerals most similar chemically and structurally to talc and pyrophyllite, have much greater values of γ⊖ (≥ 30 mJ/m2) and are hydrophyllic.

Adhesion of anionic surfactants to polymer surfaces and low-energy materials

To study the adhesion of the anionic surfactant sodium dodecyl sulfate (SDS) to various materials, a schematic molecular model of SDS was used which optimally correlates with its critical micelle concentration (c.m.c.) values under various conditions. Using the surface tension components and parameters of (a) the SDS apolar and polar moieties and (b) the polymeric surfaces of cellulose and nylon, the energy of adhesion of SDS to these polymeric surfaces as well as to a typical low-energy material (greasy dirt) in the guise of hexadecane was determined. It could be quantitatively shown (using a surface-thermodynamic approach) that SDS, in water, adheres more strongly to the low-energy (greasy dirt) compounds than to the polymeric materials. The c.m.c. of SDS was derived directly from the surface tension components and parameters of its apolar and polar moieties, and the ζ potential of its polar heads. The c.m.c. values obtained using this model correlate well with the published c.m.c. values obtained exper...

Determination of the acid-base characteristics of clay mineral surfaces by contact angle measurements-implications for the adsorption of organic solutes from aqueous media

The apolar and the polar (electron-acceptor and electron-donor, or Lewis acid-base) surface tension components and parameters of solid surfaces can be determined by contact angle measurements using at least three different liquids, of which two must be polar. With swelling clay minerals (e.g. smectite clay minerals), smooth contiguous membranes can be fabricated, upon which contact angles can be measured directly. With non-swelling clay minerals (e.g. talc), contact angles can be determined by wicking, i.e. by the measurement of the rate of capillary rise of the liquids in question through thin layers of clay powder adhering to glass plates. The apolar and polar (acid-base) surface tension components and parameters thus found for various untreated and quaternary ammonium base-treated clays allowed the determination of the net interfacial free energy of adhesion of human serum albumin onto the various clay particle surfaces immersed in water. The free energies of adhesion, thus found, correlate well with t...

IMPACT OF DIFFERENT ASBESTOS SPECIES AND OTHER MINERAL PARTICLES ON PULMONARY PATHOGENESIS

Factors that are potentially important in the pulmonary pathogenesis of asbestos and other mineral particles are: 1) morphology, 2) Fe-content, 3) solubility under intraphagosomal conditions, 4) value and sign of the surface potential of the particle, 5) hydrophobicity or hydrophilicity, 6) capacity to activate phagocytic leukocytes, and 7) duration of exposure to the particles. The order of importance of these factors in causing severe or fatal pulmonary pathogenicity is estimated to be: 1 > 3 > 7 > 6 ≫ 5 > 4 > 2. The order of pathogenicity of the minerals is estimated as: amphibole asbestos: crocidolite, tremolite, amosite > erionite > serpentine asbestos: chrysotile > talc > silica > simple metal oxides. Particle length, duration of exposure to the particles, and pre-treatment of the particles may however enhance the pathogenic potential of any of the lower-ranked particles.

Ordered and disordered organic intercalates of 8.4-Aa, synthetically hydrated kaolinite

Stable, three-dimensionally ordered complexes were formed from synthetically hydrated, highly ordered kaolinite (d(001) = 8.4 Å) and several organic compounds. Removal of the intercalated organic compound by drying or by water washing recovered the 8.4-Å hydrate with its ordered layer stacking essentially unchanged. Some of the complexes were stable for less than a day, whereas others appeared to be stable indefinitely. The compounds that formed ordered complexes were dimethylsulfoxide, form-amide, hydrazine-hydrate, 1,1-dimethylhydrazine, ethylene glycol, glycerol, and pyridine. Clay-organic complexes that were prepared from methanol, ethanol, 1- and 2-propanol, acetone, acetic acid, propionic acid, acetaldehyde, N-methylformamide, methylethyl ketone, tetrahydrofuran, and K-acetate were stable only if they were immersed in the intercalating medium and had little or no stacking order.Many of the organic compounds intercalated by the 8.4-Å hydrate are not known to be intercalated by non-hydrated kaolinite either directly or indirectly. Isolated water molecules appear to be keyed into the ditrigonal holes formed by the basal oxygen of the silicate tetrahedra of the 8.4-Å hydrate. These water molecules, referred to as “hole water,” and fluorine ions that had replaced ~20% of the inner-surface hydroxyls of the 8.4-Å phase sufficiently altered the interlayer bonding to allow an expansion of the inner-layer spaces by a variety of guest molecules. The presence of these guest molecules between the clay layers not only changed the basal spacing and perturbed the infrared (IR) bands arising from the inner-surface hydroxyls, it also shifted the position of the IR band arising from the inner hydroxyl.

MOLECULAR MOTIONS, SURFACE INTERACTIONS, AND STACKING DISORDER IN KAOLINITE INTERCALATES

Intercalates of Georgia well-crystallized kaolinite with formamide, N-methylformamide (NMF), and dimethylsulfoxide (DMSO) were prepared at room temperature by dispersing the clay in the organic liquid. Several physical and chemical properties of the intercalated organic molecules and the clay, while intercalated and after de-intercalation, were examined using nuclear magnetic resonance (NMR), infrared (IR), and electron paramagnetic resonance spectroscopy (EPR), and specific heat (Cp) measurements. The chemical bonding between the inner-surface hydroxyls and the organic molecules, as indicated by IR, was strongest for DMSO and weakest for formamide. The distortion of the kaolinite layer, as shown by EPR, also was greatest for DMSO and least for formamide. NMR T1 measurements indicated a relatively strong DMSO-kaolinite surface interaction that slowed down the methyl group reorientation comparable to that in bulk solid DMSO. T1 measurements indicated a weaker interaction for NMF. De-intercalation by mild heating did not return the kaolinite to its original structural state as shown by EPR and Cp. The greatest disorder was found for the DMSO de-intercalate and the least for the formamide de-intercalate. These experiments show that for sufficiently strong bonding between the clay inner surface and the intercalating molecule, the structure of the clay is capable of distortion, which is partly temporary and partly permanent. The permanent changes probably involve the introduction of stacking faults.

BASELINE STUDIES OF THE CLAY MINERALS SOCIETY SOURCE CLAYS: INTRODUCTION

Clay minerals share a basic set of structural and chemical characteristics (e.g. they are largely aluminosilicates with layer structures) and yet each clay mineral has its own unique set of properties that determine how it will interact with other chemical species. The variation, in both chemistry and structure, among the clays leads to their applications in extremely diverse fields. Common and important industrial applications of clays are in the manufacture of paper, paint, plastics and rubber. One of their more recent and most economically important applications is in the pet-litter industry where their adsorptive and deodorizing properties are used. Specialty uses include clay additive to chicken feed to boost nutritional uptake by the chicken, and in using clay as fillers and major ingredients in pharmaceuticals and cosmetics. Clays are used for their catalytic properties and for their ability to adsorb greases, fats and other organic materials. Those who exist with scarce resources frequently collect clays from local deposits and ingest them as a source of dietary minerals. It is difficult for a day to go by without using a product incorporating clay minerals, as we all use ceramics such …

THE DETERMINATION OF SURFACE TENSION PARAMETERS OF POWDERS BY THIN LAYER WICKING

The rate of capillary rise of a test liquid in a powder with uniform pore diameters can be used to determine the contact angle that the liquid would make on the same material as in a classical contact angle experiment. From the contact angles determined by wicking with at least three appropriate liquids, the surface tension components (Lifshitz-van der Waals and electron acceptor/electron donor) of the solid can be determined using the Young and Washburn equations. To illustrate this technique, the surface tension components of three synthetic zeolites, kaolinite, talc, pyrophyllite, calcite, and an activated charcoal have been measured.