R. F. Giese

THE HYDROPHILICITY AND HYDROPHOBICITY OF CLAY MINERALS

AbstractThe terms “hydrophobic” and “hydrophilic” are typically used in a non-specific sense and, as such, they have a limited utility. Surface thermodynamic theory, as described here, allows a natural and potentially powerful definition of these terms. The boundary between hydrophobicity and hydrophilicity occurs when the difference between the apolar attraction and the polar repulsion between molecules or particles of material (1) immersed in water (w) is equal to the cohesive polar attraction between the water molecules. Under these conditions, the interfacial free energy of interaction between particles of 1, immersed in water (ignoring electrostatic interactions), ΔG1 w1IF exactly zero. When ΔG1 w1IF is positive, the interaction of the material with water dominates and the surface of the material is hydrophilic; when ΔG1 w1IF is negative, the polar cohesive attraction between the water molecules dominates and the material is hydrophobic. Thus, the sign of defines the nature of the surface and the magnitude of may be used as the natural quantitative measure of the surface hydrophobicity or hydrophilicity.

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.

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...

Surface and Electrokinetic Properties of Clays and Other Mineral Particles, Untreated and Treated with Organic or Inorganic Cations

Abstract The apolar (Lifshitz van der Waals)component (yLW) and the polar [electron-acceptor (y ) and electron-donor (y ) ] parameters of the surface tension of more than sixty different clay and other mineral particles or surfaces have been measured, as well as their -potentials. It is essentially the y parameter that determines the hydrophobic (target;⩾y28 mJ/m1)or hydrophilicy ( y target;⩾28 mJ/mJ) character of the mineral surface. The surface properties were measured for twelve ion exchanged smectites, as a funclion of substitution with various alkali and alkalin earth cations. The hydrophobizing influence of exchanged organic cations on the surface properties of a number of clay particles is also determined. Finally, the hydrophobizing influence of plurivalent cations is demonstrated with Ca2+and La 3+for three different negatively charged mineral particles.

CHEMICAL AND MINERALOGICAL CHARACTERISTICS OF FRENCH GREEN CLAYS USED FOR HEALING

The worldwide emergence of infectious diseases, together with the increasing incidence of antibiotic-resistant bacteria, elevate the need to properly detect, prevent, and effectively treat these infections. The overuse and misuse of common antibiotics in recent decades stimulates the need to identify new inhibitory agents. Therefore, natural products like clays, that display antibacterial properties, are of particular interest.The absorptive properties of clay minerals are well documented for healing skin and gastrointestinal ailments. However, the antibacterial properties of clays have received less scientific attention. French green clays have recently been shown to heal Buruli ulcer, a necrotic or ‘flesh-eating’ infection caused by Mycobacterium ulcerans. Assessing the antibacterial properties of these clays could provide an inexpensive treatment for Buruli ulcer and other skin infections.Antimicrobial testing of the two clays on a broad-spectrum of bacterial pathogens showed that one clay promotes bacterial growth (possibly provoking a response from the natural immune system), while another kills bacteria or significantly inhibits bacterial growth. This paper compares the mineralogy and chemical composition of the two French green clays used in the treatment of Buruli ulcer.Mineralogically, the two clays are dominated by 1Md illite and Fe-smectite. Comparing the chemistry of the clay minerals and exchangeable ions, we conclude that the chemistry of the clay, and the surface properties that affect pH and oxidation state, control the chemistry of the water used to moisten the clay poultices and contribute the critical antibacterial agent(s) that ultimately debilitate the bacteria.

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.

THE ELECTROSTATIC INTERLAYER FORCES OF LAYER STRUCTURE MINERALS

Using a simple ionic model, the energy necessary to expand a layer structure by a certain distance can be calculated. This has been done for a series of 15 structures including hydroxides, 2:1 and 1:1 structures of various types. Plots of energy versus separation distance show three major groups which have common bonding properties. For large separations, the group with the strongest interlayer bonds contains the brittle micas, the hydroxides, and the 1:1 structures. Intermediate bonding structures are the normal micas and the weakest bonds occur in the zero layer charge 2:1 structures. The relative energies needed for a given separation are not constant so that for small separations the zero layer charge structures such as talc and pyrophyllite are more strongly bonded than the normal micas. These groupings correlate very well with the expandability of the structures by water and other substances. It is proposed that this approach to the study of the layer structures will provide a simple theory explaining the expansion properties of layer silicates.РезюмеИспользуя простую ионную модель,можно вычислить энергию,необходимую для расширения слоистой структуры на определенное расстояние. Это было проделано для серии из 15 структур,включая гидроокиси,структуры 2:1 и 1:1 различных типов. Графики зависимости энергии от расстояния разделения указывают на 3 главных группы,которые имеют характерные связующие свойства. При большом разделении группа с сильнейшими межслойными связями включает хрупкие слюды,гидроокиси и структуры 1:1. Структурами с промежуточными связями являются структуры нормальных слюд, и слабейшими связями обладают структуры 2:1 со слоями,имеющими нулевые заряды. Относительные величины энергии,необходимые для данного разделения,не являются постоянными.Так при небольших разделениях структуры со слоями,имеющими нулевые заряды,такие как тальк и пирофиллит,связаны сильнее,чем нормальные слюды. Это группирование очень хорошо коррелируется со способностью структур к расширению водой и другими жидкостями. Предполагается использовать этот метод для изучения слоистых структур,что обеспечит простую теорию для объяснения свойств расширения слоистых силикатов.

Synthesis of a 10-Aa hydrated kaolinite

Hydrated kaolinite (d(001) = 10 Å) can be synthesized by mild heating of a kaolinite-organic suspension, allowing time for the clay to be intercalated by the organic solvent, and then dissolving a fluoride salt in the liquid. After mild heating of the suspension, the salt and organic solvent are removed by repeated water washings. The kaolinite retains interlayer water in the form of a 10-Å kaolinite hydrate. The influence of the intercalating agent, type of salt, concentration of salt, and the time of treatment on the synthesis of 10-Å hydrate was examined for several kaolinites. The most effective salt is NH4F; much smaller yields were obtained using KF and RbF. Not all organic molecules gave high yields of the hydrate; dimethyl sulfoxide, formamide, and hydrazine worked well but N-methyl formamide did not. The reaction between clay and salt resulted in the replacement of some hydroxyls by fluoride. This replacement was rapid; after 1 min of fluoride treatment a substantial yield of hydrate was obtained. The intercalation step separated the layers and also disordered the kaolinite, facilitating the F for OH replacement at or near crystallite edges. This replacement weakens the interlayer bonding at the edges and thereby reduces the possibility of layer collapse and attendant dehydration.РезюмеГидратированный каолинит (d(001) = 10 Å) может быть синтезирован путем умеренного нагрева каолинито-органической суспензии, уели имеется достаточное время для того, чтобы органический растворитель включился в глину и для последующего растворения флюористой соли в жидкости. После умеренного нагрева суспензии, соль и органический растворитель удаляются при помощи неоднократных водных промываний. Каолинит удерживает межслойную воду в виде 10-Å каолинитового гидрата. Исследовалось влияние прослойгого вещества, типа соли, концентрации соли и времени обработки на синтез 10-Å гидрата для нескольких каолинитов. Наиболее эффективная соль это NH4F; намного меньшие результаты были получены при использовании KF и RbF. Не все органические молекулы давали высокий выход гидрата. Диметилсульфоксид, формамид и гидразин давали хорошие результаты, в то время как N-метиловый формамид не давал таких результатов. В результате реакции между глиной и солью происходило замещение некоторых гидроксилов флюоридами. Это замещение было быстрым, после 1 минуты флюористой обработки был получен значительный выход гидрата. Тонкое включение прослойки разделяло слои, а также приводило к нарушению упорядочения каолинита, облегчая замещение OH флюором на кристаллических гранях. Это замещение ослабляет межслой-ную связь на гранях и, таким образом, уменьшает возможность разрушения слоя и сопутствующей дегидратации. [E.G.]ResümeeHydratisierter Kaolinit (d(001)= 10 Å) kann durch mildes Erhitzen einer Suspension aus Kaolinit und einem organischen Lösungsmittel synthetisiert werden, wobei genügend Zeit vorhanden sein muß, damit das organische Lösungsmittel in die Zwischenschichten des Ton eindringen kann, und anschließend ein Fluorid-Salz in der Flüssigkeit aufgelöst werden kann. Nachdem die Suspension mild erhitzt wurde, werden das Salz und das organische Lösungsmittel durch wiederholtes Waschen mit Wasser entfernt. Der Kaolinit hält Zwischenschichtwasser in der Form eines 10 Å Kaolinithydrates zurück. Der Einfluß des Zwischenschicht-bildenden Agens, der Art des Salzes, der Salzkonzentration und der Behandlungszeit auf die Synthese des 10 Â Hydrates wurde für verschiedene Kaolinite untersucht. Das wirksamste Salz war NH4F; viel kleinere Ausbeuten wurden mit KF und RbF erzielt. Nicht alle organischen Moleküle ergaben hohe Hydratausbeuten. Dimethylsulfoxid, Formamid und Hydrazin hatten einen positiven Einfluß, während N-methylformamid keine Wirkung zeigte. Die Reaktion zwischen Ton und Salz führte zum Ersatz einiger Hydroxidionen durch Fluoridionen. Dieser Ersatz verlief schnell; nach einer Minute der Fluoridbehandlung wurde eine wesentliche Ausbeute an Hydrat erzielt. Der Schritt zur Bildung der Zwischenschicht trennte die Lagen und führte zu einer schlechten Ordnung des Kaoliniis, indem er den Ersatz von OH durch F an den Kristallkanten erleichterte. Dieser Ersatz scwächte die Zwischenschichtbindung an de Kanten und verringerte dadurch die Möglichkeit, daß die Schichtstruktur zerstört wird und im Zusammenhang damit eine Dehydratation stattfindet. [U.W.]RésuméUne kaolinite hydratée (d(001) = 10 Â) peut être synthétisée en chauffant légèrement une suspension organique-kaolinite, donnant du temps pour que l’argile soit intercalaté par le solvant organique, et ensuite dissolvant un sel floride dans le liquide. Après avoir légèrement échauffé la suspension, le sel et le solvant organique sont enlevés par des lavements à l’eau répétés. La kaolinite retient de l’eau intercouche sous la forme d’hydrate kaolinite 10-Å. L’influence de l’agent intercalatant, le genre de sel, la concentration du sel, et la durée du traitement sur la synthèse de l’hydrate 10-Â ont été examinés pour plusieurs kaolinites. Le sel le plus efficace est NH4F; on a obtenu des produits beaucoup moins importants en utilisant KF et RbF. On n’a pas obtenu de grandes quantités d’hydrate de toutes les molécules organiques; la sulfoxide diméthyle, la formamide et l’hydrazine marchaient bien, mais pas la formamide methyle-N. La réaction entre l’argile et le sel a résulté en le remplacement de certains hydroxyls par la fluoride. Ce remplacement est rapide; après 1 min de traitement à la floride, une quantité substantielle d’hydrate a été produite. L’étape d’intercalation a séparé les couches et a aussi désordonné la kaolinite, facilitant le remplacement de F par OH aux bords cristallites. Ce remplacement affaiblit les liens intercouche aux bords, et réduit ainsi la possibilité de l’effondrement de couches et la déshydration l’accompagnant. [D.J.]

Low temperature synthesis of a 10-A hydrate of kaolinite using dimethylsulfoxide and ammonium fluoride

In 1961 Wada succeeded in intercalating kaolinite by dry rinding with potassium acetate (KAc), thereby forming a 14product. He subsequently intercalated dickite, nacrite, and hydrated halloysite with KAc (Wada, 1965). After washing the intercalates with water to remove the KAc, he found that kaolinite, dickite, and hydrated halloysite reverted to their original spacings, but that nacrite formed a well-defined hydrate with d(001) equal to 8.35/~. Weiss et al. (1963) reported a 8.33-/~ phase formed by the water washing of a kaolinite which had been expanded by urea but did not identify this material as a hydrated phase of kaolinite. By means of KAcand urea-intercalates, van Olphen and Deeds (1963) obtained a 8.45-.~ spacing for a well-defined hydrate of what they considered to be dickite. It was later shown (Deeds et al., 1966) that the reactive mineral was really nacrite in a nacrite/dickite mixture. In the latter paper, Deeds et al. mentioned an ill-defined 7.6-/~ hydrate of kaolinite. Range et al. (1969) reported a 10.04-/~ hydrate that formed from the expansion of a disordered kaolinite by washing a hydrazine intercalate. Disregarding the ill-defined phases, there appears to be no report in the literature of a well-characterized kaolinite hydrate formed from a well-crystallized kaolinite at low temperature. In view of the problems associated with the nature and origin of hydrated halloysite, such a synthesis is of interest and is reported here.

Macroscopic-scale surface properties of streptavidin and their influence on aspecific interactions between streptavidin and dissolved biopolymers

To avoid aspecific attractions between carrier surfaces for either ligand or receptor molecules in, e.g., immunoassays, or kinetic rate constant measurements, it has long been established that a background consisting of an aspecific, very hydrophilic carrier surface is generally quite effective. However, it is not often realized that one achieves such a non-reactive background, even with electrostatically neutral materials, at the price of creating a strong (polar) hydrophilic repulsion between dissolved biopolymer (e.g., protein) molecules and the non-adsorbing carrier surface. To investigate the quantitative effects of this type of repulsion in systems involving streptavidin, the surface properties of a streptavidin-coated glass plate were determined by contact angle measurements, from which the aspecific, macroscopic-scale free energies of repulsion between a streptavidin-coated surface and dissolved proteins such as immunoglobulin-G (IgG), and human serum albumin (HSA), could be derived. Streptavidin, even at neutral pH (at which it has virtually no electric surface charge as determined by electrophoresis) is very hydrophilic and strongly repels both IgG and HSA molecules. At neutral pH, molecules such as IgG and HSA, in aqueous solution, cannot approach a streptavidin layer more closely than to approximately 3.0 nm, which suffices to prevent IgG or HSA from any aspecific adherence to the streptavidin layer (as determined by extended DLVO analysis). This aspecific repulsion however also has the (usually unsuspected) effect of causing a decreased specific attachment between ligand and receptor molecules. In addition, it decreases the measured kinetic on-rate constants, often by about two decimal orders of magnitude. However, once the surface-thermodynamic properties of all the aspecific (macroscopic-scale) and specific (microscopic-scale) entities, as well as the specific equilibrium binding constant are known, the real kinetic on-rate constant between just the ligand and the receptor determinants can be determined, yielding the value it would have if the measurement of that constant were unhindered by the repulsive interactions exerted by the background of hydrophilic carrier molecules.