Understanding the role of ion interactions in soluble salt flotation with alkylammonium and alkylsulfate collectors

There is anecdotal evidence for the significant effects of salt ions on the flotation separation of minerals using process water of high salt content. Examples include flotation of soluble salt minerals such as potash, trona and borax in brine solutions using alkylammonium and alkylsulfate collectors such as dodecylamine hydrochloride and sodium dodecylsulfate. Although some of the effects are expected, some do not seem to be encompassed by classical theories of colloid science. Several experimental and modeling techniques for determining solution viscosity, surface tension, bubble-particle attachment time, contact angle, and molecular dynamics simulation have been used to provide further information on air–solution and solid–solution interfacial phenomena, especially with respect to the interfacial water structure due to the presence of dissolved ions. In addition atomic force microscopy, and sum frequency generation vibrational spectroscopy have been used to provide further information on surface states. These studies indicate that the ion specificity effect is the most significant factor influencing flotation in brine solutions.     

Water structure in aqueous solutions of alkali halide salts: FTIR spectroscopy of the OD stretching band

Water structure making/breaking studies in solutions of five alkali halide salts (KF, KI, NaI, CsF and CsCl) in 4 wt% D(2)O in H(2)O mixtures have been performed by FTIR analysis of the OD stretching band in the full solubility range. The proposed method gives a microscopic picture of the water structure making/breaking character of the salts in terms of the hydrogen bonding between the water molecules in the solution. With the exception of CsCl, there is a very good correlation of the structure making/breaking character of the salts determined by FTIR analysis, and the viscosity coefficients of the solutions. The results fully support and explain previous studies of bubble attachment to microscopic salt particles of the above salts. The investigations support the primary importance of interfacial water structure in the explanation of the flotation of alkali halide salts in their brines.     

Water structure and its influence on the flotation of carbonate and bicarbonate salts

Interfacial water structure is a most important parameter that influences the collector adsorption by salt minerals such as borax, potash and trona. According to previous studies, salts can be classified as water structure makers and water structure breakers. Water structure making and breaking properties of salt minerals in their saturated brine solutions are essential to explain their flotation behavior. In this work, water structure making-breaking studies in solutions of carbonate and bicarbonate salts (Na(2)CO(3), K(2)CO(3), NaHCO(3) and NH(4)HCO(3)) in 4 wt% D(2)O in H(2)O mixtures have been performed by FTIR analysis of the OD stretching band. This method reveals a microscopic picture of the water structure making/breaking character of the salts in terms of the hydrogen bonding between the water molecules in solution. The results from the vibrational spectroscopic studies demonstrate that carbonate salts (Na(2)CO(3) and K(2)CO(3)) act as strong structure makers, whereas bicarbonate salts (NaHCO(3) and NH(4)HCO(3)) act as weak structure makers. In addition, the changes in the OD band parameters of carbonate and bicarbonate salt solutions are in agreement with the viscosity characteristics of their solutions.     

十二烷基吗啉选择性吸附氯化钠的分子模拟

采用分子模拟方法研究氯化钠-光卤石反浮选体系中捕收剂十二烷基吗啉(DMP)选择性吸附氯化钠的机理. 用Material Studio 4.0软件和COMPASS分子力场方法建立了DMP在氯化钠和光卤石两种矿物表面的吸附模型, 并进行动力学模拟和能量优化, 确立了DMP在两种矿物表面的最佳吸附构型. 结果表明, DMP分子通过其官能团中的O、N原子与氯化钠界面水结构中的H原子之间的氢键作用吸附在氯化钠表面, 吸附作用能为-119.49 kJ·mol-1, 而光卤石界面水结构不能保持稳定排列, 致使DMP直接作用在光卤石表面, 吸附能为-37.97 kJ·mol-1, 在两种矿物共存体系中, 这种吸附能差异导致了DMP在氯化钠表面的选择性吸附.     

Ion hydration and structural properties of water in aqueous solutions at normal and supercooled conditions: a test of the structure making and breaking concept

We study with the method of molecular dynamics simulation the structural properties of aqueous solutions of NaCl, KCl and KF salts at ambient conditions and upon supercooling at constant pressure. The calculations are performed at increasing concentration of the salt starting from c = 0.67 mol kg(-1) up to 3.96 mol kg(-1). We investigate the modifications of the hydration shells and the changes in the water structure induced by the presence of the ions. The oxygen-oxygen structure is strongly dependent on the ionic concentration while it is almost independent from the cation. The hydrogen bonding is preserved at all concentrations and temperatures. The main effect of increasing the ionic concentration is the tendency of the water structure to assume the high density liquid form predicted for pure water upon supercooling. An important consequence of our analysis is that the concept of an ion as a structure maker or a structure breaker must be revisited to take into account the other ionic species, the ionic concentration and more generally the thermodynamic conditions of the solutions.     

Dodecahydroxy-closo-dodecaborate(2-).

The cesium salt of the icosahedral borane anion dodecahydroxy-closo-dodecaborate(2-), Cs(2)[closo-B(12)(OH)(12)], Cs(2)1, was prepared by heating cesium dodecahydro-closo-dodecaborate(2-), Cs(2)[closo-B(12)H(12)], Cs(2)2, with 30% hydrogen peroxide. The other alkali metal salts A(2)1 (A = Li, Na, K, Rb) precipitated upon addition of ACl to warm aqueous solutions of Cs(2)1. The ammonium salt, [NH(4)](2)1, and the (mu-nitrido)bis(triphenylphosphonium) salt, [PPN](2)1, were obtained similarly. The [H(3)O](2)1 salt precipitated upon acidification of aqueous solutions of Cs(2)1 with hydrochloric acid. The solubility of these salts in water was determined by measuring the boron content of saturated aqueous solutions of A(2)1 (A = Li, Na, K, Rb, Cs), [H(3)O](2)1, and [NH(4)](2)1 using ICP-AES. Although these salts are derived from a dianion with twelve pendant hydroxyl groups, the alkali metal salts surprisingly displayed low water solubilities. Water solubility decreases with a decrease in the radius of A(+), except for the lithium salt, which is slightly more soluble than the potassium salt. The [H(3)O](2)1 and the [NH(4)](2)1 salts provide rare examples of water-insoluble hydronium and ammonium salts. The low water solubility of the A(2)1 salts is attributed to the dianion's pendant hydroxyl groups, which appear to function as cross-linking ligands. Four alkali metal salts, A(2)1 (A = Na, K, Rb, Cs), were characterized in the solid state by single-crystal X-ray crystallography. These data revealed intricate networks in which several anions are complexed through their hydroxyl groups to each alkali metal cation. In addition, the anions are engaged in hydrogen bonding with each other and, if present, with water of hydration. This cross-linking results in the precipitation of aggregated salts. Cation coordination numbers decrease with cation radius. Thus, cesium and rubidium are ten-coordinate, whereas potassium is seven-coordinate and sodium is six-coordinate. The geometry of anion 1(2)(-) is independent of cation identity; the B-B and B-O bond lengths of the various A(2)1 salts (A = Na, K, Rb, Cs) are identical.     

Effect of electrolyte on surface free energy components of feldspar minerals using thin-layer wicking method

Application of the thin-layer wicking (TLW) technique on powdered minerals is useful for characterizing their surfaces. Albite (Na-feldspar) and orthoclase (K-feldspar) are feldspar minerals which are frequently found in the same matrix. Despite similarities in their physicochemical properties, separation of these minerals from each other by flotation is generally possible in the presence of monovalent salts such as NaCl. Both albite and orthoclase exhibit the same microflotation properties and rather close electrokinetic profiles in the absence of salt. In this study, contact angles of albite and orthoclase determined by the TLW technique yielded close values in the absence and presence of amine collector. While the calculated surface energies and their components determined using contact angle data reveal that the energy terms remain farther apart in the absence of the collector, the differences narrow down at collector concentrations where full flotation recoveries are obtained. However, the effect of addition of NaCl on contact angles and surface free energy components at constant amine concentration indicates that albite is significantly affected by salt addition, whereas orthoclase remains marginally affected. This interesting finding is explained on the basis of ion-exchange properties, the stability of the interface, flotation data, and zeta potential data in the presence of NaCl.     

Hydrogen Bonding in Liquid Water and in the Hydration Shell of Salts

A near‐IR spectral study on pure water and aqueous salt solutions is used to investigate stoichiometric concentrations of different types of hydrogen‐bonded water species in liquid water and in water comprising the hydration shell of salts. Analysis of the thermodynamics of hydrogen‐bond formation signifies that hydrogen‐bond making and breaking processes are dominated by enthalpy with non‐negligible heat capacity effects, as revealed by the temperature dependence of standard molar enthalpies of hydrogen‐bond formation and from analysis of the linear enthalpy–entropy compensation effects. A generalized method is proposed for the simultaneous calculation of the spectrum of water in the hydration shell and hydration number of solutes. Resolved spectra of water in the hydration shell of different salts clearly differentiate hydrogen bonding of water in the hydration shell around cations and anions. A comparison of resolved liquid water spectra and resolved hydration‐shell spectra of ions highlights that the ordering of absorption frequencies of different kinds of hydrogen‐bonded water species is also preserved in the bound state with significant changes in band position, band width, and band intensity because of the polarization of water molecules in the vicinity of ions.     

Effects of Charged Surfactants on Interfacial Water Structure and Macroscopic Properties of the Air-Water Interface

Surfactants are used to control the macroscopic properties of the air-water interface. However, the link between the surfactant molecular structure and the macroscopic properties remains unclear. Using sum-frequency generation spectroscopy and molecular dynamics simulations, two ionic surfactants (dodecyl trimethylammonium bromide, DTAB, and sodium dodecyl sulphate, SDS) with the same carbon chain lengths and charge magnitude (but different signs) of head groups interact and reorient interfacial water molecules differently. DTAB forms a thicker but sparser interfacial layer than SDS. It is due to the deep penetration into the adsorption zone of Br⁻ counterions compared to smaller Na⁺ ones, and also due to the flip-flop orientation of water molecules. SDS alters two distinctive interfacial water layers into a layer where H⁺ points to the air, forming strong hydrogen bonding with the sulphate headgroup. In contrast, only weaker dipole-dipole interactions with the DTAB headgroup are formed as they reorient water molecules with H⁺ point down to the aqueous phase. Hence, with more molecules adsorbed at the interface, SDS builds up a higher interfacial pressure than DTAB, producing lower surface tension and higher foam stability at a similar bulk concentration. Our findings offer improved knowledge for understanding various processes in the industry and nature.     

Local order in aqueous NaCl solutions and pure water: X-ray scattering and molecular dynamics simulations study

The microstructures of pure water and aqueous NaCl solutions over a wide range of salt concentrations (0-4 m) under ambient conditions are characterized by X-ray scattering and molecular dynamics (MD) simulations. MD simulations are performed with the rigid SPC water model as a solvent, while the ions are treated as charged Lennard-Jones particles. Simulated data show that the first peaks in the O...O and O...H pair correlation functions clearly decrease in height with increasing salt concentration. Simultaneously, the location of the second O...O peak, the signature of the so-called tetrahedral structure of water, gradually disappears. Consequently, the degree of hydrogen bonding in liquid water decreases when compared to pure fluid. MD results also show that the hydration number around the cation decreases as the salt concentration increases, which is most likely because some water molecules in the first hydration shell are occasionally substituted by chlorine. In addition, the fraction of contact ion pairs increases and that of solvent-separated ion pairs decreases. Experimental data are analyzed to deduce the structure factors and the pair correlation functions of each system. X-ray results clearly show a perturbation of the association structure of the solvent and highlight the appearance of new interactions between ions and water. A model of intermolecular arrangement via MD results is then proposed to describe the local order in each system, as deduced from X-ray scattering data.     

The Influence of Sodium Dodecyl Sulfate on the Surface Free Energy of Cassiterite

Measurements of contact angles for water, glycerol, formamide, diiodomethane, and bromoform on a cassiterite surface covered with sodium dodecyl sulfate (SDS) film were made. The samples of cassiterite were prepared in different ways for the measurements. Using the contact angle values, the Lifshitz-van der Waals and acid-base components of cassiterite/SDS film surface free energy were determined and compared with those for a "bare" cassiterite surface. It was found that the contact angle and the components of the surface free energy depend on the pH and concentration of the SDS solution from which the adsorption layer was formed. Also, the method of preparation of cassiterite plates influences these parameters. It was also found that the hydration of the cassiterite surface in the presence of a SDS film takes place and depends on the concentration of SDS, pH, and method of sample preparation, and strongly influences the surface free energy components of cassiterite.     

盐湖卤水蒸发浓缩过程中pH值变化规律研究

实验测定了当雄错、一里坪和多格错仁3个盐湖卤水蒸发浓缩过程中pH值,结果发现卤水浓缩过程中pH值表现趋酸向变化或越碱向变化规律;卤水中氯化钠、硫酸钠和水等大量中性成分随浓缩进行不断减少,则强酸弱碱盐或强碱弱酸盐在体系中所占比例越来越大,其水解推动pH值规律性变化;上述3个盐湖卤水浓缩时pH值变化试验数据反映的规律表明,据卤水化学组成配盐结果,可预先判断浓缩过程中pH值变化趋向;氯化物型和硫酸镁亚型盐湖卤水蒸发浓缩时pH值表现逐渐下降的规律,碱金属的碳酸盐或硼酸盐占较大比例的盐湖卤水蒸发浓缩时pH值表现逐渐上升规律;卤水浓缩过程中pH值变化规律对盐湖资源开发研究具有指导作用。     

Comparative study of the hydration systems formed during interactions of hydrogen phosphate dianions with putrescine, nor-putrescine and magnesium dications

A comparative study of hydration systems, formed as a result of the interaction between hydrogen phosphate dianions and three naturally occurring cations (putrescine (Put), its nor-homologue (nPut) and magnesium), is presented. On the basis of X-ray data and IR, NMR and calorimetric measurements, we have determined how the structure and physicochemical properties of the cations influence the system of phosphate residue hydration. Our study demonstrates that the stability of the hydration systems depends not only on the character of the bonds used by water to link with other salt components (coordinate or hydrogen bonds), but also on the location of the water molecules in the crystal lattice. In addition, contrary to magnesium salts, the dehydration of diamine (Put and nPut) hydrogen phosphates is reversible. Both dehydration and rehydration processes take place in the solid state. During rehydration, the crystalline anhydrous salt absorbs water molecules from the atmosphere. This leads to the reconstruction of the hydrated salt structure; this means that the salt which is the product of rehydration is identical with that obtained by crystallization from water solution.     

Computer modeling of the structure of supramolecular systems: An example of ion exchangers

A method is proposed for the determination of the structure of supramolecular systems based on spectrum-structure correlations and quantum-chemical and molecular dynamic modeling. Application of the developed approach to the analysis of the structures carboxy- and sulfo-cation exchangers as alkali metal salts and an amino acid (glycine) suggests that the ion pairs in the studied systems are dissociated. This allows a conclusion that the retention of ions in the ion exchangers in chromatographic separation is due to not only the electrostatic interaction of the fixed and mobile ions, but also hydrogen bonding between the hydration shells of the counterions.     

Hydrogen‐bonding motifs and thermotropic polymorphism in redetermined halide salts of hexamethylenediamine

The redetermined crystal structures of hexane‐1,6‐diammonium dichloride, C₆H₁₈N₂²⁺·2Cl⁻, (I), hexane‐1,6‐diammonium dibromide, C₆H₁₈N₂²⁺·2Br⁻, (II), and hexane‐1,6‐diammonium diiodide, C₆H₁₈N₂²⁺·2I⁻, (III), are described, focusing on their hydrogen‐bonding motifs. The chloride and bromide salts are isomorphous, with both demonstrating a small deviation from planarity [173.89 (10) and 173.0 (2)°, respectively] in the central C—C—C—C torsion angle of the hydrocarbon backbone. The chloride and bromide salts also show marked similarities in their hydrogen‐bonding interactions, with subtle differences evident in the hydrogen‐bond lengths reported. Bifurcated interactions are exhibited between the N‐donor atoms and the halide acceptors in the chloride and bromide salts. The iodide salt is very different in molecular structure, packing and intermolecular interactions. The hydrocarbon chain of the iodide straddles an inversion centre and the ammonium groups on the diammonium cation of the iodide salt are offset from the planar hydrocarbon backbone by a torsion angle of 69.6 (4)°. All three salts exhibit thermotropic polymorphism, as is evident from differential scanning calorimetry analysis and variable‐temperature powder X‐ray diffraction studies.     

Tetraalkylammonium Salts as Hydrogen‐Bonding Catalysts

Although the hydrogen‐bonding ability of the α hydrogen atoms on tetraalkylammonium salts is often discussed with respect to phase‐transfer catalysts, catalysis that utilizes the hydrogen‐bond‐donor properties of tetraalkylammonium salts remains unknown. Herein, we demonstrate hydrogen‐bonding catalysis with newly designed tetraalkylammonium salt catalysts in Mannich‐type reactions. The structure and the hydrogen‐bonding ability of the new ammonium salts were investigated by X‐ray diffraction analysis and NMR titration studies.     

鎓盐的固液相转移平衡研究(Ⅰ)——溴化四丁铵和羧酸钾的平衡常数测定

目前广泛应用的液-液相转移催化反应有两个主要的缺点:一是有一些水分子会随着离子对一起萃入有机相并在其周围形成水化膜[1,2],这不仅会降低反应速度有时还会引起种种副反应[3,4]。     

Effect of hydrogen bonding on properties of hexafluorosilicates with heterocyclic cations

The known data regarding the effects of interionic hydrogen bonding on properties of onium hexafluorosilicates with heterocyclic cations are summarized. Thermal stability parameters and water solubilities for this type of salts are shown to correlate with the number of strong and medium-strength hydrogen bonds or the number of hydrogen donors in the salt structure.     

Interfacial and solution properties of tetraalkylammonium bromides and their sodium dodecyl sulfate interacted products: a detailed physicochemical study

The adsorption and solution behaviors of symmetrical tetramethyl-, tetraethyl-, tetrapropyl-, and tetrabutylammonium bromides (TMAB, TEAB, TPAB, and TBAB, respectively) were studied at the air/water interface and in the bulk aqueous environments. Their salts were prepared by reacting tetraalkylammonium bromide (TAAB) with sodium dodecyl sulfate (SDS) in a solution from which the products of the higher two homologues (tetrapropylammonium dodecyl sulfate (TPADS) and tetrabutylammonium dodecyl sulfate (TBADS)) could only be isolated as solids and for which detailed characterization has been performed. The interfacial behaviors of 1:1 molar mixtures of TAAB and SDS and the prepared TPADS and TBADS were examined. Micellization of the 1:1 mixtures along with the isolated species were studied in the presence and absence of NaBr salt. The energetics of the micellization process and the counterion binding of the micelles were evaluated. The interaction of the TAABs with SDS micelles was examined, and the results were evaluated in terms of single- and two-site binding interaction models. Of the formed tetraalkylammonium dodecyl sulfates (TAADSs), only TBADS evidenced clouding, which was investigated in detail along with 1:1 molar mixtures of TBAB and SDS in aqueous solution in the presence of additives such as NaBr, SDS, and TBAB. The solution behaviors of the TAADS and the clouding of TBADS have been rationalized in terms of a mixed micellar model.     

Competition between DPPC and SDS at the air-aqueous interface

Vibrational sum frequency generation spectroscopy is used to study the interactions of the charged soluble organic surfactant sodium dodecyl sulfate (SDS) with an insoluble 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) monolayer at the air-aqueous interface. Results indicate that the surfactant species compete for surface sites in the mixed system, with a lower monolayer number density of DPPC molecules being observed in the presence of dodecyl sulfate anions at the interface. Spectroscopic results also indicate that fewer dodecyl sulfate chains reside at the interface when the insoluble DPPC film is present. Increased conformational ordering of the acyl chains of both the DPPC molecules and the interfacial dodecyl sulfate anions is observed in the mixed system. Additionally, charged surfactant SDS promotes the alignment of the interfacial water molecules even in the presence of a DPPC monolayer.