Higher Adsorption Density Research Articles

The entrainment swelling of emulsion during lactic acid extraction by LSMs

Studies on the entrainment swelling of emulsion during batch extraction of lactic acid using a liquid surfactant membrane (LSM) system are reported. The concept of effective concentration of surfactant in membrane phase has been proposed, considering the high adsorption density of the surfactant at the droplet interfaces in the LSM system. The swelling caused by emulsification during the initial dispersion process was investigated. The swelling ratio was measured by a density method. A model for estimating the “emulsification” swelling ratio, F se, has been proposed, based on a mechanism of swelling due to the entrainment of water resulting from the interfacial turbulence and emulsification in the initial dispersion process.

The Kinetics of Adsorption of Calcium and Magnesium Ions on to an Insoluble Sodium Polyphosphate

The adsorption kinetics of calcium and magnesium on to an insoluble sodium polyphosphate were determined at 25°, 37°, and 49°C. The adsorption process has two distinct phases, an initial rapid phase lasting 10-20 min and a slow phase which occurs over several hours. The data were fitted to a two-site adsorption model by nonlinear least squares regression and the rate constants of the rapid and slow adsorption phases were estimated. Experimental adsorption rates for magnesium are lower than for calcium and this may be because the greater energy of dehydration of magnesium ions hinders adsorption. The high experimental adsorption densities for both ions suggest that the effective surface area is substantially larger than the estimate obtained by helium adsorption. It is suggested that the large apparent surface area may be due to the deposition of a gel layer of insoluble calcium or magnesium polyphosphates on the surface.

Monomolekulare Schichten nichtionischer Tenside / Non-ıonic surfactant monolayers

The foam stability of aqueous solutions of pentaethylene glycol n-decyl ether and pentaethylene glycol n-dodecyl ether surfactants was related to experimental studies carried out on thin horizontal microscopic single films of the surfactant solutions using the microinterferometric film apparatus developed by Scheludko [1]. It was observed that an initial decrease in film thickness and reduction in potential at the air/solution interface of the film, occurred at a critical surfactant concentration where black spots appeared in the film when it was observed in reflected light. The black spots appeared to expand and coalesce, until the entire sheet was thinned down to a metastable common black film. In addition, a sharp increase in foam stability occured above this surfactant concentration, which was about 0.1 c. m. c., and corresponded to a high surface adsorption density of surfactant. We believe that the cause of the phenomenon was a change in state of the surfactant monolayer as originally proposed by Scheludko and Exerowa [2, 3]. Such a change (condensation) from gaseous to liquid expanded monolayers, has already been established for ionic surfactants and may change the rheological properties of the adsorbed film, enhancing the foam stability. Finally, the foam stability was determined at a range of electrolyte (KCI) concentrations from which it could be suggested that two types of foam stabilization methods were operating. In the low electrolyte concentration, the stability appeared to be controlled by the metastable common first black film, but at high electrolyte concentrations the stability seemed to be dependent on secondary Newton black films.

Adsorption of Alkyltrimethylammonium Bromides on Silica: Calorimetric Study of Effect of Coions

Adsorption isotherms and differential enthalpies of adsorption have been determined for dodecyl- and tetradecyltrimethylammonium bromide on silica at initial pH 9 and on mica in sodium, tetramethylammonium, and calcium bromide solutions of low concentrations. The adsorption isotherms show a clear difference between the influence of the various colons at low coverage and the effect is also seen in the calorimetric results. The initial adsorption is discussed in terms of a competition between surfactant ions and ions of the same charge. The colon effect disappears at higher adsorption densities. The relation between the shape of the adsorption isotherm and the charge properties of the surface and their variation with experimental conditions are discussed. The general trend of the change of the differential adsorption enthalpies with coverage is the same for the adsorption on silica and mica except in the initial stage. As the surface coverage increases, there is a steady decrease toward more exothermic values so, as the maximum adsorption is reached, the adsorption enthalpy for tetradecyltrimethylammonium bromide on silica is -10 kJ/mol more exothermic than the enthalpy of micelle formation. The change of the adsorption enthalpy with surface coverage is discussed in terms of changes in the structure of the aggregates formed at the surface. The calorimetric results are consistent with the interpretation of the adsorption of surfactants on highly charged surfaces expressed in a newly developed thermodynamic model implying adsorption of small discrete aggregates at the lowest adsorption densities and bilayer formation in the later stages of adsorption.

Synergistic effect of mineral surface constituents in dextrin adsorption

The present work is a part of a systematic research on the interaction mechanism between polysaccharides and mineral surfaces, and deals with the adsorption of dextrin onto oxide and salt-type minerals. The minerals tested include hematite, rutile, fluorite, calcite, apatite, etc. It is observed that dextrin adsorption on these minerals is strongly pH-dependent, with high adsorption densities at pH values around the isoelectric points of the minerals, which indicates interactions with surface metal hydroxide species. X-ray photoelectron spectroscopy (XPS) and Auger Electron Spectroscopy (AES) measurements reveal chemical shifts of binding energies of core electrons, or chemical shifts of modified Auger parameters of mineral surface elements after dextrin adsorption, pointing to a chemical mechanism of interaction. Furthermore, it is observed that the reaction of minerals with moderate amounts of fatty acids prior to dextrin addition results in an increase in dextrin adsorption, whereas reaction with excessive fatty acids prior to dextrin addition results in a decrease in dextrin adsorption. In the light of the results of our previous studies, and also from the experimental observations of the present work, it is concluded that although surface hydrophobic species alone do not have any significant effect on dextrin adsorption, they synergistically enhance dextrin adsorption when present together with surface metallic sites.

Surface chemistry of ferrihydrite: Part 2. Kinetics of arsenate adsorption and coprecipitation

The kinetics of As(V) adsorption by ferrihydrite was investigated in coprecipitation and postsynthesis adsorption experiments conducted in the pH range 7.5–9.0. In coprecipitation experiments, As(V) was present in solution during the hydrolysis and precipitation of iron. In adsorption experiments, a period of rapid (<5 min) As(V) uptake from solution was followed by continued uptake for at least eight days, as As(V) diffused to adsorption sites on ferrihydrite surfaces within aggregates of colloidal particles. The time dependence of As(V) adsorption is well described by a general model for diffusion into a sphere if a subset of surface sites located near the exterior of aggregates is assumed to attain adsorptive equilibrium rapidly. The kinetics of As(V) desorption after an increase in pH were also consistent with diffusion as a rate-limiting process. Aging of pure ferrihydrite prior to As(V) adsorption caused a decrease in adsorption sites on the precipitate owing to crystallite growth. In coprecipitation experiments, the initial As(V) uptake was significantly greater than in post-synthesis adsorption experiments, and the rate of uptake was not diffusion limited because As(V) was coordinated by surface sites before crystallite growth and coagulation processes could proceed. After the initial adsorption, As(V) was slowly released from coprecipitates for at least one month, as crystallite growth caused desorption of As(V). Adsorption densities as high as 0.7 mole As(V) per mole of Fe were measured in coprecipitates, in comparison to 0.25 mole As(V) per mole of Fe in post-synthesis adsorption experiments. Despite the high Concentration of As(V) in the precipitates, EXAFS spectroscopy ( Waychunas et al., 1993) showed that neither ferric arsenate nor any other As-bearing surface precipitate or solid solution was formed. The high adsorption densities are possible because the ferrihydrite particles are extremely small, approaching the size of small dioctahedral chains at the highest As(V) adsorption density. The results suggest that the solid solution model proposed by Fox (1989, 1992) for control of arsenate and phosphate concentrations in natural waters may be invalid.

Adsorption of lead(II) on the goethite surface: Voltammetric evaluation of surface complexation parameters

The adsorption of Pb(II) to the goethite surface was investigated in dilute solutions in the pH range 6.6–8.2. Increments of goethite suspension were added to dilute (2.41 × 10 −7 M) Pb(II) solutions at constant pH in a titration procedure. Differential pulse anodic stripping voltammetry (DPASV) was used to determine the concentration of free, nonadsorbed Pb(II) in goethite suspensions without prior separation of the solution and solid phases. This procedure permits achievement of high adsorption density at relatively low total metal concentrations and correction for adsorption losses to the reaction vessel. Surface reaction parameters were obtained using different methods of interpretation of the experimental data. Surface complex formation constants for monodentate and bidentate surface complexes are FeOOH + Pb +2 ⇄ FeOOPb ++H +, log K S 1= -0.52, 2 FeOOH + Pb +2 ⇄(FeOO) 2Pb+2H +, log β S 2= -6.27 kg dm 3 Intrinsic acidity constants obtained from acid-base titrations are, respectively, log K al 5 = −6.7, and log K a2 5 = −9.0; the exchangeable H + of the goethite surface was determined as 0.135 mole/kg. The Pb(II) adsorption capacity of the goethite is 5.6 × 10 −2 mole/kg (3.8 × 10 −6 mole/m 2). The average ligand number was found to be 1.25 and constant throughout the concentration range of adsorption. The determined surface reaction parameters describe the effect of changes in pH and solution composition on Pb(II) binding and give a consistent and comprehensive thermodynamic description of the adsorption process within variations of Pb(II) concentration and particles by factors of 3000 and 1000, respectively. The methods developed here can readily be applied to the determination of adsorption parameters of natural particulate matter.

The state of fluorocarbon surfactant monolayers at the air-water interface and on mica surfaces

Monolayers of a cationic, double-chained fluorocarbon surfactant (N-(α-trimethylammonioacetyl)-O-O′-bis(1H, 1H, 2H, 2H-perfluorodecyl)- l- glutamate chloride) were studied at the air-water interface and adsorbed on the muscovite mica basal plane. The stability of the monolayer at the air-water interface was considerably higher on a 10 −2 M KBr subphase than on pure water. Fluorocarbon surfactant-coated mica surfaces were obtained either by Langmuir-Blodgett (LB) deposition or by adsorption from a polar solvent. The adsorbed layer was characterized by means of ESCA, transfer ratio and contact angle measurements. A more densely packed layer could be obtained by LB deposition than by adsorption from solution. For this surfactant the preferential conformation of the molecule in layers adsorbed from solution is different from that in LB layers. At low adsorption densities the advancing contact angle increases rapidly with the adsorbed amount. The receding angle is hardly affected until the area per molecule is less than about 140 Å 2. At higher adsorption densities, the receding contact angle is more sensitive to the adsorbed amount than the advancing angle. The reason for the low salt stability, the low adhesion force in water, and the numerous cavities forming around the contact region when surfaces coated by this fluorocarbon surfactant are brought together in water is proposed to be due to (molecular size) heterogeneities in the surfactant layer.

Adsorption-desorption behaviour of zinc(II) at iron(III) hydroxide-aqueous solution interface as influenced by pH and temperature

Batch studies were carried out to investigate the adsorption of zinc(II) from fresh waters on an iron(III) hydroxide surface maintained at the pH of zero point of charge of hydroxide (ZPC, 6·85) and also on both the acidic (5·5) and alkaline (8·2) sides of pH of ZPC, at 15 and 35°C. Zinc(II) adsorption on iron(III) hydroxide increased with an increase in pH. The rise in temperature from 15 to 35°C increased zinc(II) adsorption at pH 5·5 and 6·85, but decreased it at alkaline pH (8·2). In none of the cases did adsorption attain a maximum adsorption density. The results indicate the presence of heterogeneous sites of varying affinity on the adsorbent. Zinc(II) adsorption followed Langmuir behaviour only at small adsorption densities (less than 10 −2·95 m Zn/kg at pH 5·5) and at higher adsorption densities, the availability of strongest binding sites decreased. Nonspecifically adsorbed zinc(II) (reversible to Ba(II)) decreased with the increase in pH and temperature. Sequential desorption experiments also revealed that desorption of adsorbed zinc(II) decreased with an increase in pH.

EFFECTS OF THE EMULSIFIER IN COPPER EXTRACTION BY LIX64N-SPAN 80 LIQUID SURFACTANT MEMBRANES

Span 80-LIX65N-LIX63 liquid surfactant membranes (LSMs) were used to extract copper from sulfate solutions at a pH of 2.0. The physico-chemical effects of Span 80 (sorbitan monooleate) were investigated with the aid of measurements of emulsion stability (as determined by leakage of K ions from the internal to the external phase), membrane phase viscosity, emulsion viscosity, and inter-facial tension. As the concentration of Span 80 was increased, copper extraction rate at first increased, rose through a maximum, and then decreased. The initial increase corresponded to a significant improvement in the stability of emulsion globules. Inter-facial tension measurements indicated that in order to achieve stable emulsion globules, a high adsorption density of the emulsi-fier at the emulsion interfaces in a LSM system was needed. It was further shown that a bulk phase emulsifier-extractant interaction at high extractant concentrations had an adverse effect on the stability of emulsion globules by removing emulsi...

X-ray photoelectron spectroscopy (XPS) op adsorbed zinc on amorphous hydrous ferric oxide

The adsorption of Zn on amorphous hydrous ferric oxide (HFO) was monitored using both solution measurements of the non-absorbed Zn and X-ray photoelectron spectroscopy (XPS) measurements of the adsorbed Zn. Solution results suggest that, at low adsorption densities and/or low pH's, the mechanism of Zn adsorption may be similar to the adsorption of Cd, Cu and Pb. However, at higher adsorption densities and higher pH's, there is an apparent change in Zn speciation. Chemical shift and adsorbate thickness values derived from XPS provide information on the mechanism of the surface adsorption of Zn. The XPS studies show that submonolayer coverages of Zn are largely chemically adsorbed at solution pH's less than 6.5, while at higher pH's the Zn is partially hydrolyzed to form a surface layer containing physisorbed Zn(OH) 2 which may exceed monolayer thicknesses.

Marine scavenging: Trace metal adsorption by interfacial sediment from MANOP Site H

The adsorption of thirteen trace metals from seawater was studied on interfacial sediment from MANOP site H. The adsorption data indicate a long (~20 day) equilibration time for most metals, an increase in adsorption with an increase in pH and particle concentration, a dependence of adsorption on total metal concentration at high adsorption densities, and a lack of correlation between metal binding ability and metal hydrolysis in solution. Apparent equilibrium binding constants normalized to the total number of available sites on the solid were determined for metal binding with the interfacial sediment. The binding constants indicate that the affinity sequence for metal interactions with the interfacial sediment is: Pb > Fe > Sn ≳ Co≈ Mn > Cu > Be > Sc ≳ Zn > Ni > Cd ≳ Ba > Cs at pH 7.82 in seawater. A comparison of the binding constants for suspended particles, interfacial sediment, and surface sediment indicate that the composition of particles influences the binding ability of the particles. Biogenic particles tend to bind most metals more strongly than lithogenic or authigenic particles. Based on limited data, there is a strong positive correlation between the measured binding constants and the observed partitioning of metals between sediment and seawater.

X-ray photoelectron spectroscopy (XPS) of adsorbed zinc on amorphous hydrous ferric oxide

The adsorption of Zn on amorphous hydrous ferric oxide (HFO) was monitored using both solution measurements of the non-absorbed Zn and X-ray photoelectron spectroscopy (XPS) measurements of the adsorbed Zn. Solution results suggest that, at low adsorption densities and/or low pH's, the mechanism of Zn adsorption may be similar to the adsorption of Cd, Cu and Pb. However, at higher adsorption densities and higher pH's, there is an apparent change in Zn speciation. Chemical shift and adsorbate thickness values derived from XPS provide information on the mechanism of the surface adsorption of Zn. The XPS studies show that submonolayer coverages of Zn are largely chemically adsorbed at solution pH's less than 6.5, while at higher pH's the Zn is partially hydrolyzed to form a surface layer containing physisorbed Zn(OH)2 which may exceed monolayer thicknesses.

The effect of adsorbed humic substances on the colloid stability of haematite particles

Rate constants ( k) were measured for the Ca 2+-induced coagulation of haematite particles in the presence of different amounts of humic substances at pH 6.6. Values of k at 1 mg humics/1 were approximately one order of magnitude greater than those at 20 mg/l at all CaCl 2 concentrations studied (0.001–0.5 M). The enhanced stability does not appear to be due to charge stabilisation, since electrophoretic mobilities were almost independent of the extent of adsorption. Steric stabilisation at high adsorption density is a likely explanation. Interpretation of the results is complicated by the increase (up to 4–6 fold) in humic adsorption with CaCl 2 concentration.

Multiple-site adsorption of Cd, Cu, Zn, and Pb on amorphous iron oxyhydroxide

Adsorption of Cd, Zn, Cu, and Pb onto amorphous iron oxyhydroxide was measured as a function of pH, metal ion concentration, and adsorbent concentration. For each metal, there is a narrow pH band where fractional adsorption increases from near nil to near 100%. For fixed adsorbent concentration, the pH region of the pH-adsorption edge is independent of total adsorbate concentration when adsorption density is less than 10 −5.0, 10 −3.7, and 10 −2.3 moles adsorbate per mole Fe for adsorption of Cd, Cu, and Zn, respectively. At larger adsorption densities for these three metals and over the entire range of adsorption densities studied for Pb, the pH region of the adsorption edge becomes more alkaline as total adsorbate concentration increases. In no case did adsorption density attain a maximum, limiting value. The results suggest that the surface is composed of many groups of binding sites. The strength of binding between a given metal and the surface may vary by an order of magnitude or more from one site to another. At small adsorption densities all types of sites are available in excess, and adsorption can be described by the Langmuir isotherm. However, at higher adsorption densities, availability of the strongest binding sites decreases, leading to a decrease in the apparent adsorption equilibrium constant. This phenomenon occurs under conditions where only a few percent of all surface sites are occupied, and is inconsistent with available single-site models.

Effect of polymer adsorption on the properties of the electrical double layer

The adsorption isotherm of a neutral polymer, poly(vinyl alcohol), on the hydrated aluminium oxide gibbsite has been measured. Much higher adsorption densities were obtained than have been observed with other colloidal systems. Surface charge measurements indicate that no change either in the isoelectric point or the capacitance of the electrical double layer occurs as a result of the adsorption of the polymer. A large reduction however is observed in the value of the electrophoretic mobility of the gibbsite in the presence of poly(vinyl alcohol). This is attributed to a displacement of the plane of shear by the adsorbed polymer.

The mechanism of adsorption of fatty acids and other surfactants at the oxide—water interface

The technique of microelectrophoresis was used to obtain electrochemical evidence of any specific adsorption of saturated and unsaturated fatty acids and other surfactants at the Al 2O 3 and Fe 2O 3—water interfaces. The adsorption of alkyl sulfonates and sulfates on both of these oxide colloids resulted from simple physisorption together with subsequent hydrophobic interaction at higher adsorption densities. Lauric acid also adsorbed on alumina by simple physisorption plus hydrophobic interaction whereas on Fe 2O 3 it showed not only hydrophobic adsorption but also specific chemisorption. In contrast, potassium octyl hydroxamate showed very strong specific chemisorption but no hydrophobic interaction. Oleic acid appears to chemisorb on both Fe 2O 3 and A1 2O 3, but it is complicated by deterioration of the oleic acid.

Optical Sensitizing of Silver Halides by Dyes II. The Mechanism of Optical Sensitizing and the Quantum Equivalent

The photolysis of silver bromide sensitized with certain acid and basic dyes has been followed by chemical and photometric determination of the silver produced. It was found that in the absence of halogen acceptors the dye is progressively bleached, but in the presence of halogen acceptors it is recovered chemically unchanged. However, the dye actually combines with the bromine released but is regenerated on giving up the bromine to another less active acceptor. The maximum of sensitivity is found to occur considerably before adsorption-saturation, and at about the same adsorption-density for both photolysis and photographic sensitivity. The apparent desensitizing action of sensitizing dyes at higher adsorption-densities is ascribed to the facility of rebromination by the dye:bromine addition compound. So long as the photographic yield is proportional to the adsorption-density of the dye an initial quantum equivalent of unity is found—one quantum of light absorbed by the dye produces one silver atom. In regard to the mechanism of sensitizing, photovoltaic experiments indicate the release of photoconductance electrons. These may be emitted either by the adsorbed dye itself or by bromide ions of the silver bromide receiving energy from the photoactivated dye. Some indirect evidence supports the latter view.