Adsorption of octadecyltrimethylammonium chloride and adsolubilization on to cellulosic fibers

The coadsorption of different organic solutes on cellulosic fibers treated with octadecyltrimethylammonium chloride (ODTMA) has been studied. In the absence of ODTMA cellulosic fibers had little tendency to retain organic compounds. The enhanced solute incorporation was ascribed to the adsolubilization of these compounds on the aggregated domains of the adsorbed surfactant molecules at the solid/liquid interface. The specific shape of solute coadsorption isotherms indicated that the adsolubilization process may be regarded as a partition phenomenon between the aqueous bulk phase and the adsorbed surfactant aggregates. The decrease in solute uptake at the cellulose/water interface above the CMC of ODTMA was ascribed to micelle formation in the bulk solution and to the ensuing micellar solubilization of organic solutes. Preliminary experiments performed using vertical fixed bed columns showed that modified cellulosic fibers can be conveniently used as substrate for treating organic pollutants.     

Adsorption of cationic surfactants and subsequent adsolubilization of organic compounds onto cellulose fibers

Cationic surfactants with different hydrophobic chain length were adsorbed onto cellulose fibers in an aqueous medium. The adsorption isotherms exhibited three characteristic regions which were interpreted in terms of the mode of aggregation of the surfactant molecules at the solid–liquid interface. The hydrophobic layers were used as a reservoir to trap various slightly water soluble organic molecules. A quantitative study of these phenomena suggested typical partition behavior of the organic solutes between the aqueous phase and the surfactant layer. The surfactant chain length (from C12 to C18) was shown to play an important role in terms of the capacity to retain the organic solute and the capacity increased with the number of carbon atoms.     

Adsorption of organic compounds onto polyelectrolyte immobilized-surfactant aggregates on cellulosic fibers

The adsorption of anionic surfactants with different hydrophobic chain lengths onto cellulose fibers pretreated with a cationic polyelectrolyte has been investigated. Five steps are involved in the adsorption process, which was ascribed to the formation of monolayer and bilayer surfactant aggregates. Electrostatic interaction between the residual surface charges followed by hydrophobic interaction among the alkyl chains are considered the main factors in the adsorption process. The adsorption of the anionic surfactant was found to greatly enhance the retention of organic compounds onto the polyelectrolyte-treated cellulose. The coadsorption phenomenon, which was dependent on the saturation level of the adsorbed surfactant, has been explained in terms of the accumulation of the organic solute on the hydrophobic core generated by the adsorbed layer.     

Adsorption of a cationic surfactant onto cellulosic fibers I. Surface charge effects

The adsorption of four cationic surfactants with different alkyl chain lengths on cellulose substrates was investigated. Cellulose fibers were used as model substrates, and primary alcohol groups of cellulose glycosyl units were oxidized into carboxylic groups to obtain substrates with different surface charges. The amount of surfactant adsorbed on the fiber surface, the fiber zeta-potential, and the amount of surfactant counterions (Cl(-)) released into solution were measured as a function of the surfactant bulk concentration, its molecular structure, the substrate surface charge, and the ionic strength. The contribution of each of these parameters to the shape of the adsorption isotherms was used to verify if surfactant adsorption and self-assembly models usually used to describe the behavior of surfactant/oxide systems can be applied, and with which limitations, to describe cationic surfactant adsorption onto oppositely charged cellulose substrates.     

Sorption of a fluorescent whitening agent (Tinopal CBS) onto modified cellulose fibers in the presence of surfactants and salt

The combined effect of salt (10 mmol L(-1)) and surfactants on the sorption of the fluorescent brightener 4,4'-distyrylbiphenyl sodium sulfonate (Tinopal CBS) onto modified cellulose fibers was studied. Sorption efficiencies with both cationic and anionic surfactants were evaluated. Emission spectroscopy was used for quantitative analysis since Tinopal has an intense fluorescence. The sorption efficiency of the brightener is greater for solutions containing a cationic surfactant (DTAC) below the critical micelle concentration (cmc), while for an anionic surfactant (SDS) above its cmc the efficiency is greater. The profile of the sorption isotherms were interpreted in terms of the evolution of surfactant aggregation at the fiber/solution interface. Salt influences the efficiency of the Tinopal sorption on the modified cellulose fibers either because it decreases the cmc of the surfactants or because the ions screen the surface charges of the fiber which decreases the electrostatic interaction among the charged headgroup of the surfactant and the charged fiber surface.     

Adsorption effects in rejection of macromolecules by ultrafiltration membranes

Rejection of adsorbing solutes by ultrafiltration membranes is not adequately described by the steric rejection theory [3]. Solute adsorption (fouling) changes the shape of the rejection curve. Typically, the measured curves are steeper than the theoretical curve. The shape of the curve can be predicted qualitatively from simple theoretical considerations. For adsorbing solutes, single-solute and multiple-solute ultrafiltration experiments give different results. Relative thickness of adsorbed solute layer in a membrane pore was found to depend on (1) solute size, (2) solute hydrophobicity, (3) pH and ionic strength for a protein solute, (4) solute concentration, and (5) time of adsorption. Large differences observed between water fluxes and fluxes of very dilute polymer solutions through the same membrane are also interpreted in terms of solute adsorption.     

Solute-Solvent Interactions in the Nematic and Smectic A Phases of 4 4-di-n-Heptylazoxybenzene Studied by Deuteron NMR

The orientational order parameters have been measured by deuteron N.M.R. of both solute (at low dilution) and solvent in various binary mixtures involving the liquid crystal 4,4'-di-n-heptylazoxybenzene (HAB). The solutes studied were azoxybenzene-d₁₀ and n-heptylbenzene-d₇ which are fragments of HAB, azobenzene-d₁₀ because of its similarity to azoxybenzene, and anthracene-d₁₀ because of its known structure and symmetric shape. The major and biaxial order parameters of the solutes are analysed in terms of a molecular field model for the potential of mean torque for biaxial particles. The behaviour of the solute order parameters on approaching and entering the smectic A phase is interpreted in terms of a temperature and phase dependent partitioning of the solute between aromatic and aliphatic regions of the solvent.     

Kinetics of adsorption of polyvinylamine on cellulose fibers. II. Adsorption from electrolyte solutions

Adsorption from electrolyte solutions of fully hydrolyzed polyvinylamine on cellulose fibers was investigated by supplying the polymer to the fibers at controlled rate. This was implemented by employing a reactor only open to the fluid in which the fiber dispersion were confined and homogenized. The adsorbed layers may be defined as diffuse or dense layers. Diffuse layers are characterized by a surface coverage limited to 0.65 mg/g cellulose in salt-free solutions. Addition of NaCl or CaCl(2) to the fiber dispersion and the polymer solution promotes the adsorption rate and increases the amount of adsorption to 1.5 mg/g cellulose. For dense polymer layers, for which the coverage amounts to values close to 10 mg/g cellulose in salt-free systems, addition of electrolyte does not change the kinetic and adsorption characteristics. Insofar as the variation of the molecular areas of the polymer within the diffuse layers as a function of the ionic strength parallels the variation of the molecular characteristics of solute molecules, the formation of diffuse layers is expected to proceed by random deposition of solute molecules which later individually sustain strong reconformation. Adsorption isotherms show a limited influence of the ionic strength. Obviously, the passage from dense layers of high surface coverage to low adsorption values at equilibrium requires extended reconformation of adsorbed macromolecules and desorption of a great part of the molecules already adsorbed.     

Mobile phase effects in reversed-phase liquid chromatography: a comparison of acetonitrile/water and methanol/water solvents as studied by molecular simulation

Molecular simulations of water/acetonitrile and water/methanol mobile phases in contact with a C(18) stationary phase were carried out to examine the molecular-level effects of mobile phase composition on structure and retention in reversed-phase liquid chromatography. The simulations indicate that increases in the fraction of organic modifier increase the amount of solvent penetration into the stationary phase and that this intercalated solvent increases chain alignment. This effect is slightly more apparent for acetonitrile containing solvents. The retention mechanism of alkane solutes showed contributions from both partitioning and adsorption. Despite changes in chain structure and solvation, the molecular mechanism of retention for alkane solutes was not affected by solvent composition. The mechanism of retention for alcohol solutes was primarily adsorption at the interface between the mobile and stationary phase, but there were also contributions from interactions with surface silanols. The interaction between the solute and surface silanols become very important at high concentrations of acetonitrile.     

Scavenging of Priority Organic Pollutants from Aqueous Waste Using Granular Activated Carbon

Compliances with stringent effluent discharge limits imposed by environmental protection agencies (EPA) and the most economic way of achieving it without loss of production has led to continued refinement, recognition and development of promising wastewater treatment technologies. Many organic compounds present in industrial and domestic wastewaters are carcinogenic in nature. Removal of these organic compounds from wastewater has become a great challenge to wastewater treatment technologies, as many of them are non‐biodegradable in nature. Adsorption on granular activated carbon (GAC) has emerged an efficient and economically viable technology for removal of final traces of a broad spectrum of toxic organic compounds from domestic and industrial wastewater. In the present investigation adsorption of some priority organic pollutants, namely phenol, o‐cresol, p‐nitrophenol, m‐methoxyphenol, benzoic acid and salicylic acid on granular activated carbon, was studied in a batch system at laboratory scale. Experiments were carried out to determine adsorption isotherms and kinetics for adsorbate when present in aqueous solutions as single, bi‐ and tri‐solute systems. The commercially available bituminous coal based granular activated carbon Filtrasorb 300 (F‐300) was used as adsorbent. The results indicate that p‐nitrophenol is most strongly adsorbed as compared to other phenols studied. Aqueous phase solubility of the adsorbate plays a deciding role in multi‐component systems as more hydrophobic p‐nitrophenol adsorbs to a greater extent than less hydrophobic phenol, o‐cresol and m‐methoxyphenol. The preferential adsorption of strongly adsorbable solute over a weakly adsorbable one has been observed, as the solutes are competing for the available surface area of the adsorbent for adsorption.     

Adsorption of eicosane and 1,2-hexanediol from supercritical carbon dioxide on activated carbon and chromosorb

Supercritical adsorption isotherms were measured for eicosane and 1,2-hexanediol on activated carbon and chromosorb 101 at 324.2 K and 11.45 MPa. Adsorption isotherms of both solutes on activated carbon have typical shapes and can be modelled by Langmuir or Freundlich equations. For chromosorb 101 the equilibrium loadings are a linear function of the concentration of the solute in the fluid phase. These results suggest that for chromosorb 101 a partition of the solute between two immiscible phases rather than adsorption occurs.     

Adsorption and inactivation behavior of horseradish peroxidase on cellulosic fiber surfaces

The physical immobilization behavior of horseradish peroxidase (HRP) on cellulosic fiber surfaces was characterized using adsorption and inactivation isotherms measured by the depletion method followed by fitting of Langmuir’s and Freundlich’s models to the experimental data. The adsorption and inactivation behavior of simpler and relatively non-porous high and low crystalline cellulosic substrates (microcrystalline cellulose and regenerated cellulose) as well as more complex and porous cellulosic pulp fibers (bleached kraft softwood fibers) were investigated. The effect of the sorbent surface energy on HRP adsorption was demonstrated by increasing the hydrophobicity of the cellulosic fibers using an internal sizing agent. The influence of the fiber surface charge density on HRP adsorption was studied via modification of the cellulosic fibers using TEMPO (2,2,6,6-tetramethyl-1-piperidiniloxy radical)-mediated oxidation methods. Results showed that hydrophobic interactions had a much larger effect on HRP adsorption than electrostatic interactions. More hydrophobic fiber surfaces (lower polar surface energy) result in larger enzyme-fiber binding affinity constants and higher binding heterogeneity. It was also found that oxidation of the cellulosic fiber substrate reduces enzyme adsorption affinity but significantly increases the loading capacity per unit weight of the surface.     

Effect of temperature on competitive adsorption of the solute and the organic solvent in reversed-phase liquid chromatography

In analysis of the temperature effect on chromatographic separations the influence of the adsorption of organic solvent on the retention properties of solute is generally not taken into account. In fact, adsorption behavior of solutes is strongly affected by competitive adsorption of organic solvents, which is temperature dependent. In this work changes of adsorption equilibrium of an organic solvent as well as a solute with temperature have been analyzed. Data of the excess adsorption of methanol from aqueous solutions on octadecyl-bonded silica have been acquired at different temperature. Experiments have been performed over a relatively narrow temperature range corresponding to typical chromatographic conditions, i.e., 10-50 degrees C. The competitive adsorption equilibria of model solutes (i.e., two homologous compounds: cyclopentanone and cyclohexanone) have been measured at different temperature and composition of the mobile phase. Temperature alterations to the retention properties were found to result from combined effects of changes in adsorption behavior of the organic solvent and of the solute. The influence of temperature on the separation selectivity has been considered.     

Dilational viscoelasticity of anionic polyelectrolyte/surfactant adsorption films at the water–octane interface

In this article, the interfacial tension and interfacial dilational viscoelasticity of polystyrene sulfonate/surfactant adsorption films at the water–octane interface have been studied by spinning drop method and oscillating barriers method respectively. The experimental results show that different interfacial behaviors can be observed in different type of polyelectrolyte/surfactant systems. Polystyrene sulfonate sodium (PSS)/cationic surfactant hexadecanetrimethyl–ammonium bromide systems show the classical behavior of oppositely charged polyelectrolyte/surfactant systems and can be explained well by electrostatic interaction. In the case of PSS/anionic surfactant sodium dodecyl sulfate (SDS) systems, the coadsorption of PSS at interface through hydrophobic interaction with alkyl chain of SDS leads to the increase of interfacial tension and the decrease of dilational elasticity. For PSS/nonionic surfactant TX100 systems, PSS may form a sub-layer contiguous to the aqueous phase with partly hydrophobic polyoxyethylene chain of TX100, which has little effect on the TX100 adsorption film and interfacial tension.     

Adsorption of nonionic surfactants on cellulose surfaces: adsorbed amounts and kinetics

Kinetic and equilibrium aspects of three different poly(ethylene oxide) alkylethers (C12E5, C12E7, C14E7) near a flat cellulose surface are studied. The equilibrium adsorption isotherms look very similar for these surfactants, each showing three different regions with increasing surfactant concentration. At low surfactant content both the headgroup and the tail contribute to the adsorption. At higher surface concentrations, lateral attraction becomes prominent and leads to the formation of aggregates on the surface. The general shape of the isotherms and the magnitude of the adsorption resemble mostly those for hydrophilic surfaces, but both the ethylene oxide and the aliphatic segments determine affinity for the surface. The adsorption and desorption kinetics are strongly dependent on surfactant composition. At bulk concentrations below the CMC, the initial adsorption rate is attachment-controlled. Above the CMC, the micellar diffusion coefficient and the micellar dissociation rate play a crucial role. For the most hydrophilic surfactant, C12E7, both parameters are relatively large. In this case, the initial adsorption rate increases with increasing surfactant concentration, also above the CMC. For C12E5 and C14E7 there is no micellar contribution to the initial adsorption rate. The initial desorption kinetics are governed by monomer detachment from the surface aggregates. The desorption rate constants scale with the CMC, indicating an analogy between the surface aggregates and those formed in solution.     

Gibbs ensemble Monte Carlo simulation of adsorption for model surfactant solution in confined slit pores

An isobaric-isothermal Gibbs ensemble Monte Carlo simulation has been carried out to study the adsorption of a model surfactant/solvent mixture in slit nanopores. The adsorption isotherms, the density distributions, and the configuration snapshots were simulated to illustrate the adsorption and self-assembly behaviors of the surfactant in the confined pores. The adsorption isotherms are stepwise: a two-step curve for the smaller (30 A) pore and a three-step one for the larger (50 A) pore. The adsorption isotherms and the interfacial aggregate structure of the surfactants in the pores with various sizes show a qualitatively consistent performance with the previous experimental observation. The micelle size distributions of the adsorbed surfactant aggregates have been analyzed in order to understand the adsorption mechanism, which suggests that the step rise in the surfactant adsorption is associated with the considerable formation of the micelle aggregates in the confined pores. The effect of the interaction between the pore surface and the surfactant on the adsorption behavior has also been investigated. The simulation results indicate that a change in the interaction can modify the shape of adsorption isotherms. A nonlinear mathematical model was used to represent the multistep adsorption isotherms. A good agreement between the model fitting and the simulation data was obtained for both the amount of adsorption and the jump point concentration.     

Solubilization of phenols in anionic polyelectrolyte gels with adsorbed cationic surfactant

Solubilization isotherms for various phenols in cetylpyridinium chloride (CPC)-polyelectrolyte gel aggregates have been determined in order to compare solubilization within these aggregates with that in free micelles and to examine the effects of gel chemistry and structure on solubilization. The isotherms describing solubilization are quite similar to those found for free surfactant in solution. Solutes that are more hydrophobic give rise to larger solubilization constants with trends similar to what is seen for hydrophobic effects in adsorption from aqueous solutions onto hydrophobic solids. The solubilization constants decrease as the fraction of solute in the aggregates increases, indicating that the solutes partition into the palisade region of the aggregates. Solubilization is found to be quite insensitive to changes in gel structure (cross-linker varying from 1% to 3%) and chemistry (poly(acrylic acid) versus poly(methacrylic acid) and neutralization from 50% to 100%). However, the switch from poly(acrylic acid) to poly(methacrylic acid) did give rise to a slight decrease in magnitude of the slope of the isotherm. The most significant factors appear to be the initial concentration of surfactant in solution and the ratio of surfactant solution to gel amount. A decrease in surfactant concentration (especially combined with an increase in solution volume) gives rise to a decrease in solubilization constants.     

Separating surface and solvent effects and the notion of critical adsorption energy in the adsorption of phenolic compounds by activated carbons

A modified form of the Freundlich equation in which the solute equilibrium concentration is normalized with respect to the solute solubility is analyzed and applied to adsorption isotherms of phenol, 4-nitrophenol, 4-chlorophenol, and 2-chlorophenol at different values of pH on commercial activated carbon before and after oxidation. The analysis confirms the importance of normalizing the solute equilibrium concentration when analyzing the adsorption isotherms, and it is suggested that a parameter, K(F10), obtained by taking 10% solubility as the reference point when applying the Freundlich equation, is probably the best comparative estimate of the relative adsorption capacity of the carbon for different phenolic compounds. In combination with the Freundlich exponent, n(F), estimates of the adsorption capacity at any other reference point can then be obtained. Analysis of the experimental results also indicates a need to distinguish between two regimes of adsorption, characterized by an adsorption energy, E(ads), greater than or less than a critical value, E(ca). When E(ads) > E(ca), the shape of the adsorption isotherm is determined by solute-solid interactions. On the other hand, when E(ads) < E(ca), solute-solution interactions become more important.     

Adsorption isotherm and atomic force microscopy studies of the interactions between polymers and surfactants on steel surfaces in hydrocarbon media

The adsorption isotherms for certain polymer and surfactant molecules (and in some cases their mixtures) on stainless steel beads from isooctane have been obtained, together with corresponding adsorbed layer thicknesses, using an atomic force microscope. The polymer is a terminally functionalised (ethylene diamine), low molecular weight polyisobutylene (PIB) derivative and the surfactants are basically alkyl or alkyl phenol alkoxylate molecules, which in one case has been derivatised with an amino functionality. The results indicate the presence of multilayers at the stainless steel-isooctane interface. Theoretical analysis of the surfactant adsorption isotherms suggests molecular aggregation at the interface with an aggregation number between 2 and 6, at the highest coverages. The adsorption of the polymer is reduced in the presence of the surfactant molecules. The polymer leaches metal ions from the steel surface at higher concentrations.