Modelling adsorbent heterogeneity for adsorption from binary liquid mixtures

It is demonstrated that the detailed structure of the surface energy or selectivity distribution function is not critical to obtaining adequate analytical expressions for surface excess isotherms for adsorption from binary liquid mixtures on heterogeneous adsorbents. The gamma and the uniform selectivity distribution functions, which are very different in form, were successfully used in conjunction with the monolayer-pore filling model for adsorption on a homogeneous site to describe adsorption of various binary liquid mixtures on silica gel. Both models described the salient features of the surface characteristics of the silica gel.     

The structure of water and methanol adsorbed on silica gel by FT-NIR spectroscopy

The NIR adsorption spectra were analyzed quantitatively on the fundamental, combination and first overtone region of OH vibrations of silanol groups, water and methanol adsorbed on mesoporous silica gels. Adsorbed methanol constitutes first layer of about 3 molecules/um^–2 and second layer, the structure of which is similar to that of bulk methanol liquid. Adsorbed water consists of a first layer of about 3 molecules/nm^–2, the second layer of about 9 molecules/nm^–2 and the third layer has a structure similar to the that of bulk water. The molecular configuration at the interface is discussed.     

Wetting and adsorption on organophilic illites and swelling montmorillonites in methanol-benzene mixtures

Adsorption and immersion were studied simultaneously in methanol-benzene mixtures on non-swelling organophilic illites and swelling organophilic montmorillonites. From the combination of adsroption and calorimetric data, equations were proposed for the determination of the adsorption capacity and the molar enthalpies of wetting differences, if the adsorption layer behaved ideally. From the experimental enthalpy isotherms of wetting and the enthalpy functions related to the ideal behaviour of the layer, qualitative conclusions were drawn on the excess enthalpy of the adsorpition layer. It was again established that the adsorption layer composed of alkyl chains, benzene and methanol has a regular cluster structure in certain concentration ranges, which breaks at higher methanol concentrations. This change is reflected by a significant endothermic peak of the differential molar enthalpy.For the organophilic montmorillonites, the enthalpy of swelling was determined separately as a function of the composition. The non-swelling organophilic illites were chosen as a reference system.Dedicated to Professor Dr. Armin Weiss on the occassion of his 60th birthday.     

Adsorption of polyethylene glycol from aqueous solution on montmorillonite clays

Adsorption rates and capacities of polyethylene glycol (PEG) were investigated for five montmorillonite clays. The adsorption of PEG for all the montmorillonite clays was rapid, and equilibrium was attained within 30 min. The adsorption isotherms of PEG for all the montmorillonites conformed to the Freundlich equation. The adsorption heats were 7.3 and 11.6 kJ · mol^–1(mw.:2000), and 8.7 and 14.2 kJ · mol^–1(mw.:20000) for the montmorillonite and the bentonite II-Ca, respectively. Adsorption capacities for all the clay samples approached constants for the molecular weight of PEG over 2000, though they increased with the increase of molecular weight under 2000. The adsorption capacities were slightly influenced by a nearly neutral pH. The montmorillonite clays which had different interlayer cations showed quite different adsorption capacities. The bentonite II-Ca, the acid clay, and the activated clay showed large adsorption capacities that were 30–50 % of that of an activated carbon.     

Simulation of a monolayer adsorbed on a heterogeneous surface: effect of the size and shape of the adsorbate on the structure of the interface and the distribution of the adsorption energies

At the low temperature limit, the effect on the size and shape factors of the adsorbate molecule is shown in a simulation of a monomolecular phase adsorbed on a heterogeneous surface. This factor is reflected both in the theoretical distribution of the adsorption energies and the packing of the adsorbed phase.     

Functionalization of colloidal silica and silica surfaces via silylation reactions

A series of trialkoxysilane compounds tipped with primary amine groups were used to functionalize the surfaces of glass and colloidal silica. Streaming potential and microelectrophoretic mobility measurements were used to monitor the stability of the functionalized surfaces.Hydrolytic breakdown of the surface-to-silane coupling was induced by either successively increasing and decreasing the pH of the solution in contact with the surface, or by aging the derivatised surfaces in aqueous solution over prolonged periods of time. The chemistry of the spacer units between the trialkoxysilane group and the primary amine tip had a major influence on the subsequent hydrolytic stability. Large hydrophobic spacer groups showed small changes in the electrokinetic properties on storage, but large changes when successively titrated with acid and base through the pH range. The behavior observed with small hydrophobic spacer groups was that large changes in electrokinetic properties were obtained on storage and with pH titration.     

Melting temperature of colloidal crystals of monodisperse silica spheres in ethanol-water and ethylene glycol-water mixtures

The melting temperature (T _m) of colloidal crystals of monodisperse silica spheres in ethanol-water and ethylene glycol-water suspensions has been measured by reflection spectroscopy. A sphere of 110 nm in diameter and 0.041 in monodispersity index is used after purification and deionization processes. Transformation from the body-centered cubic lattice to the face-centered cubic lattice subphases is observed as the suspension temperature rises, which is similar to the purely aqueous suspension of the same sphere. A phase diagram including liquid-like and crystal-like structures is obtained in the presence of ion-exchange resins coexisted. The data ofT _m are analyzed successfully with the theory of Williams, Crandall, and Wojtowicz. The heat of melting decreases by the addition of ethanol or ethylene glycol in the mixtures.     

Solubilization of oleyl alcohol by pure and mixtures of surfactants

The solubilization behavior of oleyl alcohol by pure and mixtures of surfactants systems have been studied in terms of the maximum additive concentration (MAC), the solubilizing power, and the particle sizes of micelles with or without oleyl alcohol. The surfactants used are amphoteric (N,N-dimethyl-N-lauroyl lysine, DMLL; N,N,N-trimethyl-N-lauroyl lysine, TMLL; N,N-dimethyl-N-(carboxymethyl)-lauryl ammonium, DMCL), anionic (sodium dodecyl sulfate, SDS) and nonionic (alkyl polyoxyethylene ethers).The maximum additive concentration of oleyl alcohol by pure surfactants is larger by nonionic surfactants than by others.For a nonionic surfactant system mixed with DMLL, the mixing effect of surfactant on the increase in the MAC is not recognized. While, for DMLL mixed with SDS, the MAC becomes larger than that by pure surfactants. This may be attributed to the fact that the large micellar size will result in increasing the maximum additive concentration.     

Sorption properties of celluloses in binary liquid mixtures

The adsorption capacity for methanol-benzene andn-propanol-water mixtures was measured on two kinds of microcrystalline celluloses and cotton linters. Excess adsorption isotherms were obtained; the free energy of displacement was calculated from the isotherms. The enthalpy of displacement for one crystalline cellulose was measured by flow calorimetry. The entropic function of displacement was derived from the enthalpy and free energy of displacement. The preferential adsorption of methanol andn-propanol was characterized by the adsorption equilibrium constant.On leave from the Department of Physical and Colloid Chemistry, Wageningen Agricultural University, The Netherlands     

Equilibrium properties of ternary polymer solutions as explained by the renormalization group

In this paper we present a full account of renormalization group theory as applied to equilibrium properties of ternary polymer solutions containing two different polymer species and one solvent. We use a carefully constructed renormalization group mapping incorporating the screening effect by a proper choice of the renormalized segment size. The scaling functions reproducing observable quantities are calculated to tree approximation. We have evaluated a series of experiments on ternary solutions in this approximation and observe remarkably good agreement.     

Scattering function and the dynamics of phase separation in polymer mixtures under shear flow

The phenomenological mean-field theory describing concentration fluctuations and spinodal decomposition of binary mixtures of long flexible macromolecules is generalized to mixtures under steady shear flow. This shear flow leads to a partial orientation and stretching of the coils, as well as to an anisotropic deformation of concentration fluctuations. Generalizing the approach of Onuki and Kawasaki, we obtain the collective scattering function describing these concentration fluctuations in the mixture under shear flow. Both the steady-state situation in the one-phase region and the initial stages of spinodal decomposition for concentrations inside of the spinodal curve are considered.Contributed paper delivered at the Tagung der Deutschen Physikalischen Gesellschaft, Fachausschuß Polymerphysik, Berlin, March 30–April 3, 1987.     

Coagulation kinetics of amorphous colloidal silica suspensions with and without hydroxypropyl cellulose polymer

The coagulation rate constant of submicron silica has been measured as a function of solution pH, salt concentration and hydroxypropyl cellulose (HPC) polymer concentration. Results show that the colloidal stability of silica is dominated by the cation concentration in the presence of salt in the pH range 3–9.5. The stability increases as cation concentration decreases. At low salt concentration and a minimum colloid stability was found in the intermediate pH range 4–8. These results show that differences in the literature values of the critical coagulation constant by relative light-scattering experiments can be explained by the use of the coagulation rate constant analysis. When HPC polymer was present in the solution, the colloid stability of the silica increased. The adsorption of polymer stabilizes the silica suspensions, both at low pH near the isoelectric point and at high ionic strength where it coagulates without the polymer. A monolayer coverage was necessary to provide steric stabilization. At 10^–3 M KCl a smaller equilibrium concentration of HPC in solution is needed to give monolayer coverage and steric stabilization than at 1 M KCl and pH 4.2.     

Thermodynamics of adsorption of acetone on active carbon supported metal adsorbents

Adsorption of acetone on active carbon and active carbon supported metals (Ni, Cu, Zn and Cd) have been studied as a function of temperature. Thermodynamic parameters such as G ⁰, H ⁰, and S ⁰ are calculated from virial and Langmuir isotherm expressions. It is observed that active carbon supported metals have more adsorption affinity for acetone as compared to active carbon. Results show that the increase in adsorption affinity for active carbon supported metals is not due to configurational factors affecting the entropy of adsorption, but because of enhanced enthalpy of adsorption. XRD spectra show that active carbon supported metals adsorbents are amorphous and metal residues are present on the surface of active carbon in its reduced form. From adsorption data, isosteric heats and molar entropies of adsorption were calculated as a function of coverages and temperature. The values of isosteric heats of adsorption were found to be higher for active carbon supported metals, which may be due to the chemisorption of adsorbate molecules with metal sites present on the surface of active carbon. The extent of coordination of adsorbate molecules with metal sites is discussed on the basis of the acidic character of metal.     

Polymer adsorption on fine particles; the effects of particle size and its stability

The effects of particle size on polyacrylamide (PAAm,M _w =59×10⁴, 500×10⁴) adsorption were investigated using a series of well-characterized hematite (-Fe₂O₃) dispersions. The -Fe₂O₃ particles with highly monodisperse and nearly spherical shape ranged in radius from 23 nm to 300 nm. the maximum amount of PAAm adsorption (M _m ) in each system, showed a steady increase with decreasing particle radius and was influenced strongly by particle concentrations in the medium. Furthermore, it was realized that the diameter of -Fe₂O₃ particles after treatment with PAAm under different particle concentrations decreased with increasing particle concentration. The relation between particle concentration in the medium and particle size after treatment was also influenced by the medium pH, i.e., at the medium pH close to the isoelectric point of -Fe₂O₃ particles (pH^o=9.2), the particle size after treatment increased with increasing particle concentration. All these results suggest that in the system of ultra-fine particles, the mixing process between particle-particle and polymerparticle will play an important role on the conformation of adsorbed polymer layer.     

Interaction of polysilicic acid with monolayers of substances containing quaternary ammonium groups

The action of polysilicic acid on monolayers of a quaternary ammonium salt and a phospholipid spread on substrates of various pH values was studied. The films of benzyl dimethyl hexadecyl ammonium chloride were considerably affected above pH 2. No interaction was observed, at any pH, between silicic acid and dipalmitoyl phosphatidyl serine.The results support the existence of ionic interactions between dissociated silanol groups and tetra-alkylammonium groups. The involvement of hydrogen bonds is ruled out.     

Displacement processes on hydrophilic/hydrophobic surfaces in methanol-water mixtures

The adsorption from methanol/water and the enthalpy of displacement of methanol by water were studied on K-60 silica gel, on graphitized PRINTEX-80, and onn-alkylammonium vermiculites. The adsorption between the hydrophobized silicate layers was followed by x-ray measurements. The excess isotherms of alkylammonium vermiculites in methanol/water exhibit two maxima corresponding to the two steps of the individual isotherm. The equilibrium constant of the exchange of water by methanol is calculated. For calculating the adsorption capacities and molar adsorption potentials, a new equation is proposed which combines the adsorption excess quantities with free energy and enthalpy functions.     

The adsorption process of some acetic acid derivatives at the water/air interface

The adsorption of trifluoro-, trichloro-, tribromo-, and trimethylacetic acid at the water/air interface is discussed on the basis of surface tension measurements. The process of adsorption is described by Henry's and Langmuir's isotherm equations. The obtained results allow calculation of the standard free energy of adsorption of investigated molecules and the contribution to this energy of hydrophobic groups of these molecules.     

Kinetics of adsorption and desorption of sodium alkyl sulfates to and from nylon particles

The desorption rate of surfactant ions from nylon particles was investigated at the concentrations below the critical micelle concentration by applying the stopped-flow method. A mixing cell of stopped-flow spectrophotometer was modified with platinum electrodes for electric conductivity detection. The change in electric conductivity with time in the desorption process was monitored by a memory-recorder system. The surfactants used were sodium decyl, sodium dodecyl, sodium tetradecyl, and sodium hexadecyl sulfates. The desorption rate was independent of the surfactant concentration and the rate constants were obtained by applying the first-order reaction scheme. The adsorption rate constants were estimated from the experimental desorption rate constants and equilibrium constants assuming the second-order kinetics. The desorption rate constants were determined to be 1–6 sec^–1 and the adsorption rate constants to be 2–8×10⁴ mol^–1 dm³ sec^–1; the former decreased and the latter increased with increasing number of carbon atoms in alkyl chain of the surfactants.     

Formation of ultra-thin films consisting of proteins or polysaccharides and syndiotactic-rich poly-vinyl alcohol mixtures adsorbed at air-solution interface

Ultra-thin films of syndiotactic-rich poly-vinyl alcohol (s-PVA) with several proteins and polysaccharides were prepared by the bubble and frame methods using a mixed dilute aqueous solution (1.5g/dL).The mixed amount of-cyclodextrin (-CD) was the largest among these proteins and polysaccharides giving a weight ratio (-CD/s-PVA) of 1. The ratios of silk fibroin(SF), lysozyme, pepsin, and pectin tos- PVA were 0.58, 0.40, 0.35, and 0.35, respectively. For the-CD/s-PVA and SF/s-PVA blend thin films, the phase separation was observed.     

Reversible flocculation of silica suspensions by water-soluble polymers

The possibility to concentrate, then redisperse colloidal suspensions is not only of great theoretical interest, but is also relevant to the industrial process of solid-liquid separation, which must often be followed by a redispersion stage. Up to now, these consecutive operations were unrealizable in the presence of polymers, since flocculation and adsorption were generally considered irreversible in this case. Previous studies have pointed out the occurrence of two main flocculation mechanisms: charge neutralization and interparticle bridging. The use of copolymers makes it possible to take into account these two different mechanisms together. Using this fact we prepared new copolymers of acrylamide with N-vinylimidazole via radical polymerization and characterized them by light scattering, viscometry, potentiometric titrations, and UV studies. One peculiarity of the chosen system is its dependence on pH: actually the degree of neutralization of such cationic polyelectrolytes does vary with pH, especially near the pK value. This paper shows that these copolymers may induce reversible flocculation of negatively charged suspensions, e.g., silica suspensions, by simple pH adjustment. Performances of the system were followed by various physico-chemical methods. The observed results are explained in terms of flocculation mechanisms as a function of pH.