The states of water in cellulose acetate membranes

Differential scanning calorimetric melting endotherms of wet and half-dried cellulose acetate membranes and salt distribution coefficients were studied to clarify the states of water in membranes. We have suggested that (a) there are four states of water in cellulose acetate membranes; (b) these states of water are those of completely free water, free water very weakly interacting with polymer, bound water which can contain salts, and bound water which rejects salts; (c) the semipermeability of membranes depends on the ratio of four states of water in membranes.     

The thermodynamic state of water in cellulose acetate membranes

Using experimental sorption data and corresponding experimental results from calorimetric investigations, the state of water in cellulose acetate (CA) membranes is discussed by applying a theoretical treatment of sorption reported previously (1–3). The sorption of water can be attributed to a gain in surface energy at the polymer/vapor interface. Using differential thermodynamic potentials of sorbed water together with experimentally determined heat capacities of sorbed water, the thermodynamic potentials G, H, and S of sorbed water are estimated for the temperature interval ?40 to + 40°C. At constant temperature, each thermodynamic potential depends on the water content. The resulting distribution function of G indicates that the sorbed water exists in different states. Comparing the Gibbs free energy of sorbed water with that of ice or liquid water at the same temperature leads to the conclusion that none of the sorbed water freezes to ice within the temperature interval used. Based on the Gibbs free energy of water in electrolyte solutions and the distribution function of G for sorbed water, partition coefficients of salts within CA membranes may be estimated. The results are in good agreement with experimentally determined partition coefficients which are available from the literature. As the partition coefficient of a salt is directly related to the salt rejection of the membrane, this provides a method of estimating the desalination performance of a membrane from its water sorption isotherm.     

Determination of diffusion coefficients of water in cellulose acetate membranes

The determination of the diffusion coefficient for water in various porosity cellulose acetate membranes by a gravimetric method, using a humidified carrier gas, is described. It was found to be impossible to obtain meaningful results for very porous membranes, although dense membranes gave limiting values of diffusion coefficient at high carrier gas velocities. This phenomenon is explained in terms of the dissipation of the heat of sorption by the forced convection provided by the carrier gas. The variation of diffusion coefficient with concentration of water in dense cellulose acetate is explained in terms of clustering of water molecules in the polymer at high concentration.     

Characterization of water structure in cellulose acetate membranes by calorimetric measurements

The heat capacities of homogeneous and asymmetric cellulose acetate membranes have been measured at different water contents within the temperature range of ?40 to +20°C. The experimental heat capacity–temperature curves were verified by DTA measurements within a temperature range of ?120 to +20°C. The results for the partial heat capacity of water within the membranes as well as for the heat of fusion were interpreted by assuming two different states of water—unfreezing bound water due to a sorption process and unbound water due to capillary phenomena, which freezes with a freezing point depression and a reduced heat of fusion.     

Phenol transport in cellulose acetate membranes

Past detailed studies of solute transport through reverse-osmosis membranes have been conducted only with simple salts. The present work with phenol was undertaken largely because of the practical observation that the transport of low molecular weight organics is much more rapid than that of the salts. Studies of phenol sorption from dilute aqueous solution indicate that the diffusion coefficient for phenol in water-saturated 39.8 wt.-% acetyl cellulose acetate is 9.6 × 10^-10 cm.²/sec., and the equilibrium distribution coefficient between the acetate phase and water is 42. Thus, the diffusion coefficient is quite close to that measured for sodium chloride, and the higher permeability of the membranes to phenol can be attributed entirely to their greater sorption of this solute. In direct osmosis experiments performed with significant water flow a measurable interaction or positive coupling between water and phenol flows has been observed. Further evidence of flow coupling is derived from reverse osmosis experiments in which significant negative solute rejection is observed; i.e., the permeate is enriched in phenol by as much as 20%. It is shown that a solution-diffusion transport model is not adequate to rationalize the results, and a more complex transport model is apparently required.     

Ion movements in cellulose acetate membranes

The behavior of ions in a cellulose acetate membrane was discussed from the standpoint of mobilities obtained from the membrane potentials. The mobilities of univalent ions larger in radius or divalent ions relative to Na⁺ ion mobility in the membrane are much different from those in aqueous solutions. The order of the mobilities of ions in the membrane is shown quantitatively. The mobilities of the ions with radii larger than a certain value in the membrane decrease with increase in their size. This could be explained by the physical friction between the ion and the membrane wall. This friction is of importance as the ionic radius comes close to the intermolecular gaps between polymer chains. The mobilities of Na⁺ anyd 1^? ions in the membrane are (2–3) × 10^?7 cm²/sec-V and are about three to four orders of magnitude less than those in aqueous solutions.     

Thermodynamic and mechanistic characterization of water sorption in homogeneous and asymmetric cellulose acetate membranes

The water sorption isotherms for homogeneous and asymmetric cellulose acetate membranes have been measured at different temperatures. Subtle differences between the water sorption isotherms for asymmetric and homogeneous membranes have been interpreted by suggesting that capillary condensation contributes significantly to sorption in asymmetric membranes at high activities and also to an intriguing excess sorption observed in homogeneous membranes at intermediate activities. This model has been supported by the experimentally determined values of the enthalpy and entropy changes associated with sorption.     

Formation of asymmetric cellulose acetate membranes

Cellulose acetate membranes were prepared from casting solutions containing dioxane as a solvent and varying concentrations (up to 6%) of maleic acid as an additive. Coagulation took place in water at different temperatures. The effect of these variables on membrane structure and membrane properties is related to two phenomena of phase separation in the system cellulose acetate/dioxane/water, viz. gelation in the toplayer and liquid-liquid phase separation in the sublayer of the membrane. We adopted a solution transport model which correlates membrane flux with the skin thickness and membrane salt rejection with the compactness (microstructure) in the skin. Effects of variables such as maleic acid concentration and coagulation temperature on the position of curves in the phase diagram and on membrane properties are discussed. It is concluded that more work should follow on the kinetics of the phase changes mentioned.     

High flux cellulose acetate ultrafiltration membranes

Conditions for casting high flux tubular and flat cellulose acetate ultra-filtration membranes are given. The usefulness of some of the tubular membranes cast under the above conditions for ultrafiltration applications in industrial water purification and reuse is illustrated. Using ethanol-water mixture as the gelation medium in the temperature range-20° to 30°C, ultrafiltration membranes giving water fluxes in the range <4 to >400 m³/m² day at the operating pressure of 689.5 × 10³ Pa (100 psig) have been obtained.     

Modified cellulose acetate membranes for desalination

Residual hydroxyl groups of commercially available cellulose acetate (39.9% acetyl) have been reacted partially with phenyl isocyanate. The characteristics of these modified polymers have been studied. Membranes have been cast from these polymers to study their potentiality as reverse osmosis membranes. The work has been further extended to investigated the stability of these modified CA under the influence of high temperature and γ irradiation.     

Sulfonamide release from cellulose acetate membranes

Summary Cellulose acetate membranes were chemically modified by esterification with oxalyl chloride which was used as a hydrolytically labile spacer. Then five sulfonamides were attached to the acyl chloride present on the halfesterified membranes to give drug-polymer conjugates. The rate of the hydrolytic cleavage of the spacer-drug linkage was measured in simulated gastric fluids at 37°±0.5°C by determining the amount of drug released by spectrophotometric analysis. The results showed that in all cases the process followed a zero-order kinetics.     

Permeability studies with cellulose acetate membranes

Summary The diffusive permeability of cellulose diacetate membranes was measured using NaCl as solute and distribution coefficients were calculated. Water and salt permeability coefficients derived from dialysis-osmosis experiments were also given. The results obtained were discussed with the aid of scanning electron micrographs and chlorine distribution maps.     

DRY-RO membranes from cellulose acetate carbamates

Cellulose acetate carbamates (CACs) are the polymers which result when organic isocyanates are reacted with the free hydroxyl groups of cellulose acetate (CA). CACs are more hydrolytically stable and exhibit physical properties which are superior to those of their CA mixed ester analogs. Two synthetic approaches to CACs have been utilized in this study: (1) preformation, i.e., separate synthesis of such polymers prior to their inclusion in solutions for membrane casting; and (2) in situ formation, i.e., the inclusion of blocked isocyanates in standard dry process casting solutions of CA followed by thermal activation of the resultant dry membranes leading to regeneration of free isocyanate and subsequent CAC formation. Preformed CAC polymers have been prepared utilizing phenyl-, 3-chloropropyl-, 3-bromopropyl-, and 3-bromopropyl-(isothio)-, isocyanates. Polymers containing omega-halocarbamate moieties were quaternized with dimethylbenzylamine to produce ionogenic (QCAC) polymers containing quaternary ammonium groups. DRY-RO membranes from the QCACs exhibit flux/rejection values varying between 6–8 gfd at 98% rejection and 20 gfd at 90% rejection (0.5% NaCl feed at 400 psi and 25°C). In situ formation of CAC membranes has been effected with tolylene- and hexamethylene-diisocyantes, with quaternized isocyanate monomers employed for the preformed CAC polymers, and with specially tailored diisocyanates containing ionogenic groups. Crosslinking rendered all of the membranes acetone insoluble. Inasmuch as in situ formation substitutes the easy synthesis of blocked isocyanate monomers for the more difficult separate synthesis of preformed CAC polymers, it is anticipated that the former will replace the latter.     

Movements of univalent ions in cellulose acetate membranes

Membrane potentials across the asymmetric membranes of cellulose acetate with various salt rejection properties have been measured for univalent ions. The behavior of ions in the membranes is discussed from the viewpoint of relative ionic mobilities calculated from the membrane potentials. The relationship between ionic mobilities and ionic radii in the membranes having salt rejections lower than 80% is almost the same as that in aqueous solutions. This implies that the ions in these membranes behave as if they exist in bulk water. However, the ionic mobilities in the membranes having salt rejections higher than 86% differ significantly depending on the ionic radii. It seems probable that the bound water influences the ionic mobilities in these membranes.     

Performance evaluation and antifouling analyses of cellulose acetate/nanodiamond nanocomposite membranes in water treatment

In this study, nanocomposite membranes based on cellulose acetate (CA) and nanodiamond (ND) were prepared by applying phase inversion methods. In order to achieve efficient dispersion and more hydrophilic NDs, they were functionalized via heat treatment (ND‐COOH). The prepared nanocomposite membranes were characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), contact angle, porosity measurement, tensile strength, and abrasion resistance techniques. Furthermore, the governing fouling mechanisms were determined by using classic models as well as combined fouling models. Moreover, the effect of precoagulation with polyaluminum chloride (PAC) on the humic acid (HA) filtration was investigated. The obtained results showed that in the presence of ND‐COOH, the abrasion resistance of nanocomposite CA membrane was three times higher than that of pristine CA membrane. Besides, the nanocomposite membranes with 0.5 wt % of raw and functionalized ND exhibited excellent hydrophilicity and PWF. The analysis of fouling mechanism based on Hermia's model revealed that the cake formation is prevailing mechanism for CA and CA/ND (0.5 wt %) membranes while for CA/ND‐COOH (0.5 wt %) membrane, experimental results are fitted by combined cake filtration‐complete blocking (CFCB) model. It confirms that pretreatment with PAC can significantly mitigate fouling and improve HA removal. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44873.     

Membrane potential measurements on cellulose acetate membranes

Membrane potentials and apparent transport numbers of the cation are reported for cured cellulose acetate membranes bounded by HCl, NaCl, KCl and MgCl₂ solutions, using Ag/AgCl electrodes and a flow-cell method. Membranes cured at 70°, 80° and 90° are used. Bounding solution concentrations vary from 0.005 to 0.05 M at the high concentration side (bounding the dense side of the membrane), and are kept constant at 0.002 M for the low concentration solution. In the KCl 90° membrane case the low concentration solution is varied as well, from 0.0001 to 0.002 M. Results show that cured cellulose acetate membranes are permselective towards univalent cations. This is interpreted as resulting from a low cation-exchange capacity of the dense layer of the cured membrane. The permselectivity increases with increased curing temperature. Addition of a non-electrolyte to the low concentration side reverses the osmotic flow and leads to higher apparent transport numbers of the cation. It is concluded that diffusion in small pores contributes significantly to the transport of ionic solutes through uncompacted membranes.     

Permeabilities of coagulated cellulose acetate dialysis membranes

The diffusive flux of NaCl and the hydraulic flux of pure water through coagulated cellulose acetate membranes are examined. Coagulated cellulose acetate membranes (without densification by heat treatment or drying) possess higher permeability than what may be expected from the permeabilities of the dry polymer. Their overall hydraulic permeability (ultrafiltration rate of water) is greatly dependent upon the membrane casting conditions and the resulting asymmetry of the membrane. On the other hand, the asymmetry of a membrane does not play as great a role in the diffusive permeability of a solute. With homogeneous membranes, higher diffusive flux is always accompanied by higher hydraulic permeability. With asymmetric membranes, this is not always true. The diffusive permeability of NaCl and the hydraulic permeability of water through coagulated cellulose acetate membranes can be controlled nearly independently. Consequently, high diffusive (NaCl) permeability with low hydraulic water permeability and vice versa can be obtained by varying the casting conditions and also by partially saponifying the denser portion of the membrane.     

Immobilization of polymers on cellulose acetate membranes

Using models of dead-end filtration theory, the kinetics of forming dynamic layers of sulfate lignin (SL) and sodium carboxy-methylcellulose (Na-CMC) onto CA membranes during cross-flow filtration of dilute solutions of polymers was studied. It was found for both polymers (compact SL and linear Na-CMC), that the polymer layer with the least hydraulic resistance, which yields a small reduction in membrane water permeability (10–20%), but a significant increase in salt rejection, is formed, if the process kinetics corresponds to J-V linear dependence predicted by the model of 'blocking a pore by a single particle's. The results obtained may be used to define the optimum conditions for immobilizing the available catalytic active polymers on regular semipermeable membranes during the membrane filtration process.     

Transport coefficients of asymmetric cellulose acetate membranes

Using the linear relations of thermodynamics of irreversible processes, the transport coefficients l_p,l_π,l_πp and σ were measured for an asymmetric cellulose acetate membrane with NaCl, Na₂SO₄, CaCl₂, NaF, and saccharose over the concentration range (0–0.5 mol/l or l mol/l) at 20°C or 25°C. The experimental findings manifest a strong dependence of the three transport coefficients l_p,l_π, and linπp on solute concentration. This strong dependence on concentration can be attributed to a concentration gradient within the porous sublayer of the asymmetric membrane. Thereby, it is shown that the transport coefficients of an asymmetric membrane depend on the solute concentration on both sides of the membrane instead of the mean concentration c?_s, as would be the case for a homogeneous membrane.