Omar A. El Seoud

An efficient, one‐pot acylation of cellulose under homogeneous reaction conditions

Cellulose samples from cotton linters, sisal, and sugar cane bagasse have been successively acylated (acetate, propionate, butyrate, and acetate/butyrate) under homogeneous reaction conditions, in LiCl/N,N-dimethylacetamide (DMAC), by the following procedure: (i) cellulose and LiCl are heated under reduced pressure, at 110°C; (ii) cellulose is dissolved in LiCl/DMAC by heating at 155°C, followed by cooling to 40°C; (iii) the solubilized polymer is acylated at 60°C for 18 h. Attractive features of this one-pot procedure include: easy control and high reproducibility of the degree of substitution; elimination of base catalyst; negligible degradation of the natural polymer; and recovery/recycling of high purity DMAC and acid anhydride. Reaction conditions employed for the present celluloses are different from those previously used for Avicel PH 101 microcrystalline cellulose because their fibrous nature, higher indices of crystallinity and higher molecular weights retard their dissolution and decrease their rates of acylation by acid anhydrides.

Solvatochromism in pure and binary solvent mixtures: effects of the molecular structure of the zwitterionic probe

The solvatochromic behavior of 2,6-dichloro-4-(2,4,6-triphenyl-1-pyridinio)phenolate (WB) was studied by UV–visible spectrophotometry in 32 pure solvents, in binary mixtures of 1-butanol–cyclohexane (BuOH–Cyhx), and of water with methanol, ethanol, 1-propanol, 2-butoxyethanol (2-BE), acetonitrile, 1,4-dioxane and THF. The solvent polarity, ET(33) in kcal mol−1, was calculated from the position of the longest-wavelength intramolecular charge-transfer absorption band of WB and the results were compared with those for 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)phenolate [RB, ET(30)] and of 1-methyl-8-oxyquinolinium betaine [QB, ET(QB)]. For pure solvents, ET(33) is a linear function of ET(30), with a slope of practically unity. Steric crowding from the two ortho phenyl rings of RB hinders the formation of H-bonds with solvents, which results in similar susceptibilities of WB and RB to solvent acidity. For binary solvent mixtures, all plots of ET versus the mole fraction of 1-butanol or water are non-linear owing to preferential solvation of the probe by one component of the mixed solvent and, when applicable, to solvent micro-heterogeneity. Preferential solvation due to non-specific and specific probe–solvent interactions was calculated for BuOH–Cyhx and water–acetonitrile. Both solvation mechanisms contribute to the non-ideal behavior in the former binary mixture, whereas probe–solvent specific interactions dominate the solvatochromic behavior in the latter. The composition of the probe solvation shell was calculated. In aqueous alcohols, preferential solvation is by the alcohol. In water–aprotic solvent mixtures, preferential solvation of RB and WB is by the solvent which is present in lower concentration, whereas QB seems to form its own, water-rich solvation shell over a wide range of water concentration. Copyright

Effects of organized surfactant assemblies on acid-base equilibria

The solubilization of an acid or a base by an organized surfactant assembly (e.g., an aqueous micelle, a vesicle, a monolayer or a reversed micelle) usually results in a change in the pKa of the solubilized species. This change can be explained in terms of differences between the properties of the bulk solvent and of the interfacial region (e.g., changes in the microscopic polarity) and perturbation of the acid-base equilibrium by the electrostatic field effect of the charged interface. The effects of organized surfactant assemblies on acid-base equilibria in aqueous solutions have been quantitatively analyzed using two approaches. In the first, the pKa in the presence of the interface is related to that in water by taking into account the surface potential and the effect of the transfer of the probe from bulk water to the interface. In the second, the pKa values are determined by using “effective” pH values calculated by considering the possible exchange equilibria between the ions present in the bulk phase and the surfactant counterions. Literature data on the effect of aqueous micelles, oil/water microemulsions, vesicles and monolayers are discussed in terms of the particular approach used and of the dependence of the observed pKa shift on the charges of both the interface and the acid-base indicator. The determination of the pKa of an indicator in the presence of detergent aggregates in non-aqueous solvents (reversed micelles, water/oil microemulsions) is intrinsically more complex than in aqueous systems. An understanding of the peculiarities of solubilized aqueous microphases is essential to explain some of the apparent anomalies, such as the rather high sensitivity of the pKa of some indicators to the nature, concentration and volume of the solubilized buffer. The application of the ion exchange formalism to the case of reversed micelles is shown, and published micellar pKa shifts are explained in terms of the transfer of the indicator to the micellar “water pool”, as modulated by effects due to the charge of the micelle and that of the indicator. Attention is drawn to cautions which must be taken in order to obtain meaningful experimental results and to the problem of the proper choice of the buffer used.

A novel, efficient procedure for acylation of cellulose under homogeneous solution conditions

Commercially available cellulose (Avicel PH101) was successfully acylated under homogeneous solution conditions by the following novel procedure: 2.0 g of cellulose and 5.0 g of LiCl were introduced into a glass reactor, magnetic stirring was started, the pressure was reduced to 2 mmHg, the temperature was raised to 110°C in 30 min, and the reactor was kept under these conditions for another 30 min. N,N-Dimethylacetamide, 60 mL, was introduced, atmospheric pressure was restored, and the temperature was raised to 150°C in 30 min. The system was kept under these conditions for 1 h, then the temperature was decreased to 40°C; in 2 h a clear cellulose solution was obtained. Acid anhydride was added, and the solution was stirred at 60°C for additional 18 h. Acetates, propionates, butyrates, and acetate/butyrate mixed ester were prepared with excellent reproducibility of the degree of substitution, from 1 to 3. The degree of polymerization of cellulose is negligibly affected by these reaction conditions. The distribution of the acetyl moiety among the three OH groups of the anhydroglucose unit follows the order C6 > C2 > C3. Features relevant to the industrial application of this novel procedure are discussed.

Some aspects of acylation of cellulose under homogeneous solution conditions

Commercially available cellulose (Avicell PH101) was successfully acylated under homogeneous solution conditions by the following procedure: 2.0 g of cellulose were stirred with 75 mL of N,N-dimethylacetamide for 1 h at 150°C, 3.5 g of LiCl were added, the temperature was raised to 170°C, ca. 18.5 mL of the solvent were distilled and the suspension was cooled to room temperature and stirred overnight. The temperature of the clear cellulose solution was raised to 110°C, kept at that temperature for 1 h, an acid anhydride was added and the solution stirred at 110°C for additional 4 h. Acetates, propionates, butyrates, and acetate/propionate mixed ester were prepared with excellent control of the degree of substitution, DS, 1 to 3 for acetates, 2 and 3 for propionates and butyrates, and 3 for acetate/propionate. The degree of polymerization of cellulose is negligibly affected under these reaction conditions. The distribution of the acetyl moiety among the three OH groups of the anhydroglucose unit shows a preference for the C6 position.

Acid—base indicator equilibria in the presence of aerosol-OT aggregates in heptane. Ion exchange in reversed micelles

Apparent pKa values of malachite green, thymol blue, and maleic acid in the presence of Aerosol—OT reversed micelles in heptane were determined spectrophotometrically. The effects of the water/surfactant molar ratio (R), the indicator hydrophobicity, and the charge change at equilibrium were investigated. The results are discussed in terms of the solubilization sites of the indicators and the “effective” pH values therein. Thus the first two indicators are adsorbed at the surfactant/heptane interface. Using the concept of ion exchange between the solubilized hydronium ions and the detergent counterions, an equation was developed to calculate the local pH at te micelle/solvent interface. For maleic acid, which is solubilizedin the center of the water “pool,” the pH values of the starting aqueous solutions were used to calculate its pKa. The results show that the micellar effect on the apparent pKa, ΔpKa = pKa in water — pKa in the micelle, is small (<0.5 units) and is insensitive the experimental variables. The relevance of the ion exchange process to catalysis by reversed micelles is discussed.

Use of NMR to probe the structure of water at interfaces of organized assemblies

Abstract This review addresses the use of NMR spectroscopy to probe the structure of interfacial water of organized assemblies: aqueous micelles, reverse micelles, RMs, and water-in-oil microemulsions, W/O μEs. For aqueous micelles, the dependence of the 1 H NMR chemical shift of water on [surfactant] is measured in H 2 O-D 2 O mixtures. In case of RMs and W/O μEs, one determines the dependence of 1 H NMR chemical shift of solubilized H 2 O-D 2 O, and/or 1 H and 13 C chemical shifts of the surfactant headgroup on the deuterium content of solubilized water. The measured deuterium isotope effect on the appropriate chemical shift is then used to calculate the so called “deuterium/protium fractionation factor, ϕ” for interfacial water. Values of ϕ thus obtained are rationalized in terms of effects of the interface on the structure of its water of hydration, relative to that of bulk water. The important conclusions of this review are: (1) Effects of simple ions (e.g., butylsulfate or butyltrimethylammonium) on the structure of water are different from those of micellized ions (e.g., dodecylsulfate or cetyltrimethylammonium plus the associated counterions), this difference is due to electrostriction of water by the charged interface; (2) Perturbation of the structure of interfacial water is larger for ionic micelles than for the corresponding zwitterionic ones; (3) For the same class of surfactants, e.g., cationic or zwitterionic, the micelle-induced enhancement of the structure of interfacial water (relative to that of bulk water) increases as a function of increasing the hydrophobic character of the surfactant headgroup; (4) Water solubilized by RMs and W/O μEs does not seem to coexist in “layers” of different structures within the micellar water “pool”.

Cellulose dissolution in lithium chloride/N,N‐dimethylacetamide solvent system: Relevance of kinetics of decrystallization to cellulose derivatization under homogeneous solution conditions

Rate constants and activation parameters for decrystallization of Avicel PH-101 cellulose, and bagasse-based cellulose in presence of LiCl/N,N-dimethylacetamide solvent system have been determined from dependence of the index of crystallinity of cellulose, Ic, on time, under nonisothermal conditions. Calculated rate constants and activation parameters are negligibly dependent on the degree of polymerization of the natural polymer. Under experimental conditions used, derivatization of cellulose can be started after 3 h of cellulose–solvent contact. The relevance of our results to the industrial application of derivatization under homogeneous solution conditions is discussed.

Acid-base indicator equilibria in aerosol-OT reversed micelles in heptane. The use of buffers

Abstract Apparent pKa values for the second dissociation of bromocresol green and maleic acid were determined in the presence of Aerosol-OT reversed micelles in heptane. For the latter indicator, the micellar pKa is close to its value in water and is independent of the water/surfactant molar ratio (R). The pKa of bromocresol green was calculated using the starting pH values (i.e., those of the buffer solutions before solubilization in the micelle) of three different buffers (imidazole, tris (hydroxymethyl) aminomethane, and phosphate). The micellar pKa values were found to be highly dependent on the buffer and on the value of R. The results were analyzed in terms of the solubilization site of the indicator in the micelle and the “effective” pH values therein. Sodium hydrogen maleate resides in the center of the micellar water “pool” where the pH can be taken to be equal to the starting pH. On the other hand, bromocresol green is adsorbed at the micelle/heptane interface. Effective pH values at that site were calculated based on the ion exchange between the species (hydronium ions, protonated buffers) in the water pool and the sodium counterions, using an ion exchange resin as a model for the micelle. The pKa values calculated based on the effective pH values were lower than those based on the starting pH values (by 1.26 to 2.03, 1.92 to 2.36, for imidazole and tris (hydroxymethyl) aminomethane, respectively) and are insensitive to the variation of R. The reason for the dependence of the pKa values on the structure of the organic buffer and for the apparent inefficiency of the inorganic one was discussed in terms of the effective pH values that the buffer maintains at the surfactant/solvent interface. The higher the local [H+] the greater will be the attenuation of the electrostatic field effect of the −SO3− groups of the surfactant, and the smaller will be the concomitant micellar perturbation of the indicator equilibrium.

Solvatochromism in aqueous micellar solutions: effects of the molecular structures of solvatochromic probes and cationic surfactants

Solvatochromic behavior of 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)-1-phenolate (RB); 1-methyl-8-oxyquinolinium betaine (QB); sodium 1-methyl-8-oxyquinolinium betaine-5-sulfonate (QBS); and 1-methyl-3-oxypyridinium betaine (PB) was studied spectrophotometrically in micellar solutions of the following cationic surfactants: cetyltrimethylammonium chloride, cetyldimethylbenzylammonium chloride, dodecyltrimethylammonium chloride, and dodecyldimethylbenzylammonium chloride. Microscopic polarity of water at the (average) solubilization site of the solvatochromic probe, ET in kcal mol-1, was calculated from the position of the longest-wavelength absorption band of the probe. The visible spectrum of PB, the most hydrophilic probe, is not affected by surfactants because it is not included in the micellar pseudo phase. For the other three solvatochromic probes, calculated ET values depend on the structures of both the probe and the surfactant, namely, its headgroup and long-chain alkyl group. RB, the most hydrophobic probe, samples a much lower microscopic polarity than QB and QBS because it penetrates deeper into the cationic micelle. This conclusion has been confirmed by 1H NMR. Polarities measured by (zwitterionic) QB and (anionic) QBS differ because the latter probe exchanges with the surfactant counterion. Calculated ET values refer to micelle-bound probes and are, therefore, different from those reported in the literature, typically determined at [surfactant] ⩽0.05 mol L-1. Effective water concentrations at the solubilization sites of these solvatochromic probes has been calculated by using as references mixtures of water with each the following organic solvents: n-propanol and dioxane (RB); ethanol, n-propanol, acetonitrile and dioxane (QB and QBS).