Mesomorphic solutions of cellulose triacetate in trifluoroacetic acid halogenated solvent mixtures: Phase separations and factors affecting the cholesteric pitch

Cellulose triacetate (CTA) forms cholesteric mesophases in trifluoroacetic acetic acid (TFA) and mixtures of TFA and CH₂Cl₂, 1,2-dichloroethane (1,2-DCE), and CHCl₃. Cholesteric pitches and solution flow times indicate that the order of solvent powers is TFA-CH₂Cl₂ > TFA-1,2-DCE > TFA > TFA-CHCl₃, which is the order of decreasing acidity of the solvent systems. With TFA-CH₂Cl₂ as solvent, the one-fourth power of the pitch varies inversely with the CTA concentration, and increases linearly with temperature. The pitch increases exponentially with time and increases faster the more acidic the solvent. In a magnetic field a cholesteric to nematic transition occurs. A minimum in solution viscosity occurs at 34% w/v of CH₂Cl₂ for solutions in TFA-CH₂Cl₂. The miscibility gap as a function of molecular weight depends on the solvent composition and is smaller the higher the acidity of the solvent. Agreement between the experimentally observed A and B points and the theoretical points is better for the Khokhlov and Semenov theory for semiflexible chains than for the original Flory theory or the Flory-Ronca modification.     

Electrophoretic deposition of phthalocyanine in organic solutions containing trifluoroacetic acid

The absorption spectra of copper phthalocyanine (CuPc) 1,2-dichloroethane (DCE) solutions containing trifluoroacetic acid (TFAA) shows that the number of protons coordinating to the CuPc molecule was 1 and 2 for the first and second proton adducts, respectively, which indicates the formations of CuPcH(+) and CuPcH(2)(2+). This CuPc molecule may act as a catalyst to dissociate TFAA into trifluoroacetate anion (A(-)) and H(+) and form the proton adducts. The electrical conductivity dependence of the solution on CuPc concentration also supports this mechanism. A dense film of CuPc was deposited on an indium tin oxide cathode plate by electrophoresis of the solution. Similar dense films of a wide variety of phthalocyanines (MPc; M = Cu, H(2), Fe, Ni, Zn, Pb, VO) were also deposited using this method. Similar films of CuPc were also formed using dichloromethane (DCM) and 1,1,1-trichloroethane (TCE) in place of DCE. Depositions are ascribed to the migration of positively charged monomers (i.e., protonated MPc). Scanning electron microscopy revealed that these films are composed of fibrous crystallites, size of which was found to increase with the electrophoresis time, the strength of the applied electrical field and the concentration of CuPc in the bath. The influence of the dielectric constant of the organic solvent on the film growth is discussed.     

Inverse temperature-dependent pathway of cellulose decrystallization in trifluoroacetic acid

An unusual inverse temperature-dependent pathway was observed during cellulose decrystallization in trifluoroacetic acid (TFA). Decreasing the TFA treatment temperature accelerated the cellulose decrystallization process. It took only 100 min to completely decrystallize cellulose at 0 degrees C in TFA, a result not achieved in 48 h at 25 degrees C in the same medium. There was neither cellulose esterification nor a change of cellulose macrofibril morphology by TFA treatment at 0 degrees C. Our IR data suggest that TFA molecules are present as cyclic dimers when they penetrate into crystalline cellulose regions, transforming crystalline cellulose to amorphous cellulose. On the other hand, the rate of TFA penetration into the cellulose matrix was greatly retarded at higher temperatures where monomeric TFA prevails. At elevated temperatures, esterification of TFA monomers on the external surface of crystalline cellulose, agglomeration of cellulose macrofibrils, as well as water released from the esterification reaction, inhibit the diffusion rate of TFA into the cellulose crystalline region and decrease the TFA swelling capability.     

Paper chromatography of fatty acids and halogenated organic acids

A paper chromatographic method is described for the identification of the volatile fatty acids and some of their bromo- and iodo-derivatives. Some new spraying reagents are suggested which enable the different classes of acids to be detected in presence of one another.     

Some further observations on the systems cellulose/trifluoroacetic acid/CH2Cl2 and cellulose triacetate/TFA/CH2Cl2

There is little or no trifluoroacetylation of cellulose dissolved in TFA-CH₂Cl₂ admixtures. Both cellulose and cellulose triacetate (CTA)are slowly degraded in the solvent. Cellulose forms a mesophase as low as 4%(w/w)concentration, but CTA has a much higher critical concentration, 20% (w/w), in TFA-CH₂Cl₂. The cellulose behaves as a rigid rod in TFA-CH₂Cl₂ (70/30v/v) and its persistence length calculated using the lattice model approximates its chain length, presumably due to extensive interaction with the solvent. As expected, due to low polymer-solvent interactions, the persistence length of CTA in TFA-CH₂Cl₂ is only one-fourth the chain length.     

Whole-cell biodetection of halogenated organic acids

A whole-cell bacterial sensor system for short-chain halo-organic acids was constructed, using 2-chloropropionic acid (2-CPA) as a model pollutant. An Escherichia coli host was transformed with a moderate-copy plasmid containing a fusion of two foreign genetic elements: (a) a promoter-containing segment of the Pseudomonas DL-DEX (DL-2-haloacid dehalogenase) encoding gene and (b) bioluminescence (luxCDABE) genes of Photorhabdus luminescens. The resulting construct, named MT1, responded to the presence of 2-CPA by dose-dependent light emission, in a highly specific albeit a very insensitive manner. Thus, while the desired concept was successfully demonstrated, further genetic work is needed in order to make such a construct practical for environmental monitoring purposes.     

Transformation of organic and inorganic compounds in trifluoroacetic acid

It is established that the effectiveness of fluorine-containing acids in the transformation of organic and inorganic substrates is due to the ability of the acid to perform several functions: to accumulate relatively high concentrations of molecular oxygen, to activate it, and to serve as a hydrogen-containing medium.     

Lyotropic mesomorphic formation of cellulose in trifluoroacetic acid-chlorinated-alkane solvent mixtures at room temperature

Mixtures of trifluoroacetic acid (TFA)-1,2-dichloroethane (1,2-DCE); TFA-dichloromethane (CH₂Cl₂); and TFA-trichloromethane (CHCl₃) are excellent cellulose solvents at room temperature. TFA-1,2-DCE and TFA-CH₂Cl₂ are superior to pure TFA. Lyotropic cellulose mesophases were obtained in (20% w/v) solutions of cellulose in these solvent mixtures. The optical and optical rotatory powers of the solutions suggest that the lyotropic mesophase of cellulose is cholesteric. This implies that cellulose molecules are arranged in helical form in these solvent systems.     

Efficient epimerization of pyrene and other aromatic C-nucleosides with trifluoroacetic acid in dichloromethane

Trifluoroacetic acid has been found to be an efficient and non-oxidative catalyst for the epimerization of pyrene and other C-nucleosides in dichloromethane at ambient temperature. Aspects of the epimerization mechanism are also elucidated.     

Chemical transformations of CO2 in trifluoroacetic acid solutions

It is established that a conversion reaction of carbon dioxide takes place at room temperature and atmospheric pressure in aqueous solutions of trifluoroacetic acid (TFA), which leads to the formation of oxalic acid and heavier polymerized products.     

Investigation of molecular order in liquid-crystalline solutions of cellulose triacetate using PMR spectroscopy

Liquid-crystalline solutions of cellulose triacetate (CTA) in trifluoroacetic acid (TFA)–CH₂Cl₂, TFA–1.2-dichloroethane (1,2-DCE) solvent mixtures were examined by means of PMR spectroscopy. CTA forms both cholesteric and nematic phases in these solvents depending on the CTA concentration. In cholesteric solutions the CH₂Cl₂ signal is initially a singlet and then splits into a doublet. The time dependence of the splitting and the effect of CTA concentration are reported. The results suggest that the cholesteric phase slowly changes into a nematic phase in the magnetic field. The splitting of the CH₂Cl₂ proton signal into a doublet and the 1,2-DCE signal into a quartet are due to direct magnetic dipole-dipole interactions. Rotation of the sample in the magnetic field results in the disappearance of the doublet or quartet and suggests that the solvent molecules are originally oriented in the direction of the magnetic field. In the biphasic region, immediate splitting of the CH₂Cl₂ proton signal suggests that the anisotropic phase is nematic.     

Temperature-concentration conditions of liquid-crystalline-phase formation in cellulose ethers and esters in trifluoroacetic acid

The temperature-concentration conditions of formation and regions of liquid-crystalline-phase existence in concentrated solutions of methyl cellulose, hydroxypropylmethyl cellulose, cellulose diacetate, and cellulose triacetate in trifluoroacetic acid are studied via optical methods. It is shown that the type of anisotropic structures is largely determined by the pitch of the cholesteric helix. An increase in the strength of acetate films corresponding to the maximum optical anisotropy in the two-phase region of transition of cellulose acetates solutions to the LC phase is found.     

Scalar dynamic nuclear polarization of protons in trifluoroacetic acid solutions

Strongly scalar ¹H intermolecular dynamic nuclear polarization (DNP) of solvent ¹H nuclei in solutions of organic free radicals has now been observed at room temperature. Transient protonation of the CF₃COOH/pyridyl imidazoline-1-oxyl radical system accounts for the observed effects.     

Quantitative estimation of halogenated organic acids on paper chromatograms

A quantitative method is described for the estimation of the ammonium salts of the halogenated organic acids after they have been separated on a paper chromatogram. The recovery of the added acid is better than 98%.     

Convenient,metal-free ipso-nitration of arylboronic acids using nitric acid and trifluoroacetic acid

A feasible protocol for the direct ipso-nitration of arylboronic acids using trifluoroacetic acid and nitric acid as nitration reagent has been developed. Various aromatic nitro compounds are produced in moderate to good yields under the metal-free conditions. The method is operationally simple and regioselective, and might have potential application in industry.     

Surface properties of aqueous solutions of trifluoroacetic and acetic acid mixture

The coadsorption of trifluoroacetic and acetic acid at the free surface of water was investigated. Knowing the composition of mixed monolayer and the effective dipole moments of individual molecules, the surface potential was calculated. The synergetic effect of the surface properties of mixed adsorbed films was observed.     

Reaction of para-substituted aromatic acids with methyl thiocyanate in trifluoroacetic acid

We have investigated the reactions of aromatic carboxylic acids (p-XC₆H₄COOH, in which X=F, Cl, Br, I, NO₂, CH₃, H, OCH₃) with methyl thiocyanate in trifluoroacetic acid. It was shown that the starting acids are converted to S-methyl benzothioates [p-XC₆H₄COSCH₃ (40–70%)] and the nitrile of the acid [p-XC₆H₄CN (20–50%)]. In addition to starting acid, in the reaction mixtures products of the reaction of CF₃COOH with methyl thiocyanate were identified.Institute of Oil and Gas Problems, Russian Academy of Sciences, I. M. Gubkin State Academy of Oil and Gas, 117917 Moscow. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 2, pp. 412–416, February, 1992.     

Instantaneous dissolution of cellulose in organic electrolyte solutions

Herein a novel class of solvent systems for cellulose is introduced. Surprisingly, organic electrolyte solutions, which contain just a small molar fraction of ionic liquid, dissolve instantaneously large amounts of cellulose. The solvation properties of the solvent systems, required for dissolving the polymer, are discussed here.     

Acylation of alkll 1,4-dimethoxybenzenes by substituted benzoic acids in trifluoroacetic acid

A clean, convenient and highly selective acylation of alkyl 1,4-dimethoxy-benzenes, e.g., 2-methyl-1,4-dimethoxybenzene, by using substituted benzoic acids in dry trifluoroacetic acid has been achieved. The substituted benzophenones were found to be the sole product and their formation could be much reduced or even completely inhibited by adding a small amount of water. Kinetic studies show that the reaction obeys second-order kinetics with rate = k₂.[substrate]-[ArCO₂H]. The very negative reaction constant (p = -6.43) derived from the excellent Hammett correlation (r = 0.99) reveals that electron-donating substituents in benzoic acid strongly favour the protonation of the acid by trifluoroacetic acid, increasing the concentration of the acylating agents, ArtC⁺=0, and thus lead to the effective aromatic acylation.