Paulo A. R. Pires

Proton and carbon-13 NMR study of the aggregation of benzyl(2-acylaminoethyl)dimethylammonium chloride surfactants in D2O

1H and 13C NMR spectroscopy have been employed to study aggregation of the following surfactant series in D2O: RNH(CH2)2N+(CH3)2CH2C6H5 Cl−, where R is an acyl group containing 10 to 16 carbon atoms. Observed chemical shifts, δobs, and apparent transverse relaxation times, 1/T2* of the surfactant discrete groups were measured at concentrations below and above the critical micelle concentration, c.m.c. Plots of δobs and/or 1/T2* versus [surfactant] are sigmoidal, and were fitted to a model based on the mass-action law. The parameters determined by this fit were: the equilibrium constant for micelle formation, K; c.m.c. and chemical shifts of the monomer, δmon and the micelle, δmic. NMR-determined c.m.c.s are in excellent agreement with those previously determined by conductance and surface tension measurements. K increases as a function of increasing length of the surfactant hydrophobic tail. Gibbs free energies of micellization, ΔG°mic, were calculated and divided into contributions from the CH2 groups in the hydrophobic chain and (terminal CH3+head-group). Both quantities agree with those previously determined from conductivity data. Contribution of (terminal CH3+head-group) to ΔG°mic shows the importance to micellization of direct and/or water-mediated H-bonding of the surfactant amide group. Peak splitting as a function of increasing [surfactant], (δmic−δmon) and (δorganic solvent−δmon) supports a micellar structure where the benzyl group folds back toward the aggregate interior.

Kinetics and mechanism of imidazole-catalyzed acylation of cellulose in LiCl/N,N-dimethylacetamide.

Cellulose acylation by anhydrides (ethanoic to hexanoic) plus tosyl chloride, TsCl, or imidazole in LiCl/N,N-dimethylacetamide solution has been studied. Contrary to a previous claim, TsCl does not catalyze acylation. For the diazole-catalyzed reaction, N-acylimidazole is the acylating agent. Third order rate constants (k(3); 40-70 °C) have been calculated from conductivity data and split, by using information from model compounds, into contributions from the primary- (k(3;Prim(OH))) and secondary- (k(3;Sec(OH))) hydroxyl groups of cellulose. Values of k(3,Prim(OH))/k(3,Sec(OH)) are >1, and increase linearly as a function of increasing the number of carbon atoms of the acyl group. Rate constants and the degree of biopolymer substitution decrease on going from ethanoic- to butanoic-, then increase for pentanoic- and hexanoic anhydride, due to enthalpy/entropy compensation. Relative to the uncatalyzed reaction, the diazole-mediated one is associated with smaller enthalpy- and larger entropy of activation, due to difference of the acylating agent.

Acylation of cellulose in a novel solvent system: solution of dibenzyldimethylammonium fluoride in DMSO.

A novel electrolyte, dibenzyldimethylammonium fluoride has been obtained essentially anhydrous (BMAF-0.1H2O) by a simple route. Its thermal stability, relative to tetra(1-butyl)ammonium fluoride trihydrate (TBAF3H2O) has been demonstrated by thermogravimetric analysis. DMSO solution of (BMAF-0.1H2O) dissolves microcrystalline- and fibrous celluloses; the dissolved biopolymers have been acylated by ethanoic-, butanoic-, and hexanoic anhydride. The degrees of substitution of the esters are higher than with TBAF3H2O/DMSO. The reasons are discussed in terms of differences in electrolyte structure and contents of water of hydration, whose presence leads to side reactions and decreases of the basicity of (F(-)). This conclusion is corroborated by molecular dynamics simulations of the interactions of glucose dodecamer/R4NF-hydrate/DMSO. These show that the interactions oligomer-F(-)-water is operative only for TBAF3H2O/DMSO. The efficiency of BMAF-0.1H2O/DMSO is explained based on better accessibility of the biopolymer due to efficient hydrogen-bonding between its hydroxyl groups and the essentially desolvated (F(-)).

Solvation in pure liquids: what can be learned from the use of pairs of indicators?

The solvation of six solvatochromic probes in a large number of solvents (33-68) was examined at 25 degrees C. The probes employed were the following: 2,6-diphenyl-4-(2,4,6-triphenylpyridinium-1-yl) phenolate (RB); 4-[(E)2-(1-methylpyridinium-4-yl)ethenyl] phenolate, MePM; 1-methylquinolinium-8-olate, QB; 2-bromo-4-[(E)-2-(1-methylpyridinium-4-yl)ethenyl] phenolate, MePMBr, 2,6-dichloro-4-(2,4,6-triphenyl pyridinium-1-yl) phenolate (WB); and 2,6-dibromo-4-[(E)-2-(1-methylpyridinium-4-yl)ethenyl] phenolate, MePMBr(2), respectively. Of these, MePMBr is a novel compound. They can be grouped in three pairs, each with similar pK(a) in water but with different molecular properties, for example, lipophilicity and dipole moment. These pairs are formed by RB and MePM; QB and MePMBr; WB and MePMBr(2), respectively. Theoretical calculations were carried out in order to calculate their physicochemical properties including bond lengths, dihedral angles, dipole moments, and wavelength of absorption of the intramolecular charge-transfer band in four solvents, water, methanol, acetone, and DMSO, respectively. The data calculated were in excellent agreement with available experimental data, for example, bond length and dihedral angles. This gives credence to the use of the calculated properties in explaining the solvatochromic behaviors observed. The dependence of an empirical solvent polarity scale E(T)(probe) in kcal/mol on the physicochemical properties of the solvent (acidity, basicity, and dipolarity/polarizability) and those of the probes (pK(a), and dipole moment) was analyzed by using known multiparameter solvation equations. For each pair of probes, values of E(T)(probe) (for example, E(T)(MePM) versus E(T)(RB)) were found to be linearly correlated with correlation coefficients, r, between 0.9548 and 0.9860. For the mercyanine series, the values of E(T)(probe) also correlated linearly, with (r) of 0.9772 (MePMBr versus MePM) and 0.9919 (MePMBr(2) versus MePM). The response of each pair of probes (of similar pK(a)) to solvent acidity is the same, provided that solute-solvent hydrogen-bonding is not seriously affected by steric crowding (as in case of RB). We show, for the first time, that the response to solvent dipolarity/polarizability is linearly correlated to the dipole moment of the probes. The successive introduction of bromine atoms in MePM (to give MePMBr, then MePMBr(2)) leads to the following linear decrease: pK(a) in water, length of the phenolate oxygen-carbon bond, length of the central ethylenic bond, susceptibility to solvent acidity, and susceptibility to solvent dipolarity/polarizability. Thus studying the solvation of probes whose molecular structures are varied systematically produces a wealth of information on the effect of solute structure on its solvation. The results of solvation of the present probes were employed in order to test the goodness of fit of two independent sets of solvent solvatochromic parameters.

Mixed solvents for cellulose derivatization under homogeneous conditions: kinetic, spectroscopic, and theoretical studies on the acetylation of the biopolymer in binary mixtures of an ionic liquid and molecular solvents

Rate constants for the acetylation of microcrystalline cellulose (MCC), by ethanoic anhydride in the presence of increasing concentrations of the ionic liquid (IL), 1-allyl-3-methylimidazolium chloride in dipolar aprotic solvents (DAS), N,N-dimethylacetamide (DMAC), and acetonitrile (MeCN), have been calculated from conductivity data. The third order rate constants showed a linear dependence on [IL]. We explain this result by assuming that the reacting cellulose is hydrogen-bonded to the IL. This is corroborated by kinetic data of the acetylation of cyclohexylmethanol, FTIR of the latter compound and of cellobiose in mixtures of IL/DAS, and conductivity of the binary solvent mixtures in absence, and presence of MCC. Cellulose acetylation is faster in IL/DMAC than in IL/MeCN; this difference is explained based on solvatochromic data (empirical polarity and basicity) and molecular dynamics simulations. Results of the latter indicate hydrogen-bond formation between the hydroxyl groups of the anhydroglucose unit of MCC, (Cl−) of the IL, and the dipole of the DMAC. Under identical experimental conditions, acetylation in IL/DMAC is faster than that in LiCl/DMAC (2.7–8 times), due to differences in the enthalpies and entropies of activation.

Benzyl (3-Acylaminopropyl) dimethylammonium chloride surfactants : Structure and some properties of the micellar aggregates

The title cationic surfactants were synthesized by the scheme in Fig. 1, where RCO2H refers to decanoic, dodecanoic, tetradecanoic and hexadecanoic acid, respectively. In aqueous solution, the micelle/water interface may be located at the quaternary ammonium ion or at the amide group. The following pieces of evidence indicate that the interface lies at the latter site: theoretically calculated aggregation numbers and those determined by static light scattering; dependence on surfactant concentration, below and above the critical micelle concentration, cmc, of both the IR frequency of amide I band and 1H NMR chemical shifts of the discrete surfactant protons. Solution conductance and calorimetric titration have been employed to study the aggregation of these surfactants in water at 25 °C. Increasing the length of R resulted in a decrease of the cmc and the degree of counter-ion dissociation, αmic. Gibbs free energies of micelle formation were calculated and divided into contributions from the methylene groups of the hydrophobic tail, and the terminal methyl plus head-group. The former are similar to those of other surfactants, whereas the latter are more negative, i.e., the transfer of the head-group from bulk water to the micelle is more favorable. This is attributed to direct or water-mediated H-bonding of the micellized surfactant molecules, via the amide group, in agreement with the IR data presented.

Experimental and theoretical studies on solvation in aqueous solutions of ionic liquids carrying different side chains: the n-butyl-group versus the methoxyethyl group

We used solvatochromic compounds to probe solvation in mixtures of water, W, and four ionic liquids (ILs), 1-R-3-methylimidazoliumX, where R = n-butyl or methoxyethyl and X = acetate and chloride; these are denoted as (C4MeImAc), (C3OMeImAc), (C4MeImCl), and (C3OMeImCl). Our aim was to investigate the effects on solvation when an ether linkage is substituted for a –CH2– group in the IL side chain. We used the solvatochromic probes 2,6-dichloro-4-(2,4,6-triphenylpyridinium-1-yl)phenolate (WB) and 5-nitroindoline, 1-methyl-5-nitroindoline to determine the solvent polarity, ET(WB), and Lewis basicity, SB, respectively. From UV-Vis spectral data, we calculated ET(WB) as a function of the water mole fraction (χW) at different temperatures; from 15 to 60 °C for WB in IL–acetate–W; 25 °C for SB and WB in IL–chloride–W. For all IL–W mixtures, the dependence of ET(WB) on χW is non-linear and, surprisingly, shows negligible dependence on the nature of the side chain. Values of the SB of IL–W were higher for C4MeImX–W than for C3OMeImX. A rationale for these results is the deactivation of the ether oxygen due to the formation of intramolecular hydrogen bonds with the hydrogens of the imidazolium ring. Our hypothesis is confirmed by quantum chemistry and molecular dynamics calculations (energy of the conformers and radial distribution functions), density, and 1H NMR data (chemical shifts, line widths). We attributed the non-linear dependence of the solvatochromic parameters on χW to preferential solvation of the dyes. We treated ET(WB) data with a model that includes the formation of the complex solvent IL–W. Equilibrium constants for solvent exchange in the solvation layer of WB were calculated; their values showed that IL–W is the most efficient solvent species present.

FTIR and 1H NMR Studies on the Structure of Water Solubilized by Reverse Aggregates of Dodecyltrimethylammonium Bromide; Didodecyldimethylammonium Bromide, and Their Mixtures in Organic Solvents

The structure of water solubilized by the reverse aggregates of dodecyltrimethylammonium bromide, DoMe3ABr in chloroform/n-heptane; di-dodecyldimethylammonium bromide, Do2Me2ABr, in n-heptane, and mixture of the two surfactants in the latter solvent has been probed by FTIR and 1H NMR. The ν OD band of solubilized HOD (4% D2O in H2O) has been recorded as a function of [water]/[surfactant] molar ratio, W/S. Curve fitting of this band showed the presence of a small peak at (2375 ± 12) cm−1 and a major one at (2521 ± 7) cm−1; the latter corresponds to (92.5 ± 1) % of the total peak area. As a function of increasing W/S, ν OD decreases, its full width at half-height increases and its area linearly increases over the W/S range investigated. Observed 1H NMR chemical shift, δ obs, of solubilized water, and the CH 2–N+(CH3)3, CH2–N+(CH 3)3 groups of DoMe3ABr change smoothly as a function of increasing W/S. Similar trends have been observed for Do2Me2ABr-solubilized water, and for water solubilized by a mixture of DoMe3ABr plus Do2Me2ABr. δ obs for H2O-D2O mixtures solubilized by DoMe3ABr were measured as a function of the deuterium content of the aqueous nano-droplet. These data were employed to calculate the so called deuterium/protium “fractionation factor”, φ M, of the reverse aggregate-solubilized water. Plots of a function of δ obs (for HOD; CH 2–N+(CH3)3, and CH2–N+(CH 3)3) versus the atom fraction of deuterium in the aqueous nano-droplet were strictly linear, indicating that the value of φ M is unity. The results of both techniques show that reverse aggregate-solubilized water does not seem to coexist in “layers” of different structures, as suggested, e.g., by the multi-state water solubilization model.

Convenient solvatochromic probes for the determination of solvent properties: β-carotene and 2-chloro-7-nitro-9H-fluorene

Dipolaridade/polarizabilidade do solvente (SDP) tem sido calculada a partir dos espectros de UV-Vis de 2-(N,N-dimetilamino)-7-nitro-9H-fluoreno e 2-fluoro-7-nitro-9 H-fluoreno. Com base em calculos teoricos (23 solventes) e dados experimentais (56 solventes), revela-se que 2-cloro-7-nitro-9H-fluoreno (disponivel comercialmente) pode ser perfeitamente utilizado para o calculo dessa propriedade, em vez de 2-fluoro-7-nitro-9 H-fluoreno. A divisao de SDP nos seus componentes (dipolaridade (SD) e polarizabilidade (SP) do solvente) requer o uso de um composto sintetico polienico cuja sintese e trabalhosa, envolvendo 15 etapas. Nosso grupo de pesquisa mostrou recentemente que o corante natural b-caroteno pode ser convenientemente utilizado para a determinacao de SP, permitindo o calculo de SD. Utilizando estas sondas solvatocromicas, SDP, SP e SD foram calculados para uma serie de 1-bromoalcanos. Para varias series homologas, a dependencia de SDP (SD e SP para uma serie homologa) com o numero de atomos de carbono nos grupos 1-alquila ou acila foi calculada e discutida. Solvent dipolarity/polarizability (SDP) has been previously calculated from the UV-Vis spectra of 2-(N,N-dimethylamino)-7-nitro-9H-fluorene and 2-fluoro-7-nitro-9 H- fluorene. Based on theoretical calculations (23 solvents) and experimental data (56 solvents), it is shown that 2-chloro-7-nitro-9H-fluorene (commercially available) can be conveniently employed for the calculation of this property, instead of its 2-fluoro-7-nitro counterpart. The splitting of SDP into its components (solvent dipolarity (SD) and polarizability (SP)) requires the use of a synthetic polyene compound whose synthesis is laborious, involving 15 steps. Our research group has recently shown that the natural dye b-carotene can be conveniently employed for the determination of SP, allowing the calculation of SD. Using these solvatochromic probes, SDP, SP and SD for a series of 1-bromoalkanes were calculated. For several homologous series, the dependence of solvent SDP (SD and SP for one series) on the number of carbon atoms in the 1-alkyl- or acyl-group was calculated and discussed.