W. Wrzeszcz

Microheterogeneity in binary mixtures of methanol with aliphatic alcohols: ATR-IR/NIR spectroscopic, chemometrics and DFT studies

This paper reports ATR-IR and NIR spectroscopic studies on microheterogeneity in binary mixtures of methanol with five short chain aliphatic alcohols: ethanol, 1-propanol, 2-propanol, tert-butanol and cyclopentanol. The inhomogeneity distribution of molecules in the mixtures was characterized by MIR/NIR excess absorption spectra, two-dimensional correlation approach, and chemometric methods. The experimental data were compared and discussed with the theoretical calculations of homo- and heteroclusters of different size and composition. The results obtained in the present work allow us to infer that all studied mixtures deviate from the ideal and the largest effect appears at equimolar mixture. At this composition about 50% of molecules are involved in the mixed clusters. In the entire range of compositions in the mixtures coexist the clusters of pure alcohols and heteroclusters with an average mole ratio of 1 : 1. Only in methanol/tert-butanol mixture was observed an additional tert-butanol-rich cluster with an average methanol : tert-butanol mole ratio between 1 : 3 and 1 : 4. The excess absorption and two-dimensional correlation spectra reveals that methanol/ethanol mixture is the closest to the ideal one, while the largest degree of non-ideality was observed for methanol/tert-butanol mixture. The extent of deviation from the ideal mixture increases with an increase in the chain length and the order of the alcohol. These trends are correlated with decreasing degree of self-association of the alcohol.

Microheterogeneity in binary mixtures of aliphatic alcohols and alkanes: ATR-IR/NIR spectroscopic and chemometric studies

Aliphatic alcohols form homogeneous mixtures with alkanes of similar size and structure, however at a molecular level one can expect the presence of both the homo and heteroclusters leading to the local heterogeneity. Recently, we observed this phenomenon in the binary mixtures of methanol with aliphatic alcohols [RSC Adv., 2016, 6, 37195]. This paper provides new and comprehensive information on the structure of alcohol/alkane mixtures at a molecular level. Besides, we studied the relationship between the chain structure and the deviation from the ideal mixture. A particular attention was paid for the difference between the linear and cyclic alcohols and alkanes. For studies we selected two alcohols: 1-hexanol, cyclohexanol, and two alkanes: n-hexane, cyclohexane. By combining these two pairs of compounds, we obtained four different alcohol/alkane mixtures. The inhomogeneity distribution of molecules in these mixtures and deviation from the ideality was characterized by ATR-IR/NIR excess absorption spectra and chemometric methods. Obtained results allow us to conclude that the separation at a molecular level and the extent of deviation from the ideality depends on the degree of association of the alcohol and the similarity of alkyl parts. As 1-hexanol is more associated than cyclohexanol, 1-hexanol/n-hexane mixture is the closest to the ideal mixture. In contrast, cyclohexanol/n-hexane is the most non-ideal mixture since in this case the smaller degree of self-association of cyclohexanol is coupled with different structure of alcohol and alkane chains.

The infrared dielectric function of liquid triethylamine

Abstract The spectra of both components of the complex refractive index and of the complex electric permittivity for liquid triethylamine at 298 K were estimated from thin film transmission spectra in the 3200 – 650 cm −1 range. From these data, the molar vibrational polarization was determined in selected regions, and their contribution to the total value for the studied molecule was discussed. The spectra of both components of the complex polarizability of the molecule in liquid phase were also obtained in the studied region and discussed.

Thin film transmission spectra of liquid CDCl3 in the infrared

Abstract Thin film transmission spectra of neat liquid CDCl 3 were obtained in the 3500–250 cm −1 region. From these data the spectra of both components of the complex refractive index were calculated. The imaginary part was used to calculate integrated intensities and other spectral parameters for the principal bands. The obtained results were compared with literature data and discussed.

Microheterogeneity in CH3OH/CD3OH mixture

Recently, we demonstrated the presence of microheterogeneity in binary mixtures of unlike alcohols. [RSC Adv. 2016, 6, 37195-37202] The aim of this work was examination if this phenomenon occurs also in the mixture of very similar alcohols like CH3OH and CD3OH. Theoretical calculations suggest that the isotopic substitution in methyl group influences properties of the OH group. Hence, one can expect that this effect may lead to partial separation of CH3OH and CD3OH at a molecular level and it contributes to deviation from the ideal mixture. This work evidences that CH3OH/CD3OH mixture also deviates from the ideal one, but the extent of this deviation is much smaller as compared with the mixtures of other alcohols. It is of particular note that this deviation results mainly from the difference between the CH3 and CD3 groups, while the contribution from the OH groups is small. The structure of CH3OH/CD3OH mixture at a molecular level is similar to the structure of binary mixtures of other alcohols. The mixture is composed of the homoclusters of both alcohols and the mixed clusters. The homoclusters existing in the mixture are similar to those present in bulk alcohols. The highest population of the heteroclusters and the largest deviation from the ideal mixture were observed at equimolar mixture. Both the experimental and theoretical results reveal that in CH3OH/CD3OH mixture dominate the cyclic tetramers and larger clusters, while the population of the linear clusters is negligible. Though the extent and strength of hydrogen bonding in both alcohols are the same, the position and intensity of the 2ν(OH) band for CH3OH and CD3OH are different. We propose possible explanation of this observation.

The infrared dielectric function of liquid CDCl3

Abstract The spectra of both components of the complex refractive index and of the complex electric permittivity of liquid deuterochloroform (CDCl 3 ) were estimated in the infrared region from thin film transmission spectra studied previously. The contribution of principal modes to the molar vibrational polarization of the studied molecule was determined and discussed.

Microheterogeneity in binary mixtures of water with CH 3 OH and CD 3 OH: ATR-IR spectroscopic, chemometric and DFT studies

Here we report ATR-IR spectroscopic study on the separation at a molecular level (microheterogeneity) and the degree of deviation of H2O/CH3OH and H2O/CD3OH mixtures from the ideal mixture. Of particular interest is the effect of isotopic substitution in methyl group on molecular structure and interactions in both mixtures. To obtain comprehensive information from the multivariate data we applied the excess molar absorptivity spectra together with two-dimensional correlation analysis (2DCOS) and chemometric methods. In addition, the experimental results were compared and discussed with the structures of various model clusters obtained from theoretical (DFT) calculations. Our results evidence the presence of separation at a molecular level and deviation from the ideal mixture for both mixtures. The experimental and theoretical results show that the maximum of these deviations appears at equimolar mixture. Both mixtures consist of three kinds of species: homoclusters of water and methanol and mixed clusters (heteroclusters). The heteroclusters exist in the whole range of mole fractions with the maximum close to the equimolar mixture. At this mixture composition near 55-60% of molecules are involved in heteroclusters. In contrast, the homoclusters of water occur in a limited range of mole fractions (XME < 0.85-0.9). Upon mixing the molecules of methanol form weaker hydrogen bonding as compared with the pure alcohol. In contrast, the molecules of water in the mixture are involved in stronger hydrogen bonding than those in bulk water. All these results indicate that both mixtures have similar degree of deviation from the ideal mixture.