Elucidating the Association of Water in Wet 1-Octanol from Normal to High Temperature by Near- and Mid-Infrared Spectroscopy

Soft confinement of water in aliphatic alcohols: MIR/NIR spectroscopic and DFT studies

Here, we present the first examination of the state of water under a soft confinement in eight aliphatic alcohols including cyclopentanol, 1-pentanol, 1-hexanol, 1-heptanol, 1-octanol, 1-decanol, 2-octanol and 3-octanol. Due to relatively large size of the aliphatic part, water has limited solubility in all studied alcohols. Water content in saturated solutions was determined by Karl Fischer titration and correlated with the spectroscopic data. This way, we determined the molar absorptivity of the ν2+ν3 combination mode. The effect of addition of water and temperature variation was monitored by ATR-IR and NIR spectroscopy. Analysis of the experimental results was guided by DFT calculations, which provided the structures, harmonic MIR spectra and binding energies of selected alcohol-water complexes. Our studies demonstrated that the state of water in alcohols is related to its solubility, which depends on structure of solvent molecules. The solubility of water in 1-alcohols decreases on increasing of the chain length, but for long chain alcohols this effect is less evident. More apparent solubility reduction appears in going from the primary to secondary alcohols. The effective shielding of the OH group in the linear alcohols is achieved when on both sides of the OH group are ethyl or longer substituents, while the shielding by methyl groups is less efficient. Water is much better soluble in the cyclic alcohols as compared with the linear ones due to better accessibility of the OH group. The soft confinement of water in aliphatic alcohols allows for flexible structural arrangements and interactions. Even at low water content, we did not observe free molecules of water. At these conditions, the molecules of water are singly or doubly bonded to the OH groups from the alcohol. Increasing solubility of water reduces the number of the free OH groups and leads to formation of water clusters. Obtained results allow concluding that in alcohols with sizable aliphatic part the molecules of water are confined in the vicinity of the OH groups.

On the dual behaviour of water in octanol-rich aqueous n-octanol mixtures: an X-ray scattering and computer simulation study

Aqueous n-octanol (n = 1, 2, 3, and 4) mixtures from the octanol rich side are studied by X-ray scattering and computer simulation, with a focus on structural changes, particularly in what concerns the hydration of the hydroxyl-group aggregated chain-like structures, under the influence of various branching of the alkyl tails. Previous studies have indicated that hydroxyl-group chain-cluster formation is hindered in proportion to the branching number. Here, water mole fractions up to x = 0.2 are examined, i.e. up to the miscibility limit. It is found that water molecules within the hydroxyl-chain domains participate in the chain formations in a different manner for 1-octanol and the branched octanols. The hydration of the octanol hydroxyl chains is confirmed by the shifting of the scattering pre-peak position kPP to smaller values, both from measured and simulated X-ray scattering intensities, which corresponds to an increased size of the clusters. Experimental pre-peak amplitudes are seen to increase with increasing water content for 1-octanol, while this trend is reversed in all branched octanols, with the amplitudes decreasing with the increase of the branching number. Conjecturing that the amplitudes of pre-peaks are related to the density of the corresponding aggregates, these results are interpreted as water breaking large OH hydroxyl chains in 1-octanol, hence increasing the density of aggregates, while enhancing hydroxyl aggregates in branched alcohols by inserting itself into the OH chains. The analysis of the cluster distributions from computer simulations provide more details on the role of water. For cluster sizes smaller than dc = 2π/kPP, water is found to always play the role of a structure enforcer for all n-octanols, while for clusters of size dc water is always a destructor. For cluster sizes larger than dc, the role of water differs from 1-octanol and the branched ones: it acts as a structure maker or breaker in inverse proportion to the hindering of OH hydroxyl chain structures arising from the topology of the alkyl tails (branched or not).

Hydration Dynamics of Model Peptides with Different Hydrophobic Character

The multi-scale dynamics of aqueous solutions of the hydrophilic peptide N-acetyl-glycine-methylamide (NAGMA) have been investigated through extended frequency-range depolarized light scattering (EDLS), which enables the broad-band detection of collective polarizability anisotropy fluctuations. The results have been compared to those obtained for N-acetyl-leucinemethylamide (NALMA), an amphiphilic peptide which shares with NAGMA the same polar backbone, but also contains an apolar group. Our study indicates that the two model peptides induce similar effects on the fast translational dynamics of surrounding water. Both systems slow down the mobility of solvating water molecules by a factor 6–8, with respect to the bulk. Moreover, the two peptides cause a comparable far-reaching spatial perturbation extending to more than two hydration layers in diluted conditions. The observed concentration dependence of the hydration number is explained considering the random superposition of different hydration shells, while no indication of solute aggregation phenomena has been found. The results indicate that the effect on the dynamics of water solvating the amphiphilic peptide is dominated by the hydrophilic backbone. The minor impact of the hydrophobic moiety on hydration features is consistent with structural findings derived by Fourier transform infrared (FTIR) measurements, performed in attenuated total reflectance (ATR) configuration. Additionally, we give evidence that, for both systems, the relaxation mode in the GHz frequency range probed by EDLS is related to solute rotational dynamics. The rotation of NALMA occurs at higher timescales, with respect to the rotation of NAGMA; both processes are significantly slower than the structural dynamics of hydration water, suggesting that solute and solvent motions are uncoupled. Finally, our results do not indicate the presence of super-slow water (relaxation times in the order of tens of picoseconds) around the peptides investigated.

Origin and correction of magnetic field inhomogeneity at the interface in biphasic NMR samples

The use of susceptibility matching to minimize spectral distortion of biphasic samples layered in a standard 5 mm NMR tube is described. The approach uses magic angle spinning (MAS) to first extract chemical shift differences by suppressing bulk magnetization. Then, using biphasic coaxial samples, magnetic susceptibilities are matched by titration with a paramagnetic salt. The matched phases are then layered in a standard NMR tube where they can be shimmed and examined. Linewidths of two distinct spectral lines, selected to characterize homogeneity in each phase, are simultaneously optimized. Two-dimensional distortion-free, slice-resolved spectra of an octanol/water system illustrate the method. These data are obtained using a 2D stepped-gradient pulse sequence devised for this application. Advantages of this sequence over slice-selective methods are that acquisition efficiency is increased and processing requires only conventional software.