Structural aspects of aqueous tetraalkylammonium salt solutions

Nanostructure domains, voids, and low-frequency spectra in binary mixtures of N,N-dimethylacetamide and ionic liquids with varying cationic size

Classical molecular dynamics (MD) simulations were carried out on binary mixtures of N,N-dimethylacetamide (DMA) with hydroxide based ammonium ionic liquids (ILs), tetraethylammonium hydroxide (TEAH), tetrapropylammonium hydroxide (TPAH), tetrabutylammonium hydroxide (TBAH), at three different mole fractions of IL (X IL). The solvation of DMA molecules by ions of ILs was studied by the combined distribution function (CDF). CDFs show that anions have strong correlations with the DMA due to the hydrogen bonding. Increasing the DMA disrupts the nanosegregated domains and causes changes in correlations of cation-DMA and anion-DMA. Also, increased translational motion of ions, as well as the fluidity of IL and a significant improvement in self-diffusion coefficients, are observed with the presence of more DMA. The structural microheterogeneity was investigated using the Voronoi tessellation method. Domain analysis confirms the formation of discreet domains by anions at all the mole fractions. The results also complement the experimental observations, which suggest that two types of aggregations are possible in given mixtures: below and above 0.5 X IL. When the alkyl chain length on the cation increases, a notable decrease in ion translational motion was observed in the IL rich region. In the concentrated IL mixture, the self-diffusion coefficient of the cation is higher than that of the corresponding anion; further addition of IL (X IL < 0.5) results in weaker interactions between DMA and anion when compared to DMA-cation. The mean collision time of each species is found to have an inverse relation with X IL. The analysis of the vibrational density of states provides the low-frequency spectral feature of the mixtures.

Aqueous solutions of tetraalkylammonium halides: ion hydration, dynamics and ion-ion interactions in light of steric effects

Molecular simulations have allowed us to probe the atomic details of aqueous solutions of tetramethylammonium (TMA) and tetrabutylammonium (TBA) bromide, across a wide range of concentrations (0.5 to 3-4 molal). We highlight the space-filling (TMA(+)) versus penetrable (TBA(+)) nature of these polyatomic cations and its consequence for ion hydration, ion dynamics and ion-ion interactions. A well-established hydration is seen for both TMA(+) and TBA(+) throughout the concentration range studied. A clear penetration of water molecules, as well as counterions, between the hydrocarbon arms of TBA(+), which remain in an extended configuration, is seen. Global rotation of individual TBA(+) points towards isolated rather than aggregated ions (from dilute up to 1 m concentration). Only for highly concentrated solutions, in which inter-penetration of adjacent TBA(+)s cannot be avoided, does the rotational time increase dramatically. From both structural and dynamic data we conclude that there is absence of hydrophobicity-driven cation-cation aggregation in both TMABr and TBABr solutions studied. The link between these real systems and the theoretical predictions for spherical hydrophobic solutes of varying size does not seem straightforward.

Raman spectroscopic study on the hydration structures of tetraethylammonium cation in water

Conformational changes of tetraethylammonium ion (Et4N+) in aqueous solution have been studied by Raman spectroscopy as functions of pressure and concentration. The difference in the partial molar volume (Delta V(tg x tg-->tt x tt)) between the trans-gauche x trans-gauche (tg x tg) and trans-trans x trans-trans (tt x tt) conformers of Et4N+ ion has been calculated from the pressure dependence of the relative Raman intensity ratio between the conformers. We discuss about the Delta V(tg x tg-->tt x tt) with two contributions from the molecular and hydration volumes. Delta V(tg x tg-->tt x tt) is found to be negative, and this is mainly due to the large molecular volume contribution. The value of Delta V(tg x tg-->tt x tt) becomes smaller with increasing R probably due to the hydration volume contribution. In view of the pressure and concentration dependences, the water molecules around Et4N+ ions at R = 19 (R= moles of water/moles of salt) mainly form the hydrophobic hydration. The hydrophobic hydration may prefer the tt x tt conformer to the tg x tg conformer. On the other hand, the hydration structure at R = 4 no longer forms the hydrophobic hydration and changes to another type of hydration, where the medium H-bond component is the dominant species. The increase of the ion-ion interactions with decreasing R induces large fractions of the tg x tg conformer.

Solubility of sodium soaps in aqueous salt solutions

The solubility of sodium soaps in dilute aqueous salt solutions has been systematically investigated by direct visual phase behavior observations. The added electrolytes, including simple inorganic salts and bulky organic salts, influence the solubility of sodium soaps in water, as represented by the varied soap Krafft point. Two inorganic salts, sodium chloride and sodium perchlorate, demonstrate a "salting-out" property. On the other hand, tetraalkylammonium bromides show an excellent ability to depress the soap Krafft point and enhance the soap solubility in water. With increasing the tetraalkylammonium ionic size, the degree of "salting-in" of soaps in water increases. However, solubility of pure tetraalkylammonium bromide in water decreases as the length of the alkyl chains increases. Furthermore, in the ternary water-tetrapentylammonium bromide (TPeAB)-sodium myristate (NaMy) system, we observed an upper cloud point phenomenon, which greatly shrinks the 1-phase micellar solution region in the phase diagram. This miscibility gap, together with the organic salt solubility limitation, restricts the use of tetraalkylammonium bromides with alkyl chains longer than 4 carbon atoms as effective soap solubility enhancement electrolytes. We also found that for sodium soap with a longer hydrocarbon chain, more tetrabutylammonium salt is required to reduce the soap Krafft point to room temperature.