Investigation of interactions in binary and ternary systems using various techniques

Abstract

Investigation of 1-butylamine(BA)+water(W) mixtures (the system) is almost lacking in the literature. This work deals with systematic study of the system and its solutions of alkali metal bromides using various techniques. Both the components of the system have two labile hydrogens each on the same atom of their molecules and are mutually miscible in all proportions. The system also exhibits good solubility for various electrolytes; bromides of four different alkali metals have been selected for the study so that (with all other conditions same) any ‘change’ of the solution property on varying the electrolyte may be attributed to the size and other related parameters of the cation. Measurements of density, viscosity and electrical conductivity have been carried out by systematically varying composition of the system, concentration of the electrolytes and temperature, over the respective suitable ranges at appropriate intervals while dielectric relaxation (DR) spectroscopy was carried out only at 25 oC. The collected data have been interpreted and analysed according to the corresponding pertinent models and schemes. Density measurement on the system has revealed non-ideal mixing which is further substantiated by the isotherms of viscosity (ηmix), excess viscosity (ηE) and excess Gibbs free energy of activation of flow (G*E) as well as the plot of activation energy of viscous flow (Ea); all of them exhibit maxima at a common xBA=0.2. It has thus been concluded that maximum (internal) structure prevails in the system at xBA=0.2 plausibly due to replacement of weaker BA-BA H-bonds by the relatively stronger BA-W ones as well as due to further strengthening of W-W H-bonds caused by the hydrophobic interaction; it has been also inferred that this composition corresponds to ‘optimum H-bonding’ in terms of number and quality both. Same position of the viscosity maximum for different isotherms indicates that no ‘significant’ structural change occurs in the system by changing the temperature (at least within the investigated range). Overall stronger becoming of the intermolecular interactions (IMI) in the system is also indicated from negative excess molar volume (VE) over the entire composition range. The above noted excess quantities were fit to the Redlich-Kister equation to determine the corresponding coefficients. The mixtures were subjected to broadband DR measurement over the frequency range of (0.2 ≤ ν ≤ 89)/GHz. Since complete dielectric loss was not observed upto 89 GHz, the measurements for some selected mixtures were extended to 2.4 THz at which the complete loss could be achieved. The DR spectra were fit to various pertinent models; spectrum of BA got resolved into only two relaxation modes about 50 GHz and 1 THz while the system exhibit an additional mode at 10 GHz indicating (H-bonded) iii association amongst the BA molecules besides co-operative association between the BA and W. Analyses of the relaxation amplitudes have revealed that the ‘effective hydration number’ of BA molecules depends on the mixture composition starting from ~1.5 at xBA=0.03 down to 0.05 at xBA=0.80 thereby suggesting that water molecules are mutually shared by BA when the latter is in excess. Similarly ‘effective dipole moment’ determined for the system varied from 4.68 D at xBA=0.03 to 1.33 D at xBA=0.90 (the reported values for neat water and BA being 2.39 D and 1.75 D, respectively). All the solutions exhibit almost linear increase of density with the electrolyte molality (m) and the slope furnishes ‘density index’ [gρ(x1)] which can be used to quite accurately predict solution density at any mixture composition and m. The g-values follow the sequence: gρ(CsBr) ˃ gρ(KBr) ˃ gρ(NaBr) ˃ gρ(LiBr); partial molar volumes of the electrolytes at infinite dilution ( ϕ V) also follow the same sequence. Viscosity (η) of solutions generally increased with m at all the compositions and temperatures; the exceptions being solutions of KBr and CsBr in (water-rich) mixtures at xBA=0.1 and 0.2. Generally the η–m isotherms were linear and the corresponding ‘viscosity index’ [gη(x1,T)] shows dependence on temperature as well. Like the neat mixtures, each η–xBA isotherm also tends to pass through maximum at xBA=0.2 indicating that the maximum structure was maintained by the mixture upon added electrolyte. Application of a ‘modified Jones-Dole equation’ to the solutions has revealed that KBr and CsBr act as structure-breakers for the more structured (water-rich) mixtures having xBA=0.1 and 0.2; as the structure of the two mixtures is successively made to destroy by increasing temperature, the structure-breaking ability of the two electrolytes also diminished. Ea determined from the temperature-dependent viscosity measurement were quite comparable to the mixture values; Ea–xBA isotherms for all the solutions also exhibit maxima at x1≈ 0.2. All the solutions exhibit increase of electrical conductivity (κ) of the solutions with m and temperature (κ–m isotherms are linear). For a given set of conditions, both KBr and CsBr solutions exhibited quite high and comparable values of κ while LiBr solutions show the lowest values; a plausible explanation is the structure-breaking behaviour of K+ and Cs+ which tend to decrease the so-called micro-viscosity in the vicinity of their rather lesser compact solvates. Variation of κ with temperature change has been correlated with ‘thermal co-efficient of conductivity’ () which was almost independent of the electrolyte and its m but changed with composition exhibiting maximum value at xBA=0.2. For an electrolyte the molar conductivity at infinite dilution (o) changed with both xBA and T; higher values of the Walden product (WP ≡o×ηmix) were found for the water rich region. Under a given set of iv condition the WP follows the sequence: of WP(Li+) < WP(Na+) < WP(K+) ≈ WP(Cs+). From the κ–T-1 plots, values of the corresponding ‘conductivity activation energy’ (Ea) have been determined; the Ea−xBA plots tend to pass through maximum at xBA=0.2. All the electrolyte solutions were subjected to DR measurement within the frequency range from 0.2 to 89 GHz. Symmetrical DR spectra could be adequately explained by a single Cole-Cole (CC) model which furnished time of relaxation (τ), ‘amplitude’ (S) of the relaxing species and relative permittivities (εj). In a given mixture τ increased with m whereas the τ–xBA plots tend to pass through maximum at xBA ≈ 0.2; similar behaviours were also shown by η of the solutions. Thus enough evidence has become available from the study to conclude that BA+W is highly associated system having maximum ‘association & structure’ at xBA=0.2, a mixture composition that corresponds to four W molecules per BA molecule.