Effect of Adding Sodium Chloride on Tetra-n-butylammonium Chloride Semiclathrate Thermal Stability at Atmospheric Pressure

Abstract

Tetra-n-butyl ammonium chloride (TBAC) is known to form semiclathrate cages when dissolved in water. Melting points determined using differential scanning calorimetry (DSC) were used to create a phase diagram for the binary TBAC−H 2 O semiclathrate system from 0 to 8 molal TBAC at atmospheric pressure. This phase diagram shows the existence of TBAC semiclathrate cages with a congruent melting point at about 286.8 K at approximately 2 molal TBAC as well as the existence of a TBAC−H 2 O eutectic mixture of unknown composition but with fewer associated water molecules at higher concentrations of TBAC. Further, NaCl was added to this binary system to create ternary solutions of various ionic strength fractions of TBAC (Y TBAC ). The results of DSC scans were used to create a phase diagram which showed that there was no effect on the system if there was sufficient bulk water to dissolve the NaCl. However, if there was insufficient bulk water to dissolve the NaCl, then the water in the clathrate cages of the TBAC was disrupted creating both lower melting semiclathrates, with fewer water molecules within the cage, and a new TBAC−NaCl−H 2 O peritectic structure of unknown composition with an invariant melting point. Further, if NaCl was added to the binary system with no free, bulk water present (i.e., ≥ 2 molal TBAC), then this system additionally showed a new eutectic TBAC−NaCl−H 2 O structure of unknown composition with an invariant melting point.

Effect of Adding Potassium Chloride on Tetra-n-butyl Ammonium Chloride Semiclathrate Thermal Stability at Atmospheric Pressure

“…The DSC cooling thermograms for binary solutions of TBAC-H 2 O were obtained at concentrations ranging from 0 to approximately 7.775 molal TBAC, respectively (data not shown). Similar to the results of Siddiq et al, 23 the cooling thermograms showed that pure water does not have a normal freezing transition at 273.15 K. Instead, sharp freezing transitions which occurred at various temperatures near ∼251.5 K were observed. They explained this as a rapid and sudden transition due to the freezing of ultrapure, supercooled, free, bulk water.…”

“…Determination of the Purity TBAC by Precipitation with AgNO 3 . Gravimetric silver chloride analysis was utilized to determine the purity of TBAC by precipitation of AgCl of the dissolved TBAC samples using AgNO 3 using the same technique by Siddiq et al 23 2.4. Determination of the Concentration of the TBAC Stock Solution.…”

“…Such data should show how KCl affects the formation and stability of these semiclathrate complexes. These results will then be compared to those found by Siddiq et al 23 upon adding NaCl to the TBAC-H 2 O system.…”

“…It was proposed that an equilibrium exists between the fully hydrated TBAC complexes with one or more TBAC semiclathrate complexes with fewer water molecules per TBAC molecule that are less thermally stable. 23 Some thermodynamic studies have been performed involving the addition of electrolytes to binary aqueous systems to observe the effect of these electrolytes on the stability of semiclathrate complexes. 3 In industry, due to cost effects, it is unlikely that these semiclathrates will be prepared using pure water; it is more likely that the water used will contain impurities, such as simple electrolytes, which may affect the structure and stability of the semiclathrates.…”

“…However, at greater concentrations of TBAC, where there is no free, bulk water present, the addition of NaCl either disrupted the formation of the semiclathrate or formed a new semiclathrate complex that included the NaCl. 23 In this paper, to better understand how simple electrolytes will effect the TBAC-H 2 O semiclathrate system, we will study the effect of adding KCl on the formation and stability of the TBAC-H 2 O semiclathrate structures and compare it to the reported effects of adding NaCl. It is hypothesized that the potassium ion, having a larger surface-to-charge density than the sodium ion, will not interact as strongly with water molecules of the semiclathrate cage.…”