Concentration dependent speciation and mass transport properties of switchable polarity solvents

Abstract

Tertiary amine switchable polarity solvents (SPS) consisting of predominantly water, tertiary amine, and tertiary ammonium and bicarbonate ions were produced at various concentrations for three different amines: N,N-dimethylcyclohexylamine, N,N-dimethyloctylamine, and 1-cyclohexylpiperidine. These amines exhibit either osmotic or non-osmotic character as observed through forward osmosis, which led to this study to better understand speciation and its influence on water transport through a semipermeable membrane. For all concentrations, several physical properties were measured including viscosity, molecular diffusion coefficients, freezing point depression, and density. Based on these measurements, a variation on the Mark-Houwink equation was developed to predict the viscosity of any tertiary amine SPS as a function of concentration using the amine's molecular mass. The physical properties of osmotic SPS, which are identified as having an amine to carbonic acid salt ratio of $1 : 1, have consistent concentration dependence behavior over a wide range of concentrations, which suggests osmotic pressures based on low concentrations freezing point studies can be extrapolated reliably to higher concentrations. The observed physical properties also allowed the identification of solution state speciation of non-osmotic SPS, where the amine to carbonic acid salts ratio is significantly greater than one. These results indicate that, at most concentrations, the stoichiometric excess of amine is involved in solvating a proton with two amines.

1-Cyclohexylpiperidine as a thermolytic draw solute for osmotically driven membrane processes

“…This concentration correlates with the SPS's maximum osmotic pressure [17,23]. The QSAR model indicated the value of rings systems in the function of SPS but appears to have understated their importance.…”

“…This is very similar to the~8.5 kg/m 2 h water flux produced by 5 mol/kg polar CHP draw solution under the same conditions. The reduced flux of CHP relative to DMCHA was expected based on the relative mass transport properties of CHP and DMCHA [23]. While CHP provides less flux its other advantages in terms of better degassing performance, improved material compatibility, and reduced volatility more than makes up for a modest reduction in flux.…”

“…After~48 h the solution reached equilibrium with approximately 10 ml of organic material unreacted, the purge was continued for an addition~48 h to ensure the equilibrium had been met. The product aqueous layer was a yellow clear homogenous solution with a mass of 491.44 g whose material properties have been previously published [23]. Similar stock solutions were then diluted to the desired concentration and concentration validated with freezing point depression osmometry.…”

Energy requirements of the switchable polarity solvent forward osmosis (SPS-FO) water purification process

“…Additionally, lower concentration draw solutions will result in a lower water carrying capacity and increased circulation of water in the SPS FO primary process loop, which would have the effect of increasing SPS FO process costs and energy requirements as a result of increased process equipment size, pumping requirements, as well as fluid heating and cooling requirements. High concentration solutions suffer from higher viscosities [16], increasing pumping costs, and reduced mass transfer properties hindering flux kinetics given the thermodynamic driving force. Further experiments are required to find the optimal balance between high-concentration high-osmotic pressure high-carrying capacity and reduced viscosity and better mass transport properties.…”

“…Our initial demonstration of the SPS FO process used dimethylcyclohexylamine (DMCA) [4]. While DMCA is a potential SPS draw solute an SPS optimized for SPS FO process has been sought [15][16][17][18] and suggest 1-cyclohexylpiperidine is an especially promising candidate. Regardless of the SPS employed in the process DMCA is an excellent model SPS and was used to develop the process energy model.…”