Effect of Structural Differences in the Hydrophobic Alkyl Backbone and Functional Groups on the Effectiveness of Solvents for Water Desalination

Previous work from Bajpayee, Luo, Muto and Chen¹, demonstrated a novel water desalination method, directional solvent extraction (DSE), utilizing decanoic acid as the directional solvent (DS). DSE involves the direct extraction of pure water from a saline solution into DS at a moderately high temperature (~40°C -80°C) in which salts are insoluble, followed by recovery of the dissolved pure water from DS at a lower temperature by phase separation. Efficiency of this process requires that, the solubility of DS in the saline solution and in the resulting recovered pure water be negligible. The amphiphilic molecular structure of decanoic acid provides the properties of a suitable DS, which includes a polar and hydrophilic carboxylic acid group, which facilitates association with water, as well as a non-polar alkyl chain, which allows for the rejection of ions as well as the insolubility of the solvent in saline and pure water.

The objective in this project is to identify a DS with a 3X increase in effectiveness when compared to decanoic acid, which can make the DSE process more energetically efficient than a reverse osmosis process. Effectiveness is defined as the mass of pure water extracted per mass of directional solvent used in a DSE cycle and is calculated as a percentage. When using the temperature pair of 70°C and 34°C in a DSE cycle, decanoic acid has an effectiveness of approximately 0.9% to 1.7%. Our hypothesis is that effectiveness, as well as the solubility of water and salts in DS, and the solubility of DS in saline solution and pure water is tunable through varying the molecular structure of DS. As the first step, we performed a literature survey and analysis of existing solubility data of organic compounds with water, to identify key molecular factors that lead to DSs with high water extraction efficiency. Specifically, we compiled data for 28 ketones, 21 alcohols and 14 amines with varying –CH2– backbones. The data includes the mutual solubilities of the organic solvents with pure water, over the temperature range of approximately 0°C-90°C. We converted the solubility data of pure water in bulk organic solvent to mass ratios, to calculate DS effectiveness for the temperature pair of 70°C and 30°C. In addition, we converted the solubility data of organic solvent in bulk pure water to parts per million at 70°C and 30°C. Allowing for the convenient analysis of the performance of the selected solvents in a DSE cycle.

The analysis performed supports a general trend, for organic compounds with an alcohol or ketone functional group and a non-polar component (which may be a normal alkyl chain, branched alkyl chain, or cycloalkyl group), that as temperature increases the solubility of water in an organic solvent increases. Conversely, with amines, we observe that as temperature increases the solubility of water in the organic bulk decreases. Thus, desalination with amines may be possible by reversing the DSE cycle. This is an encouraging result as this suggests that the type of behavior desired of a suitable DS is not unique to decanoic acid, but rather, very common to amphiphilic solvents. Furthermore, this study supports our central hypothesis that effectiveness, as well as the solubilities of water and salts in DS, and the solubility of DS in water is tuneable by varying the molecular structure of DS. This analysis concludes that, to minimize the solubility of a DS in bulk pure water, the optimal structure of the hydrophobic backbone is an n-alkyl chain. In addition, we conclude that the optimal hydrophilic functional group position is on the terminal carbon of a DS. Since alcohols show approximately a 3X greater effectiveness than ketones with similar structures, we hypothesize, that to increase the effectiveness of a DS, the hydrophilicity of the polar component of a DS must be increased. For example, we expect that replacing the carbonyl group on the base structure of decanoic acid with an alcohol, as with the structure of 1,1-decanediol, will result in a DS with greater effectiveness than decanoic acid while maintaining low solubility in pure water.

1. Bajpayee, A., Luo, T. F., Muto, A. & Chen, G. Very low temperature membrane-free desalination by directional solvent extraction. Energ Environ Sci 4, 1672-1675 (2011).