Aqueous biphasic systems composed of alcohol-based deep eutectic solvents and inorganic salts: Application in the extraction of dyes with varying hydrophobicity

The scientific community discovered green solvents for the preconcentration and separation of various target compounds owing to challenges posed by the earliest solvents. Among these neoteric solvents are deep eutectic solvents (DESs). They have been termed “21st-century solvents” due to their distinct characteristic properties like low toxicity, high tuneability, high biodegradability, cheap nature, ease of preparation, etc. In this study, six novel DESs were synthesized from benzyltrimethylammonium chloride (BTMAC), benzyltrimethylammonium chloride (BTEAC) as hydrogen bond acceptors (HBAs), methanol (MEOH), ethanol (ETOH) and propanol (PROH) as hydrogen bond donors (HBDs) at 1:3 molar ratio. The prepared DESs were employed for the development of aqueous biphasic systems (ABSs) in combination with Na2CO3, K3PO4, K2HPO4, and K2CO3 at 298.15 K and atmospheric pressure. The salting out ability of the salts investigated follows the decreasing order: Na2CO3 > K3PO4 > K2HPO4 ≈ K2CO3. The DES based extraction platforms were applied for the partitioning of Alizarin red S (ARS), Orange II (OII), and Congo red (CR). In all the studied ABSs, the partition coefficients (Kdyes), and extraction efficiencies (%EEdyes) for the target dyes followed the sequence: CR > OII > ARS. The obtained trend was attributed to log Kow and structural differences of the investigated dyes (i.e. presence of different aromatic groups). The proposed ABSs of DES have great potential for removing various dyes completely from aqueous media.