Solubilization of Aldehydes and Amines in Aqueous C_iE_j Surfactant Aggregates: Solubilization Capacity and Aggregate Properties

Abstract

Hydroformylation of olefins to aldehydes and subsequent reductive amination of aldehydes to amines takes place in an aqueous system using a water-soluble catalyst. It is limited to short-chain molecules due to an insufficient solubility of long-chain molecules in water. A promising approach to increase the solubility of long-chain aldehydes and amines is the addition of surfactants to the aqueous phase. In this work, we thus determined the solubilization capacity (SC) of different nonionic C i E j surfactants (C 8 E 6 , C 10 E 6 , and C 10 E 8 ) toward long-chain aldehydes and amines. We used static and dynamic light scattering techniques to investigate the influence of both the surfactant and solute molecular structures on the SC as well as on the aggregation number (N agg ) and hydrodynamic radius (R h ) of mixed aggregates. Our data reveals that an optimum ratio of hydrophobic to hydrophilic chain length of C i E j surfactants exists where the SC toward long-chain aldehydes and amines possesses a maximum. Further, the size of the aggregates (N agg , R h ) passes through a minimum upon amine solubilization, while upon aldehyde solubilization, the aggregate size increases gradually. The results shown in this work give valuable insights to the solubilization of aldehydes and n-amines into nonionic C i E j surfactants and facilitate the search of suitable surfactants for hydroformylation and reductive amination as "green" solvents based on the detailed knowledge about the aggregate structure.

Influence of Hydrophobic and Hydrophilic Chain Length of CiEj Surfactants on the Solubilization of Active Pharmaceutical Ingredients

“…However, the fact that LID/C i E j surfactant aggregates are shrinking at low LID content and growing at high LID content suggests that different mechanisms of LID solubilization in LID/C i E j surfactant aggregates exist. We showed in a previous work that N agg of C i E j surfactant aggregates upon solubilization of long chain alkylamines also shows a minimum with increasing solute content . Nitrogen atoms are known to form strong hydrogen bonds with water .…”

“…API/surfactant aggregate properties were measured using composition gradient multiangle light scattering (CG-MALS). The CG-MALS setup was explained in detail elsewhere, , and only a brief description is given here. A pump and dosing unit (Calypso II), an SLS detector (DAWN HELEOS 8 + ), a DLS detector (DynaPro NanoStar), and a refractive index (RI) detector from Wyatt Technology Corporation (Santa Barbara, CA, USA) were used to measure N agg and R h simultaneously.…”

“…N agg of the surfactant molecules and the number of API molecules in an API/surfactant aggregate were obtained from SLS data. A detailed description of the used method can be found in a previous work . The amount of API and surfactant in the aqueous bulk phase were determined from SC measurements as described in Section .…”

“…A detailed description of the used method can be found in a previous work. 24 The amount of API and surfactant in the aqueous bulk phase were determined from SC measurements as described in Section 2.3. Refractive index increments of the APIs in the respective surfactant solutions (needed to obtain N agg from SLS measurements) were measured using the RI detector and are given in Table S1 in the Supporting Information, while refractive index increments of the surfactants in water were taken from a previous work.…”

“…We showed in a previous work that N agg of C i E j surfactant aggregates upon solubilization of long chain alkylamines also shows a minimum with increasing solute content. 24 Nitrogen atoms are known to form strong hydrogen bonds with water. 47 LID possesses two nitrogen atoms in its molecular structure, and therefore, the affinity of these nitrogen atoms toward water leads to an incorporation of LID at the API/surfactant aggregate/water interface.…”