Polymeric alkenoxy amino acid surfactants: IV. Effects of hydrophobic chain length and degree of polymerization of molecular micelles on chiral separation of β-blockers

Abstract

Four alkenoxy leucine-based surfactants with C8-C11 chains containing a terminal double bond, and one C11 chain surfactant with a terminal triple bond are synthesized and characterized in monomeric and polymeric forms. These polymeric pseudophases are then utilized to study the influence of chain length and DP for the enantioseparations of seven beta-blockers in MEKC. Variations in chain length and concentration of polymeric surfactants showed significant effects on the chiral resolution (Rs) and efficiency (N). A relatively large elution range combined with the highest polarity and aggregation number (A) but the lowest retention time, partial specific volume, and optical rotation generated with C8-polymeric surfactant results in simultaneous enantioseparation of all seven beta-blockers with higher N and R(s). In particular, highly hydrophobic beta-blockers are better resolved with shorter hydrocarbon chain even at higher surfactant concentration, which is unachievable with longer chain surfactant. On the other hand, polymer derived from C11-triple bond provided smaller A value compared to C11-double bond surfactant. However, chiral Rs of hydrophobic beta-blockers are still achievable with the C11-triple bond surfactant with enhanced N and shorter analysis time. In addition, effect of polymerization concentration is evaluated by polymerizing all five surfactants at five times their respective CMCs and 100 mM equivalent monomer concentrations. Polymerization of shorter chain (C8 and C9) double-bonded surfactants at five times their respective CMCs results in higher A values with better chiral Rs and N compared to the same two surfactants polymerized at 100 mM.

Review of aqueous chiral electrokinetic chromatography (EKC) with an emphasis on chiral microemulsion EKC

“…Rizvi and Shamsi [48] conducted another set of experiments using micelle polymers with a focus on alkyl chain length and degree of polymerization (DP); the analytes were again b-blocker enantiomers. Chiral surfactants with chain lengths of 8, 9, 10, and 11 (C11 monomers with a terminal double bond and a terminal triple bond) were polymerized to form micelles.…”

Recent progress in the use of ionic polymers as pseudostationary phases for EKC

“…N-Alkenoxy carbonyl leucine surfactants with C 8 -C 11 chains, as well as N-undecynoxy carbonyl leucine with a C 11 chain and a terminal triple bond, were synthesized, polymerized, characterized, and evaluated as PSPs for the chiral separation of seven b-blockers of varied hydrophobicity [44]. Predictably, the CMC, aggregation numbers, and polarity of the micelles increased as the chain length decreased.…”

“…Polymers at 25 mM equivalent monomer concentration in 25 mM ammonium acetate/25 mM triethyl amine at pH 8.8. Reprinted with permission from [44].…”

Polymeric alkenoxy amino acid surfactants: V. Comparison of carboxylate and sulfate head group polymeric surfactants for enantioseparation in MEKC

“…First, is to provide a thorough comparison of physicochemical properties (CMC, aggregation number (A), polarity, optical rotation, and partial specific volume) of the monomers and polymers of two classes of amino acid based surfactants bearing carboxylate and sulfate head groups recently synthesized in our laboratory [30,33,35]. In particular, state of the art cryogenic high-resolution scanning electron microscopy (cryo-HRSEM) is performed to investigate the aqueous phase behavior of one representative anionic polymeric surfactant from each class.…”

“…

In this work, six amino acid derived (L-leucinol, L-leucine, L-isoleucinol, L-isoleucine, L-valinol, and L-valine) polymeric chiral surfactants with carboxylate and sulfate head groups that were recently synthesized in our laboratory [30,33,35] are compared for the simultaneous enantioseparation of several groups of structurally similar analytes under neutral and basic pH conditions. The physicochemical properties of the monomers and polymers of both classes of sulfated and carboxylated surfactants are compared.

…”