A promising system of mixed single- and double-short-tailed PEO ether phosphate esters: Phase behavior and vesicle formation

Abstract

Acidic surfactants, single- and bi-2-methylheptanol polyethenoxy ether phosphate esters, H 2PO 3(OCH 2CH 2) n OCH 2CH 2CH 2CH 2CH 2CH(CH 3) 2 (u-MHPEPE) and HPO 3[(OCH 2CH 2) n OCH 2CH 2CH 2CH 2CH 2CH(CH 3) 2] 2 (d-MHPEPE), where n ≈ 4 , were synthesized. Phase behavior of u- and d-MHPEPE (u- and d-MHPEPE mixtures were abbreviated as MHPEPE) mixtures in aqueous solutions and vesicle formation were determined. Surface tension measurements showed that u-MHPEPE and MHPEPE have low surface tensions at critical micelle concentrations. γ cmc = 29.0 mN m −1 and cmc = 16.0 mmol L −1 for u-MHPEPE, MHPEPE has two transition points suggesting the mixtures of u- and d-MHPEPE with γ cmc1 = 30.5 mN m −1 and cmc1 = 4.0 mmol L −1 , and γ cmc2 = 27.3 mN m −1 and cmc2 = 42.0 mmol L −1 . These values, specific γ cmc , are much lower than those of traditionally cationic or anionic surfactants such as cetyltrimethylammonium bromide (CTAB, γ cmc = 37.1 mN m −1 at cmc = 0.92 mmol L −1 ) and sodium dodecyl sulfate (SDS, γ cmc = 39.0 mN m −1 at cmc = 8.1 mmol L −1 ). Rich phase behavior was observed with increasing MHPEPE concentration, an isotropic L 1-phase (micelle solution), an unstable emulsion-region (with time, the samples separate into two-phase), a transparently bluish and birefringent L α-phase up to 200 mmol L −1 with unilamellar and multilamellar vesicles. These unilamellar and multilamellar vesicles were demonstrated by using staining transmission electron microscopy (staining-TEM), which were compared to freeze-fracture TEM (FF-TEM). The vesicle-phase is stable for at least 1 year. Vesicle formation possibly could be explained in harmonization of the hydrophobic force of acidic surfactant tails, the hydrogen bonding (H-bonding) and the electrostatic interaction among polar headgroups of PEO ether phosphate ester. Phase transition from the flow birefringent unilamellar vesicles induced by addition of HCl, NaCl, NaOH, and increasing temperature has been observed. Surprisingly, for u-MHPEPE or d-MHPEPE in water, we just observed L 1-phase (micelle solution) with increasing u-MHPEPE or d-MHPEPE concentration.