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

Acidic surfactants, single- and bi-2-methylheptanol polyethenoxy ether phosphate esters, H2PO3(OCH2CH2)nOCH2CH2CH2CH2CH2CH(CH3)2 (u-MHPEPE) and HPO3[(OCH2CH2)nOCH2CH2CH2CH2CH2CH(CH3)2]2 (d-MHPEPE), where n≈4n≈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. γcmc=29.0 mNm−1 and cmc=16.0 mmolL−1 for u-MHPEPE, MHPEPE has two transition points suggesting the mixtures of u- and d-MHPEPE with γcmc1=30.5 mNm−1 and cmc1=4.0 mmolL−1, and γcmc2=27.3 mNm−1 and cmc2=42.0 mmolL−1. These values, specific γcmcγcmc, are much lower than those of traditionally cationic or anionic surfactants such as cetyltrimethylammonium bromide (CTAB, γcmc=37.1 mNm−1 at cmc=0.92 mmolL−1) and sodium dodecyl sulfate (SDS, γcmc=39.0 mNm−1 at cmc=8.1 mmolL−1). Rich phase behavior was observed with increasing MHPEPE concentration, an isotropic L1-phase (micelle solution), an unstable emulsion-region (with time, the samples separate into two-phase), a transparently bluish and birefringent Lα-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 L1-phase (micelle solution) with increasing u-MHPEPE or d-MHPEPE concentration.

Graphical abstractA typical FF-TEM image of multilamellar vesicles, the multilamellar vesicles are polydisperse in size. The interlamellar spacing between the bilayers is rather uniform and in the range of about 500 Å.Figure optionsDownload full-size imageDownload as PowerPoint slide