Rheological properties of lipopolymer-phospholipid mixtures at the air-water interface: A novel form of two-dimensional physical gelation

Recent surface theology and film balance experiments on monolayers of PEG l ipopolymers at the air-water interface showed that the PEG chains are able to form a quasi-two-dimensional physical polymer network if forced into a h ighly stretched brushlike configuration. To obtain a deeper understanding o f the complex film balance and theological transition behavior of lipopolym ers, we performed surface theology and film balance experiments on phosphol ipid (DMPC: 1,2-dimyristoyl-sn-glycero-3-phosphati-dylcholine)/PEG lipopoly mer (DSPE-PEG2000: 1;2-distearoyl-sn-glycero-3-phosphoethanolamine [poly-et hylene glycol) 2000]) mixtures at the air-water interface. We found that th e high-film-pressure transition observed between 40 and 100 mol % lipopolym er at about 20 mN/m, which is related to a first-order-like alkyl chain con densation, is a necessary requirement for the existence of a theological tr ansition. While the theological transition appeared at a specific area per lipopolymer of 165 Angstrom (2), thereby being independent of the amount of phospholipids incorporated, the area per lipopolymer at the high-film-pres sure transition clearly depends on the lipopolymer-phospholipid molar conce ntration. Our data clearly support the Flory model of physical gelation, wh ich predicts no thermodynamic transition at the gel point, because the isot hermal compressibility and its derivative show no discontinuity at this pai nt. The pi -A isothermal behavior at the high-film-pressure transition of t he phospholipid/lipopolymer mixtures can be interpreted if we assume that m icrophase separation occurs between phospholipids and lipopolymers. Our dat a indicate that the two-dimensional physical network of lipopolymers is for med by two different kinds of associative interactions: (1) microcondensati on of alkyl chains of lipopolymers to small clusters; (2) water molecule me diation of the interaction between adjacent PEG clusters via hydrogen bondi ng.