POLY(ETHYLENE GLYCOL) AMPHIPHILES - SURFACE BEHAVIOR OF BIOTECHNICAL SIGNIFICANCE

The interfacial behavior of surface localized, poly(ethylene glycol) ( PEG) esterified fatty acid amphiphiles was comparatively studied at mi croparticle surfaces via phase partition and at flat surfaces via in s itu null ellipsometry. Ellipsometry was performed on methylsilane (MeS i), phosphatidic acid (PA), and phosphatidylcholine (PC) coated silica slides, while human erythrocyte and PC liposomes were subjected to pa rtition in aqueous PEG, dextran two-phase systems. Analogous results f rom both methods suggest that PEG-amphiphile adsorption can be relativ ely independent of the underlying surface. Ellipsometry indicated that members of a series of PEG-fatty acid esters of the type Ci:jEO151 (1 6 less than or equal to i less than or equal to 18, 0 less than or equ al to j less than or equal to 2) adsorb similarly at MeSi-, PC-, and P A-coated surfaces, reaching a plateau (approximate to 0.1 PEG chains.n m(-2)) independent of micelle formation. When normalized for acyl tail hydrophobicity, PEG-amphiphile adsorption is relatively noncooperativ e and independent of the acyl tail; saturation is largely determined b y repulsive PEG interchain interactions. At saturation, EO151-ester la yers were 10-15 nm thick, suggesting close packed PEG molecules unfold ed normal to the target surface. Ellipsometry also indicated the avera ge PEG concentration in the layer was approximate to 0.07 g.cm(-3), an d greater than in the PEG-enriched phase of many two-phase systems. Th is suggests that the partition of PEG-coated colloids reflects interfa cial free energy differences between solution- and surface-localized, polymer-enriched phases. PEG-ester adsorption (mg.m(-2)) isotherms det ermined on flat slides via ellipsometry correlate directly with those physiological behavior of bioactive colloids. These results help expla in previous observations PEG mirror the effects of such localization o n the physiological behavior of bioactive colloids.