ABSTRACT
The interaction of water-soluble surfactants with solid polymeric or mineral sur faces has been intensively and extensively studied [1-3]. However, the potential of bilayer-forming amphiphiles as interface agents able to modify solid surfaces remains hitherto poorly explored. Bilayer-forming amphiphiles assemble on solid particles or planar surfaces either as a bilayer, a monolayer, or adhered vesicles, de pending on the nature of the amphiphile and surface and on the interactions driving amphiphile deposition [4-16]. Bilayer deposition of dioctadecyldimethylammonium bromide (DODAB) and sodium dihexadecylphosphate (DHP) from vesicles onto oppositely charged polystyrene microspheres has been reported [4]. There is
electrostatically-driven vesicle adhesion to the latex that is followed by bilayer de position [5]. Thereafter, as the amphiphile concentration increases, vesicles adhere to the bilayer-covered latex with or without disruption, depending on the nature of the synthetic amphiphile [5], In general, the interaction between bilayer vesicles and solid surfaces in the form of particulates [6-15] or planar surfaces [16, 17] still lacks characterization from the point of view of exact physical parameters. Re cently, the physical adsorption of bilayer-forming amphiphiles on hydrophilic silica, in particular, was shown to depend on previous centrifugation of the vesicle sample, the pH, the buffer, the temperature, and the physical state of the bilayer [14], In fact, establishing suitable experimental conditions for the occurrence of bilayer de position as well as an efficient and quick methodology to ascertain whether bilayer deposition has indeed taken place on a solid surface is still a problem for those in terested in producing supported bilayers. Many applications of supported bilayers in biotechnology and biophysics are presently hampered by the poor reproducibility of various bilayer deposition methods. The process of bilayer deposition on solid substrates needs to be described in terms of exact physical parameters such as the contact angle [10] or surface roughness [16],
For some synthetic amphiphile vesicles [18], it is possible to distinguish mere vesicle adhesion from bilayer deposition on a solid surface simply by surface ten sion and contact angle determinations as a function of the amphiphile concentration and time of interaction between the amphiphile dispersion and the surface [10, 19]. In this case, contact angle maximization and contact angle hysteresis minimization can be used as criteria for bilayer deposition [10, 19]. We recently determined the influence of time and DODAB concentration on assembling one bilayer or adhered vesicles on planar S1O2 surfaces [19]. Synthetic bilayer deposition was achieved at a maximum of advancing or receding contact angle and a minimum of contact angle hysteresis both for polymeric [10] and for mineral surfaces [19]. In this work, we generalize the criteria previously established for the synthetic vesicles to two phos pholipids: phosphatidylcholine (PC) and dipalmitoylphosphatidylcholine (DPPC). In addition, phospholipid concentration and time effects on the phospholipid assem bly at a S1O2 surface are presented. 2
Egg PC (type XIII-E) and DPPC of the highest purity available were from Sigma. Phospholipid concentrations were determined by inorganic phosphorus (Pj) analysis [20]. All other reagents were of analytical grade and were used without further purification. Water was Milli-Q quality.
