MOLECULAR-WEIGHT DEPENDENCE OF WETTABILITY AND MOLECULAR ADSORPTION OF POLY-L-LYSINE AT THE AIR-WATER-INTERFACE

Wettability and adsorption on substrates with low and high surface energies have been investigated using poly-L-lysine (PLL) samples to evaluate the effect of degree of polymerization (DP). Based on the experimental results, the conclusions regarding PLL wettability are that (a) the surface characteristics of PLL are molecular weight dependent, (b) DP 32 PLL takes a compact random conformation and exhibits the least amount of surface free energy, (c) high-molecular-weight PLL is effective for wettability and adhesion, and (d) the surface characteristics of PLL are conformation dependent and the beta-structure is the most effective for adhesion. The work of adhesion of PLL was lower (34-79 mJ/m(2)) on the low-energy surfaces of Teflon and polyethylene (PE) and higher (108 mJ/m(2)) on the high-energy surface of glass; the order was glass > iron > PE > Teflon. At the air-water interface the surface excess concentration of each PLL sample increased when the concentration was increased, with the maximum adsorption at DP 32 (Gamma(max) = 28.8 x 10(-11) mol/cm(2)). The increase in the adsorption of PLL is correlated with the increased hydrophobicity due to the increase in DP, and the decrease at higher DP PLLs is due to the conformational effect. (C) 1995 Academic Press, Inc.