Two-dimensional properties of surfactant-like polymer monolayrs

A functional polymer monolayer at an air/water interface has been studied systematically by substituting a hydrophobic backbone with a hydrophilic moiety (OH) by measuring the surface pressure as a function of surface concentration for different molecular weight polymers. The characteristic scaling exponent (ν) estimated from the isotherms and the overlap surface concentration suggests that the air/water interface becomes a better solvent as the amount of hydrophilic functional groups is increased. Also, the isotherms show that the area per polymer molecule increases with increasing amounts of OH groups. However, the water surface tension is most effectively reduced at the 50% substitution level, not at 100%. This optimum amount of OH group on the hydrophobic polymer is equivalent to the hydrophic/lypophic balance of a surfactant in the formation of microemulsions. Furthermore, the surface occupied area per OH group (σ_o) is estimated with the ideal gas law at a dilute surface concentrations. The results show that the σ_o is a minimum at the same substitution level at which the reduction of water surface tension is a maximum.