Adsorption behavior of surface chemically pure N-cycloalkylaldonamides at the air/water interface

Equilibrium surface tension (sigma(e)) and electric surface potential (DeltaV(e)) versus concentration isotherms of the homologous series of N-cycloalkylaldonamides synthesized from cycloalkylamines (from cyclopentyl- to cyclododecylamine) and D-glucono-1,5-lactone (c-C(n)GA) or D-glucoheptono-1,4-lactone (c-C(n)GHA) (c-n(C) = 5-12) were investigated at the air/water interface. The measurements were performed with aqueous, surface chemically pure surfactant solutions. Equilibrium surface tension vs concentration isotherms were evaluated to get the adsorption parameters, i.e., standard free energy of adsorption, DeltaG degrees (ads), saturation surface concentration, Gamma(infinity), minimum surface area demand per molecule adsorbed, A(min), and interaction parameter, H(s). Increasing the size of the cycloalkyl moiety leads to a significant increase of the minimum surface area demand per molecule adsorbed. This fact, together with a decrease of the intermolecular interaction parameter suggests that the introduction of a more bulky cycloalkyl ring (c-n(C) = 7 and 8) causes an attenuation of the hydrogen-bond network. This goes in line with the exceptional finding that the higher homologues revealed improved solubility in water. In addition, surface tension investigations suggest occurrence of a phase transition for the N-cyclooctylaldonamides at relatively small surface coverage. This observation is well supported by the surface potential measurements, for which the effect of possible changes in the molecules' surface orientation is even more pronounced. Moreover, the concentration intervals of N-cyclooctylaldonamide in which the change in orientation is observed for either the surface tension or the surface potential isotherms are in very good agreement.