Adsorption and desorption processes of polar molecules at the ice/water interface.

摘要

In this work, the nonlinear optical laser technique, second harmonic generation (SHG), is used in a total internal reflection (TIR) geometry to probe the single crystalline ice/water interface. SHG from the clean interface is observed and attributed to symmetry breaking at the boundary. Using SHG we have observed adsorption and desorption processes of two polar molecules at the equilibrium ice/water interface.; We observe a linear increase in the square root of the SHG intensity when solutions of increasing concentration of a small organic molecule, 2,2{dollar}spprime{dollar}-dihydroxy-1,1{dollar}spprime{dollar}-binaphthyl (BN) are placed in contact with the prism plane of ice. In the concentration range 0.2-7 {dollar}mu{dollar}M, we estimate that the surface coverage is most likely submonolayer. Solubility constraints do not allow higher bulk concentrations. An identical SHG adsorption isotherm plot is observed for BN adsorption at the basal plane of ice. A fit of the BN SHG isotherm to the Langmuir adsorption equation yields a free energy of adsorption of {dollar}-{dollar}13.6(8) kJ/mol. BN desorbs upon dilution of the solution contacted with the ice, leading to the conclusion that BN is reversibly adsorbed.; The adsorption and desorption processes of an antifreeze protein from Arctic winter flounder (wfAFP) at the ice/water boundary are detected using the same methods. We observe wfAFP adsorption at both the basal and prism planes of the ice/water boundary at 1,000 fold lower concentrations than in vivo. A fit of the wfAFP isotherms to the Langmuir adsorption equation yields a free energy of adsorption of {dollar}-{dollar}43(1) kJ/mol and {dollar}-{dollar}2(2) kJ/mol for the prism and basal faces, respectively. A steeper SHG adsorption isotherm at the prism face of ice is observed, indicating preferential binding. wfAFP does not desorb upon dilution of the solution contacted with ice. We argue that irreversible adsorption may be necessary for antifreeze activity.