Ice/water interface: Zeta potential, point of zero charge, and hydrophobicity

The ice/water interface is a common and important part of many biological, environmental, and technological systems. In contrast to its importance, th e system has not been extensively studied and is not well understood. There fore, in this paper the properties of the H2O ice/water and D2O ice/water i nterfaces were investigated. Although the zeta potential vs pH data points were significantly scattered, it was determined that the isoelectric point (iep) of D2O ice particles in water at 3.5 degrees C containing 10(-3) M Na Cl occurs at about pH 3.0. The negative values of the zeta potential, calcu lated from the electrophoretic mobility, seem to decrease with decreasing c ontent of NaCl, while the iep shifts to a higher pH. The point of zero char ge (pzc) of D2O ice and H2O ice, determined by changes in pH of 10(-4) M Na Cl aqueous solution at 0.5 degrees C after the ice particle addition, was f ound to be very different from the iep and equal to pH 7.0 +/- 0.5. The shi ft of the iep with NaCl concentration and the difference in the positions o f the iep and pzc on the pH scale point to complex specific adsorption of i ons at the interface. Interestingly, similar values of iep and pzc were fou nd for very different systems, such as hydrophilic ice and highly hydrophob ic hexadecane droplets in water. A comparison of the zeta potential vs pH c urves for hydrophilic ice and hydrophobic materials that do not possess dis sociative functional groups at the interface (diamond, air bubbles, bacteri a, and hexadecane) indicated that all of them have an iep near pH 3.5. Thes e results indicate that the zeta potential and surface charge data alone ca nnot be used to delineate the electrochemical properties of a given water/m oiety interface because similar electrical properties do not necessary mean a similar structure of the interfacial region. A good example is the aliph atic hydrocarbon/water interface in comparison to the ice/water interface. Although the experiments were carried out with care, both the zeta potentia l, measured with a precise ZetaPlus meter, and Delta pH values (a measure o f surface charge) vs pH were significantly scattered, and the origin of dis semination of the data points was not established. Differently charged ice particles and not fully equilibrium conditions at the ice/water interface m ay have been responsible for the dissemination of the data. (C) 1999 Academ ic Press.