Influence of the water–air interface on the apparent surface tension of aqueous solutions of hydrophilic solutes

Abstract It is well known that sugars such as sucrose and glucose, when dissolved in water, cause an increase in its measured surface tension. We determined however that polymers of sucrose and glucose, i.e. ficoll and dextran, both cause a decrease in the measured surface tension of their aqueous solutions. From the surface tension properties of solid layers of dried sucrose and glucose, and from the aqueous solubilities of these two sugars, their surface tension components and parameters in the dissolved state could be determined. From these it follows that the surface tension of these sugars in the dissolved state is about twice as high as that of water. From the surface tension data of both sugars it also follows, however, that their molecules are repelled by the water–air interface, which leaves a thin zone inside the water–air interface that is depleted of sugar molecules, thus giving rise to an apparent measured surface tension for these aqueous sugar solutions that is only a few mJ/m2 higher than that of water. The cause of the high polar components of the surface tensions of sugars in the dissolved state lies in the elevated free energy of cohesion between the electron-acceptor and electron-donor sites of molecularly dissolved sugar molecules, as manifested by a very elevated Lewis acid-base (AB) surface tension component, γAB, of about double that of water. In polymers of these sugars the strong electron-acceptor/electron-donor interactions between sugar monomers is no longer possible, or much attenuated, which lends ficoll and dextran a γAB value of only above 40% of that of water. These polymer molecules are also repelled by the water–air interface, so that the decrease in surface tension they cause in aqueous solution, is relatively modest. Hydrophilic solutes are repelled by the water–air interface and although they homogeneously pervade almost the whole volume of the aqueous solvent at very low to very high concentrations, they are absent from the depletion layer, which, for relatively low molecular weight solutes, typically is slightly thinner than 1.0 nm, closest to the air interface. Partly hydrophilic/partly hydrophobic solutes orient their hydrophobic sites to the water–air interface. The bulk solution has a (usually rather dilute) concentration of solute, not surpassing its solubility, however the solute concentration at the water–air interface can be much more concentrated than the solubility limit, e.g. by having the amphipathic solute molecules form micelle-like structures, with their hydrophilic sites directed to the water-phase, and their hydrophobic moieties protruding into the air, usually with loss of water of hydration. This causes a drastic apparent decrease in the measured surface tension of the aqueous solvent, generally already noticeable at fairly low to very low solute concentrations. Thus, whether solutes are repelled by the water–air interface (e.g. sugars and their polymers, and salts), or attracted to that interface, the measured apparent surface tension of all such aqueous solutions is not proportional to the free energy of cohesion of the bulk liquid. In general, therefore, contact angle measurements using non-homogeneous liquids, such as solutions or mixtures of liquids, are best avoided.