Previous results from this laboratory have confirmed the presence of extensive interfacial exclusion zones in water. Projecting out to a maximum of hundreds of micrometers from the surface, these zones appear to be more ordered and more stable than bulk water5–9,16
. Because they exclude particles and solutes, they have been termed “exclusion zones.” Substantial exclusion zones are seen next to many hydrophilic surfaces in aqueous systems5–8,16
The question addressed here is whether exclusion zones are found in liquids other than water, especially polar liquids. While not envisioned in contemporary thinking, the early literature contains numerous reports of broad interfacial zones in many liquids, extending up to hundreds of micrometers from the respective surfaces (c.f., Henniker, 1949). The existence of such reports raised the question whether those interfacial zones might correspond to exclusion zones.
We found that this was indeed the case. All polar liquids studied, including ethanol, methanol, isopropanol, acetic acid and DMSO showed sizable exclusion zones. Although the polar liquid exclusion zones were not as large as the zone found in water, the sizes were nevertheless appreciable, ranging from tens of micrometers to more than two hundred micrometers. As with water, the exclusion zones' presence was detectable with microspheres of negative, positive and near-neutral polarity; hence, these zones did not likely arise out of some electrostatic-repulsion phenomenon. Also as observed in the aqueous system, zone sizes were larger when larger microsphere probes were tested, a reflection, presumably of the EZ's jagged outer boundary. It appears, then, that exclusion zones are common features of many polar liquids — not a surprise given the large number of early reports indicating large interfacial zones.
Another seemingly common feature among water-based exclusion zones and polar solvent-based exclusion zones is the presence of a negative potential gradient. Consistently, the negative potential was highest near the Nafion surface and lower farther outward from the surface. Earlier results obtained with water had shown proton-borne positive charge in the zone beyond the EZ, and such positive charge was even more evident in the case of ethyl alcohol, where potential measurements showed positive electrical potential in that zone (Fig 7). Hence, the presence of interfacial charge separation might be a common feature among the different solvents.
In previous studies, we found that the charge separation in water was fueled by incident radiant energy, particularly in the infrared region of the spectrum11
. To test whether that might be the case with the polar solvents as well, we exposed the ethyl alcohol setup to infrared light. The results confirmed substantial EZ expansion in the presence of infrared, implying that the energetic basis for EZ buildup in the polar solvents was similar to that of water.
Additionally, we found a remarkable wavelength correspondence between expansion and absorption. In the case of water, the wavelength most effective for expanding the EZ was the same wavelength that water absorbs most strongly, ~3.0 μm, which corresponds to the OH-stretch17–19
. In the case of ethyl alcohol the similarity persisted. Here the wavelength absorbed most strongly, 3.4 μm, was again the wavelength most effective in producing EZ expansion. Thus, the correspondence is seen in both water and ethanol.
From this correspondence, one might speculate what happens when radiant energy is absorbed in polar liquids. The prevailing understanding is that the absorbed energy is converted into heat alone19–20
. The fact that the most strongly absorbed incident wavelengths also expand the EZ most profoundly implies that the absorbed energy may be used as well for conferring nonthermal potential energy to the system. The potential energy could exist in the form of entropy loss, i.e., as increased order in the interfacial zone, and possibly also as increased interfacial charge separation.
Among the polar solvents that showed an EZ, methanol, ethanol, isopropanol and acetic acid are protic solvents, which are presumed to strongly solvate negatively charged solutes via hydrogen bonding similar to water22–23
. On the other hand, EZ formation was also found in dimethyl sulfoxide, which is an aprotic solvent24–27
. For this solvent, EZ formation was relatively slower than the rest and became visible, expanding to approximately 200 μm in 20 minutes. The difference of behavior might arise from different mechanisms of solvation, the latter thought to arise from dipole-dipole interactions22–28
. However, the reason that dipole-dipole interaction slows down EZ buildup compared to hydrogen binding needs to be explored in future work.
In summary it appears that water is less unique than anticipated, at least in terms of exclusion-zone formation. The presence of exclusion zones seems to be a general phenomenon not only in aqueous systems but many or all polar liquid systems. Although the observed interfacial features are unexpected by contemporary standards, we stress that they are fully anticipated by the results of earlier research (Henniker, 1949). Thus, polar liquids have extensive interfacial zones that are physically and perhaps chemically different from the respective bulk fluids. How exactly these zones differ structurally remains to be elucidated.