Surfaces and interfacial water: Evidence that hydrophilic surfaces have long-range impact

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Abstract

It is generally thought that the impact of surfaces on the contiguous aqueous phase extends to a distance of no more than a few water-molecule layers. Older studies, on the other hand, suggest a more extensive impact. We report here that colloidal and molecular solutes suspended in aqueous solution are profoundly and extensively excluded from the vicinity of various hydrophilic surfaces. The width of the solute-free zone is typically several hundred microns. Such large exclusion zones were observed in the vicinity of many types of surface including artificial and natural hydrogels, biological tissues, hydrophilic polymers, monolayers, and ion-exchange beads, as well as with a variety of solutes. Using microscopic observations, as well as measurements of electrical potential and UV–Vis absorption-spectra, infrared imaging, and NMR imaging, we find that the solute-free zone is a physically distinct and less mobile phase of water that can co-exist indefinitely with the contiguous solute-containing phase. The extensiveness of this modified zone is impressive, and carries broad implication for surface–molecule interactions in many realms, including cellular recognition, biomaterial–surface antifouling, bioseparation technologies, and other areas of biology, physics and chemistry.

Introduction

Understanding the character of the near-surface aqueous zone is fundamental to an understanding how solutes interact with surfaces. Within such aqueous zones, solutes can sense surface features. Sensing interactions are generally thought to fall off within nanometers of the surface [1], although in colloidal systems studied in confined spaces, size-dependent depletion effects may extend by up to several particle diameters (e.g., [2], [3]). Beyond this limited zone, surfaces are thought to be effectively invisible to solutes.

The older literature, on the other hand, reports much longer-range impact of surfaces. A 1949 review by Henniker [4] points to numerous experimental reports showing impressive long-range surface-induced ordering of various liquids, including water. More than 100 papers are cited. A book by Nobel Laureate Albert Szent-Gyorgyi [5] builds on this concept, presuming that long-range ordering of water is the essential foundation of bioenergetics, and a similar foundational concept forms the centerpiece of several major works on cell function (e.g., Ling, 1984 [6], Pollack, 2001 [7]).

While much of this older evidence has been forgotten, the modern literature is by no means devoid of reports of long-range effects. One of them is the presence of thermal anomalies in water, which penetrate substantial distances into aqueous solution (for review, c.f. Clegg and Drost-Hansen [8]). Another is the presence of solute-free “voids” in colloidal solute suspensions, with characteristic dimensions of 100 μm [9], [10]. A third is based on interferometric measurements of polished quartz surfaces by Pashley and Kitchener [11] and Fisher et al. [12], which provide evidence that hydration could easily extend to several hundred water-molecule layers.

Two very recent papers once again raise the issue of long-range effects. On theoretical grounds Ling (2003) argues that under certain ideal conditions water ordering could extend virtually infinitely [13], while Roy et al. (2005) argue for long-range ordering based on precedent in the materials-science field [14]: In the case of semiconductor materials for example, surface substrates commonly order molten silicon into crystalline arrays; the ordering extends many molecular layers into the bulk without any transfer of substrate molecules. The same applies to the ordering of aluminum by aluminum-oxide surfaces. The commonality of such epitaxial ordering leads the authors to suggest the inevitability of similarly extensive surface-induced ordering of water molecules [14].

One obstacle to any thinking along the lines of possible long-range water ordering is how water molecules could be restricted to attain such order. Water molecules may readily adsorb onto hydrophilic surfaces through hydrogen bonding, but it is generally thought that additional ordering conferred by subsequent hydration layers that build onto the first will quickly give way due to the disruptive effects of thermal motion. Thus, while ample experimental precedent for long-range ordering is established, the reason why any such ordering could extend for long distances is not established.

To realize any such long-range stability, either hydrogen-bond energy holding molecules together would need to be higher than expected, or thermal motion tending to rip them apart would need to be weaker than expected. The latter possibility is given force by observations on colloidal microspheres in aqueous suspension: At volume ratio of ∼ 1%, microsphere suspensions show two coexisting phases, random and crystalloid. In the random phase, thermal motion is of the anticipated magnitude, but in the crystalloid phase, although microspheres are distinctly separated from one another by several micrometers, r.m.s displacements are lower by an order of magnitude [15], [16], [17]. Thus, the disruptive effects of thermal motion in ordered regions may be less than generally anticipated, and this feature might predispose molecules to long-range ordering. In other words, any observations of long-range ordering might not necessarily violate foundational principles.

One way of examining the interfacial region is to explore the local disposition of solutes. If such interfacial water is genuinely in the ordered, liquid crystalline state, it is expected to exclude many solutes [6], [18], [19]. Recently, we confirmed that colloidal solutes are excluded from the near-surface zone of various gels, by distances on the order of 100 μm [20]. Although an extensive series of controls could rule out a variety of potential artifacts or trivial explanations [20], neither the basis, nor the generality of the observed exclusion phenomenon was evident from that study. Hence, we carried out the studies reported here, which demonstrate a role of hydrophilic surfaces more profound than presently considered.

Section snippets

Generality

Examples of exclusion adjacent to a variety of surfaces are illustrated in the gallery of Fig. 1. Fig. 1a shows the disposition of microspheres in aqueous suspension some minutes after exposure to a polyacrylic-acid gel surface. Initially, the microspheres were dispersed throughout the aqueous phase. Progressively, they translated away from the gel surface, creating a particle-free zone that continued to expand at ∼ 1 μm/s, leaving a stable ∼ 250 μm-wide particle-free zone. Whereas the

Conclusions

The finding of large zones of mobility-limited water carries broad implication for biology, biotechnology, and other realms. In the cell, for example, extreme crowding implies that the aqueous phase may be entirely interfacial [7], [24], [25], and if the interfacial phase excludes solutes, then some mechanism would be required to permit solute–surface interactions, including those between substrates and enzymes. Indeed, the possibility of water impacted by surfaces raises the question of

Acknowledgments

We thank Steven Gibbs, National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL, for his assistance with the magnetic resonance experiments. We also acknowledge constructive comments along the way from Philip Ball, Mark Banaszak Holl, Frank Borg, David Burns, Charles Campbell, Angela Carden, Ben Chu, Jim Clegg, Ferenc Horkay, Jacob Israelachvili, Noah Lotan, Mickey Schurr, Jan Spitzer, Andy Symonds, Erwin Vogler, John Watterson, Philippa Wiggins, and Jennifer Whittier.

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