This article originally appeared in the spring 1998 issue of Two if by Sea, a joint newsletter of the MIT and WHOI Sea Grant programs.
It is often said that appearances are deceiving -- to understand the true essence of things, we must look not at the surface, but inside. Professor Paul Laibinis's work shows that from a chemistry standpoint, this view is not entirely correct. The behavior of a material depends on its surface properties, as well as its bulk properties. These surface characteristics, moreover, are largely dictated by their molecular-scale structure.
Professor Laibinis, of the Department of Chemical Engineering, has been interested in discerning the true nature of materials since he was young, asking such questions as: What are things made of? How do you change their properties by changing their composition? And how do you relate chemical structure to the things you see on a macroscopic scale?
His specialty has been surface chemistry or "molecular engineering," as he puts it. "In contrast to many chemical engineers, my work does not involve big industrial plants or refineries. Instead, I try to manipulate surfaces on the molecular level to get the physical properties I want."
In June, Professor Laibinis completed a two-year Doherty Professorship in Ocean Utilization sponsored by the MIT Sea Grant College Program. The work he did, in conjunction with G. Kane Jennings, a graduate student in chemical engineering, focused on a problem endemic to the ocean engineering world -- the corrosion of metals in aqueous and saline environments.
"We live in an oxidizing environment," said Professor Laibinis. "Any metal that is exposed to water, particularly salt water, will disintegrate with time. We need to minimize that corrosion to keep the properties of the metal intact and extend the lifetime of ships and other structures."
The economic impact of corrosion in the United States, according to one estimate, amounts to $300 billion per year. About 25 percent of the annual steel output in this country is used to replace corroded structures. Corrosion not only damages materials; it also threatens the environment through the distribution of leached metals into our waterways.
"When a metal is eaten away by corrosion, it has to go somewhere," Professor Laibinis said. "It ends up in rivers, lakes and oceans. The accumulation of metals can be significant in protected bodies of water like Boston Harbor and San Francisco Bay."
The answer to many of these problems, according to Professor Laibinis, may lie in thin coatings known as self-assembled monolayers. The basic idea is to design molecules that adhere strongly to a surface, creating a "barrier film" that protects the surface from corrosive agents such as oxygen, water, and halides (salts).
In particular, he is experimenting with a representative metal -- copper -- and a class of organic compounds called alkanethiols, which consist of a carbon chain (alkane) attached to a sulfur-based chemical group (thiol). Alkane-thiols spontaneously react with the surface of copper to form a coating that is roughly one ten-millionth of an inch thick. This happens because sulfur bonds strongly to the copper surface, leaving the attached carbon chain on the outside as a shield. It's a "self-assembling" system, said Professor Laibinis, because "you just put the [alkanethiol] solution in contact with the metal and the molecules figure out where to go."
The protection provided by this system is superior to that of more conventional polymer films 1,000 times thicker, he said. That's because the alkanethiol is a highly ordered, crystalline material that is less permeable to corrosive agents than a disordered polymer. Whereas the organic films can uniformly coat an object of any size or shape, polymer films often don't work well with irregularly-shaped objects.
The new system is not perfect, he admits; "the films we have don't survive forever," he said. One way to enhance longevity is to generate thicker layers that retain the crystalline structure of the alkanethiols. In this way, Professor Laibinis, Mr. Jennings and two MIT undergraduates -- Jeffrey Munro, a senior in chemical engineering, and Tseh-Hwan Wong (SB '98) -- have doubled the thickness of the films, thereby boosting their corrosion resistance by a factor of 20 or more.
The research team also intends to experiment with metals other than copper, such as steel, titanium and aluminum. "Copper has been a good material for us to work with because it's a single element and therefore a well-defined system," said Professor Laibinis. "Our assumption has been that we can apply the principles we learned here to different metals."
One drawback of organic coatings is that they won't work for high-temperature applications. While that will rule out some industrial uses, the barrier films may be employed in a variety of ocean engineering situations where high temperature exposure is not a concern. The coatings developed by the Laibinis team may eventually be applied to the exterior of ships, docks and other oceanic platforms, as well as to metal sensors that are extremely vulnerable to corrosion. The material can also be applied to already-assembled pipes as a way to reduce water pollution through inadvertent leaching.
Professor Laibinis is grateful for the Doherty Professorship, which gave him the chance to look at the problem in a different light. "This work has encouraged me to consider a range of new applications," he said. "Perhaps more importantly, I've come to see that common solutions may be found to corrosion problems that extend into many different areas and environments."
Mr. Nadis, a Cambridge-based writer, was a 1997-98 Knight Science Journalism Fellow.
A version of this article appeared in MIT Tech Talk on September 26, 1998.
