Wednesday, July 21, 2010

Scientific Exploits of Ben Franklin Part 1: The Oil Drop Experiments



Anyone who has spent any significant amount of time in Philadelphia will understand exactly what I mean when I say that the city is obsessed with Benjamin Franklin. Obsessed.  Now, don’t get me wrong--he was an amazing individual.  He was a founding father of the US, and one of the greatest statesmen and inventors of his day.  However, the number of statues and other structures (including a bridge) named after or dedicated to him sometimes seems a touch excessive.   

Many people don't realize that Ben Franklin was also one of the most important scientists of his day.  This is even more impressive due to the fact that he was entirely self-taught.  He is often referred to as "Doctor Franklin," but this is only because Oxford University gave him an honorary doctorate 1762.  Ben's scientific excellence was driving mainly by his keen mind and unrelenting curiosity. 

There are several museum exhibits in Philly about Ben’s scientific innovations and inventions.  While most people associate Ben's scientific exploits with the discovery of electricity, Ben also made some very important observations about the interactions of lipids (oils, fats, etc.) with water.  These are often overshadowed by his more risky experiment of flying a kite in a thunderstorm, but his oil-water experiments (called the "oil drop" experiments) were nonetheless an extremely important contribution to the sciences of surface chemistry and biology.  Read on to learn more...





We all know that oil and water don't mix.  It is basic human knowledge that we all pick up at some point.  Unfortunately, this can sometimes result in terrible environmental consequences and expensive clean-up costs, as seen with the BP oil spill in the Gulf of Mexico.  Most people don't realize that Ben had an important hand in pioneering the science behind our understanding of this phenomenon.  Legend has it that an old sea captain once told Ben about how pearl divers would dive with a mouthful of oil, which they would promptly spit out when they got below the surface of the water. The oil would float to the top and coat the surface of the water.  This altered the surface tension and eliminated the ripples and waves caused by the wind, improving underwater visibility.

While Ben was traveling in a convoy of ships from America to England in 1757, he made note of the fact that the ships in the back of the convoy sailed more smoothly than the ships in the front.  The captain told him, "The cooks...have, I suppose, been just emptying their greasy water through the scuppers, which has greased the sides of those ships a little."  Ben didn't quite buy the fact that this was due solely to the oil greasing the sides of the ships, but he was intrigued.  The idea of using oil to calm rough seas for seafaring or clearing the water surface for diving had actually been well-known for centuries.  It was written about by Gaius Plinius Secundus, known today as Pliny the Elder, a historian and scientist who lived in the first century AD.  People had been using this technique for thousands of years before Ben began to question it and experiment with it.  Ben was brilliant in the way that he could look at common natural phenomena that people took for granted (like lightening, oil and water, etc.) and question them.  Ben continued to observe this phenomenon on his later ocean voyages, and he wrote that he "resolved to make some experiment of the effect of oil on water" whenever he had time.           

Ben started carrying out his experiments with oil in the 1760s while living in London.  He would go to ponds while traveling around England and pour olive oil on them, noting that the waves and ripples on the pool were indeed calmed.  He wrote about his first experiment, near Clapham Common, at a pond that he "observed to be one day very rough with the wind."  He "fetched out a cruet of oil, and dropt a little of it on the water."  Ben wrote that "it spread itself with surprising swiftness upon the surface....and there the oil, though not more than a teaspoonful, produced and instant calm over a space several yards square, which spread amazingly, and extended itself gradually till it reached the lee side, making all that quarter of the pond, perhaps half and acre, as smooth as a looking glass."

Like any good scientist, Ben repeated his experiments many times over the next several years, until many of his letters regarding this topic were published in 1774 in the Philosophical Transactions of the Royal Society of London.  Importantly, he wrote about how a given quantity of olive oil would only cover a set amount of pond surface.  Twice the volume of oil would cover twice the area of pond surface.  Ben concluded that there was a limit to how thin the oil could be spread.  He carefully recorded the volumes of oil and the surface areas that they covered.  We don't actually know whether he calculated the thickness of the oil layers (it isn't recorded in his notes), but if he did, his measurements would have predicted a thickness of approximately 10 Angstroms (one Angstrom is approximately one ten-billionth of a meter).  This would have been a remarkably close measurement of the thickness of a single-molecule thick layer (called a "monolayer") of triolein, one of the major lipids in olive oil.  Had he written this down, it would have been the first recorded measurement of the size of a single molecule.  It wasn't until over 100 years later in 1890 that the scientist and eventual Nobel Prize winner John William Strutt, better known as Lord Rayleigh, repeated these experiments and measured the thickness of the triolein monolayer as 16 Angstroms.  Rayleigh was a trained physicist and a professor of Natural Science at the Royal Institute in London.  However, interestingly enough, the accuracy of Rayleigh's calculations were helped greatly by another self-taught scientist named Agnes Pockels, who herself had been denied a science education because she was a woman.           

Nevertheless, what was important about Ben's original experiments was that Ben realized and wrote about how, if he put the same amount of oil on a marble table or on a piece of glass, it didn't spread, suggesting that there was something special about the interaction between the oil and water.  He wrote that "If there be...no attraction between oil and water, oil dropt on water will not be held together by adhesion to the spot wheron it falls...it will be at liberty to expand itself..."  It is interseting to note that, in his letters, Ben had a concept of "oil particles," which today we would call oil molecules.  This was way ahead of his time, as the concept of "molecules" was not really developed until the scientist John Dalton came along in the early 1800s.      

Molecular Structure of Water.
We now understand that water (H2O) molecules (shown on the left) are polar.  The electrons (negatively charged particles) in the molecule are unevenly distributed and attracted more strongly to the oxygen (O), creating partial positive charges at hydrogen (H) ends of the molecule (shown in gray) and a partial negative charge at the oxygen (O) end (shown in red).

The organic lipid molecules that make up olive oil and other oils are non-polar. The molecules are neutral, meaning they have no net charge, and they don't like the partially charged polar water molecules coming in and trying to tug at their electrons. Instead, they stick together with other non-polar molecules.  We use the term "hydrophobic" ("water fearing") to describe non-polar molecules that don't like to mix with water.  One of the major lipids in olive oil is triolein, shown below.  Oxygen (O) is shown in red, carbon (C) in green, and hydrogen (H) in gray.  Note that most of the molecule is made of long stretches of non-polar carbon and hydrogen.  This is why oil molecules are often called "hydrocarbons."  There are a few polar oxygens, but they make up only a tiny portion of the molecule.        
       
Molecular Structure of Triolein
As we said above, polar and non-polar molecules don’t like to mix.  Today, chemists call this the "hydrophobic effect." The oil stays on top, because it is less dense that the water.  When triolein molecules come into contact with water, they orient themselves so that their most polar parts (the part with the oxygens) are facing the water while the most non-polar parts (the long carbon-hydrogen chains) are away from the water.  This allows the triolein molecules to spread out over the water into a layer that is 1 molecule thick, a monolayer.  The repulsion of the oil and water layers reduces the surface tension of the water, and prevents it from forming waves and ripples in the wind.                     
 

                               
   
Ben's experiments and observations were an important moment in the study of surface chemistry and the interactions of polar and non-polar liquids.  This has important implications for biology and the structure of cell membranes.  The cell has a lipid bilayer ("two-layer") membrane.  This membrane separates the inside (intracellular) environment from the outside (extracellular) world, both of which are full of water.  This membrane is formed from lipids that have polar head groups (shown below in blue) that orient to face the watery outside and inside environments.  The non-polar hydrophobic tails of the lipids (in black) orient to face themselves to get away from the water, forming a double layer.  


This forms a protective barrier, because polar or charged molecules can't readily cross the membrane through the hydrophobic lipids tails.  Cells use special proteins to regulate the transport of these types of molecules, including sugars and ions like sodium and chloride from salt.  This allows cells to tightly control and regulate their intracellular composition.
    
 
Ben Franklin's experiments and writings about oil and water interactions provided the foundation for the work done in the next 150 years it took for scientists to truly begin to understood these forces.  The insights that Ben provided are one of many examples of Ben's scientific contributions beyond electricity, and also one of many reasons why he is held in such high regard even today.   



Text © 2010-2013 TheMadScienceBlog


Sources and further reading:
  • W Stillwell.  An Introduction to Biological Membranes. Chapter 2: "Membrane History"  Elsevier, 2013.  Available on Google Books.
  • D-N Wang, et al.  "Benjamin Franklin, Philadelphia's Favorite Son, was a Membrane Biophysicist." Biophysical Journal.  2013.  104:287-291.  (subscription or pay per view only)
  • C Tanford.  Ben Franklin Stilled the Waves -- An Informal History of Pouring Oil on Water with Reflection on the Ups and Downs of Scientific Life in General.  Oxford, New York,  2004.  Available on Google Books.
  • Letter from Benjamin Franklin to William Brownrigg, 1773, describing many details of his oil drop experiments, available here.

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