Molecular Size

Also known as the Ben Franklin pond experiment, after a story in B.F's autobiography.

Olive oil with a known volume is dropped onto water. The water has been dusted with lycopodium powder, which floats on the surface. The oil drop expands, pushing the powder aside to form a clear circle, until the oil forms a monolayer. Measuring the area of the monolayer, dividing the volume of the drop by that area, gives the thickness of the monolayer, which is the height of the oil molecule on water.

From our demonstration movie, we found these values. The size of the patch was 62 cm diameter. This gives an area of 3000 cm2. Converting half a microliter into cubic centimeters starts with 1000 cm3 in a liter. Half a microliter is then 5 x 10-4cm3. Dividing the volume by the area gives the height of the layer as 5/3 x 10-7cm, or about 17 nanometers. This compares to an estimation of 10 nanometers, based on Ben Franklin's' experiment.

A reasonable size Langmuir trough is too small for a measurable droplet. The oil film can't form a monolayer as it runs into the sides. Using a micro-pipette to deliver e.g. half a microliter of oil requires a water surface on order of a square meter. The movie shows our current method of using a pond that large.

franklin pond frame

The pond is made of a large, black plastic trash bag. Cut off the bottom of the bag while it is still folded. Then slit one of the sides of the tube to make a sheet. Avoid touching the plastic sheet, as finger grease will show up as spots in the powder layer. On a large, level surface, arrange four 1" x 2" x 4' pieces of wood in a square. Settle the plastic sheet on this frame. Puffs of air are enough to get the sheet to lay flat and unfolded.

Once the plastic sheet is down, add water to make the pond. As long as the wood boards are touching, the square forms a border for the pond which holds water. After the demo is finished, one corner can be pulled apart to make a channel to empty the pond into a bucket.

Add enough water to cover the plastic sheet. This is where the level surface is important, to minimize the amount of water needed to make the pond.

With a modified wash bottle, puff lycopodium powder to lightly cover the water surface.

To the center of the pond, add a drop of olive oil from a micro-pipette.

franklin pond

The drop spreads quickly, then slows down. Measure its diameter right away, as there is bounce from the compressed lycopodium powder that blurs the edge.