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July 28, 2016

The proverb, "Cleanliness is next to godliness," is usually attributed to John Wesley, who included it in a 1778 sermon. His phrasing was slightly different, "Cleanliness is indeed next to godliness," and the meaning of this phrase is usually misinterpreted. The meaning is clear in Wesley's context, "Let it be observed, that slovenliness is no part of religion; that neither this, nor any text of Scripture, condemns neatness of apparel. Certainly this is a duty, not a sin."

It's been suggested that the necessity of cleanliness arose during the the time of the Black Death in mid-14th century. I wrote about the Black Death in a recent article (Yersinia pestis, February 1, 2016). While the germ theory of disease wasn't proposed until the mid-16th century, the earlier miasma theory, that vapors from decomposed matter causes illnesses, had a long history.[1]

The Greek physician, Hippocrates (c. 460-377 B.C.), thought that bad air caused pestilence, and the Roman architect, M. Vitruvius (c. 75 BC - c. 15 BC), warned of unhealthy vapors from fetid swamplands.[1] Galen (c. 130-201 C.E.) extended these bad air theories by offering a mechanism; namely, that bad air caused an imbalance in bodily humors.

While hair can be cleaned with ordinary soap and water, the soap will leave a dull residue without additional surfactant. When the British colonized India, they imported more than just spices. They also imported a primitive shampoo. The fruit pulp of plants of the genus Sapindus, of which the lychee is a member, acts as a natural shampoo. These fruits contain amphipathic glycosides called saponins that have soap-like properties.

The German inventor, Hans Schwarzkopf, perfected a liquid soap for use as shampoo in 1927. In the 1930s, Drene shampoo, the first to contain synthetic surfactants, was introduced. Industry was quick to realize the profit potential of such products, so there was significant research and development activity in these products in the mid-20th century. A major milestone was the introduction of Johnson's Baby Shampoo.

Johnson/Johnson No More Tears Ad, 1956

Johnson's Baby Shampoo, advertised as "No More Tears."

The shampoo used amphoteric cleansing agents, which are mild, so they won't sting the eyes. Johnson & Johnson quickly captured a majority of the baby shampoo market.

(From a 1956 issue of Family Circle magazine, via Wikimedia Commons.)

Johnson's Baby Shampoo, which was advertised as "No More Tears," traded a small measure of efficacy for mildness. The shampoo used amphoteric cleansing agents.[2] An amphoteric compound can act as either an acid or a base. For a shampoo, the amphoteric compound is an ampholyte, a molecule containing both acidic and basic groups.[2] Being neither acid nor base (or, both acid and base), the pH of the compound is not that different from that of ordinary water.

The shampoo that I use, Pert Plus, 2-in-1 Shampoo & Conditioner, works well on my hair; and, I use it also as a surfactant when I develop novolac-coated photosensitive printed circuit boards. This shampoo is a thick, green liquid, so there's always the problem of releasing the entire volume from the container.

This is the problem that engineers Bharat Bhushan and Philip Brown from the Ohio State University Department of Mechanical and Aerospace Engineering solved using surface modification of the shampoo container. Bhushan is a professor of mechanical engineering, and Brown is a postdoctoral research associate.

Says Bushan,
"It's what you'd call a first-world problem, right? 'I can't get all of the shampoo to come out of the bottle.' But manufacturers are really interested in this, because they make billions of bottles that end up in the garbage with product still in them."

Shampoos help water to penetrate fabrics, and this property also makes the final drops of liquid cling so tightly to the insides of bottles.[4] While coatings exist that aid in release of consumer items such as ketchup from their bottles, soap-repellent surfaces are a harder problem. Ketchup is mostly water, and water molecules will bond to each other more strongly than to plastic.[4] The "soapy" surfactant molecules in shampoo have low surface tension, so they stick to plastic more readily. Says Brown, "It was an extra challenge for us to make a surface that could repel surfactant."[4]

Shampoo running off a treated polypropylene surface

Slip-Sliding Away.

Shampoo running off a treated polypropylene surface.

(Still image from an Ohio State University video by Philip S. Brown.)

One present approach to the problem is to add a nanoscale texture to the plastic surface, a technique that's expensive, it takes times, and the resulting surface features are fragile.[3-4] The technique discovered by Bushan and Brown is simpler and less expensive than such methods, and it works well for polypropylene, a common plastic used in consumer packaging.[4] Their superoleophobic surfaces are built from microscopic y-shaped structures composed of silica nanoparticles.[3-4] These structures cradle the soap droplets on pockets of air; so, the shampoo doesn't stick to the plastic, since it never touches the plastic.[4]

Their original process involved spin-coating the plastic with a solvent–nanoparticle-polypropylene mixture at an elevated temperature, then functionalizing the exposed surface with fluorosilane to form a durable, super-repellent surface.[3] They further developed the technique so that they could spray-coat solvent and ultra-fine silica nanoparticles onto the inside of bottles.[4] When the plastic re-hardened, the silica is embedded in the surface.[4] The structures don't cover the surface completely - They're spaced a few micrometers apart. The texture result in a steepcontact angle for any droplets, so they don't wet the surface; instead, they form beads and roll right off.[4]

Figure caption

Silica nanoparticles embedded in polycarbonate form.

"Y"-shaped nanoparticles hold the liquid above the plastic surface, thereby preventing sticking.

(Microscope image by Philip S. Brown, Ohio State University.)

177 million pounds of polypropylene, identified as type-5 in the Resin Identification Coding System, were used in consumer bottles and bottle lids in the United States in 2014.[4] It's used, also for ketchup bottles, yogurt tubs, medical bottles, single-serve coffee pods, and iced coffee cups.[4] Only about a quarter of these are recycled, but the recycling process requires a thorough rinsing, a process that's becoming more wasteful in today's water-hungry world. I wrote about the world's water problems in a recent article (Future Water Scarcity, March 28, 2016).

There are other uses for such a surface treatment, such as a coating for plastic biomedical devices, such as catheters.[4] Bhushan and Brown have applied this technique for coating polycarbonate, which is used for automobile headlights.[4] Ohio State is looking to license the technique to manufacturers.[4]


  1. Carl S. Sterner, "A Brief History of Miasmic Theory," August, 2007 (PDF file).
  2. Joseph N Masci and Normand A Poirier, "Detergent composition," US Patent No. 2,999,069, September 5, 1961.
  3. Philip S. Brown and Bharat Bhushan, "Durable superoleophobic polypropylene surfaces," Philosophical Transactions of the Royal Society, vol. 374, no. 2073 (June 27, 2016), DOI: 10.1098/rsta.2016.0193.
  4. Pam Frost Gorder, "A shampoo bottle that empties completely–every last drop," Ohio State University Press Release, June 26, 2016.

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