Letters: June 2024

Blooming in industry

I read your article on Barbie’s conservation with interest. It shows the dilemma of plastics.

While the article reads like a good analytical chemistry detective story, it makes me wonder at which point the researchers sought the help of industry. The appearance of a wax-like exudate on the surface of flexible PVC articles over time is well known and referred to as blooming. The factors controlling blooming have been available in open literature for decades.

While stearic acid is widely used as a lubricant, Barbie dolls are produced from plastisols: liquid dispersions of PVC resins and other solid ingredients in liquid plasticiser. The resins required for these formulations are manufactured by emulsion polymerisation. The precipitation of low levels of residual emulsifier over time is frequently behind blooming. This is the case with stearyl alcohol.

It can be frustrating to see many publications refer to ‘industry secrets’ as reasons for extensive analytical chemistry to answer such questions. While some companies will keep the details of today’s products to themselves, the vast literature in this area points to details of numerous substances used in polymer formulation and the reasons for their use. There are also numerous publications on the plasticisation process, and these show the need for molecular interactions between polymer chains and plasticiser molecules for successful plasticisation and the retention of the plasticiser: it’s not a simple lubrication mechanism. The structure of 2-ethylhexyl phthalate will tell the chemist why it was such an ideal plasticiser, although there are now numerous alternatives.

Chris Howick CChem FRSC
Via email 

Recognising Osawa

Andy Extance’s article ‘Carbon’s allotrope explosion’ notes the 1996 Nobel prize award for the 1985 discovery of the C₆₀ buckminsterfullerene and discusses the current predictions of multiple allotropes of carbon. However, the article fails to reference the first such prediction, as early as 1970, by Eiji Osawa in the Japanese journal Kagaku (E Osawa, Kagaku, 1970, 25, 854), which was not then noticed in the west. Osawa’s sagacity and originality deserve recognition.

Leslie Glasser
Perth, Australia

Calculated analogies

Philip Ball’s article ‘Making sense of the maths’ was spot on the mark. When we study chemistry, for many aspects, the approach is usually via mathematics. We have to be able to make quantitative predictions, which necessitates mathematics. The pitfall is that we might spend many years coddled by equations without really understanding what’s happening at all.

For me, that became an issue when, after taking early retirement at the age of 51, I studied for a teaching qualification in physical sciences. How was I to teach acids and bases to children who are not mathematically inclined without being simplistic? The answer was to focus on the battle between water molecules and acids for protons. Acids and water I likened respectively to teenagers and the elderly in their relative affinities for bubble gum or false teeth. It was a useful teaching tool, which increased my level of understanding.

A blatant example of failure to understand the basics was evident in the teaching notes I spotted from an earlier physics class. The slow rate of decent of a parachute was attributed to the upward and downward forces being nearly equal. That teacher could recite Newton’s laws of motion, but she didn’t understand them. Likewise, Newton’s law of action and reaction being equal and opposite has counterintuitive consequences. It means that there is no overall change in momentum when a bomb explodes.

It is therapeutic and useful to revisit many science topics to try to express them other than through mathematics. I feel that I have a more profound understanding now than I had during my working life. Don’t think that analogies and thought experiments are second best. Albert Einstein used them and he achieved some relative success as a result.

Michael Baldwin FRSC
Via email

On the other hand

According to Karl Popper, propositions in the natural sciences cannot be proved, only disproved. So repeated assertions that Z-DNA is left-handed are therefore unhelpful.

The test of a proposition in the natural sciences is not what it explains but what it does not explain.

Studies of structural changes in DNA inside drawn, solid fibres, are numerous and instructive. For example, it has been known for many years that the A and B forms of DNA are interconvertible under mild conditions inside a drawn fibre. Therefore it has been accepted that these forms have the same handedness.

Watson Fuller’s team studied the conversion of the B form into the D form with numerous photographs and concluded that both forms have the same helical handedness. In addition, Arumugam Mahendrasingham, Watson Fuller and co-workers report the change of the B form into the Z form under mild conditions inside a drawn fibre. It would be natural to conclude that the two forms therefore have the same helical handedness.

To my knowledge, the veracity of this latter experimental finding of a change of form remains unchallenged and unexplained.

James Crawford and colleagues offered a structural solution for crystals of the DNA fragment d(CGCG), which was claimed to be left-handed. These workers make it clear, however, that they also tried A, B, C and D-DNA as search molecules, but found the best fit for a trial model of left-handed Z-DNA.

Arguably this might suggest that a fresh model, as yet untried, might give an even better fit and that their range of trial models limited their possible range of structural solutions. 

Any number of demonstrations that purport to show that Z-DNA is left-handed is not a proof that it is. A single disproof is sufficient to challenge that assertion.

A perfectly sound proposition is that the B and Z forms have the same helical handedness but differ only in that the B form has Watson–Crick base pairing and the Z form has Hoogsteen base pairing.

Clive Delmonte FRSC
Via email

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