Creative Chemistry and the Liquid State

“The Strength of Weak Ties” is the title of a landmark paper by Mark Granovetter. (American Journal of Sociology, 1973, Vol. 78, Iss. 6, pp1360-1380, accessible from JSTOR). This work is often cited in the popular literature – including two books I read recently (reviewed here and here). To get a sense of the scope of Granovetter’s article, here is the opening paragraph.

“A fundamental weakness of current sociological theory is that it does not relate micro-level interactions to macro-level patterns in any convincing way. Large-scale statistical, as well as qualitative, studies offer a good deal of insight into such macro phenomena as social mobility, community organization, and political structure. At the micro level, a large and increasing body of data and theory offers useful and illuminating ideas about what transpires within the confines of the small group. But how interaction in small groups aggregates to form large-scale patterns eludes us in most cases.”

Hopefully one concept that students took home from my Physical Chemistry II (Statistical Thermodynamics and Kinetics) course this past semester is that the liquid state is much more difficult to describe mathematically, certainly more so than gases or solids. We spent much of the semester connecting the microscopic quantum world and the macroscopic equations of classical thermodynamics via statistical mechanics. Most of our time was spent on ideal gases and how the basic equations can be modified to account for non-ideal behavior in real systems. Intermolecular forces, the weak ties of the molecular world, are particularly important in chemistry.

I would argue that most of the interesting chemistry takes place in the liquid state, where weak ties reign supreme under ambient conditions – this is certainly true of the chemistry of life on planet Earth. Gases, at least at atmospheric pressure, are too dilute. Their weak ties (intermolecular forces) are too weak, and their strong ties (covalent bonds) are too strong. Thus, while gases are very dynamic systems – they are relatively easy to describe with “static” equations when they behave close to ideal. Solids are much more restricted in dynamism and the atoms can generally be treated as relatively static – vibrations surrounding a fixed center. Ignoring plasma and other exotic phases, it is liquids that provide the blend of dynamic behavior at close quarters that lead to the most interesting, yet complex chemistry.

Now a single liquid substance can be described using the appropriate equations of state, however solutions are where all the action happens. In a solution, there are solute molecules moving around among dynamic solvent molecules. There may be multiple solutes and sometimes more than one solvent. In a cell, the “atomic unit” of an independent structure that is “alive”, the solution is very, very concentrated and chock-full of a plethora of molecules with diverse weak ties to each other. This is nowhere close to an ideal solution, and therefore very difficult to describe mathematically with an equation of state. I describe some of this complexity in a previous post about minimal cells.

A mathematical equation of state allows you to make powerful predictions about past and future states of the system being studied. But concentrated complex solutions make it very difficult to come up with equations to model the behavior of the system. Certainly the effects can be non-linear (i.e., a small tweak could lead to a large effect down the road) – almost always the case in a complex system. In The Geography of Genius, the author Eric Weiner uses this argument to explain why it is hard to predict where genius or creative clusters might arise. He also discusses the importance of weak ties in generating novelty – one measure of creativity (at least using certain “standard” tests).

The origin of life could be viewed as chemical creativity par excellence. What are the conditions required? It is hard to imagine life being created solely in the gas phase (unless under high pressure, in which case it starts to exhibit fluid or liquid-like behavior) or in the solid state (much too slow, and hard to generate diversity). While the liquid solvent does not need to be water, the unique properties of water, especially in a concentrated solution with many solutes, make it an excellent choice for interesting and complex chemistry to take place. Living cells are a crowded place, not unlike urban areas where creative clusters may arise. There is plenty of dynamic interaction in both situations, much of it in the form of weak ties and connections.

Weiner also observes that creative clusters were preceded by what seem like unfavorable conditions (political or natural disasters), and individuals we classify as “geniuses” often had difficult and challenging circumstances to overcome. Creativity arises under potentially severe constraints. Few pass through the bottleneck not just to survive, but thrive! Creative adaptations and novelty arise as the environment changes – and a drastic environment change may “speed up” the evolutionary process. It is less likely that life arose in a pleasant warm lake (current estimates suggest that the molecular concentrations are too low for prebiotic chemistry) than in the harsher conditions of a hydrothermal vent – albeit there is much destruction alongside creation under such conditions. Because of the high pressures, water remains liquid even at temperatures exceeding 100 degrees Celcius. But we haven’t pinned down where life may have arisen on our planet.

We don’t know how to describe the complex chemistry under these complicated conditions. While I am working my way towards the goal of exploring chemistry at different temperatures, pressures and solute concentrations, I have started out with models and equations that I do understand – dilute solutions under ambient conditions. Perhaps if I understand the energetics involved in those cases, I can begin to build in complexity into the theoretical model. In class, I remember a student question alluding to this complexity. I told the class that we were restricting ourselves to studying the equations of equilibrium thermodynamics, and the interesting non-equilibrium realm would be a next step for those who are interested. (I think my alluding to the more complex mathematics dampened further interest.)

But that is where all the interesting chemistry happens. Creativity is liquid!

P.S. What might help me solve this problem is liquid luck. If only I could get my hands on some Felix Felicis. You would think a chemist with a blog named Potions For Muggles would be adept at mixing chemicals. But alas, I am a computational chemistry with lousy hands in lab.