College of Engineering

Chemical Engineering

 

What is “molecular engineering?”
It’s engineering molecules, or better yet, engineering with molecules.  Normally the object of scientific inquiries, molecules—their chemical, physical, and structural properties—are the foundation for entirely new engineering technologies and for optimizing existing ones.

Engineering with molecules means selecting molecules with the right properties, then organizing them into some nanoscale level structure, that is, designing the right molecular architecture to achieve the desired product or process.  Nature creates molecular architectures in many ways, too, for example, through DNA structure and protein folding.  In these cases, molecular engineering seeks to understand these nanoscale architectures to develop ways to mimic and improve upon them.

 It’s more than being small.  All engineering disciplines have their important equations in which one can substitute ever decreasing length scales to solve them.  At the nanoscale level, these equations break down: either by giving non-sensical solutions, or by giving none at all.

At the molecular and nanoscale level, substances exhibit unique properties not seen in their macroscopic forms.  This comes about through a variety of effects as shown in the table below.

Effect	Manifestation of Effect at the Nanoscale	Applications
Random (statistical) interactions among molecules	Ensemble averaging (statistical mechanics)	Optimize chemical properties Improve fuels, etc.
Cooperative Phenomena	Non-statistical interactions combine to produce special structures   Self-assembly	Protein folding DNA structure Colloids & emulsions Complex fluids (blood)
Quantum Mechanics	Size effects lead to unique electronic structure and chemical bonding	Catalysis Fuel Cells Solar Cells Display technologies

Molecular scale:    approx. 0.3–2 nm
Nanoscale:           approx. 1–500 nm

Random Interactions

Many chemicals (fuels, ammonia, ethylene oxide, etc.) consist of molecules that interact randomly or nearly randomly with each other.  The random or near-random interactions average together to define the macroscopic properties.  The averaging process is the basis of statistical mechanics, and the average itself is referred to as an ensemble average.

An interesting feature of the ensemble average is that only in this way can the chemical properties be calculated.  Chemical properties, like viscosity, compressibility, and density, as well as thermodynamic properties, like temperature and pressure, are intensive variables and cannot be defined at the molecular level!  (An intensive variable is one that does not depend on the quantity of the substance, so long as that quantity exceeds the nanoscale limitations of the ensemble average.)

The process of statistical mechanics and ensemble averaging allows researchers to predict the macroscopic behavior of newly discovered molecules.  This ability is important in optimizing chemicals like fuels and cleaning chemicals.

Cooperative Phenomena

Imagine a large number of small interactions acting in the same direction—like many people pulling in one direction on a rope in a tug-of-war.  In this way many small interactions can build to a large effect, as occurs, for example, in protein folding and DNA structure.  The interactions are brought about by hydrogen bonding with water molecules.  Thousands of hydrogen bonds are strong enough to alter the structure of the protein so that it functions properly within the organism.  Because the cooperative phenomenon occurs in the same direction, this effect is non-statistical.

Cooperative phenomena occur in non-biological contexts, too, such as creating structure in liquids, known as colloids (cosmetics) and emulsions (milk).  They are also important in complex fluids like blood.

Quantum Mechanics

Molecular bonds occur through shared electrons at specific energies.  Altering electron energies therefore alters the nature of the chemical bond.  In nanoscale compounds electronic energy levels, so-called electronic structure, are sensitive to the size of the particle.  This is the “particle in a box” effect: a smaller box leads to higher energy levels.  Electronically conducting materials, like metals, semiconductors, and conducting polymers, are sensitive to this effect.  Thus, changing the size enables properties to be tuned to the desired effect.

The size-tuning effect is used to advantage in catalysts used in automotive catalytic converters and fuel cells.  It is important in semiconductor manufacture.  Quantum mechanical tuning is also important in solar cells and in designing conducting organic polymers for organic light emitting diodes (OLEDs).

Molecular Engineering – More than just ChemE

Molecular engineering is truly interdisciplinary, as one can easily imaging from the examples above.  By itself, chemical engineering cannot address all of the issues that make up molecular engineering.  In fact, there are many disciplines that relate to molecular engineering.  Collaborations among all of them are the key to mining the discoveries of this new paradigm.

Each discipline brings its own strengths to molecular engineering.  A summary of several disciplines is given below.



There more disciplines that can be listed on this table.  Clearly, molecular engineering is a multi-dimensional endeavor.  Regardless of one’s particular discipline, there is always a place in molecular engineering.

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Molecular Engineering