7.0: Introduction

Equilibrium by Design: Self-Assembled Monolayers

                                

Figure \(\PageIndex{1}\): A graphic with basic facts about George M. Whitesides, American chemist who pioneered the field of self-assembled monolayer (SAM) technology. This unique phenomenon has now been studied in-depth for different molecules and surfaces, finding applications in biosensors, microelectronics, and tissue engineering. Image from Wikimedia Commons with permission under Creative Commons license CC-BY.

Chemical equilibrium occurs when the forward and reverse rates of a reversible process are equal, resulting in a stable mixture of products and reactants. While we often study equilibrium in the context of chemical reactions, it also plays an important role in how molecules behave at surfaces. For Dr. George M. Whitesides, a pioneer in nanoscience and surface chemistry, equilibrium became a powerful tool for building new materials.

In the 1980s, Whitesides and his team developed a technique to create self-assembled monolayers (SAMs): ultrathin films made of molecules that spontaneously organize themselves into ordered layers on surfaces. These layers form through reversible processes, where molecules temporarily stick to a surface and sometimes detach again. Over time, a balance is reached between molecules attaching and leaving, a state known as adsorption equilibrium. Once this dynamic balance is established, other interactions, such as covalent bonding and intermolecular forces, stabilize the arrangement both between the molecules and between the molecules and the surface. Under the right conditions, the molecules align with remarkable precision, creating a well-ordered layer through the interplay of reversible and stabilizing forces at the nanoscale.

What makes this system remarkable is not just the self-assembly itself, but how it can be harnessed to create functional materials. By modifying the chemical groups on the surface, Whitesides and his team designed SAMs that interact selectively with proteins or cells, enabling precise molecular recognition. Some SAMs can even prevent fouling, a common problem in biological systems where random proteins stick to surfaces and interfere with detection. These capabilities have made SAMs valuable tools in biosensors, microelectronics, and tissue engineering.

Figure \(\PageIndex{2}\): A schematic diagram of a basic self-assembled monolayer (SAM). Molecules that are optimal for SAM formation often possess a head group, a tail, and a functional group. The head group, also often called the anchoring group, allows the molecule to bind to the surface so that it orients itself in a specific way; for example, SAMs on gold surfaces use thiol groups (-SH). The tail group, also called a linker, connects the head group to the functional group and creates stability in the SAM via covalent bonds or intermolecular forces between the tail groups. The functional group is what allows for the specific attachment or recognition of different biomolecules or cells. Image reprinted from Wikimedia Commons, with permission under open access license CC-BY.

By applying equilibrium principles to build SAMs, Whitesides was able to design functional materials with real-world applications, revolutionizing the field of surface chemistry. In this chapter, you’ll explore the core principles of chemical equilibrium, including how reversible reactions behave, how systems respond to changes, and how to apply that information.

Sources:

1. Bain, C. D.; Biebuyck, H. A.; Whitesides, G. M. Comparison of Self-Assembled Monolayers on Gold: Coadsorption of Thiols and Disulfides. Langmuir 1989, 5, 723–727.
2. Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M. Self-Assembled Monolayers of Alkanethiols on Gold: Comparisons of Monolayers Containing Mixtures of Short- and Long-Chain Constituents With CH3 and CH2OH Terminal Groups. Langmuir 1992, 8, 1330–1341.
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8. Sigal, G. B.; Bamdad, C.; Barberis, A.; Strominger, J.; Whitesides, G. M. A Self-Assembled Monolayer for the Binding and Study of Histidine-Tagged Proteins by Surface Plasmon Resonance. Anal. Chem. 1996, 68 (3), 490–497. https://doi.org/10.1021/ac9504023.
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10. George M Whitesides | Whitesides Research Group. https://www.gmwgroup.harvard.edu/people/george-m-whitesides (accessed 2025-07-31).