Biomolecular surface transformations
Diagram of a so-called "biomolecule" (see: animolecule "animated molecule" vs. animalcule "little animal") transforming on a surface, from the 2012 article “On the Thermodynamics of Biomolecule Surface Transformations. [5]
In thermodynamics, surface thermodynamics is the study of the thermodynamic behaviors of chemical species, such as atoms, molecules, bacteria, human molecules, etc., near surfaces.

The basic principle of surface thermodynamics is that a properly identified thermodynamic potential, i.e. free energy, will be minimized at equilibrium. Thus, the process under consideration, e.g. bacterial adhesion, will be favored if the process itself causes the thermodynamic function to decrease. The process will not be favored if it causes the free energy function to increase. [1]
In theory, the same situation applies to human molecules (people) attached to the surface of the earth. The details of these of these interactions, however, are more difficult to formulate mathematically. [2] The surface of the earth, from a chemical point of view, is considered as “substrate”.

The historical origin of surface thermodynamics were formulated in the second part of Willard Gibbs' 1876 On the Equilibrium of Heterogeneous Substances, on the subject he named the "theory of capillarity"; the namesake, according to A.I. Rusanov, meant to embody the classical theory of capillarity (called the traditional theory) that existed before Gibbs in the works of Pierre Laplace and Thomas Young. [6]

Bacteria-surface interactions
The two basic theoretical approaches to modeling bacterial adhesion are the “DLVO theory”, introduced in the 1940s, which takes into account the van der Waals attraction and the electrostatic repulsion, such as due layer of counterions, and the “thermodynamic model”, culled from surface chemistry, which models the interactions on changes in Gibbs free energy, i.e. the isothermal-isobaric thermodynamic potential, for the process.

The first theories on the thermodynamics of bacterial adhesion seems to have been outlined in the 1982 article “Are Solid Surfaces of Ecological Significance to Aquatic Bacteria” by American marine biologists Madilyn Fletcher and Australian microbiologist Kevin Marshall. [4]

In the thermodynamic approach to "large molecular entity" interaction with a surface, such as a bacteria molecule attaching to an aquatic surface, the attachment process is viewed as a spontaneous change, which is accompanied by a decrease in the free energy of the system. [3]

Surface interactions
Diagram shows a model of the forces involved in the attractive and repulsive interactions between a bacterium and a surface, where HB indicates the hydrophobic functional groups on the cell surface appendages that may assist in removing the layer of water adsorbed onto the substratum surface. [3]

The process of bacterial adhesion to a surface, in the thermodynamic model, is treated as an equilibrium process, described in terms of the “surface free energies” of the bacterium, substrate, and separating liquid. The estimation of the inter-facial free energies at the bacterium and substratum surfaces are generally estimated indirectly by measuring the contact angles of liquids on the test substrata and on lawns of bacterial cells. [4]

Human-surface interactions
The first models of humans attaching, reacting, or evolving on substrate were made by American anthropologist Eugene Ruyle in the 1960s, with his conception of a social “thermodynamic substratum”, and the 2007 human chemistry model of humans as “human molecules” attaching to “active sites”, such as homes, lakes, or rivers, etc., by American chemical engineer Libb Thims.

bonding sites

In the conceptual human dynamics model outlined by Thims, as shown below, two species A and B (such as two single people), on two different bonding sites, can favorably react together forming the A-B complex (such as a married couple):

A + B A-B

on a new energetically favored attachment site. [2] In more detail, to note, the general analysis of what constitutes "catalyst" vs. "substrate", the nature of the substrate or catalyst on the free energy vs. activation energy, the thermodynamic definition of sustanence (food), etc., among other difficulties, is a subject of current investigation. [2]

The following are related quotes:
“The macroscopic thermodynamics of interfaces, is sometimes called the theory of capillarity. The entire foundation of classical thermodynamics, in general, and surface thermodynamics, in particular, was laid by Gibbs, who created a ‘pure statics of the effects of temperature and heat’.”
— John Gaydos (1996), Applied Surface Thermodynamics (pg. 2)

See also
Surface chemistry

1. Absolom, Darryl. R., Lamberti, Francis V., Policova, Zdenka, Zingg, Walter, Oss, Carel J. van, and Neumnann, A. Wilhelm. (1983). “Surface Thermodynamics of Bacterial Adhesion” (PDF). Applied and Environmental Microbiology, July, pgs. 90-97.
2. (a) Thims, Libb. (2007). Human Chemistry (Volume One), (section: "Substrate and catalysts", pgs. 93-98). (preview). Morrisville, NC: LuLu.
(b) Thims, Libb. (2007).
Human Chemistry (Volume Two), (preview
). Morrisville, NC: LuLu.
3. Fletcher, Madilyn. (1996). Bacterial Adhesion: Molecular and Ecological Diversity, (pgs. 4-6). Wiley-IEEE.
4. Fletcher, Madilyn and Marshall, Kevin C. (1982). “Are Solid Surfaces of Ecological Significance to Aquatic Bacteria”, Advances in Microbial Ecology. 12: (pgs. 199-236).
5. Federici, Stefania, Oliviero, Guilio, Maiolo, Daniele, Depero, Laura E., Colombo, Italo, and Bergese, Paolo. (2012). “On the Thermodynamics of Biomolecule Surface Transformations” (abs), Journal of Colloidal and Interface Science, Feb 24.
6. Rusanov, A.I. (2012). “The Development of the Fundamental Concepts of Surface Thermodynamics” (abs), Colloid Journal, 74(2): 136-53.

Further reading
● Neumann, A. Wilhelm and Spelt, Jan K. (1996). Applied Surface Thermodynamics, 2011 2nd ed. CRC Press.
● Salager, Jean-Louis. (2011). “Review: A. Willhelm Neumann, Robert David, and Yi Zuo (eds): Applied Surface Thermodynamics, 2nd Edition. CRC Press, Taylor & Francis Group, Boca Raton, London, 2011.” Journal of Surfactants and Detergents.

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