Surface energy - meaning?

J S Webb momcfjsw at fs2.scg.man.ac.uk
Thu Sep 17 12:30:58 EST 1998


RE: Surface energy - meaning?

Miles, John, Paul (and newsgroup!),

Yes, surface energy is certainly an important determinant in adhesion
of bacteria to surfaces. When you think of the role that initially
adhering organisms have in linking the substratum with the rest of the
biofilm, it probably influences subsequent events as well (e.g.
shearing of the biofilm away from the surface). This is a complex but
important area - I'll try my best to explain!

The definition of surface energy is the work required to increase the
surface area of a substance by unit area. So what. The question is,
how does this relate to the adhesion of microorganisms to surfaces??

Surface energy derives from the unsatisfied bonding potential of
molecules at a surface,  giving rise to 'free energy'. This is in
contrast to molecules within a material which have less energy because
they are subject to interactions with like molecules in all
directions. Molecules at the surface will try to reduce this free
energy by interacting with molecules in an adjacent phase. When one of
the bulk phases is a gas, the free energy per unit area is termed the
surface energy for solids, and the surface tension in liquids. One
manifestation of surface energy is a state of tension at the surface
of a liquid, which is why work is required to increase the surface
area of a liquid, hence the above physical definition. However, when
both phases are condensed, (i.e. solid-solid, solid-liquid and
immiscible liquid-liquid interfaces) the free energy per unit area of 
the interface is called the 'interfacial energy'.

It is possible to use an 'interfacial energy balance' of all the
interfaces involved  in adhesion to tell whether adhesion will be
energetically favorable. When a bacterium attaches to a surface in a
liquid suspension, there are three interfaces involved:
cell-substratum, cell-liquid, and substratum-liquid. Each of these
will have its own interfacial energy (designated cs, cl and sl
respectively). The balance of these interfacial  energies is called
the 'free energy of adhesion' (F). Therefore   F =  cs - cl - sl. 
Thermodynamically (ignoring electrostatic interactions!), adhesion is
favored if the free energy of adhesion is negative, i.e. F < 0. A
high, positive F value is favorable for reducing adhesion.
Practically, the interfacial free energy of each  of the interfaces
can be derived from surface energy measurements of each component in
the system. Contact angle measurements can be used to determine the
surface energies of solids and bacteria, whereas in liquids they are
usually determined tensiometrically.

The term surface energy is also closely linked with surface
hydrophobicity. Whereas surface energy describes interactions with a
range of materials, surface hydrophobicity describes these
interactions with water only. Because water has a huge capacity for
bonding, a material of high surface energy (i.e. high bonding
potential) can enter into more interactions with water and
consequently will be more hydrophilic. Therefore hydrophobicity
generally decreases as surface energy increases. Hydrophilic surfaces
such as glass therefore have high surface energies, whereas
hydrophobic surfaces such as PTFE or polystyrene have low surface
energies. 

There are plenty of examples in the literature looking at the effect
of surface energy on adhesion. Generally, adhesion increases with
decreasing surface energy and increasing hydrophobicity. However, the
balance of interfacial free energies will be different in each system
and consequently there are examples where this is not the case. Each
system should therefore be considered separately.

Hope this has helped!, e-mail me if you need refs. I think initial
adhesion is an important area and would like to see more
comments/discussion on this.

Jeremy Webb

 
Jeremy Webb
1.812 Stopford Building
School of Biological Sciences
University of Manchester
Oxford Road
Manchester M13 9PT

Tel: 0161 275 5510
Fax: 0161 275 5656


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