2 The Chemical Structure of Solid Surfaces as Deduced from Contact Angles
Downloaded by UNIV OF MINNESOTA on May 11, 2013 | http://pubs.acs.org Publication Date: January 1, 1964 | doi: 10.1021/ba-1964-0043.ch002
Ν. K. A D A M Department of Chemistry, University of Southampton, Southampton, England The relation between equilibrium contact angles at a solid surface, and the adhesion between solid and liquid, is reviewed. The information deducible as to the chemical na ture of the groups exposed at the surface is summarized. This brief review surveys our knowledge, and the methods by which it has been obtained, of the chemical composition of some solid sur faces, mostly of the low-energy type, as learned from measurements of contact angles. The basic equations were known over 150 years ago, and can be found in Thomas Young s classical treatise, " O n the Cohesion of Fluids" [18]. They were given in words rather than in algebraic form; for this reason some seem to have missed the fact that Young s exceptional insight enabled him to see that the contact angle gives the relation be tween the adhesion of the liquid to the solid, and its cohesion to itself, as expressed in Equation 2. When the adhesion is less than the selfcohesion of the liquid, there is a contact angle, the larger the smaller the adhesion. When the adhesion is equal to or greater than the cohesion, the angle is zero. The equations for finite angles are T
T
y s = y
SL
Θ
(l)
(1 + cosfl)
(2)
+
y
L
cos
and W
S L
= y
L
7SJ 7LJ 7SL * free energies per square centimeter of the solid, liquid, and solid-liquid interfaces—i.e., their surface tensions; W is the work of adhesion—i.e., the work required to separate the liquid from the solid; and θ is the contact angle measured in the liquid. The name "Young s equation" has been given to both Equations 1 and 2, although it is more commonly given to 1, especially in America. But we would do more honor [1] to Thomas Young by giving his name to Equation 2, rather than to 1, since Equation 1 is obvious to anyone who regards surfaces as being in tension—the concept of surface tension a
r
e
n e
S L
1
52
In Contact Angle, Wettability, and Adhesion; Fowkes, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1964.
2.
ADAM
Surface
Structure
from
Contact
53
Angles
was generally accepted in Young s time, but the concept of free energy had not been introduced. Equation 2 is much less obvious. Moreover, 2 is far more useful than 1, since it contains on the right-hand side only measurable quantities, whereas 1 contains two solid surface ten sions, y and y , which are almost impossible to measure. The term y in Equation 1 needs clarification. A s Bangham and Razouk [6] pointed out, the vapor of the liquid will be adsorbed on the solid surface, often considerably decreasing its surface free energy. In Equation 1, and also in Dupre s Equation 3, T
s
S L
s
T
W
SL
+
=
?L
-
3
^SL
(>
Downloaded by UNIV OF MINNESOTA on May 11, 2013 | http://pubs.acs.org Publication Date: January 1, 1964 | doi: 10.1021/ba-1964-0043.ch002
which, when combined with 1 gives 2, we should write 7 s = Ύ s - ^sv where π is the surface pressure of this adsorbed film of the vapor of the liquid, and y s is the free energy of a film-free solid surface. Since the part of the surface relevant for contact angle equilibrium is infinitesimally distant from the edge of the liquid, we can safely assume that this adsorbed film is in equilibrium with the saturated vapor pressure of the liquid. We rewrite Equations 1 and 3 thus: s v
-
77
sv
=
^SL
7
+
c
L
o
s
