Robert J. Good

An equation-of-state approach to determine surface tensions of low-energy solids from contact angles

Abstract An equation of state is developed which allows the surface tension of a low-energy solid to be determined from a single contact angle formed by a liquid which is chemically inert with respect to the solid and whose liquid surface tension is known. The equation of state is obtained using two independent methods. In the first one, similar arguments to those in previous papers are used; however, the qualitative argument, based on the general appearance of plots, is replaced by computer curve fitting and statistical analysis. The second method, which has not been employed heretofore, treats the solid surface tension as an adjustable parameter. Molecular arguments in conjunction with the interaction parameter Φ are used to eliminate poor choices of the solid surface tension. The results are in excellent agreement with the first method. The range of validity of the equation of state and practical points in its application are discussed.

Thermodynamics of contact angles. I. Heterogeneous solid surfaces

Abstract A theoretical treatment of the effect of surface heterogeneity on contact angles is given, by means of a model employing the capillary rise of a liquid in contact with a stripwise heterogeneous surface. Local contortions of the liquid-vapor surface are postulated to conform to an assumed periodic shape of the three-phase line. A minimum of free energy is found in a configuration in which Youngs equation is obeyed locally. When the width of the strips is below some value of the order of 0.1 μ, the amplitude of the periodic contortion of the three-phase line is less than about 10 A, which is operationally indistinguishable from a straight line. Extension of this model is made to a patchwise heterogeneous surface, and a mechanism for hysteresis is developed. For patches smaller than about 0.1 μ, it is shown that heterogeneity should make a negligible contribution to hysteresis.

Thermodynamics of contact angles. II. Rough solid surfaces

Abstract The thermodynamics of an idealized rough surface is treated, using the geometry of a vertical plate partially immersed in a liquid. Gravity is included explicitly in the theory. The results of this treatment are more general than those of previous studies and are more easily extended to other surface topographies. Some novel results are found, such as a delineation of the conditions under which a macroscopic contact angle of 180° will result from geometric properties of the solid surface. On rough surfaces consisting of material for which, if smooth, the equilibrium contact angle would be different from 90°, the slopes of the asperities will be a very important factor in determining the effective equilibrium contact angles.

Determination of contact angles and pore sizes of porous media by column and thin layer wicking

A simple method is described for the determination of contact angles (0) on powdered materials such as clay particles. This is done by depositing the particles from a liquid suspension onto glass slides, by sedimentation, followed by drying. The dried thin-layer plates are then subjected to wicking in a number of liquids using the Washhurn equation to determine cos . However, one other unknown in the Washburn equation, i.e. the average interstitial pore radius R, must first be determined. This is done by wicking with low-energy spreading liquids, such as alkanes. It could be shown with spherical monosized polymer particles, as well as with clay particles, that spreading liquids pre-wet the surfaces of the particles over which they subsequently spread. Thus, it can be demonstrated that spreading coefficients, in the sense of Harkins, play no role in this type of spreading and cos 0 equals unity in the Washburn equation for all values of γ1, for all spreading liquids (L). Results were obtained by thin layer...

Surface free energy of solids and liquids: Thermodynamics, molecular forces, and structure

Surface chemistry constitutes a major part of science which has ubiquitous consequences in chemical processes, in engineering and technology, in biology and medicine, and, indeed, in the very existence of life. There are two properties of surfaces that are fundamentally responsible for these consequences. One is surface free energy; the other is surface charge. We owe the concepts of surface tension and surface free energy to several of the great physicists of the 19th century, such as Young (1) and Gibbs (2). Gibbs worked out the basic surface thermodynamic concepts a century ago; this was several decades before even the order of magnitude of the dimensions of molecules was established, and long before polymers were given scientific recognition. I t has been a major item on the agenda of physical science for over a century, to find out how the experimental, macroscopically observable quantities, such as surface tension, are related to atomic and molecular structure. A number of important chemists and physicists worked on this agenda, before and shortly after Word War II. I will review several aspects of this earlier work, in order to show what it was that made it possible for my research group at Cincinnati, 20 years ago,

The Modern Theory of Contact Angles and the Hydrogen Bond Components of Surface Energies

We owe a great debt to W. A. Zisman and his colleagues at the Naval Research Laboratory for their extensive, pioneering work that opened up the field of contact angles and made possible the development of the modern theory of wetting and adhesion. Their data on the wetting of solids by apolar liquids and by hydrogen bonding liquids pointed the way to the recent introduction of a theory of hydrogen bond interactions across interfaces. We will devote this chapter to a review of this new theory.

Contact Angles and the Surface Free Energy of Solids

This chapter, and the following one by Neumann and Good, deal with the measurement of contact angles in three-phase systems. The contact angle is, intrinsically, a macroscopic property, and one that should be amenable to a phenomenological treatment, e.g., to measurement without regard to its thermodynamic or microscopic interpretation. But inevitably, the desire for a thermodynamic and molecular interpretation arises. In addition, some fundamental questions about solid surfaces come up, and the problem of hysteresis must be given explicit consideration. So there is a need to go beyond phenomenology.

The Mechanism of Phase Separation of Polymers in Organic Media—Apolar and Polar Systems

Abstract The phase separation properties of nine apolar and 15 polar systems, each comprising two polymers dissolved in one organic solvent, were examined in the light of recent developments in surface thermodynamics of polar components. A clear correlation existed between the sign of the total interfacial free energy of interaction of the system and the phase separation (or miscibility). Conversely, some of the phase separation results could be used to estimate the electron-donor surface tension parameters of methyl ethyl ketone and tetrahydrofuran.

The Mechanism of Partition in Aqueous Media

Abstract From the quantitative determination of the polar surface tension parameters of Dextran 150 (DEX) and polyethylene glycol 6,000 (PEG), it is shown that both polymers are pronounced monopolar Lewis bases. By means of the theory of short-range (SR) (polar or hydrogen bonding) interactions, and also taking into account the influence of long-range Lifshitz-van der Waals (LW) interactions, it is demonstrated that DEX and PEG, immersed in water, repel each other with a sizable repulsion energy. That repulsion energy is highest when the polymers are most strongly dehydrated. Thus it becomes clear why a certain concentration of each polymer must be reached before phase separation can occur. Monopolar repulsion also accounts for the occurrence of the formation of multiple phases, provided the participating polymers all are monopoles of the same sign. The mechanism for the preferential migration of biopolymers or particles to one or the other phase is also elucidated. While in some cases (e.g., affinity par...

Theory of “Cohesive” vs “Adhesive” Separation in an Adhering System

Abstract It is shown that there is limited validitity to the doctrine that true interfacial separation, in an adhering system, is highly improbable. An analysis employing the Griffith-Irwin crack theory yields these results: The important parameters are, difference in elastic moduli, ΔE; differences in g, the energy dissipation per unit crack extension; thickness, Δ1 or δ2, of the region where dissipation occurs; and the presence or absence of strong interfacial bonds. If the forces across the interface are appreciably weaker than the cohesive forces in either phase, there is a strong minimum in g at the interface. For flaws of equal size, an interfacial flaw will be the site of initiation of failure. If strong interfacial bonds are present, then if Δg and ΔE have the same sign, failure is most probable, deep within one phase. If Δg and ΔE have opposite signs, failure may be initiated, and may propagate, at a distance δ from the interface, in the phase with lower g. This may be mistaken for weak-boundary ...