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Name : Muhammad Tri RizkiNIM : 21100113140062Geological Engineering B
Surfaces and Interfaces: General Concepts
For purposes of terminology, it is common practice to refer to that nebulous
region as a ‘‘surface’’ or an ‘‘interface.’’, In general, however, one usually finds that
the term ‘‘surface’’ is applied to the region between a condensed phase (liquid or
solid) and a gas phase or vacuum, while ‘‘interface’’ is normally applied to systems
involving two condensed phases. There are several types of interfaces that are of
great practical importance and that will be discussed in turn. These general
classifications include, solid– vacuum, liquid–vacuum, solid–gas, liquid–gas, solid–
liquid, liquid–liquid, and
solid–solid. A list of commonly encountered examples of these interfaces is given in
a table below
Interface Type Occurrence or Application
Solid–vapor Adsorption, catalysis, contamination,
gas–liquid chromatography
Solid–liquid Cleaning and detergency, adhesion,
lubrication, colloids
Liquid–vapor Coating, wetting, foams
Liquid–liquid Emulsions, detergency, tertiary oil
recoveryTABLE 2.1. Common Interfaces of Vital Natural and Technological Importance
In order for two phases to exist in contact, there must be a region through
which the intensive properties of the system change from those of one phase to those
of the other, as for example in the boundary between a solid and a liquid. In order for
such a boundary to be stable it must possess an interfacial free energy such that work
must be done to extend or enlarge the boundary or interface.
In order to define an interface and show in chemical and physical term that it
exists, it is necessary to think in terms of energy, nature will always act so as to attain
a situation of minimum total free energy. In the case of a two-phase system, if the
presence of the interface results in a higher (positive) free energy, the interface will
spontaneously be reduced to a minimum—the two phases will tend to separate to the
greatest extent possible within the constraints imposed by the container, gravitational
forces, mechanical motion, and other factors. Overall, the interfacial energy will still
be positive, but the changes caused by the alteration may prolong the ‘‘life’’ of any
‘‘excess’’ interfacial area. Such an effect may be beneficial, as in the case of a
cosmetic emulsion, or detrimental, as in a petroleum–seawater emulsion. Although
thermodynamics is almost always working to reduce interfacial area, we have access
to tools that allow us to control, to some extent, the rate at which area changes occur.
The concept of the interfacial region will be presented from a molecular (or
atomic) perspective and from the viewpoint of the thermodynamics involved. In this
way one can obtain an idea of the situations and events occurring at interfaces and
have at hand a set of basic mathematical tools for understanding the processes
involved and to aid in manipulating the events to best advantage.
As will be seen throughout, the unique characters of interfaces and interfacial
phenomena arise from the fact that atoms and molecules at interfaces, because of
their special environment, often possess energies and reactivities significantly
different from those of the same species in a bulk or solution situation. If one
visualizes a unit (an atom or molecule) of a substance in a bulk phase, it can be seen
that, on average, the unit experiences a uniform force field due to its interaction with
neighboring units (Fig. 2.1a). If the bulk phase is cleaved in vacuum, isothermally
and reversibly, along a plane that just touches the unit in question (Fig. 2.1b), and the
two new faces are separated by a distanceH, it can be seen that the forces acting on
the unit are no longer uniform.
The net increase in free energy of the system as a whole resulting from the
new situation will be proportional to the area,A,of new surface formed and the
density (i.e., number) of interfacial units. The actual change in system free energy
will also depend on the distance of separation, since unit interactions will generally
fall off by some inverse power law. When the term ‘‘specific’’ excess surface free
energy is used it refers to energy per unit area, usually in mJ m-2. It should be
remembered that the excess free energy is not equal to the total free energy of the
system, but only that part resulting from the units location at the surface.
It should be intuitively clear that atoms or molecules at a surface will
experience a net positive inward (i.e., into the bulk phase) attraction normal to the
surface, the resultant of which will be a state of lateral tension along the surface,
giving rise to the concept of ‘‘surface tension.’’ For a flat surface, the surface tension
may be defined as a force acting parallel to the surface and perpendicular to a line of
unit length anywhere in the surface (Fig. 2.2). The definition for a curved surface is
somewhat more complex, but the difference becomes significant only for a surface of
very small radius of curvature.
The specific thermodynamic definition of surface tension for a pure liquid is
given by
Where AH is the Helmholtz free energy of the system, Wis the amount of reversible
work necessary to overcome the attractive forces between the units at the new
surface, andA is the area of new surface formed. The proportionality constant σ,
termed the ‘‘surface tension,’’ is numerically equal to the specific excess surface free
energy for pure liquids at equilibrium; that is, when no adsorption of a different
material occurs at the surface. The SI (International System of Units) units of surface
tension are mN m-1, which can be interpreted as a two-dimensional analog of pressure
(mN m-2 ). As a concept, then, surface (and interfacial) tension may be viewed as a
two-dimensional negative pressure acting along the surface as opposed to the usual
positive pressures encountered in our normal experience. The work of cohesion,Wc, is
defined as the reversible work required to separate two surfaces of unit area of a
single material with surface tension σ (Fig. 2.3a).
Based on the distinction between solid and liquid surfaces the definition
applies strictly to liquid surfaces, although the concept is useful for solid surfaces as
well.
the work of cohesion is simply
Related toWc is the work of adhesion, Wa(12), defined as the reversible work required to separate unit area of interface between two different materials (1 and 2) to leave two ‘‘bare’’ surfaces of unit area (Fig. 2.3b). The work is given by
Surface Activity and Surfactant Structures
Surface-active materials (surfactants) possess a characteristic chemical
structure that consists of (1) molecular components that will have little attraction for
one surrounding (i.e., the solvent) phase, normally called the lyophobic group, and
(2) chemical units that have a strong attraction for that phase—the lyophilic group
(Fig. 3.1).
In an aqueous surfactant solution, for example, such a distortion (in this case
ordering) of the water structure by the hydrophobic group decreases the overall
entropy of the system (Fig. 3.2).
The amphiphilic structure of surfactant molecules not only results in the
adsorption of surfactant molecules at interfaces and the consequent alteration of the
corresponding interfacial energies, but it will often result in the preferential
orientation of the adsorbed molecules such that the lyophobic groups are directed
away from the bulk solvent phase (Fig. 3.3).
The chemical structures having suitable solubility properties for surfactant
activity vary with the nature of the solvent system to be employed and the conditions
of use. In water, the hydrophobic group (the ‘‘tail’’) may be, for example, a
hydrocarbon, fluorocarbon, or siloxane chain of sufficient length to produce the
desired solubility characteristics when bound to a suitable hydrophilic group. The
hydrophilic (or ‘‘head’’) group will be ionic or highly polar, so that it can act as a
solubilizing functionality.
The chemical reactions that produce most surfactants are rather simple,
understandable to anyone surviving the first year of organic chemistry. The challenge
to the producer lies in the implementation of those reactions on a scale of thousands
of kilograms, reproducibly, with high yield and high purity (or at least known levels
and types of impurity), and at the lowest cost possible.
Surfactants may be classified in several ways, depending on the intentions and
preferences of the interested party (e.g., the author). One of the more common
schemes relies on classification by the application under consideration, so that
surfactants may be classified as emulsifiers, foaming agents, wetting agents,
dispersants, or similar.
Surfactants may also be generally classified according to some physical
characteristic such as it degree of water or oil solubility, or its stability in harsh
environments. Alternatively, some specific aspect of the chemical structure of the
materials in question may serve as the primary basis for classification; an example
would be the type of linking group (oxygen, nitrogen, amide, etc.) between the
hydrophile and the hydrophobe. The four general groups of surfactants are defined as
follows:
Synthetic surfactants and the natural fatty acid soaps are amphiphilic materials
that tend to exhibit some solubility in water as well as some affinity for nonaqueous
solvents. As an illustration, consider the simple, straight-chain hydrocarbon
dodecane,
CH3(CH2)10CH
a material that is, for all practical purposes, insoluble in water. If a terminal hydrogen
in dodecane is exchanged for a hydroxyl group (-OH), the new material,n-dodecanol,
CH3(CH2)10CH2OH
still has very low solubility in water, but the tendency toward solubility has been
increased substantially and the material begins to exhibit characteristics of surface
activity. If the alcohol functionality is placed internally on the dodecane chain, as in
3-dodecanol, the resulting material will be similar to the primary alcohol but will
have slightly different solubility characteristics (slightly more soluble in water).
If the original dodecanol is oxidized to dodecanoic acid (lauric acid) is
CH3(CH2)10COOH the the compound still has limited solubility in water; however,
when the acid is neutralized with alkali it becomes water soluble—a classic soap. The
alkali carboxylate will be a reasonably good surfactant.
The solubilizing groups of modern surfactants fall into two general categories:
those that ionize in aqueous solution (or highly polar solvents) and those that do not.
Obviously, the definition of what part of a molecule is the solubilizing group depends
on the solvent system being employed. For example, in water the solubility will be
determined by the presence of an ionic or highly polar group, while in organic
systems the active group (in terms of solubility) will be the organic ‘‘tail.’’ It is
important, therefore, to define the complete system under consideration before
discussing surfactant types.
The functionality of ionic hydrophiles derives from a strongly acidic or basic
character, which, when neutralized, leads to the formation of true, highly ionizing
salts. The most common hydrophilic groups encountered in surfactants today are
illustrated in Table 3.1, where R designates some suitable hydrophobic
By far the most common hydrophobic group used in surfactants is the
hydrocarbon radical having a total of 8–20 carbon atoms. Commercially there are two
main sources for such materials that are both inexpensive enough and available in
sufficient quantity to be economically feasible: biological sources such as agriculture
and fishing, and the petroleum industry (which is, of course, ultimately biological).
Listed below and illustrated structurally in Figure 3.4 are the most important
commercial sources of hydrophobic groups, along with some relevant comments
about each.