t.geo task no. 2 nashir idzharul huda 21100113130090
TRANSCRIPT
Name : Nashir Idzharul Huda
NIM : 21100113130090
Geological 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 ApplicationSolid–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
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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.
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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
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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
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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).
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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).
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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
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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
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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.
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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.
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