CHEMICAL PROPERTIES
OF
MATERIAL
SURFACES
Marek
Kosmulski
Technical University
of
Lublin
L
ublin, Poland
m
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Zdzislaw
Kosmulski
1922-1 998
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Adsorption phenomena at solid-electrolyte solution interfaces at room temperature
and at atmospheric pressure are reviewed in this book with a special emphasis on the
mutual relationship between adsorption and surface charging. This relationship is
particularly significant for adsorption of inorganic ions on silica, metal oxides and
hydroxides, certain salts, e.g. silicates, and clay minerals. The models of surface
ionization and complexation originally developed in colloid chemistry are widely
used in different fields, including catalysis, ceramics, corrosion science, environ-
mental sciences, geology,' mineral processing, nuclear waste management, and soil
science. Association with one of the traditional branches of science (and thus
preference for particular journals
or
conferences) is only one of many factors that
have split the scientists interested in adsorption at solid-solution interfaces into
many insulated groups. For example, Western papers have rarely been cited in
former Soviet Union papers and vice versa even though English translations of the
leading Russian scientific journals are readily available. Each group has its own
goals
and methods, and specific systems
of
interest, and often ignore other systems
and methods. The generalizations formulated by different groups are frequently
based on a selective approach. They are not necessarily applicable in other systems
and sometimes contradict each other. The aim
of
this book is to systematize the
existing knowledge and to facilitate the exchange of ideas between different parts of

the scientific community.
V
vi
Preface
The point of zero charge
(PZC)
is a central concept in adsorption of charged
species. This well-known term has been given very different meanings. The
relationship between the zero points obtained by different methods and at different
conditions is discussed in this book. An up-to-date compilation of values of the
points of zero charge of various materials obtained by different methods is
presented. These materials range from simple to very complex and from well- to
ill-defined. Collections of zero points conlpiled by different authors are compared,
and the correlation between these zero points and other physical quantities is
analyzed.
Methods used in studies of adsorption of ions, their advantages and
limitations, the meaning of results, and possibilities to combine results obtained
by different methods are discussed. A large compilation of adsorption data is
presented. The results obtained in simple adsorption systems (with one specifically
adsorbed species) are sorted by the adsorbent and then by the adsorbate. Then, more
complex systems are discussed with many specifically adsorbing species.
Many materials show a certain degree of chemical dissolution. The present
survey is limited to materials whose solubility is low. This does not imply that the
solubility is always negligible in the systems of interest.
Kinetics of adsorption and adsorption of surfactants and macromolecular
species are broad fields, with their own methodologies, theories, and literature. Only
selected topics directly related to the main subject of the present book are briefly
treated.
Many recently published review articles, book chapters, and even entire books
are devoted to adsorption of ionic species. Usually they cover one adsorbent (or a

group of related adsorbents) or specific method(s). Some of these publications were
very helpful during the preparation of this book, but current original papers were the
main source of information.
ACKNOWLEDGMENTS
Substantial parts of this book were prepared at North Carolina State University,
Raleigh, NC, and at Forschungszentrunl Rossendorf, Dresden, Germany. Professors
Robert A. Osteryoung and Thomas Fanghanel are acknowledged for their
hospitality. A grant from the Alexander von Humboldt Foundation, for Chapter
5,
is gratefully acknowledged.
Contents
Preface
1’
1. INTRODUCTION
References
2. PHYSICAL PROPERTTES
OF
ADSORBENTS
1.
Crystallographic Data
11. Thermochemical Data
References
1
10
12
12
49
63
3. SURFACE CHARGING IN ABSENCE
OF

STRONGLY ADSORBING
SPECIES 65
I. Zero Charge Conditions 65
11. Correlations Between Zero Points and Other Physical Quantities 206
TII. Surface Charging in Inert Electrolytes 223
IV.
Temperature Effects 266
V.
Nonaqueous Media 282
References 293
4. STRONGLY ADSORBING SPECIES
I.
Experimental Methods
TI.
Small
Ions
111. Organic Compounds
and
their Mixtures
310
326
355
47
5
vi
i
viii
Contents
IV. Surfactants
V. Polymers

VI. Competition
VII. Kinetics
References
5. ADSORPTION MODELING
I.
Surface Sites
11. Surface Reactions and Speciation
111. Primary Surface Charging
IV. Specific Adsorption
V.
Surface Heterogeneity
References
49
1
503
510
512
555
577
579
58 5
589
666
700
704
6.
SORPTION PROPERTIES
OF
SELECTED ORGANIC MATERIALS 709
I. Activated Carbons 710

11. Latexes 714
111. Other Materials 715
References 71 5
71 7
725
731
745
Introduction
Properties of nunlerous adsorbents have been reported in the literature. This
presentation is confined to materials
0
Having the same bulk and surface chemical composition.
0
Sparingly soluble in water.
e
Showing pH dependent surface charging.
The above terms are relative, for example, some changes in surface
composition due to solvation or selective leaching are unavoidable, but this is a
part of the adsorption process. Adsorbents prepared by grafting or by adsorption of
thin films (one or a few molecular layers) of substances whose properties are
completely different from those of the support constitute an example of essential
difference between the surface and bulk properties. Such materials combine high
mechanical resistance, high surface area, and low cost
of
the support and desired
sorption properties of the film and they are widely applied in different fields, but they
are out of scope of this book.
On
the other hand, the surface properties of composite
materials with external layer at least

10
nm thick are closely related to bulk
properties
of
the coating, and a few examples of such materials will be discussed.
Solubility is another issue that requires some explanation. Materials more
soluble than silica, i.e. about mol dm-3 (this is an arbitrary choice) will not be
discussed here. There
is
no sharp border between “soluble” and “insoluble”, e.g.
dissolution of relatively soluble materials is often sufficiently slow to allow
1
2
Chapter
1
completion of sorption process before significant amount of the adsorbent is
dissolved, moreover, the presence of certain adsorbates can substantially depress the
dissolution rate and/or the equilibrium solubility. On the other hand, the solubility
of materials reputed insoluble, can be significantly enhanced in the presence of
certain complexing and/or redox agents. Kinetics of dissolution of silica is related to
surface charging and depends on pH and ionic strength [l], and
so
is the solubility of
many other materials. Chemical dissolution of oxides has been reviewed by Blesa et
al.
[2,
31
It should be emphasized that points of zero charge of relatively soluble
materials, e.g. BaO and SrO have been reported in the literature.
Gel like materials containing sufficient amount of water are much more

penetrable to ions than crystalline materials. Materials penetrable to ions are
often characterized using the methods and terminology of ion-exchange, i.e. by
their exchange capacities and diffusion coefficients of particular ions, and the
affinity of particular ions to such materials is expressed in terms of selectivity
coefficients. Misak
[4]
reviewed sorption properties of hydrous oxides from such
a perspective, but in some other publications the uptake of ions by gel like materials
is treated as adsorption. It is rather difficult to compare results interpreted in
terms of the “ion exchange” approach on the one hand and adsorption approach
on
the other. Finally, in some publications dealing with materials not penetrable
to ions a terminology borrowed from ion exchange is used: the adsorbents are
“in hydrogen form”, “in sodium form”, etc. This can be translated into the
language used in adsorption, e.g. “adsorbent in calcium form” is considered as
adsorbent with pre-adsorbed calcium. Specific approach is required
to
describe
surface charging of and adsorption
on
clay minerals on the one hand and zeolites on
the other.
Most materials discussed in this book are electrical insulators, and their surface
charge is regulated by sorption processes. However, a few oxides show sufficient
degree
of
electronic conductivity that makes it possible to polarize the surface using
an external battery. For example, the charging curves of Ir02 can be plotted as a
function of pH (when the oxide is polarized to constant potential) or as a function of
polarizing potential at constant pH

[5].
Properties and preparation of oxide
electrodes (often termed DSA, dimensionally stable anodes) were reviewed by
Trasatti
[6].
Also adsorption properties of some sulfides, e.g. natural chalcocite
[7]
can be modified by polarization by external electric potentials.
The “dry” surface chemistry, i.e. chemistry of solid-gas interfaces has its own
methodology and language. A substantial difference between wet and dry surface
chemistry is that adsorption from solution is always an exchange, the “empty”
surface is in Pact occupied by solvent. In spite of an obvious relationship between dry
and wet surfaces, only wet surface chemistry will be discussed here, although some
quantities (e.g. the BET surface area) and relationships involve results obtained for
dry surfaces. In particular, certain adsorbates considered show substantial vapor
pressure at room temperature, and sorption of their vapors has been studied. Such
results, albeit related
to
sorption of the same species from aqueous solution are
beyond the scope of the book.
With a huge amount of information, that has been published on sorption
properties of materials of interest, and in view of broad spectrum of goals
and viewpoints of the authors of the cited publications and of potential readers
of
this book, it is not easy to organize the entire material. The grouping of data
is:
i
”
”
.

Introduction
3
is based on concepts of colloid chemistry. Many results are compiled in tabular form.
They are sorted by the formal chemical formula of the adsorbent and then by the
adsorbate.
Chapter
2,
which is not directly related to surface properties presents physical
and chemical properties of the materials of interest. Not all materials described in
Chapter
2
are then directly referred to in subsequent chapters. For example mixed
oxides are not only adsorbents, but also potential products of surface reactions
involving simple oxides: crystallographic data are helpful in identifying such
products, and thermochemical data in predicting the direction of the reaction.
Chapter
2
also shows how many well-defined and potentially interesting materials
have not been studied as adsorbents and may stimulate further research. The present
author has once submitted a manuscript “Let us measure points of zero charge of
exotic oxides”, but a reviewer was against such an intriguing title and finally the title
was changed it into a more usual one. Now there is a chance to broadcast and extend
this idea: there are
so
many important and well-defined materials whose points of
zero charge are unknown. On the other hand for some materials that have been
extensively used as adsorbents crystallographic or thermochemical data are
incomplete. Availability of physical data for oxides, aluminates and silicates is
illustrated in Figs. 1.1-1.3, respectively. Symbols of elements whose simple or mixed
oxides are considered as adsorbents in this book are printed in boldface; black

background: crystallographic and thermochemical data available, gray background:
only crystallographic data available.
The organization scheme of the crystallographic and thermochemical tables
presented below is also used in the next chapters. Simple oxides, hydroxides and
oxohydroxides are listed first, in alphabetical order of the symbol of the
electropositive element and then from low to high oxidation state, and then from
low to high degree of hydration. Then results for mixed oxides (all component oxides
are insoluble with one exception
of
CO?)
are listed according to the symbol of the
most acidic element, and then according to the symbols of less acidic elements.
Aluminosilicates are listed after aluminates
as
a separate group, followed by clay
minerals (listed alphabetically by name).
A
few basic carbonates are also included in
spite of solubility of
CO2
in water. Namely, basic carbonates are potential products
of reaction
of
certain (hydr)oxides with atmospheric CO2. Also other sparingly
soluble basic salts of water soluble acids can be formed from (hydr)oxides at
sufficiently high concentrations of certain anions,
so
the example of carbonates is not
unique, but physical properties of the other basic salts are not reported
in

Chapter
2.
For derivatives of less common oxides the crystallographic data are not given
explicitly, only formulae of salts for which such data exist are reported.
The presentation of adsorption data follows the rule “from the simplest to the
most complicated”. This was achieved by organizing the adsorbates into the
following categories (listed from the simplest to the most complicated)
H+/OH-
0
Inert electrolytes
Small ions and neutral organic molecules that tend to be specifically
adsorbed
0
Surfactants
0
Polymers
P
FIG.
1
.I
Availability
of
physical data for oxides; black background: crystallographic and therniochemical data available.
ITh
1
Pa
1
U
1NpIPuIAmICrnIBk)Cf)
Es

IFm)MdINo(Lr/
FIG. 1.2
only
crystallographic data available.
Availability
of
physical data for aluminates; black background: crystallographic and therrnochemical data available, gray background:
6
Chapter 1
I
I
cz"
E
u
a'
Introduction 7
gulation involving small and large particles is often considered as adsorption
of
particles on particles. This phenomenon plays an important role in transport of toxic
waste in nature but
it
will not be discussed here. Adsorption
of
particles
on
particles
was recently reviewed by Ryan and Elimelech
[8].
In
most publications a clear distinction is made between sorption (interaction

between sorbate and pre-formed particles of the adsorbent) and coprecipitation
(particles are formed in the presence of foreign species). Sometimes, however, these
two phenomena are confused, e.g.
in
[9,
101
the terms “adsorption” and “sorption”,
respectively, are used in the title of the paper, abstract, text and figure captions,
although the description
of
the experimental procedure clearly indicates that in fact
coprecipitation was studied. Studies reporting only coprecipitation were not taken
into account in the literature survey.
The quantities, which have been directly measured are referred to rather than
those which were derived from the measured data. For example when the adsorption
of counterions was assumed to entirely balance the surface charge (measured
directly), the results are listed under “charge” rather than under “adsorption of
counterions”, even if the later was used in the title of the paper, abstract and figure
captions.
In the studies conducted in aqueous solutions, the presence and thus sorption
of
H+/OH-
ions cannot be avoided, also some concentration of inert electrolyte is
usually present, even if such electrolyte was not added
on
purpose.
To
avoid
repetitions, the studies involving multiple categories of adsorbates are listed only
under the most complex one, e.g. coadsorption of surfactants and small anions is

only listed in the section on surfactants but not in the section
on
sorption of anions.
The above classification of adsorbates is relative and subjective, it is practical,
but not generally accepted.
As
a matter of fact, the same adsorbate can belong to
different categories. This depends
on
specific system (adsorbent, solvent),
concentration. solid to liquid ratio and other experimental conditions. Moreover,
some results (e.g. Hoffmeister series) suggest that there is rather a continuum than a
sharp border between strongly and weakly adsorbing ions. In the ion-exchange
approach
(vide
ultra)
protons are treated as any other cation. In spite
of
some
shortcomings of the proposed classification, it is expedient to associate an adsorbate
with certain category typical for it and then to consider
a
few exceptions. By no
means is this classification original. The terms “potential determining ions”,
“specifically adsorbing ions”, and “inert electrolyte” are widely used in the
literature. Lyklema
[I
11
introduced a term “generic adsorption” as opposite to
“specific adsorption”.

Chapter
3
presents data on points of zero charge obtained by different methods
in the absence of strongly adsorbing species, usually at low concentrations (on the
order of
IO-’
mol dm-3) of inert electrolytes, i.e. alkali nitrates
V,
chlorates VI1 and
halides. Also corresponding salts of ammonium and its short-chain tetraalkyl
derivatives are considered as inert electrolytes. The definition
of the zero point is
anything but trivial. For materials that are not penetrable to ions, the interfacial
region as a whole is electrically neutral, but usually there is
some
excess of positive or
negative charge at the surface due to adsorption/dissociation of protons, which is
balanced by counterions (chiefly ions
of
the inert electrolyte whose sign is opposite to
the sign of the surface charge)
in
the layer of solution next to the surface. The
distribution of the counterions is governed primarily by the electrostatics. With
8
Chapter
1
purely electrostatic interaction the adsorption of coions (ions of the inert electrolyte
whose sign is like the sign of the surface charge) should be negative. The
countercharge is distributed over a thin layer of solution next to the surface, and

a sufficiently thin layer of solution carries net positive or negative charge, despite the
entire system is neutral. One way of defining the zero points is as the pH, at which
certain layer of solution next to the surface has a zero charge. The charge of such
thin layers cannot be measured directly,
so
this kind of definition must be based
on
some assumptions; specific examples are discussed in Chapter
3.
To avoid
assumptions an empirical definition may be used, linking the zero point with the
results obtained by means of specified experimental procedure. In many cited
publications more than one method was used to determine the zero point of the same
material.
The presence of equal amounts of positively and negatively charged sites more
or less evenly distributed over the surface results in zero net charge. It is often
expedient to assume that the surface charge behaves as smeared out homogeneous
charge. However in some phenomena discreteness of charge may play
a significant
role, especially if the surface shows a patchwise heterogeneity.
The materials in the compilation of zero point values are sorted by their formal
chemical formulae and then the entries are sorted by the year of publication. This
approach makes it somewhat difficult to compare results corresponding to given
crystalline modification. On the other hand
in
many publications mixtures of
different phases were used or the crystallographic data were not reported. Finally,
different compositions have been alleged for apparently the same commercial
product. Thus, it would be rather difficult to sort the data (within the same formal
chemical formula) by the structure. Commercial materials are often characterized by

trade names rather than by their crystalline structure.
Reference herein to any specific commercial product by trade name,
trademark, manufacturer, or otherwise, does not constitute or imply its recommen-
dation, or favoring. The difference in the point of zero charge
PZC
between different
crystallographic forms of the same compound has been widely discussed, e.g.
according to Parfitt
[13]
the zero point of rutile is about
1
pH unit below that of
anatase. Many collections of selected
PZC
have been published. These collections are
summarized and briefly discussed after presentation of original data.
In addition to sparingly soluble metal (hydr)oxides, salt type materials
involving two such oxides or more, and clay minerals, whose crystallographic and
thermochemical data are presented in Chapter
2,
the zero points of zeolites, clays,
and glasses are listed (in this order) after mixed oxides. Soils and other complex and
ill-defined materials are
on
the end of the list. It should be emphasized that the terms
“soil”, “sediment”, etc. have somewhat different meanings
in
different scientific and
technical disciplines. This may lead to confusion, e.g. terms “kaolin” (clay) and
“kaolinite” (clay mineral) are treated

as
synonyms in some publications. The zero
points obtained for composite materials with a layer structure (core covered by
coating) are listed separately from those in which the distribution of components is
more uniform.
The salts involving relatively soluble acid or base may show very low solubility.
A
few examples of pH dependent surface charging of such materials are discussed in
a separate section. These salts behave differently from the salts produced by
sparingly soluble acid and base, that are listed as mixed oxides. In the table of zero
Introduction
9
points the salts are sorted according to the chemical symbol of the more acidic
element, and then according to the symbol of the basic element. Sulfides are
discussed separately, and their zero points are not listed in the table.
Correlations between the PZC and other measured quantities are discussed and
illustrated by experimental results. In certain studies of surface charging or
electrokinetic behavior no zero point has been found, often because the zero point
was beyond the experimental range. Such results are collected in two separate tables
(surface charging and
5
potentials). Selected values of
5
potentials and surface charge
density
go
are presented in graphical form. They show the range of values reported in
different sources, and are not supposed to be the “most reliable” ones. In many
experimental studies the inert electrolyte concentrations are integer powers of ten,
and only such data were used in the graphs. With other concentrations it was

difficult to find another set of data obtained at the same ionic strength for
comparison.
The “inert” electrolytes are, indeed indifferent at low concentrations and near
the PZC. For example the values
of
<
potentials and
go
are often symmetrical with
respect to the PZC and insensitive to the nature of these salts. However certain
results, e.g. Hoffmeister’s series, studies
of
uptake
of
ions from solution, and even
shifts in the isoelectric point IEP and PZC suggest non-electrostatic interactions of
these ions and surfaces of materials, especially at high concentrations and far from
the PZC. These results are shown and briefly discussed next.
The compilation of PZC in this book involves results obtained in temperature
range 15-40T. In some studies the temperature was not controlled or measured
(room temperature) or at least the temperature is not reported. Detailed discussion
of temperature effects on the PZC is not intended but a few examples of studies
reporting the temperature dependence of the PZC are presented to show the general
trends. Surface charging in mixed solvents and nonaqueous media, especially in
polar solvents and water-organic mixtures is not much different from that
in
aqueous
solution.
A
few results of such studies are presented as the last section of Chapter

3.
Strongly adsorbing species are discussed in Chapter 4. Specific adsorption of
each class of compounds (small ions, surfactants, polymers) has specific terminology
and methodology. First, methods used to study specific adsorption of small ions are
discussed and the results are presented for cations and anions separately. The results
are organized according to the adsorbent in the same order as the values of PZC.
There are many publications in which more than one adsorbate was studied at
basically the same conditions. There is only one table entry for each set of such data.
The papers reporting data for multiple adsorbates are listed after the publications
dealing with single adsorbate for the same type of adsorbent (in terms of formal
chemical formula). Some readers may be interested in data for specific adsorbate
rather than specific adsorbent, e.g. when looking for the best scavenger for certain
element or species. For their convenience the Tables have indices of entries
corresponding to specific adsorbates (sorted by adsorbate). The term “neutral
organic molecules” used as a title of the next section
is
again conventional, namely,
weak carboxylic acids are discussed in the section on specific adsorption of anions,
although at experimental conditions their degree of dissociation is often low. In the
section
on
neutral organic molecules the adsorption data for amines, phenols, amino
acids, dyes, and humic substances are reported. These substances show certain
degree of acidic or basic dissociation at the experimental conditions and their
IO
Chapter 1
sorption behavior is similar to that of weak acids and bases. On the other hand,
surface charging in the presence of lower alcohols, ethers, ketones, and amides is
discussed in the section on mixed solvents. Separate sections are devoted to
adsorption of surfactants and polymers, in view of their complex solution chemistry.

The section on adsorption competition presents some results obtained in the systems
with more than one adsorbate representing the same class, e.g. adsorption from
solution containing two or more specifically adsorbing anions. Not necessarily does
a real competition occur in such systems; sometimes the presence of other solutes
even leads to a synergistic effect. On the other hand, simultaneous adsorption of
adsorbates representing different classes, e.g. specific sorption of a cation in the
presence of specifically adsorbing anion(s) or at different concentrations of inert
electrolyte is not considered in the section on adsorption competition. The
experimental results reported in Chapter
4
were obtained for different equilibration
times ranging from a few minutes to over one year. The choice of equilibration time
was often based on preliminary studies of the kinetics. Some results are presented as
a conclusion of Chapter
4.
Studies of adsorption kinetics can be also a source of
information about the sorption mechanism.
Models of adsorption are discussed in detail in Chapter 5, but some terms used
in adsorption modeling appear already in the chapters presenting the experimental
data. The concepts and models originally developed for crystalline inorganic
materials were also used to describe sorption properties of organic materials.
A few
examples are presented in Chapter 6.
The phenomena presented in this book were discussed in many reviews. For
example, Schwarz
[
131 discussed methods used to characterize the acid base
properties of catalysts. The review on sorption on solid
-
aqueous solution interface

by Parks
[
141 includes also principles of surface science. The book
Etzvironnzetztul
Chemistry
of
Ahmitzzunz
edited by Sposito reviews the solution and surface chemistry
of aluminum compounds. Chapter 3 [15] provides thermochemical data for
aluminum compounds. Chapter 5
[
161 lists the points
of
zero charge of aluminum
oxides, oxohydroxides and hydroxides with many references on adsorption of metal
cations and various anions on these materials. Unlike the present book, which is
confined to sorption from solution at room temperature, publications on
coprecipitation and adsorption from gas phase or at elevated temperatures are also
cited there. Brown et al.
[
171 reviewed on dry and wet surface chemistry of metal
oxides. Stumm
[
181 reviewed sorption
of
ions on iron and aluminum oxides. The
review by Schindler and Stumm [19] is devoted to surface charging and specific
adsorption on oxides. Schindler
[
191 published a review on similar topic in German.

Many other reviews related to specific topics are cited in respective chapters.
REFERENCES
1.
M. Lobbus,
W.
Vogelsberger,
J.
Sonnefeld, and
A.
Seidel. Langmuir 14: 43864396
2.
M.
A.
Blesa, P.
J.
Morando and A. E. Regazzoni, Chemical Dissolution of Metal Oxides,
3.
J.
A.
Salfity,
A.
E.
Regazzoni and M.
A.
Blesa. In Interfacial Dynamics
(N.
Kallay, ed.)
4.
N.
Z.

Misak. Adv. Colloid Interf. Sci. 51: 29-135 (1994).
5.
0.
A.
Petrii, Electrochem. Acta 41: 2307-2312 (1996).
(1 998).
CRC
Press,
Boca Raton 1994.
Marcel Dekker New York 1999,
pp.
513-540.
Introduction
11
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
S.

Trasatti, Croat. Chem. Acta 63: 313-329 (1990).
J.
S.
Hanson, and D. W. Fuerstenau, Colloids Surf. 26: 133-140 (1987).
J. N. Ryan, and
M.
Elimelech, Colloids Surf. A. 107: 1-56 (1996).
T. Aoki, and
M.
Munemori. Water Res. 16: 793-796 (1982).
S.
Music.
J.
Radioanal. Nucl. Chem. 99: 161-170 (1986).
J.
Lyklema. In Adsorption from Solution at Solid Liquid Interface (G.D. Parfitt and C.
H. Rochester, eds.) Academic Press, New York 1983, pp. 223-246.
G. D. Parfitt, Pure Appl. Chem. 48: 415-418 (1976).
J.
A. Schwarz,
J.
Colloid Interf. Sci. 218: 1-12 (1999).
G. A. Parks, Rev. Mineral. 23: 133-175 (1990).
B.
S.
Hemingway, and
G.
Sposito. In The Environmental Chemistry
of
Aluminum (G.

Sposito, ed.), CRC Press 1996. pp. 81-1 16.
S.
Goldberg, J. A. Davis, and J.
D.
Hem. In The Environmental Chemistry
of
Aluminum
(G. Sposito. ed.), CRC Press 1996, pp. 271-331.
G. E. Brown,
V.
E.
Henrich, W. H. Casey.
D.
L. Clark, C. Eggleston, A. Felmy,
D.
W.
Goodman,
M.
Gratzel,
G.
Maciel,
M.
I.
McCarthy,
K.
H.
Nealson,
D.
A.
Sverjensky,

M.
F.
Toney and J. M. Zachara, Chem. Rev. 99: 77-174 (1999).
W. Stumm. Colloids Surf. A 73: 1-18 (1993).
P. W. Schindler and W. Stumm. In Aquatic Surface Chemistry: Chemical Processes at
the Particle-Water Interface (W. Stumm, ed.) John Wiley 1987, pp. 83-1 10.
W. Schindler, Oster. Chem.
Z.
86: 141 (1985).
2
Physical Properties
of
Adsorbents
I.
CRYSTALLOGRAPHIC
DATA
The surface properties of various phases corresponding to the same chemical
formula can substantially differ from one crystallographic form to another.
This is well known, and the information about the crystallographic form of
the adsorbents is specified in many publications
on
adsorption, especially when
the compound of interest forms more than one phase showing a sufficient stability at
the experimental conditions. Since the present survey is devoted to adsorption
studies performed at room temperature and atmospheric pressure the phases existing
only at very high pressures
or
at very high (or very low) temperatures are not
considered.
Usually there is no doubt which phase is meant in a publication, but also no

normalized or generally accepted system has been established to name different
phases having the same chemical fornula. For example, in many publications
y
and
6
A1203 are considered two different phases, bayerite is called a-A1(OH)3, and
goethite is called a-FeOOH. However, the following excerpt:
‘*.
.
. 6-A1203
(sonletimes called
y
-A1?O3)
.
. .” was found in a recent publication in a highly
ranked scientific journal and this was supported by three literature references. In
another recent publication “y-A1(OH)3 (alumina gel, bayerite)” was studied.
Apparently these three descriptions of the adsorbent are treated as synonyms. In
another recent paper goethite is called “,6”FeOOH”. These three examples of
inconsistent nomenclature are only
a
tip of the iceberg.
12
Physical Properties
of
Adsorbents
13
There are a few examples of two or more different names for the same
crystallographic form or the same name shared by different crystallographic forms
or even different chemical compounds. Specific examples of such inconsistencies can

be found in Table 2.1. Probably the most accurate way to distinguish one phase from
another would be to specify the space group, the lengths of axes of the elementary
cell and the angles between them. This, however, is not practiced; sometimes the
crystal system is specified.
Crystal, system axes lengths
axial angles
triclinic
fl#b#c
&#PSI7
monoclinic
a#b#c
a=y=9O0#p
orthorhombic
n#b#c
a=y=+900
tetragonal
N=b#C
a=y=p=900
trigonal
cr=b=c
12Oo>a!=y=p#9O"
hexagonal
a=b#c
CY
=
/3
=
90";
y
=

120"
cubic
a=b=c
= =
/3
=
900
The above choice
of
axes for particular systems is the most common one but it is not
unique, e.g. for the trigonal and hexagonal systems it is convenient to use four axes:
three of them are in one plane symmetrically spread to
120"
and the fourth axis is
perpendicular to this plane.
Table 2.1 lists adsorbents of interest, i.e. their chemical formulas and own
names (and/or Greek letters used to distinguish given phase from the other
polymorphs) and their crystallographic data: space group number (1-230; groups
1
and 2 belong to triclinic system, 3-15 to monoclinic, 16-74 to orthorhombic, 75-142
to
tetragonal, 143-167 to trigonal, 168-194 to hexagonal and 195-230 to cubic
system), space group according to Schoenflies, space group according to Hermann-
Mauguin (sometimes the group number is unknown, in such instance the crystal-
lographic system
is
given in Table
2.1,
P
=

primitive;
F
=
face centered; R
=
rhombohedral), the number of formula units in one cell
2,
experimentally determined
specific density
d (in kg m-3 at 25°C or at the temperature given in brackets in
"C),
cell axes lengths
n,
b,
c
(in nm), and axial angles (decimal fractions are used
rather than angular minutes). The empty cells in Table 2.1 have the following
meanings: for axial angles empty cell
=
90"; for cell axes empty
b
or
c
means that
b=n
or
c=n,
respectively; empty
2
or

d
cell: data not available; there are also many phases
which to the best knowledge of the present author do not have any names, sometimes
the "name" cell contains other informations (e.g. about thermodynamic stability).
Table 2.1 reports crystallographic data of simple oxides, hydroxides and
oxohydroxides, and then for mixed oxides (as discussed in Chapter 1). The principle
of organization of Table 2.1 has been described in the Introduction. For derivatives
of less common oxides, the compounds having known crystallographic structure are
only listed, without specific information on this structure.
Reference [I] was the sole source of this data. Attempts to find a more up to
date and equally comprehensive compilation failed.
A
relatively recent compilation
[2] covers only rock-forming materials and most of the cited literature dates back to
the sixties and early seventies. Reference [3] presents a collection of crystallographic
data based on older compilations. The data from [1-31 match for a vast majority
of
(Text
continues
011
pg.
49)
14
Chapter
2
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Physical Properties

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