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Glossary
(Status: July 2003)
Here
you will find a glossary of terms in surface chemistry, which is
also intended to give a brief introduction into the most common and
important topics in this science.
Please
click on the first letter of the word you are looking for:
A
B C D
E F G
H I J
K L M
N O P
R S T
U V W
X Y Z
A
absorption:
is the entering of particles of one phase into a
different bulk phase by penetrating a
surface
absorption
kinetics:
time dependant behavior of absorption, depending on surface
characteristics, temperature, porosity and thickness of the material
activation
energy:
minimum amount of energy, that must be accommodated to a system in
order to start a process, mostly for chemical reactions
adhesion:
the chemical or mechanical bonding (strength) of one material to another
material
adsorbate: the concentrated material on a
surface or
interface
adsorbent: the surface
of a material, where a different material is being concentrated
adsorption: is the enrichment or
agglomeration of particles on a surface or
interface
advancing contact angle:
type of dynamic contact angle, which is measured:
a) by dipping in a solid geometry into a liquid from the wetting
force, due to the dynamic
Wilhelmy method,
b) by expanding the volume of a
sessile drop on a horizontal surface
c) at the forefront of a just rolling droplet on an inclined
surface,
in order to determine the dewetting
properties of a solid
aerosol: a colloidal dispersion of solid or liquid
particles in a gas
agglomerate:
spatial collection of particles, that are chemical or mechanical not
bonded
aggregate:
spatial collection of particles; aggregates are distinguished from
agglomerates by the increased difficulty in separation
amphipathic (amphiphilic): denotes a molecule combining hydrophilic and
lipophilic (hydrophobic)
properties
B
base diameter of a drop: diameter of a drop at the
generally circular area of contact (interface) to the wetted material;
for axissymmetric drops also the distance between the left and the
right three phase point
base line: is the (orthogonal)
projection line of the area of contact between the sessile
drop and the solid
bilayer: bimolecular film
bipolar:
bifunctional material with a surface that can
act as an
electron donor as well as an electron acceptor
bubble pressure method: method for
the determination dynamic
surface tensions of a liquid by measuring the maximum bubble pressure of a gas bubble in the liquid;
the gas bubble is produced on the tip of a capillary
immersed in the liquid of interest
C
calorimeter:
a device to measure the heat
capillary: very fine, most extended cavity, particularly
used for test tubes with small inner diameters
capillary pressure:
the normal stress developed on the phase
boundaries of curved objects (drops, bubbles, lamellae) which
correlate with the principal radii of curvature R1 and R2
and the surface or interfacial
tension of the liquid phase gL
: Dp
= gL
(1/R1+1/R2)
capillary forces:
denotes the kind of forces which arise from surface tension effects
in capillaries, narrow crevices and
pores
catalyst:
a substance, which increases or decreases the speed of a chemical
reaction and which remains unchanged after the chemical
reaction
chemical adsorption:
see chemisorption
chemisorption: type of adsorption characterized by a chemical reaction
between the concentrated particles and the enriched surface
circle fit: see also height/width
method; simple method to derive the contact
angle from the height/width ratio of the measured drop or bubble
contour assuming a circular shape; more precise
analysis methods are preferred (e.g. ellipse
fit, Young-Laplace fit) when
the contact angle is higher than 20°;
cluster:
spatial collection of molecules or atoms (sub units), which possess
a particular degree of order or state of symmetry and being in
exchange equilibrium with its environment; clusters in liquids and
gases show an increased rate of
fluctuations of the sub units on their surface
coadsorption: describes the simultaneous adsorption of two
or more different particles on a surface
coagulation:
clotting. process of converting a finally divided or colloidally dispersed
suspension of one substance in a liquid into larger-size particles
which do settle or precipitate under the influence of gravity, also
described as the transition of a sol into a gel under flocculation
cohesion:
the intermolecular attraction between separate molecules of a
homogeneous phase
colloid (Thomas Graham, 1861): state
of a material consisting of extremely fine dispersed particles,
(typically in a fluid), possessing diameters in one dimension in the range of
1 up to 1000 nm; see also nanoparticles
colloid chemistry: branch within the physical chemistry
concerning the colloidal state of matter
compressibility:
property of liquid, solid, and gaseous phases
to decrease their volume under applied (isotropic) pressure
concentration:
number of particles (e.g. in g or in mole) per volume of a
substance
condensation: specific:
fluidification of a gaseous phase; general:
phase transition of a gaseous or vapor phase
into a liquid or solid phase (condensed phase)
by decreasing the temperature of the system
contact angle: angle which
encloses the tangent line on the drop
shape from the three-phase point to the base
line on the solid surface; the
contact angle is a measure of the wettability of a
surface or
interface with a different liquid phase; the smaller the contact angle the
better the wettability;
contact angle hysteresis: the difference between the
advancing and receding contact angle
critical micelle
formation concentration (CMC): a characteristic concentration of
a surfactant solution, at which
the molecules of the surface active agent
starts to build agglomerates (micelles)
standing a kinetic equilibrium with the remaining single molecules of the
solution; in crossing the CMC as a phase transition point, the
slopes of several physical properties as a function of concentration
changes their values, particularly the surface
and interfacial tension show an inflexion point
at the CMC
critical surface energy:
denotes the surface energy of a
solid which is equivalent to the critical surface tension;
see also Zisman plot
critical surface tension:
the surface tension above which a liquid cannot completely wet a
solid; see also Zisman plot
D
desorption:
describes the opposite process to an adsorption, that means the removal of
aggregated particles from a surface
detergent:
synthetic washing auxiliary; surface active agent to remove
dirt or chemical contaminations from surfaces
(mostly from textiles)
diffusion:
concentration dependant migration of colloidal
particles, molecules or
ions, mainly caused by random thermal movement
diffusion coefficient: describes the rate of diffusion of
particles, depending on the particle size, viscosity and
temperature
dipole:
chem: polar molecule; phys.: electrically or
magnetically charged particle with spatially separated positive and negative charge
distributions
dipole-dipole interactions: electromagnetic attraction and
repulsion between
dipoles
dipole momentum: vector quantity
assigned to a dipole, which gives the charge and
the direction of the dipole action
dispersive contributions:
contributions to the surface energy or
surface resp. interfacial
tension, caused by dispersive
interactions on the phase boundaries
dispersive interaction:
intermolecular attractions between induced (non-permanent) dipoles
dispersion: distribution of solid particles in a
liquid
drop base diameter: see base diameter of a
drop
drop shape: optical contour
(shape projection) of a drop
drop volume: volume of a drop;
under certain conditions arising from the axis symmetry of the drop;
the volume can by calculated just from the observed drop
shape
drop volume method:
method to determine the dynamic interfacial
tension as a function of the dosing rate, drop age, temperature or
concentration
Du Noüy ring method (after P. Lecomte Du Noüy,
1919): method to determine the static surface and interfacial tension of liquids. A ring is dived into a
liquid and removed from the liquid again. The maximum force to push
or pull the arising liquid lamella out of the liquid is measured;
the method is frequently used to measure the
critical micelle formation concentration (CMC)
dynamic contact angle:
advancing and
receding contact angle(s) of a liquid front on a solid; mainly used to
determine the surface (free) energy of
solids
dynamic interfacial
tension: interfacial tension
apart the (complete) thermodynamic equilibrium
dynamic surface tension:
surface tension apart the
(complete) thermodynamic equilibrium
dynamic Wilhelmy method (after L. Wilhelmy, 1863): method to determine the
advancing and receding contact angle of prismatic or
cylindrical samples with a known geometry by pushing and pulling the sample into
resp. out of liquid
E
elasticity:
property of a substance to change it's length, volume or shape under
an applied forces and return to the original state after removing the
force
ellipse fit:
approximate method to calculate the contact angle by
assuming an elliptical shape for the drop
contour; mainly used for contact angles
between 20° and 110°
emulsion: dispersion of two
not completely miscible liquid phases, for example water
/ oil
enthalpy: the enthalpy H of a physical systems consists
of the inner energy U of the system and the volume work pV: H =
U + pV
entropy:
quantity to describe the degree molecular disorder resp. the degree
of molecular mobility of a physical system; the amount of entropy is
increasing when the molecular disorder is rising, e.g. by elevating
the temperature
equilibrium:
latin: (thermodynamic) balance
expansion: here thermal expansion; increase in volume by
elevating the temperature of a material, depends on the kind of substance,
and the starting and end temperature
expansions factor:
thermal volume expansion rate of materials
b
= 1/V·(¶V/¶T)p;
is closely correlated to the temperature coefficient of surface
tension; for many molecular liquids holds the following approximate
relation: dg(T)/dT
= 4b·g(T)
F
film: see surface film
filler:
material added to another material e.g. polymer, paper, rubber, to
increase the strengths or decrease the price of the product
flocculation: creation of flocs in a liquid by addition
of a chemical agent or by changing the physical conditions (temperature, pH)
of such flocculation capable systems; commonly denoting the
aggregation (networking) of single particles by
adsorbed
macromolecules
fluctuation:
random changes in the properties or constituents of a system
fluctuation rate: timely
rate of spontaneous occurring or induced fluctuations
of a physical system
G
gel:
dispersion of a colloidal solid in a liquid (gelatin)
goniometer:
simple optical contact angle meter with
manual analysis of the drop contour through an eye piece
H
heat of adsorption: the amount of heat evolved during an adsorption, mostly in gas / solid systems
height/width method:
analysis method for the calculation of contact angle values on drops
an bubbles according to the equation: tan Q
= 2H/W; see also circle fit
heterogeneity:
non-uniformity; used for material composed of different particles or phases;
also chemical non-uniformity on phase boundaries
high speed camera:
camera with an elevated rate of image acquisition; typically
starting with several hundreds of images/sec; particularly used in
surface chemistry to characterize fast absorption processes
on absorbent papers or non woven materials
homogeneity: denotes uniformity; used for materials composed of one
sub-structure of
the same kind
hydration: process of surrounding a charged or neutral molecule
or atom with water
molecules
hydration pressure:
is the repulsion pressure of hydrated substances in water, due to the
orientation of the water molecules, which repel mutually
hydrogen bonding: describes an important form of partial
valency bonding; occurs between one
covalently bounded hydrogen atom to an electronegative element and the lone electron pair of an
another electronegative atom; water and liquid acetic acid are
typical examples
hydrophilic: water friendly;
denotes a molecule consisting of water
soluble group(s) or a surface which is wettable
by water
hydrophobic: water repellant;
denotes a molecule consisting of water insoluble group(s)
or a surface which is non-wettable by water; see also lipophilic
hydrotropic agent: a surfactant, that enhances aqueous
solubility of another substance
hysteresis: see contact angle hysteresis
I
interaction: attraction or repulsion within or between
different particles or particles of the same kind
interface: a) general meaning:
any phase boundary between two
different condensed phases as well as different
gaseous and condensed phases;
b) narrower meaning: a phase boundary between two solids, two liquids or a
solid and a liquid
interfacial analysis: determination of the
physico-chemical properties of an
interface
interfacial chemistry:
see surface chemistry
interfacial active agent:
see surface active agent
interfacial tension:
important thermodynamic material parameter used for the
characterization of phase boundaries;
denotes the amount of work necessary to form a new piece of interface
between two phases (SI units: mN/m and mJ/m2)
J
Joule: 1 Joule is the amount of work, which is necessary
to move
the origin of force for 1 N into the direction of the force for 1 m;
SI unit of work or energy
L
lamella: concave or convex
curved liquid column (meniscus) formed on the whole perimeter of a
body during insertion to or retraction of that body from the
surrounding liquid
laminar flow: non-turbulent (irrotational) flow of fluids
lamella method: method to determine the surface
tension of more viscous liquids
from the curved shape of the occurring lamella,
frequently used for polymer melts
or hot melt adhesives also at higher temperatures
light scattering:
scattering of visible light, e.g. by colloidal
dispersed particles in a solution, can
often be identified by turbidity and opalescence just by visual
inspection.
liquid lamella: see lamella
lipophilic: used for molecules with
chemical group(s) soluble in non-polar organic
solvents and therefore insoluble in water but in liquid
alkanes and fatty oils
lipophobic: used for molecules with
chemical group(s) insoluble in
alkanes and fatty oils but soluble in water and other polar solvents
M
macroemulsion:
see emulsion
macromolecule:
denotes a molecule, which is characterized by numerous repetitions of one particular group of atoms (a monomer) mostly
concatenated or
networked by covalent bonds; the number of monomers is already as
large that the addition or removal of a few monomers does not change
the significant property of the macromolecule
maximum bubble pressure: the maximum internal pressure pmax
of a gas bubble created on a capillary tip
is achieved when the bubble reaches an almost semi-spherical shape
with the radius rB = rK (rK -
semi-spherical radius). In this case the surface
tension gLV of
the surrounding liquid can be calculated by gLV
=
Dpmax/(2
rB); see bubble pressure method
membrane:
thin, flexible film of natural or synthetic origin
micelle: an oriented molecular aggregate, often
in a spherical or rod shape; made of 10 to
100
surface active molecules formed in a solution; the hydrophilic
or lipophilic (hydrophobic)
parts of the molecule orient according to their favorite interaction
with the surrounding solvent which becomes excluded from the
interior part of the micelle.
micelle concentration: see critical
micelle formation
concentration
micellization:
denotes the formation process of micelles
microemulsion: thermodynamically stable, transparent
solution of micelles swollen with solubilizate
monodispersed particles: dispersed particles of the same size in
solution
monolayer: monomolecular film e.g. an adsorbate film of
one molecule thickness, spread on a liquid or solid surface
multilayer:
adsorbate of more than one molecule thickness spread on a mostly
solid surface
N
nanoparticles: ultra-fine dispersed,
solid particles (in the size range of colloids),
possessing a high specific surface to volume ratio, the properties of
nanoparticles are mainly determined by the surface; the preparation
and the properties of nanoparticles are investigated by the nanotechnology science and the colloid
chemistry
non-polar contributions:
see dispersive contributions
non-wetting: describes the property of a
surface, that
doesn't allow a liquid to spread when the measured contact angle is
higher or equal to 90°
O
osmosis: denotes the (liquid) material transport through a
membrane from a higher concentrated
solution to a lower concentrated
solution until there is a equilibrium of the concentration in both
solutions
P
partial wetting: incomplete wetting; the liquid doesn't
spread completely on the surface
passivation:
treatment of a metal surfaces to reduce
corrosion effects; can be achieved by decreasing the high
surface energy of metals and formation of anti-corrosive,
stable oxide layers (e.g. with chromium)
pendant drop method: precise and absolute method to determine the surface
tension of a liquid against another fluid phase with a drop hanging vertically from
the tip of a capillary; the surface tension is calculated by
evaluating the drop shape according to the
Young-Laplace equation; the method also works at
elevated temperatures and pressures
penetration: entering process of molecules or particles in another
condensed phase
permeability: capability of a material to let pass other molecules or particles
phase: closed region of a material
without discontinuous changes of the physico-chemical volume properties, e.g. density,
chemical composition
phase boundary: closed,
single or multiple connected boundary of one phase versus other
phases
phase diagram: graphical representation of the regions
and boundaries of existence for different phases
- as states of
aggregation - belonging to a given physico-chemical system
in dependence of two or more thermodynamic quantities, e.g. temperature,
pressure, concentration
physical adsorption:
see chemisorption
polar contributions:
contributions to the surface energy or
surface resp. interfacial
tension, caused by polar interactions
on the phase boundaries
polar interactions:
intermolecular attraction involving permanent electrical dipoles
(permanent - permanent, induced - permanent), but also hydrogen
bonding forces
polarization:
appearance of a dipole momentum in an
atom bond caused by
different electronegativity
polydispersed particles:
particles in a dispersion of different size; opposite of
monodispersed
pore: hole in a material
pore size: sizes of the pores in a material
porosity: describes the ratio of volumes of pores to the
material
powder contact angle method:
method to determine the contact angle of
wettable powders or fiber bundles based
on the tensiometric evaluation of the liquid adsorption
speed, see also (modified)
Washburn
method
Q
R
reverse osmosis:
describes a separation process. A solution is separated from pure
liquid by a semipermeable membrane, and a pressure applied across
the membrane drives the separation, e.g. desalination
receding contact angle:
dynamic contact angle, which is measured:
a) by pulling out a solid geometry from a liquid from the wetting
force, due to the dynamic
Wilhelmy method,
b) by soaking in a sessile drop on a horizontal surface
c) at the rear front of a just rolling droplet on an inclined
surface,
in order to determine the dewetting
properties of a solid
S
sediment:
particles in a liquid that have settled to the bottom of the liquid
column during a time
dependant process
sedimentation:
describes the process of solid particles settling in a liquid depending
on the time
semipermeable membrane:
film permeable only to small or specific molecules
sessile drop method: method to determine the
static or dynamic
contact
angle using the shape of a drop resting on a
horizontally oriented surface
single fibre contact angle method: method to
determine the advancing and receding contact angle on fibres and to
determine the wetted perimeter resp. diameter of fibres
solubility:
see solution
solution:
a) general meaning:
homogenous mixture of different substances in a liquid matrix when
even microscopic sub-volumina still possess equal composition b)
specific meaning: mixture of at least two liquid
components or of at least one solid and a liquid component
sorption: general term used to denote a specific form of adsorption
spinning drop method:
method to determine the interfacial tension
between liquids, especially in
the region of low values down to 10-6 mN/m; with
increasing speed the rotating drop becomes elongated with a prolate shape; the
diameter of the drop D correlates then to the
interfacial tension by the relation: gL
~ D3
spreading: wetting of a
surface by a liquid with a contact
angle of about 0°
stability:
the durability of a colloidal dispersion resp.
emulsion, i.e. there is no decomposition of
the dispersed resp. emulgated
phase from the surrounding phase
surface: phase
boundary between a solid and mostly a gas or
a liquid
surface active agent
(surfactant):
chemical compound with spatially separated hydrophilic and
lipophilic constituents; surfactants show adsorption
preferably on liquid surfaces and interfaces;
detergents belong to the class of surface active agents
surface analysis:
investigation of a surface in order to determine the specific
physical and chemical properties
surface chemistry: science
concerned with the formation, the chemical structure and changes as well as
the physical behavior of surfaces and interfaces
surface concentration:
concentration of an adsorbed monolayer on a
surface
surface free energy (surface
energy): denotes the total energy content of a
solid surface in equivalence to the surface
tension of a liquid; the surface free energy can be decomposed
for most practical considerations into polar
and dispersive
contributions (SI-units: mN/m and mJ/m2)
surface film (microlayer): thin
layer of molecules on liquid or solid surfaces with distinct
properties in comparison to the bulk phase; mainly arising
from adsorption (adsorptive film).
surface porosity: denotes the topography of a surface in terms of open or closed
pores
surface tension: is
equivalent to the work necessary to create a new piece of surface
(per unit area) and therefore a quantity for the energy content of a
liquid surface (SI-units: mN/m and mJ/m2)
suspension:
see dispersion
T
tangent method: method to determine the
contact
angle at the three-phase point of a sessile drop from
the drop contour in the vicinity of that point
tenside: see surface
active agent
tensiometer:
measuring device to determine the surface tension and the
interfacial tension e.g. by the Wilhelmy method
or the Du Noüy ring
method, modern electronic tensiometers also perform measurements
according to the Washburn method and more complex tasks
three phase point:
(orthogonal) projection of the closed boundary line between solid,
liquid, and gas
U
V
Van der Waals forces (after J. D. van der Waals, 1873):
atomic and molecular attraction forces arising from dipole
effects (between permanent dipoles), induction effects (between
permanent and induced dipoles) and dispersion effects (between
induced dipoles); see also Israelachvili
(1991): Chap. 11 - van der Waals Forces between Surfaces.
viscosity:
rheological quantity of fluids describing the resistance to flow
W
Washburn equation (after E.W. Washburn,
1921):
an equation describing the progress of a liquid front h(t) in a capillary tube or
in a set of cylindrical pores in a porous medium: h(t) ~ Öt
Washburn method (modified Washburn method):
measuring method to determine the wetting
speed and the contact angle of
absorbent materials (powders, fibre bundles etc). according to the Washburn
equation or any of the modifications
wetting: is the phenomenon of the adherent contact of
liquids to the surface of solids (or other liquids)
wettability: denotes the ability of liquids to
spread on
a surface; the better the wettability the lower is the
contact
angle; surfaces are called (incompletely)
wettable in case of an occurring contact
angle <90°
Wilhelmy method (after L. Wilhelmy,
1863): method to determine the static surface
tension or interfacial tension of liquids with a plate; the force
necessary to wet the vertical hanging Wilhelmy plate is measured;
also measurement of the critical micelle concentration (CMC)
Wilhelmy plate:
geometrical known plate of platinum to perform the Wilhelmy method
(see above)
work of adhesion: term used
for the intermolecular (attraction) energy between to phases per unit area being in contact to each other;
amount of energy necessary to perform adhesion
processes
work of cohesion: term used
for the intermolecular (attraction) energy between the molecules a
homogenous phase (per unit area), amount of
energy necessary to perform cohesion
processes
X
Y
Young's equation (after T. Young, 1805): relation between
the surface free energy
gS,
the interfacial free energy
gSL
of a plane solid surface, and the contact
angle Q as
well as the surface tension gLV
of a deposited liquid drop:
gS
= gSL
+ gLV
cos Q
Young-Dupré's equation (after T. Young, 1805 and A. Dupré, 1869):
relation between the work of
adhesion WA, the surface
tension gLV
and the contact angle Q
of
a sessile drop: WA= gLV
(1+cosQ)
Young-Laplace Fit:
numerical fitting procedure to obtain the parameters of the Young-Laplace equation to
a given drop contour, may include the
derivation of the surface or interfacial
tension and the contact angle
Young-Laplace equation (after P. S. de Laplace, 1806 and T. Young,
1805): relation between the occurring capillary
pressure Dp
on curved objects (like drops, bubbles, lamellae)
and the principal radii of curvature R1 und R2
of the geometrical objects as well as the surface
or interfacial tension on the phase
boundaries
gL:
Dp
=
gL
(1/R1+1/R2)
Z
Zisman plot (after W.A. Zisman, 1950):
graphical representation to determine the critical surface tension
of a solid (in fact the free surface energy) according to W.A. Zisman
(1950-52). The abscissa (x-axis) carries the surface
tensions of the used test liquids, the ordinate (y-axis) carries in
contrast the cosine of the measured contact angle cosQ.
The specific value on the abscissa on which spreading of a droplet
on a solid surface occurs the first time (cosQ=1)
is called critical
surface tension
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