----- Original Message -----
From: <[email protected]>
To: <[email protected]>
Sent: Thursday, 30 November 2000 03:17
Subject: Re: CS>Production of Silver Chloride at pH 2
> In a message dated 11/29/00 7:31:30 AM EST, [email protected] writes:
>
> << Subj: Re: CS>Production of Silver Chloride at pH 2
> Date: 11/29/00 7:31:30 AM EST
> From: [email protected] (Ivan Anderson)
> This is very complicated Roger, and is not fully understood by far
> greater minds than mine. Van de Waals force is an attractive force
that
> like particles have for each other over short distances. Collisions
> between ions or particles that have a particular vector and energy
> contributes, and there is some evidence that like charges can
attract,
> but the work in this area is particularly obscure. However, hydration
of
> ions as they leave the anode (point of highest density) limits this
> aggrigation, which is what we aim for , no?
>
> Ivan: Well, perhaps you could start by explaining why such
"like-charge"
> attractions do not result in silver ions forming compact balls of
positive
> charge in, say, silver nitrate. What special forces are present in the
> formation of CS at the anode that could possibly result in the
formation
> compact particles of 20-50 atoms (or should I say ions), where EACH of
these
> atoms has a missing electron?
Roger, you mean, can I explain why I hold such a view. I can't be
expected to explain why like charges attract (assuming it is possible)
when even the experts have trouble...I enclose a couple of papers
addressing this matter below.
> > Most of the large particles found in LVDC CS are those that have
> formed
> > dendrites on the cathode and have regained electrons, and which
are
> then
> > dislodged and re-enter the colloid.
> >
> > Ivan: So, in fact, these larger particles DO NOT HAVE INDIVIDUAL
> silver ions.
> > Isn't that what you're saying? At what CS particle size does each
> silver ion
> > within the particle begin to lose its individual charge? Why does
this
> occur?
> > Can you cite studies that provides evidence for this phenomenon?
>
> Roger, when silver ions find their way to the cathode they adhere
> loosely to it and to other ions that preceded them. Electrons are
> supplied to them from the cathode, but I have read that this is not
an
> instantaneous effect, as the electrons must travel through a mass
that
> is not as conductive as the crystaline metal would be. As this
> accululation grows it forms a tree like structure termed dendrites
(from
> the latin tree like?) and when disturbed, large particles may break
free
> and enter the sol. These particles may still have some atoms missing
> electrons or not.
> Generally these particles will settle out in short order, as Marshall
> notes, but some may remain suspended for a considerable time, and
cause
> the colloid to be somewhat turbid.
>
> Ivan: I'm sure your explanation above accounts for the EXTREMELY large
> particles observed, but it certainly does not account for large
particles
> produced when a stirred, LVDC, limited current process is used to
generate
> CS. I have used this method, and on 3 separate occasions I have
observed CS
> 'dropout' several days later. Could THESE particles have been composed
of
> many individual neutral charge silver atoms AND SOME positively
charged
> silver ions as well? Roger
Well, I am not sure that I agree with your statement, but I don't
discount the probability that some particles are composed of neutral
atoms and ions. Particles will drop out of solution if they are too
large to remain suspended, and no doubt the degree of charge has an
influence upon this.
Ivan.
>
>
> --
> The silver-list is a moderated forum for discussion of colloidal
silver.
>
> To join or quit silver-list or silver-digest send an e-mail message
to:
> [email protected] -or- [email protected]
> with the word subscribe or unsubscribe in the SUBJECT line.
>
> To post, address your message to: [email protected]
> Silver-list archive:
http://escribe.com/health/thesilverlist/index.html
> List maintainer: Mike Devour <[email protected]>
>
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Colloids: A Surprisingly Attractive CoupleThis is Google's cache of
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Colloids: A Surprisingly Attractive Couple
David G. Grier
The James Franck Institute and Department of Physics
The University of Chicago
5640 S. Ellis Ave., Chicago, IL 60637
June 9, 1998
Figure: The pair interaction potential between unconfined pairs of charged
colloidal spheres is purely repulsive, as can be seen in (a). Data are
plotted
in units of the thermal energy kB T. This pair potential was measured between
two polystyrene sulfate spheres of radius m suspended in deionized water.
Measurements were made by manipulating the spheres with optical tweezers and
tracking their motions with digital video microscopy as described in
reference
[6]. The solid line is the DLVO theory's prediction for this system. (b) When
the same pair of spheres is confined by parallel glass walls separated by m,
an attractive minimum develops in the pair potential. The unconfined
potential
from (a) is overplotted as a dashed line to emphasize the confinement's
influence. Data reproduced from reference [6]. Attractions of comparable
range
and strength have been implicated in the formation of metastable superheated
colloidal crystals such as the ones shown in (c). The scale bar indicates m.
In this case the confinement responsible for the many-sphere cohesion appears
to be provided by the spheres themselves. Photograph from reference [9].
A vast number of industrial and natural processes depend on controlling the
interactions between micrometre-sized colloidal particles dispersed in fluid
suspensions. For instance, colloidal milk fat and proteins aggregate into
either
cheese or yoghurt depending on how they are handled; colloidal paint pigments
must remain suspended for years in a can, yet should coagulate rapidly into a
tough coating when spread on a wall. For just such reasons, colloidal
interactions and their ramifications have been researched intensively over two
centuries.
And yet, over the past two decades, one of the fundamental tenets of colloid
science come under attack: contrary to the lessons of long experience it now
seems that like-charged colloidal particles sometimes attract each other.
Reports of this extraordinary effect and efforts to explain it have sparked a
rancorous debate. Its resolution may at last be at hand, in part because of
results from the numerical study by Bowen and Sharif on page XXX of this issue
[1].
Although many processes affect colloidal behaviour, the phenomena under
contention depend on three energy scales - those set by van der Waals
attraction, by the randomizing influence of thermal energy, and by the
hierarchy
of electrostatic interactions among highly-charged colloidal particles and the
singly-charged simple ions around them. The balance of these three determines
the properties of a large class of colloidal suspensions.
Van der Waals attraction arises from sympathetic fluctuations in particles'
electron distributions. It causes colloid to aggregate, and is partly
responsible for dust's tenacious grip on a television screen. But van der Waals
attraction only exceeds thermal energies for colloidal particles very near
contact, at separations smaller than a few nanometres - More widely separated
particles are saved from its grip by thermal collisions with surrounding fluid
molecules. Preventing particles from colliding therefore prevents them from
aggregating. The conventional wisdom, worked out more than 50 years ago by
Derjaguin, Landau, Verwey and Overbeek (DLVO), is that just a few static
charges
on colloidal particles' surfaces can cause repulsions strong enough to keep
them
stably separated [2].
The system of equations describing interactions among the spheres, solvent and
simple ions is so intrinsically nonlinear that it resists analytical treatment
to this day. The DLVO formulation avoids this complexity by linearizing the
equations and averaging out ionic fluctuations. Despite these simplifying
approximations, the DLVO theory accounts quite well for the stability and
properties of charge-stabilized colloidal suspensions and serves as the
standard
model for colloidal electrostatic interactions.
The problem is that some highly-charged colloidal particles do not behave as
DLVO says they should. Inspired by the alternative Sogami-Ise theory for
colloidal electrostatic interactions, Norio Ise and his collaborators carefully
investigated the structure of colloidal suspensions. Unlike the DLVO theory,
the
Sogami-Ise theory predicts that like-charged spheres can attract each other. In
observations spanning fifteen years, they recorded a variety of phenomena
apparently inconsistent with the DLVO theory, ranging from anomalously small
lattice constants in colloidal crystals [3] to large stable voids in colloidal
fluids [4]. They interpreted their observations as evidence for long-ranged
attractions between like-charged spheres. Other researchers suggested more
conventional explanations, however, and controversy ensued.
Because many-body behavior often obscures underlying pair interactions,
measurements of bulk properties leave room for disagreement. Only in the past
five years have techniques been developed capable of measuring the
fantastically
small forces between individual colloidal particles. These measurements have
begun to tell a coherent story: isolated pairs of like-charged spheres are
found
to repel each other much as predicted by the DLVO theory [5,6,7,8,9]. But
spheres confined by glass walls [6,9,10] or by a concentration of other spheres
[9] develop long-ranged attractions inconsistent with DLVO. Attractions seem to
be favored by highly charged spheres in very low salt concentrations -
circumstances under which the DLVO approximations might be expected to fail.
The apparent breakdown of linear superposition (pairs repel but groups cohere)
implicates nonlinearity as the culprit. But a slew of alternative explanations
have been proposed, including the inherently linear Sogami-Ise theory and
mechanisms involving fluctuations in the simple ion distributions. Differences
between these theories hinge on the experimentally invisible simple ions.
Bowen and Sharif's numerical study demonstrates that nonlinearity -
long-neglected - can indeed explain the observed attractions but is not the
whole story. The simple ion distributions around an isolated pair of spheres
mediates repulsive interaction, even in their nonlinear calculations. Confining
walls, however, redistribute the simple ions so as to mediate a long-wavelength
attraction. Comparable redistributions in the DLVO theory's approximations turn
out to be too subtle to induce attractions. So both nonlinearity and
confinement
are needed. Temporal fluctuations are not considered in these calculations, and
therefore are not necessary to generate attractions.
Knowing the ingredients for like-charge colloidal attractions is a big step
toward understanding their origin and predicting their ramifications. But we
still do not understand how nonlinearity and geometric confinement conspire to
produce simple ion distributions conducive to long-ranged cohesions between
neighboring spheres. Numerical simulations, like physical experiments, offer
insights into how particular systems behave under particular circumstances;
they
offer no predictive insights into the behaviour of other systems.
What geometries support long-ranged like-charged colloidal interactions? How
can
we turn them on and off? Could they affect smaller macroions such as proteins
and DNA? By identifying what processes are at work, Bowen and Sharif have
poised
us on the brink of being able to answer such questions.
Bibliography
1
W. R. Bowen and A. O. Sharif, Nature 393, XXX (1998).
2
W. B. Russel, D. A. Saville, and W. R. Schowalter, Colloidal Dispersions,
(Cambridge University Press, Cambridge, 1989).
3
T. Yoshiyama, I. Sogami, and N. Ise, Phys. Rev. Lett. 53, 2153-2156 (1984).
4
H. Yoshida, N. Ise, and T. Hashimoto, J. Chem. Phys. 103, 10146-10151 (1995).
5
J. C. Crocker and D. G. Grier, Phys. Rev. Lett. 73, 352-355 (1994).
6
J. C. Crocker and D. G. Grier, Phys. Rev. Lett. 77, 1897-1900 (1996).
7
K. Vondermassen, J. Bongers, A. Mueller, and H. Versmold, Langmuir 10,
1351-1353 (1994).
8
T. Sugimoto et al., Langmuir 13, 5528-5530 (1997).
9
A. E. Larsen and D. G. Grier, Nature 385, 230-233 (1997).
10
G. M. Kepler and S. Fraden, Phys. Rev. Lett. 73, 356-359 (1994).
About this document ...
Colloids: A Surprisingly Attractive Couple
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1998-06-09
Like-Charge Attractions in Metastable Colloidal Crystallites Accepted for
publication in NatureThis
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Next: Colloidal interactions and phase
Like-Charge Attractions in
Metastable Colloidal Crystallites
Amy E. Larsen and David G. Grier
December 2, 1996
Abstract:
Sub-micron diameter latex spheres colloidally suspended in water can form into
regular arrays known as colloidal crystals. Unlike most conventional solids,
colloidal crystals can be forced into a metastable superheated state. Analysis
of the structure and dynamics of these metastable crystals using digital video
microscopy provides clear evidence for a surprisingly strong and long-ranged
attraction between similarly charged spheres. Such unexpected and unexplained
attractions may influence the properties of natural and industrial suspensions.
Charge-stabilized colloidal suspensions of extremely uniformly-sized polymer
spheres were developed in the 1950's as media for paints and other surface
coatings. These suspensions have attracted considerable attention in recent
years for their utility as model systems with which to study the mechanisms of
structural phase transitions [1]. Depending on their concentration and chemical
environment, colloidal spheres can form up into regular crystalline arrays or
devolve into fluid disorder. Transformations between the ordered and disordered
states are phase transitions analogous to melting and freezing of atomic
matter.
Unlike atoms, however, colloidal spheres can be tracked with a conventional
light microscope. Their structural transformations thus provide unparalleled
opportunities to investigate the microscopic mechanisms of phase transitions.
This article describes, conversely, how the study of phase transitions sheds
new
light on the fundamental properties of colloidal suspensions. In particular, we
find evidence in an exotic colloidal melting transition for long-range
attractive interactions between like-charged microspheres. When combined with
direct measurements of the spheres' pairwise interaction potential, these
observations strongly suggest that such attractive interactions may be
responsible for a variety of other unexplained phenomena observed over the past
decade in bulk colloidal suspensions.
The most striking of these anomalies include large stable voids in otherwise
homogeneous suspensions [2, 3] and equilibrium phase separation between
colloidal fluids of different densities [4, 5]. Neither should be possible in a
system with purely repulsive interactions, although explanations based on
impurity effects [6] have proved difficult to exclude. Persistent quantitative
discrepancies between predicted [7, 8, 9, 10, 11] and observed [12, 13] melting
points of colloidal crystals are no less disturbing, but might reflect
relatively minor shortcomings in the theory. The long-ranged attractions we
observe would account naturally for these effects, but are qualitatively
inconsistent with the long-accepted theory for colloidal interactions. We
suggest, therefore, that the theory for colloidal interactions requires
substantial revision.
Colloidal interactions and phase transitions
Superheated crystals
Stable Facets
Interfacial kinetics
Smallest metastable clusters
Measuring the interaction
Attractive phenomenology
References
Acknowledgements
Figures
About this document ...
Next: Colloidal interactions and phase
David G. Grier
Mon Dec 2 14:09:59 CST 1996
