American Scientist, Jan-Feb 1995 v83 n1 p95(2)
Colloid-Polymer Interactions: Particulate, Amphiphilic,
and Biological Surfaces. (book reviews) Peter
K. Kilpatrick
Full Text: COPYRIGHT 1995 Sigma Xi, The
Scientific Research Society
The interactions of colloids and polymers are basic to a remarkably diverse
number of important physical processes and commercial applications, including
paints and coatings, protein isolation, water purification, paper making,
minerals and food processing, enhanced petroleum recovery, drug delivery, and
the formulation of consumer products. Improvements in these processes hinge on
discovery and research into the molecular understanding of how homo- and
copolymers interact with particulate and aggregate surfaces.
This 20-chapter book is a collection of symposium papers delivered at the
National Meeting of the American Chemical Society in
The challenge that faces a ground-breaking symposium, as well as a book of
this type, is to codify and draw meaningful generalizations from the menagerie
of experimental and theoretical observations presented. An overview chapter by
William Russel admirably accomplishes this task. This chapter entitled
"Macro-scopic Consequences of Polymer-Particle Interactions,"
provides a condensed version of an article written by Ploehn and Russel for the
journal, Advances in Chemical Engineering. It identifies three types of
interactions among homo- and copolymers with colloidal interfaces, of which the
first is adsorption, which converts the random coil nature of the polymer to a
highly distorted conformation; the second looks at nonadsorption, or depletion,
which can lead to phase separation of the colloidal particles; and the third
examines grafting, or covalent attachment, which can result in very distorted
chain conformations that depend strongly on the density of the grafted chain.
It is these interactions, which take place at the molecular level, among
polymers and colloidal surfaces that dictate the
macroscopic behavior of the colloidal dispersions that interact with polymers.
Russell surveys four major areas: specifically polymeric flocculation, which
largely results from adsorption and bridging; polymeric stabilization, which is
partially effected by adsorption of homopolymers but can be more optimally
effected by grafting of copolymers; polymer-induced phase separation, which is
primarily manifested in nonadsorbing polymer-colloid systems through a
mechanism identified as depletion; and, finally, polymer-modified rheology of
colloidal dispersions, which has long been accomplished through water-soluble
polymers but has lately been effectively accomplished by grafting homo- and
copolymers onto colloidal particles.
The rest of the book is divided into four parts that present discussions of
theory and simulation, kinetics and configurations of adsorbed chains,
flocculation and stabilization, and interactions with micelles, bilayers,
liposomes and proteins. These are imperfect divisions, of course, and many of
the papers cut across two or more of these topical areas. For example, more
than half of the papers make use of some theory or computer simulation to
illuminate an aspect of colloid and polymer interactions, such as scaling
theory, transport theory and mass-transfer correlations, Monte Carlo and
molecular simulation, thermodynamic modeling of phase behavior and partition
and virial coefficients, adsorption kinetics and isotherm modeling, and the
modeling of spectroscopic responses in colloid-polymer systems. The reader is
struck with the universality of these basic theoretical approaches as well as
the subtleties in this particular field that make it such a fruitful area of
research.
The variety of experimental approaches is equally impressive. The papers
highlight reflectometry, small-angle neutron scattering, dynamic and static
light scattering, high-resolution nuclear magnetic resonance spectroscopy,
electro-optical signal decay, turbidimetry, rheometry and viscosimetry,
electrophoretic mobility and light-scattering measurements, fluorescence
spectroscopy and dynamic quenching, microcalorimetry, electron microscopy, and
atomic-force microscopy. Thus the book is an excellent text and tutorial on the
application of most of the important theoretical and experimental approaches
used in the study of colloids, surfaces and polymers.
Although many of the papers in this book are outstanding, a brief synopsis
will be provided of two excellent contributions. The first is a review article
by Abbott, Blanckschtein and Hatton that describes their recent work on the
interactions of proteins with flexible, water-soluble polymers. In this paper,
the authors model the interactions of polyethylene oxide (PEO) and dextran with
globular proteins, including bovine serum albumin and cytochrome c, using
polymer-scaling concepts, statistical thermodynamics and a combined equation of
state and Monte Carlo approaches. The comparison of their modeling efforts with
experimental protein partitioning data among coexisting aqueous
polymer-solution phases suggests that the physical exclusion of the proteins by
the polymers through a nonadsorbing depletion mechanism is the dominant
contribution to the thermodynamics. A secondary contribution is a weak
attractive interaction between proteins and polymers that increases by an order
of magnitude with increasing protein size. The net interaction between proteins
and PEO polymers remains repulsive.
An exemplary contribution on the interactions of functionalized polymers
with surfactant aggregates is the paper by Ringsdorf, Simon and Winnik. This is
a fluorescence study of poly(N-isopropylacrylamides)
(PNIPAM), which have been functionalized with either octadecyl chains (C18) or
with pyrenyl groups that interact with phospholipid liposomes. The
C18-functionalized PNIPAM forms inter-polymeric micelles in aqueous solution.
When contacted with liposomes, the inter-polymeric micelles are disrupted and
incorporate slowly into the liposome surface. On saturation of the liposome
surface with modified polymer, coated liposomes and polymeric micelles coexist
in solution. The authors make elegant use of pyrenemonomer and excimer-emission
spectroscopy, of nonradiative, energy-transfer techniques and of
microcalorimetry.
In summary, the book is an excellent overview of the variety of approaches
currently used to study the important area of polymers interacting with
colloidal surfaces. It could serve as a text of case studies for a graduate
course in colloid and surface science or as a tutorial monograph for graduate
students in chemistry, physics, biochemistry or chemical engineering who would
like to pursue thesis research in this area. Many industrial practitioners will
also find the book extremely useful for generating ideas and for developing understanding
of this critical area.