Papers by Philip Molyneux
Journal of the Chemical Society Faraday Transactions, 1992
The effect of urea (U) on the aqueous solubility of the cosolutes (S) 4-hydroxybenzoic acid and i... more The effect of urea (U) on the aqueous solubility of the cosolutes (S) 4-hydroxybenzoic acid and its C,-C, alkyl
esters (‘alkylparabens ’) has been studied. In each case the cosolute solubility increases linearly with concentration
of U, which has been interpreted as due to association of the type SJ, where x is assumed to be either 1 or
2. The results are discussed in terms of the interaction forces (dipole-induced dipole forces, hydrogen bonds,
hydrophobic interactions) expected to occur in the association, and on the basis of some amphiphilic character
for the urea resulting from the expected ‘peripheral’ pattern of hydration around its molecule. The ‘water
structure ’ interpretation of interactions in these systems is also discussed.
In this paper the chemical implications of the propriety of
taking the logarithm of a dimensioned... more In this paper the chemical implications of the propriety of
taking the logarithm of a dimensioned quantity are discussed.
By using quantity calculus, it is shown that this is a
proper procedure, with the consequent units also being logarithmic,
and additive rather than multiplicative. This allows
equilihrium constants to have units rather than, as some
present approaches require, having to he forced into a dimensionless
form, while it also clarifies the units of a number
of derived quantities in physical chemistry such as pH, acidity
constants (i.e., pKa), standard thermodynamic functions,
and standard electrode potentials.
A small-scale filtration device (capacity 10-30 ml) is described which is based on a piston-filte... more A small-scale filtration device (capacity 10-30 ml) is described which is based on a piston-filter effect, and which is applicable to solubility measurements, recrystallisation, fractional melting, extraction, and column chromatography, as well as the general filtration of liquids. The compactness and simplicity of the device should make it particularly useful in environmental and similar fieldwork studies.
The systems studied in Part I have been further investigated by light scattering and viscometry t... more The systems studied in Part I have been further investigated by light scattering and viscometry to determine the effect of the interaction on the molecular size of the polymer. W ith anionic cosolutes the viscometric results indicate that, in general, the polymer expands; this is attributed to the coulombic repulsions between the bound anions on the polymer chain. At the higher concentrations of these cosolutes the increases in size reach essentially constant values, or actually fall with increasing concentration (with sodium phenanthroate); a lessened increase is observed, with sodium 2-naphthoate as cosolute, on the addition of a non-bound salt (sodium chloride); these effects are attributed to "screening," by the free counterions within the polymer molecule, of the coulombic repulsions between the bound anions. With non-ionic cosolutes the viscometric results show that the polymer molecule contracts; at sufficiently high concentrations of phenolic cosolutes polymer precipitation occurs. These effects seem to indicate the formation of labile "cross-links" due to hydrogen bonding between suitable groups on the bound cosolute molecules and on the polymer molecule. The light scattering data parallel those from viscometry for both types of cosolutes; they also indicate that the random-flight configuration is, in general, retained upon expansion or contraction; however, the most strongly bound non-ionic cosolutes produce deviations from this configuration, indicating some folding-up of the polymer coil due to extensive "cross-linking." No appreciable effects are produced by cationic cosolutes such as aniline hydrochloride or naphthylamine hydrochloride.
Aliphatic liquids of low refractive index (alcohols, ethers, hydroxyethers, ketones and esters) ... more Aliphatic liquids of low refractive index (alcohols, ethers, hydroxyethers, ketones and esters) have been tested qualitatively for their suitability as precipitating media with the three polymers: polyisobutene, polystyrene, and poly(methyl methacrylate); each liquid is put into one of four grades for each polymer according to its solvent power towards that polymer at 20 C and at 100 C. The results are discussed in conjunction with the relative values of the cohesive energy densities, ε, of the polymers and liquids; it is concluded that Hildebrand and Scott's rule that the values of ε1/2 (i. e. the "solubility parameter") for polymer and liquid should differ by 1.1 (cal/2 ml-I/2) if the liquid is to be on the borderline of solvency for the polymer at normal temperatures is valid for polystyrene and for poly(methylmethacrylate), but not necessarily so for polyisobutene.
The construction and operation of an absorptiometer for studying the progressive precipitation and re-solution of polymers from dilute solution over the range 20 C to 100 C at a rate of 0.3 deg min-1 are described. The instrument uses a 10 mm diameter collimated beam of green monochromatic light (λ0= 546 mμ) obtained from a compact high-pressure mercury arc, which is passed through the precipitating system held in a cuvette in a temperature control bath; the absorbance (turbidity) of the sample is measured in terms of the attenuation which has to be applied to a reference beam to reduce its intensity to that of the transmitted sample beam. The beams are detected by selenium photocells; precautions are taken to prevent any appreciable amount of forward scattered light from the sample reaching its photocell.
Studies of the progressive precipitation and re- solution of each of the three polymers (at 0.2 to 1 mg cl-1) with the absorptiometer showed that esters and ketones have too low values of ψ to reveal the molecular weight distribution for broad-distribution samples, although particular ones can be used on fractionated material, e. g. butyl acetate for polyisobutene fractions of molecular weight about one million. The experiments have shown that cyclohexanol is a suitable medium for broad-distribution polystyrene, and that propanol and butanol are similarly suitable for poly(methyl methacrylate), while with polyisobutene only mixed media (such as heptane + butanol, or butyl ether + methoxyethanol) seem to be suitable in this respect. A design for a sealed cuvette is proposed which would both enable these hygroscopic media to be used more easily, and at the same time would reduce the amount of polymer required to less than 100 micrograms. The turbidity values for complete precipitation are shown to be in reasonable concordance with the theoretically predicted values of τmax, the ratio averaging about three-quarters. The attainment of a steady turbidity at any given temperature seems to occur very much faster than can be accounted for by simple Smoluchowski aggregation of equal-size particles, which suggests that the precipitating molecules are captured either by "nuclei" or by already present aggregates. Suggestions are made for lessening the present sensitivity of the absorptiometer to instability in the arc source, and for the automatic recording of the precipitation and re- solution characteristic.
In
the
technique
of
turbidimetric
titrations
for
the
molecular-weight
analysis
of
poly... more In
the
technique
of
turbidimetric
titrations
for
the
molecular-weight
analysis
of
polymers,
the
precipitation
of
the
polymer
(phase-separation)
is
induced
by
the
addition
of
a
precipitant
to
a
very
dilute
solution
of
the
polymer;
in
the
present
paper
a
modification
of
this
technique,
in
which
the
precipitation
is
induced
by
cooling
such
a
solution,
is
proposed.
The
modification
has
the
practical
advantages
that
the
lessened
need
for
stirring
enhances
the
stability
of
the
polymer
suspen-
sion;
that
the
volume
of
the
system
stays
essentially
constant;
that
it
is
less
likely
for
there
to
be
changes
in
the
turbidity
of
the
system
after
all
of
the
polymer
has
been
precipitated
out
than
with
the
normal
turbidi-
metric
titration;
and
that
it
would
be
much
easier
to
test
the
reversibility
of
the
precipitation
than
in
such
titrations.
It
also
has
the
advantage
from
the
theoretical
viewpoint
that,
with
monodisperse
polymer
and
a
single
liquid
as
the
medium
in
which
the
polymer
is
initially
dissolved,
the
system
becomes
a
simple
binary
liquid
mixture.
TheFlory-Huggins
equationshavebeen
applied
to
the
case
of
such
a
monodisperse
polymer
at
low
concentration
in
a
single-liquid
medium,
and
the
Molyneux,
Estimation
o/
Polymer
Molecular
Weight
Averages
105
precipitation
temperatures
have
been
calculated
numerically
and
presented
graphically
for
the
ranges:
molecular
weights
from
10
4
to
10~;
volume
fractions
of
polymer
in
solution
from
10
-2
to
10-9;
and
values
for
y~
(the
entropy-change
of
dilution
parameter)
of
0.5,
1.0
and
1.5.
The
treatment
shows
that
the
ideal-
temperature,
O,
of
the
system
is
the
important
para-
meter
in
deciding
whether
the
polymer
will
all
dissolve
at
the
upper
limit
of
the
experimental
temperatures,
and
the
other
parameter
~
is
important
in
deciding
whether
all
will
then
have
precipitated
out
again
at
the
lower
limiting
temperature.
These
two
conditions
are
those
which
must
be
fulfilled
if
the
medium
is
to
be
suitable
for
revealing
the
complete
molecular-weight
distribution
of
a
broad-distribution
polymer.
In
the
subsequent
light-scattering
treatment
of
the
turbidity
of
the
suspension
of
particles
produced
by
the
phase
separation,
it
is
pointed
out
that
these
particles
are
spherical
(since
the
phase-separation
calculations
show
them
to
be
always
swollen
with
the
medium)
and,
because
of
their
composition
and
the
relative
closeness
of
the
refractive
indices
of
polymer
and
medium,
they
will
have
a
refractive
index
even
closer
to
that
of
the
surrounding
solution.
Under
these
conditions,
the
complex
Mie
relations
for
particles
of
arbitrary
size
reduce
to
a
simple
form
due
to
van
de
Hulst;
this
is
applied
to
the
aggregating
system,
for
which
it
is
shown
that
as
the
aggregation
proceeds
the
turbidity,
~,
pro-
duced
by
a
given
amount
of
polymer
in
suspension
will
pass
through
a
maximal
value
given
by:
where
n~
and
n~
are
the
refractive
indices
of
pure
medium
and
pure
polymer,
(~2)i
is
the
overall
volume
fraction
of
precipitated
polymer,
and
A0
is
the
vacuum
wavelength
of
the
incident
light.
The
significance
of
this
expression
lies
in
its
demonstration
of
the
direct
proportionality
between
~max
and
the
overall
polymer
concentration,
and
of
its
dependence
upon
four
readily
accessible
parameters
without
involving
the
size
of
the
particles
or
their
composition.
The
theoretical
treat-
ments
in
this
first
part
is
concluded
with
a
consideration
of
the
timeseale
of
the
aggregation,
based
on
the
Smoluchowski
approach,
and
with
a
discussion
of
the
basic
criteria
for
the
selection
of
suitable
media,
par-
ticularly
with
regard
to
the
choice
of
media
of
low
refractive
index
to
maximise
the
turbidity
produced
by
a
given
amount
of
precipitated
polymer.
[No abstract in original]
The
binding
by
polyvinylpyrrolidone
(PVP)
in
aqueous
solution
at
25
°C
of
each
of
t... more The
binding
by
polyvinylpyrrolidone
(PVP)
in
aqueous
solution
at
25
°C
of
each
of
the
four
homo-
logous
cosolutes:
methyl,
ethyl,
n-propyl
and
n-butyl
parahydroxybenzoates
(parabens)
has
been
studied
by
equilibrium
dialysis.
In
each
case
the
binding
isotherm
is
curved
upwards,
which
has
been
interpreted
as
a
result
of
co-operative
effects,
i.
e.
attractive
interactions
between
the
bound
cosolute
molecules.
Using
mathe-
matical
methods
developed
by
Schwarz,
the
binding
data
have
been
shown
to
conform
in
each
case
to
the
simplest
form
of
co-operativity,
in
which
the
attractive
interactions
occur
only
between
adjacent
bound
cosolute
molecules,
with
the
interaction
increasing
the
effective
value
of
the
binding
constant
by
a
multiplicative
factor
~,
the
co-operativity
parametet.
For
these
four
systems
the
parameter
~
is
found
or
have
an
essentially
constant
value
of
9.6
::E
1.2.
The
constancy
of
~
shows
that
the
alkyl
chains
are
not
directly
involved
in
the
co-operative
effects,
which
were
therefore
concluded
to
be
the
result
of
hydrogen
bonding
between
the
phenolic
hydroxyl
group
of
one
bound
cosolute
molecule
and
the
ester
group
of
its
neighbour.
The
data
have
also
been
applied
to
the
approximate
Fowler
model,
in
which
it
is
assumed
that
the
co-operative
effects
do
not
appreciably
disturb
the
random
distribution
of
bound
cosolute
molecules.
The
corresponding
test
plots
are
linear,
but
the
derived
values
of
the
co-operativity
parameter
are
widely
different
from
the
correct
values
obtained
by
the
Schwarz
method.
This
shows
that
the
basic
assumption
of
the
Fowler
model
is
incorrect
for
systems
having
these
relatively
high
degrees
of
co-
operativity
(i.e.,
:¢
~
10).
For
the
simple
co-operative
binding
model,
the
initial
slope
of
the
isotherm
is
equal
to
Kon,
where
K0
(the
intrinsic
binding
constant)
is
the
equilibrium
constant
for
the
attachment
of
a
cosolute
molecule
onto
a
site
on
an
unoccupied
chain,
and
n
(the
site
density)
is
the
limiting
value
of
binding
872
Colloid
and
l~olymer
Science,
VoL
257.
No.
8
(1979)
for
monolayer
coverage.
For
each
cosolute,
closely
similar
values
of
this
slope
are
obtained
by
the
Schwarz
method,
the
Fowler
method,
and
the
conventional
Scatchard
method
used
for
systems
not
showing
co-operative
effects.
Using
the
value
n
=
0.10
molecule
of
cosolute
per
PVP
monomer
unit
(obtained
in
previous
work
for
PVP
with
aromatic
cosolutes),
the
values
of
K0
(M
-i,
±
one
s.d.)
for
the
four
alkyl
parabens
are:
methyl,
8.2~0.5;
ethyl,
11.1:50.5;
propyl,
19.5:50.9;
butyl,
27.5:50.7.
The
values
of
log
K0
rise
essentially
linearly
with
increase
in
alkyl
chain
length,
showing
that
each
additional
methylene
group
raises
the
value
of
K0
by
an
essentially
constant
factor
of
1.5:50.1,
and
correspondingly
makes
a
constant
negative
contribution
o{
0.24:50.04
kcal
mo1-1
(1.0
±
0.2
kJ
tool
-1)
to
the
standard
free
energy
change
of
binding.
This
free
energy
contribution,
which
represents
the
strength
of
the
hydrophobic
interaction
between
the
cosolute
methylene
group
and
(at
least)
one
on
the
PVP
chain,
thus
corresponds
to
a
contribution
of
(at
most)
0.12:5
0.02
kcal
mol
-i
per
individual
methylene
group.
This
is,
as
might
be
expected,
only
one-seventh
of
that
for
the
complete
transfer
of
a
methylene
group
from
aqueous
solution
into
a
liquid
alkane
environment
(cyclohexane);
it
is,
however,
closely
similar
to
the
hydrophobic
effect
contribution
per
individual
methylene
group
of
about
0.15
kcal
mo1-1
predicted
from
the
theory
of
aqueous
solutions
developed
by
Ngmethy
and
Seheraga.
The
present
results
are
compared
with
those
reported
in
the
literature
for
the
binding
of
these
cosolutes
by
other
macromolecular
substances
including
serum
albumin,
and
by
micellar
nonionic
amphiphiles;
they
are
also
discussed
more
generally
in
relation
to
the
estimation
of
group
contributions
to
binding.
The expansion that occurs in the encompassed volume of an isolated neutral polymer molecule in aq... more The expansion that occurs in the encompassed volume of an isolated neutral polymer molecule in aqueous solution when it binds ions from the electrolyte also present in the solution is treated theoretically from the viewpoint of the "free ionic excess"; the model used is restricted to cases where there is only a small concentration of bound ions within the polymer coil. The relations obtained are applied to data reported in the first two parts on the binding of aromatic anions by polyvinylpyrrolidone in aqueous solution, for which the assumption of low bound-ion concentration is shown to hold; there is general agreement between the theoretical model and the experimental data except that the apparent binding constants for expansion are less than those determined by dialysis by a fairly constant amount (ca. 30 M' 1). Introduction In the previous parts 1 of this series we have reported experimental data on the interaction between three types of aromatic cosolutes (anionic, nonionic, and cationic) and polyvinylpyrrolidone (PVP) in aqueous solution; in this paper we develop a general theoretical treatment of the expansion in the encompassed volume of an initially neutral, random-coil polymer molecule when it binds ions predominantly of one charge (here the anions) from an electrolyte present with it in solution; the treatment is based largely upon the statistical ther-modynamic theory of dilute solutions of neutral polymers and polyelectrolytes. 3 The simplification introduced in the present treatment is that the concentration of bound ions within the volume encompassed by the polymer molecule is always small; this is shown to hold for the interaction between PVP and the aromatic anions, and hence the theoretical relations obtained are tested using the data obtained for this interaction. Theoretical Treatment, (i) Derivation of the Free Ionic Excess.—Consider an isolated polymer molecule immersed in an infinite bath of a strong electrolyte (here the cosolute NaA, molar concentration a); this state is one for which data are readily attainable by taking measurements over a range of polymer concentrations and then extrapolating to zero polymer concentration. Since in this state there is an overwhelming excess of solution over polymer then any binding of the cosolute by the polymer (or indeed any other interaction between them) will have no effect on the cosolute concentration in the bulk of the solution. In addition, since the concentration of polymer in the region encompassed by the polymer molecule is quite low (for PVP in water at ambient temperature it is about 0.4%) then we can neglect the purely diluting effect that the polymer segments have on ionic concentrations in this region. Assume that one of the ions (here, the anion A-) from the cosolute is bound by the chain according to the form of the Langmuir adsorption isotherm; also, assume that none of the cations (Na +) are directly bound by the chain itself but that they can be bound to previously attached anions, with this counter-ion binding also following the Langmuir isotherm. 4 If the binding con
Polyvinylpyrrolidone (PVP) in aqueous solution displays a strong binding affinity toward dissolve... more Polyvinylpyrrolidone (PVP) in aqueous solution displays a strong binding affinity toward dissolved aromatic compounds; the binding equilibria for such compounds (" cosolutes ") both non-ionic (nitrobenzene, phenol, benzoic acid, etc.) and anionic (sodium benzoate, sodium naphthoate, etc.), have been studied over the temperature range 3 to 60' by the equilibrium dialysis method. The binding constants a t 30°, and hence the free energies of binding, increase as the size of the aromatic system is increased; a n increase in the number of polar groups (as in the series: phenol, resorcinol, phloroglucinol) also leads to an increase in the binding constant. The thermodynamic functions obtained from the temperature dependence of the binding constants show there to be entropy gains during the binding process in almost all the systems; the greatest gains (ca. 18 e x. for benzene) are observed for the least soluble and least polar compounds, the smallest (ca. 0 e.u. for sodium benzoate) for the highly soluble and highly polar anions. The entropy gains are taken as evidence for the breakdown of so-called " iceberg " water-structure in the vicinity of the solute molecules in the course of the binding process. From a consideration of the observed entropy and enthalpy changes and from available thermodynamic data on relevant systems, it appears that the binding process involves two main effects: (1) an entropy gain due to the disordering of the " icebergs " (ie., the formation of a so-called " hydrophobicbond ") and (2) an exothermic interaction between the PVP and the aromatic *-electron system of the cosolute molecule. Infrnred spectra of dry PVP films containing varying concentrations of aromatic compounds also were taken. There are strong indications from these of the presence of polymer-cosolute hydrogen-bonding and also significant shifts in the cosolute aromatic C-H out-of-plane deformation bands in the 700-800 cm.-l region.
Abstmet-The literature data on the Arrhenius parameters for the thermal decomposition of dialkyl,... more Abstmet-The literature data on the Arrhenius parameters for the thermal decomposition of dialkyl, alkyl aralkyl and diaralkyl peroxides are compared in an attempt to remove the discrepancies between the quoted values for the activation energy, E, which is generally identified with the dissociation energy of the peroxide bond, D(RO-OR'). The collected data not only confirm there to be differences between the values of E for different peroxides (the extreme values being 31.3 f 1.0 kcaJ mole-r for diethyl peroxide, and 37.8 f I.0 kcal mole-r for di-t-butyl peroxide), but also show up a correlated variation in the pre-exponential factor, A (the corresponding extremes being about 10 " see-t, and about lOr*sec-I), involving a linear (" isokinetic ") relationship between log A and E, with an iso-kinetic temperature, T,, of 210 " , at which the rate constant, k,, is 0.1 set-I. The possible explanations for these results are. examined, but none of them either clarify the variations in the Arrhenius parameters or indicate what is the correct value for D(RO-OR').
The critical micelle concentrations (c.m.c.) at 20°C of five homologous N-alkyl, NN-dimethyl-glyc... more The critical micelle concentrations (c.m.c.) at 20°C of five homologous N-alkyl, NN-dimethyl-glycines (N-alkyl betaines) have been determined ; the variation of c.m.c. with temperature (10-57°C) for the dodecyl compound has also been studied. The results have been converted into free energy, enthalpy and entropy changes of micellization and these have been compared with corresponding values obtained from the literature data on the micellization of other amphiphiles and on related processes. The application of thermodynamics to the elucidation of micelle formation by amphiphiles in aqueous solution has attracted attention.1-7 The standard free energy change AG; of micellization is derived from the critical micelle concentration (c.m.c.), by treating the formation of micelles as analogous to phase separation; the determination of the variation of c.m.c. with temperature then enables AG," to be split into the corresponding enthalpy change A H ; and entropy change AS,. Although this approach has been applied with some success to the micellization of electrolytic amphiphiles,lp 2 its application is complicated by the likelihood of variations in the degree of association of the gegenions with the micellar surface as the length of the alkyl chain and the temperature are varied. Consequently, non-electrolyte amphiphiles such as the (non-ionic) alkyl polyoxyethylene glycol monoethers and the (zwitterionic) alkyl betaines are to be preferred for such studies. In the present paper, we report c.m.c. data for five members of a homologous series of N-alkyl betaines at 20°C, and for the effect of temperature variation on the c.m.c. of one of them. The free energy changes of micellization have been calculated and compared with those calculated from the aqueous solubility of hydrocarbons and from the adsorption of amphiphiles at the oil/water and airlwater interfaces. From the data on homologous series of various amphiphiles, AG; is resolved into the separate contributions made by the hydrophilic head group and the lipophilic chain. The temperature variation of c.m.c. for the dodecyl compound has enabled AH; and AS: to be calculated for this compound and compared with corresponding quantities for other non-electrolyte amphiphiles.
The work reported in the previous Part on the binding of phenols and 0-substituted phenols by pol... more The work reported in the previous Part on the binding of phenols and 0-substituted phenols by poly(vinylpyrro1idone) (PVP) in aqueous solution has been extended by studies of their effects on the solubility and solution viscosity behaviour of the polymer, using the same thirteen cosolutes, i.e. HOPhPhOH [where Ph = phenyl or phenylene (174-substitution in XPhY and 1,3-~ubstitutionin PhXY), E = ethylidene (CH,CH,), and G1 = 1-deoxy, 1-P-D-glucopyranosyl (' glucosyl ')I. Only four of these cosolutes precipitate the polymer, their critical precipitation concentrations c* in mol m-3, for 10 g dm-3 (1 % w/v) PVP K-90 at 20 OC, being: PhOH, 85; HOPhNO,, 17; HOPhOH, 50; HOPhOMe, 69. The viscosity measurements show that in general the intrinsic viscosity, [q], is reduced in the presence of the cosolutes (most markedly so by the precipitant cosolutes) ; parallel increases are seen in the Huggins slope parameter, k,. The importance of the acidic phenolic hydroxy group in these phenomena is shown by the small viscosity effects seen with PhOGl (which is not bound by the polymer) and with the three hydroxyethyl compounds PhOEOH, HOEOPhOEOH and Ph(OEOH),, by the low value of c* for the highly acidic cosolute HOPhNO,, and by c* decreasing with more phenolic hydrogen-bond donor/acceptor groups (PhOH-+ HOPhOMe-+ HOPhOH). The absence of precipitation with HOPhOGl, despite its phenolic hydroxy group and its high solubility, and the small viscosity effects it produces, parallel the ' inhibitory' effect of the glucosyl group seen in the binding behaviour with this cosolute and with PhOGl. With PhOMe and MeOPhOMe the solubility is evidently too low to show up any effects. Correlating the intrinsic viscosity with the binding ratio r (the average number of cosolute molecules bound per monomer unit of the chain) shows that there are two main forms of behaviour : (i) a simple linear dependence of [q] on r, which is seen with six cosolutes (HOPhNO,, PhOEOH, HOEOPhOEOH, Ph(OEOH),, PhPhOH and HOPhOH); and (ii) a linear dependence of [q] on r2, which is seen with four cosolutes (PhOH, HOPhOH, HOPhOMe and HOPhOGl). The results are interpreted on the basis of the reversible non-covalent cross-linking of the polymer chains by bound cosolute molecules, which explains the observed reductions in [q] and the parallel increases in k,, as well as the precipitation seen with four of the cosolutes. A statistical-mechanical model derived by Kuhn and coworkers, based on the cross-linking concept, is applied to the viscosity data, enabling the equilibrium constant for cross-linking to be calculated.
In an extension of previous work, and as part of an investigation of the mechanisms by which phen... more In an extension of previous work, and as part of an investigation of the mechanisms by which phenols precipitate water-soluble polymers from aqueous solution, the binding isotherms have been determined for poly-(vinylpyrrolidone) (PVP) in aqueous solution at 25 "C with the following Ph = phenyl or phenylene (1,6substitution in XPhY and 1,3-substitution in PhXY), E = ethylidene (CH,CH,) and G1 = 1-deoxy, 1-P-D-glucopyranosyl (' glucosyl ')I. The three 0-substituents used (i.e. Me, EOH and G1) have been chosen to reveal the contribution to the binding from the phenolic hydrogen without affecting that from the phenoxy or phenylenedioxy part of the cosolute molecule. The two complementary experimental techniques of equilibrium dialysis and cosolute solubility have been used for these measurements. With most of the cosolutes the binding could be interpreted in terms of the uniform site-binding model which leads to the hyperbolic (Langmuir) form of binding isotherm; with HOPhNO,, and possibly with HOPhOMe, the binding involves cooperative effects resulting from attractive interactions between bound cosolute molecules. The binding parameters are discussed on the basis of group-contribution theory, in relation to the four main types of interaction forces in these aqueous systems, i.e. hydrophobic effects, dipole/induced dipole (van der Waals-Debye) forces, dipole/dipole (van der Waals-Keesom) forces and hydrogen bonds. The correlations observed have enabled a group-contribution table to be drawn up for the binding constants of six aromatic cosolutes: PhH, PhOH, HOPhOH, PhPh, PhPhOH and HOPhPhOH. The results for the glucosides, PhOGl and HOPhOGl, showed there to be a net repulsion between the glucose ring and the PVP chain in aqueous solution.
Solubility of the cosolutes 4-hydroxybenzoic acid and its alkyl esters ("alkylparabens") in aqueous urea: evidence for 1 : 1 cosolute-urea association in solution and evaluation of the methylene group contribution to the free energy of association
Journal of the Chemical Society, Faraday Transactions, 1992
Molyneux, P. The use of the stable free radical diphenylpicrylhydrazyl (DPPH) for estimating anti... more Molyneux, P. The use of the stable free radical diphenylpicrylhydrazyl (DPPH) for estimating antioxidant activity Songklanakarin J. Sci. Technol., 2004, 26(2) : 211-219 The use of the stable free radical diphenylpicrylhydrazyl (DPPH) to estimate the activity of antioxi-dants is reviewed. Current applications of the method are examined, particularly the use of the parameter EC 50 (substrate concentration to produce 50% reduction of the DPPH). Some recommendations are made as to the most suitable ways of carrying out this assay and evaluating the data produced.
In a previous paper [P. Molyneux, "Transition-site" model for the permeation of gases and vapors ... more In a previous paper [P. Molyneux, "Transition-site" model for the permeation of gases and vapors through compact films of polymers, J. Appl. Polym. Sci. 79 (2001) a transition-site model (TSM) for the activated permeation of gases through compact amorphous solids was developed and applied to organic polymers; the present paper examines the applicability of the TSM to permeation through microporous silica. The basis of the TSM theory for amorphous solids in general is outlined; the present extension to inorganic glasses has revealed that the transition sites (TS) of this theory, which are the three-dimensional saddle-points critical in the molecular sieving action, equate to the doorways long recognized in permeation through amorphous silica and other inorganic glasses. The TSM, which views permeation as a primary process, is contrasted with the conventional sorption-diffusion model (SDM) for permeation. It is pointed out that in the SDM, the widely accepted analysis into two apparently distinct factors -sorption (equilibrium) and diffusion (kinetic) -has the fundamental flaw that these factors are not independent, since both involve the sorbed state. By contrast, the TSM focuses on the permeant molecule in only two states: as the free gas, and as inserted in a doorway D; hence the characteristics of these doorways -(unperturbed) diameter D , spacing , and the thermodynamic parameters  (force constant) and (entropy increment) for the insertion process -can be evaluated. The theory is applied to literature data [J.D. Way, D.L. Roberts, Hollow fiber inorganic membranes for gas separations, Sep. Sci. Technol. 27 (1992) 29-41; J.D. Way, A mechanistic study of molecular sieving inorganic membranes for gas separations, Final Report submitted to U.S. Department of Energy under contract DE-FG06-92-ER14290, Colorado School of Mines, Golden, CO, 1993, www.osti.gov/bridge/servlets/purl/10118702-ZAx4Au/native/1011872.pdf; M.H. Hassan, J.D. Way, P.M. Thoen, A.C. Dillon, Single component and mixed gas transport in silica hollow fiber membrane, J. Membr. Sci. 104 (1995) 27-42] on the permeation through microporous silica hollow-fiber membranes (developed by PPG Industries Inc.) of the nine gases: Ar, He, H 2 , N 2 , O 2 , CO, CO 2 , CH 4 and C 2 H 4 , over the temperature range 25-200 • C. The derived Arrhenius parameters for the permeation of these gases (excepting He) lead to estimates of the four doorway-parameters: D , 125 pm; , ca. 30 nm; Â, 0.43 nN; , 1.7 pN K −1 ; these values lie within the ranges of those obtained with the glassy organic polymers. Some "secondary effects", shown particularly by CO and CO 2 , are interpreted as host-guest interactions at the doorway. The behavior of He is anomalous, the permeation rising linearly with temperature. This study confirms that the TSM may be applied to gas permeation by activated molecular sieving for this type of inorganic membrane.
Hydrophobically substituted water-soluble polymers (HSWSP) act as associative thickeners through ... more Hydrophobically substituted water-soluble polymers (HSWSP) act as associative thickeners through the reversible crosslinking from noncovalent interactions between the various groups on the polymer chains in aqueous solution. This article shows how the intrinsic viscosity (IV) of nonionic HSWSP can be used to define the thermodynamics of these interactions. Literature data on the IV of pullulans substituted by nucleobase ester groups (thyminylbutyryl and adeninylbutyryl) (Mocanu et al., Can J Chem, 1995, 73, 1933 are used as an exemplar of these procedures. The intramolecular crosslinking in these substituted pullulans is deduced to be ''unimolecular'' (association constant K 1 ¼ 1 M À1 ), as contrasted with the ''bimolecular'' behavior expected from the stacking of the free nucleobases; evidently the crosslinking results from hydrophobic interactions between the butyryl linking groups and the main chain. The results are compared with those from other HSWSP, and from cosolute binding systems. The use of the water-octanol partition coefficients of model systems to elucidate hydrophobic interactions in HSWSP, and of denaturant cosolutes (especially urea) to diagnose the presence and strength of these interactions, are also discussed. Emphasis is placed on the need for further such studies to identify the interactions underlying the rheological behavior of the nonionic HSWSP, and of the more common ionic types.
The theory of the permeation of effectively spherical molecules of gases and vapors (permeants) t... more The theory of the permeation of effectively spherical molecules of gases and vapors (permeants) through films of compact amorphous solids such as polymers is developed using the activated-jump model (AJM). The conventional view is that permeation has to be analyzed as the resultant of sorption (solution) and diffusion effects. By contrast, in the present article, it is proposed that permeation may be viewed as a simple or fundamental process. This is suggested by a number of experimental observations: (a) the permeation correlations of Szwarc (1955-1956); (b) the "Permachor" concept of Salame (1961)(1962)(1963)(1964)(1965)(1966)(1967)(1968)(1969)(1970)(1971)(1972)(1973); (c) the "ideal" permeation behavior of water vapor through moderately polar polymers; (d) the absence of any effect of oxidation on the water-vapor permeability of polyethylene (PE); and (e) the "isokinetic" correlations between the Arrhenius parameters for permeation. The conventional AJM for diffusion is analyzed using the principle of microscopic reversibility, which shows that the average jump is characterized by a "transition site" L at its midpoint, analogous to the transition state in chemical reactions. For amorphous solids, these transition sites would be structural features, distributed at random and with their axes pointing at random. This leads to the present transition-site model (TSM) of permeation, where, at the steady state, a certain fraction of these sites will be transiently occupied by molecules of the permeant in equilibrium with the free molecules at that level. The concentration of these free molecules corresponds to the thermodynamic activity at that point, that is, for gases and vapors, the partial pressure. The ratedetermining step of the permeation process is then taken to be the release of the permeant molecule from the transition site according to classical transition-state theory. Using an idealized cubic-lattice model for the distribution of the transition sites, this is shown to lead to the observed proportionalities of the permeation rate to the area of the film, the pressure difference across it, and the reciprocal of the film thickness. It also accords with the observed Arrhenius-type dependence of the permeability coefficient on temperature, where the Arrhenius parameters relate to the thermodynamic parameters for the transfer of the permeant molecule from the gas phase and its insertion in the transition site. The Arrhenius parameters from the literature (Polymer Handbook) for 16 homopolymers-NR, PA 11, PC, PDMB, PDMS, three PEs (HDPE, LDPE, and hydrogenated polybutadiene), PETFE, PEMA, PET, PP, PTFE, PVAC, PVBZ, and PVC-with 16 "simple" permeants-H 2 , He, CH 4 , Ne, N 2 , CO, O 2 , HCl, Ar, CO 2 , SO 2 , Cl 2 , Kr, SiF 4 , Xe, and SF 6 as well as H 2 O vapor-are used as the dataset. These Arrhenius parameters are first discussed in relation to isokinetic behavior. They are then correlated according to the TSM theory with the van der Waals molecular diameter of the permeant G , and its absolute entropy S 0 . With certain exceptions, linear correlations are obtained with the 10 smaller-molecule permeants (He to CH 4 ) 981 that show that they use the same set of transition sites, below and above the glass transition temperature, with each polymer; the permeant molecules evidently behave here as "hard spheres," regardless of their other chemical characteristics. This enables estimates to be made of the four characteristic parameters for the polymer: the intersite spacing (equivalent to the lattice parameter of the idealized model and to the jump length of the AJM); the size of the transition-site aperture, L ; the force constant associated with expansion of the aperture by the permeant molecule; and the entropy increment also associated with this expansion. For most of the systems, the sitespacing is of the order of 10 nm, and the aperture L is about 200 pm. The theory provides a molecular basis for the interpretation and design of the permeation characteristics of polymers.
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Papers by Philip Molyneux
esters (‘alkylparabens ’) has been studied. In each case the cosolute solubility increases linearly with concentration
of U, which has been interpreted as due to association of the type SJ, where x is assumed to be either 1 or
2. The results are discussed in terms of the interaction forces (dipole-induced dipole forces, hydrogen bonds,
hydrophobic interactions) expected to occur in the association, and on the basis of some amphiphilic character
for the urea resulting from the expected ‘peripheral’ pattern of hydration around its molecule. The ‘water
structure ’ interpretation of interactions in these systems is also discussed.
taking the logarithm of a dimensioned quantity are discussed.
By using quantity calculus, it is shown that this is a
proper procedure, with the consequent units also being logarithmic,
and additive rather than multiplicative. This allows
equilihrium constants to have units rather than, as some
present approaches require, having to he forced into a dimensionless
form, while it also clarifies the units of a number
of derived quantities in physical chemistry such as pH, acidity
constants (i.e., pKa), standard thermodynamic functions,
and standard electrode potentials.
The construction and operation of an absorptiometer for studying the progressive precipitation and re-solution of polymers from dilute solution over the range 20 C to 100 C at a rate of 0.3 deg min-1 are described. The instrument uses a 10 mm diameter collimated beam of green monochromatic light (λ0= 546 mμ) obtained from a compact high-pressure mercury arc, which is passed through the precipitating system held in a cuvette in a temperature control bath; the absorbance (turbidity) of the sample is measured in terms of the attenuation which has to be applied to a reference beam to reduce its intensity to that of the transmitted sample beam. The beams are detected by selenium photocells; precautions are taken to prevent any appreciable amount of forward scattered light from the sample reaching its photocell.
Studies of the progressive precipitation and re- solution of each of the three polymers (at 0.2 to 1 mg cl-1) with the absorptiometer showed that esters and ketones have too low values of ψ to reveal the molecular weight distribution for broad-distribution samples, although particular ones can be used on fractionated material, e. g. butyl acetate for polyisobutene fractions of molecular weight about one million. The experiments have shown that cyclohexanol is a suitable medium for broad-distribution polystyrene, and that propanol and butanol are similarly suitable for poly(methyl methacrylate), while with polyisobutene only mixed media (such as heptane + butanol, or butyl ether + methoxyethanol) seem to be suitable in this respect. A design for a sealed cuvette is proposed which would both enable these hygroscopic media to be used more easily, and at the same time would reduce the amount of polymer required to less than 100 micrograms. The turbidity values for complete precipitation are shown to be in reasonable concordance with the theoretically predicted values of τmax, the ratio averaging about three-quarters. The attainment of a steady turbidity at any given temperature seems to occur very much faster than can be accounted for by simple Smoluchowski aggregation of equal-size particles, which suggests that the precipitating molecules are captured either by "nuclei" or by already present aggregates. Suggestions are made for lessening the present sensitivity of the absorptiometer to instability in the arc source, and for the automatic recording of the precipitation and re- solution characteristic.
the
technique
of
turbidimetric
titrations
for
the
molecular-weight
analysis
of
polymers,
the
precipitation
of
the
polymer
(phase-separation)
is
induced
by
the
addition
of
a
precipitant
to
a
very
dilute
solution
of
the
polymer;
in
the
present
paper
a
modification
of
this
technique,
in
which
the
precipitation
is
induced
by
cooling
such
a
solution,
is
proposed.
The
modification
has
the
practical
advantages
that
the
lessened
need
for
stirring
enhances
the
stability
of
the
polymer
suspen-
sion;
that
the
volume
of
the
system
stays
essentially
constant;
that
it
is
less
likely
for
there
to
be
changes
in
the
turbidity
of
the
system
after
all
of
the
polymer
has
been
precipitated
out
than
with
the
normal
turbidi-
metric
titration;
and
that
it
would
be
much
easier
to
test
the
reversibility
of
the
precipitation
than
in
such
titrations.
It
also
has
the
advantage
from
the
theoretical
viewpoint
that,
with
monodisperse
polymer
and
a
single
liquid
as
the
medium
in
which
the
polymer
is
initially
dissolved,
the
system
becomes
a
simple
binary
liquid
mixture.
TheFlory-Huggins
equationshavebeen
applied
to
the
case
of
such
a
monodisperse
polymer
at
low
concentration
in
a
single-liquid
medium,
and
the
Molyneux,
Estimation
o/
Polymer
Molecular
Weight
Averages
105
precipitation
temperatures
have
been
calculated
numerically
and
presented
graphically
for
the
ranges:
molecular
weights
from
10
4
to
10~;
volume
fractions
of
polymer
in
solution
from
10
-2
to
10-9;
and
values
for
y~
(the
entropy-change
of
dilution
parameter)
of
0.5,
1.0
and
1.5.
The
treatment
shows
that
the
ideal-
temperature,
O,
of
the
system
is
the
important
para-
meter
in
deciding
whether
the
polymer
will
all
dissolve
at
the
upper
limit
of
the
experimental
temperatures,
and
the
other
parameter
~
is
important
in
deciding
whether
all
will
then
have
precipitated
out
again
at
the
lower
limiting
temperature.
These
two
conditions
are
those
which
must
be
fulfilled
if
the
medium
is
to
be
suitable
for
revealing
the
complete
molecular-weight
distribution
of
a
broad-distribution
polymer.
In
the
subsequent
light-scattering
treatment
of
the
turbidity
of
the
suspension
of
particles
produced
by
the
phase
separation,
it
is
pointed
out
that
these
particles
are
spherical
(since
the
phase-separation
calculations
show
them
to
be
always
swollen
with
the
medium)
and,
because
of
their
composition
and
the
relative
closeness
of
the
refractive
indices
of
polymer
and
medium,
they
will
have
a
refractive
index
even
closer
to
that
of
the
surrounding
solution.
Under
these
conditions,
the
complex
Mie
relations
for
particles
of
arbitrary
size
reduce
to
a
simple
form
due
to
van
de
Hulst;
this
is
applied
to
the
aggregating
system,
for
which
it
is
shown
that
as
the
aggregation
proceeds
the
turbidity,
~,
pro-
duced
by
a
given
amount
of
polymer
in
suspension
will
pass
through
a
maximal
value
given
by:
where
n~
and
n~
are
the
refractive
indices
of
pure
medium
and
pure
polymer,
(~2)i
is
the
overall
volume
fraction
of
precipitated
polymer,
and
A0
is
the
vacuum
wavelength
of
the
incident
light.
The
significance
of
this
expression
lies
in
its
demonstration
of
the
direct
proportionality
between
~max
and
the
overall
polymer
concentration,
and
of
its
dependence
upon
four
readily
accessible
parameters
without
involving
the
size
of
the
particles
or
their
composition.
The
theoretical
treat-
ments
in
this
first
part
is
concluded
with
a
consideration
of
the
timeseale
of
the
aggregation,
based
on
the
Smoluchowski
approach,
and
with
a
discussion
of
the
basic
criteria
for
the
selection
of
suitable
media,
par-
ticularly
with
regard
to
the
choice
of
media
of
low
refractive
index
to
maximise
the
turbidity
produced
by
a
given
amount
of
precipitated
polymer.
binding
by
polyvinylpyrrolidone
(PVP)
in
aqueous
solution
at
25
°C
of
each
of
the
four
homo-
logous
cosolutes:
methyl,
ethyl,
n-propyl
and
n-butyl
parahydroxybenzoates
(parabens)
has
been
studied
by
equilibrium
dialysis.
In
each
case
the
binding
isotherm
is
curved
upwards,
which
has
been
interpreted
as
a
result
of
co-operative
effects,
i.
e.
attractive
interactions
between
the
bound
cosolute
molecules.
Using
mathe-
matical
methods
developed
by
Schwarz,
the
binding
data
have
been
shown
to
conform
in
each
case
to
the
simplest
form
of
co-operativity,
in
which
the
attractive
interactions
occur
only
between
adjacent
bound
cosolute
molecules,
with
the
interaction
increasing
the
effective
value
of
the
binding
constant
by
a
multiplicative
factor
~,
the
co-operativity
parametet.
For
these
four
systems
the
parameter
~
is
found
or
have
an
essentially
constant
value
of
9.6
::E
1.2.
The
constancy
of
~
shows
that
the
alkyl
chains
are
not
directly
involved
in
the
co-operative
effects,
which
were
therefore
concluded
to
be
the
result
of
hydrogen
bonding
between
the
phenolic
hydroxyl
group
of
one
bound
cosolute
molecule
and
the
ester
group
of
its
neighbour.
The
data
have
also
been
applied
to
the
approximate
Fowler
model,
in
which
it
is
assumed
that
the
co-operative
effects
do
not
appreciably
disturb
the
random
distribution
of
bound
cosolute
molecules.
The
corresponding
test
plots
are
linear,
but
the
derived
values
of
the
co-operativity
parameter
are
widely
different
from
the
correct
values
obtained
by
the
Schwarz
method.
This
shows
that
the
basic
assumption
of
the
Fowler
model
is
incorrect
for
systems
having
these
relatively
high
degrees
of
co-
operativity
(i.e.,
:¢
~
10).
For
the
simple
co-operative
binding
model,
the
initial
slope
of
the
isotherm
is
equal
to
Kon,
where
K0
(the
intrinsic
binding
constant)
is
the
equilibrium
constant
for
the
attachment
of
a
cosolute
molecule
onto
a
site
on
an
unoccupied
chain,
and
n
(the
site
density)
is
the
limiting
value
of
binding
872
Colloid
and
l~olymer
Science,
VoL
257.
No.
8
(1979)
for
monolayer
coverage.
For
each
cosolute,
closely
similar
values
of
this
slope
are
obtained
by
the
Schwarz
method,
the
Fowler
method,
and
the
conventional
Scatchard
method
used
for
systems
not
showing
co-operative
effects.
Using
the
value
n
=
0.10
molecule
of
cosolute
per
PVP
monomer
unit
(obtained
in
previous
work
for
PVP
with
aromatic
cosolutes),
the
values
of
K0
(M
-i,
±
one
s.d.)
for
the
four
alkyl
parabens
are:
methyl,
8.2~0.5;
ethyl,
11.1:50.5;
propyl,
19.5:50.9;
butyl,
27.5:50.7.
The
values
of
log
K0
rise
essentially
linearly
with
increase
in
alkyl
chain
length,
showing
that
each
additional
methylene
group
raises
the
value
of
K0
by
an
essentially
constant
factor
of
1.5:50.1,
and
correspondingly
makes
a
constant
negative
contribution
o{
0.24:50.04
kcal
mo1-1
(1.0
±
0.2
kJ
tool
-1)
to
the
standard
free
energy
change
of
binding.
This
free
energy
contribution,
which
represents
the
strength
of
the
hydrophobic
interaction
between
the
cosolute
methylene
group
and
(at
least)
one
on
the
PVP
chain,
thus
corresponds
to
a
contribution
of
(at
most)
0.12:5
0.02
kcal
mol
-i
per
individual
methylene
group.
This
is,
as
might
be
expected,
only
one-seventh
of
that
for
the
complete
transfer
of
a
methylene
group
from
aqueous
solution
into
a
liquid
alkane
environment
(cyclohexane);
it
is,
however,
closely
similar
to
the
hydrophobic
effect
contribution
per
individual
methylene
group
of
about
0.15
kcal
mo1-1
predicted
from
the
theory
of
aqueous
solutions
developed
by
Ngmethy
and
Seheraga.
The
present
results
are
compared
with
those
reported
in
the
literature
for
the
binding
of
these
cosolutes
by
other
macromolecular
substances
including
serum
albumin,
and
by
micellar
nonionic
amphiphiles;
they
are
also
discussed
more
generally
in
relation
to
the
estimation
of
group
contributions
to
binding.