Effect of chain length on the interaction between modified organic salts containing hydrocarbon chains and poly(N -isopropylacrylamide- co -acrylic acid) microgel particles

Article abstract of DOI:10.1021/la104411j

A series of four hydrophobically modified, diphenylazo-based organic salts have been prepared and characterized. To achieve this a C_x (x = 4, 6, 8, or 10) hydrocarbon chain was inserted between the diphenylazo moiety and the quaternary ammonium headgroup of the salt. The absorption of each of the four modified organic salts into anionic microgel particles of poly(N- isopropylacrylamide-co-acrylic acid) has been studied at pH 8. In addition, the hydrodynamic diameters and electrophoretic mobilities of the microgel particles have been studied as a function of the organic salt concentration, also at pH 8. In addition to the electrostatic attraction between the quaternary ammonium head groups of the organic salts and the anionic groups within the microgel particles, hydrophobic association between the chains of the organic salts within the microgel particles plays a role, with this effect increasing strongly from x = 4 to 10. Desorption of the x = 4 and 6 organic salts occurs readily on changing, in situ, the pH from 8 to 2.5 (and thereby eliminating the electrostatic interaction) but is only partially achieved for the x = 8 and 10 organic salts. Indeed, for the x = 10 organic salt, only about 80% of the organic salt is desorbed upon dilution of the microgel particles into a large excess of water.

Full text of DOI:10.1021/la104411j

ARTICLE
pubs.acs.org/Langmuir
Effect of Chain Length on the Interaction between Modified Organic
Salts Containing Hydrocarbon Chains and Poly(N-isopropylacryl-
amide-co-acrylic acid) Microgel Particles
Kaizhong Fan, Melanie Bradley,* Brian Vincent, and Charl F. J. Faul
School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K.
S Supporting Information
b
ABSTRACT: A series of four hydrophobically modified,
diphenylazo-based organic salts have been prepared and
characterized. To achieve this a Cx (x = 4, 6, 8, or 10)
hydrocarbon chain was inserted between the diphenylazo
moiety and the quaternary ammonium headgroup of the salt.
The absorption of each of the four modified organic salts into
anionic microgel particles of poly(N-isopropylacrylamide-co-
acrylic acid) has been studied at pH 8. In addition, the
hydrodynamic diameters and electrophoretic mobilities of
the microgel particles have been studied as a function of the
organic salt concentration, also at pH 8. In addition to the electrostatic attraction between the quaternary ammonium head groups of
the organic salts and the anionic groups within the microgel particles, hydrophobic association between the chains of the organic
salts within the microgel particles plays a role, with this effect increasing strongly from x = 4 to 10. Desorption of the x = 4 and 6
organic salts occurs readily on changing, in situ, the pH from 8 to 2.5 (and thereby eliminating the electrostatic interaction) but is
only partially achieved for the x = 8 and 10 organic salts. Indeed, for the x = 10 organic salt, only about 80% of the organic salt is
desorbed upon dilution of the microgel particles into a large excess of water.
’ INTRODUCTION
microgel particles leads to an increase in the net Hamaker
constant of the particles, as water is excluded from their interior.
At the point where aggregation occurred with each of the three
OS molecules, the diameter of the microgel particles had been
reduced from about 1200 nm, in the absence of added OS
molecules (and pH 3), to around 200 nm. At this size the van der
Waals attraction between the microgel particles is sufficient to
overcome any residual electrostatic repulsion between them,
leading to the observed particle aggregation. The calculated ratio
of absorbed OS molecules per pyridine moiety at the onset of
aggregation was significantly less than 1 (the greatest value being ∼
0.4 for the OS molecules with only one sulfonate group).
Moreover, no reversal of charge of the microgel particles could
be achieved by increasing the added OS concentration. If reversal
of charge had occurred, then the microgel particles should have
reswollen and restabilized, allowing greater amounts of OS
molecule absorption. So the low maximum absorbed amounts
of OS molecules, taken together with the fact that they are only
weakly absorbed and are hence likely to desorb on dilution of the
microgel particles, meant that using normal OS molecules of this
type in any potential applications would be severely limited.
Microgel particles have been widely studied in recent years,1
both with regard to understanding their physical properties and
also in exploiting their potential applications, for example, in drug
delivery,2,3 as biosensors,4,5 and in surface coatings.6 An impor-
tant feature of microgel particles is their stimulus-responsive
behavior, in particular they may change their size in response to
external triggers such as changes in temperature,7ꢀ9 pH,10ꢀ13 or
solvency.14,15 When microgel particles are in their swollen state,
they may absorb molecules, or even nanoparticles, provided
there is some attraction between the adsorbate and the cross-
linked polymer network of the microgel particles.
In a recent paper16 we reported on the absorption of organic
salts (OS), containing one, two, or three sulfonate groups, into
cationic poly(vinylpyridine) (PVP) microgel particles at pH 3,
where the pyridine moieties are fully protonated. The structures
of these three OS’s are shown in Figure 1.
The principle adsorption mechanism was found to be an
electrostatic one, with the adsorbed amount, at a given OS
concentration, increasing, as might be expected, with the number
of sulfonate groups per OS molecule. The adsorption isotherms,
however, were all low affinity. Moreover, for each of the OS
molecules studied there was a limiting concentration, beyond
which the PVP microgel particles aggregated. This was attributed
to the observed decrease in size of the microgel particles, with
increasing OS molecule absorption. A decrease in size of the
Received: November 5, 2010
Revised:
December 20, 2010
Published: March 16, 2011
r
2011 American Chemical Society
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ARTICLE
Figure 1. Organic salts used in a previous paper.16
Figure 2. Hydrocarbon-modified OS’s used in the present work.
In order to overcome this problem, it was felt that another
mechanism of absorption needed to be introduced, in addition to
the electrostatic one. In previous studies of the adsorption of
both ionic and nonionic surfactant molecules17ꢀ22 into microgel
particles, it had been observed that hydrophobic interactions
[between the hydrocarbon tails of the surfactant molecules and
the hydrophobic moieties in microgel particles based on poly-
(NIPAM)] contributed significantly to the adsorption mechan-
ism. In addition, when ionic surfactants of opposite charge to the
microgel particles were studied, charge reversal was observed at
sufficiently high added surfactant molecule concentrations, even
though particle aggregation was observed over a certain, lower
surfactant concentration range. It was also shown17,18 that, in the
reversed charged region, the number of adsorbed surfactant
molecules per charge group in the microgel particles was very
much greater than one. This suggested that some form of
surfactant aggregation was occurring around each charged site
of the microgel particles, as is known to occur for ionic surfactant
molecules around polyelectrolyte chains of opposite charge,
beyond the so-called “critical aggregation concentration” of
surfactant.23 This led us to the following strategy, i.e., to insert
hydrocarbon chains as spacers between the OS molecules’s
hydrophobic core and charge, thus allowing the modified OS
molecules to behave in a similar fashion to the more classical
ionic surfactants in terms of their interaction with microgel
particles containing charged groups of opposite sign. In this
way, both higher absorbed amounts of the OS molecules (in the
charge-reversed region) and stronger absorption might be
achieved.
had been successfully achieved.24 Four such modified OS mole-
cules, with different hydrocarbon chain lengths, were successfully
synthesized, and their structures are shown in Figure 2.
These molecules could be regarded as cationic surfactants with
a diphenylazo moiety attached to the end of the hydrocarbon tail.
Since these hydrophobically modified OS’s are cationic, it was
also necessary to change the microgel particles, from the cationic
PVP particles used in the previous work16 to an anionic sys-
tem. To this end, poly(N-isopropylacrylamide-co-acrylic acid)
(PNIPAM-co-AAc) microgel particles were used. These anionic
microgels have been widely studied previously.25 They are
known to be negatively charged, even under acidic conditions,
if an initiator such as potassium persulfate is used in their
synthesis, as this leads to the presence of strong acid groups
(sulfonate) at the particle periphery. However, their overall
negative charge increases strongly with increasing pH as the
acrylic acid moieties within the interior of the microgel particles
become increasingly ionized.
In the present paper, we present studies of the effect of the four
modified OS’s shown in Figure 2 on the diameter and electro-
phoretic mobility of the PNIPAM-co-AAc microgel particles at
pH 8, where the microgel particles are strongly negatively
charged. The corresponding adsorption isotherms have also
been established for the four modified OS’s at pH 8. In addition,
the effects of (i) changing the pH from 8 to 2.5 (at the same
particle concentration) and (ii) diluting the microgel particles
with water on the adsorption isotherms have been investigated.
At pH 2.5, the only anionic charge on the microgel particles
comes from the surface sulfonate groups; all the AAc moieties are
in the undissociated form. In this way, we are able to assess the
relative strengths of the electrostatic and hydrophobic interac-
tions between the modified OS’s and the microgel particles.
Furthermore, we were able to determine whether sufficiently
high absorbed amounts could be achieved and whether desorp-
tion on changing pH or upon simple dilution would be effected
or restricted, as desired for any particular application.
This concept forms the basis of the present paper. Initially
we attempted to attach a hydrocarbon chain to the OS with
one sulfonate group reported in our previous paper,16 namely,
1-tropacolin-O, the structure of which is shown in Figure 1.
However, this proved to be difficult synthetically. Hence, it was
decided to use a cationic version of a similar diphenylazo
compound, to which the attachment of hydrocarbon chains
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ARTICLE
’ EXPERIMENTAL SECTION
with water and allowing the system to stand for a given time, before
centrifuging and reanalyzing the supernatant concentration. The time
intervals chosen were 5, 15, 30, 45, 60 min.
Materials. N-Isopropylacrylamide (NIPAM) and N,N-methylene-
bisacrylamide (BA) were both from Fisher. NIPAM was recrystallized
from hexane. Potassium persulfate (KPS), an anionic initiator, was used
as received from Sigma-Aldrich. 4-Ethylaniline (98%), sodium nitrite
(NaNO2) (99.5%), phenol (99%), 1,4-dibromobutane (99%), 1,6-
dibromohexane (96%), 1,8-dibromooctane (98%), 1,10-dibromodecane
(97%), and trimethylamine (TMA) in solution in ethanol (31ꢀ35 wt %)
were all purchased from Sigma-Aldrich. Potassium iodide (KI) was
purchased from BDH Chemicals. Sodium hydroxide (NaOH), sodium
carbonate (Na2CO3), potassium carbonate (K2CO3), methanol, di-
chloromethane, and 37% hydrochloric acid (HCl) solution were all
purchased from Fisher Scientific. Except where stated, all chemicals were
used as supplied. Milli-Q water was used wherever needed.
Synthesis of PNIPAM-co-AAc Microgel Particles. NIPAM
(3.2 g, 0.028 mol) and BA (0.4 g, 2.6 mmol) were added to a 500 mL
reaction vessel and dissolved in 300 mL of Milli-Q water followed by
addition of 15 mL of Milli-Q water solution containing AAc (0.4 g, 5.56
mmol). After purging with nitrogen for 10 min, the solution was heated
to 70 ( 1 °C and stirred at 350 rpm. KPS (0.12 g, 0.44 mmol), dissolved
in Milli-Q water (7 mL), was added to the reaction flask and the solution
turned turbid within 5 min. The reaction was left to proceed for a further
10 h and then left to cool to room temperature. The resultant microgel
particle dispersion was dialyzed against Milli-Q water to remove any low
molecular weight oligomers and unreacted materials; the water was
renewed each day for 7 days.
Characterization of the Microgel Particles. The diffusion
coefficients of the microgel particles in water at ambient temperature
were determined by photon correlation spectroscopy (PCS) using a
Brookhaven Instruments Zeta Plus apparatus, fitted with a 15 mW laser
(wavelength 678 nm). The StokesꢀEinstein equation was used to
calculate the hydrodynamic diameter of the particles from the diffusion
coefficient. The electrophoretic mobility of the microgel particles in
1 mM KCl was determined by phase analysis light scattering (PALS)
using the same instrument.
1
Instrumentation. The NMR H spectrum for each of the four
organic salts in dimethyl sulfoxide-d6 (DMSO-d6) solution was deter-
mined using a JEOL ECP 400 instrument, at 20 °C.
Mass spectrometry (MS) analysis on each of the four organic salts was
carried out using an Apex 4,7 T Fourier-transform ion-cyclotron
resonance mass spectrometer (Bruker Daltonics, Coventry, UK), fitted
with an Apollo electrospray ionization source. Samples were dissolved in
a methanol/dichloromethane mixture (50:50 by vol), at a concentration
of approximately 10 μg mLꢀ1 and directly infused at 120 μL hꢀ1 into the
electrospray ionization source from a syringe pump. (These measure-
ments were kindly carried out by the mass spectrometry facility in the
School of Chemistry at the University of Bristol).
In order to determine the number of charge groups per microgel
particle, a potentiometric titration of the acrylic acid groups in the
microgel particles was carried out as follows: 10 mL of the synthesized
microgel particle dispersion was diluted with 40 mL of Milli-Q water and
adjusted to pH 1.92. This microgel dispersion was then titrated with a
solution of 0.05 M sodium hydroxide and the equilibrium pH recorded
at each stage. As a control experiment, 50 mL of Milli-Q water was
adjusted to pH 1.92 and also titrated with 0.05 M sodium hydroxide
solution.
Surface tension measurements on solutions of each of the OS
molecules in water were made using a Kruss K100 tensiometer, fitted
with a platinum ring. Plots of surface tension (γ) against log OS
concentration (c) were constructed for each OS molecule, from which
the critical micelle concentration (cmc) and the area per OS molecule
(aS) at the air/solution interface could be determined.
The saturation solubility of each of the four OS molecules in water, at
20 °C, was determined as follows. For C4-OS, C6-OS, and C8-OS, 0.001
g of the organic salt was added to 1 mL of Milli-Q water. This procedure
was repeated for each organic salt until some undissolved material was
observed, even after stirring overnight. For C10-OS, a similar procedure
was used, except that 0.001 g of the organic salt was added to 100 mL of
Milli-Q water initially.
Absorption Isotherms for the Organic Salts into the
Microgel Particles. Appropriate amounts of microgel dispersion,
organic salt solution, and Milli-Q water, each at pH 8, were mixed to give
a suitable range of initial organic salt concentrations (c0 /M), in a total
volume (V) of 10ꢀ2 L. The mixtures were allowed to stand for 48 h in
order to equilibrate. Afterward, each dispersion was centrifuged at
10 000 rpm, at 20 °C. The supernatant was collected and analyzed using
UVꢀvis spectroscopy. The equilibrium concentration (ceq/M) of the
organic salt was determined from the corresponding calibration curve.
The absorbed amount (Γ, wt/wt) of each organic salt in the microgel
particles could then be calculated from eq 1
Synthesis of Organic Salts. The method reported by Oakley24
was used to synthesize the four organic salts shown in Figure 2. Three
steps were involved in each preparation, as described below.
i. Preparation of 4-((4-Ethylphenyl)diazenyl)phenol. Three solu-
tions were prepared as follows: (A) 4-ethylaniline (24.2 g, 0.2 mol)
was dissolved in of a mixture of 250 mL of acetone and 250 mL of Milli-Q
water containing 20 mL of HCl solution (37%). The solution was
stirred for 20 min in an ice bath. (B) NaNO2 (13.8 g, 0.2 mol) was
dissolved in 200 mL of distilled water and placed in a freezer to cool to
1ꢀ3 °C. (C) Phenol (18.8 g, 0.2 mol), NaOH (8 g, 0.4 mol), and
Na2CO3 (21.2 g, 0.2 mol) were dissolved in 200 mL of Milli-Q water,
stirred for 5 min, and then also placed in the freezer to cool to 1ꢀ3 °C.
Once the solutions had cooled, solution B was slowly added to solution
A and left for 20 min to stir in an ice bath. The diazonium salt solution
generated by mixing solutions A and B was added drop by drop to the
phenol solution (solution C), ensuring at all times the reaction
temperature did not exceed 8 °C. The yellow/brown precipitate was
filtered off and dried overnight in a vacuum oven. The crude product was
recrystallized from ethanol, producing a yellow powder.
ii. Preparation of Bromoalkylated 4-((4-Ethylphenyl)diazenyl)
phenol. The product from step i was used to react with 1,4-dibromo-
butane, 1,6-dibromohexane, 1,8-dibromooctane, or 1,10-dibromode-
cane. Only the reaction with 1,4-dibromobutane is described here.
4-((4-Ethylphenyl)diazenyl)phenol (4.5 g, 0.02 mol) was dissolved in
100 mL of acetone. 1,4-Dibromobutane (21.6 g, 0.1 mol), anhydrous
K2CO3 (7 g, 0.05 mol), and KI (0.1328 g, 0.8 mmol) were all dissolved in
100 mL of acetone and then added to the previous solution. The reaction
mixture was refluxed at 75 °C and stirred for 20 h. The solution was then
allowed to cool to room temperature, after which the acetone was
removed by rotary evaporation. The resulting precipitate was washed
with 50 mL of hexane in a Buchner funnel connected to a vacuum pump.
ðc0 ꢀ ceqÞVM
Γ ¼
ð1Þ
m
where m is the mass of (dry) microgel present and M is the molar mass of
the organic salt.
The change in the absorbed amount for each of the dispersions was
then determined on changing the pH, in situ, from 8 to 2.5. This was
achieved by reanalyzing the supernatant concentration after sufficient
concentrated HCl solution had been added (in microliter quantities) to
each of the (remixed) dispersions to adjust the pH to 2.5 and the
dispersions had been allowed to stand for a further 48 h.
Similarly, the change in the absorbed amount for C10-OS, at one
initial concentration, was determined after replacing the supernatant
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Table 1. Molecular Mass (M), Critical Micelle Concentration
(cmc) in Water, Area per OS Molecule at the Air/Water
Interface (aS), and the Saturation Solubility (S) in Water, for
Each of the OS Moleculesa
M (g molꢀ1
)
calcd,
without Brꢀ
expl,
cmc
aS
(nm2)
S
OS
from MS
(mM)
(mM)
C4-OS
C6-OS
C8-OS
C10-OS
340
368
396
424
340
368
396
424
3.9
1.3
0.65
0.71
1.3
114
56
0.66
0.07
19
2.8
0.18
a Note that mass spectra (MS) are provided in the Supporting
Information.
Figure 4. Potentiometric titration of the microgel particles (4) and
Milli-Q water (b): pH against volume of 0.05 M NaOH added.
The surface tension (γ) versus ln c plot for C8-OS is shown in
Figure 3. Similar plots were obtained for the other three organic
salts. The cmc value was taken as the concentration where the
two straight lines shown in Figure 3 intersect. The cmc values
decrease with increasing hydrocarbon chain length, as expected
(Table 1). For C4-OS and C6-OS, the cmc value reduces by a
factor of ∼2 on adding two further carbon atoms. However, a
much bigger decrease (a factor of ∼9) occurs on going from C8-
OS to C10-OS. A similar disparity is seen in the solubility (S)
values given in Table 1. Clearly, adding the two extra carbon
atoms in going from C8-OS to C10-OS increases the hydro-
phobicity of the molecule considerably.
The aS value was obtained as follows. The maximum value of
the gradient of the curve [dγ/d(ln c)], just prior to the cmc (see
Figure 3. Surface tension (γ) against ln (c/M) for surfactant C8-OS at
20 °C.
Figure 3), was first determined. The Gibbs equation (eq 2) was
aw
then used to calculate the maximum adsorbed amount (Γmax
)
Then the product was washed with 50 mL of Mili-Q water several times
before being dried in a vacuum oven overnight. The precipitate was
recrystallized from ethyl acetate twice.
of the OS molecule at the air/solution interface.
ꢀ
ꢁ
1
dγ
aw
Γmax
¼
ð2Þ
2RT dðln cÞ
iii. Preparation of Cationic Organic Salts. The product from step ii (1 g,
2.77 mmol) was dissolved in 100 mL of toluene in a round-bottom
flask. Trimethylamine solution in ethanol (10 mL, 42 mM) was added and
the solution was refluxed at 90 °C for 24 h. The solution was allowed to
cool to room temperature, and then the solvent was removed by using a
rotary evaporator. The resultant solid was purified by dissolving the
precipitate in toluene and centrifuging the solution at 10 000 rpm for 20
min, retaining the solid. This process was repeated until the toluene phase
was not colored. The solid was then recrystallized from ethanol. The
purified solid was then dried in a vacuum oven overnight. The purity of the
product was confirmed by 1H NMR, for C4-OS: (300 MHz, DMSO-d6,
21 °C, TMS) δ = 1.22 (t, 3H, CH3), 1.74ꢀ1.90 (m, 4H, 2CH2),
2.64ꢀ2.75 (m, 2H, CH2), 3.03ꢀ3.12 (s, 9H, 3CH3), 3.38ꢀ3.44 (m,
2H, CH2), 4.11ꢀ4.18 (t, 2H, CH2), 7.12ꢀ7.18 (d, 2H, 2CH), 7.37ꢀ7.44
(d, 2H, 2CH), 7.72ꢀ7.81 (d, 2H, 2CH), 7.85ꢀ7.92 (d, 2H, 2CH).
C6-OS, C8-OS, and C10-OS were all prepared following the same
procedure. The 1H NMR results and sample spectra are provided in the
Supporting Information.
where R is the gas constant and T the absolute temperature. The
factor “2” in the denominator is to account for the fact that the
organic salts dissociate into two ions.
By assuming that the maximum adsorbed amount corresponds
to a close-packed monolayer of OS molecules at the air/solution
interface, the value of aS may be obtained from eq 3
1
aS ¼
ð3Þ
ΓmaxawNA
where NA is Avogadro’s number. aS for a close-packed monolayer
of linear hydrocarbon chains, oriented normal to the air/water
interface, is ∼0.2ꢀ0.6 nm2.26 The values for the organic salt
cations, listed in Table 1, are somewhat greater, as is to be
expected, since both the quaternary ammonium headgroups and
the hydrophobic tails are rather bulky. Thus, the value of
0.65 nm2 seems quite reasonable for C4-OS. It is interesting
that the aS values increase with increasing hydrocarbon chain
length in the series C4-OS to C10-OS, with the value increasing
by a factor of ∼2 in going from C8-OS to C10-OS. The reason
for this is not obvious. It may be that, because of the more
complex structure of the hydrophobic tails of these organic salts
compared to more conventional surfactant molecules with
’ RESULTS AND DISCUSSION
Characterization of the Organic Salts. The MS results
(Table 1) show that the desired OS molecules were prepared
successfully. These results were confirmed by the 1H NMR results.
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Figure 5. Hydrodynamic diameter of the microgel particles as function
of pH, at 20 °C.
Figure 7. Hydrodynamic diameter of the microgel particles as a
function of equilibrium concentration (on a log scale, for clarification)
at pH 8 and 20 °C: (4) C4-OS, (9) C6-OS, (O) C8-OS, (þ) C10-OS.
The lines are for guiding the eye only. (---) OS concentration regions
where measurements could not be made due to flocculation of the
microgel particles.
over the pH range from ∼3 to ∼6, the diameter increases; this
osmotic swelling is due to dissociation of the ꢀCOOH groups in
the acrylic acid moieties with increasing pH and the resultant
uptake of counterions (Naþ ions from the added NaOH) into
the microgel particle interior. Beyond pH ∼ 6, the hydrodynamic
diameter decreases again; this is due to the systematic increase in
the background electrolyte concentration in solution, resulting
from the incremental addition of NaOH to increase the pH.
Diameter of the Microgel Particles as a Function of pH.
Figure 6 shows the electrophoretic mobility of the PNIPAM-
co-AAc microgel particles as a function of pH, in a background KCl
concentration of 1 mmol Lꢀ1. As expected, the magnitude of the
(negative) electrophoretic mobility increases over the pH range
∼3 to ∼9, again due to the increasing dissociation of the
ꢀCOOH groups in the interior of the microgel particles. At
pH 3, where the ꢀCOOH groups are almost entirely in the
undissociated form, the small, negative electrophoretic mobility
value arises from the initiator residues (sulfonate groups) at the
periphery of the particles, which are formed during the polym-
erization process used to prepare the microgel particles.
Hydrodynamic Diameter of the Microgel Particles as a
Function of Added OS Concentration, pH 8. Figure 7 shows
the variation in the hydrodynamic diameter (d) of the microgel
particles, as a function of added OS concentration (c, on a log
scale for clarity), at pH 8, for each of the four OS molecules. From
Figure 5, it can be seen that the value of d for the microgel
particles at pH 8, in the absence of any added OS, is ∼1100 nm.
Upon adding the OS molecules the value of d decreases as a result
of the uptake of OS cations into the interior of the anionic
microgel particles. For C6-OS and C8-OS it should be noted
that, with increasing concentration, there is a region of c values
where the microgel particles flocculate, making the determina-
tion of their size by PCS impossible. These regions are indicated
by the dashed curves in Figure 7. It is of interest that, beyond
these regions where flocculation is observed, the particles
become restabilized. As will be confirmed in the next section,
in this restabilization region (for C6-OS and C8-OS) the
microgels particles change their sign and become net cationic.
For the C4-OS cations, flocculation occurred at concentrations
Figure 6. Electrophoretic mobility of the microgel particles as a
function of pH in a background concentration of KCl of 1 mM.
straight-chain hydrocarbon tails, the packing of the C10-OS tails
in the micelles and at the air/water interface is substantially
different, so that the cmc is reached well before maximum
adsorption at the air/water interface is attained.
Characterization of the Microgel Particles. Concentration
of Carboxylic Acid Groups in the Microgel Dispersion. The
potentiometric titration of the microgel dispersion (at the same
particle number concentration as used in the organic salt
absorption experiments to be discussed later) and that for
Milli-Q water are shown in Figure 4. The difference (0.49 mL)
in the volumes of added NaOH solution required to obtain the
same pH change for the microgel dispersion and for water is
reasonably constant when the pH is above 9.7. The concentra-
tion of carboxylic acid groups in the stock microgel dispersion is
calculated to be 0.000 49 mmol mLꢀ1. This figure may be
compared with the concentration of carboxylic acid groups in
the stock microgel dispersion calculated from the amount of
acrylic acid used in the preparation of the microgel particles,
namely, 0.0011 mmol mLꢀ1. Thus, only ∼45% of the acrylic acid
is actually incorporated into the microgel particles. The rest is
presumably present, after the preparation, as free (poly)acrylic
acid, which is then dialyzed out of the dispersion during cleanup.
Figure 5 shows the hydrodynamic diameter of the PNIPAM-
co-AAc microgel particles as a function of pH. It can be seen that
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Figure 8. Electrophoretic mobility of the microgel particles as a
function of equilibrium concentration (on a log scale, for clarification)
at pH 8 and 20 °C: (4) C4-OS, (9) C6-OS, (O) C8-OS, (þ) C10-OS.
The lines are for guiding the eye only. (---) OS concentration regions
where measurements could not be made since the microgel particles
flocculated.
Figure 9. Adsorbed amounts as a function of equilibrium concentration
(on a log scale, for clarification) at pH 8 and 20 °C: (4) C4-OS, (9) C6-
OS, (O) C8-OS, (þ) C10-OS. The lines are for guiding the eye only. (---)
OS concentration regions where measurements could not be made since
the microgel particles flocculated.
Electrophoretic Mobility of the Microgel Particles as a
Function of Added OS Concentration, pH 8. The electro-
phoretic mobility (u) of the microgel particles, at pH 8, in the
absence of added OS molecules is ꢀ3.2 ꢁ 10ꢀ8 m2 Vꢀ1 sꢀ1, in a
background concentration of KCl of 1 mM (see Figure 6). The
results obtained for u as a function of c (again on a log scale), at
pH 8, for each of the four OS molecules are shown in Figure 8.
The mobility data points shown in Figure 8 do not appear to
extrapolate back to the value of ꢀ3.2 ꢁ 10ꢀ8 m2 Vꢀ1 sꢀ1 in
1 mM KCl in the absence of added OS molecules, presumably
because the addition of the organic salts at concentrations greater
than 1 mM increases the ionic strength of the solution further.
The results shown in Figure 8 broadly reflect the observations
reported in the previous section concerning the dependence of d
on c, at pH 8 (Figure 7). First, only with the C6-OS and C8-OS
molecules could the net sign of the microgel particles be reversed
at sufficiently high c values, i.e., beyond a region of c where
flocculation occurred (indicated by the dashed line in Figure 8);
this reversal of sign occurs at a lower value of c for C8-OS than for
C6-OS. Second, C4-OS has the weakest effect on u; beyond the
highest range of c values reported here for C4-OS flocculation
occurs. Third, C10-OS does not reverse the charge, because the
range of c values that could be studied is limited by the low
solubility of C10-OS in water.
Absorption Isotherms for the OS Molecules in the Micro-
gel Particles at pH 8. The absorption isotherms for the four OS
molecules in the microgel particles at pH 8 are shown in Figure 9
(again with c on a log scale). The dashed lines again indicate
concentration regions (for the C6-OS and C8-OS molecules)
where measurements could not be made because the microgel
particles flocculated. In general, the absorption affinity increases
in the order C10-OS > C8-OS > C6-OS > C4-OS. This again
reflects the increase in hydrophobicity of the OS molecules. The
hydrocarbon moieties in the OS molecules may interact with the
isopropyl moieties in the NIPAM units of the microgel particles.
However, it is also likely that the OS molecules behave in a
similar fashion to traditional cationic surfactant molecules, in that
aggregates form around the anionic ꢀCOOꢀ groups inside the
microgel particles. Such aggregation has been postulated in a
previous publication from this group25 for the absorption of
beyond the highest concentration investigated, and restabiliza-
tion did not occur on increasing this concentration to higher
values, up to the solubility limit. For the C10-OS cations, the
range of concentrations available is limited by the very low
solubility of the C10-OS molecules in water; indeed, insufficient
C10-OS could be added even to reach the flocculation region.
At any given value of c, at least in the higher concentration
range, the value of d decreases in the order C8 > C6 > C4. Clearly,
this would suggest that the longer the carbon chain length, the
greater the interaction of the OS cations with the anionic
microgel particles. This would imply that there is a significant
hydrophobic contribution to this interaction, in addition to the
electrostatic interaction. This hydrophobic contribution be-
comes stronger with increasing carbon chain length. The fact
that addition of sufficient C4-OS leads to flocculation, but not
restabilization, even at the highest concentrations possible,
suggests only a rather weak hydrophobic contribution in this
case. In this regard, C4-OS behaves rather similarly to the
unmodified, anionic OS, 1-trapaeolin-O, shown in Figure 1,
whose interaction with cationic microgel particles was discussed
in our previous paper.16 There it was shown that the absorption
of this OS molecule into the microgel particles was very weak and
also that flocculation (but no restabilization) of the microgel
particles occurred at sufficiently high concentrations of the OS
molecule. Hence, it would seem that insertion of a C4 carbon
chain is insufficient to introduce significant hydrophobicity to the
OS molecule.
One would predict that insertion of a C10 carbon chain should
increase the hydrophobicity of the OS molecules considerably.
However, the additional CH2 units increase the hydrophbicity to
such an extent so that the range of c values available for C10-OS is
much smaller, because of its much more limited solubility in
water (see Table 1). Hence, studies on the effect of this OS
molecule on the hydrodynamic diameter of the microgel particles
were limited to the low c value range (see Figure 7). In this range
the effect of the added OS molecules is much less pronounced,
and no distinct trends with carbon chain length may be distin-
guished, except that C4-OS has, as expected, a much weaker
effect than C6-OS, C8-OS, or C10-OS.
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Figure 10. The number (R) of absorbed OS molecules per carboxylic
acid site in the microgel particles, as a function of equilibrium concen-
tration (on a log scale, for clarification) at pH 8 and 20 °C: (4) C4-OS,
(9) C6-OS, (O) C8-OS, (þ) C10-OS. The lines are for guiding the eye
only. (---) OS concentration regions where measurements could not be
made since the microgel particles flocculated.
Figure 11. Adsorbed amounts as a function of equilibrium concentra-
tion (on a log scale, for clarification) at pH 2.5 and 20 °C: (4) C4-OS,
(9) C6-OS, (O) C8-OS, (þ) C10-OS. The lines are for guiding the
eye only.
for the organic salts with the longer hydrocarbon chains inserted.
So there may well be within each microgel particle a distribution
of R values, with some anionic sites having no associated organic
salts located there and others a high R value locally (i.e.,
corresponding to the maximum number of organic salt cations
associated with each anionic site, i.e., the aggregation number).
Clearly the adsorbed amount would not reach a limit for C8-OS
and C10-OS until all the anionic sites in the microgel particles
had reached this maximum value of R. The fact that for C10-OS,
at the highest concentration studied, the average value for R is 2.4
(i.e., significantly greater than 1) but no charge reversal is
observed (Figure 8) is consistent with the above discussion. It
may well be that the anionic sites having no associated organic
salt, at this highest concentration studied, are situated more to
the periphery of the microgel particles, so that the particles
appear to retain their negative electrophoretic mobility at this
concentration.
Absorption Isotherms for the OS Molecules in the Micro-
gel Particles after Readjusting the pH from 8 to 2.5. Figure 11
shows the revised absorption values after changing the pH from 8
to 2.5, without changing the particle concentration. At pH 2.5
virtually all the ꢀCOOH groups within the microgel particles are
in the undissociated form, so the electrostatic interactions
between the OS molecules and the microgel particles have been
eliminated. Only the hydrophobic interactions remain. In the
case of the C4-OS and C6-OS molecules, virtually all the organic
salt molecules appear to have been desorbed, reflecting the very
weak nature of the hydrophobic contributions in these two cases.
For C8-OS and C10-OS, on the other hand, complete desorption
is not achieved. For example, for C10-OS, adjustment of the pH
from 8 to 2.5 leads to the following re-equilibration: at pH 8 the
adsorbed amount is 0.72 g/g at 0.02 mM; at pH 2.5 this becomes
0.57 g/g now at a raised concentration in solution of 0.06 mM
(i.e., just below the cmc value of 0.07 mM; see Table 1).
For both C8-OS and C10-OS, it is clear that hydrophobic
association of the cationic tails is occurring within the microgel
particles, in the absence of any electrostatic attraction. This is
consistent with the discussion in the last section in regard to the R
values achieved with these two organic salts. It is also known from
previous work21 that nonionic surfactants are absorbed by PNI-
PAM-co-AAc microgel particles, and it was suggested in that
cetyltrimethylammonium chloride molecules into PNIPAM-co-
AAc microgel particles. The exact structure of the aggregates
formed within the interior of the microgel particles is not yet
known, but clearly, although similar, they cannot be identical to
the micelles which form in free aqueous solution above the cmc.
It is quite likely with the OS molecules that any aggregates that
form involve associations between the whole tail regions, i.e., the
hydrocarbon units plus the diphenylazo units (see Figure 2).
In order to investigate whether more than one OS molecule is
associated with each ꢀCOOꢀ group inside the microgel parti-
cles, the absorption values have been converted from the g/g
values shown in Figure 9 to the number (R) of absorbed OS
molecules per anionic group in the microgel particles. The
corresponding absorption isotherms plotted in this form are
shown in Figure 10. In our previous study16 of organic salt anions
(which had not been hydrophobically modified as in the present
study) absorbing into cationic microgel particles we found that
the maximum value of R that could be achieved was 0.45. It is
clear from Figure 10 that much higher values of R (>1) can be
achieved with the current hydrophobically modified organic salts.
This is consistent with the information gained from the hydro-
dynamic diameter (Figure 7) and electrophoretic mobility
(Figure 8) results reported above, which showed that, at least
with C6-OS and C8-OS, reversal of charge can be obtained at
sufficiently high organic salt concentrations.
C6-OS is the only organic salt for which the maximum in the
adsorbed amount seems to have been reached within the range of
concentrations studied. For this organic salt the limiting value of
R reached is slightly greater than 1, implying charge reversal at
the highest concentrations studied, which is consistent with the
electrophoretic mobility measurements (Figure 8). C4-OS also
reaches a value of R ∼ 1 within the concentration range studied.
It is interesting that with C8-OS and C10-OS values of R much
greater than 1 are achieved. Moreover, at the highest concentra-
tions studied the absorbed amount (Figure 9) and, hence, also R
(Figure 10) seem to be rising steeply still. However, it should be
remembered that the value for R, determined in the way
described, is only an average value for each microgel particle. It
is highly likely that some form of organic salt association occurs
around each anionic site in the microgels particles, in particular,
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the x = 4 and 6 organic salts occurs readily on changing, in situ,
the pH from 8 to 2.5 (and thereby eliminating the electrostatic
attraction) but is only partially achieved for the x = 8 and 10
organic salts. Indeed for the x = 10 organic salt, only about 80% of
the organic salt is desorbed upon dilution of the microgel particles into
a large excess of water. Being able to introduce a controlled degree of
hydrophobicity into otherwise essentially hydrophilic organic salts in
the way described in this paper, and hence modify their uptake and
release properties into microgel particles, has clear implications for
controlled release applications of such microgel particles.
’ ASSOCIATED CONTENT
Supporting Information. 1H NMR and mass spectra for
S
b
the organic salts. This material is available free of charge via the
Internet at http://pubs.acs.org.
Figure 12. Desorption of C10-OS from the microgel particles, at an
initial equilibrium concentration of 0.02 mM and pH 8, on replacing the
supernatant with water and allowing the system to re-equilibrate for
different times prior to analysis of the new supernatant concentration.
The line is a guide to the eye.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: melanie.bradley@bristol.ac.uk.
work that some form of hydrophobically driven surfactant
aggregation process occurs within the microgel particles.
’ ACKNOWLEDGMENT
We express our thanks to AkzoNobel and EPSRC for funding
this project through the Dorothy Hodgkin postgraduate scholar-
ship scheme. M.B. acknowledges the Royal Society for the award
of a Dorothy Hodgkin Fellowship.
Desorption of the OS Molecules from the Microgel Parti-
cles after Dilution with Pure Water. In order to establish
whether complete desorption occurs when the system is diluted
with a large excess of pure water, the following experiment was
carried out with the C10-OS organic salt (this being the most
strongly adsorbed by the microgel particles at pH 8). The
adsorption equilibrium was re-established for the microgel plus
C10-OS system at pH 8, in a series of identical sample tubes. The
particles were then centrifuged in each tube and as much of the
supernatant was removed as possible. This was replaced with
water in each tube. The systems were recentrifuged at increasing
intervals from this time, and the new equilibrium concentration
of C10-OS in each tube was determined. In this way the
desorption kinetics, as well as the maximum extent of desorption,
could be established. The results are shown in Figure 12. It is
clear that for C10-OS the maximum desorbed amount is about
80% on dilution of the system into a large excess of water. This is
again indicative of the strong hydrophobic association of the
C10-OS molecules within the microgel particles. The compara-
tively long desorption time required (∼10 min) would also
support this suggestion. Finally, it is of interest to note that, as
C10-OS is a colored molecule, the microgel particles themselves
become colored when the organic salts are absorbed into them.
This color is largely retained by the microgel particles even after
80% of the organic salt has been removed from the particles by
dilution with a large excess of water and the system has been
centrifuged to observe the microgel sediment.
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’ CONCLUSIONS
Insertion of a Cx (x = 4, 6, 8, or 10) hydrocarbon chain
between the diphenylazo moiety and the quaternary ammonium
headgroup of the organic salts used in this work modifies the
hydrophobic nature of the salt. This effect increases strongly
from x = 4 to 10. Only the x = 6 and 8 cations of the organic salt
absorb sufficiently strongly to reverse the charge of the (anionic)
PNIPAM-co-AAc microgel particles at pH 8. The x = 10 organic
salt is the most hydrophobic of the series and would reverse the
charge, if its solubility in water were not so limited. Desorption of
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