Anna Jonhed,
Lars Jarnstrom
Department of Chemistry, Karlstad University, Karlstad, Sweden
Phase and Gelation Behavior of 2-Hydroxy-3(N,N-dimethyl-N-dodecylammonium)propyloxy
Starches
Some starches containing quaternary dimethylalkylammonium groups exhibit an unique phase
behavior. A solid phase or gel phase is formed upon cooling, i.e. they are temperature-responsive
polymers. The aim of this study was to investigate the phase and gelation behavior of
hydrophobically modified quaternary ammonium starch ethers in aqueous solutions. The
mechanisms behind the phase behavior and hydrophobic character were investigated by light
scattering (turbidity) and rheoiogicai measurements. A relatively large increase in the complex
viscosity at higher concentrations was observed when the solutions were cooled to room
temperature. The phase angle decreased drastically at a certain critical temperature. The decrease in
the phase angle depended on the concentration of starch in solution, higher concentrations showing
the greatest decrease and lower concentrations showing no significant change. Turbidity
measurements indicated that a solid-like highly concentrated phase was precipitated. The starch
with zero net charge showed a larger increase in turbidity than the starch with a positive net charge,
which indicates that particular precipitation is favored by a zero net charge and that the formation of
a gel network is favored by charged starch molecules.
Keywords: Modified starches; Gelation; Phase separation
1 Introduction
Interest in new "special" grades of starch in paper coating and paper surface sizing is constantly
increasing. Examples can be found in new concepts for ink-jet papers, coatings with greater fiber
coverage, etc. Several new starch grades are hydrophobically modified. These can be synthesized as
anionic [1], cationic [2], or non-ionic [3] derivatives. Two of the most promising and recently most
investigated derivatives are the substituted succinate derivatives and the derivatives of
epoxypropyldimethyl-alkylammonium chloride. The succinate derivatives have one permanent
negative charge attached to each pendant hydrophobic group and the hydrophobic quaternary
ammonium derivatives have one permanent positive charge attached to each pending hydrophobic
group. Both the succinate and the quaternary ammonium reagents form products that are used in the
paper field.
Much has been written about the advantages of using cationic starch in papermaking. Starch is
added to paper to increase its strength, both in internal and surface sizing [4]. Only a small
proportion of an unmodified starch is retained on cellulose fibers in a one-pass retention for inCorrespondence: Anna Jonhed, Department of Chemistry, Karlstad University, SE-65188
Karlstad, Sweden. Phone: + 46-54-7001571, Fax: +46-54-7002040, e-mail: anna.jonhed@kau.se.
T
emal sizing. A significantly better retention is achieved with cationic starch. The binding power of
cationic starch is higher than that of native starch because the ionic interactions of the starch with
fiber and fillers are stronger than simple hydrogen bonds. In addition, the greater stability of the
molecules of the modified starch and their inherently better rheology mean that they can be used
with higher molecular weights without runnability problems [2].
The phase behavior of hydrophobically modified non-ionic polymers has been studied for a long
time. Extensive studies have been directed towards understanding the associative behavior of
hydrophobically modified cellulose ethers [5] and the phase behavior of mixtures of such cellulose
derivatives and surfactants [6]. A diblock copolymer and a polymer with pendant hydrophobic
groups may show a self-assembly similar to that of a surfactant. The associative properties of block
copolymers and hydrophobically modified polymers have made them useful as thickeners in
waterborne suspensions [7]. Beside the micelle-like type of associative behavior, a polymer that
shows a coil-helix transition can act as a host molecule in inclusion complexes. This has been
reported for amylose in blends with hydrophobically modified cellulose ethers [8]. Many cellulose
ethers show a reversible phase separation at higher temperatures.
This paper concentrates on starches modified by the quaternary ammonium reagent, containing
methyl and dodecyl moieties. These starches are insoluble in water at low but soluble at high
temperatures. At high concentrations they form gels, and at low concentrations they form
particulate suspensions, as reported elsewhere [9]. It has been shown in technical applications, such
as the surface treatment of paper [10], that treatment by the hy-drophobically modified starches
investigated here leads to a decrease in the penetration of water and to an increase in the contact
angle of a water droplet. This is probably due to the lower solubility in water of the modified starch
grades.
A main topic of interest is to investigate the role of the hy-drophobic chain-groups and how they
affect the phase separation. It is important to understand the mechanism of phase separation in order
to be able to improve the use of starch in the surface coating of paper. In the present investigation,
turbidity and rheological measurements are used to determine the mechanism behind the phase
separation.
2 Materials and Methods 2.1 Materials
Two grades of 2-hydroxy-3-(N,N-dimethyl-N-dodecyl-ammonium)propyloxy starch and one
oxidized starch were used. All grades were based on native potato starch (supplied by Lyckeby
Starkelsen, Kristianstad, Sweden) and the degree of substitution for each starch grade is given in
Tab. 1. The hydrophobically modified starches were oxidized in the same way as the only oxidized
grade prior to hydrophobization: starch was slurried in distilled water and the temperature was
raised to 36 °C. The pH was adjusted to 9.5. Oxidization was performed by drop-wise addition of
sodium hypochlorite (technical grade) at pH 9.5 and 36 °C until a colorless sample with potassium
iodide was obtained. 1 g of sodium bisulfite per kg starch was added to terminate the reaction. The
pH was lowered to 5.5 before filtration, washing and drying. The starch grade obtained by this
oxidation is denoted Starch A.
The result of the oxidation was determined by potentio-metric titration: 5 g starch was suspended in
25 mL 0.1 M aqueous HCI and agitated for 30 min. The suspension was dewatered and dissolved in
300 mL water. After the suspension had been heated for 10 min at 95 °C, the warm suspension was
titrated with 0.1 M NaOH and the result interpreted as DS with respect to carboxylic groups
although other oxidation products may exist as well. The results in Tab. 1 were corrected for the
presence of phosphate groups. Potato starch contains mo-noester phosphate groups corresponding to
DS 0.00436 [1]. The molecular weight distribution of oxidized starch (DSCOOH = 0.03) is very broad
and the main fraction varies from about 104 to 106 [11].
The hydrophobic modification was performed by reacting a pre-oxidized starch, Starch A, with a
quaternary amine reagent (3-chloro-2-hydroxypropyldimethyldode-cylammonium chloride) at
alkaline pH and ambient temperature (pH 11.3 and 37.5 °C). When the reaction was complete, the
pH was adjusted to 9.5. In this way one positive charge was introduced per hydrophobic group. Fig.
1a shows the chemical structure of the quaternary amine used. The reaction product was carefully
washed with water in order to remove any remaining reagent and intermediates. The degree of
substitution of hydropho-bic/cationic groups was determined by analyzing the nitrogen content
using the Kjeldahl method. The Kjeldahl analyses were performed by the starch supplier, the error
range was estimated by the supplier to ± 4%. The charge densities of Starches B and C were
measured using a Particle Charge Detector (Mutek, PCD 03, Herrsching, Germany) at pH 8. The
anionic starch was titrated with poly(diallyldimethylammonium chloride) [CAS no. 26062-79-3]
[12] and the cationic starch was titrated with polyethylene sodium sulfonate [CAS no. 25053-27-4].
The resulting modified starches are amphiphilic and am-photeric (Fig. 1 b). The characteristics of
the starches are summarized in Tab. 1.
Fig. 1. a) Chemical structure of quaternary amine used for hydrophobizing starch, b) Chemical
structure (schematically) for the hydrophobically modified starch. The positions of the functional
groups are chosen arbitrarily.
Tab. 1. Degree of modification of various potato starch products. DSN is the degree of substitution
with respect to the hydrophobic part, i.e. the carbon chain, and DSCOOH is the degree of substitution
with respect to the oxidized part.
2.2 Preparation of starch solutions
Starch solutions were prepared by dispersing about 12 g starch in about 100 g distilled water. The
solution was heated to 95 °C while being stirred at 300 rpm. The starch suspensions were held at 95
°C for 30 min while stirred at 500 to 600 rpm. If not used immediately they were stored in a sealed
container at 80 °C. Each batch was investigated under a light microscope in order to ensure that the
starch was fully cooked. The pH was 8 for all the starch solutions. No salt was added to any
solution.
2.3 Determination of the overlap concentration
The viscosity (η) was measured using a conventional controlled-shear-stress rheometer (Paar
Physica, MCR 300, Graz, Austria). Rotational flow curve measurements were made with the double
gap geometry, DG 26.7 at 20 °C on Starch A. The shear rate was 10 s-1. The overlap concentration
was determined by plotting the viscosity as a function of starch concentration.
2.4 Determination of turbidity
The transmittance of the solution was measured on a conventional UV-spectrophotometer
(Shimadzu, UV2101 PC, Kyoto, Japan). Each sample was run in a cycle from 80 °C to room
temperature and back to 80 °C, with a duration of 1 h for each run. The turbidity (τ) was calculated
from the transmittance (T) of a light beam passing through a cell of length (L), according to:
2.5 Characterization by rheology
The rheological properties of the starch solutions were measured using a conventional controlledshear-stress rheometer (Paar Physica, MCR 300, Graz, Austria). Amplitude sweeps were performed
to determine the linear viscoelastic region. Oscillatory measurements were performed with a
concentric cylinder geometry in the linear viscoelastic region. The temperature was varied between
80 and 20 °C, while the amplitude (1) and the frequency (1 Hz) were held constant, in order to
determine the storage and loss moduli (G' and G"), the complex viscosity (η*) and the phase angle
(8) as a function of temperature. The absolute value of η* was plotted. The ramping time was 60
min for one cooling-reheating loop. The relation between complex viscosity and moduli is given by
where G' is the storage modulus, G" is the loss modulus and ω is the frequency of oscillation. The
phase angle 8 is defined as:
2.6 Particle size determination
The particle size distribution was measured after separation of the precipitate and re-dispersing in
water on a Coulter LS130 Fluid Model (Coulter Electronics Ltd. Lu-ton, England) at a starch
concentration of about 0.1%. Distilled water was used as fluid medium. To some samples a nonionic surfactant, decanol ethoxylate (Ethylan® 1005 from Akzo Nobel, gplace?g, Sweden), was
added during the re-dispersion process in order to make sure that the measurements were performed
on the dispersed particles and not particle aggregates. The yield of the precipitation was determined
gravimetrically after separation by centrifugation.
3 Results
These starches exhibited temperature-responsive properties, i.e. either gelation or phase separation
occurred at a certain temperature upon cooling, as detected by turbidity measurements and
rheology.
3.1 Overlap concentration
When the viscosity of the only oxidized starch grade (Starch A) was plotted vs. concentration, the
overlap concentration (c*) was revealed as the onset of a steep increase in viscosity with increasing
starch concentration, as shown in Fig. 2. This overlap concentration was determined as c* ≈ 3.5%
(w/w) at pH 8.
3.2 Turbidity measurements and particle sizing
The solution of Starch B in water showed an increase in turbidity (Τ) with decreasing temperature,
indicating a
Fig. 2. Viscosity as a function of concentration for starch A, pH=8.0 and 20 °C.
phase separation process at low temperatures. At a given temperature, the solution with a lower
starch concentration showed a higher turbidity than the solution with higher starch concentration, as
shown in Fig. 3. The solution of Starch C showed an increase in turbidity with decreasing
temperature, but in this case the solution with a lower starch concentration showed a lower turbidity
at a given temperature than the solution with higher starch concentration. The increase in turbidity
for Starch C was much weaker than for Starch B. The turbidity of starch A was unaffected by the
temperature and this is shown as a reference in Fig. 3.
The particle size measurements showed a mean particle size of 20 μm for Starch B. When adding a
non-ionic surfactant, the particle size did not change. Since the addition of the surfactant did not
reduce the measured particle size, one may conclude that the primary particle size was about 20 μm
or that the attractive forces within an aggregate were strong enough to withstand the dispersing
effect of the surfactant. For starch solutions with initial concentrations of 10% (w/w), the yield of
precipitated particles of Starch B was 15% and for Starch C 4%.
3.3 Rheological measurements
Fig. 4 shows the complex viscosity of the starch solutions as a function of temperature measured
during a cooling and reheating loop at continuous oscillatory shear. A clear hysteresis loop in
complex viscosity was observed at high concentrations of Starch C. The complex viscosity obtained
at cooling was always lower than the corresponding value obtained at re-heating. Starch A is shown
for comparison. When exhibited to the cooling ramp, the complex viscosity of Starch C started to
increase rapidly at about 45 °C, as shown in Fig. 4. Starch B and Starch A showed a lower
temperature effect on the complex viscosity. Fig. 5 shows the complex viscosity as a function of
concentration for the three starch grades measured when the cooling was allowed to take place at
rest without oscillatory shear. It is evident from Figs. 4 and 5 that the hydrophobically modified
starch C exhibited a large increase in complex viscosity with increasing concentration. The increase
in complex viscosity for Starch B after cooling without oscillatory shear was similar to that
observed for Starch A and could be explained as an Arrhenius effect. However, when the starches
were
Fig. 3. Turbidity as a function of temperature for different starch grades at 618 nm, pH=8. Starch B:
4.7% (w/w) (▲) and 7.6% (w/w) (∆); Starch C: 7.8% (w/w) (■) and 4.6% (w/w) (□); Starch A:
8.2% (w/w) (x) is shown as a reference. The figures show cooling from 80 °C to room temperature.
Fig. 4. Complex viscosity as a function of temperature for different starches and concentrations at
pH=8. Starch C: 8.9% (w/w) (♦), 7.8% (w/w) (◊), 6.5 %(w/w) (-); Starch B: 7.6 %(w/w) (∆), 6.0%
(w/w) (▲), Starch A: 9.2% (w/w) (x). The arrows indicate the temperature loop for Starch C at
8.9% (w/w) %.
Fig. 5. Complex viscosity as a function of concentration for Starch A (x), B (■), and C (∆) at 20 °C.
Fig. 6. Phase angle, 5, as a function of temperature at pH =8. Starch C:
12.1% (w/w) (∆), 6.5% (w/w) (□), Starch B: 12.2% (w/w) (♦), 7.6% (w/w) (x), 6.0% (w/w) (▲),
Starch A: 9.2% (w/w) (●). The arrows indicate the temperature loop for Starch C at 12.1% (w/w).
cooled down during oscillation, the complex viscosity at room temperature for Starch B was
substantially higher than that of Starch A.
The onset temperatures for the drop of the phase angle from 90° are shown in Fig. 6. Starch A is
shown as a reference and shows a behavior similar to that of Starch B.
For Starch C a clear hysteresis loop in phase angle was observed at high concentrations. The phase
angle obtained at cooling was always higher than the value obtained at re-heating, however, the
loop was reversible.
4 Discussion
Fig. 3 shows the turbidity as a function of temperature. Phase separation of a starch solution creates
a turbid system. The scattering of light and the turbidity depend strongly on the particle size. Starch
B showed a rapid increase in turbidity, T, with decreasing temperature. At a given temperature, the
solution with a lower starch concentration showed a higher turbidity than the solution with a higher
starch concentration. Precipitation occurred more readily at low starch concentration, similar to
what is observed for retrogradation of unmodified starches where a gel is formed in concentrated
solutions and a precipitate is formed in dilute solutions [1]. In all cases, the turbidity measurements
showed that the behavior was reversible. Thus, retrogradation cannot be the explanation for the
phase separation observed by the
turbidity measurements. The mean particle size for Starch B was 20 μm. Järnström et al. [9]
reported that the mean particle size of a precipitate formed from solutions of starch similar to Starch
B and Starch C was slightly above 1 (im. The bigger size of the precipitate observed in the present
investigation indicated that the DS and the rate of cooling affected the precipitation process.
The increase in τ was greater for Starch B than for Starch C and the yield of precipitation was
determined to 15% for Starch B whereas Starch C only had a precipitation yield of 4%. Starch C
has a higher charge density than Starch B, which has a net charge density close to zero, and this
may be one possible explanation to the difference in turbidity.
A remarkable feature was that introduction of only 3% carbon atoms from the synthetic reagent was
required to produce starch derivatives insoluble in water. This is a remarkably low value for water
solubility and can be compared to degraded starch acetates that are soluble in water to an acetyl
content of up to 25%, corresponding toDS=1.25[1].
Fig. 4 shows that the complex viscosity (η*) of starch B and C increased with decreasing
temperature, indicating the formation of a gel network in combination with the normal viscositytemperature behavior of a polymer solution, i.e. the Arrhenius behavior. The difference in η* indicates a different gelation behavior. The reference starch, Starch A, showed a behavior similar to
that of Starch B and only the Arrhenius behavior was present. On the other hand Starch C, at high
concentrations, showed an increase in originating not only from the Arrhenius behavior but also
from the gel network formation.
In polymer science, viscoelastic measurements may be used to determine the gel point as the
crossover of the viscous and elastic responses (G'=G"), i.e. the phase angle is 45°. When the phase
angle falls to a value close to zero, the formation of an elastic network structure is implied [13]. The
concentration-dependence of the gel network formation seems to be quite large. At the lower concentrations of Starch B (Fig. 6), the phase angle decreases only slightly. At higher concentrations,
the decrease in phase angle becomes more pronounced. The overlapping concentration, c*, was
determined to be about 3.5% (w/w) and was lower than the concentration at which the decrease in
δ and the increase in |η*| were observed (Fig. 2). The overlap concentration was determined for all
molecules in the starch solution, using Starch A. The partic-ulate precipitation and the gel network
formed for Starch B respective Starch C may be predominated by the amy-lose fraction. The
uniform linear nature of amylose permits intermolecular bonds similar to other linear polymers such as cellulose [1,14]. It is likely that amylose and not amylopectin predominates the
formation of the precipitate and the gel network, which may explain the discrepancy between and
the onset of rapid changes in 8 and |η*|.
At the high concentrations of Starch C, the temperature sweeps were not completely reversible,
indicating hysteresis properties of the gel or a long relaxation time compared to the time scale of the
experiments. The gelation threshold for the hydrophobically modified starches investigated was
found to be different for the different starches.
The behavior of the complex viscosity for Starch B was similar to that observed for Starch A after
cooling without shear (Fig. 5). However, Starch B showed a substantially higher complex viscosity
than Starch A when cooled under shear (Fig. 4). This suggests that the amphiphilic and amphoteric
character of Starch B introduced by the quaternary amine group does play an important role in the
mechanical behavior of the starch gel.
The phase separation and gel formation induced by the interaction of hydrophobic functional groups
must not be confused with retrogradation. The starches investigated here showed fully reversible
behavior and did not undergo retrogradation during the time-scale of the experiment.
5 Conclusions
The hydrophobically modified amphoteric starches showed a temperature-dependent phase
behavior; precipitation of a solid-like highly concentrated phase. The oxidized starch showed no
phase separation as the temperature was decreased. A zero net charge of the amphoteric starches
was shown to promote a reversible precipitation. For the positively charged grade of hydrophobically modified starch, the rheological measurements showed a decrease in the phase angle
to below 45°, which indicates the formation of a gel network. The change in phase angle was not as
pronounced for the hydrophobically modified starch with zero net charge as it was for the more
cationic starch. The temperature-induced changes in rheological properties of the oxidized starch
were reversible and the magnitude of the complex viscosity was similar to that of the
hydrophobically modified starch of zero net charge. The charge density of the hydrophobically
modified amphoteric starch apparently had an influence on whether gelation or precipitation should
be the predominating mechanism upon cooling the warm solution to room temperature. Both the
gelation and the phase separation behavior were dependent on the polymer concentration.
Acknowledgements
Lyckeby Stärkelsen, Kristianstad, Sweden is acknowledged for kindly providing starch samples and
for valuable discussions. For financial support the Swedish Pulp and Paper Research Foundation,
the Knowledge Foundation, and the Swedish Agency for Innovation Systems are gratefully
acknowledged.
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(Received: January 27, 2003)
(Revised: April 26/June 28, 2003)
(Accepted: June 30, 2003)