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Cite this: Soft Matter, 2023,
19, 3228
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A molecular–mechanical link in shear-induced
self-assembly of a functionalized biopolymeric
fluid†
Galina E. Pavlovskaya
*ab and Thomas Meersmann
ab
23
Na multiple quantum filtered (MQF) rheo-NMR methods were applied to probe the molecular
foundation for flow induced self-assembly in 0.5% k-carrageenan fluid. This method is sensitive enough
to utilize an endogenous sodium ion concentration of approximately 0.02%. Rheo-NMR experiments
were conducted at different temperatures and shear rates to explore varying molecular dynamics of the
biopolymer in the fluid under shear. The temperature in the rheo-NMR experiments was changes from
288 K to 313 K to capture transition of k-carrageenan molecules from helices to coils. At each
temperature, the fluid was also tested for flow and oscillatory shear behaviour using bulk rheometry
23
Na MQF signals were observed for the 0.5% k-carrageenan solution
Received 17th October 2022,
Accepted 30th March 2023
methods. It was found that the
DOI: 10.1039/d2sm01381a
temperatures of 303 K and above, no
only under shear and when the fluid demonstrated yielding and/or shear-thinning behaviour. At
23
Na MQF signals were observed independent of the presence or
absence of shear as the molecular phase transition to random coils occurs and the fluid becomes
rsc.li/soft-matter-journal
Newtonian.
1 Introduction
The mechanical properties of many non-Newtonian fluids are
determined by the existence of specific short range intra- and
intermolecular arrangements making fluids nano- or microstructured. In polymeric fluids, these molecular arrangements
might be created over different length scales involving various
interactions within, and between, not only the macromolecules
themselves, but also the interactions of the biomolecules with
solvent molecules, ions, and other possible constituents of
biopolymeric fluids. Most common examples of such interactions in polymeric fluids are hydrogen bondings, electrostatic
interactions and dispersive forces, often resulting in intra- and
intermolecular chain overlaps or chain cross-links in solutions
of polymers thereby affecting the rheology of the fluids.1 Moreover, changes in the molecular conformation greatly affect the
mechanical properties of the fluids, and these changes, specific
to a particular biopolymeric system, can be induced by varying
its ionic strength and temperature.1–5
a
Sir Peter Mansfield Imaging Resonance Centre, School of Medicine, University of
Nottingham, Nottingham, NG2 7RD, UK.
E-mail: galina.pavlovskaya@nottingham.ac.uk; Tel: +44115 84 68131
b
NIHR Nottingham Biomedical Research Centre, Nottingham University Hospitals
NHS Trust, Queen’s Medical Centre, Derby Road, Nottingham NG7 2UH, UK
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/
10.1039/d2sm01381a
3228 | Soft Matter, 2023, 19, 3228–3237
k-Carrageenan is a linear sulphated anionic polysaccharide
composed of repeating units of 1,3 linked b-D-galactopyranose
and a 1,4 linked 3,6-an-hydro-a-D-galactopyranose. It is produced by extraction from edible red seaweeds6 and used in food
sciences,7,8 pharmaceutical industry,9 biotechnology,10 tissue
engineering11 and medical applications.12 In aqueous solutions
and in the presence of compensating metal cations, kcarrageenan molecules change their conformation upon temperature variation.1,13 At temperatures lower than ambient
temperature, k-carrageenan molecules exist in the double helix
conformation and may form a network by an intermolecular
synergetic interaction with neighbouring molecules.13,14 When
the temperature increases to the ambient temperature range,
this synergy breaks down due to the increased molecular
motion and k-carrageenan molecules exist as separated single
helices in the solution. Upon further temperature increase, kcarrageenan helices start to unfold and transform into random
coils at around 310 K and above. Interestingly, the transition
temperature region shifts depending on the concentration and
nature of the compensating cations but not on the concentration of the polymer itself.2 The increase in the cation
concentration shifts the range to higher temperatures;15 however, sodium as a compensating cation has the weakest influence on the shift of the transition temperature.4 The transition
process is thermally reversible and is quite stable through
the entire heating/cooling 2 cooling/heating cycle.14,16
The helix to random coil transition is found in the majority
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of k-carrageenan systems, and manifests itself in the gel–sol
transition that accompanies a distinct change in material
functions.2,5,17 Controlling of the transition can be used to
selectively enhance the mechanical properties of k-carrageenan
based materials where appropriate. Such capabilities are potentially of great importance as k-carrageenan containing systems
are attractive for tissue engineering because of their ability to
form bio-compatible hydrogels.11
Additive manufacturing is often performed at temperatures
that are different from operational temperatures of the final
products.18 Examples of growing importance are the design and
production of tissue implants.19 Hence, there is an industrial
demand for non-intrusive labels that could be used to monitor
the in situ mechanical properties of final tissue implants
manufactured using k-carrageenan, especially at varying temperatures. Most of the k-carrageenan containing tissue
implants would naturally contain Na+ cations, usually at a
physiological concentration. Hence, Na+ can be used to track
changes in the mechanical properties of these materials using
23
Na magnetic resonance spectroscopy (MRS) and even 23Na
magnetic resonance imaging (MRI) if the sodium concentration
is sufficiently high.
Rheo-NMR allows one to study molecular responses to
deformations and to correlate these responses with the
mechanical properties of fluids determined through bulk rheology methods.20–22 The well-established rheo-NMR methodology
usually uses proton detection, for example in protein solution
studies,23,24 and sometimes additional labels like deuterium25
have been introduced to explore more fundamental soft condensed matter phenomena, for example shear banding in
micellear26 and other systems.27,28 Rheo-NMR can also be used
for sodium detection and it has been shown that multiple
quantum filtered (MQF) sodium (23Na) methods can be used
to observe the molecular order created in biofluids in shear
changing zones under flow.29
In this work, we applied 23Na MQF methods in combination
with rheo-NMR to monitor the shear-induced molecular order
that occurs in the 0.5% k-carrageenan fluid during the temperature ramp as k-carrageenan molecules transition from
helices to random coils. Molecular responses monitored using
23
Na MQF methods were further matched to the change in
material functions of the 0.5% k-carrageenan fluid characterised by shear and oscillatory rheometry. Note that the outcomes of this study have great potential to impact in vivo
studies of biopolymeric fluids with 23Na whole body MRS/
MRI at a body physiological sodium concentration that exceeds
the 23Na concentration used in this work by approximately
40 times.
2 Experimental section
Materials
The Na+ form of k-carrageenan was purchased from SigmaAldrich, UK, and used without further purification. An endogenous sodium concentration of 0.02%, as estimated from the
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Soft Matter
equation in the study of Gobet et al.,30 was used in the present
work and no additional sodium was added during fluid preparation. Three separate batches of 50 ml of the 0.5% kcarrageenan fluid were prepared by dissolving 0.25 g of the
polymer in 50 g of distilled water. During stirring, solutions
were covered with parafilm and heated to 333 K to ensure
complete dissolution of the added polymer. Traces of sodium
azide were added to all solutions to avoid bacteriological
contamination. After dissolution, solutions were placed in
closed vials, caps were sealed with parafilm to avoid evaporation during storage and vials were stored at room temperature.
Rheometry
An AR-G2 (TA Instruments, UK) strain-controlled rheometer
was used to collect the shear and oscillatory rheology data in
this study. Standard TA Instruments Couette geometry was
used to collect the data to match bulk rheology to rheo-NMR
outcomes. Shear rheology was performed using a stepped flow
procedure at 288 K, 295 K, 303 K and 313 K. Shear rates were
varied from 0.0001 s1 to 300 s1. Amplitude sweeps were
performed at 283 K, 288 K, 295 K and 303 K using the 0.01%
to 1000% strain range at 6.28 rad s1. Frequency sweeps
were performed at 283 K, 288 K, 295 K and 303 K in the 0.1
to 100 rad s1 radial frequency range at 1% strain. Temperature
sweeps were performed at 1% strain from 283 K to 310 K. Fluids
were first cooled to 283 K, and left at this temperature until
solutions were temperature equilibrated and then heated to
313 K at a 2 K min1 rate. The equilibrium was confirmed using
the instrument built-in functions and only after this the data
collection was performed. At each temperature, frequency
sweeps were performed in the frequency range from 0.1 to
100 rad s1. In a separate temperature ramp, shear viscosity
was measured at constant g_ = 10 s1. Temperature was changed
at a 5 K min1 rate from 283 K to 313 K. Data were analysed
using TA Instruments TRIOS software, v 4.4.0.41651. More
details on data sampling and replicated measurements are
provided in the ESI† file.
Rheo-NMR
Rheology experiments inside the 9.4T super-conducting magnet were performed using a commercially available rheo-NMR
attachment and a non-magnetic Couette cell produced by
Bruker (Germany). The Couette cell, machined from PEEK,
has an outside diameter of 19 mm, with a bob diameter of
18 mm resulting in a 1 mm gap between the stationary wall and
the rotating inner bob. The Couette cell was inserted inside a
25 mm resonator (Bruker, Germany) tuned to a 23Na resonance
frequency of 105 MHz and mounted into the centre of the 9.4T
magnet. The width of the p/2 sodium pulse was 52 ms. A variety
of shear rates were provided using an externally located motor,
remotely controlled through built-in scanner software. The
coupling of the rheo-cell to the motor was achieved using a
drive shaft inserted into the magnet bore. The outline of the
setup is shown in Fig. 1(a). Shear rates of 11.6 s1, 29 s1, 58 s1
and 87 s1 were used for the 23Na NMR experiments conducted
at temperatures of 288 K, 295 K, 303 K and 313 K. The
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Fig. 1 (a) The outline of the rheo-NMR experimental set up. Time diagrams of 23Na multiple quantum filtered (MQF) (b) triple quantum filtered
(TQF) and (c) double quantum filtered (DQF) magic angle (MA) pulse
3
sequences used in this study. The TQF sequence of 23Na (Spin I ¼ )
2
utilizes the |T3,1i coherence that is generated through quadrupolar
coupling in a molecular alignment phase or, alternatively, through quadrupolar relaxation processes caused by the slow molecular motion. This
coherence cannot be detected directly but is transformed briefly into the
triple quantum coherence, |T3,3i, for the TQF process that leaves only
|T3,1i at the beginning of the data acquisition. The same molecular
processes that generated this coherence will also reconvert it back into
the observable |T1,1i coherence that emerges over time. Similarly, the
DQF MA sequence utilizes |T2,1i. However, this coherence can only be
caused by coupling and not through relaxation. The oscillatory |T2,1i
coherence is therefore an indication for the presence of the molecular
alignment. Single quantum experiments (not depicted here) utilize a single
90 degree pulse followed by a typical free induction decay (FID) of the
observable |T1,1i coherence. Numbers and symbols displayed in the rheoNMR unit are as follows: ‘52’ are rotations per minute, ‘ccw’ is the direction
of shearing, which is counterclockwise in this experiment and ‘. . .10’ is the
gear ratio.
temperature in the Couette cell was changed by changing the
temperature in the gradient coil chiller unit. The temperature
in the gradient coils was read from the built-in sensor attached
to the gradient amplifier control console. The temperature in
the Couette cell was further checked using a thermocouple
attached in the bottom of the Couette cell. The reading of the
thermocouple was within 0.5 K of the temperature read from
the gradient console.
23
Na multiple quantum filtered (MQF) spectroscopy
23
Na MQF spectroscopy was performed using the triple quantum filtered (TQF) sequence shown in Fig. 1(b) and the double
quantum filtered (DQF) sequence shown in Fig. 1(c). The latter
one includes 57.4 degree (magic angle) r.f. pulses to filter out all
coherences except for the |T2,1i coherence (i.e. the DQF MA
sequence). Note that in the biopolymeric systems studied, the
|T2,1i coherence is only generated when some kind of ordered
phase is present that has a net alignment with respect to the
magnetic field. The net alignment causes a quadrupolar cou3
pling in the nuclear spin I ¼ system of 23Na, that, in turn,
2
causes the |T2,1i coherence to be present. Therefore, non-zero
signals obtained from the DQF MA sequence indicate the
molecular alignment. See ref. 31 and 32 for a more detailed
discussion of this fundamental physical effect utilized in this
3230 | Soft Matter, 2023, 19, 3228–3237
Paper
work. The 48-step and 36-step phase cycling for TQF and DQF
MA, respectively, were used to achieve optimal SNRs. The width
of the 23Na p/2 pulse was 83 ms at 200 W. The |T2,1i and |T3,1i
coherences are generated during the echo time t and 45
increments of the t time were used to monitor the time
evolution of these coherences in the MQF experiments in the
presence and absence of varying applied shear fields. Data were
collected in 2048 data points for each t time, and with a 100 ms
recycle delay. 960 and 1024 transients were collected at each t
value and resulted in 3 h 9 min and 3 h 21 min for full TQF and
DQF MA time evolution experiments, respectively. Error bars
were evaluated from signal-to-noise ratios (SNRs) obtained for
each individual spectrum at each t value using well-established
procedures.33
3 Results and discussion
Shear and oscillatory rheology
The shear rheology of the 0.5% aqueous k-carrageenan fluid is
displayed in Fig. 2(a). The fluid was tested at shear rates from
10 s4 to 200 s1 at 283 K, 288 K, 295 K, 303 K and 313 K. Shear
stress vs. shear rate curves were analysed using the built-in
‘‘stress vs. rate’’ routine in TRIOS software first, and the best
model was applied to extract relevant model parameters for
each replicated measurement. The extracted parameters for
each replicated measurement were grouped and further analysed with the WaveStats built-in function in IGORPro8 to result
in the averaged value and the standard deviation in each group.
If a parameter standard deviation exceeded its averaged value,
then the next best model was used to produce the averaged
parameters using the same workflow. Further details on the
analysis workflow are found in Fig. SI1, ESI,† and the averaged
parameters for each tried model are shown in Table 1. It was
found that both the Herschel–Bulkely and Power Law models
described well fluid behaviour at 283 K. As the standard
deviation was within 30% of the averaged yield stress, the fluid
was treated as a yield stress fluid at this temperature. The
standard deviations at 288 K and 295 K exceeded the averaged
yield stresses by three and two times, respectively. Therefore,
the Herschel–Bulkely model was decided to be statistically
insignificant; therefore, the fluid was treated as a Power law
fluid up to 313 K. The Power law model described the experimental behaviour of the fluid at 313 K very well, as can be seen
from both Table 1 and Fig. 2(b); however, the averaged rate
index n = 0.946 was very close to that of 1. Therefore, the
Newtonian model was also tried for the fluid at this temperature. This resulted in a smaller error for the Newtonian viscosity
as can be seen from Table 1 but the Newtonian model deviated
more from the experimental points as can be seen in Fig. 2(b).
Nonetheless, the Newtonian behaviour was within 15% of the
Power law model. Considering that the n index was almost
unity when the Power law model was used, it was decided that
the Newtonian model was an appropriate model for the fluid at
313 K. Fig. 2(b) further shows that all rheo-NMR experiments
were conducted in the stress range when a well-defined fluid
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Soft Matter
Table 1 Averaged parameters using Herschel–Bulkley (HB), Power law
(PL) and Newtonian (N) models extracted after the analysis of multiple
stress vs. rate curves displayed in Fig. 2 at all temperatures studied
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T, K
283 K
(HB)
(PL)
288 K
(HB)
(PL)
295 K
(HB)
(PL)
303 K
(PL)
313 K
(PL)
(N)
Fig. 2 Shear stress vs. shear rates of the 0.5% aqueous k-carrageenan
fluid at different temperatures: (a) full range of shear rates and temperatures probed in this work. Dashed lines with arrows indicate shear stress
and shear rate ranges used by the 23Na MQF rheo-NMR; (b) 23Na MQF
rheo-NMR shear stress vs. shear rate ranges for four temperatures. Dots in
(a) represent multiple shear stress curves collected in different shear rate
ranges at a given temperature. These replicated experiments are shown in
symbols in (b) for each temperature. Solid lines represent the predicted
shear stress vs. shear rate curves using averaged parameters displayed in
Table 1. Shading represents a 15% deviation from the predicted behaviour.
The dashed line in (b) is the Newtonian behaviour. All rheo-NMR data were
collected in the shear-thinning range.
model, namely a Power law one and a Newtonian one, can be
used to describe fluid’s flow behaviour. The temperature range
used in this study also covered a variety of molecular arrangements formed in this fluid by k-carrageenan molecules upon
heating.2,5,17 This can be correlated to the changes in the rate
indices obtained that further confirm that the fluid demonstrates shear-thinning behaviour up to 303 K (n o 1) and then
shifts to predominantly Newtonian behaviour at 313 K as the
rate index n is approaching unity, see Table 1.
To determine the linear viscoelasticity region (LVR) in the kcarrageenan fluid at temperatures used in rheo-NMR, the
amplitude sweep tests were performed. The results of these
tests are displayed in Fig. SI2, ESI.† It was determined that the
LVR region persisted up to gLVR = 20% and up to 303 K. It was
not possible to perform an amplitude test at 313 K because of
the noise. In the LVR, the storage modulus G 0 was larger than
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Yield stress, Pa
Consistency/viscosity, Pa. s
n, rate index
0.06 0.02
1.49 0.05
1.56 0.07
0.35 0.03
0.322 0.001
0.02 0.07
1.23 0.08
1.24 0.08
0.36 0.01
0.35 0.01
0.1 0.2
0.49 0.04
0.58 0.06
0.47 0.01
0.44 0.02
0.045 0.002
0.824 0.006
0.018 0.001
0.0140 0.0001
0.946 0.002
the loss modulus G00 at 283 K, 288 K and 295 K probably
indicating the more structural presence in the k-carrageenan
fluid in this temperature range. This was probably associated
with the synergetic network behaviour of semi-rigid rods as
previously has been reported for k-carrageenan solutions
of similar concentrations.14 At 303 K, G00 becomes dominant
in the LVR which most likely is associated with k-carrageenan
molecules transitioning from helices to coils as previously was
observed for k-carrageenan upon the increase in temperature.16
Transitioning of k-carrageenan molecules from helices to
coils was confirmed by two separate temperature sweeps where
both the apparent shear viscosity and the modulus were
measured at each point temperature as shown in Fig. 3. The
shear viscosity measured at g_ = 10 s1 decreased approximately
two orders of magnitude during the temperature ramp. As has
been shown previously, a similar drastic change in the viscosity
during a temperature ramp in k-carrageenan systems was
Fig. 3 Temperature sweep of the 0.5% aqueous k-carrageenan fluid.
Dynamic moduli and shear stress were measured in two separate temperatures sweeps. Dynamic data displayed for 6 and 12 rad s1 were
collected at 1% strain. One point complex viscosity was computed from
the 12 rad s1 data. Crossover was determined using TRIOS and occurred
at Tc = 301.7 K. Shear stress was measured at a constant shear rate of
10 s1 and used to calculate one point shear viscosity at each temperature.
The complex viscosity and shear viscosity were the same when G 0 became
insignificant. The symbol size in each curve roughly represents the spread
range from an averaged value at each temperature point.
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Soft Matter
associated with the helix to coil transition as confirmed by
optical rotation experiments conducted in parallel.14 At the
same time, data shown for two radial frequencies at 1% strain
demonstrate that for up to 301.7 K the storage modulus G 0
dominates over the loss modulus G00 . At temperatures above
301.7 K, the loss modulus becomes dominant with the storage
modulus becoming undetectable at temperatures above 305 K.
Tan(d) starts to increase from at right above 295 K approaching
infinity from 305 K. The complex viscosity also starts to
decrease at 295 K and reaches a plateau at above 305 K. Moreover, the complex viscosity measured at o = 12 rad s1 overlays
the apparent shear viscosity at g_ = 10 s1 at a temperatures
above 305 K, and as the Cox–Merz relationship holds at a single
value of the radial frequency and a shear rate. This is an
indication that the fluid becomes a semi-diluted polymer
solution with no entanglements between the molecules. In this
case, it appears that the loss of the elastic response also
coincides with the helix to coil molecular transition in these
k-carrageenan systems during the temperature ramp. This has
been previously reported and confirmed by optical rotation2
and calorimetric34 methods for similar k-carrageenan systems.
Extended frequency range sweeps were measured in separate experiments and their results are displayed in Fig. SI4,
ESI.† No crossover was measured within the range of probed
radial frequencies. Unfortunately, the quality of the data did
not allow for a meaningful relaxation analysis; therefore, the
complex viscosity was evaluated from the raw data in the
temperature range probed by rheo-NMR using the TRIOS
built-in Cox-Merz transformation. Where possible, average
values from replicated measurements were computed and the
results are displayed in Fig. 4. As can be seen from Fig. 4, the
differences between the complex viscosity and apparent viscosity are small. However, the complex viscosity is most likely
larger at 288 K which could be an indication of molecular
entanglements present in the fluid at this temperature.5 It was
difficult to determine whether the Cox–Merz relationship holds
at 295 K because of the quality of the data; however, it might be
a hint of the closer compared to the 288 K overlap. At 303 K and
313 K, the complex viscosity and apparent viscosity match
within 10% which is probably an indication of the break up
of molecular entanglements in the fluid. This is probably
caused by the molecular transition from helices to random
coils resulting in the fluid behaving as a semi-dilute polymer
solution. It is also worth noting that the complex viscosity starts
to increase and the onset of the increase is shown by arrows in
Fig. 4. According to the specification of the TA Instruments for
ARG2 rheometers, this is most likely associated with inertia
effects occurring in oscillatory experiments when the raw phase
exceeds 1501. This could be negated by changing to a lower
weight geometry but was not performed in this work. In all,
better quality data are required for more definitive conclusion
but our data are an indication that the helix–coil transition
most likely occurred in the fluid.
Such a drastic change in the molecular conformation will
have an effect on the molecular dynamics of sodium cations.
23
Na single quantum (SQ) and multiple quantum filtered (MQF)
3232 | Soft Matter, 2023, 19, 3228–3237
Paper
Fig. 4 Overlay of the complex viscosity (stars) and apparent viscosity
(open triangles) of the 0.5% aqueous k-carrageenan fluid in the temperature range used in 23Na rheo-NMR experiments. The Cox–Merz relationship starts to hold at 303 K. Arrows indicate the onset of inertia effects
occurring at each temperature during oscillatory measurements as the raw
phase becomes larger than 1500. Error bars in the apparent viscosity data
represent a 10% deviation from an averaged value computed at each
temperature. The shaded area in the complex viscosity data represent 10%
at 288 K, 303 K and 313 K. The shaded area was increased to 30% at 295 K
because of the lower data quality (Fig. SI4, ESI†). The complex viscosity
data at 313 K were extracted from the multiple frequency collected during
oscillation temperature sweeps and are displayed in Fig. 3 as no oscillatory
rheology is collected at 313 K.
signals were measured in the Couette cell while the fluid
sample was sheared at g_ = 58 s1 and in the absence of shear
at 288 K, 297 K 303 K and 313 K. The resulting spectra are
displayed Fig. 5. Temperature-dependent shear and oscillatory
rheology displayed in Fig. 3 indicate three distinct regions
marked in fluid behavior. The first region, where the storage
modulus dominates over the loss modulus up to 295 K, the
Fig. 5 23Na SQ, DQF MA and TQF spectra of the 0.5% aqueous
k-carrageenan solution measured at different temperatures in the Couette
cell at g_ = 58 s1 and in the absence of shear.
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Soft Matter
second one, or a transitional region, where the modulus crossover occurs at 301.7 K, and the third one, or an isotropic region,
where the fluid behaves as a pure liquid (loss of G 0 ) and
becomes Newtonian. This corresponds to the change in the
conformation of k-carrageenan molecules in solution from
helices (at 288 K) to random coils (at 313 K). Sodium spectra
obtained using single quantum (SQ), or one pulse, spectroscopy
at g_ = 58 s1 at the temperatures selected from these three
regions appear very similar to the ones recorded in the absence
of the shear as seen in Fig. 5 (left panel). However, sodium DQF
MA spectra shown in Fig. 5 (central panel), are observed only at
temperatures 288 K and 295 K corresponding to the range
where G 0 dominates and k-carrageenan molecules preserve
their helix conformation when sheared. Interestingly, when at
rest, DQF MA signals are absent. The absence of DQF MA
signals in the absence of shear at these temperatures is an
indication that the helices are randomly oriented in the fluid
and hence do not produce any aligned phase. Since the DQF
MA sequence detects sodium signals only from the ordered
phases, this implies that there is a shear-induced ordered
phase created by k-carrageenan helices as they adapt to the
applied shear, Fig. 5 (central panel). The formation of the shearinduced ordered phase was not detected when k-carrageenan
molecules were in the random coil conformation as evident
from the absence of DQF MA signals at temperatures 303 K and
above. TQF signals displayed in Fig. 5 (right panel) follow a
similar trend to DQF MA signals. In principle, TQF sodium
signals can be observed in the absence of an ordered environment; however, in this system, TQF signals followed the same
trend as DQF MA signals. Therefore, TQF signals were also
recorded only from the ordered environment created by
k-carrageenan helices in the shear field.
Molecular insights into the formation of the shear-induced
ordered phase were further gleaned using the time dependence
of the build-up of 23Na MQF rheo-NMR signals at a variety of
shear rates and temperatures. To ensure that the maximum of
the sodium signal build up was reached in each experiment,
23
Na MQF spectra were collected using the timing diagrams
displayed in Fig. 1(b and c) with 45 time increments in the t
time in the presence and the absence of the applied shear field
at different temperatures. The obtained data sets were interpreted using well-known equations that describe the time
evolution of MQF signals as a function of the multiple quantum
creation time, t.35 To reduce the number of fitting parameters,
DQF MA signals were analysed first using the following equation:35
fast
IDQFMA ðtÞ ¼ C0 sinðoeff tÞetR2
(2)
is the
where C1 is the amplitude of the TQF signal and Rslow
2
slow component of the sodium signal transverse relaxation,
s1, and the meaning of other parameters is the same as in the
description to eqn (1). This approach is well established and
has been used before.32,35
The results of this data analysis for the 23Na MQF rheo-NMR
experiments collected at 288 K at different shear rates indicated
in Fig. 2 are shown in Table 2 and are displayed in Fig. 6.
It has been suggested that at this temperature, kcarrageenan molecules exist in the helix conformation and
might have a tendency to form a network by a synergetic chain
overlap with the neighbouring molecules.4 One could assume
that the networking should contribute to a preferential molecular order in the fluid that should be detected using the 23Na
MQF spectroscopic methods. However, as can be seen from
Fig. 5 (upper panel, spectra shown in blue) and Fig. 6 (a and b,
black cross symbols), no MQF sodium signals were detected in
the absence of shear. The absence of 23Na DQF MA signals
when no shear is applied as shown in Fig. 5 and 6 provides
evidence that there is no well-defined ordered phase with a
preferential orientation with the respect to the external magnetic field that can be detected by sodium ions. The sodium
ions probe an ordered phase through the distortion of the
electron shell symmetry that interacts with the electric quadrupole moments of the sodium nuclei. Over the time course of the
NMR scan, this may result a non-zero value of the residual
quadrupolar coupling constant oeff. Note that a non-zero
quadrupolar coupling oeff in eqn (1) is needed for any DQF
MA signals to be formed. The time averaged interactions
experienced by Na+ ions in the aqueous solution typically do
not lead to a net quadrupolar coupling over the NMR relevant
time scale. A persistent non-zero coupling of the nuclear
electric quadrupolar moment with the sodium electron shell
can however be caused by an anisotropic molecular environment that breaks the spherical symmetry of the sodium ions.
An anisotropic phase is imposed by molecules that display
some kinds of net alignments rather than a random order. In
the present case, strong 23Na DQF MA signals are an indication
of the molecular order formed at the onset of shear. This
implies a shear induced phase with a likely structure that is
probably aligned with the flow direction. These shear induced
ordered domains are most likely localised to the shear changing zones that may be formed within the flowing fluid inside
(1)
where C0 is the amplitude of the DQF MA signal; oeff is the
residual quadrupolar coupling constant (QCC) in Hz and is a
measure for the anisotropy, or alignment, within the biopolymeric fluid; t is the double quantum coherence creation time,
s; and Rfast
is the fast component of sodium signal transverse
2
relaxation, s1. The obtained values of oeff and Rfast
were used
2
as non-adjustable coefficients in analysing the 23Na TQF signal
This journal is © The Royal Society of Chemistry 2023
time evolution according to:35
slow
fast
ITQF ðtÞ ¼ C1 etR2 cosðoeff tÞetR2
Table 2 Relevant fitting parameters extracted after analysing the 288 K
data displayed in Fig. 6 using eqn (1) and (2), please refer to Table SI3, ESI,
for details on other fitting parameters
g_ , s1
oeff, Hz
11.6
29
58
87
111
142
188
213
7
7
6
7
1
Rfast
2 , s
135
145
153
170
8
8
9
10
Rslow
, s1
2
46
52
46
53
3
3
3
5
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Fig. 6 Rheo-NMR data analysis at 288 K: 23Na DQFMA (a) and 23Na TQF
(b) time evolution at different shear rates. Solid lines represent fitting to
eqn (1) and (2), respectively, as described in the text. Fitting parameters and
errors are displayed in Table 2. Data collected in the absence of shear are
shown in black symbols. Red cross in (a) shows a replicated 23Na DQF MA
experiment at g_ = 58 s1 and t = 5 ms. Replicated spectra are shown in Fig.
SI4, ESI.†
the gap of the Couette cell. In a previous work, the existence of
such a localized, aligned phase was proven in the biopolymeric
flow through a cylindrical tube, utilizing DQF MA contrast in
the 23Na MRI in physiological sodium concentration.29 Unfortunately, because of the low sodium concentration in the
present system (0.02% as estimated using ref. 30) sodium
imaging was not feasible and was not performed.
The TQF results displayed in Fig. 6(b) provide further
insights into molecular dynamics influenced by shear probed
through the NMR properties of sodium ions. Strong 23Na TQF
signals, Fig. 6(b), are observed only in the presence of shear.
Meanwhile, DQF MA signals require a non-zero quadrupolar
coupling oeff (eqn (1)), and hence molecular alignment, the
explanation for the appearance of TQF signals is more complicated as these signals can be detected in the absence of the
molecular order, or when oeff = 0. The inspection of eqn (2)
reveals that quadrupolar coupling, and therefore alignment, is
indeed not the only source for TQF signals. An alternative cause
for TQF signals can be a bi-exponential transverse relaxation
that occurs when the relaxation rate constants R2slow and R2fast in
eqn (2) are not equal. Generally, for bi-exponential relaxation in
3
the nuclear spin I ¼ system of 23Na to occur, the sodium ions
2
need to be within a molecular environment that exhibits slow
3234 | Soft Matter, 2023, 19, 3228–3237
Paper
motion and that temporarily binds Na+ ions, thereby slowing
also the motional dynamics of the sodium itself. Therefore,
based on the absence of TQF signals when shear is not
imposed, as shown in Fig. 5 (right panel) and Fig. 6(b), one
can conclude that the macromolecules are not in an aligned
phase, and furthermore, the molecules experience fast molecular tumbling that causes R2fast = R2slow. The analysis of the TQF
curves obtained under shear conditions in Fig. 6(b) indicates
not only that quadrupolar coupling, and hence alignment is
present, but also the bi-exponential relaxation occurs with
significant differences between Rslow
and Rfast
2
2 , suggesting a
changed molecular dynamics of sodium ions under shear. The
structure of this shear induced phase also changes with the
increase of the applied shear rate as seen from the changed
shape of both types of MQF signals displayed in Fig. 6(a and b).
Further analysis of the shear dependence of sodium MQF
signals indicates that the parameter Rslow
is independent of the
2
applied shear rate while oeff increases linearly with the increase
of the shear rate, Fig. 7. A slight shear rate dependence may
also exist for Rfast
2 , although this may require further study.
However, a remarkable finding is that in the absence of shear,
both, the coupling oeff and the bi-exponential nature of the
relaxation disappear. While oeff = 0 indicates the absence of the
alignment, an exponential transverse relaxation indicates
the changed motional dynamics of the ions in the presence
of the biomolecules. The changed dynamics of the sodium ions
can be caused by a reduced overall tumbling of the molecules
in the aligned phase. Alternatively, an increased density of
macromolecules in the aligned phase may also alter the sodium
dynamics because there are simply more interaction events for
the ions in the vicinity of the molecules. This should also
contribute to the bi-exponential relaxation observed in the
TQF experiments, but the precise process will require further
study. Note that sodium ions in other phases, if present,
remain invisible for TQF spectroscopy.
Fig. 7 Sodium residual quadrupolar coupling constant (black stars), fast
(red triangles) and slow relaxation (blue inverted triangles) in the shear
induced phase formed in the 0.5% aqueous k-carrageenan fluid at 288 K at
different shear rates. Error bars represent errors reported in Table 2.
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Soft Matter
Fig. 8 Multiple quantum response at g_ = 58 s1 and at 288 K–298 K:
(a) 23Na DQFMA and (b) 23Na TQF time evolution. Solid lines represent data
fitting to eqn (1) and (2) for the DQF MA and TQF data, respectively.
Relevant fitting parameters are reported in Table 3; please refer to the ESI†
for additional details.
The 23Na MQF rheo-NMR data of the 0.5% aqueous kcarrageenan fluid collected in the extended temperature range
and at g_ = 58 s1 are displayed in Fig. 8.
The results of the analysis of these data using both eqn (1)
and (2) are reported in Table 3. The curve fit at 298 K can be
found in Fig. SI6, ESI.† As at 288 K, similar MQF behaviour is
observed in this fluid up to 298 K as shown in Fig. 8. Above
303 K no multiple quantum signals were observed neither in
the presence nor in the absence of the applied shear field as
shown in Fig. SI5, ESI.† We associate this behaviour with the
onset of molecular phase transition from helices to random
coils for the k-carrageenan molecules in this temperature
range. At 303 K, although k-carrageenan molecules still may
exist in the helix conformation, the helices most likely start to
unfold as their elasticity probed by oscillatory rheology is
rapidly decreasing. The application of shear at 303 K does not
create a stable ordered phase that can be detected using
sodium MQF methods, probably because of the increased
molecular motion of sodium ions. At higher temperatures, it
is unlikely that the macromolecules in the random coil conformation form a stable aligned phase; therefore, oeff = 0 and
no DQF MA signals are detected. Furthermore, although the
tumbling of the overall molecule may be reduced in the random
coil conformation, the fast internal motion becomes possible
that prevents bi-exponential transverse relaxation. Therefore,
TQF signals are also no longer present, even under shear.
Temperature dependent measurements provide further
insights into sodium relaxation and the molecular alignment
in the shear induced ordered phase. The Arrhenius plots of
1
1
sodium T2slow ¼ slow ; T2fast ¼ fast and oeff at g_ = 58 s1 in the
R2
R2
temperature range from 288 K to 298 K are displayed in Fig. 9.
appears to be largely indepenAs one can see from Fig. 9, Tslow
2
dent on the inverse of temperature, while Tfast
demonstrates
2
linear dependence on the inverse temperature with a negative
slope. The observed behavior can be explained as follows. The
molecular tumbling in the shear aligned phase is very much
reduced by the intermolecular forces that keep the molecules
aligned. The correlation times of these molecules are therefore
very long. For 23Na+ relaxation, however, the relevant correlation times are dictated by adsorption and desorption events to
and from the biomolecules in the aligned phase and are
therefore best described by the Arrhenius relationship. As the
temperature increases, the adhesion time of the sodium ions to
the aligned phase is reduced, and therefore, the 23Na relaxation
relevant correlation time is shortened. This leads to an increase
Table 3 Relevant parameters extracted after fitting the data displayed in
Fig. 8 using eqn (1) and (2); please refer to Table SI4, ESI, for additional
fitting details
Temperature, K
oeff, Hz
288
291
295
298
303
313
188
163
107
52
—
—
6
4
2
1
1
Rfast
2 , s
153
136
79
42
—
—
9
5
4
1
This journal is © The Royal Society of Chemistry 2023
Rslow
, s1
2
46
48
39
34
—
—
3
3
3
2
Fig. 9 Arrhenius plot of oeff (black stars), Tfast
(red triangles) and Tslow
2
2
(blue inverted triangles) at g_ = 58 s1 for four studied temperatures. Solid
lines represent linear fits with shading indicating a 5% range deviation from
the predicted linear behaviour. Intercepts and slopes and their errors for
each fit are shown in the legend. Further explanations are in the text. Data
to produce the plot are taken from Table 3.
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Soft Matter
in the observed relaxation time Tfast
as observed by the negative
2
slope in the inverse temperature plot in Fig. 9. Tslow
, unlike Tfast
2
2 ,
does not depend on the spectral density at zero frequency, and
hence deviates from this behavior, reminiscent to the wellknown T1, T2 dipolar relaxation behaviour as a function of
correlation time tc. Furthermore, as the temperature increases,
the k-carrageenan helices start to gain additional degrees of
freedom and do not align this readily in the shear field. Therefore,
the residual quadrupolar coupling constant oeff also decreases
during this process. It reaches a low value of oeff = 28 Hz at T =
298 K and this is probably also associated with the onset of the
unfolding of k-carrageenan helices and decreased the molecular
alignment that can be probed by anisotropic sodium dynamics.
Above this temperature, no alignment is detected upon imposition of the shear field and sodium dynamics becomes isotropic as
only single quantum sodium signals are detected, Fig. 5. This
correlates well with the bulk shear and oscillatory rheology data
displayed in Fig. 2 that also captures changes in the fluid
associated with fluid’s molecules transitioning from helices to
random coils upon the temperature ramp.
4 Conclusions
A molecular–mechanical correlation was found for the 0.5% kcarrageenan fluid with evidence of the shear-induced ordered selfassembly of k-carrageenan helices in the fluid. Any evidence of
ordered self-assembly was absent in the absence of shear and
when k-carrageenan molecules were in the random coil conformation at high temperatures. The mechanical properties of the
fluid confirmed that the viscoelasticity of the fluid was lost upon
temperature ramp and the fluid became Newtonian upon complete transition from helices to random coils. The loss of shearinduced self-assembly in this case was clearly captured using 23Na
MQF rheo-NMR methods. This particular model system, which
displays these mechanics without having an explicit liquid crystalline phase, underlines the potentially higher impact of 23Na rheoNMR for many biologically relevant fluids. For example, synovial
fluid, blood and various polysaccharide solutions used in drug
delivery contain naturally present Na+ cations that may potentially
manifest the shear-induced structure without having an explicit
liquid crystalline phase. The present work considered endogenous
sodium levels (0.02%) that are almost 45 times lower than the
physiological sodium concentration in the body (0.9%). Hence,
in vivo monitoring of the underlying shear-induced structure of
body fluids might be accomplished in vivo and may perhaps be
correlated with clinical outcomes. Moreover, most human scanners are capable of operating at the sodium frequency; therefore,
natural sodium could be used as a tracker to non-invasively
capture changes in the microstructure of body fluids and tissues
associated with clinical conditions.
Author contributions
GEP designed experiments and performed the data collection
and analysis. GEP and TM co-wrote the paper.
3236 | Soft Matter, 2023, 19, 3228–3237
Paper
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the Medical Research Council
under Grant No. G0900785 and Grant no. MC_PC_15074.
Notes and references
1 E. E. Braudo, I. R. Muratalieva, I. G. Plashchina, V. B.
Tolstoguzov and I. S. Markovich, Colloid Polym. Sci., 1991,
269, 1148–1156.
2 I. Plashchina, I. Muratalieva, E. Braudo and V. Tolstoguzov,
Carbohydr. Polym., 1986, 6, 15–34.
3 M. Nunez-Santiago, A. Tecante, C. Garnier and J. L. Doublier,
Food Hydrocolloids, 2011, 25, 32–41.
4 Y. Wang, C. Yuan, B. Cui and Y. Liu, Carbohydr. Polym.,
2018, 202, 530–535.
5 S. B. Ross-Murphy, J. Rheol., 1995, 39, 1451–1463.
6 J. Necas and L. Bartosikova, Vet. Med., 2013, 58, 187–205.
7 D. Saha and S. Bhattacharya, J. Food Sci. Technol., 2010, 47,
587–597.
8 M. G. Semenova, G. E. Pavlovskaya and V. B. Tolstoguzov,
Food Hydrocolloids, 1991, 4, 469–479.
9 D. Qureshi, S. K. Nayak, S. Maji, D. Kim, I. Banerjee and
K. Pal, Curr. Pharm. Des., 2019, 25, 1172–1186.
10 S. Rokka and P. Rantamaki, Eur. Food Res. Technol., 2010, 231, 1–12.
11 R. Yegappan, V. Selvaprithiviraj, S. Amirthalingam and
R. Jayakumar, Carbohydr. Polym., 2018, 198, 385–400.
12 K. M. Zia, S. Tabasum, M. Nasif, N. Sultan, N. Aslam, A. Noreen
and M. Zuber, Int. J. Biol. Macromol., 2017, 96, 282–301.
13 M. Semenova, I. Plashchina, E. Braudo and V. Tolstoguzov,
Carbohydr. Polym., 1988, 9, 133–145.
14 C. Rochas and M. Rinaudo, Biopolymers, 1984, 23, 735–745.
15 L. Piculell, J. Borgström, I. Chronakis, P.-O. Quist and
C. Viebke, Int. J. Biol. Macromol., 1997, 21, 141–153.
16 C. Rochas, M. Rinaudo and M. Vincendon, Biopolymers,
1980, 19, 2165–2175.
17 S. Ikeda and K. Nishinari, J. Agric. Food Chem., 2001, 49,
4436–4441.
18 Z. Liu, B. Bhandari, S. Prakash, S. Mantihal and M. Zhang,
Food Hydrocolloids, 2019, 87, 413–424.
19 T. H. Jovic, G. Kungwengwe, A. C. Mills and I. S. Whitaker,
Front. Mech. Eng., 2019, 5, 19.
20 M. Badiger, P. Rajamohanan, P. Suryavanshi, S. Ganapathy
and R. Mashelkar, Macromolecules, 2002, 35, 126–134.
21 P. T. Callaghan, Rep. Prog. Phys., 1999, 62, 599–670.
22 P. T. Callaghan, Rheo-NMR: A New Window on the Rheology of
Complex Fluids, John Wiley & Sons, Ltd., 2012.
23 D. Morimoto, E. Walinda, N. Iwakawa, M. Nishizawa,
Y. Kawata, A. Yamamoto, M. Shirakawa, U. Scheler and
K. Sugase, Anal. Chem., 2017, 89, 7286–7290.
24 N. Iwakawa, D. Morimoto, E. Walinda, Y. Kawata, M. Shirakawa
and K. Sugase, Int. J. Mol. Sci., 2017, 18, 2271.
This journal is © The Royal Society of Chemistry 2023
View Article Online
Open Access Article. Published on 03 April 2023. Downloaded on 5/5/2023 3:02:56 PM.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
Paper
25 C. Schmidt, Spectrosc. Eur., 2014, 26, 11–14.
26 M. R. Lopez-Gonzalez, W. M. Holmes and P. T. Callaghan,
Soft Matter, 2006, 2, 855–869.
27 H. Siebert, D. Grabowski and C. Schmidt, Rheol. Acta, 1997,
36, 618–627.
28 K. G. Wilmsmeyer, X. Li and L. A. Madsen, Liq. Cryst., 2018,
45, 844–856.
29 G. E. Pavlovskaya and T. Meersmann, J. Phys. Chem. Lett.,
2014, 5, 2632–2636.
30 M. Gobet, M. Mouaddab, N. Cayot, J.-M. Bonny, E. Guichard,
J.-L. Le Quéré, C. Moreau and L. Foucat, Magn. Reson. Chem.,
2009, 47, 307–312.
This journal is © The Royal Society of Chemistry 2023
Soft Matter
31 G. Jaccard, S. Wimperis and G. Bodenhausen, J. Chem. Phys.,
1986, 85, 6282–6293.
32 R. KempHarper, S. Brown, C. Hughes, P. Styles and S. Wimperis,
Prog. Nucl. Magn. Reson. Spectrosc., 1997, 30, 157–181.
33 R. Ernst, G. Bodenhausen and A. Wokaun, Principles of
nuclear magnetic resonance in one and two dimensions, Clarendon Press, Oxford University Press, Oxford, Oxfordshire,
1987.
34 M. Del Carmen Nunez-Santiago and A. Tecante, Carbohydr.
Polym., 2007, 69, 763–773.
35 G. Navon, H. Shinar, U. Eliav and Y. Seo, NMR Biomed.,
2001, 14, 112–132.
Soft Matter, 2023, 19, 3228–3237 | 3237