Unusual pH-responsive fluid based on a simple tertiary amine surfactant: The formation of vesicles and wormlike micelles

Article abstract of DOI:10.1039/c4ra08004a

A novel pH-responsive viscoelastic micellar system was prepared by N,N-dimethyl oleoaminde-propylamine without the addition of hydrotropes. The micellar system undergoes a gradual transition from vesicles to spherical micelles to wormlike micelles by adding an HCl solution. Rheology, Cryo-TEM and dynamic light scattering (DLS) results revealed that the pH-responsive flow behavior is attributed to the microstructural transition among the spherical micelles, vesicles and worm-like micelles. In this paper, we found out that a vesicle is extremely pH sensitive because the degree of protonation and the concentration of Cl^- strongly affect the pH areas which could form worm-like micelles. The notable advantages of vesicles and worm-like micelles can be utilized to prepare pH-responsive viscoelastic fluids in the desired pH areas. This journal is

Full text of DOI:10.1039/c4ra08004a

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PAPER
Unusual pH-responsive fluid based on a simple
tertiary amine surfactant: the formation of vesicles
and wormlike micelles†
Cite this: RSC Adv., 2014, 4, 51519
Hongsheng Lu,* Li Wang and Zhiyu Huang
A novel pH-responsive viscoelastic micellar system was prepared by N,N-dimethyl oleoaminde-
propylamine without the addition of hydrotropes. The micellar system undergoes a gradual transition
from vesicles to spherical micelles to wormlike micelles by adding an HCl solution. Rheology, Cryo-TEM
and dynamic light scattering (DLS) results revealed that the pH-responsive flow behavior is attributed to
the microstructural transition among the spherical micelles, vesicles and worm-like micelles. In this
paper, we found out that a vesicle is extremely pH sensitive because the degree of protonation and the
Received 2nd August 2014
Accepted 11th September 2014
ꢀ
concentration of Cl strongly affect the pH areas which could form worm-like micelles. The notable
DOI: 10.1039/c4ra08004a
advantages of vesicles and worm-like micelles can be utilized to prepare pH-responsive viscoelastic
www.rsc.org/advances
fluids in the desired pH areas.
sodium dodecyl benzenesulfonate and reported a gradual
transition from micelles to vesicles to bilayers to precipitate by
The self-assembly behavior between molecules is inuenced by manipulating the pH. Huang and co-workers introduced the
1
. Introduction
22
1,2
the magnitude of non-covalent bonding. Reasonable control pH-sensitive
potassium
phthalic
acid
into
cetyl-
of the non-covalent interactions of molecular self-assembly trimethylammonium bromide (CTAB) solutions and found that
could result in the control of the micellar structure according to the microstructure of this pH-sensitive uid can be transformed
3
the packing parameter. These non-covalent interactions between wormlike micelles and short cylindrical micelles with a
4
–6
7
23
include hydrogen bonds, electrostatic forces, hydrophobic slight change in pH. Hassan and co-workers reported the pH-
8
9
interactions, van der Waals forces, dipole–dipole interactions responsive behavior of an amino acid which was used to control
10,11
and p–p stacking.
The variations in non-covalent interac- the electrostatic interactions on a cationic micellar surface and
tions between surfactants and counterions result in the poly- thereby alter the morphology of the self-assembled structures
8
morphism of their self-assemblies such as spherical micelles, by adjusting the pH. Zakin and co-workers introduced a pH-
12
vesicles, wormlike micelles and lamellar micelles. Precise responsive thread-like micellar system by mixing alkyl bis(2-
control of micellar construction is one of the biggest challenges. hydroxyethyl)methylammonium chloride (EO12) and trans-o-
To obtain viscoelastic uids, it is necessary to control the coumaric acid (tOCA). The rheological response of this system
packing geometry of surfactant molecules under appropriate to pH is revealed by viscoelasticity at both high and low pH
a
conditions. The macroscopic properties of stimuli-responsive levels, which is caused by the dual pK of tOCA. Moreover, we
materials can be dramatically changed with minor variation of could also employ a pH-responsive surfactant. Maeda and co-
5
the stimuli, and thus show an easy way to control the packing workers demonstrated a reversible change from thread-like
geometry.
micelles to vesicles when the ionization degree of oleyldime-
13,14
15,16
Compared with some stimuli (such as photo,
CO
2
,
thylamine oxide was increased by adjusting the pH. Feng and
17,18
19,20
24
redox
and temperature ), pH is a simple and applicable co-workers developed a pH-switchable worm-like micellar
approach to control viscoelastic uids. Hitherto, there are two system, which was prepared by mixing N-erucamidopropyl-N,N-
strategies that are generally employed to formulate pH- dimethylamine and maleic acid via tuning the pH. According to
responsive viscoelastic uids. Usually, a pH-responsive hydro- these studies, pH-responsive uids can be prepared by using
trope is introduced into surfactant solutions. Kalar and co- pH-responsive hydrotropes or surfactants, and the pH area of
21
a
workers obtained pH-dependent mixtures of histidine and the uids is determined by the pK of pH-sensitive groups.
In this paper, we used DOAPA to develop a novel pH-
responsive viscoelastic uid, which shows viscoelasticity at both
low and high HCl concentrations. We analyzed the mechanism
of self-assembly behavior at both high and low HCl concentra-
tions and found that the micellar structure is a vesicle at low
School of Chemistry and Chemical Engineering, Southwest Petroleum University,
Xindu, Sichuan 610500, P. R. China. E-mail: hshlu@swpu.edu.cn
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c4ra08004a
1
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ꢀ
HCl concentrations and is a worm-like micelle at high Cl
The rheology experiment was carried out on a HAAKE RS600
concentrations. The pH-induced changes in the microstruc- rational rheometer equipped with a cone and plate geometry at
ꢁ
ꢁ
tures of the assemblies are investigated by Cryo-TEM and 25 C. The samples were equilibrated at 25 C for at least 20 min
dynamic light scattering studies. Then, we examined the inu- prior to experimentation. A solvent trap was utilized to mini-
ꢀ
ence of pH and Cl concentration on the rheological behaviors mize water evaporation. Frequency spectra were conducted in
of the obtained worm-like micellar uids.
the linear viscoelastic regime of the samples, as determined
from the dynamic strain sweep measurements.
2. Experimental
2.1 Materials
2.4 Dynamic Light Scattering (DLS) measurements
Oleic acid (>99%) and N,N-dimethyl-1,3-propanediamine A Brookhaven BI-200SM goniometer was used in the dimension
ꢁ
(
>99%) were purchased from Alfa Co., Ltd. Ammonium chloride measurement at 25 C. The solutions were ltered through a
(
NH Cl, >99%), sodium hydroxide (NaOH, >99%) and hydro- 0.45 mm membrane lter of hydrophilic MICRO PES into light
4
chloric acid (HCl, ¼37%) were purchased from Chengdu Kelong scattering cells prior to the measurements. The scattering angle
ꢁ
Chemical Factory and were used without further purication. was 90 , and the intensity autocorrelation functions were
All the solutions were prepared in distilled water. The pH was analyzed using the CONTIN method.
determined by a Leici PHS-25 pH meter.
2.5 Cryo-TEM observation
2.2 Synthesis of N,N-dimethyl oleoaminde-propylamine
Cryo-TEM samples were prepared in a vitrication system
N,N-Dimethyl oleoaminde-propylamine was synthesized
according to the reaction pathway shown in Scheme 1. An
appropriate amount of oleic acid (42.37 g, about 150 mmol) was
added to a three-necked ask. Then, N,N-dimethyl-1,3-pro-
panediamine (16.55 g, about 162 mmol) was added dropwise
under magnetic stirring and the temperature was increased
ꢁ
(CEVS) at 25 C. A micropipette was used to load 5 mL solution
onto a TEM copper grid, which was blotted with two pieces of
lter paper, resulting in the formation of thin lms suspended

on mesh holes. Aer About 5 s, the samples were quickly
plunged into a reservoir of liquid ethane (cooled by nitrogen) at
ꢀ165 ꢁC. The vitried samples were then stored in liquid
nitrogen until being transferred to a cryogenic sample holder
ꢁ
gradually to a given temperature (160 C). The mixture was
reuxed at a reaction temperature for 7 h. Unreacted N,N-
dimethyl-1,3-propanediamine was removed by vacuum distil-
lation. The product was then washed by a saturated Na CO
(
Gatan 626) and were examined with a JEOL JEM-1400 TEM (120
ꢁ
kV) at about ꢀ174 C. The phase contrast was enhanced by
under-focus. The images were recorded on a Gatanmultiscan
CCD and were processed with a Digital Micrograph.
2
3
solution three times to remove the unreacted oleic acid. The
product was dried in a vacuum oven to a constant weight at
ꢁ
5
0 C. Finally, the structure of N,N-dimethyl oleoaminde-pro-
1
1
pylamine was conrmed by HNMR (ESI Fig. 1†). The HNMR
3
. Results and discussion
spectra was recorded on a Bruker Ascend 400 spectrometer at
1
4
00 MHz. HMNR (400 MHz, CDCl
3
): 0.9 (t, 3H, alkyl-CH
), 1.56 (m, 2H, b-CH to C]O), 1.65
m, 2H, b-CH to N), 2.0 (m, 2H, CH to C]C), 2.16 (t, 2H, CH
3
), 1.26
3.1 Physical appearances
(
(
m, 20H, alkyl chain –CH
2
2
DOAPA is hardly solvated in water because of its ultra-long
hydrophobic tail and behaves as an oily paste oating on the
water surface under alkali or neutral conditions. Adding an HCl
solution dropwise protonates DOAPA and forms N,N-dimethyl
oleoaminde-propylaminehemihydrochloride (DOAPA$HCl). We
found that the solution shows viscoelasticity at both low and
2
2
2
to C]O), 2.24 (s, 6H, N–CH ), 2.38 (t, 2H, N–CH ), 3.3 (m, 2H, g-
3
2
CH to N), 5.32 (t, 2H, –HC]CH–), 7.06 (t, H, –NH).
2
2.3 Rheological measurements
The apparent viscosity of the samples was measured with a high HCl concentrations when HCl is added dropwise.
ZNN-D6B rheometer (Qingdao Tongchun Oil Instrument Co.,
The 75 mM DOAPA solution shows different ow behaviors
ꢀ
1
Ltd, China) at a shear rate ranging from 5 to 1022 s with an under varying pH values (Fig. 1). When the pH is adjusted to
ꢁ
experimental temperature of 20–80 C. The measuring rotor and 7.41 (0.03 wt% HCl), the solution samples are similar as very low
a measuring cup were made entirely from Hastelloy alloys.
viscous emulsions. In contrast, the solution samples become
transparent and have a comparatively higher viscosity when the
pH is increased to 6.82 (0.16 wt% HCl), while their viscosity
decreases dramatically under higher HCl concentrations (0.26
wt% pH 6.20). As the pH value reaches 0.45 (1.50 wt% HCl), they
again increase signicantly and reach the second maximum at
pH 0.12 (3.0 wt% HCl). At this point, the solution can support its
own weight when it is upside down. The pH-responsive ow
behavior is attributed to the microstructural transition between
spherical micelles, vesicles and worm-like micelles, which has
been conrmed in detail below.
Scheme 1 Synthetic route of DOAPA.
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Fig. 1 Viscosity as a function of HCl concentration for DOAPA solu-
ꢀ1
tions at a shear rate of 170
.
3.2 pH-switchability of the 75 mM DOAPA micellar system
The measurement of pH-switchable ability was performed at a
point where the DOAPA solution reaches the maximum viscosity
at pH 0.12 and 6.82. Interestingly, these two samples can be
repeated when HCl and NaOH are cyclically added; however, the
point where the solution reaches the maximum viscosity at pH
0.12 changes to pH 6.20. The pH-switchability of the solutions at
pH 6.82 is shown in Fig. 2a. When the pH is xed at 7.41, the
viscosity of the uid is as small as ꢂ1 mPa s, while it increases to
3
2
393 mPa s, which is approximately 10 times than that of the
solutions at pH 7.41 upon decreasing the pH to 6.82. Then, the
viscosity drops to 3 mPa s at the pH value of 6.20. Viscosity can be
3
switched between ꢂ10 and ꢂ3 mPa s by varying the pH for at Fig. 2 Switchable viscosity of the “75 mM DOAPA” micellar system
least two cycles and the sample can be turned between gel-like start at (a) pH 7.41 and (b) pH 0.12.
and water-like states. The high pH sensitivity and reversible
control of the rheological properties may facilitate the applica-
tions of such a responsive viscoelastic uid.
+
DOAPAH can be obtained with the decreasing value of the pH.
The solution becomes viscoelastic at pH 6.82 and the degree of
In general, the macroscopic properties originate from
microscopic structures. Cryo-TEM observation was adopted to
display the microstructures of the micellar solutions in the high
and low viscosity status. As shown in Fig. 3, only some small
spherical micelles were found in the micellar solutions at pH
protonation X ¼ 0.6 (Fig. 4). Hydrogen bonds are formed
6.20. On the contrary, vesicles were observed at pH 6.82.
Therefore, such a signicant change in the viscosity of the 75
mM DOAPA micellar system under different situations can be
clearly illustrated by the reversible transformation between
vesicles and spherical micelles.
The self-assembly behavior of micelles is conventionally
25
indicated by the packing parameter, P, which is a geometric
quantity dened as V/al, where a is the effective headgroup area
and V is the volume of the lipophilic chain with the maximum
effective length l. When P < 1/3, spherical micelles are formed;
when 1/3 < P < 1/2, wormlike micelles are found; and when 1/2 <
P < 1, vesicles or bilayers can be obtained. In the current work,
the effective area of the headgroup is the only variable. The
morphology of DOAPA, between nonionic (DOAPA) and cationic
+
(
DOAPAH ), is depended on the pH of the solutions. More Fig. 3 Cryo-TEM images of 75 mM DOAPA at pH 6.82 and 6.20.
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+
+
ꢁ
between the DOAPAH and DOAPA molecules (–N –H/N–). The of the concentration of NH
4
Cl at pH 6.20 and 25 C. As the
hydrogen bonding was conrmed by the IR data (ESI Fig. 2†). NH Cl concentration increases, the apparent viscosity attains a
4
+
The DOAPAH and DOAPA can form a stable tertiary amine- maximum value and then decreases. Samples close to the
cation with each other through unusually strong hydrogen maximum value of viscosity are extremely viscoelastic and
bonds and hydrophobic interactions. The protonation of almost gel-like, and the structures of the samples change from
DOAPA was entirely nished at pH 6.20, and the vesicles spherical micelles to worm-like micelles (Fig. 6). The growth of
ꢀ
changed into spherical micelles, which resulted in the solution the micelles is displayed in Scheme 3; Cl ions could reduce the
+
looking like water. A proposed mechanism for the self-assembly distance between the headgroups of DOAPAH , and can induce
+
of DOAPA and DOAPAH into vesicles is shown in Scheme 2.
the transition from spherical micelles to rod-like micelles,
As shown in Scheme 2, at pH 6.82, the stable tertiary amine- which leads to the increase in viscosity with the addition of
cation has a bigger volume for the lipophilic chain and an NH Cl. On the other hand, lengthening the hydrophobic chains
4
unchanged effective length, which induces an increase in P will promote hydrophobic interactions, leading to a linear
approaching the range of 1/2–1, indicating that the formation of structure which entangles together to form a dense network,
vesicles occurred more easily. The pH value and the degree of resulting in a rapid increase in viscosity. However, the super-
ꢀ
ionization play important roles in the formation and stability of uous absorbance of Cl on the micellar surface leads to further
vesicles. This explains the phenomenon that the enhancement reduction in the electrostatic repulsions, and the linear micelles
of viscosity only occurs at specic pH values.
may become huddled. Ultimately, the micelles undergo a tran-
The pH-switch at another point where the DOAPA solution sition from linear ones to branched ones whose framework is
reaches the maximum viscosity imparts full reversibility to the slightly weaker than the linear micelles.
process (Fig. 2b) and could be repeated for several cycles. The
We now describe the rheology of typical DOAPA–NH Cl
4
apparent viscosity can be switched between ꢂ200 and ꢂ2 mP s samples as a function of pH. In Fig. 7, data are presented for
by varying the pH for at least four cycles and the sample can be apparent viscosity as a function of pH for four samples with
turned between gel-like and water-like states. However, the c(DOAPA) ¼ 75 mM and c(NH Cl) ¼ 280 mM, 420 mM, 470 mM
4
point of maximum viscosity of the solution changes from 3.0 and 580 mM. For the 280 mM sample, the apparent viscosity
wt% HCl to pH 6.2. In the switching process, NaCl is produced increases slightly at pH 6.81, and then drops monotonically over
by repeatedly adding HCl and NaOH. Compared with the the entire range of pH (Fig. 7a). When the degree of protonation,
viscosity at pH 6.20 (Fig. 2a), the DOAPA system experiences a X, is 0.6 at pH 6.81, the microstructures of the assemblies
transition from the water-like states to the gel-like states when transform from spherical micelles to vesicles because of strong
ꢀ
Cl concentration is increased, which has considerable effect hydrogen bonds and hydrophobic interaction, resulting the
on the pH-responsiveness and rheology behavior.
increase of the apparent viscosity. This is the expected trend in
apparent viscosity vs. pH for vesicle micellar uids, and a
similar behavior is seen for all the samples below a 280 mM salt
concentration. On the other hand, the 420 and 470 mM samples
show a qualitatively different behavior. For the 420 mM sample
ꢀ
3
.3 Effect of Cl concentration on pH-responsiveness and
rheology behavior
ꢀ
We investigated the effect of Cl concentration on the rheology
of a 75 mM DOAPA solution with the pH xed between 1.37 and
(Fig. 7b), the apparent viscosity again increases to pH 1.57. For
the 470 mM sample (Fig. 7c), the apparent viscosity increases
again at pH 2.35, and the second peaks distinctly move to the
right side of the x-axis compared with the 420 mM sample. The
microstructures of the assemblies transform from spherical
micelles to vesicles at pH 6.81, but the protonation of DOAPA
entirely nishes at pH 6.20. The vesicles are broken simulta-
neously, which leads to the decrease in the apparent viscosity.
8.51 to ensure the optimal pH effect. Fig. 5 shows that the
apparent viscosity of the 75 mM DOAPA solutions is a function
ꢀ
With the decreasing pH, the Cl concentration gradually
ꢀ
increases. The worm-like micelle can be induced by Cl , which
leads to the increase in the apparent viscosity. For the 470 mM
sample (Fig. 7d), the apparent viscosity increases signicantly at
pH 7.25. The microstructures of the assemblies directly trans-
form from spherical micelles to worm-like micelles under high
ꢀ
Cl concentrations. Fig. 7 reveals an unusual increase in
viscosity with pH in some DOAPA–NH Cl samples. As the NH Cl
4
4
concentration increases above 370 mM, the increase in viscosity
occurs over a wider range of pH. As shown in Fig. 7, the second
value peak shis close to or even overlaps the rst one at higher
ꢀ
pH values by adding NH
4
Cl. An abundance of Cl can induce
viscoelastic worm-like micelles in the solutions. It also can be
provided by HCl and NH Cl, and thus this is a simple way to
control the pH range through tuning the NH Cl concentration.
Fig. 4 Species distribution of DOAPA solution within pH range of
4
5
.5–8.5.
4
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Scheme 2 Transformation between a vesicle and spherical micelle induced by pH.
ꢁ
function of shear rate at 25 C and different pH values. At pH
7
.41, the shear viscosity of the solutions is as small as water (ꢂ1
mPa s) and is constantly maintained regardless of the shear rate
or the shear stress, which is a typical behavior of Newtonian

uids. Nevertheless, as the pH decreases to 6.91, the rheogram
shows a Newtonian plateau at low shear rates and a shear-
thinning slope when the shear rate reaches a critical value,
indicating the presence of worm-like micelles, which undergo
structural change-alignment of long micelles at high shear
rates. Upon further pH decrease, the viscosity increases and all
the viscosity curves display the coexistence of a Newtonian
plateau and shear-thinning zone; the critical shear rate starts to
shi to lower values.
0
Zero-shear viscosities (h ), obtained from the extrapolation
of shear viscosity along the Newtonian plateau to zero-shear
rate, are plotted as a function of pH to examine the effect of pH
4
Fig. 5 Effect of NH Cl concentration on the viscosity of a solution
containing 75 mM DOAPA.
on viscosity. When the pH is 7.41, the value of h
0
for the solution
Cl
is very small, about 3–4 mPa s, and the aqueous DOAPA–NH
4
solution is obscure. The pH of the solution decreases with the
addition of HCl, which causes the solution to become trans-
parent immediately and the viscosity increases sharply. When
0
the pH drops from 6.61 to 5.00, h drops from 234 300 to
123 900 mPa s. The viscosity of the solution increases when its
pH continually decreases below 5.00. The abovementioned
parameters follow a similar trend as that observed in viscosity,
as shown in Fig. 7c.
The pH-induced viscoelastic nature of the micellar solutions
can also be inferred from the frequency-dependent oscillatory-
shear measurements. In general, entangled worm-like micelles
exhibit Maxwellian behavior under oscillatory shear experi-
ments. For a Maxwell uid, the frequency-dependent elastic and
0
00
viscous moduli (G and G respectively) are given by
2
ðus
R
Þ
0
G ðuÞ ¼
G
0
(1)
(2)
2
1
þ ðus
R
Þ
Fig. 6 Cryo-TEM images of 75 mM DOAPA and 75 mM DOAPA and
us
R
00
4
4
70 mM NH Cl at pH 6.20.
G ðuÞ ¼
G
0
2
1
þ ðus
R
Þ
where s
R
and u denote the relaxation time and frequency in rad
ꢀ
1
3.4 Effect of pH on rheological behavior
s
, respectively, and G0 is the plateau modulus. At a given
temperature, G is a measure of the degree of entanglements,
whereas the relaxation time gives information regarding the
The pH-responsive behaviors of the 75 mM DOAPA and 470 mM
NH Cl solutions were further conrmed by rheological results.
0
4
average micellar length. For a Maxwell uid, the relaxation time
Fig. 8a shows the variation of steady shear viscosity (h) as a
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ꢀ
Scheme 3 Transition from a spherical micelle to a wormlike micelle with increasing Cl .
0
and plateau modulus are related to the zero-shear viscosity, h0,
of the uid by the relation,
The variation of the elastic or storage modulus (G ) and the
0
0
viscous or loss modulus (G ) as a function of oscillation
frequency (u) for the DOAPA–NH Cl mixed micelles at different
4
h
0
¼ G
s
0 R
(3)
pH values are shown in Fig. 9a. At pH 6.91, the low frequency
behavior of the data is similar to that of a Maxwell uid. The
solid lines in Fig. 9a are the best t to the data using a single
relaxation time Maxwell model. Such a viscoelastic behavior
indicates the formation of an entangled worm-like micellar
network. At high frequencies, a deviation from the Maxwell
model is observed. It can be interpreted as a transition in the
0
At very high frequencies, the storage modulus G approaches
the plateau modulus G and the Maxwell uid behaves like an
0
0
00
elastic body. However, at low frequencies, G and G become
2
proportional to u and u, respectively, and the uid behaves
23
like a simple liquid.
Fig. 7 pH dependence of viscosity in the systems of 75 mM DOAPA and different NH
c) 470 mM NH Cl; (d) 580 mM NH Cl.
4 4 4
Cl concentrations: (a) 280 mM NH Cl; (b) 420 mM NH Cl;
(
4
4
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Fig. 8 Effect of pH on (a) steady rheology, and (b) zero-shear viscosity
of 75 mM DOAPA and 470 mM NH Cl micellar solutions.
4
relaxation mode from reptation at longer time scales to
2
6
“
breathing” or Rouse mode at short time scales.
0
0
In Fig. 9b, the loss modulus (G ) is plotted as a function of
the storage modulus (G ) for the DOAPA–NH Cl mixed micelles
0
4
at different pH (Cole–Cole plot). The solutions at pH 6.91 have a
good t for the Cole–Cole model in the low to medium
frequency range, but deviation occurs at high frequencies due to
27
non-reptative effects. The single-mode Maxwell constitutive
equation can be used to describe the dynamic rheological
behaviors of the DOAPA–NH Cl solutions over a comparable
4
frequency range, implying the formation of WLMs. When the
pH decreases to 6.71 or 2.5, the shape of the curves obviously
Fig. 9 Effect of pH on (a) dynamic rheology, (b) Cole–Cole plots, and
deviates from a semicircle because of the strong elastic behav- (c) plateau modulus and relaxation time of 75 mM DOAPA and 470 mM
iors, as displayed in Fig. 9b.
NH Cl micellar system.
The estimated G and s
mental data to the Maxwell model are plotted in Fig. 7c as a
function of pH. The rheological parameters G and s
follow a of G
similar trend to that observed in h , as shown in Fig. 6b. G
and vesicles and wormlike micelles. At pH 5, the complete proton-
can be used to estimate the network density of entangled ation of DOAPA led to a break in the vesicles, and G and s
increase monotonously,
4
0
R
obtained aer tting the experi-
0
R
0 R
and s at pH 6.71 could be understood as the growth of the
0
0
s
R
0
R
28
worm-like micelles or the mesh size of the network and the decreased. Below pH 5, G and s
0 R
length of the micelles, respectively. When the pH value is indicating an increase in the network density of the worm-like
decreased, the protonation of DOAPA is increased. The increase micelles.
29
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3
.5 Micelle transition induced by pH
The change in the molecular states of DOAPA with decreasing
pH value or increasing NH Cl is also a reection of the evolution
manipulate the responsive pH-range upon understanding the
rationale behind the special responsiveness of these materials,
which provides a signicant advantage in the fabrication and
development of oileld materials with desired pH areas.
4
of the self-assembled structures in the system. To verify this,
dynamic light scattering measurements were carried out. As
demonstrated in Fig. 10, the size of the particles in the 75 mM
system progressively increases with decreasing pH value or
Acknowledgements
increasing NH Cl concentration. Without the addition of The authors acknowledge the experimental support from the
4
+
NH Cl, at pH 6.20, the DOAPAH self-assemble into particles Engineering Research Center of Oileld Chemistry and the
4
with an average hydrodynamic radius of around 2 nm, which nancial support from the Education Department of Sichuan
have been previously veried to be spherical micelles (Fig. 1b). Province (13TD0025) supported by scientic research starting
In contrast, upon changing the pH to 6.71, another group of project of SWPU (No. 373).
particles with an average hydrodynamic radius of around 14 nm
were found, which have been previously veried to be spherical
micelles (Fig. 1a). When 470 mM NH
solution at pH 6.20, only an average hydrodynamic radius of
6.5 nm was obtained, which is the size range for worm-like
4
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