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Bell-shaped sol–gel–sol conversions in pHresponsive worm-based nanostructured fluid† Yongmin Zhang,*a Pengyun Ana and Xuefeng Liu*b
Received 21st December 2014 Accepted 28th January 2015 DOI: 10.1039/c4sm02845g www.rsc.org/softmatter
A pH-switchable worm system was fabricated by simply mixing two non-surface-active compounds, N-(3-(dimethylamino)propyl)palmitamide (PMA) and citric acid (HCA), at a molar ratio of 3 : 1. Such a nanostructured fluid exhibits bell-shaped sol–gel–sol transitions with sequential pH variation, reflecting continuous structural transformations from sphere to worm to no aggregates.
Nanostructured uids (NF), which are formed from low-weight molecules, have attracted considerable attention in both fundamental research and industry1–6 during the past decade. Among these, wormlike micelles (short for ‘worm’), with a diameter of several nanometres and length in the micrometer range, are a highly promising area of recent development owing to their unique aggregate structures and refreshing rheological properties, reminiscent of polymer solutions, making them potential candidates for versatile applications in biomedicine,1 clean process,1–3 drug reduction,1–4,7 templates of nano-materials,1–4 and oil recovery.1–3 Recently, the ability of worm-based NF to switch between a low-viscosity water-like solution and a viscoelastic gel is a focal point for the preparation of smart, nanostructured materials.1–3 To this end, various switching stimuli have been explored, including light,8–12 thermo,13,14 pH,15–22 redox,23 and CO2 gas.24–30 Comparatively, a pH-responsive, worm-based NF could be more readily constructed in both the laboratory and at the industrial site. However, to the best of our knowledge, existing reports have only described NFs that undergo a single sol–gel transformation of viscoelasticity in response to external stimuli. Moreover, these sol–gel transformation curves generally exhibit a Z15–17 or reverse Z18–20 shape with sequential pH variation, indicating that the viscoelasticity of worm-based NF changes
singly from low viscosity to high viscoelasticity or from high viscoelasticity to low viscosity. To date, only single transitions have been reported during increasing or decreasing pH sweeps. Nevertheless, a worm-based NF exhibits a bell-shaped variation of viscoelasticity versus pH—, i.e., a NF with sequential sol–gel– sol transformations—is preferred in some practical applications to selectively thicken an aqueous solution within a narrow pH response range, whereas there have been no reports of smart, worm-based NFs undergoing two or more sequential responsive transformations during a single stimulation process. Herein, we report the rst example of a responsive NF that displays two sequential transitions, sol–gel–sol, during a single pH sweep, i.e., a bell-shaped viscoelasticity curve as pH is varied. The pH-responsive NF was formed in situ by simply mixing two non-surface-active compounds, N-(3-(dimethylamino)propyl) palmitamide (PMA, Scheme 1) and metabolic citric acid (HCA, Scheme 1) at a precise stoichiometric ratio of 3 : 1 (referred to as “3PMA–HCA”), without specialized organic synthesis. As exhibited in Fig. 1, a transparent and homogeneous viscoelastic gel with a pH of 6.1 (3PMA–HCA-6.1) was immediately obtained when 300 mmol PMA and 100 mmol HCA were mixed in 1 L distilled water. Subsequently, the solution was
a
School of Chemical & Materials Engineering, Jiangnan University, 214122 Wuxi, People's Republic of China. E-mail: [email protected]
b
Key Laboratory of Food Colloids and Biotechnology Ministry of Education, Jiangnan University, 214122 Wuxi, People's Republic of China. E-mail: x[email protected]
† Electronic supplementary 10.1039/c4sm02845g
information
2080 | Soft Matter, 2015, 11, 2080–2084
(ESI)
available.
See
DOI:
Scheme 1 Schematic illustration of the chemical principles governing pH-switchable, worm-based viscoelastic fluid.
This journal is © The Royal Society of Chemistry 2015
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Fig. 1 Zero-shear viscosity (h0) of the 100 mM 3PMA–HCA solution measured during the sequential increase in pH at 40 C.
mechanically agitated for several minutes, which can withstand its own weight in a long timescale when tipped upside down. Individually, a 300 mM PMA aqueous solution looks like a cloudy dispersion with low viscosity, and a 100 mM HCA solution demonstrates water-like uid behaviour (Fig. S1, ESI†). Steady-state rheological measurement (Fig. 2A) of 3PMA– HCA-6.1 shows an obvious Newtonian plateau at low shear rates accompanied by shear-thinning behaviour beyond a critical shear rate, indicating the rearrangement of tangled worms under shear stress, i.e., the presence of a worm.3,31 If we extrapolate the Newtonian plateau to zero-shear rate, zero-shear viscosity (h0), as high as 2 000 000 mPa s is obtained. According to the h0 – surfactant concentration (C) curve (Fig. S2, ESI†), a clear break point is observed at 10.4 mM, which is usually dened as overlapping concentration C*. When C < C*, h0 increases linearly in accordance with the Einstein equation h0 ¼ hwater(1 + KC),31 where K is of the order of unity. When C > C*, long and exible worms develop and begin entangling with each other, forming a dynamic transient network that is responsible for the substantial viscosity enhancement of 3PMA– HCA-6.1. In this regime, h0 increases exponentially by several orders of magnitude following the scaling law h0 f Cn,31 where the power-law index n is found to be 6.63. Compared with other worm-based NFs, 3PMA–HCA-6.1 possesses a higher C*, which is unfavourable for worm network formation, but a larger n can effectively negate the unfavourably high C*. Moreover, the oscillatory data (Fig. 2B) also conrm worm formation. The storage modulus (G0 ) dominates the loss modulus (G00 ) over a wide range of frequencies (above uc 0.05 rad s1), indicative of a typical viscoelastic uid. The plateau modulus (G0, the storage modulus at high shear frequency) is 100 Pa, and the relaxation time (sR, inverse of uc) is 3.2 s. This worm-based NF can be well described by the Maxwell model for viscoelastic materials with single relaxation
This journal is © The Royal Society of Chemistry 2015
Fig. 2 Rheological experiments of 100 mM 3PMA–HCA solution at 40 C. (A) Flow curves of 3PMA–HCA solution with different pH; (B) dynamic rheology and Cole–Cole plots of 3PMA–HCA aqueous solution at pH 6.1.
times, and G0 and G00 t the Cole–Cole semicircle over a majority of frequencies, together signifying the presence of a worm network.31 Whereas if a drop of concentrated HCl or NaOH is added to 3PMA–HCA-6.1, the viscoelastic NF rapidly transforms into a owing water-like solution with pH of 4.5 (3PMA–HCA-4.5, Fig. 1) or a low-viscosity dispersion with a pH of 7.8 (3PMA– HCA-7.8, Fig. 1), accompanied by a white solid precipitation, respectively. The response time is rapid,
Published on 28 January 2015. Downloaded by Simon Fraser University on 03/06/2015 01:34:38.
COMMUNICATION
Cite this: Soft Matter, 2015, 11, 2080
View Journal | View Issue
Bell-shaped sol–gel–sol conversions in pHresponsive worm-based nanostructured fluid† Yongmin Zhang,*a Pengyun Ana and Xuefeng Liu*b
Received 21st December 2014 Accepted 28th January 2015 DOI: 10.1039/c4sm02845g www.rsc.org/softmatter
A pH-switchable worm system was fabricated by simply mixing two non-surface-active compounds, N-(3-(dimethylamino)propyl)palmitamide (PMA) and citric acid (HCA), at a molar ratio of 3 : 1. Such a nanostructured fluid exhibits bell-shaped sol–gel–sol transitions with sequential pH variation, reflecting continuous structural transformations from sphere to worm to no aggregates.
Nanostructured uids (NF), which are formed from low-weight molecules, have attracted considerable attention in both fundamental research and industry1–6 during the past decade. Among these, wormlike micelles (short for ‘worm’), with a diameter of several nanometres and length in the micrometer range, are a highly promising area of recent development owing to their unique aggregate structures and refreshing rheological properties, reminiscent of polymer solutions, making them potential candidates for versatile applications in biomedicine,1 clean process,1–3 drug reduction,1–4,7 templates of nano-materials,1–4 and oil recovery.1–3 Recently, the ability of worm-based NF to switch between a low-viscosity water-like solution and a viscoelastic gel is a focal point for the preparation of smart, nanostructured materials.1–3 To this end, various switching stimuli have been explored, including light,8–12 thermo,13,14 pH,15–22 redox,23 and CO2 gas.24–30 Comparatively, a pH-responsive, worm-based NF could be more readily constructed in both the laboratory and at the industrial site. However, to the best of our knowledge, existing reports have only described NFs that undergo a single sol–gel transformation of viscoelasticity in response to external stimuli. Moreover, these sol–gel transformation curves generally exhibit a Z15–17 or reverse Z18–20 shape with sequential pH variation, indicating that the viscoelasticity of worm-based NF changes
singly from low viscosity to high viscoelasticity or from high viscoelasticity to low viscosity. To date, only single transitions have been reported during increasing or decreasing pH sweeps. Nevertheless, a worm-based NF exhibits a bell-shaped variation of viscoelasticity versus pH—, i.e., a NF with sequential sol–gel– sol transformations—is preferred in some practical applications to selectively thicken an aqueous solution within a narrow pH response range, whereas there have been no reports of smart, worm-based NFs undergoing two or more sequential responsive transformations during a single stimulation process. Herein, we report the rst example of a responsive NF that displays two sequential transitions, sol–gel–sol, during a single pH sweep, i.e., a bell-shaped viscoelasticity curve as pH is varied. The pH-responsive NF was formed in situ by simply mixing two non-surface-active compounds, N-(3-(dimethylamino)propyl) palmitamide (PMA, Scheme 1) and metabolic citric acid (HCA, Scheme 1) at a precise stoichiometric ratio of 3 : 1 (referred to as “3PMA–HCA”), without specialized organic synthesis. As exhibited in Fig. 1, a transparent and homogeneous viscoelastic gel with a pH of 6.1 (3PMA–HCA-6.1) was immediately obtained when 300 mmol PMA and 100 mmol HCA were mixed in 1 L distilled water. Subsequently, the solution was
a
School of Chemical & Materials Engineering, Jiangnan University, 214122 Wuxi, People's Republic of China. E-mail: [email protected]
b
Key Laboratory of Food Colloids and Biotechnology Ministry of Education, Jiangnan University, 214122 Wuxi, People's Republic of China. E-mail: x[email protected]
† Electronic supplementary 10.1039/c4sm02845g
information
2080 | Soft Matter, 2015, 11, 2080–2084
(ESI)
available.
See
DOI:
Scheme 1 Schematic illustration of the chemical principles governing pH-switchable, worm-based viscoelastic fluid.
This journal is © The Royal Society of Chemistry 2015
View Article Online
Published on 28 January 2015. Downloaded by Simon Fraser University on 03/06/2015 01:34:38.
Communication
Soft Matter
Fig. 1 Zero-shear viscosity (h0) of the 100 mM 3PMA–HCA solution measured during the sequential increase in pH at 40 C.
mechanically agitated for several minutes, which can withstand its own weight in a long timescale when tipped upside down. Individually, a 300 mM PMA aqueous solution looks like a cloudy dispersion with low viscosity, and a 100 mM HCA solution demonstrates water-like uid behaviour (Fig. S1, ESI†). Steady-state rheological measurement (Fig. 2A) of 3PMA– HCA-6.1 shows an obvious Newtonian plateau at low shear rates accompanied by shear-thinning behaviour beyond a critical shear rate, indicating the rearrangement of tangled worms under shear stress, i.e., the presence of a worm.3,31 If we extrapolate the Newtonian plateau to zero-shear rate, zero-shear viscosity (h0), as high as 2 000 000 mPa s is obtained. According to the h0 – surfactant concentration (C) curve (Fig. S2, ESI†), a clear break point is observed at 10.4 mM, which is usually dened as overlapping concentration C*. When C < C*, h0 increases linearly in accordance with the Einstein equation h0 ¼ hwater(1 + KC),31 where K is of the order of unity. When C > C*, long and exible worms develop and begin entangling with each other, forming a dynamic transient network that is responsible for the substantial viscosity enhancement of 3PMA– HCA-6.1. In this regime, h0 increases exponentially by several orders of magnitude following the scaling law h0 f Cn,31 where the power-law index n is found to be 6.63. Compared with other worm-based NFs, 3PMA–HCA-6.1 possesses a higher C*, which is unfavourable for worm network formation, but a larger n can effectively negate the unfavourably high C*. Moreover, the oscillatory data (Fig. 2B) also conrm worm formation. The storage modulus (G0 ) dominates the loss modulus (G00 ) over a wide range of frequencies (above uc 0.05 rad s1), indicative of a typical viscoelastic uid. The plateau modulus (G0, the storage modulus at high shear frequency) is 100 Pa, and the relaxation time (sR, inverse of uc) is 3.2 s. This worm-based NF can be well described by the Maxwell model for viscoelastic materials with single relaxation
This journal is © The Royal Society of Chemistry 2015
Fig. 2 Rheological experiments of 100 mM 3PMA–HCA solution at 40 C. (A) Flow curves of 3PMA–HCA solution with different pH; (B) dynamic rheology and Cole–Cole plots of 3PMA–HCA aqueous solution at pH 6.1.
times, and G0 and G00 t the Cole–Cole semicircle over a majority of frequencies, together signifying the presence of a worm network.31 Whereas if a drop of concentrated HCl or NaOH is added to 3PMA–HCA-6.1, the viscoelastic NF rapidly transforms into a owing water-like solution with pH of 4.5 (3PMA–HCA-4.5, Fig. 1) or a low-viscosity dispersion with a pH of 7.8 (3PMA– HCA-7.8, Fig. 1), accompanied by a white solid precipitation, respectively. The response time is rapid,
