3,4-Dialkoxypyrrole for the Formation of Bioinspired Rose Petal-like Substrates with High Water Adhesion

Article abstract of DOI:10.1021/acs.langmuir.6b02245

Self-organization is commonly present in nature and can lead to the formation of surface structures with different wettabilities. Indeed, in nature superhydrophobic (low water adhesion) and parahydrophobic (high water adhesion) properties exist, such as in lotus leaves and red roses, respectively. The aim of this work is to prepare parahydrophobic properties by electrodeposition. For this, pyrrole derivatives with two alkoxy groups of various lengths (from 1 to 12) were synthesized in 8 steps by adapting a method developed by Merz et al. We show that the alkyl chain length has a huge influence on the polymer solubility and as a consequence on the surface morphology and hydrophobicity. Moreover, the alkyl chain length should be at least greater than eight carbons in order to obtain parahydrophobic properties. The properties are also controlled by the electrolyte nature. These materials can be used for many potential applications in water harvesting and transportation and separation membranes.

Full text of DOI:10.1021/acs.langmuir.6b02245

Article
pubs.acs.org/Langmuir
3,4-Dialkoxypyrrole for the Formation of Bioinspired Rose Petal-like
Substrates with High Water Adhesion
Claudio Mortier, Thierry Darmanin, and Frederic Guittard*
́
́
́
Universite Nice Sophia Antipolis, CNRS, LPMC, UMR 7336, 06100 Nice, France
S
* Supporting Information
ABSTRACT: Self-organization is commonly present in nature and
can lead to the formation of surface structures with different wetta-
bilities. Indeed, in nature superhydrophobic (low water adhesion) and
parahydrophobic (high water adhesion) properties exist, such as in
lotus leaves and red roses, respectively. The aim of this work is to
prepare parahydrophobic properties by electrodeposition. For this,
pyrrole derivatives with two alkoxy groups of various lengths (from
1 to 12) were synthesized in 8 steps by adapting a method developed
by Merz et al. We show that the alkyl chain length has a huge
influence on the polymer solubility and as a consequence on the
surface morphology and hydrophobicity. Moreover, the alkyl chain
length should be at least greater than eight carbons in order to obtain
parahydrophobic properties. The properties are also controlled by the
electrolyte nature. These materials can be used for many potential
applications in water harvesting and transportation and separation membranes.
due to the presence of loosely woven sheets. These properties
INTRODUCTION
■
are extremely interesting, for example, in trapping small water
droplet in dry environments25−28 and can found in many
potential applications in water transportation and harvesting,29
water/oil separation,30 and underwater locomotion.31 Hence, to
achieve parahydrophobic properties it is preferable to produce
surface structures at only one scale (nanostructures, for example)
and/or to increase the surface energy. This can be achieved using
various strategies including laser treatment,32 plasma etching,33
chemical vapor deposition,34 phase separation,35 colloidal
lithography,36 and inkjet imprinting.37
Conducting polymers can also be used to control both θw and
water adhesion. Many strategies were employed to obtain struc-
tured and organized conducting polymers.38−40 Among them,
polyaniline was largely used in solution to obtain nanostructured
polymers by self-organization.41,42 For example, rambutan,
dandelion squares, and starlike structures were reported, and
superhydrophobic properties were obtained after deposing
these materials by dip-coating, spin-coating, or spray-coating.
Using fluorinated doping agents, switchable surfaces from
superhydrophobic to superhydrophilic were reported by a simple
change in voltage.43 One-step processes, where the growth of
structured conducting polymers is induced directly on substrates,
were also reported, including preferential growth,44 grafting,45
Self-organized systems are commonly used in solution in
surfactants and membranes but can also lead to the formation
of surface structures.1,2 As a consequence, they can be extremely
useful in modifying the surface hydrophobicity. Indeed,
controlling surface hydrophobicity and water adhesion is
fundamental to various potential applications, for example, in
cookware, self-cleaning and self-healing fabrics, antifingerprint
optical devices, antibioadhesion/antibiofouling coatings, separa-
tion membranes, and sensors.3−6 In the literature, there are very
recent developments of multifunctional structured surfaces with
both special wettabilities and adhesion using different processes,
and they are usually inspired by nature.7−10 Superhydrophobic
properties, characterized by an extremely high water apparent
contact angle θw and ultralow water adhesion or hysteresis (H),
are commonly present in nature. This is the case of the famous
self-cleaning lotus leaves, and these properties give a considerable
advantage,11 for example, against other predators. For example,
flying insects possess antirefection and antifogging eyes,12 and
other insects can slide on the surface of the water with their
feet.13 It was shown that dual-scale or fractal surface structures
and materials of low surface energy are preferable to achieving
superhydrophobic properties with high robustness.14−16
By contrast, on other natural substrates, both extremely
high θw and extremely strong water adhesion,17−23 also called
parahydrophobic properties by Marmur, were observed.24 This is
the case of rose petals,17,18 peach skin,19 cicada wings21 and gecko
feet.20 Very recently, Yarger et al. also reported that embiopteran
Antipaluria urichi weaves silk fibers into sheets having extremely
strong water adhesion.22 They observed that these properties are
Special Issue: Tribute to Toyoki Kunitake, Pioneer in Molecular
Assembly
Received: June 16, 2016
Revised: July 27, 2016
© XXXX American Chemical Society
A
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vapor-phase polymerization,46 plasma polymerization,47 and
steps it is also possible to reach Py(OCn)2 with extremely long alkyl
chains.
electropolymerization.48,49
Hence, before monomer synthesis, an intermediary compound,
diethyl 1-benzyl-3,4-dihydroxy-1H-pyrrole-2,5-dicarboxylate, was syn-
thesized in four steps from iminodiacetic acid as shown in Scheme 1.
Then, the monomers were obtained by nucleophilic substitution from
diethyl 1-benzyl-3,4-dihydroxy-1H-pyrrole-2,5-dicarboxylate and, using
different bromides, deprotection of the amine function, saponification in
an acid medium, and decarboxylation on the following compounds.
The monomer mass spectra and the 1H and 13C spectra are available in
the Supporting Information.
Electropolymerization is a very fast and controllable process
for developing organized conducting polymer films. Here, a
monomer is oxidized in an electrochemical cell to induce poly-
merization and polymer deposition on a working electrode.48,49
Different conductive substrates can be used as working elec-
trodes, such as platinum, gold, titanium, stainless steel, and trans-
parent conductive glass (ITO) but not aluminum because of the
presence of the oxide layer. Complex substrates such as meshes
can also be used because polymerization starts from the
substrate. The surface structures are highly dependent on
electrochemical parameters and also on the monomer structure.
It was shown that 3,4-ethylenedioxypyrrole (EDOP) and
3,4-propylenedioxypyrrole (ProDOP) derivatives have unique
properties in term of polymerization capacity but also in terms of
their optoelectronic50−54 and wetting properties. Indeed, in
comparison with classical pyrrole derivatives, the presence of two
alkoxy groups in the 3 and 4 positions has several advantages.
First, they make polymerization possible only in the 2 and 5
positions, which greatly increases the polymer conductivity.
Moreover, their presence also reduces the monomer oxidation
potential by electrodonating effects. For example, fluorinated
EDOP was used to develop nanoporous superoleophobic
films55,56 and EDOP with branched alkyl chains and aromatic
substituents was used to obtain parahydrophobic properties.57,58
However, because of the extreme difficulty in synthesizing these
compounds, which are also very sensitive, the study of many
of these derivatives is missing in the literature. Indeed, these
derivatives are usually synthesized in eight steps from iminodi-
acetic using the process reported by Merz et al.59
The synthesis process is given below.
Nucleophilic Substitution of Diethyl 1-Benzyl-3,4-dihydroxy-1H-
pyrrole-2,5-dicarboxylate. To 500 mL of dimethylformamide (DMF)
were added 25 g of diethyl 1-benzyl-3,4-dihydroxy-1H-pyrrole-2,
5-dicarboxylate (75 mmol, 1 equiv) and 30 g of K2CO3 (0.2 mol,
3 equiv). The solution was stirred and held at 100 °C for 30 mn. Then,
the corresponding 1-iodoalkane (for n = 1 to 5) or 1-bromoalkane (for
n = 6 to 12) was added slowly (0.3 mol, 4 equiv). After 2 days at 100 °C
(70 °C for n = 1 and 2), the product was extracted with ethyl acetate.
Finally, the product was distilled at 120 °C under pressure to remove all
volatile impurities. The product could be used for the next step without
other treatment.
Diethyl 1-Benzyl-3,4-dimethoxy-1H-pyrrole-2,5-dicarboxylate.
Yield 52.6%, brown liquid. δH(200 MHz, CDCl3): 7.24 (3 H, m),
6.95 (2 H, d), 5.97 (2 H, s), 4.29 (4 H, q), 3.9 (6 H, s), 1.29 (6 H, t).
Diethyl 1-Benzyl-3,4-diethoxy-1H-pyrrole-2,5-dicarboxylate. Yield
89.7%, brown liquid. δH(200 MHz, CDCl3): 7.22 (3 H, m), 6.94 (2 H, d),
5.99 (2 H, s), 4.27 (4 H, q), 4.13 (4 H, q), 1.36 (6 H, t), 1.30 (6 H, t).
Diethyl 1-Benzyl-3,4-dipropoxy-1H-pyrrole-2,5-dicarboxylate.
Yield 85.6%, brown oil;. δH(200 MHz, CDCl3): 7.23 (3 H, m), 6.93
(2 H, d), 5.99 (2 H, s), 4.27 (4 H, q), 4.00 (4 H, t), 1.79 (4H, m), 1.29
(6 H, t), 1.02 (6 H, t).
Diethyl 1-Benzyl-3,4-dibutoxy-1H-pyrrole-2,5-dicarboxylate. Yield
98.3%, brown oil. δH(200 MHz, CDCl3): 7.23 (3 H, m), 6.92 (2 H, d),
5.98 (2 H, s), 4.29 (4 H, q), 4.04 (4 H, t), 1.76 (4H, m), 1.42 (4H, m),
1.29 (6 H, t), 0.99 (6 H, t).
Diethyl 1-Benzyl-3,4-bis(pentyloxy)-1H-pyrrole-2,5-dicarboxylate.
Yield 98.5%, yellow oil. δH(200 MHz, CDCl3): 7.26 (3 H, m), 6.92 (2 H,
d), 5.98 (2 H, s), 4.27 (4 H, q), 4.03 (4 H, t), 1.75 (4H, m), 1.33 (8H,
m), 1.29 (6 H, t), 0.92 (6 H, t).
Among all of the derivatives that can be synthesized with this
process, here we were interested in 3,4-dialkoxy-1H-pyrrole
(Scheme 1: Py(OCn)2) with n = 1 to 12. If some of these deriv-
Scheme 1. Monomers Synthesized and Studied in This Article
Diethyl 1-Benzyl-3,4-bis(hexyloxy)-1H-pyrrole-2,5-dicarboxylate.
Yield 93.5%, orange oil. δH(200 MHz, CDCl3): 7.22 (3 H, m), 6.92
(2 H, d), 5.98 (2 H, s), 4.27 (4 H, q), 4.03 (4 H, t), 1.74 (4H, m), 1.47
(4H, m), 1.43 (8 H, m), 1.31 (6 H, t), 0.93 (6 H, t).
Diethyl 1-Benzyl-3,4-bis(octyloxy)-1H-pyrrole-2,5-dicarboxylate.
Yield 98.0%, orange oil. δH(200 MHz, CDCl3): 7.22 (3 H, m), 6.93
(2 H, d), 5.98 (2 H, s), 4.27 (4 H, q), 4.02 (4 H, t), 1.77 (4H, m), 1.39
(26 H, m), 0.90 (6 H, t).
Diethyl 1-Benzyl-3,4-bis(decyloxy)-1H-pyrrole-2,5-dicarboxylate.
Yield 98.0%, orange oil. δH(200 MHz, CDCl3): 7.18 (3 H, m), 6.92
(2 H, d), 5.98 (2 H, s), 4.27 (4 H, q), 4.02 (4 H, t), 1.74 (4H, m), 1.29
(34 H, m), 0.87 (6 H, t).
Diethyl 1-Benzyl-3,4-bis(dodecyloxy)-1H-pyrrole-2,5-dicarboxy-
late. Yield 98%, orange oil. δH(200 MHz, CDCl3): 7.22 (3 H, m),
6.92 (2 H, d), 5.98 (2 H, s), 4.27 (4 H, q), 4.02 (4 H, t), 1.76 (8H, m),
1.32 (38 H, m), 0.88 (6 H, t).
Deprotection of the Amine Function and Saponification in an
Acidic Medium. Because the synthesis process is different for different
alkyl chain lengths (n), we give the process for each case. For n ≥ 6, the
extraction steps were suppressed by the amphiphilic behavior of the
products.
For n = 1 to 5. For the deprotection of the amine, the product was
dissolved in 200 mL of trifluoroacetic acid. Then, anisole (1.4 equiv) and
sulfuric acid in a catalytic proportion were added, and the mixture was
held for 1 h at 90 °C. Trifluoroacetic acid and all volatile impurities were
removed with a rotavapor. Then, the product was added to a saturated
solution of NaHCO3 and extracted with ethyl acetate or another organic
solvent and then evaporated. For the saponification, 600 mL of a
solution of NaOH at 2 N containing 50 mL of ethanol was added to the
atives were already synthesized in the literature (n = 1 and 2),56
their surface properties and especially their wetting properties
have never been explored. Indeed, because of the presence of free
NH groups, which are very polar, it is expected to lead to high
water adhesion. Here, we report for the first time a complete
study of their surface morphology and wettability of as a function
of the alkyl chain length (nine monomers were synthesized as
shown in Scheme 1).
EXPERIMENTAL SECTION
■
Synthesis. All of the starting chemical compounds used in this
study were purchased from Sigma-Aldrich. The monomers were
synthesized in eight steps from iminodiacetic acid as shown in
Scheme 2 by adapting a strategy developed by Merz et al.59 Indeed,
Merz et al. reported a process in eight steps that can be used to
obtain EDOP and ProDOP but also Py(OC1)2 and Py(OC2)2. Indeed,
for all of these molecules it is first necessary to synthesize diethyl
1-benzyl-3,4-dihydroxy-1H-pyrrole-2,5-dicarboxylate in four steps,
and then different molecules can be envisaged using different bromides
or dibromides. Here, we show also that by suppressing extraction
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Scheme 2. Synthesis of the Monomers
product, and the mixture was held for 1 day at 100 °C. Then, 300 mL of
the solution was removed with a rotavapor, and all organic impurities
were removed by extraction with ethyl acetate. The solution was
neutralized with hydrochloric acid 37% to pH 2, and the product
precipitated. The product was recovered by filtration and washed with
water. If the product does not precipitate, then the water is removed and
the product is filtered and washed with water.
For n = 6 to 12. For the deprotection of the amine, the product was
dissolved in 200 mL of trifluoroacetic acid. Then, anisole (1.4 equiv) and
sulfuric acid in a catalytic proportion were added, and the mixture was
held for 1 h at 90 °C. Trifluoroacetic acid and all volatile impurities were
removed with a rotavapor. For saponification, 600 mL of a solution of
NaOH at 2 N containing 50 mL of ethanol was added to the product,
and the mixture was held for 1 day at 100 °C. Then, 300 mL of the
solution was removed with a rotavapor, and the solution was neutralized
with hydrochloric acid 37% to pH 2. Water was removed, and the
product was filtered and washed with water.
Decarboxylation. To 100 mL of triethanolamine at 200 °C in a
nitrogen atmosphere was added 3 g of the previously obtained product
for about 1 to 2 min. Then, the product was extracted with ethyl acetate,
and the solvent was evaporated. The product was purified by column
chromatography using silica gel neutralized with triethylamine and using
cyclohexane/ethyl acetate as the eluent.
3,4-Dimethoxy-1H-pyrrole (Py(OC1)2). Yield 32%, white crystalline
solid with mp 95.5 °C. δH(200 MHz, CDCl3): 7.03 (1 H, s), 6.22
(2 H, d), 3.76 (6 H, s). δC(200 MHz, CDCl3): 138.15, 99.50, 58.45.
FTIR (KBr): νmax/cm−1 3392, 3004, 2954, 1595. 1555, 1325, 1197,
1174, 742 cm−1. MS (70 eV): m/z 127 (M+, 85), 112 (C5H6NO2•+, 100)
3,4-Diethoxy-1H-pyrrole (Py(OC2)2). Yield 27%, gray crystals with
mp 40.0 °C. δH(200 MHz, CDCl3): 7.03 (1 H, s), 6.22 (2 H, d), 3.93
(4 H, q), 1.38 (6 H, t). δC(200 MHz, CDCl3): 137.14, 100.39, 66.75,
14.99. FTIR (KBr): νmax/cm−1 3379, 2982, 2901, 2875, 1585, 1550,
1479, 1323, 1185, 1145, 1047, 777, 737, 541 cm−1. MS (70 eV):
m/z 154.9 (M+, 56), 127 (C6H9NO2•+, 34), 98.9 (C4H5NO2•+, 100).
3,4-Dipropoxy-1H-pyrrole (Py(OC3)2). Yield 33%, dark crystals
with mp 36.2 °C. δH(200 MHz, CDCl3): 7.03 (1 H, s), 6.22 (2 H, d),
3.83 (4 H, t), 1,78 (4 H, m), 0.99 (6 H, t). δC(200 MHz, CDCl3):
137.26, 100.55, 72.96, 22,54, 10.32. FTIR (KBr): νmax/cm−1 3377, 2965,
2936, 2875, 1585, 1550, 1475, 1326, 1186, 1147, 1044, 1004, 790, 738,
543 cm−1. MS (70 eV): m/z 183 (M+, 36), 141.1 (C7H11NO2•+, 21),
99 (C4H5NO2•+, 100).
3,4-Dibutoxy-1H-pyrrole (Py(OC4)2). Yield 5%, dark crystals with mp
47.6 °C. δH(200 MHz, CDCl3): 7.03 (1 H, s) 6.22 (2 H, d), 3.87 (4 H, t),
1.71 (4 H, m), 1.45 (4 H, m), 0.96 (6 H, t). δC(200 MHz, CDCl3):
137.49, 100.71, 71.28, 31.5, 19.19, 13.87. FTIR (KBr): νmax/cm−1 3380,
3173, 3158, 2953, 2869, 1587, 1551, 1478, 1469, 1327, 1184, 1144,
733 cm−1. MS (70 eV): m/z 211 (M+, 23), 98.9 (C4H5NO2•+, 100).
3,4-Bis(pentyloxy)-1H-pyrrole (Py(OC5)2). Yield 32%, dark crystals
with mp 26.1 °C. δH(200 MHz, CDCl3): 6.99 (1 H, s), 6.21 (2 H, d),
3.86 (4 H, t), 1.76 (4 H, m), 1.42 (8 H, m), 0.94 (6 H, t). δC(200 MHz,
CDCl3): 137.28, 100.50, 71.40, 28.93, 27.97, 22.30, 13.85. FTIR (KBr):
νmax/cm−1 3406, 2956, 2932, 2871, 1586, 1546, 1323, 1185, 1138,
759 cm−1. MS (70 eV): m/z 239.2 (M+, 95), 99.1 (C4H5NO2•+, 100).
3,4-Bis(hexyloxy)-1H-pyrrole (Py(OC6)2). Yield 28%, dark crystals
with mp 48.3 °C. δH(200 MHz, CDCl3): 7.01 (1 H, s), 6.21 (2 H, d),
3.86 (4 H, t), 1.76 (4 H, m), 1.41 (12 H, m), 0.92 (6 H, t). δC(200 MHz,
CDCl3): 137.23, 100.46, 71.40, 31.46, 29.20, 25.49, 22.44, 13.86. FTIR
(KBr): νmax/cm−1 338, 3180, 3155, 2952, 2922, 2870, 1588, 1550, 1477,
1466, 1325, 1185, 1144, 789 cm−1. MS (70 eV): m/z 267.2 (M+, 100),
99.1 (C4H5NO2•+, 92).
3,4-Bis(octyloxy)-1H-pyrrole (Py(OC8)2). Yield 5%, dark crystals with
mp 55.2 °C. δH(200 MHz, CDCl3): 6.97 (1 H, s) 6.21 (2 H, d), 3.85
(4 H, t), 1.79 (4 H, m), 1.34 (2,20 H, m), 0.88 (6 H, t). δC(200 MHz,
CDCl3): 137.30, 100.51, 71.44, 31.66, 29.26, 29.24, 29.1, 25.84, 22.49,
13.92. FTIR (KBr): νmax/cm−1 3380, 2922, 2852, 1590, 1550, 1466,
1326, 1186, 1145, 790 cm−1. MS (70 eV): m/z 323.3 (M+, 92.5), 281.2
(C17H31NO2•+, 82.5), 207.1 (C12H17NO22• 2+, 100).
3,4-Bis(decyloxy)-1H-pyrrole (Py(OC10)2). Yield 8%, white powder
with mp 64.0 °C. δH(200 MHz, CDCl3): 7.03 (1 H, s) 6.21 (2 H, d), 3.85
(4 H, t), 1.75 (4 H, m), 1.26 (20 H, m), 0.87 (6 H, t). δC(200 MHz,
CDCl3): 137.39, 100.61, 71.52, 31.79, 29.58, 29.51, 29.47, 29.34, 29.22,
25.90, 22.56, 13.98. FTIR (KBr): νmax/cm−1 3378, 2956, 2920, 2851,
C
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Figure 1. Ten scans of cyclic voltammetry of different monomers. Scan rate 20 mV s−1.
1591, 1551, 1475, 1467, 1326, 1186, 1145, 791 cm−1. MS (70 eV):
a carbon rod was used as a counterelectrode, and a saturated calomel
electrode (SCE) was used as a reference electrode. For the surface
characterization, the polymers were electrodeposited on gold plates
purchased from Neyco and consisted of a deposition of chromium
(20 nm) and gold (150 nm) on a silicon wafer. The mean roughness
(Ra) of the smooth gold plates is around 5 nm and has no significant
influence on the surface wettability. In a glass cell connected to the
potentiostat with the three-electrode system, 10 mL of anhydrous
acetonitrile containing 0.1 M electrolytic salt and 0.01 M monomer
were introduced under argon. First, the monomer oxidation potential
(0.86−1.08 V vs SCE following the alkyl chain length of the monomer)
was determined by cyclic voltammetry with the platinum tip as the
working electrode. Multiple potential scans were performed to study the
• 2+
m/z 379.4 (M+, 100), 281.2 (C17H31NO2•+, 56), 238.2 (C14H24NO2
,
48), 207.2 (C12H19NO2•+, 69), 99.1 (C4H5NO2•+, 76).
3,4-Bis(dodecyloxy)-1H-pyrrole (Py(OC12)2). Yield 5%, gray powder
with mp 71.1 °C. δH(200 MHz, CDCl3): 6.97 (1 H, s), 6.21 (2 H, d),
3.85 (4 H, t), 1.75 (4 H, t), 1.24 (24 H, m), 0.87 (6H, t). δC(200 MHz,
CDCl3): 137.26, 100.45, 71.39, 31.71, 29.47, 29.42, 29.42, 29.32, 29.25,
29.22, 29.15, 25.80, 22.48, 13.91. FTIR (KBr): νmax/cm−1 3377, 2952,
2920.5, 2850, 1591, 1551, 1469, 1327, 1186, 114, 791 cm−1.
Electrochemical Conditions. An Autolab potentiostat purchased
from Metrohm was used for the electrodeposition experiments.
The connection was realized via a three-electrode system. A platinum
tip was used as a working electrode for cyclic voltammetry experiments,
D
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polymer growth and to determine the polymer oxidation and reduction
potentials. Then, polymer films were electrodeposited at constant
potential on larger gold plates. Six depositions at constant potential were
performed for each monomer at several deposition charges during the
experiments: 12.5, 25, 50, 100, 200, and 400 mC cm−2. Depositions at
1 mC cm−2 were also performed to prepared smooth polymer films.
All polymer substrates were washed three times in acetonitrile to remove
all remaining electrolyte and monomer.
RESULTS AND DISCUSSION
■
Deposition Experiments. The electropolymerization
experiments were performed in anhydrous acetonitrile contain-
ing 0.01 M monomer and 0.1 M salt used as an electrolyte.
All monomers were perfectly soluble in acetonitrile except
Py(OC12)2, for which the monomer concentration is much less
than 0.01 M. First, it was necessary to determine the monomer
oxidation potential (Eoxm) by cyclic voltammetry to know the
intervals of tension where the monomer is reactive (oxidized).
Surface Characterization. Polymers were characterized by goni-
ometry, scanning electron microscopy (SEM), and optical profilometry.
The SEM images were recorded using a 6700F microscope (JEOL)
The monomer oxidation potential of each monomer (Eox
)
m
́
at the CCMA (Centre Commun de Microscopie Appliquee, Univ.
using tetrabutylammonium perchlorate (Bu4NClO4) as the salt is
the same for each monomer (between 0.89 and 0.93 V vs SCE),
which indicates that the effect of the alkyl chain length is not very
significant here.
Nice Sophia Antipolis). The mean arithmetic (Ra) and quadratic (Rq)
roughnesses were determined using a Wyko NT 1100 optical micro-
scope (Bruker) with a 50× objective and a 0.5× field of view (FOV).
VSI mode was used for smooth surfaces, and PSI mode was used for
very rough surfaces. The apparent contact angles (θ) were measured
Then, to study the polymer growth, multiple cyclic
voltammetry experiments (10 scans) were performed between
−1 V and Eoxm. The cyclic voltammograms are shown in Figure 1
except for Py(OC12)2 because its concentration was less than
0.01 M as a result of its low solubility. First, the intensity of the
polymer oxidation and reduction peaks obtained with Py(OC3)2,
Py(OC4)2, Py(OC5)2, and Py(OC6)2 is extremely low. Hence,
these polymers have a high solubility in acetonitrile, which is
problematic for obtaining stable and adherent polymer films.
The intensity of the polymer oxidation and reduction peaks is
high with Py(OC1)2 and Py(OC2)2, but the curves are not super-
posed after each scan, which indicates important steric hindrances.
By contrast, when using long alkyl chains (≥8 carbons), extremely
intense and well-defined cyclic voltammograms are obtained
with a very nice superposition of each scan. Hence, the polymer
oxidation and reduction potentials of these polymers do not
evolve as the polymer thickness increases.
using a DSA30 goniometer (Kruss) by taking the tangent at the triple-
̈
point contact line with a droplet of 2 μL. Milli-Q water (conductivity =
0.055 μS; γLV = 72.8 mN/m), diiodomethane (γLV = 50.0 mN/m),
and hexadecane (γLV = 27.6 mN/m) were used as probe liquids. For the
dynamic contact angles, a 4 μL water droplet was placed on the substrate,
and the substrate was inclined until the droplet moved. The advancing
and receding contact angles are taken just before the droplet moves.
The maximum inclination angle is called the sliding angle (α). If the
droplet does not move even for α > 90°, then the hysteresis is extremely
high and the substrate is called sticky.
The surface free energy (γSV) of the smooth surfaces and its dispersive
(γDSV) and polar (γPSV) parts were determined using the Owens−Wendt
equation: γLV(1 + cos θ) = 2(γDLV γD
) )
1/2 + 2(γPLV γP 1/2. Using three
SV SV
different liquids (water, diiodomethane, and hexadecane, for which
γLV, γDLV, and γPLV are known, γDSV and γPSV can be calculated by drawing
1/2
the function y = ax + b where y = γLV(1 + cos θ)/2(γD
)
and
LV
x = (γP
) )
LV LV
1/2/(γD 1/2. Then, γDSV = b2 and γPSV = a2 are determined.
In our case, γDSV and γPSV were directly obtained using the drop shape
The effect of the alkyl chain length of the polymer solubility
analysis software for our goniometer.
can be explained. First, it is expected that the polymer solubility
Figure 2. Apparent contact angle (θ) as a function of the deposition charge (Qs) of the polymer (PPy(OC2)2, PPy(OC8)2, PPy(OC10)10, and
PPy(OC10)12).
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decreases when the length of the alkyl chain increases, which
explains the high insolubility of Py(OC8)2, Py(OC10)2, and
Py(OC12)2. However, the other important parameter influencing
the polymer solubility is the polymer chain length. Here, because
of steric hindrance, the polymer chain length is expected to
increase when the length of the alkyl chain decreases, as observed
by cyclic voltammetry (the polymer oxidation and reduction peaks
decrease). Therefore, this is why the polymer insolubility is rela-
tively important when for an alkyl chain length of 2 ≤ n ≥ 8 where-
as the polymer solubility is much more important for 3 ≤ n ≤ 6.
Surface Wettability. Then, the polymers were electro-
deposited on large gold plates at constant potential (E = Eox
)
m
Figure 3. Picture of a water droplet deposited on PPy(OC10)2
(200 mC cm−2) before and after inclination at 90°.
in order to evaluate the surface wettability and morphology.
For the evaluation of the surface hydrophobicity of each polymer,
goniometric measurements (θw) with water were performed as a
function of the deposition charge (Qs) at up to 400 mC cm−2.
As observed by cyclic voltammetry, PPy(OC3)2, PPy(OC4)2,
PPy(OC5)2, and PPy(OC6)2 are too soluble in acetonitrile and
the polymer film do not stay on the working electrode during
the electrodeposition and washing treatments. This is also the
case for PPy(OC1)2. PPy(OC2)2 films resist but only for high
Qs ≥ 100 mC cm−2. By contrast, PPy(OC8)2, PPy(OC10)2, and
PPy(OC12)2 are much more insoluble than all of the other poly-
mers and could be electrodeposited whatever their Qs.
fibrous morphology, which explains the possibility of increased
hydrophobicity while the polymer is intrinsically hydrophilic
by trapping a small amount of air between the surface and the
droplet. PPy(OC8)2 presents an ordered nanodome systems, and
PPy(OC10)2 presents larger cauliflower-like structures. Hence,
the presence of nanodomes is not sufficient to increase the
surface hydrophobicity, but the presence of cauliflower-like
structures greatly increases it. For PPy(OC12)2, extremely large
wrinkles are observed, which are often due to high tension in the
films, but at the nanoscale the wrinkles are not highly structured.
The presence of these larges wrinkles explains the extremely
high roughness obtained with the surfaces but also their high
hydrophobicity.
The results on wettability and roughness are given in Figure 2
and Table 1. As expected, PPy(OC2)2 is hydrophilic. The surface
The changes in the surface morphology from fibrous to
cauliflower-like structures with the alkyl chain length can be
easily explained. Indeed, it was shown in the literature than the
surface morphology is highly affected by the solubility of the
oligomers formed in the first instances of electropolymeriza-
tion.60 Here, the oligomers formed with Py(OC2)2 are highly
soluble in acetonitrile, and two-dimensional growth is favored,
leading to fibrous structures. By contrast, with very long alkyl
chains, the oligomers are much more insoluble and spherical
particles are obtained (three-dimensional growth).
Table 1. Parameters for the Polymer Deposited at Qs = 100,
200, and 400 mC cm−2 in NBu4ClO4
deposition charge
θwater
[deg]
Ra
[nm]
Rq
[nm]
polymer
[mC cm−2
]
PPy(OC2)2
100
200
400
100
200
400
100
200
400
100
200
400
76.9
89.1
15
24
26
35
74.6
316
74
522
99
PPy(OC8)2
PPy(OC10)2
PPy(OC12)2
108.9
109.5
112.0
116.4
122.3
131.8
135.1
146.8
136.0
116
302
103
1626
1206
499
4116
6225
147
390
206
2362
2150
962
6822
6600
Discussion. The results can be explained by the Wenzel and
Cassie−Baxter equations.61,62 Indeed, these methods were
developed to explain the wettability of rough surfaces. These
equations are dependent on the Young angles (θY ),63 which are
w
the apparent contact angles of the same polymers but smooth,
and it was first necessary to produce smooth surfaces. Here, the
smooth polymers were obtained using an ultralow deposition
charge of 1 mC cm−2. Indeed, in the first instances of electro-
polymerization, a very thin layer of polymer covers all the
substrate, but the surface structures have no time to grow. These
θYw values can be found in Table 2, and roughness measurements
confirmed their roughness.
hydrophobicity increases and decreases with Qs. The decrease in
hydrophobicity between 200 and 400 mC cm−2 can be explained
by a high increase in the surface roughness with the Wenzel
equation. However, the increase in hydrophobicity between
100 and 200 mC cm−2 can be explained only by Cassie−Baxter
and indicates the presence of a surface morphology able to trap
air between the surface and the substrate.
PPy(OC8)2 is hydrophobic and θw is quite independent of Qs
even if the surface roughness increases. By contrast, for the
longest alkyl chains (n ≥ 10), θw increases with Qs. A maximum
value of 131.8° was reached with PPy(OC10)2 for Qs = 400 mC
cm−2, and a value of 146.8° was reached with PPy(OC12)2 for
Qs = 200 mC cm−2. Here, the polymer films are extremely rough,
especially for Qs ≥ 200 mC cm−2. Moreover, the substrates are
also extremely sticky. A water droplet placed on them remained
stuck whatever the substrate inclination, as shown in Figure 3.
The SEM images (Figure 4) show an evolution of the surface
morphology with the alkyl chain length. PPy(OC2)2 possess a
PPy(OC8)2, PPy(OC10)2, and PPy(OC12)2 are intrinsically
hydrophobic (θYw > 90°), and as expected, θYw increases and γSV
decreases with the alkyl chain length. Here, the increase in the
contact angle can now be explained by the Wenzel and Cassie−
Baxter equations. With the Wenzel equation cos θw = r cos θY ,
w
where r is a roughness parameter, it is possible to reach extremely
high θ for only θYw > 90°, which is the case here.60 However, with
the Wenzel equation relatively important but not extremely
high hysteresis is often predicted, as observed here. The Cassie−
Baxter equation, with the presence of the air fraction between
the surface and the water droplet, can allow this possibility.61
The Cassie−Baxter equation is cos θw = rff cos θYw + f − 1, with rf
being the roughness ratio of the substrate wetted by the liquid,
f being the solid fraction, and (1 − f) being the air fraction.
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Figure 4. SEM images at two magnifications (5000× and 25 000×) of the different PPy(OCn)2 species (Qs = 200 mC cm−2).
Table 2. Wettability and Roughness Parameters for the Smooth Polymer (Qs = 1 mC cm2) Obtained in Bu4NClO4
polymer
Ra [nm]
Rq [nm]
θYw [deg]
θYdiiodo [deg]
θYhexadecane [deg]
γSV [mN m−1
]
γDSV [mN m−1
]
γPSV [mN m−1
]
PPy(OC8)2
PPy(OC10)2
PPy(OC12)2
5
6
6
6
96.7
103.2
105.7
50.2
49.1
58.5
0
31.08
30.53
27.27
30.19
30.39
27.16
0.88
0.14
0.11
12
8
17.4
26.3
With this equation, it is possible to predict ultralow water
adhesion (superhydrophobic properties) if the air fraction is
very important but also extremely high water adhesion
(parahydrophobic properties) if the air fraction is less important
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or if θY greatly decreases. Hence, the extremely high water
Then, the polymer films were also deposited at constant
potential. Measurements of the water contact angles and
roughness are given in Figures 6 and 7. SEM images are also
w
adhesion of our surfaces is induced by the presence of nano-
domes or cauliflower-like structures. When a water droplet
is in contact with the surface, the solid−liquid interface is
extremely important and the liquid−vapor interface is very low
because the roughness of the surface structures is not fractal or
hierarchical.
Other Experiments. Because Py(OC10)2 gives extremely
interesting results, other experiments were performed with
this monomer. To modify both the surface morphology
and hydrophobicity, other electrolytes were used: Bu4NBF4,
Bu4NPF6, Bu4NN(CF3SO3)2, Bu4NCF3SO3, Bu4NC4F9SO3, and
Bu4NC8F17SO3. For each counterion, the same electrodeposition
parameters were used. The cyclic voltammograms as a function
of the electrolyte are given in Figure 5. These voltammograms
show relatively similar curves using each electrolyte except for
Bu4NC8F17SO3. It is not surprising to find different results with
Bu4NC8F17SO3 because the films are stabilized by the counter-
Figure 6. Graphic of water contact angles on PPy(OC10)2 (200 mC cm−2)
using different electrolytes.
−
ions of the electrolyte and C8F17SO3 counterions are much
larger than the other ones, consequently inducing higher steric
hindrance.
shown in Figures 8 and 9. Only two electrolytes (Bu4NC8F17SO3
and Bu4N(CF3SO3)2) induced a higher hydrophobicity than
Figure 5. Ten cyclic voltammetry scans of PolyPy(OC10)2 with different electrolytic salts. Scan rate 20 mV s−1.
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−
morphology and roughness. The films obtained with BF4
−
and C4F9SO3 are less rough as confirmed by profilometry
−
measurements. For the deposition performed with PF6 ,
−
−
−
C8F17SO3 , C4F9SO3 , and N(CF3SO3)2 there are the same
mean roughnesses between 417 and 514 nm, but their roughness
is much lower than with ClO4 . The morphology is relatively
−
similar in the presence of microdomes, but the number of domes
as well as the presence of nanostructures is different with these
electrolytes. This is sufficient to induce a significant change
in surface hydrophobicity. It is strange that the surface
−
−
hydrophobicity obtained with PF6 is lower than with ClO4
because the surface morphology is very similar. However, the
mean roughness of the films obtained with ClO4− is much higher.
To elucidate that, other SEM images were recorded but at a
much lower magnification. As we can see in Figure 10, wrinkles
are observed on these surfaces. The presence of wrinkles is more
Figure 7. Graphic of roughness parameters on PPy(OC10)2 (200 mC cm−2)
using different electrolytes.
did Bu4NClO4. Indeed, θ = 134.7 and 125.1°, respectively, with
Bu4NC8F17SO3 and Bu4N(CF3SO3)2.
SEM images as a function of the electrolyte are given in
Figures 8 and 9. The electrolyte clearly impacts the surface
−
important with ClO4 , which explains its high roughness and
hydrophobicity.
Figure 8. SEM images of Py(OC10)2 (Qs = 200 mC cm−2) at two magnifications (5000× and 25 000×) in different electrolytes: Bu4NPF6, Bu4NBF4, and
Bu4NN(CF3SO3)2.
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Figure 9. SEM images of Py(OC10)2 (Qs = 200 mC cm−2) at two magnifications (5000× and 25 000×) in different electrolytes: Bu4NCF3SO3,
Bu4NC4F9SO3, and Bu4NC8F17SO3.
Figure 10. SEM images of Py(OC10)2 (Qs = 200 mC cm−2) at very low magnification (100×) in different electrolytes: Bu4NClO4 and Bu4NPF6.
develop parahydrophobic properties (high water adhesion) by
electrodeposition. We showed that the alkyl chain length has a
huge influence on the polymer solubility and as a consequence on
the surface morphology and hydrophobicity. Moreover, the alkyl
CONCLUSIONS
■
Here, we have synthesized pyrrole derivatives with two alkoxy
groups of various lengths (from 1 to 12) in 8 steps by adapting a
method developed by Merz et al., and we have used them to
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.langmuir.6b02245.
1H, 13C NMR, and mass spectra of Py compounds (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: frederic.guittard@unice.fr.
Notes
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The authors declare no competing financial interest.
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DOI: 10.1021/acs.langmuir.6b02245
Langmuir XXXX, XXX, XXX−XXX