Soft nanosystems based on hydroxypiperidinium surfactants as adjuvants and micellar catalysts

Article abstract of DOI:10.1016/j.mencom.2021.04.014

Quantitative parameters characterizing aggregation behaviour and wetting effect of 1-(2-hydroxyethyl)-piperidinium surfactants have been obtained. Their ability to act as adjuvants in herbicidal compositions based on clopyralid has been revealed. The kine

Full text of DOI:10.1016/j.mencom.2021.04.014

Mendeleev
Communications
Mendeleev Commun., 2021, 31, 323–325
Soft nanosystems based on hydroxypiperidinium surfactants
as adjuvants and micellar catalysts
Alla B. Mirgorodskaya,* Rushana A. Kushnazarova, Farida G. Valeeva,
Svetlana S. Lukashenko, Anna A. Tyryshkina, Lucia Ya. Zakharova and Oleg G. Sinyashin
A. E. Arbuzov Institute of Organic and Physical Chemistry, FRC Kazan Scientific Center of the Russian
Academy of Sciences, 420088 Kazan, Russian Federation. E-mail: mirgoralla@mail.ru
DOI: 10.1016/j.mencom.2021.05.014
cleavage rate of paraoxone
Quantitative
parameters
characterizing
aggregation
0
.08
behaviour and wetting effect of 1-(2-hydroxyethyl)-
piperidinium surfactants have been obtained. Their ability to
act as adjuvants in herbicidal compositions based on clopyralid
has been revealed. The kinetics of alkaline hydrolysis of
paraoxon and armine, organophosphorus ecotoxicants, in the
surfactant solutions has been explored and it has demonstrated
more than 100-fold micellar rate enhancement.
n = 12
OH
N
Br
Br
0.06
0.04
0.02
(CH
2
)
2
OH
N
C
(CH
2
)
2
OH
n = 14
C
n
H
2n+1
16
H
33
n = 12, 14, 16
n = 16
0.00
0
.00
0.01
0.02
0.03
Сsurf /М
Keywords: hydroxypiperidinium salts, piperidinium salts, surfactants, aggregation, wetting, micellar catalysis, alkaline hydrolysis.
Compositions based on surfactants are widely used in various
fields of technology and medicine as ‘soft’ systems with
controlled behaviour. There are several basic properties of
surfactants that determine their practical application: surface
activity, ability to form nanosized aggregates, solubilization and
OH
Br
Br
Br
+
N
+
+
N
(CH2)2OH
N
Me
(CH ) OH
2
2
1
–5
wetting effects, ability to overcome biological barriers, etc.
Design of amphiphilic molecules and functionalization of their
structure by the introduction of substituents allow one to involve
various mechanisms of self-association and binding of guest
compounds and to establish the structure–property relationships.
This is the way that provides a high applied potential of
surfactant-based systems and the foundation for solving various
nanotechnological problems. These are in agreement with the
modern trends in the fabrication of multifunctional compositions
with high biotechnological potential.1
a n = 12
b n = 14
C H2n+
C16H33
C16H33
n
1
c n = 16 1a–c
2
3
drop on the test surface. Treatment of the leaves with herbicide
solution and determination of clopyralid content in them was
previously described.
7
The critical micelle concentration (CMC) is the most
important characteristic of surfactant solutions, which makes it
possible to establish the concentration limits of their functional
activity. CMC values determined by tensiometry (see Online
Supplementary Materials, Figure S1) are 11, 4.0 and 1.0 mm
for compounds 1a–c, respectively. They are in agreement with
the StauffClevens equation, which relates the CMC values
and the length of the hydrophobic surfactant tail (n):
log(CMC) = 1.19 – 0.26n (R = 0.984). However, the values of
the intercept and the slope for compounds 1a–c are somewhat
different than those typical for trimethylammonium or
methylpiperidinium surfactants, for which the free term is in
the range from 1.5 to 1.9, and the slope is ~ 0.3. Probably, the
presence of a hydroxy group introduces the specificity of the
interaction of 4-hydroxypiperidinium surfactants with water
dipoles in comparison with non-functionalized analogues.
The surface activity of surfactants allows them to be used as
adjuvants upon the fabrication of herbicidal compositions, since
it improves the wetting of the treated surface, promotes the
penetration of the active substance into the plant, and thereby
increases the effectiveness of the applied agrochemicals.1
A measure to assess the wetting effect of the surfactant solution
may be the spread area of the droplet (S) on the test surface. The
ratio of S values for the water drop in the presence of surfactant
,6
In the present work, a systematic study of 1-alkyl-1-(2-
hydroxyethyl)piperidinium bromides was carried out, whose
functional behavior depends not only on hydrophobic and
electrostatic forces, but also on the ability of these compounds to
participate in the formation of hydrogen bonds. The properties of
a number of new surfactants with two hydroxy groups in the
molecule and with a variable hydrophobic tail length (series
1
a–c) have been investigated. Their aggregation behavior,
wetting effect, ability to act as adjuvants in herbicidal
compositions based on clopyralid, as well as their catalytic effect
in the processes of decomposition of organophosphorus
ecotoxicants were studied. The properties of these surfactants
were compared with the behavior of known 1-hexadecyl-1-
8
,9
methylpiperidinium bromide
2
and 1-hexadecyl-1-(2-
hydroxyethyl)piperidinium bromide 3. For the synthesis of new
compounds 1a–c, see Online Supplementary Materials.
Surface tension measurements were performed by the ring
detachment method using KRUSS 6 tensiometer. UV-VIS
spectra of the herbicide solutions were recorded using Specord
0–12
2
50 Plus spectrophotometers. The wetting ability of surfactant
solutions was assessed by determining the spreading area of the
©
2021 Mendeleev Communications. Published by ELSEVIER B.V.
–
323 –
on behalf of the N. D. Zelinsky Institute of Organic Chemistry of the
Russian Academy of Sciences.

Mendeleev Commun., 2021, 31, 323–325
2
.ꢀ
1
2
R
O
O
R
O
O
3
P
6
2
6
NO + 2OH
P
+ O
NO2 + H2O
5
2
3
5
EtO
2
3
6
EtO
1
0
5
4
4
4a,b
a R = Et
b R = OEt
4
1
1
1
Scheme 1
Compound 1a exhibits somewhat greater efficacy. By analogy
.ꢀ
0
with current ideas about the penetration of surfactant into
1
3
bacterial cells, it can be assumed that in this case the best
geometric correspondence between transport channels in plant
membranes and the structure of surfactant is observed.
ꢁaꢂ
ꢁbꢂ
ꢁcꢂ
Another important aspect of the application of cationic
surfactants is their use as micellar catalysts that are able to affect
the rate and mechanism of a number of practically significant
o
Figure 1 The spread area of drops of 0.1% surfactant solution (25 C)
on the surface of (a) glass, (b) parafilm, (c) dandelion leaf: (1) water,
(
2) with 2, (3) with 3, (4) with 1a, (5) with 1b, and (6) with 1c.
1
4–17
chemical transformations
and, in particular, markedly
and without it is one of the indicators of the surfactant
effectiveness. The results obtained essentially depend not only
on the properties and concentration of surfactants, but also on the
properties of the surface itself. So, when the solution spreads on
the surface of the leaf, its structure will play an important role,
including hairiness and surface roughness, the presence of
hydrophobic plant waxes on the outer tissue of the plant, the
thickness of the cuticle, etc. The difficulty of conducting
experiments on plants is due to the irregularity of the surface of
the leaf and its insufficient area for applying a statistically
acceptable number of drops. We tested the wetting effect of
piperidinium surfactants on various substrates: glass, ‘parafilm’
imitating wax leaf cuticle, and on common dandelion leaves. The
data presented in Figure 1 make it possible to conclude that
addition of 0.1% of tested surfactants to water provides
improvement of surface wetting by 30–80%. There is an increase
in the efficiency of surfactant as the length of its hydrophobic tail
increases, while the presence of hydroxy groups has little effect
on their wetting.
Another factor determining the effectiveness of the surfactants
as adjuvants is their ability to transport growth regulators into a
plant. The present work has focused on the systemic herbicide
clopiralids. Spectrophotometric method (cf. ref. 7) was used for
determination of clopyralid in common dandelion leaves and in
lettuce leaves after their two-hour contact with herbicide solutions
with addition of 0.1% piperidinium surfactants, the results having
been compared with those for pure water (Figure 2). Note that
when working with dandelion, the interfering effect of the
components extracted from the leaf simultaneously with the
herbicide having absorption in the same area as clopyralid should
be taken into account. There were no interfering influences with
salad leaves.
accelerate hydrolytic processes. Their catalytic action is widely
employed in decomposition of organophosphorus ecotoxicants
and neurotoxins.
To identify the peculiarities of the effect of the structure of
hexadecyl piperidinium surfactants on the rate of nucleophilic
substitution in phosphorus acid esters, the kinetics of alkaline
hydrolysis of ethyl (4-nitrophenyl) ethylphosphonate (armine,
4a) and diethyl (4-nitrophenyl) phosphate (paraoxone, 4b) were
studied (Scheme 1).
1
8–20
It should be noted that both esters in the absence of additives
undergo hydrolysis slowly even in highly alkaline media; and
therefore, they are often used as model substrates upon the
development and testing of catalytic systems for the prevention
2
1
and treatment of organophosphorus poisoning.
Hydrolysis of phosphorus acid esters was carried out at a
temperature of 25 °C in 0.01 m NaOH with varying the surfactant
concentration in the solution from 0 to 0.05 m. The process was
spectrophotometrically monitored by control of changing the
absorption intensity at 400 nm (maximum absorption of
4-nitrophenolate ion, Specord 250 Plus). Figure 3 shows the
concentration relationship of the observed rate constant of
armine 4a and paraoxone 4b cleavage in piperidinium surfactant
solutions. It follows that hydroxy-containing surfactants exhibit
substantially greater activity than their unsubstituted analogues
with respect to both esters. The catalytic action is enhanced by
increasing the number of OH groups in the molecule, as well as
by elongation of the chain of the hydrophobic tail. The highest
effect is observed for compound 1c at a concentration of 0.05 m:
70-fold and 120-fold accelerations of the process are observed
for armine 4a and paraoxone 4b, respectively. It should be noted
that in highly alkaline media, compounds containing the
hydroxyethyl moiety at the quaternary nitrogen atom are capable
2
2
Regardless of the type of plants, the addition of surfactant
improves the penetration of clopyralid into the leaves by
of conversion into zwitterion form (pK 12.4–12.6), which also
acts as a nucleophile and interacts with the substrates, which
would contribute to increasing the observed process rate. The
second hydroxy substituent introduced into the para-position of
the piperidinium cycle is not capable of cleavage of the proton
4
0–70%. The presence of hydroxy groups in the surfactant
structure has little effect on their transport properties.
1
1
1
1
.ꢂ
.ꢁ
.ꢀ
.2
1
(a)
(b)
3
0.08
0.06
3
5
0.010
0.008
0.006
0.004
0.002
5
0
0
0
0
.ꢂ
.ꢁ
.ꢀ
.2
0
2
2
0.04
1
4
1
0.02
0.00
4
0.000
0.00
0.01
0.02
0.03
0.00
0.01
0.02
0.03
C
surf
/
M
surf
C /M
ꢃater
ꢀ
ꢁ
ꢂa
ꢂb
ꢂc
Figure 2 Increased herbicide content in lettuce leaf when used in a
composition with 0.1% piperidinium surfactants added as compared to
water (the error of experiments was < 5%.)
Figure 3 Dependence of the observed rate constant of hydrolytic cleavage
of (a) armine 4a and (b) paraoxone 4b on concentration of piperidinium
surfactants (0.01 m NaOH, 25 °C): (1) 1a, (2) 1b, (3) 1c, (4) 2, and (5) 3.
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Mendeleev Commun., 2021, 31, 323–325
5
6
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A. G. Dedov, D. Yu. Marchenko, E. A. Ivanova, R. K. Idiatulov,
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under the reaction conditions. Therefore, it does not generate an
additional nucleophilic center, and the transition from mono- to
dihydroxy-substituted surfactants is accompanied by only a
minor effect. At the same time, compounds 1b,c which are
similarly capable of generating a reactive form, not only exhibit
their effect at substantially higher concentrations than their
hexadecyl analogue, but also provide a significantly lower
catalytic effect. This allows us to say that the factors of micellar
catalysis are mainly responsible for changes in the cleavage rate
of the esters studied. In the case of hydroxypiperidinium
surfactants, the rate acceleration is due to both various types of
interactions (hydrophobic, electrostatic, specific) that provide
solubilization of the organophosphorus substrate, and
concentration of hydroxide ions at the micelle surface. The
transition to higher homologues often results in an increase in
catalytic action of cationic surfactants, which can be explained
by enhancement of solubilization activity toward hydrophobic
solutes and increase in surface potential responsible for the
binding of negatively charged nucleophiles.
In conclusion, the piperidinium surfactants studied can be
effectively used as adjuvants to improve the wetting of the plant
surface and the transport of the herbicide into it, as well as
catalysts capable of more than 100-fold accelerating the cleavage
oforganophosphorusecotoxicants.Thelengthofthehydrocarbon
tail of a surfactant plays a significant role in the first case, while
in the second, key factor is the functionalization of the head by
introducing hydroxyl substituents.
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