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zwitterionic-nonionic mixed micelles?
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Contents lists available at ScienceDirect
Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Regular Article
What happens when pesticides are solubilised in binary ionic/zwitterionic-nonionic mixed micelles? Xuzhi Hu a, Haoning Gong a, Peter Hollowell a, Mingrui Liao a, Zongyi Li a, Sean Ruane a, Huayang Liu a, Elias Pambou a, Najet Mahmoudi b, Robert M. Dalgliesh b, Faheem Padia c, Gordon Bell c, Jian R. Lu a,⇑ a b c
Biological Physics Group, School of Physics and Astronomy, University of Manchester, Oxford Road, Manchester M13 9PL, UK STFC ISIS Facility, Rutherford Appleton Laboratory, Didcot OX11 0QX, UK Syngenta, Jealott’s Hill International Research Centre, Bracknell, Berkshire RG42 6EY, UK
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 19 August 2020 Revised 20 October 2020 Accepted 21 October 2020 Available online xxxx Keywords: Pesticide Surfactants Micellar structure Solubilisation Agri-spray Nanostructure Drug delivery Pesticide delivery SANS
a b s t r a c t Hypothesis: Surfactants have been widely used as adjuvants in agri-sprays to enhance the solubility of pesticides in foliar spray deposits and their mobility through leaf cuticles. Previously, we have characterised pesticide solubilisation in nonionic surfactant micelles, but what happens when pesticides become solubilised in anionic, cationic and zwitterionic and their mixtures with nonionic surfactants remain poorly characterised. Experiments: To facilitate characterisations by SANS and NMR, we used nonionic surfactant hexaethylene glycol monododecyl ether (C12E6), anionic sodium dodecylsulphate (SDS), cationic dodecyltrimethylammonium bromide (DTAB) and zwitterionic dodecylphosphocholine (C12PC) as model adjuvant systems to solubilise 3 pesticides, Cyprodinil (CP), Azoxystrobin (AZ) and Difenoconazole (DF), representing different structural features. The investigation focused on the influence of solubilisates in driving changes to the micellar nanostructures in the absence or presence of electrolytes. NMR and NOESY were applied to investigate the solubility and location of each pesticide in the micelles. SANS was used to reveal subtle changes to the micellar structures due to pesticide solubilisation with and without electrolytes.
⇑ Corresponding author. E-mail address: [email protected] (J.R. Lu). https://doi.org/10.1016/j.jcis.2020.10.083 0021-9797/Ó 2020 Published by Elsevier Inc.
Please cite this article as: X. Hu, H. Gong, P. Hollowell et al., , Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2020.10.083
X. Hu, H. Gong, P. Hollowell et al. NMR
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Findings: Unlike nonionic surfactants, the ionic and zwitterionic surfactant micellar structures remain unchanged upon pesticide solubilisation. Electrolytes slightly elongate the ionic surfactant micelles but have no effect on nonionic and zwitterionic surfactants. Pesticide solubilisation could alter the structures of the binary mixtures of ionic/zwitterionic and ionic/nonionic micelles by causing elongation, shell shrinkage and dehydration, with the exact alteration being determined by the molar ratio in the mixture. Ó 2020 Published by Elsevier Inc.
(C12E6), this study examines how the same 3 model pesticides, Cyprodinil (CP), Azoxystrobin (AZ) and Difenoconazole (DF) become solubilised into 4 surfactant micelles and their binary mixtures with nonionic surfactants. The surfactants are nonionic C12E6, anionic sodium dodecylsulphate (SDS), cationic dodecyltrimethylammonium bromide (DTAB) and zwitterionic dodecyl phosphocholine (C12PC). The chemical structures of 3 pesticides and 4 types of surfactants are listed in Fig. 1 with different types of hydrogens marked with letters and numbers for nuclear magnetic resonance (NMR) interpretation, where A denotes hydrogens on AZ, P denotes hydrogens on CP, F denotes hydrogens on DF, C denotes hydrogens on C12E6, S denotes hydrogens on SDS, D denotes hydrogens on DTAB and N denotes hydrogens on C12PC. Some of the properties of the pesticides and surfactants are listed in Table S1 and Table S2, respectively. The Log(Pow) values given in Table S1 are in the increasing order for AZ, CP and DF, showing an increasing trend of hydrophobicity, reflecting a delicate balance between molecular structures, size, the number of aromatic rings and polar O and N atoms incorporated. Several techniques including proton nuclear magnetic resonance (1H NMR), proton nuclear Overhauser effect spectroscopy (NOESY), and small angle neutron scattering (SANS) were applied to examine the changes in the micellar structures of C12E6, SDS, DTAB and C12PC before and after pesticide solubilisation in the absence and presence of sodium chloride and the structural changes in micelles of the binary mixtures of SDS, DTAB, and C12PC with C12E6 at different mixing molar concentrations before and after pesticide saturation. Our study has illustrated the electrolyte effects on charged surfactant micellar structures and different mixing ratio effects on charged-uncharged mixed surfactant micelles upon pesticide solubilisation, providing an improved understanding on pesticide formulation and pest management from surface and colloid science.
1. Introduction Pesticide formulations and sprays used in modern agriculture and horticulture normally contain, apart from the active ingredients, a certain amount of other compounds known as adjuvants [1]. Adjuvants are used to help enhance pesticide efficiency via several different mechanisms including increased spray retention on leaves [2,3], higher solubility in foliar deposits, greater mobility in leaf cuticles [4,5,2,3], better droplet coverage [6] and pesticide transport across plant membranes [7–9]. The adjuvant comprises a large group of substances with surfactants, especially nonionic surfactants, making up the largest group [10]. Commercial products are often supplied as emulsifiable concentrates composed of several different surfactants and can be easily diluted in water upon spraying. When dispersed in water, the amphiphilicity of surfactants drives the molecules to self-assemble into micelles normally constituted of a hydrophobic core and a hydrophilic shell when above the critical aggregation concentration (CAC). The core-shell structure provides a vehicle for pesticides, which are typically insoluble in water, to penetrate or attach to. The micelles or related colloids enable the solubilised pesticides to freely move in the solvent. Thus, a better understanding of the surfactantpesticide interaction can provide constructive suggestions to make pesticide formulations more effective at pesticide loading and release. Our previous investigations [11,12] have revealed that pesticides have a tendency to penetrate into the nonionic surfactant micelles with the exact solubility and locations being determined by their amphiphilicity and structural configuration. Pesticide solubilisation can alter the effective amphiphilicity of nonionic surfactants and subsequent micellar physicochemical behaviours significantly including the phase boundary of the mixture, the shape and size of the micelles. These alterations are governed by the amount, location and amphiphilic nature of the pesticides inserted inside the micelles. Although the amount and penetration of different pesticides inside micellar core and shell vary, the dissolution of pesticides generally leads to micellar elongation, shape transition from spherical to cylindrical, dehydration of ethoxylate groups and a decline in the cloud point. Although nonionic surfactant is the most prolific adjuvant group used in pesticide formulation, it is still beneficial to seek the possibility and feasibility of introducing other types of surfactants into pesticide formulations. Charged surfactants including both anionic and cationic surfactants and zwitterionic surfactants could be potential candidates. Though some of these surfactants may bear relatively higher costs and toxicity than nonionic surfactants [13], their physicochemical properties can be easily mediated by adding charged materials including salts, polymers or proteins [14]. A careful selection of these surfactants and their usage with nonionic ones may not only mitigate the costs but also benefit future smart pesticide delivery and controlled release [15,16]. However, our understanding of the interactions of pesticides with charged surfactants and charged-nonionic surfactant mixtures remains limited. Building on our previous investigations [12] which characterised how different pesticides become solubilised into a typical nonionic surfactant hexaethylene glycol monododecyl ether
2. Experimental section 2.1. Materials and pesticidal formulations Nonionic surfactant C12E6 (99% pure, Sigma Aldrich Co. Ltd), zwitterionic surfactant C12PC (99% pure, Sigma Aldrich Co. Ltd), and D2O (99.9% atom D%, Sigma Aldrich Co. Ltd) were used without any further purification. Anionic surfactant SDS (99% pure, Sigma Aldrich Co. Ltd) and cationic surfactant DTAB (99% pure, Sigma Aldrich Co. Ltd) were purified by recrystallization. 5 g of SDS and 100 ml of ethanol were added into a 300 ml conical flask, followed by heating up to 60–70 °C under gentle swirling. De-ionised water was then dropped in whilst the temperature was kept constant, with a minimal amount of water to help dissolve all SDS. The solution was then left to cool to room temperature whilst the crystals } chnel were growing out. The crystals were then collected using a Bu funnel filtration setup. The recrystallization process was repeated 3 times to ensure no visual minimum around the CAC in the surface tension plot. Similar purification process was performed on DTAB, by first heating up 100 ml of acetone containing 5 g of DTAB, followed by adding just enough drops of absolute ethanol to enable full DTAB dissolution. The fully dissolved solution was then cooled 2
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Fig. 1. Molecular structures of 3 pesticides and 4 surfactants with specific hydrogens marked to aid the interpretation of the NMR spectra.
RST method to determine the interaction parameter, as shown by Figure S1. A detailed description of RST and RST fitting is included in Section B of the Supporting Information. NMR: Spectra were recorded on a Bruker Avance III B400 spectrometer (5 mm BBO probe) operating at 400 MHz for 1H NMR at 20 °C. The solubility of the pesticides in the micelles was then calculated using the previous method [12]. 2D NMR NOESY spectroscopy was used to locate the pesticide molecules in the micelles. Pesticide dispersions were measured on a Bruker Avance II + B500 spectrometer (5 mm BBO probe) operating at 500 MHz at 20 °C. SANS: Measurements were performed on the Larmor instrument at the ISIS Pulsed Neutron Source (STFC Rutherford Appleton Laboratory, Didcot, UK). The surfactant samples with/without solubilised pesticides in the absence or presence of NaCl were filled in 2 mm path-length quartz cells (Hellma GmbH, Type 120) and measured at 20 °C. The raw SANS data were reduced using the Mantid framework [21] and corrected following the standard procedures for the instrument. Least-squares fitting analysis to a core– shell ellipsoid model [22] using the SasView software version 4.1.2 was used to interpret the reduced data.
down to room temperature to allow recrystallization. This purification process was also repeated 3 times to achieve the high level of purity, also confirmed by the surface tension measurement. Cyprodinil (CP) (95%, Sigma Aldrich Co. Ltd), Azoxystrobin (AZ) (95%, Sigma Aldrich Co. Ltd), Difenoconazole (DF) (95%, Sigma Aldrich Co. Ltd) and sodium chloride NaCl (99% pure, Sigma Aldrich Co. Ltd) were used without any further purification. The nonionic surfactant C12E6, anionic surfactant SDS, cationic surfactant DTAB, and zwitterionic surfactant C12PC were dissolved in D2O at a concentration of 25 mM in the absence and presence of 100 mM NaCl respectively, for SANS and related studies. The CAC values for different surfactants in the absence of salt are listed in Table S2. 25 mM SDS/DTAB-C12E6 mixtures were prepared at 25/75, 50/50 and 75/25 mol% in the presence of 100 mM NaCl to test if the solutions follow the ideal mixing according to the regular solution theory (RST). A comprehensive description of this method investigating surfactant binary mixing has been covered in many open literatures [17–19]. 25 mM C12PC-C12E6 mixtures were made at the same ratios, also in the presence of 100 mM NaCl for comparison. Excessive pesticides were added to surfactant solutions as prepared above under stirring to ensure full dissolution and equilibration and the conditions and processes utilised for model pesticide formulations were kept the same as reported previously [12]. The pD values for all dispersions used in this study were mediated at 7.4.
3. Results and discussion 3.1. Pesticide solubility in C12E6/SDS/DTAB/C12PC micelles with/ without NaCl Pesticide solubility in the micellar solutions of C12E6, SDS, DTAB and C12PC in the absence or presence of sodium chloride was investigated by 1H NMR, with the pesticide solubility values shown in Fig. 2. All pesticides are well solubilised into micelles because their solubility values in the micellar solutions are significantly higher than in pure water as shown in Table S1. Clearly, pesticide solubility in different surfactant micelles varies greatly, indicating a combination of changes in surfactant head groups and chemical nature of the pesticides. In general, more hydrophobic pesticides CP and DF have higher solubility than AZ across all surfactant micelles. In our previous studies, we showed that pesticide solubility in nonionic surfactant micelles was determined by their hydrophobicity, i.e., pesticides with higher hydrophobicity have higher solubility [12]. This trend remains true for the ionic and zwitterionic surfactants as well, but only in a broad sense. Note that the Log(Pow) values for DF, CP and AZ are in the decreasing
2.2. Measurements Surface tension: A Krüss K11 tensiometer with a du Noüy ring method [20] was used to determine the CAC values for both pure and mixed surfactant systems at 20 °C. The exerted force associated with the surface tension of a sample was measured as a function of the sample concentration. Above CAC, the force is relatively constant as the interface was saturated with surfactant molecules. Below CAC, the surface tension reading rose with surfactant concentrations because the surface was no longer saturated with the surfactant molecules. Each set of experiments was performed from high to low concentrations, and the concentration where the corresponding surface tension starts to change is the CAC value. The CAC values for pure and mixed surfactant systems in the presence of salt are listed in Table S3. The CAC values were fitted using the 3
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Fig. 2. Solubility of AZ, CP and DF in C12E6, SDS, DTAB and C12PC micelles at a total surfactant concentration of 25 mM with/without NaCl (100 mM).
are listed in Table 1. The CAC values for different binary surfactant ratios listed in Table S3 and the best RST fits shown in Figure S1 are in line with similar binary surfactant mixtures as reported in open literature [19,23]. They have the values of interaction parameter, b of 0.3, 1.0 and 1.2, indicating that all 3 binary surfactant systems have weak attractive interactions. Pesticide solubility study shows two different types of responses to mixing. Solubility values in the single component micelles of C12PC and C12E6 are higher than their binary mixtures, showing ‘‘U” shape solubility profiles for the 3 pesticides against the molar ratio. In contrast, solubility values in the single component DTAB micelles are clearly lower than those from the C12E6 micelles. Mixing leads to a steady rise of solubility to the rising fraction of C12E6, but the rate of rise appears to be influenced by the hydrophobicity of the pesticide. For the most hydrophobic DF, the solubility shows an immediate rise upon mixing with the smallest molar fraction of 0.25 of C12E6. For CP with lower hydrophobicity, there is a delayed response of change in solubility with the rising molar fraction of C12E6, with noticeable rise occurring only at 0.75 as shown in Table 1. For the least hydrophobic AZ, mixing of C12E6 with DTAB does not cause measurable change at all in its solubility. In the case of SDS mixing with C12E6, the responses of solubility to mixing with C12E6 occur in both patterns as described above. Whilst the solubility of the most hydrophobic DF displays a ‘‘U” shape profile against the rising ratio of C12E6, that for CP with the medium hydrophobicity shows a steady rise with increase C12E6 fraction. For AZ with the lowest hydrophobicity, however, mixing of C12E6 even at a molar fraction of 0.25 causes a drop of solubility to the same value as that from single component C12E6, indicating the dominant impact from the nonionic surfactant.
order, thus a decreasing trend of hydrophobicity (Table S1). The order of solubility of the 3 pesticides in the SDS micelles does follow this trend exactly, i.e., the solubility for DF in SDS is 4.36 mM, that for CP is 1.56 mM and that for AZ is 1.11 mM, respectively, and the exact same order is also followed in the presence of salt. However, for the rest of the surfactant systems studied, CP has the highest solubility, followed by DF and then AZ. The reason for such small disorder remains unclear, but this may simply mean that use of Log(Pow) values as an indicator of hydrophobicity has limitations when applied to predict the solubility in surfactant micelles. The highest solubility for DF was achieved from the SDS micelles. Addition of salt made a small but measurable increase in its solubility, suggesting the influence of charge screening. Nonionic C12E6 offers the second highest DF solubility, followed by DTAB and C12PC micelles. Again, salt addition in DTAB can slightly increase DF solubility, but has little effect in the case of nonionic and zwitterionic surfactants as expected. In contrast, nonionic C12E6 micelles produced the highest CP solubility, followed by C12PC, DTAB and SDS micelles. All surfactants offered low AZ solubility, but amongst them SDS appears to be the best. Pesticides like AZ are generally difficult to dissolve. The huge variations in solubility as observed here offer useful indications for us to further explore how to design micellar environments for better solubility enhancement. CP and DF solubility in the ionic surfactant micelles of SDS and DTAB increases with salt addition whilst AZ solubility remains the lowest and does not have a clear change upon salt addition. The lack of salt effect on pesticide solubility in the nonionic and zwitterionic surfactant micelles of C12E6 and C12PC is due to the lack of changes in surfactant properties and micellar structures. In contrast, the CACs of ionic surfactants generally decrease significantly in the presence of salts while salt addition has little impact on the CACs of the nonionic and zwitterionic surfactants. In pure water, SDS and DTAB have CAC values of 8.2 mM and 14.2 mM (Table S2), respectively. Addition of salt made their CACs drop to below 4 mM. Thus, the amount of increase of solubilised CP and DF in SDS and DTAB micelles in the presence of NaCl could be attributed to the increase in the number of micelles due to CAC drop and micellar structure changes.
3.3. Pesticide locations in surfactant micelles The location of pesticide molecules in the micelles was studied by NOESY, with the NOESY spectra in the chemical shift range of 0– 8 ppm measured from SDS interactions with CP, AZ and DF being presented in Fig. 3 (a)-(c), respectively. The NOESY spectra for interactions between DTAB and C12PC with the 3 pesticides are presented in Figure S2 (a)-(f). The hydrogens present in CP were assigned as P1: ~7.61 ppm; P2: ~7.05 ppm; P3: ~6.71 ppm; P4: ~6.39 ppm; P5: ~1.67 ppm; P6: ~0.84 ppm and P7: ~2.14 ppm. The hydrogens present in AZ were assigned as A2: ~7.91 ppm; A3-A10: ~7.23–7.65 ppm and A11: ~6.22 ppm. The hydrogens present on DF were assigned as F2: ~7.94 ppm; F3: ~7.55 ppm; F4-F6:
3.2. Pesticide solubility in binary, ionic-nonionic and zwitterionicnonionic mixed surfactant micelles Pesticide solubility values from the micellar solutions of the binary SDS/DTAB/C12PC-C12E6 mixtures at different molar ratios 4
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Table 1 Solubility of AZ, CP and DF in binary SDS/DTAB/C12PC-C12E6 mixed micelles with different molar fractions of C12E6 in the presence of 100 mM NaCl. The solubility values, in mM, have an error of ± 5%. Surfactants
SDS
DTAB
C12PC
Solubilised pesticides/mM
AZ CP DF AZ CP DF AZ CP DF
Molar ratio of C12E6 against SDS/DTAB/C12PC 0/100
25/75
50/50
75/25
100/0
1.10 2.03 4.86 0.19 2.57 0.80 0.28 2.18 1.36
0.35 2.05 2.35 0.14 2.45 1.16 0.14 1.71 0.99
0.35 2.35 1.76 0.14 2.54 1.40 0.14 2.12 1.21
0.35 2.87 1.98 0.14 3.07 1.43 0.15 2.79 1.46
0.35 3.90 2.57
Fig. 3. (a)-(c): NOESY spectra for CP, AZ and DF in SDS micelles; (d): Schematic depiction of the locations for CP, AZ and DF in SDS/DTAB/C12PC micelles.
~6.87–7.16 ppm and F7: ~7.32 ppm. The hydrogens on SDS were assigned as S1: ~4.20 ppm; S2: ~1.70 ppm; S3: ~1.20 ppm and S4: ~0.80 ppm. Unlike DTAB and C12PC, however, there is no hydrogen on SDS head group. This means that our previous method [12] of looking at interactions of pesticide hydrogens with hydrogens on surfactant tail and head separately cannot be applied simply to the SDS scenario. On the other hand, surfactant head groups are always hydrated and pesticide solubility in pure water is negligible compared to its solubility in micelles. The coupling between pesticide hydrogens and surfactant head hydrogens can be replaced by the interaction between pesticide hydrogens and the tiny amount of H2O in D2O as hydration. The hydrogen present
on H2O was assigned as 4.80 ppm. P1-4/A2-A11/F2-F7 interactions with H2O (head) and S4 (tail) are shown in circles 1 and 2, respectively. CP and AZ bear comparable signal intensities when interacting with surfactant head and tail, whilst DF hardly interacts with the head. These observations indicate that the most favourable locations for CP and AZ in SDS micelles are at the interface whilst that for DF is in the core. These different locations as dissolved in the SDS micelles are schematically illustrated in Fig. 3 (d). Similarly, in the DTAB and C12PC micelles, DF locates in the core and CP and AZ stay at the interface. Thus, pesticide locations in the charged surfactant micelles are highly consistent with what have been observed when they were dissolved in nonionic surfactants 5
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the micelle is a long ellipsoid. The best fitted parameters for all SANS profiles are shown in Table 2. The shape of the SDS micelles in pure water is close to a round sphere as the ellipticity is 1.15. The shell was fitted to 3.8 Å in thickness with 82% of the total volume being occupied by water. This is consistent with the previous observation that the SDS head group is small in size but highly hydrated [26]. The net charge per micelle was found to be 8.5e and the aggregation number at 56. In general, pesticide solubilisation did not change the shell thickness, hydration or net charge. Solubilisation of AZ and CP had a negligible effect on the micellar size and aggregation number, due to the relatively low level of pesticide molecules solubilised. DF solubilisation elongated the micelle slightly and as a result, the aggregation number increased. In the presence of 100 mM NaCl, the SDS micellar length and aggregation number increased but its shell thickness and hydration were not affected. NaCl shields the charge interaction in surfactant head groups, leading to an effective net charge per micelle of 0. AZ and CP solubilisation still had little effect on micellar size or shape. However, DF solubilisation nearly doubled the micellar length. As a result, the aggregation number increased from 74 to 117. A similar trend of changes was observed for the cationic DTAB surfactant. The ellipticity for the DTAB micelles in pure water is 1.21, with the shell thickness of 4.2 Å and hydration level of 50%, consistent with the findings reported previously [27]. The aggregation number as determined for the DTAB micelles was 59 and the net charge per micelle was + 4.7. Solubilisation of AZ and DF did not alter the micellar structure while minor elongation was detected upon CP solubilisation. Again, salt addition shielded the charge interaction and slightly elongated the micellar shape, consistent with the altered interfacial packing. In the case of the zwitterionic surfactant, the structural features for C12PC have been well established [28]. The C12PC micelles are nearly spherical, with the shell thickness of 9.0 Å and hydration level of 65%. Each zwitterionic micelle carries zero net charge and is comprised of 54 molecules. Pesticide solubilisation and salt addition almost have no effect on C12PC micellar structure.
[12]. Pesticide locations in surfactant micelles are clearly determined by their hydrophobicity, with more hydrophobic pesticides staying closer to the micellar core and less hydrophobic pesticides being distributed into the shell region. The NOESY spectra measured from interactions of SDS/DTAB/ C12PC and their binary mixtures with C12E6 at different molar ratios with CP, AZ and DF in the presence of NaCl are also presented in Figures S3-S6. It is clear that interaction intensities have not changed within error by adding salts or when mixed with nonionic surfactants. The pesticide locations in micelles are not affected by electrolytes or surfactant mixing based on the NOESY spectra alone. This observation indicates that the pesticide location is dominated by the main chemical and amphiphilic nature of the pesticides and their interactions with surfactants. 3.4. Micellar structural changes upon solubilisation of pesticides for single component surfactant micelles in the absence or presence of sodium chloride Changes in the structures of the micelles before and after pesticide solubilisation in the absence or presence of sodium chloride were investigated by SANS. All SANS data were fitted into a core–shell ellipsoid model with the Hayter-Penfold Rescaled Mean Spherical Approximation (hayter_msa) structure factor [24,25]. Full details of the SANS fitting method is illustrated by Figure S7 and described in Section C of the Supporting Information, and listing of scattering length densities (SLDs) of all materials involved in this experiment is presented in Table S4. The representative SANS profiles for SDS, DTAB and C12PC micelles with or without solubilised pesticide in the absence or presence of sodium chloride are presented in Fig. 4 (a)-(f). The SANS profiles for C12E6 with or without solubilised pesticides in the absence or presence of sodium chloride are shown in Figure S8. As all surfactant molecules have the same tail length (12 units of C), the minor axis of the core radius is fixed to 16 Å. The ellipticity denotes the core major radius divided by the minor radius. For a spherical micelle, this value is 1. If the value is much higher than 1, this indicates that
Fig. 4. SANS profiles for SDS (a, b), DTAB (c, d) and C12PC (e, f) micelles with/without solubilised pesticides in the absence (a, c, e) or presence (b, d, f) of 100 mM sodium chloride. For better visualization, the scattering intensity data were shifted vertically by multiplying factors: 1, 2, 4 and 8 (from bottom to top). The continuous lines represent the best fits with structural parameters listed in Table 2. 6
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Table 2 Structural parameters for the micelles of SDS, DTAB, C12PC and C12E6 from the best fits to the SANS profiles from the core–shell ellipsoid model using the hayter_msa structure factor before and after solubilisation of pesticides AZ, CP and DF with and without 100 mM NaCl. The core inner radius was fixed as 16 Å. The ellipticity represents the ratio of core major radius against core minor radius. P/S molar ratio stands for the molar ratio of pesticide against surfactant. Surfactants
Ellipticity ± 0.05
Core SLD x10-6Å
(a)SDS +AZ +CP +DF (b)SDS + NaCl +AZ +CP +DF (c)DTAB +AZ +CP +DF (d) DTAB + NaCl +AZ +CP +DF (e)C12PC +AZ +CP +DF (f)C12PC + NaCl +AZ +CP +DF (g)C12E6 +AZ +CP +DF (h)C12E6 + NaCl +AZ +CP +DF
1.15 1.18 1.20 1.54 1.52 1.56 1.58 3.21 1.21 1.23 1.32 1.25 1.25 1.27 1.41 1.33 1.10 1.11 1.15 1.14 1.10 1.11 1.15 1.14 3.55 3.57 17.3 16.6 3.55 3.57 17.3 16.6
0.40 0.21 0.26 0.32 0.40 0.20 0.22 0.41 0.40 0.37 0.29 0.28 0.40 0.37 0.17 0.27 0.40 0.35 0.21 0.17 0.40 0.35 0.21 0.18 0.40 0.34 0.07 0 0.40 0.34 0.07 0
1
Shell thickness ± 0.5 Å
Shell hydration ± 0.05
Shell SLD x10-6Å
3.8
0.82
5.0
1
Micellar net charge (e) ± 0.3 8.5
0
4.2
0.50
3.0
+4.7
0
9.0
11.0 11.0 9.0 9.0 11.0 11.0 9.0 9.0
0.70
0.68 0.68 0.38 0.37 0.68 0.68 0.38 0.37
4.8
4.5 4.5 2.8 2.7 4.5 4.5 2.8 2.7
0
0
Aggregation number ± 5 56 56 56 61 74 74 73 117 59 59 62 59 62 62 66 62 54 54 54 54 54 54 54 54 173 174 811 739 173 174 811 739
P/S molar ratio ± 0.004 0.042 0.062 0.174 0.044 0.081 0.194 0.007 0.052 0.029 0.007 0.103 0.032 0.012 0.086 0.055 0.011 0.087 0.054
± ± ± ± ± ± ± ±
10 10 30 30 10 10 30 30
0.016 0.169 0.108 0.015 0.171 0.105
Effects of pesticide solubilisation on the structures of the binary charged-nonionic mixed surfactant micelles were also investigated by SANS. As shown in Table 3, the most hydrophobic DF resulted in the highest solubilisation in the SDS-C12E6 micelles and the largest micellar shape change as indicated by the ellipticity increase, followed by CP and AZ. As the molar fraction of C12E6 increases its influence on micellar shape, causing the amount of pesticide solubilisation to increase. Similar to pesticide solubilisation in nonionic surfactant micelles, solubilisation of pesticides also shrank and dehydrated the micellar shell with the smallest effect from AZ due to its lowest solubilisation. Although data from the binary DTAB-C12E6 and C12PC-C12E6 mixed micelles do not show a simple correlation to the order of hydrophobicity from the pesticides, the main trends are similar to the observations from the SDS-C12E6 mixed micelles as shown in Table S5.
Because of the higher surfactant concentration used in this work (25 mM against 6.7 mM used previously), the micellar length of C12E6 is higher than that obtained from our previous study [12]. This change was shown in the ellipticity derived from the core– shell model. The micellar structural changes caused by pesticide solubilisation agree well with our previous results, i.e., CP and DF solubilisation elongated the micelles, shrank and dehydrated the shell while AZ solubilisation had no observable effect. Analogous to the zwitterionic surfactant, salt addition has little influence on micellar structure or subsequent solubilisation of pesticide. 3.5. Micellar structural changes upon solubilisation of pesticides for mixed surfactant micelles The SANS profiles for the binary SDS-C12E6 micelles were measured for the molar ratio range of 25/75, 50/50 and 75/25 before and after pesticide solubilisation, as depicted in Fig. 5 (a)-(c), with the best fitted parameters listed in Table 3. The SANS profiles for the binary DTAB-C12E6 and C12PC- C12E6 micelles in the same molar ratio range before and after pesticide solubilisation together with their best fits are shown in Figure S9. As with the single component micelles, all SANS profiles measured from the binary mixtures could be well fitted well with the core–shell ellipsoid model. Clearly, when more C12E6 is added in the mixed micelles, micellar length and shell thickness tend to increase whilst shell hydration decreases. The structure of the SDS-C12E6 mixed micelles gradually transferred to C12E6 micelles as more C12E6 was introduced. The evolution of micellar size with composition in SDS-C12E6 mixed systems was not clear at low concentrations (below 10 mM) [17] but similar trend was reported at high concentration (at 25 mM) [18].
3.6. Mechanistic processes underlying pesticide solubilisation In spite of extensive use of surfactants as adjuvants in agrisprays, there is currently little scientific research reported in open literature to describe how different types of surfactant and their mixtures affect pesticide solubilisation [10,29]. Following our previous work on the investigation of the effect of pesticide solubilisation on the micellar structures of nonionic surfactants [11,12,30], this study has focused on the influence of surfactant types. 4 model surfactants were chosen in this work to have the same tail length (Fig. 1) but different head groups so that the effects of their head groups on micellar structures alone and in binary mixtures and on solubilisation of the pesticides can be examined. The concentrations of the surfactants were set at 25 mM, well above their CACs. The 3 pesticides, with different molecular structures and 7
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Fig. 5. SANS profiles measured from the binary SDS-C12E6 mixed micelles with and without of solubilised pesticides at different molar ratios of SDS/C12E6: (a) 75/25, (b) 50/50 and (c) 25/75. The continuous lines represent the best fits with structural parameters listed in Table 3. For better visualization, the SANS data were shifted vertically by multiplying factors: 1, 2, 4 and 8 (from bottom to top).
Table 3 Structural parameters obtained from the best fits to the SANS profiles measured from the binary SDS-C12E6 micelles mixed at different molar ratios of SDS:C12E6 using the core– shell ellipsoid model. The core inner radius was fixed as 16 Å. The ellipticity represents the ratio of core major radius against core minor radius. P/S molar ratio represents the molar ratio of pesticide against surfactant. Surfactants
Ellipticity ± 0.03
(a) 75 mol%SDS +AZ +CP +DF (b) 50 mol%SDS +AZ +CP +DF (c) 25 mol%SDS +AZ +CP +DF
1.27 1.29 2.69 2.93 1.76 1.76 4.95 3.20 2.60 2.62 13.71 13.30
Core SLD 10 0.40 0.34 0.22 0.01 0.40 0.34 0.19 0.11 0.40 0.34 0.15 0.07
6
Å
1
Shell thickness ± 0.5 Å
Shell hydration ± 0.05
Shell SLD 10
5.6 5.6 4.8 4.8 7.5 7.5 6.4 6.4 9.2 9.2 7.7 7.7
0.75 0.75 0.65 0.65 0.75 0.75 0.60 0.60 0.68 0.68 0.53 0.54
5.7 5.7 5.5 5.5 5.7 5.7 5.3 5.3 5.6 5.6 5.1 5.2
6
Å
1
Aggregation number ± 5 62 62 126 127 86 86 233 ± 10 143 127 127 647 ± 20 592 ± 20
P/S molar ratio ± 0.004 0.014 0.082 0.094 0.014 0.094 0.070 0.014 0.115 0.079
Binary mixing between SDS and C12E6 has revealed broadly proportional influences from components but the pattern of mixing between C12PC and C12E6 is characterised by the occurrence of a broad minimum, indicating unfavourable mixing toward solubilisation. SANS measurements revealed that whilst low pesticide solubilisation does not cause much structural change of surfactant micelles, high pesticide solubilisation tends to transform micelles into ellipsoidal shape, in much the same way as observed from nonionic surfactants. The structural changes for binary SDS-C12E6 mixed surfactant micelles due to pesticide solubilisation in the absence or presence of electrolytes are schematically shown in Fig. 6. Solubilisation of
hydrophobicity, achieved the highest solubility in the SDS micelles, followed by C12E6, C12PC and DTAB. It remains unclear as to how this order of sequence is formed, but micellar environments are thought to be the dominant factor in determining the amount and location of solubilised pesticides. NOESY spectral analysis revealed that the more hydrophobic DF molecules are dissolved in the micellar core whilst the less hydrophobic CP and AZ molecules are dispersed at the core–shell interface and shell. Different head types and the screening effect on the ionic heads from salt addition clearly affect how heads are packed in the shell and the core–shell interfacial region, affecting the amount of pesticides solubilised.
Fig. 6. Structural changes for SDS-C12E6 surfactant micelles mixed with gradually increased SDS mole fraction (from 0 to 1) in the presence of pesticide such as DF. The micelle changed from a long cylinder to a short cylinder and finally to a sphere with increased SDS mole fraction in the mixture. 8
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contains code developed with funding from the EU Horizon 2020 programme under the SINE2020 project Grant No 654000. HG and ML thank the studentships provided by China China Scholarship Council. ZL thanks BBSRC and AstraZeneca for a joint funding under BB/S018492/1.
pesticides into single component SDS micelles does not cause much structural changes but electrostatic screening reduces effective head group area. This results in slightly increased ellipticity of the micelles alone and further ellipticity increase upon pesticide solubilisation associated with increased aggregation numbers. Mixing with C12E6 shows distinct impact from the nonionic surfactant on micellar elongation and this trend is broadly followed upon pesticide solubilisation, with more hydrophobic pesticide solubilising more and causing further micellar elongation. Cationic and zwitterionic surfactants alone and their binary mixtures broadly follow the main trends set by SDS and binary SDS-C12E6 mixtures in pesticide solubilisation, but the extent is far lower. In spite of the weaker extent of changes in their associated structures, the varieties of structural changes are rather rich, inherent of the physiochemical properties of the surfactant heads and pesticides.
Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2020.10.083. References [1] M.J. Castro, C. Ojeda, A.F. Cirelli, Advances in surfactants for agrochemicals, Environ. Chem. Lett. 12 (1) (2014) 85–95. [2] L. Zheng, C. Cao, L. Cao, Z. Chen, Q. Huang, B. Song, Bounce Behavior and Regulation of Pesticide Solution Droplets on Rice Leaf Surfaces, J. Agric. Food. Chem. 66 (44) (2018) 11560–11568. [3] M. Song, D. Hu, X. Zheng, L. Wang, Z. Yu, W. An, R. Na, C. Li, N. Li, Z. Lu, Enhancing droplet deposition on wired and curved superhydrophobic leaves, ACS Nano 13 (7) (2019) 7966–7974. [4] L. Schreiber, M. Riederer, K. Schorn, Mobilities of organic compounds in reconstituted cuticular wax of barley leaves: effects of monodisperse alcohol ethoxylates on diffusion of pentachlorophenol and tetracosanoic acid, Pestic. Sci. 48 (2) (1996) 117–124. [5] L. Wang, X. Li, G. Zhang, J. Dong, J. Eastoe, Oil-in-water nanoemulsions for pesticide formulations, J. Colloid Interface Sci. 314 (1) (2007) 230–235. [6] M.B. Ellis, C. Tuck, P. Miller, How surface tension of surfactant solutions influences the characteristics of sprays produced by hydraulic nozzles used for pesticide application, Colloids Surf., A 180 (3) (2001) 267–276. [7] M. Riederer, M. Burghardt, S. Mayer, H. Obermeier, J. Schoenherr, Sorption of monodisperse alcohol ethoxylates and their effects on the mobility of 2, 4-D in isolated plant cuticles, J. Agric. Food. Chem. 43 (4) (1995) 1067–1075. [8] T. Yang, B. Zhao, A.J. Kinchla, J.M. Clark, L. He, Investigation of pesticide penetration and persistence on harvested and live basil leaves using surfaceenhanced Raman scattering mapping, J. Agric. Food. Chem. 65 (17) (2017) 3541–3550. [9] E. Pambou, X. Hu, Z. Li, M. Campana, A. Hughes, P. Li, J.R. Webster, G. Bell, J.R. Lu, Structural features of reconstituted cuticular wax films upon interaction with nonionic surfactant C12E6, Langmuir 34 (11) (2018) 3395–3404. [10] K. Krogh, B. Halling-Sørensen, B. Mogensen, K. Vejrup, Environmental properties and effects of nonionic surfactant adjuvants in pesticides: a review, Chemosphere 50 (7) (2003) 871–901. [11] F.N. Padia, M. Yaseen, B. Gore, S. Rogers, G. Bell, J.R. Lu, Influence of molecular structure on the size, shape, and nanostructure of nonionic CnEm surfactant micelles, J. Phys. Chem. B 118 (1) (2013) 179–188. [12] X. Hu, H. Gong, Z. Li, S. Ruane, H. Liu, E. Pambou, C. Bawn, S. King, K. Ma, P. Li, F. Padia, G. Bell, J.R. Lu, What happens when pesticides are solubilized in nonionic surfactant micelles, J. Colloid Interface Sci. 541 (2019) 175–182. [13] F. Pan, Z. Li, H. Gong, J.T. Petkov, J.R. Lu, Membrane-lytic actions of sulphonated methyl ester surfactants and implications to bactericidal effect and cytotoxicity, J. Colloid Interface Sci. 531 (2018) 18–27. [14] Z. Wang, P. Li, K. Ma, Y. Chen, J. Penfold, R.K. Thomas, D.W. Roberts, H. Xu, J.T. Petkov, Z. Yan, The structure of alkyl ester sulfonate surfactant micelles: The impact of different valence electrolytes and surfactant structure on micelle growth, J. Colloid Interface Sci. 557 (2019) 124–134. [15] Y. Pang, S. Wang, X. Qiu, Y. Luo, H. Lou, J. Huang, Preparation of lignin/sodium dodecyl sulfate composite nanoparticles and their application in pickering emulsion template-based microencapsulation, J. Agric. Food. Chem. 65 (50) (2017) 11011–11019. [16] Y. Li, D. Yang, S. Lu, S. Lao, X. Qiu, Modified lignin with anionic surfactant and its application in controlled release of avermectin, J. Agric. Food. Chem. 66 (13) (2018) 3457–3464. [17] J. Penfold, E. Staples, L. Thompson, I. Tucker, J. Hines, R. Thomas, J. Lu, N. Warren, Structure and composition of mixed surfactant micelles of sodium dodecyl sulfate and hexaethylene glycol monododecyl ether and of hexadecyltrimethylammonium bromide and hexaethylene glycol monododecyl ether, J. Phys. Chem. B 103 (25) (1999) 5204–5211. [18] J. Penfold, E. Staples, I. Tucker, Neutron small angle scattering studies of micellar growth in mixed anionic-nonionic surfactants, sodium dodecyl sulfate, SDS, and hexaethylene glycol monododecyl ether, C12E6, in the presence and absence of solubilized alkane, hexadecane, J. Phys. Chem. B 106 (34) (2002) 8891–8897. [19] J. Penfold, E. Staples, L. Thompson, I. Tucker, J. Hines, R. Thomas, J. Lu, Solution and adsorption behavior of the mixed surfactant system sodium dodecyl sulfate/n-hexaethylene glycol monododecyl ether, Langmuir 11 (7) (1995) 2496–2503. [20] C. Smith, Z. Li, R. Holman, F. Pan, R.A. Campbell, M. Campana, P. Li, J.R. Webster, S. Bishop, R. Narwal, Antibody adsorption on the surface of water studied by neutron reflection, mAbs, Taylor & Francis (2017) 466–475. [21] O. Arnold, J.-C. Bilheux, J. Borreguero, A. Buts, S.I. Campbell, L. Chapon, M. Doucet, N. Draper, R.F. Leal, M. Gigg, Mantid—data analysis and visualization
4. Conclusion Following our previous studies of the effects of pesticide solubilisation on the size, shape, interior structure and phase boundary of the nonionic surfactant micelles [11,12,30,31], this work has explored how pesticide solubilisation affects the micellar structures of ionic and zwitterionic surfactants and their binary mixtures with nonionic surfactant. We have demonstrated that SANS combined with NMR offers the appropriate sensitivity and resolution to follow the structural changes of single and binary surfactant micelles upon pesticide solubilisation in the absence or presence of electrolytes. Our results have revealed that pesticide solubility and location in the micelles are cooperatively affected by pesticide hydrophobicity, surfactant head type and micellar structure. In general, pesticide solubilisation can have very weak effects on the micellar structures of charged surfactants, but addition of electrolyte can enhance pesticide solubility in ionic surfactant micelles via electrostatic screening and cause small micellar elongation. In contrast, salt addition does not have any influence in nonionic or zwitterionic surfactant systems. Pesticide solubilisation can affect micellar structures of the binary charged-nonionic systems, though the extent of the influence is dependent on the amount of pesticide solubilised. This feature is similar to pesticide solubilisation in nonionic surfactant micelles, and the process is well characterised by micellar elongation, shell shrinkage and dehydration. The micellar length, shell thickness and hydration can be simply mediated by adjusting the molar ratio between charged and nonionic surfactants in the mixture. Our results clearly illustrate that anionic, cationic and zwitterionic surfactants are promising adjuvants for future agri-formulation science. A smart formulation exploiting surfactant mixing could be quite advantageous in controlled pesticide release and crop protection. Future work will examine how these pesticide formulations interact with plant waxes [9,32,33] to understand how waxes from different species control their release upon agri-spraying. Declaration of Competing Interest None. Acknowledgements We thank Syngenta for funding this work. We also thank the University of Manchester for a studentship to XH, EP, HG, ML, PH, HL, SR and ISIS Neutron Facility at Rutherford Appleton Laboratory, STFC for neutron beam times on Larmor with RB1920153. DOI for the SANS data is: https://doi.org/10.5286/ISIS.E.RB1920153. This work benefited from the use of the SasView application, originally developed under NSF Award DMR-0520547. SasView also 9
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(C12PC) surfactants studied by small-angle neutron scattering, Langmuir 31 (36) (2015) 9781–9789. A. Knowles, Recent developments of safer formulations of agrochemicals, Environmentalist 28 (1) (2008) 35–44. G. Bell, Non-ionic surfactant phase diagram prediction by recursive partitioning, Philosophical Transactions of the Royal Society A: Mathematical, Phys. Eng. Sci. 374 (2072) (2016) 20150137. X. Hu, H. Gong, Z. Li, S. Ruane, H. Liu, P. Hollowell, E. Pambou, C. Bawn, S. King, S. Rogers, K. Ma, P. Li, F. Padia, G. Bell, J.R. Lu, How does solubilisation of plant waxes into nonionic surfactant micelles affect pesticide release?, J. Colloid Interface Sci. 556 (2019) 650–657. E. Pambou, Z. Li, M. Campana, A. Hughes, L. Clifton, P. Gutfreund, J. Foundling, G. Bell, J.R. Lu, Structural features of reconstituted wheat wax films, J. R. Soc. Interface 13 (120) (2016) 20160396. X. Hu, E. Pambou, H. Gong, M. Liao, P. Hollowell, H. Liu, W. Wang, C. Bawn, J. Cooper, M. Campana, K. Ma, P. Li, J.R.P. Webster, F. Padia, G. Bell, J.R. Lu, How does substrate hydrophobicity affect the morphological features of reconstituted wax films and their interactions with nonionic surfactant and pesticide?, J. Colloid Interface Sci. 575 (2020) 245–253.
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Regular Article
What happens when pesticides are solubilised in binary ionic/zwitterionic-nonionic mixed micelles? Xuzhi Hu a, Haoning Gong a, Peter Hollowell a, Mingrui Liao a, Zongyi Li a, Sean Ruane a, Huayang Liu a, Elias Pambou a, Najet Mahmoudi b, Robert M. Dalgliesh b, Faheem Padia c, Gordon Bell c, Jian R. Lu a,⇑ a b c
Biological Physics Group, School of Physics and Astronomy, University of Manchester, Oxford Road, Manchester M13 9PL, UK STFC ISIS Facility, Rutherford Appleton Laboratory, Didcot OX11 0QX, UK Syngenta, Jealott’s Hill International Research Centre, Bracknell, Berkshire RG42 6EY, UK
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 19 August 2020 Revised 20 October 2020 Accepted 21 October 2020 Available online xxxx Keywords: Pesticide Surfactants Micellar structure Solubilisation Agri-spray Nanostructure Drug delivery Pesticide delivery SANS
a b s t r a c t Hypothesis: Surfactants have been widely used as adjuvants in agri-sprays to enhance the solubility of pesticides in foliar spray deposits and their mobility through leaf cuticles. Previously, we have characterised pesticide solubilisation in nonionic surfactant micelles, but what happens when pesticides become solubilised in anionic, cationic and zwitterionic and their mixtures with nonionic surfactants remain poorly characterised. Experiments: To facilitate characterisations by SANS and NMR, we used nonionic surfactant hexaethylene glycol monododecyl ether (C12E6), anionic sodium dodecylsulphate (SDS), cationic dodecyltrimethylammonium bromide (DTAB) and zwitterionic dodecylphosphocholine (C12PC) as model adjuvant systems to solubilise 3 pesticides, Cyprodinil (CP), Azoxystrobin (AZ) and Difenoconazole (DF), representing different structural features. The investigation focused on the influence of solubilisates in driving changes to the micellar nanostructures in the absence or presence of electrolytes. NMR and NOESY were applied to investigate the solubility and location of each pesticide in the micelles. SANS was used to reveal subtle changes to the micellar structures due to pesticide solubilisation with and without electrolytes.
⇑ Corresponding author. E-mail address: [email protected] (J.R. Lu). https://doi.org/10.1016/j.jcis.2020.10.083 0021-9797/Ó 2020 Published by Elsevier Inc.
Please cite this article as: X. Hu, H. Gong, P. Hollowell et al., , Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2020.10.083
X. Hu, H. Gong, P. Hollowell et al. NMR
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Findings: Unlike nonionic surfactants, the ionic and zwitterionic surfactant micellar structures remain unchanged upon pesticide solubilisation. Electrolytes slightly elongate the ionic surfactant micelles but have no effect on nonionic and zwitterionic surfactants. Pesticide solubilisation could alter the structures of the binary mixtures of ionic/zwitterionic and ionic/nonionic micelles by causing elongation, shell shrinkage and dehydration, with the exact alteration being determined by the molar ratio in the mixture. Ó 2020 Published by Elsevier Inc.
(C12E6), this study examines how the same 3 model pesticides, Cyprodinil (CP), Azoxystrobin (AZ) and Difenoconazole (DF) become solubilised into 4 surfactant micelles and their binary mixtures with nonionic surfactants. The surfactants are nonionic C12E6, anionic sodium dodecylsulphate (SDS), cationic dodecyltrimethylammonium bromide (DTAB) and zwitterionic dodecyl phosphocholine (C12PC). The chemical structures of 3 pesticides and 4 types of surfactants are listed in Fig. 1 with different types of hydrogens marked with letters and numbers for nuclear magnetic resonance (NMR) interpretation, where A denotes hydrogens on AZ, P denotes hydrogens on CP, F denotes hydrogens on DF, C denotes hydrogens on C12E6, S denotes hydrogens on SDS, D denotes hydrogens on DTAB and N denotes hydrogens on C12PC. Some of the properties of the pesticides and surfactants are listed in Table S1 and Table S2, respectively. The Log(Pow) values given in Table S1 are in the increasing order for AZ, CP and DF, showing an increasing trend of hydrophobicity, reflecting a delicate balance between molecular structures, size, the number of aromatic rings and polar O and N atoms incorporated. Several techniques including proton nuclear magnetic resonance (1H NMR), proton nuclear Overhauser effect spectroscopy (NOESY), and small angle neutron scattering (SANS) were applied to examine the changes in the micellar structures of C12E6, SDS, DTAB and C12PC before and after pesticide solubilisation in the absence and presence of sodium chloride and the structural changes in micelles of the binary mixtures of SDS, DTAB, and C12PC with C12E6 at different mixing molar concentrations before and after pesticide saturation. Our study has illustrated the electrolyte effects on charged surfactant micellar structures and different mixing ratio effects on charged-uncharged mixed surfactant micelles upon pesticide solubilisation, providing an improved understanding on pesticide formulation and pest management from surface and colloid science.
1. Introduction Pesticide formulations and sprays used in modern agriculture and horticulture normally contain, apart from the active ingredients, a certain amount of other compounds known as adjuvants [1]. Adjuvants are used to help enhance pesticide efficiency via several different mechanisms including increased spray retention on leaves [2,3], higher solubility in foliar deposits, greater mobility in leaf cuticles [4,5,2,3], better droplet coverage [6] and pesticide transport across plant membranes [7–9]. The adjuvant comprises a large group of substances with surfactants, especially nonionic surfactants, making up the largest group [10]. Commercial products are often supplied as emulsifiable concentrates composed of several different surfactants and can be easily diluted in water upon spraying. When dispersed in water, the amphiphilicity of surfactants drives the molecules to self-assemble into micelles normally constituted of a hydrophobic core and a hydrophilic shell when above the critical aggregation concentration (CAC). The core-shell structure provides a vehicle for pesticides, which are typically insoluble in water, to penetrate or attach to. The micelles or related colloids enable the solubilised pesticides to freely move in the solvent. Thus, a better understanding of the surfactantpesticide interaction can provide constructive suggestions to make pesticide formulations more effective at pesticide loading and release. Our previous investigations [11,12] have revealed that pesticides have a tendency to penetrate into the nonionic surfactant micelles with the exact solubility and locations being determined by their amphiphilicity and structural configuration. Pesticide solubilisation can alter the effective amphiphilicity of nonionic surfactants and subsequent micellar physicochemical behaviours significantly including the phase boundary of the mixture, the shape and size of the micelles. These alterations are governed by the amount, location and amphiphilic nature of the pesticides inserted inside the micelles. Although the amount and penetration of different pesticides inside micellar core and shell vary, the dissolution of pesticides generally leads to micellar elongation, shape transition from spherical to cylindrical, dehydration of ethoxylate groups and a decline in the cloud point. Although nonionic surfactant is the most prolific adjuvant group used in pesticide formulation, it is still beneficial to seek the possibility and feasibility of introducing other types of surfactants into pesticide formulations. Charged surfactants including both anionic and cationic surfactants and zwitterionic surfactants could be potential candidates. Though some of these surfactants may bear relatively higher costs and toxicity than nonionic surfactants [13], their physicochemical properties can be easily mediated by adding charged materials including salts, polymers or proteins [14]. A careful selection of these surfactants and their usage with nonionic ones may not only mitigate the costs but also benefit future smart pesticide delivery and controlled release [15,16]. However, our understanding of the interactions of pesticides with charged surfactants and charged-nonionic surfactant mixtures remains limited. Building on our previous investigations [12] which characterised how different pesticides become solubilised into a typical nonionic surfactant hexaethylene glycol monododecyl ether
2. Experimental section 2.1. Materials and pesticidal formulations Nonionic surfactant C12E6 (99% pure, Sigma Aldrich Co. Ltd), zwitterionic surfactant C12PC (99% pure, Sigma Aldrich Co. Ltd), and D2O (99.9% atom D%, Sigma Aldrich Co. Ltd) were used without any further purification. Anionic surfactant SDS (99% pure, Sigma Aldrich Co. Ltd) and cationic surfactant DTAB (99% pure, Sigma Aldrich Co. Ltd) were purified by recrystallization. 5 g of SDS and 100 ml of ethanol were added into a 300 ml conical flask, followed by heating up to 60–70 °C under gentle swirling. De-ionised water was then dropped in whilst the temperature was kept constant, with a minimal amount of water to help dissolve all SDS. The solution was then left to cool to room temperature whilst the crystals } chnel were growing out. The crystals were then collected using a Bu funnel filtration setup. The recrystallization process was repeated 3 times to ensure no visual minimum around the CAC in the surface tension plot. Similar purification process was performed on DTAB, by first heating up 100 ml of acetone containing 5 g of DTAB, followed by adding just enough drops of absolute ethanol to enable full DTAB dissolution. The fully dissolved solution was then cooled 2
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Fig. 1. Molecular structures of 3 pesticides and 4 surfactants with specific hydrogens marked to aid the interpretation of the NMR spectra.
RST method to determine the interaction parameter, as shown by Figure S1. A detailed description of RST and RST fitting is included in Section B of the Supporting Information. NMR: Spectra were recorded on a Bruker Avance III B400 spectrometer (5 mm BBO probe) operating at 400 MHz for 1H NMR at 20 °C. The solubility of the pesticides in the micelles was then calculated using the previous method [12]. 2D NMR NOESY spectroscopy was used to locate the pesticide molecules in the micelles. Pesticide dispersions were measured on a Bruker Avance II + B500 spectrometer (5 mm BBO probe) operating at 500 MHz at 20 °C. SANS: Measurements were performed on the Larmor instrument at the ISIS Pulsed Neutron Source (STFC Rutherford Appleton Laboratory, Didcot, UK). The surfactant samples with/without solubilised pesticides in the absence or presence of NaCl were filled in 2 mm path-length quartz cells (Hellma GmbH, Type 120) and measured at 20 °C. The raw SANS data were reduced using the Mantid framework [21] and corrected following the standard procedures for the instrument. Least-squares fitting analysis to a core– shell ellipsoid model [22] using the SasView software version 4.1.2 was used to interpret the reduced data.
down to room temperature to allow recrystallization. This purification process was also repeated 3 times to achieve the high level of purity, also confirmed by the surface tension measurement. Cyprodinil (CP) (95%, Sigma Aldrich Co. Ltd), Azoxystrobin (AZ) (95%, Sigma Aldrich Co. Ltd), Difenoconazole (DF) (95%, Sigma Aldrich Co. Ltd) and sodium chloride NaCl (99% pure, Sigma Aldrich Co. Ltd) were used without any further purification. The nonionic surfactant C12E6, anionic surfactant SDS, cationic surfactant DTAB, and zwitterionic surfactant C12PC were dissolved in D2O at a concentration of 25 mM in the absence and presence of 100 mM NaCl respectively, for SANS and related studies. The CAC values for different surfactants in the absence of salt are listed in Table S2. 25 mM SDS/DTAB-C12E6 mixtures were prepared at 25/75, 50/50 and 75/25 mol% in the presence of 100 mM NaCl to test if the solutions follow the ideal mixing according to the regular solution theory (RST). A comprehensive description of this method investigating surfactant binary mixing has been covered in many open literatures [17–19]. 25 mM C12PC-C12E6 mixtures were made at the same ratios, also in the presence of 100 mM NaCl for comparison. Excessive pesticides were added to surfactant solutions as prepared above under stirring to ensure full dissolution and equilibration and the conditions and processes utilised for model pesticide formulations were kept the same as reported previously [12]. The pD values for all dispersions used in this study were mediated at 7.4.
3. Results and discussion 3.1. Pesticide solubility in C12E6/SDS/DTAB/C12PC micelles with/ without NaCl Pesticide solubility in the micellar solutions of C12E6, SDS, DTAB and C12PC in the absence or presence of sodium chloride was investigated by 1H NMR, with the pesticide solubility values shown in Fig. 2. All pesticides are well solubilised into micelles because their solubility values in the micellar solutions are significantly higher than in pure water as shown in Table S1. Clearly, pesticide solubility in different surfactant micelles varies greatly, indicating a combination of changes in surfactant head groups and chemical nature of the pesticides. In general, more hydrophobic pesticides CP and DF have higher solubility than AZ across all surfactant micelles. In our previous studies, we showed that pesticide solubility in nonionic surfactant micelles was determined by their hydrophobicity, i.e., pesticides with higher hydrophobicity have higher solubility [12]. This trend remains true for the ionic and zwitterionic surfactants as well, but only in a broad sense. Note that the Log(Pow) values for DF, CP and AZ are in the decreasing
2.2. Measurements Surface tension: A Krüss K11 tensiometer with a du Noüy ring method [20] was used to determine the CAC values for both pure and mixed surfactant systems at 20 °C. The exerted force associated with the surface tension of a sample was measured as a function of the sample concentration. Above CAC, the force is relatively constant as the interface was saturated with surfactant molecules. Below CAC, the surface tension reading rose with surfactant concentrations because the surface was no longer saturated with the surfactant molecules. Each set of experiments was performed from high to low concentrations, and the concentration where the corresponding surface tension starts to change is the CAC value. The CAC values for pure and mixed surfactant systems in the presence of salt are listed in Table S3. The CAC values were fitted using the 3
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Fig. 2. Solubility of AZ, CP and DF in C12E6, SDS, DTAB and C12PC micelles at a total surfactant concentration of 25 mM with/without NaCl (100 mM).
are listed in Table 1. The CAC values for different binary surfactant ratios listed in Table S3 and the best RST fits shown in Figure S1 are in line with similar binary surfactant mixtures as reported in open literature [19,23]. They have the values of interaction parameter, b of 0.3, 1.0 and 1.2, indicating that all 3 binary surfactant systems have weak attractive interactions. Pesticide solubility study shows two different types of responses to mixing. Solubility values in the single component micelles of C12PC and C12E6 are higher than their binary mixtures, showing ‘‘U” shape solubility profiles for the 3 pesticides against the molar ratio. In contrast, solubility values in the single component DTAB micelles are clearly lower than those from the C12E6 micelles. Mixing leads to a steady rise of solubility to the rising fraction of C12E6, but the rate of rise appears to be influenced by the hydrophobicity of the pesticide. For the most hydrophobic DF, the solubility shows an immediate rise upon mixing with the smallest molar fraction of 0.25 of C12E6. For CP with lower hydrophobicity, there is a delayed response of change in solubility with the rising molar fraction of C12E6, with noticeable rise occurring only at 0.75 as shown in Table 1. For the least hydrophobic AZ, mixing of C12E6 with DTAB does not cause measurable change at all in its solubility. In the case of SDS mixing with C12E6, the responses of solubility to mixing with C12E6 occur in both patterns as described above. Whilst the solubility of the most hydrophobic DF displays a ‘‘U” shape profile against the rising ratio of C12E6, that for CP with the medium hydrophobicity shows a steady rise with increase C12E6 fraction. For AZ with the lowest hydrophobicity, however, mixing of C12E6 even at a molar fraction of 0.25 causes a drop of solubility to the same value as that from single component C12E6, indicating the dominant impact from the nonionic surfactant.
order, thus a decreasing trend of hydrophobicity (Table S1). The order of solubility of the 3 pesticides in the SDS micelles does follow this trend exactly, i.e., the solubility for DF in SDS is 4.36 mM, that for CP is 1.56 mM and that for AZ is 1.11 mM, respectively, and the exact same order is also followed in the presence of salt. However, for the rest of the surfactant systems studied, CP has the highest solubility, followed by DF and then AZ. The reason for such small disorder remains unclear, but this may simply mean that use of Log(Pow) values as an indicator of hydrophobicity has limitations when applied to predict the solubility in surfactant micelles. The highest solubility for DF was achieved from the SDS micelles. Addition of salt made a small but measurable increase in its solubility, suggesting the influence of charge screening. Nonionic C12E6 offers the second highest DF solubility, followed by DTAB and C12PC micelles. Again, salt addition in DTAB can slightly increase DF solubility, but has little effect in the case of nonionic and zwitterionic surfactants as expected. In contrast, nonionic C12E6 micelles produced the highest CP solubility, followed by C12PC, DTAB and SDS micelles. All surfactants offered low AZ solubility, but amongst them SDS appears to be the best. Pesticides like AZ are generally difficult to dissolve. The huge variations in solubility as observed here offer useful indications for us to further explore how to design micellar environments for better solubility enhancement. CP and DF solubility in the ionic surfactant micelles of SDS and DTAB increases with salt addition whilst AZ solubility remains the lowest and does not have a clear change upon salt addition. The lack of salt effect on pesticide solubility in the nonionic and zwitterionic surfactant micelles of C12E6 and C12PC is due to the lack of changes in surfactant properties and micellar structures. In contrast, the CACs of ionic surfactants generally decrease significantly in the presence of salts while salt addition has little impact on the CACs of the nonionic and zwitterionic surfactants. In pure water, SDS and DTAB have CAC values of 8.2 mM and 14.2 mM (Table S2), respectively. Addition of salt made their CACs drop to below 4 mM. Thus, the amount of increase of solubilised CP and DF in SDS and DTAB micelles in the presence of NaCl could be attributed to the increase in the number of micelles due to CAC drop and micellar structure changes.
3.3. Pesticide locations in surfactant micelles The location of pesticide molecules in the micelles was studied by NOESY, with the NOESY spectra in the chemical shift range of 0– 8 ppm measured from SDS interactions with CP, AZ and DF being presented in Fig. 3 (a)-(c), respectively. The NOESY spectra for interactions between DTAB and C12PC with the 3 pesticides are presented in Figure S2 (a)-(f). The hydrogens present in CP were assigned as P1: ~7.61 ppm; P2: ~7.05 ppm; P3: ~6.71 ppm; P4: ~6.39 ppm; P5: ~1.67 ppm; P6: ~0.84 ppm and P7: ~2.14 ppm. The hydrogens present in AZ were assigned as A2: ~7.91 ppm; A3-A10: ~7.23–7.65 ppm and A11: ~6.22 ppm. The hydrogens present on DF were assigned as F2: ~7.94 ppm; F3: ~7.55 ppm; F4-F6:
3.2. Pesticide solubility in binary, ionic-nonionic and zwitterionicnonionic mixed surfactant micelles Pesticide solubility values from the micellar solutions of the binary SDS/DTAB/C12PC-C12E6 mixtures at different molar ratios 4
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Table 1 Solubility of AZ, CP and DF in binary SDS/DTAB/C12PC-C12E6 mixed micelles with different molar fractions of C12E6 in the presence of 100 mM NaCl. The solubility values, in mM, have an error of ± 5%. Surfactants
SDS
DTAB
C12PC
Solubilised pesticides/mM
AZ CP DF AZ CP DF AZ CP DF
Molar ratio of C12E6 against SDS/DTAB/C12PC 0/100
25/75
50/50
75/25
100/0
1.10 2.03 4.86 0.19 2.57 0.80 0.28 2.18 1.36
0.35 2.05 2.35 0.14 2.45 1.16 0.14 1.71 0.99
0.35 2.35 1.76 0.14 2.54 1.40 0.14 2.12 1.21
0.35 2.87 1.98 0.14 3.07 1.43 0.15 2.79 1.46
0.35 3.90 2.57
Fig. 3. (a)-(c): NOESY spectra for CP, AZ and DF in SDS micelles; (d): Schematic depiction of the locations for CP, AZ and DF in SDS/DTAB/C12PC micelles.
~6.87–7.16 ppm and F7: ~7.32 ppm. The hydrogens on SDS were assigned as S1: ~4.20 ppm; S2: ~1.70 ppm; S3: ~1.20 ppm and S4: ~0.80 ppm. Unlike DTAB and C12PC, however, there is no hydrogen on SDS head group. This means that our previous method [12] of looking at interactions of pesticide hydrogens with hydrogens on surfactant tail and head separately cannot be applied simply to the SDS scenario. On the other hand, surfactant head groups are always hydrated and pesticide solubility in pure water is negligible compared to its solubility in micelles. The coupling between pesticide hydrogens and surfactant head hydrogens can be replaced by the interaction between pesticide hydrogens and the tiny amount of H2O in D2O as hydration. The hydrogen present
on H2O was assigned as 4.80 ppm. P1-4/A2-A11/F2-F7 interactions with H2O (head) and S4 (tail) are shown in circles 1 and 2, respectively. CP and AZ bear comparable signal intensities when interacting with surfactant head and tail, whilst DF hardly interacts with the head. These observations indicate that the most favourable locations for CP and AZ in SDS micelles are at the interface whilst that for DF is in the core. These different locations as dissolved in the SDS micelles are schematically illustrated in Fig. 3 (d). Similarly, in the DTAB and C12PC micelles, DF locates in the core and CP and AZ stay at the interface. Thus, pesticide locations in the charged surfactant micelles are highly consistent with what have been observed when they were dissolved in nonionic surfactants 5
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the micelle is a long ellipsoid. The best fitted parameters for all SANS profiles are shown in Table 2. The shape of the SDS micelles in pure water is close to a round sphere as the ellipticity is 1.15. The shell was fitted to 3.8 Å in thickness with 82% of the total volume being occupied by water. This is consistent with the previous observation that the SDS head group is small in size but highly hydrated [26]. The net charge per micelle was found to be 8.5e and the aggregation number at 56. In general, pesticide solubilisation did not change the shell thickness, hydration or net charge. Solubilisation of AZ and CP had a negligible effect on the micellar size and aggregation number, due to the relatively low level of pesticide molecules solubilised. DF solubilisation elongated the micelle slightly and as a result, the aggregation number increased. In the presence of 100 mM NaCl, the SDS micellar length and aggregation number increased but its shell thickness and hydration were not affected. NaCl shields the charge interaction in surfactant head groups, leading to an effective net charge per micelle of 0. AZ and CP solubilisation still had little effect on micellar size or shape. However, DF solubilisation nearly doubled the micellar length. As a result, the aggregation number increased from 74 to 117. A similar trend of changes was observed for the cationic DTAB surfactant. The ellipticity for the DTAB micelles in pure water is 1.21, with the shell thickness of 4.2 Å and hydration level of 50%, consistent with the findings reported previously [27]. The aggregation number as determined for the DTAB micelles was 59 and the net charge per micelle was + 4.7. Solubilisation of AZ and DF did not alter the micellar structure while minor elongation was detected upon CP solubilisation. Again, salt addition shielded the charge interaction and slightly elongated the micellar shape, consistent with the altered interfacial packing. In the case of the zwitterionic surfactant, the structural features for C12PC have been well established [28]. The C12PC micelles are nearly spherical, with the shell thickness of 9.0 Å and hydration level of 65%. Each zwitterionic micelle carries zero net charge and is comprised of 54 molecules. Pesticide solubilisation and salt addition almost have no effect on C12PC micellar structure.
[12]. Pesticide locations in surfactant micelles are clearly determined by their hydrophobicity, with more hydrophobic pesticides staying closer to the micellar core and less hydrophobic pesticides being distributed into the shell region. The NOESY spectra measured from interactions of SDS/DTAB/ C12PC and their binary mixtures with C12E6 at different molar ratios with CP, AZ and DF in the presence of NaCl are also presented in Figures S3-S6. It is clear that interaction intensities have not changed within error by adding salts or when mixed with nonionic surfactants. The pesticide locations in micelles are not affected by electrolytes or surfactant mixing based on the NOESY spectra alone. This observation indicates that the pesticide location is dominated by the main chemical and amphiphilic nature of the pesticides and their interactions with surfactants. 3.4. Micellar structural changes upon solubilisation of pesticides for single component surfactant micelles in the absence or presence of sodium chloride Changes in the structures of the micelles before and after pesticide solubilisation in the absence or presence of sodium chloride were investigated by SANS. All SANS data were fitted into a core–shell ellipsoid model with the Hayter-Penfold Rescaled Mean Spherical Approximation (hayter_msa) structure factor [24,25]. Full details of the SANS fitting method is illustrated by Figure S7 and described in Section C of the Supporting Information, and listing of scattering length densities (SLDs) of all materials involved in this experiment is presented in Table S4. The representative SANS profiles for SDS, DTAB and C12PC micelles with or without solubilised pesticide in the absence or presence of sodium chloride are presented in Fig. 4 (a)-(f). The SANS profiles for C12E6 with or without solubilised pesticides in the absence or presence of sodium chloride are shown in Figure S8. As all surfactant molecules have the same tail length (12 units of C), the minor axis of the core radius is fixed to 16 Å. The ellipticity denotes the core major radius divided by the minor radius. For a spherical micelle, this value is 1. If the value is much higher than 1, this indicates that
Fig. 4. SANS profiles for SDS (a, b), DTAB (c, d) and C12PC (e, f) micelles with/without solubilised pesticides in the absence (a, c, e) or presence (b, d, f) of 100 mM sodium chloride. For better visualization, the scattering intensity data were shifted vertically by multiplying factors: 1, 2, 4 and 8 (from bottom to top). The continuous lines represent the best fits with structural parameters listed in Table 2. 6
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Table 2 Structural parameters for the micelles of SDS, DTAB, C12PC and C12E6 from the best fits to the SANS profiles from the core–shell ellipsoid model using the hayter_msa structure factor before and after solubilisation of pesticides AZ, CP and DF with and without 100 mM NaCl. The core inner radius was fixed as 16 Å. The ellipticity represents the ratio of core major radius against core minor radius. P/S molar ratio stands for the molar ratio of pesticide against surfactant. Surfactants
Ellipticity ± 0.05
Core SLD x10-6Å
(a)SDS +AZ +CP +DF (b)SDS + NaCl +AZ +CP +DF (c)DTAB +AZ +CP +DF (d) DTAB + NaCl +AZ +CP +DF (e)C12PC +AZ +CP +DF (f)C12PC + NaCl +AZ +CP +DF (g)C12E6 +AZ +CP +DF (h)C12E6 + NaCl +AZ +CP +DF
1.15 1.18 1.20 1.54 1.52 1.56 1.58 3.21 1.21 1.23 1.32 1.25 1.25 1.27 1.41 1.33 1.10 1.11 1.15 1.14 1.10 1.11 1.15 1.14 3.55 3.57 17.3 16.6 3.55 3.57 17.3 16.6
0.40 0.21 0.26 0.32 0.40 0.20 0.22 0.41 0.40 0.37 0.29 0.28 0.40 0.37 0.17 0.27 0.40 0.35 0.21 0.17 0.40 0.35 0.21 0.18 0.40 0.34 0.07 0 0.40 0.34 0.07 0
1
Shell thickness ± 0.5 Å
Shell hydration ± 0.05
Shell SLD x10-6Å
3.8
0.82
5.0
1
Micellar net charge (e) ± 0.3 8.5
0
4.2
0.50
3.0
+4.7
0
9.0
11.0 11.0 9.0 9.0 11.0 11.0 9.0 9.0
0.70
0.68 0.68 0.38 0.37 0.68 0.68 0.38 0.37
4.8
4.5 4.5 2.8 2.7 4.5 4.5 2.8 2.7
0
0
Aggregation number ± 5 56 56 56 61 74 74 73 117 59 59 62 59 62 62 66 62 54 54 54 54 54 54 54 54 173 174 811 739 173 174 811 739
P/S molar ratio ± 0.004 0.042 0.062 0.174 0.044 0.081 0.194 0.007 0.052 0.029 0.007 0.103 0.032 0.012 0.086 0.055 0.011 0.087 0.054
± ± ± ± ± ± ± ±
10 10 30 30 10 10 30 30
0.016 0.169 0.108 0.015 0.171 0.105
Effects of pesticide solubilisation on the structures of the binary charged-nonionic mixed surfactant micelles were also investigated by SANS. As shown in Table 3, the most hydrophobic DF resulted in the highest solubilisation in the SDS-C12E6 micelles and the largest micellar shape change as indicated by the ellipticity increase, followed by CP and AZ. As the molar fraction of C12E6 increases its influence on micellar shape, causing the amount of pesticide solubilisation to increase. Similar to pesticide solubilisation in nonionic surfactant micelles, solubilisation of pesticides also shrank and dehydrated the micellar shell with the smallest effect from AZ due to its lowest solubilisation. Although data from the binary DTAB-C12E6 and C12PC-C12E6 mixed micelles do not show a simple correlation to the order of hydrophobicity from the pesticides, the main trends are similar to the observations from the SDS-C12E6 mixed micelles as shown in Table S5.
Because of the higher surfactant concentration used in this work (25 mM against 6.7 mM used previously), the micellar length of C12E6 is higher than that obtained from our previous study [12]. This change was shown in the ellipticity derived from the core– shell model. The micellar structural changes caused by pesticide solubilisation agree well with our previous results, i.e., CP and DF solubilisation elongated the micelles, shrank and dehydrated the shell while AZ solubilisation had no observable effect. Analogous to the zwitterionic surfactant, salt addition has little influence on micellar structure or subsequent solubilisation of pesticide. 3.5. Micellar structural changes upon solubilisation of pesticides for mixed surfactant micelles The SANS profiles for the binary SDS-C12E6 micelles were measured for the molar ratio range of 25/75, 50/50 and 75/25 before and after pesticide solubilisation, as depicted in Fig. 5 (a)-(c), with the best fitted parameters listed in Table 3. The SANS profiles for the binary DTAB-C12E6 and C12PC- C12E6 micelles in the same molar ratio range before and after pesticide solubilisation together with their best fits are shown in Figure S9. As with the single component micelles, all SANS profiles measured from the binary mixtures could be well fitted well with the core–shell ellipsoid model. Clearly, when more C12E6 is added in the mixed micelles, micellar length and shell thickness tend to increase whilst shell hydration decreases. The structure of the SDS-C12E6 mixed micelles gradually transferred to C12E6 micelles as more C12E6 was introduced. The evolution of micellar size with composition in SDS-C12E6 mixed systems was not clear at low concentrations (below 10 mM) [17] but similar trend was reported at high concentration (at 25 mM) [18].
3.6. Mechanistic processes underlying pesticide solubilisation In spite of extensive use of surfactants as adjuvants in agrisprays, there is currently little scientific research reported in open literature to describe how different types of surfactant and their mixtures affect pesticide solubilisation [10,29]. Following our previous work on the investigation of the effect of pesticide solubilisation on the micellar structures of nonionic surfactants [11,12,30], this study has focused on the influence of surfactant types. 4 model surfactants were chosen in this work to have the same tail length (Fig. 1) but different head groups so that the effects of their head groups on micellar structures alone and in binary mixtures and on solubilisation of the pesticides can be examined. The concentrations of the surfactants were set at 25 mM, well above their CACs. The 3 pesticides, with different molecular structures and 7
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Fig. 5. SANS profiles measured from the binary SDS-C12E6 mixed micelles with and without of solubilised pesticides at different molar ratios of SDS/C12E6: (a) 75/25, (b) 50/50 and (c) 25/75. The continuous lines represent the best fits with structural parameters listed in Table 3. For better visualization, the SANS data were shifted vertically by multiplying factors: 1, 2, 4 and 8 (from bottom to top).
Table 3 Structural parameters obtained from the best fits to the SANS profiles measured from the binary SDS-C12E6 micelles mixed at different molar ratios of SDS:C12E6 using the core– shell ellipsoid model. The core inner radius was fixed as 16 Å. The ellipticity represents the ratio of core major radius against core minor radius. P/S molar ratio represents the molar ratio of pesticide against surfactant. Surfactants
Ellipticity ± 0.03
(a) 75 mol%SDS +AZ +CP +DF (b) 50 mol%SDS +AZ +CP +DF (c) 25 mol%SDS +AZ +CP +DF
1.27 1.29 2.69 2.93 1.76 1.76 4.95 3.20 2.60 2.62 13.71 13.30
Core SLD 10 0.40 0.34 0.22 0.01 0.40 0.34 0.19 0.11 0.40 0.34 0.15 0.07
6
Å
1
Shell thickness ± 0.5 Å
Shell hydration ± 0.05
Shell SLD 10
5.6 5.6 4.8 4.8 7.5 7.5 6.4 6.4 9.2 9.2 7.7 7.7
0.75 0.75 0.65 0.65 0.75 0.75 0.60 0.60 0.68 0.68 0.53 0.54
5.7 5.7 5.5 5.5 5.7 5.7 5.3 5.3 5.6 5.6 5.1 5.2
6
Å
1
Aggregation number ± 5 62 62 126 127 86 86 233 ± 10 143 127 127 647 ± 20 592 ± 20
P/S molar ratio ± 0.004 0.014 0.082 0.094 0.014 0.094 0.070 0.014 0.115 0.079
Binary mixing between SDS and C12E6 has revealed broadly proportional influences from components but the pattern of mixing between C12PC and C12E6 is characterised by the occurrence of a broad minimum, indicating unfavourable mixing toward solubilisation. SANS measurements revealed that whilst low pesticide solubilisation does not cause much structural change of surfactant micelles, high pesticide solubilisation tends to transform micelles into ellipsoidal shape, in much the same way as observed from nonionic surfactants. The structural changes for binary SDS-C12E6 mixed surfactant micelles due to pesticide solubilisation in the absence or presence of electrolytes are schematically shown in Fig. 6. Solubilisation of
hydrophobicity, achieved the highest solubility in the SDS micelles, followed by C12E6, C12PC and DTAB. It remains unclear as to how this order of sequence is formed, but micellar environments are thought to be the dominant factor in determining the amount and location of solubilised pesticides. NOESY spectral analysis revealed that the more hydrophobic DF molecules are dissolved in the micellar core whilst the less hydrophobic CP and AZ molecules are dispersed at the core–shell interface and shell. Different head types and the screening effect on the ionic heads from salt addition clearly affect how heads are packed in the shell and the core–shell interfacial region, affecting the amount of pesticides solubilised.
Fig. 6. Structural changes for SDS-C12E6 surfactant micelles mixed with gradually increased SDS mole fraction (from 0 to 1) in the presence of pesticide such as DF. The micelle changed from a long cylinder to a short cylinder and finally to a sphere with increased SDS mole fraction in the mixture. 8
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contains code developed with funding from the EU Horizon 2020 programme under the SINE2020 project Grant No 654000. HG and ML thank the studentships provided by China China Scholarship Council. ZL thanks BBSRC and AstraZeneca for a joint funding under BB/S018492/1.
pesticides into single component SDS micelles does not cause much structural changes but electrostatic screening reduces effective head group area. This results in slightly increased ellipticity of the micelles alone and further ellipticity increase upon pesticide solubilisation associated with increased aggregation numbers. Mixing with C12E6 shows distinct impact from the nonionic surfactant on micellar elongation and this trend is broadly followed upon pesticide solubilisation, with more hydrophobic pesticide solubilising more and causing further micellar elongation. Cationic and zwitterionic surfactants alone and their binary mixtures broadly follow the main trends set by SDS and binary SDS-C12E6 mixtures in pesticide solubilisation, but the extent is far lower. In spite of the weaker extent of changes in their associated structures, the varieties of structural changes are rather rich, inherent of the physiochemical properties of the surfactant heads and pesticides.
Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2020.10.083. References [1] M.J. Castro, C. Ojeda, A.F. Cirelli, Advances in surfactants for agrochemicals, Environ. Chem. Lett. 12 (1) (2014) 85–95. [2] L. Zheng, C. Cao, L. Cao, Z. Chen, Q. Huang, B. Song, Bounce Behavior and Regulation of Pesticide Solution Droplets on Rice Leaf Surfaces, J. Agric. Food. Chem. 66 (44) (2018) 11560–11568. [3] M. Song, D. Hu, X. Zheng, L. Wang, Z. Yu, W. An, R. Na, C. Li, N. Li, Z. Lu, Enhancing droplet deposition on wired and curved superhydrophobic leaves, ACS Nano 13 (7) (2019) 7966–7974. [4] L. Schreiber, M. Riederer, K. Schorn, Mobilities of organic compounds in reconstituted cuticular wax of barley leaves: effects of monodisperse alcohol ethoxylates on diffusion of pentachlorophenol and tetracosanoic acid, Pestic. Sci. 48 (2) (1996) 117–124. [5] L. Wang, X. Li, G. Zhang, J. Dong, J. Eastoe, Oil-in-water nanoemulsions for pesticide formulations, J. Colloid Interface Sci. 314 (1) (2007) 230–235. [6] M.B. Ellis, C. Tuck, P. Miller, How surface tension of surfactant solutions influences the characteristics of sprays produced by hydraulic nozzles used for pesticide application, Colloids Surf., A 180 (3) (2001) 267–276. [7] M. Riederer, M. Burghardt, S. Mayer, H. Obermeier, J. Schoenherr, Sorption of monodisperse alcohol ethoxylates and their effects on the mobility of 2, 4-D in isolated plant cuticles, J. Agric. Food. Chem. 43 (4) (1995) 1067–1075. [8] T. Yang, B. Zhao, A.J. Kinchla, J.M. Clark, L. He, Investigation of pesticide penetration and persistence on harvested and live basil leaves using surfaceenhanced Raman scattering mapping, J. Agric. Food. Chem. 65 (17) (2017) 3541–3550. [9] E. Pambou, X. Hu, Z. Li, M. Campana, A. Hughes, P. Li, J.R. Webster, G. Bell, J.R. Lu, Structural features of reconstituted cuticular wax films upon interaction with nonionic surfactant C12E6, Langmuir 34 (11) (2018) 3395–3404. [10] K. Krogh, B. Halling-Sørensen, B. Mogensen, K. Vejrup, Environmental properties and effects of nonionic surfactant adjuvants in pesticides: a review, Chemosphere 50 (7) (2003) 871–901. [11] F.N. Padia, M. Yaseen, B. Gore, S. Rogers, G. Bell, J.R. Lu, Influence of molecular structure on the size, shape, and nanostructure of nonionic CnEm surfactant micelles, J. Phys. Chem. B 118 (1) (2013) 179–188. [12] X. Hu, H. Gong, Z. Li, S. Ruane, H. Liu, E. Pambou, C. Bawn, S. King, K. Ma, P. Li, F. Padia, G. Bell, J.R. Lu, What happens when pesticides are solubilized in nonionic surfactant micelles, J. Colloid Interface Sci. 541 (2019) 175–182. [13] F. Pan, Z. Li, H. Gong, J.T. Petkov, J.R. Lu, Membrane-lytic actions of sulphonated methyl ester surfactants and implications to bactericidal effect and cytotoxicity, J. Colloid Interface Sci. 531 (2018) 18–27. [14] Z. Wang, P. Li, K. Ma, Y. Chen, J. Penfold, R.K. Thomas, D.W. Roberts, H. Xu, J.T. Petkov, Z. Yan, The structure of alkyl ester sulfonate surfactant micelles: The impact of different valence electrolytes and surfactant structure on micelle growth, J. Colloid Interface Sci. 557 (2019) 124–134. [15] Y. Pang, S. Wang, X. Qiu, Y. Luo, H. Lou, J. Huang, Preparation of lignin/sodium dodecyl sulfate composite nanoparticles and their application in pickering emulsion template-based microencapsulation, J. Agric. Food. Chem. 65 (50) (2017) 11011–11019. [16] Y. Li, D. Yang, S. Lu, S. Lao, X. Qiu, Modified lignin with anionic surfactant and its application in controlled release of avermectin, J. Agric. Food. Chem. 66 (13) (2018) 3457–3464. [17] J. Penfold, E. Staples, L. Thompson, I. Tucker, J. Hines, R. Thomas, J. Lu, N. Warren, Structure and composition of mixed surfactant micelles of sodium dodecyl sulfate and hexaethylene glycol monododecyl ether and of hexadecyltrimethylammonium bromide and hexaethylene glycol monododecyl ether, J. Phys. Chem. B 103 (25) (1999) 5204–5211. [18] J. Penfold, E. Staples, I. Tucker, Neutron small angle scattering studies of micellar growth in mixed anionic-nonionic surfactants, sodium dodecyl sulfate, SDS, and hexaethylene glycol monododecyl ether, C12E6, in the presence and absence of solubilized alkane, hexadecane, J. Phys. Chem. B 106 (34) (2002) 8891–8897. [19] J. Penfold, E. Staples, L. Thompson, I. Tucker, J. Hines, R. Thomas, J. Lu, Solution and adsorption behavior of the mixed surfactant system sodium dodecyl sulfate/n-hexaethylene glycol monododecyl ether, Langmuir 11 (7) (1995) 2496–2503. [20] C. Smith, Z. Li, R. Holman, F. Pan, R.A. Campbell, M. Campana, P. Li, J.R. Webster, S. Bishop, R. Narwal, Antibody adsorption on the surface of water studied by neutron reflection, mAbs, Taylor & Francis (2017) 466–475. [21] O. Arnold, J.-C. Bilheux, J. Borreguero, A. Buts, S.I. Campbell, L. Chapon, M. Doucet, N. Draper, R.F. Leal, M. Gigg, Mantid—data analysis and visualization
4. Conclusion Following our previous studies of the effects of pesticide solubilisation on the size, shape, interior structure and phase boundary of the nonionic surfactant micelles [11,12,30,31], this work has explored how pesticide solubilisation affects the micellar structures of ionic and zwitterionic surfactants and their binary mixtures with nonionic surfactant. We have demonstrated that SANS combined with NMR offers the appropriate sensitivity and resolution to follow the structural changes of single and binary surfactant micelles upon pesticide solubilisation in the absence or presence of electrolytes. Our results have revealed that pesticide solubility and location in the micelles are cooperatively affected by pesticide hydrophobicity, surfactant head type and micellar structure. In general, pesticide solubilisation can have very weak effects on the micellar structures of charged surfactants, but addition of electrolyte can enhance pesticide solubility in ionic surfactant micelles via electrostatic screening and cause small micellar elongation. In contrast, salt addition does not have any influence in nonionic or zwitterionic surfactant systems. Pesticide solubilisation can affect micellar structures of the binary charged-nonionic systems, though the extent of the influence is dependent on the amount of pesticide solubilised. This feature is similar to pesticide solubilisation in nonionic surfactant micelles, and the process is well characterised by micellar elongation, shell shrinkage and dehydration. The micellar length, shell thickness and hydration can be simply mediated by adjusting the molar ratio between charged and nonionic surfactants in the mixture. Our results clearly illustrate that anionic, cationic and zwitterionic surfactants are promising adjuvants for future agri-formulation science. A smart formulation exploiting surfactant mixing could be quite advantageous in controlled pesticide release and crop protection. Future work will examine how these pesticide formulations interact with plant waxes [9,32,33] to understand how waxes from different species control their release upon agri-spraying. Declaration of Competing Interest None. Acknowledgements We thank Syngenta for funding this work. We also thank the University of Manchester for a studentship to XH, EP, HG, ML, PH, HL, SR and ISIS Neutron Facility at Rutherford Appleton Laboratory, STFC for neutron beam times on Larmor with RB1920153. DOI for the SANS data is: https://doi.org/10.5286/ISIS.E.RB1920153. This work benefited from the use of the SasView application, originally developed under NSF Award DMR-0520547. SasView also 9
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[22]
[23]
[24] [25] [26]
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