Saponins: A Renewable and Biodegradable Surfactant From Its

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Saponins: A Renewable and Biodegradable Surfactant From Its Microwave-Assisted Extraction to the Synthesis of Monodisperse Lattices C. Schmitt,†,‡ B. Grassl,† G. Lespes,† J. Desbrières,† V. Pellerin,† and S. Reynaud*,† †

IPREM, UMR 5254, CNRS/UPPA, 2 av.Angot, 64053 Pau cedex 9, France

J. Gigault‡ and V. A. Hackley‡ ‡

National Institute of Standards and Technology, Materials Measurement Science Division, 100 Bureau Drive, MS 8520, Gaithersburg, Maryland 20899-8520, United States ABSTRACT: Synthetic surfactants are widely used in emulsion polymerization, but it is increasingly desirable to replace them with naturally derived molecules with a reduced environmental burden. This study demonstrates the use of saponins as biodegradable and renewable surfactants for emulsion polymerization. This chemical has been extracted from soapnuts by microwave assisted extraction and characterized in terms of surfactant properties prior to emulsion polymerization. The results in terms of particle size distribution and morphology control have been compared to those obtained with classical nonionic (NP40) or anionic (SDS) industrial surfactants. Microwave-extracted saponins were able to lead to latexes as stable as standard PS latex, as shown by the CMC and CCC measurements. The saponin-stabilized PS particles have been characterized in terms of particle size and distribution by Dynamic Light Scattering and Asymmetrical Flow Field Flow Fractionation. Monomodal and monodispersed particles ranging from 250 to 480 nm in terms of diameter with a particle size distribution below 1.03 have been synthesized.

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INTRODUCTION Nonionic surfactants are a close second to ionic surfactants with about 45% of the overall industrial production.1 These surfactants constitute both hydrophilic and hydrophobic components. Typically, the hydrophilic portion consists of polyethylene glycol chains obtained by the polycondensation of ethylene oxide, while the lipophilic portion is commonly an alkyl or alkylbenzene moiety. Nonionic surfactants have been used for the last 50 years in a wide range of detergency and emulsification applications.2 For example, the U.S. demand for the polyethoxylated nonylphenol was estimated1 to be 380 million pounds in 2010. Nevertheless, recent studies have shown that some biotransformation products derived from this type of surfactant often pass through wastewater treatment plants at concentrations that are harmful to aquatic biota.3 Some studies4 report that about 60% of the total surfactant production enters into the aquatic environment. They have shown bioaccumulation by aquatic organisms,5 estrogenic activity,6 and chronic toxicity7 for this class of compounds. Protection of water quality and commercial and sport fishing industries would be beneficial for industrial users and producers of detergents to switch to a greener alternative surfactant. More specifically, synthetic surfactants are widely used in emulsion polymerization to form polymer latexes. Polymer latexes have a wide range of commercial applications in paints © 2014 American Chemical Society

and coatings, for paper and paperboard, in adhesives and sealants, and for carpet backing.1 There are some reports within the literature on the synthesis of surfactant from renewable feedstock.8,9 However, to our knowledge, there are no reports within the literature where saponins have been used to synthesize and stabilize latexes. Saponins are active natural chemical products generated from the pericarp of soapnuts. This natural surfactant has traditionally been used in India for fabric washing, bathing and in folk medicine due to the formation of lather or foam in water.10 The only published relevant scientific studies were focused on the chemical structure characterization or micelle properties of saponins obtained from various plants and regions of the world.11,12 Among plants and vegetables containing saponins, one can include soapnuts, soybean, chickpea, and so on. The overall objective of the present work was to interrogate the surfactant properties of saponins. Saponins from soapnuts were extracted via a low-energy extracted method (5 min under 50 W using a microwave reactor). The objective was then to investigate the ability of the crude extract to stabilize an emulsion polymerization reaction. The surfactant properties of Received: November 20, 2013 Revised: January 20, 2014 Published: January 21, 2014 856

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described elsewhere.13 W is the stability ratio, which is defined as the ratio of the diffusion-controlled rapid coagulation rate to the reactioncontrolled slow coagulation rate:

the extracts have been characterized in terms of critical micelle concentration (CMC), and its performance has been evaluated by taking into account the aggregate content as well as the control of the particle size and size distribution.

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W=

EXPERIMENTAL SECTION

The identification of any commercial product or trade name does not imply endorsement or recommendation by the National Institute of Standards and Technology. Materials. Styrene, sodium dodecyl sulfate (SDS), and ammonium persulfalte (APS) were purchased from Aldrich. Nonylphenol ethoxylate (NP40) was obtained from Rhodia (France). Deionized water, with a resistivity of 18.3 MΩ·cm at 25 °C, was filtered through a 0.22 μm Millipore filter prior to use. Unless otherwise noted, all chemicals were used as received. The polystyrene latex particle standards (mass fraction of 1%) were manufactured by Thermo scientific and traceable in size to the National Institute of Standards and Technologies (NIST) in the United States. The diameter is estimated at 350 nm as measured by Thermo Scientific (nanosphere size standard) with NIST traceable methodology with a coefficient of variation of 1.7% as measured by optical microscopy. Sapindus mukorossi soapnuts were purchased from Azimuts. Methods. Microwave-Assisted Extraction of Saponins. Saponins were extracted from crude Sapindus mukorossi soapnuts. The pericarp of the fruits was ground to a fine powder (grain size below 1 μm), then 5 g of saponins powder was mixed with 100 mL of deionized water in a 150 mL vial was placed in a Prolabo Microdigest 3.6 apparatus and equipped with an open reflux column. The microwave frequency was 2.45 GHz with a maximum microwave power of 250 W. An irradiation power of 20% for 5 min was employed for all experiments in order to minimize overheating. After cooling to room temperature, the yellowish liquid extract was collected by filtration and kept at 5 °C before use, without any further purification. The percentage of watersoluble matter in the ground pericarp of soapnuts was estimated by thermogravimetric analysis (TGA) and by a gravimetric method. The pH of the soapnuts extract solution remained the same before and after extraction (pH = 4.4 ± 0.1). CMC Measurement. Surface tension measurements of crude saponins extracts were used to determine the CMC at (25.0 ± 0.5) °C in water. The pendant liquid drop was used to determine the surface tension, γ (mN·m−1) by analyzing the axial symmetric shape (Laplacian profile) of the rising bubble in aqueous solutions of saponins. The measurements were performed two times on a drop tensiometer (TrackerTeclis, France) by using an appropriate cell, the standard deviation ranges from 10 to 20%. Before each measurement, the cell was cleaned with deionized water, acetone, and deionized water, and the stability of surface tension of deionized water was tested for several hours. CCC Measurement. The critical coagulation concentration (CCC)13 was determined by dynamic light scattering (DLS) measurements performed using a Zetasizer Nano (Malvern Instruments, Westborough, MA, U.S.A.) in backscatter configuration (θ = 173°) at a laser wavelength λ0 = 633 nm. Before each measurement, the cells were cleaned by filtered compressed air. The polystyrene latex was diluted into 1 mL of NaNO3 water solution of different concentrations (from 0.1 to 4 mol·L−1). The measurements were then made by collecting no less than 10 runs of submeasurements at a constant temperature of 25.0 ± 0.1, 30.0 ± 0.1, and 40.0 ± 0.1 °C. In principle, DLS characterizes the Brownian motion of particles in solution, and correlates this motion to particle size. According to the Stokes−Einstein equation, the diffusion coefficient is related to the hydrodynamic radius (rh) of the particle:

rh =

k11(fast) k11(slow)

k is dependent on the particle number concentration, and CCC is obtained by plotting the stability ratio versus the NaNO 3 concentration at a fixed temperature. Latex Synthesis. Crude microwave-assisted extracts of saponins have been employed at various concentration to carry out an emulsion polymerization of styrene in classical way in terms of monomer content (20 wt % vs water), initiator ratio (0.37 wt % vs monomer) at 70 °C under magnetic stirring. The reaction has been let overnight till quantitative yields unless noted. The concentrations of saponins have been chosen well above the critical micelle concentration (CMC). Dynamic Light Scattering. The hydrodynamic average particle size has been measured using a VASCO apparatus developed by Cordouan Technologies has been used and is suited for very thin suspension layer to avoid multiscattering phenomena and can be successfully applied to concentrated, dense, and opaque solutions (λ0 = 658 nm). Details of the treatment have been reported elsewhere.14 Asymmetrical Flow Field Flow Fractionation (AsFlFFF or A4F). The AsFlFFF system used in this study was an Eclipse 3+ (Wyatt Technology, Santa Barbara, CA) equipped with a 250 μm thick spacer. For all experiments, polyethersulfone (PES) membranes with a 10 kDa cutoff were used (Wyatt technology). Flows (i.e., main channel and cross flows) were controlled by an 1100 series isocratic pump (Agilent Technologies, Santa Clara, CA, U.S.A.) equipped with a degasser (Gastorr TG-14, Flom Co., Ltd., Tokyo, Japan). Within this study, the main flow was fixed at 0.5 mL·min−1. Injections were performed by a manual injection valve (Rheodyne 7725i, IDEX Corporation, Oak Harbor, WA) equipped with a 100 μL stainless steel sample loop. Online detectors included a 1200 series UV−vis absorbance diode array detector (DAD, Agilent Technologies) with a wavelength fixed at 274 nm and a sampling rate of 20 Hz and a MALS detector (DAWN HELIOS, Wyatt Technology). Data from different detectors was collected and analyzed using Astra version 5.3.1.18 software (Wyatt Technology). The online MALS measurements were conducted in a cell maintained at (20.0 ± 0.1) °C. Environmental Scanning Electron Microscope (ESEM). An Electroscan Corporation (ESEM E3, Wilmington, MA, U.S.A.) instrument operating at 20 or 25 keV was used in this study. The samples were mounted on double-sided adhesive carbon disks; no gold coating was required. Atomic Force Microscopy (AFM). A Dimension 3100 AFM with a Nanoscope V controller (Bruker AXS, Santa Barbara, CA) was operated in tapping mode. The AFM tips (Tap150Al-G, NanoAndMore, Lady Island, SC) had a nominal resonant frequency of 150 kHz and nominal spring constant of 5 N·m−1. Height analysis was performed using Nanoscope v7.30 software (Bruker AXS, Santa Barbara, CA). A 20 μL drop of sample was evaporated onto a Si chip. Si chips were cleaned just before use with consecutive rinsing by water, ethanol, and methanol, then drying with short blasts of compressed air.

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RESULTS AND DISCUSSION Saponins Extraction and Surface Tension Analysis. The extraction condition of saponins from soapnuts has been optimized to respect as much as possible the criteria for a sustainable chemistry in terms of solvent nature, time and heating mode. Saponins have been extracted from soapnuts using microwave irradiation to lead to an instantaneous rise of the temperature. As an example, this technique was revealed to be the “greenest” one as compared to ultrasound or supercritical fluids when used for the extraction of feedstock essential oils.15 Within the present work, the optimization of the conditions has been made by taking into account solvent

k bT 6ΠηD

where η is the viscosity of solution (Pa·s), kb (J·K−1) is Boltzmann’s constant, and T (K) the temperature of the measurement. The CCC principle is based on the kinetics of Brownian coagulation and is 857

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of the solvent. The reason why values of 70 N·m−1 are not reached will be detailed below. The second part reflected a rapid decrease in surface tension with increasing saponin extract content. The rate of decrease is dependent on the amount of surfactant adsorbed at the air/water interface. The last section of the curve represents the region where the maximum adsorption has been reached, with the unadsorbed surfactant forming micelles in solution. CMC is defined as the concentration at which surface tension is the lowest and at which micelles begin to form. In other words, the CMC is determined by the transition point between the second and the third regions of the curves. CMC values are reported in Table 1 along with the experimental conditions, such as the pH and salt concentration. The CMC of crude microwave extracted saponins at a pH of 4.4 and without any salt addition was determined to be 0.005% mass fraction. This value is 10-fold below the CMC previously reported by other authors on the same kind of saponins.20 This difference may be explained by the fact that the saponins were not purified after microwave-assisted extraction in the present study. Previous work19 already showed that additives like starch change the interfacial properties by giving rise to inclusion complexes, and lower CMCs have been obtained as a result. It is worth noticing that the CMC value is not affected within the salt concentration range used in this study as already observed elsewhere for nonionic vs ionic surfactants.21 At the same time, the CMC seems to be pH-dependent. Moreover, the surface tension remains the same above the CMC, regardless of other conditions. This behavior may be understood if structurally the carboxyl groups are located within the micelles and not exposed on the outer surface in contact with the aqueous phase. Saponins may thus be assimilated to nonionic surfactants as in accordance with previous studies.22 The results reported within Figure 1 exhibit a second CMC at extremely low concentration ((1.3 ± 0.8) × 10−8 g/g) in the case of the crude saponins at pH 4.4 with no salt addition. This CMC has been obtained from a representative set of experiments at low concentration and times of equilibrium as long as 24 h. Due to this drastic experimental condition, only the solution of crude saponins at pH 4.4 has been studied. Nevertheless, other experiments made at pH 7.9 with or without NaCl exhibit a “pseudo-plateau” around 62−65 mN· m−1 as for the crude saponins at pH 4.4, suggesting the presence of a similar second CMC. In order to understand the occurrence of this second phenomenon, it is necessary to first recall that the saponins have been extracted using a microwave without further purification. This CMC may be related to interactions as depicted in Figure 2 and occurring between surfactant molecules (saponins) and amphiphilic or hydrophilic natural polymers also extracted during the microwave process. Figure 2 describes the formation of polymer−surfactant complexes and three regions, namely A, B, and C, are identified. Region A corresponds to free molecules of saponins and polymers, both are singly dispersed. In region B, the formation

nature, extraction efficiency and time. The optimal conditions were 5% mass fraction of ground soapnuts in deionized water without organic solvent, and 5 min of microwave irradiation at a power level as low as 50 W. After filtration, the solid content of the water phase was evaluated and the extraction efficiency reached 90 ± 3%. The crude extract was then characterized and used as a surfactant water solution for emulsion with no further purification. Saponins are known to contain steroidal and triterpenoid glycosides, their chemical structures have been studied elsewhere.16,17 NMR studies already showed that the saponins chemical structure depends on the sample origin and the purification method. Purified samples do not contain only one saponin but rather saponin cocktails. More generally, saponins may be defined as triterpene glycosides composed of an oligosaccharide chain and a glycone.18 This particular structure gives interesting interfacial properties. Within the present study, we first focused on the physicochemical characterization of the crude saponin extract in relation to the surfactant properties. Although they are considered nonionic surfactants,19 saponins contain carboxyl groups on side chains and it can be assumed that the pH and the ionic force may induce electrostatic effects. Surface tension experiments have been made on the crude extract at pH 4.4 and 7.9, in the presence of sodium salt and without. The results, obtained at equilibrium, have been compared in Figure 1. The time of equilibrium ranged from 3 h at higher concentrations to more than 24 h at lower concentrations.

Figure 1. Equilibrium surface tension isotherms for microwave crude extracts of saponin at a pH of 4.4 (white triangles), at a pH of 4.4 and 0.01 mol·L−1 NaCl (black triangles), at a pH 7.9 and 0.12 mol·L−1 NaCl (black squares), and at a pH of 7.9 and 0.01 mol·L−1 NaCl (white square).

The results in Figure 1 will first be discussed for the concentration range above 5 × 10−7 g/g (equivalent to 5 × 10−5 % mass fraction). The resulting curves show three distinct parts. At lower concentrations, the surface tension tends to that Table 1. CMC Values at Various pH and NaCl Concentrations # 1 2 3 4

pH 4.4 4.4 7.9 7.9

[NaCl] (mol·L−1) 0.00 0.10 0.12 0.01

surface tension above the CMC (mN·m−1)

CMC (g/g) (5.0 (4.0 (7.0 (5.0 858

± ± ± ±

−5

0.8) × 10 0.8)× 10−5 0.8)× 10−4 0.8)× 10−4

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Table 2. Polystyrene Particle Size Obtained via Emulsion Polymerization Stabilized Either with NP40 (as Nonylphenol Ethoxylate Containing 40 Ethylene Oxide Units), with SDS (as Sodium Dodecylsulfate), or with the Microwave-Extracted Saponinsa NP40

saponins

SDS

Figure 2. Schematic representation of micelle formation in the presence of macromolecules. The solid lines I and II are representative of the CMC obtained at lower and higher concentrations, respectively.

[S]b (wt %)

DLS (dh, nm)

0.92 2.05 4.00 6.08 1.00 3.00 4.00 5.00 5.94 0.78 4.00 6.04

440 360 340 160 480 370 310 300 250 160 80 50

ESEM

A4F

430 272

360 310 280

240

a

DLS: dynamic light scattering. SEM: scan electronic microscopy. A4F: asymmetrical flow field force fractionation. bSurfactant content within the polymerization reaction as calculated versus monomer.

of polymer-bound micelles occurs. This region is defined within a limited range of saponins content and reflects the cooperative nature of formation of bound micelles. The interface B−C characterizes the domain in which the polymer is saturated with bound micelles. An increase in the saponins content results in a corresponding increase in the singly dispersed surfactant. Within region C, the formation of free micelles becomes possible. During the surface tension experiment, saponins and polymers produced from microwave extraction are diluted with respect to the ratio [saponins]/[polymers], as illustrated by the dashed line in Figure 2. The CMC obtained at lower and higher concentration could be correlated at the cross section with the dashed line (I) and (II), respectively. As compared with typical synthetic nonionic surfactants, the CMC of the microwave extracted saponins is much lower (approximately 1 order of magnitude). The saponins CMC value is not dependent on the salt concentration (up to 0.1 mol·L−1) but is pH-sensitive. The latter could be related to the chemical composition of saponins. Nevertheless, the extract may be used without further purification to carry out an emulsion polymerization as suggested by the surface tension of 36 mN·m−1 obtained at equilibrium above the CMC. It is worth noting here, that the saponins were extracted from soapnuts in 5 min under microwave irradiation to obtain a concentration 1000× above its CMC value. Latex Synthesis. Knowledge of particle size or the particle size distribution (PSD) is important for reliably characterizing the quality and stability of particulate-based systems. Examples of chemical/physical properties of polymer emulsions affected by the PSD include viscosity, suspension and emulsion stability, opacity, color, and so on. All the polymerizations reported within Table 2 were done with the same monomer-to-water and the monomer-to-initiator ratios (20 wt % and 0.37 wt %, respectively). The effects of the amount and the chemical nature of the surfactant have been investigated. To show the efficiency of crude extract of saponins, the use of this natural surfactant has been compared to another nonionic surfactant, namely, NP40, and to a commonly used ionic surfactant, namely, SDS. The surfactant content, [S] is expressed in wt %, calculated versus monomer and within the range 1 < [S] < 6, see Table 2.

The polymerizations were carried out until quantitative yields with negligible amounts of coagula were present (below 3 wt % vs the total solid content at the final conversion). In the emulsion process, particle size, and particle size distribution are influenced by the kind and the amount of emulsifiers.23,24 In the present case, all saponin-stabilized polymerizations were conducted above the CMC. An increase in surfactant concentration results in a decrease of particle size. Moreover, it is worth noticing that the results in terms of particle size are in agreement with those obtained with NP40. As a result, when the emulsion is stabilized by saponins, the particle size may be adjusted from approximately a radius of 250−450 nm with 1−6 wt % of saponins versus monomer. The hydrodynamic-average particle diameter (dh) was systematically measured in aqueous solution by DLS working on a very thin suspension layer to avoid multiple scattering phenomena. DLS shows the absence of aggregates in the emulsion medium. The particle-diameter dispersity appears extremely low; however, quantitative PSD is difficult to reach by this technique. For this reason, another technique (A4F) was performed to analyze the saponin-stabilized latex samples. When all latex particles have the same density, A4F is suitable to determine the number- averaged, the surface-averaged, the weight-averaged particle diameters, and the particle size distribution (respectively, dn, ds, dw, PSD), with dn = Σnidi/ Σni, ds = Σnidi3/Σnidi2, dw = Σnidi4/Σnidi3, and PSD = dw/dn with the amplitude of UV-signal on the fractograms proportional to nidi3. Figure 3 shows an example of the size distribution curve for saponin with 4 wt % of saponins versus monomer. For each sample, the distribution is centered on a symmetric peak. Even for the largest particle size (lowest surfactant content) PSD as low as 1.02, 1.02, and 1.03 for 4.5 and 6 wt % of saponins, respectively, are obtained. Saponin-stabilized samples have been dried by simply leaving them overnight at room temperature and atmospheric pressure. This technique uses the attractive capillary forces among colloidal particles to organize them into an ordered 2D array in a thin film. It is worth noting that when sunlight illuminates the dried films of latexes they produce iridescence. Such optical effects are well-known for near-monodisperse submicrometer859

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Figure 3. Polystyrene particles obtained with 4 wt % of saponins vs monomer. ESEM image of magnitude ×10000 (a), AFM (b), and A4F analysis (c, d). To within (d), the volume fraction has been fitted vs the particle diameter; the results may be thus easily compared to the ESEM results.

sized latexes,25−27 where the mean particle diameter is comparable to the wavelength of visible light. Dried samples have been analyzed by ESEM and AFM, and they showed good local order (Figure 3a,b). The results of microscopy analyses are in good agreement with the results obtained by DLS and A4F (Figure 3c,d). The ESEM image reveals cracks that may be explained by both the size dispersity of the particles, which may be above 5%, and the hardness of the polystyrene latex spheres, which affect the long-distance order. Other authors reported that these kind of samples may be torn by cracks resulting from volume shrinkage occurring during the film drying.28 The present results are very promising; Figure 3a,b shows ordered 2D arrays from the self-assembling of the saponin-stabilized particles, even though the solvent (water) evaporation was not done under optimized controlled conditions. As mentioned by other authors12,19 and mentioned above, saponins are considered as a nonionic surfactant even if they bear carboxyl groups. Consistent with this, the particles stabilized by saponins exhibit a size in the same order of magnitude as particles stabilized by a similar content of NP40. The smaller particle sizes were obtained when using SDS as compared to those obtained with NP40. This feature is related to the intrinsic properties of ionic versus nonionic surfactants and has already been observed and attributed to the particle

nucleation mechanism itself. Anionic surfactants can provide repulsive forces between two similarly charged electric double layers on the latex particles.23,29 By contrast, nonionic surfactants can impart two approaching particles with steric stabilization interactions. Another parameter to be considered is the solubility of the nonionic surfactants in the monomer phase, which may influence the real CMC value.29 Ozdeger et al.30 reported the emulsion polymerization of styrene and acrylate stabilized by another nonionic surfactant Triton X-405 (a polyethoxylated octylphenol containing approximately 35 ethylene oxide units). The surfactant partitioning between the oil and the aqueous phases greatly impacted the control of the particle size. In some cases, a bimodal final latex distribution was observed and this feature was attributed to two nucleation periods. This feature was not observed within the present study and monomodal samples have been obtained either by using NP40 or saponins. Latex Stability. The colloidal stability of the latexes reported herein was assessed by determination of the CCC according to a method based on DLS and described elsewhere.13 The stability of polystyrene latex particles in aqueous medium, in the presence of various concentrations of NaNO3 has been examined in order to compare latex containing 4 wt % of saponins (dh = 310 nm) to a Thermo Scientific (Nanosphere size standard) with NIST traceable 860

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derived from a natural source like soap nuts are a good candidate for use as a surfactant to facilitate emulsion polymerization with equivalent performance compared to synthetic nonionic surfactants, in terms of size distribution and stability. Moreover, these results showed that saponins represent a new biodegradable and renewable surfactant with a very low CMC, insignificant salt-sensitivity, and low pHsensitivity. Lastly, such a natural surfactant offers the advantage of avoiding the toxicity associated with common synthetic nonionic surfactants containing phenol groups. There are many technical and scale-up issues that remain to be addressed before saponin-stabilized materials can reach their full commercial potential.

methodology (dh = 380 nm). Furthermore, A4F was used to monitor the long-term stability by examining the evolution of the particle size diameter over a 3 month period. The CCC is one of the most significant characteristics of a colloidal dispersion. It is defined as the minimum concentration of ionic charge required to induce rapid coagulation of a colloidal dispersion. Two types of interactions are considered in the theoretical derivations: the van der Waals attraction interaction and the electrical repulsion interaction. Within the present work, dynamic light scattering has been employed to measure the coagulation rate constants as reported within Figure 4, with the ratio (W) of the diffusion-controlled rapid

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) U. S. Environmental Protection Agency, Action Plan for Nonylphenol (NP) and Nonylphenol Ethoxylates (NPEs); RIN 2070ZA09; 2010. (2) Dekker, M. Nonionic Surfactants; Taylor & Francis: New York and Basel, 1987; p 1136. (3) Ahel, M.; Giger, W.; Koch, M. Water Res. 1994, 28 (5), 1131− 1142. (4) Naylor, C.; Mieure, J.; Adams, W.; Weeks, J.; Castaldi, F.; Ogle, L.; Romano, R. J. Am. Oil Chem. Soc. 1992, 69 (7), 695−703. (5) Servos, M. R. Water Qual. Res. J. Can. 1999, 34 (1), 123−177. (6) Nichols, K. M.; Snyder, E. M.; Snyder, S. A.; Pierens, S. L.; MilesRichardson, S. R.; Giesy, J. P. Environ. Toxicol. Chem. 2001, 20 (3), 510−522. (7) Schwaiger, J.; Spieser, O. H.; Bauer, C.; Ferling, H.; Mallow, U.; Kalbfus, W.; Negele, R. D. Aquat. Toxicol. 2000, 51 (1), 69−78. (8) Gassama, A.; Ernenwein, C.; Youssef, A.; Agach, M.; Riguet, E.; Marinkovic, S.; Estrine, B.; Hoffmann, N. Green Chem. 2013, 15 (6), 1558−1566. (9) Gassama, A.; Ernenwein, C.; Hoffmann, N. Green Chem. 2010, 12 (5), 859−865. (10) Rao, K. N. Wealth of India; NISCOM (CSIR): New Dehli, 1999; Vol. IX (first print, 1972). (11) Huang, H. C.; Wu, M. D.; Tsai, W. J.; Liao, S. C.; Liaw, C. C.; Hsu, L. C.; Wu, Y. C.; Kuo, Y. H. Phytochemistry 2008, 69 (7), 1609− 1616. (12) Stanimirova, R.; Marinova, K.; Tcholakova, S.; Denkov, N. D.; Stoyanov, S.; Pelan, E. Langmuir 2011, 27 (20), 12486−12498. (13) Holthoff, H.; Egelhaaf, S. U.; Borkovec, M.; Schurtenberger, P.; Sticher, H. Langmuir 1996, 12 (23), 5541−5549. (14) Eyssautier, J.; Frot, D.; Barre, L. Langmuir 2012, 28 (33), 11997−12004. (15) Asfaw, N.; Licence, P.; Novitskii, A. A.; Poliakoff, M. Green Chem. 2005, 7 (5), 352−356. (16) Nord, L. I.; Kenne, L.; Jacobsson, S. P. Anal. Chim. Acta 2001, 446 (1−2), 199−209. (17) Grover, R. K.; Roy, A. D.; Roy, R.; Joshi, S. K.; Srivastava, V.; Arora, S. K. Magn. Reson. Chem. 2005, 43 (12), 1072−1076. (18) Ö sterholm, J.-E.; Cao, Y.; Klavetter, F.; Smith, P. Synth. Met. 1993, 55−57, 1034−1039. (19) Skurtys, O.; Aguilera, J. M. Food Hydrocolloids 2009, 23 (7), 1810−1817. (20) Balakrishnan, S.; Varughese, S.; Deshpande, A. P. Tenside, Surfactants, Deterg. 2006, 43 (5), 262−268. (21) Paillet, S.; Grassl, B.; Desbrières, J. Anal. Chim. Acta 2009, 636 (2), 236−241. (22) Mitra, S.; Dungan, S. R. Colloids Surf., B 2000, 17 (2), 117−133.

Figure 4. Stability ratio as a function of the temperature and salt concentration of latex stabilized by 5 wt % of saponins (crosses at 25 °C, filled triangles at 30 °C, and filled squares at 40 °C) and inverted open triangles for PS standard stabilized by SDS at 25 °C.

coagulation rate to the slow reaction-limited coagulation rate as mentioned previously in the Experimental Section. Variables such as temperature and salt concentration have been included in the study. Results reported within Figure 4 show that temperature can be considered as having a negligible effect on latex stability in the range from 25 to 40 °C when stabilized by 4 wt % saponins. The CCC of standard latex and saponins-stabilized latex has been compared at 25 °C at the same concentration and on the same day. The CCCs of both latexes are very close: 0.5 and 0.4 mol·L−1 for the standard PS and saponins-stabilized PS latexes, respectively. The latex stability has also been evaluated via the particle size measurement versus time by using the A4F technique coupled with MALS and UV detectors and by the LDS technique. These results are in good agreement and no significant change of the particle size has been noted after 3 months. The latex stability experiments confirm the great potential for saponins as effective natural surfactants, leading to good control of the particle size and excellent latex stability over time.

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CONCLUSIONS In order to respect the principles of green chemistry, a natural material like saponins has been evaluated with a goal to eventually replace commonly used synthetic surfactants. The purpose of the work summarized here was to establish the properties of saponins produced without additional purification using rapid microwave-assisted extraction (5 min, 50 W). It is worth noting that these results demonstrate that saponins 861

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dx.doi.org/10.1021/bm401708m | Biomacromolecules 2014, 15, 856−862