Water Sorption Isotherms of Surfactants: A Tool To

Article pubs.acs.org/JAFC

Water Sorption Isotherms of Surfactants: A Tool To Evaluate Humectancy Elisabeth Asmus,† Christian Popp,‡ Adrian A. Friedmann,§ Katja Arand,*,† and Markus Riederer† †

Department of Botany II, University of Würzburg, Julius-von-Sachs-Platz 3, DE-97082 Würzburg, Germany Global Formulation Technology, Syngenta Crop Protection, Breitenloh 5, CH-4333 Münchwilen, Switzerland § Syngenta Crop Protection AG, Syngenta Crop Protection, Schwarzwaldallee 215, CH-4002 Basel, Switzerland ‡

S Supporting Information *

ABSTRACT: Fundamental experimental data for moisture absorption of non-ionic polydisperse surfactants with differing ethylene oxide (EO) content and variable aliphatic portions were measured at relative humidities between 0 and 95% at 25 °C. Remarkable differences in moisture absorption were observed between surfactant classes but also within one series of surfactants differing in either EO content or the long-chain aliphatic fraction. Both the EO units as well as the entire molecular structure, including also the lipophilic domain, were discussed to account for the humectant activity of surfactants. Water sorption isotherms showed an exponential shape, which was argued to be associated with the formation of a “free” water domain. These humectant properties might be relevant to the behavior of a foliar-applied spray droplet of agrochemical formulation products because the uptake of active ingredients will be enhanced as a result of deferred crystal precipitation. KEYWORDS: non-ionic surfactants, polysorbates, Tween, oleyl alcohol ethoxylates, water sorption isotherms, humidity, humectant activity

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droplet drying and active ingredient crystallization9 or enhance cuticular swelling.19 Stevens and Bukovac20 could show within one distinct class of ethoxylated octylphenol surfactants that water absorption increases with an increasing EO content, at least at RHs above 70%. However, the humectant potential of surfactants has not yet been studied and quantified rigorously. Therefore, in the present study, the moisture sorption ability of several commonly used non-ionic polydisperse surfactants with differing EO contents and variable aliphatic chains was investigated comprehensively. From the obtained water sorption isotherms, relationships between the molecular structure and the water sorption behavior can be deduced. This knowledge can help to predict the humectant properties of a broad range of surfactants used in pesticide spray formulations as one important step toward an integrative understanding of how surfactants can influence the foliar uptake of active ingredients.

INTRODUCTION The efficacy of pesticides is strongly influenced by environmental conditions at the time of foliar application, with optimal uptake being favored by humid conditions.1 Particularly, the uptake of highly water-soluble active ingredients is enhanced under high relative humidity (RH).2 It is commonly believed that the humidity effect is basically related to the rate or extent of droplet drying, which may reduce the availability of certain active ingredients as a result of crystal precipitation.1−4 Increased active ingredient uptake and efficacy were shown to be associated with gel-like or amorphous residues without crystalline deposits.5,6 Consequently, a large variety of substances, so-called humectants, might be used to increase the “equilibrium water content and increase the drying time of an aqueous spray deposit”7 on the leaf surface.3 Humectants absorb and retain moisture from the surrounding atmosphere. A very prominent organic humectant is glycerol, which is commonly used in pharmaceutical, cosmetic, and food products to give them the desired flexibility, softness, and shelf life.8 In the context of plant protection agents, humectancy of non-ionic polyethoxylated surfactants is believed to be related to a high ethylene oxide (EO) content, because surfactants with a higher EO content enhance cuticular uptake of water-soluble active ingredients.9−12 In most ethoxylated surfactants, the hydrophilic moiety is composed of polyoxyethylene chains. Ethoxylated surfactants are thought to improve the uptake of hydrophilic and lipophilic herbicides by different modes of action.13 Low EO content surfactants having a comparably high lipophilicity enhance the uptake of lipophilic herbicides by entering the plant cuticle14−16 and altering the cuticular wax fluidity.17,18 Surfactants with a high EO content have been suggested to have humectant properties2 and thereby delay © 2016 American Chemical Society

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MATERIALS AND METHODS

Chemicals. Non-ionic sorbitan fatty acid esters (Span series) and their polyethoxylates (Tween series), oleyl alcohol ethoxylates (Genapol O series), and polyoxyethylene sorbitol hexaoleate (Atlas G1096) covering a broad range of physical and chemical properties were used in the experiments (Table 1). For generalized chemical structures of the surfactant molecules, refer to Figure S1 of the Supporting Information. Also, anhydrous glycerol was studied as a reference chemical. Received: Revised: Accepted: Published: 5310

March 24, 2016 May 24, 2016 June 7, 2016 June 7, 2016 DOI: 10.1021/acs.jafc.6b01378 J. Agric. Food Chem. 2016, 64, 5310−5316

Article

Journal of Agricultural and Food Chemistry Table 1. Selected Chemical and Physical Properties of Surfactants Used in Experiments trade name

chemical name

CAS registry number

mean MW (g mol−1)

HLBa

mean EO content

appearance

major chain length

glycerolb Span 20c Span 40d Span 60d Span 65d Span 80c Span 85c Tween 20c Tween 40d Tween 60d Tween 65d Tween 80c Tween 81c Tween 85c Genapol O-050e Genapol O-080e Genapol O-100e Genapol O-200e Atlas G1096c

propane-1,2,3-triol sorbitan monolaurate sorbitan monopalmitate sorbitan monostearate sorbitan tristearate sorbitan monooleate sorbitan trioleate polyoxyethylen sorbitan monolaurate polyoxyethylene sorbitan monopalmitate polyoxyethylene sorbitan monostearate polyoxyethylene sorbitan tristearate polyoxyethylen sorbitan monooleate polyoxyethylen sorbitan monooleate polyoxyethylen sorbitan trioleate oleyl alcohol polyglycol ether oleyl alcohol polyglycol ether oleyl alcohol polyglycol ether oleyl alcohol polyglycol ether polyoxyethylene sorbitol hexaoleate

56-81-5 1338-39-2 26266-57-9 1338-41-6 26658-19-5 1338-43-8 26266-58-0 9005-64-5 9005-66-7 9005-67-8 9005-71-4 9005-65-6 9005-65-6 9005-70-3 9009-91-0 9004-98-2 68920-66-1 68920-66-1 57171-56-9

92.09 346.46 402.57 430.61 963.54 428.60 957.49 1227.72 1283.57 1311.62 1844.54 1309.66 648.87 1838.50 488.74 620.90 709.00 1149.53 3887.59

8.4 7.3 6.8 2.3 6.8 2.3 16.7 16.0 15.7 10.7 15.7 11.3 10.8 8.4 10.9 12.0 15.1 11.3

0 0 0 0 0 0 20 20 20 20 20 5 20 5 8 10 20 50

liquid liquid pellet crystals dry powder dry powder liquid liquid liquid liquid pasty, wax-like wax-like liquid liquid liquid liquid liquid pasty pasty, wax-like wax-like liquid

(12:0) (16:0) (18:0) (18:0) (18:1) (18:1) (12:0) (16:0) (18:0) (18:0) (18:1) (18:1) (18:1) (18:1) (18:1) (18:1) (18:1) (18:1)

a

HLB values were taken from or recalculated according to Pasquali et al.29 bAppliChem GmbH (Darmstadt, Germany). cCroda (Nettetal, Germany). dSigma-Aldrich Chemie GmbH (Steinheim, Germany). eClariant (Muttenz, Switzerland). Water Sorption Isotherms. Water sorption isotherms were determined using a gravimetrical sorption test system (SPS11-10μ, ProUmid GmbH & Co. KG, Ulm, Germany). The sorption test instrument automatically determines the water uptake/release of up to 10 samples in parallel in a test atmosphere with controlled temperature and RH. The device is equipped with an analytical microbalance (precision of ±10 μg) determining the change in the sample mass at regular time intervals. When all samples are in equilibrium with the water vapor partial pressure of the test chamber, the sample masses remain stable. Subsequently, the RH is increased/decreased to the next level (Figure 1). The relative equilibrium water sorption (% of dry weight) of a specific surfactant is highly specific and independent of the initial weight, but a higher net weight requires more time for equilibration (Figure 1). Approximately 100 mg of pure substance was evenly spread over the bottom of the sample dishes to ensure a flat surface and a film

thickness comparable among all compounds. Powdery surfactants were previously melted at 50 °C. Before measurement, all samples were dried under a flow of nitrogen until they reached a constant weight. Equilibrium water sorption (mass %) was determined by increasing RH in 10% intervals from 0 to 60% and at 5% intervals from 60 to 95% and in the same way back to 0%. Individual equilibrium values for each humidity were used to generate water sorption isotherms.

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RESULTS The mass of the samples significantly increased when the RH in the sample chamber was increased from 0 to 95% (Figure 2). When the RH was subsequently decreased again from 95 to 0%, sample weights decreased without hysteresis. The resulting moisture sorption isotherms showed an exponential shape with a steeper increase starting at 60−70% RH (Figure 2). As an exception, two sorbitan fatty acid esters (Span 65 and Span 85) exhibited no significant increase in weight at 95% RH. Water sorption only reached 1.7 and 1.2% of the initial mass, respectively. In general, water sorption was low in the sorbitan fatty acid esters (Span series; Figure 2A) and much more pronounced in the polyethoxylated surfactants (Tween series and Genapol O series; panels B and C of Figure 2). Even at 95% RH, none of the sufactants sorbed more water than their initial dry weight, and therefore, the maximum mass increases were below 100%. In the group of sorbitan fatty acid esters, Span 20 reached a maximum water sorption of 42% of the initial dry weight at 95% RH (Figure 2A). In comparison, the water sorption at 95% RH was about twice as high for the polyethoxylated surfactants Tween 20 (80%; Figure 2B) and Genapol O-200 (76%; Figure 2C). The water sorption isotherm of polyoxyethylene sorbitol hexaoleate (Atlas G1096) was comparable to that of Span 20 (Figure 2D). The water absorption (mass %) of glycerol was much more pronounced than that of any surfactant investigated here. It reached a value of about 350% of the initial dry weight at 95% RH (Figure 2D). The investigated water sorption isotherms of Span 20, 40, 60, and 80 (Figure 2A) and the Genapol O series (Figure 2C) clearly differ from each other.

Figure 1. Typical sequential sorption measurement for two samples (with different initial dry weights) of Tween 20 and Tween 80 at a controlled temperature (red). The sample weight increased with moisture absorption. When equilibrium with the surrounding atmosphere was reached, the sample weight remained stable and humidity (blue) was raised to the next level. 5311

DOI: 10.1021/acs.jafc.6b01378 J. Agric. Food Chem. 2016, 64, 5310−5316

Article

Journal of Agricultural and Food Chemistry

Figure 2. Water sorption isotherms for (A) Span series, (B) Tween series, (C) Genapol O series, and (D) polyoxyethylene sorbitol hexaoleate (Atlas G1096) and glycerol expressed as moisture content (mass %) versus RH (%).

the hydration behavior of the pure surfactants and exclude side factors, which might affect the behavior of a droplet residue of a complex spray mixture. Most studied surfactants absorb water from the atmosphere, although to different extents. The sorbitan esters (Span series) lacking any EO have a significantly lower water sorption ability than the corresponding ethoxylated polysorbates (Tween series) and the oleyl alcohol polyoxyethylene ethers (Genapol O series; Figure 2). This indicates that the EO content plays a significant role for the water sorption ability of surfactants. In several experiments, cuticular uptake and efficiency of watersoluble active ingredients were enhanced by high EO content surfactants.9,11,24 Therefore, it was hypothesized that humectancy is related to a high EO content. Indeed, within the class of ethoxylated octylphenol surfactants (Triton X series), with EO contents ranging from 5 to 40, moisture content (mass %) increases with the EO content, at least at high humidities.20 This effect was also found for the Genapol O series, where water sorption also increases with the EO content (Figure S2 of the Supporting Information). However, the moisture content (mass %) of several polysorbates with identical EO content strongly differs, and the moisture content (mass %) of Atlas

Water sorption at 95% RH decreases from 42% for Span 20 to 10% for Span 80, while Span 40 and 60 have intermediate values (Figure 2A). Within the Genapol O series, water sorption systematically increases from Genapol O-050 (35.3% at 95% RH) to Genapol O-200 (70.7% at 95% RH) (Figure 2C). Similar relationships were not observed within the group of polysorbates (Tween). Water sorption isotherms for Tween 20, Tween 40, Tween 60, and Tween 80 cluster together, while Tween 65, Tween 81, and Tween 85 show a significantly lower water sorption (Figure 2B).

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DISCUSSION Surfactants are widely used in agrochemical applications.21 Non-ionic polyethoxylated surfactants are postulated to have the ability to sorb and retain water,10,20 helping to keep the active ingredient in the spray deposit in a physical state, which favors its uptake into the leaf.22,23 Therefore, the present work studied the humectant properties of selected non-ionic surfactants in a systematic approach. Comprehensive experimental data on the water sorption ability of commonly used non-ionic polydisperse surfactants at the full range of RHs up to 95% are presented. The aim was to focus systematically on 5312


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Figure 3. Comparison of water sorption isotherms for selected surfactants from different classes based on (A) moisture content (mass %) or (B) humectant activity nws (molwater molsurfactant−1).

Table 2. Humectant Activity (nws) and Humectant Activity Per Oxygen Content (nws/nO) at Five Different Humidities 30% RH

a

a

trade name

mean O content

nws

glycerol Span 20 Span 40 Span 60 Span 65 Span 80 Span 85 Tween 20 Tween 40 Tween 60 Tween 65 Tween 80 Tween 81 Tween 85 Genapol O-050 Genapol O-080 Genapol O-100 Genapol O-200 Atlas G1096

3 6 6 6 8 6 8 26 26 26 28 26 11 28 6 9 11 21 56

0.52 0.05 0 0 0 0 0.05 0.69 1.24 1.22 0.18 0.46 0 0.50 0.26 0.40 0.38 0.21 1.81

nws/nO 0.17 0.01 n.d. n.d. n.d. n.d. 0.01 0.03 0.05 0.05 0.01 0.02 n.d. 0.02 0.04 0.04 0.03 0.01 0.03

50% RH b

80% RH

90% RH

95% RH

nws

nws/nO

nws

nws/nO

nws

nws/nO

nws

nws/nO

1.30 0.10 0.17 0.06 0.09 0.15 0.10 2.77 3.25 3.24 1.54 2.25 0.57 1.70 0.63 1.00 1.18 1.06 4.80

0.43 0.08 0.02 0.03 0.01 0.02 0.02 0.11 0.13 0.12 0.06 0.09 0.05 0.06 0.11 0.11 0.11 0.05 0.09

5.08 2.23 1.64 1.54 0.33 0.76 0.36 18.72 17.50 17.13 10.24 16.98 3.85 8.91 3.07 5.71 8.36 16.37 23.65

1.69 0.37 0.27 0.26 0.04 0.13 0.04 0.72 0.67 0.66 0.37 0.65 0.35 0.32 0.51 0.63 0.76 0.78 0.42

11.00 4.65 3.56 3.06 0.61 1.56 0.48 37.34 33.24 32.92 20.02 34.03 7.83 19.63 6.89 10.67 16.41 32.39 52.68

3.67 0.78 0.59 0.51 0.08 0.26 0.06 1.44 1.28 1.27 0.72 1.31 0.71 0.70 1.15 1.19 1.49 1.54 0.94

16.84 7.43 5.44 4.51 0.84 2.42 0.64 52.40 44.52 43.76 26.11 47.03 11.44 31.36 9.28 13.61 22.89 48.47 78.00

5.61 1.24 0.91 0.75 0.10 0.40 0.08 2.02 1.71 1.68 0.93 1.81 1.04 1.12 1.55 1.51 2.08 2.31 1.39

nws = humectant activity, number of moles of water sorbed per mole of surfactant. bnws/nO = humectant activity per oxygen content of surfactant.

variation in molecular size among the studied surfactants. The order of the mole-fraction-based sorption isotherms (Figure 3B) differed from the respective mass-based isotherms (Figure 3A). Atlas G1096 has a low mass-based moisture content (mass %) (Figure 3A) but shows the highest humectant activity (nws), with 78 mol of water/mol of surfactant being sorbed at 95% RH (Table 2). This is about 1.6-fold higher than in Tween 80 and Genapol O-200 and even 4.6 times more than for glycerol. Atlas G1096 indeed takes up the most water (Figure 3B), as would have been expected from the 50 EO units. In anhydrous surfactants, the hydrophobic and hydrophilic moieties align.27 In linear non-branched surfactants, this results in a lamellar phase, where the hydroxyl functional groups or ethoxy oxygens of adjacent molecules are in close proximity.27 Single water molecules from the surrounding air absorbed in

G1096 is always significantly lower than expected from its EO content. The sorption isotherms of surfactants from different classes all bearing one or more octadecyl chains but differing in their EO content were also compared (Figure 3A). The water sorption (mass %) of Atlas G1096, which has the highest EO content (50 EO units), was lower than those of Tween 80 and Genapol O-200, which only have 20 EO units (Figure 3A). Glycerol showed by far the highest increase in mass (about 350% at 95% RH). This leads to the assumption that the water sorption capacity cannot be predicted solely by the degree of ethoxylation but that, instead, the entire molecule has to be taken into account. Therefore, we calculated the humectant activity (nws) from the number of moles of water sorbed per mole of surfactant (mole fraction)25,26 to correct the significant 5313


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Figure 4. Correlation between the (A) moisture content (mass %) or (B) humectant activity nws (molwater molsurfactant−1) and the mean EO content for all surfactants at different RHs. Parameter of the regression lines are given in Table S1 of the Supporting Information. Correlation between the (C) moisture content (mass %) or (D) humectant activity nws (molwater molsurfactant−1) and the HLB for all surfactants at different RHs. Parameter of the regression lines are given in Table S2 of the Supporting Information. Symbols indicated with an asterisk represent polyoxyethylene sorbitol hexaoleate (Atlas G1096) as an outlier.

surfactants, with an alignment of the hydrophilic and lipophilic domains. The architecture of the lipophilic portion of the surfactant molecules will strongly contribute to the availability of polar sites for hydration. Within the Span series, the oxygen content remains constant (with the exception of Span 65 and Span 85), while the lipophilic domain changes. With an increasing chain length of the fatty acid, the humectant activity (nws) of the Span surfactants and also the humectant activity per oxygen content (nws/nO) decrease (Table 2). For example, at 95% RH, each oxygen atom in Span 20 (containing a dodecylic acid) is on average associated with 1.24 water molecules, which is 1.65 times more than in Span 60 (0.75) with an octadecenoic acid, because the increasing lipophilicity of the alkyl chain impairs the accessibility of the adjacent oxygen atoms. The introduction of a double bond that leads to a cis conformation of the C18 fatty acid in Span 80 results in a further decreased humectant activity of the oxygen atoms (0.4) compared to Span 60 (0.75), with a straight conformation of the C18 fatty acid. Span 65 and

this lamellar phase can cross-link two adjacent oxygen atoms. However, the probability for a water molecule to settle far away from the alkyl chain is higher than to interact with oxygens close to the lipophilic domain.27 One water molecule attached to the facing ends of the polar chains is required per two surfactant molecules to totally cross-link the network. When the surfactant sorbs more water, the water molecules either attach to different locations along ethoxy chains or associate with other water molecules already hydrogen bound to polar groups. The latter leads to the growth of a “free” water domain.27 This process is expressed by the exponential shape of the isotherms. The flat slope at the beginning belongs to the phase where single water molecules cross-link single surfactant molecules. At higher RH and, thus, higher water contents of the surfactant, the steep increase is induced by the growth of the “free” water domain and the hydration of the inner oxygen atoms, which are closer to the lipophilic end. All used surfactants in this study have complex three-dimensional polar heads, but it is likely that molecules will arrange in the same way as the straight 5314


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and crystalline deposit containing surfactants with a strong humectant activity is visible under high humidity conditions (personal observation). The formation of aqueous phases within the deposit might prevent the active ingredient from crystal precipitation. Furthermore, also the hydration of the cuticle and, hence, the availability of the hydrophilic pathway30 for the permeation of hydrophilic solutes might be enhanced. Thus, several surfactants used in this study are supposed to enhance uptake of water-soluble active ingredients by their humectant activity. Nevertheless, a formulation consists of a complex mixture of several auxiliary ingredients, which might interact and impact the hydration. Therefore, the method for measuring water sorption isotherms introduced in this paper is a useful tool, which could be applied to complex mixtures of agrochemical formulation products to evaluate the process of water evaporation and rehydration of the foliar deposit. A comprehensive understanding of the underlying mechanism will help to improve spray formulations to optimize uptake and efficiency of plant protection agents.

Span 85 only sorb very small amounts of water (Table 2) presumably because their oxygen atoms are shielded by the three fatty acids. In the group of Genapol O, where the lipophilic portion remains constant, the humectant activity (nws) clearly increases with the EO content (Figure S2 of the Supporting Information) and also with an increasing oxygen content (Table 2). Most members of the Tween series possess a huge polar headgroup with on average 20 EO groups, resulting in a high humectant activity. Even at 30% RH, the values for Tween 20, 40, and 60 are greater than that for glycerol (Table 2). Modification of the single fatty acid domain (e.g., elongation or unsaturation) does not remarkably influence the humectant activity. However, introducing additional fatty acids decreases humectant activity probably as a result of a shielding effect (Figure 2B). The hydrophilic−lipophilic balance (HLB) is a basic proxy for the relative importance of hydrophilic and lipophilic domains in surfactant molecules.28 For an aliphatic alcohol series with a constant hydrophobic tail and differing EO contents13 and also for the Genapol O series (Figure S2 and Table S3 of the Supporting Information), HLB correlates with the degree of ethoxylation. However, when a broad range of surfactant classes is considered, the HLB value does not correlate with the level of ethoxylation because both the lipophilic and the hydrophilic domains change in different ways. Two surfactants with the same level of ethoxylation can possess different HLBs and two surfactants with a widely varying molecular structure could possess the same HLB. From these findings, Stock and Briggs23 concluded that humectancy is related to the EO content rather than to the HLB. In contrast to this, in the present work, the moisture absorption (mass %) across a broad range of surfactants correlates well with their HLB values (Figure 4C and Table S1 of the Supporting Information) and not with the EO content (Figure 4A and Table S2 of the Supporting Information). This means that the HLB can be used to rank the water sorption ability (mass %) of surfactants among different classes. Nevertheless, the HLB value does not consider sterical effects within the surfactant molecules. For example, the polyoxyethylene sorbitol hexaoleate (Atlas G1096) strongly differs from the other surfactants in size and architecture (Figure S1 of the Supporting Information) because the lipophilic domain consists of six fatty acids. Therefore, Atlas G1096 does not fit the correlation when humectant activity is plotted versus HLB over all surfactant classes (Figure 4D). Obviously, water sorption of surfactants used in this work is relatively low, and one would not expect humectant potential. Glycerol is known to be a very efficient humectant, but even at very low RHs (30%), some members of the Tween series, Atlas G1096, and, with increasing RH, also members of the Genapol O series show a higher humectant activity (nws) than glycerol. As discussed above, the exponential shape of the moisture sorption isotherms of the surfactants studied here indicates that water sorption results in the formation of “free” water domains. This will be relevant to the properties of a spray droplet residue on a leaf surface and their consequences for the availability of the active ingredient to the uptake into the leaf. Even though the concentration of surfactants in the spray solution seems quite low (about 0.5−0.05%), once the solvent has evaporated from the droplet, their concentration increases drastically and further droplet drying is reduced as a result of the humectant activity of the surfactants. A rehydration of an obviously dry

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jafc.6b01378. Regression parameter considering panels A and B of Figure 4 (Table S1), regression parameters considering panels C and D of Figure 4 (Table S2), parameter of the regression lines (Table S3), generalized chemical structures of sorbitan fatty acid esters (Span), polysorbates (Tween), polyglycol ether oleyl alcohol (Genapol O), and polyoxyethylene sorbitol hexaoleate (Atlas G1096) (Figure S1), and correlation between the moisture content (mass %) or the humectant activity (nws) (molwater molsurfactant−1) and the (A) EO content or (B) HLB of different oleyl alcohol polyglycol ethers (Genapol O series) at 90% RH (Figure S2) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +49-931-31-86219. Fax: +49-931-31-86235. Email: [email protected]. Funding

This work was supported by a grant from Syngenta Crop Protection AG. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Gudrun Reichenauer and her group at the Bavarian Centre for Applied Energy Research, Würzburg, Germany, for providing equipment, technical assistance, and helpful suggestions.

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ABBREVIATIONS USED dm, difference in mass; EO, ethylene oxide; HLB, hydrophilic− lipophilic balance; nws, humectant activity; nws/nO, humectant activity per oxygen atom of surfactant; RH, relative humidity

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REFERENCES

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Water Sorption Isotherms of Surfactants: A Tool To

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