Thermodynamics of sodium dodecyl sulphate-salicylic acid basedmicellar systems and their potential use in fruits postharvest

A. Cid a,b, J. Morales b, J.C. Mejuto b, N. Briz-Cid c, R. Rial-Otero c, J. Simal-Gándara c,⇑a REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Quinta da Torre, 2829-516 Monte de Caparica, Portugalb Physical Chemistry Department, Faculty of Sciences, University of Vigo, Ourense Campus, E-32004 Ourense, Spainc Nutrition and Bromatology Group, Analytical and Food Chemistry Department, Faculty of Sciences, University of Vigo, Ourense Campus, E-32004 Ourense, Spain

a r t i c l e i n f o

Article history:Received 22 May 2013Received in revised form 27 September 2013Accepted 13 November 2013Available online 21 November 2013

Keywords:Salicylic acidSodium dodecyl sulphateMicellization processInteractionThermodynamicsFood applicationsFruits postharvest

a b s t r a c t

Micellar systems have excellent food applications due to their capability to solubilise a large range ofhydrophilic and hydrophobic substances. In this work, the mixed micelle formation between the ionicsurfactant sodium dodecyl sulphate (SDS) and the phenolic acid salicylic acid have been studied at sev-eral temperatures in aqueous solution. The critical micelle concentration and the micellization degreewere determined by conductometric techniques and the experimental data used to calculate severaluseful thermodynamic parameters, like standard free energy, enthalpy and entropy of micelle formation.Salicylic acid helps the micellization of SDS, both by increasing the additive concentration at a constanttemperature and by increasing temperature at a constant concentration of additive. The formation ofmicelles of SDS in the presence of salicylic acid was a thermodynamically spontaneous process, and isalso entropically controlled. Salicylic acid plays the role of a stabilizer, and gives a pathway to controlthe three-dimensional water matrix structure. The driving force of the micellization process is providedby the hydrophobic interactions. The isostructural temperature was found to be 307.5 K for the mixedmicellar system. This article explores the use of SDS-salicylic acid based micellar systems for their poten-tial use in fruits postharvest.

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1. Introduction

Salicylic acid (SA) is a monohydroxybenzoic acid, a type of phe-nolic acid. SA is the pain-relieving ingredient found in aspirin, anda primary ingredient in many skin medications. It is found inplants, and helps protect the plants against bacterial and fungalinfections. It also prevents inflammation in the body. Diseases,such as Type 2 diabetes, cardiovascular disease, cancer and Alzhei-mer’s disease, are directly connected to chronic inflammation, andthose who have a high intake of plant food have a lower risk forthese chronic diseases. Some individuals are sensitive to SA andexperience headaches, nausea, rashes, itching, swelling of the faceor stomach pain when these foods are consumed; some may alsoexperience anaphylaxis. Due to its relative toxicity and risk ofbio-accumulation, its use has been limited in much the EU. TheFDA has reviewed the safety of salicylic acid and methyl salicylateand permits their use as indirect food additive in food contactmaterials (FDA, 2012a). For instance, SA is authorised for use aspreservative in adhesives and also as catalyst and cross-linkingagent for epoxy resins. In addition, SA attracted much interest be-cause several experimental studies have confirmed that SA could

be used for postharvest handling of fruits and vegetables as a foodadditive. SA treatment was demonstrated to delay fruit ripeningand/or reduce decay of several fruits and vegetables (Li & Han,2000; Srivastava & Dwivedi, 2000; Wei, Liu, Liu, & Ye, 2011; Zhang,Chen, Zhang, & Ferguson, 2003), to alleviate chilling injury of toma-to and cucumber storage at low temperature (Han, Li, & Feng,2002) or to be used as anti-browning agent in fresh-cut Chinesewater chestnut stored at low temperature (Peng & Jiang, 2006).

Micellar systems allow the coexistence of clear, isotropic andthermodynamic stable mixtures of water, oil and a surfactant,the presence of a co-surfactant being frequently necessary to en-sure the stability of the system. These systems have notable indus-trial and food applications due to their capability to solubilise alarge range of hydrophilic and hydrophobic substances (Cid et al.,2013; Esmaili et al., 2011; Fratter & Semenzato, 2011; Gaysinsky,Davidson, McClements, & Weiss, 2008; Kumar, Thakur, Mozumdar,& Patanjali, 2002; Sugiura et al., 2001). For instance, micellar sys-tems provide an effective vehicle for solubilising some water-insol-uble food flavours such as benzyl acetate, 2-phenyl ethanol andgeraniol (Kumar et al., 2002), hydrophobic antimicrobials (Gaysin-sky et al., 2008) or natural antioxidants (Esmaili et al., 2011) andalso as an entrapment medium protecting precursor lipids fromdegradation, hydrolysis and oxidation (Jabeen, Chat, Rather, &Dar, 2013). Although the use of SDS as additive is not allowed for

0308-8146/$ - see front matter ! 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.foodchem.2013.11.076

⇑ Corresponding author.E-mail address: jsimal@uvigo.es (J. Simal-Gándara).

Food Chemistry 151 (2014) 358–363

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food application in the European legislation, it is included in thelist of food additives permitted for direct addition to food for hu-man consumption of the U.S. Food and Drug Administration as a‘‘multipurpose additive’’ (FDA, 2012b). The aim of this work wasthen to evaluate the aggregation/micellar behaviour and interac-tions between sodium dodecyl sulphate (SDS) and SA. The influ-ence of the temperature and the concentration of SDS and SA onthe micellar properties were studied. SDS is widely used as a modelfor the study of micellar structures. The microstructure andproperties of SDS micelles can be tuned for food applications bychanging the concentration of salicylic acid, what produceschanges in the packing and the air–water interface of the micellarsystem, and seems also to be helpful for reducing decay in posthar-vest handling of fruits and vegetables in comparison with SA treat-ments. The final goal would be to reduce the concentration of SA tobe used both in delaying fruit ripening and/or reducing decay ofseveral fruits and vegetables, and preventing browning and/orchilling injury in fresh-cut fruits and vegetables stored at lowtemperature.

2. Materials and methods

2.1. Thermodynamics of the SDS-salicylic acid micellar system

The following reaction shows the reaction mechanism betweenSA and the micellized surfactant (Dn), where subscripts w and Mdenote aqueous and micellar pseudophases, respectively, and Ks

is the binding constant that can be calculated from the Eq. (1).

SAw þ Dn$Ks SAM

Ks ¼½SA$M½SA$w½Dn$

ð1Þ

The concentration of the micellized surfactant is that of totalsurfactant less that of the monomer, which is assumed to be givenby the critical micelle concentration (CMC) following Eq. (2).

½Dn$ ¼ ½Dt $ ' CMC ð2Þ

The micellization degree (b) was calculated from the Eq. (3),where S1 and S2 are the slopes of the conductivity–concentrationcurve in the pre-micellar and post-micellar region (Nusselder &Engberts, 1992).

b ¼ S2

S1ð3Þ

The standard Gibbs energy variation per mol of monomer (DGom)

in the aggregation process can be calculated using the Eq. (4) anal-ogy to the model mass action (Mosquera et al., 1999; Suárez,López-Fontán, Sarmiento, & Mosquera, 1999) where T is the tem-perature, R a constant and XCMC the CMC value expressed in molefraction units.

DGom ¼ ð1þ bÞRT lnðXCMCÞ ð4Þ

The standard enthalpy (DHom) and entropy (DSo

m) of micellizationprocess were obtained using the Eq. (5):

DGom ¼ DHo

m ' TDSom ð5Þ

2.2. Electrical conductivity determination

SDS and SA were both supplied by Sigma–Aldrich (Steinheim.Germany) and used without any further purification. Mill-Q sys-tem was used to purify water used to prepare all solutions.

Because of their high sensitivity (as any change in parameters,such as concentration of solution compounds, implies great

changes of the system) and high reproducibility, conductivity mea-surements provide a consistent method to determine the CMC ofan ionic surfactant-additive colloidal system (García-Río, Leis,Mejuto, Mosquera, & Rodríguez-Dafonte, 2007). Conductivity mea-surements were developed as follows. Two Crison GLP-32 conduc-tivimeters with a cell constant of 0.1 and 0.09 cm'1, respectively,were used to obtain electrical conductivity (j) measurements.Two KCl conductivity standard solutions supplied by Crison withwell-known conductivity values ([KCl] = 0.0100 mol L'1,j = 1413 lS cm'1 and [KCl] = 0.1000 mol L'1, j = 12.88 mS cm'1

at 25 "C) were used to calibrate the conductivimeters. The electri-cal conductivity for SDS micellar systems containing the surfactantat concentrations from 0.1 to 0.001 M and salicylic acid from1 ( 10'5 to 15 ( 10'5 M was determined at different temperaturesbetween 298.15 and 323.15 K. Micellar systems were homogenisedby stirring with a magnetic bar. During the measurements of elec-trical conductivity, temperature was regulated using a thermostat–cryostat Teche TE-8D RB-5, with a precision of ±0.1 "C. The uncer-tainty of the electrical conductivity measurements was ±0.1%.

3. Results and discussion

3.1. Microstructure and properties of SDS-salicylic acid based micellarsystems

The critical micelle concentration (CMC) in pure water at 25 "Cis between 8.0 and 8.3 mM (Akbas & Taliha, 2003) and the aggre-gation number at this concentration is usually considered to beabout 62 (Khan & Shah, 2008). The micelle ionisation fraction (a)is around 0.3% or 30% (Bales, Messina, Vidal, Peric, & Nascimento,1998). The CMC of the SDS/SA system was determined from theplots of the specific conductance (j) versus the concentration ofSDS (0.001–0.1 M). The relationship between j and [SDS] yieldstwo straight lines with two different slopes, corresponding to themonomeric surfactant and surfactant aggregation zones, respec-tively. The CMC coincides with the intersection point betweenthem. Thus, the zone below the CMC corresponds with a mono-meric state of the surfactant, while the zone above the CMC corre-sponds to micellar aggregates and the ratio between both slopescorresponds with the micelle ionisation fraction. On the otherhand, as the degree of micellization is a function of the tempera-ture and the nature of the solvent (Sulthana, Rao, Bhat, & Rakshit,1998), the influence of the temperature (between 298.15 and323.15 k) and the concentration of SA (1 ( 10'5–15 ( 10'5 M) onthe CMC was evaluated (Table 1). Representative plots of the spe-cific conductance (j) versus the concentration of SDS obtained atdifferent temperatures are shown in Fig. 1.

As stated by other authors, there is a quite important effect ex-erted by the temperature upon the appearance of the CMC of ionicand no-ionic surfactants (Attwood & Florence, 1983; Sulthanaet al., 1998). In this sense, when the temperature increases, theCMC of a non-ionic surfactant without additives decreases; whilstfor ionic surfactants (e.g. SDS) the CMC decreases in the region oflow temperature, declines to a minimum and then increases withthe increase of temperature (Attwood & Florence, 1983). As it canbe seen in Table 1 and Fig. 2, CMC values decrease linearly with in-crease in the temperature independently of the SA concentration(R2 > 0.88). Therefore, the behaviour showed by the SDS/SA micel-lar system agrees with the studies reported for non-ionic surfac-tant systems and confirms the non-ionic nature of the mixedmicelles system. Hence, the hydrophobic interactions are the mainreason of the appearance of the micelles. When temperature in-creases, the CMC values decrease as a consequence of the reductionof the hydration sphere of the hydrophobic groups (Attwood &Florence, 1983). With non-ionic surfactants, the CMC suffers an

A. Cid et al. / Food Chemistry 151 (2014) 358–363 359

additional decrease by rupture of the water structure and thesubsequent formation of aggregates of water molecules surround-ing the solute (Sulthana, Bhat, & Rakshit, 1996; Sulthana, Bhat, &Rakshit, 1997). In addition, a linear dependence of CMC with the

SA concentration was observed with independence of the temper-ature. R2 coefficients obtained for these correlations were 0.92,0.93, 0.99 and 0.94 at 298.15, 303.15, 313.15 and 323.15 K, respec-tively. The values of the micelle ionisation degree (b) calculated

Table 1Critical micelle concentration (CMC, mmol kg'1), micelle ionisation degree (b) and standard Gibbs energy variation per mol of monomer (DGo

m , kJ mol'1) values for the SDS/SAmicellar system at variable mole ratios as obtained from electric conductivity measurements.

Salicylic acid concentration ( 10'5 (M) Temperature (K)

298.15 303.15 313.15 323.15

CMC b DGom CMC b DGo

m CMC b DGom CMC b DGo

m

0 8.11.0 7.94 0.57 '34.45 7.84 0.57 '35.08 7.62 0.60 '37.05 7.39 0.60 '38.362.5 7.83 0.56 '34.29 7.66 0.59 '35.62 7.53 0.60 '37.10 7.22 0.61 '38.705.0 7.73 0.56 '34.34 7.53 0.57 '35.24 7.45 0.56 '36.21 7.14 0.59 '38.277.5 7.59 0.61 '35.51 7.42 0.56 '35.07 7.39 0.56 '36.25 7.01 0.59 '38.35

10.0 7.43 0.57 '34.71 7.31 0.57 '35.36 7.24 0.58 '36.80 6.91 0.59 '38.4115.0 7.40 0.57 '34.73 7.20 0.57 '35.42 7.06 0.58 '36.90 6.81 0.60 '38.71

0

10

20

30

40

50

60

0.00 0.02 0.04 0.06 0.08 0.10 0.12

Elec

tric

al c

ondu

ctiv

ity (k

) (m

S cm

-1)

SDS concentration (M)

0

2

4

6

8

10

12

14

16

0.000 0.005 0.010 0.015 0.020

Elec

tric

al c

ondu

ctiv

ity (k

) (m

S cm

-1)

SDS concentration (M)

Fig. 1. Variation of the specific conductivity (j) of the SDS/SA system in water as a function of molar concentration at different temperatures (! 298.15 K; j 303.15 K; N313.15 K; d 323.15 K). Salicylic acid concentration was 1 ( 10'5 M.

360 A. Cid et al. / Food Chemistry 151 (2014) 358–363

from Eq. (3) are also showed in Table 1. In general, the average va-lue of the micellization degree (b = 0.58) is independent of the SAconcentration and the temperature, although for temperatureshigher than 313.15 K a slight increment in the degree of micelleaggregation was observed (Fig. 3). The presence of SA implies asmall change (15% decrease) in the micelle ionisation degree fromvalues of b = 0.7 (Bales et al., 1998) to values in the range of b ) 0.6.From the CMC and b data reported in Table 1, various thermody-namic parameters were calculated. The standard Gibbs energy var-iation per mol of monomer (DGo

m), in the aggregation process wascalculated from Eq. (4). DGo

m values obtained for the SDS/SA mixedmicellar system were negative (Table 1) indicating that the micel-lization process was spontaneous. Furthermore, the micellizationprocess was favoured by high temperatures since DGo

m, in absolutevalues, increased with increasing temperatures for any concentra-tion of salicylic acid. A variation of the water structure in the Guy–Chapman layer and in the Stern layer caused by salicylic acid couldjustify the changes in the DGo

m.The standard entropy (DSo

m) and enthalpy (DHom) of the micelli-

zation process for the different additive concentrations wereobtained from Eq. (5) by plotting the DGo

m values versus tempera-ture (Table 2). The positive DHo

m values obtained for this systemwere independent of temperature and reveal the endothermicbehaviour of the surfactant aggregation process. Moreover,positive DSo

m values indicate that the micellization process wasentropically controlled. Positive DSo

m values are probably due tothe melting of ‘‘icebergs’’ or ‘‘flickering clusters’’ around the tenso-

active. This fact favours to a non-random enhance in packing ofhydrocarbon chains within the micellar core (Mosquera et al.,1999; Shireen, Rao, Bhat, & Rakshit, 1988). Hence, we conjecturethat enhancing the additive concentration (salicylic acid) showsthe way to control the three-dimensional matrix water structure.Positive values obtained for DHo

m and DSom demonstrate the impor-

tance of hydrophobic interactions as a major engine for micelliza-tion (Nusselder & Engberts, 1992; Sulthana et al., 1998). Inaddition, a linear correlation between DHo

m and DSom for the SDS/

SA mixed micellar system was found (Fig. 4), that led us to deter-mine the ‘‘isostructural temperature’’. At this temperature, micelli-zation is independent of any structural changes and only hasdependence of the enthalpic factors. In the case of the SDS/SA acidit appears at 307.5 K with a coefficient R2 = 0.9991 (Fig. 4).

This paper aims to complete the picture of the surfactant SDS byreporting the thermodynamic behaviour in aggregation with SA atdifferent concentrations and temperatures. Anyway, based onother published works, it is envisioned the potential of these sys-tems to be used to extend shelf-life of treated fruits without thesecondary negative effects of SA intolerance in sensitiveconsumers.

3.2. Potential applications in postharvest

Phenolic acids, especially hydroxycinnamic acids and hydroxy-benzoic acids, are secondary plant products and commonly foundin plant derived foodstuff. The major sources of phenolic acids

6.5

7

7.5

8

8.5

0 2 4 6 8 10 12 14 16

Criti

cal m

icel

le c

once

ntra

tion

x 10-3

(mol

kg

-1)

Salicylic acid concentration x 10-5 (M)

Fig. 2. Influence of the temperature and the salicylic acid concentration on CMC for the SDS/SA micellar system as obtained from electrical conductivity measurements (!298.15 K; j 303.15 K; N 313.15 K; d 323.15 K).

Fig. 3. Influence of the temperature on micelle ionisation degree (b) for SDS-salicylic acid mixed micellar system as obtained by electrical conductivity measurements.

A. Cid et al. / Food Chemistry 151 (2014) 358–363 361

(Radtke, Linseisen, & Wolfram, 1998) are coffee, with 92% of thecaffeic acid intake, and fruits (including fruit products and juices),with 75% of the salicylic acid and 59% of the p-coumaric acid in-take. Consequently, phenolic acids are consumed in considerableamounts with food. Dietary salicylates inhibit cycloxygenase-2and may therefore have anti-inflammatory properties similar tothose of aspirin (Scotter et al., 2007). Antioxidant and anticarcino-genic properties of phenolic acids are already proven in vitro aswell as in animal experiments. There is still a need to provewhether the fruit surface application of salicylic acid in micellarsystems helps to extend shelf-life of the stored fruits, althoughSA treatment was demonstrated to delay fruit ripening and/orreduce decay of several fruits and vegetables (Li & Han, 2000;Srivastava & Dwivedi, 2000; Wei et al., 2011; Zhang et al., 2003),alleviate chilling injury of tomatoes and cucumbers stored at lowtemperature (Han et al., 2002) or to be used as anti-browningagent in fresh-cut Chinese water chestnut stored at low tempera-ture (Peng & Jiang, 2006).

Detergents complexed with certain organic salts or acids oftenform threadlike micelles and exhibit viscoelastic behaviour as afunction of the detergent molecular weight and additive concen-tration, reflecting the mutual uncrossability of the micellar net-work (Shikata, Hirata, & Kotaka, 1989). We anticipate that thedifferent systems would lead to differences in the micellar struc-ture and lifetime of the entanglement network. The ultimate goalwill be to lengthen the lifetime of the micellar network as muchas possible to extend product shelf-life at a high quality level,and then washing the micellar network applied before eating thefruit if this material is not approved as an additive/ingredient forhuman consumption. It should be clearly indicated that is not suit-able for consumers with salicylate sensitivity, a clinical conditionof asthma and/or urticaria, but, in this case of consumers withsalicylate intolerance, fruit should be excluded from their dietsanyway (Radtke et al., 1998).

4. Conclusions

Salicylic acid eases micellization of SDS, both by increasing theadditive concentration at a constant temperature and by increasingtemperature at a constant concentration of additive. The results ofthe Gibbs free energy of micellization lead to the conclusion thatthe formation of micelles of SDS in presence of salicylic acid wasa thermodynamically spontaneous process. This was also con-firmed by the DHo

m obtained values. The DSom results indicate that

the formation of micelles in a SDS-salicylic system is entropicallycontrolled. The presence of salicylic acid gives a pathway to controlthe three-dimensional matrix water structure, and the main engineof the micellization process is provided by the hydrophobicinteractions because DHo

m and DSom are both positive. Hence, we

can conclude that the isostructural temperature taken from the lin-ear fit of DHo

m and DSom, is 307.5 K for this mixed micellar system.

The work in the end explores the use of SDS-salicylic acid basedmicellar systems for their potential use in fruits postharvest.Future studies need to check the fruit surface application of sodiumdodecyl sulphate-salicylic acid based micellar systems in compar-ison with SA treatments to demonstrate their advantages in delay-ing fruit ripening and/or reducing decay of several fruits andvegetables, and preventing browning and/or chilling injury infresh-cut fruits and vegetables stored at low temperature.

Acknowledgements

Authors are grateful to the European Union for provision ofFEDER funds. A. Cid thanks FCT-MCTES (Portugal) for his Postdoc-toral grant contract (SFRH/BPD/78849/2011).

References

Akbas, H., & Taliha, M. (2003). Effect of polyoxyethylene chain length andelectrolyte on the viscosity of mixed micelle. Turkish Journal of Chemistry, 27,357–363.

Attwood, D., & Florence, A. T. (1983). Surfactant system: Their chemistry, pharmacyand biology. London: Chapman and Hall.

Bales, B. L., Messina, L., Vidal, A., Peric, M., & Nascimento, O. R. (1998). Precisionrelative aggregation number determinations of SDS micelles using a spin probe.A model of micelle surface hydration. The Journal of Physical Chemistry B,102(50), 10347–10358.

Cid, A., Mejuto, J. C., Orellana, P. G., López-Fernández, O., Rial-Otero, R., & Simal-Gandara, J. (2013). Effects of ascorbic acid on the microstructure and propertiesof SDS micellar aggregates for potential food applications. Food ResearchInternational, 50, 143–148.

Esmaili, M., Ghaffari, S. M., Moosavi-Movahedi, Z., Atri, M. S., Sharifizadeh, A.,Farhadi, M., et al. (2011). Beta casein-micelle as a nano vehicle for solubilityenhancement of curcumin, food industry application. LWT – Food Science andTechnology, 44, 2166–2172.

FDA (2012a). Code of Federal Regulations Title 21 – Food and Drugs. Part 175 –Indirect food additives: adhesives and components of coatings. <http://cfr.regstoday.com/21cfr.aspx> Accessed 1.03.2013.

FDA (2012b). Code of Federal Regulations, Title 21- Food and Drugs. Part 172 - Foodadditives permitted for direct addition to food for human consumption. <http://cfr.regstoday.com/21cfr.aspx> Accessed 1.03.2013.

Fratter, A., & Semenzato, A. (2011). New association of surfactants for theproduction of food and cosmetic nanoemulsions: Preliminary developmentand characterization. International Journal of Cosmetic Science, 33, 443–449.

García-Río, L., Leis, J. R., Mejuto, J. C., Mosquera, V., & Rodríguez-Dafonte, P. (2007).Stability of mixed micelles of cetylpyridiniumchloride and linear primaryalkylamines. Colloids and Surfaces A, 309, 216–223.

Gaysinsky, S., Davidson, P. M., McClements, D. J., & Weiss, J. (2008). Formulation andcharacterization of phytophenol-carrying antimicrobial microemulsions. FoodBiophysics, 3, 54–65.

Han, T., Li, L. P., & Feng, S. Q. (2002). Effect of exogenous salicylic acid onphysiological parameters of cucumber and tomato fruits stored at chillinginjury temperature. Scientia Agricultura Sinica, 35, 571–575.

Jabeen, S., Chat, O. A., Rather, G. M., & Dar, A. A. (2013). Investigation of antioxidantactivity of Quercetin (2-(3, 4-dihydroxyphenyl)-3,5,7-trihydroxy-4H-chromen-4-one) in aqueous micellar media. Food Research International, 51, 294–302.

Khan, A. M., & Shah, S. S. (2008). Determination of critical micelle concentration(Cmc) of sodium dodecyl sulfate (SDS) and the effect of low concentration ofpyrene on its Cmc using ORIGIN software. Journal of the Chemical Society ofPakistan, 30(2), 186–191.

Table 2Standard enthalpy (DHo

m , kJ mol'1) and standard entropy (DSom , kJ mol'1 K'1) of

micellization obtained from the variation of standard Gibbs energy variation per molof monomer (DGo

m) with temperature.

Salicylic acid concentration ( 10'5 (M) DHm DSm R2

1.0 13.92 0.16 0.9922.5 16.53 0.17 0.9885.0 10.69 0.15 0.9777.5 1.23 0.12 0.852

10.0 9.68 0.15 0.99915.0 12.55 0.16 0.997

Fig. 4. Determination of isostructural temperature of the SDS-salicylic acid micellarsystem from electrical percolation measurements.

362 A. Cid et al. / Food Chemistry 151 (2014) 358–363

Kumar, R., Thakur, L., Mozumdar, S., & Patanjali, P. K. (2002). Solubilization of somehydrophobic food flavouring agents in single and mixed micellar systems.Journal of Surface Science and Technology, 18, 153–161.

Li, L. P., & Han, T. (2000). Effects of salicylic acid on the quality of peaches stress-induced oxidative damage in Arabidopsis involves calcium, abscisic acid,ethylene, and salicylic acid. Fruit Science, 17, 97–100.

Mosquera, V., Ruso, J. M., Attwood, D., Jones, M. N., Prieto, G., & Sarmiento, F. (1999).Thermodynamics of micellization of surfactants of low aggregation number:The aggregation of propranolol hydrochloride. Journal of Colloid and InterfaceScience, 210, 97–102.

Nusselder, J. J. H., & Engberts, J. B. F. N. (1992). Toward a better understanding of thedriving force for micelle formation and micellar growth. Journal of Colloid andInterface Science, 148, 353–361.

Peng, L., & Jiang, Y. (2006). Exogenous salicylic acid inhibits browning of fresh-cutChinese water chestnut. Food Chemistry, 94, 535–540.

Radtke, J., Linseisen, J., & Wolfram, G. (1998). Phenolsäurezufuhr Erwachsener ineinem bayerischen Teilkollektiv der Nationalen Verzehrsstudie. ZErnährungswiss, 37, 190–197.

Scotter, M. J., Roberts, D. P. T., Wilson, L. A., Howard, F. A. C., Davis, J., & Mansell, N.(2007). Free salicylic acid and acetyl salicylic acid content of foods using gaschromatography–mass spectrometry. Food Chemistry, 105, 273–279.

Shikata, T., Hirata, H., & Kotaka, T. (1989). Micelle formation of detergent moleculesin aqueous media. 3. Viscoelastic properties of aqueouscetyltrimethylammonium bromide-salicylic acid solutions. Langmuir, 5,398–405.

Shireen, B. S., Rao, P. V. C., Bhat, S. G. T., & Rakshit, A. K. (1988). The Journal of PhysicalChemistry, 92, 9653–9660.

Srivastava, M. K., & Dwivedi, U. N. (2000). Delayed ripening of banana fruit bysalicylic acid. Plant Science, 158, 87–96.

Suárez, M., López-Fontán, J. L., Sarmiento, F., & Mosquera, V. (1999).Thermodynamic study of the aggregation behaviour of sodium n-hexyl sulfatein aqueous solution. Langmuir, 15, 5265–5270.

Sugiura, S., Ichikawa, S., Sano, Y., Nakajima, M., Liu, X. Q., Seki, M., et al. (2001).Formation and characterization of reversed micelles composed ofphospholipids and fatty acids. Journal of Colloid and Interface Science, 240,566–572.

Sulthana, S. B., Bhat, S. G. T., & Rakshit, A. K. (1996). Thermodynamics ofmicellization of a non-ionic surfactant Myrj 45: Effect of additives. Colloidsand Surfaces A, 111, 57–65.

Sulthana, S. B., Bhat, S. G. T., & Rakshit, A. K. (1997). Studies of the effect of additiveson the surface and thermodynamic properties of poly(oxyethylene(10)) laurylether in aqueous solution. Langmuir, 13, 4562–4568.

Sulthana, S. B., Rao, P. V. C., Bhat, S. G. T., & Rakshit, A. K. (1998). Interfacial andthermodynamic properties of SDBS–C12E10 mixed micelles in aqueous media:Effect of additives. J. Phys. Chem. B, 102, 9653–9660.

Wei, Y., Liu, Z., Liu, D., & Ye, X. (2011). Effect of salicylic acid treatment onpostharvest quality, antioxidant activities, and free polyamines of Asparagus.Journal of Food Science, 76, S126–S132.

Zhang, Y., Chen, K. S., Zhang, S. L., & Ferguson, I. (2003). The role of salicylic acid inpostharvest ripening of kiwifruit. Postharvest Biology and Technology, 28, 67–74.

A. Cid et al. / Food Chemistry 151 (2014) 358–363 363