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Preservative solubilization induces microstructural change of Triton X-100 micelles
Journal of Molecular Liquids 216 (2016) 156–163
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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Preservative solubilization induces microstructural change of Triton X-100 micelles Urja Patel, Nilesh Dharaiya ⁎, Pratap Bahadur Department of Chemistry, Veer Narmad South Gujarat University, Surat 395007, India
a r t i c l e
i n f o
Article history: Received 14 September 2015 Received in revised form 18 December 2015 Accepted 22 December 2015 Available online xxxx Keywords: Triton X-100 Micellar growth Solubilization Parabens NOESY Scattering
a b s t r a c t The interaction of preservatives which are extensively used in food, pharmaceutical and personal care products, such as p-hydroxy benzoic acid esters (parabens) and gallic acid ester for instance propyl gallate has been investigated with commonly used non-ionic surfactant p-tert-octylphenoxy polyethylene (9.5) ether, Triton X-100 (TX-100). Solubilization of these preservatives alters the micellar behaviour of TX-100 elucidated by cloud point (CP), viscometry, dynamic light scattering (DLS), small angle neutron scattering (SANS) and NOESY measurement techniques. The changes in size/shape of TX-100 micelles depend on the hydrophilic and hydrophobic interactions with preservatives. These types of interaction facilitate the site of solubilization of preservatives in TX-100 micelles. Micellar growth of TX-100 was more pronounced for parabens as their alkyl ester chain length increases. Moreover, the micellar growth was more prominent for propyl paraben (PP) as compared to more polar propyl gallate (PG). The salt induced changes in the interaction between preservatives and TX-100 micelles have also been determined. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Parabens and gallates are well known preservatives which have long been used in industries for detaining or preventing the progress of oxidative deterioration because of their wide-ranging antimicrobial/ antifungal activity, stability and aqueous solubility [1–4]. Hence, these preservatives are extensively utilized as antioxidants in personal care products [1,5], pharmaceuticals [6], foodstuffs [7,8] and beverages [9]. Generally, parabens and gallates work in the aqueous medium and their solubilities can be altered in the presence of surfactants. Moreover, the preservative activity of the parabens and gallates significantly depends on their degree of bindings with surfactants [10–12]. Furthermore, such preservatives alter the micellar properties of aqueous surfactant systems [13,14]. Therefore, our interest has been centred to study the interaction between preservatives and surfactants in consideration of its probable application as preservatives in surfactant based formulations. Several articles have been published relating the possible mechanism of the interaction of parabens and gallates with surfactants [10–20]. Solubilization studies of parabens have been investigated in the aqueous solutions of polyoxyethylene type nonionic surfactants [11,15,18,20] and anionic surfactant, sodium dodecyl sulphate (SDS) [12,20]. The solubilization sites of parabens and PG in the surfactant micelles have been examined using fluorescence probe and 1H NMR methods [11,16]. Heins et al. [17] have determined that the antioxidant activity of gallates depends on their solubilization sites in ⁎ Corresponding author. E-mail address: [email protected] (N. Dharaiya).
http://dx.doi.org/10.1016/j.molliq.2015.12.079 0167-7322/© 2015 Elsevier B.V. All rights reserved.
micelles using electron spin resonance (ESR) technique and concluded that cetyltrimethylammonium bromide (CTAB) and Polyoxyethylene (20) cetyl ether (Brij 58) provide more antioxidant activity to gallates than SDS. Several studies have illustrated that parabens and their salts are efficient to change the geometry of surfactant micelles [13,14,21]. Khimani et al. [13] examined the different solubilization effects of methyl and butyl parabens in pluronic micelles using small angle neutron scattering (SANS). They observed that methyl paraben (MP) leads to sphere to rod micellar transition for both hydrophobic pluronic (P103) and moderately hydrophobic pluronics (P104, P105). However, butyl paraben (BP) offers the same effect for P103, but formed micellar clusters for P104 and P105 due to the inception of inter-micellar interaction. Paraben induced moderate growth of hydrophilic pluronic F127 micelles has been reported using SANS [14]. Kroflic et al. [21] studied ethyl paraben sodium salt induced elongated micelles of dodecyltrimethylammonium chloride (DTAC) with higher aggregation number whereas less hydrophobic methyl paraben sodium salt did not show such type of effect. Parabens and gallates are widely used preservatives, but they are harmful to human and animals. Parabens and gallates have been reported as endocrine disruptors and reproductive toxic [4,22,23], allergens [5,24] to humans and toxic to aquatic organisms [25,26]. Therefore, several authors have investigated green and less costly cloud point extraction (CPE) method using polyoxyethylated nonionic surfactants for separation of these types of preservatives [27–30]. Non-ionic surfactants represent an important class of amphiphiles among which Triton X series surfactants have been extensively employed in industrial and domestic fields [31]. These surfactants
U. Patel et al. / Journal of Molecular Liquids 216 (2016) 156–163
have been extensively studied for the solubilization of aromatic hydrocarbons [32–35], pesticides [36,37], phenols [38,39] and nitrotoluenes [40]. Moreover, these surfactants are used widely for the removal of hazardous substances like dyes [41,42], pesticides [43], phenols [44], and heavy metals [45] by CPE method. Our research group has investigated the locus of solubilization of phenols [38,39], aromatic acids [46], aromatic amine [47], alcohols and glycol ethers [48] and their effect considering pH, temperature and hydrophobicity on micellar characteristics of Triton X-100 (TX-100) using spectral and scattering methods. Nevertheless, the interaction of parabens and gallates with Triton X series surfactants in aqueous media and resulting micellar properties have been not studied yet. For that reason, we have investigated the effect of a homologous series on the length of the alkyl chain esters of chemicals that include methyl- (MP), ethyl- (EP), propyl- (PP), butyl- (BP), and benzyl parabens (BzP) on the micellar behaviour of TX-100. The effect of propyl paraben was compared with more hydrophilic propyl gallate (PG). The presence of salts has been found to alter the interaction between additive and TX-100 in aqueous media. We carried out cloud point, viscosity, DLS, SANS and NOESY experiments to invent the solubilization and locus induced morphological changes of TX-100 micelles.
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2.4. Turbidimetry Different solutions were prepared containing 5% TX-100 and PG concentration range of 0–55 mM in water, 1 M NaCl and 1 M NaI solutions. The transmittance (%T) was measured for these solutions after agitating for 24 h. Transmittance was measured at 420 nm by UV–Visible spectrophotometer (Thermo Scientific TM Evolution 300). The transmittance has been converted to turbidity by using formula of 100 − %T [34]. 2.5. Small angle neutron scattering (SANS) The SANS measurements were carried out using a SANS diffractometer at the Dhruva reactor, BARC, Trombay. For SANS measurements, all the solutions were prepared in D2O. Q range of 0.017–0.35 Å has been used for the scattering measurements. Data were corrected for background, empty-cell influence and sample transmission, and normalized to absolute cross-section units. The fitted parameters in the analysis were optimized using non-linear least square fitting programme to the model scattering. The detailed procedure of experiments can be found elsewhere [38,46]. 2.6. Nuclear magnetic resonance
2. Material and methods Triton X-100 (TX-100) was purchased from Sigma-Aldrich and used without further purification. Methyl paraben (MP), ethyl paraben (EP), propyl paraben (PP), butyl paraben (BP) and benzyl paraben (BzP) were also obtained from Sigma-Aldrich and used as received. Propyl gallate (PG) was purchased from SRL (India) and used without further purification.
2.1. Cloud point (CP) The cloud point (CP) was determined at a fixed concentration of TX100 (5 wt.%) at varying concentrations of parabens and gallate by gradually heating solutions in thin glass tubes immersed in a stirred heating bath. The CP has been considered as sudden attendance of turbidity in the solution by gradual increase in the temperature. The CP values were consistent within 0.5 °C.
2.2. Viscosity The viscosity measurements were achieved in a temperature controlled water bath with constancy of ± 0.1 °C. Calibrated Cannon Ubbelohde viscometers have been used with sizes 25 and 100 having constants 0.001869 and 0.01610 cSts−1, respectively. The variation in flow time was found to be ±5 s. Initially absolute viscosities of the solutions were obtained which were multiplied by viscometer constant to get kinematic viscosity in centistokes. Then viscosities in centipoise have been obtained by the multiplying the kinematic viscosity with density of solvent (water). These viscosities of solutions were divided by viscosity of water to obtain the relative viscosity (ηrel) [38].
2.3. Dynamic light scattering (DLS) The DLS experiments were carried out at a fixed scattering angle of 90° and wave length of 633 nm using Zeta sizer Nano-ZS 4800 (Malvern Instruments, UK) having He–Ne laser. Each measurement was repeated at least three times. The apparent hydrodynamic radius (Rh) of the micelles was calculated using the Stokes–Einstein equation. All samples were prepared in Millipore water and filtered through a 0.45 μm filter to avoid dust particles.
2D NMR (NOESY) experiments were performed on a Bruker AVANCE-II 400 MHz spectrometer at St. Fx University Canada. Samples were prepared in D2O. The spectrum was calibrated by setting the HDO peak at a chemical shift of 4.65 ppm at 298 K. The HDO peak due to residual water was eliminated by solvent suppression techniques. All measurements were performed at 30 °C. 3. Results and discussion Parabens possess a common aromatic ring, –OH group but different alkyl/aryl groups which define their solubility and play an important role in interaction with surfactant micelles. The structure, solubility and octanol–water partition coefficient (Po/w) of different parabens and propyl gallates used are shown in Scheme 1 and Table 1. A high Po/w, is likely to exhibit significant penetration in micellar phase [39, 48], while low Po/w value indicates that the compound remains more in aqueous bulk phase than the micellar phase. The effects of nature and concentration of these additives on phase behaviour and micellar characteristics of aqueous solution of TX-100 made from turbidimetric, scattering (DLS and SANS) and viscosity are discussed below. The interaction between the propyl paraben/gallate and TX-100 is also discussed by NOESY. 3.1. Effect of additive concentration Fundamental information regarding the interaction of preservatives with TX-100 micelles was obtained by the measurements of CP and relative viscosity. These measurements were carried out by keeping fixed concentration of TX-100 (5%) and varying concentrations of the additives. For nonionic surfactant CP is an important parameter which is very sensitive in the presence of additives even at very low concentrations [48–50]. If an additive alters CP of surfactant, it indicates changes in interaction between water and surfactant which result in changes in micellar size/shape. Changes in microstructure of micelles also lead to changes in solution viscosity. Fig. 1 shows CP, relative viscosity (ηrel) and hydrodynamic diameter (Rh) of 5% TX-100 micellar solution as a function of concentration of different additives viz. MP, EP, PP, BP and BzP. Results indicate that all additives decrease the CP and increase in viscosity of TX-100 solution but in different manners. It has been noticed that as the length of the hydrocarbon chain of paraben increases from MP to BP, the CP decreases while viscosity and micellar radius increase. BzP was also efficient and provides greater changes in micellar behaviour of TX-100 as compared to alkyl parabens.
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Scheme 1. Chemical structures of parabens and propyl gallate.
The CP of a nonionic surfactant relies on the balance between the hydrophobic and hydrophilic interactions. Generally, water molecules interact with the hydrophilic shell region (polyoxyethylene chain) of the nonionic surfactant via H-bonding which can be altered in the presence of additives [50–53]. ‘Water structure breaker’ additives increase such an interaction and enhance the hydration of micelles, while ‘water structure maker’ additives decrease the hydration of micelles. Here the change in hydration of micelles depends on the interaction of parabens with TX-100 micelles. Kandori et al. [50] explained that –OH group of phenol composes H-bond between their phenolic –OH group and the ethereal oxygen atom of the POE chain of nonionic surfactant which reduces H-bond between water and shell region of micelles. In this way parabens also decrease the hydration of TX-100 micelles. Moreover, hydrophobic interaction is likely for alkyl ester chain of parabens and hydrophobic region (core) of TX-100 micelles. Thus parabens having longer alkyl chain can easily penetrate in the micelles and reduce the hydration in shell as well as core region of TX-100 micelles. Solubilizates are located in the interior of the micelles according to their Po/w [39,40]. As the alkyl chain of parabens increases, paraben molecules penetrate deeper toward the core of micelles and induce greater changes in micellar behaviour of TX-100. BzP has phenyl ring instead of alkyl chain in other parabens thus its molecules have greater possibility to site at near core–shell interface close to phenyl groups of TX-100 micelles. Solubilization leads to changes in the packing parameter of micelles and induces micelle transitions corresponding to their location. From the above finding it has been clear that longer alkyl chain length of paraben induces growth of TX-100 micelles. Further investigation was carried out keeping the same nonpolar moiety but different polar groups in phenyl ring. Here, PG has a similar structure like PP except for two more –OH groups on the phenyl ring. PP has an aqueous solubility ~2.6 mM while PG offers higher solubility approximately ~17 mM in water at 25 °C [54–56]. PG has lower octanol–water partition coefficient than PP. These overall differences between the properties of PP and PG on the physical characteristics of TX-100 micelles are interesting.
Thus, their comparative effect on the solution behaviour of 5% TX-100 as a function of concentration was investigated by CP, viscosity and DLS measurements [Fig. 2]. The results reveal that PP gives more pronounced effect than PG. It can be understood that PP causes more dehydration and microstructural changes in TX-100 micelle than PG. Nonionic surfactants are widely used as emulsifiers, solubilizers and detergents in industrial formulation. However, all nonionic surfactants lose their stability in dispersion formulation above their CP. Therefore we studied the phase behaviour for TX-100 at different concentrations with increasing concentration of PP and PG at fixed temperature of 30 °C [Fig. 2]. Fig. 3 demonstrates that less concentration of PP is required than PG for the phase separation of TX-100 solution. Commonly additives interact with nonionic surfactant micelles on the basis of hydrophobic and hydrophilic interactions and alter phase behaviour [39, 49]. PG containing two more polar –OH groups than PP provides greater hydrophilicity and interaction with POE chain (shell) of micelles. PP is less polar than PG thus it offers more hydrophobic interaction which leads more dehydration and facilitates phase separation of TX-100 solution as compared to PG. So these phenomena are possible for the phase separation of these systems. This study reveals that PG can be used in
Table 1 Octanol/water partition coefficient (Log Po/w) and solubility in water at 25 °C of parabens and propyl gallate. Additive
Solubility (g/L)
Log Po/w
Propyl gallate Methyl paraben Ethyl paraben Propyl paraben Butyl paraben Benzyl paraben
3.5 2.5 1.7 0.5 0.2 0.1
1.80 1.96 2.47 3.04 3.57 3.61
Note: Po/w and solubility data are taken from Ref. [54–56].
Fig. 1. Effect of additives on cloud point (CP), relative viscosity (ηrel) and micellar hydrodynamic radius (Rh) of 5% TX-100 in aqueous solution in the presence of ( ) MP, (○) EP, ( ) PP, ( ) BP, and ( ) BzP.
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Fig. 2. Effect of additives on cloud point (CP), relative viscosity (ηrel) and micellar hydrodynamic radius (Rh) of 5% TX-100 in aqueous solution in the presence of ( ) PP and ( ) PG.
high concentration with any nonionic surfactant formulation as compared to PP. To find out pronounced interaction between PP/PG with TX-100, NOESY measurements have been carried out which reveal the locus of solubilization of PP and PG in TX-100 micelles. 3.2. Location of PP and PG in TX-100 micelles The NOESY spectrum offers detailed information concerning the intra- and inter-molecular interactions between the protons of additive and surfactant molecules in micellar system. The existence of cross peaks between additive and surfactant protons indicates their closeness to each other. Yuan et al. [57] using NMR and NOESY described that monomers of TX-100 are arranged in micelles in a way that the hydrophobic micellar core is surrounded by a thick shell of hydrophilic POE chain. The location of phenols [39], aromatic amines [47], alcohols and glycol ethers [48] in TX-100 micelles has been determined using NOESY experiments. Fukahori et al. [11] studied that the incorporation
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of parabens in the micelles of polyoxyethylene cetyl ether depends on the alkyl ester chain of the parabens using fluorescence probe and 1H NMR. In this study we want to compare the location of PP and PG in TX-100 micelles which reveals their effects on the micellar growth of TX-100. Fig. 4 shows the proton labelling of TX-100, PP and PG and NOESY spectra of their mixed system containing 5% TX-100 in the presence of 25 mM PP and PG. Some protons of these additives have been merged with the TX-100 protons but considerable changes in cross-peak intensity can provide the aspect of interactions. However, we have mainly considered the interaction from other unmerged protons of PP and PG which are labelled in the spectra. Fig. 4 shows the cross-peaks between alkyl chain protons (A1) of PP with the protons of methyl (T1, T2) and methylene (T3) group of TX-100 micelle core which are more intense than the alkyl chain protons (B1) of PG with same protons of TX-100. Phenyl protons (A4, A5) of PP show intense cross-peak with the protons of phenyl group (T7, T8) and POE group (T4) of TX-100 while weak cross-peak with the chain protons (T1, T2). The results indicate that PP molecules are deeply solubilized near the phenyl group region (core–shell interface) and their alkyl ester chains remain toward core in TX-100 micelles because of hydrophobic interaction between the alkyl chain of PP and hydrophobic core of TX-100 micelles. While PG has less Po/w and three polar –OH groups which allow fewer penetration of its molecules in TX-100 micelles and likely to remain in polar region (shell) of micelle. Overall finding indicates that PP penetrates more in TX-100 micelles therefore it has greater effect on the micellar behaviour relatively than PG. Heins et al. [16] have examined the interaction between PG and differently charged surfactants using 1H NMR and showed that PG is solubilized in the palisade layer of cetyltrimethylammonium bromide (CTAB) micelles, shell region of Brij 58 whereas in the Stern layer of the SDS micelles. Along these lines PG interacts more with CTAB than SDS while Brij 58 has intermediary interactions with PG. To determine the exact details concerning the PP and PG solubilized induce micellar geometry of TX-100, we have carried out some SANS measurements. 3.3. Small angle neutron scattering (SANS) Previous SANS studies indicate that TX-100 micelles are ellipsoidal having high aggregation numbers [38,58]. Here, we were interested to find out the preservative induced micellar change of TX-100. Fig. 5 shows the SANS patterns of 5% TX-100 solution in the presence of different concentrations of PG and PP. From SANS data analysis the values of semi major axis (Rb), semi minor axis (Ra), aggregation number (Nagg)
Fig. 3. Effect of PP (a) and PG (b) concentration on the phase behaviour of TX-100 at different concentrations at 30 °C.
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Fig. 4. (a) Proton labelling of (I) TX-100 (II) PP and (III) PG. (b) NOESY spectrum of 5% TX-100 in presence of 25 mM (I) PP and (II) PG at 30 °C.
and number density of micelles (nm) have been derived [Table 2]. SANS data of these systems have been fitted using ellipsoidal core–shell model. Fig. 5 indicates noticeable increase in the scattering intensity in the small Q region for the 5% TX-100 solution with solubilized preservatives. These changes show growth of TX-100 micelles. Table 2 demonstrates accurate information on increase in axial ratio, aggregation number and decrease in number density of micelles. The aggregation number of micelles, Nagg, was calculated from the following formula,
where Vm =(4/3 πa2b) is the micellar volume, and Vh is the volume of the hydrophobic part of the surfactant monomer. The value of Vh has been taken from earlier study [38]. The number density of micelles (nm) has been determined by the following expression [38]:
ð1Þ
where NA is the Avogadro's number, CMC is the critical micelle concentration and C is the concentration of surfactant in mol/L.
Nagg¼ Vm =V
h
N nm cm−3 ¼ ðC−CMCÞ A 10−3 Nagg
Fig. 5. SANS profile for 5% TX-100 solution in absence (□) and the presence of (a) PP ( ) and (b) PG ( ) at 30 °C.
ð2Þ
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Table 2 Micellar characteristics of TX-100 in the presence of PP and PG by SANS analysis. Additives
Rh (nm)
Ra (nm)
Rb (nm)
Ra/Rb
Nagg
nm (cm−3)
Shape of micelle
No additive 10 mM PP 25 mM PP 25 mM PG 40 mM PG
5.7 9.1 30.0 12.5 23.8
6.3 7.7 14.9 8.8 13.7
2.0 2.0 2.1 2.0 2.0
3.2 3.7 7.1 4.3 6.8
287 363 725 413 641
16 × 1016 13 × 1016 7 × 1016 12 × 1016 8 × 1016
Ellipsoidal Ellipsoidal Short rod-like Ellipsoidal Short rod-like
Where Rh, hydrodynamic radius; Ra, semimajor axis; Rb, semiminor axis; Ra/Rb, axial ratio (ellipticity); Nagg, aggregation number and nm, number density of micelle.
The analysis of SANS distribution shows that 5% TX-100 forms the ellipsoidal micelles having semimajor axis 1.98 nm and semiminor axis 6.35 nm of hydrophobic core. In both cases the micelle size increases with increasing concentration of additives. In the case of PP, its 10 mM concentration leads to small increase in micelle size. PP induces ellipsoidal to short rod like transition while PG increases ellipticity of TX-100 micelles in some extent at 25 mM concentration. Furthermore, PG leads to short rod-like micelle formation at 40 mM concentration. SANS plots of TX-100 with 25 mM PP and 40 mM PG have a slope of about −1 on log–log scale in low Q-region which specify the formation of rod-like micelles. In this way additive induced micellar changes are greater for PP than PG. The growth of surfactant micelles in the presence of additives depends on the locus of solubilization in micelles. PP penetrates more inside in micelle and more increase in the hydrophobicity of TX-100 micelles ensuing considerable micellar enlargement even at low concentration. These SANS results have a literal correlation with the relative viscosity and DLS measurements.
3.4. Effect of salt Commonly, electrolytes easily alter the clouding behaviour of nonionic surfactants. The electrolytes which make the water structure increase the CP whereas the water structure breaker electrolytes show decrease in the CP [49–53]. Koshy et al. [51] investigated the influence of lyotropic series of inorganic ions on the CP of TX-100 and TX-114 and reported that halogen anions offer their effect in the order: Cl− N Br− N I− on the decrease in CP. Furthermore, NaCl decreases the CPs for both surfactants while NaI enhances the CP. NaCl induces the hydrodynamic size and aggregation number of TX-100 micelles [59]. In the present study the effect of NaCl and NaI on the interaction between TX-100 and additive has been investigated using PG only. This study has been determined using turbidity, viscosity and DLS measurements. Fig. 6 shows the turbidity measurements of 5% TX-100 solution as a function of PG concentration in the presence of 1 M NaCl and 1 M NaI. In all micellar systems first turbidity increases gradually but drastically above certain concentration of PG. This sudden change in turbidity can be considered as the CP of TX-100 at 30 °C in the presence of specific concentration of PG. In the absence of salt 5% TX-100 solution shows clouding behaviour by 43 mM PG at 30 °C. However, solution of 5% TX-100 + 1 M NaCl offers clouding behaviour at very low concentration of PG about ~19 mM while 53 mM PG requires for the system containing 5% TX-100 + 1 M NaI. The effect of these salts on the relative viscosity of the TX-100 solution containing PG has been also determined [Fig. 7]. The viscosity of TX100 increases gradually with increase in concentration of PG. These electrolytes do not change the viscosity of 5% TX-100 significantly in the absence of PG. However, the viscosity of TX-100 solution increases drastically in the presence of 1 M NaCl and lower concentration of PG. However, the presence of NaI increment in the relative viscosity is less even at higher concentration of PG. Fig. 8 shows the effect of salts on the micelle size (Rh, nm) of TX-100 in the presence and absence of PG. 1 M NaCl increases the size of TX-100 micelle while 1 M NaI decreases the size in small extent.
Fig. 6. Effect of PG concentration on the turbidity of 5% TX-100 solution in the absence ( ) and presence of 1 M NaCl ( ) and 1 M NaI ( ) at 30 °C.
Micellar hydrodynamic radius of TX-100 increases from 5.5 nm to 8.6 nm in the presence of PG. The micelle size of 5% TX-100 + 15 mM PG system significantly increases with 1 M NaCl and decreases with 1 M NaI. PG molecules penetrate in TX-100 micelle and set up H-bond between the phenolic –OH group and oxygen atom of the hydrophilic POE moiety of TX-100 molecule. It causes decrease in micellar hydration resulting in decrease in CP and increase in micelle size. NaI increases the H-bonding between water and POE chain resulting in increased hydration thus high concentration of PG requires for dehydration of TX-100 micelles. NaCl reduces the H-bond interaction between water and polyoxyethylene chain of TX-100 thus less PG molecules require for dehydration of TX-100 micelles. Overall results regarding the effect of salts indicate that NaCl (water structure maker) induces the interaction between TX-100 and PG while water structure maker NaI decreases the interaction.
Fig. 7. Effect of PG concentration on the relative viscosity (ƞrel) of 5% TX-100 solution in the absence ( ) and presence of 1 M NaCl ( ) and 1 M NaI ( ) at 30 °C.
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Fig. 8. Effect of PG (15 mM) on the size (Rh) distribution curve of 5% TX-100 solution in the absence and presence of 1 M NaCl and 1 M NaI at 30 °C.
4. Conclusion The purpose of this study is to investigate the preservative induced structural changes in TX-100 micelles using phase behaviour, relative viscosity, spectral and scattering measurements. In case for parabens as the alkyl chain increases, their efficiency increase to enhance the hydrophobicity in TX-100 micelles resulting decrease in the CP and lead to micellar enlargement. The effect of polar group is also investigated by comparing PP and PG where less polar PP molecules situate deep in micelle at core–shell interphase while PG solubilized at shell region of TX100 micelles. PG has dominant hydrophilic interaction while PP has leading hydrophobic interaction with TX-100 micelles are responsible for the different effect of these preservatives on phase behaviour and micellar geometry of TX-100. Hydrophilic–hydrophobic interaction between TX-100 and preservatives can be altered by the addition of water structure maker/breaker salts. Generally the nonionic surfactants are extremely helpful in the formation of emulsions and formulations as well as the taken preservatives are also widely applicable in industrial and biological applications. Therefore the present study provides an ultimate consideration of interaction between preservatives and nonionic surfactants which will be useful in the development of industrial formulation as well as environmental friendly methods for the separation/extraction of preservatives. Acknowledgement U.P. and N.D. gratefully acknowledge UGC, New Delhi for the providing research fellowships. PB thanks Dr. K. Singh (St. FX University) and Dr. V. K. Aswal (BARC, Mumbai) for NOESY and SANS measurements. References [1] F.A. Andersen, Final amended report on the safety assessment of methylparaben, ethylparaben, propylparaben, isopropylparaben, butylparaben, isobutylparaben, and benzylparaben as used in cosmetic products, Int. J. Toxicol. 27 (4) (2008) 1. [2] T. Eklund, Inhibition of microbial growth at different pH levels by benzoic and propionic acids and esters of p-hydroxybenzoic acid, Int. J. Food Microbiol. 2 (3) (1985) 159. [3] S. Ito, S. Yazawa, Y. Nakagawa, Y. Sasaki, S. Yajima, Effects of alkyl parabens on plant pathogenic fungi, Bioorg. Med. Chem. Lett. 25 (8) (2015) 1774.
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Preservative solubilization induces microstructural change of Triton X-100 micelles Urja Patel, Nilesh Dharaiya ⁎, Pratap Bahadur Department of Chemistry, Veer Narmad South Gujarat University, Surat 395007, India
a r t i c l e
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Article history: Received 14 September 2015 Received in revised form 18 December 2015 Accepted 22 December 2015 Available online xxxx Keywords: Triton X-100 Micellar growth Solubilization Parabens NOESY Scattering
a b s t r a c t The interaction of preservatives which are extensively used in food, pharmaceutical and personal care products, such as p-hydroxy benzoic acid esters (parabens) and gallic acid ester for instance propyl gallate has been investigated with commonly used non-ionic surfactant p-tert-octylphenoxy polyethylene (9.5) ether, Triton X-100 (TX-100). Solubilization of these preservatives alters the micellar behaviour of TX-100 elucidated by cloud point (CP), viscometry, dynamic light scattering (DLS), small angle neutron scattering (SANS) and NOESY measurement techniques. The changes in size/shape of TX-100 micelles depend on the hydrophilic and hydrophobic interactions with preservatives. These types of interaction facilitate the site of solubilization of preservatives in TX-100 micelles. Micellar growth of TX-100 was more pronounced for parabens as their alkyl ester chain length increases. Moreover, the micellar growth was more prominent for propyl paraben (PP) as compared to more polar propyl gallate (PG). The salt induced changes in the interaction between preservatives and TX-100 micelles have also been determined. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Parabens and gallates are well known preservatives which have long been used in industries for detaining or preventing the progress of oxidative deterioration because of their wide-ranging antimicrobial/ antifungal activity, stability and aqueous solubility [1–4]. Hence, these preservatives are extensively utilized as antioxidants in personal care products [1,5], pharmaceuticals [6], foodstuffs [7,8] and beverages [9]. Generally, parabens and gallates work in the aqueous medium and their solubilities can be altered in the presence of surfactants. Moreover, the preservative activity of the parabens and gallates significantly depends on their degree of bindings with surfactants [10–12]. Furthermore, such preservatives alter the micellar properties of aqueous surfactant systems [13,14]. Therefore, our interest has been centred to study the interaction between preservatives and surfactants in consideration of its probable application as preservatives in surfactant based formulations. Several articles have been published relating the possible mechanism of the interaction of parabens and gallates with surfactants [10–20]. Solubilization studies of parabens have been investigated in the aqueous solutions of polyoxyethylene type nonionic surfactants [11,15,18,20] and anionic surfactant, sodium dodecyl sulphate (SDS) [12,20]. The solubilization sites of parabens and PG in the surfactant micelles have been examined using fluorescence probe and 1H NMR methods [11,16]. Heins et al. [17] have determined that the antioxidant activity of gallates depends on their solubilization sites in ⁎ Corresponding author. E-mail address: [email protected] (N. Dharaiya).
http://dx.doi.org/10.1016/j.molliq.2015.12.079 0167-7322/© 2015 Elsevier B.V. All rights reserved.
micelles using electron spin resonance (ESR) technique and concluded that cetyltrimethylammonium bromide (CTAB) and Polyoxyethylene (20) cetyl ether (Brij 58) provide more antioxidant activity to gallates than SDS. Several studies have illustrated that parabens and their salts are efficient to change the geometry of surfactant micelles [13,14,21]. Khimani et al. [13] examined the different solubilization effects of methyl and butyl parabens in pluronic micelles using small angle neutron scattering (SANS). They observed that methyl paraben (MP) leads to sphere to rod micellar transition for both hydrophobic pluronic (P103) and moderately hydrophobic pluronics (P104, P105). However, butyl paraben (BP) offers the same effect for P103, but formed micellar clusters for P104 and P105 due to the inception of inter-micellar interaction. Paraben induced moderate growth of hydrophilic pluronic F127 micelles has been reported using SANS [14]. Kroflic et al. [21] studied ethyl paraben sodium salt induced elongated micelles of dodecyltrimethylammonium chloride (DTAC) with higher aggregation number whereas less hydrophobic methyl paraben sodium salt did not show such type of effect. Parabens and gallates are widely used preservatives, but they are harmful to human and animals. Parabens and gallates have been reported as endocrine disruptors and reproductive toxic [4,22,23], allergens [5,24] to humans and toxic to aquatic organisms [25,26]. Therefore, several authors have investigated green and less costly cloud point extraction (CPE) method using polyoxyethylated nonionic surfactants for separation of these types of preservatives [27–30]. Non-ionic surfactants represent an important class of amphiphiles among which Triton X series surfactants have been extensively employed in industrial and domestic fields [31]. These surfactants
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have been extensively studied for the solubilization of aromatic hydrocarbons [32–35], pesticides [36,37], phenols [38,39] and nitrotoluenes [40]. Moreover, these surfactants are used widely for the removal of hazardous substances like dyes [41,42], pesticides [43], phenols [44], and heavy metals [45] by CPE method. Our research group has investigated the locus of solubilization of phenols [38,39], aromatic acids [46], aromatic amine [47], alcohols and glycol ethers [48] and their effect considering pH, temperature and hydrophobicity on micellar characteristics of Triton X-100 (TX-100) using spectral and scattering methods. Nevertheless, the interaction of parabens and gallates with Triton X series surfactants in aqueous media and resulting micellar properties have been not studied yet. For that reason, we have investigated the effect of a homologous series on the length of the alkyl chain esters of chemicals that include methyl- (MP), ethyl- (EP), propyl- (PP), butyl- (BP), and benzyl parabens (BzP) on the micellar behaviour of TX-100. The effect of propyl paraben was compared with more hydrophilic propyl gallate (PG). The presence of salts has been found to alter the interaction between additive and TX-100 in aqueous media. We carried out cloud point, viscosity, DLS, SANS and NOESY experiments to invent the solubilization and locus induced morphological changes of TX-100 micelles.
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2.4. Turbidimetry Different solutions were prepared containing 5% TX-100 and PG concentration range of 0–55 mM in water, 1 M NaCl and 1 M NaI solutions. The transmittance (%T) was measured for these solutions after agitating for 24 h. Transmittance was measured at 420 nm by UV–Visible spectrophotometer (Thermo Scientific TM Evolution 300). The transmittance has been converted to turbidity by using formula of 100 − %T [34]. 2.5. Small angle neutron scattering (SANS) The SANS measurements were carried out using a SANS diffractometer at the Dhruva reactor, BARC, Trombay. For SANS measurements, all the solutions were prepared in D2O. Q range of 0.017–0.35 Å has been used for the scattering measurements. Data were corrected for background, empty-cell influence and sample transmission, and normalized to absolute cross-section units. The fitted parameters in the analysis were optimized using non-linear least square fitting programme to the model scattering. The detailed procedure of experiments can be found elsewhere [38,46]. 2.6. Nuclear magnetic resonance
2. Material and methods Triton X-100 (TX-100) was purchased from Sigma-Aldrich and used without further purification. Methyl paraben (MP), ethyl paraben (EP), propyl paraben (PP), butyl paraben (BP) and benzyl paraben (BzP) were also obtained from Sigma-Aldrich and used as received. Propyl gallate (PG) was purchased from SRL (India) and used without further purification.
2.1. Cloud point (CP) The cloud point (CP) was determined at a fixed concentration of TX100 (5 wt.%) at varying concentrations of parabens and gallate by gradually heating solutions in thin glass tubes immersed in a stirred heating bath. The CP has been considered as sudden attendance of turbidity in the solution by gradual increase in the temperature. The CP values were consistent within 0.5 °C.
2.2. Viscosity The viscosity measurements were achieved in a temperature controlled water bath with constancy of ± 0.1 °C. Calibrated Cannon Ubbelohde viscometers have been used with sizes 25 and 100 having constants 0.001869 and 0.01610 cSts−1, respectively. The variation in flow time was found to be ±5 s. Initially absolute viscosities of the solutions were obtained which were multiplied by viscometer constant to get kinematic viscosity in centistokes. Then viscosities in centipoise have been obtained by the multiplying the kinematic viscosity with density of solvent (water). These viscosities of solutions were divided by viscosity of water to obtain the relative viscosity (ηrel) [38].
2.3. Dynamic light scattering (DLS) The DLS experiments were carried out at a fixed scattering angle of 90° and wave length of 633 nm using Zeta sizer Nano-ZS 4800 (Malvern Instruments, UK) having He–Ne laser. Each measurement was repeated at least three times. The apparent hydrodynamic radius (Rh) of the micelles was calculated using the Stokes–Einstein equation. All samples were prepared in Millipore water and filtered through a 0.45 μm filter to avoid dust particles.
2D NMR (NOESY) experiments were performed on a Bruker AVANCE-II 400 MHz spectrometer at St. Fx University Canada. Samples were prepared in D2O. The spectrum was calibrated by setting the HDO peak at a chemical shift of 4.65 ppm at 298 K. The HDO peak due to residual water was eliminated by solvent suppression techniques. All measurements were performed at 30 °C. 3. Results and discussion Parabens possess a common aromatic ring, –OH group but different alkyl/aryl groups which define their solubility and play an important role in interaction with surfactant micelles. The structure, solubility and octanol–water partition coefficient (Po/w) of different parabens and propyl gallates used are shown in Scheme 1 and Table 1. A high Po/w, is likely to exhibit significant penetration in micellar phase [39, 48], while low Po/w value indicates that the compound remains more in aqueous bulk phase than the micellar phase. The effects of nature and concentration of these additives on phase behaviour and micellar characteristics of aqueous solution of TX-100 made from turbidimetric, scattering (DLS and SANS) and viscosity are discussed below. The interaction between the propyl paraben/gallate and TX-100 is also discussed by NOESY. 3.1. Effect of additive concentration Fundamental information regarding the interaction of preservatives with TX-100 micelles was obtained by the measurements of CP and relative viscosity. These measurements were carried out by keeping fixed concentration of TX-100 (5%) and varying concentrations of the additives. For nonionic surfactant CP is an important parameter which is very sensitive in the presence of additives even at very low concentrations [48–50]. If an additive alters CP of surfactant, it indicates changes in interaction between water and surfactant which result in changes in micellar size/shape. Changes in microstructure of micelles also lead to changes in solution viscosity. Fig. 1 shows CP, relative viscosity (ηrel) and hydrodynamic diameter (Rh) of 5% TX-100 micellar solution as a function of concentration of different additives viz. MP, EP, PP, BP and BzP. Results indicate that all additives decrease the CP and increase in viscosity of TX-100 solution but in different manners. It has been noticed that as the length of the hydrocarbon chain of paraben increases from MP to BP, the CP decreases while viscosity and micellar radius increase. BzP was also efficient and provides greater changes in micellar behaviour of TX-100 as compared to alkyl parabens.
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Scheme 1. Chemical structures of parabens and propyl gallate.
The CP of a nonionic surfactant relies on the balance between the hydrophobic and hydrophilic interactions. Generally, water molecules interact with the hydrophilic shell region (polyoxyethylene chain) of the nonionic surfactant via H-bonding which can be altered in the presence of additives [50–53]. ‘Water structure breaker’ additives increase such an interaction and enhance the hydration of micelles, while ‘water structure maker’ additives decrease the hydration of micelles. Here the change in hydration of micelles depends on the interaction of parabens with TX-100 micelles. Kandori et al. [50] explained that –OH group of phenol composes H-bond between their phenolic –OH group and the ethereal oxygen atom of the POE chain of nonionic surfactant which reduces H-bond between water and shell region of micelles. In this way parabens also decrease the hydration of TX-100 micelles. Moreover, hydrophobic interaction is likely for alkyl ester chain of parabens and hydrophobic region (core) of TX-100 micelles. Thus parabens having longer alkyl chain can easily penetrate in the micelles and reduce the hydration in shell as well as core region of TX-100 micelles. Solubilizates are located in the interior of the micelles according to their Po/w [39,40]. As the alkyl chain of parabens increases, paraben molecules penetrate deeper toward the core of micelles and induce greater changes in micellar behaviour of TX-100. BzP has phenyl ring instead of alkyl chain in other parabens thus its molecules have greater possibility to site at near core–shell interface close to phenyl groups of TX-100 micelles. Solubilization leads to changes in the packing parameter of micelles and induces micelle transitions corresponding to their location. From the above finding it has been clear that longer alkyl chain length of paraben induces growth of TX-100 micelles. Further investigation was carried out keeping the same nonpolar moiety but different polar groups in phenyl ring. Here, PG has a similar structure like PP except for two more –OH groups on the phenyl ring. PP has an aqueous solubility ~2.6 mM while PG offers higher solubility approximately ~17 mM in water at 25 °C [54–56]. PG has lower octanol–water partition coefficient than PP. These overall differences between the properties of PP and PG on the physical characteristics of TX-100 micelles are interesting.
Thus, their comparative effect on the solution behaviour of 5% TX-100 as a function of concentration was investigated by CP, viscosity and DLS measurements [Fig. 2]. The results reveal that PP gives more pronounced effect than PG. It can be understood that PP causes more dehydration and microstructural changes in TX-100 micelle than PG. Nonionic surfactants are widely used as emulsifiers, solubilizers and detergents in industrial formulation. However, all nonionic surfactants lose their stability in dispersion formulation above their CP. Therefore we studied the phase behaviour for TX-100 at different concentrations with increasing concentration of PP and PG at fixed temperature of 30 °C [Fig. 2]. Fig. 3 demonstrates that less concentration of PP is required than PG for the phase separation of TX-100 solution. Commonly additives interact with nonionic surfactant micelles on the basis of hydrophobic and hydrophilic interactions and alter phase behaviour [39, 49]. PG containing two more polar –OH groups than PP provides greater hydrophilicity and interaction with POE chain (shell) of micelles. PP is less polar than PG thus it offers more hydrophobic interaction which leads more dehydration and facilitates phase separation of TX-100 solution as compared to PG. So these phenomena are possible for the phase separation of these systems. This study reveals that PG can be used in
Table 1 Octanol/water partition coefficient (Log Po/w) and solubility in water at 25 °C of parabens and propyl gallate. Additive
Solubility (g/L)
Log Po/w
Propyl gallate Methyl paraben Ethyl paraben Propyl paraben Butyl paraben Benzyl paraben
3.5 2.5 1.7 0.5 0.2 0.1
1.80 1.96 2.47 3.04 3.57 3.61
Note: Po/w and solubility data are taken from Ref. [54–56].
Fig. 1. Effect of additives on cloud point (CP), relative viscosity (ηrel) and micellar hydrodynamic radius (Rh) of 5% TX-100 in aqueous solution in the presence of ( ) MP, (○) EP, ( ) PP, ( ) BP, and ( ) BzP.
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Fig. 2. Effect of additives on cloud point (CP), relative viscosity (ηrel) and micellar hydrodynamic radius (Rh) of 5% TX-100 in aqueous solution in the presence of ( ) PP and ( ) PG.
high concentration with any nonionic surfactant formulation as compared to PP. To find out pronounced interaction between PP/PG with TX-100, NOESY measurements have been carried out which reveal the locus of solubilization of PP and PG in TX-100 micelles. 3.2. Location of PP and PG in TX-100 micelles The NOESY spectrum offers detailed information concerning the intra- and inter-molecular interactions between the protons of additive and surfactant molecules in micellar system. The existence of cross peaks between additive and surfactant protons indicates their closeness to each other. Yuan et al. [57] using NMR and NOESY described that monomers of TX-100 are arranged in micelles in a way that the hydrophobic micellar core is surrounded by a thick shell of hydrophilic POE chain. The location of phenols [39], aromatic amines [47], alcohols and glycol ethers [48] in TX-100 micelles has been determined using NOESY experiments. Fukahori et al. [11] studied that the incorporation
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of parabens in the micelles of polyoxyethylene cetyl ether depends on the alkyl ester chain of the parabens using fluorescence probe and 1H NMR. In this study we want to compare the location of PP and PG in TX-100 micelles which reveals their effects on the micellar growth of TX-100. Fig. 4 shows the proton labelling of TX-100, PP and PG and NOESY spectra of their mixed system containing 5% TX-100 in the presence of 25 mM PP and PG. Some protons of these additives have been merged with the TX-100 protons but considerable changes in cross-peak intensity can provide the aspect of interactions. However, we have mainly considered the interaction from other unmerged protons of PP and PG which are labelled in the spectra. Fig. 4 shows the cross-peaks between alkyl chain protons (A1) of PP with the protons of methyl (T1, T2) and methylene (T3) group of TX-100 micelle core which are more intense than the alkyl chain protons (B1) of PG with same protons of TX-100. Phenyl protons (A4, A5) of PP show intense cross-peak with the protons of phenyl group (T7, T8) and POE group (T4) of TX-100 while weak cross-peak with the chain protons (T1, T2). The results indicate that PP molecules are deeply solubilized near the phenyl group region (core–shell interface) and their alkyl ester chains remain toward core in TX-100 micelles because of hydrophobic interaction between the alkyl chain of PP and hydrophobic core of TX-100 micelles. While PG has less Po/w and three polar –OH groups which allow fewer penetration of its molecules in TX-100 micelles and likely to remain in polar region (shell) of micelle. Overall finding indicates that PP penetrates more in TX-100 micelles therefore it has greater effect on the micellar behaviour relatively than PG. Heins et al. [16] have examined the interaction between PG and differently charged surfactants using 1H NMR and showed that PG is solubilized in the palisade layer of cetyltrimethylammonium bromide (CTAB) micelles, shell region of Brij 58 whereas in the Stern layer of the SDS micelles. Along these lines PG interacts more with CTAB than SDS while Brij 58 has intermediary interactions with PG. To determine the exact details concerning the PP and PG solubilized induce micellar geometry of TX-100, we have carried out some SANS measurements. 3.3. Small angle neutron scattering (SANS) Previous SANS studies indicate that TX-100 micelles are ellipsoidal having high aggregation numbers [38,58]. Here, we were interested to find out the preservative induced micellar change of TX-100. Fig. 5 shows the SANS patterns of 5% TX-100 solution in the presence of different concentrations of PG and PP. From SANS data analysis the values of semi major axis (Rb), semi minor axis (Ra), aggregation number (Nagg)
Fig. 3. Effect of PP (a) and PG (b) concentration on the phase behaviour of TX-100 at different concentrations at 30 °C.
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Fig. 4. (a) Proton labelling of (I) TX-100 (II) PP and (III) PG. (b) NOESY spectrum of 5% TX-100 in presence of 25 mM (I) PP and (II) PG at 30 °C.
and number density of micelles (nm) have been derived [Table 2]. SANS data of these systems have been fitted using ellipsoidal core–shell model. Fig. 5 indicates noticeable increase in the scattering intensity in the small Q region for the 5% TX-100 solution with solubilized preservatives. These changes show growth of TX-100 micelles. Table 2 demonstrates accurate information on increase in axial ratio, aggregation number and decrease in number density of micelles. The aggregation number of micelles, Nagg, was calculated from the following formula,
where Vm =(4/3 πa2b) is the micellar volume, and Vh is the volume of the hydrophobic part of the surfactant monomer. The value of Vh has been taken from earlier study [38]. The number density of micelles (nm) has been determined by the following expression [38]:
ð1Þ
where NA is the Avogadro's number, CMC is the critical micelle concentration and C is the concentration of surfactant in mol/L.
Nagg¼ Vm =V
h
N nm cm−3 ¼ ðC−CMCÞ A 10−3 Nagg
Fig. 5. SANS profile for 5% TX-100 solution in absence (□) and the presence of (a) PP ( ) and (b) PG ( ) at 30 °C.
ð2Þ
U. Patel et al. / Journal of Molecular Liquids 216 (2016) 156–163
161
Table 2 Micellar characteristics of TX-100 in the presence of PP and PG by SANS analysis. Additives
Rh (nm)
Ra (nm)
Rb (nm)
Ra/Rb
Nagg
nm (cm−3)
Shape of micelle
No additive 10 mM PP 25 mM PP 25 mM PG 40 mM PG
5.7 9.1 30.0 12.5 23.8
6.3 7.7 14.9 8.8 13.7
2.0 2.0 2.1 2.0 2.0
3.2 3.7 7.1 4.3 6.8
287 363 725 413 641
16 × 1016 13 × 1016 7 × 1016 12 × 1016 8 × 1016
Ellipsoidal Ellipsoidal Short rod-like Ellipsoidal Short rod-like
Where Rh, hydrodynamic radius; Ra, semimajor axis; Rb, semiminor axis; Ra/Rb, axial ratio (ellipticity); Nagg, aggregation number and nm, number density of micelle.
The analysis of SANS distribution shows that 5% TX-100 forms the ellipsoidal micelles having semimajor axis 1.98 nm and semiminor axis 6.35 nm of hydrophobic core. In both cases the micelle size increases with increasing concentration of additives. In the case of PP, its 10 mM concentration leads to small increase in micelle size. PP induces ellipsoidal to short rod like transition while PG increases ellipticity of TX-100 micelles in some extent at 25 mM concentration. Furthermore, PG leads to short rod-like micelle formation at 40 mM concentration. SANS plots of TX-100 with 25 mM PP and 40 mM PG have a slope of about −1 on log–log scale in low Q-region which specify the formation of rod-like micelles. In this way additive induced micellar changes are greater for PP than PG. The growth of surfactant micelles in the presence of additives depends on the locus of solubilization in micelles. PP penetrates more inside in micelle and more increase in the hydrophobicity of TX-100 micelles ensuing considerable micellar enlargement even at low concentration. These SANS results have a literal correlation with the relative viscosity and DLS measurements.
3.4. Effect of salt Commonly, electrolytes easily alter the clouding behaviour of nonionic surfactants. The electrolytes which make the water structure increase the CP whereas the water structure breaker electrolytes show decrease in the CP [49–53]. Koshy et al. [51] investigated the influence of lyotropic series of inorganic ions on the CP of TX-100 and TX-114 and reported that halogen anions offer their effect in the order: Cl− N Br− N I− on the decrease in CP. Furthermore, NaCl decreases the CPs for both surfactants while NaI enhances the CP. NaCl induces the hydrodynamic size and aggregation number of TX-100 micelles [59]. In the present study the effect of NaCl and NaI on the interaction between TX-100 and additive has been investigated using PG only. This study has been determined using turbidity, viscosity and DLS measurements. Fig. 6 shows the turbidity measurements of 5% TX-100 solution as a function of PG concentration in the presence of 1 M NaCl and 1 M NaI. In all micellar systems first turbidity increases gradually but drastically above certain concentration of PG. This sudden change in turbidity can be considered as the CP of TX-100 at 30 °C in the presence of specific concentration of PG. In the absence of salt 5% TX-100 solution shows clouding behaviour by 43 mM PG at 30 °C. However, solution of 5% TX-100 + 1 M NaCl offers clouding behaviour at very low concentration of PG about ~19 mM while 53 mM PG requires for the system containing 5% TX-100 + 1 M NaI. The effect of these salts on the relative viscosity of the TX-100 solution containing PG has been also determined [Fig. 7]. The viscosity of TX100 increases gradually with increase in concentration of PG. These electrolytes do not change the viscosity of 5% TX-100 significantly in the absence of PG. However, the viscosity of TX-100 solution increases drastically in the presence of 1 M NaCl and lower concentration of PG. However, the presence of NaI increment in the relative viscosity is less even at higher concentration of PG. Fig. 8 shows the effect of salts on the micelle size (Rh, nm) of TX-100 in the presence and absence of PG. 1 M NaCl increases the size of TX-100 micelle while 1 M NaI decreases the size in small extent.
Fig. 6. Effect of PG concentration on the turbidity of 5% TX-100 solution in the absence ( ) and presence of 1 M NaCl ( ) and 1 M NaI ( ) at 30 °C.
Micellar hydrodynamic radius of TX-100 increases from 5.5 nm to 8.6 nm in the presence of PG. The micelle size of 5% TX-100 + 15 mM PG system significantly increases with 1 M NaCl and decreases with 1 M NaI. PG molecules penetrate in TX-100 micelle and set up H-bond between the phenolic –OH group and oxygen atom of the hydrophilic POE moiety of TX-100 molecule. It causes decrease in micellar hydration resulting in decrease in CP and increase in micelle size. NaI increases the H-bonding between water and POE chain resulting in increased hydration thus high concentration of PG requires for dehydration of TX-100 micelles. NaCl reduces the H-bond interaction between water and polyoxyethylene chain of TX-100 thus less PG molecules require for dehydration of TX-100 micelles. Overall results regarding the effect of salts indicate that NaCl (water structure maker) induces the interaction between TX-100 and PG while water structure maker NaI decreases the interaction.
Fig. 7. Effect of PG concentration on the relative viscosity (ƞrel) of 5% TX-100 solution in the absence ( ) and presence of 1 M NaCl ( ) and 1 M NaI ( ) at 30 °C.
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Fig. 8. Effect of PG (15 mM) on the size (Rh) distribution curve of 5% TX-100 solution in the absence and presence of 1 M NaCl and 1 M NaI at 30 °C.
4. Conclusion The purpose of this study is to investigate the preservative induced structural changes in TX-100 micelles using phase behaviour, relative viscosity, spectral and scattering measurements. In case for parabens as the alkyl chain increases, their efficiency increase to enhance the hydrophobicity in TX-100 micelles resulting decrease in the CP and lead to micellar enlargement. The effect of polar group is also investigated by comparing PP and PG where less polar PP molecules situate deep in micelle at core–shell interphase while PG solubilized at shell region of TX100 micelles. PG has dominant hydrophilic interaction while PP has leading hydrophobic interaction with TX-100 micelles are responsible for the different effect of these preservatives on phase behaviour and micellar geometry of TX-100. Hydrophilic–hydrophobic interaction between TX-100 and preservatives can be altered by the addition of water structure maker/breaker salts. Generally the nonionic surfactants are extremely helpful in the formation of emulsions and formulations as well as the taken preservatives are also widely applicable in industrial and biological applications. Therefore the present study provides an ultimate consideration of interaction between preservatives and nonionic surfactants which will be useful in the development of industrial formulation as well as environmental friendly methods for the separation/extraction of preservatives. Acknowledgement U.P. and N.D. gratefully acknowledge UGC, New Delhi for the providing research fellowships. PB thanks Dr. K. Singh (St. FX University) and Dr. V. K. Aswal (BARC, Mumbai) for NOESY and SANS measurements. References [1] F.A. Andersen, Final amended report on the safety assessment of methylparaben, ethylparaben, propylparaben, isopropylparaben, butylparaben, isobutylparaben, and benzylparaben as used in cosmetic products, Int. J. Toxicol. 27 (4) (2008) 1. [2] T. Eklund, Inhibition of microbial growth at different pH levels by benzoic and propionic acids and esters of p-hydroxybenzoic acid, Int. J. Food Microbiol. 2 (3) (1985) 159. [3] S. Ito, S. Yazawa, Y. Nakagawa, Y. Sasaki, S. Yajima, Effects of alkyl parabens on plant pathogenic fungi, Bioorg. Med. Chem. Lett. 25 (8) (2015) 1774.
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