Exploring Physicochemical Interactions of Different Salts with Sodium

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Article Cite This: ACS Omega 2018, 3, 9256−9266

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Exploring Physicochemical Interactions of Different Salts with Sodium N‑Dodecanoyl Sarcosinate in Aqueous Solution Nitai Patra,† Debes Ray,‡ Vinod Kumar Aswal,‡ and Soumen Ghosh*,† †

Centre for Surface Science, Physical Chemistry Section, Department of Chemistry, Jadavpur University, Kolkata 700032, India Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India

‡

ACS Omega 2018.3:9256-9266. Downloaded from pubs.acs.org by 185.223.165.151 on 08/21/18. For personal use only.

S Supporting Information *

ABSTRACT: Amino acid-based surfactants are used in academics and industry. Sodium N-dodecanoyl sarcosinate (SDDS) is such an amino acid-based surfactant having applications in pharmaceutical, food, and cosmetic formulations. Although the surface properties of this surfactant have been studied in the presence of univalent cationic and anionic salts, there is no report on such solution in the presence of higher valencies. In this experiment, critical micelle concentration (CMC) of SDDS from tensiometry, conductometry, and fluorimetry has been determined. In each case, CMC decreases with increasing salt concentration. Counterion binding of micelles (β), diffusion coefficient (D0), and surface properties, e.g., Gibbs free energy for micellization (ΔG0m), Gibbs surface excess (Γmax), area of exclusion per surfactant monomer (Amin), surface pressure at CMC (πcmc), etc., have been evaluated using methods such as tensiometry, conductometry, and fluorimetry. The hydrodynamic radius of SDDS in the presence of different salts was measured by the light scattering method. Aggregation number and shape of micelle have been determined by small-angle neutron scattering experiment. The nature of amphiphilic packing and the aggregation numbers of the assemblies have also been explored. The results from different experiments have been rationalized and represented systematically. human skin, and it possesses antimicrobial activities.14 It should not be used in such cosmetic products where Nnitrososarcosine, a known carcinogen, can be formed. It is also used in formulating textile treatment agent. The surfactant is commercially available as a colorless to slightly yellow liquid, as a 30% aqueous solution, or as an anhydrous white powder with 97% purity. There are limited studies on the growth of micelle in the presence of di- and trivalent cations,15−17 although studies are on the effect of mostly unicationic and anionic salts on the different aggregation properties of conventional surfactants.18−20 Instead of many applications of SDDS in biological, medicinal, and industrial fields, studies on its interfacial and bulk properties are limited. There is a report on the selfaggregation behavior of the aqueous solution of SDDS in different temperatures, pHs, acidity constants, and conformational properties by our group in the presence of unicationic and anionic salts.21 There is an important role of Mg2+ on the solubilization of cytoplasmic membrane of Escherichia coli by SDDS.22 Considering all of the uses of SDDS both in vivo and vitro,23−25 it is important enough to study the effect of higher valent cations and anions on the aggregation properties of the surfactant. Here, for the first time, we demonstrate the

1. INTRODUCTION Amino acid-based surfactants have immense importance in everyday life, in almost every industrial area and academic research.1−5 These are long-chain N-acyl amino acid-based anionic surfactants and are soluble in water. These surfactants, having long-chain hydrophobic group and the carboxylic acid as the hydrophilic head group, aggregate in aqueous solution and reduce the unfavorable interactions with water6 due to their amphiphilic nature. In the International Journal of Toxicology,7 there is a full report on the synthesis, properties, reactivity, and usages of several N-acylamino surfactant salts. There are disadvantages of using fatty acid-type surfactants owing to their tendency to form precipitate in the presence of divalent cations, such as Mg 2+ , Ca 2+ , etc. To avoid precipitation, the carboxylate groups are modified by introducing some hydrophilic chains, such as ethoxy and hydroxyl groups, etc. Sodium N-dodecanoyl sarcosinate (SDDS), used in this study, is a modified dodecanoic acid at the α-position by an amidomethyl (−CONCH3) group. It is used as a component of toothpaste8 as it does not help in the dissolution of tooth enamel and the inhibitory effect of fluoride on the dissolution process.9 It has several applications in cosmetic formulations such as hair conditioning agent, as well as foaming and cleansing agent used in shampoo, shaving, and wash products because of its soft nature, better biodegradability, antimicrobial property, and stability toward hard water.10−13 SDDS is nonirritating and nonsensitizing to the © 2018 American Chemical Society

Received: April 13, 2018 Accepted: August 1, 2018 Published: August 16, 2018 9256

DOI: 10.1021/acsomega.8b00718 ACS Omega 2018, 3, 9256−9266

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Table 1. Effect of MgCl2 on CMC, β, Interfacial and Energy Parameters, and P of the Aqueous Solution of SDDS at 298 Ka CMC (mM) [MgCl2] (mM)

Cond

Tens

Fluo

Ave

β

ΔG0m (kJ/mol)

106 Γmax (mol/m2)

Amin (nm2/mol)

ΔG0ads (kJ/mol)

P

0 0.005 0.05 0.1 0.5 3 5

12.90 9.60 7.14 5.54 2.43 0.84b

13.20 8.36 5.12 4.54 2.48 0.96 0.92

14.11 8.52 5.56 4.60 2.46 1.56 1.28

13.40 8.83 5.94 4.89 2.46 1.23 1.10

0.47 0.48 0.20 0.18 0.16 0.11b

−30.22 −32.30 −27.08 −27.38 −29.33 −28.73

2.44 1.73 1.89 1.90 1.92 2.02 2.04

0.68 0.96 0.88 0.87 0.86 0.82 0.81

−57.10 −59.52 −53.27 −54.38 −51.26 −49.08

0.31 0.22 0.24 0.24 0.24 0.25 0.26

Standard deviations in CMCave, β, and (ΔG0m and ΔG0ads) (Γmax, Amin, and P) are within 6, 8, and 7%, respectively. bObtained from Carpena’s method.

a

Table 2. Effect of Na2SO4 on CMC, β, Interfacial and Energy Parameters, and P of the Aqueous Solution of SDDS at 298 Ka CMC (mM) [Na2SO4] (mM)

Cond

Tens

Fluo

Ave

β

ΔG0m (kJ/mol)

106 Γmax (mol/m2)

Amin (nm2/mol)

ΔG0ads (kJ/mol)

P

5 10 15 25

11.42 8.51 7.78 5.80b

10.15 7.15 6.28 5.45

8.83 5.58 5.40 4.86

10.13 7.08 6.49 5.39

0.57 0.51 0.50 0.64b

−33.02 −32.86 −32.98 −27.00

1.78 1.24 1.41 1.05

0.93 1.34 1.18 1.58

−59.91 −65.68 −62.84 −60.71

0.22 0.16 0.16 0.13

Standard deviations in CMCave, β, and (ΔG0m and ΔG0ads) (Γmax, Amin, and P) are within 6, 8, and 7%, respectively. bObtained from Carpena’s method.

a

determined CMC values are close for different techniques, such as tensiometry, conductometry, and fluorimetry. The CMC data of SDDS in the presence of different cations and anions are listed in Tables 1 and 2, respectively. CMC has been observed to vary with the sensitivity of the method applied. However, the trends are found to be independent of methods. In the γ vs log[SDDS] plot, minima have been observed. As the hydrophobic part of the surfactant goes toward the surface, H-bonding network of aqueous medium is destroyed and surface tension decreases up to a specific concentration, CMC. At and above CMC, micelle is formed, and at higher concentration of surfactant, the tendency of the formation of micelle is higher and then surface tension of the whole solution may slightly increase due to the presence of a higher number of micelles in the bulk and so minima appear. CMC of SDDS decreases abruptly with the increase of MgCl2 concentration (Figure 1 and Table 1),27 but in the case of Na2SO4, the decrease is mild (Figure 2 and Table 2).28,29 The decrease in the CMC of SDDS in the presence of MgCl2 is similar to that in the presence of NaCl,13 but the rate of decrease is abrupt. This fact is due to the higher positive charge density and bigger size of hydrated Mg2+ compared to Na+.30 Some Na+ ions are replaced by highly positive Mg2+ ions at the Stern layer of SDDS micelles. Consequently, CMC decreases due to tremendous stability and association ability.31−33 At a very high concentration of MgCl2 (5 mM), there is no break in the κ vs [SDDS] plot and, that is why, no CMC is obtained from the plot at 5 mM and above. CMC measurement has been done in other MCl2 (M = Ca, Sr, and Ba) solutions at 0.1 mM, and the values are found to be increasing in nature from Mg2+ to Ba2+ (Figure S1 and Table S1). It is due to the fact that the charge density decreases from Mg2+ to Ba2+. Susan et al. showed the effect of density of charge of hydrated cations on the micellization of sodium dodecyl sulfate (SDS) solution.34 The hydration ability35 of the alkali metal cations follows the decreasing order as Mg2+ > Ca2+ > Sr2+ > Ba2+. That is why, the association capability and stabilizing ability of the micelle of SDDS decrease from Mg2+ to Ba2+ for self-association and

aggregation behavior of SDDS surfactant in the aqueous solution of salts of higher valent cations and anions. Here, aggregation occurs more rapidly and critical micellar concentration (CMC) decreases abruptly in the presence of divalent cations, which are not observed in salts of univalent cation and anion.19 It may be possible to formulate the composition of different skin care products and toothpaste using SDDS as a component along with the ions with higher valency, especially Mg2+ and Ca2+, as these cations are essential to human body. The CMC is measured tensiometrically, conductometrically, and fluorimetrically, from which average CMC is calculated. Counterion binding of micelles (β) was calculated from the conductometric plot, and standard Gibbs free-energy change (ΔG0m) was calculated from β values. Aggregation numbers, which increase with increasing salt concentration, have been evaluated fluorimetrically. Diffusion coefficient (D0) was calculated using the Stokes−Einstein equation and hydrodynamic radius obtained from the dynamic light scattering (DLS) method. Different surface properties, e.g., Gibbs surface excess (Γmax), area of exclusion per surfactant monomer (Amin), surface pressure at CMC (πcmc), etc., have been evaluated using the plot of tensiometry. The standard Gibbs free-energy change due to adsorption of SDDS was calculated for each concentration of salt. The micellar shape and aggregation number of SDDS at a higher salt concentration were determined by small-angle neutron scattering (SANS) measurement. In fluorescence quenching technique for determining the aggregation number, DLS measurement, and SANS measurement, the concentration of SDDS used was much higher compared to the CMC of SDDS, obtained at each and corresponding salt concentrations. Higher concentration of surfactant is required for better sensitivity and accuracy.26

2. RESULTS AND DISCUSSION 2.1. Critical Micellar Concentration. Different salts have been used to measure the CMC and other physical properties of SDDS solution using different techniques at 298 K. The 9257


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Figure 2. (A) Tensiometric, (B) conductometric, and (C) fluorimetric profiles of SDDS in different [Na2SO4]. Figure 1. (A) Tensiometric, (B) conductometric, and (C) fluorimetric profiles of SDDS in different [MgCl2] (some concentrations are omitted for clarity).

[salt] profile is depicted in Figure 3A,B, which also shows the decrease of CMC of SDDS with increasing [salt]. 2.2. Counterion Binding. Counterion binding (β) is the fraction of counterion bound to micelle, which was determined by the slope ratio method conductometrically. The equation is β = 1 − S2/S1, where S1 and S2 correspond to the pre- and postmicellar slopes.18 The β value decreases with increase in [MgCl2], which is quite unusual according to the literature.22 Mg2+, being highly positive in nature, can replace two Na+ ions at the palisade layer of the micelle (also discussed in the previous paragraph).40 Henceforth, the number of counterions decreases compared to only aqueous SDDS. As a result, counterion binding (β) decreases as the concentration of MgCl2 salt increases.15,31,42 When the β value in the presence of 0.1 mM MgCl2 is compared to that of other salts MCl2 (M = Ca, Sr, and Ba), a steady decrease in β value is observed from Mg2+ to Ba2+. This fact can be attributed to the hydration energy of the cations; lower the size of the cation, higher is its hydration capacity, and for this, higher hydration energy cation remains in the palisade layer.42 Arutchelvi et al. discussed the effects of different divalent cations on the counterion binding of surfactin surfactant in aqueous solution, and similar results were obtained.43 The trend of β is opposite to the β values of SDDS in alkali metal cations.21 This is due to the kosmotropic

hence the CMC value increases.36 This trend is followed by the Hofmeister series (Na+ > K+ > Mg2+ > Ca2+).37 In the presence of Na2SO4, the decrease in CMC is less compared to MgCl2; it is possibly owing to the resultant of two opposite competitive effects of Na+ and SO42−. Na+ ions have association capability and favor micellization by reducing the repulsive interactions of pure SDDS monomers, whereas SO42− ions reduce counterions from the Stern layers, following increase in the repulsive interactions. As a result, in the presence of SO42− ions, the surfactant association capability of Na+ decreases. But CMC decreases sharply with SO42− ions as counterions in the presence of cetyltrimethylammonium bromide, as shown by Naskar et al.28 In a similar way, the fixed salt concentration of 5 mM has been used to measure the CMC in case of Na3PO4, Na2S2O3, disodium ethylenediaminetetraacetate dihydrate (Na2-EDTA), Na2-succinate, Na2-oxalate, and Na-gluconate (Table S2). For Na2-EDTA solution, the CMC value is lower compared to other CMC values. This may be ascribed to its bolaamphiphilic nature.38,39 CMC and other thermodynamic parameters of SDDS in different concentrations of MgCl2 and Na2SO4 are given in Tables 1 and 2, respectively. CMC vs 9258


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Γmax = −

where

dγ dlog C

dγ 1 lim 2.303iRT C → CMC dlog C

(1)

is calculated from the slope of the linear surface

tension (γ) vs log C plot and i is the number of dissociated ions, which is 2 for SDDS. Other terms have the usual meanings.45−50 The minimum area of the surfactant head group at the saturated monolayer at the air−water interface (Amin) was calculated from the equation A min =

1018 N Γmax

(2)

where N is Avogadro’s number. In the presence of MgCl2, the Γmax value increases with increase in salt concentration, and consequently, Amin decreases (Table 1). The higher charge density of cations, Mg2+, or other alkali metal ions decreases the electrostatic repulsion of the head groups of SDDS and thereby reduces the area per headgroup (Amin).15 But for Na2SO4, an opposite trend is observed with increase in salt concentration (Table 2) due to highly polarizable SO42− ions. Amin of SDDS at the same concentration of MgCl2 (0.814) and Na2SO4 (0.933) is larger compared to the same concentration of NaCl (0.671).21 Other anions, such as phosphate, thiosulfate, succinate, and oxalate, are highly polarizable due to high negative charge density, and as a result, Amin values are high.14 The data are given in Table S2. The standard Gibbs free-energy change due to adsorption is given by the equation48,49 π 0 ΔGads = ΔGm0 − cmc Γmax (3)

Figure 3. Variation of CMC of SDDS and counterion binding (β) as a function of [salt]: (A) MgCl2 and (B) Na2SO4.

behavior of order-maker of high charged divalent cations, whereas the monovalent are chaotropic.44 All of the β values determined with change in the concentration of MgCl2 and at a fixed concentration of 0.1 mM of other cation-varied salts are listed in Tables 1 and S1, respectively. Changes of β values with [MgCl2] are given in Figure 3A. With the increase in [Na2SO4], the β vs [salt] plots are depicted in Figure 3B. In case of increasing [Na2SO4] up to 5 mM, β increases due to better Na+ condensation at the Stern layer of micelle; but beyond this concentration, counterion binding (β) decreases due to increasing polarizability of SO2− 4 ions (Figure 4). At a fixed concentration of 5 mM of Na3PO4,

where πcmc is the surface pressure and ΔG0m is the free-energy change due to micellization. Table 1 shows that ΔG0ads values increase with increasing [MgCl2]. The higher negative value of ΔG0ads indicates the higher efficiency of SDDS to adsorb at the interface in the presence of MgCl2. Tables 2 and S2 show these values in the presence of other salts. The packing parameter P and structural geometry have been calculated by the Israelachvili equation33 v P= Alc (4) where lc is the critical chain length, A is the surface area of the head group, and v is the critical volume of the critical hydrophobic chain. Both lc and v for a saturated hydrocarbon chain with Cn number of carbon atoms can be obtained from the following equations49

Figure 4. Plot of F0 vs [Q]/M for SDDS without and with 0.1 mM F MgCl2 and 5 mM Na2SO4.

lc = (0.154 + 0.1265Cn) nm

(5)

v = (0.0274 + 0.0269Cn) nm 3

(6)

Israelachvili’s theory has suggested that for spherical assemblies, P ≤ 0.333. In this study, sarcosinate ended up with P values much lower than 0.333 at all salt concentrations, concluding that spherical micelles were formed in all of the cases. This gets support as ionic surfactants in general are reported to form spherical assemblies close to their CMC values. 2.4. Energy of Micellization. The energy of micellization, Gibbs free energy (ΔG0m), is given by the following equation

Na2S2O3, Na2-EDTA, Na2-succinate, Na2-oxalate, and Nagluconate, it has been noted that β is relatively lower compared to Na2SO4 owing to the higher polarizability of the anions in the systems (Figure S2 and Table S2).21 2.3. Interfacial Parameter. The surface excess of the surfactant SDDS at CMC (Γmax) was determined from the Gibbs adsorption equation 9259


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based on counterion binding (β) and Xcmc. CMC is expressed in mole fraction scale as50,51 ΔGm0 = (1 + β)RT ln Xcmc

Table 3. Aggregation Number (Nagg), Stern−Volmer Quenching Constant (KD), and Micropolarity (I1/I3) [salt]/mM

(7)

where R is the universal gas constant and T is the temperature in absolute scale. In each case, it has been observed that the energy change is quite highly negative and comparable to each other. The ΔG0m value increases with an increase in the concentration of both MgCl2 and Na2SO4 (Tables 1 and 2) owing to decreasing β value, denoting less spontaneity of the micellization process. The negative values of ΔG0m indicate the spontaneity of the micellization process as it was seen in case of NaCl-type salt.21 In higher concentration of MgCl2, freeenergy change cannot be evaluated due to lack of β value. A similar change in ΔG0m of SDS in the presence of Al(NO3)3 was observed by Pereira et al.41 The negative value of ΔG0m is higher for SDDS in 5 mM Na2SO4 compared to SDDS in 5 mM NaCl,21 but lower compared to SDS at the same NaCl concentration.42 For other salts, the ΔG0m value does not change much (Tables S1 and S2). 2.5. Aggregation Number, Stern−Volmer Quenching Constant, and Micropolarity. Aggregation number (Nagg), the number of surfactant molecules associated in the CMC, can be determined by time-resolved fluorescence quenching method, where the equations52,53 are It = I0e{t /τ0 − n[1 − e( −kqt )]}

n = Nagg

[Q] [S] − CMC

0 0.1 0.5 3 5 Na2SO4 5 10 15 25

KD (L/mol)

I1/I3

46 ± 6 48 ± 5 50 ± 6 52 ± 5 47 ± 6 48 ± 5 49 ± 6

582 712 741 742 812

± ± ± ± ±

35 43 44 45 49

0.94 0.87 0.79 0.82 0.80

± ± ± ± ±

723 733 746 864

± ± ± ±

43 44 45 52

0.88 ± 0.05

0.05 0.05 0.05 0.05 0.05

0.87 ± 0.05 0.79 ± 0.05

There is no change in absorption intensity of pyrene when quencher is added to the probe−SDDS micelle and timeresolved lifetime (τ) changes with the addition of quencher in τ F such a way that τ0 = F0 relation followed.56 These facts prove the dynamic nature of quenching. The Stern−Volmer rate constant (KD) also increases with increasing both [MgCl2] and [Na2SO4]. This is owing to the increase in solubilization of both pyrene and quencher in the micellar region. Partition coefficient K has been calculated from the relation given below57 ρ 1 1 = + I − Imin Imax − Imin Kα[SDDS](Imax − Imin) (11)

(8)

(9)

here, It and I0 are the fluorescence intensities at time t and 0, after the excitation, respectively; τ0 is the lifetime of pyrene in micellar environment in the absence of 1-dodecylpyridinium chloride (DPC); and kq and n are the rate constant and molar ratio of quencher to micelle, respectively. Using a fitting program, the fluorescence decay curves are drawn and n values are obtained. The Nagg values are calculated by eq 9. The Stern−Volmer equation is used to get the equilibrium constant (KD) for the interaction of the pyrene probe with the micelle that determines the bimolecular quenching and unimolecular decay.6 F0 = 1 + KD[Q] F

aggregation number (Nagg)

MgCl2

where I, Imax, and Imin are fluorescence intensities of pyrene at intermediate, final, and no concentration of SDDS, respectively, and ρ and α are the density and core-forming fraction of surfactants, respectively. It has been observed that K is higher in MCl2 solution compared to aqueous SDDS (2.65 × 104 M−1), and there is an increasing order of K with increase in MgCl2 concentration (3.74 × 104 M−1). With increase in Na2SO4 concentration (2.95 × 104 M−1 in 25 mM Na2SO4), there is also a trend in increasing the K value, but the value is lower compared to that of SDDS solution in the presence of MgCl2. Change of K in both MgCl2 and Na2SO4 is consistent with the change in aggregation numbers and KD. Local polarity or micropolarity (I1/I3) is an important property because changes in micropolarity can express structural changes in micellar aggregates, and this was determined also from steady-state fluorescence quenching study. Pyrene is used as it is preferentially solubilized close to the palisade layer of the micelle; the polarity of the probe senses the level of water penetration in this region of the micelle.58 All KD, N, and I1/I3 values for SDDS−MgCl2 and SDDS−Na2SO4 systems are presented in Table 3. There are plots of I1/I3 values versus concentration of SDDS in the absence of salt and in the presence of different fixed concentrations of salts in Figures 1C, 2C, S1D,E, and S2E,F. The I1/I3 value is approximately 1.5 in the absence of SDDS, which decreases as SDDS is added and becomes constant at and after a certain concentration of SDDS. The constant value indicated the formation of micelles, uniform size distribution, and solubilization of pyrene in the hydrophobic interior of the micelles.51 The I1/I3 values in MgCl2 and Na2SO4 aqueous solutions are relatively lower (less than unity) compared to the NaCl-type salt and aqueous SDDS.21 These indicate the less

(10)

where F0 and F are the fluorescence intensities of the probe in the absence and presence of the quencher (Q). In all of the studies, concentration of SDDS was much greater than CMC. The aggregation number of SDDS increases from 46 (without salt) to 52 (5 mM MgCl2) (Table 3), and a similar increasing trend in aggregation number (74 in 1 mM and 92 in 5 mM CaCl2) is observed in the SDS system in the presence of different dicationic salts.54 The increasing trend of aggregation number in the presence of salts is because of the increase of SDDS monomers associated with micelle. There is also a transfer of pyrene molecules from bulk to micellar region.6,55 There are also similar aggregation numbers in other salts (MCl2, M = Ca and Ba) (Table S3). In case of Na2SO4, there is also increase in aggregation number, but the rate of increase is less compared to MgCl2. Aggregation numbers have been calculated also in the case of anionic polarizable anions (PO43−, S2O32−, and EDTA2−). A linear plot of F0/F vs [Q] gives one kind of quenching process in the interacting systems. 9260


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Table 4. Hydrodynamic Diameter (Dh), ζ-Potential (ζ), and Diffusion Coefficient (D0) of SDDS at Different [MgCl2] and [Na2SO4] salt

concentration/(mM)

MgCl2

0 0.005 0.05 0.1 0.5 3 5 5 10 15 25

Na2SO4

3.61 3.73 3.86 3.93 3.98 4.00 4.05 4.02 4.09 4.47 4.62

± ± ± ± ± ± ± ± ± ± ±

kT 6πηR h

−55.0 −41.5 −34.5 −30.3 −22.0 −7.56 0.04 −53.8 −43.8 −41.9 −44.2

0.21 0.23 0.22 0.22 0.24 0.20 0.24 0.25 0.25 0.27 0.27

± ± ± ± ± ± ± ± ± ± ±

1010 D0 (m2/s)

4.0 3.9 3.7 3.8 3.5 3.7 3.7 4.1 4.0 4.2 4.2

1.53 1.48 1.43 1.41 1.39 1.38 1.37 1.38 1.35 1.24 1.20

± ± ± ± ± ± ± ± ± ± ±

0.09 0.09 0.08 0.08 0.08 0.07 0.08 0.08 0.08 0.07 0.07

around the probe. The following equation shows the relationship between two terms6,64

polar environment of SDDS micelle in the aforesaid salt systems.48 2.6. Hydrodynamic Diameter (Dh), Diffusion Coefficient (D0), and ζ-Potential (ζ). The hydrodynamic diameter increases smoothly with increasing concentrations of MgCl2 and Na2SO447 (Table 4). The hydrodynamic diameter of SDDS in the absence of any salt is comparable to the values reported in the literature.59 With increasing salt concentration, the number of surfactant monomers will be higher in micelle and, as a result, the hydrodynamic radius increases. This trend is similar to the change of aggregation number. In the presence of 0.1 mM MCl2 (M = Ca, Sr, and Ba) and 5 mM anionic varied salts (Table S3), hydrodynamic radii are comparable to the values at 0.1 mM MgCl2 and 5 mM Na2SO4, respectively. The diffusion coefficient of SDDS in different salt concentrations has been calculated using the Stokes−Einstein equation60,61

D0 =

ζ (mV)

Dh/nm

η = 2.4r /(0.362 − r )

(13)

Different probes reside in different regions of the micelle, indicating different microviscosity values. Depolarization of some probes located in the micelles can also be possible by the contribution of micellar rotation. A quantitative estimation of changes in microviscosity at the micellar surface and micellar interior induced by the addition salt was performed with a fluorescence anisotropy measurement, using 1,6-diphenyl1,3,5-hexatriene (DPH) probe in micellar solution of SDDS in different aqueous salt solutions. The fluorescence anisotropy (r) can be defined as r = (Iv − GIh)/(Iv + 2GIh), and G factor is defined as G = Iv/Ih, where Iν and Ih indicate the respective fluorescence intensities of the vertically and horizontally polarized emissions when the sample is excited with vertical polarized light. The G factor denotes the ratio of the sensitivities of the detection system between vertically and horizontally polarized light with fluorescence polarization anisotropy.64,65 DPH is a nonionic and hydrophobic probe that is insoluble in water. That is why, DPH prefers palisade layer of the micelle.66 In the presence of both types of salts (MgCl2 and Na2SO4), anisotropy values increase, indicating that more probes come to micellar core from bulk. These results also indicate stronger probe−micelle interaction in the presence of salts compared to aqueous micellar solution (Table 5). Table S4 indicates that compared to 0.1 mM MgCl2, both anisotropy

(12)

where η and Rh are the viscosity coefficient and hydrodynamic radius of the solution, respectively, at temperature T. It has been observed that the diffusion coefficient of SDDS decreases with an increase in salt concentration, which follows the opposite pattern to the hydrodynamic radius. Electrostatic repulsion is higher as SDDS micelle has the same negative charge on the Stern layer. But, SDDS micelles decrease the Coulombic potential and the electrostatic repulsion decreases with increase in salt concentration. For this reason, D0 decreases with increase in salt concentration.62 A similar change in D0 was observed by Mazer et al. for SDS in the increasing NaCl concentration.63 In Table S3, it is observed that the D0 values of 5 mM anionic varied salts and 0.1 mM MCl2 (M = Ca, Sr, and Ba) are slightly lower than those of 0.1 mM MgCl2 and 5 mM Na2SO4 respectively. ζ-Potential (ζ) is the measure of repulsion of the similarly charged aggregates and stability of the colloidal system. SDDS forms anionic micelles having Na+ as positive counterions. A negative ζ is usual for the micellization in the absence of MgCl2. But, MgCl2 reduces the repulsion between the micelles for the high positive charge density of Mg2+ and consequently ζ decreases. But, with the increase in Na2SO4 salt, ζ changes slightly due to the competitive and opposite effects of sodium cation and sulfate anion.42 2.7. Steady-State Fluorescence Anisotropy and Microviscosity. A steady-state fluorescence anisotropy measurement furnishes the information on local viscosity

Table 5. Anisotropy and Microviscosity of SDDS Micellar Solution at Different [MgCl2] and [Na2SO4] salt/mM

anisotropy

microviscosity

MgCl2 0 0.005 0.05 0.1 0.5 3 5 Na2SO4 5 10 15 25 9261

0.054 0.057 0.061 0.068 0.070 0.072 0.082

± ± ± ± ± ± ±

0.005 0.004 0.005 0.006 0.006 0.005 0.006

0.42 0.45 0.49 0.55 0.57 0.59 0.70

± ± ± ± ± ± ±

0.04 0.04 0.04 0.05 0.05 0.04 0.06

0.081 0.086 0.095 0.122

± ± ± ±

0.007 0.006 0.007 0.008

0.69 0.75 0.85 1.22

± ± ± ±

0.06 0.07 0.06 0.09


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density of [SO4]2−, the repulsive interaction is more for Na2SO4 compared to MgCl2. At lower concentration of both types of salts, the micellar shape seems to be spherical as obtained by packing parameter calculation. But at higher salt concentration, the shape of micelle tends to be prolate ellipsoidal.69,70 It has been observed that fractional charge decreases with increasing salt concentration. The aggregation number of micelle increases smoothly with increasing salt concentration, and these values are consistent with the aggregation number obtained from steady-state fluorescence quenching method. All of the fitted parameters are listed in Tables 6 and S5. Aggregation numbers and other values are more or less comparable for SDDS in the presence of different salts.

and microviscosity values are higher in case of 0.1 mM MCl2 (M = Ca, Sr, and Ba). Again for other anionic varied salts, anisotropy and microviscosity values are comparable. 2.8. Small-Angle Neutron Scattering (SANS). SANS was employed to get the aggregation number of SDDS at different aqueous salt concentrations. The dimension and shape are also important parameters of the micelle. Figure 5

Table 6. Micellar Parameters Obtained from Analysis of SANS Data Using Prolate Ellipsoidal Form Factor and the Rescaled Mean Spherical Approximation Closure for the Structure Factor [salt]/mM

semimajor axis (Å)

semiminor axis (Å)

fractional charge (esu)

aggregation number (N)

MgCl2

Figure 5. Scattering intensity, d ∑ (cm−1), as a function of wave dΩ vector transfer Q (Å−1) for SDDS in the presence of (A) MgCl2 and (B) Na2SO4.

0 0.5 5 20

18.6 18.4 19.3 25.6

± ± ± ±

0.6 0.6 0.7 0.8

15.6 15.8 13.3 15.0

± ± ± ±

0.5 0.5 0.4 0.5

0.48 0.49 0.47 0.31

± ± ± ±

0.05 0.05 0.05 0.04

54 55 62 75

± ± ± ±

6 6 7 8

Na2SO4 5 25 50 100

20.7 20.8 22.1 23.7

± ± ± ±

0.7 0.7 0.7 0.8

15.3 15.8 15.8 15.9

± ± ± ±

0.5 0.5 0.5 0.5

0.47 0.46 0.45 0.41

± ± ± ±

0.05 0.05 0.05 0.05

58 61 66 72

± ± ± ±

6 6 7 8

3. CONCLUSIONS In the study of aggregation behavior of SDDS with different salts of divalent cations and di- and trivalent anions, a number of micellization parameters have been evaluated. It was observed that N-dodecanoyl sarcosinate interacts strongly with the bivalent cations. Divalent metal chlorides abruptly reduce the CMC of SDDS due to higher positive charge density of metal cations, but sodium salts of different anions reduce the CMC due to the presence of sodium cations. EDTA shows an exceptional behavior toward the micellization of the SDDS, owing to its bolaamphiphilic nature. Counterion binding follows an anomalous trend with increase in the concentration of divalent chloride. The value of β decreases with increasing the concentration of divalent metal chlorides, but increases up to 25 mM in case of sodium sulfate. The energy parameter was found to be highly favorable for micellization of SDDS in the presence of all salt solutions. In case of increasing concentration of magnesium chloride and sodium sulfate, free energy of micellization has the tendency to increase. There are minor effects of the salts on the micropolarity of the micellar systems. The aggregation number and the Stern−Volmer quenching constant were found to increase with increasing both MgCl2 and Na2SO4 salt concentrations. The hydrodynamic diameter of the micelle increased with an increase in the salt concentration, whereas the diffusion coefficient decreased. The shape of SDDS micelle determined from SANS experiment is spherical at low salt concentrations (as obtained from the packing parameter calculation), whereas it tends to prolate ellipsoidal at higher

shows the fitted SANS profiles (the scattering cross section (d∑/dΩ) vs accessible wave vector transfer (Q)) for SDDS micelles with salt concentration varying from 0 to 20 mM in case of MgCl2 (Figure 5A) and from 5 to 100 mM in case of Na2SO4 (Figure 5B). The micelles are fitted with the form factor of spherical/prolate ellipsoidal shape and structure factor, as calculated by the Hayter and Penfold analysis under mean spherical approximation for charged microions.67 In this approximation, micelles are assumed to rigid spheres of equivalent radii σ = (ab2)1/3 interacting through a screened Coulomb potential. The radius (R) for spherical micelle, semimajor axis (a), and semiminor axis (b) for ellipsoidal micelle and effective charge (Z) are the fitted parameters. The aggregation number (Nagg = 4πab2/3v, where v is the volume of a surfactant monomer) and fractional charge α (=Z/Nagg) are the calculated parameters. Through data analysis, the shape of the SDDS micelle in aqueous and less salt solution was determined as nearly spherical, but at higher salt concentrations, the shape is ellipsoidal. With increasing [salt], in the low-Q region, there is a broadening of the peak. The broadening is greater for Na2SO4 compared to that for MgCl2, suggesting the screening of electrostatic interaction.68 The strength of the repulsive interaction is set by the surface charge and the ionic strength of the medium. The addition of a salt increases the ionic strength and hence decreases the range of electrostatic interactions.60 Due to the high negative charge 9262


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quartz cuvette. Pyrene was used as fluorescence probe, and its concentration was around 2 μM. The excitation wavelength was kept at 332 nm, and emission was taken from 350 to 450 nm. The slit widths were fixed at 12 and 2.5 nm for excitation and emission, respectively. Mean CMC values of three consecutive measurements are presented in Tables 1 and 2. CPC was used to find Stern−Volmer quenching constants of different systems. Time-resolved fluorescence decay was measured by the time-correlated single photon counting technique in a HORIBA Jobin Yvon FluoroCube fluorescence lifetime system using a NanoLED of 330 nm (IBH, U.K.) as the excitation source for pyrene and a TBX photon detection module as the detector. DPC was used as a quencher to determine the aggregation number. The decay data were fitted using IBH DAS-6 decay analysis software. The lamp profile was collected by placing a dilute micellar solution of sodium dodecyl sulfate in water as a scatterer in place of the sample. The solution was added using a Hamilton microsyringe.73 The microviscosity was calculated from the equation involved with anisotropy measurement by steady-state fluorescence method.6 For fluorescence anisotropy method, 1,6-diphenyl-1,3,5-hexatriene (DPH) probe has been used. The excitation and emission wavelengths were 335 and 436 nm, respectively. The mean value of six consecutive measurements is presented for all anisotropy measurements. A fixed temperature (298 K) was maintained by a water bath circulator of accuracy ±0.1 K. 4.5. Dynamic Light Scattering (DLS). DLS measurements have been performed at 173° angle in a Malvern Zetasizer NanoZS apparatus with a He−Ne laser of wavelength (λ) 632 nm at 25 °C. The sample cells are kept in a temperature-controlled, refractive index-matched bath filled with cis−trans decahydronaphthalene. ζ-Potential measurements are taken at 25 and 90° C angle using a gold-coated copper electrode in the cell. The applied electric field strength was 80 V/cm. All solutions are filtered two to three times through Millipore membrane filters (porosity 0.25 μm) to remove dust particles before the experiment. The mean values of two repeat experiments are reported.28 4.6. Small-Angle Neutron Scattering (SANS). Smallangle neutron scattering experiments were performed using a diffractometer at the Dhruva reactor, Bhabha Atomic Research Centre, Trombay, Mumbai. The experiments were conducted with an incident wavelength of 5.2 Å and a resolution of about 15%. The angular distribution of scattered neutron was recorded using a one-dimensional position-sensitive detector. 4π sin θ The accessible wave-vector transfer is q = λ , where 2θ is the scattering angle and the range of diffractometer was 0.017− 0.035 Å−1. The samples were held in a quartz sample holder of 0.5 cm thickness. The temperature was kept fixed at 303 K. The measured SANS data were corrected and normalized to absolute units (as cross section per unit volume) using the standard procedure.28 In SANS measurement, the differential scattering cross section per unit volume (d∑/dΩ) as a function of wave-vector transfer Q is measured. In the case of a system of monodisperse particles in a medium

salt concentrations. Aggregation numbers of SDDS were calculated from SANS measurement, which were similar to those obtained from the time-resolved fluorescence quenching method. Thus, the surfactant may work well as shampoo, shaving, and different cleansing and cosmetic products in the presence of hardening materials in water.

4. EXPERIMENTAL SECTION AND METHODS 4.1. Materials. Sodium N-dodecanoyl sarcosinate (SDDS) (assay ≥97%, HPLC), purchased from Fluka (Germany) was used as received. The salts MgCl2, CaCl2, SrCl2, BaCl2, Na2SO4, Na3PO4, Na2EDTA, Na2S2O3, Na2-succinate, and Na2-oxalate were purchased from Merck (Germany), whereas Na-gluconate was obtained from Sigma-Aldrich. 1-Dodecylpyridinium chloride (DPC) (assay ≥98%) and cetylpyridinium chloride (CPC) (assay ≥99%) were obtained from SigmaAldrich. Double-distilled water (κ = 2−3 μS/cm at 30 °C) was used for all types of sample preparation. D2O (99.4 atom % D) was received from Heavy Water Division, BARC, Mumbai, for SANS experiment (Scheme 1). Scheme 1. Structure of Sodium N-Dodecanoyl Sarcosinate

4.2. Tensiometry. To measure the surface tension (γ), a calibrated Krüss (Germany) tensiometer, based on the du Noüy platinum ring detachment method, was used at the air/ solution interface of the surfactant solutions at 298 K. The temperature was controlled by a water bath circulator of accuracy ±0.1 K. A highly concentrated surfactant solution (about 10−15 times the CMC) was added dropwise using a Hamilton microsyringe to 5 mL aqueous solution of salt. A 15 min time interval for equilibration was given after surfactant addition and thorough mixing during all such measurements. The CMC values were estimated from the break points in the surface tension vs log[surfactant] plots.71 The average CMC from three measurements is presented here. The determined surface tension (γ) values were accurate within ±0.1 mN/m. 4.3. Conductometry. The specific conductance measurements were taken using a digital conductivity meter (Electronics India) in a conductivity cell of unit cell constant. The concentrated solution of surfactant was added to 8 mL of the aqueous salt solution using a Hamilton microsyringe. A sufficient time was given before each measurement for thorough mixing and for temperature equilibration. The CMC values are the mean of three measurements and determined from the break point in the specific conductance vs [surfactant] plot. Carpena’s method was used for the determination of break point (i.e., CMC) and β value at higher ionic strength of salt due to less abrupt change in conductivity as a function of [SDDS].72 The method has been discussed in detail in the Supporting Information. The temperature of the experiment was maintained at 298 K by a water bath circulator of accuracy ±0.1 K. The error in measured conductivity was within ±1%.71 4.4. Fluorimetry. Fluorescence measurements were taken using a PerkinElmer fluorimeter LS 55 of a 10 mm path length

(

d∑ )(Q ) = nV 2(ρp − ρs )2 P(Q )S(Q ) + B dΩ

(14)

where n denotes the number density of particles, ρp and ρs are, respectively, the scattering length densities of particle and 9263


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(5) Das, S.; Maiti, S.; Ghosh, S. Synthesis of two biofriendly anionic surfactants (N-n-decanoyl-L-valine and N-n-decanoyl-L-leucine) and their mixed micellization with non-ionic surfactant mega-10 in trisbuffer medium at pH 9. RSC Adv. 2014, 4, 12275−12286. (6) Das, S.; Naskar, B.; Ghosh, S. Influence of temperature and organic solvents (isopropanol and 1, 4-dioxane) on the micellization of cationic gemini surfactant (14-4-14). Soft Matter 2014, 10, 2863− 2875. (7) Lanigan, R. S. Final report on the safety of assessment of cocoyl sarcosine, lauroyl sarcosine, myristoyl sarcosine, oleoyl sarcosine, stearoyl sarcosine, sodium cocoyl sarcosinate, sodium lauroyl sarcosinate, sodium myristoyl sarcosinate, ammonium cocoyl sarcosinate and ammonium lauroyl sarcosinate. Int. J. Toxicol. 2001, 20, 1−14. (8) Dan, A.; Ghosh, S.; Moulik, S. P. Interaction of cationic hydroxyethylcellulose (JR400) and cationic hydrophobically modified hydroxyethylcellulose (LM200) with the amino-acid based anionic amphiphile Sodium N-dodecanoyl Sarcosinate (SDDS) in aqueous medium. Carbohydr. Polym. 2010, 80, 44−52. (9) Friesen, D. T.; Shanker, R.; Crew, M.; Smithey, D. T.; Curatolo, W. J.; Nightingale, J. A. S. Hydroxypropyl Methylcellulose Acetate Succinate-Based Spray-Dried Dispersion: An Overview. Mol. Pharmaceutics 2008, 5, 1003−1019. (10) Castillo, E. J.; Han, W. W.; Gerson, S. H. Use of Certain Anionic Amino Acid Based Surfactants to Enhance Antimicrobial Effectiveness of Tropically Administrable Pharmaceutical Compositions. U.S. Patent US6146622A, 2000. (11) Orr, T. V.; Sabatelli, A. D. Skin Conditioning and Method. European PatentsEP0283165A2, 1992. (12) Schmidt et al. European Patent Application No. 0194097EP assigned to Procter and Gamble mentions sodium lauroyl sarcosinate as the mild anionic surfactant utilized in an aerosol skin cleansing and moisturizer mouse, 1986. (13) Shah, R. A.; Chat, O. A.; Rather, G. M.; Dar, A. A. Volumetric properties of sodium lauroylsarcosinate in aqueous and alcohol solutions of methanol, ethanol, and 1-propanol. J. Chem. Eng. Data 2017, 62, 3015−3024. (14) Owoyomi, O.; Alo, O.; Soriyan, O.; Ogunlusi, O. Micellization thermodynamics of sodium lauroylsarcosinate in water-alcohol binary mixtures. Phys. Chem. Liq. 2014, 52, 305−319. (15) Alargova, R. G.; Ivanova, V. P.; Kralchevsky, P. A.; Mehreteab, A.; Broze, G. Growth of rod-like micelles in anionic surfactant solutions in the presence of Ca2+ counterions. Colloids Surf., A 1998, 142, 201−218. (16) Alargova, R. G.; Danov, K. D.; Kralchevsky, P. A.; Broze, G.; Mehreteab, A. Growth of giant rodlike micelles of ionic surfactant in the presence of Al3+ counterions. Langmuir 1998, 14, 4036−4049. (17) Liu, Z.; Cao, M.; Chen, Y.; Fan, Y.; Wang, D.; Xu, H.; Wang, Y. Interactions of divalent and trivalent metal counterions with anionic sulfonate gemini surfactant and induced aggregate transitions in aqueous solution. J. Phys. Chem. B 2016, 120, 4102−4113. (18) Qazi, M. J.; Liefferink, R. W.; Schlegel, S. J.; Backus, E. H. G.; Bonn, D.; Shahidzadeh, N. Influence of surfactants on sodium chloride crystallization in confinement. Langmuir 2017, 33, 4260− 4268. (19) Staszak, K.; Wieczorek, D.; Michocka, K. Effect of sodium chloride on the surface and wetting properties of aqueous solution of cocamidopropyl betaine. J. Surfactants Deterg. 2015, 18, 321−328. (20) Asadov, Z. H.; Nasibova, S. M.; Ahmadova, G. A.; Zubkov, F. I.; Rahimov, R. A. Head-group effect of surfactants of cationic type in interaction with propoxylated sodium salt of polyacrylic acid in aqueous solution. Colloids Surf., A 2017, 527, 95−100. (21) Ray, G. B.; Ghosh, S.; Moulik, S. P. Physicochemical studies on the interfacial and bulk behaviors of sodium N-dodecanoyl sarcosinate (SDDS). J. Surfactants Deterg. 2009, 12, 131−143. (22) Ren, Z. H. Mechanism of the salt effect on the micellization of an aminosulfonate amphoteric surfactant. Ind. Eng. Chem. Res. 2015, 54, 9683−9688.

solvent, and V is the volume of the particle. P(Q) is the intraparticle structure factor, while S(Q) is the interparticle structure factor. B is a constant term representing incoherent background scattering, which is mainly due to the hydrogen present in the sample. The data have been analyzed by comparing the scattering from different models to the experimental data. The calculation and modeling of different terms in equations are described in detail in the Supporting Information.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b00718. Sodium N-dodecanoyl sarcosinate (SDDS), an amino acid-based surfactant, has been studied in the presence of salts of higher valencies; critical micelle concentration (CMC) of SDDS; CMC was found to decrease with increasing salt concentration; physicochemical parameters, counterion binding of micelles (β), diffusion coefficient (D0), Gibbs surface excess (Γmax), area of exclusion per surfactant monomer (Amin), surface pressure at CMC (πcmc) and energy parameters; Gibbs free energy for micellization (ΔG0m), adsorption (ΔG0ads) etc. have been evaluated from tensiometry, conductometry, and fluorimetry; the hydrodynamic radius of SDDS in the presence of different salts was measured by the light scattering method and SANS experiment; micellar shape was envisioned from packing parameter calculation and SANS method (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Nitai Patra: 0000-0003-3968-0579 Debes Ray: 0000-0001-5564-2973 Vinod Kumar Aswal: 0000-0002-2020-9026 Soumen Ghosh: 0000-0001-7628-5768 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS N.P. thanks UGC, Govt of India, for Senior Research Fellowship and Centre for Surface Science, Jadavpur University, for laboratory facility. Special assistance from Sourav Das and Arpan Mal is cordially acknowledged.

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REFERENCES

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Exploring Physicochemical Interactions of Different Salts with Sodium

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