Effect of Amino Acids on Micellization and Micellar Parameters of

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Effect of Amino Acids on Micellization and Micellar Parameters of Anionic Surfactant Alpha Olefin Sulfonate C14−C16 in Aqueous Solutions: Surface Tension, Conductometric, Volumetric, and Fluorescence Studies Lusine R. Harutyunyan* and Romik S. Harutyunyan

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Department of Chemistry, Yerevan State University, A. Manukyan 1, 0025 Yerevan, Armenia S Supporting Information *

ABSTRACT: The effect of L-glycine, L-alanine, and L-leucine on micellization, surface activity, and colloidal parameters of an anionic surfactant sodium alpha olefin sulfonate C14−C16 in aqueous solutions at different temperatures was studied. It was shown that the hydrophobicity of the studied amino acids had great influence on behavior of physicochemical parameters. In studied systems, the electrostatic interactions between zwitterionic groups of amino acids and ions of sodium alpha olefin sulfonate C14−C16 were also important.

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INTRODUCTION Alpha olefin sulfonates (AOS) are a class of anionic surfactants, which are characterized with excellent foaming, wetting, emulsifying, dispersing, and stabilizing ability properties.1−4 It is very important that AOS have low toxicity and high biodegradability.4,5 Because of these properties, AOS are widely used in many industrial processes including emulsion polymerization, wax emulsification, and textile processing, and they are one of the main components of household-cleaning and person-care products.6−10 For example, sodium olefin sulfonates, in general, are used up to 5% in cleansers, and 16% in shampoos and bath shower products, specifically, sodium C14−16AOS is used at 3.6% in facial cleansing foams, >5−10 % in skin care preparations, and >10% in personal cleanliness products.5 At the same time, most of the person-care and cosmetic products also contain as a main component amino acids (AA).11 Therefore, study of systems containing both sodium AOS and AA has applicable importance and can be useful to expand the use of sodium AOS surfactants also in the fields of biology and drug medicine. Sodium AOS have been mainly studied in view of their application as emulsifiers and cleaning agents2−4,7,12,13 and there are a few studies in which physicochemical and colloidal properties of sodium AOS aqueous solutions both in the absence and presence of additives, mainly in the presence of gelatin, are presented.12−18 In this paper the behavior of important parameters of sodium alpha olefin sulfonate C14− C16 in aqueous solutions, such as critical micelle concentration, degree of counterion binding, aggregation number, values of standard volumes of transfer, and the effect of three AAglycine, alanine, leucine, on the parameters of sodium alpha olefin sulfonate C14−C16 in aqueous solutions at different © XXXX American Chemical Society

temperatures are presented to reveal the role of intermolecular interactions in the studied systems.

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EXPERIMENTAL SECTION Materials. Anionic surfactant alpha olefin sulfonate C14− C16 (C14−16AOS, CnH2n−1SO3Na, n = 14−16, Parchem, US, CAS no. 68439-57 and 7757-82-6) was used as it was received. Amino acids (AA) L-glycine, L-alanine, and L-leucine (BioUltra, ≥ 99.5% (NT) Sigma) were used without further purification. Pyrene (puriss. p.a., for fluorescence, >99.0% (GC) Sigma), a fluorescence probe, and cetylpyridinium bromide (Aldrich, ≥ 98.0%), a fluorescence quencher, were also used without further purification. The properties of the used chemicals are listed in Table 1. Structural formulas, values of pKa, pKb, isoelectric point pI of AA and values of pH of aqueous systems AA−C14−16AOS are presented in Table 2. From the data of Table 2 it follows that in aqueous solutions of C14−16AOS, AA are in zwitterionic form. All samples were prepared by directly mixing appropriate amount of components in double-distilled water. Methods. The measurement of the surface tension of samples (γ) was carried out with a SITA science line t60 tensiometer. The dynamic surface tension of each sample was determined by maximum bubble pressure method. The life time of the bubble was controlled in optimal diapason for 50− 55 s with a resolution of 1 ms. Temperature was controlled automatically with resolution of 0.1 K. Estimated error in γ is no more than ±2%. Critical micelle concentration (cmc) of Received: October 3, 2018 Accepted: January 23, 2019

A

DOI: 10.1021/acs.jced.8b00886 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Table 1. Specification of the Chemicals Used

chemical name

source

alpha olefin sulfonate C14− C16 L-glycine L-alanine L-leucine pyrene cetylpyridinium bromide

Parchem (US) Sigma Sigma Sigma Sigma Aldrich

purification method

final mass fraction puritya

68439-57and 7757-82-6

none

≥0.900

50-40-6 56-41-7 61-90-5 129-00-0 202869-92-9

none none none none none

≥0.995 ≥0.995 ≥0.995 ≥0.990 ≥0.990

CASRN

a

Declared by supplier.

C14−16AOS both in the absence and presence of AA in aqueous solutions at each temperature was obtained as a break point on the isotherms surface tension-logarithm of C14−16AOS concentration (Figure 1). The values of the surface tension of C14−16AOS−AA−water systems at studied temperatures are presented in Table S1 (Supporting Information). The value of surface tension at cmc (γcmc) of aqueous solutions of C14−16AOS at 298.15 K (32.51 mN·m−1 in the absence of additive) is in good agreement with literature data.14 The conductivity of samples (κ) was carried out on a conductivity/pH meter “Jenway 4330”. The temperature was controlled automatically with a precision of ±0.5 K. Estimated error in κ is no more than ±2%. The critical micelle concentration (cmc) of C14−16AOS both in the absence and presence of AA in aqueous solutions at each temperature was obtained as a break point on the isotherms conductivity− C14−16AOS concentration (Figure 2). The values of the conductivity of the C14−16AOS−AA−water systems at the studied temperatures are presented in Table S2 (Supporting Information). The degree of counterion binding (β) of C14−16AOS in the absence and in the presence of the studied AA was obtained as the ratio of slopes of the conductivity−concentration of surfactant isotherms below and above the cmc. The density of samples (ρ) was determined by a DMA-4500 densimeter. The precision of experiments is ±(5 × 10−5) g· cm−1, and the temperature was controlled automatically with a precision of ±0.1 K. Before each experiment, the densimeter was calibrated by dry air and double-distilled water at atmospheric pressure.

Figure 1. Surface tension isotherms of C14−16AOS−glycine−water system depend on C14−16AOS concentration in the absence of glycine (■) and in the presence of different concentrations of glycine (○, 0.0095 mol·kg−1; ▲, 0.0282 mol·kg−1; □, 0.0462 mol·kg−1; ⧫, 0.0774 mol·kg−1; △ 0.0968 mol·kg−1) at 298.15 K.

The aggregation number of C14−16AOS (Nagg) both in the absence and presence of AA in aqueous solutions was determined by the fluorescence quenching method suggested by Turro and Yekta.20 The dependence of the luminescence intensities relationship in the presence and in the absence of quencher (I/I0) on micelle concentration is described by the following equation: ln

Nagg I = [Q ] I0 [surfactant] − cmc

(1)

where [surfactant] is the concentration of C14−16AOS in solution, cmc is the critical micelle concentration, and [Q] is the quencher concentration, respectively. In Figure 3 isotherms ln(I/I0) = f [Q] of the C14−16AOS−glycine−water systems are presented. There is a linear dependence of ln(I/I0) = f [Q] as follows from eq 1 (average correlation coefficient is higher than 0.9990). Analogous isotherms were obtained in the presence of alanine and leucine. In fluorescence studies of aqueous-micellar solutions of C14−16AOS both in the absence and presence of AA, pyrene

Table 2. Structural Formulas, Values of pKa, pKb, Isoelectric Point pI of AA and Values of pH of Aqueous Systems AA− C14‑16AOS

B



Article

form in solution and they are in thermodynamic equilibrium with the molecules/ions of the surfactant. cmc strongly depends on the nature of the solvent, the presence of additives and their concentration, pH of solutions, ionic composition of solution, and temperature, etc. In the presence of additives, intermolecular interactions have an important role in solutions which can affect the equilibrium between monomeric molecules (ions) and micelles of surfactants. Experimentally determined values of cmc of C14−16AOS in the absence and presence of AA in aqueous solutions at different temperatures are presented in Table 3. The value of cmc of C14−16AOS in aqueous solution at 298.15 K is in good agreement with literature data.14 From the data of Table 3 it follows that the values of cmc of C14−16AOS both in the absence and presence of additives increase with the increase of temperature. The effect of temperature on the cmc of surfactants both in the absence and presence of additives must be discussed in the context of different influencing factors.23−25 The degree of hydration of hydrophilic groups decreases with the increase of temperature, which favors micellization. At the same time with the increase of temperature, the water structure around hydrophobic groups is destroyed, which does not favor micellization. It has also established that with the increase of temperature the thermal motion of the additives’ and surfactant’s molecules becomes more intensive, thereby making it difficult to form structured micelles. The kinetic energy of a system rises due to an increase of temperature, and as a result structured micelles are destroyed; therefore, the aggregation number of the micelles decreases and cmc increases.26 From the data of Table 3 it also follows that the values of cmc increase with increase of AA concentration. It is known that AA are structure-breakers26,27 and according to theory, suggested by Frank and Evans,28 structure-breakers in aqueous solutions induce the destruction of water clusters (breaking some H-bonds/destroying ordered structure), which leads to the decrease of hydrophobicity. Thus, in micellar solutions AA promote the decrease of hydrophobicity, the driving force of micellization,23,29−33 and, hence, promote the increase of cmc. In ionic surfactant− additive solutions electrostatic interactions also have a significant role. Zwitterionic groups of AA interact/bind with the alkylsulfonate anion of C14−16AOS and, therefore, inhibit micellization. As a result, the cmc of surfactant increases due to dehydration of surfactant hydrophilic groups.34 From data of Table 3 it also follows that the values of cmc C14−16AOS in the presence of the studied AA decrease in the order glycine−alanine−leucine. It is known that the longer is alkyl chain in a molecule of the AA, the higher is hydrophobicity of the AA. The length of alkyl chain increases in the order glycine−alanine−leucine; thus, in the presence of leucine less molecules of surfactant participate in hydrophobic interactions for micellization and as a result, the cmc values of C14−16AOS in the presence of leucine are smaller compared with that in the presence of glycine. The values of degree of counterion binding (β) of C14−16AOS in the absence and in the presence of studied AA in aqueous solutions at different temperatures are presented in Table 4. Obtained values of β increase with the increase of temperature both in the absence and presence of studied AA. The increase of thermal energy due to the increase of temperature enhances the ionization of the ionic surfactant C14−16AOS and, thereby, an increase in β. Values of β also increase with the increase of AA concentration. Steric

Figure 2. Conductivity isotherms of C14−16AOS−glycine−water system depend on C14−16AOS concentration in the absence of glycine (■) and in the presence of different concentrations of glycine (○, 0.0095 mol·kg−1; ▲, 0.0282 mol·kg−1; □, 0.0462 mol·kg−1; ⧫, 0.0774 mol·kg−1; △, 0.0968 mol·kg−1) at 298.15 K.

Figure 3. Isotherms ln(I/I0) = f [Q] of C14−16AOS−glycine−water systems in the absence of glycine (■) and in the presence of different concentrations of glycine (○, 0.0095 mol·kg−1; ▲, 0.0282 mol·kg−1; □, 0.0462 mol·kg−1).

was used as fluorescence probe, and cetylpyridinium bromide was used as fluorescence quencher. This pair of probe− quencher is more suitable for the determination of aggregation number of the ionic surfactants.21,22 The fluorescence spectrum of pyrene (2 × 10−6 mol·L−1) in aqueous solutions of C14−16AOS both in the absence and presence of AA was recorded on a Varian Cary Eclips luminescent spectrometer at room temperature, with excitation and emission slits of 10 and 2.5 nm, excitation wavelength of 334 nm, and a rate of scanning of 120 nm·min−1, respectively.

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RESULTS AND DISCUSSION Critical Micelle Concentration and Thermodynamics of Micellization. Critical micelle concentration (cmc) is one of the most important parameters of micellar solutions. cmc is characterized as the surfactant concentration at which micelles C



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Table 3. Values of cmc of C14−16AOS (1) in the Absence and in the Presence of AA (2) in Aqueous Solutions at Different Temperatures, at Pressure p = 0.1 MPaa 103·cmc/mol·kg−1 298.15 K [m2]/ mol·kg−1

conduc.

303.15 K

surface tension

conduc.

0.0000 0.0095 0.0282 0.0462 0.0774 0.0968

1.41 2.31 2.74 3.02 3.58 4.15

1.41 2.33 2.73 3.03 3.56 4.15

1.58 2.43 2.88 3.39 3.81 4.30

0.0095 0.0282 0.0462 0.0774 0.0968

2.18 2.61 2.98 3.40 4.03

2.18 2.61 3.00 3.41 4.01

2.30 2.84 3.21 3.69 4.19

0.0095 0.0282 0.0462 0.0774 0.0968

1.98 2.36 2.83 3.29 3.71

2.00 2.35 2.82 3.30 3.72

2.11 2.65 3.00 3.44 3.98

308.15 K

surface tension

313.15 K

conduc.

surface tension

conduc.

surface tension

1.73 2.61 3.16 3.67 4.10 4.67

1.71 2.60 3.16 3.67 4.11 4.69

1.98 2.80 3.34 3.88 4.39 4.91

1.99 2.82 3.36 3.88 4.40 4.92

2.46 3.00 3.41 3.91 4.33

2.46 3.00 3.42 3.92 4.35

2.62 3.11 3.58 4.07 4.56

2.62 3.13 3.60 4.05 4.55

2.34 2.88 3.18 3.60 4.10

2.34 2.89 3.20 3.61 4.11

2.51 2.93 3.31 3.88 4.29

2.50 2.93 3.30 3.89 4.30

Glycine 1.59 2.42 2.89 3.40 3.83 4.32 Alanine 2.31 2.82 3.20 3.70 4.19 Leucine 2.11 2.65 3.00 3.45 3.97

a Estimated error in cmc is no more than ±2%, standard uncertainties are u(T) = 0.5 K for conductivity studies and u(T) = 0.1 K for surface tension studies, u(p) = 10 kPa, u(m2) = 0.0002.

charged centers of AA become possible. An increase of AA concentration induces disruption of the H-bonded structure of water which, in turn, tends to solvate the counterions Na+ of C14−16AOS, and also there is possibility of interaction of these counterions with COO− groups of AA.30 This fact also favors an increase of β with an increase of AA concentration. From the data of Table 4 it follows that in the presence of AA values of β decrease in the order glycine−alanine−leucine as cmc. Thus, the hydrophobicity of AA also effects the β behavior. Similar results have been obtained for another anionic surfactant sodium dodecyl sulfate in the presence of AA in aqueous solutions.25,35 On the basis of the cmc and β, values of thermodynamic parameters of micellization were calculated. The free energy of micellization (ΔGm0 ) was calculated by the following equation:36,37

Table 4. Values of Degree of Counterion Binding (β) of C14−C16AOS (1) in the Absence and in the Presence of AA (2) in Aqueous Solutions at Different Temperatures, at Pressure p = 0.1 MPaa β −1

[m2]/mol·kg

298.15 K

0.0000 0.0095 0.0282 0.0462 0.0774 0.0968

0.23 0.29 0.32 0.35 0.38 0.42

0.0095 0.0282 0.0462 0.0774 0.0968

0.27 0.31 0.34 0.36 0.40

0.0095 0.0282 0.0462 0.0774 0.0968

0.26 0.29 0.32 0.35 0.38

303.15 K Glycine 0.27 0.33 0.35 0.38 0.41 0.45 Alanine 0.31 0.33 0.36 0.39 0.43 Leucine 0.30 0.32 0.35 0.38 0.40

308.15 K

313.15 K

0.30 0.35 0.37 0.40 0.44 0.47

0.32 0.38 0.41 0.44 0.47 0.50

0.33 0.35 0.38 0.42 0.46

0.36 0.40 0.42 0.45 0.48

0.32 0.34 0.37 0.40 0.43

0.35 0.38 0.41 0.43 0.45

ΔGm0 = (2 − β)RT ln χcmc

(2)

where R is the gas constant, T is the absolute temperature, χcmc is the molar fraction of surfactant at cmc, respectively. Enthalpy of micellization (ΔΗ0m) was calculated by the following equation:38 ÄÅ ÉÑ ÅÅ ÑÑ ∂ ln χ i y Å ∂ β i y Å z − ln χ jj zz ÑÑÑ cmc z ΔΗ m0 = −RT ÅÅÅ(2 − β)jjjj zz z ÑÑ cmc j ÅÅ k ∂T { pÑÑÑÑ (3) k ∂T { p ÅÅÇ Ö

Estimated error in β is no more than ±2%, standard uncertainties are u(T)= 0.5 K, u(p)= 10 kPa, u(m2)= 0.0002. a

The entropy of micellization (ΔS0m) was calculated by the following equation:36,37

difficulties in the presence of AA can enhance an increase of electrostatic repulsion between ions of C14−16AOS and, therefore, increase of β (or decrease of dissociation). As mentioned above, AA are water structure-breaking agents26,27 that favor the solvation of counterions (Na+ counterion in case of C14−16AOS) and the interactions of counterions with

ΔSm0 =

ΔΗ m0 − ΔGm0 T

(4)

Values of thermodynamic parameters of micellization both in the absence and presence of AA are presented in Table 5. D



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Table 5. Values of Thermodynamic Parameters of Micellization C14‑16AOS (1) in the Absence and in the Presence of AA (2) in Aqueous Solutions at Different Temperatures, at Pressure p = 0.1 MPaa 298.15 K

303.15 K

308.15 K

ΔG0m/ −1

kJ·mol

ΔH0m/ −1

kJ·mol

ΔG0m/ −1

kJ·mol

kJ·mol

0.0000 0.0095 0.0282 0.0462 0.0774 0.0968

−46.37 −42.79 −41.33 −40.20 −38.78 −37.25

−15.16 −23.74 −20.96 −15.95 −16.29 −22.38

−45.62 −42.28 −41.07 −39.66 −38.45 −37.02

−13.88 −22.91 −20.35 −15.00 −15.58 −21.90

0.0095 0.0282 0.0462 0.0774 0.0968

−43.49 −41.78 −40.50 −39.47 −37.84

−25.51 −24.19 −25.96 −19.33 −25.93

−42.97 −41.62 −40.37 −39.07 −37.60

−24.69 −23.74 −25.61 −18.59 −25.49

0.0095 0.0282 0.0462 0.0774 0.0968

−44.13 −42.70 −41.17 −39.80 −38.64

−20.58 −21.57 −23.94 −26.00 −30.19

−43.57 −42.17 −40.85 −39.60 −38.52

−19.62 −20.68 −23.33 −25.56 −29.93

[m2]/ mol·kg−1

ΔH0m/ −1

ΔG0m/ −1

kJ·mol

Glycine −45.17 −42.16 −40.85 −39.49 −38.06 −36.82 Alanine −42.87 −41.57 −40.24 −38.74 −37.32 Leucine −43.32 −42.00 −40.70 −39.62 −38.30

313.15 K ΔH0m/ −1

kJ·mol

ΔG0m/ −1

kJ·mol

ΔH0m/ kJ·mol−1

ΔS0m/ J· (mol·K)−1

−12.91 −22.47 −19.79 −14.43 −14.80 −21.45

−44.78 −41.77 −40.26 −38.90 −37.66 −36.48

−11.99 −21.76 −18.86 −13.43 −14.03 −20.87

104.70 83.90 78.34 65.32 61.46 49.86

−24.29 −23.39 −25.23 −17.92 −25.02

−42.52 −40.82 −39.73 −38.46 −37.26

−23.63 −22.34 −24.48 −17.30 −24.77

60.30 59.00 48.76 67.56 39.92

−18.98 −20.16 −22.89 −25.25 −29.56

−42.89 −41.58 −40.26 −39.10 −38.20

−18.15 −19.38 −22.16 −24.60 −29.33

79.60 70.88 57.80 46.32 28.34

Maximum estimation is ±0.5 kJ mol−1 in ΔG0m, ±0.5 kJ mol−1 in ΔΗ0m, and ±0.1 J. (mol·K)−1 in ΔS0m, standard uncertainties are u(T) = 0.5 K, u(p) = 10 kPa, u(m2) = 0.0002. a

Values of ΔG0m become less negative both with the increase of temperature and AA concentration, suggesting that an increase in temperature and AA concentration disfavors micellization as discussed above. Negative values of ΔΗm0 indicate that there are strong interactions between molecules of AA and the micellar system, but, at the same time there is no trend in changes of ΔΗ0m with increase of AA concentration. Similar behavior is not unusual for anionic surfactants in AA aqueous solutions as well as cationic and nonionic surfactants in AA aqueous solutions.27,39−41 Values of |TΔS0m| > |ΔΗ0m|, thus micellization is an entropy driving process due to transfer of the hydrophobic groups of C14−16AOS from bulk solution to the surface, as well as reorganization of water molecules (destruction of “ice-like” structure) as a result of AA molecules−micelles interactions. The large values of |TΔS0m| in aqueous solutions are due to the breaking of the structured water surrounding the hydrophobic chains of the surfactant molecules in the presence of AA and subsequent transfer of the hydrophobic chains from the unfriendly polar solvent environment to the friendly nonpolar interior of the micelle, and also due to increased freedom of the hydrophobic chains in the nonpolar interior of the micelle compared to the polar aqueous environment.42 According to the theory of self-organization, suggested by Nagarajan et al.,43 in the free energy of micellization the following have a contribution: (a) tail transfer of the Gibbs free energy of surfactant (ΔG0transf), which considers that there is a change of Gibbs free energy due to transfer of the surfactant alkyl chain from the bulk phase to the micelle core; (b) interfacial Gibbs free energy (ΔG0interf) at the micelle core/ solute interface, for which it is suggested that micelle formation is a new phase formation, and as a result, enhances the possibility of contact between the hydrophobic core and bulk phase; (c) Gibbs free energy of headgroups (hydrophilic groups) interactions (ΔG0electr), which represents the electro-

static repulsion between headgroups (hydrophilic groups) on the micelles surface; (d) Gibbs free energy of deformation (ΔG0def) for which it is suggested that the alkyl chain of surfactant inside the micelle has a conformation which is different from the conformation in solution in the absence of micelle due to the maximal packaging necessary inside the micelle; (e) Gibbs free energy of steric interaction (ΔG0st), which is related to the steric repulsion of headgroups (hydrophilic groups) of surfactant on the surface of micelles. Among these components the dependence of cmc on the content of the main phase is controlled by the tail transfer Gibbs free energy of surfactant (ΔG0transf), because dependence of the cmc on other components of the Gibbs free energy is negligible.43 ΔG0transf was calculated by following relationship: 0 ΔGtransf = (ΔGm0 )s − (ΔGm0 )H2O

(5)

where (ΔG0m)s and (ΔG0m)H2O are Gibbs free energies in cosolute and water, respectively. Positive values of ΔG0transf in aqueous solutions of AA at 303.15 K (Table 6) can be a result of solvophobic interactions decreasing due to increase of alkyl chain solvation and domination of interaction of alkyl chain of surfactant with hydrophobic part of cosolvent (in this case AA) which also lead to an increase of cmc.26,42,44 Values of ΔG0transf, as other discussed parameters, also decrease in the order glycine− alanine−leucine; a higher hydrophobicity of AA means a more difficult micellization. Adsorption at Air/Liquid Interface. One of the fundamental properties of surfactants is their tendency to adsorb at the air/liquid interface. Adsorption of surfactants is studied to determine (1) surfactant concentration at the interface to reveal coverage of the surface (and also charge density on surface) by surfactant molecules and evaluate applicability of the surfactant in many processes (detergation, E



Article 0 Calculated values of Γmax, Amin, Πcmc, and ΔGads of C14−16AOS in the absence and presence of AA in aqueous solutions at 303.15 K are presented in Table 7.

Table 6. Values of Tail Transfer Gibbs Free Energy (ΔG0transf) of C14−16AOS (1) in Aqueous AA (2) Solutions at Temperature T = 303.15 K and Pressure p = 0.1 MPaa [m2]/mol·kg−1

ΔG0transf/kJ·mol−1

Table 7. Values of Adsorption Parameters of C14‑16AOS (1) in the Absence and in the Presence of AA (2) in Aqueous Solutions at Temperature T = 303.15 K and Pressure p = 0.1 MPaa

Glycine 0.0000 0.0095 0.0282 0.0462 0.0774 0.0968

3.34 4.55 5.97 7.17 8.61

[m2]/ mol·kg−1

ΔG0ads/ kJ·mol−1

2.65 4.0 5.25 6.55 8.02

0.0000 0.0095 0.0282 0.0462 0.0774 0.0968

−60.10 −47.67 −46.43 −45.50 −45.50 −44.96

2.05 3.45 4.77 6.02 7.10

0.0095 0.0282 0.0462 0.0774 0.0968

−48.20 −46.75 −45.64 −44.71 −43.75

0.0095 0.0282 0.0462 0.0774 0.0968

−48.80 −47.53 −46.40 −45.53 −44.77

Alanine 0.0095 0.0282 0.0462 0.0774 0.0968 Leucine 0.0095 0.0282 0.0462 0.0774 0.0968

a 0 Maximum estimation is ±0.5 kJ mol−1 in ΔGtransf , standard uncertainties are u(T) = 0.5 K, u(p) = 10 kPa, u(m2) = 0.0002.

emulsification, foaming, etc.) depends on their concentration on the surface; (2) orientation and compact packing of surfactant molecules/micelles on the surface which are mean factors for formation of hydrophilic or hydrophobic surfaces due to adsorption; (3) changes of physicochemical parameters of a system due to adsorption, because analysis of these parameters gives important information about the nature and mechanism of the interactions. On the basis of the data of surface tension, adsorption parameters of the surfactant at the air/liquid interface in the absence and presence of AA were calculated. Gibbs adsorption equation can be presented as45 1 Γ=− dγ RT d ln c (6)

0.68 0.26 0.30 0.34 0.40 0.50

35.61 33.74 30.28 28.69 27.04 25.98

0.26 0.28 0.34 0.36 0.42

33.21 29.71 26.51 25.31 24.19

0.26 0.28 0.30 0.32 0.36

34.71 32.96 31.65 30.20 28.92

According to the literature physicochemical properties of surfactant aqueous solutions can be changed by two mechanisms:48,49 (1) specific interactions between molecules of surfactant and additive; and/or (2) structural changes of solvent. It is known25,39 that the hydrophilic headgroup of C14−16AOS has the same possibility to interact both with positively and negatively charged groups of AA. As a result of these interactions, some molecules of surfactant move from surface of solution to the bulk phase, which leads to a decrease of Γmax (and also Πcmc) in the presence of AA.45 At the same time, as it is mentioned above, AA are water structurechangers;26,27 thus, the behavior of Γmax (and adsorption parameters commonly) is a result of two factors acting together. By comparing the values of ΔG0ads (Table 7) and ΔG0m (Table 6) it is seen that values of ΔG0ads are higher than those of ΔG0m indicating that adsorption at the interface is a more favorable process than micellization in aqueous solutions of C14−16AOS both in the absence and presence of AA. Aggregation Number and Micellar Parameters. Surfactants in aqueous solutions form aggregatesmicelles or bilayers (like vesicles); micelles can be spherical, ellipsoidal, or cylindrical, while bilayers can be spherical or lamellar. As it is known, surfactants with a single alkyl chain form micelles,49 and the aggregation of surfactant molecules into micelles mainly depends on the condition of aggregates formation as well as the presence of additives.50,51 Experimentally determined values of aggregation number (Nagg)of C14−16AOS in the absence and presence of AA in

(7)

45,46

where n is equal 2. The minimum area per molecule of surfactant (Amin) was calculated by the following equation:47 (8) 18

where N is Avogadro’s number and 10 is the coefficient to convert m2 to nm2. 0 The standard free energy of adsorption (ΔGads ) was 47 calculated by the following equation: 0 ΔGads = ΔGm0 − (Πcmc/Γmax m)

Πcmc/mN m−1

Maximum estimation is ±0.5 kJ mol−1 in ΔG0ads, ±0.5 × 10−6 mol· m−2 in Γmax, ±0.4 nm2·molecule−1 in Amin, and ±0.2 mN·m−1 in Πcmc; standard uncertainties are u(T) = 0.1 K, u(p) = 10 kPa, u(m2) = 0.0002.

where dγ, R, T, and c denote the change of surface tension in solution, gas constant, absolute temperature, and concentration of surfactant in solution, respectively. The maximum density of adsorption (Γmax) was calculated by the following equation:45

A min = 1018/N Γmax

Glycine 2.46 6.26 5.65 4.91 4.13 3.27 Alanine 6.35 5.79 5.03 4.66 3.93 Leucine 6.64 6.15 5.70 5.09 4.63

Amin/ (nm2· molecule−1)

a

( )

Γmax = −1/(2.303nRT )lim itc → cmc(dγ /d log c)T

Γmax·106/ mol·m−2

(9)

where Πcmc = γ0 − γcmc, and γ0 and γcmc are the surface tension of solvent and surfactant solution at cmc, respectively. F



Article

The standard partial molar volume (Φ0ν) was obtained by the following relation:54,55

aqueous solutions are presented in Table 8. The value of Nagg of C14−16AOS in the absence of AA in aqueous solutions is in

Φv = Φ0v + Svm

Table 8. Values of Micellar Parameters of C14‑16AOS (1) in the Absence and in the Presence of AA (2) in Aqueous Solutions [m2]/ mol·kg−1

Nagg ± 2

0.0000 0.0095 0.0282 0.0462 0.0774 0.0968

128.0 125.0 119.0 111.0 94.0 86.0

0.0095 0.0282 0.0462 0.0774 0.0968

124.0 117.0 106.0 90.0 77.0

0.0095 0.0282 0.0462 0.0774 0.0968

120.0 112.0 99.0 84.0 76.0

r/Å Glycine 23.6 23.4 23.1 22.5 21.3 20.7 Alanine 23.4 22.9 22.2 21.0 19.9 Leucine 23.1 22.6 21.7 20.5 19.9

a0/Å2

P

54.8 55.2 56.0 57.5 60.7 62.4

0.385 0.383 0.377 0.367 0.348 0.334

55.2 56.4 58.2 65.6 68.0

0.383 0.375 0.363 0.322 0.311

56.0 57.2 59.6 63.1 68.0

0.377 0.369 0.354 0.335 0.311

The behavior of is regarded with respect to solute−solvent interactions.54,55 Values of Φ0ν represent the real volume of solute and changes of volume due to solute−solvent interactions.56−58 From the data of Table 10 it follows that values of Φ0ν are positive and increase with temperature indicating the existence of strong solute−solvent interactions. The increase of Φ0ν with increase of temperature can be due to the following: (1) in surfactant solutions molecules of the surfactant destroy the solvate layer of AA, therefore transfer of some molecules of solvent to bulk phase is possible; (2) at high temperatures surfactant−water interactions become weaker, which leads to positive changes of volume.54,59,60 From the data of Table 10 it also follows that values of Φ0ν increase in the order glycine−alanine−leucine due to an increase of AA hydrophobicity. In Table 10 values of Φ0ν of the AA−water system (in the absence of surfactant) are also presented. Values of Φ0ν of the AA−water system at 303.15 are in good agreement with literature data.61−63 It is known that there is a linear dependence between values of Φ0ν and number of carbon atoms in the alkyl chain of AA:55,61,64−66 Φ0ν = Φ0ν(NH+3 , COO−) + nCΦ0ν(CH 2)

1000(ρ0 − ρ) M + mρ0 ρ ρ

(12)

where nC is the number of carbon atoms in alkyl chain of AA, Φ0ν(NH+3 ,COO−) and Φν0(CH2) are the contributions of zwitterionic and methyl groups of AA in Φ0ν, respectively. Values of Φ0ν(NH+3 ,COO−) and Φ0ν(CH2) are presented in Table 11. The studied AA also contain CH2CH2−(alanine) and (CH3)2CH2(CH)2−(leucine) groups, therefore contributions of CH− and CH3− groups were also calculated based on values of Φ0ν(CH2). According to the model suggested by Hakin et al.,67,68 contributions of CH− and CH3− groups in standard partial molar volume were calculated by following equations:

good agreement with literature data.52 Values of micellar radius (r), surface area per headgroup (a0), and parameter of packing (P), which were calculated according to ref 49 are also presented in Table 8. Values of Nagg of C14−16AOS decrease with the increase of AA concentration. The behavior of Nagg can be explained due to replacement of some molecules of water by molecules of AA in the solvate layer of headgroups (hydrophilic groups) of the micelle. This fact is in agreement with the micellization behavior of C14−16AOS in the presence of studied AA (values of cmc and β of C14−16AOS in the presence of AA in aqueous solutions increase). According to the literature,53 the behavior of packing parameter makes it possible to hypothesize about the geometry of the micelles. If P < 1/3, micelles are spherical; if 1/3 < P < 1/2, micelles are nonspherical; if 1/2 < P < 1, during aggregation vesicles or bilayers are formed; if P > 1, inverted structures are formed. From the data of Table 8 it follows that aggregates of C14−16AOS both in aqueous solutions and aqueous AA solutions are spherical. Volumetric Properties. The study of volumetric properties of aqueous−micellar solutions makes it possible to analyze intermolecular interactions in detail and reveals the character of the solute−solvent interactions. Experimentally determined values of density (ρ) of C14−16AOS−AA−water system both in the premicellar and postmicellar regions at different temperatures are presented in Table 9. In Table 9 there are also presented values of apparent molar volumes (Φv) which were calculated by the following equation: Φv =

(11)

Φ0ν

Φ0ν(CH3) = 1.5Φ0ν(CH 2)

(13)

Φ0ν(CH) = 0.5Φ0ν(CH 2)

(14)

Obtained data are presented in Table 11. Values of Φ0ν(NH+3 ,COO−) higher than Φ0ν(CH2) indicate that interactions between zwitterionic groups and molecules/micelles of C14−16AOS are stronger than hydrophobic interactions between alkyl groups of AA and molecules/micelles of C14−16AOS. At the same time values of Φ0ν(CH3)2CH2(CH)2 are higher than those of Φ0ν(NH+3 ;COO−) indicating that hydrophobic interactions become dominant with increasing number of carbon atoms in the alkyl chain of AA (the longer is the alkyl chain of AA, the higher is the hydrophobicity of AA). On the basis of the values of standard partial molar volumes values of standard volumes of transfer (ΔtrΦ0ν) of AA from water to surfactant aqueous solutions were calculated by the following equation:69 Δtr Φ0ν = Φ0ν(in aqueous solution of surfactant) − Φ0ν(in water)

(10)

where M and m are the molar mass and molality of solute, ρ and ρ0 are the density of solution and solute, respectively.

(15)

In the system C14−16AOS−AA−water the following interactions are possible: (1) hydrophobic−hydrophobic interG



Article

Table 9. Values of Density (ρ) and Apparent Molar Volume (Φv) of System C14‑16AOS (1)−AA (2)−Water in the Premicellar and Postmicellar Regions at Different Temperatures, at Pressure p = 0.1 MPaa 298.15 K −1

[m2]/ mol·kg

ρ/g·cm

−3

303.15 K −1

Φv/cm ·mol 3

−3

ρ/g·cm

[m1] = 0.2 × 10 0.0000 0.0095 0.0282 0.0462 0.0774 0.0968

0.9978 0.9982 0.9989 0.9993 0.9996 0.9997

32.1611 35.9464 42.4906 51.7136 55.3452

0.9962 0.9966 0.9971 0.9974 0.9976 0.9977

0.0095 0.0282 0.0462 0.0774 0.0968

0.9982 0.9988 0.9991 0.9993 0.9994

46.8863 53.5251 60.8542 69.6262 72.4781

0.9965 0.9970 0.9972 0.9973 0.9974

0.0095 0.0282 0.0462 0.0774 0.0968

0.9984 0.9993 0.9999 1.0005 1.0007

67.8114 77.7456 85.4538 95.9914 100.9046

0.0000 0.0095 0.0282 0.0462 0.0774 0.0968

1.0005 1.0008 1.0013 1.0016 1.0018 1.0020

43.4021 46.5848 51.1206 56.8132 59.3931

0.9987 0.9989 0.9992 0.9994 0.9997 0.9998

0.0095 0.0282 0.0462 0.0774 0.0968

1.0009 1.0016 1.0021 1.0027 1.0028

46.8736 49.9325 54.2713 60.4273 65.0694

0.9990 0.9994 0.9997 1.0000 1.0001

0.0095 0.0282 0.0462 0.0774 0.0968

1.0011 1.0022 1.0031 1.0044 1.0048

67.7991 70.5911 74.5201 80.2843 86.1870

0.9995 1.0004 1.0011 1.0021 1.0025

308.15 K

Φv/cm ·mol 3

−3

0.9967 0.9975 0.9980 0.9984 0.9986 [m1] = 20 × 10−3

−1

ρ/g·cm

−3

313.15 K −1

Φv/cm ·mol 3

−3

ρ/g·cm

Φv/cm3·mol−1

−1

mol·kg (Premicellar Region) Glycine 0.9947 32.8458 0.9950 43.0883 0.9954 49.0544 0.9956 56.9799 0.9958 59.5821 0.9959 Alanine 57.5019 0.9950 60.7051 0.9953 67.4613 0.9955 74.9362 0.9957 76.7556 0.9958 Leucine 78.4265 0.9951 84.9372 0.9957 92.0745 0.9960 102.6320 0.9963 106.2608 0.9964 mol·kg−1 (Postmicellar Region) Glycine 0.9961 53.9793 0.9963 57.2923 0.9966 59.8647 0.9968 62.0819 0.9970 63.6343 0.9971 Alanine 57.4374 0.9963 64.1835 0.9966 67.3470 0.9968 72.1823 0.9971 74.5109 0.9972 Leucine 78.3549 0.9965 81.2724 0.9972 85.4060 0.9977 90.7177 0.9984 94.5704 0.9985

43.4701 50.2763 55.6606 60.9685 62.7947

0.9931 0.9933 0.9936 0.9937 0.9938 0.9939

54.1640 57.5144 62.3153 66.3043 67.0873

57.5405 67.9293 71.9153 76.3395 77.9030

0.9933 0.9936 0.9938 0.9940 0.9941

68.2584 71.6046 74.2033 77.7579 79.0641

89.1070 95.7618 103.1240 110.6273 113.7540

0.9934 0.9939 0.9941 0.9943 0.9945

99.8607 103.0628 109.8527 116.0499 117.0806

54.0650 57.3952 59.9811 63.5171 64.8169

0.9955 0.9957 0.9958 0.9959 0.9960 0.9961

54.0848 64.5849 66.5758 68.7860 69.0429

68.1170 71.4430 74.0260 76.2506 77.8097

0.9956 0.9957

78.7727 82.2293

0.9958

85.4655

89.0415 92.0981 96.4542 101.3300 106.2690

0.9958 0.9963 0.9966 0.9970 0.9971

99.6970 102.8836 107.4482 111.8682 114.7291

Maximum estimation in Φν is no more than ± (5 × 10−4) cm3·mol−1, standard uncertainties are u(T) = 0.1 K, u(p) = 10 kPa, u(m2) = 0.0002, u(m1) = 0.0002.

a

From data of Table 12 it follows that values of ΔtrΦ0ν are negative in the presence of glycine and alanine at 298.15 K both in the premicellar and postmicellar regions and become positive with the increase of temperature. At the same time in the presence of leucine values of ΔtrΦ0ν increase with the increase of temperature, but stay negative. Obtained results indicate that at low temperatures in aqueous-micellar solutions of C14−16AOS in the presence of glycine and alanine hydrophobic−hydrophobic interactions are dominant, while at high temperatures ion−ion and hydrophilic−hydrophilic interactions become dominant. But in the presence of leucine at the whole studied temperature range hydrophobic− hydrophobic interactions are dominant. The obtained behavior of ΔtrΦ0ν in the order glycine−alanine−leucine may be a result

actions between alkyl chains of C14−16AOS and AA; (2) hydrophilic−hydrophilic interactions; (3) ion−hydrophilic interactions between charged groups of AA (NH+3 ,COO−) and hydrophilic groups (headgroups) of C14−16AOS; (4) ion− ion interactions between SO−3 groups of C14−16AOS and NH+3 groups of AA, and Na+ ions of C14−16AOS and COO− groups of AA. Interactions (2) and (3) lead to positive values of ΔtrΦ0ν due to destruction of water structure around ionic and hydrophilic groups.28,70 Ion−ion interactions also lead to positive values of ΔtrΦ0ν due to decreasing electrostriction effect28,64,70,71,. Hydrophobic−hydrophobic interactions lead to negative values of ΔtrΦ0ν due to destruction of water structure around hydrophobic groups as suggested by Frank and Evans model.28 H



Article

Table 10. Values of Standard Partial Molar Volume (Φ0v ) of System C14‑16AOS (1)−AA (2)−Water in the Premicellar and Postmicellar Regions at Different Temperatures, at Pressure p = 0.1 MPaa

of high hydrophobicity of leucine compared to the hydrophobicity of glycine and alanine.

■

CONCLUSION From studies the effect of glycine, alanine, and leucine on micellization and micellar parameters of C14−16AOS in aqueous solutions, it can be concluded that detected behaviors are a result of strong intermolecular interactions. On micellization of C14−16AOS in aqueous solutions of AA hydrophobic and electrostatic interactions have a significant effect. Data of adsorption at the air/liquid interface indicate adsorption is more favorable than micellization. An increase of aggregation number of C14−16AOS in the presence of AA in aqueous solution is a result of the interaction of zwitterionic groups of AA with ionic groups of C14−16AOS. The behavior of volumetric parameters of studied systems is also a result of above-mentioned interactions.

Φ0ν/cm3.mol−1 298.15 K

303.15 K

308.15 K

313.15 K

−1

glycine alanine leucine glycine alanine leucine glycine alanine leucine

[m1] = 0 mol·kg (in Water) 43.1264 60.2933 62.7891 [m1] = 0.2 × 10−3 mol·kg−1 (Premicellar Region) 29.1727 32.9766 43.4205 45.2498 55.3234 64.5442 66.2578 75.9740 87.8137 [m1] = 20 × 10−3 mol·kg−1 (Postmicellar Region) 41.7483 53.9041 54.6336 44.4989 57.5406 68.0417 65.1517 76.4218 87.0023

52.7156 67.8243 97.2295 63.1602 78.6299 98.3819

■

Maximum estimation in Φ0ν is no more than ± (5 × 10−4) cm3· mol−1, standard uncertainties are u(T) = 0.1 K, u(p) = 10 kPa, u(m1) = 0.0002

a

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.8b00886.

Table 11. Contribution of Zwitterionic (NH+3,COO−) and Alkyl Groups of AA in Standard Partial Molar Volume of System C14‑16AOS (1)−AA (2)−Water in the Premicellar and Postmicellar Regions at Different Temperatures, at Pressure p = 0.1 MPaa

■

Φ0ν/cm3·mol−1 298.15 K

303.15 K

308.15 K

[m1] = 0.2 × 10−3 mol·kg−1 (Premicellar Region) − + NH3 ;COO 18.6687 22.6513 31.7858 CH2− 12.0963 13.7600 14.3459 CH2CH2− 24.1926 27.5200 28.6918 (CH3)2CH2(CH)2− 48.3852 55.0400 57.3836 [m1] = 20 × 10−3 mol·kg−1 (Postmicellar Region) − + NH3 ;COO 31.4219 44.4635 44.1533 CH2− 8.1619 7.7823 10.8882 CH2CH2− 16.3238 15.5646 21.7764 (CH3)2CH2(CH)2− 32.6476 31.1292 43.5528

ORCID

Lusine R. Harutyunyan: 0000-0003-1153-8212 Notes

The authors declare no competing financial interest.

■

53.2842 11.4742 22.9484 45.8968

ΔtrΦ0ν/cm3.mol−1

glycine alanine leucine glycine alanine leucine

REFERENCES

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Table 12. Values of Standard Volumes of Transfer (ΔtrΦ0ν) of System C14‑16AOS (1)-AA (2)-Water in the Pre-micellar and Post-micellar Regions at Different Temperatures, at Pressure p = 0.1 MPaa 308.15 K

AUTHOR INFORMATION

*E-mail: [email protected]. Tel.: (+374) 91519144.

38.0130 14.8186 29.6372 59.2744

Average correlation coefficient is 0.9995, standard uncertainties are u(T) = 0.1 K, u(p) = 10 kPa, u(m1) = 0.0002.

303.15 K

Experimentally determined values of surface tension (Table S1) and conductivity (Table S2) of sodium alpha olefin sulfonate C14−C16−amino acid−water systems at different temperatures (PDF)

Corresponding Author

313.15 K

a

298.15 K

ASSOCIATED CONTENT

S Supporting Information *

313.15 K

[m1] = 0.2 × 10−3 mol·kg−1 (Premicellar Region) −13.6425 −10.1498 0.03756 9.0657 −15.1478 −5.3395 3.6556 6.8734 −41.4622 −32.4263 −21.5685 −14.3410 [m1] = 20 × 10−3 mol·kg−1 (Postmicellar Region) −1.0669 10.7777 11.2506 19.5102 −15.8987 −3.1224 7.1533 17.6789 −42.5684 −31.9783 −22.3799 −13.1887

Maximum estimation in ΔtrΦ0ν is no more than ±(5 × 10−4) cm3· mol−1, standard uncertainties are u(T) = 0.1 K, u(p) = 10 kPa, u(m1) = 0.0002.

a

I



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