Micellization and Phase Behavior of Binary Mixtures of Anionic and

Article pubs.acs.org/IECR

Micellization and Phase Behavior of Binary Mixtures of Anionic and Nonionic Surfactants in Aqueous Media Rakesh Kumar Mahajan* and Durgesh Nandni Department of Chemistry, Guru Nanak Dev University, Amritsar 143 005, India S Supporting Information *

ABSTRACT: The micellization and phase behavior of the binary mixed systems comprising the anionic surfactants sodium bis(2-ethylhexyl) sulfosuccinate (AOT)/sodium alkyl phenol ether sulfate (APES) and nonionic polyoxyethylene alkyl ether surfactants (C10E8/C10E10/C13E10) have been investigated by surface tension, fluorescence spectroscopy, and cloud point measurements. Various micellar characteristics, surface active properties, thermodynamic parameters, aggregation numbers, and Stern−Volmer constants have been evaluated for these systems. Lower cmc values, negative interaction and thermodynamic parameters, higher aggregation numbers, and improved surface active properties for the proposed mixed systems indicate the existence of strong synergistic interactions and the formation of thermodynamically stable mixed micelles. Observed elevation in the cloud point (CP) shift suggests the formation of charged micelles that increases the intermicellar repulsions and poses a hindrance to the coacervation of micelles. It subsequently delays the appearance of the CP and increases the applicability of all these systems over a wide temperature range.

1. INTRODUCTION A recent upsurge in the technical applications of surfactant mixtures and the associated increase in their demand at the industrial level has necessitated the search for new surfactant mixtures possessing high surface activity and enhanced performance properties. In view of the continuously increasing demand, surfactant mixtures comprising anionic and nonionic surfactants carry immense interest for the technologist from fundamental and technology viewpoints.1−4 In these mixtures, anionic and nonionic surfactant components complement each other in terms of the enhancement of cleaning performances, solubilization capacity, and water hardness tolerance of the surfactant systems.5 Mixed micellar aggregates of these mixtures are much more flexible with regard to their physicochemical properties and areas of applications, since their functionality can be fine-tuned by simple composition modulations. The unremitting interest in mixed anionic−nonionic surfactants also stems from their tendency to efficiently solubilize hydrophobic compounds. This property is due to their synergistic behavior and has been exploited in numerous industrial applications to optimize performance, minimize the surfactant requirement, and minimize the consequent negative impact on the environment.6,7 These mixtures are, therefore, being widely used in cleansing formulations, in synthesis of nanostructure materials and pharmaceutics, etc.8−17 In view of their enormous fundamental and commercial importance, the study of anionic− nonionic mixed surfactant systems has become a topic of pursuit for surfactant chemists.17−21 Understanding of the phase behavior and micellization of these mixed surfactant systems can be helpful in designing their employment in specific applications. The present study is, therefore, focused on the investigation of the micellization and phase behavior of binary mixtures of anionic (double alkyl chain sodium dioctyl sulfosuccinate (AOT) and single alkyl chain alkyl phenol ether sulfate (APES)) and nonionic © 2012 American Chemical Society

surfactants (polyoxyethylene alkyl ether) in aqueous media employing surface tension, fluorescence, and cloud point measurements. The selection of different surfactants for this study has been carried out on the basis of their specific physicochemical properties in association with commercial importance. AOT and APES have been chosen as both are anionic surfactants with varying degrees of hydrophobicity. The selection of nonionic surfactants polyoxyethylene alkyl ethers has been made with respect to their varying ethylene oxide (EO) content and hydrophobic chain length. In terms of their commercial importance, the anionic surfactants AOT and APES used in the present study are integral components of emulsifiers, oil dispersants, and pesticides. Nonionic polyoxyethylene alkyl ether (POE; C10E8/C10E10/C13E10) surfactants are useful surfactants which attract the interest of the scientific community due to their remarkable and unique properties, especially their biodegradable nature.22,23 Being biodegradable, POE surfactants have emerged as an alternative to the alkyl phenol ethoxylates in the surfactant industry. The versatility of these surfactants permits the technologists to apply them efficiently in a variety of industrial and commercial applications such as personal care products, paint formulations, controlled drug delivery, and the agrochemical and petrochemical industries.15,16,24−30 Beside these applications, POE surfactants are also employed for electronics, cancer research, and micellar catalysis.31 Despite their wide range of possible applications, the lower consolute boundaries or cloud points exhibited by POE surfactants limit their usage at higher temperature, as clouding is to be avoided in applications such as soil remediation and cosmetics. Inclusion of ionic surfactant into the micelles of Received: Revised: Accepted: Published: 3338

October 27, 2011 December 20, 2011 January 23, 2012 January 23, 2012 dx.doi.org/10.1021/ie202463w | Ind. Eng. Chem. Res. 2012, 51, 3338−3349

Industrial & Engineering Chemistry Research

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was monitored visually during heating, and the temperature at which the solution turned turbid was taken as the cloud point (CP). As the temperature exceeded the CP, the sample was cooled below the CP and heated again to check reproducibility. Experiments were repeated thrice, and good reproducibility was obtained. The maximum uncertainty in the CP measurements was ±0.5 °C.

nonionic surfactants delays the clouding and increases surface activity, which contributes favorably to increasing the practical applications of these surfactants.19 The micellization and phase behavior of these binary systems have therefore been examined as they can prove to be powerful tools for designing their efficient employment. In the literature, a number of studies report mixed micellization of POE surfactant of −C12 alkyl chain with different ionic surfactants; however, very few reports are available on the investigation of mixed micellization behavior of mixtures of POE surfactants with −C10 and −C13 alkyl chains and anionic surfactants.32−37 To the best of our knowledge, the present study is the first report whereby micellization and phase behavior have been investigated for the mixed systems of APES and polyoxyethylene alkyl ethers.

3. RESULTS AND DISCUSSION 3.1. Critical Micellar Concentration (cmc). The cmc values for pure and mixed anionic−nonionic surfactant systems (AOT/APES + C10E8/C10E10/C13E10) have been evaluated from surface tension and fluorescence spectroscopy techniques. Tensiometric and fluorescence profiles for the pure surfactants (AOT/C10E8) and the mixed surfactant systems (AOT + C10E8) at different mole fractions of AOT (αAOT = 0.2, 0.4, 0.6, 0.8) are presented in Figure 1. The cmc values were computed

2. EXPERIMENTAL SECTION The sodium salt of dioctyl sulfosuccinate (AOT), an anionic surfactant, and hexadecylpyridinum chloride (HPyCl), a quencher, were procured from Lancaster Synthesis. Alkyl phenol ether sulfate (APES) and different polyoxyethylene alkyl ether surfactants (C10E8, C10E10, C13E10) with varying ethoxylate content and hydrophobic chain length were received from BASF as gift samples. Pyrene was obtained from Sigma-Aldrich and used as a fluorescent probe during fluorescence measurements. All products were used without further purification. Double distilled water was used for solution preparation in all measurements. A Sartorius analytical balance was used for weighing purposes, and all solutions were prepared by mass with an accuracy of ±0.0001 g. 2.1. Surface Tension Measurements. Surface tension measurements were carried out using a Krüss K12 tensiometer under atmospheric pressure by the ring method. Surfactant concentration was varied by adding concentrated surfactant solution in small installments, and the readings were noted after thorough mixing and temperature equilibration. The measured values of the surface tension were corrected according to the procedure of Harkins and Jordan (in-built instrument software). In all cases, three successive measurements were performed and the standard deviation did not exceed ±0.1 mN/m. The critical micelle concentration (cmc) values were determined from the inflection in the plot of surface tension (γ) vs log [surfactant]. 2.2. Fluorescence Measurements. Fluorescence measurements were carried out using pyrene as a fluorescent probe on a Cary Eclipse Fluorescence Spectrophotometer from Varian Ltd. at 298.15 K. Pyrene was excited at 335 nm, and emission spectra were scanned within range 350−450 nm. A very low concentration of pyrene (1 × 10−6 mol dm−3) was maintained in the samples to avoid excimer formation as well as the perturbation of the organized system. For evaluating cmc's of pure and mixed surfactant systems, the pyrene polarity index I1/I3 was measured as a function of surfactant concentration. The aggregation number was determined for different systems by observing the quenching of pyrene fluorescence by hexadecylpyridinium chloride as a quencher. Using a linear plot of ln(Io/I) vs quencher concentration, the aggregation number was calculated and the quencher concentration was adjusted to ensure a Poisson distribution. During these experiments, pyrene and surfactant concentrations were maintained at the same level as used for evaluating the cmc values. 2.3. Cloud Point Measurements. Cloud point temperatures were determined visually by immersing closed glass tubes containing sample solution in the paraffin oil bath, whose temperature was increased gradually at the rate of 1 °C min−1 with constant stirring. The appearance of the sample solution

Figure 1. Representative plots of (a) surface tension (γ) vs log [surfactant]/mol dm−3 and (b) pyrene intensity (I1/I3) ratio vs [surfactant]/mol dm−3, for pure components (AOT and C10E8) and AOT + C10E8 mixed system at various mole fractions of AOT (αAOT = 0.2, 0.4, 0.6, 0.8).

from the breakpoint in the plot of the surface tension (γ) vs log [surfactant] (Figure 1a) in surface tension studies and the ratio of intensities (I1/I3) vs [surfactant] (Figure 1b) during fluorescence measurements. Thus the obtained cmc values for pure surfactants from both techniques, i.e., surface tension and 3339

dx.doi.org/10.1021/ie202463w | Ind. Eng. Chem. Res. 2012, 51, 3338−3349


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the mixed state. The observed depression in cmc value for the proposed mixed surfactant systems can be attributed to the electrostatic stabilization of the mixed micelles due to intercalation of nonionic surfactant between the anionic surfactants. The intercalation of nonionic surfactant (POE) shields the repulsive interactions between the headgroups of anionic surfactants (AOT/APES) which consequently enhance the hydrophobicity of the mixed micellar system that causes micelle formation at low concentration. However, with an increase in the mole fraction of anionic surfactant, the surface charge density of the mixed micelles increases, which increases the cmc values of mixed surfactant systems. 3.2. Mixed Micellar Composition. The micellar mole fraction of a surfactant in mixed micelles has been evaluated in the light of regular solution theory,44 eq 2.

fluorescence, agree well with each other and the corresponding literature values38−41 (see Table S1 in the Supporting Information). Evaluated cmc values for the mixed surfactant systems obtained from both techniques lie close to each other; therefore, averages of these cmc values have been used for the further analysis42 (Table S1 in the Supporting Information). From the analysis, it has been observed that the experimental cmc values for the proposed mixed surfactant systems vary nonlinearly with change in mole fraction of anionic surfactant in the bulk solution (Figure 2). Ideality in the mixed micelles can be evaluated from the Clint equation.43 α 1 − α1 1 = 1 + cmc* cmc1 cmc2

(1)

X12 ln(cmc α1/cmc1X1) =1 (1 − X1)2 ln(cmc(1 − α1)/cmc2(1 − X1))

(2)

where X1 is the mole fraction of component 1 in the mixed micelles at the cmc, α1 is the mole fraction of component 1 (AOT/APES) in the bulk, and all other terms carry their usual meaning. The appearance of the azeotropic point in the phase diagram at a particular mixing ratio indicates maximum interactions among the components in the mixed state. Along with this, the azeotropic point also displays the effect of hydrophobicity on the strength of interactions in the mixed state45,46 Plots in Figure 3 show azeotropic points in AOT + C10E8/ C10E10/C13E10 mixed systems. These plots show a clear shift in the azeotropic point that is in accordance with the hydrophobicity of C10E8 and C10E10 components in the mixed state. Owing to its higher ethoxylate content, C10E10 is less hydrophobic than C10E8, and thereby on moving from AOT + C10E10 to the AOT + C10E8 system, the azeotropic point is shifted to a lower mole fraction of AOT. Similarly, by varying the carbon chain length from C10 to C13 at fixed EO content of n = 10, the azeotropic point shows a shift to a lower mole fraction of AOT. The effect of hydrophobicity has also been observed in APES + C10E8/C10E10/C13E10 systems (Figure 4). However, the shift in azeotropic point is less in APES + C10E10/C10E8 systems, whereas a clear shift in the azeotropic point to a lower mole fraction of APES is observed in the APES + C13E10 system (Figure 4). These observations clearly demonstrate the effect of hydrophobicity on the appearance of the azeotropic point in the mixed micelles. Figure 5 displays the variation in mixed micellar mole fraction X1 with the mole fraction of anionic surfactant αAOT/APES. These plots illustrate the contribution of individual components to the mixed micelle formation. In the case of AOT + C10E8/C10E10 systems, X1 is higher than Xideal up to mole fractions 0.60 and 0.75, respectively, while, in APES + C10E8/C10E10 systems, X1 lies lower than corresponding Xideal values over the complete mixing range. For AOT/APES + C13E10 systems, X1 remains higher over the complete mole fraction range studied. Higher X1 indicates the predominance of anionic surfactant in the mixed micelles, and lower X1 value indicates increased contribution of polyoxyethylene alkyl ether surfactants in the mixed micelles. Variation in the mixed micellar composition of AOT/APES + C10E8/C10E10 systems emanates from the difference in hydrophobic chains possessed by anionic surfactants AOT and APES. 3.3. Interaction Parameter (β). The interaction parameter β evaluates the strength and nature of interactions among

Figure 2. Plots of cmc (experimental) and cmc* (predicted from Clint equation) vs αAOT for (a) AOT + C10E8/C10E10/C13E10 and (b) APES + C10E8/C10E10/C13E10 systems.

where cmc* is the ideal state mixed cmc, α1 is the mole fraction of the first component, and cmc1 and cmc2 are cmc values for the first and second components, respectively. Experimental and ideal cmc* values were plotted against the mole fraction of the anionic surfactant AOT/APES (Figure 2). Plots reveal that cmc values for the proposed mixed systems (AOT/APES + C10E8/C10E10/C13E10) lie lower than the cmc values of the individual components as well as their ideal cmc* values in the mixed state. Lower experimental cmc values relative to the ideal cmc values indicate favorable association of the composites in 3340



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Figure 3. Plots of cmc vs αAOT, cmc vs X1, and cmc* vs αAOT for various mixed systems: (a) AOT + C10E8, (b) AOT + C10E10, and (c) AOT + C13E10.

the range −5.667 to −7.798 (Table S1 in the Supporting Information). Javadian et al.47 has reported β value variation from −0.240 to −2.170 for the mixture of the nonionic surfactant p-(1,1,3,3-tetramethylbutyl)polyoxyethylene with the cationic surfactant cetyltrimethylammonium bromide in aqueous medium. Mehta et al.48 studied the binary and ternary mixtures of hexadecyltrimethylammonium bromide, hexadecyltrimethylammonium chloride, and polyoxyethylene (20) mono-nhexadecyl ether (Brij 58) and have reported a β value of −3.0. C. Hu et al.49 have reported that the β value varies over the range from −0.167 to −1.102 for the binary surfactant systems of nonionic surfactants poly(ethylene oxide) lauryl ethers (C12E10, C12E23, C12E42) with the cationic gemini surfactant butanediyl-α, ω-bis(tetradecyldimethylammonium) (14-4-14). Thus this comparison reveals that the proposed APES + C13E10 system exhibits relatively better synergism and can be a promising candidate for the specific appliance particularly as smart materials for surfactant based industrial applications. The interaction parameter β not only provides an idea about the degree of association between two surfactants but also explains nonideal behavior in the mixed micelles and is related to the activity coefficients of the surfactants within the micelles by eqs 4 and 5.

different composites in the mixed micelles and is related to the experimental cmc by eq 3.44 β=

ln(cmc α1/cmc1 X1) (1 − X1)2

(3)

For the proposed mixed systems, the β value is negative and is found to be higher than the ln C1/C2 value (except at low mole fractions of AOT + C10E8), where C1 and C2 are the critical micellar concentrations of pure surfactants AOT/APES and POE, respectively (see Table S1 in the Supporting Information). A negative β value indicates the existence of synergistic interactions between anionic and nonionic polyoxyethylene alkyl ether surfactants in the mixed state. A negative β value is liable for the reduced electrostatic headgroup repulsions due to intercalation of POE surfactants in the mixed micelles and a consequent increase in the hydrophobicity of anionic surfactant as stated previously. In all the binary mixed systems, although the β value is negative over the complete concentration range, a decrease in the β value is observed with an increase in the mole fraction of anionic surfactant that supports the observed variation in experimental cmc values. The APES + C13E10 mixed system exhibits a maximum negative β value that indicates maximum synergism. It is attributable to the strong hydrophobic environment due to the presence of π−π interactions in the APES surfactant and the long hydrophobic chain in C13E10. Analysis of β values for different systems studied in the present study reveals that the β value for APES + C13E10 lies in 3341

f1 = exp{β(1 − X1)2 }

(4)

f2 = exp{β(X1)2 }

(5)



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Figure 4. Plots of cmc vs αAPES, cmc vs X1, and cmc* vs αAPES for various mixed systems: (a) APES + C10E8, (b) APES + C10E10, and (c) APES + C13E10.

Table 1. Interfacial Properties (Γmax, Amin, Aideal, πcmc, f1, f 2) and Thermodynamic Properties (ΔGm, ΔGads, ΔGmin, ΔGex) for Pure Surfactant (AOT) and Mixed Surfactant Systems: AOT + C10E8/C10E10/C13E10 at Various Mole Fractions of AOT (αAOT) system

αAOTa

Γmax × 10−6 (mol m−2)

Amin/Aideal (Å2)

AOT + C10E8

0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0

2.72 1.26 1.29 1.58 2.05 1.59 2.22 1.14 1.22 1.59 2.29 1.59 1.82 0.86 0.93 1.20 0.96 1.59

61.0 137.0/73.0 129.0/85.1 105.0/97.1 81.0/109.2 121.2 74.8 145.5/84.0 136.1/93.3 104.4/103.1 72.5/112.5 121.2 91.0 193.9/97.0 178.0/103.1 138.0/109.1 174.0/115.2 121.2

AOT + C10E10

AOT + C13E10

a

πcmc −ΔGm (mN m−1) (kJ mol−1) 40.6 45.5 44.8 43.9 44.6 40.1 42.6 43.8 43.3 43.7 43.3 40.1 44.7 43.8 43.5 42.9 42.7 40.1

26.84 27.28 26.91 26.57 26.17 25.04 26.54 28.96 28.33 28.20 26.56 25.04 33.11 33.54 33.9 32.89 30.29 25.04

−ΔGads (kJ mol−1)

ΔGm − ΔGads (kJ mol−1)

40.92 63.29 61.64 54.69 47.93 54.52 45.73 67.35 63.82 55.68 45.47 54.52 57.62 84.71 80.62 68.67 74.96 54.52

14.08 36.01 34.73 28.12 21.76 29.49 19.19 38.39 35.49 27.48 18.91 29.49 24.51 51.17 46.72 35.78 44.87 29.49

ΔGmin −ΔGex (kJ mol−1) (kJ mol−1) 12.79 22.77 21.99 18.47 13.90 22.78 13.74 25.68 24.42 18.48 13.03 24.25 15.45 34.22 31.73 25.10 31.86 24.25

− 0.85 0.67 0.67 0.78 − − 3.09 1.06 2.53 1.43 − − 5.58 4.77 4.04 1.01 −

f1

f2

p

− 0.36 0.56 0.72 0.84 −

− 0.89 0.88 0.80 0.62 −

0.12 0.24 0.34 0.68 − − 0.82 0.57 0.59 0.89 −

0.48 0.48 0.38 0.42 − − 0.06 0.08 0.12 0.54 −

0.81 0.31 0.32 0.40 0.52 0.32 0.56 0.29 0.31 0.40 0.58 0.32 0.47 0.22 0.24 0.31 0.24 0.32

αAOT = 0 means pure nonionic surfactant.

AOT (f1) is less than activity coefficient values for polyoxyethylene alkyl ether surfactants ( f 2) and confirms the participation of nonionic surfactant in the mixed micelle formation. However, a reverse trend is observed for APES +

Activity coefficient values less than unity for all the proposed mixed systems show deviations from ideal behavior (Tables 1 and 2). Analysis of activity coefficient values in AOT + C10E8/ C10E10 systems reveals that the activity coefficient value for 3342



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C10E8/C10E10 systems, where the activity coefficient value of anionic surfactant APES is more than those of polyoxyethylene surfactants, attributable to the strong hydrophobic π−π interactions in APES surfactant. With increase in the mole

fraction of anionic surfactant, activity coefficients of AOT/ APES increase (except in APES + C10E8), while the activity of the accompanying component remains more or less constant over the complete mole fraction range. Keen observation of the activity data also reveals that the activity of C10E8 is greater in comparison to C10E10 in AOT/APES + C10E8/C10E10 mixed systems. This observation is in accordance with the hydrophobicity of C10E8 and C10E10 surfactants, as lower EO content in C10E8 props up its participation in the mixed micelle formation. The activity coefficient of long chain POE surfactant C13E10 in the AOT + C13E10 system is less than that of the anionic surfactant AOT, while in the APES + C13E10 system, it is greater than that of the anionic surfactant APES. Lower activity of C13E10 in combination with AOT can be due to the steric hindrance in the packing of twin tailed AOT and long hydrophobic chain C13E10 in the mixed micelle. 3.4. Interfacial and Thermodynamic Properties. The surface excess concentration, Γmax, at cmc and minimum surface area per molecule, Amin, at the air/water interface for the pure and mixed surfactant systems have been simulated using Gibbs adsorption equations, eqs 6 and 7. dγ = −nRT Γmax d ln c

A min =

(6)

1 ΓmaxNA

(7)

where n is the number of species at the interface, R is the gas constant (8.314 J/mol·K), T is the absolute temperature (K), γ is the surface tension (mN/m), c is the concentration in terms of mole fraction, and NA is Avogadro’s constant. For evaluation of surface excess, the value of n was taken as 2 for pure anionic surfactant whereas an n value of 1 was used for the nonionic surfactant. For mixtures of anionic−nonionic surfactants, the value of n was taken as (2 − α2), where α2 is the mole fraction of the second component, i.e., polyoxyethylene alkyl ether

Figure 5. Plots of X1 and Xideal vs αAOT for various mixed systems: (a) AOT + C10E8/C10E10/C13E10 and (b) APES + C10E8/C10E10/C13E10.

Table 2. Interfacial Properties (Γmax, Amin, Aideal, πcmc, f1, f 2) and Thermodynamic Properties (ΔGm, ΔGads, ΔGmin, ΔGex) for Pure Surfactant (APES) and Mixed Surfactant Systems: APES + C10E8/C10E10/C13E10 at Various Mole Fractions of APES (αAPES) system

αAPESa

Γmax × 10−6 (mol m−2)

Amin/Aideal (Å2)

APES + C10E8

0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0

2.72 1.21 1.34 1.81 1.76 1.01 2.22 1.03 1.16 1.13 1.49 1.01 1.82 1.28 0.99 1.45 1.44 1.01

61.0 137.0/74.6 123.9/97.1 91.7/119.5 94.3/141.9 82.2 74.8 161.2/92.6 143.1/110.5 146.9/128.5 111.4/146.4 82.2 91.0 130.2/105.7 168.2/120.3 114.5/135.0 115.3/149.6 82.2

APES + C10E10

APES + C13E10

a

πcmc −ΔGm (mN m−1) (kJ mol−1) 40.6 41.8 41.2 41.6 41.3 41.9 42.6 42.1 42.3 42.4 42.1 41.9 44.7 41.6 40.5 37.7 40.9 41.9

26.84 27.85 28.99 29.78 30.49 33.11 26.54 28.99 29.77 29.56 29.99 31.07 33.11 35.95 36.76 32.72 34.36 33.11

−ΔGads (kJ mol−1)

ΔGm − ΔGad (kJ mol−1)

40.92 60.38 54.56 40.37 41.53 72.36 45.73 69.86 66.24 67.08 58.25 72.36 57.62 57.41 74.06 50.41 50.76 72.36

14.08 32.38 25.57 10.59 11.04 39.25 19.19 40.87 36.47 37.52 28.26 39.25 24.51 21.51 37.30 17.69 16.40 39.25

ΔGmin −ΔGex (kJ mol−1) (kJ mol−1) 12.79 25.83 23.81 17.39 18.06 15.45 13.74 30.09 26.55 27.16 20.79 31.07 15.45 24.78 33.03 24.41 22.36 15.45

− 0.345 0.554 0.713 1.478 − − 1.359 1.446 0.636 0.744 − − 4.244 5.058 4.812 3.436 −

f1

f2

p

− 0.93 0.89 0.91 0.83 − − 0.58 0.69 0.92 0.94 − − 0.05 0.06 0.10 0.26 −

− 0.82 0.66 0.42 0.15 − − 0.58 0.40 0.46 0.25 − − 0.38 0.23 0.19 0.24 −

0.81 0.31 0.34 0.46 0.45 0.26 0.56 0.26 0.29 0.29 0.38 0.26 0.47 0.33 0.25 0.37 0.37 0.26

αAPES = 0 means pure nonionic surfactant. 3343



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Table 3. Aggregation Number (Nagg) and Stern−Volmer Constant (Ksv) for Mixed Systems Evaluated at Various Mole Fractions of AOT/APES (αAOT/APES) by Fluorescence Quenching Technique AOT + C10E8

Figure 6. Plot of ln Io/I1 vs [Q] of pure AOT and AOT + C13E10 mixed system at various mole fractions of AOT (αAOT = 0.2, 0.4, 0.6, 0.8).

surfactants. The ideal mixing value of Aideal has been calculated using eq 8.

0.0 0.2 0.4 0.6 0.8 1.0

64 78 53 48 54 35

Nideal

Ksv × 104

− 2.8 58 2.5 52 1.2 47 8.4 41 6.6 − 0.4 APES + C10E8

Nagg 60 95 78 77 66 35

Nideal

Ksv × 104

− 3.7 55 3.8 50 2.2 45 2.0 40 0.9 − 0.4 APES + C10E10

Nagg

Nideal

Ksv × 104

68 48 48 32 54 35 APES

− 1.6 61 1.0 55 0.9 48 0.4 42 0.7 − 0.4 + C13E10

αAPES

Nagg

Nideal

Ksv × 104

Nagg

Nideal

Ksv × 104

Nagg

Nideal

Ksv × 104

0.0 0.2 0.4 0.6 0.8 1.0

64 82 91 108 130 50

− 61 58 56 53 −

2.8 7.8 13.1 17.4 20.5 6.2

60 72 75 58 87 50

− 58 56 54 42 −

3.7 5.1 6.6 6.8 18.8 6.2

68 63 54 67 70 50

− 64 61 57 54 −

1.6 3.1 4.0 6.6 8.9 6.2

Vo = [27.4 + 26.9(nc − 1)]2 Å3

(10)

lc = [1.54 + 1.26(nc − 1)] Å

(11)

(8)

where Z1 is the mole fraction of component 1 in the mixed monolayer; A1 and A2 designate the minimum area per molecule of anionic and polyoxyethylene alkyl ether surfactants, respectively. The surface excess concentration (Γmax) in the mixed systems of AOT + C10E8/C10E10 is higher than the corresponding value for the pure anionic surfactant AOT at higher mole fraction of AOT (Table 1). However, the AOT + C13E10 mixed system exhibits a lower value of surface excess over the complete mixing range (Table 1). A lower value of surface excess concentration exhibited by AOT + C10E8/ C10E10/C13E10 systems is attributable to the repulsive and steric forces. In the mixed systems of APES and POE surfactants (C10E8/C10E10/C13E10), surface excess values are higher than the corresponding values for pure anionic surfactant (APES) over the complete mole fraction range (Table 2). The surface excess and surface area per molecule are complementary to each other; therefore, a higher value of Amin is observed compared to Aideal at lower mole fractions for these systems. Zhou and Rosen51 and Dar et al.52 have also observed similar results and reported an expansion in Amin by the cationic surfactants in polyoxyethylene nonionic surfactant systems despite the synergistic interactions between them. However, the present study reveals that, in AOT/APES + C10E8/C10E10/ C13E10 systems, Amin values are lower than Aideal at higher mole fractions of anionic surfactants AOT/APES, while at lower mole fractions Amin values are higher than Aideal except in the AOT + C13E10 system. A lower Amin value at a higher mole fraction range indicates close packing of the two components at the interface. In the light of Israelachvili’s model,53 evaluated Amin values have been employed for the prediction of packing (p) of composites in mixed micelles as given in eq 9. p = Vo/lc A min

Nagg

AOT + C13E10

where Vo is the volume of exclusion per monomer in the aggregate and is given by Tanford’s equations.54

50

A ideal = Z1A1 + (1 − Z1)A2

αAOT

AOT + C10E10

where lc is the maximum chain length and nc is the number of carbon atoms in the hydrocarbon chain. As estimation of the exact area at the micellar surface is difficult compared to the evaluation of Amin; therefore, Amin is preferred for the calculation of the packing parameter. With regard to the packing parameter of pure surfactants, it appears that anionic surfactants AOT/APES are spherical, while polyoxyethylene surfactants (C10E8/C10E10/C13E10) are nonspherical in geometry. Regarding their mixing, mixed micelles of AOT/APES + C10E8/C10E10 systems exhibit lower values of the packing parameter at lower mole fractions, while higher valus are obtained at higher mole fractions of anionic surfactants (Tables 1 and 2). A lower value of the packing fraction ( C13E10. Figures 7 and 8 show shifts in the cloud point temperature (ΔCP = CPmix − CPpure) with varying contents of anionic and nonionic surfactants. Plots in these figures reveal an elevation in the cloud point shift of the polyoxyethylene surfactants in the presence of anionic surfactants (AOT/APES). This elevation in CP shift is attributable to the formation of surface charged micelles. The surface charge on these micelles increases with an increase in the concentration of anionic surfactants and increases the intermicellar repulsions that pose a hindrance to the coacervation of micelles. This factor subsequently delays the appearance of the CP. However, increase in the content of nonionic surfactant from 0.5 to 2 wt % causes a reduction in the CP shift as observed in Figures 7 and 8. Increased incorporation of polyoxyethylene surfactant in the micelle reduces the charge density on mixed micelle and promotes the aggregate formation and coacervation, thereby reducing the ΔCP.

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Corresponding Author

*E-mail: [email protected].

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ACKNOWLEDGMENTS D.N. is thankful to the Council of Scientific Indian Research (CSIR), New Delhi, India, for the award of a senior research fellowship.

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REFERENCES

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4. CONCLUSIONS The present study reports the investigation on the micellization and phase behavior of mixed binary systems of anionic surfactants (AOT, APES) with nonionic polyoxyethylene alkyl ether surfactants (POEs; C10E8/C10E10/C13E10) employing surface tension, fluorescence spectroscopy, and cloud point techniques. Results from surface tension and fluorescence measurements indicate synergistic interactions between the constituents in terms of a negative β parameter over the complete mixing range. Incorporation of POE surfactants (C10E8/C10E10/C13E10) into the micellar solution of anionic surfactants (AOT/APES) facilitates micelle formation by reducing the effective electrostatic and steric interactions with consequent enhancement of the surface active properties and hydrophobicity of the mixed state. Mixing also brings a variation in the packing of the surfactants and affects the micellar shape. The surfactant composition in terms of ethylene oxide and hydrophobic chain length has been found to influence the properties of these mixed systems. Among all the mixed systems investigated, the APES + C13E10 system exhibits maximum synergism with the highest negative β value on account of stronger π−π interactions (in APES surfactant) and long hydrophobic chain (in C13E10). Including positive synergism observed for these systems, cloud point measurements have illustrated that, in the mixed state, clouding is delayed due to the formation of charged micelles. Elevation in the CP will augment the applications of these systems at higher temperatures. Such mixed systems with positive synergism and enhanced surface active properties can act as smart materials for surfactant based industrial applications. As polyethoxylated surfactants are biodegradable, therefore, such mixed systems are expected to be ecologically safer than the pure anionic surfactants.

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AUTHOR INFORMATION

ASSOCIATED CONTENT

* Supporting Information S

Values of critical micellar concentrations (cmc’s) evaluated from surface tension and fluorescence measurements and the interaction parameter (β) values, and ln C1/C2 values for pure surfactants (APES/AOT) and mixed surfactant systems AOT/ 3347



Article

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Micellization and Phase Behavior of Binary Mixtures of Anionic and

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