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Mechanism on the interaction between amimo sulfonate amphoteric surfactant and sodium dodecyl benzene sulfonate in aqueous solution
Colloids and Surfaces A: Physicochem. Eng. Aspects 428 (2013) 18–24
Contents lists available at SciVerse ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Mechanism on the interaction between amimo sulfonate amphoteric surfactant and sodium dodecyl benzene sulfonate in aqueous solution Zhaohua Ren ∗ , Yue Luo, Dongpo Shi College of Chemistry and Environmental Engineering, Yangtze University, Jingzhou 434023, China
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• Micellar behavior for binary mixture • • • •
of dodecyl diamino sulfonate (C12AS) and SDBS. Rubingh model describes interaction parameters between C12AS and SDBS. Explanation of mechanism on synergistic effect between C12AS and SDBS. Effect of physicochemical property of C12AS on the formation of mixed micelle. Thermodynamic properties evaluate the tendency to the formation of mixed micelle.
a r t i c l e
i n f o
Article history: Received 2 January 2013 Received in revised form 27 February 2013 Accepted 22 March 2013 Available online 29 March 2013 Keywords: Amino sulfonate Surfactant Sodium dodecyl benzene sulfonate Mixed micelle Interaction parameter Activity coefficient Thermodynamic parameters Steric effect
a b s t r a c t The micellar properties of binary mixtures of sodium dodecyl diamino sulfonate (C12AS) and sodium dodecyl benzene sulfonate (SDBS) in aqueous solution at the pH of 6.5 have been characterized employing both conductometric techniques and isothermal titration microcalorimetry (ITC). The critical micelle concentration (cmc) values for the individual surfactants and their corresponding mixtures were determined from both the conductivity measurements and ITC. The interaction parameters between the surfactants, the activity coefficients and the molar fractions of components in the mixed micelles, and the thermodynamic parameters calculated by various approaches, like Rubingh models, were evaluated. The results show that the synergistic effect between C12AS and SDBS in all systems plays an vital role in the reduction of the overall cmc value in aqueous solution at 25 ◦ C, and that the steric effect of the head group, the lower charge density, and the capability of accepting proton on the molecule for C12AS affect the formation of mixed micelle. Thermodynamic data show that the micellization for binary mixture of C12AS and SDBS can occur spontaneously, and is both enthalpy and entropy driven process. © 2013 Elsevier B.V. All rights reserved.
1. Introduction In practical application, mixtures of surfactants, rather than individual surfatants, are used [1,2]. Mixtures of surfactants exhibit superious properties, like excellent surface/interfacial activities,
∗ Corresponding author. Tel.: +86 716 8060442; fax: +86 716 8060650. E-mail address: [email protected] (Z. Ren). 0927-7757/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2013.03.036
and are less expensive than single mixed surfactant systems. The more effective surface activities for binary mixtures of surfactants were reported in some literatures [1,3–6]. A mixed surfactants system exhibits greater surface activity, i.e., lower critical micelle concentration (cmc) values, than that obtained with any of the individual components of the mixture at the same concentration. Such effect of the mixture is said to be synergistic. The synergistic property is often exhibited due to the electrostatic attaction between the ionic headgroups of surfactants and the result of their nonideal
Z. Ren et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 428 (2013) 18–24
mixing, whereas the antagonism properties exhibit the repulsion between their molecules and the cmc value is higher than the expected. The regular solution theory (RST) is the simplest and most used approach for the mixed surfactants systems. Although the RST model was established for the nonionic surfactants, it is sometimes applicable for understanding the behavior of a binary mixture with ionic surfactants. Based on the RST, Rubingh [7] developed a model that is applicable to focus on the interaction between any two mixed surfactants system. An important parameter, the interaction parameter (ˇ12 ), was introduced in the Rubingh model and help us to predict the interaction or repulsion between the two surfactants. The negative value of ˇ12 indicates that there is some synergism between the two surfactants, whereas the positive value denotes the antagonism between the two surfactants. ˇ12 value near to zero reveals that there is little interaction between the two surfactants. Sodium dodecyl benzene sulfonate (SDBS) is an anionic surfactant having excellent surface/interfacial activities, and is usually used as a component in a mixed surfactants system. The synergistic properties of the mixtures of SDBS and some surfactants (e.g., sodium dodecyl sulfonates, cationic surfactant tetradecyl trimethyl ammonium bromide, nonionic surfactants C10-ˇ-maltoside, dodecyl betaine, and so on) were relatively entirely described in the Rosen’s book published in 2004 [1]. It is found that these surfactants, reported as above, are obviously characteritic of ionic or nonionic properties. As be compared with these surfactants, sodium n-dodecyl diamine sulfonate (C12AS) developed by our group [8] is among amino acid amphoteric surfactants and may be not obviously a ionic/nonionic property due to the hydrophilic headgroup of C12AS containing both a sulfonic group with stronger anionic property and two amino groups with weak cationic property. At present the interaction between C12AS and SDBS in aqeous solution is not found to be reported in literature. In this paper, we attempt to investigate the interacting behavior between the molecules of C12AS and SDBS in aqeous solution. The final aim of this investigation is to design a suitable component of the mixture for optimal behavior for a specific application, e.g., oil-displacing agent [9]. To obtaining the information of the interaction between surfactants and the components in the mixed micelle, we are adopting different theorys and models, e.g., RST, Rubingh’s model, and the mechanism on the interaction between C12AS and SDBS is discussed.
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containing a series of different surfactants of molar concentrations, c, were measured with DDS-307 digital conductivity meter. The cmcs of surfactants in aqeous solutions were determined by both the curve plots of the molar conductivity (/c) versus c and the thermogram obtained from the isothermal titration microcalorimetry (ITC) experiments as follow. The ITC experiments were carried out at 25 ◦ C using Nano ITC isothermal titration microcalorimeter made by American TA Instrument Co. The concentrated surfactant solution was added to DTD water in the sample cell under constant stirring 200 revolutions per minute. The concentrations of surfactant solutions using in the ITC experiments were about 20 times larger than the values of cmc obtained from the above conductometry experiments. Every titration in the ITC experiment was determined three times and the average value of the thermogram was obtained with the relative errors of less than 2%. 3. Theoretical background 3.1. Cmc of mixed surfactants Based on pseudophase separation model, nonideal mixed micellization processes can be treated by considering the chemical potentials of the individual surfactant components. For pure surfactant component i in solution: i = 0i + RT ln ciS 0i ,
(1) ciS
where i , and correspond to the chemical potential, the standard chemical potential, and the monomer concentration of surfactant i in the solution, respectively. Where the activity of surfactant i is 1. When the micelle of pure surfactant i in solution is formed, the ) and the cmc relationship of the standard chemical potential (0M i of the component i (Ci ) is as follow: 0M = 0i + RT ln Ci i
(2) (M ) i
For a mixed micelle, the chemical potential nent i in the mixed micelle can be expressed as: M = 0M + RT ln fi Xi i i
of the compo(3)
where Xi and fi are the mole fraction and the activity coefficient of surfactant i in the mixed micelle. Since at equilibrium Si = M , the i monomer concentration (ciS ) of surfactant i can be written as:
2. Experimental
ciS = fi Xi Ci
2.1. Materials
According to mass conservation, and by rearranging this Eq. (4), the mixed cmc (CM ) can be represented as [7]:
(4)
x 1 i = fi Ci CM n
C12AS developed by our group [8] has a purity of over 96%, measured with Vario EL III Automatic Elementary Analyser made by Germany Elementar Co. The chemical structure of C12AS is followed as: n-C12 H25 -NH-CH2 CH2 -NH-CH2 CH(OH)CH2 -SO3 Na. SDBS is an analytical reagent from Sinopharm Chemical Reagent Co., Ltd. Deionized triple distilled (DTD) water is used in this investigation. The conductivity of DTD water was close to 13 S/cm, measured with DDS-307 digital conductivity meter made in China.
where CM and xi are the mixed cmc and the mole fraction of surfactant i in the mixed surfactants solution, respectively. Eq. (5a) represents the nonideal behavior of mixed surfactants system. For ideal system, fi = 1, this Eq. (5a) reduces to Clint equation [10], namely: 1
2.2. Methods Solutions of C12AS, SDBS, and their mixtures with different molar ratios were confected by DTD water, respectively. The pH values of mixed surfactants solutions were found to be close to 6.5 within the range of isoelectric point for C12AS. The solutions in the conical flasks were allowed to stand in water bath of 25 ± 0.2 ◦ C over half hour. And then the conductivities, , of solutions
(5a)
i=1
C ideal
=
n x
i
i=1
Ci
(5b)
where C ideal is the mixed cmc at an ideal state. For the binary mixture of surfactant 1 and surfactant 2, Eq. (5b) is written as: 1 ideal C12
=
x1 1 − x1 + C1 C2
(6)
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Z. Ren et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 428 (2013) 18–24
Clint equation or model was established based on pseudophase separation model and is an idealization which neglects the interaction of different surfactants in the mixed state and considers the cmcs of the individual component influencing their relative tendency to the micellization. According to pseudophase thermodynamic model, for a binary surfactants system, the mole fraction (X1ideal ) of surfactant 1 in the mixed micelle at an ideal mixed state can be expressed as: x1 C2 X1ideal = (7) x1 C2 + (1 − x1 )C1 3.2. Molecular interaction in mixed surfactants systems The Clint model is useful for understanding the ideal behavior of binary surfactant systems. For the nonideal mixed system, new model based on RST is developed by Rubingh [7]. Rubingh model has been mostly used due to its simplicity, even after development of more complex models. The expression of Rubingh equation is introduced with the activity coefficient (fi ) as the above Eqs. (3)–(5). For binary nonideal mixtures, Rubingh and co-worker [7] suggested that the activity coefficients of the components could be written as: f1 = exp ˇ12 (1 − X1 )2
(8a)
f2 = exp ˇ12 X1 2
(8b)
where ˇ12 is an interaction parameter that measures the nature and extent of the interaction between the two different surfactant molecules in the mixed micelle in aqueous solution, and is a measure of deviation from the ideal behavior. The micellar mole fraction, X1 , can be calculated by solving iteratively the following equation [1,7]: M /(X C )] X1 2 ln[x1 C12 1 1 2
M /[(1 − X )C ]} (1 − X1 ) ln{(1 − x1 )C12 1 2
=1
(9)
Consequently, ˇ12 can be calculated by the equation below: ˇ12 =
M /(X C )] ln[x1 C12 1 1
(10)
(1 − X1 )2
3.3. Thermodynamic parameters of mixed micellization
Fig. 1. Plot of molar conductivity versus molar concentration at 25 ◦ C. (a) Individually C12AS or SDBS in aqueous solution and (b) the binary mixtures of C12AS and SDBS at different xC12AS .
Based on RST, the excess entropy of mixing (SE ) is zero, namely, SE = Sideal = 0 (Sideal , the change of entropy at an ideal mixed state), and the excess enthalpy of ideal mixing (Hideal ) is equal to zero. Thereafter, the relation between excess free energy (GE ), excess enthalpy (HE ), and enthalpy of micellization is written as [11]:
4. Results and discussion
GE = H E = H M = RT
n
Xi ln fi
(11)
i=1
The free energy change (GM ) in binary surfactants mixture at the nonideal state can be expressed as [11,12]: GM = RT
n
Xi ln fi Xi
(12)
i=1 M For the ideal mixture, Gideal is as below: n
M Gideal = RT ˙ Xi ln Xi
(13)
i=1
Thereafter, the entropy change (SM ) in the micellization of mixed surfactants system is: S M =
H M − GM T
(14)
4.1. Cmcs of C12AS, SDBS, and their mixtures The cmc values of C12AS, SDBS, and their mixtures in aqeous solutions were determined by the conductivity method and are graphically presented in Fig. 1. Clear break points (at cmc) were observed in the plots of molar conductivity versus molar concentration c, indicating the initiation of the micellization phenomenon. When micellar surfactant solution is titration into water in the cell, ITC records the differential enthalpy changes related with the demicellization and the dillution of surfactant molecules. The value of cmc and H M can be directly obtained from one ITC experiment [13]. An illustration of the heat flow, enthalpy change of micellization (H M ), and the plot of H M /c versus c for the binary mixture of C12AS and SDBS at 25 ◦ C is presented in Fig. 2. The dependence of the enthalpy change on surfactant concentration can be used to determine the cmc, that is, from the inflection point in the H M versus surfactant concentration curve [14]. The cmc point is indicated in Fig. 2b and c. The experimental cmcs (CM ) and the ideal cmcs (Cideal ), calculated by Eq. (6), of the mixed systems for C12AS and SDBS are listed in Table 1. As shown in Table 1, the cmc
Z. Ren et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 428 (2013) 18–24
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Table 1 Micellization parameter values for the binary mixture of C12AS and SDBS in aqueous solution at 25 ◦ C. xC12AS
0 0.20 0.40 0.60 0.80 1
CM (mmol/L) Conductivity method
ITC method
1.1 0.34 0.26 0.22 0.24 0.28
1.05 0.32 0.25 0.22 0.24 0.27
C ideal (mmol/L)
X1
X1ideal
ˇ12
f1
f2
1.1 0.694 0.507 0.399 0.329 0.28
0 0.498 0.597 0.668 0.780 1
0 0.485 0.724 0.855 0.940 1
−2.582 −2.924 −3.178 −2.657
0.488 0.622 0.705 0.879
0.493 0.353 0.242 0.199
values measured by the conductometric techniques nearly consistent with obtained from the ITC experiments, implying the validity of the two methods in this case. Generally, cationic or anionic surfactants have higher cmc than nonionic or zwitterionic surfactants, which mainly results in electrostatic repulsion between their head groups or their charge density. The charge density on the molecular chain of C12AS (q2 = 0.21 [15]) is far smaller than SDBS (q2 close to 1), which may lead to a lower cmc value of C12AS. For the mixed system of C12AS and SDBS, it is found from Fig. 3 that the experimental cmc values are lower than the ideal behavior, which confirms a constitutional interaction between the two surfactants or their nonideal behavior of mixing. In Fig. 3, the experimental CM values initially decrease with molar fraction of C12AS (xC12AS < 0.6 mol/L), which can be easily explained by the reduction of electrostatic repulsion between molecules in the mixed micelle of C12AS and SDBS due to the lower charge density of C12AS. Obviously, the molecules of C12AS intercalating into the micelle of SDBS can effectively screen the electrostatic repulsion between the ionic head groups of SDBS, which to a certain degree plays role on the decrease of CM for the mixed system. Some investigations, e.g., of Faustino et al. [16] and Vautier-Giongo et al. [17], also confirm that more molecules with lower charge density become available for intercalation in the mixed micelles, reducing electrostatic repulsion between the ionic head groups of the surfactants more effectively, thus promoting micellization. Otherwise, only depending on the intercalation of C12AS into the micelle of SDBS to screen the electrostatic repulsion between two molecules, it is not exactly possible to reduce the CM to lower than the cmc value of pure SDBS because C12AS has still a little charge and is electrically different with nonionics. Rosen [18] suggested that when the zwitterionic surfactant was capable of accepting or donating a proton, it could acquire a net positive or negative charge by the interaction with the water as a solvent, and then the interaction with an oppositely charged ionic surfactant should be strong enough to produce synergy. Upon this viewpoint, the attactive interaction between C12AS and SDBS can be depicted in Fig. 4. It can be found from Fig. 4 that the net positively charged C12AS (accepting protons from water) can be electrostatically combined with the negatively charged SDBS, consequently resulting in easily forming the mixed micelle. Thereafter, the electrostatic attraction between C12AS and SDBS may play an important role in the reduction of the CM for mixed system at xC12AS < 0.6 mol/L. And it can be also found from Fig. 4 that when the molar fraction of C12AS in binary mixture increases to not less than 0.6 mol/L, the CM values not longer continue to reduce, but increase up to the cmc value of pure C12AS. This case may result from the great steric hindrance of head group for C12AS, preventing from the formation of mixed micelle.
4.2. Molecular interaction For the mixed systems, mole fractions of mixed micelle (X1 , X1ideal at an ideal state), interaction parameter (ˇ12 ), activity
coefficients in mixed micelle (f1 and f2 ) are calculated by the above corresponding equations and are reported in Table 1 (where the subscripts 1 and 2 of parameters represent C12AS and SDBS, respectively). It can be found from Table 1 that the ˇ12 values for all mixed systems are negative, and the absolute values of ˇ12 are always greater than that of ln(C1 /C2 ) (equals to 1.368), which indicates that there is some synergistic effect between C12AS and SDBS. The synergy can be also understood from the negative deviation between the experimental cmc values and the ideally calculated cmc values in Fig. 3. As shown in Table 1 and Fig. 5, at xC12AS > 0.2 mol/L, the mole fraction of C12AS (X1 ) is always more than (1 − X1 ) of SDBS in mixed micelle and X1 increases with xC12AS , which mainly own to the stronger ability of C12AS to form micelle (C2 /C1 > 3.9), thus promoting the richer content of C12AS in mixed micelle. Fig. 5 shows that, at xC12AS < 0.24 mol/L, X1 is slightly larger than X1ideal due to the micellization ability of C12AS and the electrostatic attraction between two surfactants as shown in Fig. 4. However, when the value of xC12AS is not less than 0.24 mol/L, the X1 value is obviously far smaller than the ideal value, which may indicate that at xC12AS = 0.24 mol/L the steric effect of C12AS initially influences the formation of mixed micelle, and then at xC12AS more than 0.24 mol/L and on, the steric effect gradually predominately hinders the micellization of mixed systems. Simultaneously, with the addition of C12AS in the mixed system the ability of micellization for individual surfactant and the electrostatic interaction between surfactants play diminishing role on the formation of mixed micelle. In Fig. 6, for all mixed systems, the activity coefficients (f1 ) of C12AS in mixed micelle are continually increasing, and while there has an oppositely tendency for SDBS, which can be explained by the outstanding effect of steric hindrance. In addition, it should be not neglected that the benzene group on the molecule of SDBS sterically has a centain influence on the process of micellization for mixed surfactants systems. 4.3. Thermodynamic properties The enthalpy change of micellization (H M ) has been obtained by subtracting the initial enthalpy from the final enthalpy indicated by the vertical arrow in Fig. 2b, and can be also calculated from the experimental data obtained by the conductivity measurements. The values of different thermodynamic functions are listed in Table 2. As shown in Table 2, there is not a significant difference in the value of H M or S M obtained by the conductivity method and by the ITC method, indicating the suitability of two measurement methods for the present investigation. According the thermodyM namics, the values of H M , GM , and Gideal are negative, but M S is positive, indicating the spontaneous process of micellization for the mixed system of C12AS and SDBS. It can be observed from Table 2 and Fig. 7 that the free energy change of micellization appears to be more negative than the ideal value, favoring the mixed micelle formation. It is found from Fig. 7 that there is a largest deviation from the ideal value at xC12AS = 0.2 mol/L, which indicates that the micellization is more favorable and the mixed micelles are more stable. This case appears due to the electrostatic attraction
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Z. Ren et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 428 (2013) 18–24
Fig. 3. The critical micelle concentrations for binary mixture of C12AS and SDBS in aqueous solution at 25 ◦ C.
C12
NH2
SO3-
N H2
2
C12 SO3-
OH
C12AS
SDBS
C12 SO3H OH
C12
NH2 NH2 SO3-
SO3 H
OH
OH
C12 Fig. 4. The attractive interaction between C12AS and SDBS in aqueous solution (pH = 6.5).
Fig. 2. Titration of the micellar solutions of C12AS and SDBS into the DTD water in the sample cell at 25 ◦ C. (a) Calorimetric traces (heat flow against time) for the mixture of C12AS and SDBS at xC12AS = 0.20 mol/L. The similar curves were obtained at other values of xC12AS . (b) Process enthalpy versus molar concentration, c, for the mixture of C12AS and SDBS in the cell. The HM is represented by the length of the arrow. (c) The plot of HM /c versus c.
Fig. 5. The micelle molar fraction of C12AS in mixed micelle for all systems at 25 ◦ C.
Z. Ren et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 428 (2013) 18–24
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Table 2 Thermodynamic parameters for binary mixture of C12AS and SDBS in aqueous solution at 25 ◦ C. xC12AS
GM (KJ/mol)
M Gideal (KJ/mol)
H M (KJ/mol) Conductivity method
ITC method
Conductivity method
ITC method
0.20 0.40 0.60 0.80
−3.485 −3.415 −3.321 −2.437
−1.718 −1.671 −1.575 −1.306
−1.767 −1.744 −1.746 −1.130
−1.659 −1.614 −1.641 −1.083
5.763 5.605 5.281 4.382
6.124 6.074 5.668 4.541
Fig. 6. Activity coefficients in mixed micelle for all systems at 25 ◦ C.
S M (J/mol)
fractions of components in the mixed micelles were obtained. The cmcs of mixed solutions were found to be dependent on the mole fraction of C12AS (xC12AS ) in mixed system, with cmc values always inferior to the ideal cmc over the entire mole fraction range investigated, suggesting nonideal mixing and synergism in mixed micelle formation. The nonideal mixing and the synergy between C12AS and SDBS in aqueous solution make the lower cmcs of their mixture appear. Regular solution theory and Rubingh models has been applied in the analysis of nonideal behavior in the binary mixture of C12AS and SDBS, and the interaction parameter, ˇ12 , has been evaluated. For all mixed systems, negative ˇ12 values were obtained for the whole composition range investigated, and then the absolute values of ˇ12 are always greater than the absolute values of ln(C1 /C2 ), indicating synergistic effect of interaction between C12AS and SDBS. The interaction between C12AS and SDBS are dependent on physicochemical properties of two surfactants and mole fraction of C12AS (xC12AS ) in mixed system. When xC12AS is relatively low (<0.6 mol/L), the main causes of decreasing the mixed cmcs are two aspects: firstly, screening the electrostatic repulsion between anionic SDBS molecules by the entry of C12AS into the mixed micelle. Secondly, forming a combination between anionic SDBS and C12AS with the capability of accepting proton by the electrostatic attraction. And when xC12AS is 0.24 mol/L, the steric effect of C12AS initially affects the formation of mixed micelle, and especially when xC12AS is not less than 0.6 mol/L, the steric effect plays a predominating role on the increase of the mixed cmcs. Thermodynamic data show that micellization for the binary mixture of C12AS and SDBS occurs spontaneously once the cmc has been reached, and with the addition of C12AS, the stability of mixed micelles tends to decrease. The negative values of H M and GM indicate that the micellization of binary mixture of C12AS and SDBS is both enthalpy and entropy driven process. Acknowledgements Funding for this work was provided by the National Sciences Foundation of China (41202111) and National Major Science and Technology Special Project of China (2011ZX05011).
Fig. 7. Free energy change of micellization for all mixed systems at 25 ◦ C.
References between the head groups of C12AS and SDBS. Then, the deviation tend to decrease with the increase of xC12AS for all mixed systems, especially in the case of xC12AS > 0.6 mol/L, which results from the influence of steric effect of C12AS to the formation of the mixed micelle. 5. Conclusions At 25 ◦ C, the interaction between C12AS and SDBS in aqueous solution at the pH of 6.5 was investigated by the conductivity method and by the ITC method, and the cmcs, thermodynamic parameters for micelle formation, the interaction parameters between two molecules, the activity coefficients and the molar
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Contents lists available at SciVerse ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Mechanism on the interaction between amimo sulfonate amphoteric surfactant and sodium dodecyl benzene sulfonate in aqueous solution Zhaohua Ren ∗ , Yue Luo, Dongpo Shi College of Chemistry and Environmental Engineering, Yangtze University, Jingzhou 434023, China
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• Micellar behavior for binary mixture • • • •
of dodecyl diamino sulfonate (C12AS) and SDBS. Rubingh model describes interaction parameters between C12AS and SDBS. Explanation of mechanism on synergistic effect between C12AS and SDBS. Effect of physicochemical property of C12AS on the formation of mixed micelle. Thermodynamic properties evaluate the tendency to the formation of mixed micelle.
a r t i c l e
i n f o
Article history: Received 2 January 2013 Received in revised form 27 February 2013 Accepted 22 March 2013 Available online 29 March 2013 Keywords: Amino sulfonate Surfactant Sodium dodecyl benzene sulfonate Mixed micelle Interaction parameter Activity coefficient Thermodynamic parameters Steric effect
a b s t r a c t The micellar properties of binary mixtures of sodium dodecyl diamino sulfonate (C12AS) and sodium dodecyl benzene sulfonate (SDBS) in aqueous solution at the pH of 6.5 have been characterized employing both conductometric techniques and isothermal titration microcalorimetry (ITC). The critical micelle concentration (cmc) values for the individual surfactants and their corresponding mixtures were determined from both the conductivity measurements and ITC. The interaction parameters between the surfactants, the activity coefficients and the molar fractions of components in the mixed micelles, and the thermodynamic parameters calculated by various approaches, like Rubingh models, were evaluated. The results show that the synergistic effect between C12AS and SDBS in all systems plays an vital role in the reduction of the overall cmc value in aqueous solution at 25 ◦ C, and that the steric effect of the head group, the lower charge density, and the capability of accepting proton on the molecule for C12AS affect the formation of mixed micelle. Thermodynamic data show that the micellization for binary mixture of C12AS and SDBS can occur spontaneously, and is both enthalpy and entropy driven process. © 2013 Elsevier B.V. All rights reserved.
1. Introduction In practical application, mixtures of surfactants, rather than individual surfatants, are used [1,2]. Mixtures of surfactants exhibit superious properties, like excellent surface/interfacial activities,
∗ Corresponding author. Tel.: +86 716 8060442; fax: +86 716 8060650. E-mail address: [email protected] (Z. Ren). 0927-7757/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2013.03.036
and are less expensive than single mixed surfactant systems. The more effective surface activities for binary mixtures of surfactants were reported in some literatures [1,3–6]. A mixed surfactants system exhibits greater surface activity, i.e., lower critical micelle concentration (cmc) values, than that obtained with any of the individual components of the mixture at the same concentration. Such effect of the mixture is said to be synergistic. The synergistic property is often exhibited due to the electrostatic attaction between the ionic headgroups of surfactants and the result of their nonideal
Z. Ren et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 428 (2013) 18–24
mixing, whereas the antagonism properties exhibit the repulsion between their molecules and the cmc value is higher than the expected. The regular solution theory (RST) is the simplest and most used approach for the mixed surfactants systems. Although the RST model was established for the nonionic surfactants, it is sometimes applicable for understanding the behavior of a binary mixture with ionic surfactants. Based on the RST, Rubingh [7] developed a model that is applicable to focus on the interaction between any two mixed surfactants system. An important parameter, the interaction parameter (ˇ12 ), was introduced in the Rubingh model and help us to predict the interaction or repulsion between the two surfactants. The negative value of ˇ12 indicates that there is some synergism between the two surfactants, whereas the positive value denotes the antagonism between the two surfactants. ˇ12 value near to zero reveals that there is little interaction between the two surfactants. Sodium dodecyl benzene sulfonate (SDBS) is an anionic surfactant having excellent surface/interfacial activities, and is usually used as a component in a mixed surfactants system. The synergistic properties of the mixtures of SDBS and some surfactants (e.g., sodium dodecyl sulfonates, cationic surfactant tetradecyl trimethyl ammonium bromide, nonionic surfactants C10-ˇ-maltoside, dodecyl betaine, and so on) were relatively entirely described in the Rosen’s book published in 2004 [1]. It is found that these surfactants, reported as above, are obviously characteritic of ionic or nonionic properties. As be compared with these surfactants, sodium n-dodecyl diamine sulfonate (C12AS) developed by our group [8] is among amino acid amphoteric surfactants and may be not obviously a ionic/nonionic property due to the hydrophilic headgroup of C12AS containing both a sulfonic group with stronger anionic property and two amino groups with weak cationic property. At present the interaction between C12AS and SDBS in aqeous solution is not found to be reported in literature. In this paper, we attempt to investigate the interacting behavior between the molecules of C12AS and SDBS in aqeous solution. The final aim of this investigation is to design a suitable component of the mixture for optimal behavior for a specific application, e.g., oil-displacing agent [9]. To obtaining the information of the interaction between surfactants and the components in the mixed micelle, we are adopting different theorys and models, e.g., RST, Rubingh’s model, and the mechanism on the interaction between C12AS and SDBS is discussed.
19
containing a series of different surfactants of molar concentrations, c, were measured with DDS-307 digital conductivity meter. The cmcs of surfactants in aqeous solutions were determined by both the curve plots of the molar conductivity (/c) versus c and the thermogram obtained from the isothermal titration microcalorimetry (ITC) experiments as follow. The ITC experiments were carried out at 25 ◦ C using Nano ITC isothermal titration microcalorimeter made by American TA Instrument Co. The concentrated surfactant solution was added to DTD water in the sample cell under constant stirring 200 revolutions per minute. The concentrations of surfactant solutions using in the ITC experiments were about 20 times larger than the values of cmc obtained from the above conductometry experiments. Every titration in the ITC experiment was determined three times and the average value of the thermogram was obtained with the relative errors of less than 2%. 3. Theoretical background 3.1. Cmc of mixed surfactants Based on pseudophase separation model, nonideal mixed micellization processes can be treated by considering the chemical potentials of the individual surfactant components. For pure surfactant component i in solution: i = 0i + RT ln ciS 0i ,
(1) ciS
where i , and correspond to the chemical potential, the standard chemical potential, and the monomer concentration of surfactant i in the solution, respectively. Where the activity of surfactant i is 1. When the micelle of pure surfactant i in solution is formed, the ) and the cmc relationship of the standard chemical potential (0M i of the component i (Ci ) is as follow: 0M = 0i + RT ln Ci i
(2) (M ) i
For a mixed micelle, the chemical potential nent i in the mixed micelle can be expressed as: M = 0M + RT ln fi Xi i i
of the compo(3)
where Xi and fi are the mole fraction and the activity coefficient of surfactant i in the mixed micelle. Since at equilibrium Si = M , the i monomer concentration (ciS ) of surfactant i can be written as:
2. Experimental
ciS = fi Xi Ci
2.1. Materials
According to mass conservation, and by rearranging this Eq. (4), the mixed cmc (CM ) can be represented as [7]:
(4)
x 1 i = fi Ci CM n
C12AS developed by our group [8] has a purity of over 96%, measured with Vario EL III Automatic Elementary Analyser made by Germany Elementar Co. The chemical structure of C12AS is followed as: n-C12 H25 -NH-CH2 CH2 -NH-CH2 CH(OH)CH2 -SO3 Na. SDBS is an analytical reagent from Sinopharm Chemical Reagent Co., Ltd. Deionized triple distilled (DTD) water is used in this investigation. The conductivity of DTD water was close to 13 S/cm, measured with DDS-307 digital conductivity meter made in China.
where CM and xi are the mixed cmc and the mole fraction of surfactant i in the mixed surfactants solution, respectively. Eq. (5a) represents the nonideal behavior of mixed surfactants system. For ideal system, fi = 1, this Eq. (5a) reduces to Clint equation [10], namely: 1
2.2. Methods Solutions of C12AS, SDBS, and their mixtures with different molar ratios were confected by DTD water, respectively. The pH values of mixed surfactants solutions were found to be close to 6.5 within the range of isoelectric point for C12AS. The solutions in the conical flasks were allowed to stand in water bath of 25 ± 0.2 ◦ C over half hour. And then the conductivities, , of solutions
(5a)
i=1
C ideal
=
n x
i
i=1
Ci
(5b)
where C ideal is the mixed cmc at an ideal state. For the binary mixture of surfactant 1 and surfactant 2, Eq. (5b) is written as: 1 ideal C12
=
x1 1 − x1 + C1 C2
(6)
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Clint equation or model was established based on pseudophase separation model and is an idealization which neglects the interaction of different surfactants in the mixed state and considers the cmcs of the individual component influencing their relative tendency to the micellization. According to pseudophase thermodynamic model, for a binary surfactants system, the mole fraction (X1ideal ) of surfactant 1 in the mixed micelle at an ideal mixed state can be expressed as: x1 C2 X1ideal = (7) x1 C2 + (1 − x1 )C1 3.2. Molecular interaction in mixed surfactants systems The Clint model is useful for understanding the ideal behavior of binary surfactant systems. For the nonideal mixed system, new model based on RST is developed by Rubingh [7]. Rubingh model has been mostly used due to its simplicity, even after development of more complex models. The expression of Rubingh equation is introduced with the activity coefficient (fi ) as the above Eqs. (3)–(5). For binary nonideal mixtures, Rubingh and co-worker [7] suggested that the activity coefficients of the components could be written as: f1 = exp ˇ12 (1 − X1 )2
(8a)
f2 = exp ˇ12 X1 2
(8b)
where ˇ12 is an interaction parameter that measures the nature and extent of the interaction between the two different surfactant molecules in the mixed micelle in aqueous solution, and is a measure of deviation from the ideal behavior. The micellar mole fraction, X1 , can be calculated by solving iteratively the following equation [1,7]: M /(X C )] X1 2 ln[x1 C12 1 1 2
M /[(1 − X )C ]} (1 − X1 ) ln{(1 − x1 )C12 1 2
=1
(9)
Consequently, ˇ12 can be calculated by the equation below: ˇ12 =
M /(X C )] ln[x1 C12 1 1
(10)
(1 − X1 )2
3.3. Thermodynamic parameters of mixed micellization
Fig. 1. Plot of molar conductivity versus molar concentration at 25 ◦ C. (a) Individually C12AS or SDBS in aqueous solution and (b) the binary mixtures of C12AS and SDBS at different xC12AS .
Based on RST, the excess entropy of mixing (SE ) is zero, namely, SE = Sideal = 0 (Sideal , the change of entropy at an ideal mixed state), and the excess enthalpy of ideal mixing (Hideal ) is equal to zero. Thereafter, the relation between excess free energy (GE ), excess enthalpy (HE ), and enthalpy of micellization is written as [11]:
4. Results and discussion
GE = H E = H M = RT
n
Xi ln fi
(11)
i=1
The free energy change (GM ) in binary surfactants mixture at the nonideal state can be expressed as [11,12]: GM = RT
n
Xi ln fi Xi
(12)
i=1 M For the ideal mixture, Gideal is as below: n
M Gideal = RT ˙ Xi ln Xi
(13)
i=1
Thereafter, the entropy change (SM ) in the micellization of mixed surfactants system is: S M =
H M − GM T
(14)
4.1. Cmcs of C12AS, SDBS, and their mixtures The cmc values of C12AS, SDBS, and their mixtures in aqeous solutions were determined by the conductivity method and are graphically presented in Fig. 1. Clear break points (at cmc) were observed in the plots of molar conductivity versus molar concentration c, indicating the initiation of the micellization phenomenon. When micellar surfactant solution is titration into water in the cell, ITC records the differential enthalpy changes related with the demicellization and the dillution of surfactant molecules. The value of cmc and H M can be directly obtained from one ITC experiment [13]. An illustration of the heat flow, enthalpy change of micellization (H M ), and the plot of H M /c versus c for the binary mixture of C12AS and SDBS at 25 ◦ C is presented in Fig. 2. The dependence of the enthalpy change on surfactant concentration can be used to determine the cmc, that is, from the inflection point in the H M versus surfactant concentration curve [14]. The cmc point is indicated in Fig. 2b and c. The experimental cmcs (CM ) and the ideal cmcs (Cideal ), calculated by Eq. (6), of the mixed systems for C12AS and SDBS are listed in Table 1. As shown in Table 1, the cmc
Z. Ren et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 428 (2013) 18–24
21
Table 1 Micellization parameter values for the binary mixture of C12AS and SDBS in aqueous solution at 25 ◦ C. xC12AS
0 0.20 0.40 0.60 0.80 1
CM (mmol/L) Conductivity method
ITC method
1.1 0.34 0.26 0.22 0.24 0.28
1.05 0.32 0.25 0.22 0.24 0.27
C ideal (mmol/L)
X1
X1ideal
ˇ12
f1
f2
1.1 0.694 0.507 0.399 0.329 0.28
0 0.498 0.597 0.668 0.780 1
0 0.485 0.724 0.855 0.940 1
−2.582 −2.924 −3.178 −2.657
0.488 0.622 0.705 0.879
0.493 0.353 0.242 0.199
values measured by the conductometric techniques nearly consistent with obtained from the ITC experiments, implying the validity of the two methods in this case. Generally, cationic or anionic surfactants have higher cmc than nonionic or zwitterionic surfactants, which mainly results in electrostatic repulsion between their head groups or their charge density. The charge density on the molecular chain of C12AS (q2 = 0.21 [15]) is far smaller than SDBS (q2 close to 1), which may lead to a lower cmc value of C12AS. For the mixed system of C12AS and SDBS, it is found from Fig. 3 that the experimental cmc values are lower than the ideal behavior, which confirms a constitutional interaction between the two surfactants or their nonideal behavior of mixing. In Fig. 3, the experimental CM values initially decrease with molar fraction of C12AS (xC12AS < 0.6 mol/L), which can be easily explained by the reduction of electrostatic repulsion between molecules in the mixed micelle of C12AS and SDBS due to the lower charge density of C12AS. Obviously, the molecules of C12AS intercalating into the micelle of SDBS can effectively screen the electrostatic repulsion between the ionic head groups of SDBS, which to a certain degree plays role on the decrease of CM for the mixed system. Some investigations, e.g., of Faustino et al. [16] and Vautier-Giongo et al. [17], also confirm that more molecules with lower charge density become available for intercalation in the mixed micelles, reducing electrostatic repulsion between the ionic head groups of the surfactants more effectively, thus promoting micellization. Otherwise, only depending on the intercalation of C12AS into the micelle of SDBS to screen the electrostatic repulsion between two molecules, it is not exactly possible to reduce the CM to lower than the cmc value of pure SDBS because C12AS has still a little charge and is electrically different with nonionics. Rosen [18] suggested that when the zwitterionic surfactant was capable of accepting or donating a proton, it could acquire a net positive or negative charge by the interaction with the water as a solvent, and then the interaction with an oppositely charged ionic surfactant should be strong enough to produce synergy. Upon this viewpoint, the attactive interaction between C12AS and SDBS can be depicted in Fig. 4. It can be found from Fig. 4 that the net positively charged C12AS (accepting protons from water) can be electrostatically combined with the negatively charged SDBS, consequently resulting in easily forming the mixed micelle. Thereafter, the electrostatic attraction between C12AS and SDBS may play an important role in the reduction of the CM for mixed system at xC12AS < 0.6 mol/L. And it can be also found from Fig. 4 that when the molar fraction of C12AS in binary mixture increases to not less than 0.6 mol/L, the CM values not longer continue to reduce, but increase up to the cmc value of pure C12AS. This case may result from the great steric hindrance of head group for C12AS, preventing from the formation of mixed micelle.
4.2. Molecular interaction For the mixed systems, mole fractions of mixed micelle (X1 , X1ideal at an ideal state), interaction parameter (ˇ12 ), activity
coefficients in mixed micelle (f1 and f2 ) are calculated by the above corresponding equations and are reported in Table 1 (where the subscripts 1 and 2 of parameters represent C12AS and SDBS, respectively). It can be found from Table 1 that the ˇ12 values for all mixed systems are negative, and the absolute values of ˇ12 are always greater than that of ln(C1 /C2 ) (equals to 1.368), which indicates that there is some synergistic effect between C12AS and SDBS. The synergy can be also understood from the negative deviation between the experimental cmc values and the ideally calculated cmc values in Fig. 3. As shown in Table 1 and Fig. 5, at xC12AS > 0.2 mol/L, the mole fraction of C12AS (X1 ) is always more than (1 − X1 ) of SDBS in mixed micelle and X1 increases with xC12AS , which mainly own to the stronger ability of C12AS to form micelle (C2 /C1 > 3.9), thus promoting the richer content of C12AS in mixed micelle. Fig. 5 shows that, at xC12AS < 0.24 mol/L, X1 is slightly larger than X1ideal due to the micellization ability of C12AS and the electrostatic attraction between two surfactants as shown in Fig. 4. However, when the value of xC12AS is not less than 0.24 mol/L, the X1 value is obviously far smaller than the ideal value, which may indicate that at xC12AS = 0.24 mol/L the steric effect of C12AS initially influences the formation of mixed micelle, and then at xC12AS more than 0.24 mol/L and on, the steric effect gradually predominately hinders the micellization of mixed systems. Simultaneously, with the addition of C12AS in the mixed system the ability of micellization for individual surfactant and the electrostatic interaction between surfactants play diminishing role on the formation of mixed micelle. In Fig. 6, for all mixed systems, the activity coefficients (f1 ) of C12AS in mixed micelle are continually increasing, and while there has an oppositely tendency for SDBS, which can be explained by the outstanding effect of steric hindrance. In addition, it should be not neglected that the benzene group on the molecule of SDBS sterically has a centain influence on the process of micellization for mixed surfactants systems. 4.3. Thermodynamic properties The enthalpy change of micellization (H M ) has been obtained by subtracting the initial enthalpy from the final enthalpy indicated by the vertical arrow in Fig. 2b, and can be also calculated from the experimental data obtained by the conductivity measurements. The values of different thermodynamic functions are listed in Table 2. As shown in Table 2, there is not a significant difference in the value of H M or S M obtained by the conductivity method and by the ITC method, indicating the suitability of two measurement methods for the present investigation. According the thermodyM namics, the values of H M , GM , and Gideal are negative, but M S is positive, indicating the spontaneous process of micellization for the mixed system of C12AS and SDBS. It can be observed from Table 2 and Fig. 7 that the free energy change of micellization appears to be more negative than the ideal value, favoring the mixed micelle formation. It is found from Fig. 7 that there is a largest deviation from the ideal value at xC12AS = 0.2 mol/L, which indicates that the micellization is more favorable and the mixed micelles are more stable. This case appears due to the electrostatic attraction
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Fig. 3. The critical micelle concentrations for binary mixture of C12AS and SDBS in aqueous solution at 25 ◦ C.
C12
NH2
SO3-
N H2
2
C12 SO3-
OH
C12AS
SDBS
C12 SO3H OH
C12
NH2 NH2 SO3-
SO3 H
OH
OH
C12 Fig. 4. The attractive interaction between C12AS and SDBS in aqueous solution (pH = 6.5).
Fig. 2. Titration of the micellar solutions of C12AS and SDBS into the DTD water in the sample cell at 25 ◦ C. (a) Calorimetric traces (heat flow against time) for the mixture of C12AS and SDBS at xC12AS = 0.20 mol/L. The similar curves were obtained at other values of xC12AS . (b) Process enthalpy versus molar concentration, c, for the mixture of C12AS and SDBS in the cell. The HM is represented by the length of the arrow. (c) The plot of HM /c versus c.
Fig. 5. The micelle molar fraction of C12AS in mixed micelle for all systems at 25 ◦ C.
Z. Ren et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 428 (2013) 18–24
23
Table 2 Thermodynamic parameters for binary mixture of C12AS and SDBS in aqueous solution at 25 ◦ C. xC12AS
GM (KJ/mol)
M Gideal (KJ/mol)
H M (KJ/mol) Conductivity method
ITC method
Conductivity method
ITC method
0.20 0.40 0.60 0.80
−3.485 −3.415 −3.321 −2.437
−1.718 −1.671 −1.575 −1.306
−1.767 −1.744 −1.746 −1.130
−1.659 −1.614 −1.641 −1.083
5.763 5.605 5.281 4.382
6.124 6.074 5.668 4.541
Fig. 6. Activity coefficients in mixed micelle for all systems at 25 ◦ C.
S M (J/mol)
fractions of components in the mixed micelles were obtained. The cmcs of mixed solutions were found to be dependent on the mole fraction of C12AS (xC12AS ) in mixed system, with cmc values always inferior to the ideal cmc over the entire mole fraction range investigated, suggesting nonideal mixing and synergism in mixed micelle formation. The nonideal mixing and the synergy between C12AS and SDBS in aqueous solution make the lower cmcs of their mixture appear. Regular solution theory and Rubingh models has been applied in the analysis of nonideal behavior in the binary mixture of C12AS and SDBS, and the interaction parameter, ˇ12 , has been evaluated. For all mixed systems, negative ˇ12 values were obtained for the whole composition range investigated, and then the absolute values of ˇ12 are always greater than the absolute values of ln(C1 /C2 ), indicating synergistic effect of interaction between C12AS and SDBS. The interaction between C12AS and SDBS are dependent on physicochemical properties of two surfactants and mole fraction of C12AS (xC12AS ) in mixed system. When xC12AS is relatively low (<0.6 mol/L), the main causes of decreasing the mixed cmcs are two aspects: firstly, screening the electrostatic repulsion between anionic SDBS molecules by the entry of C12AS into the mixed micelle. Secondly, forming a combination between anionic SDBS and C12AS with the capability of accepting proton by the electrostatic attraction. And when xC12AS is 0.24 mol/L, the steric effect of C12AS initially affects the formation of mixed micelle, and especially when xC12AS is not less than 0.6 mol/L, the steric effect plays a predominating role on the increase of the mixed cmcs. Thermodynamic data show that micellization for the binary mixture of C12AS and SDBS occurs spontaneously once the cmc has been reached, and with the addition of C12AS, the stability of mixed micelles tends to decrease. The negative values of H M and GM indicate that the micellization of binary mixture of C12AS and SDBS is both enthalpy and entropy driven process. Acknowledgements Funding for this work was provided by the National Sciences Foundation of China (41202111) and National Major Science and Technology Special Project of China (2011ZX05011).
Fig. 7. Free energy change of micellization for all mixed systems at 25 ◦ C.
References between the head groups of C12AS and SDBS. Then, the deviation tend to decrease with the increase of xC12AS for all mixed systems, especially in the case of xC12AS > 0.6 mol/L, which results from the influence of steric effect of C12AS to the formation of the mixed micelle. 5. Conclusions At 25 ◦ C, the interaction between C12AS and SDBS in aqueous solution at the pH of 6.5 was investigated by the conductivity method and by the ITC method, and the cmcs, thermodynamic parameters for micelle formation, the interaction parameters between two molecules, the activity coefficients and the molar
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