Article pubs.acs.org/IECR
Mechanism of the Salt Effect on Micellization of an Aminosulfonate Amphoteric Surfactant Zhao Hua Ren* (College of Chemistry and Environmental Engineering, Yangtze University, Jingzhou 434023, China) ABSTRACT: The critical micellar concentrations (cmc’s) of sodium dodecyldiaminosulfonate amphoteric surfactant (C12AS) in aqueous solutions containing seven inorganic salts with chloride anions were measured by tensiometry using the Wilhelmy plate method at 25 °C. The mathematical relationship between the cmc of C12AS and the concentration of an inorganic salt in an aqueous solution was established. The effect of an inorganic salt on micellization of C12AS was also discussed. The results show that the dependence of the cmc of C12AS on an inorganic salt within the range of the effective concentration of the inorganic salt investigated in this experiment can be well described by the linear equation. The decrease of the cmc with an inorganic salt may be mainly attributed to the salting-out effect of the hydrophobic moiety of C12AS. Except for the salting-out effect, the causes can be partly from the vital contributions of both the electrostatic interaction between ionic heads of C12AS and the hydration of inorganic ions in the process of micellization. Also, the type of inorganic ion can affect the process of micellization. The data of Gibbs energy changes show that there is a spontaneous process of micellization, and the addition of an inorganic salt in an aqueous solution is favorable thermodynamically to the process of micellization.
1. INTRODUCTION An aminosulfonate amphoteric surfactant belongs to green chemical,1,2 which has an excellent biocompatibility and a better compatibility with nonionic, cationic, anionic, and other zwitterionic surfactants.1−3 Also amphoteric surfactants having multiple amino groups and a sulfonic group have been added successfully to surfactant formulas in enhanced oil recovery.2 Besides some merits owned by common amino acid surfactants, aminosulfonate amphoteric surfactants also have other excellent properties, such as tolerance to hard water or electrolyte.3,4 Regarding to the salt tolerance, aminosulfonate amphoteric surfactants can be used, in some occasions containing inorganic salts. For example, they can be used in the stratum water in some oilfields.2,3 The addition of an inorganic salt can effectively influence the micellization behavior and the aggregation state of the surfactants in an aqueous solution.4−13 13 Generally, it is suggested4−8 that, for ionic surfactants (including cationics and anionics), an inorganic salt (especially a counterion) can compress the thickness of the ionic atmosphere surrounding the ionic headgroup of the surfactant and can decrease the electrostatic repulsion between ionic headgroups, consequently favoring the formation of micelles and decreasing the critical micellar concentration (cmc), while for the nonionic or zwitterionic surfactants, on the addition of an inorganic salt, the cmc may be decreased mainly by the salting-in or salting-out effect, for which it is worth mentioning that in many cases the effect may not be remarkable.4,5,9−13 As reported by Ren et al.,5 Kroflic et al.,11 and Qin et al.,12 with an increase of the concentration of inorganic salts, the cmc values of zwitterionic surfactants were slightly lowered. The standpoints can be easily understood and explained theoretically. For zwitterionic surfactants (such as alkyl betaine), owing to the zero charge on their headgroups within the isoelectronic point, the change of the cmc by the addition of an inorganic salt may be attributed mainly to the effect of salting-in or salting-out of the hydrophobic groups but not to the electrostatic effect. © 2015 American Chemical Society
However, it differs from the properties of common zwitterionic surfactants (such as alkyl betaine) in that, for aminosulfonate surfactants containing multiple amino groups, the cationic groups on the molecule are usually secondary or tertiary amino groups, which makes their amphotericity have a strong dependence on the pH of the aqueous solution.3,4 On the basis of this consideration, aminosulfonate surfactants are usually named “amphoteric” surfactants.3,4 So far, the effect of inorganic salts on the cmc of aminosulfonate surfactants has not been reported. Thus, considering their “green” or environmentally friendly properties and their wide usage in many application fields,1−4 it is very necessary to focus on some problems. For example, how does the effect of an inorganic salt on micellization of an aminosulfonate surfactant work? Is the contribution of the headgroup made in the formation of micelles? The final aim of this investigation is to describe the interaction between surfactants and to design a suitable component of the mixture containing aminosulfonate-type surfactants for optimum behavior for some applications, especially relating to salt solutions. Thus, to obtain some information about the interaction between inorganic salts and surfactant molecules and to understand the micellization behavior, the cmc values of a diaminosulfonate amphoteric surfactant, sodium 3-(N-dodecylethylenediamino)-2-hydropropylsulfonate (abbreviated as C12AS) developed by our group,14 in aqueous solutions containing different inorganic salts were measured by tensiometry, and the mechanism on the interaction between the inorganic salt and surfactant in the process of micellization was discussed. Received: Revised: Accepted: Published: 9683
June 15, 2015 September 7, 2015 September 20, 2015 September 21, 2015 DOI: 10.1021/acs.iecr.5b02169 Ind. Eng. Chem. Res. 2015, 54, 9683−9688
Article
Industrial & Engineering Chemistry Research
solution.4−13 The relationship of the cmc of ionics with the counterion concentration can be described by a linear equation established by Corrin et al.5,18 For nonionic surfactants, the salting-out or salting-in effect on the hydrophobic group by the addition of an inorganic salt plays a vital role in the process of micellization. Another linear relationship of the cmc of a nonionic or zwitterionic surfactant with the concentration of the inorganic salt, [S], has been established by Shinoda et al.4,5 This relationship is as follows:
2. EXPERIMENTAL SECTION 2.1. Materials. The treated product of C12AS developed by our group14−17 has a purity of over 96% (with the rest being less than 4% water), measured with a Vario EL III Automatic Elementary Analyzer made by Germany Elementar Co., and its chemical structure is depicted in Figure 1. The inorganic salts
log cmc = K[S] + C
(1)
where K is a constant relating to the nature of the surfactant, electrolyte, temperature, solvent, and so on and C is a common constant in this equation. Shinoda et al.4,5 thought that the condition of [S] < 1 mol/L should be considered in order to fulfill the linear expression in eq 1. The plot of the logarithm of cmc of C12AS, log cmc, versus the concentration of inorganic salts, [S], is depicted in Figure 2.
Figure 1. Molecular structure of C12AS and its representation.
including NaCl, KCl, MgCl2, CaCl2, BaCl2, AlCl3, and FeCl3 are analytical reagents (>99%) from Sinopharm Chemical Reagent Co., Ltd. C12AS and inorganic salts were used as supplied. Redistilled water was used throughout all of the experiments. 2.2. Methods. Compatibility Test. Considering the solubility of C12AS in the saline water and hydrolysis of AlCl3 and FeCl3 in some solutions, compatibility tests were carried out. The prepared solutions containing inorganic salts and C12AS of 0.1 mol/L were vibrated in a SHA-C oscillator (made in China) for 6 h so that all solutions could be thoroughly mixed and then were allowed to stand in a water bath of 25 ± 0.2 °C for 18 h. The turbidities of the solutions before and after standing were respectively measured with a TN100 digital turbidity meter made by Thermo Fisher. If the relative deviation between the turbidities before and after standing is more than 2%, it shows a bad compatibility or hydrolysis of AlCl3 and FeCl3 and vice versa.5 Measurements of the cmc. The surface tensions of surfactant solutions were measured with a JK99C automatic surface tensiometer (Shanghai Zhongchen Digital Technic Apparatus Co., Ltd.) using a Wilhelmy platinum plate.5 Before measurement of the surface tension, all surfactant solutions prepared previously in conical flasks sealed with rubber plugs were allowed to stand in a HH-601A thermostatic watercirculated bath at 25 ± 0.2 °C for nearly 50 min in order to obtain a constant temperature. The experimental error for the measurement of the surface tension is ±0.2 mN/m, and the value of the surface tension obtained in this paper is an average value from twice experimental results. The cmc value of the surfactant in a salt solution was determined from the inflection point in the curve of the surface tension versus the logarithm of the surfactant concentration in solution.5,16 In addition, the pH values of all of the solutions were measured by a PHS-3G pH meter made in China. For all of the solutions, pH values of about 6.2 indicate that the addition of inorganic salts does not influence the acidicity of the solution. The pH values in salt solutions are still within the range of the isoelectric point, and the surfactants are characteristic of amphoteric properties, which is in accordance with our previous investigations.15−17
Figure 2. Variation of the logarithm of cmc of C12AS with the concentration of the inorganic salt.
For the data of each set in Figure 2, the coefficient of the linear equation as listed in Table 1 could be well obtained by the Table 1. Linear Fitting Relationship between the cmc of C12AS and the Concentration of the Inorganic Salta coefficient of the linear fitting equation as eq 1
univalent cationic bivalent cationic
trivalent cationic
range of [S] (mol/L)
R2
K
C
NaCl
−0.0746
−3.549
⩽1
0.997
KCl MgCl2
−0.0865 −0.0938
−3.555 −3.565
⩽1
