Abstract: The micellization of anionics surfactants (SDS, sodium dodecyl sulphate and SU, surfactin) and cationic surfactants (BC, benzalkonium chloride, TTAB, tetradecyltrimethylammonium bromide and HTAB, hexadecyltrimethylammonium bromide) were investigated at various temperatures using a du Nouy ring tensiometer. The cmc decreased to a certain minimum and then increased with the temperature, displaying a U-shaped behavior. This behavior was analyzed using a power-law equation. The thermodynamic parameters of micellization, ΔGm, ΔHm and ΔSm were obtained at different temperatures.
INTRODUCTION
Cationic surfactants are known to exhibit excellent antistatic effects and softness. Hydrocarbons higher than C16 are normally employed in cosmetics and toiletries (Kang et al., 2001). As for anionics, Sodium Dodecyl Sulfate (SDS) is the best known and is widely used in industry and it has been extensively studied in relation for its micellization, properties and phase behavior (Kekicheff et al., 1989; Kekicheff, 1989). Biosurfactant production from microorganisms has been studied extensively for more than a decade (Fiechter, 1992). Biosurfactants are amphiphilic in nature and reduce the surface tension of medium in which they are produced. These surface-active compounds have found many applications in industry, agriculture, mining and oil recovery, with functional properties as wetting, foaming and emulsifiers in pharmaceutical and cosmetic products. Bacillus subtilis produces a lipopeptide, called surfactin, with exceptional surface activity (Peypoux et al., 1999; Heerklotz and Seeling, 2001). The compound has been characterized as a cyclic lipopeptide containing a carboxylic acid (3-hydroxy-13-methyl tetradecanoic acid) and seven amino acid residues. Structural characteristics show the presence of a heptapeptide with an LLDLLDL chiral sequence linked, via a lactone bond, to α-hydroxy fatty acid (Peypoux et al., 1999). It has advantages over chemical surfactants in biodegradability and effectiveness at extreme temperature or pH (Randhir, 1997). If the economic problems could be solved, this compound would certainly find new applications in agriculture and environment by the petroleum industries (Peypoux et al., 1999). Last decade, it has shown diverse new activities including emulsification, foaming (Razafindralambo et al., 1998).
The capacity to aggregate in solutions is one of the characteristics of surfactants. When aggregations are formed, various physical properties of the surfactant solutions change abruptly within a narrow concentration range. Micelles are one type of aggregation and the narrow concentration range is called the Critical Micelle Concentration (cmc), above which micelles exist in the solutions. Micellization is affected by various factors including surfactant species (hydrophobic volume, chain length, head group area), temperature, pressure, ionic strength, pH, etc. For ionics and amphoterics, micellization is affected by temperature as the hydrophobic and head group interactions change relative to temperature. Accordingly, cmc versus temperature studies have already been performed to obtain information on these interactions (Miller et al., 1990). For non-ionic surfactants, the cmc decreases with increasing temperature due to an increase in hydrophobicity caused by the destruction of hydrogen bonds between water molecules and hydrophilic groups. Therefore, the log cmc of non-ionic surfactants vs. the reciprocal of temperature plot is nearly linear (Kang et al., 2001). However, for ionic surfactants, cmc decreases to a minimum value and then increases, displaying a U-shaped behavior (Stead and Taylor, 1969). The minimum is characterized by the minimum cmc, C*cmc and the temperature, T*, at C*cmc. Furthermore, it would appear that the effect of temperature on cmc can be represented by the power law |Ccmc-C*cmc| = A |T-T*| n (Kang et al., 2001). At present, it would seem that the exponent n is characteristic to the surfactant system. This article reports on the effect of temperature, i.e., C*cmc, T* and n, on the cmc of anionic surfactants such as SDS (sodium dodecyl sulphate), surfactin (a lipopeptide biosurfactant) and cationic surfactants such as BC (benzalkonium chloride), TTAB (tetradecyltrimethylammonium bromide) and HTAB (hexadecyltrimethylammonium bromide), in their aqueous region at 15, 25, 30 and 35°C were reported using surface tension measurements.
MATERIALS AND METHODS
Materials: The cationic surfactants such as BC (Sigma), TTAB (Merck), HTAB (Fluka) and anionic surfactants such as SDS (Merck) and surfactin (a lipopeptide biosurfactant produced by B. subtilis ATCC 6633), Water was doubly distilled. The materials were the highest purity.
Methods: A Du Nouy tensiometer (modle-703, Sigma) was used to determine the surface tension. Different aqueous concentrations of surfactants were prepared. Surface tension was measured with a platinum ring. The surface tension-concentration plots were used to determine Critical Micelle Concentration (cmc). The surfactant solution was mixed thoroughly using a magnetically driven stirrer. The temperature was maintained within±0.1°C.
RESULTS
Critical micelle concentrations (cmcs) and minimum cmc The surface tension of surfactants solution with different concentrations in water at 15, 25, 30 and 35°C were determined. For calculating cmc, tangents are drawn on the two portions of the plots. The cmcs of the cationic (BC, TTAB, HTAB) and anionic (SDS, Surfactin) surfactants determined at various temperature. The cmc decreased to a certain minimum point and then increased when temperature increased (Fig. 1). By fitting a polynomial function of temperature to cmc data and finding the minimum of this function, it is usually possible to determine the minimum cmc (C*cmc).
| Table 1: | T* values for all the surfactants at different temperatures |
| Table 2: | n values for all the systems |
The curves in Fig. 1 are polynomial functions fitted to the surfactants and the minimum cmc values determined from these polynomials. The temperature, T*, at which C*cmc occurred was determined.
Temperature dependence of cmc and power-law exponent: The temperature dependence of the cmc can be described by a power law between the reduced cmc and temperature, Cr and Tr, where the reduced variables are defined as Cred = Ccmc/C*cmc and Tr = T/T*. Tr and Cr can be related to each other through the following equation (Kang et al., 2001):
Where A is the constant and n is the exponent, which appear to be characteristic to the surfactant system. The curves in Fig. 2 are 6th-order polynomial functions fitted to those of surfactants cmcs (correlation coefficients, R2 = 1). The T* for these of surfactants were determined from Fig. 1 (Table 1). It was for anionic surfactants (SDS, Surfactin) around 21°C and for cationic surfactants (BC, TTAB, HTAB) around 24°C. Table 2 shows the n values of the fittings Eq. this equation: ln|Cr-1| = nln|Tr-1| + lnA to the cmc data. These quantities are like to the other reports in the literature (Kang et al., 2001).
Thermodynamics of micelle formation: Various thermodynamic quantities like the free energy ΔGm, the enthalpy ΔHm, the entropy ΔSm and the heat capacity ΔCp,m micellization were obtained by using the following relationships and temperature dependence of cmc fitted equations:
The relationship between thermodynamic parameters and temperatures were observed for all systems and this is shown in Fig. 2.
| Fig. 1: | cmc as a function of temperature for aqueous surfactants |
Relationship between enthalpy and entropy of micellization: A linear correlation between enthalpy and entropy of micellization was observed for all systems (correlation coefficient 0.999) and this is shown in Fig. 3.
DISCUSSION
The cmc of cationic (BC, TTAB, HTAB) and anionic (SDS, surfactin) surfactants were found to be dependent on temperature and followed the expected U-shaped cmc-temperature curve.
| Fig. 2: | Thermodynamic parameters of surfactants micellization at various temperatures |
The temperature dependence of the cmc for these surfactants could be represented by a power law: |Cr-1| = A|Tr-1| for these surfactants where Cr = Ccmc/C*cmc and Tr = T /T* that were in good agreement with the literature. The free energy, ΔGm, values are like with the reports in literature (Archer et al., 1984; Evans and Ninham, 1986). ΔHm values were endothermic and decreased with temperature at T<T* and were exothermic and became larger in magnitude as the temperature increased at T>T*. For these surfactants has been found that micellization is entropy-controlled at low temperatures and enthalpy-controlled at high temperatures.
| Fig. 3: | Enthalpy-entropy compensation plot for all the systems |
ΔGm is the sum of the enthalpic (ΔHm) and entropic (-TΔSm) contributions. As the temperature increased, the enthalpic contribution to the free energy increased, whereas the entropic contribution decreased (Fig. 2). Thus micellization was entropy-controlled at T<T* and enthalpy-controlled at T>T*. An equal contribution took place at 306.52 K for SDS, at 309 K for Surfactin, at 304.20 K for BC, at 300.82 K for TTAB and at 302.30 K for HTAB. These results would seem to indicate that the exponent is a characteristic of a particular surfactant system. The micellization of these surfactants was entropy-dominant at low temperatures and enthalpy-dominant at high temperatures, which has also been observed in other surfactants. The temperature at which entropy and enthalpy contributed equally to the free energy of micellization was higher for anionic surfactants than for cationic surfactants. Relationship between enthalpy and entropy of micellization were linear correlation, the slope of the curves (Fig. 3) are 288, 297, 297, 290, 296 and 295 K, for SDS, Surfactin, BC, TTAB and HTAB, respectively, which are not far away from suggested value for water systems (Sulthana et al., 1996).
ACKNOWLEDGMENTS
This study was supported by operating grants from the Vice President for Research of Mashhad University of Medical Sciences (MUMS).