Abstract:
In this research, anionic (SDS) and nonionic (TX100) surfactant mixtures (1:2,
1:1, 2:1; TX100: SDS mass ratios) were evaluated for possible synergism in Critical
Micelle Concentration (CMC). Synergism of both surfactants was sought in presence
of shale and/or oil phase. The composition of mixed micelles and the interaction
parameter,
INTRODUCTION
The use of surfactants to decontaminate groundwater aquifers, enhance residual oil recovery and in soil-clean up operations is well established and both anionic and nonionic surfactants have been used to remediate land polluted with oils and hydrocarbons as well as many other organic contaminants. A fundamental property of surfactants is their ability to form micelles (colloidal sized clusters) in solution. This property is due to the presence of both hydrophobic and hydrophilic groups in each surfactant molecule. It is the formation of micelles in solution which gives surfactants their excellent detergency and solubilization properties. Hence, the most important parameter in terms of the ability of a surfactant to mobilize or solubilize hydrophobic contaminants in contaminated soil is the surfactant Critical Micelle Concentration (CMC). In general, concentrations of surfactant in soil-water below the CMC have little or no effect on solubilization of hydrophobic materials. Only when micelles are present does significant desorption of such pollutants from soil surfaces occur (Haigh, 1996).
Nonionic surfactants are often used because of their lower CMCs as compared to ionic surfactants, their higher degree of surface-tension reduction and their relatively constant properties in the presence of salt, which result in better performance and lower concentration requirements. In particular, the non-ionic ethoxylate surfactants have been suggested for the removal of organic contaminants from soil because of their high solubilization capacity and biodegradability (Paria and Yuet, 2007). However, some concerns with these surfactants are their significant loss to soil and partitioning to organic phase during practical applications (Zhao et al., 2006, 2007). Nevertheless, under some conditions, usually at concentration well below CMC, the adsorption of these surfactants to soil can enhance the adsorption of hydrophobic contaminants to soil. This has been attributed to partition of hydrophobic contaminants into surfactant hemi-micelle formed on soil surface (Edwards et al., 1994; Sun et al., 1995; Haigh, 1996).
Anionic surfactant on the other hand, sorb less to soil but they form micelles at higher concentrations in aqueous solutions than nonionic surfactants with an equivalent hydrophobic group (Rosen, 2004) and are more prone to precipitate in presence of multivalent cations (Ca++, Mg++). Substantial loss of surfactant by such mechanisms will definitely reduce their active concentration in aqueous solution, which would greatly reduce the surfactant solubilization and flushing efficiency.
The interaction between surfactants in mixtures can produce marked interfacial effects due to change in adsorption as well as in the charge density of the surface. In most cases, when different types of surfactants are purposely mixed, what is sought is synergism, i.e., the condition when the properties of the mixture are better than those attainable with the individual components by themselves. Surfactant mixtures of practical interest include like-charge surfactants, for instance mixtures of anionic surfactants, or mixtures of cationic surfactants, but the more common case involves mixture of ionic and nonionic surfactants (Wang and Kwak, 1999). Mixing of cationic and anionic surfactants often tends to form precipitates (Joshi et al., 2005; Zhang and Somasundaran, 2006), consequently they are of no practical value. Similarly, mixtures of nonionic surfactants tend to behave ideally (Joshi et al., 2005).
A typical feature of the adsorption of ionic (anionic/cationic)-nonionic mixtures is the synergy or anti-synergism (antagonism) at interfaces. For example, the adsorption of one surfactant is either enhanced or retarded by the addition of a small amount of the other surfactant. Furthermore, mixing anionic and nonionic surfactants may raise or lower the CMC from that obtained by ideal mixing.
This study deals with an experimental study on the interaction and micellization of nonionic surfactant, Triton X 100 (TX100), with anionic surfactant, Sodium Dodecyl Sulphate (SDS). The CMC values of the corresponding binary mixtures in the whole range of composition were obtained by both surface and interfacial tension and then the result have been analyzed in terms of interaction parameter,β , using Rubingh’s regular solution theory.
Surfactant binary mixtures of nonionic surfactant, TX100 and anionic surfactant, SDS or SDBS (sodium dodecylbenzene sulfonate) have been studied (Janczuk et al., 1995; Zhu and Feng, 2003; Zhou and Zhu, 2004; Owoyomi et al., 2005; Zhao et al., 2005; Joshi et al., 2005; Yang et al., 2005; Zhao et al., 2006, 2007). The Critical Micelle Concentrations (CMCs) of mixed surfactants were sharply lower than that of sole anionic surfactant (SDS or SDBS). With the increase in mole fraction of TX100, the CMCs decrease continuously from the CMC of pure SDS/SDBS down to the CMC of pure TX100. Furthermore, the experimental CMCs were lower than the ideal CMCs. It is noteworthy however, to point out that the CMC measurements and synergism calculations in these studies were taken from surface tension measurements.
Recently, mixed surfactant was found superior to the relevant single ones mainly due to the reduction in nonionic surfactant partition and/or sorption to soil as well as the high solubilization capacity of the mixture. Yang et al. (2005) showed that the amounts of both TX100 and SDBS sorbed to Ca-montmorillonite are significant. However, the amount of either TX100 or SDBS sorbed can be decreased and minimized when they are mixed with each other. Furthermore, the extent of TX100 partitioning into the oil phase was found to decrease if the amount of SDBS increased (Zhao et al., 2006, 2007). Decreasing loss of surfactant due to partitioning and/or sorption and the greater apparent solubilization of the mixture will reduce surfactants volumes needed and thus the capital expenditure and operation cost (Zhao and Zhu, 2006).
Since, TX100 is the surfactant with the lower CMC its contribution in the mixed micelle formation is important, particularly if its composition in the aqueous phase is lower than the other surfactant with the large CMC (ex. SDS). It is expected that probable loss of TX100 monomers in aqueous phase solution upon equilibrium with soil (through adsorption mechanism) or in presence of oil phase (through partition mechanism) will affect greatly the degree of synergism in mixed micelle formation and may render the mixture exhibiting less synergism.
Accordingly, it becomes important to investigate whether the synergism of both surfactants still holds after equilibration with soil and/or contact with oil phase. In this study, it is intended to use CMCs as measured from interfacial tension trends to probe the state of interaction after equilibration with soil and/or contact with oil phase. Comparison with CMCs measured with surface tension trends is provided.
MATERIALS AND METHODS
Surfactants
Triton X-100 (TX100) extra pure, was purchased from Scharlau Chemie,
Spain. Sodium Dodecyl Sulfate (SDS) was obtained from Merck with a high
grade of purity (99%). The oil, Sarapar147, was supplied by Kota Minerals
and Chemical Sdn. Bhd. (KMC). Sarapar147 is a colourless mineral oil ranging
from C14 to C17 and is derived from petroleum crude oil. All chemicals
were used as received without further purification. Selected physicochemical
properties of the compounds are presented in Table 1.
Soil Surrogate Samples
Samples were collected from an outcrop of a local shale formation (Batu
Arang, Selangor, Malaysia). Rock samples were disintegrated into small pieces
by a jaw crusher and then ground using rock pulverizer (Fritsch, Germany). Rock
samples were air dried for 24 h followed by oven drying at 105°C for 24
h. The density of shale was measured as 2.1 g cc-1 using a calibrated
50 mL pycnometer. Dried rock samples were sieved to obtain particles less than
2 mm and larger than 1 mm in all experiments.
Preparation of Surfactant Solutions
The surfactant solutions were prepared in a standard 1000 mL volumetric
flasks. Surfactants were weighed on mass basis and emptied into the volumetric
flask and then double distilled water was used to complete the solution
to the final weight (1 kg). After the preparation of the stock solution,
it was diluted to obtain desired concentration.
| Table 1: | Physicochemical properties of chemicals |
| aMolecular weight, bCritical micelle concentration, cHydrophile-lipophile balance, dDensity, eBoiling point, $Zhou and Zhu (2004), #Provided by company; NA: Not Available | |
TX100 and SDS solutions were prepared at concentrations ranges from 0.0025 to 1 wt.% corresponding to molar concentrations of 0.039-15.47 mM for TX100 and 0.0867-34.68 mM for SDS. Mixed surfactant solutions were prepared by mixing SDS and TX100 solutions of the same weight concentrations with different volume ratios (1:2, 1:1 and 2:1 SDS:TX100). This results in a mole fraction of TX100 in the total mixtures of 0.47, 0.31 and 0.18, respectively. Mixed surfactant solutions were allowed to equilibrate for at least 5 h before any measurements were made.
Surface Tension Measurements
The surface tension technique was applied to determine the CMC in various
combinations of shale and/or surfactant systems. The surface tension measurements
were carried out with Kruss tensiometer (Kruss GmbH, Hamburg, Instrument Nr,
K6) using a platinum-iridium ring at constant temperature (25±1°C). The
tensiometer was calibrated using method described in ASTM Designation: D1331-89.
Surface and interfacial tension measurements were undertaken according to the
method described in ASTM Designation: D1331-89.
Kruss tensiometer operates on the Du Nouy principle, in which a platinum-iridium ring is suspended from a torsion balance and the force (in mN m-1) necessary to pull the ring free from the surface film is measured. Surface tension value was taken when stable reading was obtained for a given surfactant concentration, as indicated by at least three consecutive measurements having nearly the same value.
Interfacial Tension Measurements
Equal volume (15 mL) of Sarapar147 and surfactant solution was poured
into a glass beaker of 6 cm diameter and the resulting mixture used for
the interfacial tension studies. The same procedure used for the surface
tension measurement was used for the interfacial tension study except
that the balance of the tensiometer reading for zero was checked with
the platinum-iridium ring completely immersed in the surfactant solution
phase and not in the surface or interface of Sarapar147-surfactant. Hence,
the platinum ring must be completely immersed in the surfactant phase
before the platform is gradually adjusted until a force necessary to detach
the platinum ring upward from the surfactant-oil interface is exerted.
CMC Measurements
The CMC values were obtained through a conventional plot of the surface/interfacial
tension versus the surfactant concentration. The CMC concentration corresponds
to the point where the surfactant first shows the lowest surface/interfacial
tension. The surface/interfacial tension remains relatively constant after
this point.
Adsorption to Shale
Adsorption isotherms were determined by batch equilibrium adsorption procedures.
Ten gram of shale were added to a set of 60 mL surfactant solutions (surfactant
initial concentrations spans from 0.0025 to 1 wt.%) in a 100 mL glass vials
(1:6 w/v ratio) and allowed to equilibrate at 25± 1°C. The surfactant
solution-to-soil ratio was designated as 6:1 (v/w) to reach the optimal washing
performance (Chu and Chan, 2003; Urum
and Pekdemir, 2004). The vials were then agitated on a gyratory shaker at
100 rpm (wrist orbital shaker) for 3 h and allowed to rest for 16 h. Surfactant
sample aliquots were taken for surfactant concentration determination before
and after adsorption. To determine the maximum sorption of surfactants into
shale, a surface/interfacial tension method was used. Each adsorption experiment
involved 10 batch test samples in 100 mL glass vials. The amount of surfactant
adsorbed and/or abstracted was computed from the difference of CMC values before
and after adsorption. All experiments were conducted with 3 replicates at 25±
1°C and means of three replicates were used.
RESULTS AND DISCUSSION
Theory of Mixed Micelles
Surface tension measurements and Rubingh’s regular solution theory have
been proven to be remarkably successful in expressing the characteristics of
mixed surfactant solutions used to investigate the micelle formation of mixed
nonionic-anionic surfactants. The condition for synergism in surface tension
reduction efficiency is fulfilled when the total concentration of the mixed
surfactant required to reduce the surface tension of the system to a given value
is less than that of either individual surfactant. Rosen (1986)
have reached similar analogous equations for the presence of another hydrocarbon
liquid, i.e., use of interfacial tension trends for CMC measurement and examination
of synergism in mixed monolayer and mixed micelle formation.
In an ideal mixed system, the ideal CMC of mixed surfactant solutions at any mole ratio of SDS to TX100 can be predicted with ideal solution theory (Rosen, 2004):
|
|
(1) |
where, α is the mole fraction of TX100 in the mixed solutions and thus (1-α) is the mole fraction of SDS. Due to the interaction between TX100 and SDS in mixed solutions, the experimental CMC of a mixed surfactant solution (CMCexp) is always different from CMCideal. According to nonideal solution theory of Rubingh (1979), the deviation of CMCexp from CMCideal can be represented by the parameter β , which also represents the interaction between TX100 and SDS in mixed solutions. The more negative β -value of the mixed system indicates the stronger attraction and synergism between the TX100 and SDS. Synergism in mixed micelle formation exists when the CMC of a mixture is less than that of individual surfactants among the mixture. The conditions for synergism to exist in the mixture are as follows: (a)β must be negative; (b) |β|>|ln(CMCTX/CMCSDX)| (Rosen, 2004; Zhou and Zhu, 2005; Rosen, 1986; Rubingh, 1979). A negative value of β , indicates synergism in mixed micelle formation. A positive value indicates antagonism and if β = 0 then mixed micelle formation is ideal. β can be calculated as follows (Rubingh, 1979):
(2) |
where,
(3) |
Rubingh’s equation was solved iteratively to obtain the value of
(4) |
(5) |
where, fTX and fSDS are the activity coefficients of surfactants in mixed micelle. In case of ideal behaviour, fTX = fSDS =1 and hence Eq. 4 reduces to Eq. 1.
| Fig. 1: | Mixing effect on CMCs before equilibration with shale |
Evaluating Synergism Using Surface Tension
Data Before Equilibration with Shale
Figure 1 shows surface tension trends (STs) and depicts
the surface tension curves as total surfactant concentration for the mixed SDS-TX100
system in which the molar fractions of TX100 were 0, 0.18, 0.31, 0.47 and 1,
respectively. The two outer surface tension curves are for the pure components.
Surface tension between water and air was measured as 73 mN m-1.
As the surfactant solution is introduced, this value was reduced as shown in
Fig. 1. Surface tension is concentration dependent, i.e.,
as the surfactant concentration increases, surface tension decreases until the
surfactant CMC value is reached and remains constant there after wards. CMC
of pure surfactants obtained were compared with those in literature. CMC values
of SDS obtained from both surface tensions vs. concentration are similar at
0.1 wt.% (3.468 mM L-1 or 1000 mg L-1). CMC of SDS compares
well with those reported by Zhu and Feng (2003), Zhou
and Zhu (2004, 2007) and Zhao
et al. (2005).
The CMC value of TX100 obtained by surface tension measurements was about 0.025 wt.%, much lower than that obtained by SDS. This value is comparable well with values reported in previous published studies (Zhu and Feng, 2003; Zhou and Zhu, 2004, 2007; Zhao et al., 2005).
This is because anionic surfactants normally possess higher CMCs than nonionic surfactants. For nonionic surfactants, aggregation or micellization is mainly due to hydrophobic interaction among hydrocarbon chains. This is more feasible because the hydrophobic groups are easily separated from the aqueous environment, whereas for ionic surfactants, high concentrations are necessary to overcome the electrostatic repulsion between ionic head groups during aggregation (Joshi et al., 2005).
| Table 2: | Synergism data for surfactants before equilibration with shale |
Referring to Fig. 1, there is neither surface tension value nor CMC value for the mixture lower than that obtained by TX100. This is in agreement with Mata (2006), Janczuk et al. (1995) and Paria et al. (2003). The critical micelle concentrations of mixed surfactants are lower than that of sole SDS and very close to that of pure TX100. In this respect, this experimental results for fresh solutions agrees with those observed by Mata (2006), who found a decrease in CMC of mixture with increase in mole fraction of TX100, however, the CMC of the mixed system at any composition could not be reduced than that of pure TX100.
Data for all systems obtained from the surface tension measurements are shown in Table 2 along with interaction parameters. It can be said that before equilibration with shale, synergism was found between all mixtures (Table 2). As shown in Table 2A the experimental CMCs for mixtures are lower than those obtained by ideal mixing. β -values are negative as shown in Table 2B showing interaction between mixed surfactants. As mentioned before, the β -values demonstrate the extent of interaction between the two surfactants, which leads to deviation from the ideal behaviour. The larger the value of β , the stronger the mixing nonideality and the corresponding synergism.
Therefore, a bit stronger synergism is found for the lower TX100 mole fraction mixture as indicated in Table 2B. Yang et al. (2005) studied synergism of TX100 and SDBS at comparable mass ratios and found an opposite trend, i.e. the interaction increases with increasing the mass ratio of nonionic surfactant to anionic. At TX100:SDBS mass ratios of 3:7, 5:5 and 7:3, Yang et al. (2005) found respective interaction parameters 0f -2.93, -3.14 and -3.67. Zhao et al. (2006) studied the same surfactants at the same mass ratios and found interaction parameter values of -3.03, -2.89 and -3.35, respectively which do not follow the same trend. Mata (2006) found the interaction parameter for TX100 mole fractions of 5, 2 and 1 in the solution mixture to be -3.97, -4.22 and -3.10, respectively. Generally, the synergistic behavior can be explained assuming that in the mixed micelle ethoxylated chains of the nonionic surfactant coil around the charged head groups of the anionic surfactant, screening the electrostatic repulsions and thereby favoring micelle formation resulting in lower CMCs (Joshi et al., 2005).
The calculated values of the mole fraction of surfactants in the mixed micelle
| Fig. 2: | Mixing effect on CMCs after equilibration with shale |
The activity coefficient fTX and fSDS are also shown
in Table 2B. As mentioned earlier, the activity coefficient
shows the effect and contribution of individual component in mixed micelles,
hence necessary to be calculated. The much lower mole fraction
However, Rubingh (1979) and Rosen (1986, 2004) had indicated another condition for synergism in CMCs which must be fulfilled, i.e., |β |>|ln(CMCTX/CMCSDS)|. As shown in Table 2B, the second condition of synergism in CMCs is fulfilled for mixture systems with lower TX100 mole ratio (0.31 and 0.18), which showed higher synergism. However, the synergism is not fulfilled for the mixture with higher TX100 mole ratio, α TX100 = 0.47.
After Equilibration with Shale
Figure 2 shows surface tension curves of SDS-TX100 mixtures
after equilibration with shale. It is obvious that the mixture at all SDS molar
fractions did improve behavior of TX100 surfactant. The CMC of pure TX100 have
been notably reduced, i.e., all mixtures’ CMCs are similar to pure SDS CMC at
0.1 wt% and less than that of pure TX100 CMC at 1.5 wt.%. It can be said that
the presence of SDS (at molar ratios used here) did reduce sorption of TX100
to shale. This is comparable to the observations made by other researchers (Zhou
and Zhu, 2007; Yu et al., 2007). A reduction
in TX100 sorption to soil was found and hence, a reduction in CMCs of mixtures
after equilibration with soil from that of pure TX100. As shown in Table
3A, the CMCs of SDS: TX100 mixtures are less than those obtained from ideal
mixing theory. Hence synergism is maintained with regard to critical micelle
concentration even in presence of shale. β
-values are again negative as shown
in Table 3B but not as negative as the previous case indicating
less interaction and synergism. Contrary to the previous case, stronger synergism
is observed for mixtures with higher TX100 mole fraction indicating that shortage
in TX100 monomers is the cause behind decrease in synergism and interaction.
| Table 3: | Synergism data for surfactants after equilibration with shale |
Similar to the case before equilibration with shale, a deviation from the ideal
behavior can also be observed in Table 3B. For all the mixed
systems, the mole fraction of TX100 in mixed micelle,
Evaluating Synergism Using Interfacial Tension Data
Before equilibration with shale and at sub-CMC concentrations, interfacial
tension values for mixtures are similar or slightly lower than individual
surfactants (Fig. 3). However, their CMCs are lower
(0.02 wt.%) than those of individual surfactants (0.1 wt.%). After equilibration
with shale, CMC values for surfactants mixtures are closer to that of
pure SDS at 0.1 wt.% (Fig. 4).
CMC of SDS has not been changed before and after equilibration with shale indicating minimum losses of the surfactant. However, the TX100 CMC has been increased significantly at 0.25 wt.%. This significant increase in TX100 CMC is mainly due to a remarkable loss of surfactant monomers (13.5 g TX100) by both adsorption to shale and partition into the oil phase.
After equilibration with shale and in presence of oil phase, SDS-TX100 mixtures experienced low CMCs compared to TX100 revealing minor losses of mixed monomers by both mechanisms (sorption and partition). It is clear that the addition of SDS has reduced the amount of TX100 adsorbed to shale or partitioned into the oil phase. It is widely accepted among researchers that SDS surfactant is less likely to adsorb to shale and far less likely to partition into an oil phase. However, TX100 surfactant is more liable to sorb onto shale and partition into oil phase (Harusawa et al., 1980; Rosen, 2004; Zhao and Zhu, 2006; Zhao et al., 2006, 2007). Mixing SDS with TX100 may therefore retard the affinity of TX100 to sorb onto shale or partition into oil phase. It is needed to point out that sorption and/or partitioning of any surfactant proceeds through the sorption and/or partitioning of surfactant monomers and micelle formation limits surfactant adsorption and partitioning, i.e., the micelles are not directly sorbed or partitioned (Harusawa et al., 1980).
| Fig. 3: | Mixing effect on CMCs before equilibration with shale |
| Fig. 4: | Mixing effect on CMCs after equilibration with shale |
In mixed surfactant solution, the formation of mixed micelles affect the CMC, i.e., mixing will result in a lower CMC. This will reduce the monomer concentration of component surfactant in mixed solution and hence their sorption onto shale and partition to oil phase (Zhou and Zhu, 2007).
Similar results have been found by other researchers, i.e., the sorption of nonionic surfactant was strongly restricted with the presence of anionic surfactant in the mixed micelle. Yang et al. (2005) showed that the amount of TX100 was decreased when it was mixed with anionic surfactant, Sodium Dodecyl Benzene Sulfate, SDBS.
| Table 4: | CMCs obtained from IFT measurements |
| Table 5: | Synergism parameters from IFT data |
Zhao et al. (2006, 2007) concluded that the extent of TX100 partitioning into the oil phase decreased when the amount of SDBS increased. Therefore, it can be said that the presence of SDS significantly reduced adsorption and partitioning of TX100 to shale and oil phase.
As shown in Table 4, the CMCs of surfactant mixture before and after equilibration with shale are significantly lower than those obtained from ideal mixing theory. Hence, synergism is clear with respect to interfacial tension in presence and absence of shale.β -values are negative and the synergism is stronger before equilibration with shale than after equilibration with shale (Table 5).
The values of the mole fraction of surfactants in the mixed micelle
Before equilibration with shale, the mole fraction
CONCLUSION
The experimental data from surface tensions of solutions before equilibration with shale showed that CMCs of mixed surfactants are close to that of pure TX100 and much lower than that of pure SDS. However when using interfacial tension data to probe mixture interaction after contact with oil phase, the CMCs were close to SDS’s CMC and are far lower than TX100’s CMC. The CMCs of mixed surfactants as obtained from surface/interfacial tension are lower from those of ideal mixing theory, hence synergism was found.
After equilibration with shale, CMCs of mixtures as indicated by both surface and interfacial tension data were very close to that of pure SDS but much lower than that of pure TX100. More importantly, mixture CMCs are lower than those of ideal mixing theory and so synergism was obtained. However, the synergism was less significant than the one obtained in the case before equilibration with shale. For the interfacial tension data, β -values ranged from -1.286 to -2.045 after equilibration with shale meanwhile, β -values ranged from -5.803 to -5.917 before equilibration with shale. The weight of current evidence suggested that the mixture with higher TX100 mole ratio exhibited more synergism than others particularly after equilibration with shale and/or contact with oil phase. Furthermore, considering interfacial tension data to predict nonionic-anionic mixture interaction and degree of synergism in real environments are more reasonable since it takes into account probable loss of surfactant to either soil or oil phase.
ACKNOWLEDGMENT
This research was financially supported by UTM (University Teknologi Malaysia).