Conductometric and fluorometric investigations on the mixed micellar systems of cationic surfactants in aqueous media

Conductometric and fluorometric investigations on the mixed micellar systems of cationic surfactants in aqueous media

Journal of Colloid and Interface Science 304 (2006) 491–496 www.elsevier.com/locate/jcis Conductometric and fluorometric investigations on the mixed ...

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Journal of Colloid and Interface Science 304 (2006) 491–496 www.elsevier.com/locate/jcis

Conductometric and fluorometric investigations on the mixed micellar systems of cationic surfactants in aqueous media Sarah E. Moore, Michael Mohareb, Stephanie A. Moore, Rama M. Palepu ∗ Department of Chemistry, St. Francis Xavier University, Antigonish, Nova Scotia, B2G 2W5 Canada Received 17 July 2006; accepted 10 September 2006 Available online 15 September 2006

Abstract Micellar properties of binary mixtures of hexadecyldiethylethanolammonium bromide surfactant with tetradecyldimethylammonium, trimethylammonium, triphenylphosphonium, diethylethanolammonium, and pyridinium bromide surfactants have been characterized employing conductometric and fluorescence techniques. The critical micelle concentration (cmc∗ ) and the degree of counter-ion binding values (δ) of the binary systems were determined from the conductivity measurements. The results were analyzed in light of various existing theories to calculate micellar composition, activity coefficients, and the interaction parameter (β). Partial contribution of each surfactant, cmc∗1 , cmc∗2 , to the overall cmc∗ value was also evaluated. Aggregation numbers and micropolarity of the mixed micelles were determined from fluorescence measurements. The results were discussed in terms of synergetic interactions in these systems on the basis of the head group/head group and tail/tail interactions and the counter-ion binding. © 2006 Elsevier Inc. All rights reserved. Keywords: Cationic surfactants; Conductivity; Mixed micelles; Interaction parameters; Activity coefficients

1. Introduction Mixed surfactant systems are preferably used instead of single surfactant systems for industrial applications such as detergents, cosmetics and pharmaceuticals. It is well known that when compared to single surfactant systems, mixed surfactant systems have the ability to provide better performance [1–4]. Because of the presence of more than one species, it is not necessary that the surfactants in use are of high purity, and as a result, the mixed system is far less expensive. For this reason, mixed surfactant systems are widely used in industry [5,6]. Many investigations have been carried out in the literature on binary surfactant systems [7–17]. The focus has been on the overall effect of the surfactant combination and the determination of the critical micelle concentration (cmc∗ ), the degree of counter-ion binding, and to analyze the results in terms of well known theories to determine the composition and the nature of interactions of individual monomers of the two surfactants in * Corresponding author. Fax: +1 902 867 2414.

E-mail address: [email protected] (R.M. Palepu). 0021-9797/$ – see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2006.09.019

the mixed micelle [1,4]. In comparison, there is relatively little information presented in literature about the contribution of each surfactant in the monomeric and micellar phases of the mixed system. Recently Junquera and Aicart [18–20] investigated the partial contribution of cmc∗1 and cmc∗2 of the surfactants in mixed systems to the monomeric phase cmc∗ and micellar phase through aggregation number Ni∗ . The authors focused on synergistic interactions of near ideal systems using surfactants with equally sized chain lengths and very similarly structured head groups. In the present study, a total of six systems were investigated using tetradecyl and hexadecyl species as the binary components, employing both conductometric and fluorometric techniques. In five of the six systems, the hexadecyldiethylethanolammonium bromide (C16 DEEA), was investigated in combination with five tetradecyl species with different head groups namely: diethylethanolammonium bromide (C14 DEEA), pyridinium bromide (C14 PyBr), triphenylphosphonium bromide (C14 PPh3 Br), dimethylammonium bromide (C14 DABr) and trimethylammonium bromide (TTAB). The sixth system studied employed tetradecyltriphenylphosphonium bromide (C14 PPh3 Br) and tetradecylpyridinium bromide

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(C14 PyBr). These systems were chosen to study the effects of head group/head group as well as tail/tail interactions. Micellar properties were investigated as a function of concentration and composition using conductometric techniques. The contribution of each surfactant to the mixed micelles in terms of cmc∗1 and cmc∗2 to the cmc∗ of the mixture, and N1∗ and N2∗ to N ∗ where the subscripts 1 and 2 denote the C14 and C16 components in the mixed system, respectively, were evaluated employing the procedure out lined by Janquera and Aicart [18]. 2. Materials and methods 2.1. Materials The tetradecyltriphenylphosphonium bromide surfactant (C14 PPh3 Br) was obtained by Lancaster chemicals (England) and used as received. Tetradecyltrimethylammonium bromide (TTAB) (99% pure) and the fluorescence probe pyrene (99% optical grade) were obtained from Sigma–Aldrich. The pyrene was purified by repeated crystallization, followed by sublimation. The diethylethanolamine surfactants (C16 DEEA, C14 DEEA), tetradecyldimethylammonium bromide (C14 DAB) and tetradecylpyridinium bromide (C14 PyBr) were the same samples that were used in our previous investigations [21–23]. 2.2. Conductivity measurements Conductivity measurements were carried out on a CDM 83 conductivity bridge. The cell constant was 1.01 cm−1 and the operating frequency of the conductivity bridge was 1000 Hz. The conductivity experiments were taken at a constant temperature maintained within ±0.1 ◦ C. Temperature control was maintained by placing the solution to be studied in a jacketed beaker and allowing water to circulate through the beaker. All solution preparation was done using triply de-ionized water. Each conductivity measurement was estimated to have an error of ±0.5 µS cm−1 . In the present study, two different conductometric methods were employed. The first method was carried out to determine the cmc∗ of the mixed system, and involved measuring the conductivity as aliquots of the mixed surfactant system were added to triply de-ionized water. In Method II, a solution of the tetradecyl component in the premicellar range was placed in the jacketed beaker, and its concentration was taken as cmc∗1 . The tetradecyl solution was titrated with an aliquot of hexadecyl species that was dissolved in the same concentration of tetradecyl surfactant solution, and conductance was measured after each addition. The break point in the titration curve was taken as cmc∗2 .

1 × 10−6 M. The experimental details were the same as reported in earlier publications [24–27]. The I1 /I3 ratios were also taken in the absence of quencher to determine the micropolarity of the systems. 3. Results and discussion 3.1. Conductivity Conductance was measured for each system at a variety of bulk mole fractions (α) and cmc∗ values were determined for each of these mole fractions. A depiction of a typical Method I conductivity plot is given in Fig. 1 for the C14 DABr and C16 DEEA system at various mole fractions (α) of C14 DABr. Similar plots were generated for each of the six systems studied, varying the composition of the C14 component. From these conductivity plots, the degree of counter-ion binding (δ) was obtained from (δ = 1 − s2 /s1 ), where s1 and s2 are the pre- and post-micellar slopes [28,29]. The cmc∗ and counter-ion binding values determined for each system are presented in Table 1, and the uncertainties for these values are estimated to be less than ±2 × 10−3 and ±3 × 10−3 , respectively. In all systems where C16 DEEA was used as one of the components, the counter-ion binding was found to decrease as the amount of C14 in the mixed micelle increased, thus indicating a decrease in the head group repulsions, and thereby leading to an increase in the stabilization of the micelle. The Gibbs energy of micellization (G0mic ) was calculated using Eq. (1) based on the pseudophase separation model [30] Gmic = (1 + δ)RT ln Xcmc ,

(1)

where Xcmc is the cmc∗ expressed on the mole fraction scale and the G0mic values are referred to in Table 1. Conductivity plots employing Method II are given in Fig. 2 for the TTAB and C16 DEEA system. The break in each of these plots is taken to be the cmc of the hexadecyl component in the system, that is, cmc∗2 . From the value of the cmc∗1

2.3. Fluorescence measurements Fluorescence experiments were carried out with a JY Horiba Spex Fluoromax-3 fluorometer to determine the aggregation numbers of the micelles in each system. The total concentration of surfactant was of the order of 50 mM and pyrene, used as a fluorescence probe, was maintained at a concentration of

Fig. 1. Plot of specific conductance, κ, as a function of total surfactant concentration, [S]tot at 298.15 K, at various values of mole fraction, α1 , for the mixed system C14 DABr (1) and C16 DEEA (2). (Q) α1 = 0.1; (") α1 = 0.3; (2) α1 = 0.5.

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Table 1 Values of experimental mole fraction, α1 , experimental mixed micellar critical concentration, cmc, counter-ion binding, δ, and G of micellization for each of the C14 (1) and C16 (2) systems System C16 DEEA

α1

cmc (mM)

δ

Gmic (kJ/mol)

+ C14 PyrBr

0 0.10 0.30 0.50 0.70 0.90 1.00

0.91 0.61 0.67 0.78 1.19 2.00 2.83

0.66 0.63 0.56 0.41 0.40 0.51 0.76

−45.3 −46.1 −43.8 −39.0 −37.3 −38.3 −43.1

+ C14 DABr

0 0.10 0.30 0.50 0.70 0.90 1.00

0.91 0.69 0.78 0.97 1.16 1.66 2.74

0.66 0.64 0.56 0.52 0.43 0.50 0.71

−45.3 −45.9 −43.2 −41.2 −38.2 −38.7 −42.0

+ C14 PPh3 Br

0 0.05 0.10 0.30 0.50 0.70 0.90 1.00

0.91 0.62 0.59 0.51 0.46 0.40 0.44 0.57

0.66 0.65 0.59 0.56 0.49 0.40 0.40 0.44

−45.3 −46.6 −45.1 −44.8 −43.2 −41.1 −40.7 −40.9

+ TTAB

0 0.10 0.30 0.50 0.70 0.90 1.00

0.91 0.67 0.75 0.97 1.22 1.85 3.60

0.66 0.66 0.55 0.47 0.39 0.30 0.76

−45.3 −46.6 −43.1 −39.9 −37.0 −33.2 −42.0

+ C14 DEEA

0 0.10 0.30 0.50 0.70 0.90 1.00

0.91 0.69 0.76 0.94 1.20 1.71 3.00

0.66 0.64 0.57 0.51 0.43 0.52 0.70

−45.3 −45.9 −43.6 −41.1 −38.0 −39.1 −41.4

0 0.125 0.25 0.4 0.5 0.6 0.75 0.875 1

0.56 0.66 0.69 0.72 0.73 0.81 1.37 1.65 2.83

0.44 0.43 0.41 0.44 0.45 0.42 0.37 0.32 0.76

−41.0 −40.2 −39.4 −40.1 −40.4 −39.2 −36.0 −34.1 −43.1

C14 PPh3 Br + C14 PyrBr

and cmc∗2 , the value of cmc∗ = cmc∗1 + cmc∗2 for each system was calculated (Table 2) and plotted for the C14 DABr and C16 DEEA system as a function of overall bulk composition (Fig. 3). Since cmc∗1 and cmc∗2 values could not be obtained by Method I, theoretical values were found based on the relationship between cmc∗ and cmc∗1 in Method II. The relationship was used to extrapolate theoretical cmc∗1 and cmc∗2 values based on the cmc∗ values obtained by Method I. These values correspond to the open points shown in Fig. 3. It was

Fig. 2. Plot of specific conductance, κ, as a function of concentration of surfactant added at 298.15 K, at various fixed values of the premicellar TTAB (1) + C16 DEEA (2): (Q) 0.996 mM; (2) 0.498 mM; (!) 0.797 mM; (") 0.996 mM.

Fig. 3. Plot of cmc∗1 (triangles), cmc∗2 (squares), and cmc∗ (circles) as a function of mole fraction, α1 , for the mixed systems C14 DABr (1) and C16 DEEA (2): (open symbols) Method I; (solid symbols) Method II. The dotted line is cmcttl from Clint’s model.

found that Method II was applicable only if, after extrapolation from Method I data, the cmc∗1 and cmc∗2 could be fit onto the same plot for Method II within reasonable deviation. 3.2. Surfactant–surfactant interactions The experimental cmc∗ values were analyzed in terms of Clint’s model [31]. This relationship is given by the expression  αi 1 (2) = , cmc∗ cmci i

where cmci represents the cmc values of the pure components (i = 1 or 2) and αi is the mole fraction of component i in the mixture. The cmc∗ represents the calculated value of the binary mixture assuming ideal behavior. A typical plot of calculated and experimental cmc values indicating non-ideal behavior on mixed micelle formation can be seen in Fig. 3 as the dotted line. Similar behavior is also observed for all six systems studied.

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Table 2 Values of cmc∗1 , cmc∗2 , mixed micellar critical concentration, cmc∗ , and the calculated bulk mole fraction, α1 , as obtained from Method II for C14 (1) and C16 (2) system each

Table 3 Composition of mixed micelles (X1 ) the interaction parameters, β, activity coefficients, (f ) and excess free energy of mixing (GE ) obtained by Rubingh’s method

System C16 DEEA

cmc∗1 (mM)

cmc∗2 (mM)

cmc∗ (mM)

α1

System C16 DEEA

α1

χ1

+ C14 PyrBr

0 0.10 0.25 0.40 0.60 0.80 0.90 1.20 2.83

0.91 0.66 0.59 0.52 0.48 0.39 0.32 0.22 0

0.91 0.76 0.84 0.92 1.08 1.19 1.22 1.43 2.83

0 0.13 0.30 0.44 0.55 0.67 0.74 0.84 1.00

+ C14 PyrBr

0 0.10 0.30 0.50 0.70 0.90 1.00

0 0.24 −4.11 0.25 0.31 −3.07 0.31 0.38 −2.61 0.39 0.46 −1.52 0.49 0.69 −0.77 0.79 1.00 βave = −2.42

0 0.10 0.20 0.40 0.60 0.80 1.00 1.50 2.00 2.50 2.74

0.91 0.66 0.61 0.52 0.46 0.35 0.34 0.17 0.12 0.03 0

0.91 0.76 0.81 0.92 1.06 1.15 1.34 1.67 2.12 2.53 2.74

0 0.13 0.25 0.43 0.57 0.70 0.75 0.90 0.94 0.99 1.00

0 0.10 0.30 0.50 0.70 0.90 1.00

0 0.21 −3.37 0.27 0.28 −2.28 0.34 0.35 −1.63 0.42 0.46 −1.55 0.55 0.65 −1.50 0.78 1.00 βave = −2.07

0 0.05 0.10 0.20 0.25 0.40 0.57

0.91 0.57 0.46 0.27 0.19 0.03 0

0.91 0.62 0.56 0.47 0.45 0.44 0.57

0 0.08 0.18 0.42 0.56 0.92 1.00

0 0.10 0.30 0.50 0.70 0.90 1.00

0 0.30 −2.15 0.08 0.45 −1.71 0.11 0.56 −1.75 0.26 0.65 −2.28 0.43 0.78 −2.43 0.59 1.00 βave = −2.06

0 0.10 0.25 0.50 0.80 1.00 1.60 2.00 3.60

0.91 0.67 0.59 0.51 0.42 0.36 0.21 0.14 0

0.91 0.77 0.84 1.01 1.22 1.35 1.80 2.14 3.60

0 0.13 0.30 0.50 0.65 0.74 0.89 0.93 1.00

0 0.10 0.30 0.50 0.70 0.90 1.00

0 0.21 −3.89 0.21 0.28 −2.88 0.27 0.33 −2.03 0.33 0.43 −1.87 0.45 0.61 −1.77 0.68 1.00 βave = −2.49

0 0.10 0.25 0.50 1.00 1.50 2.00 2.50 3.00

0.91 0.66 0.64 0.51 0.36 0.19 0.11 0.03 0

0.91 0.76 0.89 1.01 1.36 1.70 2.11 2.53 3.00

0 0.13 0.28 0.49 0.74 0.89 0.95 0.99 1.00

0 0.10 0.30 0.50 0.70 0.90 1.00

0 0.21 −3.46 0.25 0.28 −2.56 0.32 0.35 −1.90 0.39 0.45 −1.58 0.51 0.64 −1.64 0.74 1.00 βave = −2.23

0 0.25 0.40 0.50 0.60 0.75 0.875 1.00

0 0.09 −0.45 0.69 0.21 −1.11 0.50 0.28 −1.50 0.46 0.33 −1.47 0.52 0.39 −0.16 0.94 0.57 −0.56 0.90 1.00 βave = −0.87

+ C14 DABr

+ C14 PPh3 Br

+ TTAB

+ C14 DEEA

The experimental cmc∗ values were found to be lower than the calculated (ideal) values indicating non-ideal behavior. In order to account for the deviation from ideal behavior, the data was analyzed employing Rubingh’s procedure [32–34] based on regular solution theory. From the analysis, the composition of the mixed micelles (X1 ), the interaction parameter (β), activity coefficients (fi ) and the excess free energy of mixing (Gexcess ) were obtained and presented in Table 3. The uncertainties of the X1 and β values for all systems are estimated to be less than ±2 × 10−3 , and ±5 × 10−2 , respectively. At each

+ C14 DABr

+ C14 PPh3 Br

+ TTAB

+ C14 DEEA

C14 PPh3 Br + C14 PyrBr

β

f1

f2

−GE at minimum (kJ)

0.87 0.79 0.71 0.60 0.32

3.20

0.91 0.85 0.78 0.64 0.41

2.99

0.77 0.68 0.41 0.25 0.15

4.41

0.90 0.82 0.76 0.63 0.40

3.21

0.91 0.84 0.76 0.63 0.41

3.07

0.99 0.95 0.89 0.85 0.98 0.84

1.00

α1 value for each system, the β values were negative, and found to be dependent on alpha and similar behavior was observed in many previous investigations [14]. The negative values of β indicate synergism between the monomers and a deviation from ideality. The synergism in each system is also supported by the negative deviation of the theoretical cmc∗ (Clint’s model)

S.E. Moore et al. / Journal of Colloid and Interface Science 304 (2006) 491–496

Fig. 4. Plots of X1mic vs α1 for experimental behavior (solid line) compared with Clint’s model (dotted line) for the C14 DABr/C16 DEEA system.

495

Fig. 6. Plot of N1∗ (Q), N2∗ (2), and N ∗ (") for the mixed system C14 DEEA (1) + C16 DEEA (2) as a function of α1 . Solid lines are best fits to experimental values, and Clint’s model is given by the symbol !.

Method I. Excellent agreement between values further proves the validity of the assumptions made in Method II. All systems depicted negative Gexcess values, ranging from −3.0 to −4.5 kJ/mol, which indicates the interaction of the surfactants in the mixed micelles is favorable. The favorable values may be attributed to both head group/head group and tail/tail interactions in these systems. However, for the C14 PPh3 Br/C14 PyBr system, a value of −1.0 kJ/mol was obtained. This can be attributed to the head group/head group interactions only [18]. 3.3. Fluorescence Fig. 5. Plot of X1mic vs α1 calculated by Rubingh’s analysis from Method I and Method II data for the TTAB/C16 DEEA system: (") Method I; (!) Method II.

from the experimental cmc∗ values. The synergistic effects are similar in magnitude for all the C16 DEEA systems with TTAB, C14 PyBr, C14 DEEA, C14 DABr, and C14 PPh3 Br. The slight difference in the average values of β can be attributed to the differences in head group interactions while the chain/chain interactions remain common to all. The much lower synergistic effect observed in the C14 PPh3 Br/C14 PyBr system can be explained on the bases of the presence of head group interactions only. The plots of composition of micellar phase X1mic obmic (Clint’s model) vs tained from Rubingh’s model and Xid bulk mole fraction are presented in Fig. 4. Except for the C14 PPh3 Br/C14 PyBr system, all the other systems exhibited plots with a crossover point. The micellar C14 composition is higher than expected only in the initial phase compared to the ideal values. The composition α1 at which the crossover point occurs for the C16 DEEA systems follows the order C14 PyBr > TTAB > C14 DEEA > C14 DABr > C14 PPh3 Br. The cmc∗ values obtained from Method II were also analyzed in terms of Rubingh’s method [34] to calculate the micellar composition and are plotted in Fig. 5 along with the values obtained from

Micellar aggregation numbers were determined by the static quenching method as described by Turro and Yekta [25]. From the fluorescence quenching data the aggregation number of the mixed micelles, N ∗ , for the TTAB/C16 DEEA and C14 DEEA/C16 DEEA systems were obtained. The values of partial aggregation numbers N1∗ and N2∗ of individual surfactants in the mixed micelles as well as the mole fraction X1mic in the micellar phase may be evaluated employing the following [18]: [S]mic [S]mic [S]mic tot 1 2 = = , ∗ N∗ N1 N2∗

(3)

∗ [S]mic tot = [S]tot − cmc ,

(4)

[S]mic i X1∗ =

= [S]i,tot − cmc∗i , Ni∗ . N∗

(5) (6)

mic were determined from the valThe values of [S]mic tot and [S]i ∗ ∗ ues of cmc and cmci obtained from conductivity measurements. The values of N ∗ , N1∗ , and N2∗ are given in Table A (Supplementary material) for both systems studied by fluorescence methods, and the uncertainties are estimated to be less than ±3. The values of micellar composition (X1∗ ) calculated employing Eq. (6) along with the ideal values and those obtained from

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Rubingh’s method were also presented in Table A. The plot of aggregation numbers N ∗ , N1∗ , and N2∗ for the TTAB/C16 DEEA as a function of α, is given in Fig. 6. The plot also includes the calculated values of N ∗ assuming ideal behavior, according to the following equation [35]:  X mic 1 i = , N∗ Ni

(7)

i

where Ni is the aggregation number of the pure surfactant i and Xi is the ideal mole fraction in the micellar phase. From the slopes of the plot of I0 /I vs [Q], the Stern–Volmer quenching constants were calculated. These values for the TTAB/C16 DEEA and C14 DEEA/C16 DEEA systems are presented in Table B (Supplementary material). The I1 /I3 ratios for each α C14 values are also given in Table B. Uncertainties for I1 /I3 and KSV are estimated to be less than ±0.01 and ±10, respectively. The I1 /I3 values are indicative of the micropolarity, sensed by the probe molecule in the mixed micelle. From the values of the I1 /I3 and KSV , one can conclude that the residency and the hydrophobic nature sensed by the probe varied slightly for the C14 DEEA/C16 DEEA system and remained constant for the TTAB/C16 DEEA system. 4. Conclusions Relative contributions of individual critical micelle concentration, cmc∗1 and cmc∗2 , to overall cmc∗ and the individual aggregation numbers, N1∗ and N2∗ , to the overall N ∗ were determined employing conductometric and fluorometric techniques for C16 DEEA surfactant with a variety of tetradecyl surfactants. The data was analyzed to evaluate the composition of the mixed micelles and the departure form ideal behavior. The nonideality is attributed to chain/chain and head group/head group interactions. Excellent agreement between the calculated micellar composition Xi , obtained by Rubingh’s analysis of the data from Method I and Method II substantiates the assumptions made by Janquera and Aicart in Method II are correct. These individual contribution parameters may be valuable in the analysis and authentication of various theoretical model developed for mixed micellar systems. The decrease in counterion binding values with an increase in C14 surfactants in the mixed micelles can be correlated with the synergism reflected in the β values. Acknowledgments R.P. is grateful to NSERC for the generous support in the form of a Discovery Grant. S.E.M., S.A.M., and M.M. acknowledge the USRA (2004) from Natural Science and Engineering Research Council.

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