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Protonated dye-surfactant ion pair formation between neutral red and anionic surfactants in aqueous submicellar solutions
Journal of Molecular Liquids 142 (2008) 130–135
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Journal of Molecular Liquids j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m o l l i q
Protonated dye-surfactant ion pair formation between neutral red and anionic surfactants in aqueous submicellar solutions Biren Gohain, Surashree Sarma 1, Robin K. Dutta ⁎ Department of Chemical Sciences, Tezpur University, Napaam, Tezpur 784 028, Assam, India
A R T I C L E
I N F O
Article history: Received 2 January 2008 Received in revised form 20 March 2008 Accepted 29 May 2008 Available online 6 June 2008 Keywords: Neutral red Acid–base equilibrium Dye-surfactant ion pair Hydrophobic interaction Dye-surfactant interaction
A B S T R A C T Spectral and surface tension behavior of aqueous neutral red in the presence of sodium dodecyl sulfate (SDS), sodium dodecyl benzene sulfonate (SDBS) and sodium dodecyl sulfonate (SDSN) have been studied to understand the nature of the interactions in their submicellar concentration ranges. The variations in spectra and surface tension with variation in the concentrations of the surfactants suggest the formation of a 1:1 close-packed dyesurfactant ion pair, HNR+ S− between the acid form, HNR+ of the dye and the surfactant anion at very low concentrations of the surfactant below critical micelle concentration (cmc) of the pure surfactant. The dyesurfactant ion pair behaves like a nonionic surfactant having higher efficiency and lower cmc than that of the corresponding pure anionic surfactant. The ion pairs are adsorbed on the air/water interface at very low concentrations of the surfactant. As the concentration of the surfactant increases and the ion pairs form micelles of their own, the dye in the ion pair is protonated to form H2NR2+ S−. As the cmc of the pure surfactant is approached, the protonation equilibrium gradually reverses and pure surfactant ions gradually replace the ion pairs at the interface. Finally, a homogeneous monolayer of pure surfactant anions exists at the air/water interface and the dye remain solubilized in pure micelles above the cmc of the pure surfactant. The equilibrium constants, Kc for the close-packed protonated dye-surfactant ion pair (PDSIP) formation have been determined at varying pH. The submicellar interaction has been found to be stronger with SDS than SDBS. The plots of logarithm of Kc vs. pH have been found to be quite linear which consolidates the assumption of formation of the species, H2NR2+ S−. The interaction is driven by enthalpy as well as entropy. © 2008 Elsevier B.V. All rights reserved.
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1. Introduction Neutral red (NR, Scheme 1) is used as an acid–base indicator in analytical chemistry and as a probe for gaining insight into the aqueous interfacial microenvironment of micelles, biological membranes, proteins and polyelectrolytes [1]. Reports in the literature indicate that NR exhibits erratic absorption maximum in the visible spectra corresponding to its acid form, which ranges from 535 nm to 545 nm [2]. Similarly, the reported pKa,w values of the dye range from 6.5 to 6.75 [3]. The variation in the λmax of the dye was attributed to dimerization and formation of higher aggregates of the dye by Rao and Narayana [2]. But, we indicated that the use of different buffer systems might be responsible for the variation in λmax and pKa of NR [4]. The organic buffer components, e.g., acetate, phthalate, etc., may form complex of different strengths and thus can affect the λmax and pKa of the dye to different extent. According to Ferguson and Mau, although dimmer-monomer equilibria are thought to be involved in aqueous systems of some other similar cationic dyes, viz.,
⁎ Corresponding author. Tel.: +91 3712 267007/8/9x5055; fax: +91 3712 267005. E-mail addresses: [email protected] (B. Gohain), [email protected] (S. Sarma), [email protected] (R.K. Dutta). 1 Present address: Bhabha Engineering Research Institute, Hosangabad Road, Jhatkheri, Bhupal-462026, India. 0167-7322/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.molliq.2008.05.015
acridine orange, 3,6-diaminoacridine and rhodamine B, actually involve acid–base equilibria of the dye [5]. On the other hand, several factors, viz., micellar surface potential, effective dielectric constant of the interfacial regions, interfacial solvent characteristics and specific molecular interactions between the cationic protonated moieties of the dye and the anionic surfactant headgroup contribute to pKa shifts of the dye in surfactant solutions [3,6–8]. Some dilute aqueous sulfonephthalein dyes, viz., phenol red, cresol red and cresol purple, are reported to form dye-surfactant ion pairs at submicellar concentrations where the dye exists in the ion pair in its doubly-deprotonated form [9]. Similarly, in the cases of dilute aqueous solutions of the cationic dyes, viz., phenosafranin, safranin O and safranin T with oppositely charged surfactant systems, close-packed dye-surfactant ion pairs are formed at surfactant concentrations below the cmc of the surfactant where the dye exists in the ion pair in its doubly protonated form [10]. Neutralization of the electric charge due to closepacked ion-pair formation was assumed to generate a strong hydrophobicity in the ion pair and redistribute electron densities in the dye ion, which leads to further deprotonation of anionic dye or protonation of cationic dye in the ion pairs. It was indicated earlier that aqueous NR, in the presence of submicellar sodium dodecyl sulfate (SDS), shows a shoulder in the visible region of its electronic spectra which could be attributed to a doubly protonated form of the dye in the close-packed
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Scheme 1. Schematic representation of the protonation of NR by submicellar SDS.
dye-surfactant ion pairs [4]. Thus, although the visible spectra and pKa shifts of NR are well characterized in micellar and microemulsion systems [3,4,7,8], the nature of the interaction of the dye with oppositely charged surfactant in submicellar concentrations is not yet unequivocally ascertained. While spectroscopic study can be a suitable tool to monitor protonation–deprotonation equilibrium of a dye, surface tension measurements may reflect the micelle forming behavior of the surfactants in the presence of the dye or in other words of the possible close-packed dye-surfactant ion pairs. Therefore an attempt has been made to systematically study the interactions of dilute aqueous NR with anionic surfactants in the submicellar concentrations of the surfactants from spectroscopic and surface tension measurements. The surfactants chosen for the study were SDS, sodium dodecyl benzene sulfonate (SDBS) and sodium dodecyl sulfonate (SDSN). 2. Experimental The dye neutral red (NR, 3-amino-7-dimethylamino-2-methyl phenazine hydrochloride) was a product of Spectrochem, Mumbai. It was recrystallized from water and dried before use. The purity of the dye was checked by spectrophotometry at the absorbance maximum at pH 10.0 [4]. SDS (electrophoresis grade) was obtained from Sisco Research Laboratory, India and used without further purification. SDBS and SDSN were obtained from Aldrich Chemical Company, USA and were also used as such. The cmc's of the surfactants were determined by surface tension measurement and the values agreed well with the literature values (Table 1). The buffer components (all AR grade) were obtained from Merck, Mumbai, India and used as such. Low ionic strength (I = 0.01) phosphate and formic acid buffers were used for moderate and acidic buffers, respectively [11]. The spectra were recorded on a Hitachi-U2001 UV–Visible spectrophotometer with matched pair of cells of 1 cm path length fitted in thermostated cell holder. Surface tensions for the determination of critical micelle concentrations of the surfactants were determined by using a Krüss Du Nuoy tensiometer model K9 using a platinum ring with a thermostated sample holder with temperature control by Thermo Haake K10 thermostat. The pH was measured on a Systronics India, µ-pH System.
Temperatures were maintained within ±1 K in all experiments. A fixed low concentration, viz., 5×10− 5 mol dm− 3 of NR was used in the study in order to have absorbance in a suitable range and to avoid self aggregation of the dye which is reported to occur at higher concentrations [2]. 3. Results and discussions 3.1. The spectral variations of aqueous NR in submicellar anionic surfactants The spectra of 5.00 × 10− 5 mol dm− 3 aqueous NR at a fixed pH of 2.90 in the absence and the presence of SDS of varying concentrations are shown in Fig. 1. On increasing the concentration of SDS in the submicellar concentration range, the absorbances of the 528 nm band of NR gradually decreased with a corresponding increase in the absorbance in the higher wavelength side forming a shoulder in the wavelength range of 569 to 750 nm (Fig. 1a). The intensity of the shoulder increased on increasing the surfactant concentration up to ca. 2.00 × 10− 4 mol dm− 3. On further addition of the surfactant the intensity of the shoulder started to decrease with corresponding increase of the absorbances of the 527 nm band as can be seen in Fig. 1b. The shoulder totally disappeared at SDS concentration of ca. 4.00 × 10− 4 mol dm− 3. On the other hand, on increasing the surfactant concentration above 2.00 × 10− 4 mol dm− 3, the λmax gradually shifted from 528 to 508 nm and then to 538 nm at concentration of SDS above cmc (Fig. 1c) [4,8]. The presence of the base form of the dye, i.e., NR0 can be ruled out at pH 2.90, which is about 4 pH units lower than pKa1,w of the dye. The λmax of 538 nm observed above the cmc of the surfactant is attributed to the acid form, NRH+ of the dye solubilized by the micelles. The red shift of 10 nm of the band may be due to micellization of the NRH+ form of the dye as such shifts of other cationic dyes are known [8,12]. A spectral shoulder between 569 to 750 nm of aqueous NR reported earlier in the presence of submicellar SDS at pH 7.00 was ascribed to the formation of the dicationic form, viz., H2NR2+ of the dye [4]. Difference spectra obtained by subtracting the absorbances of spectra 1 from that of spectra 4 in the submicellar concentration range at pH 2.90 also showed a λmax at 598 nm (Fig. 1). The spectra of the 5.00 × 10− 5 mol dm− 3 aqueous NR at varying pH in highly acidic medium are shown in Fig. 2. Interestingly, one can see an appearance of a band with λmax at 598 nm at pH of near
Table 1 Critical micelle concentration of the surfactants (cmc) and ion pairs (cmcip), pC20 and surface excess concentration at surface saturation (Γs) in pure water, in the presence of buffer and in the presence of buffer and NR (5.0 × 10− 5 mol dm− 3) at 298 K Surfactant
cmca/(mol dm− 3) in water
cmcipb/(mol dm− 3)
cmca/(mol dm− 3)
Γs/(mol/1000 m2)
pC20 In the presence of
SDS SDBS SDDS a b c
8.30 × 10− 3 (8.31 × 10− 3)c 1.18 × 10− 3 (1.20 × 10− 3)c 1.22 × 10− 2 (1.20 × 10− 2)c
Buffer and dye
Buffer
Buffer and dye
Buffer
Buffer and dye
Buffer
Buffer and dye
7.94 × 10− 5 6.30 × 10− 5 1.26 × 10− 4
2.51 × 10− 3 1.00 × 10− 3 3.98 × 10− 3
2.23 × 10− 3 1.12 × 10− 3 5.01 × 10− 3
3.25 4.01 3.13
3.72 4.18 3.40
1.60 × 10− 3 1.34 × 10− 3 1.31 × 10− 3
2.08 × 10− 3 1.89 × 10− 3 1.55 × 10− 3
Experimental error limits = ±3%. Experimental error limits = ±6%. The values in parentheses are literature values from Ref. [12].
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exceeding the cmc like in the cases reported earlier resulting in the disappearance of the 598 nm band [9,10]. An isosbestic point was observed at 569 nm in the concentration range of the surfactant between 4.00 × 10− 5 and 2.00 × 10− 4 mol dm− 3 which is an indicative of the presence of equilibrium between the free dye HNR+ and the protonated H2NR2+ S− form in the submicellar concentration range. The variations in the absorbances at 598 and 527 nm of aqueous NR at pH 2.90 in the presence of varying concentrations of SDBS and SDSN in the submicellar concentration ranges are shown in Fig. 3. The spectral behaviors of the dye solutions in the presence of these two surfactants have been found to be similar to that observed for SDS suggesting occurrence of similar interactions of the dye with these two surfactants also. The positions of the absorption maxima and the isosbestic points were also found to be the same as in the case of all three surfactants. The intensities of absorbance of the aqueous dye solutions at 598 nm in the presence of varying concentrations of the three dyes decrease in the order SDS N SDBS N SDSN suggesting a decrease in the strength of the interaction in the same order. The spectral variations in the 569–750 nm region with variation in the concentration of SDS were considerably reduced upon increase in the ionic strength of the solutions, e.g., in the presence of 0.1 mol dm− 3 NaBr. This observation also indicates that an increase in the ionic strength is not the reason for increase in the absorbance in the 569 nm– 750 nm region.
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3.2. Surface tension behavior of the dye-surfactant systems
Fig. 1. UV–Visible spectra of aqueous NR (5.0 × 10− 5 mol dm− 3) at SDS concentrations (×10− 4) of (a): (1) 0.0, (2) 0.4, (3) 0.8 and (4) 2.0, (b): (4) 2.0, (5) 4.0, (6) 8.0, (7) 20.0 and (8) 30.0 and (c): (8) 30.0, (9) 40.0, (10) 60.0, (11) 80.0 and (12) 300.0 mol dm− 3, respectively, at pH 2.90 and temperature 298 K. The dashed curve is the difference spectra (4–1) of aqueous NR in the presence and absence of SDS.
zero which can be attributed to the dicationic form, H2NR2+ of the dye. We have found that the three forms of aqueous neutral red, viz., NR0, HNR+ and H2NR2+ have absorption bands in the visible region with λmax at 450, 528 and 598 nm, respectively. A clear isosbestic point observed at 569 nm in the spectra can be attributed to the acid–base equilibrium between HNR+ and H2NR2+, i.e., to pKa2,w. The pKa1,w and pKa2,w of the dye have been found to be 6.75 and 0.26, respectively. The 598 nm band of the dye has been found to reverse on increasing the pH. The shoulder observed in the submicellar solutions can therefore be rightly attributed to the dicationic form, H2NR2+ of the dye. The observed shift of the λmax from 528 to 508 nm have has been attributed to HNR+ form of the dye solubilized in the micelles [3,4,7,8]. The formation of this shoulder, or in other words, the H2NR2+ form of the dye in the aqueous submicellar SDS solutions at a pH 2.90 or higher is interesting. The cmc of sodium dodecyl sulfate in pure water is 8.31 × 10− 3 mol dm− 3 [13]. Although the cmc should decrease in the presence of the buffer components and the dye, it is unlikely to decrease to about 2 × 10− 4 mol dm− 3 to cause any change to the dye to give rise to the shoulder at 598 nm. So, any type of involvement of micelles in the appearance of this band at the surfactant concentration below 4.00 × 10− 4 mol dm− 3 is unlikely. Thus the interaction between the dye and the surfactant at concentration around 2 × 10− 4 mol dm− 3 and formation of the shoulder at 598 nm may be a premicellar phenomenon. As reported earlier [10,14], it is possible that due to the presence of strong electrostatic and hydrophobic interaction between the oppositely charged dye and surfactant ions, the ions form a close-packed ion pair [15]. In these close-packed ion pairs the original charges on the ions are localized. The localization of the original positive charge of the dye cation paves way for a second protonation of the dye, i.e., a protonated dye-surfactant ion pair (PDSIP), H2NR2+ S− (Scheme 1), is formed as in the case of phenothiazine dyes with SDS [10]. This PDSIP probably breaks down as micelles are formed with the concentration of surfactant
Variations in surface tensions of the buffered aqueous surfactant solutions at pH 2.90 with variation in the surfactant concentration in the presence and absence of NR of concentration of 5.00 × 10− 5 mol dm− 3 are shown in Fig. 3. The absorbance data of the dye in the solutions at 598 nm and 527 nm are included in the figure to help in correlating the surface tension data with the absorbances. The variation in the surface tension of the buffered solution on increasing the concentration of SDS in the absence of the dye has been found to be as expected (Fig. 3a). The surface tension gradually decreased with increase in the concentration of the surfactant, reached a minimum at [SDS] = 2.51 × 10− 3 mol dm− 3 and then almost leveled off. The concentration 2.51 × 10− 3 mol dm− 3 is therefore the cmc of the surfactant in the buffered medium. The plot of the surface tension vs. [SDS] in the presence of the buffer and the dye has been found to be unusual (Fig. 3a). In the submicellar concentration range, the surface tension decreased more rapidly
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Fig. 2. The UV–Visible absorption spectra of aqueous NR (5.0 × 10− 5 mol dm− 3) at pH = 2.95 (1), 0.74 (2), 0.52 (3), 0.30 (4), 0.20 (5), 0.12 (6) and 0.05 (7) at 298 K.
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of the dye can be attributed to the formation of the close-packed dyesurfactant ion pair, HNR+ SDS−, where the ion pair itself behaves like a nonionic surfactant having an efficiency higher than SDS alone. Nonionic surfactants usually have higher efficiency than the corresponding ionic surfactants [13]. The ion-pair formation is a specific interaction between the dye and the surfactant ions and this specific interaction should exist also in the ion-pair monolayer at the air/water interface. The surface tension of a solution is affected by any change in the structure of the monolayer at the air/water interface. It was reported that presence of ptosylate causes major and unexpected changes to the structure of the monolayer of hexadecyltrimethylammonium bromide (CTAB) at the air/ water interface [16]. The area per surfactant molecule in the monolayer of CTAC with p-tosylate counterion was reported to increase by one quarter to accommodate the p-tosylate ions within the monolayer. Similarly, in the present case, the nonionic dye-surfactant ion-pair surfactant has a larger head group than that of SDS and therefore occupies a larger surface area per surfactant in the monolayer at the air/ water interface compared to that of SDS. A larger surface area per surfactant leads to a lower cmc of the ion-pair surfactant. The surface tension decreased as the concentration of SDS was increased up to ca. 1.0×10− 4 mol dm− 3 and above that again started to increase [Fig. 3a]. The surface tension reached a maximum as the concentration of SDS reached ca. 2.0 ×10− 4 mol dm− 3 and then again started to decrease with further increase in [SDS]. The first minimum of the surface tension may correspond to the cmc of the ion-pair surfactant, HNR+ S−. Although the surface tension is expected to remain almost unchanged above cmc, the surface tensions often show a minimum at the cmc in the presence of impurity, here, the dye being the impurity [17]. The cmc of the ion-pair surfactant, cmcip, for HNR+ SDS− has been estimated as 7.9×10− 5 mol dm− 3 (Table 1). It is possible that the protonation of the dye in the ion pair takes place as soon as the ion-pair surfactants start forming micelles of their own. Such ion-pair micelles can also be termed as premicelles [18]. Interestingly, the maximum of the surface tension and the maximum of the absorbance at 598 nm correspond to the same concentration of the surfactant. This suggest involvement of the same species, viz., PDSIP, H2NR2+ S− in appearance of the shoulder at 598 nm and increase in the surface tension above 1.0 ×10− 4 mol dm− 3 of SDS. The increase in surface tension may be due to replacement of some ion pairs in the air/water interface by the PDSIP. The nonionic ion-pair surfactants are transformed into cationic surfactant with a different head group on protonation of the dye in the close-packed ion pair. The increase in the surface tension is probably due to conversion of nonionic ion-pair surfactant to cationic protonated ion-pair surfactant because a cationic surfactant is expected to have lower efficiency of the compared to the corresponding nonionic surfactant. Although the ion-pair formation is a specific interaction between the dye and the surfactant ions, the protonation of the dye in the ion-pair micelles is a result of cooperative interaction since the protonation takes place only after the ion-pair micelles start forming [19,20]. The dye solutions showed similar variations in the presence of SDBS and SDSN also. With SDBS, the estimated cmcip (6.30 × 10− 5 mol dm− 3)
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Fig. 3. Plots of surface tension (mN/m) and absorbance of aqueous NR (5.0 × 10− 5 mol dm− 3) solutions as a function of logarithm of the concentrations of: (a) SDS (b) SDBS and (c) SDSN at pH 2.90 and temperature 298 K. Symbols: surface tension in the presence (Δ) and absence (○) of NR, absorbances at 527 (■) and at 598 nm (▲).
indicating a considerably higher efficiency of the surfactant in the presence of the dye. The value of pC20, which is the concentration of the surfactant required to lower the surface tension by 20 mN m− 1, is a measure of the efficiency of the surfactant. The pC20 has been found to be 3.25 and 3.72 in the absence and the presence of the dye, respectively (Table 1). This increase in the efficiency of the surfactant in the presence
Fig. 4. Illustration of the various dye-surfactant interactions at different concentration ranges of the surfactant, viz., below cmc of the ion-pair surfactant (cmcip), above cmcip but below cmc of the pure surfactant and above cmc.
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Table 2 Equilibrium constants (Kc) and thermodynamic parameters of NR-SDS and NR-SDBS systems at pH 2.90 and at different temperatures Surfactant SDS
Kc'/(dm3/mol− 1)
7
293 298 303 288 293 303
SDBS
a
Kca/(dm3/mol− 1)2
Temperature/K
1.67 × 10 1.46 × 107 1.23 × 107 4.51 × 106 4.06 × 106 2.73 × 106
9.92 × 10
− ΔG°/kJ mol− 1
4
−ΔH°/kJ mol− 1
40.5 40.9 41.1 36.7 37.1 37.3
1.72 × 104
ΔS°/J mol− 1K− 1
9.3
106.0
24.9
42.6
Experimental error limits = ±5%.
is slightly smaller than that with SDS. The increase in the surface tension at above the cmcip of HNR+ SDBS− was also less compared to that after HNR+ SDS−. On the other hand, no minimum of surface tension was observed in the NR-SDSN system. The cmcip of HNR+ SDSN− has been assumed to be the concentration at which the surface tension initially leveled off and estimated to be 1.26 × 10− 4 mol dm− 3. The lower maximum in the surface tension curve for SDBS than that for SDS may be due to lesser replacement of the ion-pair surfactants of the monolayer of the air/water interface in the case of SDBS than in the case of SDS. Absence of such maximum in the case of SDSN may be due to the absence of such substitution in the monolayer. This is expected because the PDSIP formation with SDSN is minimum among all three surfactants as has been indicated by the absorption intensities in the three cases at 598 nm (Fig. 3). When the concentration of the surfactant was increased above the concentration corresponding to the maximum surface tension, the surface tension decreased gradually but remained lower than that in the absence of the dye. The slope of the surface tension vs. surfactant concentration at surface saturation, i.e., near cmc, in the presence of the dye was smaller than that in its absence. This indicates a lower surface excess concentration, Γs of the surfactant in the presence of the dye than its absence at surface saturation. Γs is defined by
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dγ ¼ −4:606RT Γ s d log C1
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ð1Þ
where, dγ is the rate of change in surface tension with concentration at surface saturation, and C1 is the surfactant concentration at surface saturation or cmc [13]. In the case of all three surfactants, the Γs in the presence of the dye have been found to be greater than that in the absence of the dye (Table 1). This observation may be attributed to the gradual break up of the ion pairs and corresponding gradual decrease in the fraction of the ion pairs at the monolayer at the air/water interface. Overcompensation of charge on the ion-pair micelles at large excess of the surfactant may facilitate the observed reversal of deprotonation [21,22]. The cmc's of all three surfactants in the presence of the dye have been found to be almost the same as that in its absence (Table 1). Similarly, the surface tensions at the cmc, which is also a measure of the effectiveness of a surfactant, have been found to be the same in the presence as well as absence of the dye for all three surfactants. This suggests the occupation of the monolayer at the air/water interface by the pure surfactants alone at concentrations above the cmc even in the presence of NR. A summary of the different interactions, viz., the dye-surfactant ionpair formation, monolayer formation by the ion pairs at the air/water interface, micelle formation by the ion pairs, protonation of the dyes in the ion-pair micelles, formation of pure micelles with solubilized dye and replacement of the ion pair in the monolayers by pure micelles at different surfactant concentration ranges have has been illustrated in Fig. 4.
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free dye, [HNR+] and the PDSIP, H2NR2+ S−. The equilibrium of the submicellar PDSIP formation between NR and SDS can be represented by [10]: h i þ Kc H þ HNRþ þ ½S− U H2 NR2þ S−
ð2Þ
d
or h i Kc ¼ H2 NR2þ S− = Hþ HNRþ ½S−
ð3Þ
d
where [S−] and Kc are the concentration of the monomeric surfactant and the equilibrium constant of the interaction, respectively. The equilibrium constants have been determined using Eq. (4) derived from Eq. (3): Kc ¼
i h HNRþ 0 =ðe−e0 Þg= ½Hþ HNRþ 0 − H2 NR2þ S− h i ½S− 0 − H2 NR2þ S−
f d−e
0
ð4Þ
g
where [HNR+]0 and [S−]0 are the initial concentrations of the dye and the surfactant, respectively; d and ε are the absorbance and molar absorption coefficient of the PDSIP [H2NR2+ S−] at the λmax 598 nm in the presence of the surfactant, respectively and ε0 is the molar absorption coefficient of the dye at 598 nm in the absence of the surfactant. An iterative method in C language was used for computation of the equilibrium constants. Only those absorbance values of the λmax of the PDSIP in the submicellar surfactant concentration ranges were used in the calculations whose spectra passed through the isosbestic point. The equilibrium constants determined for the submicellar interactions of NR with SDS and SDBS at fixed pH of 2.90 have been shown
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3.3. The submicellar dye-surfactant equilibrium The presence of isosbestic points in the aqueous NR spectra in the submicellar solutions indicates the presence of equilibria between the
Fig. 5. Plots of log Kc vs. pH for (a) NR-SDS and (b) NR-SDBS system at 298 K. Inset: van't Hoff plot.
B. Gohain et al. / Journal of Molecular Liquids 142 (2008) 130–135
in Table 2 along with the thermodynamic parameters. The large equilibrium constant values can be compared with those observed in similar systems [10]. The equilibrium study was not performed with SDSN because the spectral variations were too less with this surfactant. ΔG° was calculated by using Eq. (5):
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1.72 × 104 mol dm− 3, respectively, at 298 K [Table 2]. The higher pHindependent equilibrium constant, K′c with SDS than that with SDBS again indicates a stronger interaction in the NR-SDS system than that in the case of NR-SDBS system. 4. Conclusion
◯
ΔG ¼ −RT ln Kc :
ð5Þ
The equilibrium constant and the ΔG° of the interaction with the two surfactants show that the interaction of PDSIP formation is stronger with SDS than that with SDBS. ΔH° and ΔS° were determined by van't Hoff plot from the ΔG° values at 293 K, 298 K and 303 K assuming ΔH° to be constant within this small temperature range. The van't Hoff plots have been shown in the insets of Fig. 5 and the ΔH° and ΔS° values have been included in Table 2. Both negative standard enthalpies and positive standard entropies observed for the present systems favor the interaction. A greater strength with SDBS then with SDS is expected due to greater hydrophobicity of SDBS than that of SDS. However, the observed greater strength with SDS than with SDBS can be attributed to the much higher positive entropy of interaction observed for the former than the latter the interaction being driven by enthalpy as well as entropy. The plots of logarithm of log Kc vs. pH at 298 K for both surfactants are shown in Fig. 5. It can be seen from the figure that log Kc increases linearly with the experimental pH in both cases. The linearity is expected as Kc is inversely proportional to [H+] in Eq. (2). The linearity of the plots also suggests the validity of Eq. (2) which is based on 1:1 dye-surfactant interaction. The intercept of the plots gives the overall equilibrium constant, Kc, which have been included in Table 2. However, the intercepts have been found to be less than the expected value of unity in all cases, which may be due to the presence of some more factors in the interaction. Thus, from a linear equation between log Kc and pH, we get −1 þ log K cV: log Kc ¼ m log Hþ
ð6Þ
The slopes for NR-SDS and NR-SDBS have been found to be 0.75 and 0.96, respectively. Assuming m to be unity (which is actually not far from unity), we get Kc ¼ K cV= Hþ
ð7Þ
which is essentially the same as Eq. (3) with h i K cV ¼ H2 NR2þ S− = HNRþ ½S− :
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
ð8Þ
Thus, the observed linear relation between the logarithm of Kc and pH in fact supports our assumption that the interaction product between NR and the submicellar anionic surfactants is doubly PDSIP, i.e., H2NR2+ S− as shown in Eq. (2). The constant K′, c where,
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K cV ¼ Kc Hþ
From the present study it is evident that the closed packed dyesurfactant ion pairs are formed between NR and anionic surfactants in submicellar aqueous solutions due to the prevailing hydrophobic and electrostatic forces in the systems. The ion pairs behave like a nonionic surfactant having greater efficiency and lower CMC than the corresponding pure anionic surfactants. The ion-pair surfactants predominate in the air/water interface at low concentrations of the anionic surfactant. Protonation of the dye in the dye-surfactant ion pair occurs as the ion-pair surfactants start forming micelle. The observed linear relationship of the logarithm of the equilibrium constant of the protonation with pH consolidates the assumption of formation of a 1:1 PDSIP. The equilibrium constants suggest stronger interaction with SDS than with SDBS. As the CMC of the pure surfactant is approached the ion pairs of the monolayer at the air/water interface are replaced by the pure surfactant ions accompanied by simultaneous transition from the PDSIP micelles to pure surfactant micelles with solubilized dye. Above the CMC of the pure surfactant, the monolayer at the air/water interface consists of only pure surfactant ions.
ð9Þ
may be considered as another equilibrium constant which is independent of the hydrogen ion concentration or pH. The values of K′c for the dye with SDS and SDBS have been found to be 9.92 × 104 and
[18] [19] [20] [21] [22]
W. Junge, G. Sconknecht, V. Forster, Biochim. Biophys. Acta 852 (1986) 93. N.V. Rao, K.L. Narayana, Indian J. Chem. 21A (1982) 995. C. Drumond, F. Grieser, T.W. Healy, J. Chem. Soc., Faraday Trans. 85 (1989) 551. R.K. Dutta, S.N. Bhat, Can. J. Chem. 71 (1993) 1785. J. Ferguson, A.W.H. Mau, Chem. Phys. Lett. 17 (1972) 543. R.K. Dutta, R. Chowdhury, S.N. Bhat, J. Chem. Soc., Faraday Trans. 91 (1995) 681. B.C. Paul, K. Ismail, Indian J. Chem. 38A (1999) 496. S.P. Moulik, B.K. Pau, D.C. Mukherjee, J. Colloid Interface Sci. 161 (1993) 72. B. Gohain, P.M. Saikia, S. Sarma, S.N. Bhat, R.K. Dutta, Phys. Chem. Chem. Phys. 4 (2002) 2617. R.K. Dutta, S.N. Bhat, Bull. Chem. Soc. Jpn. 65 (1992) 1089. D.D. Perin, Aust. J. Chem. 16 (1963) 572. K.K. Rohatgi-Mukherjee, P. Choudhuri, P.P. Bhowmik, J. Colloid Interface Sci. 106 (1985) 45. M.J. Rosen, Surfactants and Interfacial Phenomena, John Wiley and Sons, New York, 1989. A.A. Rafati, S. Azizian, M. Chahardoli, J. Mol. Liq. 137 (2008) 80. R.N. Diamond, J. Phys. Chem. 67 (1963) 2513. G.R. Bell, C.D. Bain, Z.X. Li, R.K. Thomas, D.C. Duffy, J. Penfold, J. Am. Chem. Soc. 119 (1997) 10227. Y. Moroi, Micelles, Theoretical and Applied Aspects, 1st ed., Plenum, New York, 1992, p. 17. R.L. Reeves, S.A. Harkaway, in: K.L. Mittal (Ed.), Micellization and Microemulsions, 2, Plenum Press, New York, 1977, p. 819. K. Hayakawa, R. Tanaka, J. Kurawaki, Y. Kusumoto, I. Satake, Langmuir 15 (1999) 4213. M. Novo, S. Felekyan, C.A.M. Seidel, W. Al-Soufi, J. Phys. Chem. B 111 (2007) 3614. Y. Samoshina, T. Nylander, P. Claesson, K. Schillen, I. Iliopoulos, B. Lindaman, Langmuir 21 (2005) 2855. A. Mazei, R. Meszaros, Langmuir 22 (2006) 7148.
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Journal of Molecular Liquids j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m o l l i q
Protonated dye-surfactant ion pair formation between neutral red and anionic surfactants in aqueous submicellar solutions Biren Gohain, Surashree Sarma 1, Robin K. Dutta ⁎ Department of Chemical Sciences, Tezpur University, Napaam, Tezpur 784 028, Assam, India
A R T I C L E
I N F O
Article history: Received 2 January 2008 Received in revised form 20 March 2008 Accepted 29 May 2008 Available online 6 June 2008 Keywords: Neutral red Acid–base equilibrium Dye-surfactant ion pair Hydrophobic interaction Dye-surfactant interaction
A B S T R A C T Spectral and surface tension behavior of aqueous neutral red in the presence of sodium dodecyl sulfate (SDS), sodium dodecyl benzene sulfonate (SDBS) and sodium dodecyl sulfonate (SDSN) have been studied to understand the nature of the interactions in their submicellar concentration ranges. The variations in spectra and surface tension with variation in the concentrations of the surfactants suggest the formation of a 1:1 close-packed dyesurfactant ion pair, HNR+ S− between the acid form, HNR+ of the dye and the surfactant anion at very low concentrations of the surfactant below critical micelle concentration (cmc) of the pure surfactant. The dyesurfactant ion pair behaves like a nonionic surfactant having higher efficiency and lower cmc than that of the corresponding pure anionic surfactant. The ion pairs are adsorbed on the air/water interface at very low concentrations of the surfactant. As the concentration of the surfactant increases and the ion pairs form micelles of their own, the dye in the ion pair is protonated to form H2NR2+ S−. As the cmc of the pure surfactant is approached, the protonation equilibrium gradually reverses and pure surfactant ions gradually replace the ion pairs at the interface. Finally, a homogeneous monolayer of pure surfactant anions exists at the air/water interface and the dye remain solubilized in pure micelles above the cmc of the pure surfactant. The equilibrium constants, Kc for the close-packed protonated dye-surfactant ion pair (PDSIP) formation have been determined at varying pH. The submicellar interaction has been found to be stronger with SDS than SDBS. The plots of logarithm of Kc vs. pH have been found to be quite linear which consolidates the assumption of formation of the species, H2NR2+ S−. The interaction is driven by enthalpy as well as entropy. © 2008 Elsevier B.V. All rights reserved.
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1. Introduction Neutral red (NR, Scheme 1) is used as an acid–base indicator in analytical chemistry and as a probe for gaining insight into the aqueous interfacial microenvironment of micelles, biological membranes, proteins and polyelectrolytes [1]. Reports in the literature indicate that NR exhibits erratic absorption maximum in the visible spectra corresponding to its acid form, which ranges from 535 nm to 545 nm [2]. Similarly, the reported pKa,w values of the dye range from 6.5 to 6.75 [3]. The variation in the λmax of the dye was attributed to dimerization and formation of higher aggregates of the dye by Rao and Narayana [2]. But, we indicated that the use of different buffer systems might be responsible for the variation in λmax and pKa of NR [4]. The organic buffer components, e.g., acetate, phthalate, etc., may form complex of different strengths and thus can affect the λmax and pKa of the dye to different extent. According to Ferguson and Mau, although dimmer-monomer equilibria are thought to be involved in aqueous systems of some other similar cationic dyes, viz.,
⁎ Corresponding author. Tel.: +91 3712 267007/8/9x5055; fax: +91 3712 267005. E-mail addresses: [email protected] (B. Gohain), [email protected] (S. Sarma), [email protected] (R.K. Dutta). 1 Present address: Bhabha Engineering Research Institute, Hosangabad Road, Jhatkheri, Bhupal-462026, India. 0167-7322/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.molliq.2008.05.015
acridine orange, 3,6-diaminoacridine and rhodamine B, actually involve acid–base equilibria of the dye [5]. On the other hand, several factors, viz., micellar surface potential, effective dielectric constant of the interfacial regions, interfacial solvent characteristics and specific molecular interactions between the cationic protonated moieties of the dye and the anionic surfactant headgroup contribute to pKa shifts of the dye in surfactant solutions [3,6–8]. Some dilute aqueous sulfonephthalein dyes, viz., phenol red, cresol red and cresol purple, are reported to form dye-surfactant ion pairs at submicellar concentrations where the dye exists in the ion pair in its doubly-deprotonated form [9]. Similarly, in the cases of dilute aqueous solutions of the cationic dyes, viz., phenosafranin, safranin O and safranin T with oppositely charged surfactant systems, close-packed dye-surfactant ion pairs are formed at surfactant concentrations below the cmc of the surfactant where the dye exists in the ion pair in its doubly protonated form [10]. Neutralization of the electric charge due to closepacked ion-pair formation was assumed to generate a strong hydrophobicity in the ion pair and redistribute electron densities in the dye ion, which leads to further deprotonation of anionic dye or protonation of cationic dye in the ion pairs. It was indicated earlier that aqueous NR, in the presence of submicellar sodium dodecyl sulfate (SDS), shows a shoulder in the visible region of its electronic spectra which could be attributed to a doubly protonated form of the dye in the close-packed
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131
Scheme 1. Schematic representation of the protonation of NR by submicellar SDS.
dye-surfactant ion pairs [4]. Thus, although the visible spectra and pKa shifts of NR are well characterized in micellar and microemulsion systems [3,4,7,8], the nature of the interaction of the dye with oppositely charged surfactant in submicellar concentrations is not yet unequivocally ascertained. While spectroscopic study can be a suitable tool to monitor protonation–deprotonation equilibrium of a dye, surface tension measurements may reflect the micelle forming behavior of the surfactants in the presence of the dye or in other words of the possible close-packed dye-surfactant ion pairs. Therefore an attempt has been made to systematically study the interactions of dilute aqueous NR with anionic surfactants in the submicellar concentrations of the surfactants from spectroscopic and surface tension measurements. The surfactants chosen for the study were SDS, sodium dodecyl benzene sulfonate (SDBS) and sodium dodecyl sulfonate (SDSN). 2. Experimental The dye neutral red (NR, 3-amino-7-dimethylamino-2-methyl phenazine hydrochloride) was a product of Spectrochem, Mumbai. It was recrystallized from water and dried before use. The purity of the dye was checked by spectrophotometry at the absorbance maximum at pH 10.0 [4]. SDS (electrophoresis grade) was obtained from Sisco Research Laboratory, India and used without further purification. SDBS and SDSN were obtained from Aldrich Chemical Company, USA and were also used as such. The cmc's of the surfactants were determined by surface tension measurement and the values agreed well with the literature values (Table 1). The buffer components (all AR grade) were obtained from Merck, Mumbai, India and used as such. Low ionic strength (I = 0.01) phosphate and formic acid buffers were used for moderate and acidic buffers, respectively [11]. The spectra were recorded on a Hitachi-U2001 UV–Visible spectrophotometer with matched pair of cells of 1 cm path length fitted in thermostated cell holder. Surface tensions for the determination of critical micelle concentrations of the surfactants were determined by using a Krüss Du Nuoy tensiometer model K9 using a platinum ring with a thermostated sample holder with temperature control by Thermo Haake K10 thermostat. The pH was measured on a Systronics India, µ-pH System.
Temperatures were maintained within ±1 K in all experiments. A fixed low concentration, viz., 5×10− 5 mol dm− 3 of NR was used in the study in order to have absorbance in a suitable range and to avoid self aggregation of the dye which is reported to occur at higher concentrations [2]. 3. Results and discussions 3.1. The spectral variations of aqueous NR in submicellar anionic surfactants The spectra of 5.00 × 10− 5 mol dm− 3 aqueous NR at a fixed pH of 2.90 in the absence and the presence of SDS of varying concentrations are shown in Fig. 1. On increasing the concentration of SDS in the submicellar concentration range, the absorbances of the 528 nm band of NR gradually decreased with a corresponding increase in the absorbance in the higher wavelength side forming a shoulder in the wavelength range of 569 to 750 nm (Fig. 1a). The intensity of the shoulder increased on increasing the surfactant concentration up to ca. 2.00 × 10− 4 mol dm− 3. On further addition of the surfactant the intensity of the shoulder started to decrease with corresponding increase of the absorbances of the 527 nm band as can be seen in Fig. 1b. The shoulder totally disappeared at SDS concentration of ca. 4.00 × 10− 4 mol dm− 3. On the other hand, on increasing the surfactant concentration above 2.00 × 10− 4 mol dm− 3, the λmax gradually shifted from 528 to 508 nm and then to 538 nm at concentration of SDS above cmc (Fig. 1c) [4,8]. The presence of the base form of the dye, i.e., NR0 can be ruled out at pH 2.90, which is about 4 pH units lower than pKa1,w of the dye. The λmax of 538 nm observed above the cmc of the surfactant is attributed to the acid form, NRH+ of the dye solubilized by the micelles. The red shift of 10 nm of the band may be due to micellization of the NRH+ form of the dye as such shifts of other cationic dyes are known [8,12]. A spectral shoulder between 569 to 750 nm of aqueous NR reported earlier in the presence of submicellar SDS at pH 7.00 was ascribed to the formation of the dicationic form, viz., H2NR2+ of the dye [4]. Difference spectra obtained by subtracting the absorbances of spectra 1 from that of spectra 4 in the submicellar concentration range at pH 2.90 also showed a λmax at 598 nm (Fig. 1). The spectra of the 5.00 × 10− 5 mol dm− 3 aqueous NR at varying pH in highly acidic medium are shown in Fig. 2. Interestingly, one can see an appearance of a band with λmax at 598 nm at pH of near
Table 1 Critical micelle concentration of the surfactants (cmc) and ion pairs (cmcip), pC20 and surface excess concentration at surface saturation (Γs) in pure water, in the presence of buffer and in the presence of buffer and NR (5.0 × 10− 5 mol dm− 3) at 298 K Surfactant
cmca/(mol dm− 3) in water
cmcipb/(mol dm− 3)
cmca/(mol dm− 3)
Γs/(mol/1000 m2)
pC20 In the presence of
SDS SDBS SDDS a b c
8.30 × 10− 3 (8.31 × 10− 3)c 1.18 × 10− 3 (1.20 × 10− 3)c 1.22 × 10− 2 (1.20 × 10− 2)c
Buffer and dye
Buffer
Buffer and dye
Buffer
Buffer and dye
Buffer
Buffer and dye
7.94 × 10− 5 6.30 × 10− 5 1.26 × 10− 4
2.51 × 10− 3 1.00 × 10− 3 3.98 × 10− 3
2.23 × 10− 3 1.12 × 10− 3 5.01 × 10− 3
3.25 4.01 3.13
3.72 4.18 3.40
1.60 × 10− 3 1.34 × 10− 3 1.31 × 10− 3
2.08 × 10− 3 1.89 × 10− 3 1.55 × 10− 3
Experimental error limits = ±3%. Experimental error limits = ±6%. The values in parentheses are literature values from Ref. [12].
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exceeding the cmc like in the cases reported earlier resulting in the disappearance of the 598 nm band [9,10]. An isosbestic point was observed at 569 nm in the concentration range of the surfactant between 4.00 × 10− 5 and 2.00 × 10− 4 mol dm− 3 which is an indicative of the presence of equilibrium between the free dye HNR+ and the protonated H2NR2+ S− form in the submicellar concentration range. The variations in the absorbances at 598 and 527 nm of aqueous NR at pH 2.90 in the presence of varying concentrations of SDBS and SDSN in the submicellar concentration ranges are shown in Fig. 3. The spectral behaviors of the dye solutions in the presence of these two surfactants have been found to be similar to that observed for SDS suggesting occurrence of similar interactions of the dye with these two surfactants also. The positions of the absorption maxima and the isosbestic points were also found to be the same as in the case of all three surfactants. The intensities of absorbance of the aqueous dye solutions at 598 nm in the presence of varying concentrations of the three dyes decrease in the order SDS N SDBS N SDSN suggesting a decrease in the strength of the interaction in the same order. The spectral variations in the 569–750 nm region with variation in the concentration of SDS were considerably reduced upon increase in the ionic strength of the solutions, e.g., in the presence of 0.1 mol dm− 3 NaBr. This observation also indicates that an increase in the ionic strength is not the reason for increase in the absorbance in the 569 nm– 750 nm region.
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3.2. Surface tension behavior of the dye-surfactant systems
Fig. 1. UV–Visible spectra of aqueous NR (5.0 × 10− 5 mol dm− 3) at SDS concentrations (×10− 4) of (a): (1) 0.0, (2) 0.4, (3) 0.8 and (4) 2.0, (b): (4) 2.0, (5) 4.0, (6) 8.0, (7) 20.0 and (8) 30.0 and (c): (8) 30.0, (9) 40.0, (10) 60.0, (11) 80.0 and (12) 300.0 mol dm− 3, respectively, at pH 2.90 and temperature 298 K. The dashed curve is the difference spectra (4–1) of aqueous NR in the presence and absence of SDS.
zero which can be attributed to the dicationic form, H2NR2+ of the dye. We have found that the three forms of aqueous neutral red, viz., NR0, HNR+ and H2NR2+ have absorption bands in the visible region with λmax at 450, 528 and 598 nm, respectively. A clear isosbestic point observed at 569 nm in the spectra can be attributed to the acid–base equilibrium between HNR+ and H2NR2+, i.e., to pKa2,w. The pKa1,w and pKa2,w of the dye have been found to be 6.75 and 0.26, respectively. The 598 nm band of the dye has been found to reverse on increasing the pH. The shoulder observed in the submicellar solutions can therefore be rightly attributed to the dicationic form, H2NR2+ of the dye. The observed shift of the λmax from 528 to 508 nm have has been attributed to HNR+ form of the dye solubilized in the micelles [3,4,7,8]. The formation of this shoulder, or in other words, the H2NR2+ form of the dye in the aqueous submicellar SDS solutions at a pH 2.90 or higher is interesting. The cmc of sodium dodecyl sulfate in pure water is 8.31 × 10− 3 mol dm− 3 [13]. Although the cmc should decrease in the presence of the buffer components and the dye, it is unlikely to decrease to about 2 × 10− 4 mol dm− 3 to cause any change to the dye to give rise to the shoulder at 598 nm. So, any type of involvement of micelles in the appearance of this band at the surfactant concentration below 4.00 × 10− 4 mol dm− 3 is unlikely. Thus the interaction between the dye and the surfactant at concentration around 2 × 10− 4 mol dm− 3 and formation of the shoulder at 598 nm may be a premicellar phenomenon. As reported earlier [10,14], it is possible that due to the presence of strong electrostatic and hydrophobic interaction between the oppositely charged dye and surfactant ions, the ions form a close-packed ion pair [15]. In these close-packed ion pairs the original charges on the ions are localized. The localization of the original positive charge of the dye cation paves way for a second protonation of the dye, i.e., a protonated dye-surfactant ion pair (PDSIP), H2NR2+ S− (Scheme 1), is formed as in the case of phenothiazine dyes with SDS [10]. This PDSIP probably breaks down as micelles are formed with the concentration of surfactant
Variations in surface tensions of the buffered aqueous surfactant solutions at pH 2.90 with variation in the surfactant concentration in the presence and absence of NR of concentration of 5.00 × 10− 5 mol dm− 3 are shown in Fig. 3. The absorbance data of the dye in the solutions at 598 nm and 527 nm are included in the figure to help in correlating the surface tension data with the absorbances. The variation in the surface tension of the buffered solution on increasing the concentration of SDS in the absence of the dye has been found to be as expected (Fig. 3a). The surface tension gradually decreased with increase in the concentration of the surfactant, reached a minimum at [SDS] = 2.51 × 10− 3 mol dm− 3 and then almost leveled off. The concentration 2.51 × 10− 3 mol dm− 3 is therefore the cmc of the surfactant in the buffered medium. The plot of the surface tension vs. [SDS] in the presence of the buffer and the dye has been found to be unusual (Fig. 3a). In the submicellar concentration range, the surface tension decreased more rapidly
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Fig. 2. The UV–Visible absorption spectra of aqueous NR (5.0 × 10− 5 mol dm− 3) at pH = 2.95 (1), 0.74 (2), 0.52 (3), 0.30 (4), 0.20 (5), 0.12 (6) and 0.05 (7) at 298 K.
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of the dye can be attributed to the formation of the close-packed dyesurfactant ion pair, HNR+ SDS−, where the ion pair itself behaves like a nonionic surfactant having an efficiency higher than SDS alone. Nonionic surfactants usually have higher efficiency than the corresponding ionic surfactants [13]. The ion-pair formation is a specific interaction between the dye and the surfactant ions and this specific interaction should exist also in the ion-pair monolayer at the air/water interface. The surface tension of a solution is affected by any change in the structure of the monolayer at the air/water interface. It was reported that presence of ptosylate causes major and unexpected changes to the structure of the monolayer of hexadecyltrimethylammonium bromide (CTAB) at the air/ water interface [16]. The area per surfactant molecule in the monolayer of CTAC with p-tosylate counterion was reported to increase by one quarter to accommodate the p-tosylate ions within the monolayer. Similarly, in the present case, the nonionic dye-surfactant ion-pair surfactant has a larger head group than that of SDS and therefore occupies a larger surface area per surfactant in the monolayer at the air/ water interface compared to that of SDS. A larger surface area per surfactant leads to a lower cmc of the ion-pair surfactant. The surface tension decreased as the concentration of SDS was increased up to ca. 1.0×10− 4 mol dm− 3 and above that again started to increase [Fig. 3a]. The surface tension reached a maximum as the concentration of SDS reached ca. 2.0 ×10− 4 mol dm− 3 and then again started to decrease with further increase in [SDS]. The first minimum of the surface tension may correspond to the cmc of the ion-pair surfactant, HNR+ S−. Although the surface tension is expected to remain almost unchanged above cmc, the surface tensions often show a minimum at the cmc in the presence of impurity, here, the dye being the impurity [17]. The cmc of the ion-pair surfactant, cmcip, for HNR+ SDS− has been estimated as 7.9×10− 5 mol dm− 3 (Table 1). It is possible that the protonation of the dye in the ion pair takes place as soon as the ion-pair surfactants start forming micelles of their own. Such ion-pair micelles can also be termed as premicelles [18]. Interestingly, the maximum of the surface tension and the maximum of the absorbance at 598 nm correspond to the same concentration of the surfactant. This suggest involvement of the same species, viz., PDSIP, H2NR2+ S− in appearance of the shoulder at 598 nm and increase in the surface tension above 1.0 ×10− 4 mol dm− 3 of SDS. The increase in surface tension may be due to replacement of some ion pairs in the air/water interface by the PDSIP. The nonionic ion-pair surfactants are transformed into cationic surfactant with a different head group on protonation of the dye in the close-packed ion pair. The increase in the surface tension is probably due to conversion of nonionic ion-pair surfactant to cationic protonated ion-pair surfactant because a cationic surfactant is expected to have lower efficiency of the compared to the corresponding nonionic surfactant. Although the ion-pair formation is a specific interaction between the dye and the surfactant ions, the protonation of the dye in the ion-pair micelles is a result of cooperative interaction since the protonation takes place only after the ion-pair micelles start forming [19,20]. The dye solutions showed similar variations in the presence of SDBS and SDSN also. With SDBS, the estimated cmcip (6.30 × 10− 5 mol dm− 3)
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Fig. 3. Plots of surface tension (mN/m) and absorbance of aqueous NR (5.0 × 10− 5 mol dm− 3) solutions as a function of logarithm of the concentrations of: (a) SDS (b) SDBS and (c) SDSN at pH 2.90 and temperature 298 K. Symbols: surface tension in the presence (Δ) and absence (○) of NR, absorbances at 527 (■) and at 598 nm (▲).
indicating a considerably higher efficiency of the surfactant in the presence of the dye. The value of pC20, which is the concentration of the surfactant required to lower the surface tension by 20 mN m− 1, is a measure of the efficiency of the surfactant. The pC20 has been found to be 3.25 and 3.72 in the absence and the presence of the dye, respectively (Table 1). This increase in the efficiency of the surfactant in the presence
Fig. 4. Illustration of the various dye-surfactant interactions at different concentration ranges of the surfactant, viz., below cmc of the ion-pair surfactant (cmcip), above cmcip but below cmc of the pure surfactant and above cmc.
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Table 2 Equilibrium constants (Kc) and thermodynamic parameters of NR-SDS and NR-SDBS systems at pH 2.90 and at different temperatures Surfactant SDS
Kc'/(dm3/mol− 1)
7
293 298 303 288 293 303
SDBS
a
Kca/(dm3/mol− 1)2
Temperature/K
1.67 × 10 1.46 × 107 1.23 × 107 4.51 × 106 4.06 × 106 2.73 × 106
9.92 × 10
− ΔG°/kJ mol− 1
4
−ΔH°/kJ mol− 1
40.5 40.9 41.1 36.7 37.1 37.3
1.72 × 104
ΔS°/J mol− 1K− 1
9.3
106.0
24.9
42.6
Experimental error limits = ±5%.
is slightly smaller than that with SDS. The increase in the surface tension at above the cmcip of HNR+ SDBS− was also less compared to that after HNR+ SDS−. On the other hand, no minimum of surface tension was observed in the NR-SDSN system. The cmcip of HNR+ SDSN− has been assumed to be the concentration at which the surface tension initially leveled off and estimated to be 1.26 × 10− 4 mol dm− 3. The lower maximum in the surface tension curve for SDBS than that for SDS may be due to lesser replacement of the ion-pair surfactants of the monolayer of the air/water interface in the case of SDBS than in the case of SDS. Absence of such maximum in the case of SDSN may be due to the absence of such substitution in the monolayer. This is expected because the PDSIP formation with SDSN is minimum among all three surfactants as has been indicated by the absorption intensities in the three cases at 598 nm (Fig. 3). When the concentration of the surfactant was increased above the concentration corresponding to the maximum surface tension, the surface tension decreased gradually but remained lower than that in the absence of the dye. The slope of the surface tension vs. surfactant concentration at surface saturation, i.e., near cmc, in the presence of the dye was smaller than that in its absence. This indicates a lower surface excess concentration, Γs of the surfactant in the presence of the dye than its absence at surface saturation. Γs is defined by
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dγ ¼ −4:606RT Γ s d log C1
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ð1Þ
where, dγ is the rate of change in surface tension with concentration at surface saturation, and C1 is the surfactant concentration at surface saturation or cmc [13]. In the case of all three surfactants, the Γs in the presence of the dye have been found to be greater than that in the absence of the dye (Table 1). This observation may be attributed to the gradual break up of the ion pairs and corresponding gradual decrease in the fraction of the ion pairs at the monolayer at the air/water interface. Overcompensation of charge on the ion-pair micelles at large excess of the surfactant may facilitate the observed reversal of deprotonation [21,22]. The cmc's of all three surfactants in the presence of the dye have been found to be almost the same as that in its absence (Table 1). Similarly, the surface tensions at the cmc, which is also a measure of the effectiveness of a surfactant, have been found to be the same in the presence as well as absence of the dye for all three surfactants. This suggests the occupation of the monolayer at the air/water interface by the pure surfactants alone at concentrations above the cmc even in the presence of NR. A summary of the different interactions, viz., the dye-surfactant ionpair formation, monolayer formation by the ion pairs at the air/water interface, micelle formation by the ion pairs, protonation of the dyes in the ion-pair micelles, formation of pure micelles with solubilized dye and replacement of the ion pair in the monolayers by pure micelles at different surfactant concentration ranges have has been illustrated in Fig. 4.
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free dye, [HNR+] and the PDSIP, H2NR2+ S−. The equilibrium of the submicellar PDSIP formation between NR and SDS can be represented by [10]: h i þ Kc H þ HNRþ þ ½S− U H2 NR2þ S−
ð2Þ
d
or h i Kc ¼ H2 NR2þ S− = Hþ HNRþ ½S−
ð3Þ
d
where [S−] and Kc are the concentration of the monomeric surfactant and the equilibrium constant of the interaction, respectively. The equilibrium constants have been determined using Eq. (4) derived from Eq. (3): Kc ¼
i h HNRþ 0 =ðe−e0 Þg= ½Hþ HNRþ 0 − H2 NR2þ S− h i ½S− 0 − H2 NR2þ S−
f d−e
0
ð4Þ
g
where [HNR+]0 and [S−]0 are the initial concentrations of the dye and the surfactant, respectively; d and ε are the absorbance and molar absorption coefficient of the PDSIP [H2NR2+ S−] at the λmax 598 nm in the presence of the surfactant, respectively and ε0 is the molar absorption coefficient of the dye at 598 nm in the absence of the surfactant. An iterative method in C language was used for computation of the equilibrium constants. Only those absorbance values of the λmax of the PDSIP in the submicellar surfactant concentration ranges were used in the calculations whose spectra passed through the isosbestic point. The equilibrium constants determined for the submicellar interactions of NR with SDS and SDBS at fixed pH of 2.90 have been shown
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3.3. The submicellar dye-surfactant equilibrium The presence of isosbestic points in the aqueous NR spectra in the submicellar solutions indicates the presence of equilibria between the
Fig. 5. Plots of log Kc vs. pH for (a) NR-SDS and (b) NR-SDBS system at 298 K. Inset: van't Hoff plot.
B. Gohain et al. / Journal of Molecular Liquids 142 (2008) 130–135
in Table 2 along with the thermodynamic parameters. The large equilibrium constant values can be compared with those observed in similar systems [10]. The equilibrium study was not performed with SDSN because the spectral variations were too less with this surfactant. ΔG° was calculated by using Eq. (5):
135
1.72 × 104 mol dm− 3, respectively, at 298 K [Table 2]. The higher pHindependent equilibrium constant, K′c with SDS than that with SDBS again indicates a stronger interaction in the NR-SDS system than that in the case of NR-SDBS system. 4. Conclusion
◯
ΔG ¼ −RT ln Kc :
ð5Þ
The equilibrium constant and the ΔG° of the interaction with the two surfactants show that the interaction of PDSIP formation is stronger with SDS than that with SDBS. ΔH° and ΔS° were determined by van't Hoff plot from the ΔG° values at 293 K, 298 K and 303 K assuming ΔH° to be constant within this small temperature range. The van't Hoff plots have been shown in the insets of Fig. 5 and the ΔH° and ΔS° values have been included in Table 2. Both negative standard enthalpies and positive standard entropies observed for the present systems favor the interaction. A greater strength with SDBS then with SDS is expected due to greater hydrophobicity of SDBS than that of SDS. However, the observed greater strength with SDS than with SDBS can be attributed to the much higher positive entropy of interaction observed for the former than the latter the interaction being driven by enthalpy as well as entropy. The plots of logarithm of log Kc vs. pH at 298 K for both surfactants are shown in Fig. 5. It can be seen from the figure that log Kc increases linearly with the experimental pH in both cases. The linearity is expected as Kc is inversely proportional to [H+] in Eq. (2). The linearity of the plots also suggests the validity of Eq. (2) which is based on 1:1 dye-surfactant interaction. The intercept of the plots gives the overall equilibrium constant, Kc, which have been included in Table 2. However, the intercepts have been found to be less than the expected value of unity in all cases, which may be due to the presence of some more factors in the interaction. Thus, from a linear equation between log Kc and pH, we get −1 þ log K cV: log Kc ¼ m log Hþ
ð6Þ
The slopes for NR-SDS and NR-SDBS have been found to be 0.75 and 0.96, respectively. Assuming m to be unity (which is actually not far from unity), we get Kc ¼ K cV= Hþ
ð7Þ
which is essentially the same as Eq. (3) with h i K cV ¼ H2 NR2þ S− = HNRþ ½S− :
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
ð8Þ
Thus, the observed linear relation between the logarithm of Kc and pH in fact supports our assumption that the interaction product between NR and the submicellar anionic surfactants is doubly PDSIP, i.e., H2NR2+ S− as shown in Eq. (2). The constant K′, c where,
·
K cV ¼ Kc Hþ
From the present study it is evident that the closed packed dyesurfactant ion pairs are formed between NR and anionic surfactants in submicellar aqueous solutions due to the prevailing hydrophobic and electrostatic forces in the systems. The ion pairs behave like a nonionic surfactant having greater efficiency and lower CMC than the corresponding pure anionic surfactants. The ion-pair surfactants predominate in the air/water interface at low concentrations of the anionic surfactant. Protonation of the dye in the dye-surfactant ion pair occurs as the ion-pair surfactants start forming micelle. The observed linear relationship of the logarithm of the equilibrium constant of the protonation with pH consolidates the assumption of formation of a 1:1 PDSIP. The equilibrium constants suggest stronger interaction with SDS than with SDBS. As the CMC of the pure surfactant is approached the ion pairs of the monolayer at the air/water interface are replaced by the pure surfactant ions accompanied by simultaneous transition from the PDSIP micelles to pure surfactant micelles with solubilized dye. Above the CMC of the pure surfactant, the monolayer at the air/water interface consists of only pure surfactant ions.
ð9Þ
may be considered as another equilibrium constant which is independent of the hydrogen ion concentration or pH. The values of K′c for the dye with SDS and SDBS have been found to be 9.92 × 104 and
[18] [19] [20] [21] [22]
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