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Mixed adsorption layers at the aqueous Cn TAB solution/hexane vapour interface夽 N. Mucic a,∗ , N. Moradi a , A. Javadi a , E.V. Aksenenko b , V.B. Fainerman c , R. Miller a a b c
Max-Planck Institute of Colloids & Interfaces, Potsdam/Golm, Germany Institute of Colloid Chemistry and Chemistry of Water, Kyiv (Kiev) 03680, Ukraine Donetsk Medical University, 16 Ilych Avenue, Donetsk 83003, Ukraine
h i g h l i g h t s
g r a p h i c a l
• Hexane molecules adsorb from the
Oil vapor molecules adsorb at the surface of aqueous surfactant solutions. This adsorption significantly decreases the surface tension of water and in particular of aqueous surfactant solution. Therefore, due to the oil molecules’ adsorption the physical properties of the adsorption layer change and can be represented like a mixed interfacial layer.
vapour phase at the surfactant solution surface. • The presence of hexane enhances the Cn TAB adsorption. • High Cn TAB concentration, hexane molecules are removed from the adsorption layer. • Good theoretical fitting with a model for competitive adsorption of two components.
a r t i c l e
i n f o
Article history: Received 7 January 2013 Received in revised form 11 September 2013 Accepted 17 September 2013 Available online xxx Keywords: Cationic surfactants Adsorption isotherms Water–alkane vapour interface Chain length dependence Interfacial tension Drop profile analysis tensiometry
a b s t r a c t
a b s t r a c t Surface active molecules, which are amphiphilic in aqueous solutions, have different properties at water/air and water/oil interfaces. However, their behaviour at water/oil vapour interfaces is still unknown and requires further investigations. The adsorption behaviour of the members of the homologous series Cn TAB at their aqueous solution/hexane vapour interface is studied using the drop profile analysis method. The data thus obtained are compared with results obtained earlier for the solution/air and solution/liquid hexane interfaces. The competitive adsorption of the components from both phases onto the interface is shown to take place, resulting in a significant enhancement of the Cn TAB adsorption as compared with that at the solution/air interface. The theoretical model of the mixed adsorption monolayer developed earlier, which assumes the intermolecular interaction within the adsorption layer, is shown to be capable for the quantitative description of the observed phenomenon. © 2013 Published by Elsevier B.V.
1. Introduction
夽 This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-No Derivative Works License, which permits non-commercial use, distribution, and reproduction in any medium, provided the original author and source are credited. ∗ Corresponding author. Tel.: +49 3315679259.. E-mail address: [email protected] (N. Mucic).
The surface properties of aqueous solutions of the homologous series of alkyl trimethyl ammonium bromides (Cn TAB) or chlorides (Cn TAC) have been investigated frequently due to the importance of these cationic surfactants in many applications. First investigations of the Cn TABs adsorption isotherms and foam film properties did not involve any theoretical interpretation [1]. More information
0927-7757/$ – see front matter © 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.colsurfa.2013.09.019
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of Cn TAB surface layer properties is summarised in [2]. The reorientation model, which assumes the intrinsic compressibility of adsorbed molecules, provides a good description of the adsorption behaviour of Cn TABs at the aqueous solution/air interface [3]. For aqueous solution/oil interfaces, however, there were rather few attempts to determine the adsorption parameters systematically. However, the experimental data of Cn TAB thermodynamic properties at water/oil interfaces were analysed using Langmuir and Frumkin adsorption models [4,5]. They found that the surfactant molar area and the surface concentration depend not only on the surface coverage by the surfactant molecules but also on the chain lengths of the alkane. Accordingly, the surfactants and oils with different chain lengths interact in different ways. Surfactants with longer chain length are able to squeeze out short chain oil molecules from the adsorption layer [4]. In [6] the dynamic and rheological properties of such systems were investigated. In short, dynamics of the water/oil interface is more complex than could be expected from a classical diffusion controlled adsorption or exchange of matter process of surfactants at the water/air interface. This is obviously caused by the interaction between the surfactants’ alkyl chains and the oil molecules. Hence, it is very important to get a deeper insight into the interactions between surfactant and oil molecules based on improved fundamental knowledge, having in mind the extensive industrial applications of these substances. In addition to investigations of ionic surfactants at the water/air and water/oil interface, studies were also performed for adsorption layers at the surface of an aqueous solution covered by a thin oil film [7] or in presence of alkane vapour in the gas phase [8]. More systematic studies on the dynamics and thermodynamics of surfactant adsorption layers formed from aqueous solutions in contact with alkane vapour were carried out by Javadi et al. [9,10]. The results indicate that the water/alkane vapour represents a kind of intermediate situation between the water/air and water/alkane interface because the presence of alkane molecules in the gas phase leads to a co-adsorption or competitive adsorption of the oil molecules and the surfactants. In the present work we measured the interfacial tension of aqueous solutions of the cationic surfactants Cn TAB with different chain lengths (C10 TAB, C12 TAB, C14 TAB and C16 TAB) against hexane vapour. The experiments were performed on the drop shape analysis tensiometer a bit modified to work on the water/vapour interface. The full mechanism of the oil molecules adsorption at surfactant aqueous surface is still unclear. In addition, the interaction between surfactant and oil vapour molecules can depend on whether the bulk oil phase is present or absent in the system. We tried to answer the question of the adsorbed amounts of hexane at the aqueous surfactant solution/hexane vapour interface, while the cationics Cn TAB of different alkyl chain lengths represent the surfactant molecules. We have theoretically approximated a physical picture behind the surfactant–hexane interaction at the interface and compare the findings with data measured at the water/air interface.
2. Materials and methods The substances investigated in this paper, C10 TAB (decyl trimethylammonium bromide, Mw = 280.29 g/mol), C12 TAB (dodecyl trimethylammonium bromide, Mw = 308.35 g/mol), C14 TAB (tetradecyl trimethylammonium bromide, Mw = 336.40 g/mol) and C16 TAB (hexadecyl trimethylammonium bromide, Mw = 364.46 g/mol) were purchased from Fluka (Switzerland) with a purity of >99%. The substances were additionally purified by a triple recrystallisation with an ethanol/acetone mixture. All solutions were prepared in 10 mM NaH2 PO4 /NaHPO4 phosphate
Fig. 1. Effect of hexane vapour on the dynamic interfacial tension of C10 TAB solution drops at different concentrations of 10−7 mol/l (1); 10−5 mol/l (2); 10−4 mol/l (3); 10−3 mol/l (4); 10−2 mol/l (5); 10−1 mol/l (6); 1 ml of hexane was injected at t = 300 s at the bottom of the cuvette.
buffer, pH 7, (Fluka, >99%) using ultrapure Milli-Q water (resistivity = 18.2 M cm). The experiments were performed at room temperature (23–24 ◦ C). Hexane was purchased from Fluka, distilled, purified with aluminium oxide and subsequently saturated with ultrapure Milli-Q water. The experiments were performed with the drop profile analysis tensiometer PAT-1 (SINTERFACE Technologies, Germany). In brief, the principle of the experiments consists in forming a drop in a closed cuvette (3 cm × 3 cm × 3 cm), and after a certain time (typically 300 s) a defined amount of hexane (1 ml) is injected to the bottom of the cuvette. The cell is closed such that after a few minutes the saturated hexane vapour atmosphere is established. The experimental details were explained elsewhere [10,11]. 3. Results and discussion To investigate the interaction between Cn TAB adsorption layer and hexane vapour phase we performed the experiments according to the process described in [10], where a drop of surfactant solution was initially formed in air and subsequently the atmosphere around the drop is saturated with hexane vapour. First, the interfacial tension decreases due to adsorption of the surfactant molecules from the drop bulk. Second, the formed hexane atmosphere leads to a significant additional decrease of interfacial tension. To determine the Cn TAB adsorption time sufficient to attain the equilibrium, the results published in the literature were used in the theoretical calculation. In particular, in concentrated solutions (above 5 × 10−5 mol/l) of non-ionic C10 EO4 , the dynamic surface tension attains the equilibrium during a very short time, and then remains virtually constant in the time range of 5 to 300–500 s (see [12], Fig. 9). The studies [3] of the dynamic and equilibrium interfacial tension of Cn TAB solutions without any addition of electrolytes have shown that for aqueous solutions of C14 TAB the equilibrium is attained in approximately 10 s. Similar values were found for SDS solutions with the addition of NaCl [13]: at concentrations above 10−4 mol/l and the necessary retention interval 1 to 10 s. In Figs. 1–4 the initial time range, when the interfacial tension decreases from 72 mN/m to the shown values, is omitted in these curves, because the drop profile method is incapable to follow the system evolution below 1 s. The interfacial tension of Cn TAB solutions of various concentrations at the solution/air interface, measured after 300 s from the beginning of the adsorption process (Figs. 1–4) is shown in Fig. 5. These values were also compared with the equilibrium values reported in [4], where the drop profile method was used to study the equilibrium surface and interface tension of the Cn TAB solutions (in the presence of 0.01 M phosphate buffer) at the interfaces with air and liquid hexane. In these experiments, to ensure
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Fig. 2. Effect of hexane vapour on dynamic interfacial tension of C12 TAB solution drops at different concentrations of 10−6 mol/l (1); 10−5 mol/l (2); 10−4 mol/l (3); 10−3 mol/l (4); 10−2 mol/l (5); 1 ml of hexane was injected at t = 300 s at the bottom of the cuvette.
Fig. 5. Equilibrium interfacial tension of Cn TAB solutions at the solution/air interface: solid squares, present study; open circles, data from [4]; curves calculated from Eqs. (1)–(3) with the parameters listed in Table 1.
For the Frumkin model, the equations of state and adsorption isotherm are: ˘ =−
Fig. 3. Effect of hexane vapour on dynamic interfacial tension of C14 TAB solution drops at different concentrations of 10−6 mol/l (1); 10−5 mol/l (2); 10−4 mol/l (3); 2 × 10−3 mol/l (4); 1 ml of hexane was injected at t = 300 s at the bottom of the cuvette.
equilibration, the concentrated solutions were aged during more than 1 h, while for the diluted solutions the retention interval was about 12 h. It is seen that the present data agree well with the equilibrium values obtained in [4] for C10 TAB and C12 TAB solutions. For C14 TAB and C16 TAB solutions at concentrations 10−5 mol/l and less the equilibrium is not attained, as the adsorption time of 300 s was too small. The theoretical dependencies of interfacial tension on bulk concentration of Cn TAB solutions were calculated using the Frumkin model. Note that this model was used here to describe the behaviour of ionic surfactant (instead of more rigorous equations which account for the dissociation and the presence of inorganic electrolyte [3,14]) because in our experiments the phosphate buffer (10 mM NaH2 PO4 /NaHPO4 ) was present in the solutions. These salts, being dissolved in water, undergo significant dissociation [4] which increases the adsorption activity of Cn TAB.
Fig. 4. Effect of hexane vapour on dynamic interfacial tension of C16 TAB solution drops at different concentrations of 10−7 mol/l (1); 10−6 mol/l (2); 3 × 10−6 mol/l (3); 10−5 mol/l (4); 3 × 10−5 mol/l (5); 10−4 mol/l (6); 3 × 10−4 mol/l (7): 1 ml of hexane was injected at t = 300 s at the bottom of the cuvette.
bc =
RT [ln(1 − ) + a 2 ] ω0
exp(−2a) 1−
(1)
(2)
where ˘ is the surface pressure (˘ = 0 − ), and 0 are the interfacial tensions of the solution and the solvent, respectively, ω0 is the partial molar area of the ionic surfactant at ˘ = 0, a is the intermolecular interaction constant, b is the adsorption equilibrium constant, c is the surfactant bulk concentration, = ω is the surface layer coverage by the surfactant, is the surfactant adsorption, R is the gas law constant, and T is the temperature. The molar area of the surfactant ω in Eqs. (1) and (2) depends linearly on the surface pressure [4]: ω = ω0 (1 − ε˘)
(3)
where ε is the two-dimensional relative surface layer compressibility coefficient, which characterises the intrinsic compressibility of the molecules in the surface layer. The dependencies of interfacial tension on the Cn TAB concentration as calculated from Eqs. (1)–(3) are shown as curves in Fig. 5. These curves satisfactory describe both the results obtained in the present study and the data reported in [4] where a similar buffer was used. The values a = 0 and ε = 0.005 m/mN were used for all Cn TAB studied; other model parameters are listed in Table 1. Note, the ω0 values for all the Cn TAB were found to be similar, while the adsorption equilibrium constants b become essentially higher (approximately by a factor of 7–9) with the increase of the alkyl chain length of the Cn TAB molecule by two methylene groups. Let us consider next the interfacial tension values for the adsorption of hexane from the vapour phase onto a drop of Cn TAB solution, i.e., in 500−700 s after the experiment started (see Figs. 1–4). These results are illustrated in Fig. 6. From these graphs the significant decrease of interfacial tension upon co-adsorption of hexane molecules from the vapour phase at the liquid surface can be easily seen. In Fig. 6 the interfacial tension values at the solution/hexane vapour interface are significantly lower than those at the solution/air interface, although remain higher than the values reported in [4] for the interfacial tension of Cn TAB solutions at the solution/liquid hexane interface. For high Cn TAB concentrations the ratio of the difference between the interfacial tension at the
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Table 1 Model parameters for Cn TAB adsorption layers at the solution/air and solution/hexane vapour interface; other parameters which are equal for all the substances are listed in the text. n
Cn TAB solution/air interface, Eqs. (1)–(3) b/m3 mol−1
a12
b2 /m3 mol−1
2.5 × 10 2.5 × 105 2.6 × 105 2.6 × 105
0.249 1.95 18.0 110
1.6 2.4 2.4 1.8
0.75 7.0 110 700
5
10 12 14 16
Cn TAB solution/hexane vapour interface, Eqs. (3)–(7)
ω0 /m2 /mol
The adsorption isotherm equations for the two components of the mixture were also derived in [15]. For hexane the adsorption isotherm is: d1 P1 + k1 2 =
1 exp[−2a1 1 − 2a12 2 ], (1 − 1 − 2 )
(6)
and for the water-soluble surfactants Cn TAB the adsorption isotherm reads: b2 c2 =
Fig. 6. Dependence of equilibrium interfacial tension for Cn TAB solutions at the solution/hexane vapour interface: filled symbols, experimental data; bold curves, values calculated from Eqs. (3)–(7) with the parameters listed in Table 1; thin curves, adsorption at the solution/air interface re-plotted from Fig. 5; open symbols, experimental data corrected to account for the C16 TAB depletion in the drop bulk due to adsorption (see text).
solution/air interface and that on the solution/hexane vapour interface to the difference between the interfacial tension at the solution/air interface and that on the solution/liquid hexane interface is about 0.5, while for low Cn TAB solution concentrations this ratio is smaller. To evaluate the effect caused by the simultaneous adsorption of Cn TAB from the solution and hexane from the vapour phase onto the drop surface, the model proposed in [15] was used. The equations for the mixed surface layer are listed below. The equation of state reads: −
˘ω0∗ RT
= ln(1 − 1 − 2 ) + a1 12 + a2 22 + 2a12 1 2
(4)
with ω0∗ =
ω10 1 + ω20 2 1 + 2
(5)
Here i = ωi · i are the surface coverages, i is the adsorption, ω10 is the molar area at zero surface pressure by molecules of component i (i = 1 for hexane, and i = 2 for Cn TAB). The coefficients a1 , a2 and a12 are the Frumkin interaction constants. The molar area of the components can be approximated by a linear dependence on surface pressure ˘ and the total surface coverage = 1 + 2 , similarly to Eq. (3). Note that Eq. (4) assumes ω10 ∼ = ω20 .
2 exp[−2a2 2 − 2a12 1 ], (1 − 1 − 2 )
(7)
where P1 is the partial pressure of the component 1 (hexane) in the gas phase (Torr or Pa), and d1 is its adsorption activity coefficient (1/Torr or 1/Pa, respectively). The partial pressure of saturated hexane vapour at 25 ◦ C is ca. 150 Torr (or 2 × 104 Pa). The additional term k1 2 on the left hand side in Eq. (6) for k1 > 0 accounts for the influence of adsorbed Cn TAB on the adsorption of hexane from the vapour phase. This approach was used earlier in [10] to estimate the influence of hexane vapour on the interfacial tension of C12 TAB aqueous solutions. The corresponding model parameters for the solution/air interface were found in [10] to be quite similar to those estimated above: ω0 = 2.6 × 10−5 m2 /mol, b = 2.2 m3 mol−1 , a = 0 (cf. Table 1). For the C12 TAB solution/hexane vapour interface the coefficient k1 in Eq. (6) was found to be 15. The influence of hexane vapour on the equilibrium (retention time not less than 2000s) interfacial tension of C10 EO8 solutions in a wide concentrations range was studied in [15]. The experimental results were shown to be satisfactory described by Eqs. (3)–(7) with k1 = 30. However the best fit with the experimental data was achieved when the constant b was several times increased as compared to its value for the individual C10 EO8 solution. This could be possibly ascribed to the fact that the co-adsorption of hexane from vapour phase onto the C10 EO8 solution boundary leads to an increase of the C10 EO8 surface activity. In this regard, it should be also noted that for the adsorption of C10 EO8 at the solution/liquid hexane interface the constant b becomes yet much higher than its value for the individual C10 EO8 solutions [15]. In our calculations the values of the model parameters in Eqs. (3)–(7) were: k1 = 0, a1 = 0 and a2 = 0. To calculate the hexane vapour adsorption on the drop surface, the values d1 = 6 × 10−5 1/Pa and ω10 = 3.5 × 105 m2 /mol were found to provide a good fit to the experimental data. These values are close to those used in [10], where the same experimental procedure was employed. The d1 value used in [15] was by a factor of 5 higher than that shown above, because the adsorption time at the interface with the hexane vapour in [15] was much higher than that in the experiments described here. Moreover, much more important is the fact that in [15], in contrast to the present work, the simultaneous adsorption of both components was studied: first the cell was saturated by hexane vapour, and only afterwards the drop of the aqueous C10 EO8 solution was formed in the hexane vapour atmosphere, which resulted in essentially higher hexane adsorption. Other model parameters, which were specific for each Cn TAB, are listed in Table 1; note that ω20 values were taken equal to the ω0 values estimated for the Cn TAB at the solution/air interface.
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this effect becomes pronounced only at high surfactant concentrations. At low Cn TAB concentrations a slight increase of hexane adsorption takes place with increasing Cn TAB concentration. This is caused by intermolecular attraction between the Cn TAB and hexane molecules. Therefore, at low concentrations the adsorption of Cn TAB promotes the hexane adsorption, while at higher concentrations the increased adsorption of Cn TAB results in enabling of hexane molecules to penetrate in the adsorption layer, as discussed above. 4. Conclusions
Fig. 7. Dependencies of the simultaneous adsorption of Cn TAB (bold curves) and hexane (dashed curves) at the Cn TAB solution/hexane vapour interface on the Cn TAB concentration, as calculated with Eqs. (3)–(7); the thin curves are calculated with Eqs. (1)–(3) for the Cn TAB adsorption at the solution/air interface.
It is seen from the data shown in Table 1 that the b2 value (for the Cn TAB solution/hexane vapour interface) is essentially higher than the b value (for the Cn TAB solution/air interface), in agreement with the trend observed in [15]. Also for the mixed monolayer the a12 values are high and positive, which indicates essential intermolecular (possibly hydrophobic) interaction. Fig. 6 illustrates the interfacial tension isotherms (bold curves) calculated using Eqs. (3)–(7); it is seen that the dependencies for the C10 TAB and C12 TAB solutions agree well with the experimental data, while for C14 TAB and especially C16 TAB solutions the agreement is observed only for high concentrations of the solution. As it was indicated above, at the solution/air interface for concentrations C16 TAB less than 10−5 mol/l the adsorption equilibrium cannot be reached during the available 300 s. This is also true for the solution/hexane vapour interface for 500–700 s. Besides, this behaviour for C16 TAB could be ascribed partially to the decrease of the surfactant concentration in the drop bulk due to the adsorption at the drop surface. This phenomenon was analysed in detail earlier [16]. The decrease of bulk concentration c caused by the adsorption of the surfactant can be estimated as c = ·S/V, where S and V are the surface area and volume of the drop, respectively, and the adsorption is calculated from model equation at the given experimental value. In our experiments the ratio S/V was 1.5−2.0 mm−1 [16]; therefore the initial C16 TAB concentrations 10−6 , 3 × 10−6 and 10−5 mol/l correspond to actual concentration values 3 × 10−7 , 1.4 × 10−6 and 6 × 10−6 mol/l, respectively. These results, which are shown in Fig. 6 (open symbols) are seen to be essentially closer to the theoretical curve for C16 TAB. Fig. 7 illustrates the dependencies of the equilibrium adsorption of components at the Cn TAB solution/hexane vapour and Cn TAB solution/air interfaces on the Cn TAB concentration calculated using Eqs. (3)–(7) and (1)–(3), respectively, with the parameters listed above. It is seen that for low surfactant concentration the surfactant adsorption at the solution/hexane vapour interface is much higher than that on the solution/air interface (note the logarithmic adsorption scale in the figure), and only at high Cn TAB concentrations these adsorptions become approximately equal. Accordingly, the hexane adsorption on the solution drop surface is high at low Cn TAB concentrations, and decreases with the Cn TAB concentration increase. This competitive adsorption is governed by the surface activity of the constituents, which is higher for Cn TAB than for hexane. Also, the higher is the surface activity of the surfactant, the lower are the concentrations at which this effect is observed: among all the surfactants studied, the hexane adsorption decrease appears at lower C16 TAB concentrations, while for C10 TAB solutions
The present study was dedicated to the adsorption behaviour of members of the Cn TAB homologous series (n = 10, 12, 14 and 16) at the solution/hexane vapour interface. The drop profile analysis method was employed to measure the dynamic interfacial tension: first the adsorption of Cn TAB dissolved in the drop bulk took place, and subsequently the cell in which the drop was formed was saturated by hexane vapour. From the dynamic interfacial tension curves a significant and faster decrease of the interfacial tension is seen upon releasing the hexane vapour into the atmosphere around the Cn TAB solution drop. The quasi-equilibrium values of the interfacial tension at the solution/vapour interface were compared with the results obtained for the solution/air and solution/liquid hexane interfaces. For the theoretical analysis of the results, we used the model developed in [15] valid for equilibrium conditions of the system. This model assumes the competitive adsorption of the components from both phases onto the interface: adsorption of Cn TAB from the solution in the drop bulk, and hexane adsorption from the gas phase. The model takes into account the mutual influence of components and the intermolecular interaction within the adsorbed layer. The theory is shown to be in a good agreement with the experimental results for C10 TAB and C12 TAB solutions, demonstrating the significant increase of their adsorption caused by the presence of hexane. This results in the increase of the adsorption equilibrium constant by several times, and high intermolecular interaction constant values. The results obtained for the C14 TAB and C16 TAB solutions also comply with the model predictions; however, to obtain better agreement between the theory and the experimental data at low surfactant concentration, the depletion of the surfactant solution in the drop bulk caused by the adsorption would have to be taken into account. Acknowledgements The work was financially supported by projects of the DFG (Mi418/18-1), the DLR (50WM1129), the European Space Agency (PASTA) and the COST actions CM1101 and MP1106. References [1] V. Bergeron, Disjoining pressures and film stability of alkyltrimethylammonium bromide foam films, Langmuir 13 (1997) 3474–3482. [2] V.B. Fainerman, R. Miller, Thermodynamics of adsorption of surfactants at the solution-fluid interface, in: V.B. Fainerman, D. Möbius, R. Miller (Eds.), Surfactants – Chemistry, Interfacial Properties and Application, Studies in Interface Science, vol. 13, Elsevier, 2001, pp. 99–188. [3] C. Stubenrauch, V.B. Fainerman, E.V. Aksenenko, R. Miller, Adsorption behaviour and dilatational rheology of the cationic alkyl trimethyl ammonium bromides at the water/air interface, J. Phys. Chem. 109 (2005) 1505–1509. [4] V. Pradines, V.B. Fainerman, E.V. Aksenenko, J. Krägel, N. Mucic, R. Miller, Adsorption of alkyl trimethylammonium bromides at the water/air and water/hexane interfaces, Colloids Surf. A 371 (2010) 22–28. [5] K. Medrzycka, W. Zwierzykowski, Adsorption of alkyltrimethylammonium bromides at the various interfaces, J. Colloid Interface Sci. 230 (2000) 67–72.
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Mixed adsorption layers at the aqueous Cn TAB solution/hexane vapour interface夽 N. Mucic a,∗ , N. Moradi a , A. Javadi a , E.V. Aksenenko b , V.B. Fainerman c , R. Miller a a b c
Max-Planck Institute of Colloids & Interfaces, Potsdam/Golm, Germany Institute of Colloid Chemistry and Chemistry of Water, Kyiv (Kiev) 03680, Ukraine Donetsk Medical University, 16 Ilych Avenue, Donetsk 83003, Ukraine
h i g h l i g h t s
g r a p h i c a l
• Hexane molecules adsorb from the
Oil vapor molecules adsorb at the surface of aqueous surfactant solutions. This adsorption significantly decreases the surface tension of water and in particular of aqueous surfactant solution. Therefore, due to the oil molecules’ adsorption the physical properties of the adsorption layer change and can be represented like a mixed interfacial layer.
vapour phase at the surfactant solution surface. • The presence of hexane enhances the Cn TAB adsorption. • High Cn TAB concentration, hexane molecules are removed from the adsorption layer. • Good theoretical fitting with a model for competitive adsorption of two components.
a r t i c l e
i n f o
Article history: Received 7 January 2013 Received in revised form 11 September 2013 Accepted 17 September 2013 Available online xxx Keywords: Cationic surfactants Adsorption isotherms Water–alkane vapour interface Chain length dependence Interfacial tension Drop profile analysis tensiometry
a b s t r a c t
a b s t r a c t Surface active molecules, which are amphiphilic in aqueous solutions, have different properties at water/air and water/oil interfaces. However, their behaviour at water/oil vapour interfaces is still unknown and requires further investigations. The adsorption behaviour of the members of the homologous series Cn TAB at their aqueous solution/hexane vapour interface is studied using the drop profile analysis method. The data thus obtained are compared with results obtained earlier for the solution/air and solution/liquid hexane interfaces. The competitive adsorption of the components from both phases onto the interface is shown to take place, resulting in a significant enhancement of the Cn TAB adsorption as compared with that at the solution/air interface. The theoretical model of the mixed adsorption monolayer developed earlier, which assumes the intermolecular interaction within the adsorption layer, is shown to be capable for the quantitative description of the observed phenomenon. © 2013 Published by Elsevier B.V.
1. Introduction
夽 This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-No Derivative Works License, which permits non-commercial use, distribution, and reproduction in any medium, provided the original author and source are credited. ∗ Corresponding author. Tel.: +49 3315679259.. E-mail address: [email protected] (N. Mucic).
The surface properties of aqueous solutions of the homologous series of alkyl trimethyl ammonium bromides (Cn TAB) or chlorides (Cn TAC) have been investigated frequently due to the importance of these cationic surfactants in many applications. First investigations of the Cn TABs adsorption isotherms and foam film properties did not involve any theoretical interpretation [1]. More information
0927-7757/$ – see front matter © 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.colsurfa.2013.09.019
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of Cn TAB surface layer properties is summarised in [2]. The reorientation model, which assumes the intrinsic compressibility of adsorbed molecules, provides a good description of the adsorption behaviour of Cn TABs at the aqueous solution/air interface [3]. For aqueous solution/oil interfaces, however, there were rather few attempts to determine the adsorption parameters systematically. However, the experimental data of Cn TAB thermodynamic properties at water/oil interfaces were analysed using Langmuir and Frumkin adsorption models [4,5]. They found that the surfactant molar area and the surface concentration depend not only on the surface coverage by the surfactant molecules but also on the chain lengths of the alkane. Accordingly, the surfactants and oils with different chain lengths interact in different ways. Surfactants with longer chain length are able to squeeze out short chain oil molecules from the adsorption layer [4]. In [6] the dynamic and rheological properties of such systems were investigated. In short, dynamics of the water/oil interface is more complex than could be expected from a classical diffusion controlled adsorption or exchange of matter process of surfactants at the water/air interface. This is obviously caused by the interaction between the surfactants’ alkyl chains and the oil molecules. Hence, it is very important to get a deeper insight into the interactions between surfactant and oil molecules based on improved fundamental knowledge, having in mind the extensive industrial applications of these substances. In addition to investigations of ionic surfactants at the water/air and water/oil interface, studies were also performed for adsorption layers at the surface of an aqueous solution covered by a thin oil film [7] or in presence of alkane vapour in the gas phase [8]. More systematic studies on the dynamics and thermodynamics of surfactant adsorption layers formed from aqueous solutions in contact with alkane vapour were carried out by Javadi et al. [9,10]. The results indicate that the water/alkane vapour represents a kind of intermediate situation between the water/air and water/alkane interface because the presence of alkane molecules in the gas phase leads to a co-adsorption or competitive adsorption of the oil molecules and the surfactants. In the present work we measured the interfacial tension of aqueous solutions of the cationic surfactants Cn TAB with different chain lengths (C10 TAB, C12 TAB, C14 TAB and C16 TAB) against hexane vapour. The experiments were performed on the drop shape analysis tensiometer a bit modified to work on the water/vapour interface. The full mechanism of the oil molecules adsorption at surfactant aqueous surface is still unclear. In addition, the interaction between surfactant and oil vapour molecules can depend on whether the bulk oil phase is present or absent in the system. We tried to answer the question of the adsorbed amounts of hexane at the aqueous surfactant solution/hexane vapour interface, while the cationics Cn TAB of different alkyl chain lengths represent the surfactant molecules. We have theoretically approximated a physical picture behind the surfactant–hexane interaction at the interface and compare the findings with data measured at the water/air interface.
2. Materials and methods The substances investigated in this paper, C10 TAB (decyl trimethylammonium bromide, Mw = 280.29 g/mol), C12 TAB (dodecyl trimethylammonium bromide, Mw = 308.35 g/mol), C14 TAB (tetradecyl trimethylammonium bromide, Mw = 336.40 g/mol) and C16 TAB (hexadecyl trimethylammonium bromide, Mw = 364.46 g/mol) were purchased from Fluka (Switzerland) with a purity of >99%. The substances were additionally purified by a triple recrystallisation with an ethanol/acetone mixture. All solutions were prepared in 10 mM NaH2 PO4 /NaHPO4 phosphate
Fig. 1. Effect of hexane vapour on the dynamic interfacial tension of C10 TAB solution drops at different concentrations of 10−7 mol/l (1); 10−5 mol/l (2); 10−4 mol/l (3); 10−3 mol/l (4); 10−2 mol/l (5); 10−1 mol/l (6); 1 ml of hexane was injected at t = 300 s at the bottom of the cuvette.
buffer, pH 7, (Fluka, >99%) using ultrapure Milli-Q water (resistivity = 18.2 M cm). The experiments were performed at room temperature (23–24 ◦ C). Hexane was purchased from Fluka, distilled, purified with aluminium oxide and subsequently saturated with ultrapure Milli-Q water. The experiments were performed with the drop profile analysis tensiometer PAT-1 (SINTERFACE Technologies, Germany). In brief, the principle of the experiments consists in forming a drop in a closed cuvette (3 cm × 3 cm × 3 cm), and after a certain time (typically 300 s) a defined amount of hexane (1 ml) is injected to the bottom of the cuvette. The cell is closed such that after a few minutes the saturated hexane vapour atmosphere is established. The experimental details were explained elsewhere [10,11]. 3. Results and discussion To investigate the interaction between Cn TAB adsorption layer and hexane vapour phase we performed the experiments according to the process described in [10], where a drop of surfactant solution was initially formed in air and subsequently the atmosphere around the drop is saturated with hexane vapour. First, the interfacial tension decreases due to adsorption of the surfactant molecules from the drop bulk. Second, the formed hexane atmosphere leads to a significant additional decrease of interfacial tension. To determine the Cn TAB adsorption time sufficient to attain the equilibrium, the results published in the literature were used in the theoretical calculation. In particular, in concentrated solutions (above 5 × 10−5 mol/l) of non-ionic C10 EO4 , the dynamic surface tension attains the equilibrium during a very short time, and then remains virtually constant in the time range of 5 to 300–500 s (see [12], Fig. 9). The studies [3] of the dynamic and equilibrium interfacial tension of Cn TAB solutions without any addition of electrolytes have shown that for aqueous solutions of C14 TAB the equilibrium is attained in approximately 10 s. Similar values were found for SDS solutions with the addition of NaCl [13]: at concentrations above 10−4 mol/l and the necessary retention interval 1 to 10 s. In Figs. 1–4 the initial time range, when the interfacial tension decreases from 72 mN/m to the shown values, is omitted in these curves, because the drop profile method is incapable to follow the system evolution below 1 s. The interfacial tension of Cn TAB solutions of various concentrations at the solution/air interface, measured after 300 s from the beginning of the adsorption process (Figs. 1–4) is shown in Fig. 5. These values were also compared with the equilibrium values reported in [4], where the drop profile method was used to study the equilibrium surface and interface tension of the Cn TAB solutions (in the presence of 0.01 M phosphate buffer) at the interfaces with air and liquid hexane. In these experiments, to ensure
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Fig. 2. Effect of hexane vapour on dynamic interfacial tension of C12 TAB solution drops at different concentrations of 10−6 mol/l (1); 10−5 mol/l (2); 10−4 mol/l (3); 10−3 mol/l (4); 10−2 mol/l (5); 1 ml of hexane was injected at t = 300 s at the bottom of the cuvette.
Fig. 5. Equilibrium interfacial tension of Cn TAB solutions at the solution/air interface: solid squares, present study; open circles, data from [4]; curves calculated from Eqs. (1)–(3) with the parameters listed in Table 1.
For the Frumkin model, the equations of state and adsorption isotherm are: ˘ =−
Fig. 3. Effect of hexane vapour on dynamic interfacial tension of C14 TAB solution drops at different concentrations of 10−6 mol/l (1); 10−5 mol/l (2); 10−4 mol/l (3); 2 × 10−3 mol/l (4); 1 ml of hexane was injected at t = 300 s at the bottom of the cuvette.
equilibration, the concentrated solutions were aged during more than 1 h, while for the diluted solutions the retention interval was about 12 h. It is seen that the present data agree well with the equilibrium values obtained in [4] for C10 TAB and C12 TAB solutions. For C14 TAB and C16 TAB solutions at concentrations 10−5 mol/l and less the equilibrium is not attained, as the adsorption time of 300 s was too small. The theoretical dependencies of interfacial tension on bulk concentration of Cn TAB solutions were calculated using the Frumkin model. Note that this model was used here to describe the behaviour of ionic surfactant (instead of more rigorous equations which account for the dissociation and the presence of inorganic electrolyte [3,14]) because in our experiments the phosphate buffer (10 mM NaH2 PO4 /NaHPO4 ) was present in the solutions. These salts, being dissolved in water, undergo significant dissociation [4] which increases the adsorption activity of Cn TAB.
Fig. 4. Effect of hexane vapour on dynamic interfacial tension of C16 TAB solution drops at different concentrations of 10−7 mol/l (1); 10−6 mol/l (2); 3 × 10−6 mol/l (3); 10−5 mol/l (4); 3 × 10−5 mol/l (5); 10−4 mol/l (6); 3 × 10−4 mol/l (7): 1 ml of hexane was injected at t = 300 s at the bottom of the cuvette.
bc =
RT [ln(1 − ) + a 2 ] ω0
exp(−2a) 1−
(1)
(2)
where ˘ is the surface pressure (˘ = 0 − ), and 0 are the interfacial tensions of the solution and the solvent, respectively, ω0 is the partial molar area of the ionic surfactant at ˘ = 0, a is the intermolecular interaction constant, b is the adsorption equilibrium constant, c is the surfactant bulk concentration, = ω is the surface layer coverage by the surfactant, is the surfactant adsorption, R is the gas law constant, and T is the temperature. The molar area of the surfactant ω in Eqs. (1) and (2) depends linearly on the surface pressure [4]: ω = ω0 (1 − ε˘)
(3)
where ε is the two-dimensional relative surface layer compressibility coefficient, which characterises the intrinsic compressibility of the molecules in the surface layer. The dependencies of interfacial tension on the Cn TAB concentration as calculated from Eqs. (1)–(3) are shown as curves in Fig. 5. These curves satisfactory describe both the results obtained in the present study and the data reported in [4] where a similar buffer was used. The values a = 0 and ε = 0.005 m/mN were used for all Cn TAB studied; other model parameters are listed in Table 1. Note, the ω0 values for all the Cn TAB were found to be similar, while the adsorption equilibrium constants b become essentially higher (approximately by a factor of 7–9) with the increase of the alkyl chain length of the Cn TAB molecule by two methylene groups. Let us consider next the interfacial tension values for the adsorption of hexane from the vapour phase onto a drop of Cn TAB solution, i.e., in 500−700 s after the experiment started (see Figs. 1–4). These results are illustrated in Fig. 6. From these graphs the significant decrease of interfacial tension upon co-adsorption of hexane molecules from the vapour phase at the liquid surface can be easily seen. In Fig. 6 the interfacial tension values at the solution/hexane vapour interface are significantly lower than those at the solution/air interface, although remain higher than the values reported in [4] for the interfacial tension of Cn TAB solutions at the solution/liquid hexane interface. For high Cn TAB concentrations the ratio of the difference between the interfacial tension at the
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Table 1 Model parameters for Cn TAB adsorption layers at the solution/air and solution/hexane vapour interface; other parameters which are equal for all the substances are listed in the text. n
Cn TAB solution/air interface, Eqs. (1)–(3) b/m3 mol−1
a12
b2 /m3 mol−1
2.5 × 10 2.5 × 105 2.6 × 105 2.6 × 105
0.249 1.95 18.0 110
1.6 2.4 2.4 1.8
0.75 7.0 110 700
5
10 12 14 16
Cn TAB solution/hexane vapour interface, Eqs. (3)–(7)
ω0 /m2 /mol
The adsorption isotherm equations for the two components of the mixture were also derived in [15]. For hexane the adsorption isotherm is: d1 P1 + k1 2 =
1 exp[−2a1 1 − 2a12 2 ], (1 − 1 − 2 )
(6)
and for the water-soluble surfactants Cn TAB the adsorption isotherm reads: b2 c2 =
Fig. 6. Dependence of equilibrium interfacial tension for Cn TAB solutions at the solution/hexane vapour interface: filled symbols, experimental data; bold curves, values calculated from Eqs. (3)–(7) with the parameters listed in Table 1; thin curves, adsorption at the solution/air interface re-plotted from Fig. 5; open symbols, experimental data corrected to account for the C16 TAB depletion in the drop bulk due to adsorption (see text).
solution/air interface and that on the solution/hexane vapour interface to the difference between the interfacial tension at the solution/air interface and that on the solution/liquid hexane interface is about 0.5, while for low Cn TAB solution concentrations this ratio is smaller. To evaluate the effect caused by the simultaneous adsorption of Cn TAB from the solution and hexane from the vapour phase onto the drop surface, the model proposed in [15] was used. The equations for the mixed surface layer are listed below. The equation of state reads: −
˘ω0∗ RT
= ln(1 − 1 − 2 ) + a1 12 + a2 22 + 2a12 1 2
(4)
with ω0∗ =
ω10 1 + ω20 2 1 + 2
(5)
Here i = ωi · i are the surface coverages, i is the adsorption, ω10 is the molar area at zero surface pressure by molecules of component i (i = 1 for hexane, and i = 2 for Cn TAB). The coefficients a1 , a2 and a12 are the Frumkin interaction constants. The molar area of the components can be approximated by a linear dependence on surface pressure ˘ and the total surface coverage = 1 + 2 , similarly to Eq. (3). Note that Eq. (4) assumes ω10 ∼ = ω20 .
2 exp[−2a2 2 − 2a12 1 ], (1 − 1 − 2 )
(7)
where P1 is the partial pressure of the component 1 (hexane) in the gas phase (Torr or Pa), and d1 is its adsorption activity coefficient (1/Torr or 1/Pa, respectively). The partial pressure of saturated hexane vapour at 25 ◦ C is ca. 150 Torr (or 2 × 104 Pa). The additional term k1 2 on the left hand side in Eq. (6) for k1 > 0 accounts for the influence of adsorbed Cn TAB on the adsorption of hexane from the vapour phase. This approach was used earlier in [10] to estimate the influence of hexane vapour on the interfacial tension of C12 TAB aqueous solutions. The corresponding model parameters for the solution/air interface were found in [10] to be quite similar to those estimated above: ω0 = 2.6 × 10−5 m2 /mol, b = 2.2 m3 mol−1 , a = 0 (cf. Table 1). For the C12 TAB solution/hexane vapour interface the coefficient k1 in Eq. (6) was found to be 15. The influence of hexane vapour on the equilibrium (retention time not less than 2000s) interfacial tension of C10 EO8 solutions in a wide concentrations range was studied in [15]. The experimental results were shown to be satisfactory described by Eqs. (3)–(7) with k1 = 30. However the best fit with the experimental data was achieved when the constant b was several times increased as compared to its value for the individual C10 EO8 solution. This could be possibly ascribed to the fact that the co-adsorption of hexane from vapour phase onto the C10 EO8 solution boundary leads to an increase of the C10 EO8 surface activity. In this regard, it should be also noted that for the adsorption of C10 EO8 at the solution/liquid hexane interface the constant b becomes yet much higher than its value for the individual C10 EO8 solutions [15]. In our calculations the values of the model parameters in Eqs. (3)–(7) were: k1 = 0, a1 = 0 and a2 = 0. To calculate the hexane vapour adsorption on the drop surface, the values d1 = 6 × 10−5 1/Pa and ω10 = 3.5 × 105 m2 /mol were found to provide a good fit to the experimental data. These values are close to those used in [10], where the same experimental procedure was employed. The d1 value used in [15] was by a factor of 5 higher than that shown above, because the adsorption time at the interface with the hexane vapour in [15] was much higher than that in the experiments described here. Moreover, much more important is the fact that in [15], in contrast to the present work, the simultaneous adsorption of both components was studied: first the cell was saturated by hexane vapour, and only afterwards the drop of the aqueous C10 EO8 solution was formed in the hexane vapour atmosphere, which resulted in essentially higher hexane adsorption. Other model parameters, which were specific for each Cn TAB, are listed in Table 1; note that ω20 values were taken equal to the ω0 values estimated for the Cn TAB at the solution/air interface.
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this effect becomes pronounced only at high surfactant concentrations. At low Cn TAB concentrations a slight increase of hexane adsorption takes place with increasing Cn TAB concentration. This is caused by intermolecular attraction between the Cn TAB and hexane molecules. Therefore, at low concentrations the adsorption of Cn TAB promotes the hexane adsorption, while at higher concentrations the increased adsorption of Cn TAB results in enabling of hexane molecules to penetrate in the adsorption layer, as discussed above. 4. Conclusions
Fig. 7. Dependencies of the simultaneous adsorption of Cn TAB (bold curves) and hexane (dashed curves) at the Cn TAB solution/hexane vapour interface on the Cn TAB concentration, as calculated with Eqs. (3)–(7); the thin curves are calculated with Eqs. (1)–(3) for the Cn TAB adsorption at the solution/air interface.
It is seen from the data shown in Table 1 that the b2 value (for the Cn TAB solution/hexane vapour interface) is essentially higher than the b value (for the Cn TAB solution/air interface), in agreement with the trend observed in [15]. Also for the mixed monolayer the a12 values are high and positive, which indicates essential intermolecular (possibly hydrophobic) interaction. Fig. 6 illustrates the interfacial tension isotherms (bold curves) calculated using Eqs. (3)–(7); it is seen that the dependencies for the C10 TAB and C12 TAB solutions agree well with the experimental data, while for C14 TAB and especially C16 TAB solutions the agreement is observed only for high concentrations of the solution. As it was indicated above, at the solution/air interface for concentrations C16 TAB less than 10−5 mol/l the adsorption equilibrium cannot be reached during the available 300 s. This is also true for the solution/hexane vapour interface for 500–700 s. Besides, this behaviour for C16 TAB could be ascribed partially to the decrease of the surfactant concentration in the drop bulk due to the adsorption at the drop surface. This phenomenon was analysed in detail earlier [16]. The decrease of bulk concentration c caused by the adsorption of the surfactant can be estimated as c = ·S/V, where S and V are the surface area and volume of the drop, respectively, and the adsorption is calculated from model equation at the given experimental value. In our experiments the ratio S/V was 1.5−2.0 mm−1 [16]; therefore the initial C16 TAB concentrations 10−6 , 3 × 10−6 and 10−5 mol/l correspond to actual concentration values 3 × 10−7 , 1.4 × 10−6 and 6 × 10−6 mol/l, respectively. These results, which are shown in Fig. 6 (open symbols) are seen to be essentially closer to the theoretical curve for C16 TAB. Fig. 7 illustrates the dependencies of the equilibrium adsorption of components at the Cn TAB solution/hexane vapour and Cn TAB solution/air interfaces on the Cn TAB concentration calculated using Eqs. (3)–(7) and (1)–(3), respectively, with the parameters listed above. It is seen that for low surfactant concentration the surfactant adsorption at the solution/hexane vapour interface is much higher than that on the solution/air interface (note the logarithmic adsorption scale in the figure), and only at high Cn TAB concentrations these adsorptions become approximately equal. Accordingly, the hexane adsorption on the solution drop surface is high at low Cn TAB concentrations, and decreases with the Cn TAB concentration increase. This competitive adsorption is governed by the surface activity of the constituents, which is higher for Cn TAB than for hexane. Also, the higher is the surface activity of the surfactant, the lower are the concentrations at which this effect is observed: among all the surfactants studied, the hexane adsorption decrease appears at lower C16 TAB concentrations, while for C10 TAB solutions
The present study was dedicated to the adsorption behaviour of members of the Cn TAB homologous series (n = 10, 12, 14 and 16) at the solution/hexane vapour interface. The drop profile analysis method was employed to measure the dynamic interfacial tension: first the adsorption of Cn TAB dissolved in the drop bulk took place, and subsequently the cell in which the drop was formed was saturated by hexane vapour. From the dynamic interfacial tension curves a significant and faster decrease of the interfacial tension is seen upon releasing the hexane vapour into the atmosphere around the Cn TAB solution drop. The quasi-equilibrium values of the interfacial tension at the solution/vapour interface were compared with the results obtained for the solution/air and solution/liquid hexane interfaces. For the theoretical analysis of the results, we used the model developed in [15] valid for equilibrium conditions of the system. This model assumes the competitive adsorption of the components from both phases onto the interface: adsorption of Cn TAB from the solution in the drop bulk, and hexane adsorption from the gas phase. The model takes into account the mutual influence of components and the intermolecular interaction within the adsorbed layer. The theory is shown to be in a good agreement with the experimental results for C10 TAB and C12 TAB solutions, demonstrating the significant increase of their adsorption caused by the presence of hexane. This results in the increase of the adsorption equilibrium constant by several times, and high intermolecular interaction constant values. The results obtained for the C14 TAB and C16 TAB solutions also comply with the model predictions; however, to obtain better agreement between the theory and the experimental data at low surfactant concentration, the depletion of the surfactant solution in the drop bulk caused by the adsorption would have to be taken into account. Acknowledgements The work was financially supported by projects of the DFG (Mi418/18-1), the DLR (50WM1129), the European Space Agency (PASTA) and the COST actions CM1101 and MP1106. References [1] V. Bergeron, Disjoining pressures and film stability of alkyltrimethylammonium bromide foam films, Langmuir 13 (1997) 3474–3482. [2] V.B. Fainerman, R. Miller, Thermodynamics of adsorption of surfactants at the solution-fluid interface, in: V.B. Fainerman, D. Möbius, R. Miller (Eds.), Surfactants – Chemistry, Interfacial Properties and Application, Studies in Interface Science, vol. 13, Elsevier, 2001, pp. 99–188. [3] C. Stubenrauch, V.B. Fainerman, E.V. Aksenenko, R. Miller, Adsorption behaviour and dilatational rheology of the cationic alkyl trimethyl ammonium bromides at the water/air interface, J. Phys. Chem. 109 (2005) 1505–1509. [4] V. Pradines, V.B. Fainerman, E.V. Aksenenko, J. Krägel, N. Mucic, R. Miller, Adsorption of alkyl trimethylammonium bromides at the water/air and water/hexane interfaces, Colloids Surf. A 371 (2010) 22–28. [5] K. Medrzycka, W. Zwierzykowski, Adsorption of alkyltrimethylammonium bromides at the various interfaces, J. Colloid Interface Sci. 230 (2000) 67–72.
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