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Surface rheology and its relation to foam stability in solutions of sodium decyl sulfate
Colloids and Surfaces A xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
Surface rheology and its relation to foam stability in solutions of sodium decyl sulfate Matthias J. Hofmann, Hubert Motschmann
⁎
Institut für Physikalische und Theoretische Chemie, Universität Regensburg, Universitätsstraße 31, 93053 Regensburg, Germany
A R T I C L E I N F O
A B S T R A C T
Keywords: Sodium decyl sulfate (SDeS) Oscillating bubble capillary pressure tensiometry Foam lamella stability Dilatational rheology
The anionic surfactant sodium decyl sulfate (SDeS) at concentrations below its critical micellar concentration (cmc) was studied in a surface chemically purified state at the air–water interface in terms of its static and dynamic interfacial properties. Equilibrium surface tension was determined using a pendant drop tensiometer. An oscillating bubble capillary pressure tensiometer allowed accessing the surface dilatational properties of the surfactant's adsorption layers in a frequency range from 2 to 500 Hz. The estimation of single foam lamella stability was obtained from visual observation of films formed within a rectangular glass frame in a saturated atmosphere. Upon increasing surfactant concentration, the foam lamellae were found to rupture at prolonged lifetimes. The dramatic difference in foam stability goes along with a pronounced transition from a surface elastic to a surface visco-elastic state of the adsorption layers. These findings were indicative for a correlation between foam lamella stability and surface viscoelasticity also for this model surfactant. These results may be of importance to further shed light on the processes governing foam and foam lamella stability.
1. Introduction Surfactants play a major role in the stabilization of foams and emulsions. Recently, they also found application as directional agents in the synthesis of nano-particles [1]. Sodium decyl sulfate (SDeS) as member of the commercially available homologous series of sodium-nalkyl sulfates is a frequently studied model substance for anionic surfactants. Critical micellar concentrations (cmcs) and the corresponding heats of micellation have been determined as a function of temperature [2] as well as its mixed adsorption with perfluorooctanoate towards γ-alumina [3] and mixed micellation with N,N-dimethyldodecylamine oxide [4]. Its adsorption dynamics have been studied at the water–n-hexane interface [5]. At low bulk concentrations it was found to exhibit ideal behavior [6] at both the oil–water and air–water interface [7]. Next to studying the properties of pure SDeS at different interfaces, also its interaction with other components has been of interest to previous studies. Penfold et al. studied the interaction of sodium-n-alkyl sulfate surfactants with oppositely charged polyelectrolytes at air–water interface [8]. Foam enhancement upon addition of an oppositely charged polymer to sodium-n-alkyl sulfates has been observed previously and correlated to measurements of surface viscoelasticity [9]. Mixtures of sodium alkyl sulfates (C10, C12, C14 and C16) with non-ionic surfactants of the Triton series have been studied in terms of their dynamic surface
⁎
tension properties by means of the maximum bubble pressure method [10]. The validity of a diffusion controlled adsorption regime was proven from the linear relation between dynamic surface tension γ and the reciprocal square root of adsorption time 1/ t . Electron paramagnetic resonance (EPR) spectroscopy of aqueous mixtures containing SDeS and the non-ionic polymer poly(vinylpyrrolidone) were found to form more ordered and compact micelles compared to the respective sulfonate surfactant [11]. Also the interaction between carbon-soot nano-particles and the structurally related surfactant 1-decane sodium sulfonate was subject to a dynamic surface properties study [12]. Therein, dilatational characteristics up to a perturbation frequency of 0.2 Hz and transient heights of foam columns were determined. Characterization of foams is a field of high complexity and a several characteristic numbers for the qualification of foaming and foam properties have been introduced. Karakashev et al. [13] therefore suggested a new parameter unifying characteristics during the foam formation and foam decay by definition of the so called foam production as ratio of foamability and the rate of foam decay relying on a concentration scale relative to the cmc. Foam stabilization due to the presence of the homologous series of alkyl sulfates was studied using the Ross-Miles method and foam lifetime at constant pressure [14]. Badwan et al. found the maximum foamability of the members of this series to occur at a defined temperature depending on the chain length [15]. Also the presence of long-chain alcohols was found to modify the
Corresponding author. E-mail address: [email protected] (H. Motschmann).
http://dx.doi.org/10.1016/j.colsurfa.2017.04.028 Received 28 February 2017; Received in revised form 13 April 2017; Accepted 14 April 2017 0927-7757/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Motschmann, H., Colloids and Surfaces A (2017), http://dx.doi.org/10.1016/j.colsurfa.2017.04.028
Colloids and Surfaces A xxx (xxxx) xxx–xxx
M.J. Hofmann, H. Motschmann
drainage rate in foam films and the generated foam volume [16,17]. The breakdown of foams can be accelerated by the action of bulky antifoaming agents such as tributyl phosphate or 2-ethyl-1-hexanol [18]. The two-dimensional micro-structure within plain foam films leading to film stabilization and stepwise film thinning can be determined by means of an interferometric setup [19]. Studies of this kind have been conducted earlier by Goddard and coworkers [20,21]. In this article, we measure the surface dilatational modulus in an extended frequency range from 2 to 500 Hz by means of a novel design of a capillary tensiometer and relate the complex modulus to the visually observed stability of individual foam lamellae. The aim is to establish a relation between foam stability and the surface dilatational characteristics.
Fig. 2. Metal frame mounted in a flask for the determination of foam lamella stability.
used to determine the time from lamella formation to the rupturing event. A conventional stop watch was used to measure the foam lamella life time. At each concentration, a single foam lamella was formed within the metal frame by rotating the flask, thereby moving the frame through the sample liquid. After its formation, the lamella was brought into an upright position remaining in contact with the solution as shown in Fig. 2. The measurements were carried out at room temperature and repeated 25 times for each concentration.
2. Experiment 2.1. Materials
2.4. Oscillating bubble Sodium decyl sulfate (SDeS) was obtained from Acros organics (HPLC grade). Due to the component's susceptibility to sulfate ester hydrolysis, a purification procedure to exclude the parent compound, ndecanol, as described by Lunkenheimer et al. [22] was used to ensure the state of surface chemical purity. The procedure includes repeated adsorption steps towards a large surface area followed by a suction cleaning of the compressed interfacial area. Thereupon, the preferentially adsorbed n-decanol impurities gradually become less abundant. The process is conducted in a specially designed and fully automated apparatus. Adsorption times of 600 s between the 400 subsequent expansion and compression cycles were chosen. To ensure validity of the purification procedure, it was carried out at a concentration of 30.00 mmol L−1, which is below the surfactant's critical micellar concentration (cmc). Depending on the method chosen (titration calorimetry or conductometry), this value differs between 24.5 and 36 mmol L−1 [2,23,24]. The aqueous solutions were prepared using deionized water with a resistivity of 18.2 MΩ cm. The molecular structure of SDeS is shown in Fig. 3. The studied aqueous solutions of SDeS at concentrations of 0.2, 1.5, 15.0 and 30.0 mmol L−1 were obtained from diluting the stock solution with deionized water.
For the measurement of surface rheological characteristics, an oscillating bubble device was used [26]. At each studied concentration, three independent measurements were carried out, i.e., after each measurement, the sample chamber was opened and refilled with the respective sample solution. In the following, the averaged values obtained from these three measurements will be shown. Within one measurement, the frequency scanning mode was applied, i.e., amplitude and phase angle of the pressure response were measured at a certain frequency. Before moving on the next frequency, the bubble size was corrected to half sphere geometry by an implemented control loop. All measurements were carried out at room temperature at a fixed relative deformation amplitude of 5%. 3. Results 3.1. Surface tension isotherm The experimentally obtained equilibrium surface tension isotherm of sodium decyl sulfate (SDeS) and a comparison to published literature data for the same surfactant in a purified state is shown in Fig. 3. The experimentally obtained values of equilibrium surface tension γ against the logarithm of concentration logC curves can be described by the frequently used Frumkin adsorption isotherm accounting for intermolecular interactions between the individual adsorbed decyl sulfate ions. From the fitting parameters a minimum surface area of 22.2 Å2 is obtained, which compares well with the 21.5 Å2 from the published equilibrium isotherm measured at 21.9 °C. The error due
2.2. Surface tension The equilibrium surface tension isotherm was obtained using a drop profile analysis tensiometer (PAT1M, Sinterface GmbH, Germany). Starting from the stock solution having a concentration of 30 mmol L−1, lower concentrated samples were prepared by successive dilution with deionized water. The equilibrium values of surface tension of the respective solutions were obtained from averaging the instantaneous surface tension values over 100 s starting at a surface age of 200 s measured at a temperature of 22 ± 0.5 °C. 2.3. Foam lamella stability Foam lamella stability was estimated from the life time of a macroscopic plain foam lamella (3 cm × 1.5 cm) formed at a metal frame placed in an aqueous solution of the respective surfactant concentration [25]. The life time of the studied foam lamella is defined as the time from lamella formation until its rupture. A schematic view of the home-built device is given in Fig. 2. The metal frame is contained in a flask to ensure saturation of the gas phase. Visual observation was
Fig. 3. Equilibrium surface tension isotherms of SDeS. Experimental results in comparison to literature data [23] and best fit according to the Frumkin isotherm.
Fig. 1. Molecular structure of SDeS.
2
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Fig. 4. Foam lamella life time of aqueous solutions of SDeS (Solid line as guide to the eye).
Fig. 6. Phase angle of the surface dilatational modulus E of aqueous solutions of SDeS at concentrations of 0.2, 1.5, 15.0 and 30.0 mmol L−1. The dotted lines represent the respective phase angles of the surface dilatational modulus according to the extended Lucassen-van den Tempel model (Eq. (1)). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
deviations in temperature between the experiment and published literature data is negligible.
3.2. Lamella stability
E (f ) = ϵ m
The life time of single plain foam lamellae as a function of surfactant concentration is shown in Fig. 4. It is found to increase upon increasing surfactant concentration. This behavior is in agreement with the results obtained for comparable molecular surfactants by other researchers [27,28].
with
ζ=
1 + ζ + iζ + i ·2πf ·κ 1 + 2ζ + 2ζ 2
(1)
fdiff , 2f
whereas i represents the imaginary unit with i2 =−1. The best fits to the latter equations are shown as dotted lines in Figs. 5 and 6 . There are pronounced differences in the measured values of the amplitude of the surface elastic dilatational modulus E(f, c) considering the different concentrations. Within the entire frequency range considered, the amplitude of the modulus goes through a maximum as a function of concentration. Up to a frequency of about 100 Hz, all of the measured curves feature a marked increase for all the studied concentrations. At higher frequencies in the range from 300 to 500 Hz, deviating behaviors for surfactant solutions at low and high concentrations are observed. Whereas for the low concentration solutions (0.2 and 1.5 mmol L−1) an asymptotic value is approached, an increase of the absolute value is clearly visible for the higher concentrations (15.0 and 30.0 mmol L−1) even at higher frequencies. A similar discrepancy between high and low frequency behavior of high and low concentration SDeS solutions is obvious from the phase angle data, as well. Up to frequencies of about 100 Hz, there is a uniform decrease of phase angle upon increasing frequency for all of the studied surfactant concentrations. At higher frequencies, the phase angles for the two lower concentrated solutions basically remain constant a low values. Increasing concentrations lead to larger values of phase angle at the higher frequencies in the studied range.
3.3. Oscillating bubble The values of amplitude and phase angle of the concentration dependent surface elastic dilatational modulus E(f, c) are given as a function of perturbation frequency in Figs. 5 and 6 , respectively. A theoretical description of the experimental data can be achieved using the extended Lucassen-van den Tempel model [29], where the high frequency limiting elasticity ϵm, the characteristic diffusion frequency fdiff and intrinsic surface viscosity κ are used as fitting parameters for the frequency dependent surface dilatational modulus
4. Discussion 4.1. Surface tension isotherm The measured equilibrium surface tension isotherm shows the characteristic surfactant features of decreasing surface tension upon increasing surfactant concentration. Good agreement with published literature data was found. As stated in Section 3.1, the experimentally obtained curves can be fitted using the Frumkin isotherm. The most frequently used Langmuir isotherm does not lead to sufficiently good agreement between experiment and theory. In a contribution by Lunkenheimer et al. [23] focusing on the study of so called odd-even effects in the homologous series of surface chemically purified sodiumn-alkyl sulfates, a cross-sectional area of 21.5Å2 per molecule was given
Fig. 5. Magnitude of the surface dilatational modulus E of aqueous solutions of SDeS at concentrations of 0.2, 1.5, 15.0 and 30.0 mmol L−1. The dotted lines represent the respective amplitudes of the surface dilatational modulus according to the extended Lucassen-van den Tempel model (Eq. (1)). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3
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high frequency surface dilatational rheology data. A plot of foam lamella stability against the measured phase angle at 500 Hz is given in Fig. 7. Foam stability linearly increases with the measured phase angle within experimental error. A similar conclusion concerning the correlation between foam lamella stability and visco-elastic behavior of the surfactant solutions, however only for two studied concentrations, was also drawn by Koelsch and Motschmann [27]. 5. Conclusion A transition from surface elastic to surface visco-elastic characteristics of the adsorption layers upon increasing surfactant concentration was observed. These findings on the molecular level could successfully be correlated to the accompanying rise in foam lamella stabilities from the respective aqueous solutions. Noteworthy lifetimes of the lamellae could be detected only in case of surface visco-elastic behavior of the adsorbed surfactant layer. It is to be noted that foam lamella stability is not proportionally dependent on the amplitude of the surface elastic dilatational modulus E(f, c). For instance, the 30.0 mmol L−1 solution of sodium decyl sulfate (SDeS) exhibits a higher foam lamella stability, even though the amplitude is lower than for the 0.2 mmol L−1 solution. The findings suggest that a correlation between foam lamella stability and viscoelasticity, as also observed for other classes of surfactants, was equally confirmed for the model surfactant SDeS.
Fig. 7. Foam lamella stability plotted against phase angle at the studied SDeS concentrations of at 500 Hz (Color code as in Figs. 5 and 6). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
for sodium decyl sulfate (SDeS) at maximum surface coverage. This value, however was not the result of fitting with a conventional single adsorption isotherm, but rather obtained using a two state transition model [30]. In this model, it is assumed, that surfactant adsorption is described by two different adsorption laws (Henry law for low and Langmuir law for high concentrations) depending on the surface coverage. Furthermore, a parametrized transition function describing the crossover from low to high surface pressure adsorption is required. It is to be noted, that the experimentally determined value of minimum surface area obtained from Frumkin adsorption isotherm of 21.5 Å2 is in agreement with the literature value assuming the more evolved two state adsorption model. Furthermore, the decreasing cross-sectional area of adsorbed surfactant upon increasing alkyl chain length can be interpreted in terms of a more pronounced interaction between the alkyl chains promoting film stability.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
4.2. Lamella stability The characteristic of increasing single foam lamella stability upon increasing surfactant concentration has also been observed for other surfactants such as Triton-X 100, cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), sodium tetradecyl sulfate, tetraethylammonium perfluorooctane sulfonate, decyldimethyl phosphine oxide [28] and cationic model surfactant 1-dodecyl-4-dimethylaminopyridinium bromide [27]. In this study, the same behavior is shown for an anionic surfactant.
[12]
4.3. Oscillating bubble
[20]
The observations concerning the differences in amplitude and phase characteristics of the surface elastic dilatational modulus E(f, c) for the studied lower (0.2 and 1.5 mmol L−1) and higher (15.0 and 30.0 mmol L−1) concentrations are described in Section 3.3. The characteristics of the low and high concentration solutions are referred to as surface elastic and surface visco-elastic, respectively. As observed by Koelsch et al. [27], a transition from surface elastic to surface viscoelastic adsorption layer behavior upon increasing concentration was also found for the studied solutions of SDeS.
[21] [22]
[13] [14] [15] [16] [17] [18] [19]
[23] [24] [25] [26] [27] [28] [29]
4.4. Correlation of surface rheology to foam lamella stability
[30]
In the conducted study, foam stability data are presented next to
4
A. Moulahi, F. Sediri, Optik – Int. J. Light Electron Opt. 127 (2016) 7586–7593. Z. Király, I. Dekány, J. Colloid Interface Sci. 242 (2001) 214–219. L. Nordstierna, I. Furó, P. Stilbs, Langmuir 22 (2006) 7969–7974. R. Abdel-Rahem, Tenside Surfactants Deterg. 46 (2009) 298–305. A. Sharipova, S. Aidarova, V.B. Fainerman, R. Miller, Colloids Surf. A: Physicochem. Eng. Asp. 382 (2011) 181–185. W.R. Gillap, N.D. Weiner, M. Gibaldi, J. Phys. Chem. 72 (1968) 2222–2227. W.R. Gillap, N.D. Weiner, M. Gibaldi, J. Phys. Chem. 72 (1968) 2218–2222. J. Penfold, I. Tucker, R.K. Thomas, D.J.F. Taylor, X.L. Zhang, C. Bell, C. Breward, P. Howell, Langmuir 23 (2007) 3128–3136. S.T. Regismond, F.M. Winnik, E. Goddard, Colloids Surf. A: Physicochem. Eng. Asp. 141 (1998) 165–171. V.B. Fainerman, R. Miller, Colloids Surf. A 97 (1995) 65–82. A.M. Tedeschi, E. Busi, L. Paduano, R. Basosi, G. D’Errico, Phys. Chem. Chem. Phys. 5 (2003) 5077–5083. D. Zabiegaj, E. Santini, E. GuzmáN, M. Ferrari, L. Liggieri, F. Ravera, J. Nanosci. Nanotechnol. 15 (2015) 3618–3625. S.I. Karakashev, P. Georgiev, K. Balashev, J. Colloid Interface Sci. 379 (2012) 144–147. K. Khristov, D. Exerowa, J. Dispers. Sci. Technol. 18 (1997) 561–575. A.A. Badwan, K.C. James, W.J. Pugh, J. Dispers. Sci. Technol. 11 (1990) 157–167. K.J. Mysels, K. Shinoda, S. Frankel, Soap Films: Studies of Their Thinning, Pergamon Press, New York, 1959. A. Patist, T. Axelberd, D.O. Shah, J. Colloid Interface Sci. 208 (1998) 259–265. I. Blute, M. Jansson, S.G. Oh, D.O. Shah, J. Am. Oil Chem. Soc. 71 (1994) 41–46. A.D. Nikolov, D.T. Wasan, S.E. Friberg, Colloids Surf. A: Physicochem. Eng. Asp. 118 (1996) 221–243. K.P. Ananthapadmanabhan, P.S. Leung, E.D. Goddard, Polymer Association Structures, Volume 384 of ACS Symposium Series, in: M.A. El-Nokaly (Ed.), American Chemical Society, Washington, DC, 1989, pp. 297–309. E.D. Goddard, D.B. Braun, Cosmet. Toilet. (1985) 41–47. K. Lunkenheimer, H.J. Pergande, H. Krueger, Rev. Sci. Instrum. 58 (1987) 2313–2316. K. Lunkenheimer, G. Czichocki, R. Hirte, W. Barzyk, Colloids Surf. A: Physicochem. Eng. Asp. 101 (1995) 187–197. M.H. Ropers, G. Czichocki, G. Brezesinski, J. Phys. Chem. B 107 (2003) 5281–5288. T. Gilanyi, C. Stergiopoulos, E. Wolfram, Colloid Polym. Sci. 254 (1976) 1018–1023. Dominik Stadler, Matthias Josef Hofmann, Hubert Motschmann, Mikhail Shamonin, Meas. Sci. Technol. 27 (2016) 65301. P. Koelsch, H. Motschmann, Langmuir 21 (2005) 6265–6269. H. Fruhner, K.-D. Wantke, K. Lunkenheimer, Colloids Surf. A 162 (2000) 193–202. K.-D. Wantke, H. Fruhner, J. Örtegren, Colloids Surf. A: Physicochem. Eng. Asp. 221 (2003) 185–195. K. Lunkenheimer, R. Hirte, J. Phys. Chem. 96 (1992) 8683–8686.
Contents lists available at ScienceDirect
Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
Surface rheology and its relation to foam stability in solutions of sodium decyl sulfate Matthias J. Hofmann, Hubert Motschmann
⁎
Institut für Physikalische und Theoretische Chemie, Universität Regensburg, Universitätsstraße 31, 93053 Regensburg, Germany
A R T I C L E I N F O
A B S T R A C T
Keywords: Sodium decyl sulfate (SDeS) Oscillating bubble capillary pressure tensiometry Foam lamella stability Dilatational rheology
The anionic surfactant sodium decyl sulfate (SDeS) at concentrations below its critical micellar concentration (cmc) was studied in a surface chemically purified state at the air–water interface in terms of its static and dynamic interfacial properties. Equilibrium surface tension was determined using a pendant drop tensiometer. An oscillating bubble capillary pressure tensiometer allowed accessing the surface dilatational properties of the surfactant's adsorption layers in a frequency range from 2 to 500 Hz. The estimation of single foam lamella stability was obtained from visual observation of films formed within a rectangular glass frame in a saturated atmosphere. Upon increasing surfactant concentration, the foam lamellae were found to rupture at prolonged lifetimes. The dramatic difference in foam stability goes along with a pronounced transition from a surface elastic to a surface visco-elastic state of the adsorption layers. These findings were indicative for a correlation between foam lamella stability and surface viscoelasticity also for this model surfactant. These results may be of importance to further shed light on the processes governing foam and foam lamella stability.
1. Introduction Surfactants play a major role in the stabilization of foams and emulsions. Recently, they also found application as directional agents in the synthesis of nano-particles [1]. Sodium decyl sulfate (SDeS) as member of the commercially available homologous series of sodium-nalkyl sulfates is a frequently studied model substance for anionic surfactants. Critical micellar concentrations (cmcs) and the corresponding heats of micellation have been determined as a function of temperature [2] as well as its mixed adsorption with perfluorooctanoate towards γ-alumina [3] and mixed micellation with N,N-dimethyldodecylamine oxide [4]. Its adsorption dynamics have been studied at the water–n-hexane interface [5]. At low bulk concentrations it was found to exhibit ideal behavior [6] at both the oil–water and air–water interface [7]. Next to studying the properties of pure SDeS at different interfaces, also its interaction with other components has been of interest to previous studies. Penfold et al. studied the interaction of sodium-n-alkyl sulfate surfactants with oppositely charged polyelectrolytes at air–water interface [8]. Foam enhancement upon addition of an oppositely charged polymer to sodium-n-alkyl sulfates has been observed previously and correlated to measurements of surface viscoelasticity [9]. Mixtures of sodium alkyl sulfates (C10, C12, C14 and C16) with non-ionic surfactants of the Triton series have been studied in terms of their dynamic surface
⁎
tension properties by means of the maximum bubble pressure method [10]. The validity of a diffusion controlled adsorption regime was proven from the linear relation between dynamic surface tension γ and the reciprocal square root of adsorption time 1/ t . Electron paramagnetic resonance (EPR) spectroscopy of aqueous mixtures containing SDeS and the non-ionic polymer poly(vinylpyrrolidone) were found to form more ordered and compact micelles compared to the respective sulfonate surfactant [11]. Also the interaction between carbon-soot nano-particles and the structurally related surfactant 1-decane sodium sulfonate was subject to a dynamic surface properties study [12]. Therein, dilatational characteristics up to a perturbation frequency of 0.2 Hz and transient heights of foam columns were determined. Characterization of foams is a field of high complexity and a several characteristic numbers for the qualification of foaming and foam properties have been introduced. Karakashev et al. [13] therefore suggested a new parameter unifying characteristics during the foam formation and foam decay by definition of the so called foam production as ratio of foamability and the rate of foam decay relying on a concentration scale relative to the cmc. Foam stabilization due to the presence of the homologous series of alkyl sulfates was studied using the Ross-Miles method and foam lifetime at constant pressure [14]. Badwan et al. found the maximum foamability of the members of this series to occur at a defined temperature depending on the chain length [15]. Also the presence of long-chain alcohols was found to modify the
Corresponding author. E-mail address: [email protected] (H. Motschmann).
http://dx.doi.org/10.1016/j.colsurfa.2017.04.028 Received 28 February 2017; Received in revised form 13 April 2017; Accepted 14 April 2017 0927-7757/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Motschmann, H., Colloids and Surfaces A (2017), http://dx.doi.org/10.1016/j.colsurfa.2017.04.028
Colloids and Surfaces A xxx (xxxx) xxx–xxx
M.J. Hofmann, H. Motschmann
drainage rate in foam films and the generated foam volume [16,17]. The breakdown of foams can be accelerated by the action of bulky antifoaming agents such as tributyl phosphate or 2-ethyl-1-hexanol [18]. The two-dimensional micro-structure within plain foam films leading to film stabilization and stepwise film thinning can be determined by means of an interferometric setup [19]. Studies of this kind have been conducted earlier by Goddard and coworkers [20,21]. In this article, we measure the surface dilatational modulus in an extended frequency range from 2 to 500 Hz by means of a novel design of a capillary tensiometer and relate the complex modulus to the visually observed stability of individual foam lamellae. The aim is to establish a relation between foam stability and the surface dilatational characteristics.
Fig. 2. Metal frame mounted in a flask for the determination of foam lamella stability.
used to determine the time from lamella formation to the rupturing event. A conventional stop watch was used to measure the foam lamella life time. At each concentration, a single foam lamella was formed within the metal frame by rotating the flask, thereby moving the frame through the sample liquid. After its formation, the lamella was brought into an upright position remaining in contact with the solution as shown in Fig. 2. The measurements were carried out at room temperature and repeated 25 times for each concentration.
2. Experiment 2.1. Materials
2.4. Oscillating bubble Sodium decyl sulfate (SDeS) was obtained from Acros organics (HPLC grade). Due to the component's susceptibility to sulfate ester hydrolysis, a purification procedure to exclude the parent compound, ndecanol, as described by Lunkenheimer et al. [22] was used to ensure the state of surface chemical purity. The procedure includes repeated adsorption steps towards a large surface area followed by a suction cleaning of the compressed interfacial area. Thereupon, the preferentially adsorbed n-decanol impurities gradually become less abundant. The process is conducted in a specially designed and fully automated apparatus. Adsorption times of 600 s between the 400 subsequent expansion and compression cycles were chosen. To ensure validity of the purification procedure, it was carried out at a concentration of 30.00 mmol L−1, which is below the surfactant's critical micellar concentration (cmc). Depending on the method chosen (titration calorimetry or conductometry), this value differs between 24.5 and 36 mmol L−1 [2,23,24]. The aqueous solutions were prepared using deionized water with a resistivity of 18.2 MΩ cm. The molecular structure of SDeS is shown in Fig. 3. The studied aqueous solutions of SDeS at concentrations of 0.2, 1.5, 15.0 and 30.0 mmol L−1 were obtained from diluting the stock solution with deionized water.
For the measurement of surface rheological characteristics, an oscillating bubble device was used [26]. At each studied concentration, three independent measurements were carried out, i.e., after each measurement, the sample chamber was opened and refilled with the respective sample solution. In the following, the averaged values obtained from these three measurements will be shown. Within one measurement, the frequency scanning mode was applied, i.e., amplitude and phase angle of the pressure response were measured at a certain frequency. Before moving on the next frequency, the bubble size was corrected to half sphere geometry by an implemented control loop. All measurements were carried out at room temperature at a fixed relative deformation amplitude of 5%. 3. Results 3.1. Surface tension isotherm The experimentally obtained equilibrium surface tension isotherm of sodium decyl sulfate (SDeS) and a comparison to published literature data for the same surfactant in a purified state is shown in Fig. 3. The experimentally obtained values of equilibrium surface tension γ against the logarithm of concentration logC curves can be described by the frequently used Frumkin adsorption isotherm accounting for intermolecular interactions between the individual adsorbed decyl sulfate ions. From the fitting parameters a minimum surface area of 22.2 Å2 is obtained, which compares well with the 21.5 Å2 from the published equilibrium isotherm measured at 21.9 °C. The error due
2.2. Surface tension The equilibrium surface tension isotherm was obtained using a drop profile analysis tensiometer (PAT1M, Sinterface GmbH, Germany). Starting from the stock solution having a concentration of 30 mmol L−1, lower concentrated samples were prepared by successive dilution with deionized water. The equilibrium values of surface tension of the respective solutions were obtained from averaging the instantaneous surface tension values over 100 s starting at a surface age of 200 s measured at a temperature of 22 ± 0.5 °C. 2.3. Foam lamella stability Foam lamella stability was estimated from the life time of a macroscopic plain foam lamella (3 cm × 1.5 cm) formed at a metal frame placed in an aqueous solution of the respective surfactant concentration [25]. The life time of the studied foam lamella is defined as the time from lamella formation until its rupture. A schematic view of the home-built device is given in Fig. 2. The metal frame is contained in a flask to ensure saturation of the gas phase. Visual observation was
Fig. 3. Equilibrium surface tension isotherms of SDeS. Experimental results in comparison to literature data [23] and best fit according to the Frumkin isotherm.
Fig. 1. Molecular structure of SDeS.
2
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Fig. 4. Foam lamella life time of aqueous solutions of SDeS (Solid line as guide to the eye).
Fig. 6. Phase angle of the surface dilatational modulus E of aqueous solutions of SDeS at concentrations of 0.2, 1.5, 15.0 and 30.0 mmol L−1. The dotted lines represent the respective phase angles of the surface dilatational modulus according to the extended Lucassen-van den Tempel model (Eq. (1)). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
deviations in temperature between the experiment and published literature data is negligible.
3.2. Lamella stability
E (f ) = ϵ m
The life time of single plain foam lamellae as a function of surfactant concentration is shown in Fig. 4. It is found to increase upon increasing surfactant concentration. This behavior is in agreement with the results obtained for comparable molecular surfactants by other researchers [27,28].
with
ζ=
1 + ζ + iζ + i ·2πf ·κ 1 + 2ζ + 2ζ 2
(1)
fdiff , 2f
whereas i represents the imaginary unit with i2 =−1. The best fits to the latter equations are shown as dotted lines in Figs. 5 and 6 . There are pronounced differences in the measured values of the amplitude of the surface elastic dilatational modulus E(f, c) considering the different concentrations. Within the entire frequency range considered, the amplitude of the modulus goes through a maximum as a function of concentration. Up to a frequency of about 100 Hz, all of the measured curves feature a marked increase for all the studied concentrations. At higher frequencies in the range from 300 to 500 Hz, deviating behaviors for surfactant solutions at low and high concentrations are observed. Whereas for the low concentration solutions (0.2 and 1.5 mmol L−1) an asymptotic value is approached, an increase of the absolute value is clearly visible for the higher concentrations (15.0 and 30.0 mmol L−1) even at higher frequencies. A similar discrepancy between high and low frequency behavior of high and low concentration SDeS solutions is obvious from the phase angle data, as well. Up to frequencies of about 100 Hz, there is a uniform decrease of phase angle upon increasing frequency for all of the studied surfactant concentrations. At higher frequencies, the phase angles for the two lower concentrated solutions basically remain constant a low values. Increasing concentrations lead to larger values of phase angle at the higher frequencies in the studied range.
3.3. Oscillating bubble The values of amplitude and phase angle of the concentration dependent surface elastic dilatational modulus E(f, c) are given as a function of perturbation frequency in Figs. 5 and 6 , respectively. A theoretical description of the experimental data can be achieved using the extended Lucassen-van den Tempel model [29], where the high frequency limiting elasticity ϵm, the characteristic diffusion frequency fdiff and intrinsic surface viscosity κ are used as fitting parameters for the frequency dependent surface dilatational modulus
4. Discussion 4.1. Surface tension isotherm The measured equilibrium surface tension isotherm shows the characteristic surfactant features of decreasing surface tension upon increasing surfactant concentration. Good agreement with published literature data was found. As stated in Section 3.1, the experimentally obtained curves can be fitted using the Frumkin isotherm. The most frequently used Langmuir isotherm does not lead to sufficiently good agreement between experiment and theory. In a contribution by Lunkenheimer et al. [23] focusing on the study of so called odd-even effects in the homologous series of surface chemically purified sodiumn-alkyl sulfates, a cross-sectional area of 21.5Å2 per molecule was given
Fig. 5. Magnitude of the surface dilatational modulus E of aqueous solutions of SDeS at concentrations of 0.2, 1.5, 15.0 and 30.0 mmol L−1. The dotted lines represent the respective amplitudes of the surface dilatational modulus according to the extended Lucassen-van den Tempel model (Eq. (1)). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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M.J. Hofmann, H. Motschmann
high frequency surface dilatational rheology data. A plot of foam lamella stability against the measured phase angle at 500 Hz is given in Fig. 7. Foam stability linearly increases with the measured phase angle within experimental error. A similar conclusion concerning the correlation between foam lamella stability and visco-elastic behavior of the surfactant solutions, however only for two studied concentrations, was also drawn by Koelsch and Motschmann [27]. 5. Conclusion A transition from surface elastic to surface visco-elastic characteristics of the adsorption layers upon increasing surfactant concentration was observed. These findings on the molecular level could successfully be correlated to the accompanying rise in foam lamella stabilities from the respective aqueous solutions. Noteworthy lifetimes of the lamellae could be detected only in case of surface visco-elastic behavior of the adsorbed surfactant layer. It is to be noted that foam lamella stability is not proportionally dependent on the amplitude of the surface elastic dilatational modulus E(f, c). For instance, the 30.0 mmol L−1 solution of sodium decyl sulfate (SDeS) exhibits a higher foam lamella stability, even though the amplitude is lower than for the 0.2 mmol L−1 solution. The findings suggest that a correlation between foam lamella stability and viscoelasticity, as also observed for other classes of surfactants, was equally confirmed for the model surfactant SDeS.
Fig. 7. Foam lamella stability plotted against phase angle at the studied SDeS concentrations of at 500 Hz (Color code as in Figs. 5 and 6). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
for sodium decyl sulfate (SDeS) at maximum surface coverage. This value, however was not the result of fitting with a conventional single adsorption isotherm, but rather obtained using a two state transition model [30]. In this model, it is assumed, that surfactant adsorption is described by two different adsorption laws (Henry law for low and Langmuir law for high concentrations) depending on the surface coverage. Furthermore, a parametrized transition function describing the crossover from low to high surface pressure adsorption is required. It is to be noted, that the experimentally determined value of minimum surface area obtained from Frumkin adsorption isotherm of 21.5 Å2 is in agreement with the literature value assuming the more evolved two state adsorption model. Furthermore, the decreasing cross-sectional area of adsorbed surfactant upon increasing alkyl chain length can be interpreted in terms of a more pronounced interaction between the alkyl chains promoting film stability.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
4.2. Lamella stability The characteristic of increasing single foam lamella stability upon increasing surfactant concentration has also been observed for other surfactants such as Triton-X 100, cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), sodium tetradecyl sulfate, tetraethylammonium perfluorooctane sulfonate, decyldimethyl phosphine oxide [28] and cationic model surfactant 1-dodecyl-4-dimethylaminopyridinium bromide [27]. In this study, the same behavior is shown for an anionic surfactant.
[12]
4.3. Oscillating bubble
[20]
The observations concerning the differences in amplitude and phase characteristics of the surface elastic dilatational modulus E(f, c) for the studied lower (0.2 and 1.5 mmol L−1) and higher (15.0 and 30.0 mmol L−1) concentrations are described in Section 3.3. The characteristics of the low and high concentration solutions are referred to as surface elastic and surface visco-elastic, respectively. As observed by Koelsch et al. [27], a transition from surface elastic to surface viscoelastic adsorption layer behavior upon increasing concentration was also found for the studied solutions of SDeS.
[21] [22]
[13] [14] [15] [16] [17] [18] [19]
[23] [24] [25] [26] [27] [28] [29]
4.4. Correlation of surface rheology to foam lamella stability
[30]
In the conducted study, foam stability data are presented next to
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