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Articles Anomalous Thickness Variation of Nonionic Surfactant Foam Films with Salt Concentration H. J. Mu¨ller* and Th. Rheinla¨nder Max-Planck-Institut fu¨ r Kolloid- und Grenzfla¨ chenforschung, Rudower Chaussee 5, D-12489 Berlin, Germany Received April 18, 1995. In Final Form: February 28, 1996X The thickness of equilibrium foam films of nonionic surfactants has been measured depending on the concentration of salt in the film forming solution. In the range from medium to high salt concentration, the thickness is nearly double that of the adsorption layer thickness. However, in certain intervals of salt concentration two series of peaks were observed. It is shown that the peaks in the film thickness on salt concentration dependence represent a general phenomenon for this type of surfactant. The increase of thickness always starts at a relatively well-defined critical concentration. The peak position is shifted to higher salt concentration with an increasing number of ethylene oxide units in the hydrophilic chain. The mechanism of the effect is discussed. It is shown that only a combination of a structural change of the hydrophilic chains and a change in the range of the double layer interaction is able to account for the features of the effect.
Introduction Poly(oxyethylene) alkyl ethers represent a class of nonionic surfactants which are interesting because of their wide spread application and also because of their physicochemical behavior. As the interaction of the poly(oxyethylene) chain with water is rather complex, micellar systems and mesophases of these substances show a delicate response to changes, for instance, in chemical structure, temperature, or salt concentration.1-9 Along with other methods, the measurement of the thickness of foam films stabilized with such surfactants is able to deliver information on these interactions. In measuring the thickness of foam films formed from solutions of nonionic surfactants, thick films often were found at low salt content. The thickness of such films decreases with increasing salt concentration.10-14 This indicates that a weak electrical double layer interaction also is present in films from nonionic surfactant solutions. Medium and high salt concentrations suppress the double layer repulsion. In the higher salt concentration range, X
Abstract published in Advance ACS Abstracts, April 15, 1996.
(1) Florin, E.; Kjellander, R.; Eriksson, J. C. J. Chem. Soc., Faraday Trans. 1 1984, 80, 2889. (2) Lang, J. C. Proceedings of the International School of Physics “Enrico Fermi”, 1983, Physics of Amphiphiles: Micelles Vesicles and Microemulsions, Degiorgio, V., Corti, M., Eds.; North-Holland: Amsterdam, Oxford, New York, Tokyo, 1985; p 336. (3) Leng, C. A. J. Chem. Soc., Faraday Trans. 2 1985, 81, 145. (4) Leng, C. A. Proceedings of the International School of Physics “Enrico Fermi”, 1983, Physics of Amphiphiles: Micelles Vesicles and Microemulsions, Degiorgio, V., Corti, M., Eds.; North-Holland: Amsterdam, Oxford, New York, Tokyo, 1985; p 469. (5) Saito, Y.; Sato, T. J. Phys. Chem. 1985, 89, 2110. (6) Evans, H.; Tidesley, D. J.; Leng, C. J. J. Chem. Soc., Faraday Trans. 2 1987, 83, 1525. (7) Lindman, B.; Karlstro¨m, G. Z. Phys. Chem. (Munich) 1987, 155, 199. (8) Sato, T.; Saito, Y.; Anazawa, I. J. Chem. Soc., Faraday Trans. 1 1988, 84, 275. (9) Helfrich, W. J. Phys.: Condens. Matter 1994, 6, A79. (10) Manev, E. D.; Pugh, R. J. Langmuir 1991, 7, 2253. (11) Manev, E. D.; Pugh, R. J. Colloids Surf., A 1993, 70, 289. (12) Exerowa, D.; Zacharieva, M.; Cohen, R.; Platikanov, D. Colloid Polym. Sci. 1979, 257, 1089. (13) Paluch, M. Polish J. Chem. 1980, 54, 1827. (14) Kolarov, T.; Cohen, R.; Exerowa, D. Colloids Surf. 1989, 42, 49.
therefore, the thickness of the foam films reaches roughly double that of the adsorption layer thickness and does not depend generally on salt concentration. However, maxima in the foam film thickness of surfactants of the poly(oxyethylene) alkyl ether type were found for certain ranges of salt concentration in refs 15 and 16 and 17. These maxima cannot be explained by simple electrical double layer theory, and until now, no satisfying explanation had been found. In order to obtain a more detailed picture of this phenomenon, we measured the thickness of foam films formed from solutions of poly(oxyethylene) alkyl ethers with different lengths of hydrophilic chain. The salt concentration was varied in small steps to evaluate in detail its influence on the interaction of interfaces covered with poly(oxyethylene) chains. Experimental Section Poly(oxyethylene) dodecyl ethers (C12Em) were used for the investigation. Heterogeneous products with mean numbers of ethylene oxide units in the molecule of 〈m〉 ) 8.4, 10, 15, 22, 56, and 80, were kindly supplied by Dr. R. Holzbauer from the Institute of Applied Chemistry Adlershof, Berlin. Also, homogeneous surfactants were studied with defined numbers of ethylene oxide units of m ) 7, 8, and 9 from Fluka. All surfactants were checked via thin layer chromatography and through the measurement of the surface tension/concentration isotherm, the cloud point, the electrical conductivity, and the pH. In few cases the measurement of the conductivity indicated a distinctive contamination with an electrolyte. These samples were deionized by an ion exchanger. Otherwise, the surfactants were used without purification. The sodium and potassium chloride were of analytical grade. Before use, the salt was roasted at 800 K for 1 day to remove organic contamination. Double distilled water was applied in all solutions. The thickness of the foam films was measured with the microinterferometric method as described by Sheludko and (15) Mu¨ller, H. J.; Kretzschmar, G.; Holzbauer, R. Proceedings of the International Conference on Surface Active Substances, Bad Stuer, 1985, Abh. Akad. Wiss. DDR, Abt. Math., Naturwiss., Tech.: Berlin, 1986; Vol. 1N, p 331. (16) Boomgaard, A. van den Thesis, Wageningen, 1985. (17) Boomgaard, Th. van den; Lyklema, J. Langmuir 1989, 5, 245.
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Exerowa.18,19 In a glass ring with a 4 mm diameter, horizontal microscopic films were formed (typical diameter 0.4 mm). The glass ring was situated on an inverted direct-light microscope in a closed small cell. The cell was thermostated by a jacket and its volume was saturated with vapor of the film-forming solution. A small central part of the film was illuminated and the reflected light reached a photomultiplier after passing through a optical filter with maximum permeability at 546 nm. Special computer hard- and software collected and evaluated the data. From the course of the measured reflected intensity, the socalled equivalent solution thickness, hs, was calculated as usual in such experiments.19 Thereby, the reflectivity of the film was approximated by the assumption of a homogeneous film with the refractive index of the film-forming solution. The correct thickness h was calculated then from hs by the method of Frankel and Mysels.20 Therefore, the real five-layer structure of the film (core of bulk solution, two layers of water-swollen poly(oxyethylene) chains each with thickness he and two layers of the alkyl chains with thickness ha) was taken into account:
h ) hs + 2he
n2 - n2e n2 - 1
+ 2ha
n2 - n2a n2 - 1
In the calculation, the refractive index of polyethylene glycol with 30% water was taken for ne, and that of bulk dodecane for na. As the poly(oxyethylene) chains are randomly coiled in water,21 the thicknesses, he, of the corresponding layers were calculated by multiplying the square root of the number m of ethylene oxide units in the chain with a constant taken from ref 21. For the thickness ha of the alkyl layer, the value was taken which results from the area per surfactant molecule, under the assumption that the layer has the density of bulk alkane. The thickness given for a certain composition of the filmforming solution was the mean of at least three measurements of different equilibrium films. Its accuracy was (0.1 nm in the case of thin black films. The scatter was larger for thicker films. The single standard deviation was plotted in some figures as an error bar. All measurements were performed at 295 K and natural pH (5-6). The disjoining pressure in the equilibrium film is equal to the capillary pressure in the curved meniscus surrounding the film in the ring. The capillary pressure in our experiments amounted to about 30 Pa. The surfactant concentration was chosen depending on the surfactant always slightly above the critical concentration for micelle formation in order to have comparable conditions in all experiments.
Results In this study, no difference has been noticed in the influence of sodium or potassium chloride on the thickness of foam films from solutions of the poly(oxyethylene) dodecyl ethers. This agrees with the neighboring position of these metal ions in the Hofmeister series and the nearly identical influence of these salts on the solubility of polyethylene glycol.22 Therefore, in the further discussion there is no differentiation between these two salts. The equilibrium thickness h of foam films of the surfactant C12E〈22〉 was measured for concentrations of potassium chloride between 2 × 10-4 and 4 mol/L (Figure 1). At concentrations below 0.02 mol/L KCl, thick films were found indicating some electrical double layer repulsion in the film. The film thickness decreases with increasing salt concentration. At Csalt ) 2 × 10-2 mol/L, h reaches a value of about 11 nm for the C12E〈22〉 surfactant. Further increasing of Csalt over a wide range does not change the equilibrium film thickness significantly. Such (18) Sheludko, A. J. Colloid Interface Sci. 1967, 1, 391. (19) Exerowa, D.; Zacharieva, M.; Cohen, R.; Platikanov, D. Colloid Polym. Sci. 1979, 257, 1089. (20) Frankel, S. P.; Mysels, K. J. J. Appl. Phys. 1966, 37, 3725. (21) Tanford, C.; Nozaki, Y.; Rohde, M. F. J. Phys. Chem. 1977, 81, 1555. (22) Bailey, F. E.; Koleske, J. V. In Nonionic Surfactant: Physical Chemistry; Schick, M. J., Ed.; Surfactant Sci. Ser.; Marcel Dekker: New York, 1987; Vol. 23, pp 927-970.
Figure 1. Equilibrium thickness h of foam films of C12E〈22〉 solutions depending on the concentration C of potassium chloride.
a region of nearly constant film thickness was found for all the surfactants investigated (Figures 2-4). As discussed further below, these films seem to consist mainly of two adsorbed surfactant layers and therefore will be called bilayer films in the following. The transition from film thickness controlled by electrical double layer repulsion to the range with a film thickness given roughly by the double of the adsorption layer thickness takes place between 5 × 10-3 and 10-2 mol/L salt depending on number m. However, by increasing the salt concentration from 0.9 to 1 mol/L, the film thickness increases suddenly by 2 nm in the case of C12E〈22〉 (Figure 1). The value of the thickness remains higher than that of the bilayer thickness in a certain range above this critical concentration. A similar behavior was found for the other members of this series of surfactants as shown in Figures 2 and 3. It can be noticed that the critical salt concentration for the thickness peak is shifted to smaller concentrations with decreasing number m of ethylene oxide units in the molecule. Figure 4 gives the thickness of foam films of C12E7 and of C12E〈8.4〉. A second narrow peak appears here in the curves of both of the surfactants on a salt concentration above that of the first one. The thickness increases in the peaks by 4-7 nm. The critical concentration for the peaks is shifted nearly to the range in which electrical double layer interaction governed the film thickness. The comparison between the homogeneous surfactant and the heterogeneous product shows that the distribution of the chain length in the sample tends to broaden the peaks. The second peak was found for surfactants with m not larger than 10. The dependence of film thickness on salt concentration for the surfactant C12E〈10〉 is given in Figure 5. A broad first maximum at 1.5 mol/L and a second peak at 3.1 mol/L can be seen. At very high salt concentrations (3-4 mol/L) the film thickness decreases again in the case of the surfactants with long polyethylene glycol chains. The thickness then reaches values below the bilayer level as shown in Figures 1 and 2.
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a
b
Figure 2. Influence of the salt concentration (KCl) on the film thickness for the surfactants C12E〈15〉, C12E〈22〉, and C12E〈56〉.
Figure 4. (a) Influence of NaCl on the thickness of films stabilized by the homogeneous surfactant C12E7. (b) Influence of KCl on the thickness of films stabilized by a sample of C12E〈8.4〉 with a distribution of the length of the poly(oxyethylene) chain.
agreement with Florys theory of flexible chain molecules23 which delivers proportionality to the square root for the end-end distance and the radius of gyration of randomly coiled chains. Additionally, an estimate of the bilayer film thickness is given in Figure 6 using the relations
hbilayer ) 2ha + 2he, he ) 2 fxm Figure 3. Dependence of the thickness of foam films of C12E9 on the salt concentration (NaCl).
Discussion At low salt concentration thick films were observed in this study. The thickness in this range decreases strongly with increasing salt concentration (see Figures 1-4). Such relations are normally predicted by electrical double layer theory for films with surfaces charged by adsorption of ionic surfactants. However, a similar behavior was found recently in foam films of solutions of different nonionic surfactants10-14 indicating a weak surface charge in the case of foam films from nonionic surfactant solutions, as well. As the purpose of this work was the investigation of the salt influence on film thickness at higher salt concentrations, we did not systematically investigate this range. Above the range of salt concentration where the film thickness is governed by electrical double layer interaction, the thickness is nearly constant for extended ranges of Csalt. The thickness in this range, taken from the h(Csalt)dependencies of the surfactants investigated, is shown in Figure 6. Plotting against the square root of the degree of ethoxylation yields a linear relationship. This is in
For the constant f, a value of 0.52 nm was taken from ref 21, which results from the analysis of measurements of the dimensions of micelles of CnEm surfactants. The thickness of the alkane layer ha was calculated as mentioned in the Experimental Section and varies from about 0.8 to 0.3 nm as m increases from 7 to 56. Our experimental results follow rather a relation
h′bilayer ) 2ha + 2 f′xm + a than that given above. Here the coefficient f ′ ) 0.47 nm agrees very well with that gained from micelle investigations indicating that the dimensions of the coiled poly(ethylene oxide) chains are very similar in micelles and flat adsorption layers. However, there is an excess a of around 5 nm in the case of surfactants with m g 10. This may originate from some chains which are more extended in the direction of the film interior than given by the random coil conformation. Alternatively, the excess may be caused by hydration repulsion or entropic contributions (undulation or protrusion). For the surfactants with m (23) Flory, P. J. Statistical Mechanics of Chain Molecules; Wiley: New York, 1969.
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Figure 5. Influence of NaCl on the film thickness of C12E〈10〉. Insert: influence of Na2SO4 on the film thickness of an ethoxylated nonylphenol with poly(oxypropylene) (Boomgaard et al.17).
Figure 6. Measured bilayer film thickness as a function of the degree of ethoxylation m and an estimate of this function (2ha + 2he).
from 7 to 9, the bilayer thickness is more in agreement with the picture of the zigzag conformation of the chain. The area per molecule for these surfactants, however, fits very well a dependence on m given by the random coil configuration.24 Therefore, it is likely that only a few of the chains stay in the zigzag configuration. As the normal pressure on the molecules in the adsorption layer is significantly smaller than the lateral pressure, a weak (24) van Voorst Vader, F. Trans. Faraday Soc. 1960, 56, 1078.
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steric repulsion caused by only a few molecules in the zigzag configuration may be sufficient to result in the observed thickness. The film thickness decreases below the bilayer thickness at very high salt concentrations in the case of the surfactants with a high degree of ethoxylation (m ) 15, 22, 56 in Figures 1 and 2). This may be caused by dehydration of the ethylene oxide units on this conditions. As the adsorption layers are saturated in our system, dehydration resuls only in shrinking of the poly(oxyethylene) layer and not in additional adsorption. For the surfactants with a shorter hydrophilic chain a decrease of the thickness at the highest salt concentrations was not observed (see Figure 3). The main interest of this work is directed toward the phenomenon of thickness peaks at increasing salt concentration. The investigation shows that the occurrence of thickness peaks on a variation of salt concentration is a general feature of this series of surfactants. The thickness increases in the peaks by 2-7 nm. The peak height is smaller for the heterogeneous surfactants with large m than those for the homogeneous surfactants with short chains. The width of the peaks increases with increasing m. The peak height may be larger than that given by the measurements in the case of the narrow peaks (small m) due to the difficulty in determining the salt concentration for the sharp maximum. The slope to the peak from the smaller salt concentration side is steep in all cases. The critical salt concentration Ccrit at which this steep increase happens was chosen for the characterization of the peak position. In the investigation of a nonionic surfactant with 26 ethylene oxide and 13 propylene oxide units and a nonylphenol hydrophobic part (NPE 1800), Boomgaard16,17 has also found a thickness maximum at about a concentration of 1-2 mol/L sodium chloride or bromide. However, a sudden increase of h before the peak was not noticed probably because of the fact that the film thickness was measured only for five concentrations in the range from 0 to 4 mol/L. The peak position is shifted to a higher salt concentration with increasing number m. The dependence of the peak position (Ccrit) on m is shown in Figure 7. The position of the first and the second peak is shifted to larger salt concentrations for m equal to or larger than 9 but has nearly identical values for the monodisperse surfactants with m ) 7 and 8. The critical concentration for the second peak is shifted from around 7 × 10-3 mol/L for the surfactants with m ) 7 and 8 to 3.2 mol/L in the case of m ) 10. Therefore, it is understandable that second peaks cannot be observed for the surfactants with m larger than 10 because of the solubility limit of the salt. The shift for both the first and the second peak and the other features of the thickness peaks points to a common physical background for these phenomena. To the best of our knowledge the second peak is reported here for the first time. Nevertheless, there is some indication that Boomgaard had made a similar observation. In ref 17 the foam film thickness from solutions of his NPE 1800 surfactant depending on the concentration of sodium sulfate was measured, as shown in the insert of Figure 5. After a thickness maximum is reached at 0.5 mol/L sodium sulfate, a single point (in parentheses given also in ref 17) indicates an increasing thickness again around 1.3 mol/L. The complexity of the described phenomena as shown by double peaks, narrow peak width, and shift of peak position indicates that the cause of the effect may be also complex. The film thickness maximum found in refs 16 and 17 in the case of the NPE 1800 surfactant was accounted for by an increasing undulation repulsion
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Figure 7. Critical electrolyte concentration Ccrit of the appearance of the first and second peak as a function of the degree of ethoxylation and ratio of ethylene oxide groups to salt ions in the film at Ccrit.
between the film surfaces25,26 caused by a decrease of surface tension in the same range of salt concentrtion as observed by Inanov et al.27 In Ivanov’s work the surface tension isotherm of the nonionic surfactant C12E〈18〉 was investigated at changing salt concentration and a minimum of the surface tension at cmc was found near 1.5 mol/L KCl. Thus, it is expected in refs 25 and 26 that such influence of salt on surface tension of nonionic surfactants is independent of the actual number of ethylene oxide units in the molecule and always appears at salt concentrations between 1 and 2 mol/L. It may be that there is a certain influence of increasing undulation interaction on film thickness at 1-2 mol/L salt; however, this cannot be the general cause for the peaks due to the shift of the peaks with m. For instance the thickness increases in the case of m ) 10 and m ) 8 at Csalt equal to 0.6 and 0.005 mol/L. Additionally we measured the surface tension isotherm of C12E8 without NaCl and with the addition of salt at the critical concentration of the first peak and did not find any change. Therefore, an explanation of the peak phenomenon on the basis of undulation interaction seems not probable. As poly(oxyethylene) chains are able to exist in different conformations, it should be checked whether conformational changes can cause the observed thickness peaks. However, the thickness on peak salt concentration of films from C12E7, C12E8, and C12E9 is significantly larger than the double of the length of the surfactant molecule in the most extended conformation. Therefore, changes in the chain conformation alone are not capable of causing the thickness peaks. In refs 28 and 29 ion adsorption was (25) Barnefeld, P. A.; Scheutjens, J. M. H. M.; Lyklema, J. Colloids Surf. 1991, 52, 107. (26) Lyklema, J. Proc. 6th Conf. Colloid Chem. (1992), Balatonszeplak, Hungary, p 55. (27) Ivanov, I.; Grigorov, L.; Kolarov, T. Proceedings of the International Conference on Colloid and Surface Science, Budapest, Hungary, 1975; Akade’mici Kiado’: Budapest, 1976; Vol. 2, p 169. (28) Ingram, B. T. Trans. Faraday Soc. 1972, 68, 2230.
Mu¨ ller and Rheinla¨ nder
discussed as the reason of a thickness peak of films of decyl methyl sulfoxide and decyltrimethylammonium decyl sulfate with increasing salt concentration. The ion adsorption should increase the electrical double layer potential and therefore cause an increasing film thickness until, at further increasing ion concentration, the Debey length is shortened. However, in the case of the peaks of surfactants with large m at high salt concentrations (2 to 3 mol/L) the Debey length is too short to explain the observed thickness increase. Therefore neither changes in the dimension of the layer of the hydrophilic chains nor an increase of the electrical double layer charge alone is able to give the general explanation of the phenomenon. A clue toward the mechanism of the effect may be given by the fact that for the surfactants with m equal or more than 10 the relation between the number of ethylene oxide groups in a film and of salt ions on a critical salt concentration for the appearance of the peak is nearly constant (see Figure 7, lower curve). This means that the peaks appear if a certain relation of ethylene oxide groups to ions is reached. In this connection it should be mentioned that the formation of complexes was observed with nonionic surfactants of the type investigated here.30 Reaching a certain (stoichiometric) salt concentration may cause a sudden change in the state of the layer of the hydrophilic chains. The volume phase behavior of mixtures with water of the surfactants used in this work is well investigated.31,32 The phase diagram of C12E8, for instance, shows changes from hexagonal to cubic to lamellar phases in the range from 40 to 70 wt % surfactant. Yet no detailed information about influence of salt on the phase behavior in this range is available. From the results given in this paper, we assume the following mechanism: At the critical salt concentration a specific interaction between the ethylene oxide groups and the salt cations takes place. This leads to a change in the configuration of the hydrophilic chains and in a redistribution of the ions, as well. The second fact leads to a more extended double layer interaction in the case of surfactants with small m and small critical salt concentration. At high values of m the change of double layer interaction is negligible because of the high critical salt concentration. First results of measurements of film thickness on disjoining pressure dependence in our lab done with the “porous plate method”33 also point in the direction of such a mixed mechanism. Conclusions The results show that the appearance of peaks in the dependence of foam film equilibrium thickness on salt concentration is a general effect for the poly(oxyethylene) alkyl ether surfactants. Some common features of the peaks, such as the steep increase in thickness approaching the peak from the low salt concentration side and the shift of the peak position with number of ethylene oxide units in the hydrophilic chain, indicate that the phenomena are caused by a common effect. The steep increase in thickness at a well-defined salt concentration and the relative constant relation of ions and ethylene oxide groups (29) Buscall, R,.; Ottewill, R. H. In Monolayers; Goddart, Ed.; Adv. Chem. Ser. 144; American Chemical Society: Washington, DC, 1975; p 83. (30) Cross, J. In Nonionic Surfactant: Chemical Analysis; Schick, M. J., Ed.; Surfactant Sci. Ser.; Marcel Dekker: New York, 1987; Vol. 19, pp 31-75. (31) Mitchell, D. T.; Tiddy, G. J. T.; Waring, L.; Bostock, T.; McDonald, M. P. J. Chem. Soc., Faraday Trans. 1 1983, 79, 975-1000. (32) Clerc, M.; Laggner, P.; Levelut, A. M.; Rapp, G. J. Phys. II 1995, 5, 901-917. (33) Exerowa, D.; Kolarov, T.; Khristov, K. Colloids Surf. 1987, 22, 171-185.
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in the film at this concentration (in the case of m g 10, see Figure 7) point to the following mechanism: If in the film with increasing salt concentration the relation between the number of ethylene oxide units and the number of ions reaches a certain value, then a change in the interaction between the ethylene oxide groups and the ions takes place. This results in an expansion of the diameter of the poly(oxyethylene) layer. If the position of the peak is at high salt concentration (m g 10) this is the whole effect and results in a height of the peak of around 2 nm above the bilayer thickness. If the salt concentration at peak position is smaller, a change of the distribution of the ions in the hydrophilic part of the adsorption layer causes an additional increase of thickness by a more extended electrical double layer repulsion.
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However, this model has to be confirmed by additional investigations, and some peculiarities of the phenomenon are far from clear. In this connection we have started measurements of the film thickness on disjoining pressure dependence to evaluate the mechanism of the effect. The specific influence of salt on the state of the poly(oxyethylene) layer found in this work should be reflected also in the phase behavior of volume phases.
Acknowledgment. We thank the Max Planck Society for fellowship support for T.R. and Mrs. E. Ru¨tze for helpful assistance. LA950314D
