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Articles Trisiloxane Surfactants: Surface/Interfacial Tension Dynamics and Spreading on Hydrophobic Surfaces T. Svitova,* H. Hoffmann,† and Randal M. Hill‡ Institute of Physical Chemistry, Russian Academy of Sciences, Leninsky Prospect 31, 117915 Moscow, Russian Federation, Bayreuth, Universitatstrasse 30, Postfach 101251, D-8580 Bayreuth, Germany, and Central Research and Development, Dow Corning Corporation, 2200 West Salzberg Road, Midland, Michigan 48686-0994 Received June 26, 1995. In Final Form: October 20, 1995X Dynamics of surface (at the solution/air interface) and interfacial (at the solution/n-dodecane interface) tension of nonionic siloxane surfactants, some of which are known as “superwetter”, and ethoxylated isododecyl alcohols was studied by the drop volume method. The influence of surfactant concentration and hydrophilicity (length of the ethoxy chain) on surface/interfacial tension dynamics and spreading of aqueous solutions on the liquid hydrocarbon surface was investigated. Surface and interfacial tension fall rates were estimated on the basis of the Hua and Rosen approach (Hua, X. Y.; Rosen, M. J. J. Colloid Interface Sci. 1988, 124 (2), 652). It was found that concentrated solutions of surfactants with intermediate ethoxy chain length show unusually high surface/interfacial tension fall rates. These solutions spread very fast on a liquid hydrocarbon surface: a drop of aqueous solution with a volume of about 3 µL forms a thin spreading film with an area of several square centimeters in 5-10 s. The rate of spreading and the resulting film thickness were found to depend on the surfactant concentration and the hydrophilicity and hydrocarbon subphase chain length. A good correlation between surface/interfacial tension fall rate, rate of spreading, and the dynamic spreading coefficient was found. Diffusion coefficient values were calculated according to the method of Fainerman et al. (Fainerman, V.; Makievski, A.; Miller, R. Colloids Surf. A 1994, 87, 61), and it was found that for the siloxane surfactant with eight ethoxy groups the diffusion coefficient values are 1 order of magnitude higher than that of the hydrocarbon analogue with 5 ethoxy-groups. An increase of the ethoxy chain length for siloxane as well as for hydrocarbon surfactants causes a decrease of the diffusion coefficient and the surface/interfacial tension fall rate and leads to a suppression of surfactant spreading ability.
Introduction In many cases, for instance, during wetting and spreading, emulsification, foaming, and dispersion formation, the processes in the presence of the surfactants take place under nonequilibrium conditions, and in these cases the dynamic properties of the surfactant adsorption layer are of great importance. One of the methods to investigate amphiphile adsorption kinetics at freshly formed interfaces is to measure the dynamic interfacial tension. Different dynamic tension techniques are available now, and the theory has reached a level where a quantitative description is possible.1-8 A new method for measuring dynamic tensions by a growing drop technique is described in refs 4 and 5. One can find the reviews of modern dynamic tension methods in refs 6 and 7. * Corresponding author. E-mail:
[email protected]. † Universitat Bayreuth. ‡ Dow Corning Corporation. X Abstract published in Advance ACS Abstracts, February 1, 1996. (1) Miller, R.; Kretzchmar, G. Adv. Coll. Interface Sci. 1991, 37, 97. (2) Krotov, V. V.; Rusanov, A. I. Kolloidn. Zh. 1977, 39, 58. (3) van den Tempel, M.; Lucassen-Reynder, E. Adv. Colloid Interface Sci. 1983, 18, 281. (4) MacLeod, C. A.; Radke, C. J. J. Colloid Interface Sci. 1993, 160, 435. (5) MacLeod, C. A.; Radke, C. J. J. Colloid Interface Sci. 1994, 166, 73. (6) Miller, R.; Joos P.; Fainerman, V. P. Adv. Colloid Interface Sci. 1994, 49, 249. (7) Chang, C.-H.; Franses, E. I. Colloids Surf., A: Physicochem. Eng. Aspects 1995, 100, 1. (8) Fainerman, V. B.; Makievski, A. V.; Miller, R. Colloids Surf., A: Physicochem. Eng. Aspects, 1994, 87, 61.
It is worth noting that most of the dynamic tension work mentioned here was performed for dilute surfactant solutions, and it has been shown that diffusion and twodimensional phase transitions due to surfactant molecule reorientation play an important role in such cases.1-3,8-11 It was also found that usually at surfactant concentrations above the cmc, surface/interfacial tension only slightly depends on surface age.12 In ref 13 a theoretical analysis of adsorption kinetics from micellar solutions was performed and it was shown that the presence of aggregates can influence the rate of adsorption. The authors of ref 14 have analyzed the experimental data of Triton X 100 surface tension dynamics at concentrations above the cmc, and they have proposed a way to evaluate the rate of demicellization on the basis of these data. For the processes of wetting and spreading of surfactant solutions, occurring under nonequilibrium conditions, as was mentioned above, as far as these processes obey the Young equation and Neuman inequality, surface/interfacial tension dynamics must be one of the most important factors, determining spreading dynamics. In work ref 15 it was found that retention of nonionic surfactant solutions (9) Miller, R.; Schano, K.-H.; Hofmann, A. Colloids Surf., A: Physicochem. Eng. Aspects 1994, 92, 189. (10) Svitova, T.; Smirnova, Yu.; Yakubov, G. Colloids Surf., A: Physicochem. Eng. Aspects, in press. (11) Svitova, T.; Smirnova, Yu.; Churaev, N.; Rusanov, A. Kolloidn. Zh. 1994, 56 (3), 441. (12) Davies, J. T.; Rideal, E. K. In Interfacial Phenomena; Academic Press: New York, 1963. (13) Miller, R. Colloid Polym. Sci. 1981, 259, 1124. (14) Rillaerts, E.; Joss, P. J. Phys. Chem. 1982, 86, 3471. (15) Anderson, N. H.; Hall, D. J. Adjuvants Agrochem. 1989, 2, 51.
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Langmuir, Vol. 12, No. 7, 1996 1713 Table 1. Objects of Investigation abbrev
source
chemical structure
M(D′E8OH)M M(D′E12OH)M M(D′E7OH)M M(D′E16OH)M
Trisiloxane Surfactants D-8 Dow Corning D-12 Corporation PR-7 Goldschmidt PR-28 Goldschmidt
(Me3SiO)2Si(Me)(CH2)3(OCH2CH2)8OH (Me3SiO)2Si(Me)(CH2)3(OCH2CH2)12OH (Me3SiO)2Si(Me)(CH2)3(OCH2CH2)7OH (Me3SiO)2Si(Me)(CH2)3(OCH2CH2)16OH
ethoxylated i-C12 alcohol, EO n ) 4.9 ethoxylated i-C12 alcohol, EO n ) 9.8 ethoxylated i-C12 alcohol, EO n ) 14.6
Hydrocarbon Surfactants i-C12 EO4.9 Hoechst i-C12 EO9.8 Hoechst i-C12 EO14.6 Hoechst
(CH3)2C10H19O(CH2CH2O)4.9OH (CH3)2C10H19O(CH2CH2O)9.8OH (CH3)2C10H19O(CH2CH2O)14.6OH
was related to dynamic, not to equilibrium, surface tension values and depended more on solution concentration than on the chemical structure of the surfactants. A number of siloxane surfactants exhibit “superwetting” or “superspreading”;16-19 according to ref 18 a surfactant is a superspreader if the addition of a small amount, say less than 0.1%, to a small droplet of water enables it, when placed on a hydrophobic surface (they mean a solid surface), to spread into a thin, wetting film within tens of seconds. The interesting work was done to study the spreading behavior of the pure siloxane surfactants on high- and low-energy surfaces.20,21 These investigations showed the importance of the aggregate organization and atmospheric humidity for spreading of amphiphilic molecules on a low-energy surface. Some authors attributed the special properties of the trisiloxane surfactants to the unique “T”, hammer-like or “umbrella” shape of these surfactants.16,17 Recently the results of systematic studies of siloxane surfactant aqueous solutions spreading on Parafilm (a paraffin wax) surface were published.18 In ref 18 it has been shown that for linear as well as “hammer”-shaped siloxane surfactants, spreading ability is related to dispersed particle size and to the transport rate of surfactant from dispersed particles to the interfaces at the spreading front. The authors concluded that the silicone hydrophobic moiety, the presence of water vapor, and a dispersed surfactant-rich phase are necessary for superspreading, but the molecular geometry of surfactant is not a critical factor. The spreading mechanism is, however, still largely unexplained. The authors18 have proposed that a thin pre-existing high-tension film is formed at the leading edge of the spreading drop, and so spreading is driven by a Marangoni effect, but the mechanism of this precursor film formation is unclear. They also studied a series of aqueous dispersions of both hammer-like and linear hydrocarbon polyoxyethylene surfactants and found that none were superspreaders. The explanations of superspreading, proposed in refs 1619, apparently should be applied to each surfactant, and thus the main question, namely, what are the peculiarities of siloxane surfactants determining their unusual spreading behavior, is still open. In the present work dynamics of surface (at the solution/ air interface) and interfacial (at the solution/n-dodecane interface) tension was studied for nonionic trisiloxane surfactants with different ethoxy chain length, EO ) 8, 12, and 16, and ethoxylated isododecyl alcohols with ethoxy group numbers 4.9, 9.8, and 14.6. The influence of the surfactant concentration and chemical structure on the (16) Ananthapadmanabham, K. P.; Goddard, E. E.; Chandar, P. Colloids Surf. 1990, 44, 281. (17) Roggenbuck, F. C.; Rowe, L.; Penner, D.; Petroff, L.; Burow, R. Weed Technol. 1990, 4, 576. (18) Zhu, S.; Miller, W. G.; Scriven, L. E.; Davis, H. T. Colloids Surf., A: Physicochem. Eng. Aspects 1994, 90, 63. (19) Lin, Z.; Hill R. M.; Davis H. T.; Ward M. D. Langmuir 1994, 10, 4060. (20) Tiberg, F.; Cazabat, A.-M. Europhys. Lett. 1994, 25 (3), 205. (21) Tiberg, F.; Cazabat, A.-M. Langmuir 1994, 10, 2301.
surface/interfacial tension dynamics was studied. These results were compared with the spreading behavior of these solutions on a liquid hydrocarbon surface. Thus we intended to clarify the relation between interfacial tension dynamics of surfactant solutions and their spreading on a uniform hydrophobic fluid surface and to make a new short step toward understanding the mechanism of the superspreading phenomenon. Experimental Section Materials. Four trisiloxane surfactants were studied (see Table 1). These products were of 90% purity. The ethoxylated isododecyl alcohols with ethoxy chain length n ) 4.9, 9.8, and 14.6, narrow ethoxy chain length distribution, of 98% purity, were kindly supplied by Hoechst Company, Germany. All the surfactants were used without special purification. Distilled and deionized water was used for surfactant solution preparation. n-hydrocarbons (Fluka) and paraffin oil (Fluka), of spectroscopic grade purity, were used for interfacial tension measurements and spreading behavior studies. The reason why we chose these substances as subphases for spreading behavior investigations was to permit us to measure interfacial tension on the surfaces of solution/subphase and subphase/air and therefore be able to calculate both the dynamic and the equilibrium spreading coefficients. At the same time, the liquid hydrocarbon surface is always horizontal, molecularly smooth, and homogeneous, and its surface properties are independent of surface prehistory and treatments. These properties are often difficult to reproduce for solid hydrophobic surfaces, always having some roughness and heterogeneity which produces a significant influence on the character and rate of spreading.18 Methods. Dynamic surface/interfacial tension measurements were performed by using the drop volume technique; a TVT-1 drop volume tensiometer, Lauda, Germany, was used for these measurements. The detailed description of this apparatus can be found in refs 9, and 22. Drop volume measurements were mainly made in the dynamic regime, using Dyn Mode of the apparatus, 9-15 measurements of drop volume were performed for each drop formation time, and the average drop volume values were used for the calculation of surface/interfacial tension values. The mean reproducibility of the drop formation time was (0.2 s, and drop volume deviations did not exceed (0.3 µL from one set of measurements to another; this provided the mean reproducibility of the interfacial tension as (0.5 mN/m. We also used the quasi-static regime for measurements of dynamic surface tension of dilute D-8 solutions. This method was developed by Addison and Tornberg23 and consists of growing a drop of a given volume Vo at the tip of a capillary in the shortest possible formation time. In this case, the drop volume Vo and dropping time t do not have to be corrected. The mean reproducibility of the drop formation time for dilute surfactant solutions was (0.5 s at short dropping times, but it became worse (the deviations was comparable with the dropping time) with increasing dropping times. For concentrated solutions, the dynamic drop volume is quite close to the equilibrium one; the quasi-static regime gives inadmissibly large deviations in dropping time, exceeding this (22) Miller, R.; Hofmann, A.; Hartman, R.; Shano, K.-H.; Halbig, A. Adv. Mater. 1992, 5 (4), 370. (23) Addison, C. C. J. Chem. Soc., 1946, 579. Tornberg, E. J. Colloid Interface Sci. 1978, 64, 391.
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Figure 2. Dynamic surface tension at the D-8 aqueous solution/ air interface, 25 °C. 1, 0.05% (wt/wt); 2,3, 0.1% (wt/wt) (2, quasistatic mode; 3, dynamic mode); 4, 0. 25% (wt/wt); 5, 0.5% (wt/ wt) D-8. On the basis of the approach of Miller et al.,9,22 taking into account the dependence of the drop volume on the flow rate
Ve ) V(t) - (R + βF), F ) V(t)/t
(1)
we have calculated R and β coefficients from our experimental data as the slope and intercept with the Y axis of the ∆V(F) dependence for pure water/air and water/n-dodecane systems. The corrected drop volume for surfactant solutions was calculated using eq 1 with the thus obtained R and β coefficients. The surface/interfacial tension was then calculated using the following well-known equation:27
γ ) Vdyn cor ∆ρ g F/R
Figure 1. Dependence of the dynamic surface tension on (a) the flow rate at the 0.25% (wt/wt) D-8 aqueous solution/air interface, 25 °C. 1, rcap ) 1.055 mm; 2, rcap ) 1.71 mm; (b) time dependence at the 0.025% (wt/wt) D-8 aqueous solution/air interface, rcap ) 1.38 mm, 25 °C. 1, dynamic mode; 2, quasistatic mode. time, we were forced to use the dynamic regime for the investigation of the solution dynamic tension. To choose the right way of accounting for hydrodynamic drop volume perturbation under dynamic conditions, we have analyzed the different modes of hydrodynamic correction, developed at present for pure liquids and surfactant solutions,9,24-26 and we have selected the hydrodynamic correction equation, proposed by Miller et al.9,22 This hydrodynamic correction mode was chosen because (i) it was developed especially for the drop volume method, using a TVT apparatus; (ii) it seems to be based on quite simple and physically proven assumptions; (iii) it provides a good correlation between the results, obtained using different capillary tip radii, for pure solvents as well as for surfactant solutions (Figure 1a); (iv) in the cases when it was possible, we have compared the results of quasi-static and thus corrected dynamic measurements, and very good agreement was found (Figure 1b). (24) Jho, C.; Burke, R. J. Colloid Interface Sci. 1983, 95 (1), 9. (25) van Hunsel, J.; Joss, P. Colloid Polym. Sci. 1989, 267 (11), 1026. (26) Kloubek, J.; Friml, K.; Krejci, F. Czech. Chem. Commun. 1976, 41, 1845.
(2)
in which ∆F is the density difference between the studied liquid and air or oil, g is the acceleration of gravity, R is the capillary tip radius, and F is the correction factor, tabulated in ref 27 . Figure 1a illustrates the surface tension vs flow rate dependencies, obtained for 0.25% D-8 aqueous solution using capillaries of different radii. As it is seen the dependence of the surface tension on the flow rate is normal and there is a good correlation for the results, corresponding to the different capillary sizes. The comparison of the dynamic surface tension measurements, performed using dynamic (curve 1) and quasi-static (curve 2) regimes, for 0.025% D-8 solution is presented in Figure 1b. It is seen that there is very good coincidence between these two sets of measurements. Note that for the dynamic regime the time scale corresponds to the surface age or diffusion time,8,9,22 equal to 3/7 of the drop formation time for a continuously growing drop; for the quasi-static regime, the time scale corresponds to a dropping time. The same coincidence of dynamic and quasistatic results was obtained for the 0.1% D-8 solution (Figure 2, curves 2 and 3) for short dropping times; this fact proves that the hydrodynamic correction mode used here gives accurate drop volume values that correspond to reality. Equilibrium interfacial tension measurements at the D-8 aqueous solution/n-hydrocarbon interface were performed by using a spinning drop tensiometer, Kruss, Germany. Spreading of surfactant solutions on hydrocarbon subphases was studied by visual observations without humidity control at room temperature. A drop of surfactant solution was put on the subphase surface, and the maximum radius of the spread drop (27) Padday, J. In Surface and Colloid Science; Matijevic, E., Ed.; Wiley-Interscience: New York, 1969.
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Figure 3. Dynamic surface tension at the PR-28 and PR-7 aqueous solution/air interfaces, 25 °C. 1, 0.25% (wt/wt) PR-28; 2, 0.05% (wt/wt); 3, 0.25% (wt/wt) PR-7.
Figure 4. Dynamic surface tension at the D-12 aqueous solution/air interface, 25 °C. 1, 0.25% (wt/wt); 2, 0.5% (wt/wt) D-12. and the time of spreading to maximum radius were estimated. Mean values for 5-7 measurements were calculated for each solution. Spreading coefficients were calculated according to28
K ) γB - γA - γAB
(3)
where γΒ is the hydrocarbon surface tension, γΑ is the solution surface tension, and γ ΑΒ is the solution/hydrocarbon interfacial tension.
Results Surface/Interfacial Tension Dynamics. Figures 2-5 show the plots of the dynamic surface tension for D-8, D-12, PR-7, PR-28, and i-C12EO4.9 aqueous solutions at 25 °C, which the capillary radius was 1.055 mm. Data (28) Thorpeis Dictionary of Applied Chemistry, 4th ed.; Longmans, Green and Co.: London, New York, Toronto, 1954; X1, p 348.
Figure 5. Dynamic surface tension at the i-C12EO4.9 aqueous solution/air interface, 25 °C. 1, 0.01% (wt/wt); 2, 0.1% (wt/wt); 3, 0.25% (wt/wt); 4, 0.5% (wt/wt) i-C12EO4.9.
are shown for several surfactant concentrations. In all cases, the surfactant concentration was near or above the critical micelle concentration (cmc), which for nonionic surfactants is usually about 0.01% (wt/wt)29 and is equal30 to 0.007% (wt/wt) (1.24 × 10-4 mol/kg30) for D-8 and increases from 7 × 10-5 M to 5 × 10-4 M for ethoxylated isododecyl alcohols with an increase in the ethoxy chain length from 5 to 15. In these plots the X-axis time scale as mentioned above corresponds to the diffusion time8,9,22 (except curve 2 of Figure 2, where it corresponds to the dropping time, measured in the quasi-static regime). At low surfactant concentration, 0.01-0.025% (wt/wt) one can see the usual surface tension dependencies on time, namely, the surface tension decreases slowly with increasing surface age. The higher the solution concentration, the less pronounced the dependence of the surface tension on time, for example, the dynamic surface tension of 0.5% D-8 (curve 5 of Figure 2) and PR-7 (curve 3, Figure 3) aqueous solutions decreases about 0.25-1.0 mN/m during all the time of the observations. For D-12 (Figure 4), having an ethoxy chain longer than that of D-8 and PR-7, the decrease of the surface tension with time is still well pronounced at a solution concentration of 0.5% (wt/ wt). For PR-28, the most hydrophilic trisiloxane surfactant studied here, having 16 ethoxy groups, the surface tension dynamics at the 0.5% (wt/wt) solution/air interface has its usual character and the surface tension slowly decreases with surface age, according to curve 1 of Figure 3. As is seen from Figure 5, the same regularity is observed for ethoxylated isododecyl alcohol i-C12EO4.9, the increase of the surfactant concentration from 0.01% to 0.5% (wt/ wt) leads to the suppression of the surface tension vs time dependence. Next, Figure 6 shows the results of dynamic surface tension measurements of 0.5% (wt/wt) aqueous solutions of the ethoxylated isododecyl alcohols with different numbers of ethoxy groups. It is seen from this figure that for these surfactants under certain conditions the dynamic (29) Shinoda, K.; Nakagawa, T.; Tamamushi, B.-I.; Isemura, T. Colloidal Surfactants; Academic Press: New York, 1963; Schonfeldt, N. Grenzflachenaktive Athylenoxid-Addukte; Wissenschaftliche Verlagsgesellschaft MBH: Stuttgart, 1976. (30) Hill, R.; He, M.; Davis, H. T.; Sciven, L. E. Langmuir 1994 10 (6), 1724.
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Figure 6. Dynamic surface tension at the i-C12EOn aqueous solution/air interface, 25 °C. Surfactant concentration, 0.5% (wt/wt). 1, i-C12EO14.6; 2, i-C12EO9.8; 3, i-C12 EO4.9.
surface tension is nearly constant with an increase in the surface age, although some temporary oscillations, reproducible from one set of measurements to another, were observed. Similar drop volume bifurcations were observed in ref 31, and the authors have proposed that this may be caused by capillary waves, arising at the drop surface under specific conditions. Note that with increasing ethoxy chain length both dynamic and equilibrium surface tension values increase. The same regularity is observed for trisiloxane surfactants, as is seen from Figures 2-4. The results of equilibrium interfacial tension measurements at the 0.5% (wt/wt) D-8 aqueous solution/nhydrocarbon interface are presented in Figure 7. One can see that the equilibrium interfacial tension rises almost linearly with an increase in the hydrocarbon chain length. For 0.5% (wt/wt) D-8/n-hexane system the interfacial tension reaches a very low value, 0.03 mN/m, and the formation of a thin layer of an intermediate phase, probably a microemulsion, surrounding the hexane drop in the surfactant solution was observed during interfacial tension measurements on the spinning drop tensiometer. In the systems with long-chain hydrocarbons this phenomenon was not observed and the interfacial tension value for 0.5% (wt/wt) D-8/tetradecane system is an order of magnitude higher than that with hexane. The dynamic interfacial tension was measured by using the drop volume method for 0.25% (wt/wt) D-8 and 0.5% (wt/wt) D-12 solutions against n-dodecane, and the results of these measurements are presented in Figure 8. It is seen that for both solutions the interfacial tension decreases with a time increase. The dependence of the interfacial tension on time is more significant for the 0.5% D-12 solution; for the D-8 solution, the interfacial tension only slightly decreases with time, and the same regularity was observed at the solution/air interface. Unfortunately, we could not measure the interfacial tension for the 0.5% (wt/wt) D-8 solution because it was very low (the equilibrium value is 0.23 mN/m, as is seen from Figure 7), below the TVT 1 apparatus limits; in this case we observed nonstop flow of the D-8 solution. On the other hand, for (31) Fainerman, V. B.; Miller, R. Colloids Surf., A: Physicochem. Eng. Aspects 1995, 97, 255.
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Figure 7. The dependence of the interfacial tension at the 0.5% (wt/wt) D-8 aqueous solution/hydocarbon interface on the hydrocarbon chain length, 25 °C.
Figure 8. Dynamic interfacial tension at the D-n aqueous solution/dodecane interface, 25 °C. 1, 0.25% (wt/wt) D-8; 2, 0.5% (wt/wt) D-12.
the spinning drop method there is a minimum time, about 1 min, below which measurements cannot be performed, and it is impossible to compare the measurements performed on the TVT and spinning drop apparatuses due to the differences in time scale of these methods. The results of the interfacial tension measurements in i-C12EOn/n-dodecane systems, presented in Figure 9, show that the interfacial tension decreases with time in the i-C12EO4.9 /n-docecane system; in the i-C12EO14.6/n-docecane system the dynamic interfacial tension is more or less constant, and in the i-C12EO9.8/n-docecane system the interfacial tension even slightly increases with time. Note that in distinction with the i-C12EOn solution/air interface, where we have observed an increase of the surface tension with an ethoxy chain length increase, at the i-C12EOn/ndodecane interface the dynamic as well as the equilibrium interfacial tension was found to be minimum in i-C12EO9.8/ n-docecane system. The same dependence of the inter-
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Figure 9. Dynamic interfacial tension at the i-C12EOn aqueous solution/dodecane interface, 25 °C. 1, 0.5% (wt/wt) i-C12EO14.6; 2, 0.25% (wt/wt) i-C12EO4.9; 3, 0.5% (wt/wt) i-C12 EO9.8.
facial tension on ethoxy chain length with a minimum was observed in ref 32 for the ethoxylated isononylphenol solution/octane systems. Spreading Behavior. The spreading behavior of surfactant solutions on the hydrocarbon surface was studied in the open air at room temperature. First of all we studied the behavior of a pure water drop on the surface of different hydrocarbons. It was observed that a small drop of water, about 5 µL, being put on an octane and a decane surface, drowned nearly immediately. At the surface of dodecane and longer-chain hydrocarbons a small drop of water can float a long time, held by surface tension like a thin steel needle at the surface of pure water during the well-known demonstration of surface tension action. The same behavior was observed for dilute (0.01-0.025% (wt/wt)) trisiloxane and i-C12 EOn surfactant solutions and 0.5% (wt/wt) PR-28 and i-C12EO14.6 solutions. More concentrated D-8, D-12, PR-7, i-C12EO4.9, and i-C12EO9.8 solutions spread at the dodecane and longer-chain hydrocarbon surface, the rate of spreading depends on the solution concentration, the hydrocarbon chain length, and the surfactant ethoxy chain length. Note that a droplet of the pure “dry” siloxane surfactants D-8 and D-12 did not spread significantly on the liquid hydrocarbon surface and it could float on the surface for a few minutes, but after contact with the humid atmosphere it started to spread. This observation is in accordance with the spreading behavior of pure D-8 on solid low-energy surfaces,20,21 and we can say that in our case the presence of water is necessary for fast spreading on the fluid surface and likewise on solid ones. As for solid surfaces,18-21 for our case the main driving term for spreading is the difference in chemical potential between the edge of the film and the main drop. Taking that into account, according to Figures 1-6, 8, and 9, at freshly-created interfaces the tension is higher than at aged ones, we can say that this spreading is caused by a Marangoni effect. The results are summarized in Table 2. Analysis of Table 2 data shows that the spreading of surfactant solutions on the hydrocarbon surface occurs only in the cases when the equilibrium spreading coef(32) Svitova, T.; Smirnova, Yu.; Pisarev, S. Kolloidn. Zh. 1994, 56 (3), 436.
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ficient value is positive; this is in good agreement with the theory of spreading.24 On the other hand, Kspr is positive for a D-8 solution with decane and undecane, but aqueous surfactant solution drops, weighing 3-5 mg, drown upon being put on the surface of these hydrocarbons, and spreading cannot be observed. The smaller drop of 0.5% (wt/wt) D-8 solution, about 1 mg, spreads on the undecane surface, and the rate of spreading is 5.2 mm2/s. In all cases we saw that in the open air a spreading film was unstable and tended to contract or to break into tiny droplets of surfactant-rich phase, perhaps this occurred due to water evaporation. At 100% humidity the films were stable and the spreading rate was even greater in supersaturated air; the same trends were observed in ref 18 when these solutions were spread on a solid hydrophobic surface. It is seen from Table 2 that there is not a good correlation between the equilibrium spreading coefficient value and the rate of spreading; for instance, the equilibrium spreading coefficient for i-C12EO4.9/paraffin oil is 4 times smaller than that for D-8/paraffin oil, but the rates of spreading are comparable. The maximum rate of spreading of a 0.5% (wt/wt) D-8 solution and the minimum resulting film thickness were observed on tetradecane and paraffin oil surfaces. These hydrocarbons have very close surface tension values, 30.6 and 31.5 mN/m, respectively. The maximum dependence of the radial spreading velocity on the surface energy was found for a D-8 solution spreading on various monolayers immobilized on quartzAu resonator surfaces.19 The influence of the subphase critical surface tension on the spreading behavior of pure D-8 on solid low-energy surfaces was also observed in ref 21. The authors have remarked that in this case the decrease of the solid surface critical tension of ∼1-3 mN/m led to a total depression of the D-8 spreading. It appears that spreading on a low-energy surface is governed by the very delicate balance of the surface energy excesses on the three-phase contact line and thus the surface/ interfacial tension dynamics may play an important role in this process. To clarify the relationship between the surface/interfacial tension dynamics and the spreading behavior of the surfactant solutions on the hydrophobic surfaces we have analyzed the results of dynamic surface/ interfacial tension measurements on the basis of the approach of Hua and Rosen,33 as is described further in the Discussion. Discussion Hua and Rosen have studied dynamic surface tension33 and adsorption dynamic behavior for 15 highly purified surfactants34 by use of the maximum bubble pressure method. They have proposed the division of the dynamic surface tension vs log time curves into four stages: an induction region, a fast fall region, a mesoequilibrium region, and an equilibrium region. All four of these regions can be observed when dilute surfactant solutions are under study.10,11,33 As we deal with surfactant solutions near or above the cmc, the regions of fast fall, mesoequilibrium, and equilibrium can be observed in Figures 1-6, 8, and 9. In the cases when our curves could be satisfactorily fitted by single-exponential decay we had defined the mesoequilibrium surface/interfacial tension as the limiting surface/interfacial tension value of the fitting curves. The fitting curves have been used to estimate the surface/ (33) Hua, X. Y.; Rosen, M. J. J. Colloid Interface Sci. 1988, 124 (2), 652. (34) Rosen, M. J.; Hua, X. Y. J. Colloid Interface Sci. 1990, 139 (2), 397.
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Table 2. Spreading of Surfactant Solutions on Fluid Hydrocarbons surfactant
conc, % wt
subphase
rate of spreading, mm2/s
D-8 D-8 D-8 D-8 D-8 D-8 D-8 D-8 D-12 PR-7 PR-7 PR-28 i-C12EO4.9 i-C12EO9.8 i-C12EO14.6
0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5
C10H22 C11H24 C12H26 C14H30 C16H34 paraffin oil C10H21OH C12H25OH C12H26 C12H26 paraffin oil paraffin oil paraffin oil paraffin oil paraffin oil
drown drown 70 240 160 230 1.4 40 50 68 220 no spreading 80 46 no spreading
Keqsp, mN/m 7.7 8.2 8.7 9.3 9.4 9.5
resulting film thickness, µm
3.5 1.2 1.8 1.8 340 8 3.5 4.0 1.4
4.5 8.5 9.2 -4.5 2.1 0.2 -8.6
5.0 8.1
Table 3. Parameters of Spreading and Surface Tension Dynamics system
surfactant conc, %
R1/2, mN/(m s)
rate of spreading, mm2/s
0.025 0.05 0.05a 0.1 0.25 0.5 0.25 0.5 0.05 0.25 0.5 0.01 0.1 0.25 0.5 1.0 0.5 0.5 0.25 0.25 0.5
224 484 90 1.9 × 103 3.4 × 107 5.9 × 107 7.8 × 103 1.1 × 104 430 8.6 × 103 3.4 25.3 161 2.9 × 103 5.6 × 105 7.8 × 105 4.6 × 104 70 1.2 × 103 1.1 × 106 270
no spreading 15 no spreading 28 40 70 26 50 14 68 no spreading no spreading 2 17.6 80 280 no spreading
D-8/air
D-12/air PR-7/air PR-28/air C12EO4.9/air
C12EO14.6/air C12EO14.6/C12H26 C12EO4.9/C12H26 D-8/C12H26 D12/C12H26 a
K0spr, mN/m
Keqspr, mN/m
-6.55 1.1 -2.5 3.2 3.4 4.2 1.6 2.1 1.0 5.0 -18.1 -26.4 0.03 0.08 0.12
3.5 7.6 6.4 7.8 8.6 9.1 3.5 5.1 7.3 8.2 -12.3 -12.6 0.05 0.8 1.03
-9.0
-6.8
Two weeks after preparation.
interfacial tension values at t f 0 and γtf0 and thus to calculate the dynamic spreading coefficients, K0spr, at t f 0. According to ref 33, the surface tension fall rate R1/2 at t1/2 ) t* is determined as
R1/2 )
Πm (γ0 - γm) ) 2t1/2 2t*
(4)
where γm is the mesoequilibrium tension, γ0 is the pure solvent tension, t* is constant, having the meaning of the time at which the dynamic surface pressure Π ) γ0 - γt reaches 1/2 of the mesoequilibrium value. The constant t* can be evaluated by plotting log[(γ0 - γt)/(γt - γm)] vs log t, where γt is the dynamic surface tension. We perceived that Hua and Rosen’s analysis has quite an empirical character and other parameters they have proposed to define33,34 do not have deep physical sense. Nevertheless we thought that the surface tension fall rate thus calculated may be used as a measure of the dynamic surface activity and thus will be useful for comparison of surface/interfacial tension dynamics of different surfactant solutions. In Table 3 the surface/interfacial tension fall rate, calculated according to eq 4, is compared with the rate of spreading and spreading coefficients of studied surfactant solutions on a dodecane surface. It is seen from Table 3 that an increase of the surfactant concentration leads to a significant rise of the surface tension fall rate. The highest value of R1/2 is observed for
a 0.5% D-8 solution at the boundary with air and a 0.25% solution with dodecane. It is noticeable that an increase of the ethoxy chain length causes a decrease of R1/2 for trisiloxane as well as for hydrocarbon surfactants. Note also that in all cases R1/2 at the boundary with air is significantly higher than that in the solution/dodecane systems. In so far as we studied nonionic surfactants, which are not individual substances but a mixture of homologues with different ethoxy chain length, this may be explained by partial dissolution and distribution of surfactant homologues with short ethoxy chains between an oil and an aqueous phase, occurring in the solution/ dodecane systems and thus retarding interfacial adsorbed layer equilibration. It is seen that D-8 solutions at a concentration of 0.05% (wt/wt) and above have a positive spreading coefficient as at equilibrium so as at t f 0 and for these solutions there is a good correlation between the surface tension fall rate and the rate of spreading. Knowing the area of the spreading film and assuming that the area per molecule in the spreading film cannot be smaller than that in the saturated adsorbed layer, equal to 59 Å2 for D-8 and 47 Å2 for C12EO4.6,17,35 we could estimate a ratio between amount of surfactant molecules adsorbed on a totally spread film surface and the total amount of surfactant molecules in the drop. This value is almost constant for D-8 solutions of different concentrations and equal to 0.4(35) He, M.; Hill, R. M.; Lin, Z.; Sciven, L. E.; Davis, H. T. J. Phys. Chem. 1993, 97 (34), 8820.
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Trisiloxane Surfactants
0.45; that means that under the conditions studied a spreading film does not reach the full possible coverage of interfaces and that more than half of the surfactant still remains in the bulk of the film. This value is about 1 order of magnitude lower for C12EO4.6 than for D-8 and is 0.03-0.07 in its dependence on the surfactant concentration. Thus, C12EO4.6 is not as effective a spreading agent as D-8, is less than 1/10 of the total C12EO4.6 amount, present in solution, and participates in adsorption layer formation at solution/air and solution/dodecane interfaces in a maximally spread film. The solutions, having a negative dynamic spreading coefficient, as is seen from Table 3, do not spread on the dodecane surface. In Table 3 are presented the results for a dilute (0.05% (wt/wt)) solution 1 day (row 2) and 2 weeks (row 3) after preparation. The change, occurring after aging the 0.05% D-8 solution (0.05a) is caused by a decomposition of the trisiloxane surfactant due to mixing in distilled water, as it was noticed in ref 35; the lower the surfactant concentration, the faster decomposition occurs. The surfactant decomposition leads to a significant decrease of the surface tension fall rate and simultaneously to a loss of superspreading properties. We should like to emphasize that more concentrated D-8 solutions were significantly more stable and no noticeable changes in dynamic surface/interfacial tension were observed within at least 2 weeks after preparation. The deviations of dynamic tension values of freshly prepared and two week old 0.25% D-8 solutions fell into measurement accuracy limits. For a 0.25% (wt/wt) PR-7 aqueous solution we have observed a total loss of spreading ability 1 week after preparation. It was senseless to measure the surface/ interfacial tension for this aged solution because the separation of decomposition products was observed in the bottom of a test tube. Unfortunately, we did not check the pH of D-8 and PR-7 solutions, but we can propose, according to ref 35, that the pH of PR-7 solutions was shifted from its optimal value of around 7, where trisiloxane surfactants are more stable in aqueous solution and that this was the reason of very fast decomposition of this product. In Table 3 the analogous results of spreading behavior and dynamic surface tension analysis for ethoxylated alcohol aqueous solutions are also listed. As is seen from Table 3, for these surfactants a good correlation between the rate of spreading, the surface tension fall rate, and the dynamic spreading coefficient is also observed. The comparison of the data for D-8 and i-C12EO4.9 shows that at the same concentration D-8 exhibits better superspreading properties, and this corresponds to the higher values of the dynamic spreading coefficient and surface/interfacial tension fall rate. We propose that in the cases under study the adsorption barrier is not a leading factor in adsorption dynamics. To estimate the role of diffusive mass transport in the adsorption process it is necessary to know the aggregate size of the studied surfactants, taking into account that we dealt with rather concentrated solutions above the cmc. We have used freeze-fracture transmission electron microscopy to estimate the aggregate size in 0.025% and 0.25% (wt/wt) D-8 aqueous solutions. The micrographs of carbon-platinum replicas, obtained from these solutions, are presented in Figure 10 a,b. It is seen that in both solutions small aggregates exist with a mean size of ca. 40 nm and there are not strong differences in individual aggregate size in dilute and more concentrated solutions, but in the concentrated solution some clusters of aggregates can be observed. The size of D-8 aggregates is larger than that of normal micelles and is comparable to
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Langmuir, Vol. 12, No. 7, 1996 1719
Figure 10. Electron micrographs of D-8 aqueous solutions: (a), 0.025% and (b), 0.25 % (wt/wt); bar ) 200 nm.
the size of double-tailed surfactant unilamellar vesicles,36 and so we can propose, in accordance with refs 30 and 35 that D-8 forms unilamellar vesicles in aqueous solution at the concentrations of 0.01-0.25% (wt/wt). Regarding these aggregates as solid spheres, fulfilled by water, the diffusion coefficient D of these particles can be calculated from the Einstein formula:
D ) kT/6πηr
(5)
and for D-8 individual aggregates with a mean diameter of 40 nm the diffusion coefficient in water is 1.3 × 10-11 m2/s. Recently8 a new approach, based on the asymptotic solutions of the Ward and Torday equation extended for the case of a continuously growing drop (or bubble), was developed, and it permits one to estimate the diffusion coefficient values at t f ∞ by use of the following equation:
( ) dγ dt-1/2
tf∞
)
( )
RTΓ2 π c0 4D
1/2
(6)
where C0 is the surfactant bulk concentration. Γ is the dynamic surface concentration, which was estimated by using the Frumkin equation:
γ0 - γ ) -ΓmaxRT ln(1 - Γ/Γmax)
(7)
According to ref 35 Γmax ) 2.8 × 10-6 mol/m2 for D-8, 1.95 × 10-6 mol/m2 for D-12, and 3.5 × 10-6 mol/m2 for (36) Svitova, T.; Smirnova, Yu.; Pisarev, S.; Berezina, N. Colloids Surf., A: Physicochem. Eng. Aspects 1995, 98, 107.
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Langmuir, Vol. 12, No. 7, 1996
Svitova et al.
Table 4. Diffusion Coefficients of the Surfactants at t f ∞ system D-8/air
D-12/air PR-28/air C12EO4.9/air D-8/C12H26 D-12/C12H26 C12EO4.9/C12H26
surfactant conc, %
Dtf∞, m2/s
0.025 0.05 0.05b 0.25 0.25 0.5 0.01 0.1 0.25 0.25 0.5 0.25
2.5 × 10-10 1.3 × 10-10 8.0 × 10-11 6.8 × 10-11 1.3 × 10 -12 7.1 × 10 -13 7.0 × 10-11 2.1 × 10-11 6.5 × 10-12 2.7 × 10-12 1 × 10-13 8.3 × 10-13
-10, from ref 20; D -11, a D apparent ) 1 × 10 self-diffusion ) 3.2 × 10 from ref 17; DEinstein ) 1.3 × 10-11, calculated using eq 9. b Two weeks after preparation.
i-C12EO4.9. The diffusion coefficients were calculated from the slope of the γ (t-1/2) dependencies extrapolated to t f ∞, and they are listed in Table 4. In this table the values of the D-8 self-diffusion coefficient Dself-diffusion from ref 17 and the apparent diffusion coefficient Dapparent, obtained in ref 20 for this surfactant spreading on a low-energy solid surface, are presented. It is seen from this table that the diffusion coefficients thus calculated decrease with surfactant concentration increase. Keeping in mind that the Ward-Torday approach was developed for monomer solutions, we can say that the thus calculated diffusion coefficient does not correspond to the diffusion coefficient of the surfactant monomer, which is independent of concentration, but these are effective or average values, characterizing joint mass transfer in a mixture of monomers and aggregates. The decrease of these values with an increase of concentration, here observed, is caused, according to ref 13, by the increase in aggregate concentration at constant monomer concentration equal to the cmc. Unfortunately, it was impossible to separate the terms corresponding to monomers and aggregates using the approach proposed by Miller13 because too many parameters included in the mass balance equation were unknown. Note that in all cases the diffusion coefficients, obtained for a boundary solution/air, are about 1 order of magnitude higher than that in the solution/dodecane systems. As it was mentioned above when we regarded the interfacial tension fall rate, this may be caused by surfactant distribution between aqueous and oil phases. The aging of the 0.05% D-8 solution (row 3, 0.05a) leads to a decrease of the diffusion coefficient and as was mentioned above to a loss of super spreading properties inspite of the fact that the D-8 diffusion coefficient in the aged 0.05% solution was still higher than that in the 0.25% solution. It is possible to say that there is not a visible correlation between the diffusivity and spreading ability for the same surfactant solutions, having different concentration. On the other hand, comparing the diffusion coefficients and the spreading velocity of different surfactants at the same concentration, one can see that the higher the diffusion coefficient the faster spreading occurs. Moreover, D-8 has diffusion coefficient values about 1 order of magnitude higher than that of its hydrocarbon analogue i-C12EO4.9 in all surfactant concentration ranges studied. The diffusion coefficient of D-8 is also significantly higher than that of D-12 inspite of the fact that D-12 forms normal micelles in aqueous solution with a mean size of ∼10 nm and has to be more diffusive than D-8, forming vesicles with a diameter of 40 nm. Note that D-8 diffusion coefficients, derived from surface tension dynamic data, are higher than Einstein diffusion coefficients for spheres
with a diameter of 40 nm and than the D-8 self-diffusion coefficient, mentioned in ref 17, and that these values are of the same order of magnitude as was found in ref 20 from spreading experiment data. To check the diffusion in the bulk of these surfactant solutions dynamic light scattering measurements were carried out using a Brookhaven BI-9000 correlator at 90°. The apparatus is equipped with a 623.8 nm He/Ne laser, and the size distribution of the particles was calculated by an inverse Laplace transformation. We have obtained the mean values of the particle diameter to be 196 nm for 0.1% D-8, 302 nm for 1.0% D-8, 10 nm for 1.0% D-12, and 330 nm for 1.0% i-C12EO4.9 aqueous solutions. Thus the diffusion coefficients of spherical aggregates in the bulk of D-8 aqueous solutions are 20-30 times lower than that in solutions of D-12 and comparable with that in i-C12EO4.9 solutions, while from dynamic surface tension measurements we have gotten an opposite picture. An increase of surfactant hydrophilicity (ethoxy chain length) suppresses diffusion coefficient values as well as the spreading ability of trisiloxane and hydrocarbon surfactants. As it was mentioned in ref 20 the effect maybe related to the increasing difficulty in forming a dense zerocurvature bilayer structure when the length of the ethoxy chain increases. Ananthapadmanabham et al.16 studied the kinetics of the adsorption of some silicone surfactants on liquid/air and solid/liquid interfaces. They found the superspreader SS1, having the same chemical structure as D-8 and a mean number of ethoxy groups of 7.5, when depleted from the liquid/air interface, is replenished more rapidly than other silicone surfactants studied. They also concluded that the SS1 dispersions have a higher mobility and a higher solid-liquid adsorption rate than other silicone surfactant solutions at comparable concentrations. These results are in good agreement with our observations. At the present stage it is difficult to explain this unusually high mobility of D-8 in aqueous solutions, manifesting itself as a high rate of adsorption at interfaces with air and oil. We can only speculate that this may be connected with a low cohesive energy and a high flexibility of trisiloxane surfactant molecules,18,20 and thus we propose that for these reasons the lifetime of silicone surfactant molecules, entrapped in bilayer aggregates, is lower than that in a hydrocarbon surfactant aggregate. According to ref 13, in the case when the aggregation number is very high (for D-8 it corresponds to about 8000) and thus the diffusion coefficient of the aggregates has to be hundreds of times lower than that of the monomers, the terms, connected with the aggregate formationdissolution processes, strongly influence the adsorption rate. In such cases the rate constants of aggregate formation and dissolution are very important factors for adsorption dynamics. In ref 14 it was shown that it is possible to estimate the rate of demicellization from the dynamic surface tension when the data for the solution at the cmc are available. So, this may become the direction of our further investigations. We perceive that all the questions, arising with regard to superspreading processes on fluid surfaces, cannot be totally resolved in the framework of the present work and that further detailed investigation is necessary. Conclusion Studies of surface/interfacial tension dynamics and spreading behavior of aqueous siloxane and hydrocarbon surfactant solutions on fluid hydrocarbon surfaces were performed. Analysis of surface/interfacial tension dynamics has shown that surfactants exhibit an unusually fast surface/
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Trisiloxane Surfactants
interfacial tension fall rate at the concentrations corresponding to superspreading on hydrophobic surfaces. The diffusion coefficient values were calculated from surface/interfacial tension dynamics data, and it was found that for the superspreading surfactant D-8 the diffusion coefficient values thus obtained are 1 order of magnitude higher than the diffusion coefficients of the hydrocarbon analogue. A decrease of diffusion coefficients with a surfactant concentration increase was observed. For the first time liquid hydrophobic subphases were used to study spreading of aqueous surfactant solutions, which permitted us to evaluate directly dynamic and equilibrium spreading coefficient values from dynamic tension measurements. It was found that fast spreading of surfactant solutions on a hydrocarbon surface occurred in the cases when both equilibrium spreading coefficient and dynamic spreading coefficient values were positive. The positive equilibrium spreading coefficient is necessary but not sufficient to ensure that fast spreading would take place. In distinction with solid low-energy subphases, where as it was found in refs 15 and 18 a superspreading of hydrocarbon surfactants does not occur, on fluid hydro-
Langmuir, Vol. 12, No. 7, 1996 1721
carbon surfaces superspreading of nonionic hydrocarbon surfactant solutions was observed. The rate of spreading depends on the surfactant nature, structure (hydrophobicity), and concentration and the subphase nature. The increase of the surfactant hydrophilicity (ethoxy chain length) suppresses the superspreading ability of siloxane and hydrocarbon surfactants. A good correlation between the surface/interfacial tension fall rate and the rate of spreading was found. Acknowledgment. The authors thank Dow Corning Corporation for financial support of this work and kindly supplied samples of the trisiloxane surfactants. This work was partially supported by Russian Fundamental Research Foundation, Grant 93-03 4467. T.S. thanks Dr. Andre Stuermer (Bayreuth University, Germany) for the help with the spreading behavior observations and light scattering measurements and Sergey Pisarev (Institute of Physical Chemistry, RAS, Moscow) for electron micrograph preparation. LA9505172
