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Enhanced Spreading of Aqueous Films Containing Ethoxylated Alcohol Surfactants on Solid Substrates T. Stoebe,† Zuxuan Lin,† Randal M. Hill,‡ Michael D. Ward,*,† and H. Ted Davis*,† Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Avenue SE, Minneapolis, Minnesota 55455, the University of Minnesota NSF Center for Interfacial Engineering, and the Dow Corning Corporation, 2200 West Salzburg Road, Midland, Michigan 48686-0994 Received July 2, 1997. In Final Form: October 2, 1997X The spreading behavior of aqueous solutions containing the ethoxylated alcohol surfactants CH3(CH2)9(OCH2CH2)3OH and CH3(CH2)11(OCH2)n-OH (n ) 3-6) on solid substrates has been investigated at different surfactant concentrations and substrate surface energies. The trends in the dynamic spreading behavior differ from those expected on the basis of the thermodynamic spreading coefficients calculated from static surface tensions. The data demonstrate that the spreading rates do not depend upon any identifiable aqueous phase surfactant microstructure. However, the spreading rate dependence upon the length of the hydrophilic poly(oxyethylene) chain suggests an interplay between surfactant adsorption on the substrate surface and the aggregation of this surfactant. The results suggest that the onset of turbidity, and an optimal surfactant hydrophilic/hydrophobic balance, are important for achieving high spreading rates.
Introduction The spreading of fluids over solid substrates is a ubiquitous process of fundamental technological importance. Wetting and spreading play a critical role in applications such as coatings, cosmetics, agrochemicals, lubrication, dispersants, and many others.1-3 Wetting dynamics are not yet fully understood, however. In principle, a liquid may spontaneously wet a solid substrate if the spreading coefficient (S ) γSV - γSL - γLV, where γSV, γSL, and γLV refer to the solid-vapor, solid-liquid, and liquid-vapor interfacial free energies, respectively) is positive.4 Added surfactant can promote wetting by reducing the liquid-vapor and/or liquid-solid interfacial tension, yielding a more positive spreading coefficient. However, favorable thermodynamics are a necessary, but not sufficient, condition for efficient wetting, as S does not address kinetic aspects that may influence the spreading rate. Indeed, detailed models of wetting dynamics have not yet been formulated.5 Consequently, agents capable of promoting spreading in specific applications are developed primarily by screening likely candidates rather than by rational design. The need to gain greater understanding of the relationship between surfactant structure and wetting dynamics prompted us to develop a methodology for systematic investigation of the influence of substrate surface energy * Authors to whom correspondence should be addressed. † Department of Chemical Engineering and Materials Science, University of Minnesota. ‡ Dow Corning Corporation. X Abstract published in Advance ACS Abstracts, November 15, 1997. (1) Karsa, D. R. In Industrial Applications of Surfactants; Karsa, D. R., Ed.; The Royal Society of Chemistry: London, 1987. (2) Smid-Korbar, J.; Kristl, J.; Stare, M. Int. J. Cosmet. Sci. 1990, 12, 135. (3) (a) Zabkiewicz, J. A.; Gaskin, R. E. Adjuvants and Agrochemicals; Chow, N. P., Grant, C. A., Hinshalwood, A. M., Simundsson, E., Eds.; CRC Press: Boca Raton, FL, 1989; Vol. 1, (Mode of Action and Physiological Activity) p 142. (b) Knoche, M.; Tamura, H.; Bukovac, M. J. J. Agric. Food Chem. 1991, 39, 202. (4) Zisman, W. A. in Contact Angle, Wettability and Adhesion; Fowkes, F. M., Ed.; Advances in Chemistry Series 43; American Chemical Society: Washington, DC, 1964; p 1. (5) Marmur, A. Adv. Colloid Interface Sci. 1983, 19, 75; de Gennes, P. G. Rev. Mod. Phys. 1985, 57, 827.
S0743-7463(97)00702-6 CCC: $14.00
on the spreading behavior of aqueous solutions containing various surfactants. Substrates with different surface energies can be fabricated readily by introducing appropriately functionalized organosulfur reagents to gold surfaces, which leads to well-defined alkanethiol monolayers.6 Specifically, the surface energies can be adjusted systematically from very hydrophobic to strongly hydrophilic simply by varying the proportions of hydrophobic (CH3) and hydrophilic (OH) terminal groups exposed at the monolayer upper surface. Recently, we reported apparent universal spreading behavior for aqueous media containing nonionic surfactants.7 This behavior was described as “surfactant-enhanced spreading”, which is characterized by rapid wetting of hydrophobic substrates, an initial linear dependence of the wetted area on time, a maximum in spreading rates on surfaces of intermediate hydrophobicity, and in some cases, a maximum in spreading rate at surfactant concentrations well above the critical micelle/critical precipitation concentration. Despite these investigations, many aspects of surfactant-enhanced spreading remain elusive. Although the surfactant aqueous phase microstructure is expected to affect spreading rates, no specific microstructure has been identified as being responsible for enhanced spreading. Ethoxylated alcohol surfactant CmEn (Cm ) CH3(CH2)m-1, En ) (OCH2CH2)nOH) systems have been extensively investigated, and many aqueous phase diagrams have been reported.8-11 In order to advance the understanding of the role of surfactant molecular structure and aqueous phase microstructure in spreading dynamics we have examined the spreading behavior of aqueous solutions and dispersions of C10E3, C12E3, C12E4, C12E5, and C12E6 (6) (a) Blackman, L. C. F.; Dewar, M. J. S. J. Chem. Soc. 1957, 162. (b) Blackman, L. C. F.; Dewar, M. J. S. J. Chem. Soc. 1957, 171. (c) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7155. (d) Sanassy, P.; Evans, S. D. Langmuir 1993, 6, 1024. (e) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 9, 1024. (f) Holmes-Farley, S. R.; Bain, C. D.; Whitesides, G. M. Langmuir 1988, 4, 921. (7) Stoebe, T.; Lin, Z.; Hill, R. M.; Ward, M. D.; Davis, H. T. Langmuir 1996, 12, 337. (8) Mitchell, J. D.; Tiddy, G. J. T.; Waring, L.; Bostock, T.; McDonald, M. P. J. Chem. Soc., Faraday Trans. 1 1983, 975. (9) Strey, R.; Schomaker, R.; Roux, D.; Nallet, F.; Olsson, U. J. Chem. Soc., Faraday Trans. 1990, 86, 2253. (10) Lang, J. C.; Morgan, R. D. J. Chem. Phys. 1980, 73, 5849. (11) Ali, A. A.; Mulley, B. A. J. Pharm. Pharmacol. 1978, 30, 205.
© 1997 American Chemical Society
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Langmuir, Vol. 13, No. 26, 1997 7271 of time.12 The area covered by the droplet generally increased linearly with time, and the spreading rate (in mm2/s) typically was determined from the slope of this dependence. The data typically exhibited a linear regression coefficient of R > 0.995. Multiple measurements performed under identical conditions afforded spreading rates that were reproducible to within (10%.
Figure 1. (a) Schematic representation of a liquid droplet on a solid substrate and the contact angle, θ. Substrates exhibiting large values for θ (θ g 90°, cos θ e 0) are termed very hydrophobic. Decreasing values of θ corresponding to substrates that are increasingly hydrophilic. (b) Schematic representation of a self-assembled alkanethiol mixed monolayer modified gold surface. The A ()CH3) terminal groups are very hydrophobic. Incorporation of B ()OH) terminated alkanethiols yields increasingly hydrophilic substrates. Table 1. Contact Angles Measured between 3.0 µL of Water Drops and Mixed Monolayer Modifed Substrates with Varying Concentrations of Hydrophilic (A ) OH) and Hydrophobic (B ) CH3) Terminal Groupsa solution composition HS(CH2)15OH:HS(CH2)15CH3
θ
cos θ
0:100 30:70 50:50 60:40 70:30 100:0 Parafilm
112 93 74 57 37 24 99
-0.37 -0.05 0.28 0.55 0.80 0.91 -0.15
a The contact angle measured on Parafilm is included for comparison.
The ethoxylated alcohol surfactants were obtained from Nikko Chemicals Co. Ltd. (Tokyo, Japan) and are stated to be greater than 98% pure with little polydispersity in the length of the poly(oxyethylene) chains. These surfactants were used without further purification. The aqueous mixtures were prepared using Milli-Q reagent (18 MΩ) water. The mixtures were vigorously hand shaken to disperse the surfactant and were allowed to stand for a time sufficient to allow any foam generated by the shaking to subside before experiments were performed. Only relatively fresh (less than 3 days old) mixtures were investigated. The quartz substrates were sufficiently rough to appear translucent. Average roughness was estimated to be ≈0.30 ( 0.10 µm by stylus profilometry. The roughness affected both surface energy characterization and spreading rate.4 Spreading rates on rough crystals were found to be higher than those on smooth crystals, presumably due to the increased capillarity associated with the rough surface. Substrate roughness also leads to substantial contact angle hysteresis. Although the measured contact angles were found to be quite reproducible, the absolute error may be significant and the resulting energy scale should be regarded as somewhat qualitative. Differences in the adsorption kinetics of the two organosulfur reagents prevented use of their solution composition to establish the monolayer compositions and their corresponding surface energies.
Results at various surfactant concentration and on substrates of varying surface energies. These systems exhibit the characteristics of surfactant-enhanced spreading and demonstrate an interesting spreading rate dependence upon poly(oxyethylene) chain length. The maximal spreading rates observed appear to be associated with the onset of turbidity and exhibit a maximum with respect to poly(oxyethylene) chain length. Experimental Section The apparatus and procedure for measuring spreading rates have been described in a previous report.7 The substrates comprised quartz disks onto which 12 mm diameter gold surfaces (≈2000 Å thick) were deposited onto titanium underlayers (≈200 Å thick). Substrate surface energy was controlled by the deposition of mixed organosulfur monolayers onto the gold surfaces. The monolayers were created by immersing the goldcoated substrates in 1 mM ethanol solutions with different relative concentrations of the thiols, with HS(CH2)14CH2OH providing hydrophilic (OH) functional groups and HS(CH2)15CH3 providing hydrophobic (CH3) functional groups (Figure 1a). After the substrate was rinsed and dried, the surface energy could be characterized in terms of the apparent contact angle, θ, formed between the monolayer-coated substrate and pure water (Milli-Q 18 MΩ reagent water) (Figure 1b).4 This procedure provided a series of substrates ranging from very hydrophobic (θ > 90°, cos θ < 0) to very hydrophilic (θ ≈ 20°, cos θ ≈ 0.9) (Table 1). Spreading experiments were performed in a closed environmental chamber at >95% relative humidity. Droplets with a volume of 2.0 ( 0.1 µL, formed at the custom-made delivery tip of a precision micrometer syringe, were transferred to the substrate surface by bringing the droplet into contact with the substrate and quickly retracting the syringe tip. The spreading event was recorded onto video tape, which was subsequently digitized using a personal-computer-based frame grabber. The digitized movie was analyzed using a macro written for NIH Image to determine the area covered by the droplet as a function
Spreading of C10E3/H2O Dispersions. At surfactant concentrations examined here, the C10E3/H2O mixtures form turbid two-phase (W + LR) dispersions and are well above the reported critical micelle concentration (CMC) (CMC for C10E3 ) 6.0 × 10-4 M ) 0.017 wt %) (Table 2).11,13 These dispersions do not spontaneously wet the most hydrophobic substrates (Figure 2). However, rapid wetting was observed on more hydrophilic substrates (cos θ > 0.25), with spreading rate maxima observed on surfaces of intermediate hydrophobicity near cos θ ≈ 0.55.14 The positions of the spreading rate maxima were not significantly affected by surfactant concentration. However, the spreading rate depends upon surfactant concentration, with a pronounced maximum near 0.4 wt % (Figure 3). Because these dispersions are well above the CMC, it seems unlikely that this concentration dependence is due to marked changes in the static interfacial tensions, indicating the importance of dynamic processes in rapid spreading. The spreading behavior (and maximal rate) of these dispersions is very similar to that exhibited by “superspreading” trisiloxanes M(D′E8OMe)M (M ) (CH3)3SiO, En ) (OCH2CH2)n, Me ) CH3, and D′ ) -Si-CH3R), the principal distinction being the increased ability of the (12) NIH Image is a public domain image analysis program written by Wayne Rasband at the U.S. National Institutes of Health and available from the Internet by anonymous ftp from zippy.nimh.nih.gov or on floppy disk from NTIS, 5285 Port Royal Rd., Springfield, VA 22161, part number PB93-504868. A macro written to automate the spreading rate data analysis is available from the authors upon request. (13) Rosen, M. J.; Cohen, A. N.; Dahanayake, M.; Hua, X. Y. J. Phys. Chem. 1982, 86, 541. (14) In a previous report,7 triethylene glycol monodecyl ether (C10E3) was mistaken for triethylene glycol monododecyl ether (C12E3) leading to incorrectly labeled graphs. This report corrects this error and presents a more comprehensive data set. The behavior exhibited by the C12En/ HO (n ) 3, 4, 5, and 6) systems now appears more consistent with behavior exhibited by trisiloxane surfactant aqueous systems (also presented in7), and suggests that the effects of increased poly(oxyethyelene) chain length may be fairly general.
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Table 2. Parameters Related to the Aqueous Phase and Spreading Behavior Exhibited by Ethoxylated Alcohol Surfactants surfactant
CMC11,13
maximum spreading rate (mm2/s)
concentration at maximum rate (wt %)
aqueous phase8,11,13
surface tension (mN/m)11,13
C10E3 C12E3 C12E4 C12E5 C12E6
0.017 0.0016 0.0023 0.0026 0.0031
75 15 48 45 20
0.4 >2 >2 0.1 0.2
W + LR (turbid) W + LR (turbid) W + LR (turbid) L1 (clear) L1 (clear)
28.4 29.2 31 32.5
Figure 2. Spreading rate dependence on substrate surface energy (at 22 °C and > 95% relative humidity) for aqueous dispersions of C10E3. Substrate surface energy was modified by the deposition of mixed organosulfur monolayers on rough goldcoated quartz crystals and characterized in terms of the cosine of the observed contact angle of 18 MΩ deionized water. The error in the spreading rate measurement is estimated to be roughly (10%.
Figure 3. Spreading rate dependence on the concentration of aqueous dispersions and solutions of C10E3, C12E3, C12E4, C12E5, and C12E6. For clarity, only the results on the cos θ ) 0.55 substrates are presented.
trisiloxane systems to wet substrates of greater hydrophobicity.7,15 Spreading of C12E3/H2O Dispersions. The C12E3/ H2O aqueous mixtures investigated also formed turbid two-phase (W + LR) dispersions,8 similar to the C10E3/ H2O test mixtures. The C12E3/H2O dispersions were observed to spread effectively on substrates with surface energies comparable to those observed for C10E3/H2O dispersions (Figure 4a). The dependence of the spreading rate of C10E3/H2O dispersion on surface energy qualitatively resembles that of C10E3/H2O dispersions, although the maximal rates are significantly lower. However, C12E3/ H2O dispersions did not exhibit spreading rate maxima in terms of surfactant concentration, even up to 2.0 wt %. Rather, the spreading rates increased in a nearly linear manner over this range of surfactant concentration (Figure 3). Measurements performed above 2.0 wt % C12E3 were complicated by rapid macroscopic phase separation. (15) (a) Ananthapadmanabhan, K. P.; Goddard, E. D.; Chandar, P. Colloids Surf. 1990, 44, 281. (b) Zhu, X. Ph.D. Thesis, University of Minnesota, 1992. (c) Zhu, X.; Miller, W. G.; Scriven, L. E.; Davis, H. T. Colloids Surf. 1994, 90, 63.
Figure 4. Spreading rate dependence on substrate surface energy (at 22 °C and >95% relative humidity) for aqueous dispersions and solutions of (a) C12E3, (b) C12E4, (c) C12E5, and (d) C12E6.
Spreading of C12E4/H2O Dispersions. The C12E4/ H2O mixtures also form turbid two-phase (W + LR) dispersions.8 The maximal spreading rates exhibited by the C12E4/H2O dispersions (Figure 4b) on the organosulfur monolayers are significantly larger than those achieved with C12E3. A spreading rate maximum once again is observed on substrates of intermediate hydrophobicity (cos θ ≈ 0.55), but it is somewhat sharper due to comparatively lower spreading rates on the more hydrophobic surfaces. Therefore, increasing the poly(oxyethylene) (hydrophilic) chain length appears to inhibit spreading on the more hydrophobic substrates. As in the case of the C12E3/H2O dispersions, the spreading rate did not exhibit a maximum with respect to surfactant concentration. However, spreading rates did not increase greatly above 0.7 wt % C12E4 (Figure 3). Spreading of C12E5/H2O Solutions. The longer
Ethoxylated Alcohol Surfactants on Solid Substrate
hydrophilic moiety of C12E5 favors the formation of micelles. Consequently, aqueous mixtures of this surfactant formed nonturbid micellar solutions (L1 phase) at the concentrations investigated, which were well above the CMC (Table 2).8,13 However, the largest rates exhibited by these C12E5/H2O solutions (Figure 4c) are comparable to those exhibited by the C12E4/H2O dispersions, which form turbid dispersions and are not dominated by the L1 phase. These data demonstrate that the enhanced spreading characteristics, while apparently influenced by the presence of dispersed or soluble aggregates, are not strongly correlated with any specific, identifiable microstructure. The maximum in spreading rate with respect to substrate surface energy is shifted to somewhat more hydrophilic substrates compared to C12E4/H2O, continuing the trend of poorer wetting of more hydrophobic substrates with increasing length of the hydrophilic poly(oxyethylene) chain. Unlike the C12E3/H2O and C12E4/H2O systems, a pronounced maximum in spreading rate was observed at 0.2 wt % C12E5 (Figure 4). It is interesting to note that much less C12E5 is necessary to achieve spreading rates comparable to the maximal values exhibited by the C12E4/ H2O dispersions (0.2 wt % C12E5 as a micellar solution vs >0.7 wt % C12E4 as a dispersion). Spreading of C12E6/H2O Solutions. As in the case of the C12E5/H2O system, aqueous mixtures of C12E6 form nonturbid micellar solutions (L1 phase) at the concentrations investigated. The increased hydrophilic chain length is expected to result in lower aggregation numbers and smaller micelles.16 The largest spreading rates exhibited by the C12E6/H2O solutions were significantly lower than those exhibited by the C12E5/H2O solutions or C12E4/H2O dispersions (Figure 4d). The spreading rate profiles shifted to higher substrate surface energy, continuing the trend of poorer wetting of more hydrophobic substrates with increasing length of the hydrophilic poly(oxyethylene) chain. A maximum in the spreading rate with respect to surfactant concentration again was apparent, albeit it shifted to higher surfactant concentration relative to C12E5/ H2O (Figure 3). Discussion The term superspreading has been used to describe surfactants that promote spreading on very hydrophobic substrates such as Parafilm,15 which exhibits a large contact angle with water (θ ≈ 99°; cos θ ≈ -0.15). By this definition, the CmEn surfactants investigated here are not superspreaders as aqueous solutions containing these reagents did not spread on Parafilm or highly hydrophobic organosulfur monolayers. However, rapid wetting by aqueous dispersions and solutions of the CmEn surfactants did occur on moderately hydrophobic substrates (substrates subtending contact angles > 40° with pure water). The distinction, which we previously observed for some ethoxylated trisiloxane surfactants, prompted us to define this behavior more generally as surfactant-enhanced spreading. In general, the CmEn/H2O mixtures exhibited spreading characteristics which were similar to those exhibited by trisiloxane surfactant systems. These characteristics include (1) maximal rates of spreading that are too high to be explained by surface diffusion of the liquid over a dry substrate17,18 or movement of a threephase contact line under the constraint of the no-slip boundary condition,19,20 (2) spreading rate maxima with (16) Rosen, M. J. Surfactants and Interfacial Phenomena; John Wiley & Sons: New York, 1989; p 114. (17) Tetetzke, G. F.; Davis, H. T.; Scriven, L. E. Chem. Eng. Commun. 1987, 55, 41. (18) Tiberg, F.; Cazabat, A.-M. Europhys. Lett. 1994, 25, 205. (19) Huh, C.; Scriven, L. E. J. Colloid Interface Sci. 1971, 35, 85.
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respect to surfactant concentration and substrate surface energy, and (3) a correlation of rapid spreading to the onset of turbidity. Furthermore, the spreading rates observed here for CmEn surfactant dispersions and solutions are comparable to the highest rates attained by the superspreading systems.7 These similarities suggest that similar mechanisms drive the rapid wetting of these structurally different systems. Although the mechanism responsible for surfactantenhanced spreading has not been established fully, it is reasonable to suggest that it involves Marangoni flow, in which spatial differences in surface tension create a tractive force that results in fluid motion in the adjacent bulk liquid in the direction of increasing surface tension.21,22 The gradient could be produced by contact between the surfactant-containing droplet and a preexisting water layer on the substrate,23,24 and augmented by dynamic processes described below. Marangoni flow is known to be much more rapid than spreading by surface diffusion over a dry substrate.23 Such flow could produce the high spreading rates we observed for aqueous surfactant dispersions and solutions and is consistent with the covered area initially increasing linearly with time. Contact of the liquid droplet with a pre-existing water layer would relax the no-slip boundary condition at the junction of the advancing edge of the droplet, permitting Marangoni flow and high spreading rates. This is illustrated in Figure 5, which depicts the leading edge region of an advancing droplet in contact with a preexisting water layer present on hydrophilic and intermediate energy surfaces. The contact boundary of the leading edge of the droplet and the pre-existing water layer is continuous and may not be very distinct. However, the surfactant concentrations at the air-water interfaces of the droplet and the region just beyond this contact boundary will differ throughout the spreading event due to continuous advancement of the contact boundary across the pre-existing water layer, which initially is devoid of surfactant. The most notable observation in our studies has been the dependence of the spreading rates on substrate surface energy, in which a spreading rate maxima typically occurs at moderate substrate surface energies. A very hydrophobic surface will not favor a pre-existing water layer, thereby preventing creation of a surface tension gradient required for Marangoni flow. An increasing tendency of the poly(oxyethylene) hydrophile to adsorb on the substrate with increasing substrate surface energy could explain the decrease in spreading rate on the high-energy side of the observed maxima. If surfactant adsorbs in this orientation, at or just beyond the advancing edge of the drop (in the region defined by the pre-existing water layer), the spreading drop would be pinned by the resulting hydrophobic barrier. We suggest that this process could be one of the inhibitive processes responsible for the maximum in spreading rate versus substrate surface energy. The rate of desorption of the surfactant from the substrate may influence the rate of spreading if this mechanism contributes to the spreading event. We note that studies of surfactant-enhanced spreading on a fluid mineral oil substrate did not exhibit spreading rate maxima with respect to surfactant concentration. This supports the contention that surfactant adsorption on the (20) Blake, T. D. Colloids Surf. 1990, 47, 135. (21) Hardy, W. B. Collected Works; Cambridge University Press: Cambridge, 1936. (22) Thomson, J. Philos. Mag. 1855, 10 (4th Ser.), 330. (23) Troian, S. M.; Herbolzheimer, E.; Safran, S. A. Phys. Rev. Lett. 1990, 65, 333. (24) Elander, G.; Sackmann, E. J. J. Phys. II 1994, 4, 455.
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Figure 5. Schematic representation of the processes that may be responsible for surfactant-enhanced spreading. In (a) and (b) the drop introduced to the surface actually is in contact with a pre-existing water layer. The contact line defining the boundary of the leading edge of the droplet and the pre-existing water layer is continuous and may not be very distinct. However, the surfactant concentrations at the air-water interfaces of the droplet and the region just beyond this contact line will differ throughout the spreading event due to continuous advancement of the contact line across the pre-existing water layer, which initially is devoid of surfactant. A surface tension gradient (∆g) between a pre-existing water film and the surfactant/water droplet favors Marangoni flow and rapid spreading of the aqueous droplet. (a) On highly hydrophilic substrates the pre-existing water layer will be present. However, spreading on these surfaces is observed to be slow. This can be attributed to adsorption of the surfactant hydrophiles on the substrate, which forces the hydrophobe to point away from the substrate surface and presents a hydrophobic barrier to drop advancement. The surfactant molecule, with its hydrophile bound to the substrate, may pin the drop at the contact line joining the leading edge of the drop and pre-existing water film. The kinetics of desorption of the surfactant hydrophile from the substrate may govern the spreading rate. Surfactant aggregates may adsorb on hydrophilic substrates intact as there is no driving force to adsorb the hydrophobes on these surfaces. Consequently, aggregate disintegration may be slower than on hydrophobic substrates. (b) On surfaces of intermediate surface energy a pre-existing water film necessary for Marangoni flow is still likely, but the tendency for the hydrophobe of the surfactant to adsorb on the substrate will increase. This can instigate aggregate disintegration, resulting in the efficient delivery of surfactant to the air-water interface when this occurs in the vicinity of the leading edge of the advancing drop. This is illustrated here for a flat-layered microstructure unraveling at the leading edge, although the specific microstructure of the aggregates responsible for enhanced spreading have not been identified. Direct surfactant adsorption on the substrate in the region just beyond the leading edge of the drop, depicted here by the arrow, will also reduce the concentration of surfactant at the opposing air-water interface and augment the surface tension gradient. The difference in thickness of the pre-existing water layer in (a) and (b) is only for clarity. (c) On a highly hydrophobic substrate a pre-existing water layer is considerably less likely, prohibiting the creation of the surface tension gradient necessary for Marangonic flow. Very hydrophobic surfaces do not favor formation of a continuous water film, which prevents the creation of the surface tension gradient necessary for Marangoni flow and accounts for the negligible spreading on these surfaces. Spreading on these surfaces also is less thermodynamically favorable owing to a less positive spreading coefficient.
Ethoxylated Alcohol Surfactants on Solid Substrate
solid substrate surface, which would be more significant at higher surfactant concentrations, inhibits spreading. The influence of adsorption processes on spreading of aqueous films has been demonstrated previously for ionic surfactants.25 The maximum observed at moderate surface energies can be attributed to these substrates having the capability to support a sufficiently continuous pre-existing water layer so that the surface tension gradient can be established for Marangoni flow, combined with a decreased susceptibility of these sufaces to adsorption of the polar poly(oxyethylene) hydrophile which obviates the autophobicity apparent on the more hydrophilic surfaces. Direct surfactant adsorption, via the hydrophobe, on the substrate in the vicinity of the leading edge of the advancing drop will deplete the surfactant at the opposing air-water interface, thereby augmenting the surface tension gradient and increasing Marangoni flow. The maxima in spreading rate with respect to surfactant concentration occur above the respective critical aggregation concentrations. Numerous companion studies in our laboratory have indicated that rapid spreading is associated with the presence of dispersed surfactant aggregrates or micelles, the former evident by the turbidity of the mixtures. We have not been able to identify specific surfactant microstructures that may be responsible for spreading, although we surmise, on the basis of other studies in our laboratory, that the surfactant-rich phase has a bilayer microstructure. The observation of decreasing spreading rates on the more hydrophobic substrates with increasing poly(oxyethylene) chain length for the C12En surfactants is consistent with trends in static surface tension (Table 2),13 in which the liquid-vapor interfacial tension for C12En/H2O mixtures increases with increasing length. This implies a less positive spreading coefficient for C12En with larger n such that spreading should be inhibited as chain length is increased (assuming an unchanging liquid-solid interfacial tension). However, this trend is not evident for the spreading rate maxima observed on the higher energy surfaces, for which the highest rates appear to occur between n ) 4 and n ) 5 (Table 2). While C12E4 forms two-phase dispersions, C12E5 forms clear micellar solutions. This suggests that the highest spreading rates may be associated with the onset of turbidity when the higher surface energy substrates are used, similar to behavior exhibited by superspreading trisiloxane systems.7 Parallel investigations of the spreading of turbid aqueous dispersions of trisiloxane surfactants on mineral oil surfaces, described in a companion paper,26 have revealed that disintegration of surfactant aggregates occurred concommittantly with stepwise motion of the leading edge of the aqueous film. This suggests that similar events occur for turbid surfactant dispersions during spreading on solid substrates. If aggregate disintegration occurs just behind the contact boundary of the advancing aqueous film and the pre-existing water layer, this process can deliver a large amount of surfactant to both the expanding air-liquid and solid-liquid interfaces. Surfactant delivered to the air-liquid interface behind the advancing drop will produce high interfacial concentrations of surfactant in this region, thereby maintaining the surface tension gradient between this (25) Frank, B.; Garoff, S. Langmuir 1995, 11, 87. (26) Stoebe, T.; Hill, R. M.; Ward, M. D.; Davis, H. T. Langmuir 1997, 13, 7282.
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region and the pre-existing water layer region as the drop advances. Marangoni flow can be sustained under this condition. Different kinetics on different substrates can explain, at least partially, the influence of substrate surface energy on spreading rates. A surfactant aggregate can adsorb on a hydrophilic surface while remaining intact. In contrast, adsorption on a hydrophobic surface must be accompanied by substantial reorganization in order for surfactant hydrophobes to bind to the surface, leading to disintegration of the aggregate. Consequently, the decreasing spreading rate with increasing substrate surface energy may reflect diminished aggregate disintegration, which in turn diminishes surfactant delivery to the airliquid interface. We note that, in the CmEn surfactant dispersions and other systems we have investigated, the surfactant-rich phase typically has a bilayer microstructure. The flat geometry of bilayer microstructures may facilitate movement of surfactant into the two-dimensional air-liquid interface and adsorption onto the substrate surface. The observation that aggregate disintegration is related to spreading would suggest that larger aggregates would favor this process. However, slower spreading rates were observed for C12E3/H2O, which is expected to favor larger aggregates and micelles compared to the other surfactants investigated. The presence of a spreading rate maximum with respect to surfactant concentration also appears to be related to turbidity. The clear micellar C12E5 and C12E6 systems exhibit such a maximum while the turbid C12E3 and C12E4 systems do not. This trend is not general, however, since turbid C10E3 dispersions display a pronounced spreading rate maximum with respect to surfactant concentration. Clearly, the role of aggregates is rather complex and the kinetics of dissociation and transport of the aggregate may be critical. Conclusion The ethoxylated alcohol surfactants investigated (C10E3, C12E3, C12E4, C12E5, and C12E6) promote spreading on hydrophobic solid substrates and exhibit the characteristics assigned previously to surfactant-enhanced spreading. The complexity of the spreading process and the role of surface energy and aggregation are evident from the dependence of spreading rates on the length of the poly(oxyethylene) hydrophilic chain. On hydrophobic substrates the spreading rates decreased with increasing poly(oxyethylene) chain length whereas on more hydrophilic substrates no such trend was observed. Although no specific microstructure has yet been identified as being responsible for surfactant-enhanced spreading, the largest spreading rates appear to be associated with the onset of turbidity. This suggests that an optimal balance between hydrophobe and hydrophile is necessary to achieve large spreading rates. These results demonstrate that surfactant-enhanced spreading is not limited to the narrow class of trisiloxane surfactants. Acknowledgment. This work was supported by the Center for Interfacial Engineering (CIE), an NSF Engineering Research Center. The authors are grateful to Dr. Robert Stevens (Air Products) for helpful discussions and to Dr. L. M. Frostman and C. M. Yip (University of Minnesota) for providing the organosulfur reagents. LA970702A
