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Direct Study of C12E5 Aggregation on Mica by Atomic Force Microscopy Imaging and Force Measurements Jinping Dong and Guangzhao Mao* Department of Chemical Engineering and Materials Science, Wayne State University, 5050 Anthony Wayne Drive, Detroit, Michigan 48202 Received January 27, 2000. In Final Form: May 12, 2000 Atomic force microscopy (AFM) was used to study the aggregation structure and kinetics of nonionic surfactant penta(oxyethylene) dodecyl ether (C12E5) on mica as a function of temperature. Surface forces and topographical images of surfactant aggregates at the liquid/solid interface were captured in the vicinity of 21 °C. The surfactant molecular aggregates were imaged by choosing an imaging force in the steric repulsion region so that the AFM tip was just outside the surface of the aggregates. At below 21 °C, C12E5 adsorbed as fragments of tens of nanometers in size. The fragments consist of 1-2 nm thick strongly adsorbed species and weakly adsorbed species extending as far as 15 nm from the substrate surface. The fragments gradually increased in number and attached to each other to form clusters connected in one dimension (strings) and two dimensions (networks). With increasing surface coverage, the surface force changed from a purely attractive one to one showing a repulsive force barrier. At above 21 °C, a smooth and continuous layer structure formed instantaneously. Scan damage in the form of ever-expanding holes was observed in the surfactant layers. The force versus distance profiles suggest continuous bilayers formed on both mica and AFM tip with a thickness of 4 nm. The abrupt changes in the aggregate structure and adsorption rate point to a phase transition at the surfaces, which originates from the temperatureinduced dehydration of the headgroup as in the cloud point phenomenon. This correlation was also supported by the evidence that the adsorption of C12E5 was suppressed by the addition of a salting-in compound, sodium perchlorate.
Introduction Nonionic surfactants are used in dispersions, flotation, tertiary oil recovery, and household and industrial cleaning.1 Nonionic surfactants have advantages over ionic ones because of their hard-water stability, mildness, and biodegradability. The above applications primarily rely on the adsorption of nonionic surfactants at the liquid/ solid interface. Compared to ionic surfactants, the driving force behind the adsorption of nonionic surfactants is more complex and often consists of a combination of substrate/ surfactant, surfactant/solvent, and surfactant/surfactant interactions. Fortunately the molecular structure of surfactant self-associating aggregates can be probed directly. The successful attempts in resolving surfactant micellar structure using in situ atomic force microscopy (AFM) enable a systematic approach to the molecular nature of many long-standing issues concerning the surfactant adsorption at the liquid/solid interface, such as the existence and shape of hemimicelles.2 The structural change due to temperature and the addition of salts, alcohols, and ionic cosurfactants can be studied in situ at the molecular level. In bulk, additives influence properties such as the mutual solubility between nonionic surfactants and water, characterized by the cloud point. The cloud point (or the lower consolute temperature) is the temperature at which the surfactant begins to lose water solubility, and the clear solution changes into a cloudy dispersion. Additives such as anionic surfactants and salts were found to influence the cloud point of nonionic surfactants.3 The additives which raise the cloud point * To whom correspondence should be addressed. Phone: (313) 577-3804. Fax: (313) 577-3810. E-mail: gzmao@ chem1.eng.wayne.edu. (1) Schick, M. J., Ed. Nonionic Surfactants; Marcel Dekker: New York, 1967. (2) Manne, S.; Cleveland, J. P.; Gaub, H. E.; Stucky, G. D.; Hansma, P. K. Langmuir 1994, 10, 4409.
are called salting-in or hydrotropic additives, and the additives with the opposite effect are called salting-out or lyotropic additives. One school of thought argues that, unlike anionic surfactants and some hydrotropes, salts affect the cloud point mainly by modifying the properties of water rather than the interfacial properties of nonionic surfactants.4 The temperature-induced phase separation has been used in solvent and hard-metal ion extraction.5 Enhanced detergency has been found to occur near the cloud point.6 However, the surface phase transition and the exact structures responsible for the variation in the adsorption behavior near the cloud point have not been understood. Much of our understanding of nonionic surfactants is based on the study of polyoxyethylene alcohols.7 Recently Patrick et al.8 imaged the self-assembled structure of nonionic surfactants, poly(oxyethylene)n dodecyl ether (C12En), on highly oriented pyrolytic graphite (HOPG) using soft-contact AFM. In soft-contact AFM, an imaging force in the steric repulsion region just outside the surface of the aggregate is employed. Stripes with characteristic periodicity of 55 Å were observed on C12E5-10 series which were attributed to hemicylindrical hemimicelles. We chose C12E5 as a model compound because of its well-researched phase and interfacial behavior. Its critical micelle concentration (cmc) was determined to be 0.064 mM by (3) Collins, K. D.; Washabaugh, M. W. Q. Rev. Biophys. 1985, 24, 247. Lindman, B.; Carlsson, A.; Karlstro¨m, G.; Malmsten, M. Adv. Colloid Interface Sci. 1990, 32, 183. (4) Franks, F. In Water-A Comprehensive Treatise; Franks, F., Ed.; Plenum: New York, 1973; Vol. 2, p 1. Goel, S. K. J. Colloid Interface Sci. 1999, 212, 604. (5) Kang, S. I.; Czech, A.; Czech, B. P.; Stewart, L. E.; Bartsch, R. A. Anal. Chem. 1985, 57, 1713. (6) Corkill, J. M.; Goodman, J. F.; Tate, J. R. Trans. Faraday Soc. 1967, 62, 979. Goel, S. K. J. Surfactants Deterg. 1998, 1 (2), 213. (7) Schick, M. J., Ed. Nonionic Surfactants: Physical Chemistry; Marcel Dekker: New York, 1987. (8) Patrick, H. N.; Warr, G. G.; Manne, S.; Aksay, I. A. Langmuir 1997, 13, 4349.
10.1021/la000103v CCC: $19.00 © 2000 American Chemical Society Published on Web 07/11/2000
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Deguchi et al.9 The phase diagram of the water-C12E5 system was determined by Harusawa et al.,10 and more recently by Strey et al.11 The cloud point was found to be 26.5 °C in the former study, and 31.9 °C in the later study as the concentration limit was lowered. At 20 °C with increasing concentration, the system evolves from an isotropic micellar solution to a short span starting from 46% w/w in the hexagonal phase, and a large region starting from 59% w/w in the lamellar phase. The headgroup area of C12E5 in the lamellar phase is 45 Å2.11 C12E5 separates as a solid phase at 21.5 °C in the surfactant-rich region. A recent AFM study has found that C10E5 adsorbs on a hydrophobic surface HOPG, and on a hydroxylated, hydrophilic surface silicon nitride, but only weakly on a non-hydroxylated, hydrophilic surface mica.12 The same trend has generally been observed on similar oxyethylene surfactants. On hydroxylated surfaces, bilayers, defective bilayers, or bilayerlike aggregates of surfactants have been observed by neutron reflection,13 ellipsometry,14 fluorescence,15 HPLC,16 and AFM force measurement17 close to and above the cmc. The bilayer thickness of C12E5 was determined to be 40-50 Å in the neutron studies,13 where the thickness of the ethylene oxide layer and the hydrocarbon layer each was around 16 Å. The fully stretched length of C12E5 is calculated to be 32 Å, where 14 Å belongs to the hydrocarbon and 18 Å belongs to the ethylene oxide chain. The hydrocarbon chains were therefore interdigitated or compressed in these studies. In another study, a more vertically compressed, interdigitated bilayer was observed with a thickness of 30 Å before the final equilibrium was reached.17 Levitz et al.15 studied the adsorption kinetics of poly(oxyethylene) alkylphenol Triton 100. With increasing surface coverage, the surface aggregates first increase in size, then in number density, and finally in size again, which results in a transformation from a fragmented structure to a continuous bilayer structure. On the mica surface, no adsorption was generally concluded due to the weak interaction of oxyethylene surfactants with the substrate. But in cases where adsorption was detected, several structures have been postulated: a flat configuration, submonolayers with the hydrophobic moiety toward the mica,18 and a layer of cylindrical or wormlike micelles extending from the mica surface.19 In this study, soft-contact AFM was used to image the aggregate structure of C12E5 on mica, and to measure the force versus distance curves simultaneously. We found that the adsorption of C12E5 on mica changed abruptly from a fragmented structure to a continuous structure when the temperature was raised above 21 °C. Upon the addition of a hydrotropic salt, sodium perchlorate (Na(9) Deguchi, K.; Meguro, K. J. Colloid Interface Sci. 1972, 38, 596. (10) Harusawa, F.; Nanamura, S.; Mitsui, T. Colloid Polym. Sci. 1974, 252, 613. (11) Strey, R.; Schoma¨cker, R.; Roux, D.; Nallet, F.; Olsson, U. J. Chem. Soc., Faraday Trans. 1990, 86, 2253. (12) Grant, L. M.; Ducker, W. A. J. Phys. Chem. B 1997, 101, 5337. (13) Lee, E. M.; Thomas, R. K.; Cummins, P. G.; Staples, E. J.; Penfold, J.; Rennie, A. R. Chem. Phys. Lett. 1989, 162, 196,. McDermott, D. C.; Lu, J. R.; Lee, E. M.; Thomas, R. K.; Rennie, A. R. Langmuir 1992, 8, 1204. Bo¨hmer, M. R.; Koopal, L. K.; Janssen, R.; Lee, E. M.; Thomas, R. K. Langmuir 1992, 8, 2228. (14) Tiberg, F.; Jo¨nsson, B.; Tang, J.-a.; Lindman, B. Langmuir 1994, 10, 2294. (15) Levitz, P.; Van Damme, H., Keravis, D. J. Phys. Chem. 1984, 88, 2228. (16) Desbene P. L.; Portet, F.; Treiner, C. J. Colloid Interface Sci. 1997, 190, 350. (17) Rutland, M. W.; Senden, T. J. Langmuir 1993, 9, 412. (18) Rutland, M. W.; Christenson, H. K. Langmuir 1990, 6, 1083. (19) Giasson, S.; Kuhl, T. L.; Israelachvili, J. N. Langmuir 1998, 14, 891.
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ClO4), particles were observed up to 35 °C. We suggest that this abrupt structural change is related to the cloud point phenomenon, where deteriorating solvent quality with increasing temperature results in the condensation of continuous C12E5 bilayers on the mica surface. Materials and Methods Materials. GC grade penta(oxyethylene) dodecyl ether (98%) (C12E5), was purchased from Fluka and used as received. HPLC grade NaClO4 was purchased from Fisher Scientific and used as received. Water was deionized to 18 MΩ cm resistivity (Nanopure system, Barnstead). Water saturated with the ambient atmosphere of CO2 has a pH of 6. The concentration of C12E5 is 0.11 mM, approximately twice the cmc. Grade 2, muscovite mica was purchased from Mica New York Corp., and hand-cleaved just before use. ZYH grade HOPG (12 × 12 × 2 mm3) was purchased from Advanced Ceramics Co., and hand-cleaved with an adhesive tape until a smooth surface was obtained. Atomic Force Microscopy. A nanoscope IIIa (Digital Instruments, Inc.) was used. Freshly cleaved substrate was mounted onto a stainless steel disk, and scanned in an aqueous solution. The solution was injected through a silicone rubber tubing into a fluid cell (Digital Instruments, Inc.), sealed by an O-ring. An E-scanner with a maximum scan area of 16 × 16 µm2 was used. The scanner was calibrated by a standard 1 µm unit length gold calibration ruling, HOPG, and mica. Silicon nitride (Si3N4) integral tips (NP type) were used with a factory-specified spring constant of 0.22 N/m, length of 120 µm, width of 15 µm, and nominal tip radius of curvature of 20-40 nm. Images unless specified were captured in the deflection mode with low feedback gain values. Height images were captured with high feedback gains. Deflection images were not filtered. Height images were flattened to remove background slopes. The scan rate was between 3 and 12 Hz. The images of surfactant aggregates were captured using the soft-contact technique, where the imaging force was maintained at a value just below the breakthrough force. The force calibration plot is converted to a force and surface separation or distance plot by defining the point of zero force and the point of zero separation; i.e., the tip and the substrate surface are in adhesive contact.20 Zero force is determined by identifying the region at large separation where the deflection is constant. A systematic error may occur if a long-range force is not taken into account. Zero separation is determined from the constant compliance region at high force where deflection is linear with the expansion of the piezoelectric crystal. An uncertainty may be due to the incomplete removal of adsorbates. The exact value of the spring constant is unknown and therefore the forces are only approximate. We chose a nominal spring constant of 0.22 N/m in all force curves. Only approaching force curves are reported here. The tip to substrate velocity was set at 0.1 µm/s. The Nanoscope IIIa we used does not have a temperature control unit. Therefore, the temperature reported here was either the room temperature adjusted by a thermostat or the solution temperature varied by placing the solution either in a refrigerator or in a hot water bath prior to injection. When the temperature was controlled by the thermostat, the AFM results were obtained at a precise temperature. We also reported results where the temperature changed with time using the second method. In this case the temperature was estimated roughly by measuring the inlet and outlet solution temperatures.
Results and Discussion The cloud point measurement was conducted according to ASTM D2024-65 (1980). C12E5 solution 1% w/w ()24.6 mM) was slowly heated to a cloudy solution and cooled gradually. The temperature at which the solution became clear was taken as the cloud point. The cloud point for 1% w/w C12E5 is 31.5 ( 0.5 °C, and in the presence of 0.5 M NaClO4 it is 41.0 ( 0.5 °C. We conducted AFM measurements on a few wellestablished systems, including mica/Si3N4 tip in water (20) Prater, C. B.; Maivald, P. G.; Kjoller, K. J.; Heaton, M. G. Probing Nano-scale Forces with the Atomic Force Microscope; Application Note No. 8 from Digital Instruments, 1995; see also references therein.
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Figure 1. Surface force as a function of surface separation distance as a Si3N4 tip approaches a mica substrate in deionized water. The jump-in is indicated by the arrow.
and HOPG/Si3N4 tip in C12E5 solution, to ensure the correctness of our experimental procedures and data acquisition techniques. Figure 1 is the force vs distance curve between bare mica and a Si3N4 tip in pure water at 20 °C. There exists a weak and long-range repulsion with a maximum of 0.18 nN at 7.4 nm, followed by a strong attraction near the surface with a minimum of -0.4 nN before contact. Others have attributed this to the classical DLVO interactions between negatively charged mica and the Si3N4 tip.21 The force curve and AFM image of C12E5 adsorption on HOPG in 0.11 mM solution at 20 °C are shown in Figure 2, and they are similar to published results.8 The stripes were postulated to be hemicylindrical hemimicelles with a periodic separation of 6 nm. The steric barrier started at 6 nm and maximized at just below 5 nm, followed by a jump-in to the underneath substrate. The desorption during jump-in was slow as indicated by the number of data points captured before the final tip/ substrate contact was made. We also noticed that the height of the steric force barrier gradually decreases with time, and regains its initial value after reinjection of the surfactant solution. We attributed this to the low volume (∼0.1 mL) to area (>2 cm2) ratio of surfactant adsorption in an AFM fluid cell. This decrease in forces was only observed when the substrate was HOPG, indicating a competitive adsorption on the glass and O-ring surface of the fluid cell. At the beginning of the AFM study on the C12E5 adsorption on mica, we were puzzled by variation in results from one day to another. In the end, we discovered that the data variation from one type to another is due to roomtemperature fluctuation from one day to another. Indeed after a careful examination of the temperature and AFM data correlation, we found that the adsorbate structure, amount, and thickness and the adsorption kinetics changed abruptly at 21 °C, incidentally, close to the average room temperature of the laboratory. When the room temperature was set at 18-19 °C, particulate features were observed. The number of particles increased with time as shown in Figure 3a a few minutes after injection and Figure 3b 10-20 min later. As time elapsed, some particles attached to each other and formed stringlike features (Figure 3c). Assuming the particles are touching each other in the strings, the diameter of the particles was estimated to be 20-25 nm, which eliminates the possibility of a simple, spherical micellar structure. Using the area per C12E5 headgroup of 45 Å2, the number of surfactants in one fragment layer is estimated to be between 700 and 1090. In about 3 h, the mica surface was almost covered by particles connected with each other to form a two-dimensional network of particles (Figure 3d). A height image is displayed in Figure 3d to show the particles as brighter protrusions and uncovered regions as darker holes. It should be pointed
Figure 2. (a) Surface force as a function of surface separation as a Si3N4 tip approaches an HOPG substrate in 0.11 mM C12E5 solution. The jump-in is indicated by the arrow. The set point force in imaging hemicylindrical hemimicelles is indicated by SP. (b) AFM image of C12E5 hemicylindrical hemimicelles on HOPG in 0.11 mM C12E5 solution with a scan area of 200 × 200 nm2. The average distance between stripes is 60 Å.
out that, at such a high surface coverage, the particles do not coalesce to form a continuous layer. The height of particles was consistently measured by AFM section analysis to be 1-2 nm regardless of the surface coverage in all these images. Adsorption kinetics of isolated aggregates similar to that of joined aggregates and to that of an aggregate network has been observed in the adsorption of a cationic gemini surfactant, 1,2-bis(ndodecyldimethylammonium)ethane dibromide, on mica using AFM.22 However, in their case a flat bilayer was obtained after 90 min of equilibrium, which was never observed in the case of C12E5 on mica at below 21 °C. At low surface coverage (corresponding to Figure 3a-c), similar to that between mica and the Si3N4 tip in water, a pure attraction was measured before the contact of hard surfaces, and no secondary repulsive force barriers were exhibited. As the surface coverage increased and reached the maximum amount as shown in Figure 3d, the secondary repulsion due to the steric effect was detected as shown in Figure 3e. The steric repulsion reached a maximum of 0.66 nN at 2.1 nm, consistent with the height of particles measured in the section analysis. Below this (21) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Langmuir 1992, 8, 1831. Teschke, O.; de Souza, E. F.; Ceotto, G. Langmuir 1999, 15, 4935. (22) Fielden, M. L.; Claesson, P. M.; Verrall, R. E. Langmuir 1999, 15, 3924.
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e Figure 3. Adsorption of C12E5 on mica in 0.11 mM solution at 18 °C. (a)-(d) are AFM images of C12E5 particles with increasing time. Scan size 3 × 3 µm2 (a), 2 × 2 µm2 (b), and 1 × 1 µm2 (c, d). Particles increased in numbers from (a) to (b), and formed one-dimensional strings in (c) and two-dimensional networks in (d). (d) is a height image. (e) is a plot of surface force as a function of surface separation as a Si3N4 tip approaches a mica substrate. The jump-in events are indicated by the arrows. The set point force in imaging particles is indicated by SP.
primary break in the force curve, the separation was not immediately reduced to zero, indicating that there are strongly adsorbed surfactant molecules trapped between the tip and mica. The secondary break due to a small jump-in at 6.7 nm may indicate a minority of weakly adsorbed species on the mica surface. This adsorbed species on average may extend as far as 15 nm into the aqueous solution. The thicker species were not imaged by AFM due to the small break-through force.
When the room temperature was set at 22-23 °C, a different surface morphology was observed. The surface after injection was immediately covered by a smooth layer as shown in Figure 4a. This is different from the lower temperature case where the adsorption is much slower and takes a longer time to reach equilibrium. After repeated scanning, holes appeared in the layer and grew larger with time from part b to part c of Figure 4. A height image was presented in Figure 4c to show the scan damage
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d
Figure 4. Adsorption of C12E5 on mica in 0.11 mM solution at above 22 °C. (a)-(c) are AFM images of C12E5 continuous layers on mica with increasing time. Scan size 1 × 1 µm2 (a), 2 × 2 µm2 (b), and 2 × 2 µm2 (c). Holes were generated and extended during scanning. (c) is a height image. (d) is a plot of surface force as a function of surface separation as a Si3N4 tip approaches a mica substrate. The jump-in is indicated by the arrow. The set point force in imaging bilayers is indicated by SP.
as depressed, darker regions. Figure 4c is different from Figure 3d in that Figure 3d consists of joined but physically distinctive particles and appears to be grainier, while Figure 4c shows a smooth and continuous domain with isolated holes. The depth of the holes was measured by AFM section analysis to be at most 0.5 nm, which is only a fraction of the total film thickness indicated by the force curve (Figure 4d). Therefore, the surfactant coating was only partially removed during scanning. In Figure 4d, the steric repulsion started at about 8 nm and rose smoothly to a maximum of 1.5 nN at 3.5 nm. The tip then pushed away the adsorbed species and jumped into contact with the substrate surface. The jump-in is faster than in Figure 3e, as no data points were recorded in the jumping-in process. AFM images and force measurements point to a continuous layer structure in the case of C12E5 adsorption at above 21 °C. We also tried to observe the structural transition in situ by gradually increasing and decreasing the solution temperature. In one experiment, the solution was placed in the refrigerator at 4 °C, and particles were found on the mica surface. The same solution was heated to above 30 °C; the layer structure emerged. We also observed a transition from a particulate structure (Figure 5a) to a
layer structure (Figure 5b) in situ as the solution temperature increased slightly in the vicinity of 21 °C. In Figure 5b, the layer structure is inferred from the force curves and the holes in an otherwise flat coating. The adsorption of 0.11 mM C12E5 was studied in the presence of 0.5 M NaClO4, a salting-in additive. The temperature was raised to as high as 35 °C by heating the solution just before injection; only particles were found on the mica surfaces. Our current system prevents a precise control at higher temperatures. Since the cloud point of 1% w/w C12E5 in 0.5 M NaClO4 is 41 °C, it is apparent that raising the cloud point also raises the particle to layer transition temperature at the surfaces. Our study indicates that C12E5 adsorption on the mica surface increases with increasing temperature beyond 21 °C. At below 21 °C, the adsorption increases slowly with time. At above 21 °C, the adsorption is instantaneous and remains constant. How do these results compare with those of adsorption studies conducted on similar surfactants previously? In other studies, polyoxyethylene alcohols adsorbed weakly on mica. The different results are summarized here. Surface force measurements conducted by Rutland et al.18 showed weak but finite adsorption in 10-4 or 4.5 × 10-3 M Na2SO4 with C12E5 concentrations
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Figure 5. AFM images of in situ structural transition from fragments (a) to continuous layers (b) as temperature increased above 21 °C.. Scan size ) 5 × 5 µm2 (a) and 2 × 2 µm2 (b). (b) is a height image to show holes due to repeated scans.
as low as 15(cmc) at T ) 22 °C. Long-range repulsive forces were observed at concentrations >100(cmc) after 24 h of equilibration time, reminiscent of the particle aggregation in our study. In our study, the adsorption of C12E5 occurred at lower concentration, and two types of adsorbed structures were observed near 22 °C. The adsorption of sodium ions on mica or the higher volume to surface ratio in the surface forces apparatus may account for the difference. It should be noted that the jump into the primary contact observed in our study also points to a weak interaction between C12E5 and mica. Surface force measurements by Giasson et al. showed weak and long-range repulsion (5-30 nm) between mica surfaces across 1.8(cmc) C12E4 solution in 10-4 M Na2SO4 at 18 °C.19 The surfaces jumped into contact from 4.6 nm, the diameter of a C12E4 micelle. They attributed the longrange repulsion to a layer of cylindrical or wormlike micelles extending from the mica surface into the water phase, and the short-range repulsion to a structural rearrangement. Compared to our study, the steric forces measured in our study are shorter-ranged and more uniform, which exclude significant aggregation toward the water phase in the C12E5 system. However, at below 21 °C in the case of a high surface coverage (Figure 3e), the repulsive force extended to 15 nm. The cloud point of C12E4 is 4 °C23 lower than that of C12E5, which may attribute to the difference. In another study by Grant et al. using AFM force measurements, no adsorption was observed on mica at 4.9(cmc) C10E5 at 25 ( 2 °C.12 It should be noted that the cloud point of C10E5 is 44 °C,24 higher than that of C12E5. However, a zwitterionic surfactant, dodecyldimethylammoniopropanesulfonate (DDAPS), was found to form spherical micelles on mica. The adsorption on the Si3N4 AFM tip is also possible. Is it possible that the observed phenomena are solely due to C12E5 on the Si3N4 tip? We noticed that the AFM images varied at different substrate surface locations using the same tip. If the only significant adsorption of C12E5 is on the Si3N4 tip, then the same features with the tip geometrical dimensions should show up everywhere in the tip scans. In fact, from the relatively sharp images obtained in the experiments, we can exclude significant three-dimensional C12E5 aggregation on the tip. In ad(23) Michell, D. J.; Tiddy, G. J. T.; Waring, L.; Bostock, T.; McDonald, M. P. J. Chem. Soc., Faraday Trans. 1 1983, 79, 975. (24) Lang, J. C.; Morgan, R. G. J. Chem. Phys. 1980, 73, 5849.
dition, at below 21 °C, within 3 h, adsorption on the Si3N4 tip is not significant due to the lack of a repulsive barrier in the force curves. From the various holes created and extended during scanning in an otherwise featureless background (Figure 4b,c), we concluded that the layer structure forms on the mica surface. Therefore, the adsorption of C12E5 on the Si3N4 tip, if any, cannot account for all the observed structure. Is it possible that C12E5 also forms a layer structure on the Si3N4 tip? Rutland et al.17 measured force curves between two silica surfaces in 0.1 mM C12E5 and 0.3 mM NaCl solution at 22 ( 3 °C and found a steric barrier starting at around 8 nm with the steepest increase at around 5 nm. They attributed this to the formation of intercalated bilayers on silica surfaces. On the basis of similarity in hydroxylated nature between silica and oxidized Si3N4 and the similarity between Figure 4c and that in Rutland’s paper, it is likely that C12E5 forms a bilayer structure both on mica and on the AFM tip at above 21 °C. The uncompressed bilayer thickness of C12E5 is therefore estimated to be around 4 nm, agreeing with measurements from neutron studies of an uncompressed bilayer.13 In conclusion, it is most probable that C12E5 does not aggregate significantly in the low to medium surface coverage range at below 21 °C and forms a layer structure at above 21 °C on the Si3N4 tip. It is difficult to determine the exact aggregate structure at below 21 °C. The measured particle thickness at below 21 °C, 1-2 nm, corresponds to a monolayer or hemimicellar structure. The lateral dimension of particles as measured in Figure 3c excludes a simple micellar structure. The height was obtained at an imaging force near the jump-in point, and therefore it is possible that 1-2 nm corresponds to a compressed structure or a structure whose weakly adsorbed top portion is removed. The repulsion tail extending as far as 15 nm further suggests the presence of a weakly adsorbed, extended structure. A monolayer structure of hydrophilically anchored C12E5 is not possible because it is not stable and no hydrophobic interactions were detected. A monolayer of the opposite C12E5 orientation is also unlikely to form on a hydrophilic mica. A likely structure is a fragmented, loosely formed bilayer or multilayer structure as shown in Figure 6a. The bulkier oxyethylene chains due to a higher degree of hydration may prevent close packing of C12E5 molecules. At above 21 °C, the most likely structure is the bilayer structure with interdigitated hydrocarbon chains depicted in Figure
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Figure 6. Proposed aggregate structure of C12E5 adsorption on mica: (a) loosely bound bilayer and multilayer fragments at below 21 °C and (b) continuous bilayers of interdigitated hydrocarbon chains at above 21 °C. The oxyethylene groups are depicted as filled circles, and the hydrocarbons are depicted as wiggling lines.
6b. The sizes of the surface aggregates in both cases are larger than the micelles in solution due to the nucleation effect as surfactants interact with each other on mica surfaces. What is the driving force behind the enhanced C12E5 adsorption on mica with temperature? It is well-known that increasing the temperature increases the surfactant adsorption at above the cmc. Increasing the temperature reduces the solvation of the headgroup and the solubility of nonionic surfactants, and in turn increases their surface activity. Mahdi et al. attributed the maximum in the permeate concentration of C8EO4.5 aqueous solution to the enhanced adsorption near the cloud point at a cellulosic-coated hydrophilic membrane surface.25 Kronberg and Silveston also demonstrated that the adsorption of nonionic surfactants on hydrophobic surfaces increases with temperature.26 Results of HPLC experiments with nonylphenol ethoxylates were rationalized in terms of a model which accentuates the decreasing water structuring around the hydrophilic parts of the surfactant. The disappearance of water structuring with temperature promotes alternative interactions between the solute and other components of the system, such as surrounding hydrophobic solid surfaces and other surfactants in existing micelles. Corkill et al. showed that the adsorption isotherm of C8E3 on graphitized carbon black (Graphon) changes from a Langmuirian to an S shape when temperature is raised from 4.5 to 25 °C. The isotherms show a strong increase in adsorption in the proximity but lower than the cloud point. Kumagai et al. studied the adsorption of C12E5 on carbon black and found similar behavior around the cloud point.27 Below the cloud point, the adsorption was Langmuirian with an area per molecule of 53 Å2; above the cloud point, the adsorption increased enormously. In both cases, the carbon black particles were well dispersed, indicating a hydrophilic surface. The abrupt change in the surfactant aggregation was attributed to the formation of another phase close to the solid surface due to the nucleation and condensation of surfactant aggregates at the surfaces.28 Both the forces between adsorbent and adsorbate as well as those between adsorbate and solvent contribute to the adsorption process. But in this case, the enhanced adsorption is more due to the dehydration of the oxyethylene chain than due to the (25) Mahdi, S. M.; Sko¨ld, R. O. Colloids Surf. 1992, 66, 203. (26) Kronberg, B.; Silveston, R. Prog. Colloid Polym. Sci. 1990, 83, 75. (27) Kumagai, S.; Fukushima, S. In Colloid and Interface Science; Kerker, M., Ed.; Academic Press: New York, 1976; Vol. 4, p 91. (28) Corkill, J. M.; Goodman, J. F.; Tate, J. R. Trans. Faraday Soc. 1966, 62, 979.
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physical adsorption. Our results correlate well with the phenomena described above in various surfactant/surface combinations. It shows that the enhanced adsorption coincides with a structural change of surfactant aggregates, possibly from a fragmented, loose bilayer to a continuous interdigitated bilayer. More importantly, the changes in the adsorption kinetics from a gradual deposition of individual particles to an instant condensation of macroscale layers indicate a phase transition on surfaces similar to the phase separation near the cloud point in bulk. The surface phase transition temperature is 10 °C below the cloud point. A new surfactant-rich macroscopic phase is formed above 21 °C at the mica surface, while at below 21 °C, the surface coating is formed by the diffusion and adsorption from a dispersed phase. This aggregate structural change is responsible for the enhanced surfactant performance at near their cloud points. The softcontact imaging technique has demonstrated that it is possible to identify the surfactant molecular aggregates near the liquid/solid interface which are responsible for unique phenomena in surfactant technologies. Conclusions The experiments reported are an attempt to characterize the aggregate structure and to understand the molecular mechanisms of nonionic surfactant adsorption at the liquid/solid interface by soft-contact AFM. It shows that the steric repulsion is as suitable for imaging soft and mobile surfactant aggregates as the electrostatic repulsion between charged surfactants. The adsorption of a nonionic surfactant, C12E5, has been studied as a function of temperature and salt. Our results suggest that there exists a phase transition temperature where the surfactant surface aggregates change their structure abruptly. At below 21 °C, C12E5 forms a fragmented structure which was postulated to be loosely bound bilayer or multilayer fragments. In time, the fragments slowly attached to each other to form strings and networks. At above 21 °C, uniform and continuous bilayers were formed instantaneously on mica and most likely the AFM tip, which give rise to well-defined steric force versus distance profiles. The addition of 0.5 M sodium perchlorate raises both the cloud point of C12E5 in solution and the particle to layer transition temperature at the surfaces. The proximity of this surface phase transition to the cloud point transition suggests that they are closely related, both due to the dehydration of the oxyethylene chain. The phase transition also results in the different adsorption kinetics. The sizes of the surface aggregates in both cases are larger than the micelles in solution due to the nucleation effect as surfactants interact with each other on mica surfaces. To further confirm the structure and structural transformation in the adsorption of C12E5, we plan to use a more precise temperature control method, to determine the adsorption on Si3N4 independently, and to determine the transition temperature of C12E5 in the presence of other salting-in and salting-out additives. Acknowledgment. G.M. acknowledges the Career Program of the National Science Foundation (Grant CTS9703102) for partial support of this research. J.D. is supported by a graduate research fellowship from the Institute for Manufacturing Research at Wayne State University. We also thank Dr. Randal M. Hill and Dr. Yihan Liu from Dow Corning for discussions and help with the AFM experiments. LA000103V
