6944
Langmuir 2006, 22, 6944-6950
Compression of Langmuir Films Composed of Fine Particles: Collapse Mechanism and Wettability Sa´ndor Borda´cs, Attila Agod, and Zolta´n Ho´rvo¨lgyi* Department of Physical Chemistry, Budapest UniVersity of Technology and Economics, Budapest H-1521, Hungary ReceiVed March 14, 2006. In Final Form: May 27, 2006 The collapse mechanism of microparticulate Langmuir films was studied experimentally in the present work. Using a Wilhelmy film balance, surface pressure vs area isotherms were determined, and the particle removal during the compression was examined by video-microscope and by naked eye. Upon compressing partially wettable 75 µm diameter surface modified glass beads at liquid (water or aqueous surfactant solution)-air (or n-octane) interfaces, different collapse mechanisms were visualized depending on the wettability of the particles. At low contact angles (below 40°) irreversible particle removal was observed as a consequence of a particulate line-by-line collapse mechanism. At higher contact angles a buckling-type collapse mechanism was revealed without particle removal from the liquid interface. In the case of irreversible particle removal we assessed the contact angles from the nondissipative part of the isotherm. These values were found to be in reasonable agreement with those determined directly on the beads.
Introduction Particles are trapped at liquid-fluid interfaces due to their partial wettability by the immiscible phases. The energy well where they settle can be very deep and the trapping energy can be higher by some orders of magnitude than kT.1 The array of fine particles at liquid-fluid interfaces has been intensively studied from different points of view. The particles can act as amphiphilic agents at interfaces, i.e., frequently stabilize emulsions and foams.2-8 The organization of particles into different structures at interfaces also attracted significant attention, since this phenomenon can serve as a model for colloid aggregations, flotation,9 phase transitions,10-16 and interfacial thermodynamics and interactions.17-28 Moreover, the nanoparticulate arrays at liquid-fluid interfaces can provide an entry into the fabrication * Corresponding author. e-mail:
[email protected]. (1) Pieranski, P. Phys. ReV. Lett. 1980, 45, 569. (2) Pickering S. U. J. Chem. Soc. 1907, 91, 2010. (3) Binks, B. P. Curr. Opin. Colloid Interface Sci. 2002, 7 (1-2), 21. (4) Horozov, T. S.; Binks, B. P. Angew. Chem., Int. Ed. 2006, 45 (5), 773. (5) Binks, B. P.; Horozov, T. S. Angew. Chem., Int. Ed. 2005, 44 (24), 3722. (6) Horozov, T. S.; Aveyard, R.; Clint, J. H. Langmuir 2005, 21, 2330. (7) Wang, D. Y.; Duan, H. W.; Mo¨hwald, H. Soft Matter 2005, 1 (6), 412. (8) Horozov, T. S.; Aveyard, R.; Clint, J. H.; Neumann, B. Langmuir 2005, 21(6), 2330. (9) Nguyen, A. V. J. Colloid Interface Sci. 2002, 249, 147. (10) Hurd, A. J.; Schaefer, D. W. Phys. ReV. Lett. 1985, 54, 1043. (11) Robinson, D. J.; Earnshaw, J. C. Phys. ReV. A 1992, 46 (4), 2065. (12) Ho´rvo¨lgyi, Z.; Ma´te´, M.; Zrı´nyi, M. Colloids Surf. A: Physicochem. Eng. Asp. 1994, 84 (2-3), 207. (13) Martı´nez-Lo´pez, F.; Cabrerizo-Vı´lchez, M. A.; Hidalgo-AÄ lvarez, R. J. Colloid Interface Sci. 2000, 232, 303. (14) Moncho-Jorda´, A.; Martı´nez-Lo´pez, F.; Gonza´lez, A. E.; Hidalgo-A Ä lvarez, R. Langmuir 2002, 18, 9183. (15) Gonza´lez, A. E.; Martinez-Lo´pez, F.; Moncho-Jorda´, A., Hidalgo-AÄ lvarez, R. Physica A 2004, 333, 257-268. (16) Armstrong, A. J.; Mockler, R. C.; O’Sullivan, W. J. J. Phys.: Condens. Matter 1989, 1, 1707. (17) Menon, V. B.; Nikolov, A. D.; Wasan, D. T. J. Colloid Interface Sci. 1988, 124 (1), 317. (18) Levine, S.; Bowen, B. D. Colloids Surf. A: Physicochem. Eng. Asp. 1993, 70, 33. (19) Fainerman, V. B.; Kovalchuk, V. I.; Lucassen-Reynders, E. H.; Grigoriev, D. O.; Ferri, J. K.; Leser, M. E.; Michel, M.; Miller, R.; Mo¨hwald, H. Langmuir 2006, 22, 1701. (20) Hurd, A. J. J. Phys. A 1985, 18, L1055. (21) Stamou, D.; Duschl, C.; Johannsmann, D. Phys. ReV. E 2000, 62, 5263. (22) Kralchevsky, P. A.; Denkov, N. D. Curr. Opin. Colloid Interface Sci. 2001, 6, 383.
of nanostructural, solid-supported films.29-36 The wettability of particles plays an important role in the aforementioned processes and phenomena, because it can relate to the particle-particle (p-p) colloid and capillary interactions. So the determination of contact angles for fine particles is the main topic of recent and previous papers.37-61 (23) Kralchevsky, P. A.; Nagayama, K. Capillary forces between particles of irregular contact line. In Particles at Fluid Interfaces and Membranes; Elsevier: Amsterdam, 2001; Chapter 12, p 503. (24) Kralchevsky, P. A.; Denkov, N. D.; Danov, K. D. Langmuir 2001, 17, 7694. (25) Nikolaides, M. G.; Bausch, A. R.; Hsu, M. F.; Dinsmore, A. D.; Brenner, M. P.; Gay, C.; Weitz, D. A. Nature 2002, 420, 299. (26) Danov, K. D.; Kralchevsky, P. A.; Boneva, M. P. Langmuir 2004, 20, 6139. (27) Danov, K. D.; Kralchevsky, P. A.; Naydenov, B. N. J. Colloid Interface Sci. 2005, 287, 121. (28) Olugebefola, S. C.; Park, S. Y.; Banerjee, P.; Mayes, A. M.; Santini, C. M. B.; Iyer, J.; Hammond, P. T. Langmuir 2002, 18, 1098. (29) Kotov, N. A.; Meldrum, F. C.; Wu, C.; Fendler, J. H. J. Phys. Chem. 1994, 98, 2735. (30) Fendler, J. H.; Meldrum, F. C. AdV. Mater. 1995, 7, 607. (31) Achermann, M.; Petruska, M. A.; Crooker, S. A.; Klimov, V. I. J. Phys. Chem. B 2003, 107, 13782. (32) Szekeres, M.; Kamalin, O.; Schoonheydt, R. A.; Wostyn, K.; Clays, K.; Persoons, A.; De´ka´ny, I. J. Mater. Chem. 2002, 12 (11), 3268. (33) Szekeres, M.; Kamalin, O.; Grobet, P. G.; Schoonheydt, R. A.; Wostyn, K.; Clays, K.; Persoons, A.; De´ka´ny, I. Colloids Surf. A: Physicochem. Eng. Asp. 2003, 227, 77. (34) Reculusa, S.; Masse´, P.; Ravaine, S. J. Colloid Interface Sci. 2004, 279, 471. (35) Reculusa, S.; Ravaine, S. Appl. Surf. Sci. 2005, 246, 409. (36) Dea´k, A.; Sze´kely, I.; Ka´lma´n, E.; Keresztes, Zs.; Kova´cs, A. L.; Ho´rvo¨lgyi, Z. Thin Solid Films 2005, 484, 310. (37) Garhsva, S.; Contreras, S.; Goldfarb, J. Colloid Polym. Sci. 1978, 256(3), 241. (38) Tschajlovska, S. D.; Aleksandrova, L. B. Chem. Technol. 1978, 30, 301. (39) Hansford, D. T.; Grant, D. J. W.; Newton, J. M. J. Chem. Soc. Faraday Trans. 1980, 76, 2417. (40) Bayramli, E.; Mason, S. G. Colloid Polym. Sci. 1982, 260, 452. (41) Ba´n, S.; Wolfram, E.; Rohrsetzer, S. Colloids Surf. 1987, 22, 301. (42) Crawford, R.; Koopal, L. K.; Ralston, J. Colloids Surf. 1987, 27, 57. (43) Yang, Y.-W.; Zografi, G.; Miller, E. E. J. Colloid Interface Sci. 1988, 122(1), 24. (44) Yang, Y.-W.; Zografi, G.; Miller, E. E. J. Colloid Interface Sci. 1988, 122(1), 35. (45) Hanning, R. N.; Rutter, P. R. Int. J. Miner. Process. 1989, 27, 133. (46) Diggins, D.; Fokkink, L. G. J.; Ralston, J. Colloids Surf. 1990, 44, 299. (47) Huethorst, J. A. M.; Leenaars, A. F. M. Colloids Surf. 1990, 50, 101. (48) Ramesh, R.; Somasundaran, P. J. Colloid Interface Sci. 1990, 139(1), 291. (49) Clint, J. H.; Taylor, S. E. Colloids Surf. 1992, 65, 61.
10.1021/la060696v CCC: $33.50 © 2006 American Chemical Society Published on Web 07/11/2006
Compression of Langmuir Films of Fine Particles
The idea of contact angle determinations from an “apparent boundary tension” measured in the presence of partly wetted micron-sized particles arose in the 1960s.62 The expression of “apparent boundary tension” (originating from an earlier work of Gomm et al.62) means that the interfacial particles cannot change the surface tension (boundary tension) of liquid-fluid interface and only the measurable value shows deviation from its original value. The authors used a “Du-Nouy film balance” to compress the monoparticulate film into a close-packed array. They obtained reasonable contact angles at the water-air interface for hydrophilic spheres in the contact angle range of 40-50°, but their measurements revealed erroneous results for hydrophobic spheres, with contact angles being higher than 100°. For the latter case, the compression resulted in wrinkling of the film. The investigations at water-oil (petroleum white spirit) interfaces, however, showed reasonable results, though upon compressing the layer of hydrophilic spheres beyond the close-packed state, a further decrease of the apparent boundary tension and wrinkling of the film were observed. The results of the aforementioned workspractically speakingsremained without echo, though in the following years several papers about the film balance investigations of particulate films were put forward in the literature.63-67 These studies, however, focused on the p-p interactions rather than the wettability of particles. Thermodynamic Background of Particle Removal from the Interface. The compressed particulate films show a measurable (effective) surface pressure basically due to the p-p interactions. It has been shown that the kinetic energy term originating from the Brownian motion is very low beside the interaction energies and its effect is negligible on the pressure.68 The idea of contact angle determinations from the effective (apparent) surface (or interfacial) tensions (henceforth surface tension) of particulate films presented itself again at the beginning of the 1990s.49-50 According to these papers contact angles can be derived from the analysis of surface pressure (Π) vs surface area (A) isotherms of monoparticulate films obtained by film balance investigations. Information on particle wettability can be obtained by assuming the identity of Er and Wr.49 Er is the work necessary for removal of a particle from the layer, obtained from the experimentally determined Π vs A isotherms by (50) Clint, J. H.; Quirke, N. Colloids Surf. A: Physicochem. Eng. Asp. 1993, 78, 277. (51) Aveyard, R.; Binks, B. P.; Fletcher, P. D. I.; Rutherford, C. E. Colloids Surf. A: Physicochem. Eng. Asp. 1994, 83, 89. (52) Ho´rvo¨lgyi, Z.; Ne´meth, S.; Fendler, J. H. Langmuir 1996, 12, 2 (4), 997. (53) Hadjiiski, A. Dimova, R.; Denkov, N. D.; Ivanov, I. B.; Borwankar, R. Langmuir 1996, 12, 6665. (54) Ecke, S.; Preuss M.; Butt H. J. J. Adhesion Sci. Technol. 1999, 13, 1181. (55) Ho´rvo¨lgyi, Z.; Ma´te´, M.; Da´niel, A.; Szalma, J. Colloids Surf. A: Physicochem. Eng. Asp. 1999, 156, 501. (56) Chibowski, E.; Perea-Carpio, R. AdV. Colloid Interface Sci. 2002, 98, 245. (57) Paunov, V. N. Langmuir 2003, 19 (19), 7970. (58) Gillies, G.; Buscher, K.; Preuss, M.; Kappl, M.; Butt, H. J.; Graf, K. J. Phys.-Condens. Matter 2005, 17 (9), S445. (59) Fiegel, J.; Jina, F.; Hanes, J.; Stebe, K. J. Colloid Interface Sci. 2005, 291, 507. (60) Lazghab, M.; Saleh, K.; Pezron, I.; Guigon, P.; Komunjer, L. Powder Technol. 2005, 157, 79. (61) Cui, Z.-G.; Binks, B. P.; Clint J. H. Langmuir 2005, 21, 8319. (62) Gomm, A. S.; Hauxwell, F.; Moilliet, J. L. In Wetting; S. C. I. Monograph No. 25; Society of Chemical Industry: London, 1967; Vol. 25, p 213. (63) Schuller, H. Kolloid Z. Z. Polym. 1967, 216-217, 380. (64) Sheppard, E.; Tcheurekdjian, N. Kolloid Z. Z. Polym. 1968, 225, 162. (65) Sheppard, E.; Tcheurekdjian, N. J. Colloid Interface Sci. 1968, 28, 481. (66) Doroszkowski, A.; Lambourne, R. J. Polym. Sci., Part C 1971, 34, 253. (67) Garvey, M. J.; Mitchell, D.; Smith, A. L. Colloid Polym. Sci. 1979, 257, 70. (68) Tolnai, G.; Agod, A.; Kabai-Faix, M.; Kova´cs, A. L.; Ramsden, J. J.; Ho´rvo¨lgyi, Z. J. Phys. Chem. B 2003, 107, 11109.
Langmuir, Vol. 22, No. 16, 2006 6945
Er ) ΠcAc
(1)
where Πc is the collapse pressure and Ac is the surface area of a particle at the collapse pressure (collapse area). The adhesion work for a spherical particle, Wr, is defined by
Wr ) γR2π(1 ( cos Θ)2
(2)
where R is the radius of the particle; γ is the liquid-fluid surface tension; π is the geometric constant (or Ludolph number), approximately 3.1415; Θ is the contact angle; and the ( sign in the expression means that the particle moves into the upper (+) or the lower (-) phase during the collapse of the compressed layer. Equations 1 and 2 provide an experimental approach, therefore, for the determination of the wetting (contact angle) of the spherical particles in a monodisperse sample. The contact angle can alternatively be evaluated, even in the absence of information on the radius and density of the spheres, from49,62
Θ ) arcos ([(Πc2(3)1/2/πγ)1/2 - 1]
(3)
for monoparticulate layers that are hexagonal close-packed. Later a repulsion energy term (Vrep) was included to refine the expression of the invested work.50 Hence the following formula
γR2π(1 - cos(Θ))2 ) Vrep + ΠcAc
(4)
was suggested for the determination of contact angles considering the equality of the adhesion and invested work (for a particle) that provides a more realistic picture about the phenomenon. Namely, the particles can be removed from their layer (into the lower phase) as a result of a lateral compression, if the repulsive energy just exceeds the value of adhesion work (neglecting any other energy dissipations). Nonequilibrium Feature of the Particle Removal from the Interface. In an ideal, equilibrated situation, every particle would have the same potential energy, so when they would reach the collapse point they should leave the surface at the same time. This would be a “real” collapse phenomenon.69 In real cases, however, the collapse is a long-term phenomenon because the particles have different potential energies in the layer due basically to the lattice defects and/or polydispersity of the particulate sample.69-71 Hence, ΠcAc in eq 4 includes an excess energy that is necessary for the continual removal of particles after the experimentally observable collapse point (Figure 1). This is the dissipative part of surface pressure-surface area isotherm (beyond the collapse point). One has to take into consideration, however, that the longterm collapse is a dissipative process, and the energy partition between the reversible and irreversible terms is not known. During the particle removal, the contact angle hysteresis, via the stickand-slip motion of the three-phase contact line,72 can lead to significant energy dissipation. Hence, the extra energy term is related to a permanent process of the expansion and compression of the layer as a consequence of the continuous particle removal.69 The nonequilibrium feature of compression manifests itself in further experimental evidence. The surface pressure increases beyond the collapse point in many cases, and the collapse pressure (69) Agod, A.; Dea´k, A.; Hild, E.; Ka´lma´n, E.; Kova´cs, A. L.; Tolnai, Gy.; Ho´rvo¨lgyi, Z. J. Adhes. 2004, 80 (10-11), 1055. (70) Agod, A.; Tolnai, G.; Esmail, N. Ho´rvo¨lgyi, Z. Prog. Colloid Polym. Sci. 2004, 125, 54. (71) Pugnaloni, L. A.; Ettelaie, R.; Dickinson, E. Langmuir 2004, 20, 6096. (72) Blake, T. D. Dynamic Contact Angles and Wetting Kinetics: Wettability; Berg, J. C., Ed.; Surfactant Sci. Ser., Vol. 49; Marcel Dekker: New York, 1993; p 251.
6946 Langmuir, Vol. 22, No. 16, 2006
Figure 1. Surface pressure (Π) vs surface area isotherm obtained for micron-sized particles and the characteristic parameters: Πc, collapse pressure; Ac, collapse area for a particle; Ao, contact cross sectional area for a particle (which relates to particle-particle distance in the secondary potential energy minimum for weakly cohesive films52,68). The introduced work in the nondissipative part of the isotherm corresponds to the area of the AoAcΠc,ext triangle.
can depend on the amount of spread of the particles.73 The latter was attributed to a surface pressure gradient along the layer.73 Can the Particles Be Removed from the Interface by a Lateral Compression? The compression of the monoparticulate film in a trough leads to the wrinkling (folding) of the layer in certain cases, as observed by many authors.52,62,74-75 Cohesive films of particles always show wrinkling, especially in the vicinity of the moving barrier.52 It is accepted in the recent literature that if the layer wrinkles, the particles cannot be expelled out of the interface by their lateral compression, because the theoretical collapse pressure is higher than the surface tension of the liquidfluid interface.75-76 Moreover, it has been emphasized that wrinkling is a general mechanism of collapse and particle removal from the interface cannot take place in film balance experiments.76 So, an experimentally found lower collapse pressure than that of the liquid surface tension can solely be attributed to contamination or any other effect.76 We suppose, however, that the collapse pressure should be sensitive to the wettability of particles in the lower contact angle range. The main purpose of the present study is to show experimentally that the wrinklingtype collapse is not a general mechanism and particles can be irreversibly removed from the interface in certain cases. We chose such a model system to allow us to study the phenomenon carefully by optical microscopy or even by the unaided eye and to carry out reliable wetting characterization of model particles by simple optical microscopy examinations. We will study the compression of microparticulate films at one- and two-liquid interfaces and show that different types of collapse mechanisms can exist depending on the particle wettability and surface tension of liquid-fluid phases. Experimental Section Materials. Glass beads as model particles (Supelco; 75 ( 5 µm diameter; density obtained by picnometer, 2.57 g/cm3) were used for the particulate layer formation. The required surface modification of beads was accomplished by trimethylsilyl N,N-dimethylcarbamate (73) Ma´te´, M.; Fendler, J. H.; Ramsden, J. J.; Szalma, J.; Ho´rvo¨lgyi, Z. Langmuir 1998, 14, 6501. (74) Aveyard, R.; Clint, J. H.; Nees, D.; Quirke, N. Langmuir 2000, 16, 8820. (75) Fenwick, N. I. D.; Bresme, F.; Quirke, N. J. Chem. Phys. 2001, 114(16), 7274. (76) Powell, C.; Fenwick, N.; Bresme, F.; Quirke, N. Colloids Surf. A: Physicochem. Eng. Asp. 2002, 206, 241.
Borda´ cs et al. (>97%, GC grade, Fluka) as an n-hexane (Merck, “for synthesis”, 99%) solution. For the interfacial investigations deionized water (18.2 MΩ cm), purified with a Millipore Simplicity 185 filtration system and n-octane (Merck, “for synthesis”, 99%) were applied. The surface tension of the liquid-fluid interface was modified by Triton X-100 (spec, Reanal) nonionic surfactant; a 0.001% aqueous solution was used in the experiments. Surface Modification of Particles. The surface of the commercially available glass beads is rather hydrophilic; the particles cannot form a stable film at liquid-fluid interfaces and can spontaneously sink into the aqueous phase. We prepared two samples of surface modified particles, having a lower and a higher characterizing water contact angle (sample A and B, respectively). The particles were pretreated by stirring them in the aqueous solution of 1 M HCl (Reanal, 37%, a.r.) for 1 h and then washed with water until neutral pH was reached and finally with ethanol (99.7%, Reanal). A total of 1 g of 75 µm diameter glass beads was silylated in 40 mL of hexane solutions of trimethylsilyl-N,N-dimethylcarbamate. The duration of silylation was 3 min for both samples. The concentration of silylating agent was 0.005 v/v% for sample A and 0.05 v/v% for sample B. After the silylation, the particles were washed in hexane three times and dried at room temperature. For the preparation of sample A, the reaction was stopped by addition of some amount of ethanol into the reaction vessel. For the preparation of sample A, the reaction was stopped by addition of some amount of absolute ethanol into the reaction vessel. Determination of One- and Two-Liquid Contact Angles of Model Particles. For characterizing the particle wettabilities, oneliquid measurements were carried out by employing the “parallelplate” method.52 In this method one ca. 10 µL water drop was formed on the center of a microscope slide, and then a small amount of glass beads was sprinkled around the liquid patch. A microscope coverglass was then pressed onto the liquid drop, whereby the particles were collected at the liquid-air boundary layer. The beads in the boundary layer were photographed from the top (Figure 2a). The extent of immersion into liquid was directly proportional to the wetting angle formed on the beads. The contact angle was determined by the following relationship: Θ ) arccos
(H -D/2D/2)
(5)
where H is the immersion depth of sphere into the liquid and D is the diameter of the bead (also see Figure 2a). It should be noted that this method can only provide values for the advancing wetting angle but not for the critical value which forms immediately prior to the beginning of the three-phase contact line motion. There may be a strong fluctuation of the liquid-air interface due to trapping the silanized glass spheres,77 which probably results in a lower (close to Young’s) value of the wetting angle. This method has been developed for two-liquid cases. Following the one-liquid-procedure, one ca. 20 µL octane drop was formed on the surface of the microscope slide close to the cover-glass. The octane shows a complete wetting on glass; hence, its spontaneous imbibition can easily happen between the plates, forming a two-liquid interface. The effect of prewetting was demonstrated by using glass plates rendered weakly hydrophobic by silanization. First the silylated plate was placed on the bottom of a glass cuvette and then one ca. 20 µL water drop was formed on the plate. Then a suitable amount of octane was poured in the vessel in order to form the two-liquid interface. This procedure allows us to measure the receding two-liquid contact angle (across the aqueous phase). Next a new water drop was formed on the octane-covered interface, resulting in advancing contact angles (also see Figure 2b). The wettability experiments were performed at 23 ( 1 °C. Measurements of Surface Tensions. Prior to the film balance examinations, the surface tensions of liquid-fluid interfaces were determined by the modified Wilhelmy plate method. We employed (77) Scheludko, A.; Toshev, B. V.; Bojadjiev D. T. J. Chem. Soc., Faraday Trans. 1976, 72, 2815.
Compression of Langmuir Films of Fine Particles
Langmuir, Vol. 22, No. 16, 2006 6947 Table 1. Contact Angle and Surface Tension Values Obtained at 23 ( 1 °C contact angle,a deg sample A ()b water-air water-n-octane 0.001% Triton X-100 solution-air 0.001% Triton X-100 solution-n-octane
sample B ()
surface tension, mN/m
40 ( 6 (1) 62 ( 5 (5) 71.7 ( 0.5 46 ( 5 (2w and 2o) 46.5 ( 0.5 40 ( 6 (3) 53 ( 5 (6) 51 ( 0.5 44.5 ( 5 (4)
-
21.4 ( 0.5
a The contact angles were always determined via the aqueous phase. The number in the parentheses in the sample columns identifies the investigated systems in the film balance experiments [(1), for example, corresponds to the film balance examination of sample A at the waterair interface; 2w and 2o mean prewetting by water and by n-octane, respectively]. b
Figure 2. (a) Demonstration of contact angle measurements by the “parallel-plate” method: D, sphere diameter; H, immersion depth into the aqueous phase (particle at water-air interface). (b) Demonstration of the effect of prewetting on the contact angles: the drop on the left-hand side was formed at the solid-air interface before adding octane (receding contact angle 37°), and the drop on the right-hand side was formed at the solid-octane interface (advancing contact angle 63°). this technique without pulling the plate out of the interface, i.e., like one does in the Wilhelmy film balance experiments. In this case the bottom of the plate is positioned exactly in the boundary layer, at the level of the unperturbed liquid-fluid interface. The measurements were performed at 23 ( 1 °C. Film Balance Experiments. The pressure-area isotherms were measured in a miniature (homemade), computer-controlled Wilhelmy film balance. The instrument is suitable for one- and two-liquid examinations, as well. The size of the rectangular-shaped trough is 4 cm × 9 cm and its bottom is made of a transparent glass sheet, allowing us to perform optical investigations during the compression. Because of the small working area of the miniature film balance, we took into consideration the curvature of the liquid surface in the calculation of real areas. With regard to this effect, the Laplace equation was solved, and the boundary conditions were determined using that the derivative of the surface is equal to the tangent of the contact angle. In the experiments, a specified amount of glass beads was scattered carefully onto the aqueous phase-air interface (one-liquid study) or onto the aqueous phase-octane interface (two-liquid study). The model particles showed a complete wetting against octane; hence, those fall across the octane-air interface and were trapped in the boundary layer of aqueous phase and octane. For the investigation of the effect of prewetting, the particles were scattered first onto the aqueous phase and then a suitable amount of octane was layered on them. After a 5-min equilibration, the compression was actuated at a constant rate of 0.5 cm2/min. The behavior of the particulate film was visualized by a CCD camera and by the naked eye. From the pressure-area isotherms the collapse pressure, collapse area, and the contact cross sectional area (Ao) were determined (also see Figure 1). This latter one corresponds to a p-p distance at the secondary
energy minimum and was defined earlier for weakly cohesive particulate films.52,68 As was suggested previously,52 the particles having lower water contact angles (below ca. 60-65°) can be in the secondary energy minimum of their pair-potential energy curve (flocculated state: weakly cohesive film) at the water-air interface. One can suppose the same situation for the water-liquid interface at such low contact angles. The contact cross sectional area can be determined from the isotherm analogously to the headgroup area of molecular Langmuir films,52,68 as shown in Figure 1. By compressing the film beyond this point, the potential energy of particles will increase due to the increasing p-p repulsion. The measurements were carried out at 23 ( 1 °C.
Results Particle Wettabilities and Surface Tensions of Model Liquids. The contact angle and surface tension values are collected in Table 1. As can be seen, the surfactant in the aqueous phase significantly decreased both the one- and two-liquid surface tensions, but its effect on the contact angles was only remarkable for sample B particles. The effect of prewetting is visualized in Figure 2b. If the surface modified glass was prewetted by octane, a significantly higher two-liquid contact angle was observed (an advancing value of the water-octane contact angle). Visual Observations of the Particulate Films during Compression. Sprinkling solid particles onto the liquid surface resulted in the initial formation of well-separated particles. Subsequently, prevailing long-range capillary interactions78 facilitated the interfacial association of beads. As was shown earlier,52 the water-air interface is practically flat around the spheres having this particle size and density. The inclination of the liquid-fluid interface from the plane is only 0.02-0.04° directly at the bead surface.52 For the present case the capillary interaction energies defined for one pair of floating spheres (at zero particle-particle distances)78 are in the range from (-)10-15 to (-)10-16 J.52 These values are lower by some orders of magnitude than the adhesion energy [(1-2) × 10-10 J/particle for the present cases]; consequently, the flotation-type capillary interparticle energies cannot perturb practically our examinations. As required, compression of the aggregates resulted, in every case, in the formation of contiguous and nearly hexagonal-ordered monoparticulate layers (Figure 3a). Approaching the collapse, two distinct types of film behavior were observed. In some cases, the examinations revealed a strictly line-by-line irreversible particle removal in the vicinity of the moving barrier. For these cases we observed a contiguous (78) Chan, D. Y. C.; Henry, J. D., Jr.; White, L. R. J. Colloid Interface Sci. 1981, 79, 410.
6948 Langmuir, Vol. 22, No. 16, 2006
Borda´ cs et al.
Figure 4. (a) Characterizing isotherms for the water-air (sample 1) and water-octane (sample 2o) interfaces. (b) Extrapolated values of collapse pressures as a function of the surface tension of the liquid-fluid interface in the case of the investigated systems (cf. Table 1).
Figure 3. (a) Compressed particulate film at the water-air interface. (b) Visualization of irreversible particle removal: the “line-by-line” collapse mechanism (water-air interface, sample 1). The arrow shows the particulate carpet at the bottom of the trough.
particulate “carpet” at the bottom of the trough (Figure 3b) behind the moving barrier, confirming the existence of a line-by-line particle removal (systems 1, 2w, and 3). For other systems, however, we observed wrinkling (buckling) of particulate film especially in the vicinity of the moving barrier and at the Wilhelmy plate (systems 2o, 4, 5, and 6). In certain cases a complex collapse mechanism appeared: beside the wrinkling, individual particles were expelled out of the interface at different points of the film. This phenomenon was observed in the case of system 6 (sample B at the surfactant solution-air interface). Evaluation of Surface Pressure vs Surface Area Isotherms. Characterizing Π-A isotherms are shown for sample A particles at the water-air and water-octane interfaces, respectively (Figure 4a). The intersection of two straight lines fitted to the isotherm, as depicted in Figure 1, will provide the collapse pressure. We measured steeper isotherms for sample A particles than for particles of sample B. The steepness of the isotherms was found to be higher for one-liquid interfaces than for the corresponding two-liquid interfaces.
We assigned the real (“intrinsic”) values of collapse pressures of the investigated samples for one- and two-liquid interfaces, as well, and plotted these values as a function of the surface tension (Figure 4b). The “intrinsic” collapse pressure was determined by extrapolating linearly the isotherm to zero area (see Figure 1). In the case of particle removal, we suppose that a further increase in the surface pressure beyond the collapse point can solely be attributed to a pressure gradient along the layer. In this case particles with different potential energies can be removed at the same time (line-by-line mechanism), so there is no particle selection during the process. We also show the type of collapse mechanism (buckling or line-by-line) in Figure 4b. As can be seen, the experimentally determined collapse pressure values were always lower than the corresponding surface tension values, independent of the type of collapse mechanism. Though, we found higher collapse pressure values for the wrinkling (buckling) mechanism (data above the broking line), but those correspond to higher contact angles. There is only one exception: systems 2w and 2o show an opposite behavior. We observed a higher collapse pressure for the case of irreversible particle removal (prewetting by water) than for buckling-type collapse (prewetting by octane).
Discussion Collapse Mechanism. The collapse mechanism of the particulate Langmuir film is basically affected by two main parameters from thermodynamic point of view, contact angle and surface tension, and both of them are included in the
Compression of Langmuir Films of Fine Particles
Langmuir, Vol. 22, No. 16, 2006 6949
expression of adhesion work (eq 2). It can be expected that the irreversible particle removal takes place in the case of low adhesion energy and a relatively high value of surface tension. If so, the energy required for increasing the liquid-fluid surface is high; hence, the system will avoid that route. As can be seen from eq 2, very low values of contact angle even in the case of high surface tension can fulfill the requirement of low valued adhesion energy. In other words, the phenomenon is more sensitive to the contact angles than to the surface tension. The contact anglessvia the cohesiveness of the layerscan also influence the collapse mechanism indirectly. Namely, for anisotropic compression, at the lowest contact angles there can be a negligible pressure difference in the layer and the particles can be expelled out of the interface individually. The particle removal in this case is under the control of the local properties of the film (defects, fluctuations, etc.). Contrarily, at higher contact angles that provoke some cohesiveness in the film, the pressure gradient becomes significant, which means that the potential energy of particles will decrease (particulate) line-by-line from the moving barrier toward the pressure sensor. The aforementioned decrease in the pair-potential energies (and surface pressures) can be a result of the increasing p-p distances toward the Wilhelmy plate. If we consider the first two (i and j) particulate lines in front of the moving barrier and say
∆Vmin(i,j) > ∆Emax(i)
(6)
where ∆Vmin(i,j) is the minimum of potential energy difference between the particulate rows and ∆Emax(i) is the maximum of the adhesion energy difference between any two particles in row i, then there can be a chance for a line-by-line collapse mechanism, provided the surface tension and contact angle values are optimal for the irreversible particle removal. According to this concept, the collapse mechanism can show a change, keeping the surface tension constant, from the individual particle removal (at very low contact angles) via a line-by-line mechanism (at not too high contact angles) toward buckling (at higher contact angles). In the transition range of contact angles, complex mechanisms can appear. The experimental results depicted in Figure 4b are in good agreement with this concept. At the same surface tension, particles with lower contact angles show a line-by-line collapse mechanism (systems 1 and 3), while particles with higher contact angles generate buckling mechanism (systems 5 and 6). For two-liquid interfaces, the buckling mechanism appears in the case of prewetting by octane (system 2o), but collapse happens by a line-by-line mechanism in the case of prewetting by water (system 2w). In the latter case, receding contact angles exist, so the particles can expel out of the interface irreversibly. The typical buckling mechanism was observed for the aqueous surfactant solution-octane interface (low surface tension, system 4) and for system 5 (relatively high contact angle). It is worth mentioning that the collapse pressure in the first case (17.2 mN/m) is very close to the theoretically expected interfacial tension (21.4 mN/m), though irreversible particle removal happens. It is interesting to note that individual particle removal and buckling were observed at the same time in certain experiments (system 6) showing that the parameter dependence of the collapse mechanism can be more complicated than that delineated above. Beyond the thermodynamic reasons, there can be geometrical viewpoints. The polydispersity of samples, for instance, results in different immersion depth and far from ideal arrangement of particles at the liquid-fluid interfaces, ensuring special conditions for particle removal. Relating to the relevance of structure formation, further experimental evidence can be mentioned. Previously, a clearly
Table 2. Calculated and Measured Contact Angles in the Case of Irreversible (Line-by-Line) Particle Removal contact angles, deg calculated interfaces water-air aqueous 0.001% Triton X-100 solution-air water-n-octane
sample (system) measured
eq 4
nondissipative
A (1) A (3)
40 ( 6 40 ( 6
74 ( 15 69 ( 15
34 ( 10 28 ( 10
A (2w)
46 ( 5
72 ( 15
49 ( 10
line-by-line collapse mechanism was not observed by studying micron-sized particles in film balances.52 In those experiments, however, a much higher rate of compression (15 cm2/min52) was applied than in the present case (0.5 cm2/min). The lower the rate of compression, the better the ordering of the particulate film that can be reached, as shown previously by computer simulations.70 For the buckling-type collapse mechanism, we always found a lower collapse pressure than that predicted from theory.75,76 A possible reason can be that the p-p distances parallel to the barrier might be larger than that perpendicular to it due to the uniaxial compression of the particulate film. In case of domain structured layers, this can result in a hexagonal to rhombohedral transition inside the domains.74 The restructuring of the particles in the layer to form hexagonal order again is not a continuous process (if it happens at all) due to geometrical reasons. This phenomenon implies that the surface pressure is not a scalar but varies in different directions, causing difficulties in measuring the correct value of surface pressure. Another possible reason for the difference between the measured and the theoretical surface pressures of the buckling layers is the cohesiveness of the film. If the Wilhelmy plate (as it happened in our experiments) is not exactly perpendicular to the moving barrier, the particles cannot easily rearrange themselves behind the plate during the compression. This results in a difference between the surface pressures on the opposite sides of the plate. On one side the layer may wrinkle, having a surface pressure equal to the surface pressure, while on the other side the pressure is lower; thus, the total measured surface pressure is lower than the surface tension. Contact Angles from Film Balance Measurements. Summarizing the results from the contact angles points of view, we can establish that irreversible particle removal can happen in certain cases during the compression of particulate Langmuir films. In these cases contact angles can be determined from the pressure-area isotherms according to eq 4. The measured and calculated values are collected in Table 2 for the appropriate systems. As can be seen, the calculated values are significantly higher than the measured ones due to the dissipative nature of particle removal from the interface, as was mentioned in a previous section of this paper. Considering that particles can have considerable potential energy reaching the collapse point (during the quasi-nondissipative part of compression), we can calculate the contact angles for comparison only from the Vrep term of eq 4. Because of the surface pressure gradient, we obtain the introduced work by taking the area of such a triangle, which is determined by points of Ac, Ao, and the extrapolated (intrinsic) value of the collapse pressure (also see Figure 1). The resultant values also appear in Table 2. As can be seen the “nondissipative” values are in reasonable agreement with the measured ones.
Conclusions The main purpose of the present work was to study experimentally the collapse mechanism of particulate Langmuir
6950 Langmuir, Vol. 22, No. 16, 2006
films in film balances. By compressing partially wettable 75 µm diameter surface modified glass beads at liquid-air and liquidliquid interfaces, different collapse mechanisms were visualized, depending on the wettability of particles. It was suggested that irreversible particle removal could take place in the case of low adhesion energy and relatively high value of surface tension. The experimental results revealed that there is no general mechanism of layer collapse. According to this concept, it was found that the collapse mechanism could change, upon keeping the surface tension constant and increasing the contact angle, from the individual particle removal via a line-by-line mechanism to buckling. The line-by-line collapse mechanism was interpreted in terms of the cohesiveness of the particulate layer and the rate of compression. Without cohesiveness at very low contact angles particles can be expelled out of the interface individually, due to local effects: the polydispersity of particles and structural defects in the compressed layer. At higher contact angles (but below ca. 40°) and at more significant cohesiveness, a surface pressure gradient arises along the film that facilitates the line-by-line collapse mechanism during the irreversible particle removal. At an elevated rate of compression,
Borda´ cs et al.
the number of structural defects that form in the layer increases the effect of local parameters on the mechanism of collapse, facilitating individual particle removal. At higher contact angles (above ca. 40°) a buckling-type collapse mechanism was revealed without particle removal from the liquid interface, as expected from the theory. In this work we distinguished two main (a dissipative and a quasi-nondissipative) parts of surface pressure-area isotherms. After the beginning of the collapse, considerable proportion of the introduced work can dissipate as a result of the dynamics of particle removal (dissipative part of isotherm). Below the collapse pressure, however, the introduced work is mainly applied for increasing the potential energy of particles (nondissipative part of isotherm). In the case of irreversible particle removal, we assessed the contact angles from the nondissipative part of isotherms. These values were found to be in reasonable agreement to those determined directly on the beads. Acknowledgment. This work was supported by the Hungarian National Scientific Foundation for Research (OTKA T049156). LA060696V
