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Confirmation of the Heterocoagulation Theory of Flotation L. Alexandrova,† R. J. Pugh,*,‡ F. Tiberg,‡ and L. Grigorov§ Institute for Surface Chemistry, Box 5607, S-114 86 Stockholm, Sweden, and Department of Inorganic Chemistry, University of Sofia, 1 J. Bourchier Avenue, 1126 Sofia, Bulgaria Received November 18, 1998. In Final Form: April 7, 1999 To model the flotation process, we have used the microscopic method developed by Scheludko et al.,11 to study the stability of an aqueous thin film containing tetradecyltrimethylammonium bromide (C14TAB) between an air bubble and a silica substrate. The experiments were performed at a range of C14TAB concentrations and pH values. Spontaneous rupture of the thin aqueous film was interpreted in terms of the earlier proposed heterocoagulation mechanism and resulted from the preferential adsorption of relatively low surfactant concentrations at the vapor/solution interface causing a net positive charge while the solution/ silica interface remained negatively charged. This attractive electrostatic interaction was sufficient to overcome the van der Waals repulsion. At higher amine concentrations, the negative charge at the solution/ silica interface was reversed. Finally, on approaching the critical micelle concentration (cmc), both interfaces were sufficiently positively charged to cause the restabilization of the film by electrostatic repulsion. In addition, in dilute solution, during the three-phase-contact (TPC) expansion or dewetting step following film rupture, it was suggested that the movement of TPC across the silica substrate leads to transfer of amine from the vapor/solution interface to the vapor/silica. This process resembles a Langmuir-Blodgett deposition process and emphasizes the importance of the solution/vapor interface in the dewetting process.
Introduction At present, the froth flotation process is quite well understood as an optimal combination of (1) a collision between an air bubble and a mineral particle, and (2) the attachment and detachment of mineral particles and bubbles.1 The attachment step involves three-phasecontact (TPC) formation, and this is achieved in practice by an appropriate choice of flotation collectors, pH values, activators, depressants, frothers, and so forth. One of the roles of the collector is to hydrophobize the mineral interface, and for a particle to become attached to an air bubble, the following two consecutive processes must occur successfully: (1) rupture of the thin film between the particle and the bubble surface to produce a contact point, and (2) expansion of the TPC perimeter to produce a sufficiently large contact area to ensure a strong attachment force. For the flotation of silicate minerals, long-chain nitrogencontaining organic surfactants are frequently used as collectors.1,2,6 This has resulted in many fundamental thinfilm and adsorption studies carried out on silica, mica, or quartz in amine solution. Details on film rupture and TPC expansion for long-chain primary weakly ionizable amines (such as dodecylamine hydrochloride) on quartz and mica substrates have previously been reported in several publications.3,4,5 Results from these studies have confirmed * Corresponding author. † Visiting scientist from Institute of Biophysics, Bulgarian Academy of Sciences, Blok 21, Acad. G. Bonchev Street, 1113 Sofia, Bulgaria. ‡ Institute for Surface Chemistry. § University of Sofia. (1) Schulze, H. Physico-Chemical Elementary Process in Flotation Analysis from Point of View of Colloid Science; Elsevier: Amsterdam, 1984. (2) Leja, J. Surface Chemistry of Froth Flotation; Plenum Publishing: New York, 1982. (3) Tchaliovska, S.; Herder, P.; Pugh, R. J.; Stenius, P.; Eriksson, J. K. Langmuir 1990, 6, 1535 (4) Pugh, R. J.; Rutland, M.; Manev, E.; Claesson, P. Int. J. Mineral Processing 1996, 46, 245. (5) Yoon, R. H.; Jordan, J. L. J. Colloid Interface Sci. 1991, 146, 565. (6) Gaudin, A. M. Flotation, 2nd ed.; McGraw-Hill: New York, 1957.
that both the lifetime of the thin films and the TPC expansion rate are strongly affected by the collector concentrations and pH values. The aim of this study is to use a strongly ionizable amine to investigate, in further detail, the influence of flotation collectors on the physicochemical parameter involved in the elementary act of flotation. An interesting, but as yet not fully explained, enigma in the froth flotation process is that effective flotation can be readily achieved at extremely low surface coverages of collector as determined from adsorption isotherms in dilute solution under equilibrium conditions. In fact, flotation in amine solution readily occurs at concentrations as low as 10-7 to 10-6 M, which corresponds to very low surfactant adsorption densities.1,2,6,7 These densities are certainly insufficient to produce an hydrophobic surface. However, in dilute amine solution, the preferential adsorption of relatively low surfactant concentrations at the vapor/ solution interface will cause a net positive charge which will result in film rupture due to heterocoagulation with the negatively charged solution/silica interface. In addition, in the TPC expansion process, collector can readily transfer from the vapor/solution interface. This increases the supply of surfactant molecules to the TPC line, where increased deposition can occur on the vapor/silica interface. Experimental Section Materials and Reagents. For the aqueous thin-film experiments, a smooth, flat, polished silica substrate was used. The surface charge of silica is induced by the ionization of silanol groups (pKa values 6.4 to 8.4). Since the charge on silica increases with pH, from the isoelectric point (pHiep) around 2, the adsorption of amine must be strongly influenced by electrostatics forces which will also vary throughout the pH range. It is therefore important to study these pH effects on the thin film. However, in the case of weakly ionizable amine collectors such as dodecylamine (pKb ) 10.6), which has been used in several earlier studies,3,4,5 acid-base equilibria complicate the solution chemistry and hydrolysis occurs in alkaline-producing nonionic amine species. In the present study, a strongly ionizable amine (7) Wark, I. W. J. Phys. Chem. 1936, 40, 661.
10.1021/la981618d CCC: $18.00 © 1999 American Chemical Society Published on Web 09/10/1999
Confirmation of Heterocoagulation Theory surfactant (tetradecyltrimethylammonium bromide) was used to avoid this situation. This surfactant was supplied by Sigma Chemical Co. and was used without further purification. The critical micelle concentration (cmc) of the surfactant was reported at 3.5 × 10-3 M according to Wangnerud and Olofsson.8 Highly purified water, obtained from a Milli-Q plus 185 system, was used in all the experiments. Receding Contact Angle Measurements. Receding contact angles were determined from the attachment of a silica glass sphere (ballotini, 100-300 µm) to a pendant drop of solution using the microscope technique developed by Mingins and Scheludko in 1979.9 The silica glass spheres, supplied by VEB Glaswerke Ilmenau, Germany, were chosen because they have adsorption characteristics similar to those of the silica plate used in the thin-film experiments. Therefore, the data obtained by the two techniques could be compared. Because the contact angle technique has been used infrequently in the 20 years following the original publication, a brief description is given below. Initially, the glass spheres were cleaned with hot sodium hydroxide followed by hot bichromate mixture, and afterward washed repeatedly with Milli-Q water. A suspension was then prepared from the spheres in the aqueous surfactant solution. After the spheres were dispersed in solution, the dispersion was allowed to stand for over 1 h with continuous stirring in a glass beaker to ensure that equilibrium adsorption occurred on the silica/solution interface. A pendant drop containing the dispersion was then prepared in a small cylindrical glass cell (2 mm radius) which has previously been used in the investigation of foam films.10 This was achieved by dipping the cell into the dispersion, leading to the formation of a pendant drop in which the particles were then allowed to sediment to the lower surface. The cell was then attached to the stage of a photometallographic microscope for observation in reflected light with a magnification factor of ≈100. The microscope has a long-focus objective with the lowest possible background of reflected light, and it was focused on the lower surface of the drop at its most convex part. This enabled the particles to be observed as they collected at the interface. As the particles sediment to the interface, they become visible under the microscope as a set of white rings (ghosts). Those that became attached formed a TPC. This could be distinguished by a black disk at the center with a radius corresponding to the wetting perimeter. The part of the spherical particle that projects from the liquid strongly scatters the light reflected from it, resulting in a black disk with a central bright spot. Attached particles could thus be clearly distinguished from unattached particles, and the diameter of the wetting perimeter of the attached particles could be measured. In the original publication by Mingins and Scheludko,9 glass spheres of up to 44 microns in size were used, and it was found that attachment starts for particles >10 microns at a wetting angle of 20-40°. In the present study, larger particles (100-300 µm) were used and attachment was observed to occur at lower wetting angles. The radii of the particles (R) and the radii of the TPC wetting perimeter (Rtpc) were determined by direct observation with the microscope and recorded by photographs. The equilibrium receding contact angle of the particles was estimated according to the approximate formula Rtpc/R ≈ sin θ. For comparison purposes, the receding contact angle was also determined on a silica substrate by the sessile-drop technique, using the Rame-Hart goniometer and following standard procedures. Film Lifetime and Kinetics of the TPC Line Formed after Rupture of the Thin Aqueous Films. The experimental method employed was based on the original microscopy technique developed by Scheludko et al.11 The method consists of forcing a vapor/solution meniscus from a glass capillary (radius Rc) against a polished substrate, which is submerged in surfactant solution. The apparatus is shown in Figure 1(A). A liquid film (8) Wangnerud, P.; Olofsson, G. J. Colloid Interface Sci. 1992, 153, 392. (9) Mingins, I.; Scheludko, A. J. Chem. Soc., Faraday Trans. 1979, 75, 1. (10) Pugh, R. J.; Manev, E. Langmuir 1992, 8, 2253. (11) Scheludko, A.; Tchaliovska, S.; Fabricant, A. Faraday Discuss. 1970, 112, 1A, 1.
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Figure 1. (A) Scheludko-Tchaliovska apparatus for studying the contact between an air bubble formed in a capillary tubing, a, and a solid surface, c, in a surfactant solution, b. d is a mercury pump controlling the pressure via a valve, e. The position of the capillary meniscus in the manometer, f, is measured with a cathetometer. (B) Approach of a gas bubble before and after the breakup of the film. Rc is the radius of the glass capillary; Rtpc is the radius of the TPC. is formed by thinning to a critical thickness value. The film lifetime was recorded until a TPC nucleus was formed. The TPC arises following the formation of a primary hole, indicating film rupture as illustrated in Figure 1(B). The primary hole expands at constant pressure, but the expansion velocity diminishes drastically within milliseconds and does not tend to approach zero until an equilibrium has been established. At this moment, the meniscus takes an equilibrium shape. To summarize, essentially two kinds of rate processes can be distinguished: (1) The drainage of the liquid film forming between bubble and particle, leading to rupture and a contact point. This process is opposed by a line tension associated with the wetting perimeter; and (2) The expansion of the TPC perimeter following film rupture. A plane parallel, optically polished silica plate was used in the experiments. Earlier experience in the field of wettability has shown that the reproducibility of the experimental results requires special care.12 The plate and cell were therefore left for 1 h until the adsorption at the vapor/surfactant solution and silica/surfactant solution interfaces had equilibrated. The determination of the film lifetime and rate of expansion of the TPC (dRtpc/dT) can be achieved by recording the process either photographically when it is fast or visually on an eyepiece microscope when it is slow. Because the TPC rate is a function of Rtpc for times >80 ms after point contact,1 we found it convenient to express the values in terms of Rtpc determined 0.1 s after the point of contact was observed. This parameter was designated R0.1. It may be noted that several researchers prefer to express the TPC expansion velocity in terms of the so-called timedependent contact angle (dynamic contact angle), because the two parameters are theoretically related.13 Surface Charge Measurements. Electrokinetic measurements were performed with a Zeta Sizer MK 4 (Malvern Instruments, Ltd, UK). The instrument calibration was checked before each experiment using the supplied latex standard and was always within 6% of the quoted value. (12) Alexandrova, L.; Grigorov, L. Colloids Surf. 1998, 131, 265. (13) Scheludko, A.; Toshev, B. V.; Bojadjiev, D. T. J. Chem. Soc., Faraday Trans. 1976, 72, 2815.
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Figure 2. Dependence of the equilibrium receding contact angle (9) on the C14TAB concentration measured on the silica glass spheres using the microscopic technique according to Mingins and Scheludko.9 A and A′ give the average values of the attached and unattached particles. (b) measured by the sessile drop technique using the Rame-Hart goniometer.
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Figure 3. Dependence of equilibrium receding wetting angle θ for attached particles on their radius (R) confirming the random character of the distribution in 10-4 M C14TAB.
Results and Discussions Influence of Surfactant Concentration on the Receding Contact Angle and Thin Film Lifetime at pH 6.5. The interaction between an air bubble and a mineral surface controls the stability of the thin aqueous film. In fact, from details of this interaction, together with adsorption data, a considerable amount of information can be deduced on the effect of adsorbed surfactant on the adsorbed layer structure and the hydrophobicity of the surface. The receding contact angle presented as a function of amine concentration at pH of 6.5 is shown in Figure 2. This plot reveals a strong dependence on the amine concentration throughout the range from 10-7 to 4 × 10-3 M. In the low amine concentration (10-7 M) the particles are completely unattached, indicating wetting, but they become attached with increases in concentration. A maximum contact angle value of 42-45° is observed between 10-5 and 10-3 M in concentration. The values remain reasonably constant. However, at higher concentration (above 10-3 M), the contact angle decreases. This indicates a tendency for the film to restabilize until a completely hydrophilic surface is obtained slightly above the cmc. Throughout the concentration range from 10-6 to 10-3 M, the contact angles formed with the particles are generally about 15 to 20° higher than the data obtained from the sessile-drop experiments carried out on the silica plate. An explanation for this difference is suggested in the later part of this paper. However, the general pattern of the contact angle behavior, in terms of the hydrophilic/ hydrophobic changes, is more or less in agreement with several previous studies carried out with cationic surfactants on quartz, silica, and mica surfaces (determined by the sessile-drop method, as previously summarized in references).14,15 It was of interest to note that as the amine concentrations increased on approaching the cmc, in addition to decreasing in contact angle. Also, the ratio of attached to unattached particles at the interface decreased until a concentration was reached in which approximately only half of the sedimented particles were attached to the (14) (a) Zorin, Z. M.; Churaev, N. V.; Esiponova, N. E.; Sergeeva, I. P. J. Colloid Interface Sci. 1992, 152, 170. (b) Churaev, N. V.; Esiponova, N. E.; Zorin, Z. M. J. Colloid Interface Sci. 1996, 177, 589. (c) Churaev, N. V. Adv. Colloid Interface Sci. 1995, 58, 87. (15) Aronson, M. P.; Princen, H. M. Colloid Sci. 1978, 256, 140.
Figure 4. Dependence of the film lifetime on the C14TAB concentration.
interface. This gives two separate distributions of contact angles, corresponding to attached and unattached particles (two discrete values of wetting angle distribution were obtained, each having a mean value). One population was found to have an average contact angle much greater that of the other family. For example, in 2.5 × 10-3 M amine solutions, two values of 32° (corresponding to the attached) and 6° (corresponding to the unattached) were determined for the two populations. These results are designated by the symbols A and A′, respectively, in Figure 2. As previously discussed by Mingins and Scheludko,9 the origin of this behavior lies in the process of formation of the TPC line, which is opposed by the line tension associated with the wetting perimeter. From a force balance on the particles, an expression for the limiting size of attachment was derived. This effect will be discussed in more detail in a further publication. Also, for each concentration of amine, the ratio of the radius of the wetting perimeter to the radius of the particles was not the same for particles of different sizes; this has been explained by scatter in the wetting angle. Because of this scatter, a statistical treatment of the results is necessary as described in the original publication. From the present study, a typical set of data is presented in Figure 3 for 10-4 M amine solution, where a plot in the form of 2R versus θ is shown. This plot shows the scatter around the average value of θ, and statistical analysis confirms that the distribution of the wetting angle for the attached particle is independent of particle size, in
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Figure 6. Adsorption isotherms for C14TAB at the vapor/ solution and solution/silica interfaces. These data were calculated from previously reported surface tension data22 and previously reported ellipsometry measurements.8
Figure 5. Distribution of film lifetime at three different amine concentrations: (A) 10-5 M C14TAB, (B) 10-4 M C14TAB, (C) 10-3 M C14TAB. tc indicates the average values.
agreement with the original studies of Mingins and Scheludko.9 In Figure 4, a plot is shown relating the average film lifetime to the concentration of surfactant at pH 6.5. Each point on this plot represents an average result obtained from mutiple experiment measurements, and the scatter can be explained by the heterogeneity of the mineral surface. Typical sets of data, showing the distribution of lifetimes at three different amine concentrations, are shown in Figure 5. From Figure 4, it can be seen that initially, stable films are formed in very dilute surfactant (2 × 10-7 M or 1/10000 cmc) solution, but beyond 10-6 M, metastable films with lifetimes of about 0.5 s are formed. Complete rupture begins occurring at about 10-5 to 10-4 M (about 1/100 × cmc). However, on passing through this instability region and reaching higher concentrations, the lifetime begins to increase, and stable films begin to form
upon approaching the cmc. At the higher concentrations, film lifetimes of about 20 s are recorded. Adsorption of Amine at the Vapor/Solution and Solution/Silica Interface. These data are presented in Figure 6 in the form of two isotherms. The adsorption of C14TAB at the vapor/solution interface was calculated by applying the Gibbs equation to previously reported surface tension data following the approach of Bergeron.22 The adsorption of C14TAB at the solution/silica interface has been determined by Wangnerud and Olofsson8 using ellipsometry. Clearly these plots indicate preferential adsorption of amine occurring at the vapor/solution interface in dilute surfactant solution because of the strong hydrophobic interaction compared with the electrostatic adsorption of ions at the solution/silica interface. Adsorption at the solution/silica interface begins to occur at relatively higher bulk concentrations compared to adsorption at the vapor/solution interface. Electrostatic Interaction between the Vapor/ Solution and Solution/Silica Interfaces. Before discussing a possible explanation for the thin-film stability, we must first point out that this is a complex asymmetric system because the surfactant does not adsorb to the same extent at the vapor/solution and solution/silica interfaces. Experimental results can therefore be discussed in terms of electrostatic heterocoagulation theory, where the interactions can be attractive or repulsive depending on differences in adsorbed amine and charge at the two interfaces. This situation is quite different from the typical symmetric films which are usually encountered in colloidal systems. Also, because of the dielectric properties of the silica/water/vapor system, the Lifshitz theory predicts a (16) Sharma, B. G.; Basu, S.; Sharma, M. M. Langmuir 1996, 6506. (17) Pagac, E. S.; Prieve, D. C.; Tilton, R. D. Langmuir 1998, 14, 2333. (18) Nguyen, A. V.; Stechemesser, H.; Zobel, G.; Schulze, H. Proceedings of the XX IMPC, Aachen, Sept 21-26, 1997; p 31. (19) Sandvick, K. L.; Digre, M. Inst. Min. Metall. (London) 1968, 77, 61. (20) Ter-Minassian-Saraga, L. Contact Angles, Wettability, and Adhesion; Gould, R. F., Ed.; Advances in Chemistry Series 43; American Chemical Society: Washington, DC, 1994; p 233. (21) Ter-Minassian-Saraga, L. Advances in Chemistry Series 43; American Chemical Society: Washington, DC, 1964, p 232. (22) Bergeron, V. Langmuir 1997, 13, 3474.
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Figure 7. The surface potential at the vapor/solution interface and the solution/silica interface (ζ potential measurements) versus the concentration of C14TAB.
negative Hamaker constant causing a repulsive interaction with A ) -0.8 × 10-20 J.23 Finally, the characteristics of the film can be changed by the fact that the silica/ aqueous solution interface is initially hydrophilic, but becomes first hydrophobic, and then hydrophilic again with an increase in the adsorbed amine concentration, while the vapor/water interface is initially hydrophobic and becomes more hydrophilic with an increase in adsorbed amine. Under these circumstances, it is necessary to first consider the magnitude of the charge at the vapor/solution and solution/silica interfaces in the amine solution. In Figure 7, the surface potential (ζ potential) at the solution/ silica interface obtained from measurements is shown. In addition, the surface potential at the vapor/solution interface is plotted. These values were calculated from the Gouy-Chapman model:
Ψ0 )
(
2RT asinh zF
zFΓxπ
)
x2r0RTc1000
(1)
where F is the Faraday constant; z is the number of elementary charge of ion (in our case, z ) 1); Γ is adsorption (mol/cm2); r is the dielectric constant; 0 is the dielectric permittivity of a vacuum; and c is the concentration in mol/L in bulk solution. The curves show that the vapor/solution interface has a fairly strong positive charge (about 300 mV) throughout a range of amine concentrations, while the solution/silica interface is weakly negatively charged in the low and intermediate concentration range, with charge reversal (- to +) occurring at about 10-4 M. In Figure 8, a series of the potential energy of interaction curves are shown corresponding to the range of relevant amine concentration and film stability/instability regimes. Based on this information, the stability of the thin film can be explained as follows. In the absence of amine or at very low concentrations, the stability can be explained by the repulsive van der Waals interaction, and assuming a negative charge at both the vapor/solution and solution/ silica interfaces gives rise to a weak repulsive electrostatic double-layer force. In fact, a negative charge (-30 mV to -50 mV) at the vapor/water interface and at the solution/ silica interface (-20m V to -40mV) has frequently been reported in the literature. In the fairly dilute amine solution (10-6 M) at pH 6.5, the adsorption of the amine (23) Israelachvili, J. Intermolecular and Surface Forces, 2nd ed.; Academic Press: New York, 1985.
Figure 8. Interaction energy curves between an air bubble and a flat silica plate in C14TAB solution. Linearized PoissonBoltzmann calculations with Hamaker constant ) -0.8 × 10-20 J (24) 10-4 M 1:1 electrolyte. (a) Vapor/solution interface +300mV and the solution/silica interface +60 mV. (b) Vapor/ solution interface +300mV and the solution/silica interface +20 mV. (c) Vapor/solution interface +300mV and the solution/silica interface -20 mV.
at the vapor/solution interface will begin to reverse this charge. However, at these concentrations very little amine will adsorb at the solution/silica interface so that the surface will retain a negative charge up to 10-4 M (about -20 to -30 mV, according to the ζ potential measurements). Spontaneous thin-film ruture within this critical concentration range can be interpreted in terms of the heterocoagulation. This electrostatic interaction must be sufficient to overcome the van der Waals repulsion, and it appears to be confirmed by the potential energy interaction curves in Figure 8. When the amine concentration is increased to beyond 10-4 M solution, adsorption on the silica surface begins to occur more strongly because of the binding of the positively charged headgroups to the negative surface sites produced by dissociation of silanol groups. This causes charge reversal (- to +) at the solution/silica interface and results in a repulsive interaction, and this is again confirmed from the potential energy curves. From ellipsometry8 it was estimated that at pH 6, only about 0.3 mg/m2 (about 1 molecule/170 Å2) adsorbed from 1 × 10-3 M solution. At higher amine concentrations (2 × 10-3 M solution), about 0.8 mg/m2 (about 1 molecule/65 Å2) adsorbed. Finally, on or near the cmc, saturation occurs, producing a plateau on the isotherm at 1.5 mg/m2 (about 1 molecule/35 Å2). According to these data, it is unlikely that a monolayer is formed at the solution/silica interface. The buildup of positive charge can be explained by the adsorption of surface micelles or bilayer fragments on the silica substrate reversing the charge. This picture appears to be in general agreement with ideas reported in several recent publications16,17 which suggest that the traditional picture of the bilayer on a solid substrate is oversimplified and that so-called aggregate adsorption takes place in small patches or isolated aggregates before the completion of the bilayer. On the silica surface at pH 6.5, with only a limited number of widely distributed neutralized sites, additional surfactant will tend to aggregate (through the hydrophobic tails). An aggregated surface micellar layer, rather than a classical continuous structure formed on the silica surface, can be suggested. At these higher surfactant concentrations, repulsion can be explained by electrostatic interactions from the positive part of the adsorption bilayer and the positively charged adsorption at the vapor/solution
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Figure 9. Schematic diagram describing the surfactant distribution and structure of the wetting film. (A) 10-7 M partially stable wetting film due to a weak negative charge on both interfaces and the repulsive van der Waals forces. (B) 10-6-10-3 M unstable due to adsorption of amine leading to compensation of negative charges at the vapor/solution interface and heterocoagulation overcoming the repulsive van der Waals forces. (C) >10-3 M cluster of adsorbed aggregates and beginning of charge reversal and partial restabilization. (D) 3 × 10-3 M stable film due to the strong positive charge on both silica substrate and vapor/solution interface causing electrostatic repulsion and repulsive van der Waals forces.
Figure 10. TPC velocity (expressed as R0.1, the radius of the TPC perimeter after 0.1 s) versus C14TAB concentration.
interface. Models for this proposed heterocoagulation mechanism are illustrated in Figure 9. Influence of Amine Concentration on the TPC Expansion at pH 6.5. In Figure 10, the plot of R0.1 versus amine concentration is shown. This plot indicates a gradual increase in R0.1 with increasing surfactant concentration up to 10-4 M. Beyond this concentration, a gradual decrease of R0.1 takes place. It is this region, on approaching the cmc, that corresponds to restabilization of the film. According to Schultze,1 the three forces which are important in determing the TPC velocity are the driving capillary forces, the inertial forces, and the frictional forces (surface viscosity). Also, the surface viscosity term is strongly influenced by the disturbance of the adsorption/desorption equilibrium during the TPC motion. In considering the TPC velocity, it was suggested earlier that relatively lower concentrations of unassociated surfactant coverages can give higher TPC mobility, while
Figure 11. Dependence of the film lifetime (2), equilibrium receding contact angle (b) measured by the silica spheres using the microscopic technique according to Mingins and Scheludko9 and TPC velocity (expressed as R0.1, the radius of the TPC perimeter after 0.1 s) (9) versus pH in 10-4 M C14TAB.
lower TPC mobility was reported for aggregated multilayers.1 However, the mechanism of movement of the TPC is complex because in addition to the above factors, surface roughness and heterogeneity specific to the substrate are important. In a recent study involving the spontaneous movement of the TPC line across the surface of a spherical glass particle, a correlation between the TPC velocity and the degree of hydrophobicity was reported,18 but in this case the hydrophobicity of the surface was achieved by methylation of the silica surface. Under these circumstances it was unnecessary to consider the adsorption/ desorption discontinuity on the two sides of the wetting line. Influence of pH on Receding Contact Angle, ThinFilm Lifetime, and TPC Velocity. In Figure 11, the
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Figure 12. Schematic diagram describing the proposed mechanism of expansion of TPC after point contact formation. Rapid transport of surfactant molecules from the dynamic vapor/solution interface to the vapor/silica interface involves a LangmuirBlodgett type deposition mechanism and results in an increase in the amount of adsorbed amine at the vapor/silica interface.
results, which include the receding equilibrium contact angle, film lifetime, and TPC velocity, are presented for experiments in 10-4 M C14TAB (< 1/10 × cmc) at a range of pH values. A fairly pronounced increase in contact angle is shown to occur from about pH 2.5 to 4, followed by a more gradual increase to pH 11. This behavior can be explained by an increase in the adsorbed amine as the surface charge on the silica/solution interface is increased with pH from the pHiep. As the surface density of amine is further increased, charge reversal can occur, leading to heterocoagulation between the positively charged vapor/ solution interface and the negatively charged solution/ silica substrate. This behavior correlates with the decrease in film lifetime from pH 4, and the most unstable films (with lifetimes 10-3 M) approaching the cmc, adsorption at the solution/silica interface began to play an important role leading to patches or aggregates; or, partial bilayer formation occurred resulting from a hydrophilic molecule forming as on the hydrophobic layer (before the completion of the monolayer). This causes a reversal of charge at the solution/silica interfaces, leading to an increase in the film lifetime and a decrease in the
Confirmation of Heterocoagulation Theory
contact angle resulting from electrostatic repulsion forces. Interaction energy curves for the relevant charged interfaces are presented and tend to confirm the theory. Experiments carried out in 10-4 M amine or 1/10 × cmc throughout the pH range show an increased contact angle and a decrease in film lifetime with a corresponding increase in pH. This corresponds to an increase in adsorption of amine at the vapor/solution interface, causing charge reversal. Again, a heterocoagulation model can explain the thin-film instability. However, in the three-phase boundary expansion step (initiated after film rupture), an increase in TPC expansion velocity was found to occur with increase in amine concentration and increase in pH. Because the amount adsorbed at the solution/silica interface (in dilute solution), from which the liquid was receding, was relatively low, it was suggested that a transfer of collector from the vapor/ solution interface to the vapor/silica interface occurred, producing an increase in adsorption density. This process
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resembles Langmuir-Blodgett deposition and occurs at extremely low pressures at the solution/vapor interface. A rapid transport of surfactant molecules along the vapor/ solution interface would be essential for this mechanism. This study appears to reinforce the work of Tchaliovska et al.,3 who strongly emphasized the importance of the solution/vapor interface which aids the deposition adsorption process in two ways. First, surfactant ions are transported by means of surface streaming induced by a surface tension gradient, and second, the surfactant may form a condensed monolayer under conditions prevailing at the TPC line, which can be transferred subsequently to the silica surface upon moving the contact line layer. Acknowledgment. L.A. thanks Akzo Nobel Surface Chemistry AB, Sweden, and the Bo Rydin Foundation, Sweden, for partial funding of her research studies at the Institute for Surface Chemistry. LA981618D
