Langmuir 1990,6, 1535-1543
1535
Studies of the Contact Interaction between an Air Bubble and a Mica Surface Submerged in Dodecylammonium Chloride Solution Slavka Tchaliovska,tJ Peter Herder,t Robert Pugh,t Per Stenius,? and Jan Christer Eriksson* Department of Physical Chemistry, Royal Institute of Technology, S-100 44 Stockholm, Sweden Received December 16, 1988. I n Final Form: January 25, 1990 A microscope-based observation method developed by Scheludko and Tchaliovska has been employed to study the wetting properties of hydrophobed as well as initially clean, freshly cleaved muscovite mica submerged in dodecyl ammonium chloride (DAC) solution. The observations made and the results obtained on thin-film lifetimes, contact angle hysteresis, and rates of expansion of the meniscus perimeter reflect the importance of hydrophobic attraction forces as well as of attractive and repulsive electrostatic forces. For clean mica in the very dilute regime, extensive hydrophobation occurs only from the three-phase contact line where a condensed surfactant monolayer apparently forms as a result of electrostatic and hydrophobic attraction. For hydrophobed mica, there is a transition from water-like wetting behavior with marked contact angle hysteresis to hysteresis-free wetting, taking place at about CDAC = 1pmol/L. At higher surfactant concentrations, CDAC > 1 mmol/L, the wetting properties of initially clean mica and hydrophobed mica, respectively, are quite similar. These latter results are in agreement with parallel surface force measurements. It is inferred that a high flotation recovery of mica can only be achieved when the mica surface is well-covered by a dense, hydrophobic surfactant monolayer. This provides a rationale for "bubble conditioning" prior to the flotation proper whereby surfactant deposition takes place from the three-phase boundary.
Background By means of surface force measurements using the Israelachvili apparatus, it has been demonstrated that there is a comparatively strong and long-range attractive force between two hydrophobed mica surfaces immersed in and likewise in the case of a clean mica surface approaching a hydrophobed mica surface.6 So far, the origin of this hydrophobic attraction has not been fully clarified, b u t experimental as well as theoretical investigations are in progress with t h e purpose of elucidating this question. A preliminary view is that hydrogen-bonding effects, when combined with the local ordering constraints imposed by a rigid, hydrophobic wall on the adjacent water molecules, can give rise to these rather strong and surprisingly long-range forces.6 This would imply that the free energy disadvantage associated with the contact between water and a hydrophobic surface in a sense is propagated some 50 nm out from the surface and that the overlap between such interfacial zones yields a reduction of the overall excess of the free energy (Le., of the film tension), resulting in attraction. Regardless of the rather fundamental scientific issue about the exact cause of the hydrophobic attraction, the
* Author t o whom correspondence should be addressed. Department of Physical Chemistry, Royal Institute of Technology, S-100 44 Stockholm, Sweden. t Institute for Surface Chemistry, Box 5607, S-114 86 Stockholm, Sweden. On temporary leave from the Department of Physical Chemistry, Sofia University, 1 Anton Ivanov Av., 1126 Sofia, Bulgaria. (1) Israelachvili, J. N.; Pashley, R. M. J. Colloid Interface Sci. 1984, 98,500. (2) Claesson, P.M.; Blom, C. E.; Herder, P. C.; Ninham, B. W. J. Colloid Interface Sci. 1986, 114, 234. (3) Rabinovich, Y. I.; Derjaguin, B. V. Colloids Surf. 1988,30, 243. (4) Claesson, P. M.; Christenson, H. J. Phys. Chem. 1988, 92, 1650. (5) Claesson, P. M.; Herder, P. C.; Blom, C. E.; Ninham, B. W. J. Colloid Interface Sci. 1987,118,68. (6) Eriksson, J. C.; Ljunggren, S.;Claesson, P. M. J. Chem. SOC., Faruday Trans. 2 1989,85, 163.
*
question arises to what extent forces of this nature are responsible for the primary processes in the conventional froth flotation of mineral ores. A successful ore flotation depends critically on the capillary and surface-chemical events which take place when an air bubble contacts a collector-covered mineral s u r f a ~ e The . ~ ~ collector ~ evidently acts as a hydrophobizing agent, and hence it is not unreasonable to presume, as has sometimes been done in the past, that hydrophobic attraction plays an essential part a t the thinning and rupture of the water solution film which at first separates the mineral surface and the air bubble? Accordingly, the lifetime of this film would largely be determined by the net attractive force operating before the critical thickness is reached.1° After the rupture of the film, t h e kinetics of t h e three-phase boundary movement across the hydrophobed mineral surface might be of considerable importance."J2 In order to study these primary events of froth flotation, we have carried out a study, using microscopy, of the contact between air and a clean or separately hydrophobed mica surface submerged in water and dilute dodecylammonium chloride (DAC) solutions of various concentrations. The state of the DAC adsorption layers on mica has been explored previously by the conventional solution method13 and also through ESCA analysis14and surface force measurements.15 Furthermore, as to the hydrophobed mica substrate, corresponding surface force (7) Scheludko, A. Adu. Colloid Interface Sci. 1967, 1 , 391. (8) Scheludko, A.; Radoev, B.; Fabrikant, A. Ann. Uniu. Sofia, Fac. Chemie 1968/69,63,43. (9) Pugh, R. J. Colloids Surf. 1986, 18, 19. (10)Scheludko, A. Kolloid-Z. Z. Polym. 1963, 191, 52. (11) Scheludko, A.;Tchaliovska, S.; Fabrikant, A. Spec. Discuss, Furaday SOC. 1970,1,112. (12) Tchaliovska, S. Thesis, Sofia University, 1986. (13) Cases, J. M.; Mutaftschiev, B. Surf. Sei. 1968, 9, 57. (14) Herder, P. C.; Claesson, P. M.; Blom, C. E. J. Colloid Interface Sci. 1987, 119, 155. (15)Herder, P. C. J. Colloid Interface Sci. 1990, 134, 336.
0 1990 American Chemical Society 0143-1463J90~24Q6~1535$Q2.5Q/Q
Tchaliouska et al.
1536 Langmuir, Vol. 6, No. 10, 1990
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Figure 1. ScheludkwTchaliovska apparatus for studying the contact between an air bubble, formed in a capillary tubing a, and a solid surfacee. d is a mercury pump controlling the presure via a valve e. The position of the capillary meniscus in the manometer f is measured with a cathetometer. The microscope M is most conveniently furnished with video accessories. measurements16 and recent theoretical calculations1'JS provide a good background about the state of the DAC monolayers formed. It appears from the present paper that this auxiliary knowledge is essential for t h e general understanding of the phenomena observed in the wetting investigations. T h e experimental method employed b y us is a microscopy technique developed by Scheludko and Tchaliovska12 which enables careful observations and measurements of (i) the lifetime ( 7 ) of thin water films on solid substrates, (ii) the kinetics of moving the film/ surface-meniscus contact line, and (iii) the contact angle hysteresis when changing the direction of motion of the contact line. This method has been used previously in a similar study of t h e events occurring when an air/ solution interface of a dilute surfactant solution is brought in close contact with a quartz glass surface.12 An analogue method has also been applied recently when studying the wetting of a solid by two immiscible liquids.'g However, to make use of DAC solutions and mica, the substrate offers the additional advantage that the state of the surfactant adsorption layer is characterized more completely through previous ESCA and surface force investigations. Moreover, freshly cleaved mica is a molecularly smooth substrate.
Methods and Materials In these experiments, freshly cleaved mica samples were used as substrates in the form of thin sheets measuring about 10 x 10 mm in size. The muscovite mica was purchased from Brown Mica Co., Sydney, Australia. As was mentioned above, the quantification of the wetting properties of the samples was accomplished by means of the method developed by Scheludko and Tchaliovska12on the hasis of earlier works by ScheludkoZ0and Platikanov.zl In this method, the approach of the solution/air interface toward a solid substrate is carefully geared by using a mercury screw pump (Figure 1). The formation of a thin film and the expansion of the meniscus perimeter are monitored by means of a metallographic microscope and an attached video (or film) camera. By changing the meniscus pressure, the wetting perimeter can he forced to move, either outwards or inwards, and receding as well as advancing contact angles below about 75" can he determined with a high precision. Contact angle hysteresis is readily detected hy recording the change in pressure, Ap8 needed to (161 Herder, P. C . J. Colloid Inrerfoco Sri. 1990. 134,346. (17) Erikwon, J. C.; Ljunggren, S . Colloids Sur/. 19R9. .W.179. t 18) Eriksson. J. C . :Ljunggren. S . plngr. Colloid Poljm. Sci. 19R8. 76. 188 !I91 Foixer, R. T. J. Colloid Inler/ore Sri. 1981. 116. 109. (20)Scheludko. A. Thesis. Institute of Physical Chemistry of the Academy of Sciences of the USSR, Moscow. IYBI. 121, Plal~kanuv.D. J. Ph,r. Chem. 1964.6h. 'XI9
Figure 2. Approach of an air huhhle before (a, left) and after (b, right) the breakup of the film. R is the radius of the capillary tubing, OR the contact angle against the tube wall. and 8. the contact angle against the solid substrate. alter the direction of motion of the meniscus perimeter. Furthermore, the lifetime, T, before rupture of the thin film can he measured as well as the rate of expansion of the three-phase boundary. In early works,'2 these latter parameters have proved to be extremely sensitive to the chemical state of the substrate surface. The central part of the equipment used is shown in Figure 1. A liquid meniscus is pushed through the capillary tubing a onto the solid surface e. The pressure is controlled with a mercury pump d and is measured by means of a manometer f using a conventional cathetometer. When the capillary pressure p , = 2 y / R cos 8~ (Figure 2a) is reached, a thin liquid film is formed. Eventually,this first film may rupture and a secondary thin film or a regular three-phase contact boundary is generated. The thin liquid film and the wetting perimeter can be observed through the metallographic microscope M and recorded by using a video cameraltape recorder setup. Hence the lifetime, T , of the thin film can be determined as well as the rate of expansion of the circular contact line. Flotation. A modified Hallimond tube similar to the one described by Fuerstenau et aLZ2was used in the flotation experiments, Bubbles were generated by flowing nitrogen through a glass frit at a controlled flow rate. The flotation procedure was as follows: a standard quantity of ground mica (2 g) and some fresh water were added to the flotation cell. The solution (170 mL), containing known amounts of collector, was poured carefully over the wet mineral grains before conditioning took place by agitation with a magnetic stirrer. Two series of flotation experiments were carried out: (i) after conditioning the system at the solid/solution interface only and (ii) after conditioning by means of forming solid/air/ solution contacts. This was accomplished by passing a gentle flow of nitrogen bubbles through the suspension during the conditioning. After completion of the conditioning step, the particles were allowed to settle, the supernatant solution was discarded, and a fresh quantity of solution was introduced into the cell. Nitrogen gas was then introduced at a predetermined rate (50 mL/min), and the time of bubbling was maintained at 5 min for all flotation tests. A t the end of the period, the gas flow was terminated and the amount of floated mineral was determined hy gravimetric analysis. Materials. Dodecylammonium chloride (DAC) was ohtained from Eastman Kodak. Surface tension measurements gave no indication of impurities in the DAC solutions, and the critical micelle concentration (cmc) found (14 mmol/L) was in good agreement with values in the literature. The water used in the experiments was treated by the following consecutive steps: decalcination and prefiltration srith activated charcoal followed by treatment with a reversed osmosis unit, two mixedbed ion changers, activated charcoal, Zetapore filter (0.1 pm), Organex, and a final filtration with a Zetapore filter (0.2 pm). All purification units were Millipore products except for the filters (Cuno). Dioctadecyldimethylammonium bromide (DDOAB) was supplied by Eastman Kodak in recrystallized form. The hydrophobed mica samples were prepared by means of LangmuirBlodgett deposition from a DDOAB monolayer spread on a surface balance manufactured by KSV Chemicals, Helsinki, Finland. The barrier was made of a hydrophobic polymer with a water contact angle to 60". During deposition, the barrier was (22) Fuerstenau, D. W.;Metzher, P. H.: Seele, D. G.Eng. Min. 1957, 158,93.
Langmuir, Vol. 6, No. 10, 1990 1537
Wetting Properties of Mica
Table I. Data Obtained for Hydrophobed Mica Samples CDAC,~
mol
L-’
hwYb nm
0 10-1 10” 3 x 10” 10-5 10-4 10-3
10-2
7:
s
0.4 2.7 0.4 0.6 1.3 139 65 21
5 8
ArlAt,d cm
r: cm
1.6 1.3 1.4 2.5 2.5 2.0 2.4
0.081 0.084 0.100 0.115 0.120 0.105 0.115
m
8,j deg
54 57 56 90 90 90 90
---
8,,f deg
90 90 65
--9090
-
90 -90
comment
slow reduction of hysteresis with time no hysteresis no hysteresis droplets on film surface after 10-20 s no hysteresis droplets on film surface after 10-20 s no hysteresis droplets on film surface after 10-20 s stable film
*
Surfactant concentration. Equilibrium thickness of corresponding free soap film. Lifetime before rupture. Perimeter expansion rate for At = 0.04 s. e Radius of thin film. f Receding contact angle. 8 Advancing contact angle. a
Table 11. Data Obtained for Initially Pure Mica Samples comment
0 10-7 10” 3 x 10” 10-5 10-4 10-3
10-2
m
98 58 27
0.21 0.9 5 10 m
2.4 2.3 3.8
0.03 0.12 0.13 0.13
--
19 70 90 90
52 90 -90 -90
stable film patchwise film rupture upon moving the patchwise film rupture upon moving the patchwise film rupture upon moving the patchwise film rupture upon moving the no hysteresis, occasionally stable film no hysteresis, occasionally stable film stable film
perimeter perimeter perimeter perimeter
Surfactant concentration. Equilibrium thickness of thin film.Z4 Lifetime before rupture. Perimeter expansion rate for At = 0.04 s. e Radius of thin film. f Receding contact angle. 8 Advancing contact angle. a
coupled to a feedback mechanism, in order to keep the surface pressure constant a t 20 mN/m-’. T h e advancing water contact angle was found t o be 98O, i.e., slightly higher than obtained previously (94’) using a slightly different deposition procedure.2 According to available ESCA results, this implies that the surface area occupied per each DDOA+ ion is about 50 A2,which is close to the surface area of each negative site on the mica surface.23
Theory To summarize, essentially two kinds of rate processes can be distinguished: (i) the thinning of the liquid film to its critical thickness and (ii) the formation and expansion of the three-phase contact after film rupture. Before film rupture, the capillary pressure is where R denotes the radius of the capillary tubing. After the rupture, it is
r cos OR - r sin 6, (2) R2 - r2 where OR is the contact angle against the capillary tube wall whereas Or denotes the contact angle against the solid substrate (cf. Figure 2b). Combining eqs 1 and 2 and assuming OR = 0 yield Py
= 27
sin Or = r / R (3) Thus the stable radius, r, of the wetting perimeter, which can easily be measured, gives the contact angle 6, against the solid substrate. In our equipment, the radius R of the capillary tubing was equal to 1.0 mm. Note that according to eq 3 the accuracy of the 6, determination i s worse when 0, approaches 90’. Slowly decreasing the capillary pressure by Ap causes the wetting perimeter to move slightly inwards. The decrease in pressure, Ap, needed to bring about the movement of the perimeter yields information about the advancing contact angle 6,:12 (23)Claesson, P. M.Thesis, The Royal Institute of Technology, 1986. (24)Claesson, P.M.;Herder, P. C.; Stenius, P.; Eriksson, J. C.; Pashley, R. M. J . Colloid Interface Sci. 1986,109, 31.
A P ( R -~ r2) + sin 6, (4) 27r Thus, when Ap = 0 there is no contact angle hysteresis. sin Oa =
Results The results of our measurements are summarized in Tables I and 11. Hydrophobed Mica. For the hydrophobed mica (Table I), its general wetting behavior toward the DAC solution of different concentrations can be characterized in broad terms as follows. A t low CDAC (
