Influence of Polyelectrolyte Layers Deposited on Mica Surface on

Mar 27, 2007 - Dynamic adsorption layer formation and time of bubble attachment to a mica surface in solutions of cationic surfactants (CnTABr)...
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J. Phys. Chem. C 2007, 111, 5743-5749

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Influence of Polyelectrolyte Layers Deposited on Mica Surface on Wetting Film Stability and Bubble Attachment M. Krasowska, M. Kolasinska, P. Warszynski,* and K. Malysa Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, ul. Niezapominajek 8, 30-239 Cracow, Poland ReceiVed: December 11, 2006; In Final Form: February 5, 2007

The degree of hydrophobicity of surfaces is considered a key factor determining the stability of wetting films. They are expected to be stable at hydrophilic surfaces, while at hydrophobic surfaces wetting films should burst leading to three phase contact (TPC) formation. We demonstrated, by studying collisions of air bubbles with mica surfaces covered with polyelectrolyte (PE) layers, that for weakly hydrophobic surfaces (contact angle below 50°) the electric charge of the surface becomes a decisive factor determining the wetting film stability. Comparison of collisions of bubbles in the cationic surfactant solution (DTABr) with mica surfaces located “close” to (10 mm) and “far” from (300 mm) the point of bubble formation indicated the importance of the motion-induced nonequilibrium distribution of surfactant at the bubble surface, which influenced the stability of wetting film formed after the bubble collisions with unmodified and modified mica surfaces.

1. Introduction Wettability of solids is of crucial importance for many processes, especially those aimed at separation, and it is usually quantified in terms of the contact angle (θ) values. The contact angle can be influenced by chemical and/or physical modification of the solid surfaces. In an arbitrary classification solids with θ < 90° are called hydrophilic, while θ > 90° indicates hydrophobic surfaces. Degree of hydrophobicity is a key factor in determining the probability of flotation separation, but one should remember that under dynamic condition of the flotation cell the apparent contact angle (static value) does not need to be attained there. Generally particles showing high contact angles (hydrophobic) can be attached to colliding air bubbles (i.e., thin liquid film between the air bubble and particle should rupture, forming a three phase contact), while the hydrophilic ones (showing low contact) should not be adhered to them. This difference in probability of attachment of the hydrophobic and hydrophilic particles to air bubbles are related to the stability of the wetting film on hydrophobic and hydrophilic substrates. Surface charge is a parameter of crucial importance for stability of the wetting films. According to the DLVO theory, electric double layer repulsion and van der Waals attraction are the main interactions determining thin liquid film stability. Repulsion arises from the electric interactions of surface charges (of identical sign) at interfaces and their range is on the order 1-100 nm, while the van der Waals attractions have a range about 1-10 nm. When a wetting film is formed on a hydrophilic solid surface, the thin liquid layer drains until the wetting film thickness is in the range of the electrostatic interactions and the stability of the film depends on the disjoining pressure acting in the film. Positive disjoining pressure, i.e., repulsive electric interactions between surfaces, can stabilize the wetting film. If we consider that the potential at the gas/water interface (bubble surface) is negative, ca. -35 mV1 to -65 eV,2-4 then, in order to produce an electrostatic repulsion, the double layer potential * Corresponding author. E-mail: [email protected].

at the solid/liquid interface should be also negative. As most of the natural solids are negatively charged, it is commonly accepted that the wetting films at such solids are stable. The opposite situation, of electrostatic attraction, can occur due to either (i) a change of the bubble surface potential (for instance, by cationic surfactant adsorption) or (ii) a change of the surface charge (for instance, via Al3+ ions adsorption5), silanization or methylation,5-8 or polyelectrolyte adsorption9-12 at the solid surface. In such cases (when the interfaces are oppositely charged) the disjoining pressure becomes negative and the wetting film is not stable. The paper deals with the effect of charge modification, either bubble or solid surface, on the stability of the wetting films and attachment of the bubble colliding with the liquid/solid interface. Negatively charged bubble surface was turned positive by adsorption of cationic surfactant, while the solid (mica) surface charge was modified by deposition of polyelectrolytes (PEs). The importance of motion-induced nonequilibrium adsorption coverage at the surface of the rising bubble for the three phase contact (TPC) formation at the modified solid surfaces was studied and described. 2. Materials Freshly cleaved mica (supplied by Continental Trade Ltd. Poland) was chosen as a model solid surface, having an ideal smoothness and being totally wetted by water (contact angle about 0°). Surface properties of mica plates used in experiments were very well characterized,13 and its zeta (ζ) potential was determined to be negative in the range of -100 to -120 mV in water or in dilute electrolyte solution.14-16 The surface charge of mica surface can be modified by covering its surface with different layers, using various deposition techniques.15 The method used in our experiments was electrostatically driven “layer-by-layer” (“LbL”) sequential adsorption of polyions from their solutions.17,18 Poly(4-styrenesulfonate) (PSS, Mw ) 70 000 g/mol) was used as a polyanion and poly(diallyldimethylammonium chloride) (PDADMAC, Mw ) 100 000-

10.1021/jp068486e CCC: $37.00 © 2007 American Chemical Society Published on Web 03/27/2007

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Figure 1. Bubble bouncing and shape pulsations during collision with water/mica interface. Each subsequent frame shows the bubble shape and position at time intervals of 0.845 ms.

200 000 g/mol) was used as a polycation. Both PEs were received from Aldrich. Adsorption of polyelectrolytes at freshly cleaved mica surfaces was performed with NaCl solution of 0.15 M ionic strength and PE concentration of 0.5 g/L. Formation of polyelectrolyte adsorption layers was monitored by changes of contact angle. The adsorption time of 20 min allowed for formation of a saturated layer of polyelectrolyte. Dodecyltrimethylammonium bromide (DTABr) from Fluka was used as a cationic surfactant for modification of the charge of rising bubble surface. Four-fold distilled water was used for the solution preparation. The conductivity and surface tension (t ) 22 ( 0.1 °C) of 4-fold distilled water were 0.2-0.4 µS and 72.4 mN/m, respectively. 3. Methods The experimental setup used in the bubble collision measurements was presented in our previous papers.19,20 It main parts were as follows: (i) a square glass column (cross section 50 mm × 50 mm) of different lengths (40 mm or 350 mm); (ii) glass capillary of 0.075 mm inner diameter mounted at the bottom of the column; (iii) syringe pump (ColeParmer) with glass high precision syringes (Hamilton); (iv) high-speed camera (Weinberger, SpeedCam 512+) for recording the bubble collisions with the solid plates; (v) PC with image analysis software. The solid plates studied were mounted just below the water or solution surface at a distance ca. 10 or 300 mm from a point of the bubble formation (capillary orifice). The time of the bubble formation (interface expansion) at the capillary orifice was ca. 1.6 s. Further details of the setup and the experimental procedure are given in refs 19 and 20. A sessile drop imaging analysis was used to determine the contact angle values. The measurements were carried out in a thermostated chamber (22 ( 22 °C) with regulated humidity to prevent the drop evaporation. Liquid droplets of constant volume were slowly created at the tip of the pipet and placed on the studied surfaces. The CCD camera recorded the image of the sessile drop. The value of the equilibrium contact angle was

obtained by fitting the solution of the Young-Laplace equation to the drop profile and calculating the slope of the profile derivative at the three phase contact line. The details of experimental setup and drop shape analysis are given elsewhere.21 4. Results and Discussion 4.1. Bubble Collisions with Mica/Modified Mica Surfaces in Distilled Water. Figure 1 presents sequences of photos illustrating phenomena occurring during collisions of the rising bubble with freshly cleaved mica surface located at a distance L ) 300 mm from the capillary orifice. As seen there, the colliding bubble bounced backward, changing its shape rapidly. The bubble bounced a few times with diminishing amplitude due to the energy dissipation and finally stayed “arrested” beneath the mica plate (see Figure 1). Even after a long time of the bubble being trapped beneath the mica, there was no three phase contact (TPC) formation and bubble attachment to the mica surface. Quantitative data on the bubble velocity variations during the collisions with hydrophilic mica surface are presented in Figure 2. As seen there, the bubble approached the water/ mica interface with constant velocity of 35 cm/s and bounced, reaching a velocity ca. -30 cm/s, within the time period of 3-4 ms only. Note that positive values of the bubble velocity correspond to the “approach”, while negative correspond to the backward motion (“bounce”). There were a few “approachbounce” cycles, and each subsequent cycle had a smaller amplitude due to the energy dissipation. The bubble motion was fully stopped after ca. 96-97 ms from the moment of the first collision, and the bubble stayed “trapped” beneath the plate without TPC formation (see photo inserted in Figure 2). The fact that the bubble was not attached means that the liquid layer (wetting film) separating the bubble and the mica plate was of high stability and did not rupture. In distilled water the mica surface is negatively charged (ζ-potential within the range -100 to 120 mV).14,16 The bubble surface is also negatively charged, showing surface potential

Wetting Film Stability and Bubble Attachment

Figure 2. Variations of bubble velocity during collision with water/ mica interface in distilled water 300 mm above the capillary orifice.

Figure 3. Variations of bubble velocity during collision with mica/ PDADMAC (open diamonds) and mica/PDADMAC/PSS (filled circles) interfaces in distilled water 300 mm above the capillary orifice.

in the range of -351 to 65 mV.2-4 Thus, both interfaces of the wetting film formed between the bubble and mica surface were negatively charged and there were repulsive electrostatic interactions, which ensured the film stability. As a result of the high stability of the wetting film formed, the TPC was not formed. Therefore, in a subsequent series of experiments the mica surface charge was reversed due to adsorption of the cationic polyelectrolyte PDADMAC (poly(diallyldimethylammonium chloride)). Figure 3 presents quantitative data on variations of the bubble velocity during collisions with mica/ PDADMAC surface. Comparing data presented in Figures 2 and 3, one can observe that the velocity variations during the “approach-bouncing” cycles were practically identical for the bare and PDADMAC-covered mica surfaces. However, a crucial difference was found when the bubble motion was stopped and the bubble stayed arrested beneath the plates. In the case of PDADMAC-covered mica surface the TPC was formed (see Figure 3 and the photo inserted there). Thus, due to charge inversion of the mica surface the wetting film stability was diminished and the TPC was formed at tTPC ) 126 ms, from the moment of the first collision. In this system the interfaces of the wetting film formed were oppositely charged, so electrostatic interactions were attractive and therefore the film was not stable. Data of Figures 2 and 3 show the importance of the solid surface charge for TPC formation and bubble attachment. However, one could also claim that facilitation of TPC formation and bubble attachment resulted from interactions of polymeric

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Figure 4. Variations of bubble velocity during collision with 1 × 10-5 M DTABr/mica interface 300 mm above the capillary orifice.

chains of the PDADMAC molecules with the surface of the colliding bubble. To check if the role of the surface charge is really decisive for TPC formation, a layer of anionic polyelectrolyte, PSS (poly(4-styrenesulfonate)), was deposited on mica/ PDADMAC surface. In such a way the mica plate had its surface negatively charged again. Filled circles in Figure 3 represent velocity variations of the bubble colliding with the mica/ PDADMAC/PSS plate. As seen, the velocity variations were identical as in the case with mica/PDADMAC, but when the bubble motion was stopped, there was no TPC formation at the mica/PDADMAC/PSS plate. Here again, both interfaces of the wetting film formed (between mica/PDADMAC/PSS and the bubble top pole surface) were negatively charged. Thus, there were repulsive electrostatic interaction and high stability of the film. In our opinion these data show convincingly the importance of the electric charge of the interacting interfaces for stability of the intervening liquid layer (wetting film). It is worth to point out that the contact angle for a mica surface covered with PDADMAC was ca. 45 ( 5°, while for mica with bilayer PDADMAC/PSS adsorbed the contact angle was 20 ( 5°, so in both cases the bubble collided with surfaces which according to the common arbitrary classification had rather hydrophilic character. 4.2. Bubble Collisions with Mica/Modified Mica Surfaces in DTABr Solution. Surfactant molecules present in solution adsorb on the bubble surface. Thus, in principle, the negatively charged bubble surface can be inversed due to formation of an adsorbed layer by molecules of cationic surfactant. The ζ-potential at the bubble surface in equilibrium can be evaluated using the theoretical model of ionic surfactant adsorption at the air/surfactant solution interface.22,23 Taking into account that the electric potential of the bubble surface in distilled water (at the gas/distilled water interface) is ca. -65 eV,2-4 the bubble surface potential in the solution of 10-5 M CTABr was estimated as +60 mV. On the other hand, the ζ-potential of mica surface in that surfactant concentration is around -100 mV.15 In Figures 4 and 5, there are presented quantitative data on variations of the local velocity of the bubble colliding in 1 × 10-5 M DTABr solution with bare mica surface (Figure 4) and mica covered by PE layer (Figure 5) plates located at the distance L ) 300 mm. As can be noted, after a series of “approach-bounce” cycles, the bubble did not attach either to bare mica or to mica/PDADMAC/PSS surfaces but stayed “trapped” beneath the plates. Only in the case of the collision with mica/PDADMAC surface the TPC was formed after 124 ms from the first collision (Figure 5). Analyzing data

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Figure 6. Schematic view of distribution of surfactant molecules at the surface of the rising bubble. Figure 5. Variations of bubble velocity during collision with mica/ PDADMAC surface (open diamonds) and mica/PDADMAC/PSS surface (filled circles) in 1 × 10-5 M DTABr solution. The plates were mounted 300 mm above the capillary orifice.

presented in Figures 4 and 5, we can say that (i) the TPC was not formed and the bubble was not attached during collisions with negatively charged surfacessbare mica and mica/PDADMAC/PSSswhile (ii) in the case of positively charged mica/ PDADMAC surface the TPC was formed and the bubble was attached. At first, these results could seem rather surprising and contradictory to the results described above on the importance of electric surface potential in the stability of the wetting films since one could expect that the bubble surface was positively charged in the cationic surfactant solution. However, these results indicate that the top pole of the bubble colliding with the solid plates was not reversed due to cationic surfactant adsorption and it preserved its negative surface charge. Let us discuss in detail the state of adsorption layer at the interface of the rising bubble. During bubble formation the molecules of surface-active substances cover the bubble surface uniformly. Equilibrium adsorption coverage Θeq is attained only when the adsorption kinetics is faster than the velocity of the bubble surface expansion. When the detached bubble starts to rise, then a nonuniform adsorption coverage is induced, which retards fluidity of the liquid/gas interface and causes significant decrease of the bubble terminal velocity.24-27 The surfactant coverage at the upper pole of a bubble is lower than the equilibrium adsorption coverage Θt < Θeq, and there is a continuous supply of surfactant from the bulk phase to the bubble surface (adsorption flux). The adsorption coverage on the rear part of the rising bubble is higher than the equilibrium one, Θb > Θeq, which initiates desorption of the surfactant molecules. This nonuniform surfactant coverage at the bubble surface (see Figure 6) means the existence of the surface tension gradient and retardation of the bubble surface mobility due to the Marangoni effect.26 At a certain distance from the point of bubble formation (capillary orifice), a steady-state dynamic structure of the adsorption layer is established27,28 and the bubble reaches a constant, terminal velocity. Dukhin and Deryaguin showed29-31 that for the systems in which the ratio Γeq/c > 10-4 cm, where Γeq is the equilibrium surface concentration (surface excess) and c is the surfactant concentration in solution bulk, the top part of the bubble surface is practically devoid of any surfactant; i.e., Γtop ≈ 0. Later, it was shown32,33 that even in the case when Γeq/c < 10-4 cm the surfactant surface concentration at the top pole of the rising bubble remains significantly lower than the equilibrium one. The concept of nonequilibrium surfactant distribution was experimentally proven by recording the difference in the bubble lifetime at the free surface of various

surfactant solutions situated at different locations.34,35 When a distance traveled by the bubble through the solution was sufficient for the nonequilibrium surfactant distribution to develop, the shortening of the bubble lifetime was observed. At the lower surface of foam film formed from the top pole of the bubble, surfactant coverage was much smaller than in equilibrium and that led to lower film stability. When the solution surface was close to the point of bubble formations both interfaces constituting the foam film had adsorption coverage close to the equilibrium onesthen the symmetrical film formed was of much higher stability.34,35 Thus, in our case of the bubble rising in DTABr solution the bubble top pole was not inversed because there were no DTABr molecules adsorbed. This explains why there was no TPC formation when the mica surface was negatively charged (bare mica and mica/PDADMAC/PSS): the wetting film formed was stable because both its interfaces were of identical sign. The TPC was formed with positively charged mica/PDADMAC surface because the interfaces constituting the wetting film were oppositely charged. A bubble detaching from the capillary rises initially with local velocity lower than the bubble terminal velocity and nonuniform surfactant molecule distribution over the bubble surface is not yet established there.36,37 As shown in refs 34 and 35, during the initial stage of motion of the detached bubble adsorption coverage was still retained at the top pole of the bubble. Thus, depending on the location of the interface with respect to the point of the bubble formation, two limiting situations can occur (see Table 1): (i) at location “close” there will be adsorbed surfactant molecules at the top pole of the bubble, while (ii) at location “far” the bubble top pole will be devoid of surfactant molecules. This means that the bubble colliding in DTABr solution (cationic surfactant) with mica surfaces located “close” (10 mm above the capillary orifice) would have also adsorption coverage at its top pole, i.e., that the bubble surface would be still positively charged. Thus, for negatively charged solid surfaces there should be wetting film rupture and bubble attachment, while attachment should not be observed in the case of the positively charged solid. The situation should be reversed in the case of the bubble collision with plates located “far”. Figures 7 and 8 show the velocity variations during the bubble collision with bare (Figure 7) and modified by PE deposition (Figure 8) mica surfaces located 10 mm above the capillary in 1 × 10-5 M DTABr solution. As seen in all cases, the bubble approach velocity prior to the first collision was smaller (ca. 24 cm/s) than the terminal velocity, but similarly as in the case of the solid plates located “far”, there were a few “approachbounce” cycles prior to the full damping of the bubble motion. The effect of surface charge of the solid plates was found to

Wetting Film Stability and Bubble Attachment

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TABLE 1: Schematic Illustration of the Electrical Charge Distribution at the Rising Bubble Surface and Liquid/Solid (Modified Mica) Interface for Two Different Locations of the Solid Plates from the Capillary Orifice

exist at times longer than 120 ms from the moment of the first collision. The wetting film ruptured and the TPC was formed in the case of bare mica (tTPC ) 127 ms; Figure 7) and mica/ PDADMAC/PSS (tTPC ) 142 ms; Figure 8) plates, i.e., for the surfaces bearing negative electrical charge. In the case of the positively charged mica/PDADMAC surface (Figure 8) there was no TPC formation; i.e., the wetting film was stable. Thus, the situation was completely reversed in the case when the plates were located “far”, and in our opinion it proves that there were still adsorbed molecules of DTABr at the top pole of the bubble and therefore the surface was positively charged. In Table 1 there are sketches illustrating charges on interacting surfaces of the solid plates and the top pole of the bubbles for locations “close” to and “far” from the liquid/gas interface. There

are also presented photos showing that in the case of the negatively charged solid surfaces the bubble was attached only when the plates were in the location “close” (positively charged bubble top pole). At the location “far” the bubble attachment occurred only for positively charged solid surfaces: the top pole of the rising bubble was negatively charged. These data indicate that when the steady-state distribution of the adsorption layer was established, then the top pole of the rising bubble was devoid of positively charged surfactant cations. Both interfaces of the wetting film formed were negatively charged and the electrostatic repulsive interaction assured the film stability. When the plates were at the location “close”, the motion-induced steady-state adsorption layer was not yet established and the top pole of the bubble rising in DTABr was still positively

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Figure 7. Variations of the bubble velocity during collision with mica surface in 1 × 10-5 M DTABr solution. The plate was mounted 10 mm above the capillary orifice.

Figure 8. Variations of bubble velocity during collision with mica/ PDADMAC surface (open diamonds) and mica/PDADMAC/PSS surface (filled circles) in 1 × 10-5 M DTABr solution. The plates were mounted 10 mm above the capillary orifice.

charged. Therefore, the attachment occurred (see Table 1) at negatively charged mica/PDADMAC/PSS and bare mica surfaces. The fact that at the location “close” the wetting film between the bubble and the hydrophilic (contact angle ca. 21° for a droplet of DTABr solution) mica/PDADMAC/PSS surface was not stable, but was stable in the case of positively charged mica/PDADMAC of higher hydrophobicity (contact angle ca. 44° for droplet of DTABr solution) shows the great importance of electrical surface charge for stability of the wetting film formed.

Krasowska et al. was stable and the bubble remained arrested beneath the surface without the TPC formation. Since the bare water/air interface is negative, our observations indicating the strong effect of the surface charge on the wetting film stability are in agreement with DLVO theory. The rising bubble after collision with liquid/ solid interface bounced several times until it remained immobile underneath the interface. Each subsequent “approach-bounce” cycle had diminishing amplitude due to energy dissipation. Then, depending on the charge of the surface, the bubble arrested or the wetting film between the bubble and the solid surface ruptured and the TPC was formed. In the aqueous solution of cationic surfactant (DTABr) the stability of the wetting film strongly depended on the distance the bubble traveled from the point of its formation. When the interface was located 300 mm from the point of the bubble formation, then identical behavior as in water was observed. The collisions of the rising bubble led to the three phase contact formation only for a positively charged solid surface (mica covered with adsorbed PDADMAC layer). For negatively charged interface (unmodified mica surface and mica surface with adsorbed PDADMAC/PSS bilayer) there was no the TPC formation: the wetting films were stable. On the other hand, when the liquid/solid interface bubbles were 10 mm from the capillary, then the stability of wetting films was reversedsthe stable film was formed only for the bubble colliding with positively charged solid surface. That phenomenon confirms the existence of motion-induced, nonequilibrium distribution of surfactant molecules at the surface of a rising bubble. Adsorption of cationic surfactant leads to a positive surface charge at the water/air interface. However, as a result of viscous drag exerted on the surface of the rising bubble, the nonequilibrium distribution of surfactant over the bubble surface develops and the top pole of the bubble becomes practically free of surfactant molecules. Therefore, the dependence of stability of wetting film formed during the collision of a bubble with the mica surface located 300 mm from the point of bubble formation is the same as in water. When the distance traveled by the bubble is not sufficient for the redistribution of surfactant, its concentration at the bubble top pole is sufficient to render it positively charged, and the stable wetting film is formed only for a positive mica surface with an adsorbed layer of PDADMAC. It is worth mentioning that in that case the degree of hydrophobicity played a secondary role as the wetting film between the bubble and the more hydrophilic mica/PDADMAC/PSS negatively charged surface was not stable. Acknowledgment. This work was partially supported by the NANOCAPS EC Project (NMP4-CT-2003-001428) and MNiSW Grant 3 T09A 164 27. References and Notes

5. Conclusions Dynamic phenomena occurring during bubble collisions with hydrophilic and weakly hydrophobic surfaces, in distilled water and in cationic surfactant (DTABr) solutions, were studied to determine the effect of surface charge on three phase contact formation and the bubble attachment to the solid surface. It was found that the collisions of the rising bubble in water led to three phase contact formation only when the hydrophilic, negatively charged mica surface was turned positive by adsorption of polycation PDADMAC (poly(dimethylamonium chloride)) layer. For unmodified mica surface and mica surface with adsorbed PDADMAC/PSS bilayer, the wetting film formed between the bubble and the negatively charged solid surface

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