Langmuir 2004, 20, 123-128
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Oil Coating on Hydrophilic Surfaces from Emulsions and under Shear Flow Wafa Essafi,*,† Philippe Poulin,† Ste´phanie Chiron,‡ and Bruno Bavouzet‡ Centre de Recherche Paul Pascal, Avenue Albert Schweitzer, 33600 Pessac, France, and RHODIA, Centre de Recherche d’Aubervilliers, 52 rue de la Haie Coq-F, 93308 Aubervilliers, France Received May 27, 2003. In Final Form: September 28, 2003 We study the formation of silicone oil coating on negatively charged hydrophilic surfaces via emulsion deposition. Cationic surfactants usually adsorb and form bilayers onto negative surfaces. As a result, direct emulsions stabilized with cationic surfactants are paradoxically poorly efficient at coating negative substrates. We show in this work an alternative and new method, still based on electrostatic attractions, to coat negative substrates. Our method consists of using emulsions stabilized with nonionic surfactants and of adding to the oil cationic additives that are non-water-soluble and of high molecular weight to minimize their solubilization in the micelles formed by the neutral surfactant. The positively charged droplets stick efficiently onto the substrates. They form monolayer and uniform coatings. We study the kinetics and the density of the resulting coating using a flow cell experiment. This technique allows us to finely analyze the influence of several physicochemical parameters.
I. Introduction The formation of oil coatings on hydrophilic solid surfaces is a critical issue in a wide range of applications such as cosmetics (shampoo), softeners, bitumen, and so forth. For ecological, economical, comfort, or health constraints, the direct deposition of the coating through an organic medium has to be avoided. In these conditions, materials emulsified in water are greatly preferable. Unfortunately, the use of oil in water emulsions (direct emulsions) usually makes coatings more difficult. For example, it is known that direct emulsions stabilized with cationic surfactants are paradoxically poorly efficient at coating hydrophilic substrates that carry negative charges, as widely found in natural or synthetic interfaces. Indeed, cationic surfactants dissolved in the aqueous continuous medium diffuse quickly and adsorb onto the substrate to form positive bilayers. This charge inversion makes the substrate repulsive for the droplets that finally do not deposit. We show in this work an alternative and new method, even though it is still based on electrostatic interactions. Our method to coat negative substrates consists of using emulsions stabilized with nonionic surfactants and of adding to the oil cationic additives that are non-watersoluble. The used additive is a silicone oil derivative with pH-sensitive amino groups. The obtained positively charged droplets stick efficiently onto the substrates. They form monolayer and uniform coatings. We study the kinetics of formation and the density of the coating using a flow cell experiment. This technique allows us to analyze the influence of several physical-chemical parameters: oil concentration, concentration of cationic additives, pH, flow rate, and so forth. The coating process can be governed by the diffusion1 of the oil droplet to the surface in the case of the dilute * To whom correspondence should be addressed. Present address: LPMC/CNRS/UMR 6622, Parc Valrose 06108 Nice-France. † Centre de Recherche Paul Pascal. ‡ RHODIA. (1) Hinrichsen, E. L.; Feder, J.; Jossang, T. J. Stat. Phys. 1986, 44, 793-827.
regime and by the reaction2 between the oil droplet and the surface in the case of the concentrated regime. Since the interactions between the droplets and the substrates are purely attractive, one could expect the coating kinetics to be only limited by the diffusion of the droplets. Surprisingly, our method reveals that the coating is kinetically limited by the reaction between the substrate and the oil droplets. Possible reasons for this behavior are briefly discussed. We finally conclude and give some extensions of this work. II. Materials and Methods 1. Emulsions. The neutral oil used for this study is poly(dimethylsiloxane) of chemical formula (CH3)3-Si-O-[Si(CH3)2O]n-Si(CH3)3, abbreviated as silicone oil and purchased from Fluka. Its density is equal to 0.968, and the viscosity at 25 °C is about 100 mPa s. The charged oil is aminopolydimethylsilane of the chemical formula (CH3)3-Si-[Si(CH3)2-O]148-[Si(CH3)((CH2)3-NH-(CH2)2-NH2)-O]2-Si-(CH3)3 and was obtained from RHODIA Co. The used surfactant is Synperonic A7, a commercial mixture of nonionic surfactants kindly provided by ICI Surfactants. Even though we do not know the exact proportion of this mixture, we note that Synperonic A7 is comprised of alcohol ethoxylate molecules of C13/C15 alkyl chains fully saturated and branched primary. The cationic surfactant used in section III.5 is tetradecyl trimethylammonium bromide, CH3-(CH2)13-N+-(CH3)3 Br(TTAB); it originates from Aldrich, and its critical micellar concentration (cmc) is about 10-3 M. Direct emulsions of oil in water are prepared by incorporating 20% of the oil phase which is an adequate mixture of charged oil with neutral oil, in the aqueous phase containing 0.025% in mass of Synperonic A7. The resulting crude polydisperse emulsion is passed through a cross-flow membrane3,4 made of polycarbonate with a pore size of 1 µm to produce monodisperse emulsions characterized by a Malvern Mastersizer giving a droplet size diameter of 1 µm and a polydispersity of 20%. (2) Dabros, T.; Van de Ven, T. G. M. J. Colloid Interface Sci. 1982, 89, 232-244. (3) Katoch, R.; Asano, Y.; Furuya, A.; Sotoyama, K.; Tomita, M. J. Membr. Sci. 1996, 113, 131. (4) Zhang, L.; Hu, J.; Lu, Z. J. Colloid Interface Sci. 1997, 190, 7680.
10.1021/la0349166 CCC: $27.50 © 2004 American Chemical Society Published on Web 11/27/2003
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Figure 1. The experimental setup: The emulsion sample is injected from a reservoir to pass into the flow cell where the coating will form. The corresponding views are visualized by a microscope linked to a camera and a monitor and recorded by a video recorder. The resulting oil emulsion is stable thanks to the presence of the neutral Synperonic A7 surfactant (amphiphilic molecule having an hydrophilic part and hydrophobic part). These surfactant molecules adsorb on the oil/water interface, create a barrier, and prevent the fusion of the oil droplets. 2. The Substrate. The material used as a model anionic hydrophilic collector substrate is glass. The glass plates were cleaned according to a procedure described in refs 5 and 6. 3. The Flow Cell. The flow deposition experiments were carried out in a parallel plate flow cell, made of two glass plates, spaced by a Teflon spacer. The experimental dimensions of the cell are 55 × 13 × 0.6 mm3. The cell is supplied by a floodgate, an entrance glass tube, and an outgoing glass tube. The flow coming from a reservoir made of a syringe supplies the cell continuously. A syringe pump delivers the required flow rate. It can vary from 5 to 500 mL/h, yielding a Reynolds number between 0.1 and 10, well within the range of laminar flow. 4. The Experimental Setup. The experimental setup is shown in Figure 1. The cell is placed on the stage of a microscope, equipped with a 40× objective. The coating forms preferentially on the bottom side of the upper plate, being the focus plane of the optical microscope. The microscope is linked to a CCD camera and a monitor in order to visualize the images that are recorded by a video recorder. They are then digitized and treated. 5. Data Treatment. We used the NIH software for the image treatment. Each image is built of 512 × 512 pixels. The light intensity on each pixel is characterized on a gray scale by a value between 0 (black) and 255 (white). The level of light of the droplets being markedly different from that of the background, we can easily define a threshold to distinguish the area covered by the droplets. We set all the pixels of the background to 0 and all the pixels of the stuck droplets to 255 and divide the mean gray value of the image by 255 to obtain the coating density.
III. Results and Discussion In this work, we are interested in forming a monolayer of oil droplets on hydrophilic surfaces via aqueous media (from emulsions) and under the presence of a shear flow. The attractive interaction between the oil droplet and the surface is of electrostatic nature. The emulsion used to coat the surface is made of oil phase dispersed in the aqueous phase and stabilized by the neutral surfactant Synperonic A7. The oil phase is a mixture of cationic aminopolydimethylsilane oil and neutral silicone oil. The oil droplet is charged positively owing to the presence of (5) Brzoska, J. B.; Shahidzadeh, N.; Rondelez, F. Nature 1992, 360, 719. (6) Davidovits, J. V. Thesis, Universite´ de Paris 6, Paris, France, 1998.
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the cationic silicone oil at the oil/water interface, and it is so stuck on the hydrophilic glass collector which is charged negatively. Figure 2 shows some views of a coating at different times obtained on hydrophilic glass from an emulsion made of 0.2% of oil in the aqueous phase where the concentration of the Synperonic A7 surfactant is 0.02%. The oil is a mixture of 5% of the aminopolydimethylsilane and 95% of the silicone oil. The pH is 7.4, and the flow rate is 50 mL/h. This coating is resistant to rinsing even at hard conditions. Figure 3 shows the view of the final coating at t ) 3600 s after rinsing with water at a flow rate of 500 mL/h for 30 min. The surface coating density remains constant after the rinsing. In this example, the surface coating density is equal to 35% before and after rinsing. Moreover, this coating is selective; it can be performed only on hydrophilic surfaces and not on hydrophobic surfaces. Figure 4 shows the view of the coating on hydrophobic glass made by chemical grafting of organic molecules of octadecyl trichlorosilane,7 at 3600 s from an emulsion where the oil phase is made of a mixture of 5% of aminopolydimethylsilane and 95% of the silicone oil. The oil concentration is 0.2%, the Synperonic A7 concentration is 0.025% in mass, the pH is 7.4, and the flow rate is 50 mL/h. It emerges that the surface coating density does not exceed 2% at 3600 s on the hydrophobic glass. We were then interested in studying the effect of several physical-chemical parameters on the kinetics of coating formation: 1. Effect of the Shear Rate. Figure 5 shows the evolution of the coating density as a function of time for different shear rates from 50 to 200 mL/h from an emulsion made of 5% of aminopolydimethylsilane and 95% of the silicone oil. The oil concentration is 0.2%, the Synperonic A7 concentration is 0.025% in mass, and the pH is 7.4. For all the shear rates, no oil droplets are removed when they are stuck (no desorption) and no structuration of the coating appears even at high shear rate. Our experimental results show that the shear rate has no effect on the coating kinetics. This means that the kinetics is not limited by the diffusion of the oil droplets toward the surface but only by the reaction between the oil droplet and the surface. This limitation involves a kinetic barrier. This barrier cannot be ascribed to electrostatic interactions since the net interaction between the negative substrate and the positively charge droplets is attractive. Instead, local hydrodynamic interactions due to flow heterogeneities can induce an effective repulsion between the droplets and the substrate. This scenario is more likely in our opinion. Considering nonperfectly homogeneous conditions, one could imagine that some collisions can be inefficient. This behavior means that we are dealing here with a regime that can be considered as concentrated. In this regime, according to the Dabros and Van de Ven model2 based on the Langmuir formalism, the surface coating density S(t) is expressed as a function of an adsorption rate constant kad and a desorption rate constant kdes. It varies with time as follows:
S(t) )
S(∞) [1 - exp(-(kadcp + kdes)t)] kdes 1+ kadcp
(1)
(7) Brzoska, J. B.; Ben Azouz, I.; Rondelez, F. Langmuir 1994, 10, 4367.
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Figure 2. Views of a coating at different times on hydrophilic glass from an emulsion made of 5% aminopolydimethylsilane and 95% silicone oil. Experimental conditions: oil concentration ) 0.2%, Synperonic A7 concentration ) 0.025% in mass, pH ) 7.4, and flow rate ) 50 mL/h (the black bar corresponds to 18.7 µm).
Figure 3. View of the coating after rinsing with water at a flow rate of 500 mL/h for 30 min. The initial coating was first performed in 3600 s from an emulsion made of 5% aminopolydimethylsilane and 95% silicone oil. Oil concentration ) 0.2%, Synperonic A7 concentration ) 0.025% in mass, pH ) 7.4, and flow rate ) 50 mL/h (the black bar corresponds to 13.7 µm).
cp is the oil particle concentration, and S(∞) is the surface fraction covered at infinite time. It can be much lower than 1 because of geometrical constraints, shadow effects (due to hydrodynamical interactions between the deposited
Figure 4. View of the coating on hydrophobic glass at t ) 3600 s, from an emulsion made of 5% aminopolydimethylsilane and 95% silicone oil. Oil concentration ) 0.2%, Synperonic A7 concentration ) 0.025% in mass, pH ) 7.4, and flow rate ) 50 mL/h (the black bar corresponds to 13.7 µm).
oil droplets and the flowing ones8), repulsive interactions between particles, and also microheterogeneity of the surface. (8) Pagonabarraga, I.; Bafaluy, J.; Rubi, J. M. Phys. Rev. Lett. 1995, 75, 461-464.
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Figure 5. Effect of the shear rate on the kinetics of the coating from an emulsion made of 5% aminopolydimethylsilane and 95% silicone oil. Experimental conditions: oil concentration ) 0.2%, Synperonic A7 concentration ) 0.025% in mass, and pH ) 7.4.
Figure 7. Evolution of the coating density as a function of time for different oil concentrations from an emulsion made of 5% aminopolydimethylsilane and 95% silicone oil. Experimental conditions: Synperonic A7 concentration ) 0.025% in mass, pH ) 7.4, and flow rate ) 50 mL/h.
Figure 6. Fit of the experimental data according to eq 2 (the Langmuir fit) for the emulsion made of 5% aminopolydimethylsilane and 95% silicone oil. Experimental conditions: oil concentration ) 0.2%, Synperonic A7 concentration ) 0.025% in mass, pH ) 7.4, and flow rate ) 50 mL/h.
Since there is no desorption of the oil droplets from the surface during the coating process, the experimental results can be compared to the Van de Ven model in the particular case where kdes ) 0 (the Langmuir model). The surface coating density is expressed simply as follows:
S(t) ) S(∞)[1 - exp(-(kadcp)t)]
(2)
We fit our experimental data according to eq 2 (the Langmuir model), by using two adjustable parameters kad and S(∞). Figure 6 shows the validity of this model to analyze the experimental data. The fit gives a value for the adsorption rate kad of 0.0062 s-1 and 0.36 for S(∞). The reaction-limited character of the kinetics means that the sticking probability of the droplets upon collision with the substrates is less than 1. 2. Effect of the Oil Concentration. Figure 7 shows the evolution of the coating on the hydrophilic glass as a function of time t, from emulsions of different oil concentrations, where all the other physical-chemical parameters are kept constant. The emulsions are made of 5% of aminopolydimethylsilane and 95% of the silicone oil. The Synperonic A7 concentration is 0.025% in mass, the pH is 7.4, and the flow rate is 50 mL/h. It appears that the kinetics of the coating increases as the oil concentration increases. The experimental data fit well eq 2 (the Langmuir model) where the coating rate is proportional to the oil particle concentration while the rate constant kad remains constant.
Figure 8. Evolution of the coating density as a function of time for different pH values from an emulsion made of 5% aminopolydimethylsilane and 95% silicone oil. Experimental conditions: oil concentration ) 0.2%, Synperonic A7 concentration ) 0.025% in mass, pH ) 7.4, and flow rate ) 50 mL/h. The insert corresponds to the evolution of the zeta potential of the aminopolydimethylsilane oil droplets as a function of the pH; the corresponding emulsion is stabilized with Synperonic A7.
So an easy way especially in industrial applications to increase the coating kinetics by keeping all the other physical-chemical parameters constant is to increase the oil concentration of the emulsions. 3. Effect of the pH. First, we should stress that the charge of the charged aminopolydimethylsilane oil depends on the pH of the media according to the insert in Figure 8 representing the evolution of the zeta potential of an emulsion made of 100% of aminopolydimethylsilane oil as a function of pH. At pH < 9.2, the oil is charged positively and its charge decreases as the pH increases to reach a zero charge for pH ) 9.2. For pH > 9.2, the oil is charged negatively and its negative charge increases as the pH increases. Figure 8 shows the evolution of the surface coating density as a function of time t, for different pH values. The experimental conditions are as follows: the oil phase is made of 5% of aminopolydimethylsilane and 95% of the silicone oil. The oil concentration is 0.2%, the Synperonic A7 concentration is 0.025% in mass, and the flow rate is 50 mL/h.
Oil Coating on Hydrophilic Surfaces
Figure 9. Evolution of the coating density as a function of time for different concentrations of the charged aminopolydimethylsilane oil. Experimental conditions: oil concentration ) 0.2%, Synperonic A7 concentration ) 0.025% in mass, pH ) 7.4, and flow rate ) 50 mL/h.
It appears that the coating kinetics decreases as the pH value increases. The oil droplets are less positively charged and so less attractive to the substrate as pH increases. The experimental data are fitted according to eq 2 (the Langmuir fit), and both the rate constant kad and the surface coating density at infinite time S(∞) decrease as the pH increases (kad ) 0.0062 s-1 and S(∞) ) 0.36 at pH ) 7.4, kad ) 0.00296 s-1 and S(∞) ) 0.121 at pH ) 8.35). Finally, note that for pH ) 9.6, both the oil droplets and the surface are negatively charged, resulting thereby in an uncovered surface (S(∞) ) 0.011 at pH ) 9.6). 4. Effect of the Charged Aminopolydimethylsilane Oil Concentration. Figure 9 shows the evolution of the coating density as a function of time for different concentrations of the charged aminopolydimethylsilane oil in the oil phase. The experimental conditions are as follows: the oil concentration is 0.2%, the Synperonic A7 concentration is 0.025% in mass, the pH is 7.4, and the flow rate is 50 mL/h. It appears that the kinetics of the coating increases as the concentration of the charged aminopolydimethylsilane oil in the oil phase increases. The experimental data fit well with eq 2 (the Langmuir fit) giving the following parameters: kad ) 0.0062 s-1 and S(∞) ) 0.36 for an oil phase mixture of 5% of aminopolydimethylsilane and 95% of silicone oil; kad ) 0.00068 s-1 and S(∞) ) 0.27 for an oil phase mixture of 0.5% of aminopolydimethylsilane and 99.5% of silicone oil. We remark that the rate constant kad is very sensitive to the concentration of the charged oil (aminopolydimethylsilane). Note that the rate constant kad gives an average value of the effective reaction between the oil droplet and the surface and its value (kad) increases as the charge of the oil droplet increases (i.e., as the concentration of the cationic oil increases in the oil phase). Finally, we should stress that there is an optimum in the increase of the concentration of the aminopolydimethylsilane oil in the oil phase in order to increase the kinetics of the coating. Indeed, if the concentration of the aminopolydimethylsilane oil is increased to 100%, there is no coating on the surface. In the pure state, this oil is highly positively charged and some oil chains in the molecular state can diffuse to the aqueous phase and so adsorb on the surface which becomes inaccessible to the oil droplets. 5. Effect of the Presence of a Competing Cationic Surfactant. We are interested in studying the effect of addition of a cationic surfactant (TTAB) in order to
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Figure 10. Evolution of the coating density as a function of time for different added concentrations of a cationic surfactant, tetradecyl trimethylammonium bromide. Experimental conditions: the oil phase is made of a mixture of 5% aminopolydimethylsilane and 95% silicone oil; oil concentration ) 0.2%; Synperonic A7 concentration ) 0.025% in mass; pH ) 7.4; flow rate ) 50 mL/h. Table 1 cationic surfactant concn (M)
rate constant kad (s-1)
S(∞)
0 0.1 × 10-5 0.5 × 10-5 1.45 × 10-5
0.0062 0.0080 0.0211 0.5558
0.36 0.29 0.20 0.04
investigate the competition between the oil droplet and the surfactant to adsorb on the surface. Figure 10 shows the evolution of the surface coating density by the oil as a function of time for different added concentrations of cationic surfactant. The experimental conditions are as follows: The oil phase is made of a mixture of 5% of aminopolydimethylsilane and 95% of the silicone oil. The oil concentration is 0.2%, the Synperonic A7 concentration is 0.025% in mass, the pH is 7.4, and the flow rate is 50 mL/h. First, we should stress that the concentrations of the added cationic surfactant are so low that we operate practically at surfactant concentrations in the aqueous phase of
