Journal of Colloid and Interface Science 337 (2009) 555–562 Contents lists available at ScienceDirect Journal of Colloid and Interface Science www.elsevier.com/locate/jcis Dynamic contact angles on PTFE surface by aqueous surfactant solution in the absence and presence of electrolytes Rajib Ghosh Chaudhuri, Santanu Paria * Department of Chemical Engineering, National Institute of Technology, Rourkela 769 008, Orissa, India a r t i c l e i n f o Article history: Received 8 January 2009 Accepted 13 May 2009 Available online 21 May 2009 Keywords: Teflon Dynamic contact angle Wetting Electrolyte a b s t r a c t This study presents the experimental results on dynamic contact angles of pure surfactants and surfactants with electrolyte solutions on PTFE (Teflon) surface. Dynamic advancing (hA) and receding (hR) contact angles measurements by the Wilhelmy plate technique were carried out for aqueous solution of three different surfactants Triton X-100 (TX-100), sodium dodecylbenzene sulfonate (SDBS), and cetyltrimethylammonium bromide (CTAB). The same measurements in the presence of different electrolytes NaCl, Na2SO4, and CaCl2 for ionic surfactants (SDBS and CTAB) were also carried out to see the change in contact angle and wetting behavior. The presence of electrolytes changes the advancing contact angle as well as wetting properties of hydrophobic solid surface significantly even at very low surfactant concentration. Counter ion valency of the electrolyte is more important in reducing advancing contact angle on hydrophobic PTFE surface at very low concentration of ionic surfactants from CMC. Pure surfactants and ionic surfactants in the presence of electrolytes show a linear relationship between the adhesional tension and surface tension at air–water interface with different slope and intercept. Ó 2009 Elsevier Inc. All rights reserved. 1. Introduction The wetting properties of solid materials are of both fundamental and practical importance due to their wide range of applications such as detergency [1], liquid surface coating [2–4], flotation [5,6], chemical reactions at solid–liquid interface [7], agrochemicals, flows in reservoirs [8–11], and mass transfer in packed column [12]. Understanding and characterizing the wettability of solid surfaces are thus highly essential. Proper wetting of hydrophobic solids with aqueous solutions becomes difficult due to low surface energy of the hydrophobic surfaces. Since water has a high surface tension (72.8 mN/m) it does not spontaneously spread over a solid whose surface free energy is less than 72.8 mN/m [13]. Thus, for wetting of a solid by water which has surface free energy smaller than 72.8 mN/m, we need to add surfactants to reduce the surface tension. Wettability of a liquid on a solid surface generally depends on the physical and chemical surface characteristic of the solid, contact angle and the surface tension of the liquid. The contact angle can be measured on a flat surface by different common methods [14,15]. The contact angle, which is measured under the condition where the three-phase contact line is moving with respect to the surface, is referred to as ‘‘dynamic contact angle” (DCA). In DCA measurement, the advancing contact angle (hA) can be measured during solid plate immersion, and the receding one (hR) during the emersion process. When hA – hR the system is * Corresponding author. Fax: +91 661 246 2999. E-mail addresses: santanuparia@yahoo.com, sparia@nitrkl.ac.in (S. Paria). 0021-9797/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2009.05.033 said to exhibit contact angle hysteresis (H), which is attributed to chemical heterogeneities [16,17] or roughness [18,19]. Miyama et al. (1997) [20] mentioned that when the hysteresis occurs due to the motion of the surface with respect to the water line is called ‘‘dynamic hysteresis”, and when it occurs due to the intrinsic change of surface configuration or surface caused by wetting of the surface by water can be termed as ‘‘intrinsic hysteresis”. H can be calculated by subtracting the maximum advancing contact angle with the minimum receding contact angle (hA  hR), or in the form of (cos hR  cos hA). But in the case of dynamic contact angle, the hysteresis value depends on the spreading velocity of the contact line (three-phase line) [15] and flow flied [21,22]. If the measurement speed is very slow, dynamic values tend to equilibrium contact angle [21]. Although the fundamental studies of equilibrium wetting and contact angle have been explored well [23–25], but the dynamic process, which has many practical applications is not well understood [21,26]. The equilibrium contact angle is related to advancing and receding contact angle according to [20,27] he ¼ 0:5 cos hA þ 0:5 cos hR ð1Þ The equilibrium contact angle (he) is represented in terms of interfacial tensions by well-known Young’s equation, which depends on liquid–air, solid–air, and solid–liquid interfacial tension cLG cos he ¼ cSG  cSL ð2Þ where cSL, cLG, and cSG are the interfacial tensions between the solid–liquid, the liquid–gas, and the solid–gas, respectively. 556 R.G. Chaudhuri, S. Paria / Journal of Colloid and Interface Science 337 (2009) 555–562 Adsorption of surfactants at air–liquid interface reduces the liquid surface tension, which may lead to better wettability of solid surfaces by the liquid; however, the adsorption of surfactants at solid–liquid interface not always reduces the solid–water interfacial tension. Therefore, it is very difficult to establish the conditions of spreading of liquid solution of the surface-active agents over the solid surface [28]. Zisman [29] showed that there is a linear relationship between the cos h and the surface tension for pure liquid and aqueous surfactant solutions. But, many other researchers have found that there is a linear relationship between cLG cos h (adhesional tension) and surface tension of aqueous solution, cLG for hydrophobic solids [28,30–32]. They have also found that the slope of the straight line is 1, which implies a similar adsorption density at solid–liquid and air–liquid interfaces. Wetting behaviors of solid surfaces in the presence of surfactant solution are dependent on surfactant physical chemistry, concentration at gas–liquid interfaces, and concentration along gas (or liquid)–liquid–solid contact lines [33]. Vogler (1992) found that wetting on different surfaces was highly dependent on the particular surfactant species and that both advancing and receding contact angles could either increase or decrease depending on interactions between the surfactant and the solid surface [34]. From the thermodynamic point of view when the surfactant molecules are adsorbed at the interface (gas–liquid or solid–liquid), depending on the extent of adsorption surface tension of liquid and solid–liquid interfacial tension is reduced, which cause contact angle decrease for hydrophobic low energetic solid–solution–air systems, in the case of hydrophilic solid–solution–air systems surfactant adsorption at solid liquid interface does not always reduce the solid liquid interfacial tension [35,36]. From the literature it is very clear that the influence of surfactants on contact angle and wetting on solid–liquid system has been studied by many investigators [31,33,36], but studies on the influence of electrolytes in dilute surfactant solution are rare. We have found only one very recent literature on the effect of low concentration NaCl on organic liquid (hexadecane) contact angle in ionic surfactant solutions [37]. The presence of electrolytes decreases the surface tension and CMC, as well as increases the adsorption density at both air–liquid and solid–liquid interfaces at low surfactant concentration solution. An understanding of the effect of the addition of electrolytes to aqueous surfactant solutions is of importance to a wide range of applications such as in pharmaceuticals, nanomaterials synthesis, and aqueous surface cleaning. So, it is worthy to study the effect of electrolytes on contact angle and wetting to correlate with adsorption density on both solid–liquid and air–liquid interfaces. In this investigation, we have studied dynamic contact angle of three surfactant solutions (TX-100, SDBS, and CTAB) on the flat hydrophobic (Teflon) surface, and influence of electrolytes (in a wide concentration range) on ionic surfactants at very low surfactant concentration (below CMC). Furthermore, mostly the reported contact angle studies on Teflon surface are static by goniometric method; we have carried out the experiments under dynamic condition by Wilhelmy balance method and compared our results with those of the reported studies. 2. Experimental 2.1. Materials The surfactants TX-100 and CTAB were taken from Loba Chemie Pvt. Ltd., India, with 98% and 99.5% purity, respectively. SDBS was taken from Sigma Aldrich, Germany (Technical grade, Cat no. 28995-7). NaCl and CaCl2 were taken from E. Merck. (India) with 99.5% purity and Na2SO4 was taken from Ranbaxy fine chemical Pvt. Ltd., India, with the same purity. All surfactants and electro- lytes were used as received without any further purification. Ultra pure water was used for the experiments of 18.2 MXcm resistivity and pH 6.4–6.5 (Sartorius, Germany). A surface tensiometer, Data Physics, Germany (DCAT-11EC) was used for measuring the surface tension and dynamic contact angle. The Teflon plate of dimension 25.192 mm  1.0625 mm was cut from a sheet and used for dynamic contact angle measurement without any further treatment. The thickness and width of the plate were measured at four different points using a digital slide caliper and the average values were taken. 2.2. Methods The surface tension and contact angle values were measured by Wilhelmy plate technique. For surface tension and contact angle measurement, Platinum and Teflon plates were used, respectively. All the measurements were repeated three to four times and their average values were taken for all calculations. Due to the adsorption of the surfactant on the surface of the plates, there was a fluctuation in reading between first and second measurements, to avoid that, the surface was cleaned properly after each measurement. The surface was first washed with pure water, acetone, treated with freshly prepared chromic acid, and then sonicated for 5 min. in pure water. Platinum plate was burned after acetone wash under alcohol flame to remove the adsorbed surfactants completely. All the experiments were carried out at constant temperature (28 ± 0.5 °C). Motor speed of 1 and 0.2 mm/s, and immersion depth of the plates 3 and 5 mm were maintained, respectively, throughout the experiments during the surface tension and contact angle measurements. 3. Results and discussion 3.1. Surface tension and molecular area at air–water interface Surface tension of all the surfactants with the variation of surfactant concentration was measured to determine the critical micellar concentration (CMC) and molecular density at air–water interface. The CMC values and the molecular density at air–water interface are presented in Table 1. The minimum surface tension values, cCMC for TX-100, SDBS, and CTAB are 31.5, 33.75, and 32.75 mN/m, respectively. The surface excess is a useful measure of effectiveness of adsorption at the interface. The effectiveness of adsorption is an important factor for determining the properties of surfactants such as wetting, contact angle, foaming, and emulsification. Surface excess (C) in mole/m2 and surface area (Amin) in nm2 for each surfactant can be calculated by using the following formulae: 1 dc n  2:203RT d log C 1 ¼ NA Cmax C¼ ð3Þ Amin ð4Þ where R is universal gas constant (8314 m3 Pa/kg mole K), T is absolute temperature, and NA is Avogadro number (6.023  1023). Amin is minimum surface area of a molecule occupied at the surface in nm2. The value of n is 1 for nonionic surfactants and 2 for 1:1 ionic surfactants. Normally, C is measured as Cmax where there is linear dependence between surface tension and log C, so that we obtain Amin for surfactant molecule. The value dc/d log C can be obtained from the slope of surface tension (c) vs. log C plot at a constant temperature 28 °C. The surface excess and molecular areas for three surfactants with the literature values are given in Table 1. It has been found from the literature that the reported values are of wide ranges and it is also difficult to obtain a similar temperature to com- 557 R.G. Chaudhuri, S. Paria / Journal of Colloid and Interface Science 337 (2009) 555–562 Table 1 CMC, surface tension at CMC, and comparison between experimental and literature values of surface excess and molecular area of different surfactant solutions. Surfactant TX-100 SDBS CTAB a b Exp. Cmax (mole/m2)  106 cCMC (mN/m) CMC (mM) 0.15 1.2 0.93 31.5 33.75 32.75 2.444 2.407 1.704 Exp. Amin (nm2) 0.68 0.69 0.96 Literature Cmax (mole/m2)  106 a 2.8 (at 20 °C) 2.41 (at 30 °C)b 1.8 (at 30 °C)a Literature Amin (nm2) 0.61a 0.69b 0.91a Ref. [35]. Ref. [38]. pare. The values we have compared are available close to our condition. The values available for CTAB and SDBS are close to our experimental values and measurement temperature. For TX-100 our value is close to the value reported [35] although there is a difference in temperature. 3.2. Effect of immersion speed, surfactant concentration, and type on hysteresis During the measurement of contact angles, it is also very important to take into consideration deviation of receding angle from advancing angle. In this case, due to immersion speed and roughness of the plate, we found that the receding angle is always less than the advancing angle. The hysteresis (in the form of (a) 0.44 Hysteresis (H) 0.42 0.4 0.38 0.36 0.34 0.32 0 0.5 1 1.5 2 2.5 3 3.5 4 (cos hR  cos hA)) values with the different immersion speed for Teflon surface are shown in Fig. 1a. Whenever the motor speed (i.e., immersion speed) is low, the contact time between the plate surface and liquid is more. Therefore, there is sufficient time available for adsorption–desorption process to reach the surface in equilibrium with the surfactant solution, ultimately the receding angle reaches toward equilibrium contact angle. At this condition, both the angles should be very close. Hysteresis is minimum at 0.2 mm/s speed, above that there is a rise and consequently becomes constant with increasing immersion speed. Hysteresis value depends not only on the surface immersion speed, but also on the concentration of the surfactant solution. Fig. 1b shows the change in hysteresis with the variation of normalized surfactant concentrations (Csurf/CMC) and surfactant types (nonionic, cationic, and anionic). In the presence of pure water hysteresis may be due to the contribution of both dynamic and intrinsic factor, since the surface is hydrophobic intrinsic contribution may be smaller. With the increasing surfactant concentration hysteresis decreases and ultimately becomes constant above the CMC of the surfactant solution. With the increase in surfactant concentration of the solution, the adsorption rate of surfactant on the plate also increases; as a result, the surface becomes more heterogeneous in molecular dimension within that limiting time. Since the surface heterogeneity changes with the change in surfactant concentration, so hysteresis value changes with the concentration change till significant adsorption is there. Thus, the effect of adsorption again supports the higher hysteresis at higher motor speed since contact time is too less to reach the adsorption equilibrium at higher motor speed. In the presence of surfactants both the dynamic and intrinsic hysteresis are important. Hysteresis also decreases in the presence of electrolytes than the pure surfactant alone at that concentration. Motor Speed (mm/sec) (b) 3.3. Effect of surfactant concentration on contact angle and adhesion tension 1.2 SDBS TX-100 CTAB Hysteresis (H) 1 0.8 0.6 0.4 0.2 0 0.5 1 1.5 2 CSurf /CMC Fig. 1. Contact angle hysteresis on Teflon surface (a) with plate immersion speed for TX-100 solution (b) in the presence of TX-100, SDBS, and CTAB with 0.2 mm/s immersion speed. The change in advancing and receding contact angles with surfactant concentration on Teflon surface is shown in Fig. 2a, b, c for TX-100, SDBS, and CTAB, respectively. The advancing and receding contact angles on Teflon surface in the presence of pure water are 115.3° and 45.68°, respectively. The reported values of advancing angle are in the range of 108–116° [31] and our value is very close to the value 116° reported by Busscher et al. (1983) [36]. In the presence of TX-100 hA decreases very sharply even at very low concentration. At 0.1 mM concentration of surfactant, advancing contact angle decreased to 85.21° and beyond this concentration contact angle is almost constant. Whereas hR increased up to 71.8° at 0.01 mM concentration and becomes constant beyond that concentration. In the presence of SDBS hA decreases gradually to a value of 87.28° at 1.5 mM concentration and beyond that there is no significant change in contact angle. For hR, just opposite trend is observed, there is an increase in contact angle up to a value of 60.63° at 1.8 mM concentration. For CTAB hA decreases gradually and significantly up to 0.75 mM with a value of 84.06°. This value is close to the reported value 83° [13]. In the case of hR there is a 558 R.G. Chaudhuri, S. Paria / Journal of Colloid and Interface Science 337 (2009) 555–562 120 For the Teflon surface we have found that there is no linear relationship between cos hA (cosine of advancing angle) and surface tension as shown by Bernett and Zisman (1959) [29]. Consequently, similar to other researchers [28,31,32], there is a linear relationship followed between cLG cos h (adhesion tension) and cLG according to TX-100 110 θA 100 θR 90 80 cLG cos h ¼ acLG þ b 70 60 50 40 0 0.05 0.1 0.15 0.2 0.25 θA Contact Angle ( θ , θ ) A R 110 (c) 0.35 θR 100 90 80 70 60 50 40 0 120 0.5 1 1.5 CTAB θA θR 110 Contact Angle ( θ , θ ) A R 0.3 SDBS 100 2 3.4. Adsorption at Teflon–water and air–water interfaces 90 The change in contact angle and surface tension (cLG and cSL) at both the interfaces with the change in surfactant concentration is due to the adsorption of surfactants. By combining and rearranging Young’s equation (Eq. (2)) and Gibbs surface excess equation (Eq. (3)) for solid–air, solid–water, and water–air interfaces we can write 80 70 60 50 40 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 Surfactant Concentration (mM) Fig. 2. Advancing and receding contact angles of (a) TX-100, (b) SDBS, and (c) CTAB solutions on Teflon surface. sudden increase at very low concentration (0.01 mM) to 67.15° and after that the change is not very significant. All the three surfactants show that the change in hA is constant above the CMC, and hA values above the CMC are very close (within the variation of ±3°) having the highest value in SDBS. Decrease in contact angle in the presence of surfactants is due to the increase in hydrophilicity of the surface since the expected orientations of the surfactant molecules are similar in nature on hydrophobic surface after the adsorption through tailgroups. Since all the surfactants are having different head and tailgroups, they should show different adsorption densities on the solid surface. Nonionic surfactant may have more adsorption density on PTFE surface than ionic due to the absence of electrical repulsive force between headgroups, and between the ionic surfactants, CTAB may have more adsorption density due to the presence of longer tail length. This may be the reason why SDBS is having little higher contact angle value than CTAB. The surface tension at air–water interface also influence the contact angle, lower surface tension gives lower contact angle. Between these three surfactants the surface tension values at CMC are also very close. Ultimately, similar adsorption pattern and close surface tension values at CMC may be the reason of not showing significant difference in contact angle. Finally, the resultant contact angle is obtained after combining all the effects. CSG  CSL dðcSV  cSL Þ dðcLG cos hÞ ¼ ¼ dcLG dcLG CLG ð6Þ where CSG, CSL, and CLG represents the surface excess of surfactants at respective interfaces. Assuming CSG  0 we can say the ratio of CSL to CLG is the slope of Eq. (5). The value of slope from the fitting of experimental data is 0.83, which indicates that at a given bulk surfactant concentration the excess concentration at solid–water LG (b) 120 ð5Þ where a and b are constants. The value of a depends on the solid surface property. Bargeman and van Voorst Vader (1973) [32] have proposed that for nonpolar solids and surfactant system the value is 1. Fig. 3 shows the linear fit of all the experimental data and also there is no significant difference between three different surfactants. Table 2 shows the values of a and b for different surfactants. Our average value of a (0.83) that is lower than 1, and b is also lower (31.2) than others, 46.7 [31], and 40.6 [32]. Substituting the value of cos h = 1 in the above-mentioned equation we can estimate the liquid surface tension required to give zero degree contact angle or critical surface tension (cc), equal to 17.05 mN/m. The obtained value is lower than those reported by Szymczyk et al. (2006) 23.63 mN/m [28] and Bargeman and van Voorst Vader (1973) 20.3 mN/m [32] and close to that reported by Pyter et al. (1982) 16.5–19.5 [39] using hydrocarbon surfactants in water. Szymczyk et al. (2006) [28] also obtained different cc (19 mN/m) using pure alkanes than from the aqueous surfactant solution on Teflon surface. Although the reported values are in a wide range, our value also fall in that range. γ .cosθA Contact Angle ( θ , θ ) A R (a) 5 TX-100 0 SDBS -5 CTAB -10 -15 -20 -25 -30 -35 30 40 50 60 γ 70 80 LG Fig. 3. Plot of adhesional tension (cLG cos hA) vs. surface tension (cLG) for three different surfactants (TX-100, SDBS, CTAB) on Teflon surface. 559 R.G. Chaudhuri, S. Paria / Journal of Colloid and Interface Science 337 (2009) 555–562 Table 2 Values of a and b for different pure surfactants. Surfactant a b TX-100 SDBS CTAB 0.83 0.82 0.85 31.4 30.3 31.8 Average 0.83 31.2 interface is 0.83 times of that at water–air interface. Although several workers have found that for low surface energy solids, solid–water and air–water interface surface excess are same [28,31,32,40,41], there are also reported studies of unequal adsorption between hydrophobic solid–water and air–water interfaces such as Nylon and PMMA [28,39]. It has also been mentioned that the slope of cLG cos h–cLG curve equal to 1 is a necessary condition but not a sufficient condition for equal adsorption at solid–liquid and air–liquid interfaces [41,42]. Another condition, the curve of cos h–1/cLG should be linear and intercept on the cosh axis equal to 1 should be fulfilled. In our study we found that the curve cos h–1/cLG is linear as shown in Fig. 4 with an intercept on the cos h axis equal to 0.77 ± 0.035. This also confirms that the adsorption at two interfaces is not equal. Further, it can also be shown that when the surface excess is equal at both the interfaces, the work of adhesion is independent of surfactant concentration, but when it is unequal the work of adhesion will depend on surfactant concentration. The work of adhesion can be represented as W A ¼ cSG  cSL þ cLG ð7Þ Differentiating with respect to cLG we can write dW A dcSG dcSL ¼  þ1 dcLG dcLG dcLG ð8Þ From the Gibbs adsorption equation we can write, dcSG/dcLG = CSG/ CLG and dcSL/dcLG = CSL/CLG. Now if CSG = 0 and CSL/CLG = 1, dWA/ dcLG = 0. So, we can say that the work of adhesion will not change with the concentration of surfactant. In this study CSL/CLG < 1, and dWA/dcLG – 0 shows the work of adhesion well changed with the concentration of surfactants. The differences in results with others may be attributed in terms of the following reasons. In our study we have measured the advancing angle by the Wilhelmy method under dynamic condition where three-phase contact line is moving but others [28,31,32,40,41] have measured by sessile drop method where three-phase contact line is not moving. They have measured the contact angle after 1–10 min of (equilibrium time) the drop was settled onto the PTFE surface. In our case, for 5 mm immersion depth with 0.2 mm/s immersion speed, the maximum contact time of solid–liquid interface during advancing angle measurement is 25 s. The contact time may be too less to achieve complete adsorption of surfactants on Teflon surface. No such reported study was found on surfactant adsorption kinetics on Teflon surface to compare, but the adsorption of nonionic surfactants on a hydrophobic surface (polystyrene) shows the equilibrium time is approximately 400 s [43]. We also did the surfactant adsorption kinetics study on PTFE powder for SDBS and TX-100. We found 5 min was required to reach the equilibrium for both the surfactants. Thus, it indicates our contact time is less for reaching the equilibrium. Another reason for the difference in the result may be the presence of other impurity in our Teflon surface. 3.5. Effect of surfactant concentration on the work of adhesion Further, the work of adhesion (WA) is calculated for advancing contact angle and plotted with log C in Fig. 5 using equation W A ¼ cSG  cSL þ cLG ¼ cLG ð1 þ cos hA Þ ð9Þ where cSG  cSL = cLG cos hA. For all three surfactants, the work of adhesion decreases with increasing concentration of surfactants and the change is sharp in the low concentration. Here, as contact angle decreases in the presence of surfactants wetting should increase and that should reflect in increased work of adhesion. But, initially contact angle on Teflon surface in the presence of water and low surfactant concentration is above 90°, where cos hA values are negative, so (1 + cos hA) term increases gradually with the decrease of contact angle, at the same time surface tension also decreases gradually. The decrease in surface tension is more than the increase in (1 + cos hA) term; as a result the work of adhesion values decreases with increase in concentration. Contradictory to our results Szymczyk and Janczuk (2006, 2007, 2008) [28,30,41] found that work of adhesion is independent on the types of surfactants, their concentration, and composition in the mixture on the Teflon surface. As it is discussed before that the work of adhesion is independent of concentration when surface excess at solid–water and air–water is equal, since we obtained unequal surface excess on both the surfaces, the work of adhesion is dependent on surfactant concentration. 0.2 45 Work of Adhesion (WA) 0.1 cos θA 0 -0.1 -0.2 TX-100 -0.3 SDBS CTAB -0.4 -0.5 0.01 0.015 0.02 0.025 1/γ 0.03 0.035 LG Fig. 4. Plot of cos hA vs. reciprocal of surface tension (1/cLG) of aqueous surfactant solutions on Teflon surface. 40 35 TX-100 SDBS CTAB 30 25 -3.5 -3 -2.5 -2 -1.5 -1 -0.5 0 0.5 log C Fig. 5. Plot of the work of adhesion (WA) with log C for three different aqueous surfactant solutions (TX-100, SDBS, CTAB) on Teflon surface. 560 R.G. Chaudhuri, S. Paria / Journal of Colloid and Interface Science 337 (2009) 555–562 3.6. Polar component of aqueous solution 34 1.4 Surface tension 1.2 qffiffiffiffiffiffiffiffiffi cSL ¼ cSG þ cLG  2 cdL cdS 0.8 Combining this equation with Eq. (7) we can write 1 CMC ð10Þ 33 CMC 32 31 0.6 30 qffiffiffiffiffiffiffiffiffi W A ¼ 2 cdS cdL ð11Þ where cdS and cdL are the dispersive contributions to the surface tensions of solid and liquid phases, respectively. For the hydrophobic Teflon surface the polar component of surface tension is negligible and cSG ¼ cdS ¼ 20:24 mN=m [31]. Now cdL can be calculated from Eq. (11) and the polar component of the surface tension of surfactant solution, cpL can be calculated using equation cpL ¼ cLG  cdL ð12Þ cSL calculated from equation 1 is plotted as a function of cpL in Fig. 6. Fig. 6 shows there is a linear relationship between cSL and cpL for all three surfactants cSL ¼ 0:986cpL þ 0:644 ð13Þ From the equation it appears that the increase in polar component of the surfactant solution will increase the solid–water interfacial tension. 3.7. Effect of electrolytes on CMC When the ionic surfactants are adsorbed at air–liquid or solid– liquid interface the repulsive force exists between the headgroups of the adsorbed molecules for single surfactant solution. The presence of electrolyte reduces the repulsive interaction between the headgroups of ionic surfactant molecules at interface and also repulsive force between the charged headgroups of monomer surfactant molecules in the solution. So due to the decreased in repulsive force, CMC value of the surfactant solution also decreases in the presence of electrolyte. Fig. 7 shows decrease in CMC values of SDBS at different electrolyte concentrations and minimum surface tension achieved (cCMC) at that concentration. Here, we found a linear correlation between log(CMC) and log(CMC + salt conc.) 0.4 29 0.2 0 0 20 40 60 80 100 120 Surface Tension (mN/m) According to Fowkes (1963) [44] interfacial tension can be expressed in terms of individual surface tension of the surfaces and London–van der Waals dispersion force component of the surfaces as 28 140 NaCl Concentration (mM) Fig. 7. The change in CMC and surface tension (cCMC) of SDBS solution in the presence of NaCl. with slope 2.99 and intercept 6.64 for SDBS in the presence of NaCl as proposed by Attwood (1969) [45]. log CMC ¼ c  d logðCMC þ C salt Þ ð14Þ where c, d are constants and Csalt is the salt concentration. Attwood (1969) [45] found similar linear relationship with slope 0.54 and intercept 6.4 for other anionic surfactant (SDS) in the presence of NaCl. In this section we have studied the effect of different electrolytes on surface tension, and advancing contact angle in the presence of ionic surfactants. The electrolyte concentration increased gradually for a particular surfactant concentration, and the change in contact angle and surface tension was measured. When the surface tension was reached to a constant value it was assumed that CMC reached at that particular surfactant and electrolyte concentration. For SDBS three electrolytes were studied, NaCl and CaCl2 to see the effect of cation valence among two electrolytes, and Na2SO4 for anion valence. 3.8. Effect of electrolytes on contact angle and wetting Fig. 8 shows the change in advancing contact angle with increasing NaCl concentration in the presence of different initial SDBS concentrations. Although there are some irregularities in 55 50 120 A Advancing Angle ( θ ) 45 γ SL 40 35 30 CTAB 25 TX-100 SDBS 20 110 0.5 SDBS-NaCl 0.05 SDBS-Na2 SO4 0.1 SDBS-NaCl 0.05 SDBS-NaCl 0.01 CTAB-NaCl 0.01 CTAB-Na 2 SO4 0.05 SDBS-CaCl2 100 90 80 15 15 20 25 30 35 40 45 50 55 P γ L Fig. 6. Relationship between Teflon–water interfacial tension in the presence of surfactants (cSL) and the polar component of the surface tension ðcpL Þ. 0.01 0.1 1 10 100 Electyrolyte Concentration (mM) Fig. 8. Plot of advancing contact angle (hA) for different electrolytes (NaCl, CaCl2, Na2SO4) in the presence of SDBS and CTAB solution on the Teflon surface. 561 R.G. Chaudhuri, S. Paria / Journal of Colloid and Interface Science 337 (2009) 555–562 60 Work of Adhesion (WA) 50 Work of Adhesion (WA ) 55 50 45 0.5 mM 45 0.1 mM SDBS-NaCl 40 0.05 mM 35 0.05 mM 0.05 mM 30 0.01 mM Electrolyte Concentration (mM) 25 40 SDBS-NaCl 0.01 0.1 1 10 100 0.01 mM SDBS-NaCl SDBS-CaCl2 SDBS-Na2SO4 CTAB-NaCl CTAB-Na2SO4 35 30 25 0 50 100 150 200 Electrolyte Concentration (mM) Fig. 9. Plot of the work of adhesion (WA) in the presence of electrolytes for ionic surfactants (SDBS, CTAB) on the Teflon surface. (a) 10 0.5 mM SDBS-NaCl 0.1 mM SDBS-NaCl 0.05 mM SDBS-NaCl 0.05 mM SDBS-CaCl 2 5 A -5 γ cosθ LG 0.05 mM SDBS-Na 2SO4 0 -10 -15 25 30 35 40 γ (b) 45 50 55 LG 0 0.01 mM CTAB-Na2SO4 0.01 mM CTAB-NaCl A -5 LG γ cosθ the curve at low NaCl concentration for both the surfactants, still the decreasing trend is clear for advancing angle. At low surfactant concentration change is significant with the electrolyte (NaCl) concentration. The change in contact angle in the presence of maximum NaCl concentration and in the absence of NaCl is more when surfactant concentration is low. Specifically, at 0.05 mM SDBS concentration (24 times below CMC) in the presence of 200 mM NaCl advancing contact angle decreased from 100.11° to 82.31°; 0.1 mM (12 times below CMC) in the presence of 200 mM NaCl the change is 100.67–84.50°; and 0.5 mM (2.4 times below CMC) in the presence of 50 mM NaCl the change is 93.28–83.48° due to closer packing of adsorbed surfactant molecules on the Teflon–water and air–water interfaces. By comparing the contact angle data in the absence of electrolyte it is observed that even at low surfactant concentration by adding electrolyte we can achieve a similar or even lower minimum contact angle that in the presence of pure surfactant with out electrolyte. From Fig. 9, it can be seen that the work of adhesion is decreased for all the concentrations and at high NaCl concentration it reached to a plateau. Initially the difference is more. Since for pure surfactant the work of adhesion was dependent on surfactant concentration and at a constant surfactant concentration by adding increasing electrolytes concentration we can see a similar effect to that of increasing surfactant concentration. To see the effect of counter ion and co-ion valance on contact angle and wetting we have varied the concentrations of Na2SO4 and CaCl2 at the same SDBS concentration (0.05 mM). We have observed that in the presence of CaCl2 at 1 mM concentration minimum contact angle is reached (78.43°), and in the presence of 100 mM Na2SO4 minimum angle is 86.97° above that concentration again there is a small increase contact angle. So, it is worthy to mention that CaCl2 is most effective in reducing the contact angle on Teflon surface in the presence of SDBS, even at very low concentration. After comparing the effect of NaCl and Na2SO4 it is observed that when Na+ concentration is the same (at 100 mM Na2SO4 and 200 mM NaCl) Na2SO4 is showing little higher contact is probably angle (86.97°) than NaCl (82.31°). So, bivalence SO2 4 having some negative effect on the adsorption of anionic surfactant on Teflon surface, which reflect in increase in contact angle. Fig. 10(a) shows that there is a linear relationship between cLG cos h  cLG for all the initial surfactant concentrations and in the presence of all the electrolytes (NaCl, Na2SO4, and CaCl2), but the values of the constants a and b (in the presence of electrolytes termed as ae and be) are different from the pure surfactants. In the -10 -15 30 35 40 45 γ 50 55 LG Fig. 10. Plot of adhesional tension (cLG cos hA) vs. cLG on Teflon surface. (a) Effect of electrolytes solution (NaCl, CaCl2, Na2SO4) in the presence of different SDBS concentration. (b) Effect of electrolytes solution (NaCl, Na2SO4) in the presence of different CTAB concentration. presence of electrolytes, the constant values are lower than the pure surfactants, ae = 0.629 and be = 23.11. The value of slope 0.629 indicates that at a given bulk surfactant concentration in the presence of electrolytes the excess concentration at Teflon– water interface is 0.629 times of that at air–water interface. In the presence of electrolytes, in general, surface excess of ionic surfactants at air–water interface increases due to reduction in repulsion between the surfactant headgroups [35,46]. In the presence of electrolytes if CLG increases significantly but the change is not proportionately for CSL then the surface excess ratio ((CSL/CLG)e) will be less than that in the absence of electrolyte. Since the surface is hydrophobic in nature surface charge will be close to zero the change in surface potential in the presence of electrolyte will be very less, the adsorption density will increase only due to decrease in repulsion between the headgroups. For cationic surfactant (CTAB) two electrolytes (NaCl and Na2SO4) were studied at 0.01 mM CTAB concentration. It is found that for both the cases contact angle is changing till 100 mM of electrolytes concentration with the values of 96.25° and 94.04° for NaCl and Na2SO4, respectively. The values are higher than that of pure CTAB. Both the electrolytes are showing a linear relationship of cLG cos hA–cLG, shows in Fig. 10(b) with a different slope (ae = 0.467) and intercept (be = 12.43) than in the presence of SDBS. Similar to pure surfactants we have calculated polar compo- 562 R.G. Chaudhuri, S. Paria / Journal of Colloid and Interface Science 337 (2009) 555–562 35 Finally, considering all the above-mentioned points, it is concluded that for achieving a certain extent of contact angle or wetting, for different applications, low surfactant concentration in the presence of electrolytes may reduce the surfactant consumption and cost of the process instead of using high concentrated surfactant solution. 30 25 0.05 mM SDBS-Na SO4 SL 2 Acknowledgment γ 0.05 mM SDBS-CaCl 2 20 0.05 mM SDBS-NaCl 0.1 mM SDBS-NaCl 0.5 mM SDBS-NaCl 0.01 mM CTAB-NaCl 0.01 mM CTAB-Na SO4 15 The financial support from University Grants Commission (U.G.C), Grant No. F. 32-96/2006 (SR), New Delhi, India, for this project is gratefully acknowledged. 2 10 10 15 20 25 30 35 40 References 45 γ P L Fig. 11. Relationship between Teflon–water interfacial tension in the presence of different electrolytes and surfactants (cSL) and the polar component of the surface tension ðcpL Þ. nent of the surface tension of surfactant solution in the presence of electrolytes and plotted cSL vs. cpL in Fig. 11. The figure shows that there is a linear relationship and independent of types of surfactants, and types of electrolytes. The slope and the constant value are also very close to those of pure surfactant: cSL ¼ 0:983cpL þ 0:649 ð15Þ The equation clearly shows that cSL value can be changed by changing cpL and the change can be done by changing the surfactant concentration or at a particular surfactant concentration by adding electrolyte. For a particular solid in an aqueous medium it follows the same equation for different surfactants or surfactant in the presence of electrolytes. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] 4. 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