26
Langmuir 1989,5, 26-29
10. The quantum yield of the reduction process was determined from the absorption a t 850 nm (€ = 1.12 X lo4 M-' cm-'), which was recorded immediately after the laser pulse excitation (anthracene in cyclohexane was used as a referen~e'~).The observed quantum yield for the reduction of phenosafranin in 1 mM colloidal CdS suspension was 0.02. This value is similar to the quantum yields obtained for the reduction of zwitterionic viologen and oxazine dyes by CdS and CdSe colloids (4 I0.1).12J9
Acknowledgment. The research described herein was supported by the Office of Basic Energy Sciences of the U S . Department of Energy. This is NDRL-3100 from the Notre Dame Radiation Laboratory. Registry No. PHNS, 81-93-6;PHNS'-, 63317-02-2;PHNS2-, 63317-03-3;TiO,, 13463-67-7; CdS, 1306-23-6. (19) DimitrijeviE, N. M.; Kamat, P. V. Langmuir 1987, 3, 1004-1009.
The Effect of Triton X-100 and Ethanol on the Wettability of Quartz+ Gaspar Gonzglezh and A. M. Travalloni-Louvisse Petrobras Research Center, Cidade Universitciria, Ilha do Funddo, Quadra 7, Rio de Janeiro, RJ, Brazil Received January 8, 1988. In Final Form: July 29, 1988 The effect of the nonionic surface-active agent (SAA) Triton X-100 on the wetting behavior of glass and quartz was studied with water and 10% and 25% aqueous ethanol solutions as the liquid phase. The effect of the surfactant adsorption on the solid-liquid and solid-vapor interfaces was also studied. The results indicate that the hydrophobization of the surface occurred at a very low concentration of surfactant and that the reverse transition to zero contact angle was effective at a concentration well below the critical micelle concentration (cmc) of the surfactant. The results also showed that ethanol reduced the surface tension of aqueous solutions without changing the surfactant cmc and reduced the adsorption of the surfactant at the solid-solution interface.
Introduction Nonionic surface-active agents (SAA) like polyoxyethylenic or polyoxypropylenic (or both) derivatives of alkylphenols are commonly used as surfactants in many industrial processes such as chemical demulsification of crude oi1,lP2powder dispersion: solubilization,4etc. In all these processes surfactant solutions will encounter and interact with solid surfaces, and as a result, surfactant molecules may adsorb on the solid particles and modify their wettability. There are a number of articles on the solution behavior of nonionic surfactants6p6and on their adsorption to the air-water i n t e r f a ~ e . ~Various ~~ studies have also been published on the adsorption of these surfactants by solids or mineral particles,@JO clays,11or latexes.12 However, there are few works on the changes in the wettability of solid surfaces induced by these parameters. Doren13examined the effect of Triton X-100 on the flotation of quartz particles and in a subsequent paper extended these studies to other minerals and other oxyethylenic surfactants.14 In a recent article, Healy et a1.16 studied the changes in the contact angle of quartz induced by various commercial polydisperse polyoxyethylenic surfactants. In this study we examine the wetting behavior of quartz in the presence of a nonionic surfactant and ethanol as well as the parameters associated with the contact angle changes in this system. The original objective of the study was to determine wettability changes in the quartz (or silica)/water interface, which have an important influence on the oil recovery and treatment processes for which nonionic surfactants are used very frequently. 'Presented at the Second Meeting of the Southern Hemisphere on Mineral Technology,Rio de Janeiro, June, 1987. 0743-7463/89/2405-0026$01.50/0
Experimental Section The solid samples used in this study were quartz, silica, and glass. For the adsorption tests a sample of silica (Merk)was used with a specific surface area of 388 m2g-l, measured by using the BET method. For the contact angle measurements, the solid specimenswere microscope glass slides and pieces of q&z. Prior to the contact angle measurementa the solid specimens were cleaned they were waked in concentrated sulfuric acid containing potassium dichromate and carefully rinsed with copious amounts of double distilled water. The surface tension of Triton X-100 solutions was measured by using a du Nouy ring tensiometer (Fisher Scientific Co.) at 25 "C. Cloud points were determined by visual observation of a 1%surfactant solution contained in a test tube. The tubes were placed in a transparent water bath, and the temperature increased at a rate of 3 "C/min. (1) Jones, T. J.; Neustadter, E. L.; Whittingham, K. P. J. Can. Pet. Technol. 1978,17, 100. (2) Thompson, D. G.; Taylor, A. S.; Graham, D. E. Colloids Surf. 1985, ,c
,"C
10, 110.
(3) Mathai, K. G.; Ottewill, R. H. Trans. Faraday SOC.1966,62,750. ( 4 ) Buzier, M.; Ravey, J. C. J. Colloid Interface Sci. 1983, 91, 20. (5) Arai, H. J. Colloid Interface Sci. 1967,23, 348. (6) Schot, H.; Raoyce, E.; Suk, K. M. J. Colloid Interface Sci. 1984, 98,196. (7) Tajima, K.; Iwahashi, M.; Sasaki, T. Bull. Chem. SOC.Jpn. 1971, 44, 325. (8) Nishikido, N. J. Colloid Interface Sci. 1986, 112, 87. (9) Ottewill, R. H. In Nonionic Surfactants;Schick, M. J., Ed.; Marcel Dekker: New York, 1966. (10) Rouquerol, J.; Partika, S.; Rouquerol, F. In Adsorption at the
Gap-Solid and Liquid-Solid Interface;Rouquerol, J. Sing, K. S.W., Eds.; Elsevier: Amsterdam, 1982; p 69. (11) Nava, N.; Bella, B. D. Water Res. 1985,19, 815. (12) Kronberg, B.; Stenius, P.; Shoraell, Y. Colloids Surf. 1984,12,113. (13) Doren, A.; Vargas, D.; Goldfarb, J. Trans. Inst. Min. Met. 1975, 8, c33. (14) Doren, A.; Van Lierde, A.; De Cuypier, J. 13th International Mineral Processing Congress, Wroclaw, Poland, 1979. (15) Scales, P. J.; Grieser, F.; Furlong, P. N.; Healy, T. W. Colloids Surf. 1986, 21, 55.
0 1989 American Chemical Society
Effect of Triton X-100 on Quartz Wettability
I
Langmuir, Vol. 5, No. 1, 1989 27
I
-
N
'E I
-In 2 0 e +
-
a 2
L
-g
10
4. N 0 -5
-3
-4
-2
-1
log Cs (moles dni3)
Figure 2. Adsorption isotherms for Triton X-100on silica in water and ethanol solutions. Symbols as in Figure 1. -5
-4
-3
-2
log C, (moles d m 3 )
Figure 1. Surface tension versus logarithm of the surfactant concentration for Triton X-100in water (a),10% v/v ethanol (b), and 25% v/v ethanol (c).
I
: a
u)
SO
w W
CMC
n
I
The amount of surfactant absorbed by silica was determined by measuring the difference between the initial and final concentration of 25 m L of SAA solution after 3 h of contact with 1 g of silica. The SAA concentration was determined spectrophotometrically, measuring the absorbance at 281 nm in IO-mm optical path quartz cells by using a double-beam Varian 634 spectrophotometer. The contact angles at the quartz (or glassbaqueous solution-air interface were measured by using a modified captive bubble apparatus.lB
Results Adsorption of Triton X-100to the Air-Water Interface. Figure 1 shows the surface tension of aqueous solutions of Triton X-100 and the effect of ethanol at 25 OC. The critical micelle concentration (crnc) for the surfactant in water was 2.8 X mol dm-3. This figure agrees with results reported by other authors. The graph shows a rather shallow minimum in the cmc region which is usually interpreted as evidence of some degree of polydispersity.l' Curves b and c show the effect of ethanol at 10% and 25% (in volume). The reduction of surface tension for low SAA concentration corresponds to the effect of ethanol. For higher concentrations, the surfactant is preferentially absorbed at the liquid surface, displacing the alcohol from the interface. However, as the slope of the y-log C,curve in the cmc region is lower in the presence of alcohol, it means that the displacement is not complete. Nishikido8 studied this effect for various oxyethylenic surfactants and concluded that the ratio between the maximum adsorption of SAA in the presence of alcohols and its corresponding value in pure water decreased linearly with the mole fraction of the alcohol. The results of Figure 1 also indicate that ethanol does not modify to a great extent the cmc of Triton X-100. Opposite effects are responsible for this behavior, the solvent effect which would increase the cmc and the adsorption of the ethanol on the surface of the micelles which would result in a reduction of the cmc.18 Longer chain alcohols may penetrate into the micelle structure, causing the latter effect to predominate.
Adsorption of Triton X-100to the Silica-Water Interface. The adsorption of Triton X-100 onto silica is shown in Figure 2. The results indicate that there are three well-defined regions in the isotherm. Initially, for low concentrations of surfactant, the adsorption is rather low. Following this region, there is a sudden increase in adsorption and finally a saturation plateau at an equilibrium concentration of 6 X 10"' mol dm"', for the case of pure aqueous solutions. The amount adsorbed in the plateau is 1.3pmol m-z. This value is lower than the results reported by Doren et al. for quartz but agrees with results of Astonlg and van der Boomgaard.20 From this value of adsorption a molecular cross section of 1.2 nm2 may be calculated for Triton X-100 at the silica-water interface. The area per molecule in a compact monolayer at the air-water interface, calculated by applying the Gibbs equationz1to the results of Figure 1, is 2.22 nm2. These values indicate that the adsorption plateau at the solidsolution interface is attained when a bilayer of surfactant is formed on the solid. This association process takes place at the interface before the formation of micelles in solution. Furthermore, the adsorption stops when a bilayer is formed, which indicates that multilayers do not form on the solid substrate. The isotherms do not show a maxi-
(16)Adamson, A. W. Physical Chemistry of Surfaces, 3rd ed.; Wiley: New York, 1976; p 343. (17)Becher, P.In Nonionic Surfactants; Schick, M. J., Ed.; Marcel Decker: New York, 1966. (18)Shinoda, K.;Nakagawa, T.; Tamamushi, B. I.; Isemura, T. Colloidal Surfactants. Some Physico-chemical Properties; Academic: New York, 1963.
(19) Aston, J. R.; Furlong, D. N., Grieser, P. J.; Scales, R. J.; Warr, G. G. In Adsorption at the Gas-Solid and Liquid-Solid Interface; Rouquerol, J., Sing K. S. W., Eds.; Elsevier: Amsterdam, 1982;p 97. (20)Van der Boomgaard, Th.; Thadros, Th. F.; Lyklema, J. J. Colloid Interface Sci. 1987, 116, 8. (21)Defay, R.;Prigogine, 1. Tension Superficielle et Adsorption; Editions Desoer: Liege, 1951;p 72.
-7
-e
-5
-4
-3
log C, (moles drn-')
Figure 3. Receding contact angle at the quartz-aqueous solu-
tionah interface versus logarithm of the surfactant concentrations in water and ethanol solutions. Symbols as in Figure 1.
Gonzcilez and Traualloni-Louvisse
28 Langmuir, Vol. 5, No. 1, 1989 Table I. Effect of Ethanol on the Cloud Point of Triton
x-100 ethanol, 70 (v/v)
cloud point, “C
0 5
62 72 80 100
10
13
mum in the cmc region as reported by other authors for samples presenting relatively high degree of polydispersity. Ethanol at low concentrations does not modify the adsorption plateau. However, the saturation is attained at a different equilibrium concentration due to the changes in the solubility behavior of the surfactant in the presence of alcohol. Only when the concentration of ethanol is 25% (6 M) is the adsorption maximum reduced to 0.8 pmol m-’. Contact Angle Measurements. The values of receding contact angle as a function of the concentration of Triton X-100 in water (curve a) or in the presence of ethanol (curves b and c) are shown in Figure 3. The shape of the curves corresponds to a typical wetting isotherm for polar high-energy solids.22 In this case the contact angle increases for low surfactant concentration, reflecting the adsorption of single molecules on the polar groups of the solid. After reaching a maximum, the contact angle decreases as a consequence of the formation of a second layer of surfactant with their polar groups oriented toward the liquid phase. The addition of ethanol reduces the values of the contact angles, but the shapes of the wetting isotherms are maintained. Furthermore, the curves reflect the modifications observed for the adsorption isotherms; i.e., the surfactant concentration for which 8~ starts to decrease is higher in the presence of ethanol. Similarly, the contact angle also decreases at a higher concentration, reflecting the fact that the bilayer at the solid-solution interface is formed at a higher concentration when the alcohol is present. This last observation is less obvious than the previous one in the results shown in Figure 3. However, when the surfactant equilibrium concentration increases from 3 X mol dm-3, 6~ decreases to 7 X from 38’ to 27’ in pure water while it remains constant within the experimental error for the solutions containing ethanol. For a further increase in surfactant concentration, 8~ goes to 21’ in pure water, which represents a reduction of 45% ; the corresponding change in the presence of alcohol is only 2290. In the absence of surfactant, ethanol wets quartz completely, giving a zero contact angle for any concentration of alcohol. Cloud Point. Aqueous solutions of Triton X-100 present a cloud point of 62 ‘C,and the addition of ethanol increases this temperature as indicated in Table I. This effect is related to a better dissolution of the nonionic surfactant in water/ethanol mixtures and is usually called the “solvent effect”.
Discussion As expected, Triton X-100 modifies the wettability of quartz. For a concentration as low as 2 x lo-’ mol dm-3, the water contact angle reaches approximately 20’. This effect is associated with the adsorption of free surfactant molecules through interactions with the surface hydroxyls or silanol groups of the solid surface. A plateau of about mol ~ l m - ~ . 38’ is observed between 3 X lo* and 3 X When the surfactant concentration is further increased a reduction of the contact angle is observed due to the aggregation of surfactant molecules at the solid-solution (22) Yarar, B. 2nd Latin American Flotation Congress, Concepcion, Chile, 1985; pp C2.1-C2.35.
interface. This association process is caused by the same hydrophobic effect responsible for the formation of micelles= in aqueous surfactant solutions, but it precedes the cmc. Scamehorn, Schechter, and WadeNhave shown that for the adsorption of ionic surfactants on oppositely charged mineral particles initially the proceeds by the formation of patches on the charged groups of the solid surface. As the concentration increases a second layer of surfactant is formed on these sites before the formation of a complete monolayer on the solid surface and at a concentration well below the cmc. For a higher surfactant concentration this structure covers the whole solid surface. Schechter et aLZ3designated these bilayer structures “admicelles”,to distinguish them from the “hemimicelles” used by Gaudin%to describe the formation of a monolayer of surfactant on the solid surface. From the results of Figures 1-3 it is possible to infer that the adsorption of Triton X-100 on silica follows a mechanism similar to that described by the above authors. The initial driving force for the adsorption process in this case would be the interaction between the oxyethylenic portion of the surfactant molecule with the surface hydroxyls of the solid surface through hydrogen bonds. The hydrophobizationof the solid surface caused by the surfactant may be described by Young’s equation, given by eq 1, or its derivative, eq 2. y L V cos 8 = ySV - ySL (1) d(yLVCOS 8) d(YSV- ysL) (2) d log CS d log Cs
On the other hand, the Gibbs adsorption isotherm, eq 3, gives the relation between the surface tension, the amount adsorbed per unit area (FJ, and the chemical potential (pi) of component i. dri = CI’idpi (3) i
If eq 1 is applied to the solid-vapor, solid-liquid, and liquid-vapor interfaces, for a two-component system, the following three relationships may be assuming that the activity coefficients equal 1. -dyLV = 2.3RTrLVd log Cs (4) -dySL = 2.3RTrSL d log Cs (5) -dysv = 2.3RTrSVd log CS - h d r
(6) In eq 6, the term h d r allows for the possibility of having a wetting film of thickness h and disjoining pressure P at the solid-vapor interface. A t equilibrium, r must equal the excess capillary pressure in the air bubble (assumed spherical) used to measure the contact angle:
(7) where rb represent the bubble radius. Introducing eq 3-7 into the derivative form of Young’s equation and reordering yield
Equation 8 gives the unknown quantity YSv as a function (23) Harwell, J.; Hoskins, J. C.; Schechter, R. S.; Wade, W. H. Langrnuir 1985, 1, 251. (24) Scamehorn, J . F.; Schechter, R. S.; Wade, W. S. J. Colloid Interface Sci. 1982, 85, 463. (25) Gaudin, A. M.; Fuerstenau, D. W. Trans AIME 1966,202,958. (26) Billet, D. F.; Hough. D.B.: Ottewill, R. H. J.Electroanal. Chem. 1976, 74, 107.
Effect of Triton X-100 on Quartz Wettability
Langmuir, Vol. 5, No. 1, 1989 29
an aqueous solution containing two surface-active compounds (alcohol,a, and surfactant, s) it is possible to derive from eq 329
-2,o.
Figure 4. Adsorption-relatedparameters versw logarithm of the surfactant concentration in distilled water. 4 represents in each cos 0 (curve b), (rLV/2.3R7')[(d cos fl)/(d case rsL(curve a), rLV log c ) ] (curve c), and YSv (curve d).
of the adsorption at each of the other interfaces, the contact angle, and the wetting film thickness when this film is present. When the wetting film breaks (h = 0) to conduce to a contact angle situation we have
The three terms on the right-hand side of eq 9 were measured experimentally in this work. The silica sample used for the adsorption tests was, in fact, different from the solid, impervious pieces of quartz used for the wettability studies. However, recent results2' indicate that contact angles measured on solid plates and in powders of the same material are identical. This means that, at least concerning the aggregation state of both samples, the experimental results may be used to obtain an estimation of the unknown quantity Ysv. The results of Figure 4 reveal that YSv is higher than YsL and also higher than rLV for relatively high surfactant concentration. Wolframz8 highlighted this point on the basis of an analysis of the derivatives of Young's equation with respect to the concentration of SAA and ascribed it to migration of surfactant across the air-water interface toward the solid-vapor interface. However, for a receding meniscus, the high value of Ysv should not be surprising because the deposition of Langmuir-Blodgett films on solid substrates is based precisely on this effect. Furthermore, as stated in a recent article, this result also reflects the depletion of the hydration sheath of the oxyethylenic portion of the surfactant molecule at the solid-vapor interfa~e.'~ The effect of ethanol on the properties of Triton X-100 is more important in solution than at the interfaces. For (27) Crawford, R.; Koopal, K. L.; Ralston, J. Colloids Surf. 1987,27, 57.
(28) Wolfram, E. Lecture Notes, University of Bristol, 1981.
Equation 10 indicates that the addition of a surfactant will further reduce the surface tension of an ethanol solution, because it will still adsorb to the aqueous ethanol solution-vapor interface. The saturation cross section areas for the surfactant molecules obtained by the application of Gibbs adsorption isotherm at the cmc of the three curves of Figure 1 are not very different, indicating that the decrease in surface tension is achieved mainly by replacement of ethanol by Triton X-100at the interface. The shifts observed for the adsorption a t the solid-solution interface are related to the solvent effect of the ethanol on the surfactant as illustrated by the changes in the cloud point shown in Table I. On the other hand, short-chain alcohols also adsorb at the solid solution interface. Most of the experimental studies have been carried out on hydrophobic solids for which the hydrocarbon end of the alcohol remains in contact with the surfa~e.~"However, when polar groups are present, adsorption occurs preferentially through interactions between these groups and the OH group of the alcohol.3l For silica or quartz, the main interactions are hyrogen bonds between the alcohol and the surface hydroxyl groups of the solid.32 This observation is further supported by results reported by Codes,%which indicate that the adsorption of methanol vapor on silica is considerably reduced when part of the silanol groups is suppressed by reaction with trimethylchlorosilane. In this context, the adsorption of Triton X-100,which also involves the formation of hydrogen bonds between the surface hydroxyls and the oxygen atoms of the oxyethylenic group of the surfactant, must proceed through the replacement of the ethanol by surfactant molecules on the solid surface. The results of Figure 2 show that when the concentration of ethanol is 25% (6 mol dm-3) the adsorption maximum is reduced to 0.8 pmol m-2, indicating that for this rather high concentration the surfactant does not completely dislocate the alcohol from the solid surface. The effect of ethanol on the contact angle basically agrees with the previous description. Consideringthat the adsorption at the interfaces is not seriously altered, there should be little change in the wettability of the solid. The reduction in the contact angles obtained for low and medium concentrations of SAA in the presence of ethanol is obviously linked to the reduction in yLv caused by the alcohol. An analysis similar to that illustrated in Figure 4 conduces to similar results except that curve b differs due to the changes in yLv caused by the ethanol, at low SAA concentrations. Registry No. S O 2 , 7631-86-9; quartz, 14808-60-7;Triton X-100, 9002-93-1; ethanol, 64-17-5. (29) Overbeek, J. Th. G. Faraday Discuss. Chem. Soc. 1978, 65,7. (30) Everet, D.H.;Fletcher, J. P. J. Chem. Soc., Faraday Trans. 1, 1986,82, 2605. (31) Ottewill, R.H.; Vincent, B. J. Chem. Soc., Faraday Trans. 1 1972, 68,1190. (32) Klimenko, N. A.; Koganoski, A. M. Kolloidn. Zh.1973,23,452. (33) Cortes, J.; Jenser, M.; Araya, J. J. Chem. Soc., Faraday Trans. 1 1986,82, 1351.
