Relation between the Variation of the Contact Angle and the Adhesion Force between Silica Surfaces

Journal of Colloid and Interface Science 216, 432– 435 (1999) Article ID jcis.1999.6324, available online at http://www.idealibrary.com on

NOTE Relation between the Variation of the Contact Angle and the Adhesion Force between Silica Surfaces EXPERIMENTAL SECTION Adhesion force and contact angle measurements between an aqueous solution of the nonionic surfactant TN 101 and silica surface have been studied. The variation of the contact angle is a function of the adsorption of the surfactant at the liquid–vapor, solid–liquid, and solid–vapor interfaces. The contact angle is maximum well below the cmc, when a monolayer of surfactant is formed at the silica surface. To this maximum contact angle corresponds a maximum adhesion force between the silica surfaces. This force is due to the hydrophobic interaction between the alkyl chain of the surfactant adsorbed at the silica surface. © 1999 Academic Press

INTRODUCTION According to the DLVO theory (1–2), the stability of colloidal suspensions in a dielectric medium is determined by the repulsive ion– electrostatic force between the overlapping electrical double layers (F e ) and the attractive London–van der Waals (dispersion) force (F d ). Thus the net interaction force (F t ) is given by the algebraic sum

F t 5 F e 1 F d.

The repulsive force decays approximately exponentially with the distance of separation, while the attractive force decays with a power law. Despite its honored success, application of the classical DLVO theory has its limitations. When the dielectric medium in which colloidal particles are suspended is water, the theory generally fails to predict the stability of very hydrophilic and very hydrophobic suspensions. It is now known that other forces exist and can explain the stability of these suspensions (3– 6). For the case of hydrophobic surfaces, the extraneous force is called hydrophobic force (7–11). It has been shown that this force is attractive and can be 100 times larger than the London–van der Waals force. This hydrophobic force can be expressed by a simple or a double-exponential function. The purpose of the present investigation was to determine the variation of the adhesion force, which is defined as the maximum force required to pull two surfaces apart after an initial contact, due to the hydrophobic interaction between a silica plate and silica sphere using an AFM (12–14). These adhesion force measurements were compared with the variation of the wettability of the silica surface induced by the adsorption of an alkylphenol nonionic surfactant TN 101. The nonionic surfactants like polyoxyethylenic derivatives of alkylphenols are commonly used as surfactants in many industrial processes such as powder dispersion, detergency, flotation, and tertiary oil recovery. In all these processes surfactant solutions interact with the solid surfaces and as a result, surfactant molecules may adsorb on the solid surfaces and modify their wettability (15–18). 0021-9797/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.

The silica dioxide used in the present study was supplied by Rhoˆne-Poulenc, Aubervilliers Laboratory, France. It was obtained by precipitation from sodium silicate solution. The mean particle size, observed in a scanning electron micrograph was about 0.1 mm. The specific surface area measured by nitrogen adsorption at 77 K (BET method, a m (N 2) 5 16.2 Å 2) was found to be 40 m 2 g 21. The TN 101 surfactant is a nonionic surfactant containing an average of 10 oxyethylene units per molecule. It was manufactured and supplied by SEPPIC (France) with high grade of purity (the curve of surface tension g versus the logarithm of the concentration ln c shows no minimum near the critical micelle concentration (cmc) region). The surfactant solutions were prepared with water which was distilled after deionization on ion exchange resins. The surface tension of water was always tested before preparing the solutions. The force measurements were taken with a Digital Instruments, Inc. (Santa Barbara, CA) Nanoscope III atomic force microscope. The adhesion force measurements were performed by determining the deflection of the cantiliver, which, as the spring constant is known, gives the force. The spring constants of the cantilever used in these experiments were determined by the method of Cleveland et al. (19) and was found to be 0.54 N m 21. The receding contact angle was measured at 25°C under atmospheric pressure, by the Kruss processor tensiometer K12 using the Wilhelmy plate method. The apparatus was controlled by a computer which also analyzed the results. The temperature was controlled within 60.1°C with thermostated water, from a Haake-C thermostat, circulating through a jacket surrounding the vessel containing the surfactant solution.

RESULTS AND DISCUSSION Figure 1 shows the variation of the surface tension of the aqueous solutions of TN 101 versus the logarithm of the surfactant concentration. The surface tension decreases until the cmc is reached, at 1.2 3 10 24 mol l 21. At the cmc a compact monolayer of surfactant molecules is formed. Above the cmc value the surface tension of the aqueous solution of the surfactant remains constant and equal to about 31.1 mN m 21. From the value of the surface tension at the cmc, we can calculate the area per molecule of the surfactant in the compact monolayer at the solution–air interface, by applying the Gibbs equation (20). The area per molecule (a 0 ) at this interface is about 118 Å 2. The adsorption isotherm of the nonionic surfactant TN 101 onto silica is shown in Fig. 2. The results indicate that there are three well-defined regions in the isotherm. At the low surface coverage, i.e., for the low concentrations of the surfactant, the adsorption is very weak. Following this region, there is an increase of the amount adsorbed until a saturation plateau is reached, at an equilibrium concentration of about 1.5 3 10 24 mol l 21. The amount adsorbed at the saturation plateau is about 2.8 mmol m 22. From the amount adsorbed at the plateau, an area per molecule of the nonionic surfactant at the silica– solution interface can be calculated, a 1

432

a1 5

1 , G maxN

433

NOTE

g LVcos u 5 g SV 2 g SL,

[1]

where g LV is the surface tension of the liquid, g SV the surface tension of the solid, g SL the interfacial tension between the solid and the liquid, and u the contact angle at the solid–liquid–vapor interface, or its derivative, d~ g LVcos u ! d~ g SV 2 g SL! 5 , d ln c s d ln c s

[2]

where ln c s is the logarithm of the surfactant concentration. On the other hand, the Gibbs equation gives the relation between the surface tension (d g i ), the amount adsorbed (G i ), and the chemical potential ( m i ) of the component i, 2d g i 5

O

G id m i,

[3]

i

where m i 5 m i0 1 RT ln( f i c i ). If the above equation is applied to the liquid–vapor (LV), solid–liquid (SL), and solid–vapor (SV) interfaces, the following relationships may be obtained, assuming that the activity coefficient ( f i ) is equal to 1.

FIG. 1. Surface tension versus the logarithm of the concentration of TN 101 in water at 25°C.

where G max is the amount adsorbed at the saturation plateau and N is Avogadro’s number. The area per molecule calculated from the above equation is about 61 Å 2. The value of a 1 is about two times less than the value of a 0 . The comparison between a 0 and a 1 indicate that the adsorption of the surfactant at the saturation plateau is attained when a statistical bilayer of surfactant is formed on the silica surface (6, 21). The variation of the wettability of the silica surface upon adsorption of the nonionic surfactant TN 101 was determined by the measurements of the receding contact angles by the Kruss processor tensiometer K12 using the Wilhelmy plate method. The values of the contact angle as a function of the logarithm of the concentration of the surfactant are shown in Figure 2. The contact angle, u, increases for low surfactant concentration, reflecting the adsorption of the surfactant molecules onto the silica surface via the interaction between the hydrophilic surface and the polar head group of the surfactant. The alkyl chain is oriented toward the solution phase. Therefore the silica surface becomes more hydrophobic with the adsorption of the surfactant. The contact angle reaches a maximum at the surfactant concentration of about 3.7 3 10 25 mol l 21 , well below the cmc value. This concentration corresponds to an amount adsorbed of 1.35 mmol m 22 . The area per surfactant molecule calculated from this value is about 123 Å 2 , closely related to the value of the compact monolayer at the air–solution interface, i.e., 118 Å 2 . Therefore, at this concentration, a monolayer of surfactants is believed to be formed at the silica–solution interface with the alkyl chain oriented toward the solution. This explains why the contact angle is maximum and equal to 66°. After reaching a maximum, the contact angle decreases because the adsorption of the surfactant continues by the hydrophobic interaction between the alkyl chain of the surfactant. In this case the polar headgroup of the surfactant is oriented toward the solution. The silica surface becomes again more and more hydrophilic until the complete formation of a bilayer at the silica–solution interface, around the cmc value. The contact angle reaches a minimum, 10°. The variation of the contact angle, i.e., the hydrophilic– hydrophobic properties of the silica surface, upon the adsorption of a surfactant may be described by Young’s equation,

2d g LV 5 RTGLVd ln c s

[4]

2d g SL 5 RTGSLd ln c s

[5]

2d g SV 5 RTGSVd ln c s

[6]

If we introduce Eqs. [3]–[5] into Eq. [2], we obtain G SV 5 G SL 1 G LVcos u 2

g LV RT

S

d cos u d ln c s

D

.

[7]

Equation [7] gives the unknown quantity G SV as a function of the adsorption of the surfactant at each of the other interfaces and the contact angle. Therefore,

FIG. 2. Adsorption isotherm (G SL) and contact angle (u) for TN 101 on silica in water at 25°C.

434

NOTE

from the contact angle measurement and the amounts adsorbed at the solid– liquid (G SL) and liquid–vapor (G LV) interfaces, the adsorption of the nonionic surfactant TN 101 at the silica–vapor interface may be calculated. The three terms on the right-hand side of Eq. [7] were determined experimentally from the surface tension measurements (Fig. 1), the adsorption isotherm (Fig. 2), and the contact angle measurements (Fig. 2). The variation of G SV versus the logarithm of the surfactant concentration is shown in Fig. 3. The results reveal that at the concentration below 1 3 10 25 mol l 21, the adsorption of the surfactant at the liquid–vapor interface is about 1.8 mmol m 22 while it is weak for the solid–liquid and solid–vapor interface. Above 1 3 10 25 mol l 21, G LV remains roughly constant while G SL and G SV increase. For relatively high surfactant concentration, G SV is higher than G LV and also higher than G SL. The concentration (3.7 3 10 25 mol l 21) at which G SV becomes higher than G SL corresponds to the maximum value of the contact angle. The adsorption of the surfactant at the solid–vapor interface can be ascribed to the migration of the surfactant molecules across the solution–air interface toward the solid–vapor interface. Therefore, the variation of the contact angle between the surfactant solution and the silica surface is not only due to the adsorption of the surfactant at the solid–solution interface. Adsorption of the surfactant at the solid–vapor interface must play an important role. The adhesion force between the silica plate and the sphere has been measured by an AFM. Without any addition of the nonionic surfactant, the force measurements reveal an absence of a jump in to contact and therefore an absence of the adhesion between the silica plate and the sphere. The attractive force does not dominate the net interaction between the two surfaces because of the additional hydration force at small separation which masks the attractive van der Waals force (22–23). This force is believed to derive from the bound water molecules hydrating the surface. When the surfactant concentration was increased to 3.7 3 10 25 mol l 21, a strong adhesion force F ad/R 5 88 mN m 21 is observed; see Fig. 4. This adhesion force corresponds to approximately the maximum of the contact angle value (66°). It is expected as we have previously shown that at this concentration, a monolayer of the surfactant is formed at the solid–liquid interface with the hydrophobic moiety oriented toward the liquid phase. As a consequence, the silica surface becomes completely hydrophobic. Therefore the measured adhesion force is largely due to hydrophobic interac-

FIG. 4.

Adhesion force versus the surfactant concentration of TN 101.

tion. The London–van der Waals (dispersion) and the ion– electrostatic forces calculated using the Hamaker constant, A 5 1.2 10 220 J for silica in water (23, 24) and a Stern potential, c s 5 260 mV, respectively, are much less than the measured force. From the adhesion force and the contact angle value, it is possible to calculate the surface free energy (g SV) of the silica surface coated with a monolayer of the nonionic surfactant to be 7 mJ m 22. This value is less than the surface free energies of many hydrocarbon surfaces, which are in the range of 22–30 mJ m 22. Around this concentration, the adhesion force is very sensitive to the change of the surfactant concentration. This is obvious, since the amount adsorbed is an extremely steep function of the concentration. As the concentration was increased to 8 3 10 25 mol l 21 or decreased to 6 3 10 26 mol l 21, the adhesion force decreases and becomes negligible because the contact angle decreases in both cases, which means that the silica surface becomes more hydrophilic. When the concentration was increased to the cmc of the nonionic surfactant (1, 2 3 10 24 mol l 21), the nature of the interaction changed significantly; a strong repulsive barrier appeared between the silica surfaces. At the cmc, the silica surface is completely hydrophilic because of the formation of a bilayer surfactant. Then, when the silica surfaces comes into contact, the interpenetration of two adsorbed bilayers of surfactants can result in a loss of transformational freedom and so to a loss of entropy and leads to repulsion between the surfaces (25–27). The variation of the adhesion force due to the hydrophobic interaction between the alkyl chain of the nonionic surfactant was followed by turbidity measurements of a silica suspension (28). The results show that the turbidity is constant at low and high concentration but decreases at concentration around 3.7 3 10 25 mol l 21 which corresponds to the maximum contact angle and then to the maximum value of the hydrophobic interaction. Around this concentration, the suspension coagulated.

REFERENCES FIG. 3. Adsorption isotherms (G LV), (G SL), and (G SV) for TN 101 in water at 25°C.

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