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Wetting Effects Due to Surfactant Carryover through the Three-Phase Contact Line Jonny Eriksson,* Fredrik Tiberg, and Boris Zhmud Institute for Surface Chemistry YKI, Box 5607, Stockholm 114 86, Sweden Received March 19, 2001. In Final Form: August 24, 2001 Surfactant transport through the three-phase contact line is studied in relation to the wetting-enhancing effect of surfactants. A specially designed experimental setup is used to simultaneously monitor interfacial adsorption and wetting tension during forced immersion of a hydrophobic or hydrophilic plate into aqueous surfactant solutions. The adsorption at the liquid/solid interface has been measured by ellipsometry, the wetting tension has been determined according to the Wilhelmy plate method, and the adsorption to the liquid/vapor interface has been evaluated from surface tension measurements. The relative importance of different adsorption modes could thereby be grasped. For hydrophobic substrates, the dynamic wetting behavior is strongly affected by surfactant carryover through the advancing three-phase contact line, which appears to be the dominant mode of surfactant transport to the solid/liquid interface. Conversely, for hydrophilic substrates, three-phase contact line carryover of surfactant does not appear to be so important, the predominant transport mode being bulk diffusion.
Introduction Adsorption, wetting, and capillary flow are intimately related phenomena of large importance to numerous applications including flotation, detergency, enhanced oil recovery, painting and printing with waterborne systems, and the rapidly growing microfluidics. Control of these processes requires an understanding of dynamic phenomena occurring at the solid/liquid and liquid/vapor interfaces. The case in point is inkjet printing where limited spreading but rapid absorption of the ink vehicle is desired. Wetting behavior of solids is governed by the balance of interfacial tensions acting on the three-phase contact line. For smooth and chemically homogeneous substrates, this is accounted for by the Young equation. Adsorption leads to a buildup of the surface pressure, which in its turn causes a shift of the wetting tension balance. To predict the effect of added surfactants on wetting, in a general case, one needs to know the adsorption isotherms for all three interfaces. Furthermore, bulk depletion effects associated with the interface expansion during wetting need to be taken into account, a fact sometimes overlooked in experimental wetting studies. Drop spreading, capillary flow, and other dynamic wetting phenomena have been attracting a lot of attention from the scientific community during recent years.1-13 * Corresponding author. E-mail:
[email protected]. (1) Damania, B. S.; Bose, A. J. Colloid Interface Sci. 1986, 113, 321335. (2) Joanny, J. F. J. Colloid Interface Sci. 1989, 128, 407-415. (3) Gau, C. S.; Zografi, G. J. Colloid Interface Sci. 1990, 140, 1-9. (4) Tiberg, F.; Cazabat, A. M. Langmuir 1994, 10, 2301-2306. (5) Zhu, S.; Miller, W. G.; Scriven, L. E.; Davis, H. T. Colloids Surf., A 1994, 90, 63-78. (6) Frank, B.; Garoff, S. Colloids Surf., A 1996, 116, 31-41. (7) Stoebe, T.; Lin, Z.; Hill, R. M.; Ward, M. D.; Davis, H. T. Langmuir 1996, 12, 337-344. (8) Stoebe, T.; Lin, Z.; Hill, R. M.; Ward, M. D.; Davis, H. T. Langmuir 1997, 13, 7270-7275. (9) Hill, R. M. Curr. Opin. Colloid Interface Sci. 1998, 3, 247-254. (10) von Bahr, M.; Tiberg, F.; Zhmud, B. V. Langmuir 1999, 15, 70697075. (11) Starov, V. M.; Kosvintsev, S. R.; Velarde, M. G. J. Colloid Interface Sci. 2000, 227, 185-190. (12) Tiberg, F.; Zhmud, B.; Hallstensson, K.; von Bahr, M. Phys. Chem. Chem Phys. 2000, 2, 5189-5196.
But despite some obvious experimental and theoretical advances with respect to the understanding of the basic physicochemical factors and mechanisms controlling these processes, many questions still remain unanswered. Thus, to our knowledge, no systematic studies have ever been done to evaluate the true significance of the three-phase contact line transport phenomena in surfactant-enhanced wetting. This is at issue in the present, largely experimental study. By judiciously combining wetting force, surface tension, and ellipsometric adsorption measurements, the relative importance of different adsorption modes and surfactant transport mechanisms in surfactantpromoted wetting of hydrophobic and hydrophilic substrates is judged, bringing the issue to a close. Experimental Section Materials. Surfactant. Monodisperse hexaethylene glycol mono-n-tetradecyl ether, C14E6 (Nikko Chemicals), was used as the surfactant. For this surfactant, the value of the refractive index increment, dn/dc ) 0.135, which is needed for interpretation of ellipsometric data, is well-known from the previous studies.14 The critical micellar concentration (cmc) lies in the range from 0.006 to 0.01 mM.15,16 Substrates. Polished silicon wafers (dimensions 30 × 12.5 × 0.4 mm) were used as substrates. The substrates were cleaned in a mixture of 25% NH4OH, 30% H2O2, and H2O (1:1:5 by volume) at 80 °C for 10 min, rinsed by water, and then cleaned in a mixture of 25% HCl, 30% H2O2, and H2O (1:1:5 by volume) at 80 °C for 10 min and rinsed in plenty of water again. To produce hydrophobic substrates, the cleaned wafers were exposed to vapors of dimethyldichlorosilane for 24 h and then rinsed in toluene, ethanol, and water and dried at 200 °C for 1 h. All substrates were stored in ethanol until needed. Water contact angles measured by the Wilhelmy plate method were as follows: for the hydrophilic substrates, about 10° (advancing) and 0° (receding), and for the hydrophobic ones, about 106° (advancing) (13) von Bahr, M.; Kizling, J.; Zhmud, B.; Tiberg, F. In Advances in Printing Science and Technology; Bristow, J. A., Ed.; Pira International Ltd: Surrey, U.K., 2001; Vol. 27, pp 87-102. (14) Tiberg, F.; Jo¨nsson, B.; Tang, J.; Lindman, B. Langmuir 1994, 10, 2294-2300. (15) Zhmud, B. V.; Tiberg, F.; Kizling, J. Langmuir 2000, 16, 76857690. (16) Mukerjee, P.; Mysels, K. J. Critical Micelle Concentration of Aqueous Surfactant Systems; National Bureau of Standards (U.S.): Washington, DC, 1971.
10.1021/la0104119 CCC: $20.00 © 2001 American Chemical Society Published on Web 10/18/2001
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Figure 1. The scheme of the experimental setup for ellipsometric measurements of adsorption to a moving substrate. The adsorption is measured 2 mm below the advancing three-phase contact line. The same setup was used for the wetting force measurements. and 100° (receding). Milli-Q quality water was used in all experiments. Methods. Ellipsometry. A simplified scheme of the experimental setup used for studying adsorption from the three-phase contact line (tpl) can be seen in Figure 1. Adsorption to the immersing wafer was measured using the ellipsometric technique, the polarized light probe being focused on a fixed spot about 2 mm below the tpl. The movement of the substrate was controlled by a high precision dc motor. The adsorbed amount was obtained as a function of the immersion depth. An Optrel Multiscope null-ellipsometry instrument equipped with a laser having a wavelength of 5320 Å was used. The angle of incidence of the laser beam was 67.6°. The volume of the solution was 6 mL, and the temperature was 25 °C. The area of the liquid/vapor interface was 250 mm2. The adsorbed amount of surfactants was determined using the standard procedure due to Feijter.17 Before the surfactant adsorption experiments, a blank experiment had been run using pure water in order to characterize the properties of the clean substrate. During the experiment, the upper layer of water was continuously aspirated to keep the surface of water as clean as possible by removing any contaminants accumulated there. Surfactant solutions were prepared by adding a required amount of stock solution to the measuring cuvette. Prior to the adsorption measurement, the solution was stirred for 1 min and then left to equilibrate for 10 min. To facilitate reproducible measurements, this same timetable was followed rigorously in all experiments. As seen in Figure 2, 10 min was not enough for complete equilibration of the liquid/vapor interface. However, since the pre-equilibration time was known, the initial adsorption could be readily evaluated from the surface tension relaxation data. Estimated initial adsorption values are given in Table 1. The estimates were obtained on the basis of the equation
Figure 2. Surface tension relaxation dynamics for different concentrations of C14E6 surfactant. The relaxation dynamics reflects the characteristic times needed for the adsorbed layers to equilibrate at a given bulk concentration of the surfactant. The measurements were done using the Wilhelmy plate method.
Figure 3. Adsorption from the bulk solution to the motionless hydrophobic substrate at different bulk concentrations of surfactant. Table 1. Degree of Completion of the Adsorbed Monolayer at the lv Interface at the Beginning of the Adsorption Experiment and in Equilibrium -1] degree of completion, θ concn, surface tension, γ/[mN m c0/[mM] after 10 min equilibrium after 10 min equilibrium
0.001 0.002 0.005 0.010
65.0 52.3 34.7 31.5
45.8 40.4 33.1 31.8
0.580 0.913 0.990 0.993
0.961 0.980 0.992 0.993
(2)
to the equilibrium surface tension (γ) versus concentration (c0) data (see Table 1 as well). Thus, the initial conditions were welldefined. To get reference equilibrium adsorption values, adsorption from the bulk solution to the motionless substrate was also measured (Figures 3 and 4). In this way, the kinetics of adsorption from the bulk solution could be studied independently. Adsorption of poly(ethylene glycol) type surfactants has been shown to obey mixed, activation-diffusion-controlled kinetics.18,19 To achieve release from diffusion limitations, that is, to get the upper bound of the bulk adsorption rate, the solution was continuously stirred during the measurement. Wetting Force Measurements. The substrates were immersed into the surfactant solution under the same experimental conditions as used in the adsorption experiments. During the immersion, the wetting force was measured by the Wilhelmy plate method using a Kru¨ss K12 tensiometer.
(17) Feijter, J. A. d.; Benjamins, J.; Veer, F. A. Biopolymers 1978, 17, 1759-1772.
(18) Zhmud, B.; Tiberg, F.; Kizling, J. Colloids Surf., A, submitted. (19) Lin, S. Y.; Tsay, R. Y.; Lin, L. W.; Chen, S. I. Langmuir 1996, 12, 6530-6536.
[
θ ) 1 - exp -
γ0 - γ Γm lv RT
]
(1)
where θ is the degree of completion of the adsorbed monolayer at the liquid/vapor (lv) interface, Γm lv is the monolayer capacity, γ0 and γ are the surface tensions of pure water and surfactant solution, respectively, R is the gas constant, and T is the absolute -2 temperature. The value of Γm lv ) 3.26 µmol m , together with the Langmuir adsorption constant, KL ) 2.46 × 107 dm3 mol-1, was found by fitting the Szyszkowski equation,
γ ) γ0 - Γm lv RT ln(1 + KLc0)
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Figure 4. Comparison of adsorption kinetics for the hydrophilic and hydrophobic substrates. Adsorption from bulk solution to motionless substrates is considered. The surfactant concentration is 0.02 mM.
Results and Discussion General Analysis. As mention before, wetting-enhancing action of surfactants is explained by the surface pressure buildup occurring when surfactant is adsorbed to the solid/liquid (sl) and liquid/vapor (lv) interfaces. Adsorption of surfactant to the surface of the immersing substrate can in principle occur either by spillover of surfactant from the lv interface onto the sl interface (a conjecturable mechanism of capillary rise of surfactant solutions in hydrophobic capillaries12) or by diffusional transport of surfactant from the bulk solution to the sl interface. Here, an attempt is made to distinguish the relative importance of these different transport routes and to estimate the characteristic mass-transfer rates in the cases of hydrophobic and hydrophilic substrates. The first route involves transfer of surfactant from the lv interface to the sl interface. The kinetics of this process depends in a nontrivial way on the mobility of the adsorbed layer, the rate constants of the local adsorption processes, the diffusivity of the surfactant, and also the history of the system in question. As the lv interface is getting depleted of surfactant, a diffusional flux develops trying to restore the original surface excess of surfactant at the lv interface. Thus, the flow behavior of complex solutions in capillary systems has been found to vary strongly depending on the equilibration time of the lv interface. Similar effects have been observed in the studies on drop spreading of surfactant solutions at planar surfaces.10
Eriksson et al.
The second route involves direct adsorption of surfactant from the adjacent solution layer, known as the subsurface, to the solid surface. As the subsurface is getting depleted, a diffusional flux develops pumping surfactant from the bulk to the sl interface. It is useful to formulate general mass-transfer equations describing both cases. Let us consider what happens when a thin plate of width l is immersed into surfactant solution. A rectangular coordinate system with the z-axis originating at the lv interface and directed downward is a natural choice for modeling the system (see the sketch). If the plate velocity is constant and equal to v, the plate is immersed at a depth z for time t ) z/v. If the immersion rate is low enough, the disturbance of solution by the moving plate can be neglected. This eliminates the forced convection term, thereby drastically simplifying the mathematical picture of the mass transfer. Under the no-convection condition, the only mechanism of surfactant redistribution is diffusion. Let us formulate boundary conditions for the diffusion problem in question. If the solution has been equilibrated before the plate touches the liquid surface, the initial distribution of surfactant is given by
c(r,0) ) c0
r∈V
(3)
and the equilibrium adsorption to the lv interface is
Γeq lv ) F(c0)
(4)
where F is a functional representation of the adsorption isotherm. For the sake of simplicity, cell walls are assumed to be nonadsorbing; this restriction can easily be dropped if necessary. As soon as immersion begins, the surfactant starts to adsorb to the plate surface (sl interface). If adsorption occurs by the tpl transfer, then as immersion goes on, the lv interface is getting depleted of surfactant, and hence the surfactant starts to diffuse from the underlying solution to the top. A straightforward modification of the results obtained in ref 12 allows one to write the following equation,
{
[
Γsl(z) ) Γm sl 1 - exp -
]}
k+ slΓlv(t - z/v) vΓm sl
(5)
relating adsorption to the sl interface to the surface excess of surfactant at the lv interface. Here, Γm sl is the monois the tpl transfer rate layer adsorption capacity, and k+ sl constant. Furthermore, writing down the mass conservation equation,
2l
∫Γsl(z) dz ) S[Γlv(0) - Γlv(t)] + [c(r,0) - c(r,t)] dV
(6)
where S is the area of the lv interface, dz ) vdt is the immersion depth, and 2l is the perimeter of the plate, one gets
dΓlv ∂c 2lv ) D |z)0 Γ | dt ∂z S sl z)0
(7)
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Immediately after the plate has touched the surface (at t ) 0),
{
[ ]}
eq 2lvΓm k+ ∂c sl slΓlv D |z)0 ) 1 - exp ∂z S vΓm sl
(8)
and over a short initial period of time, the depletion of the lv interface is negligible, and hence Γsl should not change much with time. Apparently, this is what is observed at high concentrations of surfactant. Finally, assuming that adsorption to the lv interface is diffusion-controlled, one can easily find the concentration of surfactant in the subsurface region by inverting eq 4,
c(0,t) ) F-1[Γlv(t)]
(9)
Together with no-flux constraints that need to be set at cell walls, eqs 3, 5, 6, and 7-9 compose the necessary boundary conditions for the diffusion problem. In the opposite limit, where tpl transfer is of no importance, the diffusion problem should be coupled with another boundary condition, the classical one in this case,
[
dΓ/dt Dn‚∇c ) 0
at adsorbing interfaces (10) at nonadsorbing ones
where n is the unit normal erected from the corresponding interface. The initial state of the lv interface may still exert some influence of the surfactant redistribution in the system by virtue of eq 6. In particular, as adsorption to the sl interface gradually depletes the bulk solution, surfactant will start to desorb from the lv interface. A cautionary note needs to be stated regarding a few tacit assumptions underlying the present formulation. First, the concentration gradient over the lv interface is neglected, although it is this gradient that forces surfactant to move to the sl interface as the plate is being immersed. A justification for neglecting the surface gradient while formulating the boundary conditions for the diffusion problem is that the adsorbed layer at the lv interface is mobile which permits fast equalization of the surface excess throughout the interface. On the contrary, there may be a significant concentration gradient over the plate surface, but inasmuch as adsorption to the sl interface is assumed to be irreversible and the adsorbed layer immobile, the surface diffusion is not possible. Finally, as noted before, any collateral effects caused by convection in the cell are neglected. It is interesting to investigate a limiting case where the lv interface is strongly depleted. Over a reasonably short period of time, until the diffusion zone has reached the bottom of the cell, the concentration profile near a completely depleted interface can approximately be described by the formula corresponding to the semi-infinite diffusion,
[ ]
c(z,t) ) c0 erf
z 2xDt
(11)
Since Γlv is close to zero, the tpl transfer has to be compensated by the diffusional flux, which gives
Γsl )
xDt
c0S 4lv
(12)
that is, Γsl should decrease proportionally to the reciprocal square root of time.
Figure 5. Adsorption of surfactant to the hydrophobic substrate at different bulk concentrations of C14E6. The adsorption values vary depending on the immersion depth.
In another limiting case, where the diffusional transport is extremely slow, Γsl is to be determined from a transcendental equation,
{
[
Γsl ) Γm sl 1 - exp -
k+ sl vΓm sl
(Γ
eq lv
-
2lvt Γ S sl
)
]}
(13)
In this case, Γsl also decreases with time. In particular, if k+ sl is small enough, one simply gets
Γsl )
eq Sk+ slΓlv
v(S + 2k+ sllt)
(14)
Effect of Surfactant Concentration. Figure 5 shows how the adsorption to the hydrophobic substrate depends on the bulk concentration of surfactant. The rate of substrate immersion was 0.2 mm s-1. The fact that the adsorbed amount decreases with increasing the immersion depth needs to be discussed in detail. This decrease is only seen at relatively low surfactant concentrations, when the diffusional transport is not intensive enough to restore the equilibrium level of surfactant at the lv interface during the time of the experiment. For the highest surfactant concentrations, the adsorbed amount was constant and equal to 3.1 µmol m-2, which is close to the equilibrium adsorption value measured in a time-resolved adsorption experiment (see Figure 3). The rate of direct adsorption from bulk solution to the sl interface is much slower than it would need to be in order to produce the surface excess values presented in Figure 5. As can be seen in Figure 3, the direct adsorption from the bulk contributes only about 0.013 µmol m-2 s-1 in the case of 0.01 mM solution and 0.037 µmol m-2 s-1 in the case of 0.1 mM solution. By design, the adsorption time is equal to the immersion time. For the immersion rate of 0.2 mm s-1, the latter is about 10 s by the time the first point is measured. This means that the direct adsorption cannot contribute more than about 10% to the measured surface excess. The actual contribution is even lower due to the fact that in the immersion experiment the surfactant solution was not stirred. Thus, one can conclude that the surfactant adsorbed at the sl interface in the experiments shown in Figure 5 arrives there mainly by tpl transfer from the lv interface. At low surfactant concentrations, the variation in the adsorbed amount with the immersion depth reflects the depletion of the lv interface. The observed values are quite reasonable, since the area of the newly generated sl
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Figure 6. Adsorption of surfactant to the hydrophilic substrate at different bulk concentrations of C14E6. The variations in the adsorbed amounts with increasing the immersion depth are much smaller than those for the hydrophobic substrate (cf. Figure 5).
interface (375 mm2 at the deepest immersion) is comparable with the area of the lv interface (250 mm2). In fact, one can easily estimate the depletion value as far as the adsorption kinetics for adsorption from the bulk solution to the lv interface are known (see Figure 2). For more concentrated solutions, the readsorption at the lv interface is sufficiently fast to keep the adsorbed amount constant during immersion, and as a result, the surface excess on the sl interface does not change with immersion. In Figure 6, the adsorption onto a hydrophilic substrate is shown. The adsorbed amount does not change much during the immersion and is fairly small, about 0.2 µmol m-2 for 0.01 mM solution, as compared to the saturation value, 4.6 µmol m-2 (see Figure 4). The latter is higher than that for the hydrophobic surface, because a bilayer rather than a monolayer is formed. The principal difference between these two cases is schematically shown in the following picture:
Although one cannot exclude the possibility that the lv interface provides surfactant for the expanding sl interface, the intensity of tpl carryover in this case must be low. Otherwise, some depletion of the lv interface would occur, and the amount of surfactant transferred to the sl interface would be decreasing during immersion. The adsorption from the bulk solution to motionless substrates also proved to be significantly slower for the hydrophilic substrate as compared to the hydrophobic one (see Figure 4). Apparently, the formation of a bilayer or surface micelles is not as fast as the formation of a monolayer. Effect of Immersion Rate. If the hypothesis that the tpl transfer is the dominant mode of surfactant transport to the substrate surface is valid, there should not be seen any effect of the immersion rate on the adsorption values, provided that k+ sl is sufficiently large, while the bulk diffusion is sufficiently slow (cf. eq 13). To validate this
Eriksson et al.
Figure 7. Effect of the substrate immersion rate on adsorption to the hydrophobic substrate.
Figure 8. Effect of the substrate immersion rate on adsorption to the hydrophilic substrate.
conclusion, experiments at two different immersion rates were carried out. The results obtained are represented in Figures 7 and 8. In the case of the hydrophobic substrate (Figure 7), the adsorption measurements were carried out at the surfactant concentrations of 0.002 and 0.005 mM and the immersion rates of 0.2 and 0.7 mm s-1. Under such conditions, the dynamic surface tension relaxation time for the lv interface is of the order of 1000 s, while the maximum immersion time does not exceed 100 s. Therefore, the experiment time is not enough for the bulk diffusion to fully replenish the depleted lv interface with surfactant. As a result, despite the fact that the substrate exposure time has changed by a factor of more than 3, there is no significant difference in the adsorption values measured at the sl interface at these two different immersion rates. This also indicates that in the time scale of the experiment the tpl transfer is a sufficiently fast process. Under such circumstances, the rate-determining step is the diffusion of surfactant from the bulk solution to the lv interface. In the case of the hydrophilic substrate (Figure 8), much higher surfactant concentrations were needed for measurable adsorption levels to be obtained; the concentrations used were 0.05 and 0.1 mM, and the immersion rates were 0.2 and 0.6 mm s-1. In contrast to the previous case, a significant effect of the immersion rate on the adsorbed amounts has been observed. The reason for this can be ambiguous: both the tpl transfer and diffusion from the bulk might contribute to surfactant transport to the sl interface. The adsorption kinetics appears to be activationcontrolled in this case, and the immersion time is rather short for local adsorption equilibria to be attained.
Wetting Effects Due to Surfactant Carryover
Figure 9. Variation in the wetting force during immersion of the hydrophobic substrate. The wetting force at different surfactant concentrations was measured by the Wilhelmy plate method.
Figure 10. Variation in the wetting force during immersion of the hydrophilic substrate. The wetting force at different surfactant concentrations was measured by the Wilhelmy plate method.
However, a close similarity between the adsorption kinetics measured for immersing substrates and that measured for motionless substrates placed in the bulk solution suggests that in the case of the hydrophilic substrate the dominant surfactant transport mode is the diffusional transport from the bulk. Wetting Force Measurements. Results from the wetting force measurements are shown in Figures 9 and 10. In the absence of surfactant, that is, when pure water is used as a wetting liquid, the measured force appears to be constant, for both the hydrophobic and hydrophilic substrates. As expected, in the hydrophobic case (Figure 9) the force is negative, which suggests that the spreading, associated with forced expansion of lv and sl interfaces during substrate immersion, is energetically unfavorable, that is, γsv - (γlv + γsl) < 0, while in the hydrophilic case (Figure 10) the force is positive, that is, γsv - (γlv + γsl) > 0 and the spreading is spontaneous. Addition of surfactant affects the wetting balance in different ways for hydrophobic and hydrophilic substrates. For the hydrophobic substrate (see Figure 9), surfactant enhances wetting. Thus, the wetting force measured at the highest surfactant concentration, 0.1 mM, is positive and fairly constant. This agrees with the conclusion drawn while discussing the adsorption data that at such a high surfactant concentration the lv interface manages to regain the surfactant lost due to carryover to the sl interface extremely fast. In other words, the lv interface never gets depleted, which guarantees that a constant amount of surfactant is transferred to the sl interface. In
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its turn, this guarantees a constancy of the surface tensions, γlv and γsl, and therefore of the wetting balance as a whole. At lower surfactant concentrations, the wetting force decreases during immersion, which correlates with the decreasing adsorbed amounts in Figure 5. According to the Gibbs adsorption equation, a decrease in the adsorbed amounts means an increase in the surface tensions, γlv and γsl, which entails a drop in the wetting force. For the hydrophilic substrate, the picture is completely different. The wetting force decreases while increasing the surfactant concentration up to the cmc and does not change after that. Furthermore, the wetting force remains constant during immersion. This suggests that the lv interface is primarily responsible for the observed effects. Really, it is the surface tension, γlv, of the lv interface that decreases steadily with increasing the surfactant concentration and levels off after the cmc has been exceeded. On the basis of the adsorption data, it has been concluded that for the hydrophilic substrate the lv interface is not the primary supplier of surfactant for the sl interface, which means that the lv interface never gets depleted. This implies that γlv remains constant during substrate immersion, which explains the constancy of the wetting force. One more note needs to be stated concerning the data in Figure 10. When looking more closely at the wetting curve measured at 0.001 mM concentration, one can still notice a very small decrease in the wetting force during the immersion. This is explained by the fact that the lv interface failed to equilibrate fully before the measurement (see Figure 2). Concluding Remarks What has not so far been discussed in this work is the possibility of adsorption of surfactant to the solid/vapor interface in front of the moving three-phase contact line. As claimed by Starov et al.,11 this controls the spreading dynamics of droplets of anionic surfactants on hydrophobic surfaces. Furthermore, Joanny2 advanced an opinion that in a system where the affinity for adsorption to the lv and sl interfaces is similar, the Marangoni effect might be in control of the spreading. The existence of such phenomena was beyond the scope of the present study. Instead, the significance of tpl transfer in surfactant-enhanced wetting of hydrophobic substrates is emphasized. When a surfactant solution advances over a hydrophobic substrate, adsorbate can spill from the lv interface on the sl interface. Redistribution of surfactant at the lv interface is very fast as compared to the bulk diffusion; hence, the diffusion of surfactant to the lv interface is the rate-determining stage of the spreading process. On the contrary, when a surfactant solution advances over a hydrophilic substrate, the adsorption of surfactant to the sl interface obeys activationcontrolled kinetics, and the dominant transport route is the diffusion from the bulk solution directly to the sl interface. As a result, the lv interface and the sl interface are largely independent from each other until the bulk solution is depleted of surfactant. The wetting balance is primarily influenced by adsorption of surfactant to the lv interface. This is especially true as applied to the dropspreading case,10 where the lv interface is expanding as a drop spreads over a substrate. Acknowledgment. The financial support for this research was provided by the Foundation for Strategic Research (SSF) within the framework of the Colloid and Interface Technology program. LA0104119
