Langmuir 2005, 21, 1875-1883
1875
Forces between Silica Surfaces with Adsorbed Cationic Surfactants: Influence of Salt and Added Nonionic Surfactants Johanna Stiernstedt,† Johan C. Fro¨berg,‡,§ Fredrik Tiberg,‡,| and Mark W. Rutland*,† Department of Chemistry, Surface Chemistry, Royal Institute of Technology, SE-100 44 Stockholm, Sweden, and Institute for Surface Chemistry, Box 5607, SE-114 86 Stockholm, Sweden Received September 7, 2004. In Final Form: November 11, 2004 Forces have been measured between silica surfaces with adsorbed surfactants by means of a bimorph surface force apparatus. The surfactants used are the cationic surfactant tetradecyltrimethylammonium bromide (TTAB) and the nonionic surfactant hexakis(ethylene glycol) mono-n-tetradecyl ether (C14E6) as well as mixtures of these two surfactants. The measurements were made at elevated pH, and the effect of salt was studied. At high pH the glass surface is highly charged, which increases the adsorption of TTAB. Despite the low adsorption generally seen for nonionic surfactants on silica at high pH, addition of C14E6 has a considerable effect on the surface forces between two glass surfaces in a TTAB solution. The barrier force is hardly affected, but the adhesion is reduced remarkably. Also, addition of salt decreases the adhesion, but increases the barrier force. In the presence of salt, addition of C14E6 also increases the thickness of the adsorbed layer. The force barrier height is also shown to be related to literature values for surface pressure data in these systems.
Introduction The adsorption of surfactants to solid surfaces is of great interest in many industrial processes. One application is the addition of avivage in yarn making. Avivage generally consists of a mixture of cationic and nonionic surfactants, and its main role is to reduce the friction between the fibers. It is, however, crucial that the friction not become too low, as the fibers will then separate and no yarn can be formed. The relationship between the nature of adsorbed surfactant aggregates and parameters such as micellar charge, surface charge, and salt concentration is furthermore a question of fundamental interest, and varying the surfactant composition provides an additional complexity. Adsorption from surfactant systems containing mixtures of ionic and nonionic surfactants is also of interest in other contexts, for example deinking or detergency. The structure of surfactant aggregates in bulk is relatively well-known and has been shown to be dependent on the relation between the size of the headgroup and the hydrocarbon chain in terms of the critical packing parameter, CPP.1 Spherical and cylindrical aggregates have a diameter of roughly twice the length of the hydrocarbon chain in its most extended conformation, whereas bilayers have a thickness of approximately 1.6 times the chain length.2 TTAB has a 1.9 nm long hydrocarbon chain and forms spherical aggregates in bulk. A smooth macroscopic surface, such as those employed in the MASIF measurements, is flat compared to the size of * To whom all correspondence should be addressed. Phone: +46 8 790 99 14. Fax: +46 8 20 89 98. E-mail: mark.rutland@ surfchem.kth.se. † Royal Institute of Technology. ‡ Institute for Surface Chemistry. § Present address: Ho ¨ gskoleverket, Box 7851, SE-103 99 Stockholm, Sweden. | Present address: Camurus AB, So ¨ lvegatan 41, SE-223 70 Lund, Sweden. (1) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525-1568.
the surfactants, and thus, geometrically the largest adsorbed amount is obtained in bilayer adsorption. A question of debate has been as to whether surfactants adsorb as bilayers or if the type of aggregates formed in bulk is also important for the structure of the adsorbed layer. The adsorption of cetyltrimethylammonium bromide (CTAB), which has two more carbons in its hydrocarbon chain than TTAB, on mica has been extensively measured with the surface force apparatus (SFA)3-7 and has been found to form bilayers. Mica is highly negatively charged, and thus it can template the cationic surfactants into planar aggregates. However, surfactants with shorter chain length give the aggregates higher curvature, and TTAB8-10 and DTAB11 (dodecyltriammonium bromide) have both been found to form stable cylindrical aggregates on mica. These studies were made with atomic force microscopy (AFM) imaging. Though extremely informative, the AFM images only give information about the outer structure and do not shed light on the structure of the inner surfactant layer closest to the solid surface, which may well be planar. The significance of the surface is seen when changing from mica to silica, which has a much lower surface charge (2) Evans, D. F.; Wennerstro¨m, H. Colloidal Domain: Where Physics, Chemistry, Biology, and Technology Meet; 2nd ed.; Wiley-VCH: New York, 1999. (3) Pashley, R. M.; Ninham, B. W. J. Phys. Chem. 1987, 91, 29022904. (4) Pashley, R. M.; McGuiggan, P. M.; Horn, R. G.; Ninham, B. W. J. Colloid Interface Sci. 1988, 126, 569-578. (5) Pashley, R. M.; Israelachvili, J. N. Colloids Surf. 1981, 2, 169187. (6) Kekicheff, P.; Christenson, H. K.; Ninham, B. W. Colloids Surf. 1989, 40, 31-41. (7) Chen, Y. L.; Chen, S.; Frank, C.; Israelachvili, J. J. Colloid Interface Sci. 1992, 153, 244-265. (8) Manne, S.; Gaub, H. E. Science 1995, 270, 1480-1482. (9) Patrick, H. N.; Warr, G. G.; Manne, S.; Aksay, I. A. Langmuir 1999, 15, 1685-1692. (10) Schulz, J. C.; Warr, G. G. Langmuir 2000, 16, 2995-2996. (11) Ducker, W. A.; Wanless, E. J. Langmuir 1996, 12, 5915-5920.
10.1021/la047763a CCC: $30.25 © 2005 American Chemical Society Published on Web 01/22/2005
1876
Langmuir, Vol. 21, No. 5, 2005
at ambient pH. From analysis of surface forces, the evidence for bilayer formation of CTAB on silica is inconclusive,12,13 and the structure has been interpreted as patchy bilayers. Also, neutron reflection studies concluded that the surface coverage was not high enough for a full bilayer, but proposed to be strongly flattened surface micelles.14 Later, AFM images have shown spherical aggregates of CTAB, TTAB, and DTAB on silica.8,15,16 Addition of salt reduces the curvature of bulk aggregates by screening the electrostatic repulsion between the headgroups and the spherical aggregates of CTAB, TTAB, and DTAB change to cylindrical with addition of salt.17 In addition, salt reduces the space between the aggregates16 and increases the adsorbed amount. The surfactant aggregate structure is also dependent on the type of counterion,16 so that the greater the counterion binding, the lower the aggregate curvature. The initial adsorption of cationic surfactants to silica is due to electrostatic attraction, giving higher adsorbed amounts at higher pH when the silica has higher charge density. However, the chemistry of the silica is also important for the adsorption mechanism; when one cationic surfactant adsorbs onto a charged silanol group, the local electric field is perturbed such that a neighboring silanol group may then dissociate to form a new charged group.18 Hence, the cationic surfactants in these systems have, in addition to the hydrophobic driving force always present in surfactant systems, also an electrostatic driving force to adsorb next to an already adsorbed surfactant. This behavior of the silica enhances patchwise or aggregate adsorption.19-21 Nonionic surfactants, on the other hand, are thought to form hydrogen bonds with the hydroxyl groups on the substrate surface (mainly silica), which early findings by Rupprecht22,23 suggested and numerous studies have supported using a range of techniques such as force measurements,24-27 AFM imaging,28 ellipsometry,29 and neutron reflection.30-33 The adsorption of nonionic surfactants is very small up to the critical aggregation (12) Rutland, M. W.; Parker, J. L. Langmuir 1994, 10, 1110-1121. (13) Parker, J. L.; Yaminsky, V. V.; Claesson, P. M. J. Phys. Chem. 1993, 97, 7706-7710. (14) Fragneto, G.; Thomas, R. K.; Rennie, A. R.; Penfold, J. Langmuir 1996, 12, 6036-6043. (15) Subramanian, V.; Ducker, W. A. Langmuir 2000, 16, 44474454. (16) Velegol, S. B.; Fleming, B. D.; Biggs, S.; Wanless, E. J.; Tilton, R. D. Langmuir 2000, 16, 2548-2556. (17) Schulz, J. C.; Warr, G. G.; Butler, P. D.; Hamilton, W. A. Phys. Rev. E 2001, 63, art. no. 041604. (18) Iler, R. K. The Chemistry of Silica; John Wiley & Sons: New York, 1979. (19) Gao, Y. Y.; Du, J. H.; Gu, T. R. J. Chem. Soc., Faraday Trans. 1 1987, 83, 2671-2679. (20) Partyka, S.; Lindheimer, M.; Faucompre, B. Colloids Surf. A 1993, 76, 267-281. (21) Rupprecht, H.; Gu, T. Colloid Polym. Sci. 1991, 269, 506-522. (22) Rupprecht, H. Kolloid Z. Z. Polym. 1971, 249, 1127-1132. (23) Rupprecht, H. Prog. Colloid Polym. Sci. 1978, 65, 29-44. (24) Rutland, M. W.; Senden, T. J. Langmuir 1993, 9, 412-418. (25) Rutland, M. W. Colloids Surf. A 1994, 83, 121-128. (26) Giasson, S.; Kuhl, T. L.; Israelachvili, J. N. Langmuir 1998, 14, 891-898. (27) Tiberg, F.; Ederth, T. J. Phys. Chem. B 2000, 104, 9689-9695. (28) Grant, L. M.; Tiberg, F.; Ducker, W. A. J. Phys. Chem. B 1998, 102, 4288-4294. (29) Tiberg, F. Division of Physical Chemistry 1; Lund University: Lund, 1994. (30) Lee, E. M.; Thomas, R. K.; Cummins, P. G.; Staples, E. J.; Penfold, J.; Rennie, A. R. Chem. Phys. Lett. 1989, 162, 196-202. (31) Bohmer, M. R.; Koopal, L. K.; Janssen, R.; Lee, E. M.; Thomas, R. K.; Rennie, A. R. Langmuir 1992, 8, 2228-2239. (32) Penfold, J.; Staples, E. J.; Tucker, I.; Thompson, L. J. Langmuir 1997, 13, 6638-6643. (33) Penfold, J.; Staples, E.; Tucker, I. Langmuir 2002, 18, 29672970.
Stiernstedt et al.
concentration, normally slightly below the critical micelle concentration (cmc), when aggregates are formed34-36 and the adsorbed amount increases rapidly. Much higher adsorbed amounts of nonionic surfactants on silica are seen at low pH when the silica surface is less charged.29,33,34 On mica, the adsorption is also very spare,26,37,38 the reason being the absence of surface hydroxyl groups. The surface aggregates formed by nonionic surfactants depend on the relation between the length of the hydrocarbon chain and the ethylene oxide chain, and C14E6 has been shown to form spherical aggregates on silica.28 Surface force measurements in nonionic surfactant solutions show low barrier force and adhesion just above the cmc at pH 6.24 At pH 4, however, the barrier force is substantial,27 but the adhesion values are very low. Compared to cationic surfactants, the adsorption of nonionic surfactants is much “weaker”. A range of methods can be used to independently measure the adsorbed amount on an isolated surface. Ellipsometry studies have shown that the plateau adsorbed amount of C14E6 on silica at pH 5 is 5 µmol/m2 39 and that the adsorbed amount of TTAB on silica is 4 µmol/ m2 at pH 4.8 40 and 5.5 µmol/m2 at pH 9.7.41 With neutron reflection, a great deal of work has been done on the adsorption and structure of CTAB and C12E6 mixtures,32,42-45 which have shown that the CTAB and C12E6 molecules are evenly distributed on silica at low pH (pH ) 2.4), whereas at higher pH (pH ) 7.0) the cationic surfactant is overrepresented in the layer closest to the silica surface. At pH 2.4 the adsorbed amount is slightly less than at pH 7.0, but in both cases the adsorbed amounts are significantly larger than those obtained for single surfactants, 9.3 and 10.4 µmol/m2, respectively.32 Ideal mixing in the adsorbed layer was observed45 for solutions rich in cationic surfactant, whereas solutions rich in nonionic surfactant had an unexpectedly high amount of cationic surfactant in the adsorbed layer. The highest adsorbed amounts were seen for roughly 1:1 mixtures of cationic and nonionic surfactants. Proximal Adsorption However, isolated surfaces, as studied above, cannot be treated in the same way as two interacting surfaces. The adsorbed amount on one surface in the vicinity of another surface is dependent on the distance between the surfaces and the change in adsorbed amount, ∆Γi, follows:46-49 (34) Rubio, J.; Kitchener, J. A. J. Colloid Interface Sci. 1976, 57, 132-142. (35) Clunie, J. S.; Ingram, B. T. In Adsorption from Solution at the Solid/Liquid Interface; Parfitt, G. D., Rochester, C. H., Eds.; Academic Press: London, 1983; pp 105-152. (36) Partyka, S.; Zaini, S.; Lindheimer, M.; Brun, B. Colloids Surf. 1984, 12, 255-270. (37) Dong, J. P.; Mao, G. Z. Langmuir 2000, 16, 6641-6647. (38) Rutland, M. W.; Christenson, H. K. Langmuir 1990, 6, 10831087. (39) Tiberg, F.; Jonsson, B.; Tang, J.; Lindman, B. Langmuir 1994, 10, 2294-2300. (40) Wa¨ngnerud, P.; Olofsson, G. J. Colloid Interface Sci. 1992, 153, 392-398. (41) Wa¨ngnerud, P.; Jonsson, B. Langmuir 1994, 10, 3268-3278. (42) McDermott, D. C.; Kanelleas, D.; Thomas, R. K.; Rennie, A. R.; Satija, S. K.; Majkrzak, C. F. Langmuir 1993, 9, 2404-2407. (43) Penfold, J.; Staples, E. J.; Tucker, I.; Thompson, L. J.; Thomas, R. K. Physica B 1998, 248, 223-228. (44) Penfold, J.; Staples, E.; Tucker, I.; Thomas, R. K. J. Colloid Interface Sci. 1998, 201, 223-232. (45) Penfold, J.; Staples, E. J.; Tucker, I.; Thomas, R. K. Langmuir 2000, 16, 8879-8883. (46) Pethica, B. A. Colloids Surf. A 1995, 105, 257-264. (47) Podgornik, R.; Parsegian, V. A. J. Phys. Chem. 1995, 99, 94919496. (48) Yaminsky, V. V.; Ninham, B. W.; Christenson, H. K.; Pashley, R. M. Langmuir 1996, 12, 1936-1943.
Surface Forces in Surfactant Mixtures
∆Γi ) Γi(s) - Γi(∞) ) -
Langmuir, Vol. 21, No. 5, 2005 1877
( )
1 ∂Ea 2 ∂µi
(1)
T,p,µj,s
derived from the Gibbs adsorption equation. Γi is the adsorbed amount of component i on each surface, s, is the distance between the surfaces, Ea is the interaction energy per unit area, and µi is the chemical potential. This equation is valid when the temperature, T, pressure, p, chemical potential of the other components, µj, and the distance between the surfaces are constant. For an ionic surfactant, which adsorbs together with the counterion, there is an additional factor 1/2 on the right-hand side. An experimental measure of the interaction energy can be obtained from surface force measurements by applying the Derjaguin approximation:50
Ea )
F(s) 2πR
(2)
where F(s) is the measured force at a given separation and R is the mean radius of curvature. The implications of the equation are that at low surfactant concentrations the adsorbed amount increases as the surfaces come into contact, when the hydrophobic tails can escape from the aqueous environment. At concentrations above the cmc, on the other hand, the adsorbed amount decreases; i.e., surfactants are expelled from the contact zone. This distance dependence has been recently measured and called proximal adsorption,51,52 since the adsorbed amount depends on the proximity of one surface to another and has been invoked in earlier publications.46-49 An implication of the proximal adsorption arguments is that the presence of an imaging AFM tip may actually affect the adsorbed amount and hence, also the adsorbate structure! In this work we attempt to determine the effect of varying the nonionic-cationic surfactant composition on the adsorbed aggregate structure and features, such as barrier force and adhesion, through measurements of the surface forces. Experimental Section Tetradecyltrimethylammonium bromide (TTAB) of purum grade was purchased from Fluka and recrystallized from water. Monodisperse hexakis(ethylene glycol) mono-n-tetradecyl ether (C14E6) was bought from Nikkol and used without further purification. The cmc for the surfactants, without added salt, are 3.5 × 10-3 and 1 × 10-5 M, respectively.53,54 Water was purified with a Millipore MilliQ water purification system, and NaBr was roasted before use at 500 °C to remove any organic contaminants. The pH was set to 8.5 using NaOH, suprapur, to ensure a high surface charge but to avoid any issue of silicate gel formation. The surface forces were measured with the bimorph surface force instrument, MASIF, which is described fully elsewhere.55,56 Briefly, two glass surfaces are mounted in a chamber, one below the other. The surfaces are pushed together and subsequently retracted; meanwhile the forces between the surfaces are (49) Christenson, H. K.; Yaminsky, V. V. Colloids Surf. A 1997, 130, 67-74. (50) Derjaguin, B. V. Kolloid-Z 1934, 69, 155-164. (51) Lokar, W. J.; Ducker, W. A. Langmuir 2002, 18, 3167-3175. (52) Subramanian, V.; Ducker, W. J. Phys. Chem. B 2001, 105, 13891402. (53) Mukerjee, P.; Mysels, K. J. Critical Micelle Concentrations of Aqueous Surfactant Systems; U.S. Department of Commerce, National Bureau of Standards: Washington, DC, 1970; Vol. NSRDS-NBS 36. (54) Sepulveda, L.; Cortes, J. J. Phys. Chem. 1985, 89, 5322-5324. (55) Claesson, P. M.; Ederth, T.; Bergeron, V.; Rutland, M. W. Adv. Colloid Interface Sci. 1996, 67, 119-183. (56) Parker, J. L. Langmuir 1992, 8, 551-556.
measured. The measurements are made at a rate of approximately 10 nm/s. Glass surfaces of spherical curvature used in the surface force measurements were made by melting one end of a 2 cm long glass rod of 1 mm radius, which gives a hemisphere with a radius of about 2 mm. The upper sphere is mounted on a piezoelectric tube, which is used to drive the surfaces together and is mounted on a motorized stage, providing coarse positioning. The lower sphere is mounted on a bimorph force sensor, which is protected from the liquid in the chamber with a Teflon sheath. It allows forces to be measured with a resolution of 30 nN. The chamber is made from steel and Teflon, with silica windows, and its volume is approximately 10 mL. The motion of the piezoelectric tube is independently monitored using a linearly variable displacement transducer (LVDT) to compensate for the nonlinearity in the expansion of the piezo with applied voltage. The only calibration needed is the sensitivity of the LVDT (which is determined interferometrically). The radii of the spheres and the spring constant of the bimorph force sensor are measured after each experiment, converting deflection to force normalized by radius. The spring constant was determined by placing small weights on the bimorph force sensor and measuring the deflection by means of a horizontal microscope. The radii of the spheres, R1 and R2, were measured with a micrometer, and the radius of curvature, R, was calculated by
R)
R1R2 R1 + R2
(3)
As with force measurements performed with AFM the zero of separation is not absolute but is defined as the point where the surfaces move at the same rate as the piezo, often referred to as the region of constant compliance or hard wall contact. Hence, the surface separation reported in this paper is not necessarily the separation between the silica surfaces. If material is trapped between the surfaces, the actual surface separation is larger than the apparent surface separation derived from the hard wall contact. To obtain a value of the surface potential, the force curves were fitted with DLVO theory.57,58 The van der Waals attraction was estimated with a Hamaker constant of 8 × 10-21 in all fits, and the Poisson-Boltzmann equation was solved numerically, using the algorithm by Chan et al.,59 which provides the electrical double layer repulsion. However, caution should be taken when interpreting the values of the surface potential and the Debye length in solutions above the cmc. When the micelles contain charged surfactants, that application of the PB-equation is strictly not valid.3
Results The forces between two glass surfaces in a dilute electrolyte solution are well-described by DLVO57,58 theory down to quite small surface separations, which is seen in Figure 1, open symbols. The solid line is a fit of DLVO theory to the data, with the boundary condition of constant charge, and returns an apparent surface potential, ψ0, of -85 mV and a Debye length, κ-1, of 45 nm. This Debye length agrees well with the concentration of approximately 0.05 mM NaOH needed to raise the pH. At large surface separations the repulsive double-layer force dominates, and according to the DLVO theory, the surfaces should come into adhesive contact due to the action of van der Waals forces. However, this is not observed and instead an exponential repulsion appears at small surface separations. This additional repulsion is typical for glass and (57) Verwey, E. J. W.; Overbeek, J. T. G. Theory of the Stability of Lyophobic Colloids: The Interaction of Sol Particles Having an Electric Double Layer; Elsevier: New York, 1948. (58) Derjaguin, B. V.; Landau, L. Acta Physicochim. URSS 1941, 14, 633-662. (59) Chan, D. Y. C.; Pashley, R. M.; White, L. R. J. Colloid Interface Sci. 1980, 77, 283-285.
1878
Langmuir, Vol. 21, No. 5, 2005
Figure 1. Surface forces measured on approach between two glass spheres immersed in water (open circles) and in 0.5 mM TTAB (closed diamonds). The solid lines are fits of DLVO theory to the data with ψ0 ) -85 mV and κ-1 ) 45 nm for water, and ψ0 ) 33 mV and κ-1 ) 13 nm for 0.5 mM TTAB.
silica surfaces in aqueous solutions and has been ascribed to hydration forces.60 The hydration force is believed to arise due to ordering of the water molecules at the surface due to hydration of adsorbed ions and hydrogen bonding to silanol groups. Another hypothesis is that it is due to the formation of a gel layer on the silica surface,61 but there is little evidence to support this view.62 Pure TTAB Solutions. Addition of low concentrations of cationic surfactant removes the hydration repulsion as the surfactants adsorb to the glass surface with the hydrocarbon tails toward the solution. The surfaces jump into adhesive contact at small surface separations and the forces follow the predictions of DLVO theory very well, as also seen in Figure 1, closed symbols, which depict the interaction force in 0.5 mM TTAB. The two solid lines are fits of DLVO theory to the data with boundary conditions of constant charge (upper) and constant potential (lower). In this fit the apparent surface potential is 33 mV and the Debye length is 13 nm, which agrees well with the calculated Debye length assuming full dissociation of the surfactants and the counterions. The apparent noise is exaggerated by the logarithmic scale and results from the much lower absolute force (closer to instrument resolution). This is compensated for by an increased resolution in fitting at lower potentials. We note here that no changes were observed in the force curves with repeated contact. The force curves themselves took 90 s, and the system was allowed to relax for at least 30 s between runs. This observation is true for all subsequent systems in this work. The implication of this is that the surfactant layer is able to completely heal over this time scale, which is consistent with earlier measurements in related systems.12 At higher concentrations of cationic surfactant (Figure 2), long-ranged double-layer forces and additional shortrange force barriers are observed, indicating the formation of positively charged surfactant aggregates on the surface. The barrier force, Fb, is the force required to push out the surfactant aggregates from the contact region. The two solid lines are fits of DLVO theory to the 3.5 mM TTAB data with boundary conditions of constant charge (upper) and constant potential (lower), returning an apparent surface potential of 55 mV and a Debye length of 5.1 nm, (60) Horn, R. G.; Smith, D. T.; Haller, W. Chem. Phys. Lett. 1989, 162, 404-408. (61) Adler, J. J.; Rabinovich, Y. I.; Moudgil, B. M. J. Colloid Interface Sci. 2001, 237, 249-258. (62) Grabbe, A.; Horn, R. G. J. Colloid Interface Sci. 1993, 157, 375383.
Stiernstedt et al.
Figure 2. Surface forces measured on approach between two glass spheres immersed in TTAB solutions (closed diamonds, 0.5 mM; squares, 2 mM; crosses, 3.5 mM). The inset shows the force curves on a logarithmic scale. The solid line is a fit of DLVO theory to the 3.5 mM TTAB data with ψ0 ) 55 mV and κ-1 ) 5.1 nm.
Figure 3. Two outward runs (surfaces moved apart after contact) in TTAB solution at the cmc. The lower curve (crosses) shows the adhesive contact, which is a result of overcoming the force barrier and expelling the aggregates from the contact zone. The arrow indicates the jump apart after the adhesive contact. The upper curve (closed diamonds) shows that there is no adhesion at all between unruptured aggregates, i.e., when the force barrier has not been overcome.
which agrees well with the TTAB concentration of 3.5 mM assuming full dissociation of the surfactants and the counterions. Due to the presence of the surfactant aggregates, the plane of charge had to be moved and is assumed to be located 4 nm from hard wall contact. The data clearly display that the glass surface is neutralized by the cationic surfactants, and subsequently recharged, since the double layer repulsion is absent at 0.5 mM (close to neutral surfaces) and reappears above this concentration. We note that the sign of the charge cannot be obtained from the fits directly; the trend with concentration is required to infer this. Figure 3 shows two outward runs (surfaces moved apart after contact) in TTAB solution at the cmc. The lower curve shows the adhesive contact, which is a result of overcoming the force barrier and expelling the aggregates from the contact zone. The surfaces jump from contact at about -5 mN/m to a separation of about 400 nm. The jump is an instability of the spring, so no information can be obtained between these points. The upper curve, on the other hand, shows that there is no adhesion at all between unruptured aggregates, i.e., when the force
Surface Forces in Surfactant Mixtures
Figure 4. Barrier force as a function of TTAB concentration for solutions with 10 mM added NaBr (open squares and dashed line) and without added salt (closed diamonds and solid line). The lines are only drawn to guide the eye.
Figure 5. Pull-off force as a function of TTAB concentration for solutions with 10 mM added NaBr (open squares and dashed line) and without added salt (closed diamonds and solid line). The lines are only drawn to guide the eye.
barrier has not been overcome. This adhesion behavior is consistent with previous work on related systems.12 Figure 4 shows the barrier force for increasing TTAB concentrations. Below the cmc, the force barrier height increases as the surfactant concentration increases, due to increased adsorption, as seen previously.12,13,52,63 Above the cmc when surfactant aggregates have formed at the surface, the force barrier height is constant. At low surface coverage there is no force barrier; instead, the surfaces jump into adhesive contact. The pull-off force, a measure of the adhesion, which is displayed in Figure 5, increases with increasing TTAB concentration as a hydrophobic layer is built up on the surfaces. However, the adhesion reaches a maximum, and as the surfactant aggregates are formed at higher surfactant concentrations, the adhesion is reduced.12,13 Note that the pull-off forces plotted here are the pull-off force in contact after any “steric” force barrier has been overcome. This adhesion value is also independent of the applied force. Addition of salt increases the surface charge of the glass,18 which in turn leads to increased adsorption of cationic species. It also reduces the curvature of the surfactant aggregates, which thus favors adsorption. This increased adsorption is confirmed by the increased barrier height when salt is present (cf. Figure 4). Further, salt (63) Imae, T.; Kato, M.; Rutland, M. Langmuir 2000, 16, 1937-1942.
Langmuir, Vol. 21, No. 5, 2005 1879
Figure 6. Force curves on approach for solutions of TTAB and C14E6, with the total concentration of surfactant being 3.5 mM, no added salt (closed triangles, 10% C14E6; open circles, 90% C14E6). The inset shows the force curves on a logarithmic scale. The solid lines are fits of DLVO theory to the data with ψ0 ) 54 mV and κ-1 ) 5.4 nm for 10% C14E6 (the steep solid line in the inset) and ψ0 ) 53 mV and κ-1 ) 15.2 nm for 90% C14E6 (the flatter line in the inset).
decreases the cmc of the surfactant as well as the adhesion, as seen in Figure 5. Mixed Surfactants. Presented in Figure 6 are the forces after addition of C14E6, the total surfactant concentration is 3.5 mM for all mixtures. Both the 10% C14E6 mixture, closed triangles, and the 90% C14E6 mixture, open circles, show long-ranged double-layer forces and additional short-ranged force barriers, originating from the presence of positively charged surfactant aggregates on the surfaces. The steep solid line in the inset is a fit of DLVO theory to the 10% C14E6 data, with the plane of charge moved 4 nm from hard wall contact. In this fit the apparent surface potential is 54 mV and the Debye length is 5.4 nm, as expected for a TTAB concentration of 3.15 mM. The flatter solid line in the inset is a fit of DLVO theory to the 90% C14E6 data, returning an apparent surface potential of 53 mV and a Debye length of 15.2 nm. The forces in both mixtures are very similar to the force profile in pure TTAB at the cmc, shown in Figure 2. As expected, the 90% C14E6 mixture has a longer Debye length resulting from the lower ion concentration. The apparent surface potential is maintained, so the apparent surface charge necessarily decreases to 2.9 mC/m2. The 90% C14E6 mixture contains only 0.35 mM TTAB, which is far below the cmc of the pure surfactant. The forces in this mixture are not at all like the force profile in pure TTAB at 0.5 mM, where the surfaces jump directly into adhesive contact. Instead, the short-range force barrier appears, showing the existence of surfactant aggregates on the surfaces. On retraction, however, the pull-off force, as displayed in Figure 7, decreases with addition of C14E6. Figure 8 presents how C14E6 affects the measured forces at higher salt concentration. The left solid line in the inset is a fit of DLVO theory to the 3.5 mM TTAB data, returning an apparent surface potential of 28 mV and a Debye length of 2.6 nm. The plane of charge has been set to 4 nm. The solid line in the middle is a fit with the same potential, but the plane of charge offset 5 nm, and it fits both the 10 and 90% C14E6 mixtures reasonably well. The force barrier becomes thicker with added nonionic surfactant, which effect is more pronounced at the higher salt concentration and is presumably related to steric repulsion of protruding ethylene oxide chains. Alternatively the C14E6 curves could also be fitted with DLVO theory,
1880
Langmuir, Vol. 21, No. 5, 2005
Stiernstedt et al. Table 1. Surface Potentials and Charges Obtained from Fitting surfactant amount of concn (mM) C14E6 (%)
Figure 7. Barrier force, Fb, and pull-off force, Fpo, as a function of C14E6 content for mixtures of TTAB and C14E6, the total surfactant concentration being 3.5 mM. Both data with 10 mM added NaBr (open squares and dashed lines) and without added salt (closed diamonds and solid lines) are presented. The lines are only drawn to guide the eye.
surface potential (mV)
Debye surface plane of lengtha chargeb charge (nm) (mC/m2) (nm)
0 0.5 2 2 2 3.5 3.5 3.5 3.5 3.5
0 0 0 0 0 0 10 10 90 90
No Salt -85 33 70 60 45 55 54 34 53 44
45 13 6.7 6.7 6.7 5.1 5.4 5.4 15.2 15.2
-1.8 2.0 10.1 8.0 5.4 9.1 8.4 4.8 2.9 2.3
0 0 4 5.5 8 4 4 7 4 7
0 0.5 2 3.5 3.5 3.5
0 0 0 0 10 90
10 mM Salt -35 28 28 28 28 19
3.5 3.1 2.8 2.6 2.6 2.6
-7.6 6.7 7.4 7.9 7.9 5.2
0 4 4 4 5 7
a All Debye lengths agree well with expected values assuming full dissociation of the ions. b The surface charge is calculated using the Grahame equation.
Figure 8. Force curves on approach for solutions of TTAB and C14E6, with the total concentration of surfactant being 3.5 mM and 10 mM added NaBr (crosses, 0% C14E6; closed triangles, 10% C14E6; open circles, 90% C14E6). The inset shows the force curves on a logarithmic scale. The solid lines are fits of DLVO theory to the data with ψ0 ) 28 mV and κ-1 ) 2.6 nm for 0% C14E6 (the left solid line). The 10 and 90% C14E6 solutions show rather similar long-range forces, which lay between the middle (ψ0 ) 28 mV and κ-1 ) 2.6 nm) and the right (ψ0 ) 19 mV and κ-1 ) 2.6 nm) solid lines. The plane of charge has been offset 4, 5, and 7 nm in the fits from left to right.
returning an apparent surface potential of 19 mV and a Debye length of 3 nm (the solid line to the right in Figure 8) and then the plane of charge is offset 7 nm. A summary of all DLVO fits is presented in Table 1. Surface Charge. The apparent surface potential, ψ0, as obtained from fitting the experimental data to the DLVO theory, is a useful tool for comparing different surfaces. It is, however, dependent on the salt concentration. A complementary characteristic is the surface charge, σ, which is related to the surface potential through the Grahame equation.64 The apparent surface charge of the outer adsorbed layer gives a clue to its composition and adsorbed amount. As mentioned in the Introduction, addition of salt increases the adsorbed amount and also the surface charge. However, the apparent surface charge achieved from fits with DLVO theory seems to decrease with addition of salt. This decrease is within the customary (64) Israelachvili, J. Intermolecular and Surface Forces, 2nd ed.; Academic Press: San Diego, CA, 1992.
Figure 9. Apparent surface charge as a function of the amount of C14E6 for mixtures of TTAB and C14E6 (open squares and dashed line, 10 mM added NaBr; closed diamonds and solid line, without added salt). The total surfactant concentration was 3.5 mM for all mixtures.
10% error, and if the salt induces changes in the aggregate structure and the position of the plane of charge, that might be enough to change the fit, which is thoroughly discussed later. Nonionic surfactant, when added, coadsorbs and replaces some of the TTAB molecules. As a consequence, the surface charge decreases. This decrease in apparent surface charge, as seen in Figure 9, is clear evidence that mixed surfactant aggregates form. Discussion The forces between two glass surfaces in TTAB solutions are shown in Figure 2. Surfactant aggregates are built up with increasing surfactant concentration, which is seen as additional repulsive forces close to zero separation. The thickness of the adsorbed surfactant layer is measured when the surfactants are pushed away, which manifests as a step in the force profile, the thickness of which corresponds to twice the layer thickness. As with AFM, the zero separation is given from the constant compliance region, and, if it is unclear if all material has been pushed
Surface Forces in Surfactant Mixtures
out, then it is not possible to determine the absolute value of the adsorbed layer thickness. Consequently, uncertainty exists as to whether the glass surfaces come into contact, or if a monolayer on each surface remains. Thus, other information, such as the adhesion value, and the shape and thickness of the force barrier, needs to be evaluated. The thickness of the push-trough step, seen in Figure 2, is about 4 nm, which is two times the hydrocarbon chain length and agrees with the diameter of spherical aggregates. This thickness therefore implies that half of each aggregate is expelled and that the surfaces come into monolayer-monolayer contact. The presence of adhesion between the spheres on retraction would further suggest monolayer contact, as there is no adhesion between glass surfaces in water. In addition, previous work on CTAB covered glass surfaces12,13,63 implies very strongly that the surfaces come into monolayer contact, given that the applied force is high enough to overcome the force barrier. There is, though, a compression of the barrier prior to the push-through (cf. Figure 2, after the DLVO fit has reached its maximum value), rendering the total thickness of the expulsion event greater than two molecule lengths, and thus more material than one layer from each side is probably pushed out. In addition, no further compression occurs after the push-through, and there is no load dependence of the pull-off force once the force barrier has been overcome. This absence of compressibility suggests strongly that there is little or no surfactant left between the surfaces (or that the nature of the contact is welldefined and rigid). The adhesion values are also lower, by a factor 2, than for the above-mentioned work on CTAB.12,13 The adhesion is a result of hydrophobic contact, so it is indeed expected that TTAB, which has a shorter hydrocarbon chain, should have lower adhesion than CTAB. The difference in adhesion is, however, rather large, implying that the surfaces may even achieve glass-glass contact. Even though glass-glass contact per se is not adhesive in water, in a surfactant solution adhesion could arise due to hydrophobic contact of surfactants across the gap in an annulus around the contact region.65 Another feature, supporting the idea of glass-glass contact, is that the push-through distance in the present study is much larger than for CTAB on silica, despite the fact that the CTAB molecule is longer than TTAB. Both CTAB and TTAB form spherical aggregates on glass, so there should not be big differences in the aggregate thickness. Still, the expelled TTAB layer is thicker than for CTAB, so this must imply that the surfaces in TTAB solution come closer to glass-glass contact than the surfaces in CTAB. The implication of the differences between CTAB and TTAB (thicker removed layer for TTAB, compressive force after collapse for CTAB) is that, despite their extreme similarity both in composition and adsorbate structure, there is nonetheless a significant difference between them. A clue to this lies in the melting temperatures of the hydrocarbon chains. The C14 chain melts at 6 °C 66 and for TTAB the hydrocarbon is expected to be very liquidlike. In contrast, the C16 chain melts at 18 °C,66 which is very close to room temperature, and may explain why the aggregates are harder to remove. In support of this observation Atkin et al.67 have also noticed a difference between the adsorption dynamics of CTAB and that of TTAB and DTAB (C12), which behave qualitatively similarly. (65) Parker, J. L.; Rutland, M. W. Langmuir 1993, 9, 1965-1967. (66) Weast, R. C., Astle, M. J., Eds. CRC Handbook of Data on Organic Compounds; CRC Press: Boca Raton, FL, 1985.
Langmuir, Vol. 21, No. 5, 2005 1881
Similarly, our results on TTAB are more similar to related measurements by Subramanian and Ducker52 on DTAB. As discussed in the Introduction, it is a conclusion of this extraordinarily thorough work on proximal adsorption that the entire adsorbed amount of DTAB, above the cmc, is expelled, as the surfaces come into contact, implying of course that the surfaces achieve glass-glass contact. As noted in Figure 2, the force at 2 mM (open squares) is more repulsive than at 3.5 mM (crosses), which leads to a higher apparent surface charge at 2 mM than at 3.5 mM when fitted with DLVO theory (Table 1). These data are highly reproducible, but it seems, however, unlikely since the surface charge originates from adsorbed surfactant and the adsorbed amount is lower at 2 mM. The lower adsorbed amount is confirmed by the lower force barrier and is well-known from adsorption studies.40,41,67,68 For the 2 mM case the plane of charge must be moved 1.5 nm further out with respect to hard wall contact, to receive higher surface charge for the higher concentration. This implies that the final contact position is different for the two concentrations and that in fact more surfactant is left between the surfaces at the higher concentration. The surface charge is expected to be higher at 3.5 mM, so the offset of 1.5 nm is the smallest possible movement. Therefore, the surfactant layer left in contact is probably at least 1.5 nm thick. The zero separation is an arbitrary measure of hard wall contact, and it is almost certainly not located at the same point with respect to the glass surface for different concentrations of surfactant. Any uncertainty in the zero separation is also the reason proximal adsorption calculation cannot be 100% quantitative, as the method relies on adhesion originating at the same contact plane independently of the surfactant concentration. The above discussion leads to no absolute conclusion but implies strongly that the final contact is considerably less than monolayer-monolayer, but nonetheless not glass-glass. Position of Plane of Charge. As aggregates form on the surface, the surface charge reverses and the plane of charge moves away from the glass surface, which must be taken into consideration when fits with DLVO theory are made. When curved aggregates are present, of course, the concept of a plane of charge breaks down, so an effective plane must be defined. Although some movement of the effective charge plane may occur as aggregates are compressed, the plane of charge has been taken to lie at 4 nm unless otherwise stated. However, it must be stressed that this is completely arbitrary and that equally good fits can be obtained at 7 nm with commensurately lower values of the potentials. The absolute value of the location of the effective charge plane is thus of limited use in determining the final contact, but the range of possible values once again supports the conclusion that hard wall contact is somewhere between glass-glass contact and monolayer-monolayer contact. Reduced Adhesion above cmc. The pull-off force has a maximum just below the cmc. Above that concentration, the adhesion decreases rapidly up to cmc and continues to decrease more slowly above the cmc. The reason for this decrease in adhesion is not completely clear. There are two ways of rationalizing this. First, it should be recognized that the adhesion is a measure of the change (67) Atkin, R.; Craig, V. S. J.; Wanless, E. J.; Biggs, S. J. Colloid Interface Sci. 2003, 266, 236-244. (68) Atkin, R.; Craig, V. S. J.; Biggs, S. Langmuir 2000, 16, 93749380.
1882
Langmuir, Vol. 21, No. 5, 2005
Stiernstedt et al.
Figure 10. Barrier force (data from Figure 4) as a function of TTAB concentration with 10 mM added NaBr (open squares and dashed line) and without added salt (closed diamonds and solid line), and surface pressure, calculated according to eq 4, as a function of TTAB concentration with 10 mM added NaBr (crosses) and without added salt (plus signs).
in interfacial energy in going from two surfaces in contact to two surfaces exposed to solution. Above the cmc, the surface energy of the isolated surface exposed to the solution is lower than that below the cmc since the latter is more hydrophobic. Thus, less work is needed to achieve the state of isolated surfaces from an adhesive contact above the cmc, leading to a smaller pull-off force. The pull-off force can also be rationalized in terms of the proximal adsorption concept discussed earlier; for ∆Γ to be constant and equal to the entire adsorbed amount there is necessarily a monotonic decrease in the adhesion with increasing chemical potential. Maximum adhesion occurs when there is no adsorption or desorption as the surfaces come into contact, which was shown to be the case just below the cmc. Above the cmc the chemical potential increases slowly,69 and thus, the adhesion decreases slowly. The adhesion also decreases with addition of salt and C14E6. Interestingly, both addition of salt and C14E6 reduces the cmc. Surface Pressure Π. The values of the surface pressure, for TTAB on silica, plotted in Figure 10, have been calculated from adsorption data measured by Wa¨ngnerud et al.40,41 using the equation
∫0c
Π ) RT
b
Γij d ln c
(4)
which is valid for an ideal case below the cmc since the chemical potential then is proportional to ln(c). The force barrier and the surface pressure of Wa¨ngnerud clearly follow the same trend; however, the magnitudes differ by a factor of roughly 2π. The barrier force, when present, thus seems to be a useful measure of the adsorbed amount. The small discrepancy between the surface pressure, and the force barrier at low concentrations of TTAB is attributed to the absence of barrier force when the surfaces jump into adhesive contact, even though there indeed is a surface pressure due to the adsorption. Further, the surface pressure describes all the surfactant aggregates on one single surface, whereas the barrier force is exerted by the surfactants that are pushed out when two surfaces come into contact. For comparison, it has been shown that the surface pressure of a monolayer of polymer on a hydrophobic surface corresponds to half the barrier force of two such surfaces at monolayer-monolayer contact.70,71 (69) de Lisi, R.; Fisicaro, E.; Milioto, S. J. Solution Chem. 1988, 17, 1015-1041.
Mixed Surfactants. The barrier force stays constant in the mixtures, all having a total surfactant concentration of 3.5 mM, which we have shown to be a good measure of the surface pressure. The barrier force stays high also for the 90% C14E6 mixture, both with and without salt, even though the concentration of TTAB is well below the cmc for pure TTAB. At such low concentrations TTAB alone would be expected to adsorb only sparsely and jump into adhesive contact.12,13,52,65 Instead, there is a force barrier before the adhesive contact showing that surfactant aggregates indeed have formed at the surfaces. The existence of this force barrier in mixtures with low concentrations of TTAB is related to the low cmc of C14E6. The cmc of the mixed systems are considerably lower than for pure TTAB; if x1 is the mole fraction of surfactant 1 in the solution, then the cmc of the mixture72 is predicted to be
x1 1 - x1 1 ) + cmcmix cmc1 cmc2
(5)
This is a very simple model, which assumes ideal mixing. The mixing of TTAB and C14E6 is almost certainly not ideal. However, when there is a large difference between cmc1 and cmc2, the surfactant with the lower cmc will dominate for almost all compositions. The mixtures in the present study would, according to eq 5, have cmcs of 0.097 and 0.011 mM for 10 and 90% C14E6 mixture, respectively; corresponding cmcs in the presence of salt are 0.096 and 0.011 mM. Deviation from eq 5 is only expected if there is a large interaction parameter (β) associated with the system.72 Related measurements show that β is small for cationic/nonionic surfactant mixtures. Moreover when the cmc values are so disparate, the correction for β is not significant. Even though there is very limited adsorption of pure C14E6 at silica at high pH,29,33,34 there is a large adsorbed amount of mixed TTAB and C14E6. Thus, the cationic surfactants, which adsorb sparsely at very low concentrations, act as anchors, allowing the nonionic surfactant to coadsorb into aggregates of similar structure to the case of pure TTAB above the cmc. If there were decreased adsorption in the mixed systems, the force barrier would be expected to decrease. Previously it has been observed that the adsorption occurs at lower concentrations in mixtures of ionic and nonionic surfactants compared to the pure ionic surfactant.73-75 The formation of mixed surfactant aggregates is also confirmed by the decreasing surface charge with increasing amount C14E6. Generally the adsorbed amount increases when mixing surfactants,32,42-45 as long as there is not repulsion between one of the surfactants and the surface.76-78 A thicker surfactant layer is expected for C14E6, as the ethylene oxide chain is much larger than the TTAB (70) Yaminsky, V. V.; Ninham, B. W.; Stewart, A. M. Langmuir 1996, 12, 836-850. (71) Eskilsson, K.; Ninham, B. W.; Tiberg, F.; Yaminsky, V. V. Langmuir 1999, 15, 3242-3249. (72) Rubingh, D. N. In Solution Chemistry of Surfactants; Mittal, K. L., Ed.; Plenum Press: New York, 1979; Vol. 1, pp 337-354. (73) Huang, L.; Maltesh, C.; Somasundaran, P. J. Colloid Interface Sci. 1996, 177, 222-228. (74) Esumi, K.; Nagahama, T.; Meguro, K. Colloid Polym. Sci. 1991, 269, 1274-1280. (75) Somasundaran, P.; Huang, L. Pol. J. Chem. 1997, 71, 568-582. (76) Ivanova, N. I.; Shchukin, E. D. Colloids Surf. A 1993, 76, 109113. (77) Ivanova, N. I.; Volchkova, I. L.; Shchukin, E. D. Colloids Surf. A 1995, 101, 239-243. (78) Thibaut, A.; Misselyn-Bauduin, A. M.; Grandjean, J.; Broze, G.; Jerome, R. Langmuir 2000, 16, 9192-9198.
Surface Forces in Surfactant Mixtures
headgroup; however, it is also much more flexible. The push-through distance does not change when nonionic surfactant is added, but the bottom of the barrier becomes thicker and there is thus a larger compression of the aggregates, as expected with the more flexible headgroup. The adhesion becomes even more reduced with addition of C14E6 than in the pure TTAB case above the cmc, Interestingly, the adhesion is also reduced by addition of salt, which like C14E6 decreases the cmc. Salt decreases the cmc of TTAB by roughly a factor 2, whereas 10% C14E6 decreases the cmc 36 times. However, salt does not significantly affect the cmc for the surfactant mixtures since it is so strongly dominated by C14E6, which is not salt-dependent. There is a pronounced difference in the pull-off force in the pure TTAB solutions, where the cmc (and thus also the critical surface aggregation concentration) is dependent on the salt, which is seen in Figure 7. In the mixtures however the pull-off forces (and barrier forces) agree well. This is because the salt concentration does not affect the effective cmc, and the concentration used is well above this value. Therefore we assume that the surfaces come into the same type of contact in the mixtures as for pure TTAB well above its cmc and that TTAB dominates this. Conclusions A clear relationship has been observed between the barrier force and the adsorbed amount, and the existence of large force barriers in the mixed systems then provides strong evidence that mixed surfactant aggregates do form, with essentially the same density. Even when only small amounts of TTAB are present, there is an essentially unchanged (large) adsorbed amount, which is predominantly C14E6. This is a dramatic difference from the adsorption behavior measured earlier in the absence of cationic surfactant, which shows no adsorption of nonionic
Langmuir, Vol. 21, No. 5, 2005 1883
surfactant on silica at high pH. Interestingly, the nature of the contact does not seem to change with addition of C14E6, which suggests both that the aggregates do not change dramatically in structure and also that the inner layer is composed largely of TTAB. Further, we find evidence that there appears to be a fundamental difference between the desorption behavior of CTAB (previously widely used) and that of shorter chained molecules. This is an important point since CTAB is often used as a model for cationic surfactants. In addition, this study suggests that considerably more material is expelled from the contact zone during the pushout step than inferred from previous studies, implying that much less than a monolayer on each surface remains in contact. Partly, this difference is due to the fact that, in many other studies, for example CTAB adsorbed on mica, bilayer aggregates are studied, which are almost certainly not representative of typical aggregate structures in practical systems. In contrast to the findings for CTAB, the proximal adsorption argument tends to lead to the conclusion that the entire adsorbed amount desorbs as the surfaces come into contact and that the surfaces achieve glass-glass contact. However, we have shown that the surfaces have not come to glass-glass contact; instead, the achieved contact is most probably somewhere between glass-glass and monolayer-monolayer contact Acknowledgment. This work was done in the framework of the Colloid and Interface Technology program (CIT) sponsored by the Swedish Foundation for Strategic Research (SSF). BiMaC, the Biofibre Materials Centre, is also thanked for financial support. Professors Jan Christer Eriksson and William Ducker are gratefully acknowledged for useful discussions. LA047763A
