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Langmuir 1999, 15, 3924-3934
Investigating the Adsorption of the Gemini Surfactant “12-2-12” onto Mica Using Atomic Force Microscopy and Surface Force Apparatus Measurements Matthew L. Fielden,* Per M. Claesson, and Ronald E. Verrall† Department of Chemistry, Surface Chemistry, Royal Institute of Technology, SE-100 44 Stockholm, Sweden, and Institute for Surface Chemistry, P.O. Box 5607, SE-114 86 Stockholm, Sweden Received September 29, 1998. In Final Form: February 16, 1999 The adsorption of the cationic gemini surfactant 1,2-bis(n-dodecyldimethylammonium)ethane dibromide on mica was followed by measuring forces between mica surfaces and by atomic force microscopy (AFM) imaging. The surface charge was found to be neutralized at total surfactant concentrations between 8 × 10-7 and 5 × 10-6 M, depending on equilibration time, as judged by the elimination of the repulsive electrostatic double-layer force. At around this concentration, monolayer aggregates of the surfactant started to form on the surface, varying in size between 8 and 130 nm across and between 0.5 and 0.6 nm high. The coverage rapidly increased with a small increase in surfactant concentration, as seen by AFM images. In the concentration range (5 × 10-6)-(1 × 10-4) M the surfactant continued to adsorb steadily as judged by the increase in the double-layer repulsion between surfaces. The hydrophobicity of the surfaces was confirmed by the magnitude of the force required to separate the surfaces, which increased from 20 mN/m in pure water to 120 mN/m at 4.6 × 10-6 M, up to 270 mN/m at 9.0 × 10-6 M; it then stayed virtually constant. AFM imaging showed that, in this range, a significant amount of the surfactant adsorbed on top of the monolayer, although neither technique suggested that the adsorbing material aggregated into bilayer patches. At 1.8 × 10-4 M, a full bilayer formed on each surface, causing an increase in the compressed layer thickness from 1 to 4 nm, and a reduction in pull-off force to 5-10 mN/m. In this concentration range, the nucleation and growth of the complete bilayer was directly observed with AFM. It appeared to occur as isotactic growth of patches, which were initially around 70 nm in size and evenly distributed. These patches grew and joined together to form a flat bilayer over a time scale of around 2 h. Between 1.8 × 10-4 and 7.6 × 10-4 M, a non-DLVO (Derjaguin-Landau-Verwey-Overbeek) force was observed between 12 and 7 nm, followed by an attractive force which pulled the surfaces into bilayer-bilayer contact. The extra repulsive force, which has not been observed previously with cationic surfactant bilayers, was probably due to additional surfactant adsorbed outside the bilayer. AFM imaging confirmed that an extra layer was present above the critical micellar concentration (cmc), as indicated by a significant increase in surface roughness from 0.5 nm to 7-8 nm.
Introduction The adsorption of surfactants on solid surfaces is a topic which has received steady attention in recent years. One of the principle investigative techniques has been the surface force apparatus (see ref 1) for studying adsorption on muscovite mica. This has been a productive method of investigation, which has also introduced a number of dilemmas, in particular, the mysterious hydrophobic force (e.g., ref 2). The surface force apparatus (SFA) operates by measuring the separation and force between semicylindrical mica surfaces approaching each other in a crossed-cylindrical geometry. The optical nature of the distance measurement provides a high degree of accuracy in the direction perpendicular to the surface; however, the large radius (∼2 cm) of the surfaces precludes any assessment of the effect of nonuniformities in the plane parallel to the surface. When these limitations are taken into account, the SFA allows the determination of the * To whom correspondence should be addressed at the current address: Department of Organic and Molecular Inorganic Chemistry, Groningen University, Nijenborgh 4, 9747 AG, Groningen, The Netherlands. † Department of Chemistry, University of Saskatchewan, 110 Science Place, Saskatoon SK S7N 5C9, Canada. (1) Kjellin, U. R. M.; Claesson, P. M. In Surfactant Science Series; Binks, B. P., Ed.; Marcel Dekker: New York, 1998; Vol. in press. (2) Israelachvili, J. N.; Pashley, R. M. J. Colloid Interface Sci. 1984, 98, 500.
concentration at which the surface charge is neutralized, by the elimination of the electrostatic double-layer force, and can indicate the appearance of monolayers and bilayers by increases in the compressed layer thickness and also changes in the force required to separate the surfaces. The force between glass surfaces in cetyltrimethylammonium bromide (CTAB) solutions has been assessed by Parker and co-workers,3,4 and some important differences in adsorption due to the dissimilar surface properties between mica and glass were found. However, the technique used to study adsorption on glass did not allow the measurement of absolute separation, so the assessment of compressed layer thicknesses was complicated. The recent application of atomic force microscopy to the study of surfactant aggregate structures on mica, graphite, and silica surfaces has opened a new door to the study of surfactant adsorption processes.6-9 A rich variety of adsorbed aggregate structures has been observed with different surfactants and surfaces, analogous to the (3) Parker, J. L.; Yaminsky, V. V.; Claesson, P. M. J. Phys. Chem. 1993, 97, 7706. (4) Rutland, M. W.; Parker, J. L. Langmuir 1994, 10, 110. (5) Sharma, B. G.; Basu, S.; Sharma, M. M. Langmuir 1996, 12, 6506. (6) Ducker, W. A.; Wanless, E. J. Langmuir 1999, 15, 160. (7) Manne, S.; Gaub, H. E. Science 1995, 270, 1480. (8) Zana, R. J. Colloid Interface Sci. 1980, 78, 330. (9) Zana, R.; Benrraou, M.; Rueff, R. Langmuir 1991, 7, 1072.
10.1021/la981342+ CCC: $18.00 © 1999 American Chemical Society Published on Web 05/06/1999
Adsorption of Gemini Surfactant onto Mica
different micellar and lamellar structures observed above the critical micelle concentration (cmc) in solution. To this point, the bulk of the work has concentrated on characterizing aggregate structure above the “bilayer” concentration, where the repulsive electrical double-layer force (in the case of ionic surfactants) allows “noncontact” imaging of the surface. This method avoids the mechanical stresses placed on the surfaces when imaging in contact. Only one attempt has been made so far to investigate the mechanism of surfactant adsorption on solid surfaces. In that case5 the adsorption of CTAB on mica was studied as a function of concentration and pH. Above pH 7, it was found that large “curvy” stripes appeared and grew in density with increasing surfactant concentration, until the entire surface was covered. At lower pH values, patches of surfactant adsorbed, but did not join together. It should be stated that the periodicity of the stripes was roughly 10 times higher than that observed by Ducker and Wanless6 for the same system, and by Manne and coworkers for the C14 analogue (TTAB).7 For these surfactants the adsorbed aggregate structure was hemicylindrical, with a radius approximately equivalent to the length of the surfactant molecule. The recent synthesis of so-called “gemini” or dimeric surfactants has provided a new method of controlling surfactant properties. These surfactants consist of two surfactant head-tail pairs (monomers) joined together, usually through the headgroup, by a “spacer” of varying length and chemical character. Some of these molecules have been shown to be superior to the analogous monomeric surfactants in depressing surface tension and also tend to micellize at much lower concentrations. Several studies, notably of Zana et al.,8-13 have uncovered some interesting properties of a variety of these type of surfactants. The most commonly studied type to date is the one having two quaternary ammonium headgroups.14 For example, the surfactant 1,2-bis(n-dodecyldimethylammonium)ethane dibromide, or 12-2-12 (two 12-carbon tails and a 2-carbon spacer) when placed under shear, has been found to form tangled micellar networks in narrow gaps at concentrations much lower than those for which similar networks are observed in bulk solutions. They have also measured binding isotherms on silica of a number of C12 quaternary ammonium geminis with alkyl spacers varying between 2 and 10 methylene groups in length.13 The aggregate geometry of these surfactants, like all others, is a function of the “surfactant packing parameter”.15 For the C12 quaternary ammonium series, for example, spacer lengths of less than 4 produce cylindrical micelles, whereas spacer lengths above 4 produce spherical or spheroidal micelles. Asymmetry in tail length also produces more highly curved aggregates.16 Manne and co-workers16 studied the influence of spacer length and tail symmetry on adsorbed aggregate geometry and found the behavior to be slightly different, but analogous to that in solution, that is, an increased aggregate curvature for a decreasing surfactant packing parameter. (10) Alami, E.; Beinert, G.; Marie, P.; Zana, R. Langmuir 1993, 9, 1465. (11) Danino, D.; Talmon, Y.; Zana, R. Langmuir 1995, 11, 1448. (12) Zana, R.; In, M; Le´vy, H. Langmuir 1997, 13, 5552. (13) Chorro, C.; Chorro, M.; Dolladille, O.; Partyka, S.; Zana, R. J. Colloid Interface Sci. 1998, 199, 177. (14) Devı´nsky, F.; Lacko, I.; Bittererova´, F.; Tomeckova´, L. J. Colloid Interface Sci. 1986, 114, 314. (15) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. Biochim. Phys. Acta 1977, 470, 185. (16) Manne, S.; Scha¨ffer, T. E.; Huo, Q.; Hansma, P. K.; Morse, D. E.; Stucky, G. D.; Aksay, I. A. Langmuir 1997, 13, 6382.
Langmuir, Vol. 15, No. 11, 1999 3925
Figure 1. Chemical structure of 12-2-12.
The aim of this study is to follow the adsorption of 122-12 by surface force and AFM measurements. By combining these two techniques, we hope to deepen the understanding of the mechanisms of surfactant adsorption as well as broaden the knowledge of a novel class of surfactants. Experimental Section The 1,2-bis(n-dodecyldimethylammonium)ethane dibromide surfactant (Figure 1) was synthesized as follows. Reagent grade N,N,N′,N′-tetramethylethylenediamine (Aldrich) was reacted with ≈2.2 equiv of 1-bromo-n-dodecane (Aldrich) in dry acetonitrile under reflux in a nitrogen atmosphere at 70-80 °C for 48 h. The solvent was removed by rotary evaporation under vacuum. The salt was recrystallized from an ethyl acetate-acetonitrile mixture (at least three times) until the surface tension showed no minimum in the post-cmc region. The Krafft point was 19 ( 1 °C, and was determined by measuring solution conductivity as a function of temperature. Water was prepared by passage through a Millipore Milli-RO Plus system, followed by a Milli-Q 185 system. Immediately prior to the measurement, the water was degassed under vacuum using a water jet pump for at least 1 h. A Mark IV surface force apparatus (SFA) was used for all measurements. The SFA has been described in great detail elsewhere.17,18 The principle of the technique is simply to approach two smooth surfaces to each other, one of which is attached to a spring, and simultaneously measure the distance of separation between them. The distance measurement is achieved by optical interferometry. This technique requires that the surfaces be partially reflecting so that a standing interference pattern arises when white light is directed perpendicular through the surfaces. Briefly, muscovite mica surfaces of around 1-2 cm2 (New York Mica Co., New York) are cleaved to 1-3-µm thickness and coated on one side with a thin layer of silver via vapor deposition. Two mica pieces are then glued (Shell Epikote 1004) with the silvered side down onto silica disks having a cylindrical curvature of ≈2 cm. When the different wavelengths of the light in a spectrometer are separated, a pattern of “fringes” can be seen. From the position and shape of the fringes, the distance between the two surfaces can be calculated. Measurements in liquids are made possible by the use of a passivated stainless steel chamber, which has an approximate volume of 350 mL. The construction of the apparatus was conducted in a laminar flow cabinet using tools to avoid contact of parts with the hands. To measure in aqueous solutions, degassed and purified water (Millipore Milli-Q 185) was transferred into the chamber under pressure (N2) through a Luehr-lock port in the bottom of the chamber. The concentration of solutions was varied by injecting the appropriate volume of a stock solution via a syringe, through a Gelman 0.2-µm in-line particle filter and a different Luehrlock port in the bottom of the chamber. The final measuring solutions were mixed by withdrawing and reinjecting 5-10 mL of solution several times. The mixing stage was completed after 10 min of “settling” time by moving the lower surface up and down with the motor for a further 10 min. In the current experiment, equilibration times of 1-2 h were employed to allow (17) Israelachvili, J. N.; Adams, G. E. J. Chem. Soc., Faraday Trans. 1 1978, 74, 975. (18) Parker, J., L.; Christenson, H. K.; Ninham, B. W. Rev. Sci. Instr. 1989, 60, 3135.
3926 Langmuir, Vol. 15, No. 11, 1999
Figure 2. Interaction force measured between mica surfaces immersed in water as a function of separation. The lines represent DLVO theory, using a nonlinear numerical solution to the PB equation and a nonretarded van der Waals force. Ψ0 ) -160 mV, 1/κ ) 51.4 nm, and A ) 2.2 × 10-20 J. Solid and dashed lines represent constant potential and constant charge fits, respectively, a protocol followed throughout the remaining figures. a comparison to AFM measurements, for which longer equilibration times were limited by the time availability on the instrument. The atomic force microscope (Nanoscope III, Digital Instruments, Santa Barbara CA) has also been described in great detail.19 The basic principle is that a sharp tip attached to a weak spring is scanned over a surface using a piezoelectric crystal. The deflection of the spring determines the height information and is measured using the light-lever technique, that is a laser beam is focused on the back of the spring and detected using split photodiodes. The use of mechanical forces to provide surface information allows the observation of processes at solid-liquid interfaces. The fluid cell is made of glass and contains a groove in which sits a rubber O-ring that seals the cell. The small liquid volume (0.1 mL) is replaced by injecting a new solution. In the current experiment, all images were performed in “contact mode” in liquid using standard silicon nitride cantilevers. The definition of contact mode is that the tip is rastered across the surface, and the sample height is adjusted so as to maintain a constant cantilever deflection. The surface can thus be imaged while the tip and surface are in adhesive contact, or a small distance out of contact if the tip-sample force is sufficiently repulsive. The weakest cantilever (k ) 0.06 N/m) was used, to minimize the mechanical damage to the surface while imaging in adhesive contact. Scan speeds were varied between 1 and 10 Hz to ensure no artifacts due to imaging speed. Mica surfaces were cleaved immediately prior to immersion in water, and the fluid cell with tubing and syringe were cleaned by rinsing with copious amounts of ethanol, acetone, and water. All solutions were filtered upon injection into the fluid cell through a 0.2-µm Gelman particle filter. After each increase in surfactant concentration, the system was left to equilibrate for ≈1 h. The pH of all solutions under investigation was 5.7-5.8, and the temperature of all measurements was 21-23 °C or 294-296 K.
Results Measurement of Forces between Mica Surfaces in Surfactant Solutions. The interaction between two mica surfaces in purified water is well-described by DLVO theory (Figure 2). To fit the data, a nonretarded Hamaker constant of 2.2 × 10-20 J was employed,17 along with a nonlinear numerical solution to the Poisson-Boltzmann equation,20 employing a surface potential of -160 mV. The rather high fitted ionic strength of 3.5 × 10-5 M can be explained by considering the difficulties of measuring (19) Binnig, G.; Quate, C.; Gerber, G. Phys. Rev. Lett. 1986, 56, 930. (20) Miklavcic, S. J. J. Phys. Chem. 1994, 98, 2602.
Fielden et al.
Figure 3. Interaction forces measured between mica surfaces as a function of separation in aqueous solutions of 12-2-12. Concentrations were 8.4 × 10-7 M ([), 2.5 × 10-6 M (]) measured after equilibration overnight, and 9.2 × 10-7 M (9) after 2 h of equilibration. Zero force is represented by the dashed line. The solid line is a DLVO fit at constant charge, Ψ0 ) -100 mV, 1/κ ) 51.4 nm, and A ) 2.2 × 10-20 J.
forces in water where the force persists out to very long separations. A consequence of this difficulty is that introducing the surfactant to low concentrations does not change the apparent Debye length. The pull-off force was measured as 15-23 mN/m, within the expected range in pure water. The forces measured after introducing the surfactant to a concentration of 1 × 10-6 M vary with equilibration time. In the case where the system is equilibrated overnight (Figure 3), the double-layer repulsive force is eliminated, leaving an attractive force which dominates the interaction from a separation of 20 nm, greater than that expected to result from a van der Waals force (between bare mica surfaces). The absence of a double-layer force suggests that the surface charge has been neutralized by adsorption of the surfactant, and the strong attraction may be attributed to a hydrophobic force. The force required to separate the surfaces was 115 mN/m, demonstrating that the surfaces have become somewhat hydrophobic. The layer thickness was very small (
