7930
Langmuir 2002, 18, 7930-7935
Adsorption and Frictional Properties of Gemini Surfactants at Solid Surfaces K. Boschkova,*,†,‡ A. Feiler,† B. Kronberg,‡ and J. J. R. Stålgren† Department of Chemistry, Surface Chemistry, Royal Institute of Technology, Drottning Kristinas va¨ g 51, SE-100 44 Stockholm, Sweden, and Institute for Surface Chemistry, Box 5607, SE-114 86 Stockholm, Sweden Received December 7, 2001. In Final Form: July 29, 2002 The adsorption and frictional properties of gemini surfactants at hydrophilic gold surfaces were measured using QCM-D (quartz crystal microbalance dissipation) and AFM. The molecular packing of a series of gemini surfactants was determined from QCM-D measurements, and the frictional behaviors of the surfactant films were characterized by employing atomic force microscopy (AFM). The results show that by changing the length of the spacer group from 3 to 12 a systematic change in the molecular packing at the surface is obtained. Furthermore, the molecular packing is seen to correlate to the frictional behavior of the surfactant film. A linear relation between the spacer group length, the adsorbed amount, and the frictional properties of the layer at the solid surface is found. This is discussed in terms of the critical packing parameter (CPP) of the surfactant, and a relation between CPP and frictional behavior is proposed. No correlation between spacer length and viscoelasticity of the adsorbed surfactant layer was detected using QCM-D. This indicates that the resolution of the dissipation factor from QCM-D measurements is not sufficient to describe the viscoelastic character of the thin surfactant film.
Introduction Dimeric, or gemini, surfactants are composed of two amphiphilic moieties connected at the level of or close to headgroups by a spacer group. The dimeric surfactants have a lower cmc (critical micelle concentration) compared to those for their single chained analogues. This results in better utilization regarding foamability, antibacterial activity, solubilization, and wetting. In addition, some of the gemini surfactants display very interesting rheological properties even at very low concentrations.1 The reported studies on dimeric surfactants mainly cover the behavior at the air/solution interface,2,3 micellization,4,5 microstructure,6,7 phase behavior,8 and adsorption at solid surfaces.9-12 The main purpose of this study was to investigate whether the molecular packing of surfactants, which can be estimated by the critical packing parameter (CPP), could be correlated with the frictional properties of the thin surfactant film. The frictional properties were characterized using atomic force microscopy (AFM). † ‡
Royal Institute of Technology. Institute for Surface Chemistry.
(1) Oda, R.; Weber, V.; Lindner, P.; Pine, D. J.; Mendes, E.; Schosseler, F. Langmuir 2000, 16, 4859. (2) Alami, E.; Beinert, G.; Marie, P.; Zana, R. Langmuir 1993, 9, 1465. (3) Rosen, M. J.; Song, L. D. J. Colloid Interface Sci. 1996, 179, 261. (4) Zana, R.; Benrraou, M.; Rueff, R. Langmuir 1991, 7, 1072. (5) Aswal, V. K.; De, S.; Goyal, P. S.; Bhattacharya, S.; Heenan, R. K. Phys. Rev. E 1999, 59. (6) Bernheim-Groswasser, A.; Zana, R.; Talmon, Y. J. Phys. Chem. B 2000, 104, 12192. (7) Danino, D.; Talmon, Y.; Zana, R. Langmuir 1995, 11, 1448. (8) Knaebel, A.; Oda, E.; Candau, S. J. Langmuir 2000, 16, 2489. Oda, E.; Panizza, P.; Schmutz, M.; Lequeux, F. Langmuir 1997, 13, 6407. (9) Seredyuk, V.; Alami, E.; Nyde´n, M.; Holmberg, K. Langmuir 2001, 17, 5160. (10) Chorro, C.; Chorro, M.; Dolladille, O.; Partyka, S.; Zana, R. J. Colloid Interface Sci. 1998, 199, 169. (11) Manne, S.; Scha¨ffer, T. E.; Huo, Q.; Hansma, P. K.; Morse, D. E.; Stucky, G. D.; Aksay, I. A. Langmuir 1997, 13, 6382. (12) Fielden, M.; Claesson, P. M.; Verrall, R. E. Langmuir 1999, 15, 3924.
The surfactant packing parameter or shape factor, v/lmaxa,13 is a relation between the volume of the hydrophobic part of the headgroup, v, and the extended length, lmax, and a is the optimal headgroup area of a given surfactant molecule. This simple model is useful for predicting the features in surfactant self-assembly: a micellar system displays a shape factor of less than 1/3, a hexagonal system displays a value between 1/3 and 1/2, and a lamellar phase has a packing parameter around 1. In the following, we denote this quantity the critical packing parameter (CPP). For gemini surfactants, it is not straightforward to calculate the exact CPP. However, the trends are easily predicted from the length of the spacer group; in this case, a short spacer (such as 12-3-12) will give a higher CPP value than a longer spacer (such as 12-12-12). Another purpose of this study was to identify the maximum resolution/sensitivity of the quartz crystal microbalance (dissipation), QCM-D, technique in surfactant adsorption studies. This is a very important area, and yet there are no studies addressing this issue. In particular, we wanted to determine whether the dissipation parameter obtained from QCM-D experiments could be used to determine the viscoelastic properties of the surfactant layer. We chose a series of gemini surfactants, since their tunable molecular geometry enables a systematic change in the molecular packing by varying the length of a polymethylene spacer (CH2)s. In this study, an alkanediyl R,ω-bis(alkyldimethylammonium bromide) or (CnH2n+1)[N+(CH3)2](CH2)s[N+(CH3)2](CmH2m+1) 2Br- (m ) n) was used, which will be referred to as m-s-m in the following. A series consisting of four different spacers, namely 123-12, 12-6-12, 12-8-12, and 12-12-12 were investigated. Experimental Section Materials. The gemini surfactants used were a gift from Prof. Verrall, University of Saskatchewan, and synthesized as previ(13) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 1 1976, 72, 1525.
10.1021/la0117754 CCC: $22.00 © 2002 American Chemical Society Published on Web 09/17/2002
Properties of Gemini Surfactants at Solid Surfaces
Langmuir, Vol. 18, No. 21, 2002 7931
ously explained.12 All surfactants had been purified (recrystallized) until no minimum in the surface tension isotherm was detected in the pre-cmc region. In the experiments, the surfactant concentration was kept constant at 2 mM, which for the geminis used here is about 2-7 times the cmc (critical micelle concentration).2 All solutions were prepared from 20 mM batch solutions and equilibrated for at least 48 h after dilution. The glassware used for the surfactant solutions was carefully cleaned with bichromate sulfuric acid and then rinsed with excess Milli-Q water. The water purification unit consists of a Milli-RO 10 Plus followed by a Milli-Q plus185 unit. The outgoing water is filtered through a 0.2 µm filter. Quartz Crystal Microbalance. The quartz crystal microbalance used is a QCM-D from Q-sense, Gothenburg, Sweden. The technique is described in detail elsewhere.14 The device allows for simultaneous measurement of changes in resonance frequency and energy dissipation. The change in resonance frequency is directly related to adsorbed amount using the Sauerbrey relation (eq 1),15 whereas the change in dissipation is a measure of the viscoelastic properties of the adsorbed layer.
∆f n
∆m ) -C
(1)
where ∆m is the adsorbed mass, C is a constant characteristic of the crystal, in our case, 0.178 mg/m2 Hz, ∆f is the change in frequency, and n is the shear wavenumber. This relation is based on the assumption that the deposited mass forms a thin rigid film and that the mass sensitivity is uniform over the entire surface. Equation 1 has been supported by experimental data up to mass loadings of approximately 2%.16 There are various models for converting the frequency shift to mass loadings, and up to approximately 5%, these models give similar results.17 The energy dissipation is measured on the basis of the principle that when the driving power to a piezoelectric oscillator is switched off, the voltage over the crystal decays exponentially and a damped oscillating signal is recorded. Hence, before disconnection of the driving oscillator, we obtain f, and D is obtained after the disconnection. The dissipation factor is defined by
D)
Edissipated 2πEstored
(2)
where Edissipated is the energy dissipated during one oscillation and Estored is the energy stored in the oscillating system. The surfaces used for these experiments were hydrophilic gold surfaces with a 3D surface roughness of 2 ( 0.2 nm, which was determined using a Profilometer (Zygo View 5010). All measurements were conducted at 25 ( 0.02 °C. The QCM cell, crystal, and the tubing were cleaned using a 2% HellmanEx II solution for 1 h; the system was then rinsed with water, and the crystal was also dismounted and cleaned with ethanol. The system was then rinsed with excess water while monitoring the frequency and dissipation values. The injection time was kept constant at approximately 90 s, and at each injection 2.0 mL of surfactant solution was supplied to the cell. In total, two injections, with 30 min of equilibrium time for each injection, were made to make sure that the surface was saturated by surfactant. At least three independent measurements were performed (using different crystals) for each surface composition and surfactant, and the results are shown as mean values from all measurements. AFM Measurements. An atomic force microscope (Multimode SPM, Nanoscope IIIA; Digital Instruments) equipped with a liquid cell was used for sliding friction measurements; the details of the technique are described elsewhere.18 Tipless, long, wide, triangular cantilevers were used for the friction measurements. (14) Rodahl, M.; Ho¨o¨k, F.; Krozer, A.; Brzezinski, P.; Kasemo, B. Rev. Sci. Instrum. 1995, 66, 3924. (15) Sauerbrey, G. Z. Phys. 1959, 155, 206. (16) Pulker, H. K. Z. Angew. Phys. 1966, 20, 537. (17) Mecea, V.; Bucur, R. V. Thin Solid Films 1979, 60, 73. (18) Feiler, A.; Larson, I.; Jenkins, P.; Attard, P. Langmuir 2000, 16, 10269.
Figure 1. Changes in the first overtone resonance frequency (open circles) and dissipation (filled circles) during adsorption of a 2 mM 12-8-12 surfactant onto a hydrophilic gold surface (injection at arrow number 1, after 10 min). An exchange of the surfactant solution for a new identical solution was done at arrow number 2 (after 40 min). The measuring chamber was flushed with water at arrows 3 and 4 (after 70 and 85 min, respectively). The normal spring constant was determined with a resonance technique and found to be 0.1 N/m. The torsional spring constant was measured according to the method of Feiler et al.19 and was found to be 2 × 10-9 N m. A spherical tungsten particle with a diameter of 20 µm was attached to the cantilever using a small amount of a two-part epoxy resin (Araldite). Before the measurements, the liquid cell, tubing, gold surface, and tip were rinsed with filtered ethanol and Milli-Q water. A rubber O-ring (polyethylene) was used to seal the cell and substrate. Initially both normal forces and friction forces were monitored in Milli-Q water in the absence of surfactant. The gold surface was then exposed to the dimeric surfactant solution for about 15 min prior to the measurements (from the QCM-D, it is seen that the surfactant adsorption has reached equilibrium considerably faster). The surfactants were injected sequentially starting with the 12-3-12 surfactant and ending with the 12-12-12 surfactant. The cell and surfaces were rinsed with excess Milli-Q water after the injection of each surfactant. The same relative difference in friction between the 12-3-12 and the 12-8-12 surfactants was obtained by performing the friction experiments starting with the 12-3-12 surfactant followed, after rinsing, by the 12-8-12 surfactant as compared with the series 12-3-12, 12-6-12, and 12-8-12 (with rinsing in between). This supports the idea that the surfactant is removed from the surface by rinsing with excess water. Approximately 5 mL of surfactant solution was flushed through the cell in order to fully saturate the surface and minimize depletion effects. The friction measurements were carried out starting with low applied loads followed by sequentially increasing the load and then decreasing the load. Three consecutive measurements were made for each load both on compression and decompression. The results are displayed as mean values of frictional data from a friction loop over a scan size of 5 µm with a scanning frequency of 1 Hz. A program, AFMLAB v1.04, specifically developed for AFM friction analysis was used for evaluation of friction coefficients.
Results and Discussion Adsorption Measurements Using QCM-D. Results from a typical QCM measurement are displayed in Figure 1. The resonance frequency of the crystal in water is (19) Feiler, A.; Attard, P.; Larson, I. Rev. Sci. Instrum. 2000, 71, 2746.
7932
Langmuir, Vol. 18, No. 21, 2002
Figure 2. Mean values of frequency shifts from the first overtone, f15MHz, as a function of spacer length for a series of 2 mM gemini surfactants, 12-s-12. Results for s ) 3, 6, 8, and 12 are displayed. The error bars indicate the spread in the results. The second axis displays the frequency shifts converted to adsorbed amount expressed in milligrams per meter squared.
subtracted from the curve, thereby giving rise to an initial signal of 0 Hz upon starting the experiment. At arrow number 1 in Figure 1 the first injection is made, in this case a 2 mM 12-8-12 solution, giving rise to a decrease in the resonance frequency of slightly less than 30 Hz. Note that the transient peak observed at the injection time is due to pressure and temperature changes and it does not contain any information relevant to this study. A second injection is made at arrow number 2. This does not result in any further change of mass or dissipation change. Next, the cell is rinsed with water (arrow numbers 3 and 4), which results in an increase in resonance frequency to 13 Hz. The viscoelastic character of the film is also shown in Figure 1, as indicated by the dissipation value. The dissipation increases slightly upon surfactant adsorption, which shows that the coupling between the surface and the nearby solution has increased. Upon rinsing, the dissipation decreases again, but not down to its initial value. Frequency Shift and Adsorbed Amount. Mean values of the frequency shifts determined from the first overtone as a result of adsorption of dimeric surfactants at hydrophilic gold surfaces are displayed in Figure 2. The surfactant with the shortest spacer, 12-3-12, shows a frequency shift of 33.5 ( 0.7 Hz, whereas 12-12-12 displays a significantly lower frequency shift of 24.8 ( 1.6 Hz. The 12-6-12 and 12-8-12 systems behave quite similarly with frequency shifts of 29 ( 3 and 28 ( 2 Hz, respectively. The frequency shift can be converted to adsorbed amount by using the Sauerbrey relation (eq 1), resulting in the adsorbed amount expressed in milligrams per meter squared, shown in Figure 2. The adsorbed amount expressed in milligrams per meter squared shows an approximately linear decrease as a function of spacer length, which is consistent with previous adsorption studies employing silica particles.10 Effect of Rinsing. After two injections with surfactant solutions, a third injection was made with water. This resulted in an immediate rise of the frequency response (this is illustrated in Figure 1), which is attributed to a rapid removal (partial) of the surfactant from the surface. For the gemini surfactants with the shorter spacers, 12-
Boschkova et al.
Figure 3. Frequency response after rinsing with water normalized against frequency response upon adsorption for a series of 2 mM gemini surfactants, 12-s-12, where results for s ) 3, 6, 8, and 12 are displayed.
3-12 and 12-6-12, it is observed that 14 ( 5% remains at the surface after rinsing, as displayed in Figure 3, whereas 12-8-12 and 12-12-12 have somewhat more surfactant remaining at the surface after rinsing, 30 ( 8% and 25 ( 1%, respectively. It is noted that it is difficult to rinse away the surfactant with the procedure used here (2 × 2 mL of water). This is mainly due to the small volumes of water in each rinsing step and problems with liquid exchange in the cell (no stirring and entrapment of liquid around the O-ring). If the injection is made with too high a pressure, it is easy to lose the resonance frequency of the crystal and also the accuracy of the measurement. The latter problem is due to the fact that a high rinsing pressure gives in some cases a small displacement of the crystal and thereby the reference frequency is lost. Thus, this rinsing step is by no means comparable to the rinsing step in the AFM equipment. Dissipation. The dissipation factor provides information about how rapidly the energy of the oscillating crystal is given up to the surroundings. Firmly attached, smooth, and homogeneous layers give rise to a low dissipation, whereas loosely bound, rough, and extended layers containing much solvent result in a high dissipation. Adsorption of a monolayer of surfactant at the surface will in general give rise to a small change in the dissipation factor, typically within (0-1) × 10-6,20 in contrast to adsorption of intact vesicles, which typically give a dissipation factor of approximately 3 × 10-6.20,21 Surprisingly, there seems to be no correlation between spacer length and viscoelasticity after formation of the layer; spacers n ) 3-12 behave very similarly with a dissipation value of around 0.5 × 10-6. Intuitively, a more closely packed surfactant layer gives rise to a higher surface elasticity modulus, which is seen from the AFM measurements and also observed from thin film rheology measurements.22 This makes us conclude that the resolution of the dissipation factor in the QCM measurements is not sufficient to resolve the effect of changing the packing in a thin surfactant film. (20) Keller, C. A.; Kasemo, B. Biophys. J. 1998, 75, 1397. (21) Johnsson, M.; Bergstrand, N.; Edwards, K.; Stålgren, J. J. R. Langmuir 2001, 17, 3902. (22) Boschkova, K.; Kronberg, B.; Rutland, M.; Imae, T. Tribol. Int. 2001, 34, 815.
Properties of Gemini Surfactants at Solid Surfaces
Langmuir, Vol. 18, No. 21, 2002 7933
Table 1. Area per Surfactant and Friction Coefficient versus Spacer Length for a Series of 2 mM Gemini Surfactants, 12-s-12, Where Results from s ) 3, 6, 8, and 12 Are Displayed gemini
area per surfactanta (Å)
friction coefficientb
12-3-12 12-6-12 12-8-12 12-12-12
106 ( 2 131 ( 8 140 ( 4 171 ( 6
0.003 ( 0.001 0.09 ( 0.01 0.10 ( 0.01 0.3 ( 0.02
a The area per surfactant is calculated from the frequency shift displayed in Figure 2 using eq 3, and it assumes a bilayer adsorption model. b The friction coefficients are extracted from data in Figure 4.
Area per Molecule Determination. The packing density of surfactants at the surface, expressed as area per surfactant, is shown Table 1, where the area per surfactant, A (m2), in a bilayer or micellar layer is calculated from the frequency shift using
f15MHz 2 )C N A 3M QCM A
(3)
where f15MHz is the frequency change deduced from the first overtone, (Hz), M is the molar weight of the surfactant, 12-s-12, with counterions included (629-755 g/mol), CQCM is a constant (0.178 mg/m2Hz), NA is Avogadro’s constant, and a bilayer/micellar layer at the surface is accounted for by multiplying the area per surfactant by a factor of 2. The area per surfactant deduced from the QCM measurements is much smaller than previously obtained at the air/water interface. For instance, we obtain an area per surfactant of 53 ( 1 Å2 for 12-3-12 assuming a monolayer adsorption, compared with 105 Å2 at the air/ liquid interface determined from surface tension measurements on 12-3-12.2 The 12-12-12 displays an area per surfactant of 86 ( 3 Å2 assuming a monolayer adsorption from QCM measurements, compared with 226 Å2 at the air/water interface. From this we conclude that full bilayers or bilayer aggregates are formed at the solid/ liquid interface. This motivates the assumption in eq 2, where a bilayer/micellar layer at the surface is accounted for by multiplying the area per surfactant by a factor of 2. This results in rather similar values for the area per surfactant molecule at the solid/liquid interface and at the air/water interface, especially for the 12-3-12 surfactant where the area per surfactant is 106 ( 2 Å2 in our study, compared with 105 Å2 at the air/water interface. This is also supported by AFM images of 12-4-12 (2.2 mM) at the mica surfaces, where tightly packed cylindrical aggregates with a spacing of 42 ( 4 Å are observed.11 We thus conclude that the high adsorbed amount obtained from the QCM measurements is in quantitative agreement with previous studies on other interfaces. Counterion Binding. In the determination of packing density expressed as area per surfactant, one has to make an assumption about how to treat the mass of the counterion. The data in Table 1 assume a perfect degree of counterion binding; that is, all surfactant brings down two bromide ions. This is not correct. Hence, the calculated areas per molecule should be regarded as apparent values showing the correct trends for the relation between surfactant structure and packing density. It should be noted that the frequency and the adsorbed amount calculated in milligrams per meter squared are quantitatively worthy. Aswal et al.23 found that the degree of counterion binding increases with decreasing spacer
Figure 4. Friction-load measurements between a tungsten particle and a gold surface in a 2 mM dimeric surfactant aqueous solution. Mean values of friction force as a function of spacer length for a series of gemini surfactants 12-s-12, where results for s ) 3, 6, 8, and 12 are displayed. The error bars indicate the spread in the results.
length for the series 16-s-16, where s ) 5-12. An association of 0.86 was found for 16-5-16 whereas a longer spacer such as that in the 16-12-16 system displayed an association of 0.68. This means that the packing density, that is, the area per surfactant, must be adjusted up to 30%. Also, the binding of the counterion is believed to be different in the inner and outer surfactant layer, where the inner layer most probably has less access to bromide ions. This is indirectly supported by normal force measurements on DTAB,24 where a bilayer-bilayer/micellarmicellar contact (for 0.3 times the cmc) has a thickness of 60 Å and upon further compression results in a monolayer-monolayer contact of 15 Å thickness, which is a less dense layer than a bilayer at each surface (15 × 2 ) 30 Å instead of 60 Å). Frictional Properties of Gemini Surfactant Layers Using AFM. Friction versus load measurements between a tungsten sphere and a gold surface in the presence of gemini surfactant solutions (2 mM) of varying spacer length is shown in Figure 4. All curves show an almost linear dependence on load, at least above 20 nN. A clear trend is observed between the friction force and the spacer length of the gemini surfactant, namely that the friction force at any given load increases with spacer length. At higher loads, above 90 nN (not shown), a large increase in the friction force was observed for all surfactant films expect for 12-3-12. It is likely that this is due to the penetration of the probe through the surfactant film. However, upon decompression, the surfactant film reforms and the unloading and loading curves overlap. The latter observation makes us conclude that the wear of the opposing surfaces in the system is negligible. Also, this suggests that the relaxation time of the surfactant layer is fast (seconds). This is in contrast to a surfactant system forming high viscous lamellar liquid crystals where the time to recover is considerably higher than that in the present case, because of viscosity effects. The effect of the driving frequency on the frictional properties of gemini 12-12-12 is shown in Figure 5. The measurements were performed at 82 nN (which is just (23) Aswal, V. K.; De, S.; Goyal, P. S.; Bhattacharya, S.; Heenan, R. K. Phys. Rev. E 1998, 57, 776. (24) Boschkova, K.; Kronberg, B.; Stålgren, J. J. R.; Persson, K.; Ratoi Salagean, M. Langmuir, submitted.
7934
Langmuir, Vol. 18, No. 21, 2002
Figure 5. Effect of scan size on the friction force for 2 mM 12-12-12 at 82 nN with a fixed scanning frequency of 1 Hz. The scan size is converted to scan rate (displayed).
Figure 6. Friction coefficients (filled symbols, from data in Figure 4) and adsorbed amounts (unfilled symbols, from data in Figure 2) for a 2 mM aqueous solution of gemini surfactant on gold surfaces. The error bars indicate the spread in the results.
after the transition from low to high friction). It is seen that there is a critical sliding frequency where the frictional force is reduced to almost zero. This is explained as a transition from the boundary lubrication regime to full film lubrication. The friction coefficients obtained from the data in Figure 5 are given in Table 1. The gemini surfactant with a short spacer has the lowest friction coefficient. The spacers s ) 6 and s ) 8 show a similar friction behavior and have a 30-fold increase in friction compared with that of the s ) 3 one. The s ) 12 gave a 100 fold increase in friction coefficient compared to that for s ) 3. In Figure 6 the adsorbed amount, determined from adsorption measurements, is plotted versus friction coefficient. A clear trend of the spacer length and hence the CPP of the surfactant system with the friction coefficient is observed. It may be, therefore, possible to predict the friction properties of a surfactant film from the geometry of the surfactant. In this study a surfactant with a high
Boschkova et al.
CPP (for CPP e 1) acts as a much better lubricant than a surfactant with a low CPP. We will discuss two different models that would explain the friction results above. In the first model we notice that the 12-12-12 surfactant has the lowest CPP of the measured gemini surfactants despite the largest hydrophobic group. Thus, the 12-12-12 surfactant most easily forms micelles in solution as well as at the surface. In contrast, a surfactant with a short spacer group (12-2-12) is known to form bilayers at surfaces.11 Possibly, the trends in friction can be explained in terms of the rigidity of the layer, where the 12-3-12 surfactant forms a laterally tightly packed layer with high resistance against compression and thereby giving rise to low friction force. In contrast, the surfactant with a longer spacer displays a less closely packed layer with more flexible properties, where the hydrocarbon chains are more likely to entangle, which may give rise to the high frictional force. This is indirectly supported from the increasing load measurements, in which the 12-3-12 surfactant film remained intact over the entire load regime, whereas the other films were penetrated by the probe at larger applied loads (this is not shown in Figure 4). The other model is an analogy to the stability of a thin aqueous film separating two emulsion droplets, which prevents coalescence of an emulsion. A theory of thin film stability of emulsion coalescence has been proposed by Kabalnov and Wennerstro¨m.25 Here the authors discuss the stability of a thin oil film separating two water droplets in a water-in-oil emulsion (or conversely a thin water film separating two oil droplets in a oil-in-water emulsion). The film is stabilized by surfactants, and any instability is characterized by the ease of formation of a water bridge (a hole) in the oil film in the first case. Thus, surfactants having a molecular structure such that a water bridge can easily form (low CPP) are poor emulsifiers, whereas good emulsifiers are surfactants having a molecular structure which prevents the formation of a bridge (high CPP). There is a large free energy of forming a hole in the oil film if the surfactant has a high CPP, whereas there is a low free energy of forming a hole if the surfactant CPP is low. Hence, the stability of the oil film increases with an increase of the surfactant’s CPP. We propose that the film formation and film stability of gemini surfactants between two solid surfaces can be viewed similarly; that is, the film formation is rendered more difficult if there is a high probability of forming a hole in the film, that is, in systems where the CPP is low. Thermal and mechanical fluctuations lead to spontaneous formation of holes, or channels of water, between the surfaces, thereby increasing the adhesion between the two surfaces. Hence, poor film-forming properties are analogous to hole formation in the surfactant film separating the surfaces. Thus, it is expected that both film-forming ability and film stability increase with increasing CPP. Finally, we would like to emphasize the uniqueness of these surfactants in that the CPP decreases as the hydrophobic moiety increases in size. This is opposite to the results for normal surfactants. This unique feature makes these surfactants very suitable for lubrication purposes. Here normally two opposite criteria are requested. These are (i) high mobility of the surfactant in order to obtain a fast adsorption so that a surface will regain its surfactant layer after being perturbed by the relative motion of the surfaces and (ii) high lateral cohesion within the adsorbed surfactant layer. The first criterion (25) Kabalnov, A.; Wennerstro¨m, H. Langmuir 1996, 12, 276.
Properties of Gemini Surfactants at Solid Surfaces
is fulfilled by surfactants with a low CPP while the second criterion is normally fulfilled by surfactants with a high CPP. The short spacer gemini surfactants studied here, however, fulfill both of these criteria, hence rendering them good candidates for future technical and academic research. Conclusions The results show that, by changing the length of the spacer group (of a dimeric surfactant) from 3 to 12, a systematic change in the molecular packing at the surface is obtained. Furthermore, the molecular packing is shown to correlate to the frictional behavior of the surfactant film. A linear relation between the spacer group, the adsorbed amount, and the frictional properties of the surfactant layer at the solid surface is found. We would like to propose that there is a linear relation between CPP and the frictional properties of a thin surfactant film (CPP e 1). The behavior could be explained in terms of the rigidity of the layer, where s ) 3 has a laterally tightly packed layer with high resistance against compression and thereby giving rise to low friction force. The surfactant with a longer spacer displays a less close packed layer, where the hydrocarbon chains are more interpenetrating, giving rise to a high frictional force. On the other hand, the high friction force observed for longer spacers could be a result of the formation of defects, holes, in the
Langmuir, Vol. 18, No. 21, 2002 7935
lubricating film. The hole formation is facilitated as the CPP of the surfactant system decreases. The gemini 1212-12 has a lower CPP than 12-3-12 and is therefore more prone to form small holelike structures, which will rupture the lubricating film. We suggest viewing the results in the light of the theory of thin film stability, as proposed by Kabalnov and Wennerstro¨m25 for the stability of emulsions. The good reproducibility shows that it is straightforward to extract information about adsorbed amount from the QCM measurements whereas interpreting data in terms of area per surfactant and dissipation is more difficult. No correlation between spacer length and viscoelasticity of the formed surfactant layer is found from the QCM measurements (as interpreted from changes in the dissipation factor) despite the large difference in packing density as determined from the frequency shift. Acknowledgment. We thank Prof. P. Claesson for valuable discussions. Prof. Ronald Verrall is thanked for providing us with the dimeric surfactants. K.B. acknowledges financial support from the Swedish Research Council for Engineering Sciences. J.J.R.S. acknowledges financial support from the SSF Colloid and Interface Technology Program. LA0117754
