Organized Structure of Lithium Perfluorooctanesulfonate at the Graphite–Solution Interface

JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.

191, 303–311 (1997)

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Organized Structure of Lithium Perfluorooctanesulfonate at the Graphite–Solution Interface Reuben Lamont and William Ducker 1 Department of Chemistry, University of Otago, P.O. Box 56, Dunedin, New Zealand Received October 2, 1996; accepted May 9, 1997

The structure of aggregates of lithium perfluorooctanesulfonate (LiFOS) adsorbed to the interface between graphite and aqueous solution have been measured. This fluorocarbon surfactant produces aggregates which are long (Ç100 nm) and thin (Ç5 nm), and about one molecule (Ç1.3 nm) deep. The aggregates lie in straight, parallel arrays on the surface with a characteristic repeat distance, or period, perpendicular to the long axis. As the bulk concentration of LiCl is increased, the period decreases, but as the bulk concentration of LiFOS is increased, the period increases. The decrease in period on addition of salt is similar to that observed for sodium dodecyl sulfate (SDS) and is explicable in terms of electrostatic forces. The increase in period on addition of surfactant is in sharp contrast to the behavior of SDS and may be due to a higher surfactant packing-parameter for LiFOS. q 1997 Academic Press Key Words: hemimicelle; surfactant; adsorption; self-assembly; atomic force microscope (AFM).

INTRODUCTION

An important property of surfactants is that they partition to an interface. The result of this partitioning is that surfactants are effective at very low bulk concentrations, and thus produce results cheaply. Another important property of surfactants is that they form organized structures such as micelles and vesicles in bulk solution. Recently it has been demonstrated that adsorbed surfactants also display distinct organization at interfaces (1). Work is now underway to systematically study the relationship between the forces acting on individual surfactants, and the shape of the adsorbed surfactant aggregate. Electrostatic interactions have been examined in the sodium dodecyl sulfate (SDS) –graphite system by systematically varying the concentration of SDS (2), NaCl (2), MnCl2 (3), or MgCl2 (3) in solution. Structures observed under all these conditions are consistent with the formation 1 To whom correspondence should be addressed at Department of Chemistry, Virginia Tech., Blacksburg, VA. WDUCKER@CHEMSERVER. CHEM.VT.EDU.

of hemicylindrical aggregates, and, for Na / and Mn 2/ , the period of the aggregate scales with the solution Debye length. This has been interpreted in a model where the aggregate separation is a compromise between a repulsive electrostatic force between adsorbed aggregates, and an attractive force due to a lowering of the interfacial energy when surfactant aggregates cover regions of the (hydrophobic) graphite which were formerly in contact with water. The forces between the solid substrate and water also influence the aggregate structure. This has been examined in a system containing a zwitterionic surfactant, dodecyldimethylammoniopropanesulfonate (DDAPS) (4). DDAPS is net uncharged, so the strong effect of long-range monopolar electrostatic forces is excluded. DDAPS forms cylindrical aggregates on (hydrophobic) graphite and spherical aggregates on (relatively hydrophilic) mica and silicon nitride. It has been hypothesized that the lower curvature is a result of minimizing the area of contact between water and the hydrophobic substrate. Mixtures of two surfactants have also been studied (5). Dodecyltrimethylammonium bromide (DTAB) forms cylindrical aggregates on mica, and when it is mixed with the zwitterionic DDAPS, it is possible to form surface aggregates where the length of the aggregate depends on the fraction of surfactant in the bulk mixture. In this system, the importance of the surface is manifest in the relative partitioning of the cationic surfactant to the anionic mica surface. In this work, we have studied a fluorocarbon surfactant, lithium perfluorooctanesulfonate (LiFOS), in order to examine the effect of a change in surfactant-tail chemistry on surface aggregation. LiFOS is similar to the well studied SDS surfactant (see Fig. 1), and has approximately the same critical micelle concentration (cmc) at 257C: 6.3–7.1 mM (6, 7) compared to 8.1 mM for SDS (8). They are not structural analogues: SDS is a sulfate and LiFOS is a sulfonate; SDS has 12 carbons and LiFOS has 8; and they have different counterions. LiFOS was chosen because the similarity in cmc indicates a similarity in the free energy of transfer of the entire hydrophobic chain from water to the micelle interior. This is because each fluorocarbon unit is

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0021-9797/97 $25.00 Copyright q 1997 by Academic Press All rights of reproduction in any form reserved.

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FIG. 1. SDS (top) and LiFOS (bottom): a fluorinated chain with the same cmc is wider, shorter, and less flexible.

more hydrophobic. LiFOS and SDS also have different counterions and slightly different headgroups, but this switch of counterion is forced by the high Krafft temperature of NaFOS (7). A brief study of lithium dodecylsulfate (LiDS) shows that it does form the same cylindrical aggregates on graphite as SDS (3). For aggregation in bulk, the difference between alkane sulfates and sulfonates is small: sulfates have similar aggregation to sulfonates with one less methylene unit (9). Here we show that at the graphite–solution interface LiFOS forms aggregates of the same general shape as SDS, that is long thin aggregates, but that the detailed behavior is quite different. LiFOS aggregates are aligned over smaller distances, and the period of aggregation increases with concentration whereas the period decreases for SDS.

Contact Angle Measurements

EXPERIMENTAL

Sample Preparation and Characterization Water was prepared by distillation then passage through a Milli-Q RG system consisting of charcoal filters, ion-exchange media and a 0.2 micrometer filter. The resulting water has a conductivity of 18 MV cm01 , and a surface tension of 72.4 mJ m02 at 22.07C. Lithium chloride (BDH, Poole, UK, 99%) was roasted at 5007C for 15 h in air to decompose organic contaminants. Perfluorononane (Aldrich, Milwau-

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kee, MO, 97%) was used without further purification. Adhesive tape was used to cleave a fresh sample of graphite for each experiment from a Pyrolytic Graphite Monochromator (grade ZYH, Union Carbide, Cleveland, OH). HFOS was prepared by ion-exchange of KFOS (PCR Inc., Gainsville, FL) on the H / form of a Dowex column at 657C, then LiFOS was made by neutralization with LiOH, 98% (Riedelde Hae¨n, Seelze, Germany). The resulting LiFOS was recrystallized twice from a mixture of distilled ethanol and distilled chloroform. The bulk aggregation of LiFOS was examined in D2O at 307C by 19F NMR on a 300 MHz VXRS300 spectrometer (Varian, Palo Alto, CA), and by conductivity measurement in H2O at 257C (Suntex Instruments, model SC-170 Taipei, Taiwan).

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Droplet contact angles on graphite were measured using a protractor connected to a microscope with a freely rotating cross-hair eye-piece. Samples were prepared in a laminar flow cabinet, then sealed in a container during microscopy. A solution droplet of about 2 mm radius was placed on the graphite substrate then the volume was increased or decreased using a syringe. The advancing angle was measured when the perimeter of the droplet increased, and the receding angle was measured when the perimeter decreased.

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Microscopy Images were captured using a Nanoscope III AFM (10) (Digital Instruments, CA) using silicon ultralevers (Park Scientific, CA) with a spring constants of 0.12 { 0.02 N m01 , as determined by the measuring the resonant frequency of loaded and unloaded cantilevers (11). The ultralevers were irradiated for 40 minutes ( Ç9 mW/cm2 at 253.7 nm) in a laminar flow cabinet before use. Unless otherwise stated, the images presented are deflection images (showing the error in the feedback signal) with low integral and proportional gains, and scan rates of about 10 Hz. No filtering of images was performed other than that inherent in the feedback loop. Distances in lateral dimensions were calibrated by imaging a standard grid (2160 lines/mm), and distances normal to the surface were calibrated by measuring etch pits (180 nm deep). All measurements were performed in the temperature range 22 { 27C and in equilibrium with single phase surfactant solutions. The 1 mW laser used to detect the bending of the cantilever has a minimal effect on the temperature of the sample under the cantilever. When a thermister is imaged in water, the temperature of the thermister rises by only 0.17C when the laser is turned on. There is no effect on the surfactant morphology when the laser is turned off for differing periods between imaging. Before the images were captured the graphite substrate was left to equilibrate in the solution of interest for at least 30 min, unless stated otherwise. Solutions were changed by flushing the AFM cell with about 20 times its volume of new solution over 5 min. For the quickly adsorbing surfactant and the concentrations used here, this meant that the cell equilibrium concentration was very close to that of the flushing solution. Imaging was performed at a force which was insufficient to observe the graphite lattice, but with sufficient gradient to obtain high resolution of the adsorbed surfactant aggregates. All quantitative data on periods was taken from images in which the fast scan axis was perpendicular to the aggregate axis, and the angle of tilt between features on ‘‘up’’ and ‘‘down’’ scans was less than 107. However, it must be stressed that the same morphology was observed at all scan angles. The forces between the tip and sample were also measured using a Nanoscope III AFM and analyzed as described previously (12). It is important to note that the zero of distance is defined to occur when the gradient of the force has a high and constant (negative) value which implies that the tip is in contact with the sample. Very strongly adsorbed material may not be displaced in a particular measurement and therefore there may be a systematic error in evaluating the zero of separation for each complete measurement of force as a function of separation. The zero of force is defined to occur when the gradient in force is very low at a large separation.

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FIG. 2. Change in 19F NMR chemical shift as a function of inverse concentration for LiFOS solutions at 307C. Data are shown for the five easily resolved peaks in the spectrum which are numbered starting at the sulfonate group (i.e., C8 is the terminal CF3 ). The intersection in the lines of best fit indicate the cmc and range between 7.1 and 7.5 mM for different groups.

RESULTS

Bulk Aggregation The critical micelle concentration (cmc) in aqueous solution was measured in two ways: from the change in chemical environment indicated by the 19F NMR chemical shift ( d ), and from conductivity measurements. A plot of changes in d vs 1/concentration (Fig. 2) reveals two linear regimes, which is consistent with a model in which concmonomer 1 ( dmonomer 0 dmicelle ) is constant above the cmc, and yields of cmc of 7.3 mM at 307C. A plot of the conductivity versus solution concentration at 257C exhibits a distinct break indicative of the cmc at 6.6 mM, in reasonable agreement with literature values of 6.3 mM using the same method (6) and 7.1 mM from surface tension measurements (7). Following the procedure of Evans (13), we have used the conductivity data to estimate the fraction of surfactant anions which are associated with counterions in the micelles, a. This requires knowledge of the viscous drag on the aggregates which is usually estimated in terms of the aggregation number, N. If it is assumed that the aggregates are spherical, N can be estimated from the length of the molecule and the surface area of the headgroup. When these are determined from a molecular model and values of the surface tension at the air–water interface (7), respectively, we obtain N Å 37 and a Å 71. Although there are many assumptions in determining N, a is very insensitive to the value of N (e.g., a 20% discrepancy in the headgroup area leads to only a 2.4% error in a ).

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FIG. 3. Contact angle of LiFOS solutions on graphite. Both advancing and receding angles decrease monotonically form the pure water solution to a steady minimum above the cmc.

Surface Aggregation: Structure Normal to the Interface Solution contact angle studies are used to determine changes in interfacial energy via the Young equation (14). The values for advancing and receding angles for aqueous LiFOS solution on graphite are shown in Fig. 3. In water, both the advancing and receding angles are very high but decrease rapidly over the range 1–3 mM LiFOS. A constant minimum value occurs from just below the cmc to the highest concentration measured. The decrease in angle implies a decrease in ggraphite – solution caused by adsorption of surfactant with the headgroups exposed to the solution. To examine the interaction of the fluorocarbon chain with graphite, we have also measured the contact angle of nperfluorononane on graphite. A droplet of this fluorocarbon wets graphite in air, but when the droplet is in distilled water, the advancing angle is 767 and the receding angle is 607. Measurement of the force between a silicon AFM tip and a graphite surface are consistent with the contact angle measurements. Figure 4 shows the measured force as the tip approaches the sample. If we assume that the oxide-passivated silicon tip is negatively charged in water due to dissociation of surface hydroxyl groups, then adsorption of the similarly charged surfactant will be limited to the graphite substrate. Figure 4a shows that in 2.9 mM LiFOS there is a long-ranged repulsive force. The surface aggregates are probably charged because of counterion dissociation (as was found for bulk aggregates) so an electrostatic double-layer force is expected between the tip and the aggregates. The measured force is exponential with a decay length of 7.8 nm at separations greater than one decay length. This is somewhat larger than the calculated value of 5.9 nm for a 1:1 electrolyte solution. A more thorough comparison with theory is not possible because we do not know the potential on

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either the tip or the aggregate, or even the shape of the tip. At separations smaller than one decay length, the decay becomes steeper as expected for double-layer forces, but may also include contributions due to confining the hydrated surfactant to a smaller volume and/or dehydrating the surfactant. At about 1 nm, there is a mechanical instability when the gradient of the attractive force exceeds the spring constant. For subsequent measurements on the same approach, the system behaves as it did in pure water, indicating that the surfactant has been displaced. When the concentration of LiFOS is increased, the repulsive force is still exponential, but the decay length decreases in qualitative agreement with the trend expected from the solution Debye length. (For a 1:1 electrolyte, the Debye lengths are 3.0 and 2.0 nm for 10.1 mM and 14.2 mM solutions and the decay lengths of our measured forces are 2.0 and 1.6 nm, respectively.) The force at which the instability occurs also increases with concentration. At 14.2 mM, the separation at which the instability occurs is much larger. This may be because the surfactant is displaced at that load, and does not necessarily indicate a layer which is 4 nm thick. One interesting feature that frequently occurred was a small barrier at 0.2 nm separation. Possible reasons for this are a layer of solvent molecules, or stick-slip of the tip on the graphite surface. At even higher concentrations of LiFOS a distinct instability in force does not always occur. Force curves are reversible when the load is decreased providing that the tip does not pass through an instability. After an instability, a negative load is required to remove the tip from the surface (i.e., the contact is adhesive). In summary, addition of surfactant caused a switch from an attractive to a repulsive force between the tip and graphite, indicating that surfactant adsorption caused the graphite to switch from having a lower energy in the presence of the tip to a lower energy in the presence of water. This additional force appears to have an electrostatic component, but may also have contributions due to specific interactions between water and the surfactant, and is consistent with a structure with headgroups facing the solution. Surface Aggregation: Structure Parallel to the Interface Figure 5 shows an image of the LiFOS adsorbed to the surface of graphite at a bulk concentration of 14.6 mM, about 2 h after the graphite was exposed to the surfactant solution, and immediately after imaging at a force sufficient to resolve the graphite lattice. This picture shows two significant features: the adsorbed surfactant does not cover the entire surface of the graphite during scanning, and the aggregates organize to produce long parallel periodic features on the surface. The organization is similar to that observed for SDS, but the incomplete coverage is in sharp contrast. It is impossible to determine whether or not the incomplete

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FIG. 4. Force on a silicon AFM-tip on approach to graphite substrate in LiFOS solution. (a) In water, the force is attractive until the tip contacts the graphite, but in 2.9 mM LiFOS there is a long-range repulsion, presumably due to electrostatic forces. At about 1 nm separation, there is an instability, and the tip jumps in toward the surface. The images were captured on the repulsive force at greater separations than the instability. (b) At higher LiFOS concentration, the repulsive force has a smaller decay length and higher maximum force.

coverage is due to the action of the tip, but one immediate consequence is that it allows us to accurately measure the thickness of the adsorbed layer. A cross section through part of Fig. 5 reveals that when the tip images the periodic structures, it is about 1.5 nm above the substrate. The extended length of the LiFOS molecule is 1.3 nm (Fig. 1), so a tip–graphite separation of 1.5 nm when the tip sits just above an aggregate suggests that some of the molecules in the aggregate are approximately perpendicular to the solid

substrate and is consistent with a hemicylindrical structure. This thickness value is also similar to the separation at which the instability is measured in 2.9–10 mM solution. After more than about 2 h, the long periodic structures are observed over almost the entire surface, as shown in Fig. 6a. When the concentration of LiFOS is changed, the structure remains periodic, but the period changes. Figure 6b shows a larger period at a higher concentration (57.6 mM) than that in Fig. 6a (14.2 mM). We can accurately measure the

FIG. 5. AFM image of LiFOS adsorbed to graphite from 14.6 mM solution recorded about 2 h after exposure, showing (a) the incomplete coverage (height image) and (b) the periodic structure (deflection image). Both images were captured simultaneously and display the feedback value (low frequencies) and the instantaneous deflection (high frequencies) respectively.

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FIG. 6. (a) Aggregation of LiFOS in 14.2 mM LiFOS solution, showing high coverage after about 3 h. Compared to SDS, there are more aggregate termini per unit length. (Examples are the pair in the top left section, which are marked by an arrow, and the pair immediately to the right of the scale bar.) (b) 57.6 mM LiFOS solution.

periods by taking a Fourier transform of the image, and a summary of the periods from (300 nm) 2 images over a range of concentrations is shown in Fig. 7. In all of these measurements a single dominant period (plus overtones) was observed. Each point represents an average (and the error bars the standard deviation) of many measurements on different regions and at different times. It is clear from this figure that the period increases with concentration, in sharp contrast to the behavior of sodium dodecyl sulfate (SDS) (2). Since we have imaged the aggregates at a variety of forces, we have also checked for the possibility that the period is a function of the force applied by the tip. In a single image,

FIG. 7. Adsorbate period as a function of LiFOS concentration.

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we have varied the force from zero to a force greater than that which displaces the surface aggregates. This was done either manually in steps, or by ramping the setpoint with a function generator. At 45 mM there was no measurable change in the period as a function of the applied force over the entire force range ( Ç1 nN) in which the aggregates could be measured. This test was done at a relatively high concentration because the range of measurable force increases with concentration. In both SDS and LiFOS, the long axes of the aggregates are aligned parallel. This alignment occurs in one of only three directions, each oriented at 607 relative to the others. Another feature which distinguishes LiFOS from SDS aggregates on graphite is the small area (grain size) over which the long-axis of each aggregates is aligned. In SDS the grain size is very large ( ú (500 nm) 2 ) while in LiFOS, the grain size is smaller, typically Ç (100 nm) 2 . The grain size in LiFOS was polydisperse, and Fig. 8 shows a case where there was a particularly high density of grains. The termini of the aggregates at the grain boundaries are one example of defects in the aggregate structure. LiFOS surface aggregates also have a relatively high density of isolated defects. In Fig. 6a there is a particularly clear pair of aggregate termini in the top left corner and also another pair immediately to the right of the scale bar. The existence of a higher density of aggregate termini gives the aggregate a higher overall curvature. It is not clear whether this is a property of LiFOS, or perhaps the result of a small amount of impurity. To examine the influence of electrostatic forces on surface

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FIG. 10. Effect of bulk LiCl concentration on the period of adsorbed LiFOS aggregates in 15 mM LiFOS.

FIG. 8. AFM image in 14.2 mM LiFOS showing many grains oriented at 607, and the small length of the aggregates relative to those formed by SDS.

period. This summary, shown in Fig. 10, demonstrates that the period decreases with LiCl concentration, and thus that the behavior is similar to that observed for SDS.

aggregation, we have studied the behavior when the concentration of the surfactant counterion, Li / , is varied at constant FOS 0 concentration. In 15 mM FOS 0 , the morphology is unchanged over the range 0–207 mM LiCl. Figure 9 shows an image in 45 mM LiCl. This uniformity again allows us to summarize the observed changes in terms of the aggregate

DISCUSSION

FIG. 9. AFM image of LiFOS adsorbed to graphite in equilibrium with a solution containing 15.5 mM LiFOS and 45 mM LiCl.

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Comparison of SDS and LiFOS: Adsorption to Graphite Previous work on the adsorption of SDS to graphite showed that the period of SDS aggregates decreased on addition of either SDS or NaCl. This was interpreted using a model in which the SDS formed very long cylindrical aggregates on the substrate, and the period was the sum of the aggregate diameter and the interaggregate spacing. The period decreased linearly as a function of Debye length in either NaCl (2) or MnCl2 (3) solutions, and this was interpreted in terms of a decrease in interaggregate spacing due to screening of the repulsive electrostatic forces between the aggregates. For LiFOS, the period also decreases on addition of counterions (Li / ), suggesting a similar mechanism, but we do not have enough data in an appropriate regime to determine the relationship to the solution Debye length. We can consider the aggregate spacing to be determined by a balance between repulsive electrostatic forces such as electrostatics, hydration, protrusion and undulation forces, and attractive forces such as surface energy terms and van der Waals forces. Here we will concentrate on repulsive electrostatic forces and an attractive force due to the reduction in free energy when the aggregates move closer together and decrease the area of hydrophobic graphite exposed to solution. (A rough estimate of the attractive force can be obtained as follows. When an extra aggregate adsorbs above the cmc and the aggregates on the surface become more closely spaced, an area of graphite –solution interface is

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exchanged for graphite/ surface –micelle interface. Additional surface –micelle /solution interface is created (which has p / 2 times the graphite/ surface –micelle area for hemicylinders) but we will neglect this term above the cmc because it is likely to be similar to the surface energy of micelles which adsorb from solution. The attractive force between adsorbed hemicylinders is thus proportional to ggraphite – water 0 ggraphite – hemicylinder . If we neglect headgroup interactions we can approximate this to gfluorocarbon – water 1 cos u using the Young equation. u is the contact angle for a fluorocarbon droplet on graphite in water, which we have measured to be 767 when the droplet is advancing and 607 when the droplet recedes. Using the value of gfluorocarbon – water Å 0.05 J m02 ( 15 ) we obtain an energy /area or force /length of 0.012 –0.025 Nm01 which pulls the aggregates together.) Both force and conductivity measurements suggest that the LiFOS aggregates are charged. As more counterions (Li / ) are added the interaggregate repulsion is screened and perhaps the charge is reduced by increased counterion binding. The separation between aggregates decreases until the electrostatic force is large enough to oppose the attractive force due to the surface energy. Between 15 and 45 mM LiCl, the measured period plateaus at 5.3 nm and this may represent the diameter of the aggregates in 14.5 mM LiFOS. The behavior on changing the surfactant concentration is qualitatively different for LiFOS and SDS: for LiFOS, the period increases with surfactant concentration. This is shown more clearly in Fig. 11 where the effect of adding Li / (either as LiCl or LiFOS) to a 14.5 mM solution is shown. If we assume that the effects of Li / and FOS 0 are additive, we can determine the effect of FOS 0 as the difference between

the two curves in Fig. 11. Adding FOS 0 appears to swell the adsorbed aggregates. Comparison of SDS and LiFOS: Molecular Properties Fontell and Lindman have discussed differences between hydrocarbon and fluorocarbon surfactant properties (16). A study of the variation of the cmc of surfactants as a function of chain-length shows that the cmc of a perfluorinated surfactant is the same as for alkyl surfactant with a chain 1.5 times as long. This allows an estimation of the free energy required to transfer one CF2 group from an aqueous environment into a micelle (7). The free energy for {CF2{ is about 1.6kT or about 1.5 times the value for a single {CH2{ group (7). The two main forces acting on a surfactant aggregate in bulk solution are the drive to reduce the contact of water with the hydrophobic chains, and the repulsive electrostatic interactions between similarly charged headgroups. The aggregate curvature is thus a compromise between maximizing the separation between headgroups, and minimizing the exposure of water to the hydrophobic groups in the gaps between the headgroups (17). Considering this effect alone, we would expect a perfluorinated surfactant aggregate to have a smaller headgroup spacing than the equivalent hydrocarbon surfactant, and thus a lower curvature. Other influences of the fluorocarbon chain are that the anion is a weaker base due to inductive effects, a larger cross-sectional area and volume due to the greater van der Waals radius of fluorine, and a greater preference for the trans rather than the cis configuration of the carbon backbone due to steric hindrance of the larger fluorine atoms. Both the alkyl sulfate and the perfluorinated sulfonate are very weak bases, so this effect is probably not important, but the preference for the trans state will lead to a longer average distance between head and tail. The most important effects are likely to be the smaller effective headgroup area and the larger hydrophobic volume, suggesting that fluorocarbon aggregates should produce a less curved structure than hydrocarbon aggregates of the same chain length. CONCLUSION

FIG. 11. The effect of total bulk FOS 0 concentration on aggregation. The horizontal axis shows the concentration of LiCl (circles) or LiFOS (squares) added to a 15 mM LiFOS solution. If the effect of each ion is additive, the difference between data points lying at the same position on the horizontal axis (same Li / concentration) shows the effect of the FOS 0 ion: FOS 0 ions swell the aggregates in the dimension parallel to the interface and normal to the aggregate axis.

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If we combine the period measurements with the properties of the surfactants, there is a simple explanation for the observed response of the aggregates to an increase in bulk surfactant concentration. In both SDS and LiFOS, the response to an increase in solution concentration is to cover more of the hydrophobic graphite with aggregates. This can be achieved in at least two ways: by the aggregates moving closer together, or by each aggregate covering more of the surface. Which of these two produces the lowest energy structure depends on the details of the surfactant molecule. For SDS, most of the additional coverage occurs via a closer

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molecules. Each terminus adds to the aggregate curvature, a feature which is inconsistent with a larger packing parameter if the headgroups also cover the hydrophobic core at the aggregate termini. ACKNOWLEDGMENTS Our thanks go to Erica Wanless for useful discussions and to George Petersen for giving us the ultralevers. This work was funded in part by the NZ Lottery Science Commission (Ap38518) and by an Otago Research Grant.

REFERENCES

FIG. 12. Schematic figure showing our model for the effect of electrolyte and surfactant on the cross section through an aggregate of anionic surfactant adsorbed to the graphite-solution interface. For SDS aggregates, an increase in salt or surfactant concentration causes the aggregate period to decrease. The main contribution to this is probably a decrease in interaggregate separation. For LiFOS aggregates, an increase in salt concentration causes the aggregates to move closer (as for SDS). When the LIFOS concentration is increased, the aggregates probably also move closer together because of screening due to the Li / ions, but this effect is masked by an increase in the diameter of the aggregates.

packing of the aggregates (Fig. 12). In this regard, NaCl is similar to SDS in achieving the dense coverage by reducing the electrostatic repulsion. For LiFOS, rather than the aggregates just moving closer, they swell significantly (i.e., achieve a larger diameter). This can be rationalized on the basis of a higher packing parameter of the LiFOS molecule (higher gwater – surfactant tail and hydrophobic chain volume per CX2 unit), which allows it to more easily form aggregates of larger diameter (Fig. 12). What remains now is to see whether or not this model has broad applicability to other surfactant systems. One result which remains unexplained is the observation that LiFOS molecules form shorter aggregates than SDS

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