Langmuir 1994,10, 988-990
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Forces between Macroscopic Surfaces in a Sponge Phase Plamen Petrov,*i+Ulf 01sson,+Hugo Christenson,$ Stanley Miklavic,g and Hhkan Wennerstromt Division of Physical Chemistry 1, Chemical Center, P.O.Box 124,S-221 00 Lund, Sweden, Experimental Surface Physics, RSPhysS, Australian National University, Canberra, ACT 0200,Australia, and Division of Food Technology, Chemical Center, P.O. Box 124,S-221 00 Lund, Sweden Received December 13, 1993. In Final Form: February 7, 1994' We present measurements of forces between macroscopic mica surfaces immersed in a series of AOTNaNOs-water samples of a sponge (L3)phase. We observe a long ranged oscillatory force which exhibits two distinct regions. At large separations, 40-1200 A, the period of the oscillations is concentration dependent and the effect is due to structure in the bulk solution. In a confined space, at separations smaller than 300A, there is a to ological transition to a layer structure with a force having huge repulsive barriers with a period of 30-35 ,! virtually independent of concentration.
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Introduction The self-assembly of amphiphilic molecules leads, under certain conditions, t o the formation of infinite monolayer or bilayer films with multiply connected topology t h a t extends in three dimensions.' T h e films can pack into ordered structures as in bicontinuous cubic phase^.^?^ Alternatively, they form liquid isotropic phases with a disordered structure, like a bicontinuous microemulsion with a monolayer film or a sponge (L3) phase with a bilayer film."' Even though these are macroscopicallydisordered, there is a single aggregate and one would expect long range correlations in the system. Another interesting aspect of these systems with a three-dimensional infinite aggregate is their properties a t interfaces/surfaces. The threedimensional organization is incompatible with the twodimensional nature of an interface (or surface). Clearly there has t o be structural changes in the aggregate when it meets a surface. When measuring the force between two macroscopic surfaces in a sponge phase, we can expect forces due both t o the structure in the bulk solution and to surface induced structures. In the present communication we present the first surface force data for solutions in a sponge phase using the ternary system water-AOTNaN03. This work is a part of our continuing investigations of sponge phases and microemulsions.
Experimental Section Sodium bis(2-ethylhexyl)sulfosuccinate (AOT) was obtained from Sigma (99% pure) and used without further purification. Deionized and distilled water used in all experiments was passed through a Millipore Water System consisting of an ion-exchange cartridge, organex-Q, activated charcoal filter, and nucleopore filters. NaNO3 was obtained from Merck (pro analysi, >99%). t Division of Physical Chemistry 1,Chemical Center.
Experimental Surface Physics, Australian National University. f Division of Food Technology, Chemical Center. * Abstract published in Advance ACS Abstracts, March 15,1994. 4
(1)Scriven, L. E. Nature 1976,263, 123. (2)Andemson, S.;Hyde, S. T.; Larsson, K.; Lidin, S. Chem. Rev. 1988,
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(3)Anderson, D. M.; Davis, H. T.; Scriven, L. E.; Nitache, J. C. C. Adu. Chem. Phys. 1990, 77,337. (4)Porte, G.; Marignan, J.; Bassereau, P.; May, R. J. Phys. (Paris) 1988, 49,511. (5)Anderson, D.; WennerstrGm, H.; Olsson, U. J. Phys. Chem. 1989, 93, 4243. (6) Strey, R.;Winkler, J.; Magid, L. J. Phys. Chem. 1991, 95, 7502. (7)Safran, S.A. In Structure and Dynamics of Strongly Interacting Colloids and Supramolecular Aggregates in Solution; Chen, S., Huang, J. S., Tartaglia, P., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1992;p 237.
The nitrate was used rather than the more studiedNaCl system&" as the former does not affect the silver layer on the back side of mica surfaces.12 The phase diagram with NaN03does not differ significantly from that observed with NaCl. The samples were prepared by weighing the components and mixing until homogeneity. Their compositions (weight percent) were as follows: (1)10% AOT, 2.8% NaN03; (2) 20% AOT, 3.15% NaNOs; (3) 30% AOT, 3.5% NaN03; (4) 40% AOT, 4% NaNOa; (5)50% AOT, 4.4% NaN03. The samples were checked between crossed polarizersto confiim that they were in a single phase. All solutions were filtered through Acrodisc filters (0.2-pm pore size). Forcesbetween mica surfacesare measured using Israelachvili's technique, described in full el~ewhere.'~J~ Equally thin sheets of molecularly smooth ruby mica (from Associated Commodity Corporation Ltd. (N.Y.)) are cleaved and silvered on one side. The prepared surfaces are glued with an epoxy resin (Epikote 1 0 4 from Shell Chemical Co.) with the silvered side down onto two supporting silica disks which are then mounted on a piezoelectrictube and double cantilever spring in crossedcylinder geometry. The force between the surfaces, F,is calculated from the deflection of the spring from its equilibrium position. The separation is monitored interferometrically. The measured forces are given in FIR units ( R is the mean radius of curvature of the surfaces) which can be related to the interaction free energy per unit area between plane surfaces using the Derjaguin approximation.ls There is a natural mechanical instability in the system which sets in whenever the gradient of the force (aFlaD) overcomes the spring constant. One surface jumps to the next stable position where aFJaDis again less than the spring constant. In some cases we observed that the surfaces could pass a stable position due to inertia or other nonequilibrium effects. With the SFA assembled, the surface-surfacecontact in air was determinedafter which the solution (-30 cm9 was injected. The first force run was performed a few hours after the injection, as soon as the thermal drifts were low enough (Cf5 "in) to ensure stable calibration. Subsequentmeasurements were done (8)Fontell, K. In Colloid dispersions and micellar behauiour; ACS Washington,DC, 1976;
Symposium Series;American Chemical Society: p 270.
(9)Balinov, B.; Olsson, U.; Sderman, 0. J. Phys. Chem. 1991, 96, 5931. (10)Skouri, M.; Marignan, J.; Appell, J.; Porte, G . J. Phys. N 1991, 1,1121. (11)Strey, R.;Jahn, W.; Skouri, M.; Porte, G.; Marignan, J.; Olsson, U. In Structure and Dynamics of Strongly Interacting Colloids and Supramoleculor Aggregatesinsolution; Chen,S., Huang,J. S.,Tartaglia, P., Eds.;Kluwer AcademicPublishers: Dordrecht,The Netherlands, 1992; p 351. (12)Shubin, V. E.; KBkicheff, P. J. Colloid Interface Sci. 1993,155, 108. (13)Israelachvili,J. N.; Adams, G. E. J. Chem. Soc., Faraday Trans. 1 1978, 74,975. (14)Parker, J.L.;Christenson, H. K.; Ninham, B. W. Reu. Sci. Znstrum. 1989,60, 3135. (15)Derjaguin, B. V. Kolloid 2. 1934,69, 155.
0743-7463/94/2410-0988$04.50/00 1994 American Chemical Society
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Separation, A Figure 1. Normalized force versus distance between mica surfaces in an La phase of 20% (a, top) and 40% (b, bottom) AOT. Forcesat short separations are shown in the inserts. Arrows indicate the positions of inward jumps. (Notethat for one or two force barriers closest to the surface the points only represent an estimate of the force.) over a period of several days and no time or history dependence of the forces was observed. Each individual study was repeated several times at different contact positions and excellent reproducibility was found. All the measurements took place in a thermally insulated room, kept at a constant temperature of 22 OC. Measurements were carried out very slowly to ensure that the system had sufficient time to establish equilibrium after a change of separation.16J' The total time to bring the surfaces from 3000 A down to contact was about 1.5-2 h doing steps of 15-20 A in constant intervals of time. Measurements at higher compression rates did not lead to qualitatively different results and even at very high rates one could observe the main features of the force profile. This can be related to the fact that La phase behaves as a Newtonian liquid over a wide range of shear rates and has a relatively low viscosity (3-10 cP).'~The inward jumps from one stable position to another took tens of seconds in the most concentrated system and the measurement was resumed when no further change of the separation was taking place.
Results Parts a and b of Figure 1 show the measured force (normalized by the radius of curvature R) between two curved mica surfacesimmersed in a sponge phase containing 20% and 40% AOT, respectively. Each curve represents data from at least two different force runs. In both cases a force is observed at separations of more than lo00A. This is a remarkably long range consideringthat (16) Horn, R. G.; Hin, S. J.; Hadziioannou,G.; Frank, C. W.; Catala, J. M.J. Chem. Phys. 1989,90,6767. (17) KBkicheff, P.; Richetti, P.; Christenson, H. K. Langmuir 1991,7, 1874. (18) Snabre, P.; Porte, G. Europhys. Lett. 1990,13,641.
the sample is an isotropic solution of a short chain surfactant at a high electrolyte concentration. Furthermore the force is of an oscillatory nature indicating that it is caused by self-assembly structures present between the surfaces. On separation the force curve follows the same profile as recorded on approach. Although the two force curves differ substantially quantitatively, we can in both cases identify two regimes each with virtually constant periods of the oscillations. The oscillatory force observed at large separations, 1400 A, has a longperiod (>MA) which is stronglyconcentration dependent. The period at short separations is in the range 30-35 A with only a weak concentrationdependence. Quite often, particularly at low volume fractions, two or three of the short range oscillations are jumped over. The samples with 30% and 50% AOT show the same qualitative behavior as those with 20% and 40%, while for the 10%AOT sample we were unable to detect any long range oscillations. Discussion The isotropicspongephase can be thought of as a melted bilayer cubic phase. In the present system a sponge-tocubic phase transition occurs around 60 w t % AOT.889 Although there is no long range order in the sponge phase, there is every reason to expect short range order. To characterize the short range order, we need, at least, two lengths. The positional correlation function is oscillatory with a period A1 and decays in amplitude with a decay length A2. When confined between two mica surfaces, the sponge phase is confronted with two constraints influencing ita structure. It has to meet the flat polar surface and, in addition, the structure between the surfaceshas to be such that the phase meets the two surfaces in a symmetrical way. Sufficientlyfar outsidea singlesurfacethe localstructure of the self-assembled aggregate should be that of the bulk phase. However, the surface could still have an effect both by influencing the orientation of the aggregate and by fixing the phase of structural oscillations. When two such surfaces approach one another, these density oscillations interfere and one should obtain an alternating repulsive and attractive force with a period A1 corresponding to that in the bulk phase along the relevant direction. Oscillatory forces with such an origin were previously reported for simple and both micellar23and reversed micellar" solutions. The observed concentration dependence of A1 is shown in Figure 2. For an infinite film structure, the structural length scale is inversely proportional to the amount of interfacial area per unit volume. We thus expect A1 a c#~-l, where 9 is the volume fraction of surfactant, with some deviation at higher concentrations due to the finite bilayer thickness. The fitted line in Figure 2 shows that this inverse dependence is observed to a good approximation also for the period of the force curve. However, we note that the actual value of A1 differs significantly (-40% greater) from the characteristic length determined from the peak position in t h e smallangle scattering function of an equivalent system.lOJ1 (19) Horn,R. G.; Israelachvili,J. N. Chem. Phys. Lett. 1980, 71,192. (20) Christeneon, H. K.; Horn, R. G.; Ieraelachvili, J. N. J. Colloid Interface Sci. 1982, 88,79. (21) Christeneon, H. K. J. Chem. Phys. 1983, 78,6906. (22) Attard, P.; Parker, J. L. J. Phys. Chem. 1992,96,6086. (23) Richetti, P.; KBkicheff, P. Phys. Rev. Lett. 1992, 68, 1961. (24) Parker, J. L.; Richetti, P.; KBkicheff, P.; Sarman, S. Phys. Rev. Lett. 1992, 68, 1966.
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Volume fraction AOT Figure 2. Period XI (peak to peak) of the force oscillation in sponge phaae as a function of surfactant volume fraction 4. The solid line is a hyperbolic fit to data points, XI = -36 + 58.6/4.
In the force curves of Figure 1there is a transition region between 500 and 200 A and the short period of the oscillations observed at smaller separations indicates that another mechanism operates at short range. In fact, the
force curve shows clear analogies with those observed for the lamellar phases of sodium dodecyl sulfate/water/lpentanol by K6kicheff et a1.l' and sodium octylbenzenesulfonate/ l-pentanol/n-decanelwater.mThe most likely interpretation is that the strong confinement has induced a topological change in the bilayer so that these are stacked between the surfaces as in a lamellar phase. Due to the high salt content, double layer forces are weak and the repeat distance in the stack is small even at relatively high water contents in the bulk phase. An analogous observation of surface induced structure in a disordered bulk solution was recently reported by Chen et al.28 In conclusion we report in this paper the observation of a long range oscillatory structural force for a system containing one macroscopic continuous aggregate. The force is measurable at separations above loo0 A and the period reflects a structure present in the bulk phase. At shorter separations there is a change in the local bilayer topology and the aggregate seems to consist of stacked bilayers in the confined space between the surfaces. We will continue these studies to verify that the observed effects are also relevant for other sponge phases. This work should also lead to a quantitative analysis of the forces using current models for the sponge phase.2'7 (25) Klkicheff, P.; Christeneon, H. K. Phys. Rev. Lett. 1989,sS, 2823. (26) Chen, Y.L.; Xu, Z.; Ieraelachvili, J. Longmuir 1992,8, 2966. (27) Schmid,F.;Schick,M.Phys.Rev.E Stat.Phys.,Plasmas,Fluids, Relot. Znterdiecip. Top. 1993,48, 1882.
