Langmuir 1992,8, 757-759
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Direct Measurements of the Attraction between Solvophobic Surfaces in Ethylene Glycol and Mixtures with Water John L. Parker* and Per M. Claessont Department of Applied Maths, Research School of Physical Sciences, Australian National University, G.P.O.Box 4, Canberra, A.C.T. 2601, Australia Received October 29, 1991. In Final Form: December 17, 1991 The attraction between two macroscopic hydrophobic surfaces extends to extraordinary separations in aqueous systems. Whether similar forces exist in nonaqueous solvophobic systems forms the subject matter of this paper. A fluorocarbon surfactant monolayer adsorbed from ethylene glycol solutions has a low surface energy with a contact angle of 96' measured against ethylene glycol. Force measurements between two such monolayers reveal a force with a range not larger than expected for a van der Waals interaction. A dramatic increase in range and strength of the attractive force is observed with increasing water content, and at 51 5% the force is almost identical to that observed between hydrophobic surfaces in water. These observations indicate that the molecular nature of the interface and intervening liquid must be considered in a theoretical treatment of the interaction.
Introduction received any attention is the possible existence of a similar long-range force in nonaqueous systems. Hence, it is not One of the most surprising results to emerge from the known whether this force is peculiar to aqueous systems direct surface force measurement technique is the strength or not. and range of the attractive force between two hydrophobic The forces measured between surfaces in water with surfaces in aqueous solutions. At large surface separations low contact angles (8< 60') are well described by a classical the force is characterized by an exponential attraction van der Waals interaction.12 Long-range attractive forces with a very long decay length (5-10nm).1-4 I t can extend are only observed at high contact angle (8> 80') surfaces. to over 100nm and is 2 orders of magnitude stronger than Fluorocarbon surfaces are solvophobic(i.e., they have large the van der Waals intera~tion.~ These forces have been contact angles) with a number of liquids. For instance a observed between hydrophobic surfaces prepared in a sessile droplet of ethylene glycol (EG)on a fluorocarbon number of different ways. Surfactant adsorption from surface, used in this study, has an advancing contact angle solution,' Langmuir-Blodgett d e p ~ s i t i o n , and ~ - ~covalent 96' and a receding angle of 80°, which is not too of modifi~ation.~Jj The effects of a variety of salts have been dissimilar from the contact angles measured for water on studied, but interpretation of these results is complicated by ion adsorption to the surface and surfactant e ~ c h a n g e . ~ ~a similar surface of 110' and 60'. Studies of aggregationbehavior in ethylene glycol-water A phenomenologicalmodelloand one based on electrostatic mixtures have revealed a decreasing driving force for dipolar correlations" have been proposed to account for aggregation on increasing ethylene glycol ~ 0 n t e n t . lThis ~ these observations. Recent measurements8t9have elimto a lower solvophobic interaction in ethylene glycol is due inated dipolar correlations as a possible mechanism, and than in water. In water, the hydrophobic interaction which as a result we are left without an adequate understanding. drives aggregation is due to the clathrate-like liquid When strongly hydrophobic surfaces are brought into contact, a vapor cavity forms around the contact r e g i ~ n . ~ structure which exists around an individual hydrophobic moiety in solution. This should not be confused with the This cavitation phenomenon indicates that the aqueous long-range force measured between macroscopic hydrofilm between the surfaces is metastable, and it has been phobic surfaces. Intuitively the two may be linked in some proposed that this metastability may be related to the way, but this has yet to be established. long-range nature of the interaction, but exactly how remains to be elucidated. There are a large number of Met hods experimental findings, but one key issue which has not We have measured the force-distance profiles between macroscopic solvophobic surfaces in pure ethylene glycol and over + On leave from The Surface Force Group,Department of Physical a range of water contents. The surfaces were prepared by Chemistry, The Royal Institute of Technology, 5-10044 Stockholm, adsorption from a 1.3 X 10-5 M solution of N-[a-(trimethylamand The Institute for Surface Chemistry, 5-114 86 Stockholm, monio)acetyl]-O,O-bis(1H,1H,2H,2H-perfluorodecyl)-~-glutaSweden. mate chloride (Sogo Ltd.,Japan; used without further purifi(1) Pashley, R. M.; Mcguiggan, P. M.; Ninham, B. W.; Evans, D. F. Science 1985,229, 1088. cation; henceforth referred to as 'fluorocarbon surfactant") in (2) Claesson, P. M.: Blom, C. E.; Herder. P. C.; Ninham, B. W. J. pure ethylene glycol (Fluka; distilled twice before use). Water Colloid Interface Sci. 1986, 114, 234. was purified by distillation followed by treatment with a Mil(3) Chiitenson, H. K.; Claesson, P. M. Science 1988,239, 390. lipore MilliQ plus unit and final distillation. Forces were (4) Claeseon, P. M.; Christenson, H. K. J. Phys. Chem. 1988,92,3531. measured with the MkIV surface force apparatus1' (SFA),and (5) Parker, J. L.; Cho, D. L.; Claesson, P. M. J.Phys. Chem. 1989,93, 6121. the water content was varied by injection of a fixed amount of (6) Rabinovich, Ye. I.; Derajaguin, B. V. Colloids Surf. 1988,30, 243. pure water into the chamber of the apparatus. As a result the (7) Christenson, H. K.; Claesson,P. M.; Berg, J.;Herder, P. C. J . Phys. surfactant concentration drops with increasing water content. Chem. 1989,93,6121.
(8) Christenson, H. K.; Fang, J.; Ninham, B. W.; Parker, J. L. J.Phys. Chem. 1990,94,8004. (9) Parker, J. L. Manuscript in preparation. (10) Erikeson; J. C.; Ljunggren, S.; Claesson, P. M. J. Chem. SOC., Faraday Trans. 2 1989,85, 163. (11) Attard, P. J. Phys. Chem. 1989,93,6441.
(12) Berg, J.; Claesson, P. M. Thin Solid Films 1991, 178, 261. (13) Wiirnheim, T.; Jonsson, A. J. Colloid Interface Sci. 1988, 125, 627. (14) Parker, J. L.; Christenson, H. K.; Ninham, B. W. Reo. Sci. Instrum. 1989, 60,3135.
0743-7463/92/2408-0757$03.00/00 1992 American Chemical Society
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758 Langmuir, Vol. 8,No. 3, 1992
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Computer Figure 1. Constant deflection system illustrated schematically. The system, based on the MkIV surface force apparatus, employs a force sensor and magnetic force transducer. The signal from the force sensor is amplified and applied to'two coils in a closed feedback loop. The magnetic field generated by the coils exerts a force on a small magnet attached to the end of the force sensor. In a closed feedback loop this force cancels the force between the surfaces. The force sensor is then maintained at a nominally null deflection. Force measurements were carried using a constant deflection measurement technique.15 This is the first application of this technique, and a brief description follows. Freshly cleaved mica sheets are silvered and glued silver side down onto two silica cylinders with a radius of 2 cm and mounted in the SFA. One surface is mounted a t the end of a piezoelectric tube which is used to control the surface separation, and the other surface is mounted on a bimorph force sensor. Figure 1shows a schematic diagram of the constant deflection system. It is based upon the combination of the bimorph force sensor developed by ParkeP and the magnetic force control system developed by Stewart and Chri~tenson.'~The bimorph produces a charge proportional to an applied load. This charge is detected electronically with a sensitivity corresponding to a displacement of 0.05 nm and a force resolution of N. The magnetic force system utilizes the magnetic field generated by two coils mounted outside the apparatus to apply an external force to a magnet mounted at the end of the force measuring spring. A combination of these two techniques allows the spring deflection to be both controlled and measured electronically with unsurpassed resolution. In these experiments we have chosen to maintain a constant spring deflection with a feedback loop. (Full details will be described elsewhere.18) This is achieved by controlling the current in the coils so that the force applied to the spring exactly cancels the force measured by the bimorph sensor. In this way the instabilities which occur with a simple cantilever spring when the slope of the force law exceeds the spring constant are eliminated. The effective spring constant of the system depends on the characteristics of the feedback loop, and for slowly varying forcesthe spring deflection is nulled completely. With this system the lowest spring constant is due to the elastic deformation of the glue, and this sets the upper limit to the slope of an attractive force which can be measured.15 The constant deflection system has a significant advantage over previoustechniques in that the force can be measured slowly ~
~
(15) Parker, J. L.;Stewart, A. Prog. Colloid Polym. Sci., in press. (16) Parker, J. L.Langmuir, in press. (17) Stewart, A. M.;Christenson, H. K. Meas. Sci. Technol. 1990, 1 , 1301. (18) Parker, J. L.;Stewart, A. M. Manuscript in preparation.
1
Figure 2. Force as a function of surface separation between two mica sheets coated with a monolayer of a fluorocarbon surfactant in ethylene glycol and a t various water contents (A). The dotted line is a calculation of the van der Waals interaction for two bare mica surfaces in water and represents an upper bound for dispersion interactions. The same data as in (A) are plotted on a logarithmic scale in (B) and fitted with a single exponential (FIR = -A exp(-D1X)). in a highly viscous medium and any inaccuracy in estimating the hydrodynamic force in such a medium is eliminated. The motion of one of the surfaces is uncoupled from the other, and if.one assumeslinearity in the piezoelectricdeviceused to controlsurface separation, then force-distance profiles can be recorded by computer in a continuous manner, and the interferometer is used simply for determining contact positions and layer thickness, and for calibration of the motion control and coil sensitivity.
Results A series of experiments were performed in order to determine the optimum concentrations for the formation of a tightly packed monolayer of fluorocarbon surfactant on mica surfaces. A t low surfactant concentrations M) a partial monolayer forms, whereas at high concentrations a charged bilayer forms M) (these results will be presented elsewhere). A good monolayer was found to form at a concentration of -2 X M fluorocarbon surfactant in ethylene glycol. The monolayers adsorbed a t this solution concentration have a thickness of 2 nm when measured with respect to the contact position in air. Figure 2 shows the measured force profiles in this solution and with increasingwater content. In pure ethylene glycol the range of the force is comparable to that expected for the van der Waals interaction. Figure 2 shows the van der Waals interaction expected for two mica surfaces interacting across an aqueous interlayer. The Hamaker constant for fluorocarbon surfactant coated mica surfaces is expected to be much lower; and so the line in Figure 2 represents an upper bound on the van der Waals contribution. Dramatic changes occur in the force law on addition of water. At large surface separations all the force curves obey the empirical relationship
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Langmuir, Vol. 8,No. 3, 1992 759
FIR = -A exp(-DIX) The force becomes progressively more long ranged, and the decay length increases (see parameters in Figure 2.) until at 50% water content the force has a range and strength which is comparable to the force measured between hydrophobic surfaces in pure water. In additional experiments the reversibility of the dramatic effects of water was investigated. When the water content was decreased from 50 76, the range of the attractive interaction decreased with roughly the same dependence as the increase in range with increasing water content.
Discussion Ethylene glycol is not too dissimilar to water, but only water can form a tetrahedral hydrogen-bonding network. Both solvents are hydrogen-bonding dipolar fluids. (HzO, 1.333 D; EG, 2.28 D). EG has a higher heat of vaporization (EG, 58.65 kJ mol-'; HzO, 40.7 kJ mol-') and a higher boiling point, but the cohesive energy densities of the two fluids are very similar. It is useful to compare the bulk physical properties because classical theories of surface interactions depend on these quantities. Theoretical descriptions of electrostatic, van der Waals, and dipolar correlation forces distinguish different fluids only on the basis of bulk properties. For instance, if the force was due to an electrostatic correlation mechanism, then one would expect to see such a force in both fluids. The fact that we observe such a dramatic difference between water and EG does not however rule out classical theories by itself. It is possible that the force propagates with a classical mechanism, but its origin must lie in a molecular description of the surface or the fluid. The dramatic change in the decay length of the attractive interaction on addition of water must be associated with either a change in the molecular structure (morphology) of the surface or a change in the property of the liquid which is not so obvious from comparison of the bulk properties. One change in surface morphology which one could envisage is the adsorption of an extremely sparse second layer of surfactant to form a partial bilayer. The presence of such a layer would reduce the hydrophobicity and the range of the interaction. Addition of extra solvent
could cause this layer to desorb and the hydrophobicity of the surface and the range of the surface force to increase. However, there is no experimental evidence for the existence of such a layer. The thickness of the adsorbed layer corresponds to a single monolayer; there is no measurable double-layer repulsion or steric force due to a second adsorbed layer. Furthermore, in such a scenario one would not expect the measured force to be reversible with increasing and decreasing water content. There are a number of other morphological surface changes possible,crystallization and formation of domains, etc., induced by the addition of water, but it is difficult to see why such changes should occur and how they could cause such a dramatic increase in the range of the force without resorting to a nonclassical description for the propagation of the force. If the force is not due to a change in surface structure, then we must consider the properties of the intervening fluid between the surfaces. Cavitation is observed in EG (and in all EGwater mixtures) after the solvophobic surfaces have come into contact. The presence of cavitation indicates that the film between the surfaces is metastable. A t first sight, the presence of cavitation and the absence of a long-range force appear to rule out any explanation of the force based on the film meta~tability.~ However, the viscosity of ethylene glycol is 20 times higher than the viscosity of water, and this will certainly affect the dynamics of cavity formation and the nature and behavior of the metastable film. To conclude, the strong dependence of the range of the attraction on the water content in EGwater mixtures appears to rule out classical theories as the source for the extraordinary behavior of such surfaces in water, and we must look for an explanation on a molecular level. Unfortunately these results cannot distinguish whether surface morphology or some property of the fluid next to a solvophobic interface is responsible for the long-range attractive force in aqueous systems. Registry No. EG,107-21-1; N-[a-(trimethy1a"onio)acetylO,O-bia(LH,LH,W,W-perfluorodecyl)-L-glutamate chloride, 8818538-0.
