J. Phys. Chem. 1992,96,67254728
6725
Hydrophobic Attraction: A Reexamination of Electrolyte Effects Hugo K. Christenson,* Departments of Food Technology and Physical Chemistry I , Chemical Centre, Lund University, S-221 00 Lund, Sweden
Per M. Claesson, Institute for Surface Chemistry, Box 5607, S-I1486 Stockholm, and Department of Physical Chemistry, Royal Institute of Technology. S-IO044 Stockholm, Sweden
and John L. Parker Department of Applied Mathematics, Research School of Physical Sciences, Australian National University, Canberra, A.C.T. 2601, Australia (Received: December 17, 1991)
We present a reanalysis of measurements of the effect of monovalent electrolyte on the attraction between macroscopic hydrophobic surfaces, both published (Claesson, et al. J. Colloid Interface Sci. 1986,114,234.Christenson, et al. J . Phys. Chem. 1989,93,1472) and unpublished data. We suggest that it is possible to reconcile the influence of monovalent electrolyte with that of divalent electrolyte (Christenson, et al. J . Phys. Chem. 1990,94,8004), and a unified picture of the effects of moderate ( 4 0 - 2 M) amounts of electrolyte on the long-range (D 1 15-20 nm) part of the hydrophobic attraction emerges. With increasing electrolyte concentration the preexponential of the attraction decreases, but the decay length remains unaffected. We argue that the decrease in the strength of the attraction is most likely due to an increase in surface charge rather than the presence of electrolyte in the interlayer of water between the surfaces. We emphasize that the short-range (D I20 nm) interaction is not affected by electrolyte to the same extent as the long-range part of the attraction.
Introduction The very long-range attraction that acts between macroscopic hydrophobic surfaces across aqueous solutions is currently a subject of considerable interest, both experimental and theoretical. Various laboratories around the world have reported an attractive force F i n pure water that appears to obey an exponential relationship at separations beyond 15-20 nm:I4
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F / R = A exp(-D/Do) (1) where R is the mean radius of curvature of the surfaces (R 2 cm). The value of the preexponential factor A has in most cases been found to be between -2 and -3 mN/m and the decay length Do between 12 and 16 nm. Such a hydrophobic attraction has been measured between hydrophobic surfaces prepared by a number of different methods: deposition of Langmuir-Blodgett (LB) films of fluorocarbon or hydrocarbon surfactants on mica,'9Z4 silica hydrophobed by methylati~n,~ and mica rendered hydrophobic by water plasma activation followed by silylati~n.~ In a few cases forces of similar decay and magnitude have been measured between surfaces prepared by in situ adsorption from solution of surfactants to mica.4 Despite several recent attempts7+ to provide a theoretical model or explanation for the hydrophobic attraction, the origin of the interaction remains unknown.I0 The above measurements of long-range hydrophobic forces appear to be qualifativelydifferent from earlier observations of very strongly attractive forces of much shorter range (115 nm) and decay length (1-1.5 nm) between surfactant monolayers adsorbed to mica surfaces from s ~ l u t i o n . ~ In * - the ~ ~ more recent measurements of the long-range hydrophobic attraction, it has not been possible to obtain reliable data on the attraction at separations below 15 nm although the force does change its distance dependence and becomes more strongly attractive at smaller separations. Some results can be approximately fitted to an exponential with a decay length of 2-3 nm at separations below 15-20 nm.2 The very strongly attractive forces found at these separations and resultant substantial surface deformations make accurate measurements in this regime difficult. 'To whom correspondence should be addressed at the Department of Applied Mathematics, Research School of Physical Sciences, Australian National University, Canberra, A C T . 2601, Australia.
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Considerable confusion appears to have been generated in the scientific community by the publication of the many results on the hydrophobic attraction. The precise decay length and magnitude of the attraction appear to be very sensitive to the exact surface preparation procedure (e.g., surface pressure during LB d e p o ~ i t i o n , ~plasma * ~ J ~ power during activation of mica for subsequent silylation,Is etc.). In particular, it appears that conventional measures of surface hydrophobicity such as the advancing contact angle of water on the surface cannot on their own yield reliable predictions of the range of the hydrophobic attraction.2J6 Many measurements of the force between hydrophobic surfaces have been carried out with charged surfaces.4~5~11~1~14~16 Since it is only the total force that is measured with the surface force technique, the repulsive double-layer force present in such cases has to be subtracted from the measured force. This procedure leads to considerable uncertainty, particularly in cases where the hydrophobic attraction is very long-range. For the same reason, several studies concerned with the effect of dissolved electrolyte on the measured interaction have resulted in a number of o b servations that appear to be contradictory. In a study of fluorocarbon surfaces it was found that addition of monovalent ions (tetrapentylammonium bromide) resulted in a sizable doublelayer repulsion between the initially neutral ~ u r f a c e s .The ~ measured force could be fitted in the vicinity of the force maximum to the sum of a double-layer repulsion under constant surface charge conditions and a hydrophobic attraction whose decay length decreased with electrolyte concentration but whose strength increased. It was, however, clear that certain features of the measured forces could not be explained by this simple model, in particular the existence of a secondary minimumat concentrations of IC2M. By contrast, an earlier study showed only a minor effect M KBr on the attraction between hydrocarbon surface^,'^ at although the exponential decay length inferred from these measurements was only of the order of 5 nm. Later measurements with divalent ions showed that the decay length of the hydrophobic attraction was largely independent of electrolyte concentration but that the preexponential decreased with increasing ionic strength.I0 This conclusion did not suffer from the uncertainty of having to subtract a double-layer repulsion from the measured interaction to yield the hydrophobic attraction. The short Debye length (
