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Langmuir 1992,8, 2080
Comment on "Interactions between Hydrophobic Surfaces. Dependence on Temperature and Alkyl Chain Length" The long-range attraction measured between hydrophobic surfaces in water and aqueous solutions1+ is currently generating much interest, particularly because there is as yet no convincing explanation of its cause. A recent article in Langmuir reports on measurements of the hydrophobic attraction at three different temperatures in the range 25-50 "Ce7While these results form an important and interesting contribution to our knowledge about the hydrophobic attraction, we cannot agree with the authors' conclusions. Experimentally, it has been found that the force F (normalizedby the radius of curvature R of the surfaces) between hydrophobic surfaces immersed in pure water is given by FIR = A exp(-DID,) (1) for surface separations D greater than 15-20 nm. Here A has typical values of -2 to -3 mN/m and the decay length DOis about 12-16 nm, for surfaces prepared by LangmuirBlodgett deposition of hydrocarbon and fluorocarbonsurfactants on mica.lI2 A similar decay length is found with surfaces prepared by silylation of silica3 or of plasmatreated mica surface^.^ In general, the values of A and DO vary somewhat depending on the type of hydrophobic surfaceused, but the values are quite reproduciblebetween different experiments as long as the surface preparation technique is the same (cf. the force curves measured between surfaces prepared by LB deposition of dimethyldioctadecylammonium ions in refs 2 and 6). Information on the temperature dependence of the hydrophobic attraction is of great importance as it might shed much needed light on the mechanism behind the phenomenon. It should in principle be possible to gauge the relative importance of enthalpic and entropic contributions to the interaction if the force as a function of temperature could be measured sufficiently accurately. Suchan analysishas been used in the case of the interaction between adsorbed layers of poly(ethy1ene oxide) surfactants across water.8 Tsao et d7have prepared their hydrophobic surfaces by a new method involving adsorption of double-chain cationic surfactants from cyclohexane onto mica. Three different types of surfaces were produced, consisting of hydrocarbonchainsof 16-,la-,and 20-atomlengths. These surfaces are referred to by their abbreviations DHDA (dihexadecyldimethylammonium), DODA (dioctadecyldimethylammonium),and DEDA (dieicosyldimethylammonium). Measurements were made at 25, 40, and 50 "C. In contrast to what the authors state in the abstract and conclusion sections of their paper, we believe that their results ( i ) do not rule out D relationship between the hydrophobic attraction and cavitation and ( i i )show that any temperature dependence of the hydrophobic attraction in the range 25-50" C cannot at present be t Permanent address: Institute of Physical Chemistry, Academy of Sciences, Moscow, Russia. (1) Christenson, H. K.; Claesson, P. M. Science 1988,239,390. (2) Claesson, P. M.; Christenson, H. K. J. Phys. Chem. 1988,92,1650. (3) Rabinovich, Ya. I.; Derjaguin, B. V. Colloids Surf. 1988,30, 243. (4) Parker, J. L.; Cho, D. L.; Claesson, P. M. J.Phys. Chem. 1989,93,
6121. (5) Christenson, H. K.;Claesson, P. M.; Berg, J.; Herder, P. C. J.Phys. Chem. 1989,93, 1472. (6) Christenson, H. K.; Fang, J.; Ninham, B. W.; Parker, J. L. J. Phys. Chen. 1990,94,8004. (7) Tsao, Y.; Yang, S. X.; Evans, D. F.; WennerstrBm, H. Langmuir 1991, 7, 3154.
(8)Claesson, P. M.; Kjellander, R.; Stenius, P.; Christenson, H. K. J. Chem. SOC.,Fraraday Trans 1 1986,82, 2735.
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resolved experimentally. Furthermore, (iii) statements by the authors about the relationship between the crystallinity of hydrocarbon chains and the strength of the hydrophobic attractionare in clear disagreement with several studies in the literature. Cavitation has been inferred or directly observed between hydrophobic surfaces in contact or directly after separation from c o n t a ~ t . l ~This ~ J ~type of cavity formation is not a boiling point depression. A necessary condition for cavitation to occur between hydrophobic surfaces in contact is that the surface energy of the hydrophobic surface against vapor is lower than against water; i.e. the contact angle is greater than 90'. (A complete analysis of the thermodynamics of cavitation between hydrophobic surfaces is given by Yushchenko et al.") In the absence of information on the contact angle and ita temperature dependence, no statement can be made about cavitation and its possiblerelationship to the hydrophobicattraction. We emphasize that refs 1and 2 did not suggest that the formation of cavities plays a role in generating the hydrophobic attraction, merely that cavitation and the attraction may have a common origin in the metastability of thin water films between hydrophobic surfaces. Comparison of the measurements with the DEDAcoated surfaces at 25 and 50 "C (Figures 4 and 6 of ref 7) showsthat there is, within error, no change in the measured force at long range (for separations beyond 200A)for these surfaces. In other words,the hydrophobic attraction, when measured between stable surfaces,has no observable temperature dependence. This contradicts the second and third conclusions of Tsao et al. In their discussion about the importance of the crystallinity of chains for the existence of a long-range attraction, the authors have not considered all those systems where a long-range hydrophobic attraction has been measured without any crystalline order of hydrocarbon chains. These include a number of chemically modified surfaces where decay lengths of up to 12 nm have been found,3s4 as well as surfaces with very short chain^.^^^ The longer the hydrocarbon chains that make up the surface, the more hydrophobic it is likely to be and the less it will be affected by temperature through adsorption or desorption. This does not necessarily mean that the crystallinity of chains directly influences the strength of the attraction. From the data presented by Tsao et al. we conclude that the hydrophobic attraction is not strongly temperature dependent and that the present experimental resolution does not permit more quantitative conclusions to be drawn. The results do not rule out a possible relationship between the hydrophobic attraction and cavitation. When viewed alongside other resulta from the literature, they do not suggest any direct relationship between the crystallinity of hydrocarbon chains and the strength of the attraction. Hugo K. Christenson,' John L. Parker, and Vasily V. Yaminskyt Department of Applied Mathematics, Research School of Physical Sciences, Australian National University, Canberra ACT 2601, Australia Received March 10,1992. In Final Form: May 7, 1992 (9) Pashley, R. M.; McGuiggan, P. M.; Ninham, B. W.; Evans,D. F. Science 1985,229, 1088. (10)Rabinovich,Y a I.; Derjaguin,B.V.; ChuraevAdu. ColloidInterface Sci. 1982, 16, 63. (11) Yushchenko, V. S.; Yaminsky, V. V.; Shchukin,E. D. J . Colloid Interface Sci. 1983, 96, 307.
0 1992 American Chemical Society
