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of this type of bond, a decrease in the wavenumber of the S-H stretching vibration, an increase of the half-width of the band, and an increase of the integral intensity were observed. However, the shift of the band to lower wavenumbers (AvSH20-30 cm-I) relative to the band of the S-H stretching vibration of the mercaptan molecule adsorbed on SiOH groups suggests a very weak interaction through the S-H group.
Conclusions Ethanethiol and butanethiol act as electron pair donor molecules upon adsorption on Si02, H-ZSMS, and Na-ZSMS. Judged from the perturbation caused by the adsorption, the thiol molecules act as considerably weaker EPD than alcohols, but, except for part of the butanethiol molecules, they did not act as Bronsted acid donating protons to the surface. In contrast to alcohols, the larger sulfur seems to stabilize noncyclic hydrogen bonding at hydroxyl groups. The form of adsorption of the thiols on alkali-metal cation
exchanged ZSMS (Le., via the lone electron pair of the sulfur atom) was similar to that previously reported for alcohols. On H-ZSMS three forms of adsorption were determined: (i) linear (asymmetric) hydrogen bonding to the SiOHAl site, (ii) cyclic hydrogen bonding to the SiOHAl site, and (iii) protonization at the SiOHAl site. At the higher equilibrium pressure (1 mbar) clusters of thiol molecules were observed which were held together by weak hydrogen bonding. The unusual strong hydrogen bonding in these clusters, as seen with protonated water14J5and alcohol,*6 was not observed. Acknowledgment. This work was supparted by the Fonds zur Forderung der Wissenschaftlichen Forschung under project P 73 12 CHE. C.L.G. acknowledges partial support from CONICET (Argentina). We are grateful to Mobil Oil Corp. for supplying us with the H-ZSMS sample. Registry No. HSEt, 75-08-1; HSBu, 109-79-5.
Molecular Mechanisms Associated with Adhesion and Contact Angle Hysteresis of Monolayer Surfaces Y. L. Chen, C. A. Helm,+ and J. N. Israelachvili* Department of Chemical and Nuclear Engineering, and Materials Department, University of California, Santa Barbara, California 93106 (Received: April 5, 1991)
Experiments were carried out on a variety of surfactant-coated mica surfaces using the surface forces apparatus technique and contact angle measurements. The experiments were designed to clarify the molecular mechanisms underlying adhesion hysteresis (during loading-unloading cycles) and contact angle hysteresis (of advancing/receding liquids), and to explore any possible relationship between these two energy-dissipating phenomena. We found that hysteresis effects are not simply due to surface imperfections,such as roughness or chemical heterogeneity. Even surfaces that are initially smooth and chemically homogeneous can exhibit large adhesion and contact angle hysteresis effects. Our results indicate that, for such surfaces, hysteresis arises because of molecular rearrangements occurring at solidsolid or solid-liquid interfaces afer they have come into contact. This results in a lower surface free energy during the approach of two surfaces (or during spreading) than during separation (or retraction). We have studied a number of factors that enhance hysteresis: (i) increasing the freedom of the surface molecules to reorder, (ii) increasing the load and time surfaces are allowed to remain in contact, and (iii) increasing the rate of separation (or retraction). These findings highlight the inherent nonequilibrium nature of most loading-unloading and wetting-dewetting cycles and suggest ways for reducing the energy-dissipating hysteresis associated with such processes. Our results further indicate that the adhesion or pull-off force F between two curved surfaces of radius R is related to the surface energy y by the Johnson-Kendall-Roberts theory, for example, F = 3uRy for a sphere on a flat surface, but only when the separation occurs under equilibrium conditions. Preliminary results also indicate a correlation between adhesion hysteresis and friction/stiction.
Introduction Adhesion and Wetting (Contact Angle) Hysteresis. Most real processes involving adhesion and wetting are hysteretic or energy-dissipating even though they are usually described in terms of (ideally) reversible thermodynamic functions such adhesion free energy, reversible work of adhesion, surface tension, interfacial tension, etc. [Throughout the text the following convention is adopted:” the surface energy per unit area of the solid-vapor or liquid-vapor interface is denoted by ys or yL,which by definition is half the adhesion energy or cohesion energy, W, this beiig the (positive) work done on separating two unit areas of the surfaces from contact in vapor. For two different surfaces in contact, such as a solid-liquid or a solid l s o l i d 2 interface, the interfacial energy is defined by y I 2= y I + y2- W, where the adhesion energy between the two surfaces is related to the surface energies of the two media by the approximate relation: W = In cases where subscripts A and R are used, e.g., WA, these refer to advancing (loading) and receding (unloading) values, respectively.] For example, the energy change, or work done, on
’ Permanent address: Institut fur Physikalische Chemie, Johannes Gutenberg Universitat, Jakob-Welder-Weg 1 I , D-6500,Mainz, Germany. 0022-3654/91/2095-lO736$02.50/0
separating two surfaces from adhesive contact is generally not fully recoverable by bringing the two surfaces back into contact again. This may be referred to as adhesion hysteresis and expressed as wR
receding (separating)
>
wA
advancing
(approaching)
or A W = (WR- WA) > 0
(1)
where WRand WAare the adhesion energies for receding (separating) and advancing (approaching) two solid surfaces, respectively. Adhesion hysteresis is responsible for such phenomena as “rolling” friction and “elastoplastic” adhesive contacts2 during loading-unloading and adhesion-decohesion cycles. Hysteresis effects are also commonly observed in wetting/ dewetting phenomena (Figure l).3 For example, when a liquid (1) Bowden, F. P.; Tabor, D. Friction and Lubricution; Methuen: London, 1967. (2) Greenwood, J. A.; Johnson, K. L. Philos. Mug. A 1981, 43, 697. Michel, F.; Shanahan, M. E. R. C. R.Acad. Sci. Paris 1990, 310 (11). 17. Maugis, D. J . Mufer. Sci. 1985, 20, 3041.
0 1991 American Chemical Society
Adhesion on Monolayer Surfaces
The Journal of Physical Chemistry, Vol. 95, No. 26, 1991 10737
Hacroscopic jumps
Holecular jumps (peeling)
Approach
Figure 1. Examples of wetting and contact angle hysteresis. (A) Solid surface in equilibrium with the vapor phase. On wetting the surface, a contact angle e, is observed, on dewetting it decreases to eR. (B) A liquid droplet resting on a flat solid surface cannot be described as a true equilibrium situation: at the three-phase contact boundary the vertical component of the liquid stress, yL sin 8, is balanced by high local stresses on the solid which induce elastic or plastic deformations (inset) and/or chemical rearrangements to relax these stresses. (C) and (D) Contact angle hysteresis is usually explained by the inherent roughness (left side) and/or chemical heterogeneity (right side) of surfaces. (E) Interdiffusion and interdigitation at an interface may induce roughness and chemical heterogeneity even though initially both surfaces are perfectly smooth and homogeneous. Such surface restructuring effects can occur both on the macroscopic, microscopic, and molecular scales. Here we will mainly be concerned with effects occurring at the molecular level.
spreads and then retracts from a surface the advancing contact angle 8 A is generally larger than the receding angle 8 R (Figure 1A). Since the contact angle, 8, is related to the liquid-vapor surface tension, yL,and the solid-liquid adhesion energy, W,by the DuprE equation (Figure 1B) (1 + cos =w (2) we may conclude that wetting hysteresis or contact angle hysteresis (8, > 8,) implies that either yL:A > y L , R or that W R > W A . Since there is no reason or any previous evidence for a liquid surface having different surface tensions for increasing and decreasing areas (so long as the liquid is pure), we may conclude that wetting or contact angle hysteresis actually implies adhesion hysteresis, viz. eq 1. In all the above cases at least one of the surfaces is always a solid. In the case of solidsolid contacts, the hysteresis has generally been attributed to viscoelastic bulk deformations of the contacting materials or to plastic deformations of locally contacting asperities.’I2 In the case of solid-liquid contacts, hysteresis has usually been attributed to surface roughness or to chemical heterogeneity (Figure 1C,D),3though there have been reports of significant hysteresis on molecularly smooth chemically homogeneous surface^.^ This paper focuses on two other, less studied but possibly more important, mechanisms that can give rise to hysteresis. These may be conveniently referred to as (i) mechanical hysteresis, arising from intrinsic mechanical irreversibility of many adhesion/decohesion processes (Figure lB, inset; and Figure 2), and (ii) chemical hysteresis, arising from the intrinsic chemical irreversibility at the surfaces associated with rearrangements of (3) Miller, C. A.; Neogi, P. Interfacial Phenomena; Marcel Dekker: Basel, 1985. (4) Schwartz, A. M. J. Colloid Interface Sci. 1980, 75, 404.
I
F i m e 2. Origin of mechanical adhesion hysteresis during 1 le approach
and separation of two solid surfaces. Top: In all realistic situations the force between two solid surfaces is never measured at the surfaces themselves, at S, but at some other point, say S’, to which the force is elastically transmitted via the backing material supporting the surfaces. Center (left): Macroscopic “magnet” analogy of two approaching surfaces, where the lower is fixed and where the other is supported at the end of a spring of stiffness Ks. Bottom: Force-distance curve for two attracting surfaces (or magnets), showing the path taken by the upper surface on approach and separation. On approach, an instability occurs at D = DA,where the surfaces spontaneously jump into “contact” at D = D,,. On separation, another instability occurs where the surfacesjump apart from -Do to DR. Center (right): On a molecular level, the separation of two surfaces is accompanied by the spontaneous popping of molecular bonds, which is analogous to the jump apart of two macroscopic surfaces.
chemical groups occurring during the necessarily finite time it takes to go through any adhesion/decohesion or wetting/dewetting process (Figure 1E). Henceforth we shall use the term approach-separation to refer quite generally to any cyclic process, such as adhesion-decohesion, loading-unloading, advancing-receding, and wetting-dewetting cycles. As will be argued below, because of natural constraints offinite time and the finite elasticity (or compliance) of materials most approachseparation cycles are thermodynamically irreversible, and therefore energy dissipating. By thermodynamic irreversibility we simply mean that over any reasonable time period one cannot go through the approachseparation cycle via a continuous series of equilibrium states because some of these are connected via spontaneous-and therefore thermodynamically irreversibletransitions. During such transitions there is an absence of mechanical and/or chemical equilibrium. In many cases the two will be intimately related and occur at the same time (and there may also be an absence of thermal equilibrium), but the above distinction is nevertheless a useful one since there appears to be two fairly distinct molecular processes that give rise to them. These will now be considered in turn. Mechanical Hysteresis. Consider two solid surfaces a distance D apart (Figure 2) interacting with each other via an attractive potential with a hard-wall repulsion at some cutoff separation, Do. Let the materials supporting the surfaces have a bulk elastic modulus K (in units of N m-2) so that depending on the system geometry the surfaces may be considered to be supported by a simple spring of effective “spring constant” Ks (in units of N m-1).5 (5) Pethica, J. B.; Sutton, A. B. J. Vac. Sci. Technol. A 1980,6 (4), 2490. Landman, U.; Luedtke, W. D.; Burnham, N. A.; Colton, R. J. Science 1990, 248,454.
10738 The Journal of Physical Chemistry, Vol. 95, No. 26, 1991
When the surfaces are brought toward each other, a mechanical instability occurs at some finite separation, D,, from which the two surfaces will jump into contact. This instability occurs when the gradient of the attractive force, dF/dD(also in units of N m-I), exceeds KS.’ Likewise, on separation from adhesive contact, there will be a spontaneous jump apart from Do to DR (Figure 2). Separation jumps are generally greater than approaching jumps. Such spontaneous jumps occur at both the macroscopic and atomic levels. For example, they occur when two macroscopic (I? = 1 cm) surfaces are brought together in surface forces experiments;6 they occur when STM or AFM tips ( R = 10 nm) and they occur when individual bonds approach a flat are broken (or ”popped”) during fracture and crack propagation in solids.* But such mechanical instabilities will not occur if the attractive forces are weak or if the backing material supporting the surfaces is very rigid (high K ) . However, in many practical cases these conditions are not met and the adhesion-decohesion cycle is inherently hysteretic regardless of how smooth the surfaces or how perfectly elastic the materials. Due to the spontaneous jumps in and out of contact, the process cannot be reversible in a thermodynamic sense, and the adhesion (free) energy needed to separate the surfaces from contact must be greater than that on approach. It is important to note that this irreversibility does not imply that the surfaces become damaged or changed in any way, or that the molecular configuration is different at the end from what it is at the beginning of the cycle. Energy is always dissipated in the form of heat when two surfaces or molecules impact each other. Before continuing with other types of hysteresis, it is worth mentioning what the true equilibrium situation is in the presence of mechanical instabilities. Referring to Figure 2, if the two surfaces are brought to some arbitrary separation, D’, between D, and DR, and left to equilibrate at a finite temperature for an infinite length of time, the equilibrium separation will actually be a bimodal Boltzmann distribution of distances, peaking at D = D’(the separated state) and D = Do (the contact state). Every now and then a spontaneous thermally induced fluctuation will occur, taking the surfaces from the separated to the contacting state, or vice versa. Over a long enough period of time the surfaces will have moved back and forth, sampling the whole of “ergodic” space. The resulting distribution defines the true thermodynamic equilibrium “state” of the system. It is also worth noting that (i) there is no one unique thermodynamically equilibrium separation, and (ii) if D’lies between D, and DR,the time-averaged distribution will center around two discrete separations D’and Do which is entirely analogous to a two-phase (e.g., solid-gas) system even though only two surfaces, or particles, are involved. Chemical Hysteresis. When two surfaces come into contact, the molecules at the interfaces relax and/or rearrange to a new equilibrium configuration that is different from that when the surfaces were isolated (Figure 1E). These rearrangements may involve simple positional and orientational changes of the surface molecules, as occurs when the molecules of two homopolymer surfaces slowly intermix by interdiffusion9 and reptation.I0 In more complex situations, new molecular groups that were previously buried below the surfaces may appear and intermix at the interface. This commonly occurs with surfaces whose molecules have both polar and nonpolar groups, for example, copolymer surfaces,” surfactant s ~ r f a c e s , ~and * ~ ’protein ~ surfaces.14 All (6) Horn, R.; Israelachvili, J. N. Chem. Phys. Letf. 1980, 71, 192. (7) Weisenhorn, A. L.; Hansma, P. K.; Albrecht, T. R.; Quate, C. F. Appl. Phys. Lett. 1989, 54, 26. Hansma, P. K.; Elings, V . B.; Marti, 0.;Bracker, C. E. Science 1988, 243, 1586. (8) Lawn, B. R.; Wilshaw, T. R. Fracture of Brittle Solids; Cambridge University Press: London, 1975. Sahimi, M.; Goddard, J . G. Phys. Reo. B 1986, 33, 7848. (9) E M , M. D.; Gent, A. N. J . Polym. Sci.: Polym. Phys. 1984.22, 1953; 1985, 23, 1823. Shanahan, M. E. R.; Schreck, P.;Schultz, J. C.R . Acad. Sei. Paris 1988, 306 (II), 1325. Okawa, A. BSc. Thesis, Department of Materials Science, University of Utah, June 1983. Holly, F. J.; Refojo, M. J. J . Biomed. Mater. Res. 1975, 9, 3 15. (IO) Klein, J. J . Chem. Soc., Faraday Trans. I 1983, 7 9 , 99; Makromol. Chem. Macromol. Svmp. 1986, I , 125.
Chen et al. these effects act to enhance the adhesion or cohesion of the contacting surfaces. What distinguishes chemical hysteresis from mechanical hysteresis is that during chemical hysteresis the chemical groups at the surfaces are different on separation from on approach. However, as with mechanical hysteresis, if the cycle were to be carried out infinitely slowly it should be reversible. Previous Work. A number of have studied how molecular reorientations at surfaces correlate with the contact angle hysteresis of water on a polymer surface and showed that the increased watersurface adhesion energy on receding is related to the exposure of polar groups following the initial spreading (advance) of the liquid. Qualitatively similar findings were reported by Chen and co-workers13 on the adhesion of two surfactant-coated mica surfaces. The surfaces were initially hydrophobic on approach (high 6) but had become significantly more hydrophilic on separation (low 0) due to the reorientation of the surfactant molecules (flip-flop). As might be expected, these effects depended on the contact time and the presence of water (relative humidity) as indeed are most adhesion processes. Present Approach. While interdigitation effects appear to be well-known in polymer and protein science, to our knowledge the adhesion has not been studied systematically as a function of contact time, contact pressure, separation speed, polymer type, temperature, etc. [In this paper terms such as “interdigitation”, “interdiffusion”, “interpenetration”, etc., are used somewhat loosely to cover any molecular restructuring occurring at or across an interface (see, for example, Figure 1E) but without reference to any one particular mechanism. This may involve one or more of the following (i) diffusive interpenetration of chain segments via reptation or Rouse motion, (ii) reorientations of bipolar molecular groups at surfaces without involving any translational displacement, and (iii) exchange of different molecular species with others that were previously buried in the bulk.] In our studies we have chosen to study these effects using molecularly smooth mica surfaces onto which well-characterized surfactant monolayers were adsorbed, either by self-assembly or by the Langmuir-Blodgett technique. Different types of surfactants and deposition techniques were used to provide monolayers with a wide variety of properties such as surface coverage, surface roughness, phase state, etc. The surface forces apparatus technique was used for measuring adhesion forces, adhesion energies (by two independent methods), and the geometry of adhering surfaces. We also investigated factors which favor hysteresis. These include the length of time two surfaces were in contact before they were separated, the temperature and load, the rates of loading and unloading, the surface coverage of the deposited monolayers (or chains), the effects of different headgroups, etc. Quantitative studies of all these effects provided a fairly consistent picture of the mechanisms underlying adhesion hysteresis. Complementary contact angle measurements were conducted to correlate the adhesion data with 6. Organic molecules are known to have a high affinity for hydrocarbon surfaces (with small contact angles, 6). Low molecular weight hydrocarbons (below dodecane) are also known to penetrate into the hydrocarbon regions of surfactant monolayers and lipid bilayers,I5 where they alter the packing of the molecules and the structure of the chains. It was therefore interesting to see how such penetration might affect the adhesion properties of monolayers exposed to organic vapors. If the separation processes were all occurring at thermodynamic equilibrium, then the values expected for the surface energies y should be the same on approach and separation, and correspond ( 1 1) Yasuda, H.; Sharma, A. K.; Yasuda, T. J . Polym. Sci., Polym. Phys. 1981, 19, 1285. (12) Langmuir, I. Science 1938,87,493. Wasserman, S . R.; Tao, Y.-T.; Whitesides, G . M.h n g m u i r 1985, 5 , 1074. (13) Chen, Y . L. E.; Gee, M. L.; Helm, C. A,; Israelachvili, J. N.; McGuiggan, P. M. J . Phys. Chem. 1989, 93, 7057. (14) Andrade, J . D.; Smith, L. M.; Gregonis, D. E. In Surface and Interfacial Aspects of Biomedical Polymers; Andrade, J. D., Ed.: Plenum: New York, 1985; Vol. 1, p 249. (15) Gruen, D. W. R.; Haydon, D. A. Biophys. J . 1981, 33, 167.
The Journal of Physical Chemistry, Vol. 95, No. 26, 1991 10739
Adhesion on Monolayer Surfaces to the well-known literature values for hydrocarbon surfaces, obtained from surface tension, critical wetting and contact angle measurements.16 These vary from about y = 23 mJ/m2 for surfaces composed mainly of CH3 groups, to about y = 31 mJ/m2 for surfaces composed mainly of CH2groups. Note, however, that the phase state of hydrocarbon chains has little effect on y, as can be ascertained from the similar values for liquid hexadecane (27 mJ/m2) and solid paraffin wax (25-30 mJ/m2) at the same temperature of 25 O C . 1 6 Thus, we would expect the equilibrium values of y to fall within the range 23-31 mJ/m2.
Theoretical Background Here we briefly describe the two standard continuum theories underlying wetting and adhesion, starting with the former. The theoretical background for the contact angles of a liquid on a solid surface (Figure 1B) is based on a straightforward balance of forces or minimization of surface energies” and leads to the Dupre equation, eq 2, or the Young-Dupre equation: 7s = YSL + Y L cos @
(3)
Note, however, that the normal component of the tension, yL sin 0 in Figure l B , must be balanced by high local stresses on the solid at the three-phase boundary, a matter that is not straightforward to resolve within a continuum framework. Turning now to adhesion, modern theories of the adhesion mechanics of two contacting solid surfaces are based on the theory of Johnson, Kendall, and Roberts (known as the JKR theory).’*Jg In the JKR theory two spheres of radii R I and R2,bulk elastic moduli K,and surface energy y per unit area will flatten when in contact. The contact area will increase under an external load or force, F, such that at mechanical equilibrium the contact radius a is given by a3 = 
