Alkane adsorption from vapour onto hydrophobic solid/vapour andhydrophobic solid/water interfaces

Robert Aveyard, Ben D. Beake and John H. Clint

Surfactant Science Group, Chemistry Department, T he University of Hull, Cottingham Road,Hull, UK HU6 7RX

Received 10th February 1999, Accepted 17th March 1999

In connection with our interest in the e†ect of adsorption of alkanes on the wettability by water ofhydrophobic solid surfaces, we have determined adsorption isotherms for a range of alkane vapours ontomethyl-terminated self-assembled alkanethiol monolayers. The adsorption has been measured using a quartzcrystal microbalance. The isotherms are well-Ðtted using the isotherm equation proposed by Aranovich andDonohue. It is found that alkane adsorption is only slightly a†ected by the alkanethiol chain length (between10 and 18). Mixed alkanethiol layers (10] 18) however lead to enhanced alkane adsorption. We havecompared adsorption of alkanes onto the crystalline thiol layers with that observed for liquid-like surfactantmonolayers on water, where mixing of alkane with surfactant alkyl chains is known to occur. Using surfacepressures of ““ saturatedÏÏ alkane layers on the solid surfaces, together with various measured contact angles, wehave obtained surface pressures of alkanes at the hydrophobic solid/water interface. The adsorption of alkanesonto solid surfaces, the solid/water interface and into surfactant monolayers has been treated successfully usingsimple theory of van der Waals forces together with reasonable values of Hamaker constants.

Introduction

Our interest in the adsorption of hydrocarbon vapours ontothe surfaces of hydrophobic solids and of aqueous surfactantsolutions stems from earlier work on the mechanisms bywhich liquid hydrocarbons containing dispersed hydrophobicsolid particles can rupture thin soap Ðlms and hence act asfoam breakers.1,2 We show in Fig. 1 part of an oil dropletunder water with a small (spherical) solid particle bridging theoil/water and air/water interfaces as the oil dropletapproaches the air/water surface from the water phase.Neglecting dynamic e†ects and supposing the entry of the oildrop into the air/water surface in the absence of the solid par-ticle is thermodynamically feasible2,3 it might be expected thatdrop entry could be prevented (i.e. that a thin aqueous Ðlmcould be stabilised) if the sum of the contact angles haw and

of, respectively, the air/water and oil/water interfaces withhowthe solid (Fig. 1) is less than 180 ¡. If (haw ] how) [ 180 ¡however, the two 3-phase contact lines would coincide atsome stage and the oil/water/vapour Ðlm would be unstable.

Fig. 1 Solid particle bridging an oil droplet and an air/water inter-face. If the sum of the contact angles and of the air/water andhaw howoil/water interfaces with the particle is less than 180 ¡ the thin oil/water/vapour Ðlm will be stable (but see text).

We have shown that it is indeed possible for solid particles toprevent entry of oil drops into the surfaces of surfactant solu-tions. The sum of angles required for this however turns outto be considerably less than 180 ¡, probably as a result of thedynamic process of particle de-wetting when the solid surfaceengages that interface as the drop approaches the air/waterinterface.

In a previous study3 we measured rest times (before entryinto the air/water interface) of oil drops containing dispersedhydrophobic spherical solid particles placed under a sur-factant solution surface. The vapour space above the aqueoussolution was kept saturated with the vapour of the hydrocar-bon (heptane). Alkanes are known to adsorb on (i.e. mix with)close-packed surfactant layers at the air/water surface.4,5 Thislowers the surface tension of the solution and consequentlya†ects the contact angle, of the solution with the solid.hawSimilarly, adsorption of hydrocarbon on the solid/vapour andsolid/water interfaces will inÑuence contact angles. In thepresent paper we describe a study of the adsorption ofvapours of homologous alkanes onto self-assembled alkane-thiol monolayers on gold using a quartz crystal microbalance(QCM). From the adsorption so obtained, together with themeasured contact angles of water and liquid alkanes on theself-assembled monolayers (SAMs), it has been possible toestimate the surface pressure of alkanes adsorbed at the solid/water interface in systems containing saturated alkanevapours.

Some recent work on vapour adsorption onto hydrophobicsurfaces6h8 has been motivated in part by the potential forusing SAMs in chemical sensors.5 Matsuura and co-workers6have examined the adsorption kinetics of a variety of organicmolecules (alkanoic acids, pentylamine, aniline, ethylene-diamine, n-octane and ethanol) at very low partial pressuresusing a super-sensitive QCM and determined their selectivityfor di†erent functionalised surfaces. Thomas et al. havereported7 a similar study of the adsorption of small organicmolecules (acetone, i-octane, n-propanol, diisopropylmethyl-

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phosphate, trichloroethylene, toluene and water) onto methyland carboxylate coordinated Cu2`-terminated self-assembledmonolayers. Recently Karpovich and Blanchard8 have pre-sented QCM and ellipsometric data for the adsorption ofcyclohexane, methanol, n-hexane, acetonitrile, acetone andwater onto octadecanethiol and 11-mercapto-1-undecanol sur-faces.

In our work we have employed methyl-terminated mono-layer surfaces di†ering in the length of the alkyl chain in orderto determine if alkane molecules can inter-penetrate the singlecomponent SAMs. In addition we have also examined theadsorption of n-alkanes onto mixed self-assembled mono-layers composed of dodecanethiol and octadecanethiol and wecontrast this behaviour with that observed using the single-component monolayers. We were interested in the possibilityof alkane adsorption within the putatively ““ liquid-like ÏÏ9 outerlayer of this mixed-chain length monolayer which should bereÑected in enhanced adsorption.

To summarise then, we have been concerned with theadsorption of alkane vapours on hydrophobic solid/vapourand hydrophobic solid/water interfaces and the e†ects of theadsorption on the solid wettability by water. We also compareisotherms for adsorption onto solid surfaces with those foradsorption into close-packed surfactant monolayers at the air/water interface.

ExperimentalMaterials

Decanethiol (96%), dodecanethiol (98%) and octadecanethiol(98%) were from Aldrich and used as supplied. Hexa-decanethiol (Avocado, practical grade) was recrystallised fromethanol. n-Hexane, n-octane, n-decane, n-dodecane, n-tetradecane, n-hexadecane and squalane were all high-purityreagents ([99%, Aldrich) and were passed through chromato-graphic alumina before use to remove polar impurities.Ethanol ([99.7%, B.D.H.) was used as supplied. Water puri-Ðed by reverse osmosis was passed through a Milli-Q reagentwater system.

Preparation of hydrophobic surfaces

The QCM electrodes were cleaned by sequential rinsing in hotchloroformÈethanol (50/50 v/v), and cold ethanol, followed bydrying in a stream of high-purity nitrogen before 4 h immer-sion in the 1 mM solution of the alkanethiol in ethanol. Afterthe self-assembly process, the electrodes were thoroughlyrinsed in ethanol and dried in nitrogen before use. We foundthat the coated electrodes could be reused (after rinsing inethanol and drying in a stream of nitrogen) and gave repro-ducible (^2 Hz) frequency changes on alkane adsorption. Anexception was on exposure to hexane vapour at or very nearto saturation pressure, which led to slightly larger frequencychanges in subsequent adsorption runs due to (probable)damage of the monolayer ; these crystals were discarded.

To prepare mixed monolayers with di†ering surface concen-tration of the two components, freshly cleaned quartz crystalswere immersed for 4 h in ethanolic solutions containing mix-tures of octadecanethiol and dodecanethiol over a(C18) (C12)range of compositions. The total concentration of the two alk-anethiol components in solution was maintained at 1 mM.

Gold substrates for wettability measurements were pre-pared by evaporation of gold (ca. 2000 onto a thin adlayerÓ)of freshly evaporated titanium (ca. 100 on clean cutÓ)(2.5] 1.5 cm) glass microscope slides. All glassware wascleaned by sequential rinsing in alcoholic potassium hydrox-ide, Milli-Q water, dilute hydrochloric acid and Milli-Q waterbefore oven-drying. Gold-coated microscope slides wererinsed in ethanol and immersed in 1 mM solution of the alka-nethiol in ethanol for 4 h. After the self-assembly, the slides

were thoroughly rinsed in ethanol and dried in nitrogenbefore use.

QCM measurements

AT-cut 10 MHz polished quartz crystals were obtained fromInternational Crystal Manufacturing (Oklahoma City, USA).The crystals had evaporated gold electrodes (1000 thick)Ówith a chromium underlayer (50 as an adhesion promoter.Ó)A QCM oscillator has been designed and constructed in ourlaboratory. The oscillating frequency of the crystal was moni-tored with an Amplicon GT200 high-resolution frequencycounter. The temperature was controlled by immersion of theadsorption vessel (Fig. 2) in a thermostatting bath at 25.0 ¡C.Required partial vapour pressures of alkanes were obtainedby mixing with squalane, which is e†ectively involatile. Theactivity coefficients in the liquid mixtures, which were neededto calculate the alkane vapour pressures, were obtained fromthe literature.10 Once the adsorption vessel was thermallyequilibrated, (as indicated by a stable QCM baselinefrequency) an alkaneÈsqualane mixture was admitted andrapidly vapourised by causing it to wick round Ðlter paperinside the vessel. The kinetics of the alkane adsorption werethen followed by changes in the QCM frequency with time.The very low vapour pressure of hexadecane led to a rate ofadsorption comparable to the stability of the QCM and soonly data for the lower n-alkanes are reported here.

Ellipsometry

Ellipsometric measurements were carried out with a PlasmosSD 2300 ellipsometer operating at 632.8 nm (HeÈNe laser)with an incidence angle of 60¡. The adsorption cell, which wasdesigned to enable simultaneous ellipsometric and QCM mea-surements of the kinetics of alkane adsorption, was used forthe calibration procedure.

Contact angles

All angles were static advancing angles and are quoted to^2¡. Contact angles of liquid alkanes with the SAM surfaces

were measured with the alkanethiol surface vertical in a(how)cuvette partially Ðlled with alkane (Fig. 3). With the hydro-phobic plate in place, a small additional volume of alkane wasadded to obtain an advancing angle, which was measuredusing a Kruss G1 goniometer 1 min later with the syringe tipstill in the oil. The presence or absence of a nearby cottonwool plug soaked in oil to ensure saturation of the vapourspace had no e†ect on the observed values. Contact angles ofwater with the SAM surface under liquid alkane were(how)obtained either by introducing water drops into an oil layer,

Fig. 2 The QCM cell houses the hydrophobically coated quartzcrystal Q. The side arms S allow introduction of the oil samples and aÐlter paper F, saturated with the oil, around the wall of the cell facili-tates liquid equilibrium with the vapour phase. The leads L are con-nected to the oscillator and frequency counter. The cell is immersed ina water thermostat W.

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Fig. 3 Measurement of contact angles of e.g. an oil/water interfacewith a hydrophobic plate. (a) The plate is held vertically in a cuvetteand the oil/water interface can be made to advance over the solid byinjection of water into the aqueous layer. (b) The plate is horizontal ina dish and covered with oil. A drop of water is placed on the plateunder oil and the 3-phase contact line is made to advance over thesurface by adding more water to the drop.

allowing them to fall under gravity onto the horizontal surface(Fig. 3b), or by addition of a small volume of water to anaqueous layer under oil in a cuvette with a vertical alkanethiolsurface (Fig. 3a). No signiÐcant di†erences were observed inangles obtained using the two methods. Wettability withwater was determined by addition of a small volume of(haw)water to a sessile water drop on a horizontal alkanethiolsurface. Measurements were taken 1 min later with the syringetip still in the drop, following the method used by Drelich andco-workers11 (the drop was large enough for the e†ect of thesyringe to be negligible). We encountered experimental diffi-culties in obtaining reliable advancing contact angles forwater drops in the presence of alkane vapour by this(hwv)method (presumably due to the inability to saturate thevapour space at the three-phase contact region with alkanevapour (from a liquid reservoir). Instead was calculatedhwvfrom the other two measured angles, and usinghov how ,YoungÏs equation (see later). Measurements were taken underambient laboratory conditions (20È22 ¡C). Tensions used inthe calculation of were literature values12,13 relating tohwv20 ¡C.

Surface energies from contact angles

Using the contact angles of water, decane, dodecane, tetra-decane and hexadecane with the octadecanethiol surfaces, thedispersive and polar contributions to the surface energy of thesolid have been determined as mN m~1 andcsd \ 19.6^ 0.1

mN m~1, respectively, using a computer programcsp \ 0.13based on the combining rules of Fowkes,14 which wereapplied by Owens and Wendt to solid surfaces.15 The disper-sion value compares well with that of mNcsd\ 19.3 ^ 0.6m~1 obtained by Bain and Whitesides on SAMS formed fromCH3(CH2)21SH.16

Calibration of the QCM

The QCM frequency change on adsorption is determined bythe mass adsorbed per unit actual area of the electroactivesurface, i.e. the area that would be obtained from standard gasadsorption methods. Ellipsometry provides a measure of

mean thickness of the adsorbed layer. Therefore the tech-niques used together can produce a roughness factor (actualarea/projected or geometrical area) provided the density of theadsorbed layer is known. For the calibration of the QCM fre-quency response we assume that the adsorbed (decane) layerhas liquid density (0.726 g cm~3 at 25 ¡C). As an illustrativeexample, decane adsorption at gives a change in thep/po \ 0.7ellipsometric parameter D of 0.51¡ and a QCM frequencydecrease of 28 Hz (mean values from 6 determinations). Appli-cation of McCrackin et al.Ïs equations17 gives the change in Dto be 0.71¡ nm~1 at this angle using the refractive index ofliquid decane (n \ 1.41)¤ and optical properties of the baregold (complex refractive index, n \ 0.17È3.1i). A calibrationfactor of the change in mass per unit (actual) area with changein frequency, *m/*f \ 18.2 lg m~2 Hz~1 is obtained. Thiscan be compared with that from the Sauerbrey equation18

*fm\ [[2nf 02(kq oq)~1@2]*m

where *m is the mass change of the crystal per unit geometri-cal surface area, is its fundamental resonance frequency inf0vacuum, and g cm~1 s~2 and gkq\ 2.947 ] 1011 oq \ 2.648cm~3 are the shear modulus and the density of the quartz,respectively. The parameter n has a value of 2 when both sidesof the crystal are exposed. Application of the Sauerbrey equa-tion yields *m/*f \ 22.1 lg m~2 Hz~1 for an ideally Ñatsurface. From our calibration the roughness factor, R, of thesurface can be estimated to be 1.22 ^ 0.05. This is in goodaccord with previous estimates of the roughness of gold elec-trodes, R\ 1.22, obtained by electrochemical methods,19 andR\ 1.2, obtained from studies of benzene adsorption,20although it should be noted that values for the roughnessof evaporated gold surfaces between 1.2 and 2 have beenreported.21

Results and discussionAdsorption of alkanes at the solid/vapour interface

A typical QCM frequencyÈtime plot showing the kinetics ofadsorption is shown in Fig. 4. The equilibrium (maximum)frequency changes resulting from alkane adsorption onto theoctadecanethiol thiol) have been converted to adsorbed(C18amounts using the calibration factor, obtained as discussedearlier from the comparison with ellipsometric data. Isothermsfor a range of alkanes adsorbed onto thiol surfaces at 298C18K are shown in Fig. 5 ; we plot adsorptions (C) against pres-

Fig. 4 A relative change of resonance frequency for a quartz crystalcoated with n-octadecanethiol. At time 220 s a decane] squalanemixture was introduced to the adsorption cell to provide a decanerelative pressure p/po \ 0.93.

¤ Although the refractive index of the decane adlayer may di†erslightly from that of the bulk liquid, we have ascertained that smallvariations around this value do not greatly a†ect the calibration.

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Fig. 5 Isotherms for the adsorption of alkanes on thiolated goldC18surfaces plotted with abscissa (a) pressure and (b) relative pressure.

sure (Fig. 5a) and against relative pressure (Fig. 5b). For agiven relative pressure the shorter alkanes adsorb more, on amolar basis, than the longer ones. Increasing the alkane chainlength gives a greater decrease in conformational entropy onadsorption, leading to lower adsorption of longeralkanes.22,23 The chain length dependence at high relativepressure reÑects the wetting behaviour of the alkanes. Hexaneand octane, which give sharp rises in adsorption close to satu-ration, are wetting liquids, whereas the higher alkanes are not.The slope of an isotherm as saturation is approached may beused to show whether partial or complete wetting occurs.22,23The hexane and octane isotherms do not appear to intersectthe p/po \ 1 axis. In contrast, the decane, dodecane and tetra-decane isotherms appear to intersect the saturation axis atÐnite adsorbed amounts and these alkanes can be classiÐed aspartially wetting.

A close-packed alkane monolayer with molecules orientedperpendicular to the surface (with cross-sectional area about0.2 nm2) would have a surface concentration of around 5 mol-ecule nm~2. Our data (Fig. 5) therefore appear to be consis-tent with predominantly perpendicular adsorption at highpartial pressures. The isotherms for hexane and octane onlystart to rise dramatically beyond a surface concentration ofabout 5 molecule nm~2 which implies that condensation onlyoccurs once a close-packed perpendicular monolayer ofalkane molecules is formed, as has been reported for theadsorption of alkanes onto silica.21,23

A useful empirical method of Ðtting multilayer adsorptionisotherms has been proposed by Aranovich andDonohue.24h26 The isotherm equation for a system with aweakly attractive surface (as is the case here) reads

C\ CmA1 [

ppo

B~d(1)

Here is the initial slope of a plot of C vs. and d is aCm p/po ,constant between zero and unity ; is the (hypothetical)Cmsaturation monolayer coverage that would be obtained ifadsorption proceeded in the same way as in dilute monolayersup to We show the adsorption isotherm for hexanep/po \ 1.on a surface in Fig. 6 ; the full line is obtained fromC18-coated

Fig. 6 Isotherm for the adsorption of n-hexane onto a thiolC18surface. Points are experimental and the full line is generated by eqn.(1) with molecule nm~2 and d \ 0.35 (see Table 1).Cm\ 2.83

eqn. (1). Values of and d from the Ðtting of the isothermsCmfor all the alkanes are given in Table 1 and the Ðts to theexperimental data are good in all cases.

The surface pressure of an adsorbed layer is readily(Psv)obtained as a function of C by use of the standard relationship

P \ RTP0

pCd ln(p/po) (2)

The ideal 2-D gas equation, was used to estimateC\ PsvRT ,the surface pressure at the Ðrst experimental data point.Sample plots of surface pressure vs. C are shown in Fig. 7 forhexane and decane. It is possible by extrapolation of plots of

vs. to obtain the surface pressure of an adsorbedPsv ln(p/po)Ðlm at saturation Alternatively, the Aranovich and(Psv)sat .Donohue treatment of adsorption leads to the expression forthe saturation surface pressure

(Psv)sat\ Cm RT /(1 [ d) (3)

We give values of in Table 2, where it is seen that there(Psv)satis good agreement between the values obtained by the 2methods. We note that for spreading alkanes (hexane andoctane), although at unit relative pressure the adsorption canbecome ““ inÐnite ÏÏ there is nonetheless a limiting (maximum)surface pressure. This is because when the alkane Ðlm on thesolid can be regarded as macroscopic, increasing the Ðlm

Table 1 Parameters from the Aranovich and Donohue treatment ofadsorption isotherms for alkanes adsorbed on thiol surfaces atC18298 K

Cm Cm *akoN alkane /lmol m~2 d /molecule nm~2 /kJ mol~1

6 4.7 0.35 2.8 21.638 3.5 0.40 2.1 16.52

10 3.5 0.29 2.1 10.7312 4.0 0.24 2.4 4.5714 3.9 0.17 2.4 [1.49

Fig. 7 Surface pressure of alkanes adsorbed onto thiol surfacesC18at 298 K as a function of surface concentration.

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Table 2 Surface pressures at saturation, for monolayers of(Psv)sat ,alkanes adsorbed on thiol-coated surfaces at 298 KC18(Psv)sat/mN m~1

N alkane From extrapolation From eqn. (3)

6 17.2 17.68 14.5 14.2

10 12.4 12.012 12.8 12.814 11.4 11.4

thickness does not change interfacial tensions.

Nature of self-assembled thin Ðlms in relation to alkaneadsorption

The alkanethiol surfaces were prepared under conditions (air-saturated solution of 1 mM alkanethiol in ethanol for 4 h)reported to produce monolayer coverage.27 Although an insitu QCM study of the self-assembly process has suggestedthat even at this short immersion time multilayers mayform.20 We have found that after rinsing in ethanol a QCMfrequency decrease consistent with the formation of an octade-canethiol monolayer was obtained. Contact angle data (seelater) indicate that the monolayer is free of physisorbedmaterial. As mentioned, the value of the dispersion componentof the surface free energy (19.6^ 0.1 mN m~1) determined bythe method of Fowkes14 and of Owens and Wendt15 usingn-alkanes and water as probe liquids, agrees closely with thatdetermined by Bain and Whitesides studies on

formed using longer adsorption times.16CH3(CH2)21SHThe baseline frequency between successive alkane adsorp-

tion runs was constant (within the stability of the QCM), andthe coated electrodes could be re-used and gave reproduciblefrequency changes (^2 Hz) on alkane adsorption. This behav-iour suggests that the single-component monolayer is e†ec-tively impenetrable by alkanes and adsorption appears to be afully reversible process.

To investigate further whether inter-penetration of thealkane vapour into self-assembled monolayers occurs to anysigniÐcant extent, we have examined the adsorption of decaneonto SAMs di†ering only in the length of their alkyl chain. Itis known that adsorption of alkanes onto monolayers of sur-factants at the air/solution interface (see below) shows amarked dependence on chain length of surfactant4 whichresults from the alkane (such as decane) mixing with the sur-factant chains.28 In the present study however we Ðnd that theisotherms for the adsorption of decane onto decanethiol,dodecanethiol and octadecanethiol surfaces (Fig. 8a) are vir-tually identical, implying that decane adsorbs on top of theSAMs rather than mixing with them. However there is someevidence that smaller molecules (e.g. hexane) can penetrateoctadecanethiol surfaces on gold to some extent.8 Weoccasionally found that allowing hexane to condense on themonolayer (from pure vapour) led to increased adsorbedamounts in subsequent runs. It seems possible that this mayhave led to some damage of the octadecanethiol monolayer.Ong et al. have reported, on the basis of sum-frequencyspectra, that good non-polar solvents such as hexane may beable to penetrate defects in octadecanethiol monolayers.29

The adsorption isotherm for decane on a mixed C12 ] C18thiol layer is depicted in Fig. 8a, where it is seen that adsorp-tion is enhanced relative to that for the single chain lengthSAMs. We have also studied the adsorption of alkanes at arelative pressure of 0.98 onto hydrophobic surfaces composedof mixtures of and thiols, and it appears that there isC12 C18an optimum composition of the monolayers at which alkaneadsorption is maximum (Fig. 8b). We suppose that thisenhancement is a result of the outer region of a mixed mono-

Fig. 8 E†ects of thiol chain length and thiol mixing on the adsorp-tion of alkanes. (a) Isotherms on and thiols and aC10 , C12 C18mixture of with (ratio in preparation solution ofC12 C18 C12 : C184.5). (b) Adsorption of alkanes at a relative pressure of 0.98 on sur-faces of mixtures of and thiols.C12 C18

layer being more liquid-like than the pure monolayers, wherechain length has little e†ect on adsorption.

Comparison with adsorption in surfactant monolayers at theair/water surface

Measurements of adsorption of alkanes into close-packed sur-factant monolayers at the air/water interface have beenreported in the literature.4,5 We show isotherms for theadsorption of octane and dodecane (in Fig. 9a and b,respectively) into close-packed monolayers of surfactants ofthe type with n \ 10 and 12 andC

nH2n`1(OCH2CH2)mOH

(respectively) m\ 7 and 5 (denoted and At lowC10E7 C12E5).relative pressures adsorption of a given alkane is similar forboth the surfactants and close to that for the thiol-coatedC18solid surface.

The limiting slope of an isotherm at low alkane vapourpressure p yields the standard free energy of adsorption, *ako,into an inÐnitely dilute alkane monolayer according to

*ako \ [RT ln(C/p)p?0 \ [RT ln(Cm/po)p?0 (4)

The limiting slopes have been obtained by Ðtting adsorptiondata in the range of relative pressure between zero and 0.5 toquadratic equations (see Table 1). With C expressed in mol-ecule nm~2 and p in Pa, the standard states for the adsorptionprocess are, for the surface 1 molecule nm~2 and for the bulkvapour 1 Pa. Values of for a range of alkanes are plotted*akoagainst alkane chain length N in Fig. 10. The free energies aree†ectively the same for a given N for adsorption into the 2surfactant monolayers, and are more positive than for adsorp-tion onto the thiol SAM. The methylene group incrementC18is kJ mol~1 for adsorption into the surfactant mono-[2.75layers and kJ mol~1 for adsorption onto the thiol[2.91 C18layer. The areas per surfactant molecule in the close-packedsurfactant layers are of the order of 0.5È0.6 nm2 (as a result ofthe large head groups), so the thickness of the chain region,which is liquid in nature, is expected to be only around 0.5 nmcompared with around four times this for the SAMs.

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Fig. 9 Comparison of adsorption of (a) octane (b) dodecane ontothiol layers and nonionic surfactant monolayers at the air/waterC18interface at 298 K.

At high relative pressures alkane adsorption (octane anddodecane) at the solid surfaces is considerably higher thanthat into the surfactant layers (Fig. 9). In the surfactantsystems there is partial mixing of alkane molecules with theliquid-like surfactant chains.28 In the case of the thiolC18layers adsorption at the highest relative pressures appears tobe multilayer adsorption on top of the alkyl chains.

Relationship between surface pressures and contact angles

We have shown how is obtained from adsorption iso-Psvtherms. The surface pressures of alkanes at the solid/water

Fig. 10 Standard free energies of adsorption of alkanes, chain lengthN, onto thiol coated solid surfaces and into close-packed mono-C18layers of the surfactants and at the air/water interface.C10E7 C12E5

Fig. 11 Relevant contact angles in oil (alkane)/water/hydrophobicsolid systems. The arrows denote the direction of action of forces dueto the various interfacial tensions.

interface, cannot be obtained directly, but they can bePsw ,derived from a knowledge of appropriate contact angles (h)and With reference to Fig. 11a YoungÏs equation may bePsv .expressed

csv\ csw] cwv cos hwv (5)

where the subscripts sv, sw and wv refer to the solid/vapour,solid/water and water/vapour interfaces, respectively. If wedenote by the di†erence in a quantity in the presence andDairNabsence of saturated vapour of an alkane, with chain lengthN, we have from eqn. (5)

DairN (csv [ csw) \ (Psv)sat [ (Psw)sat\ DairN (cwv cos hwv) (6)

We have been unable to determine in the presence ofhwvalkane vapour reproducibly, but the quantity can be obtainedfrom a knowledge of and (see Fig. 11b and c) both ofhov howwhich are easily measurable. YoungÏs equations for thesystems depicted in Fig. 11b and c are

csv\ cso ] cov cos hov (7)

cso\ csw] cow cos how (8)

From eqn. (5), (7) and (8) we have for hwvhwv \ cos~1[(cov cos hov ] cow cos how)/cwv] (9)

In Table 3 we give values of and and of andcov cow hov howfor systems with a range of alkanes, and calculated values ofThe presence of alkane causes an increase in of abouthwv . hwv4¡ ; there is only little e†ect of alkane chain length on Thehwv .

changes in contact angle are a consequence of alkane adsorp-tion not only at the sv interface but also at the sw interface.Adsorption at the wv interface is known to be negligible fordecane and higher homologues.

Values of saturated spreading pressures of the alkanes atthe sw interface, calculated using eqn. (6), are given in Table 4.

Table 3 Tensions and contact angles in systems with thiol-coated surfacesaC18N alkane cov/mN m~1 hov/degrees cow/mN m~1 how/degrees hwv/degrees

10 23.76 35.7 52.36 169.8 116.212 25.38 40.5 52.76 163.3 115.414 26.54 43.0 53.32 161.0 115.216 27.44 46.5 53.77 157.9 115.1

in the absence of alkane vapour is 111¡. Measurements were made at room temperature (20È22 ¡C).a hwv

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Table 4 Surface pressures, and in saturated monolayers of partially wetting alkanes at the solid/vapour and solid/water inter-(Psv)sat (Psw)sat ,faces, respectively, in systems with thiolÈcoated surfaces at room temperatureC18(Psv)sat/ (Psv)sat [ (Psw)sat/ (Psw)sat/ (Psw)sat/(Psv)sat (Psw)sat/(Psv)satN alkane mN m~1 mN m~1 mN m~1 Experiment Theory

10 12.2 6.1 6.1 0.50 0.5012 12.8 5.1 7.7 0.60 0.5214 11.4 4.9 6.5 0.57 0.53

It is clear that adsorption at the sw interface is very signiÐ-cant, giving rise to surface pressures of around 6È8 mN m~1.This is at Ðrst sight perhaps surprising since the alkanes arevirtually insoluble in water. It appears that alkane moleculesdi†use along the solid surface under the aqueous phase. Pre-sumably contact angles are determined by interfacial tensionsin the vicinity of the 3-phase contact line, and it may be ofcourse that the alkanes do not cover the whole of the sw inter-face.

Prediction of adsorption in terms of Hamaker constants

The form of the adsorption isotherms for n-alkanes, adsorbedabove monolayer coverage, from vapour onto coated sub-strates can be calculated from macroscopic considerations ofthe dispersion forces involved. For example, for a four-layermodel consisting of vapour (1), adsorbed alkane (2), alkylthiol(3) and gold (4), the disjoining pressure P(z) of the adsorbedÐlm is given30,31 as a function of z, its thickness by the expres-sion

P(z)\A124 [ A1236p(z] d)3

]A1236pz3

(10)

where d is the thickness of the alkylthiol layer. The Hamakerconstant is for the interaction of vapour with gold acrossA124adsorbed alkane, and is for the interaction of vapourA123with alkylthiol across alkane. These Hamaker constants canbe expressed in terms of the Hamaker constants for individualphases by the use of combining rules. Approximate expres-sions are30

A123 \ (JA11[ JA22 )(JA33[ JA22 ) (11)

A124 \ (JA11[ JA22 )(JA44[ JA22 ) (12)

If we assume that the vapour behaves as an ideal gas, thedisjoining pressure can also be expressed32 in terms of therelative pressure :

P(z)\ [(kT /vm)ln(p/po) (13)

where is the molecular volume of the adsorbate. Combin-lming eqn. (10) and (13) we have

A124[ A1236p(z] d)3

]A1236pz3

\ [(kT /vm)ln(p/po) (14)

In this way the relationship between z and (i.e. thep/p0isotherm) is obtained. The surface concentration, C, is calcu-lated from z assuming the adsorbed layer has liquid density.

We show in Fig. 12 Ðts to the higher pressure region of theisotherms for octane and dodecane on thiol surfaces ;C18Hamaker constants used for the individual phases are given inTable 5. It is seen that the experimental data are wellaccounted for by the simple Hamaker approach, the alkanechain length e†ect being well predicted. The di†erence inadsorption of a given alkane on solid-like thiol layers and ona mobile surfactant layer on a water substrate is also(C12E5)adequately explained using the approach (Fig. 13). The e†ec-tive Hamaker constant for the hydrated ethyleneoxy head-group region of the close-packed surfactant layer was selectedas follows. We have found that there is a linear relationshipbetween the dispersion force contribution to solid surface ten-sions and the Hamaker constants for a wide variety of poly-

meric materials. On this basis the Hamaker constant for solidethylene oxide (high molar mass polymer) is approximately8 ] 10~20 J. The Hamaker constant for the hydrated head-group layer has been taken to be 6] 10~20 J, intermediate

Fig. 12 Experimental adsorption isotherms for n-octane (Ðlledcircles) and n-dodecane (open circles) on gold coated with thiol,C18compared with theoretical predictions (upper curve, octane and lowercurve dodecane). Hamaker constants used in the calculations arelisted in Table 5.

Table 5 Hamaker constants used in dispersion force calculations

Material 1020A/J

Water 3.73Octane 4.53Dodecane 5.04C12 alkyl chainsa 5.54C18 thiol chainsb 7.1Gold 40

a Estimated value for ““ liquid ÏÏ alkane (which has the same ratioC24of to groups) by extrapolation of values for shorter n-CH2 CH3alkanes. b Value taken to be equal to that of crystalline paraffin (ref.30).

Fig. 13 Adsorption of dodecane on thiol coated gold (ÐlledC18circles) and on a monolayer of adsorbed at the air/water inter-C12E5face (open circles), compared with theoretical predictions. Hamakerconstants used in the calculations are listed in Table 5.

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between the values for water (3.73] 10~20 J) and pure ethyl-ene oxide.

In the 4-layer Hamaker calculations of the type describedabove, changing phase 1 from vapour to water changes A123and by the same ratio. Since, in eqn. (14) the Ðrst termA124on the left hand side is much smaller than the second term, ata constant pressure, changing from vapour to water willchange z3 in approximately the same ratio. As a result theratio of the saturated surface pressures, should(Psw)sat/(Psv)satbe in the ratio Theo-[A123(1\ water)/A123(1\ vapour)]1@3.retically predicted values of are shown in the(Psw)sat/(Psv)satÐnal column of Table 4, and are in reasonable accord with thevalues obtained from experiment.

Conclusions

A quartz crystal microbalance has been used to measure theadsorption of n-alkanes from vapour, over a range of partialpressures, onto methyl-terminated self-assembled alkanethiolmonolayers. The isotherms are Ðtted well using the isothermequation proposed by Aranovich and Donohue.24h26 Wehave also attempted to Ðt data for the higher relative pres-sures (above monolayer coverage) by considering van derWaals forces using accepted Hamaker constants. Alkaneadsorption is independent of the thiol chain length for singlechain length coatings in the range from to ForC10 C18 .mixed coatings however, alkane adsorption is enhanced, andthere is an optimum proportion of long and short thiol chainswhich gives maximum alkane adsorption at a given relativepressure.

A comparison has been made of alkane adsorption ontosolid surfaces of SAMs and into close-packed nonionic sur-factant monolayers on water. It is known that adsorption insurfactant monolayers on water involves partial mixing withthe surfactant chains. For the systems considered, adsorptionat low relative pressures is similar for the solid/vapour sur-faces and the surfactant monolayers on water ; for a givenalkane the standard free energies of adsorption into inÐnitelydilute monolayers are very close. At high relative pressureshowever adsorption onto the SAMs is much larger than thatinto surfactant monolayers. This di†erence is predicted by thesimple Hamaker calculations.

From a knowledge of saturated surface pressures at svinterfaces and various measured contact angles we have beenable to obtain values for the surface pressures of alkanes atthe sw interface. These turn out, perhaps surprisingly at Ðrstsight, to be as large as between 50 and 60% of those for the svinterfaces. Again however, the Ðndings are consistent withwhat is predicted from the operation of van der Waals forces.

Acknowledgement

authors gratefully acknowledge the provision by theTheEPSRC of a ROPA grant to carry out this work.

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