organic chemistry in two dimensions : su rface-fu ... · organic chemistry in two dimensions : su...
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ORGANIC CHEMISTRY IN TWODIMENSIONS : SU RFACE-FU NCTIONALIZEDPOLYMERS AND SELF.ASSEMBLEDMONOLAYER FILMS1
George M. Whitesides and Gregory S. Ferguson,z Harvard University
Organic chemistry is largely derived from studies of the reactivity and prop-enies of moiecules in homogeneous solution. and much of the intuidon oforganic chemists is based on the behavior of molecules in solution. Surfacesand intertacesr (that is, quasi rwodimensional assemblies of molecules orfunctional groups) provide environmen$ that can be quite different from thoseof solutions. and chemical intuition derived from solution is often wrong whenapplied to processes occurring at surfaces. The centrai focus of our programin organic surface chemistry is on new science: that is. understanding andcontrolling the phenomena characteristic of surfaces. interfaces. and thinfilms. A charm of surface chemistry is. however. its ability to combine newscience with relevance to a wide range of technological problems.''s and wehope to contribute to these appiied areas as well.6
Underlying our program in surface chemistry is a broad interest in the prop-erties of organic surfaces as components of materials. In panicular. we hopeto develop the abiiity to ration alize and predict the macroscoprc propeniesof surtaces-wening, adhesion, friction-by knowing their microscopic, mo-lecular-level structures. The issue of stmcture/properry relationships in solidslies at the base of much of the current research in areas such as matenaisscience. condensed maner and device physics, and polymer physical chem-istry. Surface science spans these fields and is curently a research area ofparticularly great activitv.s'7-6 The appeal of surface chemistry as an avenueinto detailed understanding of the relations between microscopic and mac-roscopic properties of maner is that interfaces are more accessible to analysisand more easily modified by synthesis $att are the interiors of solids.
Organic chemistry has played a surprisingly small role in interfacial science.tAlthough organic chemistry offers, in principal, the ability to introduce a widerange of functional and structural groups into surfaces, in practice it has beendifrcult to rationalize, much less design and synthesize. ordered two-dimen-sional arrays of organic moieties.a We have taken a physical-organic approachto the study of organic interfacial chemistlv: We formulare a hypothesis re-
CF{EMTRACTS-ORGANIC CHEMISTRY 1: 171- 1 87 ( I 988 )0895-444518E/$.m O 1986 Chemtracts
iTAY/JUNE It'88 / SURFACEfUilCTTONALIZED PC'LYTERS ANO SELFASIIETBI-ED FIL'IS 171
lating molectrlar-scale stnrctlre to macroscopic ProPerty' synthuize and char-
orrrirc interfaca having structures aPProPriate to testing that hypothesis,
measure the'propenies it interest, and iruerpret the information concerning
strucrure *a pioperties in terms of the original hypothesis. The physical-
organic paradigFl ior the study of complex pailerns of strucnrre and reactiviry
is iundam"ntally a qualitadve one, often relying more on analogy than on
numerical calcuiations based on fundamental theory. It has, however, pro-
vided one of rhe most durable and useful methods of understanding compli-
cated systems. Physical-organic chemistry counts among its many successes
the correlation of organic structures with reactivities in solution, the ration-
alization of areas tu.l .t photochemistry and catalysis' and the inference of
the properties and stnrctures of reactive intermediates;D we believe it will
also be immensely valuable in understanding surfaces.
SYNTHESIS OF SURFACES AND INTERFACES
We have reiied on two seParate ryPes of experimental systems in our studies
(Scheme 1):
r-j
€
-
Self-Asscmbly
Oxidat ion
+
H e C r O lpE 4 -PE-CO2H"
1) PlasmaPE + -PE-.H"
2) BHe'
1. Slrfrct-Functirindbed Orgrnic Pollmcrs, Espcddly'Polyethylene Car'
boxylic Acidl (PE-CO2[I). Thesc s),stems are PrePared by .oxidizing poly-
ethytene (PE) films with chromic acid and using the carborylic acid groups
in6oduccd onto the surface as the staning point for more elaborate chemical
modification (Scheme 1).$s The chromic acid oxidation has the advantages
of restraining the funstionality to a very thin (less than 10 A in depth) layer
along the surfacc @ntour of the polyurer and of generating a set of func-
tionalities limited to carboxylic acids and ketones and/or aldehydes. PE-CO:H
is convenient to prepare and snrdy and is an excellent material for exploratory
snrdies. It also provides an entry into the examination of propenies rePre-
PC'LY'TERS AT0) SEU!TSSENI-ED FII.G / CHEII|TRACTS-ORGANIC CHEMISTRY
N=Au I HS-R-X
s-R-xJ-r'-r'
S, R-X
- Rr-x'
Si / SlOz ClrSl-R-X
Schcme 1. Scfiematic representation of the two mdtods used for production of functionalizEd surfaces' Left:
spontaneous self-assembly of an onented monolayer film by adsorption of organosulfur compounds on gold or
af'fyf ricfrbrosilanes on silicon/silicon dioxide. Rigtrc Oxidative funstionalizEtion of polydlylene ftlms.
172 SURFACE.FU}GNOil L.trED
sentadve of a "real" material. It is, however, a complex, microscopicallyheterogeneous and strucrurally illdefined material.rT2' self'Assembled Adsorbed Monolayer Films.l&p we and others have fo-ctsed on rwb classes of monollyersi organosulfur compounds (especiait), ;;-ganic thiols) adsorbed on gold,r5'{)-'rs and alkyl siloxane monolayen preparedby reaction of alkyl trichlorosilanes with surfaces containing hydroryl groupsand/or adsorbed water.'tsn Both of these systems, and otheis rilated to rhem,are excellent models for interprering the characterisrics of pE-CO2H and itsderivatives. Immersion of a silicon iuf.r coated wirh a thin film (:i000 i;of evaporated gold in a solution of a faqv thiol for t hour at room remperarureresults in the formation of a highly ordered, quasicrvstailine monolaver offatty thiol anached to tbe gold suriace by ruirui-gold coordi""',#i;l;r:1)' The essential Processes occurring during the adsorption and organzationof the thiol on the gold surface are stitl iniompleteli understood, but rhevare cenainly reiated to the familiar, if complex, coordination ;;;';?thiols and gold(O) or gold(I).sra
One of the most anracdve features of organic chemistry is the wide variarionin the strucure of organic molecules thaican be produced through synthesis.A challenge to our program in organic surface ihemistry has been ro bnngthese synthetic techniques to bear on two separare classes of problems insurface chemistry: first, the introduction of small fragments having desiredfunctionality onto surfaces through chemicai reacrion: second. the prepara-tion/assembly of these fragments in extended macroscopic arravs with controiover position and orientadon. The nvo approaches we have followed-oneleading to PE-CO2H and its derivatives. and the other to self-assembledorganic monolayers-are quite diferent. The former inroduces funcrionaigrouPs onto a preformed heterogeneous material (Scheme 2). This procedureis convenient and exPenmentati relevant to a broad range of polymer tech-nologies, but it requires the srudy and analysis of matenals rhat are intnnsicallvstructurallv ill-defined. The latter prepares well-defineo. approp";;.1:, fu;;:
PE-HII
III
I
CrO3/H2SO4
ROH<t---
H'
LiAtH4orBHg
-> PE-CH2OHPE-CO2B PE-C02H
l*,,II
PE-COCIJ rcoc'
PE-CH2OCOR
RoH // \
nnrxz/ \
PE-CO2R PE-CONHR
Sghcme 2 Representative reaction seguences used to convert the surfaceof polyethylene fitm (pE-H) to "polyethylene carborylic acid,,(pE-corH) andderivatives.
ttiAY/JUNE I98S / SURFACE-FTJilCNOilALJZED POLYTGRS Ar€ SEI.FAS{itr8LED FIL.TIS 17g
tionalized molecules, which are then allowed to self-assemble on a reactive
surface and form highly ordered two-dimensional stru$ures. Self-assembly
will, we beiieve, bec6me a mainstay of ordered monolayer formadon,"t and
will eventually prove invaluable in ratronal suategies for modification of the
propenies of interfaces. Prepanng these svstems is, however, expenmentally
roi. complex than generating functionalized polymer surfaces'
CHARACTERIZATION OF SURFACES
We have used rhe usuai array of specroscopic techniques to characterize
surtaces: ailenuared total reflectance-infrared (ATR-IR) and polarized in-
frared external reflective specrroscopy (PIERS), X-rav photoelectron sPec-
troscopy (xPS), elecrron spln resonance (ESR), fluorescence. electron
microscopy, and ellipsometry are all usefui (Table). We have, however, aiso
Table. selected Methods for Analysis of surtaccs and Intertaces
Technique Application and Depth Sensed
Scanning tunneling microscopy (STM)Low-angle X'raY scattenng
Electron microscopy (SEM, TEM);eiectron difiractton
Contact angle (HrO)X-ray photoelectron spectroscopy ;
(XPS); auger spectroscoPyEllipsometry
Attenuated total reflectance'infrareO I(ATH-|R) |
Polarized infrared extemal reflecttve )spectroscopy (PIERS) |
Rutherford backscattenng (RBS) I
Fluorescence spectroscopy
Electron spin resonance (ESR)spectroscopy
Individual atomic positions on surfacesElec-tron density map of the surface of
very flat solidsSurface morphology and degree of
crystalline orderPolanty of top -10 A'Atomic and chemical composition of top
-so ADetermination of film thickness with a
resolution of 2 A
Vibrational analysis of top -1000 A
Atomic composition as a function ofdepth with resolution of hunclredso f A
Assay for density of functionality aftercovalent attachment of fluorescentprooes
Location and mobility of paramagneticcenters (e.9., TMPO) in interfaces
been able to apply to problems in the physical-organic chemistry of surfaces
rwo techniqu"i ior cbaractenzation that are less familiar to the spectroscopiccommunity. Ttre first is the measurement and interpretation of liquid-soiidcontact angles. This technique has proven to be the most surface'sensitiveand most convenienr (if not the most easiiy interpreted) method that we have
available to charact eiue organic interfaces.!0-35'{r'4r It is especially useful in
characterizing the soli&water interface. The sccond technique involves stud-ies of chemicai reactivity at interfaces. This approach is especially useful when
applied using simple, high-yreld reactions that are well undentood in ho-mogeneous, liquid phase chemistry. Ionization and esterification of carboxylicasids and saponificltion of carboxylic acid esters are esPecially diagnostic.r
The combination of measurement of contact angle with studies of ionizationof functional groups has resulted in a technique we call "contact angle titra-tion":+hat is, study of the vanation in the contact angle with the pH of the
SURFACE-FUI{CflOilAUZED P(TLYIERS AXO SELFA$iEilBt€D FILilS / CHEMTRACTS-ORGANIC CHEMISTRY174
aqueoui drop (Scheme 3). This technique increases the information derivedfrom the measurement of contact angies. Traditional approaches to studyingcontact ange generate only nro.numben (the advancing conract angle 0"-anJthe receding contact angle g,)." Receding conracr angies are presendy vervdifficult to interpret. EEorts to characterize complex, heierogenious interfacesusing only advancing contact angles are unlikely to be verv broadly userul.By measunng contact angle as a function of pH, however, one can often inferthe existence, environment, and narure of ionizable groups present at theinterface.
Contact angle titration is based on the observation of variations in contactangle with pH at surfaces connining ionizable groups.rs This variadon plau-
eo
t20
90
60
30
ot?o
90
60
30
0
* O O O $ S o O o PE-CONHoPE.H
PE-COOCH3
PE-CH20H
IPE-CONHCHzCHzN)
\ PE[>C=OJ[CHzNH2J
PE.C OOH
o48t?pH
Sclrcmc 3. Dependence of 0. on pH for surface-functlonalized polyethylene film. Iop: Using unbufiered aqueoussofut tons.Buf iers: (D0.1 Mphosphatebuf ier ; (o) a i lo thers(0.05M),pH1,0.1 NHCI; gHz,male icac id;pH3,tartaric acid; pH 4, succinic acid; pH 5, acetic acid; pH 6, maleic acid: pH 7 and g, HEPES; pH g and 10, CHES,pH 11, Fiethylamine; pH 12, phosphate; pH 13,0.1 N NaOH. The crosshatctred and labeled "assorted bufiers"at pH 8 include data for phosphates MoPS, HEPES, TAps, TFlls, and triethanolamine.
ASSORTED BUFFERS
MAY/JUNE I9E8 / SURFACE.FIJi|CNOTAUZED PC'LYrER9 A}IO SELFASTiETBIID FILTS 175
sibly reflects ionization of the functional group: The charged form of an acid
or base is more rr'var"prrnic than the unclargla form. and the contact angle
with water is rower. Although this simpre expranation is fundamentally cor-
rect. and the technique is a ver.V useful one in companng acidities' its simplicity
hides a number of iomPlexides'rus
one interesting aspect of conuct angre tiration concerns the intuitive concept
of the ..quanuty" of a functional group Present at an interface' our inidal
inruition .on."Lirig ,rrrr".. functionai group chemistry was that' in most
sysrems likely r"i.i*Ji"d ."p"rimentatiy, th" number of functional groups
present on a rePresentative area of surface would be small compared with
the quantity of . r""g.nt Pr:s.elt in the volume of soiution used in expenments
on that s,rrface. Tffi Ueiiet is largely incorrect for measurements of contact
angles: The number of functional Eoups Present at high density on a surface
is comparable to that preseat in titutibni used for contact angle titration in
unbufiered systems. The difierence berween the titration curves obtained using
bufiered and unbuffered sorutions (Scheme 3) exempiifies the phenomenon.3z
Explanation of this observation helps to ciarif.v the concept of "concentration"
in a heterogeneous system consisung of a iurface *d: :ontaaing
liquid
phase. we .onsil; ,i. spreading of-an aqueous drop at an interface to be
determined in part by the extent -of
ionization of the functionality present at
that interface. Let us examine the "concenrration" of this functionaliqv in a
sysrem consistrng of a 1-p.l gtop in contact with a denvatized polvethvlene
surface (a l-p.l drop typically covers an area of -1 mm:)' The density of
functionat groups-"'" riti r*i".. can be in the order of 6 x 10r4/cm: for a
surface with typical roughness;33 at this density' the concentration of reagent
in solution in the contacting drop required to- react stoichiometricallv with
that funcrionatity is -0.1 m-M.rt For an unbufiered aqueous solution and a
monoprodc acidjbase reaction. a concentration of acid or base > 0' 1 mM
(i.e., pH < 4 or pH > 10) is thus require.d to.achieve a stoichiometnc reaction'
clearly, the difierence in contact angle duation.crrrves obtained using buffered
and unbuffered solutions is due ,o ,irf".. functionaliry that is itself sufficiently
concentrated in the system comprising surface and drop to bufier the pH of
the aqueous solurion in the ranie pH :-? Tlur. the qualitative idea that a .
monolayer of organic functiona-liry is insignificantly small in quantitv com-
pared with the fuictionaliry preseniin solution is incorrect, if one is concerned
with small volumes of soludon'
A second interesting issue concerns the detailed interpretadon of the contact
angle titration .o*Ir. h particular, we ask how should the solid-iiquid in-
terfacial free enerSy "Ysr be related to the functional groups present on the
surface? The fundamental relation connecdng the conracr angle to igterfacial
free energy tem$ is Young's equation (Eq' 1)'5t
Tsu
cos0 =
^lr s L
Jsv - 'Ysr
' lw( 1 )
176 ITRFACE.FUrCNOilALtrED POLYTERS AXO ITELFASSEMLED FII'IS / CHEMTRACTS{RGANIC CHEMISTFIY
For aqueous solutions constituted with appropriate buffers, the liquid-vaporinterfacial free enerily ̂ lw is the same as that for pure water. vanationJininterfacial free energies are thus related to the obsCrved value of g pr,,',*iivby the terrns 15v and "y51. These tenns, in turn, depend on a number of factors:&g tyP!, densiry, and distribution of funaional groups present at the solid-vaPor (liquid) interface; their extent of ionization; the roughness of the sur-face; tbe relative humidiry of the vapor.
As a fint approximation, we have proposed that the interfacial free energycan be expressed as a linear combinatibn of funcdonal group contnbutions.multiplied by the normalized fraction 9i of these groups on the surfacei2r4(Eq. 2). The parameters "yisr and 1,.5y refled intrinsic
(2a)
(2b)
hydrophiiiciry and group size or:rea.data with contact angles indicate that
Comparisons of infrared specroscopicthis gypg of analysis is approximatelv
=?=?
'Isr
"Isv
9i 1,se
9i lisv
Vo
o
S
oeI' a - A -
- r,lL911-tfl - v r r ! l E
! - A - v I
I I - + 7
: - - T I I II - - -
- I 4
_ _ _ I
?rs ry/sv
Figurc' scfiematic representation of an ideal 1np leftl and real (top right, bottomldrop of tiquid (L) in contactwith a solict (s) and vapor (v) with contact angle 0. The symbols inirre upper right picture represent (c) watermolecules' (A) dissolved solutes (phosphate, buner salts), io, e) polar surface groups (co2H, cor, c=o,. . .),(t) nonpolar surface grcups (CH;, 9H., .). t f,p of liquid (,bo6m; not drawn to scate), the ,.precu6or fitrn,,,extends microns beyond the edge of thi drop in ."rt",n orcumstances.
tI
MAY/JUNE l!)gE / SURFACE RJ[crloilAU"Fn POLYI|ERS atrto sELFAssEtBLED FtLfls 1TT
correct for PE-CO2H and some of its acidic derivatives,rs but that interactions
berween groups, ana perrraps interfacial heterogeneity, make the problem
more complex than can be discribed completely Jr*g tiris simple approach'35
Rn unaerltanding of these interactions and complexities remains to be es-
tablished.
Ath i rd important issueis themeaningofhysteres is in themeasurementofconuct
"ngl"r- For derivatines of PE-CO'H, 0" aPPear-s to provide a simple'
semiquantitadvely interpretable measure of interfacial group character and
density. connct angles are, however, a simple Parameter derived from ob-
servations of a complex realiry (Figure)' Advancing and receding contacl
angles difier on many surfaces, anOltt the derivatives of PE-CO2H (partic-
utarly polar derivatives) display very large hystereses in their contact ansies:
0, is frequently 0 even for'syste*i tt"t'ing fairly large vaiues of 0"' Large
hysteresis is usually interpr.,id -,o indicats a heterogeneous system far from
thermodynu.i. equilibrium.se Yet analyses of 0" based on Young's equation'
- .qu"rion *ru,n]ng th;odynamic equilibrium, seem to give interpretable
and reasonable results. It is not crear how one shourd rreat a system that is
nor at thermodynamic equilibnum. but for which physical measurements cor-
relate with those expeded based on physical-organic analogies to Processes
occurring at equilibrium in solution'
RESULTS
Both functionatized polvethvlene and its derivatives, and self-assembled mon'
olayer fiims. provide systems with which to examine reactions occumng at
interfaces and to test hypotheses concerning stnrcture/reactivit;- and struc-
rure/properry relationships. In so doing' we find that many of the results we
obtain can be rationaiized by anaiogy to phenomena in solution (often with
cbaracteristic difierences thai can uJinterpreted to comPare and contrast the
environments provided by homoseneous solutions and interfaces)' we also
frequently encoun,., un.ipectedlh.no*.na, which suggest that any models
of organic reactivity at interfaces. based exclusively on analogies with solution'
are not complete. The studies that follow provide examples'
Surface Acidities. Scheme 3 indicates that carboxylic acids and many' but
not all, amines show inflections in plots of 0, vs the pH of the drop used in
measuring the conract angie.s?r5 Asiuming that the midpoint of the inflectton
.orr.rporids to half-ionizltion of the functional group (an assumption sup-
ported by independent ATR-IR measurem.nts oJ.arboxylic acid surfaces),31
we infer that acidiries of functional groups at an interface and in solution are
very difierent. For example, the value of pH lol " solution in connct with a
surface required to achiive half-ionizarion of the carboxylic aod grouPs at
that surfari ott be as high as 12. What is the oriFn of this very large aPParent
decrease in acidity of &rUorylic acids (and con"sponding increase in the
apparent acidiry of ammoniurn-ions)? We believe thai the origin of these shifts
can ultimately be anributed to the locally low dielectric constant at the Poly-
ethylene-water interface,x{-CI but rationeli-a3isn of these anomalous values
of p& is not Yet comPlete.
Relstions between Func{ional Group Hydrophilicity and Wettebility of In'
terfaces. We assumed at the outset of our studies that more hydrophilic
interfacial groups (as measured by some convenient Parameter such as the
Hansch ,r, i"o*eiet',) would lead to more wettable surfaces. In fact, ex-
perimental observations reiadng wenability to functional group hydrophiiiciry
SUFFACE.FUI{CTIOIIAIJZED POLYTERS AI{D SEI-FASSETELED RtrIS / CHEMTRACTS{RGANIC CHEMISTRY178
difier significantly from those expected (Scheme a). As the value of n for thefunctional group on the surface decreases. 0o also decreases, but only up toa point. Beyond thar point, further increases in functionai group hydrophilicityresult in no further increase in wettability: that is. the hydrophilicirv of thesurface "sarurates."35 We postulate that the origin of this efiect lies in con-densation of water vapor at polar solid-vapor interfaces (Scheme 5). Nonpolarinterfaces condense relatively little water. All of our experiments invoivingcontad angJes with water nre carried out at LAUVo relarive humidiry in orderto Elssure that the system is as close to therrnodynamic equilibrium as possible.Polar functional groups at interfaces are undoubtedly associated with hy-drating warer adsorbed from the vapor phase. We postulate that. beyond acertain vaiue of the Hansch tr pzuameter. the polar surface functional groupsbecome completely surrounded by condensed. hydrating water, producing asolid-vapor interface whose polarity is essentially independent of the under-lying functionai group. Under these circumstances. the wettabilitv of thesurface is determined primarily by the area fraction of the surface convertedto polar funcdonaliry, and then hydrated by condensed water.
160
120
/ro|t t ' ?-o-
ooo
o o 6o
' 2 n '1
Schcme 4. Contact angles of water for derwatves of PE-CO.H, PE-R, witha range of fryctrophilicities of the trc,up R. lr is the Hansch parameter, a me€lsureof functional group hydrophilioty, derived trom the equilibrium constant forpartioning between aqueous and hyclrocarbon phases (rnset).
These observations and interpretations imply the existence of a thin. con-densed water film on polar surfaces. The nature of this film, and especiallythe relAtion of its stnrcnrre to that of bulk water, remains an important and
complex problem.
The Range of Interactions Determining Wetting. Scheme 4 displays an aston-ishing observation: Although a surface incorporating amides (PE'CONH:) is
relatively hydrophilic, the analogous primary amide PE-CONHC3H' is morehydrophobic than unfunctionalized polyethylene. Some of the apparent hy-
9Oo
-3. 4
\ (
MceHrzI t t {x ro -cxro8c ,r,?. /
/ /
?rr/ t o iC O N H C a H , . ^ \ /
lo /o 05, /PE-Hc - / , . \ o \ //o^\ 6 /3(:Ar/r**
II/TAY/JUNE 1988 I SURFACE.FI,NCTIOTAU:ZED FOLYTERS ANO SELFASSEilBLED FIL.TIS 179
oHz
Hro
P
Scheme 5. Schematic illustration of the degree of hydration of a functional
group p at the solid+rapor interface. When P is nonpolar, the equilibnum lies
io tne bft; when P is polar, it lies to the righl
drophobiciry of PE-CONHCTH7 and its analogs undoubtedly reflects the mt-
croscopic roughness of the surface of these. materials (generated during the
oxidanve surfice functionalization). Nonetheless. we find that it takes oniy
a small hydrophiiic or hydrophobic group to determine the wenability of a
surface. Furthermore, a smail hydrophobic group is capable of completelv
masking an underlying, intnnsically hydrophilic core functionaliry' Thus. for
example, replacement of a terminal CHi grouP in one of the well-defined.
seif-assemblid monolayer systems by a CH2OH group changes the monolaver
from being very hydrophobic to very hydrophilic,'r and reacvlation of the
terminal n-ydroiyt (CHTOCOR) once again makes it very hydrophobic- The
interactions that determine macroscopic wettability are, aPParently, verv short
in range.r0We believe, in fact, that measurement of contact angie is the most
surface-sensitive rechnique presently available for examining the sclid-liquid
interface. The great advantages of wening as a probe of surface stnrcture
(reladve, for eiample, to XPS) are that its measurement is very simple,
convenient, and ineipensive, and that it is inrinsically appiicable to the soli#
iiquid interface and to heterogeneous, noncrystalline surfaces. Its disadvan-
mges are that contact angle measurements are information Poor, that they
require a liqui&solid interface, and that their physical basis is complex and
still incompletely understood.
Designed Interfaces. The materials PE-CO-X are convenient but heteroge-
neous. The best characterized and strucnrrally best defined organic interfaces
now available are rhose formed by adsorbing long-chain alkvl thiols on gold,
or by atlowing long-chain alkyl trichlorosilanes to react with surface hydroxyl
groups and adsorbed water prescnt on the surface of glass or silica. Both of
these systems have the alkyl groups in completely rraru'extended confor-
mationi, provided that the terminal functional group is relatively srnall. For
organic ttriots on gold, the chains are tilted -30o from the normal to the metal
surfacc;€'rs for attcyl siloxanes on silicon/silicon dioxide, they are aPProxr-
mately perpendicoi"t to the subsrate surface (Scheme 6).e Transmissionelectron microscopy indicates that the thiol/gold system has at least micro-
srystralline order in the plane of the monolayer.azThese ordered monolayer systems permit an exquisite degree of control over
sttucture and dimensionality at the interface. As one example, consider a
monolayer formed by adsorption of HS(CH?),9OH on gold. Formation of
such a monolayer is experimintally very straightforward: one simply dips thegold-coated substrate into a solution of the c,,rrr-thioalcohol in a solvent such
as acetonitrile for t hour at room temperature, withdraws it, and washes it
briefly. At the conclusion of this procedure, the entire accessible surface of
suRFAcE-FuilcnoilALeED FoLynERs Ar{o SELFASSETELED FrLrs r CHEMTRACTS-ORGAN|C cHEMlsrFY180
it-IIIIII
LrI
IIL
OH'"OH
) )
) )
) )
) )a{-l'-o'l't lt l
tsl-o,sl
(o'''ro'lr
( (
a((aaa
Scheme 6. Schematic illustration of conformation and packing order in mon-olayers of organic thiols on gold and alkyl siloxanes on silicon/silicon dioxide.The monolayer is composed of three important regions: the head groups(portion binding to solid substrate), the polymethytene chains (for formation ofvan der Waals surface), and the tail groups (termrnal functronality that deter-mines the character of the soli#liquid and solid-vapor interfaces).
$!1c 7' Stylized illustrauons of monolayer structures.'r Proposed structures of (A) pure monotayer ofHs(cHJ'eoH; (B) monolayer composed of 50o/o HS(CHJ,eOH and 50o/o HS(CHJ,,OH; (C) pure monolayer ofHS(CHJ'OH' Struaures we believe do not occur in the systems studied here: (D) disordered monolayer and (E)monolayer containing a mixture of components and strowing phase separation into islands.
variabletunctionality
polymethylenechains
suPPort-organicinterface
\ H S H SAu Au Au
O H
looHtf,,r,, l l ,J-.I U '::"'SH
ITIAY/JUNE 1988 / SURFACEfiJrcnor r r?FF FoLyrcns Al|O SETFASSEilBLED FtLrS 1g1
the gold is covered with a uniform monolayer?3 A thick, and the exposed
surface is a densely packed monolayer of hydroryl groups. The thickness of
the monolayer is "aiity
controlled at the scale of angstroms by varying the
number of methylene groups in the thiol chain; the surface ProPerties ate
independently controllable through variations in the terminal functionai
group. If mixtures of rwo difierent terninally funcdonalized thiols are used'
monolayers can be made having the rwo mixed on the surface (Scheme 7)'
CURRENT PROBLEMS
The physicakrganic chemistry of surfaces promises to provide new materiais
based on rationil synthetic modificadon of surfaces and interfaces' new an-
alytical methods with which to characterize surfaces, and deeper levels of
understanding of familiar processes such as dissolution. wetting, adsorption.
and adhesiorioccurring arlnrerfaces and in solutions. The phenomena.being
obsewed are, however, usually more complex than those occurring in ho-
mogeneous solution, and are, consequently, still incompietely understood at
even the simplest levels. Tbe fieid presents a number of fascinating funda-
mentai problems in interfacial chemistry, among which we place the following:
1. Molecular-Level Order. How should the order in these svstems be de-
fined and measured? One advantage of a two-dimensional system is that
it is, in principle, less complex structurally than a three-dimensional
svstem: The components of a two-dimensional system are by definition
restricted to a plane rather than free to nanslate and rotate in three
dimensions. In practice. however, the problem of defining order in
surface -fu nction alize d po I ymers and seif- assem b le d monoi aye rs rem ains
very complex. All of ih.s. svstems are, in realiry, only quasi rwo-di-
mensionai. Matenals such as functionalized polyethylene are obviously
mictoscopically rough and heterogeneous and have functionaliry dis-
tributed nonuniformiy in a thin interfacial layer. Contact of these
systems with a liquid phase may result in interfacial swelling and
reconsrruction. Seli-assembled monolavers are bener defined strucrur-
ally, but even with these sysrems, subtle issues of order in the plane of
thi monolayer. ar the gold-monolayer and monolayer-liquid interfaces
and berween adja..niorganic molecules require the development of
new analytical techniques and new criteria for order.
2. Kinetics vs Thermodynamics. The extent to which any of the systems
currently studied :re ar thermodynamic equilibrium. and the influence
of deparnrres from equilibrium on their behavior, is almost completely' uncertain at Present.
3. Wetting. Despite interesting and provocative theoretical contributions
to the ih"ory'of wening inlenain idealized systems,e'6a-7t there is no
usefully detailed theory of wening relevant to real, misroscopically het-
.rog"n.ous surfaces. The current rationalizadon of hysteresis in the
measgrement of conracr angles is especially unsatisfaaory. Detaiied
examination of hysteresis, bJth theoretically and experimentally would
be particularly uieful, because hysteresis apPears to be very sensidve
to order; an understanding of the relation between interfacial stnrcture
and hysteresis might provide a new avenue of approach to this important
subject.4. IVlolecular Design of Monolayers. Essentially all work so far carried
out with self-asslmbled monolayers has focused on derivadves of fary
ltUFFACE-Ffrt{61OtrA!.trED FOLYUeRS At{O SELFASSETBLED RIJS t CHEMTRACTS-ORGANIC CHEMISTRY1&l
acids. These systems have the two virnres that they are easy to manip-ulate synthetically and that they do, for whatever reason, form *.it-ordered monolayers. They are, however. not stable at even modesdvelevated temperatures and have no strong intermolecular interactionscontributing to order in the plane of the monolayer or to thermal oroxidarive stability. It is important to develop molecrrlar stnrctures otherthan fattv acids that form ordered, stable rwodimensional sheet struc-nrres.
CONCLUSIONS
Organic chemistry at interfaces is a field offering major oppornrnities for borhthe conduct of basic science and the developmenr of new technology. It alsoprovides, through the synthesis of extended funcrionalized interfacej, a bridgebenveen the science of isolated molecules and the ssience and technology 6fmateriais. Since chemical reactivity and we$abiliw provide whar we beiievewiil prove to be invaluable probes of interfacial structure for organic svsrems.these s)'stems are particularly attractive for srudies aimed at understandinethe characteristics of soli&liquid interfaces.
Surface'funcdonaiized polymers (of which the best developed is PE-CO'H)are proving to be convenient svstems with which to conduct exploratorv work.They are easily prepared and manipulated. and because thev presenr solid-vaPor interfaces that have low surface free energies, thev are relativelv re-sistant to contamination by atmosphenc contaminanc. Funher. since theyare physically robust, surface-modified polvmers can be used to examinecomplex materials problems such as biocompadbilirv,D.s adhesion.srgas per-meation.s friction,s3 and the influence of bending, stretching,s and su*o..reconstructions on interfacial propenies.
Self-assembled monolayers will, we believe, prove to be the ultimare cor-nerstone of the basic science in organic surface chemisrry. They will cenainlyalso find technological application in areas such as promotion of adhesion,inhibition of corrosion, and control of friction, and they may prove importantin the production of sensors and microelectronic devices. The remarkableease with which very complex monolayer stnrcrures can be assembled frommolecules of very modest complexiry will be invaluable in studying the prop-ernes of organized molecular assemblies. The best defined of these suitemsis presently obtained by adsorption of orfunctionalized fatty thiols on gold.although organosilicon compounds on silicon dioxide and glass may ultimitelyprove equally ordered. Alkyl thiols on gold have as their major advantagethe comparibility of the thiol moiety with a wide range of organic functionalgrouPs, and the fact that these systerm lead to highly ordered monolayers.Silanes on silica are more economical, bener adapted to the formation ofmultilayer stnrctures, and more robust srnrctrrrally.
Given the astonishing sensitiviry of wenabiliry to local surface structure. itssrudy should provide a range of imponant new q?es of informadon aboutinterfaces, especially solid-liquid interfaces. Designing and interpreting theseexperirnents will require a physical-organic approach-the sysrems beingstudied are too complex to be defined using conventional, spectroscopy-basedphysical chemistry. Because werdng is directly relevant to a broad range oftechnological problems, these studies should be exceptionally valuable inapplicarions. The experimental techniques required to srudy wening are vervsimple. Surface science based on studies of wettabilir,v should rhus be acces-sible even to those without routine access to the instrumenmtion of high-
ir Y/JUNE l98E / suRFAcE-FuilctoNALuED poLyrEns Ailo sELFAssEilBLED Frlrs 1&l
ACKNOWLEDGMENTS
u, 4739 -$. Adams' N'K' Adv' Chem'
Chem. Ser', 1964, No'
16. Ishida.H. Rubber Chem'19tr7, 24, L.
PC'LYTERS AND SELHSSEilBLED
Scr., 1964, No. 43,S?;'-Adamson' A'W'' Ling' l ' Adv'
43. 57.
Tech. Rev.,Il|tnr 60, 497; Debe' M'K' Prog' Surl' Sci"
FIIJS / CHEMTRACTS-ORGANIC CHEMISTFY
vacuum physics. A more realistic theoretical basis for wening is desperately
needed. Current fieatments do not deal adequately with su.bjects such as
molecular-scaie heterogeneity, hysteress, . deviations from thermodynamic
equitibrium, *otl. na-rure 6i tnl interactions berween sorids and liquids.
The research carried out in our Eoup has been the.result of-skilled experi-
menrarion and ;;;G;rfrl and lreatiue design and interpretarion by a num-
ber of individuals, including Jim Rasmussen, Randy H-olmes-Fariey' Tom
McCarthy, Cotin il"in, n"rry Troughton'-Paul Laibinis' Hans Biebuyck' Lou
Suong, Stephen Wasserman, and l-luis M. Scarmoutzos' We are also grateful
toourco l leaguesPete rpen t ran(D iv i s ion : {npp l l :dSc ience ,Harvard ) ,Rarph Nuzzo (AT&T Bell Labor"rri.r), and Maiii wrighton (Mrr) for im-
pori"n, contributions to our understanding of surfaces.
REFERENCES AND NOTES
l . T h e w o r k d e s c r i b e d i n t h i s p a P e r w i r s s u p p o l l d l n p a r t b y t h e o 6 c e o f N a v a lResearch and the Defensc ndvlced Rescarch Projects Agency' and by the NSF
through g-nl. oruw (cT{E g54S702) and to the Harvard Materials Research
I-aboratory (DMR 86-14003)'
2. NIH Postdoctoral Fellow, 1988-1989'
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MAY/JUNE 1988 ,' SURFACE.FUilCNOXALEED PCILYTERS AflD SELFAIISEilBLED FIL.iISi 187