formular organic

7/29/2019 formular organic

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636 Acc. Chem. Res. 1993,26,636-643

Liquid InterfacesKENNETH . EISENTHAL

Chemistry Dep artmen t, Columbia Unive rsity, New York, New York 10027Received March 30, 1993

Interfaces are the boundary regions that separatedifferent bulk regions of ma tter. Th ey have specialchemical, physical, and biological properties t ha t havefascinated and drawn the attention of scientists andengineers from many different fie1ds.l W hat ma kesthe interface unique is the asymmetry in forces th at isexperienced by m olecules and atomic species locatedthere together with the almost two dimensional geo-metry of the interface. Th e chemical composition, thegeometrical arrangem ent of th e species, he equilibriumconstants, pH, the motion of molecules, the thermo -dynamics and kinetics of ground- and excited-statechemical change, energy relaxation, and the phases an dphase transitions of long-chain amphiphilicmonolayersare among the fundamental manifestations of theunique characteristics of an interface .Despite its importance and widespread interest, itremains difficult to probe t he chem istry and physics ofinterfaces, and in particular liquid interfaces (vapor/liquid, liquid/liquid, iquid/solid) with trad itional spec-troscopic techniques. Th e reason is th at th e over-whelmingly large number of solute molecules in th ebulk can dom inate the signal originating from solutemolecules in the interface. One way to circum vent thislimiting characteristic of trad itional m ethods is to usethe techniqu es of second harmonic generation (SHG )and sum frequency generation (SFG), which canselectively probe an interface that separates cen-trosymmetric The key factor is tha t SHGand SFG are electric dipole forbidden in centrosym-metric media (bulk liquids, gases, centrosymmetricsolids) bu t are allowed at th e interface where inversionsym metry is broken. In second harm onic generationthe incident light at a frequency o nteracts with theinterface to generate radiation a t 20, whereas the sumfrequency generation the in cident light, which consistsof two sou rces, one a t 01 and the other at 02, generateradiation a t the sum frequency 01 + 02. The secondharm onic or sum freque ncy light field E ( o ~ + o ~ )an bethought of as arising from the n onlinear polarization,Pf+o,,nduced in the interface by the incident lightfields E(q) an d E(o2),

where xz+u2 s the second-order nonlinear suscepti-bility which contains all the inform ation on the m ol-ecules at the interface. In this Accoun t the focus ofattention will be t he work in our laboratory using secondharmonic generation (SHG) and more recently sumfrequency generation (SFG ) to probe equilibrium andKenneth B. Elsenthal ecehreda B.S. in chemistry rom Brooklyn College andan M.A. In physics and a Ph.D. in chemical physics from Harvard University.He M postdo ctoral esearch at UCLA and was at IBM Almaden before comingto Columbia University in 1975.

0001-4842/93/0126-0636$04.00/0

dynam ic properties of liquid interfaces. Although thisAccount is restricted to work in our laboratory, thereader is referred to reviews th at reflect the impressiveam oun t of fine research going on in th is field.=Orientational Structure and Population atInterfaces

When I first learned about the interface specificityof SHG from a talk given by Y. R. Shen a t a meetingin Alex andria, Egy pt, organized by A. H. Zewail, theidea tha t raised my tem perature was the possibility toactu ally look at th e surfa ce of a liqu id, e.g., a solutio nor neat liquid/gas interface, something tha t had notbeen d one before by a spe ctroscopic technique forreasons already noted. Th e system we chose to look atwas the vapor/aqueous phenol solution interface.6 Th esecond harm onic experim ental setu p is shown below.

LaserDetector

7We chose phenol because it had been extensively studie dby surface tension and o ther m ethods and we wantedto compare SHG results with the results of othermethods. Theoretical predictions indicated that th emag nitude of the second harmo nic field which isthe squ are root of the second harmonic signall% , houldbe proportional to th e num ber of phenol molecules inthe surface area irradiated by the incident beam.However, there were all sorts of approxim ations behindthis theoretical prediction. To find out if E% wasproportional to the phenol interface population, notingthat the SHG from the interface water could beneglected a t the wavelengths used in our experiments,we me asured its adsorption isotherm, i.e., E% vs bulkphenol concentration. We compared this curve withthe phenol excess surface population, obtained fromsurface tension m easurem ents, which is very close tobeing equal to the interface population for a highlysurface active species,such as phen01.~We were pleased

(1) Adamson, A. W. 1982. Physical C hemistry o f Surfaces, 4th ed.;(2) Shen, Y. R. Annu. Rev. Phys. Chem. 1989,40,327-50.(3) Richmond,G. .;Robinson, J. M.; Shannon, V. L. Bo g. Surf . Sci.1988,28,1-70.(4) Heinz, T.F. Nonlinear Surface Electromagnetic Phenomena;Ponath, H. E., Stegeman, G. I., Eds.; North-Holland: Amsterdam, 1991;(5) Eisenthal, K. B.Annu. Rev. Phys. Chem. 1992,43,627.(6) Hicks, J.M.; emnitz, K.; Heinz, T. F.; Eisenthal, K. B. J . Phys.( 7 )Paluch, M.; Filek, M. J . Colloid Interface Sei. 1980, 73, 282.

Wiley: N ew York.

pp 353-416.Chem. 1986,90,560-2.

0 1993 Am erican Chemical Society


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Liquid Interfacesto learn that the two isotherms from low surfacecoverages (-0.05 monolayer) to beyond a monolayercoverage were identical. We now know tha t SH G canbe used not only to obtain interface populations butalso to obtain the free energies of m olecular adsorptionth at determine the interface population. Furtherm ore,we saw th at since it is a spectroscopic method, it candifferentiate am ong the various interface species, th atit is more sensitive than normal surface tension andother interface techniques, and t ha t it can probe buriedinterfaces (liquid/solid and solid/solid interfaces) tha tnormal methods cannot.To com plement the interface population measure-ments obtained from the am plitude of the SH lectricfield, we can use the p olarization of th e SH ight to gaininformation on the orientation of molecules in theinterface.2 We learned th at the orientation of phenolwas 50 f 5" using a simple model! and th a t itsorientation did not change as its surface densityincreased, suggesting that the phenol orientation isdetermine d chiefly by its interactions w ith the subp hasewater molecules and not by lateral phenol/phenolinteractions.

Although the SH polarization measurements yieldthe molecular orientation, they cannot give the polaralignme nt of t he m olecules a t the interface; i.e., are themolecules oriented with their dipole moments up ordown with respect to the surface normal? For the vapor/aqueous phenol interface we are confident that thedipole mom ent is oriented "up", which corresponds tothe hydroxy end of phenol pointing toward the bulkwater phase and the hydrophobic phenyl ring towardthe vapor. How ever, we surmise this by chemicalreasoning and not by experimental measurement. Weof course want a mea surem ent th at will yield the polaralignment for any polar molecule of interest at anyinterface of interest. T he determination of the polarordering became possible when it was recognized th atthe sign (more generally the phase) of in eq 1 sdirectly related to the polar alignm ent of th e interfacemolecules.8 an interference methodwas used and was first applied to the vapor/aqueousphenol interface.8 T he results confirmed th e intuitivenotion t ha t the hydroxy group is directed toward thebulk water. We were now able to ask questions aboutother molecules; the ones we chose were the para-subs tituted phenols. In particular, what are the align-men ts of p-bromophenol a nd p-nitropheno l, two mol-ecules which both have their perm anent dipole mom entspointing in the opposite sense to phenol? Th e results

To obtain th e phase of

Acc. Chem. Res., Vol.26, No. 12, 1993 637

Air 96SolutionOH NQBrOH

indicate tha t it is the hydroxy group th at determinesthe polar alignment; all of these phenols have theirhydroxy group pointing toward the bulk water.9(8 ) Remnitz, K.; Bhattacharyya, K .; Hicks, J. M.; Pinto, G. R.; Heinz,(9)Hicks, J. M.; emnitz, K.; Bhattacharyya, K.; Eisenthal, K. B.T. F.; Eisenthal, K. B. Chem. Phys. Le t t . 1986,131, 85-90.Unpublished results.

W hat about the ubiquitous vap orb eat water inter-face? Although the various surface potential mea-s u r e m e n t ~ ~ ~ - ~ ~re not in agreement, the variouscalculations1G22of the molecular water orientationindicate th at the oxygen up orientation is the preferredone. From the phase measurements it was inferredth at the preferred o rientation of the water molecule isthe one where its dipole mom ent is directed toward thebulk water, Le., the oxygen is up.23H\ Vapor

Diwle Moment/ 'i Neat Water" H

The implication of the preferred alignment of theinterface water molecules is that there are a largernumber oriented with their perm anent dipole momentsdown than up. W hat the net population difference isa t the interface cannot be determined from the phasemeasurements since the n et population is determinedby the energy difference between water oriented "up",i.e., in the upp er qu ad ra nt (0-90") versus "down", i.e.,the lower angular quad ran t (90-180"). One way to gaininformation on th e average "up" versus "down" energydifference is to measure th e tem perature dependenceof the SH signal. Although our earlier SH mea-surementsZ4a ndicated a temp erature depende nce overthe temperature range from 15"C to 80 "C , more recentexperiments in our laboratory and SF experiments24bover a similar range indicate a change of less tha n 1 0%in the SH and SF signals. One of two differ entpossibilities consistent with th e very weak tem peratu redependence and the relatively weak SH signal, con-sidering the very high number dens ity of water, is th atthere is a wide distribution of water orientations dueto a very small orientational energy barrier. T he effectof temp erature in the range studied would not alter theoverall orien tationa l distribution. Alternatively thestructural demands at the interface, e.g., hydrogenbondin g as well as electrostatic interac tions, could resultin a favorable structure w here the w ater molecules wereoriented close to parallel to the interface. Th is wouldyield a small SH signal due to cancellations fromoppositely oriented molecules, though the SF signalwould be fairly large because the 0-H groups pointingou t of th e water would be frequency shifted from thosein the w ater and would therefore not cancel each other.In any event, more extensive temperature dependentstudie s are necessary to differentiate th e low- and high-energy possibilities.

(10) andles, J. E. B. Phys. Chem. Liq. 1977, 7, 107.(11) ley, D.D.; vans, M. G. Tr am . Faraday SOC. 938, 4, 1093.(12) aloman, M. J . Phys. Chem. 1970, 4,2519.(13)Gomer, R.; Tryson, G. J.Chem. Phys. 1977,66,4413.(14) orazio, A.;Farrell, J. R.; McTigue, P. J . E l e c t r o a d . C he m.1985,193, 03.(15) hou,Y.; tell, G.;Friedman, H. L. J . Chem.Phys.1988,89,3836.(16)Weyl, W. A. . Colloid Sci. 1961, , 389.(17) tillinger, F.H.; en-Naim,A. J . Chem. Phys. 1967,47,4431-47.(18) letcher, N. H. hilos. Mag. 1968,18, 287.(19) ownsend, R. M.; Gryko, J.; Rice, S. A. J . Chem. Phys. 1985,82,4391-2.(20) roxton, C. A. hysica A 1981, MA, 39-59.(21)Wilson, M. A.; ohorille, A.; Pratt, L. R. J. Phys. Chem. 1987,91,(22)Matsumoto, M .; Kataoka, Y. J . Chem.Phys. 1988,88, 233.(23) oh, M. C.;Hicks, J. M .; Kemnitz, K.; Pinto, G. R.; Heinz, T. F.;Bhattacharyya, K.; Eisenthal, K. B. J . Phys. Chem. 1988, 92, 5074-5.(24) a) Goh, M. C.; Eisenthal, K. B. Chem. Phys. Le t t . 1989, 157,101-4. (b) Shen, Y.R. Unpublished results.

4873.


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638 Acc. Chem. Res., Vol. 26, N o. 12, 1993 EisenthalCNfrequencyshi f l 5o15 & 0 0 CN p olarization

e 8

88

ee-- 0 -

F- 06%cl e3 5 -i e

0

1 1 100I I I I I0.0 0.1 0.2 0.3 0.4 0. 5 0.6CH,CN Concentration in w ater (Mole Fraction)Figure 1. Comp ilation of frequency shifts of the resona nce peakwith respect toneat bulk acetonitrile absorption and polarizationdat a for the CN transition of acetonitrile in the interface versusmole fraction of acetonitrile in the bulk water solution.Surprising Phase Transition at the Air/Acetonitrile-Water Solution Interface

Infrared-visible sum frequency generation is a pow-erful analytical method for probing molecules atinterfaces because it h as th e fingerprint sensitivity ofvibrational spectroscopy.2 It thu s provides a valuablecomplement to second harm onic studies of interfacialmolecular properties. Information on th e orientationand environm ental interactions of specific vibrationalchromophores in a molecule can be obtained frommeasurements of the polarization of the SF light andfrom the frequencies of th e vibrational transition^.^-^Using the SFG method th e orientational structure andenvironm ent of CD3CN a t th e air/acetonitrile-watersolution interface was investigated.% A t ow acetonitrilebulk concentrations the C z N vibrational frequencywas blue shifted with respect to n eat bulk acetonitrile,which is consistent with th e effecta of hyd rogen bondingand solvation on the vibrational frequencies of nitrilesin bulk aqueous or alcohol solutions. T he orientationof CD& N using a simple model was found to be about40 o th e interface norm al below a CD3CN .bulk molefraction of 0.07. Surprisingly, an abru pt change in bot hthe orientation and chemical environment was observedabove this. T he frequency of the cyano SF signalshowed a sharp red shift t o a value close to th at of thebulk acetonitrile interface, and at this same concen-tration a sharp change in the polarization of the SFoccurred, corresponding to an orientation shift fromabout 40 o more than 70 (Figure 1). This sharpchange was attributed to a phase transition in th einterface, which appears t o be the first case of a phasetransition involving small mutually soluble moleculesa t a liquid interface.Chemical Equilibria at Liquid Interfaces

In our first att em pt to investigate how m olecularforces a t the liquid interface affect chemical equilibria,we used SHG to probe the acid-base reaction of(25) hang,D.;Gutow,J.H.; Eisenthal, K. B.J.Chem.Phys. 1993,98,(26) Schulman, J. H.; Hughes, A. Biochem.J. 1935,29, 1243.(27) ethica, B. A. Trans. Faraday SOC.955,51, 1402.

5099.

I I0 2 4 6 8 1 0 1 2 1 4

PHFigure 2. s-Polarized surface second harmonic field. F-[111/*(2w)] f an aqueous solution of p-nitrophenol as a functionof pH a t 22 C.

p-nitrophenol at the air/water interface.28 The SHsignal, which at low p H values was 100 times strongerth an th at of neat water, was observed to decrease asthe bulk p H increased, (Figure 2). Th e surprising resultoccurred a t high pH values (-12), where it was foundth at th e S H signal had decreased to the value for theneat aidw ater interface. We thus learned tha t the anion(nitrophenolate) is not present at he air/water interface,but remains in the bulk water because the free energyof the bulk solvated anion is much lower,Clearly it is not possible to measure the interfaceacid-base equilibrium c onstan t when we cannot detectthe p opulation of the basic form. T o overcome thisproblem, we lowered the surface free energy of thecharged form by attaching hydrophobic groups to it,thereb y forcing it to the interface. Th e systems chosenfor study were para-substituted alkyl phenols anda n i l i n e ~ . ~ ~ - ~ ~or th e case of p-alkylphenols the bulkpH was adjusted so tha t the bulk contained only thephenolate form. It was found using SHG th at not untilth e alkyl chain consisted of a t least five carbons couldthe alkyl phenolate a t the interface be detected. Thedependence of the S H field on carbon chain length isshown in Figure 3 for both alkylphenolate and alky-lanilinium ions. The increment in the SH field as thecarbon chain length increases reflects the increase inthe phenolate interface population. T he interfacepopulation is determined, for a given bulk concentra-tion , by the free energy of adsorp tion. If we view theadsorption process as the reaction of a bulk moleculewith an em pty surface site to give a filled site, anddesorption as the reverse process, then we obtain theLangmuir adsorption equation,where N , is the numb er/cm2of adsorb ed solutes, N -is the maximum num ber in a m onolayer, C is the bulksolute concentration, and a = 55.5 exp(AGOms/RT),where 55.5 is the m olarity of bulk water an d AGOADSsthe adsorption free energy. For the adsorption of acharged molecule the AGOADSconsists of two parts. One

(28)Bhattacharyya, K.; itzmann, E. V.; Eisenthal, K. B. J . Chem.(29) Bhattacharyya, K.; Castro, A.; Sitzmann, E. V.; Eisenthal, K. B.(30)Bhattacharyya, K.; astro, A.; Eisenthal, K. B. J.Chem. Phys.

P h p . 1987,87, 1442-3.J. Chem. Phys. 1988,89, 3376-7.1991, 95, 1310.


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Liquid Interfaces Ace. Chem. Res., Vol.26,N o. 12, 1993 639CH3I

An i l i n iums

I. 5 NH3"

IJ I I I L1 . 0 1 rf

Phenolatesf

0-I* e * * f0.0 I l0 2 4 6 aCarbon Chain Length

Figure 3. s-Polarized second harmonic field as a function ofcarbon chain length for phenolate and anilinium ions.is an electrostatic pa rt, AGOEL,which is the reversibleelectrical work to bring a charge from th e bu lk, wherethe electrostatic potential is 0, to th e interface, whereth e electrostatic potential is WO). The remaining partis referred to as the chemical part, AGOCHEM,AGO,, = AGO,,, + AGOEL = AGOCHEM + NAZeiP(0)

(3 )where N A s Avogadro's num ber an d z is the valency ofthe adsorbed ion.For all of the alkylphenolates and alkylaniliniumsstudied (carbon chain lengths from 6 through 8), headsorption isotherm s yielded excellent fits to a Lang-muir adsorption equation. Th e relation between thealkylphenolate and alkylanilinium interface chargedensities and the interface potential 9(0) as obtainedusing the Gouy-Chapman model' of the chargedinterface, a model which is known to be app licable unde rthe c onditions of our experiment^.^' From the fit areobtained the A G O C H E M and the interface potential(Figure 4).With the interface potential WO ) , the surface pop-ulation of hydronium ions, which is needed for thedetermination of the equilibrium constant, can becalculated using the Boltzmann equation,

(H30+) ,= (H30+),e"*/kT (4)where the subscripts s and B refer to the surface andbulk regions. Th e equilibrium PKA a t the interface,

( 5 )is obtained from SH measurements that yield thepopulation ratio (HA )/(A-) together w ith a determi-nation of the interface charge density (A-), which thenyields the interface pH, u singeq 4. With this approachthe interface pKa of hexadecylanilinium C H 3 (C H h -C&NH3+ was found to be 3.6, which is a facto r of 50

PK, = PH, +W ( H A ) / ( A - ) l

(31)Davis, J. T. o c . R.SOC. ondon 1951, -4208,224.

A GO - - 6.0W d" , , , , , r0

0 1 2 3 4 5 6Concentration("I)Figure 4. p-Hexylanilinium adsorption isotherm. The linerepresents the best fit to the Langmuir equation including theelectrostatic term.

r' iAqueousSolution

more acidic tha n th e bulk equilibria where th e pK, is5.3. In con trast to th e anilinium case, it was found forthe long-chain pheno l CH ~(C H Z)& ~H & Hhat theacidity decreased by a factor of 100. The pK, at theinterface is 12 versus 10 in the bulk. Th e observeddecrease in acidity of th e phenol derivative is attributedto th e lower free energy of th e neutral acidic form (H A)of the molecule versus its charged basic form (A-) a tth e interface, whereas the acidic form of aniline (H A+ )is charged and has a higher free energy than its neutralbasic form at the interface.Acid-Base Equilibria at a SolidjWaterInterface: Polarization of Water Molecules

Inseeking to extend ourstud ies of acid-base reactio nsand e lectrostatic potentials to other liquid interfaces,we decided to investigate the silica/water interfacebecause of i t s widespread in terest and im p ~ r t a n c e .~ ~FusedSilicaAqueousSolution

Th e acidic moieties are the silanol groups (SiOH ) ha tterm inate the silica surface. Th e approach we usedwas the same as for th e alkylphenols and alkylanilinesat t he aid wa ter interface, namely, to m easure the SHsignal as a function of bulk pH and to relate this to theratio of th e charged to the uncharged populations. Ascan be seen from Figure 5 there are two plateaus in th eSH electric fieldvs bulk pH , which suggests the presenceof two silanol sites having different pK, values. Thevery large increase ( lo2) n the SH signal (-P~O(~)sthe pH increases indicates that the nonlinear polar-izability of th e SiO- form is much greater th an t ha t ofthe neutral acidic form S O H . These various inferencesseemed just fine until we m easured the dependence of(32)Ong,S.; hao,X.;Eisenthal,K. . hem. Phys. Let t . 1992,191,327.


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640 Acc. Chem. Res., Vol. 26 , No. 12, 1993 Eisenthalfound that including the water polarization contribu-tion, which dominates the SH signal at alkaline p Hvalues, yields quantitative agreement with the elec-trolyte and tem perature depend ent results. Briefly theSH signal decreases with increasing electrolyte con-centration d ue to the more rapid decay of the electricfield (increased Debye shielding) with distance fromthe interface. Th us, fewer water molecules experiencethe polarizing electric field and th e S H decreases. T heincrease in SH w ith higher temperature is due to thedecrease in the dielectric con stant, which increases thedistance over which the electrostatic field extends.Fitting of the S H signalvs bulk pH with the polarizationmodel yielded two silanol sites, one having a pK, of 4.5occupying 20% of the sites and the other occupying80% of the sites having a pK, of 8.5. One of thepromising results of this stu dy is th at S HG can be usedto measure th e surface electrostatic potential a t chargedliquid interfaces.Polarization of Water Molecules at an Air/Charged Monolayer Interface: Measurement ofInterface Potential

The silica/water results suggest that if we had acharged monolayer a t the air/w ater interface tha t hada smaller second-order nonlinearity, then we should beable to observe the polarization of the bulk watermanifested in the x (3 ) erm and thus be able to obtainthe interface electrostatic potential. Indeed it wasfound tha t the third-order contribution ( x ( ~ ) )s amajorpart of the SH signal from the negatively chargedmon olayer, n-docosyl potassium sulfo nate , CH3-(CH2)21S03---K+, and from the positively charg edmonolayer n-docosyltrimethylammonium bromideC H ~ ( C H ~ ) ~ ~ N + ( C H ~ ) Y B ~ -t aidwater interface^.^^T h einterface electric potential was determined to extendfrom -50 mV to -265 mV for the negatively chargedmon ola yer CH ~( C H ~) Z I S O ~-nd from +50 mV to +265mV for the positively charged monolayer CH3(CH&IN+-(CH3)3depending on monolayer charge density an d bulkelectrolyte concentration.Dynamics of Molecular Adsorption to anAir/Water Interface

How long does it take a solute molecule in the bulkto diffuse to the interface, and in point of fact is theadsorption kinetics governed by diffusion? One wayto investigate such dynam ic processes a t an air/liquidinterface is to use a liquid jet. One of th e useful aspectsof a liquid jet is that the formation of the air/liquidinterface immediately after the m oving liquid exits thenozzle is probably not the equilibrium aidliquid in-terface. Th e population an d structure of the initiallyformed aidliquid interface is determined by the in-teractions of the liquid molecules with the nozzlematerial and the liquid motions involved in formingthe jet. The equilibrium aidliquid jet interface prop-erties take time to develop. Measurements of the S Hsignal a t various distances from th e jet nozzle combinedwith knowledge of the jet velocity yield the time-depe nden t change in the air/liquid interface. Using anaqueous nitrophen ol solution flowing through a glassnozzle to generate the l iquid je t , the k inet ics of

(33) Zhao, X.;Ong,S.; isenthal,K. B. h e m .Phys . Le t t . 1993,202,513.

I I I I2 4 6 6 10 12pH in th e Bulk

Figure 5. Second harmonic electric field (in arbitrary units)from the SiOdH 20 interface as a function of pH in the bulksolution. Th e dots are experimental points, while th e solid lineis the theoretical fit with constant capacitance model. Th epercentag e of site I, 6 = 20 % with pK,(l) = 4.5; he percentageof site 11,1- 6 = 80% with pK,(2) = 8.5.the SH signal on the bulk electrolyte concentration. A ta given bulk pH we would expect that the number ofSiO- moieties would increase as electrolyte co ncentra-tion increases due to the increased Debye screening.Since we inferred tha t a(,)or SiO- is greater th an SiOHfrom the higher S H signal a t high pH , we would predictthat the SH signal should increase as electrolyteincreases. However, we found th at th e S H signaldecreased as the electrolyte concentration increased.We were troubled further by the results of the tem-perature dependence of the SH signal. A t high pH(>12.5),where all (>99.9%) of th e silanols are ionized,we would predict th at the S H signal should stay thesame or decrease as the temperature was raised,depend ing on the sign of th e entha lpy change for theacid-base reaction. We find tha t the S H signal increaseswith temperature a t high pH , contrary to expectations.From the electrolyte and temperature results weconcluded tha t the SH signal could not be due simplyto the populations of the SiO -and SiO H forms. It turnsout th at th e S H is generated not only by a second-orderprocess th at depends on the populations and nonlinearpolarizabilities d2) f all of the interfacial species,a&, CY:&,gbH,but also by a third-order processarising from the very large electric field EO rising fromthe charged silica interface (SiO-),

E,, - P z = x(~)E&,E, (6 )Th e fieldEO xtends into the bulk solution and polarizesthe w ater molecules, both by a third-order nonlinearpolarizability d3 ) nd by the alignment of watermolecules th at breaks the bulk inversion symmetry.Depending on the electrolyte concentration, the fieldEo extends from tens t o hundreds of angstroms into thebulk region. T he tota l second harmonic signal is givenby

E,, a P,, =X ( 2 ) ~ p ,~+ ( o ) E , E , (7 )where +(O) is the electrostatic potential a t the interfaceand results from the integration of the third-orderpolarization Pi: from the bulk to the surface. It was


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Liquid Interfacesadsorption of nitrophenol to the air/solution je t inter-face was 0btained.3~ It was determined that a time-

Acc. Chem . Res. , Vol. 26, No. 12, 1993 64 13,3-diethyloxadicarbocyanine odide (DODCI) at theair/water interface.23 It was found th at the cis-transisomerization on th e low est excited singlet surface ofDOD CI is considerably faster a t the interface (220 ps )tha n in bulk aqueous solution (520 ps). Whether thereaction potential surface is significantly different atth e interface and thu s effects a faster barrier crossingor the anticipated lower friction at the interface ispredom inant is not clear. A result th at points to frictionbeing the key factor was the observation that thereaction a t the interface became faster when methanolwas added to the solution and adsorbed to the interfacewhereas the rate becomes slower in the bulk withincreasing me thanol c on ~ en t ra t io n . ~~he decrease inrate in th e bulk could be attrib uted to the lower polarityof methanol, which results in an increase in barrierheight, and not to an increase in viscosity since theviscosity a t high m ethanol concentrations is less tha nthe viscosity of n eat w ater. If th e lower polarity of theinterface also results in a n increased reaction barrier,then we conclude tha t it is the lower friction at theinterface th at is responsible for the faster isomerizationrate.

J e tSolution 7-------dependent Langmuir model yielded an excellent fit tothe data. Th e adsorption rate constant is 4.4 f 0.2 Xlo4s-l,and for desorption the rate co nstant is 6 f s-I.Th e adsorption free energy calculated from the kineticdata was found to be the same as tha t obtained froman equilibrium measurement of th e sta tic air/aqueousnitrophenol solution. Th is agreement supports thevalidity of the kinetic model since the free energy mustbe the same whether measured by a dynam ic methodth at yields the forward and backward rate con stants orby an equilibrium constant measurement t ha t can onlyyield the ratio of the rate constants. W ith respect tothe question of whether the kinetics was diffusioncontrolled, the answer is no. T he rate con stants werefound to be time ind ependent and considerably slowerth an th at predicted by a diffusion process. Althoughall the dynam ics up to the vicinity of the in terface mu stsurely be diffusive, it was suggested tha t an adso rptionactiva tion barrrier of a few kilocalories/mole governedthe adsorption kinetics.

PositionPhotoisomerization Dynamics

T he liquid-phase kinetics of a unimolecular reactionsuch as an excited-state structural change can bedescribed in many cases in terms of the barrierseparating the reactant and product states, and thefrictional force exerted by th e solvent as the m oleculemoves on th e reaction pote ntial surface. Experim entaland theoretical studie s of photoisomerization in bulksolution have active areas of research that seek tound erstan d th e role of th e solvent in so-called simpleunimolecular reactions.36 It is natural to extend thesestudies to iquid interfaces to determine how the uniquefeature s of th e interface asymm etry affect the b arrierto chemical reactions a nd th e friction op posing barriercrossing.Although there have been only a few studies ofphotoisomerization dynam ics at interfaces, the resultshave been markedly different from those for their bulksolution c ~ u n t e r p a r t s . ~ ~ ~ ~he first photoisomerizationstudy at a liquid interface was on the dye molecule

(34) Castro, A,; Ong, S.;Eisenthal,K. B. Chem. Phys . Let t . 1989,163,(35) For a review, see: Waldeck, D. Chem. Rev. 1991, 91,415.(36) Meech, S. R.; Yoshihara, K. Chem. Phys. Le t t . 1990,174,423-7.(37) Meech, S. R.; Yoshihara, K. J . Phys. Chem. 1990, 94, 4913-20.(38) This reference was deleted on revision.

412-6.

Molecular Rotation at an Air/Water InterfaceThe investigation of rotational motions at liquidinterfaces has been limited generally to fluorescentprobes covalently attached to insoluble long-chaina m p h i p h i l e ~ . ~ ~ ~s a result the rotational motions arehindered by the bonding of the probe to the am phiphileand do not yield the free rotational motions of thefluorescent probe. T he reason for tying th e probe toan insoluble monolayer is based on th e need to avoidthe dominating fluorescence th at would result from t heprobe bulk population. It has been possible in somecases where the fluorescent probe adsorption to th einterface was very large to use a sufficiently small bulk

population so that the bulk fluorescence could beneglected.40bUnfo rtunately th is is not th e usual case.It is possible, however, to selectively study therotational motions of interface molecules using SHGeven when there is a large population of th e m oleculesof interes t in the bulk solution. T he method was firstappl ied to Rhodamine 6G at the ai r/water in t e r f a ~ e . ~ ~A picosecond pump pulse was used to perturb theequilibrium orientational distribution by photoselectiveexcitation (Figure 6). T he time-depend ent change dueto the rotational motions of the ground and excitedRho dam ine 6G molecules moving toward the ir respec-tive equilibrium orientations, as well as the decay ofth e photoexcited m olecules, was observed by measure-

m ent of the S H signal from a probe pulse tha t was timedelayed with respect to the pum p pulse. Th e measuredSH kinetics was attributed to rotational m otions basedon th e observed polarization dependence of the kinetics.By controlling the p olarization of th e inciden t probepulse an d using an analyzer to select agiven polarizationof the SH light generated a t the interface, the kineticsof different susceptibility elements x::; were deter-(39) Sitzmann, E. V. ; Eisenthal, K. B. Unpublished results.(40) (a) Anfinrud, P. A.; Hart, . E.; Struve, W. S. J . Phys . Chem.1988,92,4067. (b) Wirth, M. J.; Burbage, J. D. J. Phys. Chem. 1992,96,9022.(41) Castro, A.; Sitzmann, E. V.; Zhang, D.; Eisenthal, K. B. J.Phys.Chem. 1991, 95, 6752.


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642 Acc. Chem. Res. , Vol. 26, No. 12, 1993 Eisenthal

probe

%I

tlinearly orcircularlypolarized

Y

surfacef lplaneFigure 6. Schematic experimental setup. Th e arrows representthe m olecular transition dipoles arranged t o a common origin.Th e inearly or circularly polarized pump disturbs th e equilibriumorientational distribution at the interface. Th e relaxation of th eperturbed distribution is monitored by observing the SH signal( 2 0 ) generated by a time-delayed probe (0).

0 1 2 3 4 5 6Time (nanoseconds)Figure 7. &L an d xt:L components of the second harmonicsignal rom the surface of a 20pM Rhodamine 6G aqueous solutionversus probe delay time.mined. They were found to differ as anticipated fororientation relaxation kinetics (Figure 7). A t the lowconcentrations used there was no change in th e kineticson doubling the concentration, indicating that thedynamics is not due to intermolecular energy transfe r.T he time scale of the rotations is in the range of severalhundred picoseconds, which appea rs to be slower thanth e 200 ps in bu lk w ater.42 However, different orien-tational moments are being probed by SH than influorescence.A model of the interface motion is needed to ex tractthe rotational constants. Unlike the bulk solution, the

(42) Berndt, K.; urr, H.; alme, D. p t . Common. 1982, 42 , 419.

rotation of the interface molecules is determined notonly by the collision-induced time-varying torques b utalso by the asymm etric force field at t he interface th atfavors certain molecular orientations. T he molecularmotions can be thou ght of as otational diffusion in anexternal force field.In addition to the apparently slower time scale forrotations, it was inferred from the SH measurementsthat the equilibrium orientation of the photoexcitedmolecules is different from that of the ground-statemolecules possibly due to th e change in dipole mom entand charge distribution. In addition, the rotationalmotions were attributed to out-of-plane molecularmotions, chiefly, i.e., motions perpendicular to theinterface plane, on the basis of th e observation tha t thekinetics was the sam e whether the pum p pulse prop-agating normal t o the interface was linearly polarizedor circularly polarized. T he linearly polarized pum plight should induce an anisotropy in the plane of theinterface whereas circularly polarized pump light wouldnot. * Th e absence of any observed difference indicatesth at th e relaxation of this in-plane anisotropy was fasterthan the picosecond time resolution or that the effectof th e anisotropy on th e nonlinear susceptibility ele-ments m easured was small. Furth er experiments onrotations a t nterfaces will hopefully clarify hese effectsand in general provide information about the orien-tational force field at an interface and th e friction a tinterfaces and its connection with a hydrodynamicdescription of molecular motions in this unique regionof the liquid.Summary

What I have tried to do in the se few pages is to kindlefurther interest and activity in the study of theequilibrium and dynam ic processes a t iquid interfaces,in par t by demonstrating t he power of second harmonicand sum frequency spectroscopy to probe these ubiq-uitous and often elusive regions of ma tter. From thestudies outlined in this Account we see that we cangain new fun dam ental information on the orientationof molecules, the ir energetics of adso rption , the ch angesin chemical equilibria at interfaces , and th e effects ofhydrogen bonding and solvation on the vibrationalspectrum, as well as hydrogen bonding and solvationeffects on the stru cture and unexpected phase transitionof molecules at interfaces. When there a re chargedspecies at the interface, the resulting polarization ofbulk water molecules can be investigated using thesemethods. T he time-dependent phenomena includingthe kinetics of adsorption a nd desorp tion, the rotationof molecules in the anisotropic field of an interface,and th e photochemistry of an excited-state isomeriza-tion can also be studied by SHG nd S FG techniques.Besides he desire of scientists to probe the fundam entallaws th at govern the intersting world of interfaces, thereis keen interest due to their important role in envi-ronm ental, medical, and technology issues. Theseinclude the effects of pollutants on the abundant lifeprocesses th at occur a t he aid wa ter interfaces of oceans,lakes, and rivers, as well as the impact of these air/water interfaces on the earths weather an d atmosphericchemistry. Similarly th e aqueous/solid interface iscrucial to our understanding the multitude of naturalphenomena that occur in the soil as well as the


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Liquid Interfacesdevelopm ent of new knowledge necessary to de al withthe massive effects of pollution in our soils and groundwaters. In my view the investigation of interfac es is inits earliest stages of development and, with t he aPPli-cation of modern spectroscopicmethods a nd theoreticalmodeling, great advances can be anticipa ted not only

Acc. Chem. Res., Vol. 26, No. 12, 1993 643in the fundamental science but in its application toimportant world problems.

M y special thanks has to go to the members of my groupand collaborators f or venturing into this exciting areastarting with our first efforts in 1986. M y special thanks toDr. J.Gutow for his comments and assistance.

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