Table Of ContentProgr Colloid Polym icS )7991( V
(cid:14)9 galreVffpoknietS 7991 PREFACE
This volume is loosely based upon presentation of these various lighting the importance of electric
papers presented and topics discussed approaches has been attempted. dipoles in the genesis of interfacial
at several recent international meet- Invaluable support for these meet- activity. Murray presents some novel
ings. The first of these meetings was ings was provided by the Division of Langmuir trough studies applied both
the international symposium on Inter- Colloid and Surface Chemistry of the to oil-water and to air-water inter-
facial Structure held in conjunction American Chemical Society, Eastman faces. Andelman and Diamant present
with the 210th National Meeting of the Kodak Company, 3M Specialty an elegant variational study of the
American Chemical Society on August Chemicals Division, the Petroleum derivation of kinetics at liquid-liquid
20-25, 1995 in Chicago. The second of Research Fund of the American interfaces, and their work will interest-
these was the 1996 Chemistry at Inter- Chemical Society, Chemical Transport ingly be compared to the experimental
faces Gordon Research Conference & Separations Division of the National study of Pitt at the end of this volume
held July 21-26, 1996 in Meriden, New Science Foundation, and the Chemis- and the lattice-gas simulations of Khan
Hampshire. Additional contributors try Division of the Army Research and Shnidman. Stauffer alludes to the
to this volume were selected because Office. This support was very helpful inclusion of amphiphiles in an Ising-
of work they presented at the llth in assisting with the travel expenses of like treatment, and this allusion is
Surfactants in Solution conference distinguished scientists from Asia, the illustrated by Khan and Shnidman
held June 9-13, 1996 in Jerusalem or middle east, and Europe, and in mak- later in this volume.
because their work was known to the ing it possible for a significant number The second section on Vesicles,
Editor. All of the papers included in of graduate students and postdoctoral Bilayers, and Membranes addresses
this volume, however, are of more research associates to attend these the most biologically relevant area for
recent vintage, and were written in the meetings. amphiphiles. Chaimovich introduces
fall and early winter of 1996. The sub- The contents of this volume have the section with an interesting survey
ject of this volume is limited in that it been distributed among six sections. of chemical factors affecting transport
addresses amphiphiles at liquid/air, The first of these sections, Liquid/ through liposomes and bilayers. Beyer
liquid/liquid, and liquid/solid inter- Liquid Interfaces, is the least well introduces a mechanism for bilayer
faces, with little attention paid to characterized interface in colloid sci- formation from micelles, and Klopfer
vapor/solid interfaces. This subject is ence. The first two papers by Brevet and Vanderlick present a molecular
an important focal point for examining and Girault and by Richmond and dynamics study illustrating how small
physical phenomena that occur in co-workers give a comprehensive nonionic amphiphilic structure can
colloidal systems such as micellar solu- review of the great progress made lead to the selection of bilayer struc-
tions, microemulsions, emulsions, recently in the application of two- ture over micellar structure. McIntosh
dispersions, slurries, etc., as well as photon processes in the analysis of and Simon present a comprehensive
flows at and wetting of macroscopic molecular orientation and conforma- review of work done on the experi-
interfaces. This volume hopefully will tion at oil-water interfaces. The paper mental characterization of fluctua-
serve to summarize our current under- by Volkov and Deamer reviews tions in lipid bilayers, and Berkowitz
standing of interfacial structure at the theoretical aspects of charge transfer and coworkers articulate various
molecular level in these systems, and at oil-water interfaces, and discusses approaches to defining the role of
the relation of this structure to chemi- experimental interfacial catalytic sys- hydration forces in the interactions of
cal and physicochemical phenomena. tems. Pohorille, Wilson, and Chipot such bilayers. Stouch reviews the key
Simulations and experiments are provide a comprehensive review of physical features affecting transport
becoming complementary approaches simulation studies of water-oil inter- and partitioning in bilayers, and
to investigating the structure of these faces and present some exciting new atomic-level molecular dynamics
interfacial systems, and a balanced results on surface segregation, high- simulations of such bilayers. Green
VI
and Lu review studies applied to trying assembled monolayers on electrodes of the minimal functional require-
to model transport through membrane derivatized with thiolates and contain- ments for such protein, with a view to
pores. ing preformed binding sites. Some- expediting the design and synthesis of
Micellar Aggregation is the focal what similarly, the paper by Baszkin less expensive replacements for
point of the third section. Care and and coworkers in the last section inves- human lung surfactant. Siepmann
coworkers lead off with a paper model- tigates the incorporation of cyclodex- reviews vapor-liquid phase equilibria
ing micellar aggregation on a lattice. trins into phospholipid monoiayers. and examines by Monte Carlo methods
They examine relatively short-chain Bizzotto and Lipkowski comprehen- the suitability of two different force
surfactants, and calculate the ther- sively review nonionic amphiphile fields in modeling such equilibria.
modynamics, shapes, and aggregate- adsorption on electrodes and show Goedel shows how hydrophobic
size distributions. Mattice and cowor- how hemimicellar and miceilar forma- polymers may be assembled at the
kers present a comprehensive review tion, adsorption, and desorption pro- air-water interface to prepare models
of their work modeling the formation cesses may be studied experimentally. of polymer "melt brushes". The
of association colloids of triblock The fifth section on Adsorption at experimental approach is viable for
copolymers. They also use a lattice Solid/Liquid Interfaces commences preparing nanometer thick coatings.
approach. Shelley, Sprik, and Klein with an important review by Thomas Baszkin and coworkers examine the
outline key issues to consider in on the resolution of surfactant struc- miscibility of two phospholipids with
molecular dynamics modeling of ture by neutron reflectivity. Manne lipid-derivatized fl-cyclodextrin and
micellization and other structured follows with a review of recent, excit- with lipid-derivatized poly(ethylene
surfactant-water phases. They find ing force microscopy results that go oxide). While Murray's paper on pro-
that inclusion of polarizability results further to upset some of the dogma on tein dynamics appears in the first sec-
in counter ions being heavily solvated, surfactant hemimicelle formation, tion, it is equally directed to air-water
but that the water-hydrocarbon inter- and yield some direct morphological interfaces, and it will be interesting
face and the electrical double layer evidence for hemimicellar structure. to see if these protein dynamics can
at the surfactant water interface is Balazs and coworkers present a self- meaningfully be treated by models
clearly distinguishable irrespective of consistent field treatment of polymers developed for more classical surfac-
polarizabilty assumptions. Mavelli tethered to solid surfaces, and explore tants. Abbott reviews recent applica-
presents an exciting new application association structures formed by these tions of electrochemically active sur-
of stochastic dynamical modeling in polymers in various solvents. Grainger factants in modifying interfacial ten-
illustrating surfactant aggregation. reviews polymeric thin film formation sion, and illustrates some exciting
Texter and coworkers present a review by adsorption from solution using electrochemical engineering in deduc-
of experimental studies directed at chemisorption and using stratified ing structural features that facilitate
examining the role of cosurfactants in polyelectrolyte layers. Khan and control of surface adsorption. This
modifying reverse micelle aggregation Shnidman present a lattice gas treat- work also defines a mechanism
and the onset of percolation processes ment of capillary dynamics, and whereby localized release of surfac-
therein. include the effects of amphiphiles. tants may be utilized in the localized
The fourth and fifth sections deal Some formal similarity with the treat- permeabilization of membranes. The
with amphiphiles at solid-liquid inter- ments of Daimant and Andelman and paper by Pitt presents exciting experi-
faces. Section four on Amph!philes at of Stauffer in the first section exist in mental results for different classes of
Electrode Surfaces contains a collec- the formalism adopted to attack these surfactants in the development of an
tion of important applications. The flow problems. Cohen and coworkers understanding of key features that
first by Rusling illustrates how surfac- present a review of small angle x-ray control dynamic interracial tension
tant films on electrode surfaces may scattering in the analysis of adsorbed and the factors important in modifying
be fabricated to electrocatalyze a vari- layers surrounding colloidal particles. the efficiency of dynamical surface
ety of reactions. Koglin and coworkers Section six is devoted to tension lowering. Correlation of these
provide some exciting new surface Amphiphiles at Vapor-Liquid Inter- data with various models put forth by
enhanced Raman spectroscopy results faces. Zasadzinski and coworkers Diamant and Andelman earlier in the
that contravene some of the reigning present a review of lung surfactant volume will help refine our perspec-
dogma on how cationic surfactants protein and the effects of such protein tive of the modeling of these systems.
assemble on anionic surfaces. Kaifer on altering phase structures in palmitic
presents a concise review of self- acid monolayers. They articulate some John Texter
Polym Colloid Progr icS )7991( VII
Steinkopff © galreV 7991
Preface ......................................... V
diuqiL/diuqiL Interfaces
R E Brevet, H. H. Girault: Optical SHG measurements of amphiphiles at liquid/liquid interfaces 1
J. C. Conboy, M. C. Messmer, An investigation of surfactant behavior at the liquid/liquid interface with
R. A. Walker, G. L. Richmond: sumfrequency vibrational spectroscopy ..................... 10
A. G. Volkov, D. W. Deamer: Redox chemistry at liquid/liquid interfaces ................... 21
A. Pohorille, M. A. Wilson, C. Chipot: Interaction of alcohols and anesthetics with the water-hexane interface: a
molecular dynamics study ............................. 29
B. S. Murray: Dynamics of proteins at air-water and oil-water interfaces using novel
Langmuir trough methods ............................. 41
H. Diamant, D. Andelman: Adsorption kinetics of surfactants at fluid-fluid interfaces ......... 51
D. Stauffer: Oil-water interfaces in the Ising-model ..................... 60
,selciseV Bilayers, dna Membranes
H. Chaimovich, I. M. Cuccovia: Quantitative analysis ofr eagent distribution and reaction rates in vesicles 67
K. Beyer: Packing and bilayer-micelle transitions in mixed surfactant-lipid systems
as studied by solid state NMR ........................... 78
K. J. Klopfer, T. K. Vanderlick: Self-assembly of volatile amphiphiles ...................... 87
T. J. Mclntosh, S. A. Simon: Experimental tests for thermally-induced fluctuations in lipid bilayers 95
L. Perera, U. Essmann, The role ofw ater in the hydration force - molecular dynamics simulations 107
M. L. Berkowitz:
T. R. Stouch: Solute transport and partitioning in lipid bilayers: molecular dynamics
simulations ....................................... 116
J. Lu, M. E. Green: Simulation ofw ater in a pore with charges: application to a gating mecha-
nism for ion channels ................................ 121
Micellar Aggregation
C. M. Care,T. Dalby, J-C. Desplat: Micelle formation in a lattice model of an amphiphile and solvent mixture 130
M. Nguyen-Misra, S. Misra, Y. Wang, Simulation of self-assembly in solution by triblock copolymers with sticky
K. Rodrigues, W. L. Mattice: blocks at their ends .................................. 138
J. C. Shelley, M. Sprik, M. L. Klein: Structure and electrostatics of the surfactant-water interface ....... 146
E Mavelli: Stochastic simulations of surfactant aggregation kinetics .......... 155
J. Texter, B. Antalek, E. Garcia, Cosurfactant facilitated transport in reverse microemulsions ....... 160
A. J. Williams:
selihpihpmA at Surfaces Electrode
J. E Rusling: Catalytically active, ordered films of proteins, surfactants and polyelectro-
lytes on electrodes .................................. 170
VIII
A. Tarazona, S. Kreisig, E. Koglin, Adsorption properties of two cationic surfactant classes on silver surfaces
M. J. Schwuger: studied by means of SERS spectroscopy and ab initio calculations .... 181
A. E. Kaifer: Electrodes derivatized with mono- and multilayer assemblies containing
preformed binding sites ............................... 193
D. Bizzotto, J. Lipkowski: Amphiphiles at electrified interfaces ...................... 201
noitprosdA at Interfaces Solid/Liquid
R. K. Thomas: Neutron reflection from surfactants adsorbed at the solid/liquid interface 216
S. Manne: Visualizing self-assembly: force microscopy of ionic surfactant aggregates
at solid-liquid interfaces ............................... 226
A. C. Balazs, C. Singh, E. Zhulina, Forming patterned films with tethered polymers ............... 234
D. Gersappe, G. Pickett:
D. .W Grainger: Synthetic polymer ultrathin films for modifying surface properties .... 243
A. A. Khan, Y. Shnidman: Molecular mean-field models of capillary dynamics ............. 251
H. Bianco,Y. Cohen, M. Narkis: Probing the structure of inhomogeneous colloidal particles by small-angle
x-ray scattering .................................... 261
selihpihpmA at diuqiL-ropaV Interfaces
M. M. Lipp, K.Y.C. Lee, Protein and lipid interactions in lung surfactant monolayers ........ 268
J. A. Zasadzinski, A. J. Waring:
J. I. Siepmann: Monte Carlo calculations, for vapor-liquid phase equilibria in Langmuir
monolayers ....................................... 280
.W A. Goedel: Hydrophobic polymers tethered to the water surface ............ 286
.V Rosilio, A. Kasselouri, G. Albrecht, The effect of the chemical nature of grafted chains on the interfacial mis-
A. Baszkin: cibility of amphiphiles ................................ 294
N. L. Abbott: Active control of interfacial properties of aqueous solutions using ferro-
cenyl surfactants ................................... 300
A. R. Pitt: The efficiency of dynamic surface tension reductions within homologous
series of surfactants in aqueous gelatin solution ................ 307
S. Boussaad, R. M. Leblanc: Pure and mixed chlorophyll a Langmuir and Langmuir-Blodgett films:
Structure, electrical and optical properties ................... 318
rohtuA Index ...................................... 327
Subject Index ...................................... 327
Progr diolloC Polym icS )7991( 9-1:301
(cid:14)9 galreVffpoknietS 7991
Optical GHS measurements
P.F. Brevet
H.H. Girault of amphiphiles at liquid/liquid
interfaces
Abstract Surface second harmonic properties of the amphiphile
:devieceR 6 rebmeceD 6991
generation (SSHG) is described as monolayers are shown to principally
:detpeccA 21 rebmeceD 6991
a powerful surface tool to investigate depend on the hydrophobic-hydro-
amphiphiles at liquid/liquid inter- philic forces. Photoisomerization
faces. In particular, the molecular experiments are reported and they
picture of the interface which is show that the interfacial molecular
retrieved from these measurements is friction si dramatically different from
emphasized. In a first part, a theoret- the friction found in bulk aqueous
ical analysis of the origin of the SH solution. Chemical reactions, and in
signal from these systems is discussed. particular acid/base equilibria, are
The dipole electric contribution, also discussed and the role of the
which is highly surface specific, is external potential at polarized
shown to be the dominant one at interfaces in the apparent pKa value
liquid/liquid interfaces, the volume measured is underlined. Finally,
contributions of higher orders in the charge-transfer reactions at polarized
multiple expansion of the nonlinear liquid/liquid interfaces are presented.
polarization being overwhelming at
.rD P.F. teverB (N~). H.H. Girault pure solvant air/liquid interfaces only. Key words ITIES SSHG
eriotarobaL eimihcortcelE'd In the second part, the experimental amphiphiles - photochemistry -
elocE euqinhcetyloP
results available to date are reported. surface chemistry
elar6d6F ed ennasuaL
5101 ,ennasuaL dnalreztiwS The orientation and solvation
may be used as mimetic models for biological cell mem-
noitcudortnI
branes 6.
Second-harmonic generation is a nonlinear optical
The study of surfaces and interfaces has always been ham- process through which two photons at a fundamental
pered by the search of appropriate experimental tools. frequency are converted to one photon at twice the funda-
Optical surface second-harmonic generation (SSHG) has mental frequency. In the electric dipole approximation,
been shown to be a powerful method and has thus been this process is forbidden in media with inversion symmetry
widely applied over the last decades in surface science. and therefore only occurs at interfaces where the inversion
Phenomena like adsorption processes, surface reconstruc- symmetry is broken. As a main drawback to such a surface
tion or interracial chemical reactivity have been investi- specificity, the process is very weak, its efficiency being in
gated at both solid and liquid surfaces and have already the range of 10-12%. However, with the availability of
been extensiVely reviewed in the past -1-5. At liquid/ high intensity lasers and highly sensitive detectors, signal
liquid interfaces experimental results are far less numer- levels of few photons per pulse are routinely detected in
ous, despite the growing interest for these interfaces which laboratoriesl The suitability of the technique to the study
2 P.F. Brevet and H.H. Girault
Optical SHG measurements of selihpihpma
of liquid surfaces has been recognized for a long time, since measurement of this dielectric constant and this question
the experiments of C.C. Wang at the air/water interface 7 has been discussed in detail by Zhang et al. 11. The
but its use to liquid/liquid interfaces, and in particular at phenomenological approach, first suggested by Mizrahi
interfaces between two immiscible electrolyte solutions and Sipe 12, 13 and recently reformulated for a three-
(ITIES), has only developed in the last ten years 6. layer model ,41-, has been derived from Heinz's model, the
In this review, we focus on the recent advances of sheet of nonlinear polarization being embedded in a thin
SSHG at liquid/liquid interfaces. We first present the strata of linear material which is in turn taken between
theoretical background with the fundamental equations two-half spaces of linear optical dielectric media. In this
necessary to analyse the data. Then, we discuss the origin model, the SH waves generated by the sheet of nonlinear
of the SH response at liquid/liquid interfaces before pre- polarization within the thin linear slab are transmitted to
senting a review on experimental results at liquid/liquid either half space through Fresnel coefficients. Hence, the
interfaces. We first describe the structure of amphiphile SH intensity If is given by 14, 15
monolayers at free interfaces and ITIES, in particular,
the interracial orientation and solvation properties of 2)o Re )~i~/x( )2( 2~O%e
If l "?e ~oZ
the monolayers. We then present dynamics studies = : (1~o)2 ,
C0~4 3 Re2(x/~l ~6cos Oal 2
like isomerization processes, chemical equilibria and
(1)
finally charge-transfer reactions across liquid/liquid
interfaces.
where I~' is the intensity of the incoming fundamental
wave, ef the optical dielectric constant of medium i at
frequency 2f = )o2 and 0 ~ the angle of refraction of the
ehT HS esnopser from diuqil/diuqil interfaces
harmonic wave within the thin slab containing the nonlin-
ear polarization sheet and of optical dielectric constant e a.
The question of the origin of the nonlinear optical re-
Equation )1( is given in SI units where the surface suscepti-
sponse of an interface has been described with two models
bility tensor has the units of m V-2. Also, we have
in the past: the model of the thick slab and the model of the
nonlinear polarization sheet. The two models yield similar e~ "~e,z" .(2). O~eO,e = a~lxe~,xxz (2) sin 72 sin F
results although the model of the nonlinear polarization
sheet, owing to its phenomenological aspect, is usually (2) (2) (2) 2
+ (ai2Zeff, xx z + ai3Zeff, zx x + ai4zeff, ZZZ) SOC ? COS/~
preferred.
The first model, which we call the model of the thick (2) (2)
+ ai5 ,ffe~) zxx sin2 7 COS/"
slab, appeared with the early developments of nonlinear
optics. Indeed, in the paper of Bloembergen and Pershan where the a u, i = ,1 2 and j = 1 ... 5 are the geometrical
in 1968 8, a section was already devoted to the problem factors embedding both the Fresnel coefficients and the
of SH response from a thick nonlinear slab embedded in angular dependence 153. The angles ~/and F are, respec-
a linear optical medium. Here it is understood that the tively, the angle of polarization of the incoming funda-
thickness of the slab is larger than the optical wavelength. mental and outgoing harmonic waves. Finally, we add that
From the equations derived for the SH field amplitude i= 1 in reflection and i=2 in transmission. The SH re-
transmitted in the linear media, the limiting case of a van- sponse of a liquid interface, owing to its in-plane isotropy,
ishing thickness yielded results applicable to the SH re- has thus a characteristic shape as shown in Fig. .1 From
sponse at the boundary between two centrosymmetric the curve obtained for the harmonic light S-polarized, one
media. The problem was soon revisited from another point usually extracts the component Z~,xx~ (2) _ -, whereas from the
of view. Since the SH generation is expected to arise from curve obtained for the harmonic light P-polarized, one
a very thin strata of material, thin as compared to the extracts the components Z(~2)zxx )2(.zzz,~x*dna If the S-polar-
wavelength of light, the nonlinear polarization induced by ized curve has always a similar shape owing to the factor
the fundamental wave may be taken as a sheet of vanishing sin 2y, the P-polarized one may take a different shape
thickness. In this case, the mathematical description for depending on the relative weight of the three components
the nonlinear polarization is a Dirac delta function in the of the tensor 16. In Eq. (2), we have used an effective
direction normal to the boundary surface. Heinz and co- tensor. Indeed, it has been shown that at air/liquid interfa-
workers first used this approach and their results com- ces, and the air/water interface in particular, contributions
pared well with the previous model 9. One of the major from the next order in the multipole expansion of the
issues which quickly arises is then the value of the optical nonlinear polarization could overwhelm the electric dipole
dielectric constant of the nonlinear layer. Guyot-Sionnest surface sensitive contribution -17, 18. These contribu-
et al. 10 have stressed the need for an independent tions arise from the strong field gradients present at the
Progr Colloid Polym icS )7991( 1:301 9 3
(cid:14)9 Steinkopff Verlag 7991
ethane and air/hexane interfaces and the values of the
1.0--
BInllln|l I
three components of the macroscopic susceptibility tensor
0.8- lm I compared to their counterparts at the water/1,2-dichloro-
I I
ethane and water/hexane interfaces 17. It is then demon-
0.6 strated that the SH signal from air/liquid interfaces is
gl overwhelmingly of electric quadrupole origin, and there-
0.4
fore nonspecific to the surface, whereas the response from
liquid/liquid interfaces is mainly attributed to the surface
0.2
specific dipole electric contribution. The main reason lies
in the weakness of the hyperpolarisability tensor of the
0.0
I I I I solvant molecules, and subsequently, in the weakness of
0 20 40 60 80 the surface electric dipole susceptibility tensor. Hence,
Input noitasiraloP Angle )seergeD(
terms from higher orders in the multipole expansion have
to be included. The two main volume contributions are
Fig. 1 Theoretical curve, sa calculated from .qE ,)2( for an air/liquid
or a liquid/liquid interface for the S-Polarized (solid curve) and the thus arising from the electric field gradient present at the
P-polarized (dotted curve) SH wave in reflection as a function of the interface owing to the strong mismatch of the optical
input polarization angle dielectric constants between air and the liquid phase and
to the gradient of the electric quadrupole susceptibility
interface and the electric polarization takes the following tensor across the air/liquid interface. At liquid/liquid in-
form: terfaces, the mismatch of the optical dielectric constants
and of the electric quadrupole susceptibility tensors is
dramatically reduced and the SH signal is again surface
P(e) = (cid:1)89 )2(Z(og : 671~~E - "V Q(2) + ~#. V x M(2)), )3(
specific. However, in this case, the signal levels are rather
10)
weak. A second survey of bare liquid/liquid interfaces has
where Q(2) is the second-order electric quadrupole tensor
been performed at the n-alkane/water interfaces, for the
and M )2( the second-order magnetization tensor. At
alkanes ranging from heptane to decane 19. In order to
liquid/liquid interfaces however, the volume contributions
avoid the problem of weak signal levels, the experiment
cancel owing to the better matching of the optical dielec-
has been conducted in the total internal reflection mode
tric constants of the two media and the SSHG technique is
(TIR) thus dramatically enhancing the signal. In this study,
again surface specific. This is an interesting feature which
based on the magnitude of the ratio between the two
allows very low surface coverages to be studied. The com- components Z(e2~), XXZ and Ze~,Z X)~2 ( . , a close correlation is
plete expression for the three nonvanishing independent
made between the violation of Kleinman symmetry rule,
tensor elements are thus 15: i.e. the loss of the equality between )2( ~and ,ffeZ )2( zx:~ .,and
)~eff, XX,~
(2) (2) (2) . (2) the molecular order at the interface. It is thus suggested
eff, XXZ = Zs,XXZ "~ ZQZ,XZXZ -- )~QI,XZXZ ' that the odd n-alkanes/water interfaces are less ordered
bil, .viuqe than the even n-alkanes/water ones, the ratio between
)2( ., )2( +~tTil equiv "+
)~eff, ZX:~ = )~s,ZXX 7i2 ! )2(( and Z~2),zxx being around 0.1 for octane and decane
eft, XXZ
and 1.3-1.5 for heptane and nonane. It is interesting to
.(2) -- X(2)
note here that the value for this ratio is also found to be 0.1
~- .'I~Q2,ZZXX QI,ZZXX '
(bi2 .viuqev(~3ialib
for hexane 17. In this work, the alternation between the
/~(2) (2) 7equiv.) odd and even alkanes for the ratio between the elements
eff, ZZZ ~ )~s,ZZL -}- ---- ~,til -rJ" i2
,k ai5 ai4aisJ ,rfe~) (2) ~XX - and ,uoX )2( z,~ _ .. is correlated with a similar alternation
.(2) .(2) found in the heats of fusion.
)4(
~" .~Q2,ZZZZ -- ~QI,ZZZZ , Before closing this section, the case of polarized interfa-
where bu, ! = ,1 2 and j = ,1 2 are geometrical factors and ces has to be introduced since SSHG at interfaces between
,.v~q~7 .~,uq~7 two parameters related to the quadrupole two immiscible electrolyte solutions (ITIES) constitues
electric susceptibility tensor. one of the main trends of nonlinear optical applications at
Several experiments have been devoted to the study of liquid/liquid interfaces. It has been shown for metals, that
the structure of bare liquid/liquid interfaces. As shown upon polarization by an externally applied electric poten-
above, diffe, rent contributions may interfere in the SH tial, a specific SH response was generated from the coup-
generation and a careful analysis is required in order to ling between the static dc-field established across the
assess the surface specificity of the technique. Such a work interface and the fundamental electromagnetic wave 20.
has been carried out at the air/water, air/1,2-dichloro- The main property of this contribution is that it evolves
4 P.F. Brevet and H.H. Girault
Optical SHG measurements of selihpihpma
with the square of the static applied dc-field. At 'zo
O zO
metal/electrolyte interfaces, this dc-field may be quite large
since on the metal side, the potential drop is restricted to
a very thin layer of a few atomic units at the surface. This
property has been used to study adsorption processes, for
example 21. On the contrary, in electrolytes the space
charge region may be much wider, in the range of tens of
nanometers, owing to the reduced number of charges
available in the solution. To date, this contribution to the
HO YO
total SH response has not been directly observed at polar-
ized liquid/liquid interfaces yet 22, only at the air/water
interface in presence of charged monolayers 23. This
question is of great importance to SSHG studies since the
inversion symmetry may be broken within the diffuse
Fig. 2 Euler angles ,~q( 0 and )p~ gnicnerefer the molecular frame, with
layer, hence the region probed with the technique may the zO axis taken along the molecular axis, to the laboratory dexif
expand further towards the bulk solutions. frame where the OZ axis si along the surface normal
factor is often omitted. It has also been demonstrated that
Orientation and solvation of amphiphiles
it would only lead to a rescaling of the susceptibility tensor
at liquid/liquid interfaces
~X )2 in isotropic media. Equation )5( is only tractable when
the molecular symmetry strongly reduces the number of
The study of amphiphile ordering at interfaces is necessary
independent nonvanishing components of the molecular
to understand many phenomena, like microemulsions,
hyperpolarizability tensor. In many instances, the sym-
foams or interracial reactivity. It is expected that the pref-
metry point group of the amphiphile may be reduced to
erential orientation taken by these compounds at interfa-
the v2C point group, hence reducing the number of inde-
ces is entirely determined by their interactions with the
pendent nonvanishing tensor elements to only three,
two solvants forming the interface and the intermolecular
namely fl .... fl=x and fl=x. Assuming a random distribu-
repulsion or attraction within the monolayer. As men-
tion for the angle 5q then yields:
tioned above, the SH response at liquid/liquid interfaces is
dominated by electric dipole contributions and is therefore )2( sN
Zs.xxz = ~ ( sinz 0 cos O)fi=z - (sin 2 ~/cos 0 sin 2 0)
surface specific. Neglecting the contribution from the sol-
vant molecules, which usually only have a weak nonlinear
x (fi=x +2fl=x) + (cos 0){/=z , (7a)
optical activity, the passage from the macroscopic suscep-
tibility tensor )2~(; to the microscopic molecular hyper-
)2( sN
polarizability p of the adsorbate is obtained by merely Zs,zxx = ~ ( sine 0 cos O)fl=~ - (sin 2 ~ cos 0 sin e 0)
taking the SHG response of the amphiphile monolayer as
the superposition of the contribution from each single x (fi=~ +2fix=) + <cos O)fiz= , (7b)
moiety. Hence, it yields
)2(~) sN (cos3 0)fi= ~ + (sin 2 0 cos 0 sin 2 0)
)2() ~N L(2) s,ZZZ -- g0
s = -- (T)p, )5(
Og x (/3=~ + 2fl=x) . (7c)
where ~N is the number of molecules per unit surface and
The extraction of the molecular parameters requires an
T the tensor referencing the adsorbate in the fixed lab-
assumption on the orientation distribution for the 0 angle
oratory frame with the three Euler angles ,Sq 0 and 0, see
and on the relative magnitude of the different elements of
Fig. .2 Here, the brackets emphasize the averaging over the
the molecular hyperpolarizability. This is achieved with
different molecular orientations. L )2( is the local field cor-
the knowledge of the symmetry of the molecular electronic
rection factor and its expression is
transitions lying in the vicinity of the fundamental and the
harmonic wavelengths. Finally, working with relative in-
tensities on the macroscopic susceptibility tensor, one can
extract the orientation angle ,0 usually determined for
in the Lorentz model. In liquids, where optical dielectric a narrow distribution, and the ratio of the two dominant
constants are not too large and isotropic, this correction hyperpolarizability tensor elements.
rgorP Colloid Polym icS )7991( 9-1:301 5
(cid:14)9 galreVffpoknietS 7991
The first determination of a molecular orientation ap- than the one observed in monolayers where tail-tail inter-
peared in 1983 for p-nitrobenzoic acid at the air/silica and actions dominate. In particular, no two-dimensional phase
ethanol/silica interfaces 24 but only in 1988 at the transitions can occur, like in monolayers of pentadecanoic
liquid/liquid interface for sodium 1-dodecylnaphtalene-4- acid at the air/water interface 27. Similar studies have
sulfonate (SDNS) at the octane/water and the carbon been conducted at the polarized water/1,2-dichloroethane
tetrachloride/water interfaces 25. The fundamental interface. The orientation angle of 4-octyl-oxybenzoic acid
wavelength of 235 nm was used and the harmonic light ,)ABO( n-octyl-4-hydroxybenzoate (OHB) and 4-(4'-dodecyl-
collected at 662 nm. At these wavelengths, it was noted oxyazobenzene),benzoic acid (DBA) were thus reported in
that only one molecular tensor element was dominant, a series of experiments by Corn's group 28-30 and the
namely fi .... the element along the molecular C2-axis. The measured values were 43 ~ 40 ~ and 92 ~ for OBA, OHB and
orientation angle of SDNS was measured at 12 ~ at the DBA, respectively. The difference in the orientation angle
decane/water interface and at 83 ~ at the water/carbon of these compounds stems from the different hydrophilic-
tetrachloride interface, the hydrophilic sulfonate head ity and hydrophobicity of the groups on both sides of the
pointing into the aqueous phase. Such a considerable nonlinear active chromophore, the benzene ring for OBA
change was attributed to the interactions between the and OHB and the azobenzene group for DBA, as already
organic solvant and SDNS. The solvant is screening the mentioned, at free interfaces. Also, no reorientation was
tail-tail interactions within the monolayer and conse- observed as the applied potential between the two phases
quently relaxing the constraints on the orientation of was swept, indicating that the strength of these adsor-
SDNS. Also, the larger angle found at the water/carbon bate-solvant interactions are much larger that the elec-
tetrachloride interface was correlated with the decrease in trostatic interactions.
the surface tension as compared with the water/decane SSHG may also be used as a tool to study adsorption
interface and the question of the influence of the sharpness processes. From Eq. (5), we note that the susceptibility
of the interface on the orientation of the amphiphile raised. tensor si proportional to the number of molecules per unit
The screening effect of the intermolecuar interactions with- surface. A simple analysis of the SH intensity as a function
in the monolayer by the solvant has been studied in greater of the bulk solution concentration thus leads to adsorption
detail for several phenol derivatives 16. In this series of isotherms. This has been performed at the air/water inter-
experiments, p-nitrophenol, phenol and p-propylphenol face for phenol or SO2, for example 31, 32, but the SH
were studied at the hexane/water interface and the results intensity may be monitored as a function of the applied
compared with similar data obtained at the air/water potential across the interface. These measurements have
interface. All experiments were performed at full mono- been performed for both ONS and DBA, yielding the
layer coverage and showed that the hexane phase was surface coverage as a function of potential. In the case of
screening the interaction between the aqueous phase and ONS, the increase in the surface coverage has also been
the substituent group placed in the para position: the nitro correlated with the drop in the surface tension 28. An
group in p-nitrophenol, the hydrogen atom in phenol and expression for the surface coverage may be found using the
the propyl group in p-propylphenol. Indeed, at the Frumkin isotherm. It yields:
air/water interface, the three moieties take an angle deter-
mined by |he hydrophilicity strength of the substituent ln(l_~ ) =ln(aoNs~ AG O bF cO
\aDcE/ --R-T- + wb~(~-~ -q~o) -~, )8(
group, namely 84 ~ for the p-nitrophenol, 34 ~ for phenol
and 93 ~ for the p-propylphenol. At the hexane/water inter- where O si the surface coverage, wb~ - 0b~ is the potential
face, the organic phase was shown to completely screen drop across the interface, AG o is the free energy of adsorp-
this interaclfion, the angle of orientation being measured at tion when wSq = ,0b~ F si Faraday's constant, sNoa and ECDa
34 ~ for the three compounds. It was also noted that the are, respectively, the activities of ONS in the organic phase
hexane phase allowed more flexibility for the most hydro- and the organic solvent itself. The parameter b gives the
phobic group, as seen from the larger angle taken by portion of the electric potential felt by ONS and c si a Frum-
p-propylphenol at the hexane/water interface as compared kin interaction parameter. Hence, in Eq. ,)8( b provides
to the air/water interface. These results were in complete information on the position of ONS relative to the interface.
agreement with the ones reported for SDNS. Comparison In these measurements, the value of b = 0.67 was determined
with surface tension measurements then led to the con- indicating an intermediate position for ONS, neither com-
clusion that the surface coverage at full monolayer was pletely in the aqueous phase nor in the organic phase.
completely determined by the angle taken by the com- Solvation of amphiphiles is of great interest since it
pounds at the interface. For these small compounds, the directly gives insight into the molecular environment of
angle has been found to be invariant with the surface the molecules. Depending on the position of the moiety
coverage 26. This behavior is thus completely different along the direction normal to the interface, the molecule
6 .F.P teverB dna H.H. tluariG
lacitpO GHS stnemerusaem of selihpihpma
will undergo a transition from solvation in one bulk solu- order to understand the processes ocurring at these inter-
tion to the other. One way of determining the molecular faces, an insight into the solvant properties like surface
environment is to use the potential-dependent adsorption roughness, capillary waves or molecular friction are neces-
properties of amphiphiles as explained above for the case sary. Different types of motion may then be studied, like
of ONS. However, a more direct method consists in re- orientation relaxation or isomerization. The experimental
cording the absorption spectrum of the molecules. Mol- procedure si rather similar in both cases. For photo-
ecules with large solvatochromic shifts present different isomerization studies, a first laser pulse photoexcites the
absorption maxima, depending on the solvent polarity. At probe molecule from its ground state to one of its excited
the interface, it is known that the absorption maximum state. The molecule then undergoes motion along the reac-
takes a different value from that in the bulk, owing to the tion coordinate, involving torsion of part of the molecule,
modified solvation experienced by adsorbates in this re- before relaxing to the ground state through internal con-
gion 16. Such measurements can be conducted at version. A second pulse, delayed from the first one with an
liquid/liquid interfaces where a full spectrum is recorded optical delay path, then probes the molecule by SSHG as it
as a function of the fundamental wavelength. Another way relaxes.
to obtain the information is to compare the ratio of the The first experiments on photoisomerization at liquid/
two dominant elements of the hyperpolarizability tensor at liquid interfaces were reported by .A Shiet al. 36 for
different interfaces. This latter method was performed at Malachite Green. In this work, a femtosecond laser with
the heptane/water interface for p-nitrophenol and at the pulses of 031 sf duration is used to sample the Malachite
hexane/water interface for p-nitrophenol, phenol and p- Green isomerization at the different aqueous interfaces.
propylphenol, the data being compared with the air/water The relaxation of Malachite Green after photoexcitation is
interface in both cases. A change in the ratio of the two measured to be 0.2 ps at the air/water interface, a signifi-
dominant elements is attributed to a change in solvation. cantly shorter time scale than at liquid/liquid interfaces.
At the heptane/water interface, it was found that the At the octane/water interface, the relaxation time is found
solvation of p-nitrophenol was changing whereas at the to be 0.3 and 6.3 ps at the pentadecane/water interface. In
hexane/water interface this was surprisingly not the case. bulk water, where the shortest time scale is found, a value
Owing to the hydrophilicity of the nitro group, the moiety of 7.0 ps is measured. These results suggest that the Mala-
lies flat at the air/aqueous interface and therefore the chite Green photoisomerization process occurs through
benzene ring is highly solvated. The presence of the or- twisting of the two dimethylaniline groups projecting into
ganic phase does not perturb the hydration at the the water phase whereas the rotation around the axis
hexane/water interface and the change observed at the going through the central carbon and the phenyl ring is
heptane/water interface may stem from the structure of the not significant (see Fig. .)3 The slower isomerization time
interface itself 19, 33. On the contrary, for the aromatic obtained at liquid/liquid interfaces si therefore consistent
ring of phenol and p-propylphenol, it is found to clearly lie with an increased structuring of the water phase at the
out of the water phase and thus to undergo a change interface as compared to the bulk water phase. However,
of solvation when going from the air/water to the one must be cautious in generalizing these results. It is
hexane/water interface 16. Molecular dynamics simula- indeed known that the photoisomerization of the dye
tions of phenol and p-pentylphenol at the liquid/liquid 3,3'-diethyloxadicarbocyanine iodide (DODCI) is faster at
interface have yielded similar results, both for the orienta-
tion angle and the solvation properties ,43-1 35. It is im-
portant to note here, that SHG is sensitive to electronic .giF 3 citamehcS fo eht noitaziremozi ssecorp ni etihcalaM neerG
transitions and therefore only to a restricted portion of the at eht retaw/enakla .ecafretni ehT lynehp yteiom sedurtorp into eht
nonlinear active molecules. In the case of long alkyl chain enakla esahp saerehw eht negortin gniniatnoc spuorg era no eht
suoeuqa .edis From eht latnemirepxe ,stluser eht lanoisrot noitom
molecules, this implies that the carbon chains are not
si enogrednu along eht owt sworra
probed and therefore may take a different angle from the
one measured for the nonlinear chromophore. An illustra-
tion of this question may be found in molecular dynamics
calculations performed for p-pentylphenol 35.
enaklA esahP
/
scimanyD at liquid/liquid interfaces
We have described above the orientation and the solvation
of amphiphiles at liquid/liquid interfaces. However, in 2)3HC(N 2)3HC(N
