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Langmuir 1991, 7, 1222-1224
Probing the Structure of the Adsorption Layer of Soluble Amphiphilic Molecules at the Air/Water Interface Viola Vogel, C. S. Mullin, and Y. R. Shen’ Department of Physics, University of California, and Center for Advanced Materials, Lawrence Berkeley Laboratory, Berkeley, California 94720 Received August 2,1990.I n Final Form: November 14,1990 Optical second harmonic generation (SHG)together with surface tension measurements are used to investigate soluble amphiphilic molecules adsorbed at the air/water interface from solution. It is found that the adsorbed molecules form a single polar-oriented monolayer which, at saturation, resembles a close-packedmonolayer of insolublemolecules. SHG is particularlysensitiveto the noninversionsymmetry of the interfacial region and therefore provides an inherent discrimination against isotropically oriented molecules in the subsurface and the solution. Soaps have been used since ancient times to reduce the surface tension of water by adsorption of soap molecules from the bulk to the air/water interface. Only in this century have investigations on the composition and structure of such an interfacial layer of adsorbed molecules been undertaken. Most experiments aimed at finding the number of excess molecules in the interfacial layer. It was often deduced from surface pressure measurements with the thermodynamic analysis introduced by Gibbs (see e.g. refs 1and 2). The first attempt to directly measure the excess molecules was by the microtome method. It measures the weight of a uniform surface layer 0.1 mm thick sliced off from the top of the liquid. The weight increase due to the adsorbed molecular layer, and hence the difference in concentration between the interfacial layer and the bulk, is then determined.3 Later, radiotracer techniques were applied to study the dynamics and equilibrium of surface a d s o r p t i ~ n . ~However, * ~ * ~ ~ neither of these methods provides information about the distribution of excess molecules along the surface normal and their polar-ordering. Accordingly, the following question remains unanswered: Do the excess molecules a t the interface form a single monomolecular layer of totally or partially polar-oriented molecules, or does the interfacial layer also contain a subsurface layer with totally or partially polar oriented solute molecules? Recently, techniques more sensitive to the surface molecular orientation, such as surface potentialgJO light reflection spectroscopy,ll neutron specular reflection,12 and optical second harmonic generation,I3-l6 have been (1) Davies, J. T.; Rideal, E. K. In Interfacial Phenomena, Academic Press: New York, 1963; Chapter 4, pp 154-216, and references therein. (2) Chattoraj,D. K.; Birdi, K. S. In Adsorption and the Gibbs Surface Excess: Plenum Press: New York, 1984; Chapter 3, _pp _ 39-82, and references therein. (3) McBain, W.; Swain, R. C. Proc. R. SOC.London, A 1936,154,608. (4) Salley, D. J.; Weith, A. J., Jr.; Argyle, A. A.; Dixon, J. K. Proc. R. SOC.London, A 1960,203,42. (5) Nielsson, G. J. Phys. Chem. 1957,611, 1135. (6) Matuura, R.; Kimizuka, H.; Miyamato, S.; Shimazowa, R. Bull. Chem. SOC.Jpn. 1968,31,532. (7) Tajima, K.: Muramatsu,.M.;. Sasaki.T. Bull. Chem. SOC. J m .1970, 43, 1991.- . . (8) Tajima, K. Bull. Chem. SOC. Jpn. 1970,43, 3063. (9) Paluch, M.; Filek, M. J. Colloid Interface Sci. 1980, 73, 283. (10) Dynarowicz, P. Colloids Surf. 1989, 42, 39. (11) Kozarac, Z.; Dhathathreyan, A.; Mebius, D. Colloid Polym. Sci. 1989,267, 722. (12) Lee,E. M.;Thomas, R. K.; Penfold,J.; Ward, R. C. J. Phys. Chem. 1989.93. 381. (13) L i n g , T.; Shen, Y. R.; Kim, M. W.; Valint, P.; Bock, J. Phys. Rev. A: Gen. Phys. 1986,31,537.
0743-7463/91/2407-1222$02.50/0
used to study surface adsorption of amphiphilic molecules with charged and uncharged head groups from aqueous solution. Aside from the above-mentioned question, there exists the controversy of whether the total number of excess molecules of a saturated adsorbate layer of soluble surfactants is the same: 1arger,13J7or smaller7m8incomparison with that of a close-packed insoluble monolayer of homologous molecules. This controversy arises partially from the difficulty in measuring the absolute number of soluble molecules adsorbed to the interface. The aim of this paper is to answer more definitely the questions raised above. We chose to use optical second harmonic generation (SHG) as the technique to study and compare surface molecular layers formed at the air/water interfaces by soluble and insoluble sodium naphthalenesulfonate molecules, CnHzn+l-CloH8-S03Na (C,NS) with n = 6 and 18,respectively. The SHG process is forbidden in a bulk with inversion symmetry and is therefore highly surface specific. It is sensitive to the polar ordering and arrangement of molecules in a surface or interfacial layer and can be used to probe the total number of polar-oriented molecules in that layer. SHG as a surface analytical tool has been described elsewhere.l8Jg The SH response to the adsorption of molecules at an interface aJlows us to deduce the nonlinear susceptibility tensor x@)(20) for the adsorbed molecular layer. As we shall see, for the surfactant molecules studied in our experiment, we have found that )(’; = N,(& (1) where N, is the surface density and is the orientationaveraged second-order nonlinear polarizability of the ad%orbedmolecules. The various nonvanishing elements of x ( ~can ) in principle be obtained from SHG measurements with different input-output polarization combinations. The ratio of these elements yields informatip about the average orientation of the molecules. If (8) is known, then x ( ~is) a direct measure of the surface density N,. SHG is used here to study C6NS and Cl8NS adsorbed at the airlwater interface. In the presence of excess salt,
(B)
(14) Hicks, J. M.; Kemnitz, K.; Eisenthal, K. B.; Heinz, T. F. J. Phys. Chem. 1986,90, 560. (15) Bhattachacharyya, K.; Castro, A.; Sitzmann,E. V.; Eisenthal, K. B. J . Chem. Phys. 1988,89, 3376. (16) Zhao, X.; Goh, M. C.; Eisenthal, K. B. J. Phys. Chem. 1990,94, 2222. (17) Defay, R.; Prigogine, I. Trans. Faraday SOC. 1950,46, 199. (18) Shen, Y. R. In The Principles of Nonlinear Optics; John Wiley & Sons: New York, 1984. (19) Shen, Y. R.Nature 1989,337,519. Annu.Reu.Phys. Chem. 1989, 40, 327, and references therein.
0 1991 American Chemical Society
Langmuir, Vol. 7, No. 6,1991 1223
Structure of Soluble Amphiphilic Molecules 31
..
I
1-
2-
1' V
0
1
2
Surface Density N, [nm**]
Figure 1. Second-ordersusceptibility Xpm") versus the surface density of insoluble C18NS molecules at the air/water interface (0.35 M NaCl solution with pH = 5.6 at 20 "C). The monolayer is close-packedat the surface densityof 2.7 moleculea/nm2(=0.36 nm* molecule) as indicated by the broken line. The accuracy of
xpm( l) is &2%.
the former is soluble in water, but the latter forms insoluble monolayers on water. The second-order optical nonlinearities of CGNS and C18NS are essentially th_esame, since the alkane chains contribute negligibly to j3. It is well established that insoluble amphiphilic molecules form polar-ordered monolayers at an air/ water interface. All experiments were done on a Teflon trough that allowed simultaneous measurements of SHG and surface pressure versus surface area. They were carried out with 0.35 M NaCl in the aqueous solution to provide excess counterions. At such a NaCl concentration, the critical micelle concentration for CGNS is found to be close to 600 pM.A reflection geometry was used for the SHG experiment. The pump beam, with an energy of 30 mJ/pulse and a repetition rate of 10 Hz, was from a frequency-doubled Nd:YAG and was focused to a 3-mm spot at the interface. Figure 1depicts the SH response from a C18NS monolayer spread on water. The surface susceptibility Xpm") is plotted against the surface density N , of the C18NS molecules, where the subindices p and m refer to the p-polarization of the SH output and a linear polarization a t 45O from the incident plane of the fundamental input, is linearly prorespectively. The data show that xpmf2) portional to N , for surface pressures from 3 mN/m up to 33 mN/m, where the monolayer is close-packed with a limiting area of All"and 1/N8" = 0.36 nm2 per molecule. The nonlinear susceptibility elements obtained with other polarization combinations also exhibit the linear proportionality to N,. This indicates that the molecular orientation remains unchanged above the surface pressure of 3 mN/m. Below 3 mN/m, x ~ ~becomes ( ~ ) vanishingly small, presumably due to a change of the molecular orientation into a face-flat position as will be discussed elsewhere.20 The above results have two important implications. First, the local field effect arising from the interaction between C18NS molecules is negligible since, otherwise, x ( ~would ) be nonlinear in NE. Second, x@) is a linear measure of the total number of polar-oriented molecules adsorbed a t the interface as denoted in eq 1. To study the adsorption of soluble CGNS molgcules to the_air/water interface with SHG, we notice that B(C6NS) = fl(Cl8NS). If the chromophore orientation of CGNS and C18NS at the interface is also the same, then the measured x ( ~from ) SHG can directly be used to determine (20) Mullin, C. S.;Vogel, V.; Shen, Y. R.; Kim, M. W. In preparation.
'
0
.
I
.
250
,
.
500
Bulk Concentration
,
750
.
, 1000
[pM]
Figure 2. Second-order susceptibility Xpm(') versus the bulk concentration of the soluble C6NS molecules (0.35 M NaCl solution with pH = 5.6 at 20 "C). xP(') of a saturated adsorption layer is indicated by the broken line. the surface density of the polar-oriented CGNS molecules in the interfacial layer. This is indeed the case, as confirmed experimentally. The orientation-dependent ratio Xpm(2)/Xsm(2) ( 8 referring to s-polarization for the SH output), which should be sensitive to the molecular orientation, is the same for CGNS and C18NS interfacial layers.21 We have studied SHG from the air/water interface of a CGNS solution. In Figure 2, the measured xpm@) is plotted against the bulk concentration Cb of CGNS in the solution. It is seen that Xpmf2) increases with cb and approaches saturation. A comparison with Figure 1reveals that x ~ ~ ( ~ ) ( C G at N Ssaturation ) equals ~pm(~'(C18NS) of a close-packed monolayer. This indicates that in both cases, one with soluble and the other with insoluble molecules, the interfacial layer contains the same number of polar-ordered naphthalenesulfonate molecules. It is likely that the polar-ordered CGNS molecules also appear at the interface as a single close-packed monolayer. However, the possibility still exists that the interfacial layer is composed of a partially polar-ordered CGNS surface monolayer and some polar-ordered molecules in the subsurface region. The following experiment was carried out to reject the possibility of polar ordering in the subsurface. A monolayer of insoluble C18NS molecules was spread on top of the CGNS solution. For low surface densities of C18NS, we should have CGNS and C18NS molecules coadsorbed at the interface. Reducing the surface area forces the soluble CGNS molecules to submerge into the water. Eventually, only the C18NS molecules would float on the surface and form a close-packed, totally polar-oriented monolayer. We found that whether the close-packed monolayer of C18NS was on salt water or on the CGNS solution, the nonlinear optical responses xpm(2) are the same. This indicates that beneath a polar-ordered CGNS monolayer at the surface of a CGNS solution, there should not be a subsurface layer of CGNS with partial polarordering. In a separate experiment, an insoluble monolayer of eicosanol (CZO-OH), C ~ O H ~ ~was O Hspread , on top of the CGNS solution. The OH head group has a different polarity than that of the naphthalenesulfonate head group. Thus the polar-ordering of CGNS in the subsurface layer ~~
(21) Adsorbed molecules appearing with a constant tilt angle over a wide range of surface concentrationsseem to be fairly common. Thie has
been observed both in the case of insoluble monolayera (see for example, Berkovic,G.; Rasing, T.;Shen, Y. R. J. Opt. SOC.Am.B 1987,4,946) and in the case of soluble monolayera (see ref 14).
1224 Langmuir, Vol. 7, No. 6, 1991
0.1
Vogel et al.
0.2
Area per C20-OH Molecule
0.6
a
-
0.2
-
[I"]
. ..
-
0.4
0.4
0.3
a
.
a.
a
8 0
* 0
0.0 0.1
0.2
0.3
0.4
Area per C20-OH Molecule [nmz]
Figure 3. Experimental results of (a) surface pressure and (b) second-order susceptibilityversus mean area per C20-OH molecule for a C20-OH monolayer on plain water (0.35 M NaCl solution with pH = 5.6 at 20 "C) ( 0 )and on solutions with C6NS bulk concentrationsof 200 r M (m) and 600 r M (0).The secondorder susceptibility is normalized with respect to the signal from a close-packedC6NS adsorbate layer in the absence of C20-OH molecules. The fluctuations in the nonlinear response from the interface of a C20-OHmonolayer spread on a C6NS solution can presumably be attributed to phase separation of soluble and insoluble molecules in the surface monolayer.
underneath a C20-OH monolayer, if present, could be different. Figure 3a depicts the measured surface tension ( A ) versus the mean area per C20-OH molecule (A) for three different CGNS concentrations in the solution, Cb = 0,200, and 600 pM. It is seen that at low surface densities of CGNS, the surface tensions for the three cases are very different because different numbers of CGNS molecules are coadsorbed with C20-OH at the interface. Upon compression to reduce the surface area, however, the curves with Cb # 0 asymptotically approach the one with Cb = 0. This indicates that the adsorbed CGNS molecules can be squeezed back into the water22eventually leaving only a
close-packed C20-OH monolayer at the interface, and that CGNS molecules do not form an ordered subsurface layer underneath the C20-OH monolayer. This conclusion is supported by the SHG results shown in Figure 3b, where Xpm(2) is plotted against A for cb = 0, 200, and 600 pM. Although the values of xpm(2) are different for the C20-OH monolayer ,on water and_on the CGNS solution a t large A [note that j3 (CGNS) > j3 (C20-OH)], they become nearly equal toward the limiting value of A at which the C20-OH molecules form a close-packed monolayer. The result shows that as the CGNS molecules are driven back into water by compression,they do not form any partially polarordered subsurface layer underneath the C20-OH monolayer. In another experiment to be reported elsewhere,20we have measured the surface pressure as a function of the bulk concentration of CGNS in the aqueous solution. Using Gibbs' approach, the total number of excess molecules in the interfacial layer can be deduced. This number was found to be the same within 5% of the number of polaroriented molecules a t the interface detected by a simultaneous SHG measurement. Thus even if there is an isotropic subsurface layer of CGNS, its concentration cannot be more than 5% of that of a full monolayer. In summary, we have found that naphthalenesulfonate molecules (CGNS)on adsorption to the air/water interface form a single, totally polar-ordered monolayer. No subsurface layer of CGNS seems to be present. At saturation, the mean surface area per adsorbed CGNS molecule is the same as that of a close-packed insoluble C18NS monolayer. Over a wide range of surface densities up to a full surface coverage,the mean chromophore tilt angle remains constant and is the same for the soluble (CGNS) and insoluble (C18NS) monolayers. Therefore, a saturated monolayer of soluble CGNS formed at the air/water interface by adsorption from solution appears to be very much the same as that of insoluble Cl8NS monolayers formed by spreading on water.
Acknowledgment. We thank Dr. M. W. Kim of Exxon Research and Engineering Company, Annandale, NJ, for helpful discussions and support of this work and for providing the naphthalenesulfonate molecules. We also appreciate very useful comments from Professor K. B. Eisenthal. V.V. gratefully acknowledges the Feodor-Lynen Fellowship from the Alexander-von-Humboldt Foundation. The work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Science Division of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098and also partidy by Exxon Research and Engineering Cooperation. Registry No. CGNS, 17803-00-8;C18NS, 103837-95-2;C20OH, 629-96-9. (22) The process of squeezing coadsorbed soluble molecules back into solution by monolayer compression of ineoluble surface moleculea ham also been observed with SHG by Eisenthal and co-workers (ref 16).
