monomolecular surface film

Molecular aspects of the system water/monomolecular surface film and the occurrence of a new anomalous dispersion regime at 1.43 GHz...
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J. Phys. Chem. 1983, 87, 5251-5258

plots are shown in column b of Table I, which coincide fairly well with the values obtained from the potential modulation method. It has been well-known that the dye-sensitized photocurrent can be stabilized by addition of hydroquinone to the dye solution.i7,is This is attributable to the high rah (17) Tributsch, H. Ber. Bunsenges. Phys. Chem. 1969, 73, 582. (18) Pettineer. B.: Schouuel. H.-R.: Gerischer. H. Ber. Bunsenpes. Phys. Chem. i973, 77, 960.- ' I

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constant for electron transfer to the oxidized dye as determined above. Acknowledgment. The authors are indebted to Dr. A. Yoshimura (Faculty of General Education, Osaka University) for the flash photolysis experiments and for useful discussions. Registry No. KI, 7681-11-0;ZnO, 1314-13-2;Rose Bengal, 11121-48-5;Rhodamine B, 81-88-9; L-tryptophan, 73-22-3; hydroquinone, 123-31-9.

Molecular Aspects of the System Water/Monomolecular Surface Film and the Occurrence of a New Anomalous Dispersion Regime at 1.43 GHz Helnrlch Huhnerfuss' Institut fur Organische Chemie und Biochemie. 2000 Hamburg 13, FRG

and Werner Alpers Institut fur Meereskunde, Universitat Hamburg, and Max-Planck-Institut fur Meteorologie, 2000 Hamburg 13, FRG (Received: December 14, 1982; In Final Form: May 2, 1983)

The physicochemical implications of airborne microwave radiometer measurements at 1.43 GHz and at 2.65 GHz over a sea surface covered with a monomolecular 9-octadecen-l-ol,Z isomer (oleyl alcohol), surface film are discussed. These measurements indicate that "icelike" clathrate structures with average 0..SOdistances m and a free activation enthalpy AG = 5.53 kJ/mol are induced by the monolayer in a of about 5.8 X water layer of d I190 ym thickness. This "penetration depth" is further substantiated by evaluating about 30 experiments published elsewhere, from which comparable penetration depths can be deduced. In light of the molecular interactions between the surface film and the adjacent water layer, and in the framework of the Debye relaxation theory, the occurrence of a new additional dispersion regime at a frequency f, = 1.43 GHz (relaxation time T, = 1.11X lO-'Os) and the anomalously high dielectric constant E* I5.2 X lo4 at this frequency can be explained. It is suggested that anomalies of the upper water layer may occur, when a water-structuring influence by a surface-active compound (or a solid) and microscopic local disturbances of the surface film order are present. The ordering effects within the system surface film/water and the relaxation processes of disturbances of this order are investigated by laboratory measurements in an automatic Langmuir trough. The thus determined relaxation time for disturbances of the surface film order T, = 16 min is about loi3 times larger than the relaxation time T , of the water molecules.

Introduction In the years 1909-1936 several scientists attempted to determine the anomalous dispersion regime of water in the frequency range between 0.1 and 20 GH2.l But the first results reported between 1909 and 1921 were very inconsistent and showed various anomalous dispersion regimes ranging from 252 MHz (wavelength X = 1.188 m) to 1.43 GHz (X = 0.21 m).I However, some years later (1927-1936) other authors' could not confirm these anomalous dispersion regimes when performing experiments in the frequency range 83 MHz (A = 3.6 m) to 2.22 GHz (A = 0.135 m) using improved instruments. Today it is well established2 that anomalous dispersion of pure water occurs a t about 14.5 GHz (X = 0.021 m) for a water temperature of 287 K. Some investigators conjectured that the occasionally observed higher dielectric constants were caused by

boundary layer forces and they tried to verify this hypothesis by performing high-precision dielectric constant measurements of thin water layers. But no values significantly different from those for bulk water were obtained,3 with the exception of data for very thin water layers of thickness d I loTam, which a3proach the value for ice,4 being only slightly higher than those for bulk water. In our view, the occasionally measured anomalous dispersion regimes of water a t frequencies below 14.5 GHz are probably due to interfacial phenomena. Either contamination of the free-water surface with surface-active molecules or long-range ordering effects a t the solid boundaries of the measuring instrument presumably affected the measurements. In a recent airborne remote sensing experiment performed over the North Sea, Alpers et al.5 demonstrated

(1) 'Gmelins Handbuch der anorganischen Chemie, Sauerstoff, 8.Auflage/Lieferung 5, Verlag Chemie, Weinheim, 1963. (2) E. H.Grant, T. Y. Buchanan, and H. F. Cook, J.Chem. Phys., 26, 156 (1957).

(3) H. Kallmann and K. E. Dorsch, 2.Phys. Chem., 126, 305 (1927). (4) W. G. Palmer, Proc. R. SOC.London, Ser. A , 106, 55 (1924). (5) W. Alpers, H.-J. C. Blume, W. D. Garrett, and H. Huhnerfuss, Int. J. Remote Sensing, 3, 457 (1982).

0022-3654/83/2087-525 1$01.50/0 0 1983 American Chemical Society

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?

The Journal of Physical Chemistry, Vol. 87, No. 25, 1983

Huhnerfuss and Alpers

\ I

70

+

0

0

10

2’0

’

30

40

5’0

60[~OOxhml7

Figure 2. Surface pressure (mN m-’) plotted vs. area/molecule (100 X nm’). Compression and dilatation curve (indicated by -) of an oleyl alcohol surface film, measured in an automatically working Langmuir trough. Point 1 indicates a constant area of 0.25 nm*/molecule, at which compression was stopped and the time dependence of surface 0 ~ , ~ , , , , . . . , , , , , , , , , , , ! . , , , , , , , , , , , , , pressure decrease was measured. 335%

m

610

TIME

LM

(SECONDS PAST MIDNIGHTI

Flgure 1. Brightness temperature as measured by the L-band (1.43 GHz) and S-band (2.65 GHz) radiometer while flying over the slick area and adjacent clean ocean areas during run 4 (1 s corresponds to 75-m flight distance). The recorded drop of the L-band brightness temperature to 0 K is an instrumental effect due to the too fast rate of decrease of T , (“overshooting”). The real minimum value is estimated to lie between 5.4 K (background temperature) and 10 K.

that a monomolecular surface film floating on the sea surface strongly modifies the dielectric properties of water in the vicinity of the air-sea interface. This result was obtained by measuring the emission of the “clean” sea surface and a water surface covered by an artificial monomolecular 9-octadecen-1-01, 2 isomer (oleyl alcohol), surface film in two microwave frequency bands (1.43 and 2.65 GHz) from an airplane. This substance was often used on sea surfaces for simulating the influence of natural and man-made surface films on air-sea interaction processes.6 This paper interprets the data set obtained by Alpers et aL5 in light of the molecular arrangements within the surface film and the adjacent “vicinal” water layer. Furthermore, additional laboratory experiments on the relaxation properties of monomolecular oleyl alcohol films are presented, which give further evidence for the interactions within the whole system surface film/vicinal water layer. Description of t h e Experiment Aircraft Experiment. During the MARSEN (marine remote sensing) experiment of 1979 in the North Sea, a monomolecular surface film experiment was conducted near the German North Sea Research Platform (Forschungsplattform “Nordsee”, 54O 42’ N; 7O 10’ E) on Sept 22, 1979. The film material, (2)-9-octadecen-l-ol (oleyl alcohol), was disseminated from a helicopter in small frozen chunks,6 which melted and spread over the sea surface forming a surface film (“slick”)covering a 1.5-km2 area of the sea. Oleyl alcohol was used as a model substance on account of its physicochemical characteristics, which are known to be similar to those of natural surface films.6 A NASA Wallops Flight Facility (WFF) P-3A research aircraft overflew the slick 12 times as well as the adjacent “clean” sea areas at a height of 170 m.5 The P-3A aircraft was equipped with an L-band (1.43 GHz) and an S-band (2.65 GHz) radiometer. The antenna of the two microwave radiometers pointed vertically downward operating at horizontal polarization. The brightness temperature of the (6) H. Hiihnerfuss and W. D. Garrett, J. Geophys. Res., 86,439(1981).

sea surface TBwas measured in both microwave channels as a function of time. The meteorological and oceanographic conditions encountered during the experiment were as follows: water temperature, 287.2 K; wind speed, 4.1 m s-l; and significant wave height, 1.47 m. An example of an optimal data set measured from the NASA airplane during pass 4 is shown in Figure 1. During this flight track the antenna footprint (approximately 60 m in diameter) was completely within the surface film area a considerable time. During five other passes qualitatively similar results were obtained, though for some runs the footprint of the sensor contained both slick-covered and a slick-free sea surface. Figure 1 shows no modification of the brightness temperature at 2.65 GHz by the monomolecular surface film, but a strong decrease at 1.43 GHz. In the latter case, the measured brightness temperature T Bdecreased from 92 to 0 K. However, the value 0 K is not real and is caused by instrumental deficiencies. The response of the instrument was such that for a large rate of decrease in brightness temperature “overshooting” occurred because of the small time constant of the instrument. The correct value is estimated to be about 10 K and cannot fall below 5.4 K, which is the background temperature originating from the intervening atmosphere and the sky.5 Relaxation Measurements in the Laboratory. In order to determine the relaxation effects within the monomolecular oleyl alcohol surface film, laboratory measurements were performed in an automatic Langmuir trough type A, Fa. Lauda, Lauda (Federal Republic of germ an^)^ at 288 K, i.e., at almost the same temperature as encountered on the ocean surface. The compression and dilatation curve of (2)-9-octadecen-l-o1 is shown in Figure 2. The relaxation process was investigated by compressing the surface film, until monomolecular coverage was achieved at 0.25 nm2/molecule (point 1 in Figure 2), keeping the surface film material within this constant area, and measuring the time dependence of the surface pressure decrease. The results of the Langmuir-trough relaxation measurements are shown in Figure 3. The thus determined relaxation time of the (2)-9-octadecen-l-o1 surface film molecules is T,,,~ = 16 min. Theory a n d Evaluation of t h e Data The derivation of the theory is based upon the dielectric relaxation theory of Debye.s The physical parameter (7) H. Huhnerfuss and W. Walter, J. Colloid Interfuce Sci., in press. (8) P. Debye, ‘Polare Molekeln”, S. Hirzel Verlag, Leipzig, 1929.

The Journal of Physical Chemistry, Vol. 87, No. 25, 1983 5253

Molecular Aspects of Water/Monomoiecular Surface Film [ m Nm‘l]

20

1. & + ,’

i

’***

0 ’ 0

,

’

:

’

20

:

’

40

:

:

’

’

:

’

100

80

60

: ’ 120 [min]

Figure 3. Time dependence of surface pressure decrease at 288 K and a constant area A = 0.25 nm*/molecule. The relaxatbn tlme ( l / e - time) was determined to be T,,,,,~ = 16 min. 1

?

~

ANOMALOUS DISPERSION REGIME

I 1 1

remaining high-frequency real dielectric constant E‘, arises from the atomic and electronic polarizations. Typical values for water arelo E‘, = 80 and E’, = 4.5. The imaginary part E” is significantly different from zero only in the “domain of anomalous dispersion”, which implies that only in this frequency band energy is absorbed. At a water temperature of 287.2 K as encountered during this experiment, the center of the anomalous dispersion regime (maximum of the e’’ curve) of clean water is observed a t a frequency of 14.5 GHz. In the presence of a monomolecular surface film intensive hydrophobic and hydrophilic interactions with the adjacent water layer O C C U ~ . ~ ~Various - ~ ~ spectroscopic studies14 have shown that the effect of a surface film is largely to hinder the rotational motion and the mobility of the water molecules and enhance the degree of the 0-H.. -0hydrogen bonding between water m~lecules.’~J~J~ The reduction in mobility of the water molecules is associated with a decrease of the diffusion coefficient Do of pure water. If it is assumed that a surface film induced (potential) activation energy AI3 and the thermal energy k T govern the energy distribution of the water molecules, Boltzmann statistics can be applied to this system and the diffusion coefficient D, of a surface film covered water surface can be expressed by

D, = Do exp[-AE/kTl

(5)

Often the diffusion properties are expressed in terms of a relaxation time 7. The modification of the relaxation time by the slick is given by log f

fo

7,

Figure 4. Frequency dependence of the dielectric constant of clean water in the microwave frequency band. The anomalous dispersion regime is centered around 9 GHz for 273 K and around 17 GHz for 293 K water temperature.

measured by a microwave radiometer is the brightness temperature TB. TB is related to the molecular water temperature T and the emissivity e by T B = eT + To (1) where To is the background temperature, which contains contributions from the intervening atmosphere and the sky.g In this experiment Towas 5.4 K. e is a function of the complex dielectric constant E*of the upper water layer of the ocean5 E* = 6’ -jet‘ (2)

I

1 - e*1/2 2 e = l - / 1 €*I12

+

+ It*l - 21e*1’/2 cos $0/2 1 + lE*l + 21€*11/2cos $0/2 1

=Iwhere

$0

(3)

is the loss angle, defined by tan

Q

= -c“/E’

(4)

The qualitative dependence of the real part E’ and the imaginary part E” of this complex dielectric constant c* on frequency is depicted in Figure 4. If an alternating electric field is applied to the medium, the dipoles are able to follow the field oscillations at low frequencies, thus giving rise to a large real dielectric constant E’,. However, when the frequency becomes sufficiently high, the dipoles are unable to reorient in the rapidly changing field; i.e., this effect cannot contribute to the dielectric constant. The (9) D. H. Staelin, IEEE Trans. Antennas Propag., AP-29,683 (1981).

=

T~

exp[AE/kTl

(6)

where ro and 7,are the relaxation times for slick-free and slick-covered water, respectively. The polarized water layer adjacent to the monomolecular surface film can be described by an additional real dielectric constant AE* ~*,li~k = E*H~O + At*

(7)

which implies that a new region of anomalous dispersion has to be p ~ s t u l a t e d .We ~ write A€* in the form given by the Debye theorya

+

Ac* = A E ’ , ( ~ j f / f J 1

(8)

where f, is the relaxation frequency corresponding to 7,. Since in our airborne microwave radiometer experiment the brightness temperature at 1.43 GHz nearly decreased to the background level To,it can be safely assumed that this frequency coincides with the center of the new anomalous dispersion regime (peak frequency f, of the e’’ curve, Figure 4). Under the assumption fc = f, = 1.43 GHz we can calculate E*from eq 1 and 2: E* 2 5.2 x 104 (Note that a t the center of the anomalous dispersion Re E* = Im E* and, therefore, $0 = 45O.) The relaxation time of the water molecules adjacent to the oleyl alcohol surface film calculated from f, is 7, = 1.11 x 10-10 s i.e., there is a shift of 1 order of magnitude compared to (10) L. A. Klein and C. T. Swift, IEEE Trans. Antennas Propag., AP-25, 104 (1977). (11) G. Nemethy and H. A. Scheraga,J.Phys. Chem., 66,1773 (1962). (12) A. Suggett, Pharmacochem. Libr., 1, 95 (1977). (13) E. Wicke, Angew. Chem., 78, 1 (1966). (14) F. Franks, “Water-A Comprehensive Treatise”, Vol. 4, Plenum Press, New York, 1975. (15) G. Nemethy, Angew. Chem., 79, 260 (1967).

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The Journal of Physical Chemistry, Vol. 87, No. 25, 1983

the value of pure water, 70 = 1.19 X 10-l’ s. The thickness of the emitting surface layer d, Le., the depth at which the surface film induced activation energy A E decreased to l / e (37%), is given by5J6 d = X ( ~ T . ( E *sin ~ ’ /p/2)-l ~ Inserting the value calculated for e* and p = 45O we obtain for f c = 1.43 GHz d 5 1.9 X m. From a thermodynamic point of view, the expression “surface activation energy AE”, which is often used in the literature in this form, ought to be substituted by the term “free activation enthalpy AG”. It can be deduced from the values obtained by our measurements that clathrate-type arrangements of the water molecules are formed, which implies an increase of volume. Therefore, in the presence of surface films the energy term associated with volume changes has to be added to the internal activation energy AU and we thus obtain the activation enthalpy AH. In the upper water layer, which is influenced by the surface fii, the activation enthalpy AH is connected with the free activation enthalpy AG by the Gibbs-Helmholtz equation AG = AH- TAS where A S is the entropy change in the surface layer due to the surface film. The free activation enthalpy AG is then given by5 AG = k T In ( f o / f J (9) The enthalpy change per mole is, therefore AG = 2.32RT = 5.53 kJ/mol where R is the gas constant. The average intermolecular distance r between the water molecules in the surface layer d, which is influenced by the monomolecular surface film, is given by5

r = (1j101ep/AG)1/2= 5.8 X

m

Huhnerfuss and Alpers

calculated value r = 5.8 X m may, therefore, support this clathrate hypothesis, even if this value is supposed to have error bars of about f l X 10-lom. The occurrence of an additional anomalous dispersion regime at 1.43 GHz of a water surface covered with a monomolecular surface film agrees well with the abovedescribed theory of dielectric relaxation. The observed shift of the relaxation time T, = 1.19 x s of pure water at 287.2 K to 7g = 1.11X s in the presence of an oleyl alcohol surface film answers the question raised by Drost-Hansen in 1969: l8 “When discussing structure near an interface, the implication is only that the structure under consideration may have a lifetime which is notably longer than the corresponding lifetime of the structures entity in the bulk of the liquid. It remains to be determined whether this means an increase in the relaxation time by an order of magnitude (say, from to 10-lo sec), or an increase corresponding to the increase in (dielectric) relaxation time on going from water to ice. Undoubtedly, future refinements of dielectric and NMR studies will throw considerable light on this question.” However, the relatively large penetration depth d and the unusually high dielectric constant e* = 5.2 X lo4,which were calculated from the measured data, need some further discussion. Penetration Depth. At first glance the obtained value for the penetration depth d I 190 ym appears to be surprisingly high. However, an extensive evaluation of the literature showed that there is an overwhelming amount of experimental evidence supporting our conclusions: in Table I various measurements are summarized, which show long-range effects within interfacial water layers at the boundary water/organic substance (mostly mono- or bilayer), and in Table I1 examples are given for long-range

(10)

where ljlol is the dipole moment of a monomeric water molecule, yo = 1.84 X esu, and ep is the partial charge, Cpo ~ = full electric which according to Franks17is ep = 0 . 3 5 ~ charge = 4.8 X esu).

Discussion The values obtained for the free activation enthalpy, AG = 5.53 kJ/mol, and for the intermolecular distance between the water molecules in the surface layer, r = 5.8 X m, are within 1 order of magnitude of the values expected for such systems. The AG value reflects the formation of weak hydrogen bonds within the upper water layer, which are known to exhibit AG values between 3 and 10 kJ/mol. Note that our AG value is a weighted average over a surface layer of 190-ym thickness. We expect that in the upper layer a much stronger hydrogen-bond formation is induced by the surface film than in the lower part of the surface layer. The intermolecular distance between the water molecules r = 5.8 x m as inferred from our microwave measurements is about twice as large as the published values of 0. -0distances of associated water molecules, and 2.8 X m.17 normally ranging between 2.7 X However, according to Wicke,13 clathrate-type arrangements of the water molecules may be induced by surface-active substances. The cavity diameter varies between m, thus considerably augmenting 5.2 x 10-loand 5.9 x the average 0. -0distances in such systems. The above

-

-

(16) G. P. DeLoor in ‘Remote Sensing for Environmental Studies”, E. Shanda, Ed., Springer-Verlag, West Berlin, 1976. (17) F. Franks, “Water-A Comprehensive Treatise”, Vol. 1,Plenum Press, New York, 1972.

(18) W. Drost-Hansen, Ind. Eng. Chem., 61, 10 (1969). (19) H. L. Rosano, J . Colloid Interface Sci., 23, 73 (1967). (20) R. Merigoux, C. R. Hebd. Seances Acad. Sci., 203, 848 (1936). (21) H. L. Rosano, S. H. Chen, and J. H. Whittam in “Monolayers”, E. D. Goddard, Ed., American Chemical Society, Washington, DC, 1975, Adv. Chem. Ser. No. 144. (22) C. Y. Pak and N. L. Gershfeld, Nature (London),214,888 (1967). (23) J. H. Schulman and T. Teorell, Trans. Faraday SOC.,34, 1337 (1938). (24) M. Blank and V. K. LaMer, J. Phys. Chem., 61, 1611 (1957). (25) D. J. Crisp, Trans. Faraday Soc., 42,619 (1946). (26) G. A. Johnson, S. M. A. Lecchini, E. G. Smith, J. Clifford, and B. A. Pethica, Discuss.Faraday SOC.,42, 120 (1966). (27) H. G. L. Coster and R. Simons, Biochim. Biophys. Acta, 203, 17 (1970). (28) B. V. Derjagin, 2. Phys., 84, 657 (1933). (29) W. B. Russel, J. Fluid Mech., 92, 401 (1979). (30) K. Rodel, Tenside Deterg., 18, 141 (1981). (31) L. S. Palmer, A. Cunliffe, and J. M. Hough, Nature (London), 170, 796 (1952). (32) L. I. Weber and G. Lewin, Kolloid-Z., 50, 197 (1930). (33) B. V. Derjagin and E. Obuchov, Acta Physicochim. URSS, 5 , 1 (19381. \____,.

(34) S. Lenher, J. Chem. Soc., 272 (1927). (35) M. Tschapek and I. Natale, Kolloid 2. Z. Polym., 211, 46 (1966). (36) W. Drost--Hansen,Report no. F 59-G-2 Pan American Petroleum Corp. Research Department, 1959, cited in W. Drost-Hansen, Ind. Eng. Chem., 61, 10 (1969). (37) W. Drost-Hansen, ReDort no. F 59-G-6 Pan American Petroleum Corp. Research Department,-l959, cited in W. Drost-Hansen, Ind. Eng. Chem., 61, 10 (1969). (38) J. M. Macauly, Nature (London), 138, 587 (1936). (39) J. A. Schufle, C.-T. Huang, and W. Drost-Hansen, J . Colloid Interface Sci., 54, 184 (1976). (40) R. Dordick, L. Korson, and W. Drost-Hansen, J. Colloid Interface Sci., 72, 206 (1979). (41) G. Peschel and K. H. Adlfinger, Naturwissenschaften, 56, 558 (1969). (42) G. Peschel and K. H. Adlfinger, 2. Naturforsch. A,-26,707 (1971). (43) G. Peschel and K. H. Adlfinger, 2. Naturforsch. A , 24, 1113 (1969). (44) B. V. Derjagin, Discuss.Faraday SOC.,42, 109 (1966).

Molecular Aspects of Water/Monomolecular Surface Film

The Journal of Physical Chemistry, Vol. 87, No. 25, 1983 5255

TABLE I: Experimental Evidence for L o n g R a n g e Ordering Effects within Interfacial Water Layers a t t h e Boundary Organic Substance/Watera ref

method

boundary layer

19 20

diffusion transp drag technique passive microwave isothermal surface distillation

Na,SO,/l-butanol oleic acid oleyl alcohol bovine serum albumin oleic acid decyl alcohol m e t h y l laurate stearyl alcohol oleic acid cyclohexyl myristate oleic acid, oleyl alcohol, ethyl myritate, e t h y l laurate poly(vinylacetate), poly(vinylto1uene) m e m b r a n e bilayer saponin charged macromolecules sodium alkylbenzenesulfonate

5 21

22 23 24 25 26 27 28 29 30

drag drag drag drag

technique technique technique technique

NMR dielectric relaxation m o d u l u s of rigidity viscosity IR spectroscopy

penetration depth, pm 300 200

< 190 125 32.8 2.15 12.8 108 t 2 27-96b 30‘ all 25‘

-

0.1-19d 4

> 1.5e -1 >O.lf

‘

N o t exactly Depending o n viscosity. Only references were evaluated which r e p o r t e p e n e t r a t i o n d e p t h s d > 0.1 p m . Depending o n particle size a n d temperature. e Exact thickness could n o t be determined, m e t h o d only for determined. d < 1 . 5 p m reliable. f E x a c t thickness could n o t be determined, m e t h o d o n l y for d < 0.1 p m reliable. a

TABLE 11: Experimental Evidence for L o n g R a n g e Ordering Effects within Interfacial Water Layers a t the Boundary Solid Surface/Watera

ref 31 32 33 34 35 36 37 38 39

method dielectric c o n s t a n t adhesion a t glass disjoining pressure adsorption equilibr surf conductivity conductivity conductivity viscosity conductivity

40 41

viscosity viscosity

28

m o d u l u s of rigidity

42 43

disjoining pressure disjoining pressure m o d u l u s of rigidity viscosity air-bubble flow

44 45 46

solid b o u n d a r y sheets of mica glass mica o r steel plates glass glass t u b e quartz particles quartz particles glass plates P y r e x glass P y r e x glass quartz plate/> convex glass convex/ plate fused silica c o n v / p l q u a r t z plates glass glass plates glass t u b e

penetration depth, ,urn 2-5

1.5 0.1 p m . Presumably printer’s error; we calculate 0.05 p m f r o m the m a x i m u m n u m b e r of water molecule layers given in earlier papers (cited in ref 34).

effects a t the boundary waterlsolid interface. In both cases, only references reporting penetration depths d 2 0.1 pm were considered. The maximum value, to our knowledge, described in the literature was d = 300 pm.19 Thus, about 30 papers representing a variety of different measurements and methods are cited in Tables I and 11, which clearly demonstrate long-range effects within the water layer adjacent to organic or solid boundaries. Note, that “classical” theories include penetration depths of only m due to direct interactions between the hydrophilic group of the surface-active compound and adjacent water molecules by forming “shells” of structured water around the head group. The above-cited papers, however, give (45) C. Terzaghi, 2.Angew. Math. Mech., 4, 107 (1924). (46) B. V. Dejagin and M. M. Samygin, Akad. Nauk SSSR, Soueshch. Vyazkosti Zhidk. Kolloidn. Rastuorou, 1, 59 (1941), cited in J. C. Henniker, Reu. Mod. Phys., 21, 322 (1949).

experimental evidence for penetration depths, which are 102-105 times larger. These papers have obviously not received sufficient attention, perhaps because they were published in very different and partly not very accessible periodicals. In consideration of the above results and the data given in Tables I and 11,the long-range effects within the system monolayer/vicinal water layer appear to be well established. However, the physicochemical nature of this phenomenon needs some further discussion. It is unlikely that the hydrophilic part of the surface-active substance contributes very much to a long-range ordering within the upper water layer, since in this case preferably “high-energy interactions” (hydrogen bonding, ion/dipole, and dipole/dipole interactions) are known to penetrate the uppermost water column. Several hundred papers are devoted to investigations on the interaction between charged or noncharged hydrophilic groups and the underlying water (Rao and Berne47found 128 papers by a computer search, which only dealt with theoretical aspeds). The authors nearly univocally postulate short-range influences of the hydrophilic group on the water molecules. Therefore, it can be assumed that the observed longrange effect is mostly due to the hydrophobic part of the surface-active compound. This assumption is supported by theories of London,48Nemethy and S ~ h e r a g a , ~ ~ ~ ~ ~ , F ~ w k e s ,and ~ ~ other , ~ ~ authors (for reviews see T a n f ~ r d ~ ~ ) . Furthermore, solution experiments with alkanes in water, which were reviewed by Wicke,13 showed that the interaction of these exclusively hydrophobic compounds with water induced extensive solvent ordering effects around the alkane solute, which was clearly revealed by a negative entropy term and an exothermic reaction. The contribution of the hydrophobic part of surfaceactive substances t~ long-range interactions with the vicinal water layer can also be concluded from viscosity measurements performed by Nutting and hark in^^^ and by Boyd and hark in^,^^ who observed an increase of surface (47) M. Rao and B. J. Berne, J. Phys. Chem., 85, 1498 (1981). (48) F. London, 2.Phys., 63, 245 (1930). (49) G. Nemethy and H. A. Scheraga, J. Chem. Phys., 36,3382 (1962). (50) G. Nemethy and H. A. Scheraga, J. Chem. Phys., 36,3401 (1962). (51) F. M. Fowkes, J. Phys. Chem., 66, 1863 (1962). (52) F. M. Fowkes, J. Phys. Chem., 84, 510 (1980). (53) C. Tanford, Science, 200, 1012 (1978). (54) G. C. Nutting and W. D. Harkins, J. Am. Chem. Soc., 62, 3155 (1940). (55) E. Boyd and W. D. Harkins, J. Am. Chem. SOC.,61, 1188 (19391.

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viscosity with increasing alkyl chain length in the homologous series of alkanols and carboxylic acids, respectively. Note that in both cases the hydrophilic group was kept constant and only the alkyl chain length of the hydrophobic group was varied. Since the viscosity is linearly correlated with the relaxation time -r,8the results of Harkins and co-workers are consistent with the above assumptions concerning the contribution of the hydrophobic part of surface-active compounds to long-range ordering effects in vicinal water layers. However, the physical nature of these “hydrophobic interactions” remains subject to considerable debate. As a first approximation, for distances d I0.05 pm the well-known London equations can be applied stating that the attractive potential between two atoms is proportional to d4. But since the finite time needed for the transmission of the electromagnetic field from the first atom to the second, and vice versa, has not been taken into account, the attractive potential between two atoms decays more rapidly than given by London’s equations and approaches a d-7 law for larger distances d 1 0.05 pm. Therefore, penetration depths of about 100 pm can hardly be explained by the classical approach of London dispersion forces. Another type of long-range force that may develop in the case of large particles is that arising from the interaction of long wavelength oscillations. London56 has suggested that in such a case a rather long-range dispersion type effect might be possible. Long oscillators, as in systems with conjuggted double bonds, can be represented by monopoles of opposite sign suitably located in the molecule. According to London (see Verwey and Overbeek57) monopole forces are not necessarily confined to conjugated systems, but may also be present in “diluted” double bonds and thus possibly apply to our oleyl alcohol surface film. Even if the latter aspect may be somewhat speculative, it has to be stressed that in classical theories attention has been so focused on the dominant role of the outermost layer of molecules that the inevitable polarizing effect of this oriented layer upon the molecules immediately adjacent to it has been overlooked. The concept of Alpers et al.5 was, therefore, that, although the powerful forces are very short-range, they are transmitted by successive polarization of neighboring molecules to an impressive depth. As already pointed out by Schufle et al.,39such collective, cooperative, long-range structuring effects ought to be due to “low-energy interactions” between the water molecules and the surface. Another effect, possibly explaining the observed longrange effects, has been mentioned by Cini (Cini, personal communication). His approach is based on diffusional interactions within the system monolayer/water. However, further investigations are necessary to verify Cini’s approach. In light of the above arguments it appears to be necessary to modify the classical view of the influence of a monolayer on the underlying water surface. This implies the modification of some classical formulas for the determination of several physicochemical parameters by introducing additional terms, which take into account the adjacent water layer. This has already been considered by Nutting and Harkins” with regard to the determination of the surface viscosity of such systems. They stressed the need for expanding the formula of J ~ l by y including ~ ~ a (56) F. London, J. Phys. Chem., 46, 305 (1942). (57) E. J. W. Verwey and J. Th. Overbeek, Trans. Faraday SOC.,42B, 117 (1946).

Huhnerfuss and Alpers

L L

-

d

1

i

20

40

I

60 r g u o

i-

Figure 5. Typical Cole-Cole diagram for ideal consistency with the Debye theory. For details see ref 60.

} k

‘0-H

d

‘0

K O ’ H

I

H

ca 2.5nrn

orientated water

- 0” H

J

Flgure 6. Interaction between a monomolecular surface film, consisting of a hydrophobic and a hydrophilic part, and the adjacent water molecules.

second term for the drag effect of the film on the water layer. Furthermore, our surface potential measurements revealed (Huhnerfuss, in preparation) that the assumption of Helmholtz (see G a i n e ~ that ~ ~ )alterations of the water structure due to compression of the monolayer can be neglected is no longer valid. Consequently, the classical Helmholtz equation has to be expanded. Dielectric Constant. Consistency with the Debye relaxation theory8 usually is checked by plotting t” vs. t’. If the obtained curve describes a Cole-Cole half-circle,21as depicted in Figure 5, the results can be described by the Debye theory. In our case, the extremely high dielectric constant is orders of magnitude above the Cole-Cole half-circle and cannot be included in Figure 5. Significant deviations from the Cole-Cole half-circle of this kind indicate that the Debye theory is not adequate for explaining the observed effects. As depicted in Figure 6, the effect of a surface film is largely to enhance the degree of O-H...O hydrogen bonding between water r n o l e ~ u l e s . ~ ~This - ~ ~may J ~ even lead to icelike clathrate structures13 with cavity diameters of about 5.2 X 10-lo-5.9 X m. However, this icelike structure of the water molecules alone cannot account for the extremely high dielectric constant, since high-precision measurements of pure ice did not give significant deviations from the Debye the0ry.l But if “pollutants” are injected into the ice lattice structure, both t” and t’ may rapidly increase with decreasing frequency61and thus exhibit an anomalous dispersion. The frequency at which deviation from the Cole-Cole half-circle starts to become significant is dependent on temperature. Granicher, Schemer, and Steinemand2 generated a disturbed ice lattice by partially including HF molecules in the system, such that some 0 positions of the lattice were substituted by F- ions. Systematic measurements in the frequency range between 0 and lo5 Hz (temperature, 273-243 K) (58) M. Joly, Kolloid-Z., 89, 26 (1939). (59) G. L. Gaines, “Insoluble Monolayers at Liquid-Gas Interfaces”, Interscience, New York, 1966. (60) K. S. Cole and R. H. Cole, J. Chem. Phys., 9, 341 (1941). (61) R. P. Auty and R. H. Cole, J . Chem. Phys., 20, 1309 (1952). (62) H. Granicher, P. Schemer, and A. Steinemann, f f e l u . Phys. Acta, 27, 217 (1954).

Molecular Aspects of Water/Monomolecular Surface Film

The Journal of Physical Chemistry, Vol. 87, No. 25, 1983 5257

revealed that concentrations of loT5M H F already lead to molecular surface film on the adjacent water layer under significant deviations from the Cole-Cole half-circle. static conditions”J2J3J5 is locally in part counterbalanced Maximum e’ values obtained during such experiments were by a positive entropy effect. e’ = 15000 at f = 100 Hz and 268 K. Furthermore, 6” To our knowledge a correct theory for inferring exact already exhibited strong deviations from the theoretical quantitative entropy values from the hysteresis effect of Debye value at very low H F concentrations. Langmuir curves as depicted in Figure 2 does not exist at Strong deviations of E* from theory (30 times higher present. A promising qualitative approach was suggested values) are also observed, if gaseous substances are inby Pruss.% The work of compression/mol E is given by cluded in ice. O ~ l a t k aconjectured ~~ that space charges E = F(dA)NL (11) are induced in such a “disturbed” ice lattice, if an alterwhere dA is the change of area/molecule, F the surface nating electromagnetic field is applied. The reason for pressure, and NLthe Loschmidt number. Pruss showed these space charges was assumed to be a special sort of for di- and trisubstituted glyceride derivatives that E inionic conductivity due to disturbances of the lattice. creases with increasing interaction of the hydrophilic part In light of the above-summarized results on disturbance with the adjacent water layer. E can be determined for of ice structure and its effect on the dielectric constant e*, the compression (Ecomp) and the dilatation (Edll)curve. it can be assumed that similar disturbance effects of the The quotient of both values, the reversibility R , is given surface film induced icelike water structure occur. Such by disturbances can be caused principally by three different effects: R = 100(Edil/Ecomp)% (12) (1) Due to irregularities within the monomolecular Thus, a qualitative comparison of the entropy effect of surface film (islands of compressed molecules, patches of surface films of different chemical structure is obtainable. different molecular structure, e.g., kink and gauche blocks, However, care has to be taken that the measurements are etc.) local disturbances of the film-induced electrical field performed by using the same constant compression and may occur. dilatation velocities ucompand udfi in each experiment, since (2) The special hydration atmospheres, surrounding the the hysteresis depends on these velocities. Greater comions in aqueous salt solutions like seawater, supply point pression velocities lead to increased irregularities, i.e., more sources of disturbances. In this case, the observed high positive entropy effects. In the case of the oleyl alcohol dielectric constant would only be valid for seawater meafilm used in this investigation a value for the resurements and aqueous ion solutions. However, H ~ r n e ~ surface ~ versibility R = 74.1% is determined at 288 K (see Figure pointed out that due to the special water structure of the 2; Udil and ucomp were 0.016 nm2/(molecule.min). (slick-free!) boundary layer ionic species may preferentially It is worth noting that the relaxation time 7c,mpfor the be excluded in this upper part of water. But it is quesrearrangement of the film-forming molecules after comtionable whether this repulsion effect can also be transpression is orders of magnitude greater than the relaxation ferred to slick-covered water surfaces. time of the water molecules in the surface layer, which is (3) It is well established that gases, soluted in ice, are 7, = 1.11X s. If the compression of the surface film the most intensive sources for deviations of dielectric is stopped at any A value, it takes several minutes, or even constant measurements of ice, if these gases are able to be hours, until the equilibrium surface pressure Fequis centers for point charges.65 Basically, this effect ought achieved. In the case of the oleyl alcohol, F, was reached to be independent of the presence or absence of a slick. after 115 min (288 K; A = 0.25 nm2; see Figure 3). The However, if microbubbles (the size of which depends upon relaxation time 7c0mpis defined as the time after which the the presence of slicks) or the above-discussed film-induced surface pressure dropped to l / e (37%) of the difference clathrate structures enclose such gas molecules, a possible after stopping between the initial surface pressure Fcomp influence cannot be excluded. the compression and the equilibrium surface pressure Fqu. Disturbance Due to Irregularities within the Surface From the data shown in Figure 3 a relaxation time 7c,mp Film. Much experimental evidence is available, which = 16 min is obtained. So, in addition to the locally difclearly shows that from a microscopic (molecular) point ferent entropy regimes, a t least two systems of very sigof view several monomolecular surface films exhibit irnificantly different relaxation times are interacting. regularities under static conditions; e.g., surface-active The above-described mechanisms ought to be valid for compounds with very long saturated alkyl chains tend to both seawater and salt-free solutions, though there might form islands. Under dynamic conditions as encountered be slight differences. Therefore, if these mechanisms are on the wavy sea surface, almost all surface films are miresponsible for the observed anomaly of the dielectric croscopically irregular due to continuous compression and constant, only slight differences have to be expected bedilatation. tween measurements performed on seawater and salt-free This effect can be simulated in an automatic Langmuir water surfaces. trough. An example for a compression/dilatation LangDisturbances Due to Hydrated Ions. The hydrated-ion muir curve of a 9-octadecen-l-ol,Z isomer (oleyl alcohol), disturbance term may also interfere with the surface film surface film is shown in Figure 2. It can be concluded from induced icelike water structure but is not a necessary the difference in the shape of compression and dilatation precondition for obtaining the above anomaly of the dicurves that “different arrangements” of the molecules exist electric constant. As was clearly shown by Cini et al.67and at wavy water surfaces with compression and dilatation other investigators,68 anomalies were also observed at zones. Since different arrangements must be concerned surface film covered water surfaces, when ions were absent. with the entropy S, the hysteresis of the compression/ A possible mechanism for the interaction between the dilatation curves is presumably correlated with the change hydrated ions and the water layer adjacent to a surface film of entropy A S of the system. Consequently, the wellknown negative entropy effect, -AS, of a regular mono(63) G. Oplatka, Xelu. Phys. Acta, 6, 198 (1933). (64) R. A. Horne, J . Geophys. Res., 77, 5170 (1972). (65) H. Brommels, SOC.Fenn. Comment., AI Nr.19, 1 (1922/23).

(66) H.-D. Pruss, Ph.D. Thesis, University Koln, Koln, Federal Republic of Germany, 1969. (67) R. Cini, G. Loglio, and A. Ficalbi, Nature (London),223, 1148 (1969). (68) W. Drost-Hansen, J . Geophys. Res., 77, 5123 (1972).

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Huhnerfuss and Alpers

or colloidal particles was suggested and discussed by sev287-289,302-305,317-319, and 332-335 K.68 It has been eral authors on account of biological r e l e ~ a n c e . ~ ~ - ~ lconjectured that at these temperature ranges sudden However, it has to be questioned whether their theory changes of structure within the water layer occur, similar developed for an ionic double layer can be easily applied to phase alterations. Since the water temperature ento systems discussed in the present paper. The electric countered in this experiment was 287.2 K, it cannot be double layer principle involves colloidal particles or other excluded that the observed anomaly in dielectric behavior solid interfaces, which are electrically charged by fixed or is confined to the well-established thermal anomaly regime adsorbed ions and which are surrounded by hydrated between 287 and 289 K. This has to be clarified by adcounterions. In our case, however, a noncharged oleyl ditional experiments. alcohol monolayer is causing the anomaly and only hySummary drated ions arising from the bulk seawater (the “counterion part” of the theory) might play a role. The modification of the brightness temperature of the Disturbances Due to Gas Molecules. Cini et scrusea surface a t 1.43 GHz due to the presence of monomopulously took care that no gas molecules were left in the lecular oleyl alcohol surface films provides a basis for the water used for their experiments: the water was distilled calculation of the relaxation time 7, = 1.11X s, the from a KMn04 solution and then twice distilled under N2 free activation enthalpy AG = 5.53 kJ/mol, and the avin a quartz distiller, without boiling, using infrared heating. erage intermolecular distance of the water molecules r = Water was then transferred to another quartz apparatus, 5.8 X m within the vicinal water layer. It is concluded which was sealed directly to the part of the apparatus used from the obtained values that icelike clathrate structures for subsequent measurements. In order to outgas both the are induced by the surface film in a water layer of d 5 190 liquid and the walls of the container completely, the water km thickness. The obtained value for the penetration was reduced to half its volume through evaporation under depth is supported by various experiments reported in the a vacuum. literature. It is suggested that the hydrophobic part of the It can be safely assumed that after such precautions Cini surface-active compound contributes most to the observed et al.67in fact obtained gas-free water and that the anomlong-range effect within the upper water layer. alies in surface tension, which they observed in the presIn light of the molecular interactions between the ence of surface-active substances, were not due to the monolayer and the upper water surface, the occurrence of action of any gas molecule. It can be concluded from these the new additional anomalous dispersion regime at 1.43 results that disturbance effects due to the presence of gas GHz and the anomalously high dielectric constant at this molecules can at best be considered an additional term, frequency e* = 5.2 x lo4 can be explained. It is suggested however, not a precondition for the occurrence of anomthat such anomalies of the upper water layer may occur, alies. if (a) a water structuring influence of a surface-active Thermal Anomaly. In addition to the above-developed compound or a similarly active solid surface takes effect hypothesis on “disturbance effects”, another phenomenon and (b) microscopic local disturbances occur. Such dismay also contribute to the anomalously high dielectric turbances are expected always to arise under dynamic constant: There is much experimental evidence that peconditions, and under static conditions, if local irregularriodic alterations (“anomalies”)of several physical paramities of the surface film are present, and/or due to the eters occur as a function of temperature. E.g., when the action of hydrated ions and/or gas molecules. temperature is systematically varied, anomalies of surface Additional experiments have to be performed, in order tension,67electrical condu~tivity,~~ optical properties,@and to clarify whether the observed anomaly of the dielectric surface v i s c ~ s i t i e are s ~ ~observed ~ ~ ~ at water surfaces covconstant of the film-covered water surface is confined to ered with a monomolecular surface film. If the water the temperature range near 287-289 K, at which several surface is scrupulously cleaned, the thermal anomalies physical properties of water are known to exhibit a thermal disappear. Cini et al.67demonstrated the occurrence of anomaly. thermal anomalies in the surface tension of water in the presence of traces of surface-active substances like hepAcknowledgment. This research was supported by the tanol and the disappearance of the thermal anomalies on Deutsche Forschungsgemeinschaft (German Science removal of the surface-active substance by selective adFoundation) through the Sonderforschungsbereich 94, sorption on activated charcoal. Meeresforschung, Hamburg (Federal Republic of GerThe same surface tension anomalies were also measured many). We thank the Henkel KGaA, Dusseldorf (Federal by using narrow glass capillaries.68 Glass surfaces are Republic of Germany), which generously provided the known to exhibit similar ordering influences on adjacent (Z)-9-octadecen-l-ol (oleyl alcohol) at no cost; Frau Heike water layers as monomolecular surface films (see Table 11). Dannhauer, who prepared the frozen chunks during the The local disturbances, which appear to be a precondition experiment on the sea surface; and Frau Francy Serwe for for the occurrence of thermal anomalies, are, in this case, her engaged assistance during the Langmuir-trough meapresumably caused by the well-known microscopic irregsurements. Especially the cooperation of Dr. P. Lange ularities of the glass surface.75 (BAW, Hamburg, Federal Republic of Germany), Dr. H.-J. The thus far available data suggest that anomalies can C. Blume (NASA Langley Research Center, Hampton, predominantly be expected in the temperature ranges near VA), and Dr. W. D. Garrett (NRL, Washington, DC) during the field experiments and discussions with Dr. G. (69) C. T. O’Konski, J . Phys. Chern., 64, 605 (1960). P. DeLoor (TNO, Den Haag, Netherlands) concerning the (70) H. P. Schwan, G. Schwarz, J. Maczuk, and H. Pauly, J. Phys. theoretical aspects of dielectric relaxation are gratefully Chern., 66, 2626 (1962). acknowledged. With regard to the “long-range ordering (71) G. Schwarz, J. Phys. Chern., 66, 2636 (1962). (72) A. A. Trapeznikov, “Proceedings of the 2nd International Coneffects” of monolayers, the suggestions and critical remarks gress of Surface Activity”, Butterworths, London, 1957, p 109. of Prof. Drost-Hansen (University of Miami, Coral Gables, (73) R. P. Enever and N. Pilpel, Trans. Faraday Sot., 63, 781 (1967). FL) and of Prof. Cini (University of Florence, Italy) are (74) L. E. Copeland, W. D. Harkins, and G. E. Boyd, J. Chern. Phys., very much appreciated. 10, 357 (1942). (75) K. Hiltrop, Ph.D. Thesis, University and GH Paderborn, PaderRegistry No. Water, 7732-18-5; (Z)-9-octa.decen-l-ol,143-28-2. born, Federal Republic of Germany, 1979.