628
Langmuir 987, 3, 628-631
creases. This is different from anothermicrostructural-effect associated with the molecular discrete nature of liquid. This effect shows up in the range of short distances, H 5 10-15 A, where the interaction oscillations have a period close to the diameter of the molecules.21 In our experiments, we have not detected the oscillating microstructural component of force. This may be attributed to both an insufficient sensitivity of the apparatus employed and some roughness of the samples. The macrostructural component of disjoining pressure may be of various natures: it may be dependent on variations in the architecture of the hydrogen bonds, due to the solid wall, and on the orientation of dipole liquid molecules. The supposition on the presence of two different components of structural forces is substantiated, in our opinion, by the fact that in the case of silica surfaces the forces, in accordance with rep2 depend very sensibly on temperature. These forces would practically disappear a t t = 70 “C, whereas the forces acting between the mica surfaces, as measured in ref 9, practically did not depend on temper(21)Israelachvili, J. N.; Pashley, R. M. Nature (London) 1983,306, 249. (22) Derjaguin, B. V.; Zorin, Z. M.; Churaev, N. V.; Shishin, V. A. In Wetting, Spreading and Adhesion; Academic: London, 1977; p 201.
ature (at least, in the range t 5 50 “C). Introduction of the term “structural forces” to designate the macrostructural and microstructural forces seems to be much more expedient than the term “hydrated forces”, for it is indicative of the common source of the forces: the overlapping of the boundary layers of liquids (not only of water) possessing a modified structure.
Conclusion Thus, we have calculated the dependences of the surface potential and the glass charge on the distance between surfaces in KCI solutions. When the pH value ranges from 4 to 6 at C = 10-2-10-5 mol/L, the surface potential proves to be practically independent of the gap width. The sums of the electrostatic and the molecular force calculated by taking into account the aforesaid were subtracted from the experimentally measured force of the interaction of glass filaments. As a result, we have calculated the dependence of the structural component of the forces on the distance between samples. The calculated dependence is of a monotonous (exponential) character, which allows these forces to be qualified as a macrostructural component of the surface forces. Acknowledgment. We express our gratitude to V. M. Muller for his discussion of the present work.
Structural Component of Disjoining Pressure in Wetting Films of Nitrobenzene Formed on the Lyophilic Surface of Quartz? B. V. Derjaguin,” Yu. M. Popovsky, and A. A. Goryuk Department of Surface Phenomena, Institute of Physical Chemistry, Academy of Sciences of USSR, Moscow 117915, USSR, and University, Odessa, USSR Received February 6, 1987 It has been shown that a structural component of disjoining pressure, IIs,decreasing according to the exponential law as the layer thickness increases, arises in the wetting films of nitrobenzene formed on the lyophilic surface of quartz. A family of isotherms, &, have been obtained within a temperature range 293-333 K. The dependence of parameters on temperature has been determined, the parameters being characteristic of the transition of a wetting film into a thermodynamically nonequilibrium state. The electrostatic and the molecular components of disjoining pressure have earlier been investigated in ref 1 and 2. The structural component II,, arising when the interphase boundary layers overlap, whose structure is different from that of bulk l i q ~ i dhas , ~ been considerably less investigated. The existence of the structural component of disjoining pressure has been shown experimentally for a-films of water, formed on the quartz surface? and for free and wetting films of nematic liquid crystal^.^ An attempt was made to calculate theoretically the structural component, II,, for water films on the lecithin surface.6 The exponential dependence of the structural component of disjoining pressure on thickness, obtained in the cited work, qualitatively agrees with the data.4 ‘Presented at the “VIIIth Conference on Surface Forces”, Dec 3-5, 1985, hloscow; Professor B. 1’. Derjaguin, Chairman.
0743-7463/87/2403-0628$01.50/0
Yet the theoretical diagram used ref 6 contains a number of simplifications that considerably reduce its merits. The structural component of disjoining pressure was theoret(1)Derjaguin, B. V. Kolloidn. Zh. 1941, 7,285-287; Zh. Eksp. Teor. Fiz. 1941,11,802-821;15,633-636 Acta Phys. Chem. USSR 1939,lO(3), 333-346; Trans. Faraday Sac. 1940,36,203-215,730-731. Derjaguin, B. V.; Landau, L. D. Acta Phys. Chem. USSR 1941,14 (6),633-662. (2)Dzyaloshinsky, I. E.; Lifshits, E. M.; Pitaevsky, L. P. Zh. Eksp. Teor. Fiz. 1959,37,229-243;Uspekh. Fiz. Nauk 1961,73,381-402;Ado. Phys. 1961,10,165-187. (3)Popovskij, Yu. M.In Research in Surface Forces; Plenum: New York, 1971; Vol. 3, pp 129-134. Derjaguin, B. V.;Popovsky, Yu. M. Kolloidn. Zh. 1982,44, 863-870. (4)Derjaguin, B. V.; Zorin, 2. M. Zh. Fis. Khim. 1955,29,1010,1755. Derjaguin, B. V.; Zorin, Z. M. Proc. Int. Congr. Surf. Activity, 2nd; London, 1957;Ershova, G.P.; p 145. Zorin, Z. M.;Novikova, A. V. In Surface Forces in Thin Films; Nauka: Moscow, 1979;pp 168-173. (5) Gorokhov, V. M.; Shcherbakov, L. M., theses of the reports held at the Union Scientific Conference, Liquid Crystals and Their Practical Applications; Ivanovo, 1985;Vol. 1, book 2, p 212. (6) Marcelja, S.; Radic, N. Chem. Phys. Lett. 1976,42,129-134.
0 1987 American Chemical Society
Langmuir, Vol. 3, No. 5, 1987 629
Disjoining Pressure in Wetting Films of Nitrobenzene Table 1 ~~
~
t, "C
20
30
40
50
60
h, nm
A
P
A
P
A
P
A
P
A
P
20 25 30 35 40 45 50 55 60
14.59 17.91 21.10 24.04 26.73 29.13 31.20 32.89 34.13
6.52 6.31 6.06 5.79 5.49 5.17 4.84 4.50 4.16
13.81 16.97 19.94 22.70 25.20 27.41 29.29 30.80 31.86
6.52 6.33 6.09 5.83 5.54 5.24 4.92 4.59 4.25
13.03 16.00 18.79 21.30 23.68 25.72 27.43 28.77 29.67
6.54 6.35 6.12 5.87 5.59 5.30 4.99 4.68 4.36
12.21 16.06 17.66 20.00 22.20 24.07 25.62 26.80 27.56
6.56 6.37 6.16 5.91 5.65 5.37 5.07 4.77 4.47
11.50 14.10 16.54 18.76 20.75 22.46 23.85 24.89 25.52
6.57 6.40 6.19 5.96 5.71 5.44 5.16 4.87 4.58
-
ically calculated for comparatively thick films of the nematic phase of liquid crystals, h 1 pm, in ref 7 and 8 within the framework of the continual theory of liquid crystals. A satisfactory agreement with experiment was thus obtained. However, the continual theory is probably inapplicable to the orientation-ordered wetting films of nonmesogenic liquids having thickness within the range h 10 to 60 nm. The structural component, II,, has not up to now been directly experimentally investigated in such films. Using the theory of a self-consistent field as the basis: B. V. Derjaguin assessed the structural component of disjoining pressure for thin, nonsymmetrical wetting films. It has been shown that a positive component of disjoining pressure arises as a film gets thinner and the near-to-wall orientation-ordered layers overlap. However, no quantitative results have been obtained. As has been shown earlier,'O some nonmesogen polar organic liquids, monosubstitutes of benzene (e.g., nitrobenzene, aniline, etc.), on the quartz surface form the orientation-ordered, near-to-wall layers ( h 50 nm), whose symmetry and character of ordering are similar to those of nematic liquid crystals. The symmetrical layers of these liquids having thickness 2h I100 nm, confined between quartz surfaces, are uniform with regard to their properties, and are characterized by the homotropic orientation of molecules. The wetting films of such liquids, formed on the lyophilic surface of quartz, are a convenient object for investigation of the structural component of disjoining pressure, inasmuch as their stability is totally determined by the values of II,. Such dielectric films do not contain any electrostatic component of disjoining pressure, while the molecular component for these is negative. For thicknesses h > 30 nm, the molecular component of disjoining pressure is low, and experiment gives directly the values of n,, while for the range of thickness h 15-30 nm the molecular component of disjoining pressure may be taken into account with the known formula for retarded interactions.'l The wetting films of nitrobenzene, formed on the lyophilized surface of quartz, were chosen as the object to be investigated. Chemically pure graded nitrobenzene, additionally purified by vacuum distillation, was used; special attention was paid to purification and lyophilization of the surface of a quartz plate. For this purpose, after usual
-
-
-
(7) Proust, J. E.; Ter-Minassian-Saraga, L. Colloid Polym. Sci. 1978, 256, 784-792. (8) Samsonov, V. M.; Filippov, S. V.; Shcherbakov, L. M. Theses of the reports held at the Union Scientific Conference, Liquid Crystals and Their Practical Application; Ivanovo, 1985; Vol 1, book 1, p 11. (9) Chandrasekhar, S. Liquid Crystals; Cambridge University Press: Cambridge, London, New York, Melbourne. (10) Derjaguin, B. V.; Popovskij, Yu.M.; Altoiz, B. A. J. Colloid Interface Sci. 1983, 96,492-503. (11) Derjaguin, B. V.; Churaev, N. V. T h e Wetting Films; Nauka: MOSCOW, 1984; pp 9-20.
~
purification operations (rinsing with hot chromic acid and distilled and bidistilled water and drying in vacuum), the working surface of a plate was treated by a hydrogen-air flame jet of a quartz microburner. This enabled one to obtain stable wetting nitrobenzene films on quartz without using surfactants. A porous filter method was applied to measuring the disjoining pressure of nitrobenzene wetting films.12 The film thickness h was measured with the aid of an ellipsometric microscope assembled according to a scheme using a fixed Senarmon's compensator (6' = 45O), K2K.I3 Monochromatic light (A = 546 nm) was directed to an object at an angle p = 60". The accuracy of measuring the polarized asimuth was A€' = fO.lO, while that of an analyzer was about AA = f0.02". The decay asimuths of the polarizer Po and the analyzer A, were read off visually according to a two-zone scheme.I3 The values of Po and A. were measured in each zone 5-10 times and averaged, the error in this case being equal to about A0.25O and fO.15O for Po and Ao, respectively. The ellipsometric angles @ and AI3 were calculated from the known values of decay asimuths Po and A, and the compensator asimuth 0 = 45O as @ = arctan (-tan
A = arccos
[
P( tan P,,")'I2
sin (Po" - P,,') sin (Ao" + A,,') 2 tan \k cos Pi cos Po" sin (A{ - Ad)
1
(1)
Optical quartz (\k = 6.92', A = 0") and polished silicon (\k = 25.0°, 4 = 180°)were used as check (control) samples. The main ellipsometry equation tan \k exp (iA) = R p / R ,
(2)
where Ppand R, are the generalized Fresnel coefficients, correlates the experimental parameters, \k and 4, with the optical characteristics of a reflecting sample, and with the wetting film thickness, h. In the case under consideration, the object being examined is a dielectric-quartz, having a refraction coefficient n, = 1.46, coated by a nitrobenzene layer, n2 = 1.553 (at t = 20 "C, X = 546.1 nm). The dependence of the functions \k(h) and A(h) are calculated by Z. M. Zorin using the known McCrakine program, at the Institute of Physical Chemistry, Academy of Sciences of USSR. Knowing the experimentalvalues of \k and A, it was easy to determine the nitrobenzene film thickness, h, using the (12) Shishin, V. A.; Zorin, Z. M.; Churaev, N. V. Kolloidn. Zh. 1977, 39,400-405. (13) Azzam, R. M. A.; Bashara, N. M. Ellipsometry and Polarized Light; North Holland: Amsterdam, New York;Oxford, 1971.
630 Langmuir, Vol. 3, No. 5, 1987
Derjaguin et al.
1200-
t
1000.
7*0
\
\
830.
600 *
200'
0
20
30,
/-m----G-
,
1.m
k,WM
&) I
/
2 0 3 0
4 0 5 0
Figure 2. Isotherm of the disjoining pressure of a nitrobenzene film at t = 25 "C.
-@O
t
i
3, P,
Figure 1. Isotherm of the disjoining pressure of a nitrobenzene film at t = 25 "C. I200
calculated nomograms for @(h)and A(h). From the values of @(h)and A(h) given in Table I for different temperatures, it is apparent that the ellipsometric angle, @, is less sensitive to a variation in h than the parameter A. Therefore, the film thicknesses were further calculated by using the A(h) dependence. However, a combined 9-A diagram was used for checking the correctness of the measurement results. The experimental values of 9 and A thus obtained were found to fit satisfactorily that diagram. The experimental error (involved in the measurement and the subsequent calculation of the values of 9 and A) was found within the ranges 69 = f0.06" and 6A = f0.47", which resulted in an error of determination of the layer thickness 6h = 0.5-1.0 nm. A setup provided the possibility for examining simultaneously a wetting film by the aid of an ellipsometric and an optical microscope (the latter having a magnification of about X60). This enables one to control more effectively the stability and the degree of purity of the film and differentiate its thermodynamic instability, determinable by the sign of the derivative dIIldh, from the instability of a film, which is associated with lyophobicity of separate areas of a quartz surface. A porous plate having a hole about 0.46 mm in diameter was placed into an air thermostat, which enables one to effect measurements within a temperature range 20-60 "C. The preset temperature was maintained at an accuracy of about 0.2-0.5 "C. The stability of temperature within the thermostat volume was controlled by using a differential thermocouple, whose thermojunctions were attached to the upper and the lower planes of a porous cell. The equilibrium thickness of the film was established approximately in 20 min, which was controlled (checked) by an ellipsometer. In Figure 1 is represented by a continuous line (curve 1)the experimental isotherm of the disjoining pressure of a nitrobenzene film, II, at t = 25 "C; while a dotted line, calculated according to ref 11, shows the molecular component, 11, (curve 2). The isotherm of the structural component of disjoining pressure n, = n + n, (3)
1000
800
600
400
20 30 40 50 60 h , nm Figure 3. Family of isotherms of the structural component of disjoining pressure: (1)t = 40 "C; (2) t = 50 "C; (3) t = 60 O C .
is indicated by a dot-and-dash line (curve 3). The horizontal lengths correspond to the film thickness measurement errors. In Figure 2 are represented in semilogarithmic coordinates (for the same temperature t = 25 "C) II(h) (curve 1) and II,(h) isotherms (curve 2). As is seen from Figure 2, the points on curve 1 satisfactorily fit a straight line within a range of thicknesses of 35-50 nm, whereas systematic deviations are observed with smaller thicknesses, the deviations being associated with the effect of the The molecular component of disjoining pressure, I,. II,(h) isotherm, obtained by taking into account the molecular component, is rectilinear in semilogarithmic coordinates till the thickness h 23 nm is attained. This approximately corresponds to the transition of a wetting nitrobenzene film into the thermodynamically nonequilibrium state and to its breaking up (rupture). This enables one to approximate the isotherm of the
-
Langmuir 1987, 3, 631-634
t'O
Figure 4. Dependence of II= f ( t ) ;(2) nmru = f(t).
and h- on temperature: (1)hdn
structural component of disjoining pressure by an exponential dependence,
II, = noexp (-ah)
(4)
while the total isotherm is approximated by the expression
II = II, exp (-ah)
- Bh-4
(5)
where B is the constant of retarded dispersion interactions. In Figure 3 are presented the isotherms of the structural component of disjoining pressure, measured at 40,50, and 60 "C. The lower limits of the thickness of a wetting nitrobenzene film for each temperature correspond within the limits of experimental errors to the extremum point of the total isotherm of disjoining pressure, (5). On passing
631
through this point, the value of the derivative (dII/dh), becomes positive, and the film breaks up due to its thermodynamic instability. The moment of transition of the wetting nitrobenzene film into its thermodynamically unstable state was ascertained by a considerable increase in the ellipsometric angle \k, as well as by direct examination with an optical microscope. On attaining the nonequilibrium state, a considerable number of microruptures were formed over the whole film surface. The positions of microruptures were statistically varied over the entire film surface, when it formed again (having the same thickness). Yet in the cases where the film instability was caused by lyophobicity of separate areas of a quartz plate, the film rupture spots did not change in the course of repeated experiments. In Figure 4 are represented the minimum film thickness vs. temperature dependences, hminand IImax,calculated with eq 5. For comparison, the ranges of experimental values,,,,II are indicated by vertical lines. As appears from Figure 4, the film is rather unstable at t > 60 "C, and it cannot exist at higher temperatures. The experimental results derived from the examination of wetting nitrobenzene films on a quartz surface, presented in this paper, definitely prove that stability of such films is completely due to the effect of the structural component of disjoining pressure. Moreover, in distinction from the electrostatic and the molecular component of disjoining pressure, the structural component is sensitive to temperature and decreases as the temperature is raised. Registry No. Nitrobenzene, 98-95-3;quartz, 60676-86-0.
Evaluation of the Thickness of Nonfreezing Water Films from the Measurement of Thermocrystallization and Thermocapillary Flowst B. V. Derjaguin, N. V. Churaev,* 0. A. Kiseleva, and V. D. Sobolev The Department of Surface Phenomena, The Institute for Physical Chemistry, USSR Academy of Sciences, Moscow 117915, USSR Received March 10, 1987 The thicknesses h of nonfreezing adsorbed water films on the surface of a quartz capillary between two ice menisci were obtained by measuring the thermocrystallization film flow rate under the effect of a temperature gradient. The values of h decrease from 5-6 nm at -1 "C to 1-1.5 nm at -6 "C. In this temperature range the thermocrystallization flow rate is much higher than the thermocapillary flow rate of the liquid films. When the pore space of porous bodies is filled but incompletely with ice, the surface of particles, which is not in contact with ice, is coated due to the adsorption of vapor by the polymolecular water films that remain in the liquid, nonfrozen state.lY2 When a temperature gradient is imposed on a frozen, porous body, the moisture is transferred into the cold side due to diffusion of vapor and film flow. In the first series of experiments,lV2it has been established that in capillaries about 10 pm in radius the contribution of the film thermocrystallization flow is commensurable with that of the vapor diffusion. In the present work are given the results of the further experiments carried out within a wider temperature range 'Presented a t the "VIIIth Conference on Surface Forces", Dec 3-5, 1985, Moscow; Professor B. V. Derjaguin, Chairman.
and including research on the same quartz capillaries of both the thermocrystallization and the thermocapillary flow of water films. The films were formed on the capillary walls between two menisci (of ice or water) maintained at different temperatures on a setup enabling a permanent temperature gradient to be preserved for a long period of time.3 As was done earlier,1-3the film transfer rates, V,, were determined as a difference between the measured menisci shift velocity, V, and the vapor diffusion rate, V,, calcu(1) Derjaguin, B. V.; Churaev, N. V.; Sobolev, V. D.; Barer, S. S. J. colloid Interface sei. 1981. 84. 182. (2) Barer, S . S.; Kiseleva, 0.'A.; Sobolev, V. D.; Churaev, N. V. Kolloidn. Zh. 1981, 43, 627. (3),Kiseleva,0. A.; Rabinovich, Ya. I.; Sobolev, V. D.; Churaev, N. V. Kollordn. Zh. 1979, 41, 1074.
0743-7463/87/2403-0631$01.50/0 0 1987 American Chemical Society
