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“additional” surface potential is caused by the contribution of the surface dipoles. Thus, it was shown that significant electric field created by surface dipoles exists near the surface of the phospholipid membrane. Just this field leads to strong “hydration” repulsion between two membranes at small separations. At present we are not aware of the value of the parameter lo entering the water dielectric function t ( q ) . Therefore we must use the experimental value of the decay length X N 2.5-3 A.7 Then from comparison of eq 36 with the experimental data it follows that lo E X = 2.5-3 A. The preexponential factor which we estimate for the pure water case is (see eq 36) P1 (5 X 1010)(d,/lo)2dyn/cm2 with ed, being the normal component of the head group dipole moment (we use S N 60 A2 for the lateral area of a lipid molecule). A precise value of d, is not known now but the estimation d, N X seems to be reasonable. Thus, we obtain P, in the range 101o-lO1l dyn/cm2, which is slightly above the measured interval. This overestimation can be attributed to the use of the simple linearized (with respect to the electric field) theory. Note that the increase of the intermembrane repulsion with the increase of parameter 1 (see eq 39) enables us to understand the measured decrease of P, in the course of a bilayer phase transition from the ‘‘liquid’’ to “solid”state? The point is that this transition leads to the pushing out of the water from the head group region and so to a decrease in l. (It is possible that thermal fluctuations of bilayer considered in ref 15 are also responsible for this effect.)
Thus, we have shown that structural (hydration) forces can be accounted for as a result of the dipole-dipole interaction of polar surfaces in media with nonlocal polarization. Nevertheless we cannot exclude the possibility of another (nonelectrostatic) origin of structural forces. For example, the general order parameter introduced in ref 9 could be connected with the orientational order parameter which was considered in some papers (see, e.g., ref 38,39) dealing with the problem of a melting phase transition. Indirect confirmation for our theory could be produced by careful measurement of the dependencies of the “polarization” (q1-l) and “screening” (q2-I)lengths on the electrolyte concentration [see (18) (19)]. If our theory of hydration forces were confirmed, it would be possible to determine both parameters (e1 and lo)of water dielectric response by comparison of measured values of q l , q2, and Powith our eq 18, 19, and 36. Acknowledgment. We are grateful to I. E. Dzyaloshinsky, A. Z. Patashinsky, B. V. Derjaguin, S. A. Leikin, I. L. Fabelinsky, Ya. A. Chizmadzev, N. B. Churaev, I. G. Abidor, and especially A. A. Kornyshev for many illuminating discussions. Registry No. Water, 7732-18-5. (38)Mitus, A. S.;Patashinsky, A. Z. Zh. Eksp. Teor. Fiz. 1981,80, 1554. (39)Bruinsma, R.;Nelson, D. R. Phys. Reu. B 1981,23,402.Nelson, D. R.;Sachdev, S. Phys. Reo. B 1985,32,4592. (40)Dzhavakhidze, P. G.;Kornyshev, A. A.; Levadny, V. G. Phys. Lett. A 1986,118A, 203. (41)Kornyshev, A. A.;Vorotyntsev, M. A. Can. J. Chem. 1981,59, 2031.
Temperature Dependence of the Stability of Natural Diamond Dispersions in Electrolyte Solutionst Yu. M. Chernoberezhskii,* V. I. Kuchuk, 0. V. Klochkova, and E. V. Golikova Leningrad Technological Institute of Cellulose-Paper Industry, 198092 Leningrad, USSR Received January 15, 1987 The flow ultramicroscopy method was used to investigate the aggregative stability and coagulation of diamond hydrosols having a particle size of about 0.5 pm at different pH values and electrolyte concentrations (LiC1and AlClJ. The determining role of structural forces in hydrosol stability has been demonstrated. An increase in temperature causes destruction of the boundary layers of water and a decrease in structural repulsion. It has been shown that the change of the structure of boundary layers with temperature is reversible. The existence of structural changes in boundary layers (BL) of a liquid near the surface of a solid body is beyond all doubt at present. When considering disperse particle interactions it was necessary to take into account structural and modified BL, which added a lot to the physical foundation of the stability theory. Derjaguin and Churaev introduced the concept of structural components of interaction forces between converging surfaces with BL overla~ped.’-~ The concept was based on experimental studies proving the existence of bound water layer^.^ Recently there appeared a great number of theoretical and experimental studies devoted to the investigation of ‘Presented at the ”VIIIth Conference on Surface Forces”, Dec 3-5, 1985,Moscow; Professor B.V. Derjaguin, Chairman.
structural forces. Some progress has been achieved in understanding their nature and regularities in their changes. Thus, based on studies6-16the law of structural (1)Derjaguin, B. V.; Churaev, N. V. J.Colloid Interface Sci. 1974,49, 49. (2)Derjaguin, B. V. Chem. Scr. 1976,9,95. (3)Derjaguin, B.V.; Churaev, N. V. Croat. Chem. Acta 1977,50,187. (4)Churaev, N. V. Kolloidn. Zh. 1984,46,301.Churaev, N. V.;Derjaguin, B. V. J. Colloid Interface Sci. 1985,103, 542. (5) Derjaguin, B. V. In Works of Allunion Conference of Colloid Chemistry; Acad. Nauk USSR: Kiev, 1952;p 26; Wear, 1958,1, 277; Discuss. Faraday SOC. 1966,42,109. (6) Marcelja, S.; RadiB, N. Chem. Phys. Lett. 1976,42,129. (7)Peschel, G.;Belouschek, P. 2. Phys. Chem. N.F. 1977,108, 145. (8)Peschel, G.;Belouschek, P.; Muller, M. M.; Muller, M. R.; Konig, R. Colloid Polym. Sci. 1982,260,441. (9)Israelachvili, J. N.J. Chem. Soc., Faraday Trans. I 1978,74,975.
0743-7463/87 /2403-0654$0l.50/0 0 1987 American Chemical Society
Langmuir, Vol. 3, No. 5, 1987 655
Stability of Natural Diamond Dispersions
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tructible at higher electrolyte concentrations as the temperature increases. Figure 4 represents the dependence of diamond particle aggregation degree on the period of observation at pH 3. As follows from the results of the investigation of diamond dispersion stability in LiCl solutions at room temperature,%this pH value corresponds to the maximum stability of the system. With LiCl concentration equal to 1 X mol/L, diamond dispersion is stable ( m = 1)at all temperatures. However, an increase in aggregation degree at higher temperatures is observed as electrolyte concentration increases (Figure 4b,c). It should be noted that with CLicl= 5 X 10-1 mol/L, the values of aggregation degree when the temperature ranges from 40 to 50 "C remain practically the same ( m = 1.8). Data on coagulation at pH 6 (Figure 5) do not show any appreciable variations. As is the case with pH 3, with 1 X mol/L concentration of LiC1, and at higher temperatures, aggregation degree does not change. At higher concentrations up to lo-' mol/L and growing temperature a considerable increase in aggregation degree ( m = 1.6) is observed at 40 "C compared to the value at room temperature (m = 1.2). Further temperature growth to 50 "C, however, does not lead to changes in the system's aggregate equilibrium. With 5 x lo-' mol/L concentration of LiC1, (33) Golikova, E. V.; Klochkova, 0. V.; Kuchuk, V. I.; Chernoberezhskii, Yu. M. Kolloidn. Zh., in press.
an increase in temperature leads to deeper BL changes: not only the aggregation degree sharply increases, but the nature of aggregation also changes compared to the investigated system's behavior with 1 X 10-1 mol/L concentration of LiC1, m,-t curves coinciding at 40 and 50 "C. From the calculations based on DLVO theory, forces of molecular attraction should prevail at all distances between diamond particles at 20 "C and concentration of 1:1 electrolyte more than 1 X mol/L (pH 2,3, and 6), and therefore the observed stability of diamond dispersion at room temperature can be explained only by the prevailing contribution of the structural component of the interaction energy over the molecular component. Changes in BL structure due to temperature growth are reflected to a great extent in the stability of the system under investigation. However, comparing the data obtained at different pH values, one can see that the least degree of aggregation still corresponds to pH 3. This, in our opinion, is evidence of the maximum thickness and strength of BL on the surface of diamond particles at the stated pH value. On the basis of the previous study of diamond dispersion stability in A1C13solutions at 20 it could be supposed that under the given conditions BL are sufficiently developed and the temperature effect would be more noticeable. Besides, the aluminum ion effect in A1Cl3 solutions-in contrast to that in LiCl solutions-might result not only from ion implantation into the BL structure but also from the changing properties of the diamond
658 Langmuir, Vol. 3, No. 5, 1987
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