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Langmuir 1987, 3, 612-620
break-up of the boundary layer structure under the influence of incorporated hydrated molecules and ions does distinctly show up. An increase in the concentration of solutions causes a decrease in the difference between the dissolving power of water in fine pores and in bulk. The discussed results have demonstrated sufficiently good agreement between the investigation of the physical properties of the boundary layers of water and the structural forces. At high surface hydrophilicity, the structural forces are positive and cause the repulsion of surfaces, the viscosity and density are enhanced, and the dissolving power is reduced. In the case of hydrophobic surfaces, these effects have opposite signs. A wide intermediate range is disposed between these two limit cases, where the degree of the structural peculiarities of the boundary layers may be relatively low.
The structural changes in the boundary layers of liquids, the structural and microstructural forces, represent a physcical reality. However, the conditions under which the structural effects can actually show up will have to be clearly defined, and these should not be used for clarification of deviations from the existing theories in the cases where the structural effects had not to be present a t all. The maxima of structural effects are to be expected in two extreme cases-that of the highly lyophobic and that of the highly lyophilic surface. Further efforts should be aimed at the development of a quantitative theory of structural forces. This will permit their incorporation in the general theory of long-range surface forces and disjoining pressure. Registry No. H20, 7732-18-5.
Thermodynamic and Kinetic Aspects of the Stabilization of Microscopic Liquid Films by the Adsorbed Layers of Macromolecular Surfactants? V. G. Babak Chair of Physical and Colloid Chemistry, All- Union Correspondence Institute of Food Industry, Moscow, USSR Received December 3. 1986 The effect of kinetic factors on the steric stabilization of disperse systems by adsorbed layers (AL) of macromolecular surfactants is discussed in this paper. We have demonstrated that the repulsion force f between two molecularly smooth curved mica surfaces in polymer solution measured as a function of tke film thickness Hf,the adhesive force f a between the fluid and solid particles, and the lifetime T* of the emulsion and foam films stabilized with the AL of macromolecules is substantially influenced by the AL formation time t f even after tf > lo5 s of observation. The experimental dependencies of f p , fa, and T* on tf provide information about the mechanism of polymer adsorption, the conformational rearrangement of macromolecules in the AL, and the interactions between the AL of macromolecules in microscopic liquid films (MLF). We have also elucidated the effect of the physicochemical parameters (the bulk polymer concentration, C,; the electrolyte concentration, Gel; the pH of the solution; the surface area of MLF, 5’; the acetate group content of water-soluble macromolecules able to form hydrophobic bonds; the concentration of the thickening (tannin)and the plasticizing (ethanol)agents) on the stability of the disperse-phaseparticles against aggregation and coalescence. Introduction The interaction between the adsorbed layers (AL) of macromolecules in microscopic liquid films (MLF) appearing in the contact region between the disperse-phase particles determines the stability and the structural-mechanical (rheological) properties of polymer-containing disperse systems. As a general rule the adsorption of the macromolecules at an interface, unlike the adsorption of low molecular weight surfactants, is irreversible,’,’ and therefore the equilibrium concentration of the macromolecular component in MLF is not accessible (at least during the time intervals of film existence in the modeling experiments). This allows us to neglect the adsorption component of the disjoining pressure which is proportional to the difference between the adsorption values in the bulk solution and in the film according to the Derjaguin and Churaev t h e ~ r y .We ~ may also neglect the molecular (van Presented a t the “VIIIth Conference on Surface Forces”, Dec. 3 4 , 1985. Moscow; Professor B. V. Derjaguin, Chairman.
0743-746318712403-0612$01.50/0
der Waals) attraction between the disperse phases in MLF, taking into account the relatively great value of film thickness compared with the macromolecule’s size. At the same time, the nonreversible character of polymer adsorption requires the consideration of the so-called “steric” component of the disjoining pressure due to the contact interaction between two AL of macromolecules in MLF. The theoretical description of the steric interaction is based on two alternative approaches which may be conventionally designated as thermodynamic and structural-mechanical. According to a thermodynamic approach (the HVO theory4 or other variants of thermodynamic theories of (1) Lankveld, J. M. G. Meded. Landbouwhogesch. Wageningen 1970, 70 (21), 114. (2) Lyklema, J.; Vliet, T. v. Faraday Discuss.Chem. SOC.1978, 65, 25. (3) Derjaguin, B. V.; Churaev, N. V. Dokl. Akad. Nauk SSSR 1975,
222, 554. (4) Hesselink, 1971, 75, 2094.
F. Th.; Vrij, A.; Overbeek, J. Th. G. J . Phys. Chem.
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Stabilization of Microscopic Liquid Films steric stabilization5)the mutual repulsion between two AL of macromolecules in the film arises from the increase in free energy which results from the effect of volume restriction or from the increase in energy of mixing within the interaction region of the adsorbed macromolecules. Direct measurements of the repulsion energy between AL of macromolecules as a function of film thickness Hf6-12 have demonstrated the qualitative accordance between HVO theory and experiment, at least for the relatively small deformation of AL.12 Note that the HVO theory explains also the effect of mutual attraction (adhesion) between two AL of macromolecules due to the decrease of “goodness” of the solvent.l0 It is noteworthy that the HVO theory, as well as other thermodynamic theories considering only flexible-chain macromolecules, neglects the influence on the stabilizing effect of the intermolecular interactions and structure formation in the AL of macromolecules and therefore cannot be applied unreservedly to polymers (e.g., poly(viny1 alcohol) having a hypermolecular structure13due to the hydrophobic and hydrogen bonds within individual macromolecules and those coupling the macromolecules of the adsorbed layers. Meanwhile, it is the processes of structure formation in the AL of macromolecules that, according to Rhebinder,14J5 convert the adsorbed layers into a structural-mechanical barrier (SMB) with anomalously high viscosity and mechanical strength which hinders both the thinning of MLF and the convergence of the interphase surfaces to the distance ( 10 nm) at which molecular attraction results in an irreversible thinning and rupture of liquid films. Taking into account that structure formation in the AL depends on time, the notion of a SMB as a factor of stability of dispersion systems grounds the necessity of studying kinetic characteristics of the mechanism of stabilization of dispersion systems by macromolecules and orients the experimentalists to realize the modeling of transition processes accompanyingthe contact interaction between the disperse phase particles. In real technological processes of emulsifying, stirring, and transportation of the dispersions, the formation and the destruction of MLF appearing in the contact region between the disperse-phase particles occur under nonequilibrium conditions by the process of adsorption, conformational rearrangement of macromolecules, and structurization in the adsorption layers. Under such conditions, due regard for the kinetic factors is of basic importance for elucidating the stability of concentrated dispersions, since the role of the kinetic factors, unlike the thermodynamic factors of stability, grows as the system becomes more and more in nonequilibrium; i.e., the rate N
(5) Sato, T.; Ruch, R. In Stabilization of Colloidal Dispersions by Polymer Adsorption; Schick, M. J., Fowkes, F. M., Eds.; Marcel Dekker: New York and Basel, 1980; Vol. 9, p 155. (6) Sonntag, H. IV Znt. Tagung iiber Grenzflechenaktiue Stoffe; Akademie-Verlag: Berlin, 1977; p 517. (7) Vliet, T. v. Meded. Landbouwhogesch. Wageningen 1977, 77 (l), 126. (8) Baran, A. A. Teor. E x s p . K h i m . 1979, 19, 534. (9) Knapschinsky, L.; Katz, W.; Ehmke, B.; Sonntag, H. Colloid Polym. SCL.1982, 260, 1153. (10) Rabinovich, Ya. I.; Derjaguin, B. V.; Churaev, N. V. Adu. Colloid Interface Sci. 1982, 16, 63. (11)Klein, J. J. Chem. Soc., Faraday Trans. 1 1983, 79, 99. (12) Babak, V. G. Kolloid. Zh. USSR 1986, 48, 641. (13) Efremov,I. F.; Kovilov, A. E. In Physico-chemical Mechanics and Liophility of Disperse Systems; Naukova Dumka: Kiev, 1978; Vol. 10, p 100 (in Russian). (14) Rhebinder, P. A. Zzu. Akad. N a u k SSSP,Ser. Chim. 1939,5,639 (in Russian). (15) Rhebinder, P. A. Surface Phenomens in Disperse Systems. Colloid Chemistry; Nauka: Moscow, 1978; p 368.
Table I. Some Physicochemical Parameters of Poly(viny1 alcohol) (PVA), Poly(acry1ic acid) (PAA), and Acrylic Acid-Vinyl Acetate Copolymer (CP) Samples sample PVA-1 PVA-3 PAA-1 PAA-3 PAA-4 CP-1 CP-3 CP-5 CP-26
content of acetate groups, % 1
26 0 0 0 1
3 5 26
hEs
mol wt, macromolecule size, M x io6 ( r 2 ) I j 2nm , 0.62 25 0.50 22 0.55 21 1.70 42 2.10 47 1.20 33 1.40 37 1.25 36 1.50 39
(4)
P Y
t L
dynamometer(?) t l a e r ( 8 )
?I
31 81
Figure 1. Scheme of the device for measuring contact interaction between the fluid and solid surfaces (for explanation see the text).
of a change in the external perturbating factors acting on the dispersion system increases. That is the reason why the results obtained in so-called “equilibrium” conditions cannot be applied unreservedly to real technological processes. The objective of the present work is to point out the role that is played by the kinetic factors in the process of steric interaction between the adsorbed layers (AL) of macromolecules in polymer-stabilized disperse systems and the possibility of obtaining more information about the mechanism of the stabilizing effect of the AL from a study of the transition processes in the microscopic liquid films stabilized by polymers.
Experimental Section Materials. Mica specimens (10 pm thick, 1 X 1.5 cm2 sheets) were cleaved off a bulky crystal open to air in a dust-proof box. The sheets were tightly pressed to the cylindrical surface of radius
R = 1cm of titanium miniholders and fixed with clamps (no glue used). In the middle part of the holder surfaces, 3 X 5 mm2 rectangular peepholes were bored through which to inspect by reflected light the surface contact of the two mica specimens. Measurements employed outgassed bidistilled water purified additionally of dust by the procedure described in ref 16 and 12 via passing it through columns with carbon filters and through a system of Millipore filters with the minimal size of 10 nm. In a similar way dust was eliminated from 0.1 M KC1 solutions. Note that when unpurified bidistilled water was used, the mica sheets failed to come closer than 50 nm to each other. The polymer solutions were prepared by using unpurified bidistilled water. Purification from dust consisted of centrifuging 5% heptane emulsions in a polymer solution with the initial size of the drops close to 50 fim at a rate of 8000 rpm for 30 min. Use was made of poly(viny1alcohol) (PVA) with different acetate group content n, poly(acry1ic acid) (PAA), and acrylic acid-vinyl acetate copolymers (CP). Table I lists the parameters which characterize (16) Pashley, R. M.; Israelachvili, J. N. Colloids Surf. 1981, 1, 169.
614 Langmuir, Vol. 3, No. 5, 1987
Babak
150 Mica sheets
dynamometer
0 0' 2 0
C
@
t I-
I
10-
I
0
Electric current intensity, I
I
-
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force
I I I
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E !I
Compression f o r c e
Pull-oii
t - 0
L
S
A
m 0
5
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Figure 2. Illustration of the measurement of film thickness H f as a function of the electric current intensity I in MES dyna0
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-
(17) Babak, V. G.; Chlenov, V. A. J . Dispersion Sci. Technol. 1983,4, 221. (18) Babak, V. G.; Monissova, R. A. J . Dispersion Sci. Technol. 1985, 6, 539.
(19) Babak, V. G. Kolloid Zh. USSR 1985,47, 435, 582. (20) Israelachvili, J. N.; Adams, G. E. J . C h e n . Soc., Faraday Trans. I 1978, 74, 975.
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