16 Chemisorption and Dewetting off
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Glass and Silica
L I S B E T H TER-MINASSIAN-SARAGA Laboratoire de Chimie Physique Faculté des Sciences, Paris, France The adsorption of dodecyltrimethylammonium bromide (LNBr) from aqueous solutions on silica and glass slides dewetted by these solutions was measured by using molecules labeled with radioisotopes (C and B r ). At low L N B r concentrations the counterion, Br , was not adsorbed. It was assumed that the LN ions were chemisorbed by cation exchange between the aqueous L N B r solution and the ≡SiOH groups at the surface of silica or glass. The adhesion free energy was measured for these systems. The results were interpreted by assuming that the adhesion free energy may be related to a process of cation exchange between the solid surfaces and a L N B r monolayer adsorbed at the surface of the solution that dewets the solid. 14
82
-
+
The chemisorption, at the surface of a solid (wet slide or suspension), of surfactants which are completely dissociated when in aqueous solution may depend on the character of the active groups of the solid (acid or basic). According to Carman [8] and many other authors [17], the surface hydration of collodial amorphous silica results in the formation of one hydroxyl for every silicon atom at the surface; the other three valencies of this atom are saturated by oxygen atoms, in such a manner that the tetrahedral coordination is preserved. When hydrated silica is brought in contact with water, hydrated silicon atoms or silanol groups may dissociate and form electrically charged ionic sites. The existence and the sign of the electrical charges are revealed by electrokinetic phenomena [21]. Glass [22], vitreous silica [3, 23], and quartz [19] bear negative charges. The pK of dissociation of silanol groups is not known. But for the first dissociation of silicic acid: 232
In Contact Angle, Wettability, and Adhesion; Fowkes, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1964.
76.
TER-MINASSIAN-SARAGA
Chemisorption
H Si0
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4
4
-
H SiO; + H 3
and
Dewetting
233
+
the value of pK is 9.8 [11,17]. It maybe inferred that silica behaves as a very weak colloidal acid. The exchange of mineral cations between silica and many aqueous solutions has been studied recently as a function of pH [1,12]. It has been shown [1] that the experimental results conform to the law of mass action. When brought in contact with aqueous solutions of organic salts (dodecyltrimethylammonium bromide [3] or dodecylammonium chloride [19]) of increasing concentrations, the surface electric charge of silica may decrease to zero and even become positive. The decrease of the electric charge at the silica-solution interface may proceed by the following mechanisms: The long-chain organic cations of the salts are attracted by electrostatic forces [24] and combine with a negatively charged (ionized silanol) site which becomes neutralized, while another cation (which may be a H * ion) is released from the diffuse Gouy layer. A cation exchange [19] between the solution and the undissociated (neutral) silanol groups at the surface of silica leads to a decrease of charge in the diffuse Gouy layer, if the groups resulting from this exchange dissociate to a lesser extent than the original silanol groups of the silica-water interface. In this process too a H * ion is released into the solution. As the end result of both processes is adsorption, we call it "chemisorption." It leads to the "neutralization" of the ionized or ionizable groups of the colloid (silanol groups for silica surfaces). In this process H * ions participate. It should, therefore, be dependent on the pH of the aqueous phase. The positive electric charge at the surface of silica in contact with the above-mentioned solutions has been ascribed either to the physical adsorption of a second layer of long-chain ions (and of their counterions) on top of chemically bound ions to silica [19,24] or to the chemisorption on silica from solution [18] of multivalent positively charged micelles. Greenberg [12] has found by measuring the capacity of cation exchange, with calcium hydroxide, that a maximum of eight silanol groups per 100 sq.A. may be accounted for at the surface of amorphous silica gels. The capacity of exchange was less (about half of the first value) when measured with sodium hydroxide. However, areas per molecule of stearic acid and n-octadecane adsorbed on dry nonporous aerosil from solutions in heptane [5] as well as areas per cation-exchange site of clay minerals [6], which have a planar symmetry, measured by Brooks, lie within the range of 60 to 140 sq. A . Also in this range (40 to 80 sq.A.) lie the values of areas per silanol site, which may be calculated from the results of Young [31]. A peculiar type of adsorption [26] may take place while a glass slide is raised out of a solution of a substituted quaternary ammonium salt, which is a dewetting agent for glass. Adsorbed molecules at the solutionair interface are transferred to the solid surface and are added to the molecules already adsorbed on the solid directly from the solution. Tenebre [26] has called this transfer "complementary adsorption." The densities of molecules adsorbed from the bulk of the solution and those transferred from the free surface of the solution were measured q
q
q
In Contact Angle, Wettability, and Adhesion; Fowkes, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1964.
ADVANCES IN CHEMISTRY SERIES
234
separately. It was found that 1.4 x 1 0 molecules of tetradecyltrimethylammonium bromide were transferred to 1 sq.cm. of glass from the s u r face of a solution (2 χ 1 0 " M ) . The area per molecule was therefore 70 sq.A. The free energy of adhesion, τ, glass solution [26], has been related to the surface density of transfer red molecules by using the usual defini tion of the free energy of adhesion, τ = y - y = y ^ cos θ [2,16] and the Gibbs equation as 14
4
s
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δ
- δ
dT
s
_
i f
A given capacity of exchange for the silica surface: Ν = Γ°
+ Γ°
SiOH
+
Γ °
SiOLN
.
(13)
SiO~
Constant pH of the aqueous phase μ + = constant Η
(14) μ O H - = constant Constant activity and chemical potential in the bulk phases of water and silica, leading to: μ
S i 0 H
= constant (from 6 ) f
(15) μ
S i 0
- = constant (from 7 and from 14) T
When the concentration of L N B r is varied in the aqueous phase at constant pH, conditions 14, 15, and 9 lead to the following expression for the change in the chemical potential of the superificial = SiOLN groups: T
^ M SiOLN
=
d
^ L N +
In Contact Angle, Wettability, and Adhesion; Fowkes, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1964.
r _ are zero). The capacity of exchange is equal to o
+
γ
s i 0
Ν = Γ°
+ Γ°
SiOH
^
SiOLN
The measured chemisorption is equal to the number of = SiOLN groups δ
= Γ° S
(23) V
SiOLN
7
For the S/A interface the variation of the interfacial tension with concentration of L N B r at constant temperature and pH (Relation 5) has the following expression: -
=
CoH
^ S i O H
+
r
SiOLN
^ S i O L N
^
Substituting Equations 14, 16, 22 , and 23 into 24 we get an equation analogous to Equation 20. T
(- d y / k T d l n c s
L N B r
) = δ
(25)
δ
Subtracting 20 from 25, the slope of the tangent to the curve of adhesion tension vs. the logarithm of the concentration of L N B r , is ob tained at constant temperature and pH. The expression for this slope is: (- d r / k T d In
C L N B r
) = (ô
s
- ô
s / L
)
(26)
Comparison of this relation to that suggested by Tenebre (Equation 1) shows that the parameter a is equal to 1.
In Contact Angle, Wettability, and Adhesion; Fowkes, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1964.
ADVANCES IN CHEMISTRY SERIES
248
Relations 20, 25, and 26 have been obtained for a system constituted of silica and plain aqueous solutions of L N B r . The same result can be arrived at when sodium hydroxide is d i s solved in the aqueous phase and maintained at a constant concentration (1.2 x ΙΟ* M) while c is variable. It has been shown by Ahrland, Grenthe, and Noren [1] that at the corresponding high pH all the = SiOH groups are transformed into = SiONa groups. The exchange equilibria (Equations 9 and 22) then become equilibria of exchange between the pseudo-silicate groups, = SiONa, and the aqueous solution of L N B r (Figures 6 and 7). There fore if the symbol H is replaced by Na, the expression obtained at last for the coefficient (- d r / k T d ln C u ^ r ) is again Equation 26.
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2
lNBr
Nomenclature μ . = molecular chemical potential of species i μ· = molecular chemical potential (standard) of species i Γ ° = surface excess per unit area of species i ([15], equation 9.30.1) in Gouy layer or on solid surface Ν = total number of acid sites (silanol) on 1 sq. cm. of silica surface f i = activity coefficient of species i in aqueous phase Conclusions By using molecules labeled with radioisotopes it is possible to de termine the amount of substituted ammonium ions chemically adsorbed (by cation exchange) on glass or silica surfaces from an aqueous solu tion or from the monolayer adsorbed at the surface of the solution. The isotherms of adsorption of trimethyldodecylammonium ions on both solids have a two-stepped shape corresponding to an expanded layer at low surface coverages and a compressed layer at high surface coverages. In the expanded layer the paraffin chains of the ions lie on the surface of the solid and may have a screening effect on several nearby silanols, which become inaccessible to other adsorbing ions. In the compressed layer the chains are gradually standing up. At maximum coverage the number of chemisorbed ions equals the number of silanol groups on the solid s surface. Each silanol group occupies 65 sq.A. on the silica surface and 52.5 sq.A. on the glass surface. The free energy of adhesion or adhesion tension for the system s i l i c a - L N B r aqueous solutions has been measured. The model, which explains the above-mentioned peculiarities of the isotherm of chemi sorption at the surface of the solids, is confirmed by the results of adhesion tension measurements. 1
Acknowledgment The author is indebted to Yolande Hendricks for many stimulating discussions and to A . S. Michaels for helpful suggestions.
In Contact Angle, Wettability, and Adhesion; Fowkes, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1964.
76.
TER-MINASSIAN-SARAGA
Chemisorption and Dewetting
249
Literature Cited
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(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29) (30) (31) (32)
Ahrland, S., Grenthe, I., Noren, B., Acta Chem. Scand. 14, 1059 (1960). Bartell, F. E., Bartell, L. S., J. Am. Chem. Soc. 56, 2205 (1934). Baruch-Weil, M., Ann. Phys. (13) 4, 1159 (1959). Bolt, G. H., J. Phys. Chem. 60, 325 (1956). Brooks, C. S., J. Colloid Sci. 13, 522 (1958). Brooks, C. S., J. Phys. Chem. 64, 532 (1960). Bungenberg de Yong., H. G., "Colloid Science," H. R. Kruyt, ed., Vol. II, p. 304, Elsevier, Amsterdam, 1949. Carman, P. C., Trans. Faraday Soc. 36, 964 (1940). Ellison, A. H., J. Phys. Chem. 66, 1867 (1962). Fleury, Ph., unpublished results. Greenberg, S. Α., J. Am. Chem. Soc. 80, 6508 (1958). Greenberg, S. Α., J. Phys. Chem. 60, 325 (1956). Guastalla, J., "Proceedings of 2nd International Congress," p. 143, Butter worth, London, 1957. Guastalla, L., Compt. Rend. 243, 1314 (1956). Guggenheim, Ε. Α., "Thermodynamics," p. 326, North Holland Publishing Co., Amsterdam, 1949. Harkins, W. D., Fowkes, F. M., J. Am. Chem. Soc. 60, 1511 (1938). Her, R. K., "Colloid Chemistry of Silica and Silicates," Cornell University Press, Ithaca, N.Y., 1955. Ibid., p. 251. O'Connor, D. J., Buchannan, A. S., Trans. Faraday Soc. 52, 397 (1956). Okersee, C., de Boer, J. H., "Silice," Colloque, de l'Association Belge pour Favoriser l'Etude des Verres et des Composes Silicieux, Brussels, Bel gium, 1960. Overbeek, J. Th., "Colloid Science," H. R. Kruyt, ed., Vol. I. p. 194, Elsevier, Amsterdam, 1952. Rutgers, A. J., de Smet, M., Trans. Faraday Soc. 41, 758 (1945). Rutten, F. (Van), J. Chim. Phys. 53, 3 (1956). Tamamushi, B., Kolloid Z. 150, 44 (1957). Tamamushi, B., Tamaki, D., Trans. Faraday Soc. 55, 1013 (1959). Tenebre, L., Mem. Serv. Chim. Etat. 40, 77 (1955); J. Chim. Phys. 53, 6 (1956). Ter-Minassian-Saraga, L., Compt. Rend. 249, 1652 (1959). Ibid., 252, 1596 (1961). Ter-Minassian-Saraga, L., J. Chim. Phys. 57, 10 (1960). Wilhelmy, L., Ann. Physik, 9, 475 (1902). Young, G. J., J. Colloid Sci. 13, 67 (1958). Zutrauen, H. Α., J. Chim. Phys. 53, 54 (1956).
Received April 3, 1963.
In Contact Angle, Wettability, and Adhesion; Fowkes, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1964.
