1188
Langmuir 1988,4, 1188-1193
In order to check whether the above picture makes sense, we carried out a similar set of experiments with a mixture of two samples of PEO (SE-8 and PD, respectively) in a 1:l weight ratio. All experimental conditions were exactly the same as those for Figures 4 and 6. If our explanation is correct, we should again expect a maximum in both aH(0)and as a function of pH and substantial relaxation in the neighborhood of the maximum. The result, presented in Figure 7, shows exactly that pattern; the maximum is quite large, and for pH >9 both curves drop very rapidly. There is still some doubt as to the importance of the flow field in the relaxation process.18 We therefore checked for a particular experiment (polydisperse, PVP, pH 8.3) whether the relaxation would proceed differently in the absence of flow. The result is shown in Figure 8 where we plot V,(t) while switching the pressure off and on several times. It seems from this result that relaxation continues even in the absence of flow, which supports the idea that the flow field is of little importance even when the polymer is relatively weakly bound. (18)Lee, J. J.; Fuller, G. G. J . Colloid Interface Sci. 1985, 103, 569.
Conclusions Thickness relaxations observed in the streaming potential experiment developed by us show very strong dependence on the polydispersity of the polymer sample, especially when the segments are strongly bound to the surface. This could be concluded from data sets obtained with polydisperse (PEO, PVP) and monodisperse (PEO) samples a t various pH values. The results are consistent with an essentially diffusion-controlled enrichment of the surface with small molecules a t low pH and a thermodynamically driven preference for the large molecules to adsorb at neutral and high pH. The role of the flow field seems to be minor, except a t pH >10 where the adsorption becomes extremely weak. Acknowledgment. We thank Prof. J. Lyklema for his encouraging discussions. We are indebted to A. J. van der Linde for his help with the streaming potential measurements. H. Tamai also wishes to thank the Wageningen Agricultural University for the financial support that enabled him to stay in the Netherlands. Registry No. PVP, 9003-39-8; PEO, 25322-68-3.
Interactions of Poly(dimethylsi1oxane) with Lewis Bases Sydney Ross* and Nguyet Nguyen Department of Chemistry, Rensselaer Polytechnic Institute, Troy, New York 12180-3590 Received March 14, 1988 The surface activity of poly(dimethylsi1oxane) in a synthetic ester lubricant is traced to an acid-base interaction between solute and solvent, which is enough to confer a degree of solubility on the poly(dimethylsiloxane). The acid character of poly(dimethylsi1oxane) is revealed by its interaction with a basic this interaction further increases the solubility of the second solute, namely, N-phenyl-1-naphthylamine; poly(dimethylsi1oxane) in the lubricant and so reduces its surface activity. The relative measures of the surface activity of the polymer are provided by the concentration required to reach the 50% point of transition between the regimes of Rybczynski-Hadamardand of Stokes in the rate of rise of a bubble in the solution. Shifts of the NMR spectra when poly(dimethylsi1oxane) interacts with a “soft”base (triethylamine) and with a ”hard” base (ethyl acetate) confirm that poly(dimethylsi1oxane) is a Lewis acid and a hard acid at that. The silicon atom is the source of its hard acid character because of ita small size, slight polarizability, and empty d-orbitals, which can accept electrons from a base. Introduction The problem of the foaming of aircraft lubricants was a serious one for the defenders during the Battle of Britain in the second world war. Before takeoff, the pilots of intercepter planes (Spitfires) would pump gasoline into the cold oil so as to reduce its viscosity in order to make the motors start without delay; only, on rising to a high altitude, to suffer a loss of lubricant through the breather holes, because of the expansion of the volume of the oil by entrapment of vaporized gasoline. The technical problem thus posed was how to separate the entrained vapor from the liquid phase by immediate rupture of the foam lamellae. Various foam inhibitors were tried, but none proved as effective as either fluorinated hydrocarbons or poly(dimethylsi1oxane). The properties that make these compounds so effective are low volatility, low surface tension, and insolubility in hydrocarbon oil. These properties give the foam inhibitor a positive surface-entering coefficient’ with respect to the oil medium. A positive 0743-7463/88/2404-1188$01.50/0
surface-entering coefficient corresponds exactly to a negative spreading coefficient of the medium with respect to the insoluble droplet. In practice that results in a mechanical action, namely, the retraction of the oil in the foam lamella from the insoluble droplet situated on its surface. The process is called dewetting. The force of its mechanical action or, as some conceive it, the bridging of the liquid lamella by a droplet whose adhesion to the medium is too weak to hold the composite structure together, causes it to collapse. The problem arose anew when synthetic esters, such as trimethylolpropane heptanoate, were used as lubricants in the place of hydrocarbon oils. The presence of poly(dimethylsiloxane), instead of removing the difficulty, seemed to augment it. Work done in this laboratory under contract with the U.S. Air Force found that poly(di(1) Robinson, J. V.; Woods, W. W. J . SOC.Chem. Ind., London 1948, 67, 361.
0 1988 American Chemical Society
Langmuir, Vol. 4, No. 5, 1988 1189
Poly(dimethylsi1oxane) Interactions with Lewis Bases
methylsiloxane), which acts as a foam inhibitor in hydrocarbon oils, actually promotes foaminess when mixed with synthetic ester lubricants. According to a principle enunciated by Ross and Nishioka? foam inhibition gives way to foam stabilization when a potential inhibitor becomes, for any reason, soluble in the medium. The present work is based on the hypothesis that the solubility of poly(dimethylsi1oxane) in an organic ester is due to an electron donor-electron acceptor, or Lewis acid-Lewis base, interaction between solute and solvent. Esters are Lewis bases: so we sought evidence to demonstrate that poly(dimethylsi1oxane) is a Lewis acid or an electron-pair acceptor. For this purpose we used two techniques: (a) the rate of rise of a single bubble in the solution, which detects the onset and development of surface activity as a function of the concentration of solute, and (b) the shift of the NMR frequency of the proton of a Lewis base due to interaction with a Lewis acid. Changes of the rate of rise of a single bubble in a solution as a function of concentration of solute were shown by Suzin and Ross3 to be a means to detect surface activity of the solute. The method is well suited to oil solutions, where the lowering of the surface tension is not as sensitive an indicator of surface activity as it is in aqueous solutions. The method was developed further by Furler and ROSS: who widened its scope to include larger Reynolds numbers, thus extending the detection of surface activity of a solute in a solution to lower viscosities and higher temperatures. Using this technique, we can trace the onset and development of the surface activity of poly(dimethylsi1oxane) as a function of its concentration in the ester lubricant, both at room temperature and at higher temperatures. We then tried the effect of having present in the solution small amounts of a base stronger than the solvent, namely, N-phenyl-1-naphthylamine.The result of this experiment was to shift the onset and development of the surface activity of the poly(dimethylsi1oxane) to higher concentrations. We ascribe this result to an increase in the solubility of the poly(dimethylsi1oxane) due to the increased basicity of the medium. An increase in solubility is usually accompanied by a decrease in surface activity (Lundelius rule). NMR shifts of bases on addition of polydimethylsiloxane also revealed an interaction. The peak frequencies of a proton shift as a result of its nucleus experiencing changes of “deshielding”, that is, the degree of the pulling away of its electron by changes of the electronegativity of an atom attached to it. The discovery of the present work is that bases show a shift of their NMR spectra when mixed with poly(dimethylsi1oxane). A “hard” base5 showed a larger shift than did a “soft” base, demonstrating that poly(dimethylsiloxane) is a hard acid, i.e., that the bonding of a base to the polymer is predominantly electrovalent in nature rather than covalent. Theory of t h e Method of Bubble Rise The theory of the rate of bubble rise a t moderate Reynolds numbers was presented by Furler and Ross.4 For a bubble of radius a , rising with a velocity V in a liquid of density p and viscosity q, the drag coefficient C D and the Reynolds number Re are defined as CD = 8 a g / 3 V (1) Re = 2 a V p / q (2) Ross, S.; Nishioka, G. M. J . Phys. Chem. 1975,79,1561. (3)Suzin, Y.;Ross, S. J . Colloid Interface Sci. 1985,103, 548. (4)Furler, G.;Ross, S. Langmuir 1986,2, 68. (5)Pearson, R. G. J.Am. Chem. SOC.1963,85,3533.
(2)
The rate of rise of a spherical bubble a t low Reynolds numbers (Re < 0.5) is given by the Rybczynski-Hadamard equation: CD Re = 16 (3) Equation 3 describes a rate of rise faster by a factor of 1.5 than that described by the Stokes law, which is CD Re = 24 (4) A transition from the behavior described by eq 3 to that described by eq 4 is effected by the presence of a surface-active solute, which introduces a dynamically varying surface tension at the bubble surface; this, in turn, inhibits momentum transfer across the liquid-gas interface, by means of a Marangoni effect. Equations 3 and 4 are modified for higher Reynolds numbers. For 0.75 < Re < 20 the rate of rise of a spherical bubble whose dynamic air-liquid interface is completely characterized by a constant surface tension is described by the following equation:
CD Re = -0.244/Re
+ 16.78 + 1.021Re - 0.023Re2
(5)
That is, eq 3 is modified for higher Reynolds numbers to eq 5. These equations hold for the same kind of air-liquid interface and differ only in the range of Reynolds number to which each applies. Stokes law too is limited to values of Re < ca. 1 and requires modification for higher Reynolds numbers. Kuerten et al.6 give empirical formulas for rates of motion of solid spherical particles a t higher Reynolds numbers. The following equation is based on numerous measurements of the drag coefficients of solid spheres in fluid media a t 0.75 < Re < 20: CD Re = 24 + 2Re (6) Experimental Section Apparatus and Procedure. Apparatus used to measure the rate of bubble rise at different temperatures has been described previ~usly.~ The procedure to obtain the size of a bubble is to measure the length of the air slug in a calibrated capillary tube before the air is released into the solution, using a cathetometer with a precision of 0.01 mm. The volume of the spherical air bubble is the same as the volume of the air slug before its release. The volumes of the meniscuses at the top and bottom of the slug are taken into account by assuming them to be hemispheres. The radius of the spherical bubble is then calculated by the equation u3 = r2(3h - 2r) (7) where h is the height of the slug and r is the radius of the capillary tube. A length of 42.0 cm is marked on the containing cylinder, the lower mark at approximately 15 cm from the bottom of the cylinder. The time for each released bubble to traverse the distance is measured with a stopwatch (i0.l s). The retarding effect of the walls of a cylindrical container on the velocity of a sphere moving axially can be expressed by a multiplying factor to convert the velocity in a vessel with a particular ratio of d/D to that in an infinite medium. A number of theoretical and empirical wall-effect equations have been proposed. This one is given by Ladenburg and Faxen7 V = Vm(l+ 2.l(d/D)) (8) where Vis the corrected rate of rise, V , the measured rate of rise, d the bubble diameter, and D the diameter of the cylindrical container. Materials: 2,2-diheptanoyloxymethyl-n-butylheptanoate (trimethylylolpropane heptanoate); Mobil Ester P-41,Mobil Lot A13B, Eastman Chemical Co.; N-phenyl-1-naphthylamine, Kodak Co., No. 351; poly(dimethylsiloxane),SF96-1000,General ~~~~~
~
~
(6)Kuerten, H.;Raasch, J.; Rumpf, H. Chem.-Ing.-Tech.1966,38,941. (7)Bird, R.B.; Stewart, W. E.; Lightfoot, E. N. Transport Phenomena; Wiley: New York, 1960; p 206.
Ross and Nguyen
1190 Langmuir, Vol. 4, No. 5, 1988
1 \
7.0
3
0.5 ppm
n
-v:E 0
6.0
W
x
.y 5.0 0 0 -
al > 4.0 -0
al
+ 0 3.0
FL 0
02.0
1.0
+,5.5 -1
4.5
v
I
I
6.5
7.5
Bubble D i a m e t e r Squared
I
8.5
(IO
.3
'
2 \
Electric Co.; and n-hexane, certified ACS reagent grade, Fisher Scientific Co. A stock solution of poly(dimethylsi1oxane) in n-hexane at a concentration of 0.161% (w/v) was prepared. An aliquot was transferred by pipet into a sample of the oil to obtain a known concentration between 0.10 and 20.55 ppm. The aliquots appear to dissolve readily in the oil; each mixture was then stirred for 2 h to ensure a homogeneous solution.
Results and Discussion To obtain the rate of bubble rise in any solution a t any temperature, 10 separate measurements were made. Since it is impossible to duplicate a bubble of the same size each time, these data give the variation of bubble velocity with diameter. The Rybczynski-Hadamard theory underlying eq 3 embodies an implicit assumption that the velocity of the bubble varies with the square of its diameter, which is also a condition for the Stokes law to hold. The first treatment of data, therefore, is to plot the corrected velocity, V, against the square of the diameter of the bubble, 4az. These plots are shown in Figure 1: they demonstrate that an implicit assumption of the Rybczynski-Hadamard theory is supported by the observations. The second treatment of our data for each system investigated is to reduce the number of measured bubble radii to that of a single representative value. This can readily be done by using the linear relation, obtained by least squares, between the square of the bubble diameter and the velocity of rise (see Figure 1 for a selection of typical data) and then interpolating the velocity value corresponding to any desired radius of the bubble (we have taken a = 0.042 cm). Drag coefficients can then be calculated by eq l. For the comparison on the theoretical side, we make use of Ryskin and Leal's tabulation8 of drag coefficients, CD, a t various values of Re and W , taking W = 0 for our near-spherical bubbles. The relation between G.;Leal, L. G.J . Fluid Mech. 1984, 148, 1.
5
6
7
.8
Reynolds Number, Re
*cm ,# Figure 2. Limits (solid lines) and the transitional states (dotted
Figure 1. Linear relations between the corrected velocity of bubble rise (cm/s) and the square of the diameter (cm2)of the bubble for a series of concentrations of poly(dimethylsi1oxane) in a mixture of 1% (w/w) N-phenyl-1-naphthylaminein trimethylolpropane heptanoate oil at the temperatures 24.8 "C (dashed and dotted lines) and 74.8 "C (solid lines).
(8) Ryskin,
4
lines) of the variation of the drag coefficient, C, with the Reynolds number, Re, on logarithmic scales. Solid lines are theoretical equations; points are observational data. The lower solid line refers to the Rybczynski-Hadamard law, eq 3; the higher solid line refers to the Stokes law, eq 4. The system is the solution of 1% (w/w) N-phenyl-1-naphthylaminein trimethylolpropane heptanoate with various concentrationsof poly(dimethylsiloxane), at a temperature of 24.8 "C.
CDand Re (W = 0) for 0.75 > Re > 20 is given by eq 5 and for Re (W = 0) < 0.75 by eq 3. For Reynolds numbers less than 0.75, the ratio m of the drag coefficient of a completely "rigid" interface to that of a completely fluid interface can be written as m = CD(rigid)/CD(fluid)= 24/16 = 1.5 (9) The ratio m is equal to 1.5 in the range of Reynolds numbers in which Stokes law and the Rybczynski-Hadamard equation are valid, that is, in the range of Reynolds numbers less than 0.75. For Reynolds numbers larger than 0.75, the ratio m increases gradually as the Reynolds number increases. At Re = 7.0, the ratio of "rigid" CD to "fluid" CD is 1.67, calculated from eq 5 and 6. Figures 2 and 3 show the variation, as observed experimentally, of drag coefficients with Reynolds numbers on log-log scales for a range of concentrations of poly(dimethylsiloxane) in trimethylolpropane heptanoate oil. Figure 2 is a plot of this variation for the Stokes and the Rybczynski-Hadamard regime, and Figure 3 is a plot for a higher range of Reynolds numbers where these simpler theories are no longer valid. The significance of these data is most readily apprehended by reducing the quantity of observed Reynolds numbers to that of a single representative value. This can readily be done by using the linear relation obtained by least squares between the logarithm of the Reynolds number and the logarithm of the measured drag coefficient (see Figures 2 and 3 for a selection of typical data) and then interpolating the value of the drag coefficient corresponding to any desired Reynolds number. We have taken Reynolds numbers of 0.5 for data at 24.8 "C and of 7.0 for data at 74.8 "C. To provide a more graphic representation of the transition between the fluid and rigid states as the concentration of surface-active solute is increased, we divide the observed drag coefficients at a chosen Reynolds number by the theoretical value for a completely fluid interface,
Langmuir, Vol. 4, No.5, 1988 1191
Poly(dimethylsi1oxane) Interactions with Lewis Bases
4
L
090
I
10 -'
I
10
10
Corlcentration (ppm) Figure 4. Transition of behavior from a fluid to a rigid interface
with various concentrations of poly(dimethylsi1oxane) in trimethylolpropane heptanoate, at Reynolds numbers of 0.5 and 7.0. 4-
\ 3 5 7 IO
1.70
.-a,
.-
0
'c Lc
E 1 50
Reynolds Number, Re
0
Figure 3. Limits (solid lines) and the transitional states (dotted lines) of the variation of the drag coefficient, CD, with the Reynolds number, Re, on logarithmic scales. Solid lines are theoretical equations; points are observational data. The lower solid line refers to the Rybczynski-Hadamard law, eq 3; the higher solid line refers to the Stokes law, eq 4. The system is the solution of 1% (w/w) N-phenyl-1-naphthylaminein trimethylolpropane heptanoate with various concentrations of poly(dimethylsi1oxane) at a temperature of 74.8 "C.
0
P 1 30 U a,
N .-1.10
E L 0
Z 0.90
Table I. Compatison of Bubble Rise Results of the Trimethylolgropane Heptanoate Oil and the Solution of n -Hexane and Trimethylolpropane Heptanoate Oil
?lP, TMP TMP +
cm2/s 0.300 0.285
v,cm/s 1.903 2.017
Re
CD
cD,Tb
0.533 30.33 30.02 0.594 27.01 26.92
CD/CD,Th
1.01 1.00
-'
10
10
10
Coricen t r a tion ( p p m ) Figure 5. Transition of behavior from a fluid to a rigid interface with various concentrations of poly(dimethylsi1oxane) in the solution of l % (w/w) N-phenyl-l-naphthylaminein trimethylolpropane heptanoate, at Reynolds numbers of 0.5 and 7.0.
n-hexane
using for that purpose either the theory of RybczynskiHadamard as expressed by eq 3 for Re = 0.5 or the theory of Ryskin and Leal as expressed by eq 5 for Re = 7.0. This ratio is then plotted against the logarithm of the concentration of poly(dimethylsiloxane), as in Figures 4-8. The results of bubble-rise experiments with trimethylolpropane heptanoate alone and with a solution of n-hexane in trimethylolpropane heptanoate at 24.8 "C show that n-hexane is not a surface-active constituent in the solution, as the surface of the air bubble is fluid both in the solvent alone as well as in the solution (see Table I). This result allows us to use n-hexane as a solvent to make a stock solution of poly(dimethylsi1oxane) without fear that the solvent of the stock solution is surface active in solutions made up with it in trimethylolpropane heptanoate. Solutions of poly(dimethylsi1oxane) in trimethylolpropane heptanoate have a tfansition region of concentration in which the interface changes from fluid to rigid, as is shown in Figure 4 for Reynolds numbers of 0.5 and 7.0, obtained a t temperatures of 24.8 and 74.8 O C , respectively. Poly(dimethylsi1oxane) is seen to be more surface active at the higher temperature. Figures 5 and 6 show that the additional presence of a basic solute, N-phenyl-1-naphthylamine, reduces the surface activity
0.90
I
10 -'
10
Co rlcent ra t io n ( p pm)
10'
Figure 6. Transition of behavior from a fluid to a rigid interface with various concentrations of poly(dimethylsi1oxane) in the solution of 5% (w/w) N-phenyl-l-naphthylaminein trimethylolpropane heptanoate, at Reynolds numbers of 0.5 and 7.0. of the poly(dimethylsi1oxane) in trimethylolpropane heptanoate. In this respect a concentration of 1% N phenyl-1-naphthylamine is more effective than a concentration of 5 % . The concentration of poly(dimethylsi1oxane) required to reach the halfway stage, or 50% tigidication, can be
1192 Langmuir,Vol. 4,No.5, 1988
Ross and Nguyen Table 11. Surface Activity of Poly(dimethylsi1oxane) in the Presence of N-Phenyl-1-naphthylamine PDMS concn at 7 0 of 50% transition, N-phenyl- 1 PPm temp, “C Re naphthylamine 1.6 24.8 0.5 0 1.0 74.8 7.0 0 2.3 24.8 0.5 1 1.8 74.8 7.0 1 2.4 24.8 0.5 5 1.6 74.8 7.0 5 ~
10
-’
13
10
’
Concen t r o t I C i ( p p m ) Figure 7. Transition of behavior from a fluid to a rigid interface with various concentrations of poly(dimethylsi1oxane)in trimethylolpropane heptanoate oil and in mixtures of 0% ( O ) , 1% (*), and 5% (a) (w/w) N-phenyl-1-naphthylaminein the oil, at a Reynolds number of 0.5.
z 0 90 10
10
10
coA c e n t ra t ion ( p pm) Figure 8. Transition of behavior from a fluid to a rigid interface with various concentrations of poly(dimethylsi1oxane)in trimethylolpropane hepanoate oil and in mixtures of 0% ( O ) , 1% (a), and 5% (*) (w/w) N-phenyl-1-naphthylaminein the oil, at a Reynolds number of 7.0. ->
taken as an inverse measure of the effective surface activity of poly(dimethylsi1oxane) dissolved in trimethylolpropane heptanoate, for comparisons under different conditions of temperature and in the presence of N-phenyl-lnaphthylamine at different concentrations (1% and 5% ). Table I1 contains the 50% values under these conditions. In every one of the solutions tested, the poly(dimethy1siloxane) is more surface active at the higher temperature. This result bears a strong analogy to the surface activity of poly(ethy1ene oxide) adducts in aqueous solution, which are also more surface active at higher temperatures. The reason for the latter behavior is found in the retrograde solubility of these compounds with temperature, as evidenced by the appearance of a cloud point at a critical temperature. The solubility of poly(ethy1ene oxide) adducts in water is the result of hydrogen bonding, in which the ether oxygens of the polymer act as Lewis bases and the water acts as a Lewis acid. As the temperature is raised, the acid-base interaction between solute and solvent is lessened by the increase in thermal energy. In accord with Lundelius’s general rule, the reduction of solubility is accompanied by an increase in surface activity. The same circumstances would account for the higher degree of surface activity with temperature of poly(dimethylsiloxane) in an ester solvent, where the acid-base
Table 111. Chemical Shift (in ppm) of the Proton of Ethyl Acetate and of Triethylamine due to the Presence of Poly(dimethylsi1oxane) A B C D E ethyl acetate + PDMS 0.19 0.15 0.17 triethylamine + PDMS 0.04 0.05
interaction is diminished by the thermal energy conferred on the molecules by raising the temperature. Also, the finding that a larger concentration of poly(dimethylsiloxane) is required to reach the 50% transition in the presence of N-phenyl-1-naphthylamine, that is, a reduction of the surface activity, shows that an interaction has taken place between the two solutes. In this respect the 5% solution of N-phenyl-1-naphthylamine is no more effective than the 1% solution. The interaction between poly(dimethylsi1oxane) and N-phenyl-1-naphthylamineis probably linked to their respective Lewis acidity and basicity. N-Phenyl-1naphthylamine is obviously a base by virtue of its amine group. To gain further insight into the postulated acidic character of poly(dimethylsiloxane), its interactions with known bases were followed by means of the shift of their NMR peaks. The bases selected were ethyl acetate and triethylamine. The Drago coefficients of ethyl acetate are C = 1.74 and E = 0.975, with a CIE ratio of 1.78; the Drago coefficients of triethylamine are C = 11.09 and E = 0.991, with a C / E ratio of 11.19.9 The ratio C / E measures the relative “softness” of a base, that is, its tendency to interact covalently rather than electrovalently. The numbers quoted above show that triethylamine is a much softer base than ethyl acetate. The NMR shift of a proton in ethyl acetate was found to be notably larger than the corresponding shift of a proton in triethylamine when their interactions with poly(dimethylsi1oxane) were compared. For a 1:l (v/v) mixture of ethyl acetate and poly(dimethylsiloxane), the ethyl acetate peaks shift to a higher field by about 0.15 ppm, while for a 1:l (v/v) mixture of triethylamine and poly(dimethylsiloxane), the triethylamine peaks shift by only about 0.05 ppm (see Table 111). This result indicates a stronger interaction of the poly(dimethylsi1oxane) with ethyl acetate. According to P e a r ~ o nstronger ,~ interactions take place between “like” acids and bases, that is, soft with soft, hard with hard; therefore, poly(dimethylsi1oxane) may be classified as a hard acid. The sample of poly(dimethylsi1oxane) used in these experiments is “end-capped”; that is, the terminal groups of the linear polymer are methoxy rather than hydroxy. The Lewis acid character of the polymer is therefore not due to SiOH groups; it resides in the silicon atoms. The hard acid character of poly(dimethylsi1oxane)has not been ascertained before to our knowledge, although it is not unexpected, as silicon is listed by Pearsons as a hard acid (9) Drago, R. S. Struct. Bonding (Berlin) 1973,15,73; Coord. Chem. Rev. 1980, 33, 251.
Langmuir 1988,4, 1193-1197
I
0I Figure 9. Schematic diagram of proposed chemisorption bond
between Lewis acid poly(dimethylsi1oxane)polymer and Lewis acid silanolate group on a silica substrate. constituent because of its small size, slight polarizability, and its empty d-orbitals, which can accept electrons from a base. The Lewis acid character of poly(dimethylsi1oxane)explains its solubility in the basic synthetic ester lubricant trimethylolpropane heptanoate compared to its insolubility in a hydrocarbon lubricant as well as its solubility in benzene and toluene, both Lewis bases. It also explains the widely used process by which silica is made hydrophobic by interaction with poly(dimethylsiloxane), which is an important step in the manufacture of agents to combat foam in black liquor in paper making and for other sources of troublesome foam in various industries. Silica normally has a hydrophilic surface, which is to say it is perfectly wetted by water, and separates readily into its individual particles (which are aggregates of primary particles) without clumping, clotting, or agglomerating when shaken with water. After treatment with poly(dimethylsiloxane) the surface of each particle is drastically altered. The particle is no longer wetted by water but floats on the surface, just as would an oiled needle, in spite of its density being greater than that of water. This behavior points to an angle of contact larger than 90°. A prerequisite for producing such large angles of contact is that the silica be first treated with a solution of caustic soda, which has the effect of creating silanolate groups on
1193
the surface: -Si-O-. The higher the pH at which the caustic treatment is performed, the more effective is the foam-inhibiting composition made from the silica so treated.l0 Patterson has shown a three-way correlation with the pH of the caustic treatment.ll As pH is increased above a value of 7, the amount of bound polymer on the silica surface so treated increases, the contact angle of black liquor on the surface of the silica increases, and the more effective a foam-inhibiting agent is the final manufactured composition containing the hydrophobic silica. These results support an interpretation that the interaction between the silica surface and poly(dimethy1siloxane) is one between a Lewis acid and a Lewis base. The caustic treatment creating silanolate groups on the silica surface introduces anions, which are clearly electron donors and capable of interacting with the electron-accepting silicon atoms of the poly(dimethylsi1oxane). The interaction is therefore an example of chemisorption. The polymer becomes attached by multiple bonds to the silica surface (Figure 9). These bonds are separately weak enough to form and break in a dynamic equilibrium with the bulk phase, but since not all of them break simultaneously, the polymer remains tenaciously attached to the surface and resists elutriation by solvents. The nature of the bonding of poly(dimethylsi1oxane)to a silica surface has long been uncertain.12 The present finding of the Lewis acid character of poly(dimethy1siloxane) provides a definitive answer. Registry No. TMP, 78-16-0; N-phenyl-1-naphthylamine, 90-30-2. (10) Miller, J. R.; Pierce, R. H.; Linton, R. W.; Wills, J. H. U.S.Patent 3 714068 to Philadelphia Quartz Company, Jan 30, 1973. (11) Patterson, R. E. TAPPI Non-Woven Conference, Nashville, TN, April 5-8, 1988. Preprinted paper "Influence of Silica Properties on Performance of Antifoams in Pulp and Paper Applications". (12) Ross, S.; Nishioka, G. M. In Emulsions, Latices, and Dispersions; Becher, P.; Yudenfreund, M. N., Eds.; Marcel Dekker: New York, 1978; p 254.
1291Mossbauer Studies of Iodine Sorbed on Charcoal? M. L. Hyder,* R. L. Postles, and D. G. Karraker E. I. du Pont de Nemours & Company, Savannah River Laboratory, Aiken, South Carolina 29808 Received June 16, 1987. I n Final Form: May 25, 1988 lZ9I2sorbed on several different samples of commercial charcoal was investigated by 1291Mossbauer spectroscopy. The spectra obtained were similar for all types of charcoal studied. They show two quadrupole split patterns: for the first (A), 6 = 1.42 mm/s, e2qQ12,/h= -2470 MHz, and 7) = 0.09; for the second (B), 6 = 0.17 mm/s, e2qQ12,/h= -1230 MHz, and 7 = 0.20. The data are interpreted in terms of the sorption of I through one iodine atom at approximately a right angle to the carbon surface by comparison with the iizsIMossbauer spectrum and crystal structure of the 12-hexamethylenetetraminecharge-transfercomplex. In this interpretation, the spectrum A corresponds to the iodine atom nearest to the carbon surface and spectrum B to the remote iodine atom. The A atom is electronically positive, the B atom is negative, and the charcoal surface contributes 0.22e to the iodine molecule.
Introduction Speciality charcoals and some other sorbents are commonly used to remove radioactive iodine from the ventilation systems of nuclear reactors and other nuclear fa'This paper was prepared in connection with work done under Contract No. DE-AC09-76SR00001 with the U.S.Department of Energy.
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cilities. At the Savannah River Plant (SRP), the exhaust air from the nuclear reactor buildings is passed through beds of granular charcoal before being released to the atmosphere. The purpose of these beds is to minimize the release of fission product radioiodine to the environment in the event of fuel damage or a reactor accident. Iodine uptake by iodine has been found to be very fast and efficient, and the iodine is very strongly held by the 0 1988 American Chemical Society
