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Some surface studies of antifoams
JOUI~NAL OF COLLOID SCIENCE 11, 419-427 (1956)
SOME SURFACE STUDIES OF ANTIFOAMS J. G. Hawke and A. E. AIexander School of Applied Chemistry, New South Wales University of Technology, Sydney, Auslralia Received April 5, 1956 INTRODUCTION
In order to overcome the problem of foaming in steam boilers several types of chemical compounds have been developed in recent years, one large group of such compounds being broadly classed as polyamides (1, 2). Compared with materials previously used, such as castor oil, they are substantially superior in stability and effectiveness, the amounts required being only of the order of a few p.p.m. A number of theories of their mode of action have: been put forward, based largely on laboratory boiler tests (1, 2), although a few surface studies have been reported (3). An examination of these theories in the light of general surface phenomena made it appear that certain aspects at least were open to question. The present investigation of the film-forming properties of a number of these polyamides was therefore undertaken, and the results certainly substantiate these earlier doubts. Some new suggestions are advanced based on the present findings, but it is clear that much further work needs to be done before the basic physicochemical phenomena involved in the activity of these antifoams can be established on a firm basis. EXPERIMENTAL
Two series of compounds have been investigated, namely, the diacylated C~H4 piperazines C.tt2.+ICON
/
\
\
/
NCOC~H2~+I with n = 11,13, 15,17, and
C2H~ the diethylene triamines R. CO .NH(CH2)z.NtI(Ctt2)2.NH. CO. R with R --- 15 and 17 (both the stearyl and oleyl derivatives in the latter case). These compounds were kindly donated to us by the late Dr. A. L. Jacoby, and ful ! analytical data have been given in his earlier publications (2, 3). Monolayers were spread from solution in benzene containing about 2 % ethyl acetate to assist spreading. In some cases addition of up to 10 % 419
z~20
3. G. HAW-KE AND A. E. ALEXA/qDER
chloroform was necessary to effect solution. Surface pressure (II) vs. area (A) curves were usually obtained on a simple type of film balance (4) with a hanging mica strip instead of the horizontal float. This apparatus gave an accuracy of ca. 0.1 dyne/cm. At the higher temperatures some measuremeats were made with a du Nofiy surface tension apparatus. Surface potentials (AV) were measured in the usual manner with an ionizing air electrode (5), the surface viscosity and rigidity being assessed qualitatively by blowing talcum powder on the surface. Surface moments (p) were calculated by means of the usual equation A V = 4mL/A. Equilibrium spreading pressures (IIz) were obtained by spreading a few crystals on the previously cleaned surface and measuring the change in surface tension either with the hanging plate or du Nofiy apparatus. RESULTS
The results for the spread monolayers of the various piperazines are given in Figs. 1-3. Figure 1 Shows the variation in the H-A, A V - A and p-A relationship with chain length, Fig. 2 the variation with temperature for the dimyristyl compound. The results in Fig. 2 were obtained by the hanging plate method except for those at 72°C., which were obtained from the du Nofiy apparatus. The effect of acidity was examined with several compounds with extremely similar results, those for the dimyristyl compound being given in Fig. 3.
2~0
~oo
20
I,
do
i.~o
AREA/MO&~C¢~.E A~
I~'Ia. 1. H-A, ~I/--A, and ~-A curves for the various piperazines on 0.01 N ItC1. (1) II-A for distearyl, dipalmityl, and dimyristyl compounds (mean value). (~) II-A for dilauryl compound.
421
SOME SURFACE STUDIES OF ANTI:FOAMS
JO
Io ~o
90
Iio
tao
t5o
AREA/MoLECUL E
17o
/gO
A°~
FIG. 2. II-A curves for dimyristyl piperazine on 0.01 N HC1 at various temperatures.
20
IO
@
so
o
iso
13o
AREA / MOLECULE
JTo
19o
A °a
FIG. 3. II-A curves for dimyristyl piperazine at various acidities at room temperatures. (1) 0.01 N HC1. (2) 0.1 N HC1.
(3) N HC]. (4) 5.3 N HC]. (5) ~0.6 N HC].
422
J. G. HAWKE AND A. E. ALEXANDER 60
20
2O
.40
60
ao AREA/MOLECULE
Ioo
120
A
FIG. 4. I1-_/1curves for distearyl and dioleyl diethylene triamines on 0.01 N and 5.3 N HC1 at room temperatures. (1) Distearyl compound (0.01 N HC1). (2) Distearyl compound (5.3 N HC1). (8) Dioleyl compound (0.01 N HC1,) (4) Dioleyl compound (5.3 N HC1). ~0
4O
®
ioo
~oo
300
400
$oo
"4REA/MOLECUL E A'2
FIG. 5. II-A curves for dipalmityl diethylene triamine on 0.01 N HCI at various temperatures. (1) 13.3°C.
(z) 32.0oc. (s) 45.0oc. (4) 55.o°c.
SOME SURFACE STUDIES OF ANTIFOAMS
zJ~
46
C~
2~
5"0
I
I
70
90
TEMP
....
/10
0 C
FIG. 6. Equilibrium spreading pressures (]:Is) as ~ function of temperature for two piperazines and two tri~mines. /k Dioleyl diethylene triamine. + Distearyl diethylene triamine. O Dimyristyl piperazine. X DistearyI piperazine.
Figures 4 and 5 show the results for the monolayers of the triamines. The results for the equilibrium spreading pressures (II~) at various temperatures are given in Fig. 6, two piperazines (dimyristyl and distearyl) and two triamines (dioleyl and distearyl) being chosen for this purpose. Discussion Before considering the mechanism of antifoam action it is necessary to consider the results for the spread monolayers, in order to determine, as far as possible, the molecular configuration at the air/water interface.
1. The Structure of the Spread Monolayem At room temperature (18.5-20.5°C.) all the piperazines gave monolayers of the liquid expanded type, quite fluid at all areas. This behavior shows at once that appreciable hydrogen bonding between the head groups is absent (6), which is not surprising in view of their chemical nature. The expansion on strong acids is again not unexpected, being shown by compounds such as ethers and acids (7). At the cohering point of the monolayer the configuration of the head group is probably as shown below:
42~
J. G. HAWK:E AND A. E. ALEXANDER
\ /
/ CH~
CH~
\
CH~
\
//
/ C--N--CH~--CH~--N--C
CH~
%
O
o
(a) Head group in elevation (i.e., viewed parallel to surface)
/ C--N
CH~--CH~
\
\ /
N--C
CII2--CH~ (b) Head group in plan (i.e., viewed perpendicular to surface) Calculations from a scale drawing of the structure, on the assumption of planar nitrogen and with covalent and van der Waals' radii as given by Pauling (8), gave about 75 A? as the area of the head group including the first few CH2 groups (the remaining part of the chain could, if necessary, fold back inside the area of the ring). This value is clearly very much less than the experimental limiting area of about 120 A.~--a behavior which is typical of liquid expanded films in general. The results for the surface moment (~) show that at areas below about i00 A. 2 some head-group reorientation is occurring, but it does not seem possible to relate this to any specific change ~uch as a tilting of the head group by rotation around the N--C bonds. At the higher temperatures the piperazines give gaseous films, in the case of the dimyristyl compound, for example, the results at 72°C. obeying the equation H(A-31) -- 503, this being the well-known equation of state for an imperfect gaseous film. The monolayers given by the triamines are seen to differ considerably from those of the piperazines. At room temperature the dioleyl compound gives a typical liquid expanded film with cohering area of about Ii0 A. ~. The distearyl compound, on the other hand, gives a very rigid solid film, with an extrapolated area at zero compression of about 40 A. ~. Such a difference between a saturated and its cis mono-unsaturated relative has been noted in many other systems. The behavior of the distearyl compound strongly suggests marked hydrogen bonding between the head
\
groups, probably through the
/
/
C~O
and H--N
\
groups. (The observed
tendency for the dilute benzene solutions to form gels probably arises from the same cause.) Calculation from a scale model (Catalin Molecular Models) showed
SOME SURFACE STUDIES OF ANTIFOAMS
425
that the head groups, if fully extended parallel to the surface, would occupy about 65 A. 2, so that with the distearyl compound the orientation of the head group must be substantially different. However, examination of the model showed that, by suitable rotation, the head group can be folded up in such a manner as to fit almost exactly into the area of the two paraffin chains (i.e., ca 40 A.2). The two important features of the high-temperature results are the physical nature of the films and the magnitude of the equilibrium spreading pressures. The fluid nature even of the distearyl triamine at temperatures near the boiling point of water shows that the tendency for head group hydrogen bonding is largely broken down by thermal agitation under such conditions. The magnitude of the equilibrium spreading pressures at the high temperatures should be noted and also their variation with the chemical nature of the head group and comparative independence of chain length and unsaturation (see Fig. 6). 2. The Mechanism of Antifoam Action
The bearing of the present results upon the mechanism of antifoam action can now be considered. The theory suggested by Jacoby and Thompson for compounds of the polyamide type is summarized by them as follows (9); "In the presence of an adsorbed layer of surface-active insoluble material, the collapse of a foam bubble may be accompanied by a syneresis (10) or formation in the adsorption layers of dehydrated aggregates of the surface-active material. It has been observed that certain monolayers, although they may actually stabilize the bubble film while they are in the liquid state, lose this ability at once when the solid (brittle) state is attained (10). This indicates that one of the conditions for stabilization is the great mobility of the molecules of the adsorption layers, and if this mobility is lost by attainment of the solid state, the adsorption layers may contribute to the rupture of the bubble film. It is likely that the syneresis described above results essentially in the formation of patches of monolayer in the brittle state which are incapable of redispersion at a rate equal to or greater than the velocity of destruction of the bubble film." "The theory of antifoam action referred to above suggests that hydrogen bonding promotes antifoam action by enhancing the syncretic effect and creating a greater tendency for the monolayer to reach the solid or brittle state, as shown by Alexander (11)." The monolayer results obtained in the present investigations, in particular the fluidity and the expanded nature of all films at the higher temperatures, makes the above explanation improbable. The fact that the melting point of all the compounds lies well below the operating temperatures of normal commercial boilers provides additional support to this argument.
426
J. G. HAWKE AI~D A. E. ALEXAN'DER
A tentative alternate theory is advanced below but much further work on surface behavior at high temperatures will be necessary before any finality is likely to be reached. The important factors in promoting antifoam action are believed to be: a). A high insolubility in water at the normal operating temperatures (i.e., > 100°C. and usually much higher). b). A moderate degree of surface activity at the steam/water interface. c). The ability to form fluid films at the steam/water interface (i.e., at the high temperatures existing in boilers). d). A low tendency for adsorption on the suspended particles in the boiler as well as on the heating surfaces. e). High chemical stability in the boiler. Of these factor (e) is obvious and will not be considered further. The possession of high insolubility (as judged by monolayer standards which are of course quite different from bulk solubility standards) ensures that the antifoam is virtually entirely concentrated in the region where it is most essential, i.e., at the free steam/foam surface. This would arise from the steam bubbles' coming into contact with the colloidal particles of antifoaln (the antifoam is usually added in dioxane solution to the feedwater), resulting in some surface spreading and subsequent transport to the uppermost foam surface. The surface spreading due to the lowering of the steam/water interracial tension could have a number of effects, although their relative importance is hard to assess at this stage. A moderate degree of surface activity (say, c a . 20 dynes/cm.) would probably suffice to displace most solid suspended particles from the interface (unless they were unusually hydrophohic), and it would appear that such interracial particles are one of the major factors in promoting excessive foaming in boilers. Further, the lowering in surface tension would damp out violent movements at the surface when the bubble collapses, and would also assist draining by allowing easier deformation of two bubbles in close contact. A low tendency for adsorption on the suspended particles is essential to keep them as hydrophilic as possible. Reactive soluble molecules, such as fatty acids present in oily contamination, would tend to make such particles hydrophobic and thus to increase their tendency to concentrate at the foam surface. SUMMARY
The surface behavior at the air/water interface of a number of paraffinchain compounds used as antifoams in steam boilers has been studied. The compounds were chiefly diacy]ated piperazines (Cn, C13, C1~, and C17), together with dipalmityl, disteary], and dioleyl diethylene triamines. The effects of acidity of the substrate and temperature were examined using spread monolayers, and in addition equilibrium spreading pres-
SOME SURFACE STUDIES OF ANTIFOAMS
427
sures were m e a s u r e d a t h i g h e r t e m p e r a t u r e s (up t o 95°C.). T h e results show t h a t c e r t a i n a s p e c t s of c u r r e n t theories of a n t i f o a m a c t i o n need c o r r e c t i o n a n d some a l t e r n a t i v e suggestions are a d v a n c e d . ACKNOWLEDGMENTS Part of this work was carried out (by J.G.H.) at the University of New England and we wish to acknowledge the assistance given during that period by Mr. V. R. Stimson. ~:~EFERENCES 1. JACOB¥, A. L., AND THOMPSON, W. It., Proc. 7th Ann. Water Conf. Engrs. West. Penn. 1947, 31. 2. JAcoBY, A. L., J. Phys. & Colloid Chem. 52, 689 (1948). 3. JAco~Y, A. L., ANO JOI~NSON, C. E., J. Phys. & Colloid Chem. 53, 9 (1949). 4. ALeXANDEr, A. E., Nature 159, 304 (1947). 5. AL~XANDEI~, A. E., AND JOHNSON, P., "Colloid Science." Oxford University Press, 1949. 6. ALEXANDER,A. E., Proc. Roy. Soc. (London) A179, 470 (1942). 7. SCgULMAN,J. H., AND HVG~ES, A. It., Proc. Roy. Soc. (London) A138, 436 (1932). 8. PAULING, L., "The Nature of the Chemical Bond." Cornell University Press, Ithaca, 1948. 9. Reference 2, p. 692. 10. TRAP~Z~IXOV, A. A., AND REBI~DE~, P. A., Compt. rend. acad. sci. U.R.S.S. 18, 427 (1938). 11. ALEXANDER,A. E., Proc. Roy. Soc. (London) A179, 486 (1942).
SOME SURFACE STUDIES OF ANTIFOAMS J. G. Hawke and A. E. AIexander School of Applied Chemistry, New South Wales University of Technology, Sydney, Auslralia Received April 5, 1956 INTRODUCTION
In order to overcome the problem of foaming in steam boilers several types of chemical compounds have been developed in recent years, one large group of such compounds being broadly classed as polyamides (1, 2). Compared with materials previously used, such as castor oil, they are substantially superior in stability and effectiveness, the amounts required being only of the order of a few p.p.m. A number of theories of their mode of action have: been put forward, based largely on laboratory boiler tests (1, 2), although a few surface studies have been reported (3). An examination of these theories in the light of general surface phenomena made it appear that certain aspects at least were open to question. The present investigation of the film-forming properties of a number of these polyamides was therefore undertaken, and the results certainly substantiate these earlier doubts. Some new suggestions are advanced based on the present findings, but it is clear that much further work needs to be done before the basic physicochemical phenomena involved in the activity of these antifoams can be established on a firm basis. EXPERIMENTAL
Two series of compounds have been investigated, namely, the diacylated C~H4 piperazines C.tt2.+ICON
/
\
\
/
NCOC~H2~+I with n = 11,13, 15,17, and
C2H~ the diethylene triamines R. CO .NH(CH2)z.NtI(Ctt2)2.NH. CO. R with R --- 15 and 17 (both the stearyl and oleyl derivatives in the latter case). These compounds were kindly donated to us by the late Dr. A. L. Jacoby, and ful ! analytical data have been given in his earlier publications (2, 3). Monolayers were spread from solution in benzene containing about 2 % ethyl acetate to assist spreading. In some cases addition of up to 10 % 419
z~20
3. G. HAW-KE AND A. E. ALEXA/qDER
chloroform was necessary to effect solution. Surface pressure (II) vs. area (A) curves were usually obtained on a simple type of film balance (4) with a hanging mica strip instead of the horizontal float. This apparatus gave an accuracy of ca. 0.1 dyne/cm. At the higher temperatures some measuremeats were made with a du Nofiy surface tension apparatus. Surface potentials (AV) were measured in the usual manner with an ionizing air electrode (5), the surface viscosity and rigidity being assessed qualitatively by blowing talcum powder on the surface. Surface moments (p) were calculated by means of the usual equation A V = 4mL/A. Equilibrium spreading pressures (IIz) were obtained by spreading a few crystals on the previously cleaned surface and measuring the change in surface tension either with the hanging plate or du Nofiy apparatus. RESULTS
The results for the spread monolayers of the various piperazines are given in Figs. 1-3. Figure 1 Shows the variation in the H-A, A V - A and p-A relationship with chain length, Fig. 2 the variation with temperature for the dimyristyl compound. The results in Fig. 2 were obtained by the hanging plate method except for those at 72°C., which were obtained from the du Nofiy apparatus. The effect of acidity was examined with several compounds with extremely similar results, those for the dimyristyl compound being given in Fig. 3.
2~0
~oo
20
I,
do
i.~o
AREA/MO&~C¢~.E A~
I~'Ia. 1. H-A, ~I/--A, and ~-A curves for the various piperazines on 0.01 N ItC1. (1) II-A for distearyl, dipalmityl, and dimyristyl compounds (mean value). (~) II-A for dilauryl compound.
421
SOME SURFACE STUDIES OF ANTI:FOAMS
JO
Io ~o
90
Iio
tao
t5o
AREA/MoLECUL E
17o
/gO
A°~
FIG. 2. II-A curves for dimyristyl piperazine on 0.01 N HC1 at various temperatures.
20
IO
@
so
o
iso
13o
AREA / MOLECULE
JTo
19o
A °a
FIG. 3. II-A curves for dimyristyl piperazine at various acidities at room temperatures. (1) 0.01 N HC1. (2) 0.1 N HC1.
(3) N HC]. (4) 5.3 N HC]. (5) ~0.6 N HC].
422
J. G. HAWKE AND A. E. ALEXANDER 60
20
2O
.40
60
ao AREA/MOLECULE
Ioo
120
A
FIG. 4. I1-_/1curves for distearyl and dioleyl diethylene triamines on 0.01 N and 5.3 N HC1 at room temperatures. (1) Distearyl compound (0.01 N HC1). (2) Distearyl compound (5.3 N HC1). (8) Dioleyl compound (0.01 N HC1,) (4) Dioleyl compound (5.3 N HC1). ~0
4O
®
ioo
~oo
300
400
$oo
"4REA/MOLECUL E A'2
FIG. 5. II-A curves for dipalmityl diethylene triamine on 0.01 N HCI at various temperatures. (1) 13.3°C.
(z) 32.0oc. (s) 45.0oc. (4) 55.o°c.
SOME SURFACE STUDIES OF ANTIFOAMS
zJ~
46
C~
2~
5"0
I
I
70
90
TEMP
....
/10
0 C
FIG. 6. Equilibrium spreading pressures (]:Is) as ~ function of temperature for two piperazines and two tri~mines. /k Dioleyl diethylene triamine. + Distearyl diethylene triamine. O Dimyristyl piperazine. X DistearyI piperazine.
Figures 4 and 5 show the results for the monolayers of the triamines. The results for the equilibrium spreading pressures (II~) at various temperatures are given in Fig. 6, two piperazines (dimyristyl and distearyl) and two triamines (dioleyl and distearyl) being chosen for this purpose. Discussion Before considering the mechanism of antifoam action it is necessary to consider the results for the spread monolayers, in order to determine, as far as possible, the molecular configuration at the air/water interface.
1. The Structure of the Spread Monolayem At room temperature (18.5-20.5°C.) all the piperazines gave monolayers of the liquid expanded type, quite fluid at all areas. This behavior shows at once that appreciable hydrogen bonding between the head groups is absent (6), which is not surprising in view of their chemical nature. The expansion on strong acids is again not unexpected, being shown by compounds such as ethers and acids (7). At the cohering point of the monolayer the configuration of the head group is probably as shown below:
42~
J. G. HAWK:E AND A. E. ALEXANDER
\ /
/ CH~
CH~
\
CH~
\
//
/ C--N--CH~--CH~--N--C
CH~
%
O
o
(a) Head group in elevation (i.e., viewed parallel to surface)
/ C--N
CH~--CH~
\
\ /
N--C
CII2--CH~ (b) Head group in plan (i.e., viewed perpendicular to surface) Calculations from a scale drawing of the structure, on the assumption of planar nitrogen and with covalent and van der Waals' radii as given by Pauling (8), gave about 75 A? as the area of the head group including the first few CH2 groups (the remaining part of the chain could, if necessary, fold back inside the area of the ring). This value is clearly very much less than the experimental limiting area of about 120 A.~--a behavior which is typical of liquid expanded films in general. The results for the surface moment (~) show that at areas below about i00 A. 2 some head-group reorientation is occurring, but it does not seem possible to relate this to any specific change ~uch as a tilting of the head group by rotation around the N--C bonds. At the higher temperatures the piperazines give gaseous films, in the case of the dimyristyl compound, for example, the results at 72°C. obeying the equation H(A-31) -- 503, this being the well-known equation of state for an imperfect gaseous film. The monolayers given by the triamines are seen to differ considerably from those of the piperazines. At room temperature the dioleyl compound gives a typical liquid expanded film with cohering area of about Ii0 A. ~. The distearyl compound, on the other hand, gives a very rigid solid film, with an extrapolated area at zero compression of about 40 A. ~. Such a difference between a saturated and its cis mono-unsaturated relative has been noted in many other systems. The behavior of the distearyl compound strongly suggests marked hydrogen bonding between the head
\
groups, probably through the
/
/
C~O
and H--N
\
groups. (The observed
tendency for the dilute benzene solutions to form gels probably arises from the same cause.) Calculation from a scale model (Catalin Molecular Models) showed
SOME SURFACE STUDIES OF ANTIFOAMS
425
that the head groups, if fully extended parallel to the surface, would occupy about 65 A. 2, so that with the distearyl compound the orientation of the head group must be substantially different. However, examination of the model showed that, by suitable rotation, the head group can be folded up in such a manner as to fit almost exactly into the area of the two paraffin chains (i.e., ca 40 A.2). The two important features of the high-temperature results are the physical nature of the films and the magnitude of the equilibrium spreading pressures. The fluid nature even of the distearyl triamine at temperatures near the boiling point of water shows that the tendency for head group hydrogen bonding is largely broken down by thermal agitation under such conditions. The magnitude of the equilibrium spreading pressures at the high temperatures should be noted and also their variation with the chemical nature of the head group and comparative independence of chain length and unsaturation (see Fig. 6). 2. The Mechanism of Antifoam Action
The bearing of the present results upon the mechanism of antifoam action can now be considered. The theory suggested by Jacoby and Thompson for compounds of the polyamide type is summarized by them as follows (9); "In the presence of an adsorbed layer of surface-active insoluble material, the collapse of a foam bubble may be accompanied by a syneresis (10) or formation in the adsorption layers of dehydrated aggregates of the surface-active material. It has been observed that certain monolayers, although they may actually stabilize the bubble film while they are in the liquid state, lose this ability at once when the solid (brittle) state is attained (10). This indicates that one of the conditions for stabilization is the great mobility of the molecules of the adsorption layers, and if this mobility is lost by attainment of the solid state, the adsorption layers may contribute to the rupture of the bubble film. It is likely that the syneresis described above results essentially in the formation of patches of monolayer in the brittle state which are incapable of redispersion at a rate equal to or greater than the velocity of destruction of the bubble film." "The theory of antifoam action referred to above suggests that hydrogen bonding promotes antifoam action by enhancing the syncretic effect and creating a greater tendency for the monolayer to reach the solid or brittle state, as shown by Alexander (11)." The monolayer results obtained in the present investigations, in particular the fluidity and the expanded nature of all films at the higher temperatures, makes the above explanation improbable. The fact that the melting point of all the compounds lies well below the operating temperatures of normal commercial boilers provides additional support to this argument.
426
J. G. HAWKE AI~D A. E. ALEXAN'DER
A tentative alternate theory is advanced below but much further work on surface behavior at high temperatures will be necessary before any finality is likely to be reached. The important factors in promoting antifoam action are believed to be: a). A high insolubility in water at the normal operating temperatures (i.e., > 100°C. and usually much higher). b). A moderate degree of surface activity at the steam/water interface. c). The ability to form fluid films at the steam/water interface (i.e., at the high temperatures existing in boilers). d). A low tendency for adsorption on the suspended particles in the boiler as well as on the heating surfaces. e). High chemical stability in the boiler. Of these factor (e) is obvious and will not be considered further. The possession of high insolubility (as judged by monolayer standards which are of course quite different from bulk solubility standards) ensures that the antifoam is virtually entirely concentrated in the region where it is most essential, i.e., at the free steam/foam surface. This would arise from the steam bubbles' coming into contact with the colloidal particles of antifoaln (the antifoam is usually added in dioxane solution to the feedwater), resulting in some surface spreading and subsequent transport to the uppermost foam surface. The surface spreading due to the lowering of the steam/water interracial tension could have a number of effects, although their relative importance is hard to assess at this stage. A moderate degree of surface activity (say, c a . 20 dynes/cm.) would probably suffice to displace most solid suspended particles from the interface (unless they were unusually hydrophohic), and it would appear that such interracial particles are one of the major factors in promoting excessive foaming in boilers. Further, the lowering in surface tension would damp out violent movements at the surface when the bubble collapses, and would also assist draining by allowing easier deformation of two bubbles in close contact. A low tendency for adsorption on the suspended particles is essential to keep them as hydrophilic as possible. Reactive soluble molecules, such as fatty acids present in oily contamination, would tend to make such particles hydrophobic and thus to increase their tendency to concentrate at the foam surface. SUMMARY
The surface behavior at the air/water interface of a number of paraffinchain compounds used as antifoams in steam boilers has been studied. The compounds were chiefly diacy]ated piperazines (Cn, C13, C1~, and C17), together with dipalmityl, disteary], and dioleyl diethylene triamines. The effects of acidity of the substrate and temperature were examined using spread monolayers, and in addition equilibrium spreading pres-
SOME SURFACE STUDIES OF ANTIFOAMS
427
sures were m e a s u r e d a t h i g h e r t e m p e r a t u r e s (up t o 95°C.). T h e results show t h a t c e r t a i n a s p e c t s of c u r r e n t theories of a n t i f o a m a c t i o n need c o r r e c t i o n a n d some a l t e r n a t i v e suggestions are a d v a n c e d . ACKNOWLEDGMENTS Part of this work was carried out (by J.G.H.) at the University of New England and we wish to acknowledge the assistance given during that period by Mr. V. R. Stimson. ~:~EFERENCES 1. JACOB¥, A. L., AND THOMPSON, W. It., Proc. 7th Ann. Water Conf. Engrs. West. Penn. 1947, 31. 2. JAcoBY, A. L., J. Phys. & Colloid Chem. 52, 689 (1948). 3. JAco~Y, A. L., ANO JOI~NSON, C. E., J. Phys. & Colloid Chem. 53, 9 (1949). 4. ALeXANDEr, A. E., Nature 159, 304 (1947). 5. AL~XANDEI~, A. E., AND JOHNSON, P., "Colloid Science." Oxford University Press, 1949. 6. ALEXANDER,A. E., Proc. Roy. Soc. (London) A179, 470 (1942). 7. SCgULMAN,J. H., AND HVG~ES, A. It., Proc. Roy. Soc. (London) A138, 436 (1932). 8. PAULING, L., "The Nature of the Chemical Bond." Cornell University Press, Ithaca, 1948. 9. Reference 2, p. 692. 10. TRAP~Z~IXOV, A. A., AND REBI~DE~, P. A., Compt. rend. acad. sci. U.R.S.S. 18, 427 (1938). 11. ALEXANDER,A. E., Proc. Roy. Soc. (London) A179, 486 (1942).