T H E STRUCTURE OF T H E ADSORPTION LAYER AND THE ADHESION OF MICROSCOPIC PARTICLES' A. YON
BUZAGH
Royal Hungarian University, Budapest, Hungary Received August 1 , 1959
This paper gives a summary of the newer experiments that have been carried out by the author and his collaborators on the adhesion of microscopic particles and, in connection with this, on surface films. First of all, it may be mentioned that some years ago a method was worked out by means of which the forces operating in coagulation may be measured directly. This method is based on allowing microscopic particles, for example, quartz particles, to settle on a plate of the same substance, and on measuring the exact force required to tear the particles off the plate. The angle of inclination of the plate at which the particles just begin to move is measured. This angle is called the angle of adhesion. From the experiments carried out by this method, the fundamental knowledge was gained that the adhesion is caused not by gravitational force, but by the interaction of the surface films of the adsorption layers. The chemical substance of the particles and of the adhesive plate plays a rBle only so far as this is one of the factors that determine the properties of the surface films. The specific adhesion of the particles of the same size is different in different liquids, but is characteristic for the chemical structure of the adhering particles. 14 the same way, the specific adhesion represents a characteristic size for the chemical structure of the adhering particles. From experiments made on different substances, it has become clear that the adhesion in different liquids is closely connected with the lyophilic properties of the substances. Table 1 gives some experimental results that have been obtained on different microscopic particles of the same size in different liquids. From these experiments it follows that an antagonism exists between the specific adhesion in dynes per square centimeter and the lyophilic character. This is especially pronounced in the cases of quartz and carbon. Quartz is very lyophilic in water, but very lyophobic in organic liquids. Accordingly, quartz shows a considerably smaller adhesion in water than in organic liquids. With carbon the case is reversed. Since 1 Presented at the Sixteenth Colloid Symposium, held at Stanford University, California, July 64,1939. 1003
1004
A. VON
BUZLGH
carbon is less lyophilic in water than in organic liquids, its adhesion in organic liquids is less than in water. This procedure was used for the investigation of the structure of surface layers, since it enables us to extend our knowledge of the peculiarities arising in the adhesion of microscopic particles to plates. Adhesion studies in connection with cataphoretic measurements led directly to the question as to what conclusions 'may be drawn from the variations in the adhesion caused by the influence of different substances on the detailed structure of the adsorption film in solid adsorbents. For, from the statement that the TABLE 1 +des i n different liquids Adhesion of microscopic
-
LIQUIDS
CIFIC AD.
LIQUIDB
XIIC 'AD.
=BIOS
WION
dun-
ivnu wr r g .
par
u.
cm.
em.
Quartz. . . . . . . .. f
__ WE-
L)pE-
Water Ethyl alcohol Chloroform Benzine Benzene Toluene
0.3 2.9 3.1 3.2 3.1 3.3
Water Ethyl alcohol Ether
0.5 3.0 3.4
Water Ethyl alcohol Benzene
4.7
Water Ethyl alcohol Benzene
2.8 3.4 3.8
i
Water Ethyl alcohol Benzene
2.8 3.4 3.8
Coal (anthracite). . . . . . . . .
i
Water Ethyl alcohol Benzine Benzene
3.8 2.4 1.2 1.1
Graphite. . . . . .
Water Ethyl alcohol Benzine Benzene
3.7 2.5 1.4 0.8
Gold.. . . . . . . . . .
Water Benzene
5.8 3.5
Pyrites. . . . . . . .
5.0 5.1
-
adhesion of particles in liquids is caused by an interaction of the adsorption films, it follows necessarily that all influences that alter the structure and composition of the adsorption films are also of decisive importance for the adhesion. As regards the alteration in the adhesion through the influence of different substances, three groups of extreme cases must be distinguished: the influence of strong electrolytes, that of weak electrolytes and non-electrolytes, and that of colloids. The adhesion in solutions of strong electrolytes is due to the interionic (electrostatic) forces acting between the ions of the electric double layer.
1005
ADHESION OF MICROBCOPIC PARTICLES
The alteration in the adhesion is determined by the two characteristic variables of electrolytic systems,-the electric charge and the thickness of the electric double layer. With regard to the effect of different electrolytes on the adhesion, the following quantitative law was found to hold: In solutions of strong neutral salts, in which the activity coefficient of the effective ions has the same value, the adhesion is influenced to the same degree. The well-known activity coefficient law of Ostwald also holds good for the adhesion of microscopic particles. In order to make a further, more detailed investigation of the correlation between adhesion and coagulation, I determined the coagulation values of different electrolytes on different suspensions-especially on monodisperse quartz suspensions-by measuring the speed of sedimentation. Parallel to these experiments, the adhesion of quartz particles on a quartz plate TABLE 2 ELECTEOLYTE
I
COAQUL4TION
vAmm
ACTIVITY COEFalCIINT
ADEmION
dunes per
NaCl . . . . . . . . . . . . . . . . . . . . . . . . . . . . . KC1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NanS04. . . . . . . . . . . . . . . . . . . . . . . . . . . NaaPO4. . . . . . . . . . . . . . . . . . . . . . . . . . . KaFe (CN)s . . . . . . . . . . . . . . . . . . . . . . . K4Fe (CN)s. . . . . . . . . . . . . . . . . . . . . . . CaCla. . . . . . . . . . . . . . . . . . . . . . . . . . . . . BaCL . . . . . . . . . . . . . . . . . . . . . . . . . . . MgSOi . . . . . . . . . . . . . . . . . . . . . . . . . . . . AlCls. . . . . . . . . . . . . . . .,..,,.
42 40 17 28 5.5 8.5 0.7 0.5 0.32 0.00012
0.78 0.79 0.83 0.76 0.89 0.84 0.81 0.84 0.85 0.96
8q.
em.
1.88 1.84 1.96 1.80 1.92 1.87 1.98 1.94 1.82 2.05
was measured under the same conditions. The results of these experiments are given in table 2. From these results it will be seen that, in solutions of different neutral salts, the same adhesion and the same value of the activity coefficient correspond to the coagulation value. It is true that these values are different in the case of different suspensions. This result makes it very probable that, in the case of weakly solvated disperse systems, the more important and characteristic part of the system is the dispersion medium. A disperse system is stable if the disperse particles and the dispersion medium form a harmonious unity, but this is possible only if the particles or micelles are in the same physical state as t>tedispersion medium, this state being conditioned by the physical structure of the adsorption film;that is to say) in a state that renders possible the building up of the particles. If this condition is not fulfilled and the structural harmony between the adsorption films and the dispersion medium is disturbed, the system coagulates, since the dispersion medium eliminates the disperse particles. According
1006
A. VON BUZ&GH
to this theory, the active partner in the coagulation is the dispersion medium. The force of attraction among the particles plays a rBle only after the coagulation, insofar as it determines the physical properties of the coagulum.
TABLE 3 Adhaion of quartz particles to a quartz plate in alcohole ALCOBOL
P
CONCEXTBAllON
mdca pcr likr
Methyl alcohol.. , . , , , . , . . . . . .
0 1.24 2.48 4.96 7.44 9.6 12.4 14.8 17.3 18.6 22.3
0.087 0.105 0.107 0,115 0.201 0.309 0.500 0.630 0.772 0.920
17.4 17.8 18.3 18.9 20.6 21.5 23.5 25.3 27.4 28.6 32.8
Ethyl alcohol.. . . . . . . . . . . . . . . . . .
0 0.83 1.66 3.32 4.98 8.29 9.96 10.77 11.62 13.28
0.064 0.096 0.101 0.107 0.130 0.216 0.743 0.809 0.874 0.951
17.4 17.8 18.3 19.4 20.7 24.0 25.5 28.6 29.3 32.9
0 0.67 1.34 4.02 6.70 8.04 9.38 10.72
0.064 0.111 0.130 0.146 0.500 0.908 0.924 0.966
17.4 18.0 28.7 22.4 27.3 30.9
Propyl alcohc
.................
0.064
35.5
41.8
This principle proved fruitful also in the case of weak electrolytes and non-electrolytes. The adhesion in solutions of non-electrolytes is caused by the action of intermolecular forces (dipole forces, van der Waals forces) between the molecules of the adsorption films. According to this, organic compounds of different types have a different influence on the adhesion of microscopic particles. Compounds of analogous structure, for example, the members of homologous series, act qualitatively in the same way, that
ADHESION OF MICROSCOPIC PARTICLES
1007
is, they influence the adhesion of the microscopic particles with increasing concentration in much the same way; they show a difference only in the intensity of their effect. The experiments with non-electrolytes,-different homologous and isomeric carbon compounds,-were of special interest in connection with the above question: namely, to what extent the structure of the adsorption layer plays a-rBle in the adhesion, and also to what extent conclusions may be drawn as to the structure of the adsorption layer from alterations in the adhesion. At the same time, these experiments gave further information as to the influence of the size and shape of the molecule on the adhesion. Among other experiments, those with the homologous series of the alcohols were very instructive. The experimental results in table 3 show the adhesion of quartz particles on a quartz plate in solutions of methyl,
e
MOLES PER LITER
FIQ.1. The adhesion of quartz particles to a quartz plate in solutions of alcohols
ethyl, and propyl alcohols of different molecular conpentrations. The experiments were carried out with monodisperse quartz powder (size of particles 14.2p ) . It will be seen that sin a of the angle of adhesion increases with increasing alcohol concentration and with increasing molecular weight of the alcohol. This peculiar form of the adhesion curve, figure 1, is caused by the increasing dehydration of the adsorption layers with the increasing concentration of the solution. With increasing alcohol concentration the adsorption film becomes constantly richer in alcohol molecules, and thus the intermolecular forces between the adsorbed alcohol molecules play an increasingly important rBle. That in this case the adhesion is in fact due to the intermolecular forces between the adsorbed molecules is proved by the discovery that the adhesion bears a close relation to the dielectric molecular polarization of the
1008
A. VON B U Z ~ G H
mixed liquids. The dielectric polarization of the mixed liquids, PI,2, was calculated from the well-known formula:
+
- 1.NA,*= D-
~ M N*M, ~ d D f 2 in which D is the dielectric constant of the mixture and d is its density. From table 3, and especially from figure 2, it will be seen that in the case of different alcohols the same adhesion corresponds to the same P value. The sin a values of the three homologous compounds lie on a common sin a-P1 ,S curve. The above regularity was found to exist only in the case of compounds of analogous structure and chiefly in the case of homologous compounds.
/I I
J
10
20
30
40
50
+P Fm.2. The relation between adhesion and the dielectric molecular polarization for three alcohols
Compounds of different mole&lar structure but of the same molecular polarization have a different effect on the adhesion. This fact indicates that when alteration of the adhesion is affected by the addition of nonelectrolytes, not only the dielectric properties of the liquid medium but also 0 t h properties, especially the molecular constitution and the size and shape of the dissolved molecules, are of great importance. Besides the properties of the dissolved portion, those of the solid phase are also important. In this respect CJSO the electrical properties proved to be the most decisive factors. This is especially pronounced in the case of solutions of polar and asymmetrical compounds, for instance, in those of the aliphatic acids. Experiments carried out with quartz showed especially clearly to what a great extent the original nature of the electric charge influences the whole character of the concentration curve of the
ADHESION OF MICROSCOPIC PARTICLES
1009
adhesion. Figure 3 shows the effect of water-soluble aliphatic acids on the adhedoh of quartz particles with a negative charge to a negatively charged quartz surface. I n the case of low concentrations we are dealing essentially with an electric action, a discharge. With high concentrations strong dehydration takes place; accordingly in this case also the adhesion shows a considerable increase, as in the case of the alcohols. On the other hand, we find quite different conditions in the case of positively charged quartz. The quartz particles and the adhesive plate were given a positive charge by means of an aluminum salt, and the adhesion was measured under the same conditions as those of the negatively charged quartz. The results are shown in figure 4.
H
'r(
4 f
5
IO
c, MOLES
15
20
PER LITER
FIG. 3. The adhesion of quartz particles with a negative charge to a negatively charged quartz surface in solutions of aliphatic acids
Three concentration zones can be distinguished. With low concentrations we have a simple ion action. This is shown by the speed of the cataphoretic motion on the lower curve. Parallel to the recharging, first a decrease and then an increase in the adhesion take place. The minimum adhesion coincides with the maximum of the negative electrokinetic potential. At higher concentrations, however, the change of the electrokinetic potential has a noticeable influence on the adhesion; for with higher concentrations, the adhesion is independent of the concentration, although the electrokinetic potential decreases further with increasing concentration. From the first it seemed a probable hypothesis that these phenomena might have their origin in an alteration of the dipole double layer formed on the quartz surface by the orientation of the aliphatic acid molecules. To a great extent the abovementioned experimental results seem to in-
1010
A . VON BUZdGH
dicate the following change in the adsorption layer: First of all, a monomolecular film is formed, in which the molecules of the carboxyl group ad-
e
*
nCooH
CW~COOH
c,n.coou c,n,coon
FIG.4. The adhesion and the cataphoretic mobility of quartz particles with a positive charge in solutions of aliphatic acids
H
H
CHz
CHz
H-
H-
H
coo- coo- coo- coo- cooI I I I l CHI
CHI
CHI
CHI CHI CHI CHI CHI .................................................. CHa
CHI
CHa
CHs
CHI
CHI
CHa
CHa
CHa
CH:
CHz
CHn
CHz
CHz
CHz
CHz
CHI
CHn
CHa
CHr
I I 1 coo- coo- coo- coo- coo-
I
1
I I I 1 I coo- coo- coo- coo- coo-
,++++++++++ +++++++++ (b) (a) FIG.5. Orientation of aliphatic acid molecules on the quartz surface
here to the positive quartz surface. Condition a (figure 5 ) corresponds to the isoelectric point. Later a second layer, b, is deposited, in which the carboxyl groups occupy the outer position. The second layer gives the
ADHESION OF MICROSCOPIC PARTICLES
1011
surface a negative charge, the value of which is determined by the dissociation of the carboxyl group. The thickness of this bimolecular layer chiefly determines the adhesion, as is evidenced by the fact that the adhesion is independent of the concentration in more concentrated solutions. The most decisive experiments in this connection, however, were those made on the molecular films of stearic and palmitic acids. Following the method of Langmuir and Blodgett, films built up alternately of stearic and palmitic acids were produced on a quartz plate positively charged with aluminum ions, and on this plate the adhesion of quartz particles, also positively charged, was measured in water. The results are shown in figure 6. On the monomolecular film the value of the angle of adhesion is 22’. If the plate is once more dipped in water and drawn through the surface
I
E Film
Ill llllllliil TI TI 11I1 lll~lllli 1I IllIllllllllJ111llll11 COOH
#J3”
+ + t , t + + * * t t + t t * * * * * * ,
\\\\\\\\\\\\\\\\\\\\\\\\‘~
FIG.6. Films of stearic and palmitic acids on a quartz plate positively charged with aluminum ions
film of stearic acid, a second film is built up on the plate, in which the COOH groups are on the outside. On this second film, the angle of adhesion is 53’. If a third film is built up, the adhesion again decreases, and the angle of adhesion shows a value of 20.5’; on the fourth film, the angle of adhesion again shows the same value as on the second film. That is, on these alternate films the adhesion changes alternately. So these experiments show clearly that, as regaids the change in the adhesion in solution of polar compounds, the orientation of the adsorbed dipoles plays a decisive rOle. It should be mentioned here that such alternate films exhibit a periodic change in their activity in the case of heterogeneous catalysis. Quartz plates were covered with polymolecular stearic acid films, and the rate of decomposition of hydrogen peroxide was measured in the presence of these plates. As the numerical values in table 4 show, the rate of decomposition
1012
A. YON
BUZXGH
QUANTITY OB
DECOYPOBED WITHIN
a HE.
mded w liter
Without quartz plates.. . . . . . . . . . . , . . . . . . . . . . . . . , , , . . . . . . . . . . , . In the presence of plates covered with a single film. . . . . . . . , . . . , In the presence of plates covered with a twofold film. . . . , . , . . . . In the presence of plates covered with a threefold film.. . , . . . . . , In the presence of plates covered with a fourfold film.. . . . . . . . . .
0.05
---f C
0.1
BELATIN
I;
-
0.0004 0.0069 0.0134 0.0056
0.0138
PH
FIG.7 FIG8 FIG.7. The effect of gelatingn the adhesion of quartz particles FIG.8. The adhesion of quartz particles in gelatin solutions of different pH values
conditions in the case of the albumins are more complicated, since here an interaction of the adsorption films of the adsorbed colloid must be taken into account. Nevertheless, in this field also several regularities have been detected. In the first place it waa found that the different albumins have a very
1013
ADHESION OF MICROSCOPIC PARTICLES
individual effect on the adhesion of microscopic particles, especially on quartz particles. In addition to the individual properties and the physical condition of the albumins, the electric condition of the quartz surface is of great importance. This is clearly shown, among other things, by the concentration functions of the adhesion. These functions yield us information principally with regard to the sensitizing and protective action of the colloids. In figure 7 the upper curve refers to positively charged quartz particles. With increasing concentration the adhesion shows a TABLE 5 Connection between the building up f collagen $hers and the adhesion i n solutions electrolytes ELECTBOLYTE
CONCENTBATION OF DLECTBOLYTE
TEE ADHESION (BIN (1) OF QUARTZ PABTICLm I N COLLAQEN BOLUI1ONII
BUILDINQ UP OW FIBERII
mdsa p e liter
Sodium chloride. . . , .
0 0.005 0.01 0.05 0.10 0.15 0.30 0.40
0.27 0.31 0.32 0.46 0.59 0.64 0.66 0.72 0.73
0 0.0001 0.0002
0.27 0,321 0.34
0.u)
0.0005
Barium chloride.. . . .
0.001 0.002 0.005 0.007 0.01 0.02 0.03 0.04
0.58; 0.53 0.64 0.72 isoelectric point
0.42
Faint Very noticeable Strong Faint
Faint Strong Strong Faint Very faint Faint Strong
steady decrease; this signifies a protective effect. On the other hand, the lower curve, which ha8 been obtained on negatively charged quartz, shows first a sensitizing effect, the protective action manifesting itself only with higher concentrations. It is true that the shape of these concentration functions depends on the physical state of the albumin. The electrical condition is especially important. This is evidenced by the fact that the adhesion exhibits a maximum at the isoelectric point of the albumin. This is shown, for example, by the adhesion of quartz particles in gelatin and'in albumin
1014
A. VON B U Z h H
solutions of different pH values. The isoelectric point is especially sharply defined in both cases (figure 8). This method is therefore very suitable for the determination of the isoelectric point of colloids. The circumstance that the adhesion of solid particles in solutions of albumins is due principally to the interaction of the adsorbed micelles led to the problem of the structure of the biogels. One of several important questions is: What circumstances determine the building up of fibers and fibrous gels out of the submicroscopic particles? A concrete case, for example, is that of the collagen fibers. It is generally known that by the action of electrolytes under certain circumstances fibrous gel is built up out of diluted collagen solutions which may be obtained from rats’ tails by means of acetic acid. To detect the conditions under which collagen fibers are built up, various electrolyte solutions of different concentrations were added to the collagen solution and observations were made by means of a microscope aa to the concentration at which the building up of the fibers takes place. Parallel with these observations, the adhesion of quartz particles in these solutions waa determined under the same conditions, and their cataphoretic mobility was also measured. These experiments (table 5 ) showed that the fibers never form at the isoelectric point, that is, never at the maximum of the adhesion. The building up of the fibers takes place only near the isoelectric point, that is, in the state of the solution in which the attractive forces between the micelles are not great enough to prevent their orientation during coagulation. This effect is seen very clearly in the case of barium chloride solutions, where the sign of the electric charge on the micelles is reversed. As may be seen, the building up of fibers takes place both to the left and to the right of the isoelectric point, but not at the isoelectric point itself (table 5 ) . From such observation of the behavior of adsorbed albumin films on adhesion it is believed that further information may be obtained concerning the nature and properties of the biogels. It is hoped that this summary gives a satisfactory survey of the phenomenon of adhesion and that it serves to indicate the fruitfulness of this field for biology and technics. I n conclusion the author wishes to express his great appreciation of the invitation to give a paper before the Colloid Symposium and of the opportunity thus afforded to make the acquaintance of his American colleagues.
