- Email: [email protected]
The behavior of linear polymers at interfaces
THE BEHAVIOR OF LINEAR POLYMERS AT INTERFACES Hiroshi Hotta The Institute of Scientific and Industrial Research, Osaka University, Sakai, Osaka, Japan Received July 30, 1954 ABSTRACT Monolayers of synthetic linear polymers were studied by means of measurements of interfaciai pressure and potential at air/water and oil/water interfaces. The polymers examined were: poly-N-vinyl pyrrolidone, polyvinyl acetate, polyvinyl stearate, polymethyl methaerylate, polymethacrylic acid, copolymers of polymethacrylic acid and diethylaminoethyl vinyl ether, poly-e-caproamide, polyethylene terephthalate, and polyacrylonitrile. All the polymers except polyvinyl acetate give films of the condensed type at the air/water interface, whereas all the polymers except polyethylene terephthalate and polyacrylonitrile give films of the expanded type at the oil/water interface. The film of the condensed type gives a constant surface moment up to a collapse point, and the true molecular weight can be obtained from the surface pressure. The film of the expanded t:tpe gives a constant surface moment at larger areas, but the moment changes at a certain area. In this case, a smaller molecular weight than the true one is obtained from the Surface pressure. The film of the expanded type at the oil/water interface has segments in the main chain as a statistical kinetic unit, each consisting of about 48 skeletal atoms, and is discussed in the light of the Singer equation of state. The value of surface moment is determined chiefly by the orientation of the C ~ O bond. The behavior of an ampholyte was studied by the measurement of interracial potential. The results showed that the amine groups could not be ionized more than about 20 mole per cent owing to the strong cohesion between major nonionized carboxyl groups at the air/ water interface, but such a restriction was not found at the oil/water interface owing to the sufficiently expanded state. The extension of these conclusions to proteins is suggested. INTRODUCTION Surface films of s y n t h e t i c h i g h p o l y m e r s were s t u d i e d s y s t e m a t i c a l l y first b y C r i s p in 1946 (1, 2). A s t h e c o n s t i t u t i o n s of s y n t h e t i c h i g h p o l y m e r s are n o t only g e n e r a l l y well k n o w n b u t also can b e v a r i e d as desired, t h e y are well s u i t e d to a n i n v e s t i g a t i o n of surface b e h a v i o r . F u r t h e r m o r e , t h e i r s t u d y m a y p r o v i d e a useful k e y to t h e b e h a v i o r of organic s u b s t a n c e s in n a t u r e . W e h a v e carried o u t for some y e a r s a series of studies of s y n t h e t i c linear p o l y m e r s b y m e a n s of m e a s u r e m e n t s of i n t e r r a c i a l pressure a n d p o t e n t i a l a t a i r / w a t e r a n d o i l / w a t e r interfaces to o b t a i n i n f o r m a t i o n on t h e g e n e r a l b e h a v i o r of l i n e a r p o l y m e r s a t interfaces. T h e c o p o l y m e r s especially gave 5o4
BEHAVIOR OF LINEAR POLYMERS AT INTERFACES
505
many interesting results. Although the results for each polymer have been published (3, 4, 5, 6, 7, 8), we will discuss the relationships between all these results in the present paper. EXPERIMENTAL
The surface pressure and potential of films spread at the air/water interface were measured simultaneously by the hanging plate method and the vibrating electrode method, respectively, using standard techniques. The low surface pressure associated with the determination of molecular weight was measured by a surface balance of the Langmuir-Adam type (3). Films were spread at a petroleum ether (b.p. 100°-140°C.)/water interface, using a micrometer syringe according to Alexander and Teorell's method (9). Only a film of polyethylene terephthalate was spread at the benzene/water interface. The interracial pressure and potential were measured by the ring method (4, 10) and the vibrating electrode method (7, 11), respectively. Measurements were made at room temperature without any special temperature regulation, but the variation of temperature during the determination of a single curve never exceeded one degree. Distilled water was used aS the substrate for nonelectrolyte. The pH for the polyelectrolyte was adjusted with hydrochloric acid or sodium hydroxide, and measured with pH test paper. Detailed descriptions of apparatus and procedure were given in the previous papers (3, 4, 7). The polymers and the solvents used for spreading are shown in Table I. TABLE I
Principal Polymers Used in the Present Paper Abbreviations PVP
Amilan
Polyraers Poly-N-vinyl pyrrolidone Polyvinyl acetate Polyvinyl stearate Polymethyl methacrylate Poly+caproamide
PMA
Polymethacrylic acid
PMA-DAV
Copolymers of PMA with diethylaminoethyl vinyl ether Polyethylene terephthalate Polyacrylonitrile
PVAc PVS PMM
PET PAN
Spreading solvents Water + pyridine (4/1) Benzene Benzene Benzene Water + c o n c . H~SO~ + isopropyl alcohol (5/1/4) Water + pyridine (4/1) Water + pyridine (4/1) Benzene 4- cresol (4/1) Dimethylformamide
References At A/W At O/W 8 8 3 3 5
4 4 5
4, 5
4, 5
5, 6
7
6
7
8
8
--
5
506
ttIROSHI HOTTA
8
.6
c
.E
6
2 ~
z
o
0
on
~0
40
60
BO
I00
Area in A~ per residue
FIG. 1. T h e surface pressure-area curve of various polymers at t h e a i r / w a t e r interface.
The abbreviation for each polymer in Table I is used throughout in the present paper. The detailed precautions for each polymer are given in the references listed in Table I. The compositions of copolymers were evaluated from the results of elemental analyses. The air/water and oil/water interfaces are abbreviated hereafter as A / W and O/W interfaces, respectively. RESULTS
The interracial pressure (Tr)-area (A) curves of the nonelectrolytic ['-- CH2--CH---] linear p°lymers °f the type °f L ]R J (R representing a side chain) at the A/W and O/W interfaces are shown in Figs. 1 and 4, respectively. In the case of the polyelectrolyte, only the result under the condition of no effective charge is shown in these figures. In Fig. 4, kink points are shown by arrows. The characteristics of each polymer are described in full in the references shown in Table I. The area for copolymers was calculated as the mean area per vinyl unit from their composition. The surface moment was calculated from the observed surface potential using Helmholtz's formula in the usual manner (3). DIscussioN
When we investigate the conditions under which polymers can be spread as a monomolecular film at an interface, the interaction between polymer and upper or lower phase, chiefly the hydrophility and oleophility of the polymer, should be considered first. However, once molecules are
BEHAVIOR OF LINEAR POLYMERS AT INTERFACES
507
spread at the interface as the result of a counterbalance between such interactions with the phases and the intra- and intermolecular forces, the state of the film is chiefly determined by the latter. The various factors may be classified as follows: the effects of medium include (1) the effect of aqueous phase, and (2) the effect of oil phase; and the effects of intraand ~ntermolecular actions include (3) the cohesion between nonpo]ar groups, (4) the sterie effect of a side chain, (5) the interaction between polar groups, and (6) the interaction between ionized groups. To interpret how the state of the film is influenced by these various effects, we shall discuss first the behavior of nonelectrolytic linear polymers in Sections A and B and then that of polyelectrolytes in Section C.
A. The Behavior of Nonelectrolytic Polymers at the Air~Water Interface (a) Surface Pressure. As shown in Fig. 1, most of the linear polymers give films of the condensed type. They have a specific limiting area depending on the nature of the side chain lying at the interface, although the main chain is constant (--CH2--CIt--). They have a common limit-
I
ing area (about 10 A. ~ per residue) in the case of no side chain lying at the interface. The disposition of the side chain, whether at the interface or not, is more important with copolymers than with homopolymers. That is, the copolymers of vinyl acetate and vinyl stearate (VS), which lay their side chains at the interface, give irregular results and have a smaller limiting area than that of PVS (3), whereas the PMA-DAV copolymers, which lay no side chain at the interface, give reproducible results and have the same limiting area as that of PMA under the condition of no effective charge (6). The latter area is also nearly equal to that of PAN at the O/W interface, as shown later in Fig. 4. On the other hand, PVAc gives a film of the expanded type, but the film of its copolymers with VS is considerably condensed even if it contains only 10 mole per cent VS (3). It is clear in such a case t h a t the cohesion between nonpolar groups makes a film condensed. But, in the case of PMA, the action of hydrogen bonds between the nonionized carboxylic groups contributes considerably to condensation (6, 25). It will be clear later why the film of PVAc is of the expanded type. It has been confirmed by many investigators that the molecular weight of high polymers determined from surface pressure at the A / W interface agrees satisfactorily with that determined by other methods in the case of natural materials such as protein (12). On the other hand, it has been reported that the monolayer method is unsuitable for some synthetic polymers such as PVAc (13). We obtained the experimental results shown in Table II, that the monolayer methodI gives fairly good agreement with
508
HIROSHI HOTTA
TABLE II Molecular Weight of PVAc and PVS Polymer
Sample
Molecular weight from Surface pressure
J:a
10,000
Viscosity 58,820
Osmotic pressure --
PVAc
tb
PVS
~d e
7,020 7,920 9,240 41,000
69,350 110,500 ---
--110,000 4- 5,000 70,000
the bulk method in the case of PVS, whereas it gives a far smaller result than the bulk method in the case of PVAc (3). The reason for the disagreement with PVAc is as follows. The molecule of PVS is considered to behave as a rigid island in the film of the condensed type at the A / W interface, because of the strong cohesion between the long side chains of the stearyl group (as shown later in Table III). In such a case, where one molecule corresponds to one rigid aggregate, the molecular weight can be determined by the monolayer method, and the result agrees satisfactorily with that by other methods. The good agreement between the monolayer and bulk methods with proteins is because of some rigid structure in the protein, arising from intramolecular hydrogen bonding, interaction between polar groups, or other chemical bonds. On the other hand, the molecule of PVAe is considered to be fairly loosely packed and is subject to micro-Brownian motion owing to its weak intramolecular cohesion (Table III). Therefore, the film is of the expanded type. In such a case, where one molecule corresponds to more than one kinetic unit, only the molecular weight of the submolecule is determined by the monolayer method, and the result is far smaller than the true one, as shown in Table II. (b) Surface Moment. The observed value of the surface moment of polymers varies with area up to a certain area, shown as A .... in Table III. PVP is the extreme example for a small A . . . . and does not give a reliable surface moment nearly up to its kink point of surface pressure (8). The maximum area obtainable for a reliable value is denoted hereafter b y A ..... When we notice the surface moments in the range of area for reliable potentials shown in Fig. 2, they are constant up to the kink point of surface pressure except for PVAc. The case of PVS is somewhat different because there is no constant region but a continuous linear change. In Fig. 3 for Amilan, the surface viscosity, measured by the oscillating disc method in our laboratory, also varies up to A m~ and rises rapidly below A .... (14). It is supposed from these facts that the film of polymer is not always compressed uniformly in all parts of a trough, with a moving barrier, at areas larger than Am~x but t h a t compression becomes uniform
TABLE III
Surface Moment and Corresponding Model at the Air/Water Interface Amax (A.* Surface moment per (roD.) residue)
Model
Polymer
Polyvinyl a alcohol
C*
I
0
\
40
50
90
65
175 X 2
115
250
25
tI CHa
I
C*
\
PMA b
/
C~-~O
0
I
H
/ PET
\
Ring*--C*
/7
% O
C*--Ring*
O CHa
\
C*
I
PMM
C
CHa
//\/ 0
0 Ctta
l
C*
°
C
280 ~
0 PVAc
0
C*
CH3 O--C
340 d
70
280 c
55
460
45
0 CI~Ha~
L
PVS
C*
Polyocta- a decyl methacrylate
CHa
\/%
C
0
I
1
C*
0
\/ o,
II
0 F r o m Ref. 2. b I n the nonionized state. At 25 A2 per residue. d In the expanded state.
0 C18Ha7
510
HIROSHI HOTTA 400 PVAc E
PET
300 PMM
200 E
PMA
I00
0
I
I
0
I
20
I
I
i
40
60
I
I
I
80
I00
Aree in A~ per residue Fig.
2
FIO. 2. The surface m o m e n t - a r e a curve of various polymers at the a i r / w a t e r interface. 0.20
Io
250 E
= g
-&
o.J6 8
c_
~-A .c_
6
If
\ .oo
0.12
0.08
8 0.04~ co
Oo'
20
40
60
80
I00
~0.00 120
Area inA 2 per residue
FIG. 3. The surface pressure, moment, and viscosity of Amilan at the a i r / w a t e r interface as the function of area per residue.
at A . . . . Then, the reproducible surface moment and viscosity can be obtained at A . . . . Therefore, A .... is also the appropriate measure to s t u d y the surface behavior of polymers, which m a y be affected not only by the specific property of the residue but also by its degree of polymerization. Since the polymer molecule has a rigid structure in a film of the condensed type, as mentioned above, the form of each residue is unchanged with compression. Therefore, they have their surface moment as shown in Figs. 2 and 3. On the other hand, since the polymer molecule has a loosely packed structure in a film of the expanded type, such as PVAc as mentioned
BEHAVIOR OF LINEAR POLYMERS AT INTERFACES
511
above, the form of each residue is transformed to a more packed form from a certain area as compression occurs. Therefore, the surface moment has a bending point in the curve as shown in the case of PVAe in Fig. 2, even ff a kink point does not appear in the case of surface pressure. This is also confirmed by the fact that the area at this bending point for PVAc agrees closely with that for the kink point of interracial pressure for PVS shown in Fig. 4. In spite of their similar polar groups, the polymers shown in Fig. 2 have different surface moments, as shown in Table III, in which Crisp's data are also included (2). On the other hand, the bond moments of C ~ O and C--O bonds in vacuo are 2.5 and 0.86 D., respectively (15). In the light of the moment and the orientation of these polar bonds at the A/W interface, the most plausible models consistent with the observed surface moments are proposed in Table III, in which the atom of the main chain is shown by a star. In these models polyvinyl alcohol, which has no C~-~-O bond, has the lowest value, and the polymers which have a C~---O bond have a higher value with increasing inclination of the Cm~-O. Furthermore, the various lbniting areas shown ia Fig. 1 and the surface properties of PVAc mentioned above are also understood in tile light of these models (5). Therefore, this table would be useful for estimating the inclination of the C-~-O bond for an unknown polymer. The interaction between polar groups is a function of the magnitude and direction of dipole moment and of the distance between the groups. But these variables at the interfaces cannot be estimated directly from the data in bulk phase owing to the specific orientation at the interfaces. Furthermore, the orientation is different even between the A/W and O/W interfaces (7, 16). For example, according to Wesson's Table, the dipole moment of all carboxylic acids and their esters is about 1.7 D. (17), while they have different surface moments as mentioned above. On the other hand, polyacrylate and polymethacrylate show a quite different surface behavior (1, 2). The expanding property of PVAc is due not only to the short side chain but also to the large distance between polar groups as shown in Table III. With PAN the dipoles can approach closely parallel to each other because there is no side chain and, in addition, the bond moments are large, 3.6 D. (15, 17). Consequently, it cannot be spread at the A/W interface, and is spread as a film of the condensed type even at the O/W interface, as shown in Fig. 4. B. The Behavior of Noneleetrolytic Polymers at the Oil~Water Interface Since the effect of cohesion between nonpolar groups is predominant in most cases at the A / W interface, we investigated behavior at the O/W interface in order to bring out other effects. That is, on dissolving hydrocarbon chains in a hydrocarbon phase, the nonpolar main chain can move
512
HIROSI-II tIOTTA
'2tinp A ...S PV
i
1 PVAc
PMA-DAV\\\ ,%
-
0
20
40
Area
in
60
A.2 per
80
I00
residue
Fi~. 4
FIG. 4. The interracial pressure-area curve of various polymers at the petroleum ether/water interface.
freely at the interface owing to liberation from lateral cohesion. For example, whereas the different behavior between PVAc and PVS at the A/W interface might be attributed to different cohesion of side chains as mentioned above, this cohesion would be eliminated at the O/W interface. In fact, both polymers show entirely identical pressure-area curves up to 2 dynes per cm. at the O/W interface, as shown i~ Fig. 4. In such a case, the Flory-ttuggins' treatment of flexible linear polymers in bulk solution may be applied to the behavior at the interface. Singer has made the necessary calculation (18), the equation of state at the interface being given to a close approximation by: ~r = ~
x
2
zA /
)
'
where v, A, x, A0, and z are interfacial pressure, apparent area per residue, the degree of polymerization, the actual area occupied by each residue (the size of a surface site), and the co-ordination number around a surface site, respectively. Since z should be 2 in the ease of a rigid molecule, Davies defined the increment of z from 2 as surface flexibility (19). His results for synthetic polyamino-acid and protein are shown for comparison in Table IV.
BEHAVIOR
OF LINEAR
513
POLYMERS AT INTERFACES
TABLE IV Polymer
PVP b PVAc PVS FMM Amilan PMA-DAV ~ FMA ~ PAN PET Polyamino-acidl Human methemoglobin]
in Eq. [4]
18.5 19 19 23.5 6.9 23.5 26.5 36.5 13 ---
~tom
in Eq, [5]
37 38 38 47 48 47 53 73 123.5 e ---
(~s
in Eq. [6]
0.6 0.5 0.5 0 0 0 --0.2 - 0.7 --5.8 ---
At O/W
Za
At A/W
3.60 3.60 3.60 3.75 3.75 3.75 3.75 2.05
-2.20 2.02 ------
2.02
--
3.33 2.12
-2.015
a A t 0.1 dyne per cm. (Ao = 5 A. ~ per skeletal a t o m ) . b On t h e acidic s u b s t r a t e . c A t p H 5. d A t p H 1.6. e Assumed on t h e basis of a ring group being e q u i v a l e n t to 3.5 s~raight chain atoms in a. I F r o m Ref. 20. I n t h e l i m i t w h e r e A >> A0, E q . [1] c a n b e r e w r i t t e n a s
I2] where
b= Ao(2-2x2z~zx)
[3]
I n f a c t , w h e n w e p l o t t h e p r o d u c t of ~ a n d A a g a i n s t t h e r e c i p r o c a l of A , a curve as shown in Fig. 5 can be obtained. Then, on putting
w e c a n d e t e r m i n e t h e v a l u e of n b y t h e e x t r a p o l a t i o n of s u c h a c u r v e t o a n i n f i n i t e a r e a . H o w e v e r , t h i s v a l u e is f a r s m a l l e r t h a n t h a t a t t h e A / W i n t e r f a c e a s w e l l a s t h e t r u e v a l u e of x. F o r e x a m p l e , t h e v a l u e s of n a n d x f o r P V A c a r e 19 a n d a b o u t l 0 s, r e s p e c t i v e l y (see T a b l e s I I a n d I V ) . I t is c o n c l u d e d f r o m t h i s f a c t t h a t w e m u s t c o n s i d e r a n - m e r s e g m e n t a s a s t a t i s t i c a l k i n e t i c u n i t a t t h e O / W i n t e r f a c e i n s t e a d of x. T h e v a l u e s of n for various polymers obtained by us are shown in Table IV together with t h e v a l u e s of n~o,,, w h i c h is d e f i n e d a s n~tom = a n,
[5]
514
I-IIROSHI H O T T A
8O
== "to
o 60
,=,40
'o ,= 2 0 PAN
¢
......
0
I.
1
e I
I
500200150
'0 I
I
I
1008070 60
I
50
Area in A2 per residue
FI~. 5. The product of interracial pressure (~) and area per residue (A) plotted against the reciprocal of A at the petroleum ether/water interface. where a is the number of atoms constituting the skeleton of the main chain per residue. In this case, the length of this segment can be expressed on a universal scale. In respect to a, a question rises how m a n y carbon atoms of straight chain might be equivalent to a benzene ring in P E T . Since it is reported t h a t ' a benzene ring is equivalent to 3.5 straight-chain carbon atoms in the effect on the critical micelle concentration for micelle formation (20), we used for convenience this value in Table IV. Flory had estimated from the activation energy for viscous flow that there are approximately 32 atoms in each segment of flow of polyesters (21). Then, when it is assumed in Eq. [1] that there are segments consisting of n a t o m skeletal atoms in the very dilute state, and that each skeletal atom occupies apparently one-a'th of the area per residue (.4) and actually 5A 2 as the size of a surface site (A0), we can obtain the value of z at 0.1 dyne per cm., as shown in Table IV. If the side chain affects the behavior of the main chain, the effect may appear in z. For comparison, the values of z for PVAc and PVS at the A / W interface were calculated from the data for Table II (3), and are shown together in Table IV. On the other hand, to estimate the contribution of a side chain to natom, we calculate the value of a~ shown in Table IV, which is defined by 48 a~ = - - - a, n
[6]
on assuming that all segments in the perfectly expanded state should have the length of 48 skeletal atoms. Negative values of a~ occur with larger values of n,,ol~ and correspond to the degree o f condensation of the monolayer. On summarizing these results, it is concluded that there is no serious effect of the side chain on z in the sufficiently expanded state. In other
515
B E H A V I O R OF L I N E A R P O L Y M E R S A T I N T E R I ~ A C E S
words, the above assumptions in our theory are correct for this state. The value of as in this state is also equal to about 0.5 for a massive side chain independent of its structure. On the other hand, a low value of z corresponds with a negative value of a~. The low value of z for PET is due to the complete rigidity of the benzene ring, which is assumed to be equivalent to a flexible chain consisting of 3.5 atoms. From the viewpoint of a~, the group - - C - ~
~ - - C - - should be considered to be kinetically
a
rigid
unit of flexibility equivalent to --0.3 (= 5.5 -- 5.8) carbon atoms instead of 5.5 (= 3.5 + 2). The behavior of PAN has been already interpreted in terms of the strong interaction between polar groups. However, its film may be expanded if we can select a suitable solvent having a higher dielectric constant, as the oil phase. Proteins show the very condensed behavior even at the O/W interface (22), but to some extent are more expanded than at the A/W interface (19). This difference is shown in z in Table IV. This fact shows, as pointed out already by Davies and also by us, that the links holding the native protein hi its characteristic configuration are due not only to cohesion between nonpolar parts, but also, mainly, to interaction between polar groups and intramolecular chemical bonds. The difference between PVAc and PVS at the A/W interface is also shown in Table IV. When we compare them with human methemoglobin at both interfaces, on the basis of z, the degree of expansion of PVAc at the A/W interface becomes clear. Although Davies' results are quoted in the same table, we must remember the difference between natom and x on comparing all values in Table IV. Although we have hitherto tried various quantitative interpretations of the "expanded" and "condensed" states, surface elasticity seems to be also the appropriate measure. Bateman and Chambers defined the quantity
M=
-A
dZ dA
[7]
as surface elasticity (23) and we calculate it at 0.5 dyne per cm., using the data in Fig. 4. For a plot of log M versus log nat. . . . the points lie perfectly on a straight line expressed by the equation 303 M - (n~,om)1.5'
[8]
as shown in Fig. 6, except for the polymers containing a ring group. However, we must bear in mind that the value of M reflects the uncertainty in dr the determination of ~-~. When the values of n~tomfor polymers deviating from the straight line are calculated by Eq. [8], a ring group in PET and PVP becomes equivalent to - 1 . 4 and 0.5 skeletal atom of the main
516
HIROSHI HOTTA 0.2
•
PVAc, PVS ~:
A
O.O a.
PVP \ on pH 3 . 5 ~ "
0
P M M ~ Amilan
PMA-DAV~ on p H 4 . 8 0
PMA on pHI.6
~E ,-I
-02
PET
-0.4
i
1.5
I
I
1.7
I
1.9
w 2.1
Log %tom
FIG. 6. The log of M at 0.5 dyne per cm. in Eq. [7] plotted against the log of natom for various polymers at the oil/water interface. The straight line is expressed by Eq. [8]. chain respectively. I t is concluded that the ring group in a main chain should be considered in a different way from the ring group in a side chain. When the kink points in Fig. 4 are compared with the corresponding points in Fig. 1, it is found t h a t a classification can be drawn up in terms of vinyl units: that is, (1) polymers which lay no side chain at the interface have a kink point at 9 ~ 10 A. 2, per vinyl unit as mentioned in Section A; (2) polymers which lay a side chain, consisting of a chain group alone, at the interface have a kink point at about 19 A? per vinyl unit, which is equal to the second kink point of PVP; (3) polymers which lay a ring group as the side chain at the interface have a kink point at about 24 A. 2 per vinyl unit. However, these values hold for a condensed structure, and not always for a structure at a larger area. Therefore, it is easily understood by comparison with the model of PVAc at the larger area in Table I I I that PVS has a kink point at 38 A. ~ in spite of its being in group II. T h a t is, there is a transformation not accompanied with a kink point of surface pressure as mentioned in the case of the surface moment of PVAc. On the other hand, PVP has a second kink point. The appearance of a kink point depends on the degree of hydrophility of the group, which is transformed at this point. The degree of hydrophility of a polar group may be measured b y a study of the adsorption of water on the polymer (24).
B E H A V I O R OF L I N E A R P O L Y M E R S A T I N T E R F A C E S
517
C. The Effect of Substrate (pit) Now, we shall consider the effect of substrate. Although there are various problems concerned with some primary or secondary chemical reaction, namely, oxidation, hydrolysis, complex formation,: penetration, and adsorption, the most fundamental problem for physical chemistry is the effect of hydrogen ion and neutral salt in a substrate. Among these, only the effect of the pH of the substrate on a polyampholyte, namely, the effect of ionizatLon on its surface behavior will be discussed here. To make this effect clear, we studied the monolayer of PMA and of the copolymers of methacrylic acid with diethylaminoethyl vinyl ether (DAV) at the A/W (6) and O/W (7) interfaces. The mole percentage of DAV for the copolymers K and L is given in Table V. The surface moment of copolymer L at both interfaces is shown in Fig. 7. When we plot the interfacial moment of all samples in the constant range at the larger area at both interfaces against the pH of the substrate, the relationship between them becomes as shown in Fig. 8. The trend of PMA in Fig. 8 corresponds closely to the change of the ionization and viscosity of its solution with pH, which was interpreted in detail by Katchalsky (25). Therefore, the surface behavior can also be entirely understood by his interpretation of the relationship between the shape of the molecule and the degree of ionization in solution. The constant range of interracial moment corresponds to the nonionized state, and the range of decreasing moment is due to the ionization of carboxylic groups. In the case of copolymers, it is considered that their carboxylic groups are in the nonionized state in the constant range of interfacial moments, that the degree of ionization of DAY groups is not changed in the same range, and that their interfacial moments decrease with the ionization of carboxylic groups. The ionization of carboxylic groups, which is pronounced even in sample K, is so predominant in the high pH range that the interfacial moment becomes negative, but we can13ot determine its exact magnitude owing to partial dissolution into the Substrate. Therefore, the points in the negative area in Fig. 8 indicate no absolute value but only qualitatively the inversion of sign. The molecular interpretation of this behavior was given in detail in the previous papers (6, 7). On assuming the perfect ionization of DAV groups and the perfect nonionization of carboxylic groups in the constant range, the interfacial moments of DAV groups are calculated as shown in Table V from the mole percentage of DAV and the interfacial moment of nonionized carboxylic groups at the corresponding interface shown in the same table. On the other hand, the interfacial moment of ClsHaTN(CHa)a+ was reported to be 600 roD. or more and equal at both interfaces (11). Therefore, only the value of sample K at the A / W interface in Table V might be inconsistent with the above fact, owing to the incorrect assumption of the perfect ionization of DAV groups. It is concluded from these facts
518
HIROSHI
ttOTTA
TABLE V The Interracial Moments of Nonionized Carboxylic Group and Ionized DA V Group in PMA and Its Copolymers with DA V at Both Interfaces Estimated from the Data in Fig. 8 Sample
Mole percentage of DAV in copolymer
Interracial moment in roD. of Nonionized carboxyllc group Ionized DAV group At A/W At O/W At A/W At O/W
K 31 (90)a (15)~ 450 L 20 (90)a (15)~ 600 PMA 0 9O 15 -The assumed value used for the estimation of the DAV group.
600 600 --
that the DAV groups of both copolymers are perfectly ionized to the expanded state at the O / W interface, whereas they cannot be ionized more than about 20 mole per cent of total ionizable groups in the condensed state at the A / W interface. T h a t is, there exists a certain minimum distance to which ionized groups can approach in the condensed state of the polyampholyte, owing to the mutual repression of ionization by any closer approach. The residual nonionized DAV groups appear to have a surface moment of about 90 roD. at the A / W interface. Since such a restriction is due to the strong cohesion between nonionized carboxylic groups mentioned already, this is not found for P M A owing to the absence of such a nonionized opposite component. McBain and Peaker have already suggested that the film of stearic acid cannot be much ionized in the solid state (26). The interfacial moment-area curves of t h r e e samples a t both interfaces converge to about 90 ~-~ 100 roD. at 10A3 per residue with a few exceptions as shown in Fig. 7. The trend of curves is also understood in the light of the above conclusion. As the mutual repression of ionization by the approach of already ionized groups takes place with compression, the apparent mean interfacial moment per residue decreases and the difference in all cases disappears at the collapse point, at which the state of the molecules becomes identical with that at the A / W interface. However, the surface moment of sample L at p H 4.9 is constant up to the collapse point in Fig. 7. The reason is that a repression does not take place at this pH, which corresponds to the isoelectric point of this polyampholyte. This is also concluded from the fact that the value of n~tom at this p H is identical to that of other similar nonelectrolytie linear polymers in Table IV. All the conclusions from the results of interfacial moment are perfectly consistent with those from the interracial pressure at both interfaces (6, 7). The p H (pH 3.5), from which the interfaeial moment of P M A begins to decrease at the O / W interface, is lower than that (pH 4.0) at the A / W interface in Fig. 8. Furthermore, the change is sharper at the latter than at the former because the cohesive force connecting ionizable groups is stronger at the latter.
519
BEHAVIOR OF LINEAR POLYMERS AT INTERFACES 240
2OO p
E
H
;oH 1.4~ 2.4, I and ~.8
4
160
H4.4
pilL4 and
"~ ~ ~ - - - . . . . . . . .i
5.5
E 120 o
o
pH4.9 ~
~_
pH 4.7
40
I
Oc)
I
I
I
20
I
l
40
I
60
I
1
80
I00
Ares i. h~ per residue
FIG. 7. The interfacial m o m e n t - a r e s curve of sample L at the a i r / w a t e r ( oil/water ( - - - ) interfaces on the s u b s t r a t e of various pH's.
) and
250
200
E 150
o E
I00 -
,I
b
PMA . .X. . . . . " x - & ~ , ~ )
~
~ ....
g -50
I
1 2
t.
I 4
The
I
6 pH
i
i
8
I0
of the substrate
FIG. 8. The interfaeial moment of copolymers at t h e larger area at b o t h interfaces p l o t t e d against t h e p H of the substrate. Symbols in the figure are as follows: Sample
At A/W
L PMA
A X
At O/W
[] ®
520
ItIROSHI HOTTA
Glazer and Dogan investigated the state of ionization of an insoluble monolayer by the measurement of maximum surface potential at the A/W interface (27). Although they found that the value of pK from their surface technique agrees fairly well with that from the measurement in bulk solution in the case of proteins, but not in the case of small molecules such as stearic acid and octadecylamine, they could not explain the latter disagreement satisfactorily. Nevertheless, these facts may be understood in the light of our present investigation. That is, the method of maximum surface potential is applicable to those compounds that have a constant interracial moment up to a collapse point, owing to a rigid structure. It is unsuitable, however, for such compounds as stearic acid, which have a varying interracial moment up to the collapse point, owing to a loosely packed structure. For the latter, we must consider the interracial moment at the larger ares instead of that at a collapse point as shown in Fig. 8. The value of pK from the measurement in bulk solution in such a case corresponds to that for an expanded film. Some investigators consider that the pH under the interface is different from that in bulk solution (28). Nevertheless, the trend in bulk solution is almost directly applicable to that at the interface as mentioned above. It is because conditions for both cases are identical in respect to the atmosphere in the vicinity of the ionizable groups, except for the characteristic orientation and concentration at the interface. In this case, we must of course keep the difference between expanded and condensed films in mind. SUMMARY
We now summarize the above discussion according to the classification mentioned at the outset. For the effects of medium: (1) The most important effect of the aqueous phase is due to hydrogen ion and neutral salt; and (2) The most important effect of the oil phase is the liberation of the cohesive forces between nonpolar parts. For the effects of intra- and intermolecular actions: (3) The cohesion between nonpolar groups is predominant in most cases at the air/water interface and is absent at the oil/water interface. (4) The steric effect of side chains is very effective when they lie at the interface. (5) The interaction between polar groups is not relieved by the petroleum ether phase, and cannot be estimated directly from the magnitude of the dipole moment in the bulk phase alone. (6) The interaction between ionized groups should not be considered without considering the type of film.
BEHAVIOR OF LINEAR POLYMERS AT INTERFACES
521
ACKNOWLEDGMENTS The author expresses his hearty thanks to Professor T. Isemura for his kind guidance throughout the present work, and to Professor S. Murahashi and Mr. S. Otsuka in the Institute, who kindly supplied valuable samples to him. REFERENCES 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.
C~IsP, D. J., J. Colloid Sci. 1, 49 (1946). CRISF, D. J., J. Colloid Sci. 1,161 (1946). IS~MTJRA,T., ttOTTA, H., AND Mlwn, T., Bull. Chem. Soc. Japan 26,380 (1953). HOTTA, H., Bull. Chem. Soe. Japan 26,386 (1953). ttO~TA, H., Bull. Chem. Soc. Japan 27, 80 (1954). ISEMTJRA,T., HOTTA,-H., ~ D OTS~J~:A,S., Bull. Chem. Soe. Japan 27, 93 (1954}, HOTTA, tt., Bull. Chem. Soc. Japan 9.7, 412 (1954). HOTTA, H., Bull. Chem. Soc. Japan, in press. ALEXANDER, A. E., AND TEOR~L~, T., Trans. Faraday Soc. 35, 727 (1939). ALEX~tNDER, A. E., "Surface Chemistry" [Supplement of Research (London)], p. 123 (1949). DAVIES, J. T., Nature 167, 193 (1951); Z. Elektrochem. 55, 559 (1951). ABRIBAT, M., ~Nl) POURADIER, J., Compt. rend. 9.27, 1101 (1948); BULL, H. B., J. Biol. Chem. 185, 27 (1950). BENSON, G. C., AND MCINTOSH, R. L., J. Colloid Sci. 3, 323 (1948). ISE~UI~.~, T., ANn HAMAGUCHI,I~., Bull. Chem. Soc. Japan 9.7,125 (1954). SMVTH, C. P., J. Am. Chem. Soe. 60, 183 (1938). DAVIES, J. T., Trans. Faraday Soc. 49,949 (1953). WESSON, L. G., "Table of Electric Dipole Moment," The Technology Press, M.I.T., 1948. SINGEn, S. J., J. Chem. Phys. 16, 872 (1948). DAvIEs, J. T., Biochim. et Biophys. Aeta 11, 165 (1953). PAQUETTE, 1:~. G., LINGAFELTER,]~. C., ANn TARTAR, H. V., J. Am. Chem. Soc. 65, 686 (1943). FLoRY, P. J., J. Am. Chem. Soc. 62, 1057 (1940). C~IE~S~AN, D. F., Bioehem. J. 50, 667 (1952). BATEMAN, J. ]3., AND CHAMBERS, L. A., J. Chem. Phys. 7, 244 (1939). McLAREN, A. D., ANn I~OWEN, J. W., J. Polymer Sci. 7,289 (1951). I(ATCHALSKY,A., J. Polymer Sci. 7,393 (1951). McBAIN, J. W., A~I) PEAleR, C. R., Proe. Roy. Soe. (London) 125A, 394 (1929). GLAZE~, J., AND I)OGAN, M. Z., Trans. Faraday Soc. 49,448 (1953). HAVlNGA, E., AI~I)HERToG-PoLAK, M. DEN, Rec. tray. chim. 71, 64 (1952).
BEHAVIOR OF LINEAR POLYMERS AT INTERFACES
505
many interesting results. Although the results for each polymer have been published (3, 4, 5, 6, 7, 8), we will discuss the relationships between all these results in the present paper. EXPERIMENTAL
The surface pressure and potential of films spread at the air/water interface were measured simultaneously by the hanging plate method and the vibrating electrode method, respectively, using standard techniques. The low surface pressure associated with the determination of molecular weight was measured by a surface balance of the Langmuir-Adam type (3). Films were spread at a petroleum ether (b.p. 100°-140°C.)/water interface, using a micrometer syringe according to Alexander and Teorell's method (9). Only a film of polyethylene terephthalate was spread at the benzene/water interface. The interracial pressure and potential were measured by the ring method (4, 10) and the vibrating electrode method (7, 11), respectively. Measurements were made at room temperature without any special temperature regulation, but the variation of temperature during the determination of a single curve never exceeded one degree. Distilled water was used aS the substrate for nonelectrolyte. The pH for the polyelectrolyte was adjusted with hydrochloric acid or sodium hydroxide, and measured with pH test paper. Detailed descriptions of apparatus and procedure were given in the previous papers (3, 4, 7). The polymers and the solvents used for spreading are shown in Table I. TABLE I
Principal Polymers Used in the Present Paper Abbreviations PVP
Amilan
Polyraers Poly-N-vinyl pyrrolidone Polyvinyl acetate Polyvinyl stearate Polymethyl methacrylate Poly+caproamide
PMA
Polymethacrylic acid
PMA-DAV
Copolymers of PMA with diethylaminoethyl vinyl ether Polyethylene terephthalate Polyacrylonitrile
PVAc PVS PMM
PET PAN
Spreading solvents Water + pyridine (4/1) Benzene Benzene Benzene Water + c o n c . H~SO~ + isopropyl alcohol (5/1/4) Water + pyridine (4/1) Water + pyridine (4/1) Benzene 4- cresol (4/1) Dimethylformamide
References At A/W At O/W 8 8 3 3 5
4 4 5
4, 5
4, 5
5, 6
7
6
7
8
8
--
5
506
ttIROSHI HOTTA
8
.6
c
.E
6
2 ~
z
o
0
on
~0
40
60
BO
I00
Area in A~ per residue
FIG. 1. T h e surface pressure-area curve of various polymers at t h e a i r / w a t e r interface.
The abbreviation for each polymer in Table I is used throughout in the present paper. The detailed precautions for each polymer are given in the references listed in Table I. The compositions of copolymers were evaluated from the results of elemental analyses. The air/water and oil/water interfaces are abbreviated hereafter as A / W and O/W interfaces, respectively. RESULTS
The interracial pressure (Tr)-area (A) curves of the nonelectrolytic ['-- CH2--CH---] linear p°lymers °f the type °f L ]R J (R representing a side chain) at the A/W and O/W interfaces are shown in Figs. 1 and 4, respectively. In the case of the polyelectrolyte, only the result under the condition of no effective charge is shown in these figures. In Fig. 4, kink points are shown by arrows. The characteristics of each polymer are described in full in the references shown in Table I. The area for copolymers was calculated as the mean area per vinyl unit from their composition. The surface moment was calculated from the observed surface potential using Helmholtz's formula in the usual manner (3). DIscussioN
When we investigate the conditions under which polymers can be spread as a monomolecular film at an interface, the interaction between polymer and upper or lower phase, chiefly the hydrophility and oleophility of the polymer, should be considered first. However, once molecules are
BEHAVIOR OF LINEAR POLYMERS AT INTERFACES
507
spread at the interface as the result of a counterbalance between such interactions with the phases and the intra- and intermolecular forces, the state of the film is chiefly determined by the latter. The various factors may be classified as follows: the effects of medium include (1) the effect of aqueous phase, and (2) the effect of oil phase; and the effects of intraand ~ntermolecular actions include (3) the cohesion between nonpo]ar groups, (4) the sterie effect of a side chain, (5) the interaction between polar groups, and (6) the interaction between ionized groups. To interpret how the state of the film is influenced by these various effects, we shall discuss first the behavior of nonelectrolytic linear polymers in Sections A and B and then that of polyelectrolytes in Section C.
A. The Behavior of Nonelectrolytic Polymers at the Air~Water Interface (a) Surface Pressure. As shown in Fig. 1, most of the linear polymers give films of the condensed type. They have a specific limiting area depending on the nature of the side chain lying at the interface, although the main chain is constant (--CH2--CIt--). They have a common limit-
I
ing area (about 10 A. ~ per residue) in the case of no side chain lying at the interface. The disposition of the side chain, whether at the interface or not, is more important with copolymers than with homopolymers. That is, the copolymers of vinyl acetate and vinyl stearate (VS), which lay their side chains at the interface, give irregular results and have a smaller limiting area than that of PVS (3), whereas the PMA-DAV copolymers, which lay no side chain at the interface, give reproducible results and have the same limiting area as that of PMA under the condition of no effective charge (6). The latter area is also nearly equal to that of PAN at the O/W interface, as shown later in Fig. 4. On the other hand, PVAc gives a film of the expanded type, but the film of its copolymers with VS is considerably condensed even if it contains only 10 mole per cent VS (3). It is clear in such a case t h a t the cohesion between nonpolar groups makes a film condensed. But, in the case of PMA, the action of hydrogen bonds between the nonionized carboxylic groups contributes considerably to condensation (6, 25). It will be clear later why the film of PVAc is of the expanded type. It has been confirmed by many investigators that the molecular weight of high polymers determined from surface pressure at the A / W interface agrees satisfactorily with that determined by other methods in the case of natural materials such as protein (12). On the other hand, it has been reported that the monolayer method is unsuitable for some synthetic polymers such as PVAc (13). We obtained the experimental results shown in Table II, that the monolayer methodI gives fairly good agreement with
508
HIROSHI HOTTA
TABLE II Molecular Weight of PVAc and PVS Polymer
Sample
Molecular weight from Surface pressure
J:a
10,000
Viscosity 58,820
Osmotic pressure --
PVAc
tb
PVS
~d e
7,020 7,920 9,240 41,000
69,350 110,500 ---
--110,000 4- 5,000 70,000
the bulk method in the case of PVS, whereas it gives a far smaller result than the bulk method in the case of PVAc (3). The reason for the disagreement with PVAc is as follows. The molecule of PVS is considered to behave as a rigid island in the film of the condensed type at the A / W interface, because of the strong cohesion between the long side chains of the stearyl group (as shown later in Table III). In such a case, where one molecule corresponds to one rigid aggregate, the molecular weight can be determined by the monolayer method, and the result agrees satisfactorily with that by other methods. The good agreement between the monolayer and bulk methods with proteins is because of some rigid structure in the protein, arising from intramolecular hydrogen bonding, interaction between polar groups, or other chemical bonds. On the other hand, the molecule of PVAe is considered to be fairly loosely packed and is subject to micro-Brownian motion owing to its weak intramolecular cohesion (Table III). Therefore, the film is of the expanded type. In such a case, where one molecule corresponds to more than one kinetic unit, only the molecular weight of the submolecule is determined by the monolayer method, and the result is far smaller than the true one, as shown in Table II. (b) Surface Moment. The observed value of the surface moment of polymers varies with area up to a certain area, shown as A .... in Table III. PVP is the extreme example for a small A . . . . and does not give a reliable surface moment nearly up to its kink point of surface pressure (8). The maximum area obtainable for a reliable value is denoted hereafter b y A ..... When we notice the surface moments in the range of area for reliable potentials shown in Fig. 2, they are constant up to the kink point of surface pressure except for PVAc. The case of PVS is somewhat different because there is no constant region but a continuous linear change. In Fig. 3 for Amilan, the surface viscosity, measured by the oscillating disc method in our laboratory, also varies up to A m~ and rises rapidly below A .... (14). It is supposed from these facts that the film of polymer is not always compressed uniformly in all parts of a trough, with a moving barrier, at areas larger than Am~x but t h a t compression becomes uniform
TABLE III
Surface Moment and Corresponding Model at the Air/Water Interface Amax (A.* Surface moment per (roD.) residue)
Model
Polymer
Polyvinyl a alcohol
C*
I
0
\
40
50
90
65
175 X 2
115
250
25
tI CHa
I
C*
\
PMA b
/
C~-~O
0
I
H
/ PET
\
Ring*--C*
/7
% O
C*--Ring*
O CHa
\
C*
I
PMM
C
CHa
//\/ 0
0 Ctta
l
C*
°
C
280 ~
0 PVAc
0
C*
CH3 O--C
340 d
70
280 c
55
460
45
0 CI~Ha~
L
PVS
C*
Polyocta- a decyl methacrylate
CHa
\/%
C
0
I
1
C*
0
\/ o,
II
0 F r o m Ref. 2. b I n the nonionized state. At 25 A2 per residue. d In the expanded state.
0 C18Ha7
510
HIROSHI HOTTA 400 PVAc E
PET
300 PMM
200 E
PMA
I00
0
I
I
0
I
20
I
I
i
40
60
I
I
I
80
I00
Aree in A~ per residue Fig.
2
FIO. 2. The surface m o m e n t - a r e a curve of various polymers at the a i r / w a t e r interface. 0.20
Io
250 E
= g
-&
o.J6 8
c_
~-A .c_
6
If
\ .oo
0.12
0.08
8 0.04~ co
Oo'
20
40
60
80
I00
~0.00 120
Area inA 2 per residue
FIG. 3. The surface pressure, moment, and viscosity of Amilan at the a i r / w a t e r interface as the function of area per residue.
at A . . . . Then, the reproducible surface moment and viscosity can be obtained at A . . . . Therefore, A .... is also the appropriate measure to s t u d y the surface behavior of polymers, which m a y be affected not only by the specific property of the residue but also by its degree of polymerization. Since the polymer molecule has a rigid structure in a film of the condensed type, as mentioned above, the form of each residue is unchanged with compression. Therefore, they have their surface moment as shown in Figs. 2 and 3. On the other hand, since the polymer molecule has a loosely packed structure in a film of the expanded type, such as PVAc as mentioned
BEHAVIOR OF LINEAR POLYMERS AT INTERFACES
511
above, the form of each residue is transformed to a more packed form from a certain area as compression occurs. Therefore, the surface moment has a bending point in the curve as shown in the case of PVAe in Fig. 2, even ff a kink point does not appear in the case of surface pressure. This is also confirmed by the fact that the area at this bending point for PVAc agrees closely with that for the kink point of interracial pressure for PVS shown in Fig. 4. In spite of their similar polar groups, the polymers shown in Fig. 2 have different surface moments, as shown in Table III, in which Crisp's data are also included (2). On the other hand, the bond moments of C ~ O and C--O bonds in vacuo are 2.5 and 0.86 D., respectively (15). In the light of the moment and the orientation of these polar bonds at the A/W interface, the most plausible models consistent with the observed surface moments are proposed in Table III, in which the atom of the main chain is shown by a star. In these models polyvinyl alcohol, which has no C~-~-O bond, has the lowest value, and the polymers which have a C~---O bond have a higher value with increasing inclination of the Cm~-O. Furthermore, the various lbniting areas shown ia Fig. 1 and the surface properties of PVAc mentioned above are also understood in tile light of these models (5). Therefore, this table would be useful for estimating the inclination of the C-~-O bond for an unknown polymer. The interaction between polar groups is a function of the magnitude and direction of dipole moment and of the distance between the groups. But these variables at the interfaces cannot be estimated directly from the data in bulk phase owing to the specific orientation at the interfaces. Furthermore, the orientation is different even between the A/W and O/W interfaces (7, 16). For example, according to Wesson's Table, the dipole moment of all carboxylic acids and their esters is about 1.7 D. (17), while they have different surface moments as mentioned above. On the other hand, polyacrylate and polymethacrylate show a quite different surface behavior (1, 2). The expanding property of PVAc is due not only to the short side chain but also to the large distance between polar groups as shown in Table III. With PAN the dipoles can approach closely parallel to each other because there is no side chain and, in addition, the bond moments are large, 3.6 D. (15, 17). Consequently, it cannot be spread at the A/W interface, and is spread as a film of the condensed type even at the O/W interface, as shown in Fig. 4. B. The Behavior of Noneleetrolytic Polymers at the Oil~Water Interface Since the effect of cohesion between nonpolar groups is predominant in most cases at the A / W interface, we investigated behavior at the O/W interface in order to bring out other effects. That is, on dissolving hydrocarbon chains in a hydrocarbon phase, the nonpolar main chain can move
512
HIROSI-II tIOTTA
'2tinp A ...S PV
i
1 PVAc
PMA-DAV\\\ ,%
-
0
20
40
Area
in
60
A.2 per
80
I00
residue
Fi~. 4
FIG. 4. The interracial pressure-area curve of various polymers at the petroleum ether/water interface.
freely at the interface owing to liberation from lateral cohesion. For example, whereas the different behavior between PVAc and PVS at the A/W interface might be attributed to different cohesion of side chains as mentioned above, this cohesion would be eliminated at the O/W interface. In fact, both polymers show entirely identical pressure-area curves up to 2 dynes per cm. at the O/W interface, as shown i~ Fig. 4. In such a case, the Flory-ttuggins' treatment of flexible linear polymers in bulk solution may be applied to the behavior at the interface. Singer has made the necessary calculation (18), the equation of state at the interface being given to a close approximation by: ~r = ~
x
2
zA /
)
'
where v, A, x, A0, and z are interfacial pressure, apparent area per residue, the degree of polymerization, the actual area occupied by each residue (the size of a surface site), and the co-ordination number around a surface site, respectively. Since z should be 2 in the ease of a rigid molecule, Davies defined the increment of z from 2 as surface flexibility (19). His results for synthetic polyamino-acid and protein are shown for comparison in Table IV.
BEHAVIOR
OF LINEAR
513
POLYMERS AT INTERFACES
TABLE IV Polymer
PVP b PVAc PVS FMM Amilan PMA-DAV ~ FMA ~ PAN PET Polyamino-acidl Human methemoglobin]
in Eq. [4]
18.5 19 19 23.5 6.9 23.5 26.5 36.5 13 ---
~tom
in Eq, [5]
37 38 38 47 48 47 53 73 123.5 e ---
(~s
in Eq. [6]
0.6 0.5 0.5 0 0 0 --0.2 - 0.7 --5.8 ---
At O/W
Za
At A/W
3.60 3.60 3.60 3.75 3.75 3.75 3.75 2.05
-2.20 2.02 ------
2.02
--
3.33 2.12
-2.015
a A t 0.1 dyne per cm. (Ao = 5 A. ~ per skeletal a t o m ) . b On t h e acidic s u b s t r a t e . c A t p H 5. d A t p H 1.6. e Assumed on t h e basis of a ring group being e q u i v a l e n t to 3.5 s~raight chain atoms in a. I F r o m Ref. 20. I n t h e l i m i t w h e r e A >> A0, E q . [1] c a n b e r e w r i t t e n a s
I2] where
b= Ao(2-2x2z~zx)
[3]
I n f a c t , w h e n w e p l o t t h e p r o d u c t of ~ a n d A a g a i n s t t h e r e c i p r o c a l of A , a curve as shown in Fig. 5 can be obtained. Then, on putting
w e c a n d e t e r m i n e t h e v a l u e of n b y t h e e x t r a p o l a t i o n of s u c h a c u r v e t o a n i n f i n i t e a r e a . H o w e v e r , t h i s v a l u e is f a r s m a l l e r t h a n t h a t a t t h e A / W i n t e r f a c e a s w e l l a s t h e t r u e v a l u e of x. F o r e x a m p l e , t h e v a l u e s of n a n d x f o r P V A c a r e 19 a n d a b o u t l 0 s, r e s p e c t i v e l y (see T a b l e s I I a n d I V ) . I t is c o n c l u d e d f r o m t h i s f a c t t h a t w e m u s t c o n s i d e r a n - m e r s e g m e n t a s a s t a t i s t i c a l k i n e t i c u n i t a t t h e O / W i n t e r f a c e i n s t e a d of x. T h e v a l u e s of n for various polymers obtained by us are shown in Table IV together with t h e v a l u e s of n~o,,, w h i c h is d e f i n e d a s n~tom = a n,
[5]
514
I-IIROSHI H O T T A
8O
== "to
o 60
,=,40
'o ,= 2 0 PAN
¢
......
0
I.
1
e I
I
500200150
'0 I
I
I
1008070 60
I
50
Area in A2 per residue
FI~. 5. The product of interracial pressure (~) and area per residue (A) plotted against the reciprocal of A at the petroleum ether/water interface. where a is the number of atoms constituting the skeleton of the main chain per residue. In this case, the length of this segment can be expressed on a universal scale. In respect to a, a question rises how m a n y carbon atoms of straight chain might be equivalent to a benzene ring in P E T . Since it is reported t h a t ' a benzene ring is equivalent to 3.5 straight-chain carbon atoms in the effect on the critical micelle concentration for micelle formation (20), we used for convenience this value in Table IV. Flory had estimated from the activation energy for viscous flow that there are approximately 32 atoms in each segment of flow of polyesters (21). Then, when it is assumed in Eq. [1] that there are segments consisting of n a t o m skeletal atoms in the very dilute state, and that each skeletal atom occupies apparently one-a'th of the area per residue (.4) and actually 5A 2 as the size of a surface site (A0), we can obtain the value of z at 0.1 dyne per cm., as shown in Table IV. If the side chain affects the behavior of the main chain, the effect may appear in z. For comparison, the values of z for PVAc and PVS at the A / W interface were calculated from the data for Table II (3), and are shown together in Table IV. On the other hand, to estimate the contribution of a side chain to natom, we calculate the value of a~ shown in Table IV, which is defined by 48 a~ = - - - a, n
[6]
on assuming that all segments in the perfectly expanded state should have the length of 48 skeletal atoms. Negative values of a~ occur with larger values of n,,ol~ and correspond to the degree o f condensation of the monolayer. On summarizing these results, it is concluded that there is no serious effect of the side chain on z in the sufficiently expanded state. In other
515
B E H A V I O R OF L I N E A R P O L Y M E R S A T I N T E R I ~ A C E S
words, the above assumptions in our theory are correct for this state. The value of as in this state is also equal to about 0.5 for a massive side chain independent of its structure. On the other hand, a low value of z corresponds with a negative value of a~. The low value of z for PET is due to the complete rigidity of the benzene ring, which is assumed to be equivalent to a flexible chain consisting of 3.5 atoms. From the viewpoint of a~, the group - - C - ~
~ - - C - - should be considered to be kinetically
a
rigid
unit of flexibility equivalent to --0.3 (= 5.5 -- 5.8) carbon atoms instead of 5.5 (= 3.5 + 2). The behavior of PAN has been already interpreted in terms of the strong interaction between polar groups. However, its film may be expanded if we can select a suitable solvent having a higher dielectric constant, as the oil phase. Proteins show the very condensed behavior even at the O/W interface (22), but to some extent are more expanded than at the A/W interface (19). This difference is shown in z in Table IV. This fact shows, as pointed out already by Davies and also by us, that the links holding the native protein hi its characteristic configuration are due not only to cohesion between nonpolar parts, but also, mainly, to interaction between polar groups and intramolecular chemical bonds. The difference between PVAc and PVS at the A/W interface is also shown in Table IV. When we compare them with human methemoglobin at both interfaces, on the basis of z, the degree of expansion of PVAc at the A/W interface becomes clear. Although Davies' results are quoted in the same table, we must remember the difference between natom and x on comparing all values in Table IV. Although we have hitherto tried various quantitative interpretations of the "expanded" and "condensed" states, surface elasticity seems to be also the appropriate measure. Bateman and Chambers defined the quantity
M=
-A
dZ dA
[7]
as surface elasticity (23) and we calculate it at 0.5 dyne per cm., using the data in Fig. 4. For a plot of log M versus log nat. . . . the points lie perfectly on a straight line expressed by the equation 303 M - (n~,om)1.5'
[8]
as shown in Fig. 6, except for the polymers containing a ring group. However, we must bear in mind that the value of M reflects the uncertainty in dr the determination of ~-~. When the values of n~tomfor polymers deviating from the straight line are calculated by Eq. [8], a ring group in PET and PVP becomes equivalent to - 1 . 4 and 0.5 skeletal atom of the main
516
HIROSHI HOTTA 0.2
•
PVAc, PVS ~:
A
O.O a.
PVP \ on pH 3 . 5 ~ "
0
P M M ~ Amilan
PMA-DAV~ on p H 4 . 8 0
PMA on pHI.6
~E ,-I
-02
PET
-0.4
i
1.5
I
I
1.7
I
1.9
w 2.1
Log %tom
FIG. 6. The log of M at 0.5 dyne per cm. in Eq. [7] plotted against the log of natom for various polymers at the oil/water interface. The straight line is expressed by Eq. [8]. chain respectively. I t is concluded that the ring group in a main chain should be considered in a different way from the ring group in a side chain. When the kink points in Fig. 4 are compared with the corresponding points in Fig. 1, it is found t h a t a classification can be drawn up in terms of vinyl units: that is, (1) polymers which lay no side chain at the interface have a kink point at 9 ~ 10 A. 2, per vinyl unit as mentioned in Section A; (2) polymers which lay a side chain, consisting of a chain group alone, at the interface have a kink point at about 19 A? per vinyl unit, which is equal to the second kink point of PVP; (3) polymers which lay a ring group as the side chain at the interface have a kink point at about 24 A. 2 per vinyl unit. However, these values hold for a condensed structure, and not always for a structure at a larger area. Therefore, it is easily understood by comparison with the model of PVAc at the larger area in Table I I I that PVS has a kink point at 38 A. ~ in spite of its being in group II. T h a t is, there is a transformation not accompanied with a kink point of surface pressure as mentioned in the case of the surface moment of PVAc. On the other hand, PVP has a second kink point. The appearance of a kink point depends on the degree of hydrophility of the group, which is transformed at this point. The degree of hydrophility of a polar group may be measured b y a study of the adsorption of water on the polymer (24).
B E H A V I O R OF L I N E A R P O L Y M E R S A T I N T E R F A C E S
517
C. The Effect of Substrate (pit) Now, we shall consider the effect of substrate. Although there are various problems concerned with some primary or secondary chemical reaction, namely, oxidation, hydrolysis, complex formation,: penetration, and adsorption, the most fundamental problem for physical chemistry is the effect of hydrogen ion and neutral salt in a substrate. Among these, only the effect of the pH of the substrate on a polyampholyte, namely, the effect of ionizatLon on its surface behavior will be discussed here. To make this effect clear, we studied the monolayer of PMA and of the copolymers of methacrylic acid with diethylaminoethyl vinyl ether (DAV) at the A/W (6) and O/W (7) interfaces. The mole percentage of DAV for the copolymers K and L is given in Table V. The surface moment of copolymer L at both interfaces is shown in Fig. 7. When we plot the interfacial moment of all samples in the constant range at the larger area at both interfaces against the pH of the substrate, the relationship between them becomes as shown in Fig. 8. The trend of PMA in Fig. 8 corresponds closely to the change of the ionization and viscosity of its solution with pH, which was interpreted in detail by Katchalsky (25). Therefore, the surface behavior can also be entirely understood by his interpretation of the relationship between the shape of the molecule and the degree of ionization in solution. The constant range of interracial moment corresponds to the nonionized state, and the range of decreasing moment is due to the ionization of carboxylic groups. In the case of copolymers, it is considered that their carboxylic groups are in the nonionized state in the constant range of interfacial moments, that the degree of ionization of DAY groups is not changed in the same range, and that their interfacial moments decrease with the ionization of carboxylic groups. The ionization of carboxylic groups, which is pronounced even in sample K, is so predominant in the high pH range that the interfacial moment becomes negative, but we can13ot determine its exact magnitude owing to partial dissolution into the Substrate. Therefore, the points in the negative area in Fig. 8 indicate no absolute value but only qualitatively the inversion of sign. The molecular interpretation of this behavior was given in detail in the previous papers (6, 7). On assuming the perfect ionization of DAV groups and the perfect nonionization of carboxylic groups in the constant range, the interfacial moments of DAV groups are calculated as shown in Table V from the mole percentage of DAV and the interfacial moment of nonionized carboxylic groups at the corresponding interface shown in the same table. On the other hand, the interfacial moment of ClsHaTN(CHa)a+ was reported to be 600 roD. or more and equal at both interfaces (11). Therefore, only the value of sample K at the A / W interface in Table V might be inconsistent with the above fact, owing to the incorrect assumption of the perfect ionization of DAV groups. It is concluded from these facts
518
HIROSHI
ttOTTA
TABLE V The Interracial Moments of Nonionized Carboxylic Group and Ionized DA V Group in PMA and Its Copolymers with DA V at Both Interfaces Estimated from the Data in Fig. 8 Sample
Mole percentage of DAV in copolymer
Interracial moment in roD. of Nonionized carboxyllc group Ionized DAV group At A/W At O/W At A/W At O/W
K 31 (90)a (15)~ 450 L 20 (90)a (15)~ 600 PMA 0 9O 15 -The assumed value used for the estimation of the DAV group.
600 600 --
that the DAV groups of both copolymers are perfectly ionized to the expanded state at the O / W interface, whereas they cannot be ionized more than about 20 mole per cent of total ionizable groups in the condensed state at the A / W interface. T h a t is, there exists a certain minimum distance to which ionized groups can approach in the condensed state of the polyampholyte, owing to the mutual repression of ionization by any closer approach. The residual nonionized DAV groups appear to have a surface moment of about 90 roD. at the A / W interface. Since such a restriction is due to the strong cohesion between nonionized carboxylic groups mentioned already, this is not found for P M A owing to the absence of such a nonionized opposite component. McBain and Peaker have already suggested that the film of stearic acid cannot be much ionized in the solid state (26). The interfacial moment-area curves of t h r e e samples a t both interfaces converge to about 90 ~-~ 100 roD. at 10A3 per residue with a few exceptions as shown in Fig. 7. The trend of curves is also understood in the light of the above conclusion. As the mutual repression of ionization by the approach of already ionized groups takes place with compression, the apparent mean interfacial moment per residue decreases and the difference in all cases disappears at the collapse point, at which the state of the molecules becomes identical with that at the A / W interface. However, the surface moment of sample L at p H 4.9 is constant up to the collapse point in Fig. 7. The reason is that a repression does not take place at this pH, which corresponds to the isoelectric point of this polyampholyte. This is also concluded from the fact that the value of n~tom at this p H is identical to that of other similar nonelectrolytie linear polymers in Table IV. All the conclusions from the results of interfacial moment are perfectly consistent with those from the interracial pressure at both interfaces (6, 7). The p H (pH 3.5), from which the interfaeial moment of P M A begins to decrease at the O / W interface, is lower than that (pH 4.0) at the A / W interface in Fig. 8. Furthermore, the change is sharper at the latter than at the former because the cohesive force connecting ionizable groups is stronger at the latter.
519
BEHAVIOR OF LINEAR POLYMERS AT INTERFACES 240
2OO p
E
H
;oH 1.4~ 2.4, I and ~.8
4
160
H4.4
pilL4 and
"~ ~ ~ - - - . . . . . . . .i
5.5
E 120 o
o
pH4.9 ~
~_
pH 4.7
40
I
Oc)
I
I
I
20
I
l
40
I
60
I
1
80
I00
Ares i. h~ per residue
FIG. 7. The interfacial m o m e n t - a r e s curve of sample L at the a i r / w a t e r ( oil/water ( - - - ) interfaces on the s u b s t r a t e of various pH's.
) and
250
200
E 150
o E
I00 -
,I
b
PMA . .X. . . . . " x - & ~ , ~ )
~
~ ....
g -50
I
1 2
t.
I 4
The
I
6 pH
i
i
8
I0
of the substrate
FIG. 8. The interfaeial moment of copolymers at t h e larger area at b o t h interfaces p l o t t e d against t h e p H of the substrate. Symbols in the figure are as follows: Sample
At A/W
L PMA
A X
At O/W
[] ®
520
ItIROSHI HOTTA
Glazer and Dogan investigated the state of ionization of an insoluble monolayer by the measurement of maximum surface potential at the A/W interface (27). Although they found that the value of pK from their surface technique agrees fairly well with that from the measurement in bulk solution in the case of proteins, but not in the case of small molecules such as stearic acid and octadecylamine, they could not explain the latter disagreement satisfactorily. Nevertheless, these facts may be understood in the light of our present investigation. That is, the method of maximum surface potential is applicable to those compounds that have a constant interracial moment up to a collapse point, owing to a rigid structure. It is unsuitable, however, for such compounds as stearic acid, which have a varying interracial moment up to the collapse point, owing to a loosely packed structure. For the latter, we must consider the interracial moment at the larger ares instead of that at a collapse point as shown in Fig. 8. The value of pK from the measurement in bulk solution in such a case corresponds to that for an expanded film. Some investigators consider that the pH under the interface is different from that in bulk solution (28). Nevertheless, the trend in bulk solution is almost directly applicable to that at the interface as mentioned above. It is because conditions for both cases are identical in respect to the atmosphere in the vicinity of the ionizable groups, except for the characteristic orientation and concentration at the interface. In this case, we must of course keep the difference between expanded and condensed films in mind. SUMMARY
We now summarize the above discussion according to the classification mentioned at the outset. For the effects of medium: (1) The most important effect of the aqueous phase is due to hydrogen ion and neutral salt; and (2) The most important effect of the oil phase is the liberation of the cohesive forces between nonpolar parts. For the effects of intra- and intermolecular actions: (3) The cohesion between nonpolar groups is predominant in most cases at the air/water interface and is absent at the oil/water interface. (4) The steric effect of side chains is very effective when they lie at the interface. (5) The interaction between polar groups is not relieved by the petroleum ether phase, and cannot be estimated directly from the magnitude of the dipole moment in the bulk phase alone. (6) The interaction between ionized groups should not be considered without considering the type of film.
BEHAVIOR OF LINEAR POLYMERS AT INTERFACES
521
ACKNOWLEDGMENTS The author expresses his hearty thanks to Professor T. Isemura for his kind guidance throughout the present work, and to Professor S. Murahashi and Mr. S. Otsuka in the Institute, who kindly supplied valuable samples to him. REFERENCES 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.
C~IsP, D. J., J. Colloid Sci. 1, 49 (1946). CRISF, D. J., J. Colloid Sci. 1,161 (1946). IS~MTJRA,T., ttOTTA, H., AND Mlwn, T., Bull. Chem. Soc. Japan 26,380 (1953). HOTTA, H., Bull. Chem. Soe. Japan 26,386 (1953). ttO~TA, H., Bull. Chem. Soc. Japan 27, 80 (1954). ISEMTJRA,T., HOTTA,-H., ~ D OTS~J~:A,S., Bull. Chem. Soe. Japan 27, 93 (1954}, HOTTA, tt., Bull. Chem. Soc. Japan 9.7, 412 (1954). HOTTA, H., Bull. Chem. Soc. Japan, in press. ALEXANDER, A. E., AND TEOR~L~, T., Trans. Faraday Soc. 35, 727 (1939). ALEX~tNDER, A. E., "Surface Chemistry" [Supplement of Research (London)], p. 123 (1949). DAVIES, J. T., Nature 167, 193 (1951); Z. Elektrochem. 55, 559 (1951). ABRIBAT, M., ~Nl) POURADIER, J., Compt. rend. 9.27, 1101 (1948); BULL, H. B., J. Biol. Chem. 185, 27 (1950). BENSON, G. C., AND MCINTOSH, R. L., J. Colloid Sci. 3, 323 (1948). ISE~UI~.~, T., ANn HAMAGUCHI,I~., Bull. Chem. Soc. Japan 9.7,125 (1954). SMVTH, C. P., J. Am. Chem. Soe. 60, 183 (1938). DAVIES, J. T., Trans. Faraday Soc. 49,949 (1953). WESSON, L. G., "Table of Electric Dipole Moment," The Technology Press, M.I.T., 1948. SINGEn, S. J., J. Chem. Phys. 16, 872 (1948). DAvIEs, J. T., Biochim. et Biophys. Aeta 11, 165 (1953). PAQUETTE, 1:~. G., LINGAFELTER,]~. C., ANn TARTAR, H. V., J. Am. Chem. Soc. 65, 686 (1943). FLoRY, P. J., J. Am. Chem. Soc. 62, 1057 (1940). C~IE~S~AN, D. F., Bioehem. J. 50, 667 (1952). BATEMAN, J. ]3., AND CHAMBERS, L. A., J. Chem. Phys. 7, 244 (1939). McLAREN, A. D., ANn I~OWEN, J. W., J. Polymer Sci. 7,289 (1951). I(ATCHALSKY,A., J. Polymer Sci. 7,393 (1951). McBAIN, J. W., A~I) PEAleR, C. R., Proe. Roy. Soe. (London) 125A, 394 (1929). GLAZE~, J., AND I)OGAN, M. Z., Trans. Faraday Soc. 49,448 (1953). HAVlNGA, E., AI~I)HERToG-PoLAK, M. DEN, Rec. tray. chim. 71, 64 (1952).