3537
J. Phys. Chem. 1992, 96, 3537-3542
Polymer Swelling. 14. Sorption of Polyhaloalkane Liquids by Poly(styrene-co -divinylbenzene) L.A. Errede 3M Corporate Research Laboratories, 3M Center, Bldg. 201 -2N-22, St. Paul, Minnesota 55133 (Received: September 16, 1991; In Final Form: December 30, 1991)
The number, a=,,,of adsorbed molecules per phenyl group of ply(styrene-cedivinylbenzene) at liquid saturation was determined for liquids that comprise series of the types CH,-,Z,, C2H6-,Z1,and Z(CH2),Z, where Z is either chloro or bromo, z is the values were compared with the a,,,,values already reported for number of such substituents, and n is 1-10, These for liquids with the same Z and n was in all cases less than 1.5 the corresponding Z(CH2),,H liquids. The ratio aZ,n/aZ,r,n and usually less than 1.2. It was inferred from these results that the mode of adsorption to the phenyl group of such polymers at liquid saturation is primarily monodentate rather than polydentate, even when z is 5 or n is IO.
Introduction Earlier p~blicationsl-~ from this laboratory, concerning dynamic sorption of liquids by atactic poly(styrene-co-divinylbenzene) [hereinafter referred to as either poly(Sty-co-DVB) or (Sty) I-x(DVB),] particles enmeshed in poly(tetrafluorcethy1ene [PTFE] microfibers, reported that it was possible to determine the number, a, of adsorbed molecules per accessible phenyl group of (Sty),,(DVB), at liquid saturation (or in solution when x is 06) by measuring the weight of sorbed liquid per unit weight of enmeshed sorbent particles at a series of levels of x . The results observed in those studies using homologous series of liquids of the type ZCRR’R”, in which the adsorptive substituent Z (Le., a phenyl group or halogen atom) is kept constant and the alkyl portion is varied systematically, showed that az increases with the affinity of Z for the phenyl substituents in the polymer and that it decreases with the ‘bulkiness” or CRR’R”, owing presumably to steric hindrance due to space limitations at an adsorption site [e.& a phenyl group in poly(Sty-co-DVB)]. For example, it has been shownS that the aZ,,, values for Z(CH2),,H liquids with n = 2-8 and the az,, values for the ZCH3-,(CH3), liquids with m = 0-3, decrease logarithmically with n and with m,respectively, as noted in eqs 1 and 2. Here
- 2)
(1)
= log a2.0- A ’ z ( ~ )
(2)
log aZ,,, = log log
a2.m
az,2- AZ(n
the constants ( Y ~ and , ~ aZ,o are respectively the a values extrapolated to ZCH2CH3in the Z(CH2),H series and to ZCH3 in the ZCH3-,(CH3), series; the constant A Z (eq 1) is the decrementation constant per methylene group inserted at the w-carbon atom (and therefore reflects only its steric contribution), and A’, (eq 2) is the decrementation constant per methylene group inserted a t the a-carbon atom (and therefore reflects both the electronic and steric contributions). Thus, if the steric contributions are assumed to be similar, the difference (A’Z,-A Z ) is the portion of A’z attributable to the electronic contribution in the corresponding ZCH3-,(CH3), series. The constants az,2and A Z (eq 1) and the constants az,oand A’z (eq 2) are characteristic of Z as summarized in Table I. It was also noted’” that negative deviation from the linearity expressed by eq 1 for a homologous series of the type Z(CH2),,H cccurs at n’ (n’being a value of n greater than 6) and that the magnitude of this deviation, A(log cyZ,,,) increases linearly thereafter with the square root of the difference ( n - n’). Similar Errcde, L. A. J. Phys. Chem. 1989, 93, 2668. Erredc, L. A. J . Phys. Chem. 1990, 94,466. Errede, L. A. J. Phys. Chem. 1990, 94. 3851. Errede, L. A. J. Phys. Chem. 1990, 94, 4338. (5) Errcdc, L. A. J. Phys. Chem. 1991, 95, 1836. (6) Efrede, L. A. (a) Polymers, in press. (b) Molecular Interpretations of Sorption in Polymers. Adu. Po/ym. Sci. 1991, 99, 1. (1) (2) (3) (4)
TABLE I: Adsorption Constants for ZCHt,(CH3),
and Z(CH2),H
Liauids“
H I Br
1.00 3.6 3.0 3.5 2.0
0.127 0.096 0.180 0.140
2.64 2.26 2.23 1.55
0.085 0.063 0.075 0.051
0.27 0.21 0.21 0.13
6 8 8 8 7
n’also increases linearly with the square r w t of the difference (n - n? as noted in eq 3, it was possible to ascertain the relative influence of substituent Z at the a-carbon atom upon the magnitude of self-association in Z(CH2),,H liquids with n > n’, using eq 4. Here
log a Z , n = BAFn - Fd)
(4)
BZ is the correlation constant, normalized to 1 when Z is H, and F,,,is the force F corresponding to n’ where A(log aZ,,,) first becomes significant in a given Z(CH2),,H series. The data for B,, collected in Table I, show that the magnitude of self-association is greatest when Z is H and least when Z is phenyl, Le., the (7) Orwoll, R. A.; Flory, P. J. J. Am. Chem. Soc. 1967, 89, 6822. (8) Botherel, P. J. Colloid Sci. 1968, 27, 529. (9) Tancrede, P.; Patterson, D.; Botherel, P. J. Chem. Soc.,Furuduy Truns. 2 1977, 73, 29. (IO) Fowkes, F. W . J. Phys. Chem. 1980, 84, 510. ( I 1) Errede, L. A. Macromolecules 1986, 19, 1525.
0022-3654/92/2096-3537$03.00/00 1992 American Chemical Society
Errede
3538 The Journal of Physical Chemistry, Vol. 96, No. 8. 1992 disruptive influence on self-association over the adjacent polymethylene chain appears to vary with the bulkiness of the Z substituent as well as the electronic character of that substituent. The above relationships (eqs 1-4) were established using liquids that comprise homologous series of alkanes that have only one Z substituent, Le., C,,Hznc,Z, and therefore can adsorb to a phenyl group of poly(Sty-co-DVB) only in the monodentate mode. Interpretation of such relationships becomes more difficult, however, when the sorbed liquid is a multisubstituted haloalkane, CnH2n+2-rZ,rwhich might associate with several (up to z) adsorption sites simultaneously. Since a is defined as the number of adsorbed molecules per accessible phenyl group at liquid saturation, not the number of adsorbed functionul groups (Z) per phenyl group, the ratio of aZ,nfor a given monosubstituted haloalkane, C,,HznclZ, to a=,, for the corresponding multisubstituted haloalkane, CnHzn+z-rZz, i.e., (hereinafter referred to by I $ J , can be used as a criterion for adjudicating the mode of adsorption. The purpose of this publication is 2-fold: (1) to report the results observed in such comparisons using haloalkanes that comprise series of the types CHe,Z,, CzHb,Z,, and Z(CH2),Z, where Z is chloro or bromo, z is the number of such substituents, and n is a number from 1 to 10; (2) to report how the constants established for eqs 1-4 are affected by accumulation of halogen atoms on methane and ethane.
Experimental Section The preparation of microporous composite films [consisting of (Sty),-,(DVB), particles, 80% by weight, enmeshed in PTFE microfibers], and the use of these films to establish the relative swelling power of the test liquid, are described in earlier public a t i o n ~ . ~Briefly -~ this involves allowing swatches, cut from each of six such composite films that contain polymer particles with known DVB mole fraction x , to swell to saturation in a given test liquid. The weight of liquid sorbed by the enmeshed particles in each sample is used to calculate the corresponding specific volume, S in mL/g, of sorbed test liquid. The six S values obtained thereby are plotted as a function of the cube root of the number (A) of backbone carbon atoms in the polystyrene segments between cross-link junctions. The relative swelling power, C, of the test liquid is established from the slope of the straight line obtained thereby, as expressed by eq 5. The cross-link density, above which
S = C(A1/3 - A01/3)
(5)
S is virtually zero, is given by the reciprocal of b. The square of the correlation coefficient ( r ) for the straight line of best fit through the set of six data points for a given test liquid (eq 5) was in all cases greater than 0.99. The values of C range from 0 to about 3.5 for the test liquids studied thus far. These values are reproducible to within fO.O1; therefore, the percent error varies inversely with C. The test liquids used in these studies were obtained from commercial sources, and almost all samples were of reagent grade or better. The same set of composite film samples used in our previous measurements of sorption by poly(Sty-co-DVB) particles was used again in this study; thus, these swatches have now undergone without significant change more than 2000 swelling, cleaning, and drying cycles using a wide variety of test liquids. The number (a)of adsorbed molecules per accessible phenyl group of (Sty)l-,(DVB), at liquid saturation, or in solution when x is zero, is calculated from the observed value for C by means of eq 6. Here 104 and M a r e the formula weights of styrene and a = 104Cd/M (6) of the test liquid, respectively, and d is the density of that liquid. The ratio I$z,n(as defined in the Introduction) was established value already for the reference by dividing the q n value established in these studies monohaloalkane by the aZ,z,n for the corresponding polyhaloalkane having the same number of carbon atoms. The Flory-Huggins interaction parameter (x)at 23 OC for such polystyrene-liquid (PS-L) systems12 is calculated from the ob-
TABLE 11: Sorption of Polybaloalkane Liquids RCH,,Z," liquid CHjCI CHIC12 CHC13 CCl4 CH3Br CH2Br2 CHBr, CHjCH2CI CHjCHCl2 CHjCCIj CH2CICH2CI CHCl2CH2Cl CHC12CHCIz CC13CH2C1 CCI3CHC12 CH3CH2Br CH2BrCH2Br CHBr2CHBr2
d
x0'13
c
1.33 1.48 1.59
1.70 1.75 1.80
1.99 2.32 1.97
2.50 2.89
1.75 1.85
1.78 2.19
1.177 1.339 1.256 1.435 1.586 1.598 1.680 1.460 2.179 2.96
1.70 1.90 1.78 1.75 1.90 1.83 1.90 1.65 1.80 2.01
1.68 1.79 1.79 2.04 2.29 2.25 2.52 1.62 1.67 1.94
a2.r.n
[3.21] 3.27 2.99 2.14 (2.721 2.66 2.60 [2.23] 2.25 1.86 2.36 2.28 2.24 2.23 2.03 2.26 2.01 1.73
XI
0.29 0.08 0.30 0.41 0.16 0.47 0.34 0.34 0.26 0.10 0.13 4.04 0.51 0.48 0.44
4 1.oob 0.98 1.07 1.50 1.OOb 1.02 1.05 1.OOb 0.99 1.20 0.94 0.97 1.00 1.00 1.10 1.00* 1.12 1.30
" d is the density of the test liquid; A,,') and Care as defined in eq 5; the absorption parameter as defined in eq 6; xI is the interaction parameter at u = 1 as defined in eq 7; q+ is the ratio of a2$for the reference liquid (either CHJZ or CH3CH2Z)to azs,,for the test liquid. *The reference liquid and the test-liquid are the same. aZ,i,n is
served C (eq 5 ) and volume fraction (u) of polymer in the system by means of eq 7 . The xo values of these PS-L systems at the
xu = 0.49 + 1.010 - 0.610C
(7)
polymer volume fraction level u = 1 (Le., x i ) are recorded in the various tables of adsorption data shown in the text. The corresponding xu values at any other level of u of such systems may be calculatedI2 whenever necessary using the reported xI value and eq 8. xu = 0.49 u(xI - 0.49) (8)
+
Results and Discussion (A) Sorption of CHcZZ, and C2H6rZz Liquids. The chloroor bromomethanes and ethanes evaluated thus far are listed in Table 11, along with the corresponding adsorption data. The &,, ratios, calculated from the adsorption parameters established for the given test liquid and the corresponding reference-liquid, either CH3Z or CH3CHzZ,show clearly that these ratios do not increase incrementally with the number (z) of halo substituents in the test liquid [note that solids cannot be used in these studies of liquid sorption by polymers]. In fact the I$,,l ratios for the CH,+,Z, liquids have values that are typically not greater than 1.10. The same can be said for the C2H6-,C1, liquids even when z is as great as 5, and for the C2H6,Brr liquids even when z is as great as 4. The above results are interpreted to mean that association of polyhalomethanes and ethanes to poly(Sty-co-DVB) at liquid saturation occurs primarily in the monodentate mode, Le., only one of the available halogen substituents on a given adsorbed polyhaloalkane molecule is presumed to be "complexed" with the adsorption site, as shown schematically in Figure la. In this figure only the halogen substituent that actually forms a liaison with the phenyl group of the polymer is designated by the letter Z. The symbols R1, R2, and R3on the a-carbon atom represent hydrogen, noncomplexed halogen, or an alkyl (or haloalkyl) group. The largest positive differences in I$,,, from a value of one were observed for CCll and for CHBr2CHBr2. Even these differences, however, are relatively small [i.e., (&, - 1) = < 0.51. Although this might support the point of view that at any given time a fraction of the adsorbed molecules is complexed with the polymer in a polydentate mode, an alternate interpretation also is tenable, namely, that this small difference is attributable to the combined effects of steric hindrance and electronic interaction characteristic of polyhalocarbons. As noted in the Introduction, progressive (12)
Errede, L. A . J. Appl. Polym. Sci., in press.
The Journal of Physical Chemistry, Vol. 96, No. 8, 1992 3539
Polymer Swelling
TABLE 111: Sorption of CI(CH2),CI Liquids'
H\
-C
(>"=3 \
CH,
-
Figure 1. Schematic representation of association between R3CZ and the pendent phenyl group of poly(Sty-co-DVB) at liquid saturation showing the assumed liaison via a pair of nonbonded electrons of halogen subelectrons of the phenyl group (a) when two or more R stituent Z and i~ groups are alkyl and (b) when R3C is H(CH2),.
replacement of the hydrogen atoms in ZCH3 by methyl substituents causes the a, values for the corresponding ZCH3,(CH3),,, liquids to decrease logarithmically as expressed by eq 2. In that series, the cumulative effects of steric hindrance and electropositive perturbation operate in the same directive sense to decrease the value of the adsorption parameter a,,,.In polyhaloalkane liquids that comprise series of the type ZCH,R',R (where R' is a halogen atom that is not complexed with the adsorption site and R is an alkyl or haloalkyl group); however, the steric and electronic effects do not operate in the same directive sense, i.e., the increase in steric hindrance with z causes CY^,,,^ to decrease accordingly, but the increase in electropositive perturbation causes Q ~ , , ,to~ increase, so that the net result reflects the difference of these two opposing influences. The magnitude of the net effect depends also upon the nature of the R group in the RCH3,Z, molecule, as may be inferred from the data collected in Table 11. When R is H or CH,, the ( Y ~ , , , ~ values for the RCHZ2 liquids are equal approximately to the aZ,],, values for the corresponding RCH2Z liquids (Figure 2), presumably because the steric, statistical, and electronic contributions virtually cancel one another. When R is C1 or CCl,, however, aC1,2,n values for the RCHZ, liquids are significantly less than the aCl,l,n for the corresponding RCH2Z liquids (Figure 2), implying that in such cases the steric contribution dominates over the electronic contribution. The difference between 0~c1.3~ for RCC13 for RCHC12 liquids is even more marked than liquids and aCI,2,n the differences noted above for the RCHZ2 and RCH2Z liquids. In such cases the marked decrease in the adsorption parameter caused by the addition of the third halogen at the a-carbon atom may reflect not only the increase in steric hindrance but also the decrease in polarizability of chlorine substituents when virtually all of the a-carbon atoms have been replaced by chlorine atoms. Progressive replacement of hydrogen atoms on the beta-carbon atom by halogen atoms also affects the azm of the liquid obtained thereby in accOrdance with the net contributions to steric hindrance and to decreased electron availability on the a-carbon atom. It is assumed here that the halogen atom adsorbed is one of those on carbon atoms bearing the fewer halogen substituents. Thus, the acI,Iavalues for the RCH2CI liquids vary with R in the order C1 > H > CH2Cl > CHCCl, > CH3 > CC13 (Table 11), Le., the respective a values are in the ratios 1.02:1.00:0.73:0.71:0.69:0.60. It is concluded, therefore, that at any monent only one of the halogen atoms in CH4-zZ, or C2CH6-,Z, molecules is actually involved in liaison with an accessible phenyl group of poly(Styco-DVB) at liquid saturation, and that the remaining halogen atoms of the adsorbed molecule are in dynamic association with nonadsorbed molecules, which results in a net force away from the adsorption site, such that the adsorbed molecules attain a vertical orientation as in Figure la.
n 1 2 3 4 5 6 7 8 9
IO
d
&!I3
1.33 1.256 1.191 1.16 1.101 1.068 1.041 1.025 1.017 0.999
1.70 1.82 1.70 1.70 1.75 1.70 1.80 1.80
C 1.99 1.75 1.94 1.82 1.83 1.76 [1.69] [1.58] 1.45 1.38
11 12
"d,
C, a,, and
an
XI
3.24
0.29 0.43 0.32 0.39 0.38 0.43 0.47 0.54 0.62 0.66
2.30 2.13 1.73 1.48 1.25 [1.08] [0.92] 0.78 0.66 [0.56] [0.48]
x , are as defined in the footnotes of Table 11.
TABLE I V Sorption of Br(CH2),Br Liquids n d hi13 C a. 1 2.497 1.75 1.78 2.66 2 2.179 1.80 1.67 2.01 3 1.982 1.80 1.91 1.95 4 1.808 1.85 1.82 1.58 5 1.702 1.92 2.02 1.55 6 1.58 1.83 1.91 1.29 7 1.51 1.78 1.92 1.17 8 1.44 1.80 1.79 0.99 9 1.35 1.85 1.70 0.84 10 [0.75] 11 [0.65] 12 [OS71
"d,
C, a,, and
XI
0.41 0.48 0.33 0.39 0.27 0.33 0.33 0.41 0.46
x , are as defined in the footnotes of Table 11.
It was noted in the Introduction that aZ,nfor Z(CH2),H molecules decrease logarithmically with n as expressed by eq 1. It is reasonable to assume on the basis of analogy and parallelism that azU for CHZ2(CH2),,H molecules and az3,,for CZ3(CH2),,H molecules will also decrease logarithmically with n as expressed by eqs 1A and lB, respectively. Here the constants CY^,,,^ and
(~z,3,1 are
log
az.2.n =
log
aZ.3,n
log
= log
-
- 1)
(1A)
- AZ,3(n
- l)
(1B)
a ~ . 2 , 1A ~ . 2 ( n a2,3,1
the a values for CHZ2CH3and CZ3CH3,respectively (Figure 2), and the constants 4, and AZ,3 are the decrementation values in log a units per added methylene group characteristic of the respective groups CHZ, and CZ3 in the a position (Figure 2). If one now assumes that these decrementation constants will be about equal to the corresponding AZ,,constants (Table I), then the approximate (Y~,,,~ and ( Y Z , ~values .~ for the CHZ2(CH2),H and the CZ3(CH2),H liquids with n < 9 can be estimated using eqs 1A and 1B. Hopefully these estimated values can be verified experimentally, when and if the liquids that comprise the above series becomes available for sorption studies. (B) Sorption of Cl(CH2),CI and Br(CH2),,Br Liquids. Having established that multichloro- and -bromo-substituted methanes and ethanes are adsorbed to poly(Sty-cc+DVB) at liquid saturation in the monodentate mode, it is now necessary to establish whether or not this is also true for the corresponding Z(CH2),Z liquids with n > 2 (Figure lb). Here the alpha and omega Z substituents are chemically indistinguishable, i.e., the two halogen atoms are not affected electronically or sterically by adjacent halogen atoms, which was not the case with CH,-,Z, or C2HbrZz molecules. (a) Relative Swelling Power, C. The sorption data for the Cl(CH,),CI and Br(CH2),Br liquids studied thus far are collected in Tables 111 andIV, respectively. The relative swelling powers (C as defined in eq 5 ) are correlated with n in Figure 3 and 4, respectively. Both correlations exhibit weak odd-even alternation patterns as noted in the corresponding Z(CH2),H series (see especially Figure 2 of ref 3). The odd-even alternation pattern exhibited by Z(CH2),Z liquids is more pronounced when Z is bromo (Figure 4) than it is when Z is chloro (Figure 3), whereas the opposite is true for the Z(CH2),H liquids. Although odd-even
3540 The Journal of Physical Chemistry, Vol. 96, No. 8, 1992 R H,&H
R H-LrZ
I
2
R 2-;
C" /
I
I
2
2
2
2'5[
I
a,
I
I c
3.5
Errede
I I
I I
l-
I
.
I
% '\
-
-
I
I
10-
I
-
8 HCBr, I
:."I 1.5
'.
-\
L
I
--
05
-
-
CHJ
~
1
2
3
number of Z substituents on the alpha carbon atom Figure 2. Correlation of a, for RCH3_,Z, with R , Z, and z.
C"
--.
t
*.'
0.6 0.5
0.4 ~
O3
2
3 4
5
6
7
8
910111213
Figure 5. Correlation of u, with n for CI(CH2),CI. The line of best fit through the data points is given by log a,,= log 3.4 - 0.0714n.
l . 1
O'O
1
n
0.5
and in Figures 2 of refs 3 and 4 (for the latter series). 1 1 2 3 4 5 6 7 8 9101112 (b) Mode of Adsorption of Z(CH2),Z to Polystyrene. The n
Figure 3. Correlation of C,, with n for CI(CH,),CI.
alternation of physical properties in homologous series of this sort have been observed frequently in the past,I3-l5the cause of such alternations is still not well under~tood.~ Maxima are exhibited at about n = 5 in the patterns for the Br(CH2),Br liquids (Figure 4), and at n = 3 in the Br(CH2),H liquids (see Figure 2 of ref 4), whereas no such maxima are exhibited in the patterns for the corresponding chloro series of liquids (Figure 3; see also Figure 2 of ref 3). These differences are presently unexplained. Since x decreases with the observed C (eq 9 ) , x varies with n of Z(CH2),Z and Z(CH2),H liquids in the inverse manner to that for C shown here in Figure 3 and 4 (for the former series) (13) Bauer, N.; Fajans. K.; Lewis, S.Z. Refractometry. In: Weisberger, A., Ed.; Techniques of Organic Chemistry, Physicol Methods, 3rd ed.; Interscience Publishers: New York, 1960; Vol. I , Part 2, Chapter 18. pp 1139-1282. ( I 4) Fajans, K . Chem. Eng. News, 1947, 27, No. 13, 900. ( I 5) Mukerjee, P. Kolloid-Z. Z . Polym. 1970, 236. 76.
adsorption parameters (azq) for the two Z(CH2),,Z series, which were calculated from the corresponding relative swelling powers (C,,,), are collected in Tables 111 and IV. These data show that both sets of (YZ,~,,values decrease monotonically with n despite that the corresponding ssts of CZ,2,nvalues for the Cl(CH2),Cl and Br(CH,),Br liquids do not. This confirms that the molar volume (M/d; eq 6) of the liquid is responsible for a large part of the anomalies in patterns exhibited in Figures 3 and 4. The plots of the logarithms of a- vs n (Figures 5 and 6) show linear relationships for both series of Z(CH2),Z liquids with n < 11. The open circles in Figures 5 and 6 represent estimated values, it being assumed that it is justified to extrapolate the observed linearity for the data collected for liquids with n < 11 to the a values for those liquids with n = 12 (the justification for such extrapolation is discussed in Section B.d). These values are also recorded in Tables 111 and IV for the sake of completeness, but they are placed in brackets to emphasize that they were established by extrapolation rather than by direct measurement. The same is true for the values for the C1(CH2),C1 liquids with n = 7 and 8 (open circles in Figure 4), which were obtained by interpolation.
The Journal of Physical Chemistry, Vol. 96, No. 8,1992 3541
Polymer Swelling
a" 4.0
0, "
I
.'
:::I
o,2
0.01 0.9 OB 07 06 05
a'
\
2 is Chloro
I
1
I
2
I
3
' 5'
4
6 n
7
I
8
' ' '
I
9 1 0 1 1 1 2
0, n 1. ._ 3-
04
e-.
0.8
n Figure 6. Correlation of a, with n for Br(CH2),Br. The line of best fit through the data points is given by log a, = log 2.9 - 0.058411.
:
F
:
Having determined the aZ.2,n values for these two sets of Z(CH,),Z liquids, it is now possible to establish how $,, (i.e., the ratio of am already reported" for Z(CH2),H liquids, to the azlsn for the corresponding Z(CH2),,Z liquids, reported here in Tables 111 and IV varies with n. Such correlations (Figure 7) show that 4,,, for the liquids with n < 9, are equal to about 1 in both the chloro and bromo series and that they decrease monotonically thereafter. More precisely the line of best fit (Figure 7), through the 4*,, data for the liquids with n < 9, is given by 42."
= 92.0 - A':(n)
(9)
where r&o is the value of aZ,n/aZ,2,n extrapolated to n = 0 (i.e., 0.99 when Z is chloro and 1.13 when Z is bromo) and A': is the corresponding decrementation constant (i.e., 0.009 when Z is chloro and 0.020 when Z is bromo). These decrementation constants (A':) are proportional to the small positive differences (AZ - AZ,2)in the corresponding decrementation constants (eq 1) for the respective Z(CH2),H and Z(CH2),Z series of liquids. In view of the above results, it is concluded that the mode of adsorption of Cl(CH,),Cl and Br(CH2),Br molecules to poly(Sty-co-DVB) at liquid saturation occurs primarily via the monodentate mode, Le., one halogen substituent of the Z(CH2),Z molecules is associated with the A electrons of a phenyl group, and the other, at the w position of the polymethylene chain, extends above the adsorption site as illustrated schematically in Figure lb. The w-halogen atom is presumed to be in dynamic association with the nonadsorbed molecules of the liquid, and the sum effect of these forces on the adsorbed molecule is a resultant vector, the direction of which is normal to the plane of the adsorption site on the polymer. That the decrementation constant (AZ,2;eq 1) for Z(CH2),Z liquids is smaller than the decrementation constant ( A Z ) for the corresponding series of Z(CH2),H liquids implies that the force of association of adsorbed molecules with solvent molecules is greater when substituent R in the w position (Figure 1b) is Br or Cl than it is when R is H. This stronger pull on an adsorbed Z(CH2),Z molecule causes the height ( h ) of the imaginary inverted cone of effective occupied space above the adsorption site to increase, and its diameter (6)to decrease accordingly, thus decreasing the decrementation in log a per added methylene group, which is proportional to the ratio d / h . That the height ( h ) to which a long chain segment extends above the plane of its adsorption site depends on the efficacy of interaction with the liquid is not a new concept. Eirich,16and Quinn," (16) Rowland, F. W., Eirich, F. R . J . Polym. Sci.; Part A 1966,4, 2033; 2401. (17) Petzky, W. J.; Quinn. J. A. Science 1%9, 751. Yavorsky, D. Y. Ph.D. Thesis, Submitted to the Graduate Faculty of the University of Pennsylvania, 1981.
'
---
2 is Bromo
o,2
0.0
5 - _ _
8 ,-
6 7 8 9 101112 n Figure 7. Correlation of &, with n for Z(CH2),H and Z(CH,),Z, where &, is the ratio of aZ,"for Z(CH2),H to C V ~ , ~for , , the corresponding Z(CH2),Z. The lines of best fit through the data points for those liquids with n < 9 are given by &,, = 0.99 - 0.009n, when Z is CI, and by &,, = 1.13 - 0.02011,when Z is Br.
1
2
3
4
5
and other^,^^^'^ who studied the effect of polymer or long-chain molecules adsorbed to the microcapillary walls of microporous films on the permeability of these films, have shown that the rate of flow of a given liquid through such microporous media varies inversely with the number of backbone atoms in the segment that extends above the molecular portion of the macromolecule adsorbed to the surface (i.e., the height above the adsorption site). Recently Anderson?O who made similar studies using A-B block polymers for which the B segments had a very strong affinity for the microporous film and the A segments had a very weak affinity, has shown that the rate of flow through such systems varies with the relative affinity of the liquid for the A segments of the block polymer, i.e., the height to which the A segment extends above the anchor site varies with the affinity of the liquid for that segment. The monodentate mode of adsorption of CnH2n+2-2Z2 and Z(CH2),,Z molecules by poly(Sty-ceDVB) at liquid saturation may not prevail at all levels of sorption in PS-L systems. It is clear that as the sorbed-but-not-adsorbed molecules are eliminated by evaporation, the opportunity for bidentate adsorption will increase. It is highly probable, therefore, that after elimination of nonadsorbed molecules, the residual volatile molecules of the PS-L system in its rubbery state may in fact be adsorbed to polystyrene in the bidentate mode. This possibility is supported by our preliminary results of desorption studies using C1CH2CH2Cl and Cl(CH2),Cl liquids.*l Our desorption studies22using ZR liquids show that the residual compositions that mark sequentially complete elimination of nonadsorbed molecules (Le., a:), incipient transition from the rubbery state to the glassy state (Le., ai),and completion of this transition (i.e., aJ all vary linearly with a,(eq 6) for PS-L system at liquid saturation.21 The desorption results using Cl(CH2)2C1and C1(CH2)$l, however, show that the respective compositions a l , a; and agare about half those expected (18) Errede, L. A. J . Colloid Interface Sci. 1984, 100, 414. (19) Errede, L. A. J . Membr. Science 1984, 20. 45. (20) Webber, R. M.; Anderson, J. L.; John, M. S. Macromolecules 1990, 23, 1026. (21) Errede, L. A,, results to be published. (22) Errede, L. A. J . Polym. Sci.: Polym. Chem. Ed. 1990, 28, 851.
3542
J. Phys. Chem. 1992, 96, 3542-3547 the norm exhibited by the linear polymethylene liquids with n > 7 (up to n = 13). This implies that the presence of the methyl substituents precludes the possibility of correlated molecular orientation along the six adjacent polymethylene units on both sides of the methyl substituent, presumably owing to disruption of geometric alignment of the adjacent six contiguous methylene groups required for measurable self-association. Similar results were observed in this laboratory*’ in our ongoing studies of poly(Sty-co-DVB) swelling in symmetrical ethers H(CH2),,0(CH2),H, and in w-substituted halo esters Z(CH2),,CO2(CH2),,,H. We noted that the replacement of a methylene group by 0 or C 0 2 eliminates all evidence of selfassociation in the corresponding plots of log a,,vs n, even when the total number of atoms along the chain (exclusive of the two terminal atoms) is as much as 15. This observation is consistent with the point of view that replacement of a hydrogen atom or methylene group in polymethylene liquids by Z, 0, CO, COz, or CRR’ suppresses correlated molecular orientation along the adjacent (CH,),, chain and that the range of the suppressant effect extends at least over the adjacent six methylene groups. This implies that self-association owing to correlated molecular orientation of the internal chain of polymethylene groups in Z(CH2),Z liquids will not be significant unless the value of n is greater than 12. Registry No. Sty-co-DVB,9003-70-7; Met, 74-88-4; EtI, 75-03-6; MePh, 108-88-3; EtPh, 100-41-4; CH3CI, 74-87-3; CHCI,, 67-66-3; CCI,, 56-23-5; CH3Br,74-83-9; CHBr,, 75-25-2; CH,CH2CI,75-00-3; CHjCHCI2, 75-34-3; CH3CCI3, 71-55-6; CHC12CH2C1, 79-00-5; CHC12CHC12,79-34-5;CCl$H2CI, 630-20-6; CCI,CHCI,, 76-01-7; CHICH2Br,74-96-4; CHBr2CHBr2,79-27-6; CICH2CI,75-09-2; CI(CH2)2C1, 107-06-2; CI(CH2),CI, 142-28-9; CI(CH2)dCI, 110-56-5; CI(CH2)5CI, 628-76-2; CI(CH2)6CI, 2163-00-0; CI(CH2)$I, 821-76-1; CI(CH2)&I, 2162-99-4; CI(CH2)9CI, 821-99-8; Cl(CH,)I&I, 2162-98-3; CI(CH2)IICI, 822-07-1; CI(CHz)12CI,3922-28-9; BrCH2Br, 74-95-3; Br(CHz)2Br, 106-93-4; Br(CH2),Br, 109-64-8; Br(CH2),Br, 110-52-1; Br(CH2)SBr, 11 1-24-0; Br(CH2)6Br,629-03-8; Br(CH2),Br, 4549-3 1-9; Br(CH2)8Br, 4549-32-0; Br(CH,),Br, 4549-33-1; Br(CH2),,Br, 4101-68-2; Br(C3344-70-5. H2)11Brr16696-65-4; BT(CH~)~,B~,
for such PS-L systems assuming monodentate adsorption,2’ which implies that the mode of adsorption at these levels of liquid sorption be bidentate. (a) Self-Association of Nonadsorbed Molecules. As noted in the Introduction, negative deviation from the linearity expressed by eq 1 is exhibited by Z(CH2),,H liquids when n is greater than n’, that latter being a numerical value that depends on Z (see Figures 3 of refs 3 and 4), and that the cause of this deviation is self-associationof the sorbed liquid owing to cumulative London eq 3) between polymethylene portions of dispersion forces (F,,; Z(CH2),,H molecules with n > n’. In the case that Z is H, n’is 6, but in the cases that Z is chloro or bromo, n’is 8. Lateral self-association along the polymethylene chain7-I0 causes the effective bulkiness to adsorbed Z(CH2),,H molecules to increase, and therefore causes a greater decrease in A log a per added methylene group (Az;eq 4) than that established for the liquids with n < n’. This association to form the partial complex Z(CH2),,H:[H(CH2),,Z], is equivalent to increasing the mass of R at the terminal position of the adsorbed Z(CH2),R molecule (shown schematically in Figure lb) such that the diameter d of the inverted cone of effective occupied volume space per molecule above the adsorption site is increased, and therefore aZ,,, is decreased accordingly. In contrast to the above results using Z(CH2),,H liquids, the corresponding plots of log CY^,^,,, vs n for the Z(CH2),Z liquids (Figures 5 and 6) do not exhibit deviation from the linearity expressed by eq 1 even when n is 10. It is inferred from these results that suppression of self-association along the polymethylene portion of Z(CH2),Z molecules is attributable to the presence of the Z substituent in the w-position (i.e., the suppression of selfassociation on the adjacent six methylene groups may indeed be additive as was assumed to be true in section Bed). This conclusion is consistent with the observations made by the earlier investigator^,^-"^ who studied H(CH2),,H liquids as described briefly in the Introduction. They noted that replacement of a hydrogen atom by a methyl group anywhere along the middle portion of the polymethylene chain eliminates the deviation from
Surface Potential in Charged Synthetic Amphiphile Vesictes A. M. Carmona-Ribeiro*,+and B. R. Midmore Department of Chemistry, University of Reading, Reading, RG6 2AD, UK (Received: July 3, 1991; In Final Form: December 26, 1991)
The surface potential in large dioctadecyldimethylammonium chloride (DODAC) and dihexadecyl phosphate (DHP) bilayer vesicles at low ionic strength is determined. Salt and pH effects on vesicle size and {-‘-potential are investigated using photon correlation spectroscopy and particle microelectrophoresis, respectively. A large effect of amphiphile concentration on electrophoretic mobilities ( E M ) is described. The EM values were extrapolated to zero amphiphile concentration and the is, the {-potential was calculated using the O’Brien and White theory. The larger the harmonic z-average diameter (0,) smaller is the {-potential. pH and salt effects on size are in agreement with calculations from monolayer data using a self-assembly model. {-potentials determined for a range of salt and pH qualitatively agree with Stern potentials estimated from flocculation data using the DLVO.
Introduction Depending on the hydrophobic/ hydrophilic balance, amphiphile molecules assemble in aqueous solution to form a variety of structures as micelles, rods, disks, vesicles, liposomes, or hexagonal phases.’q2 Amphiphiles with long alkyl double chains like the phospholipids3 or some synthetic amphiphiles4-* form bilayer systems when dispersed in water solution. The nature, size, and properties of amphiphile assemblies in aqueous solution depend Address correspondence to this author at the Universidade de Sao Paulo. ‘On leave from the Departamento de Bioquimica, lnstituto de Quimica, Universidade de Sao Paulo, CP 20780, Sao Paulo, Brazil.
0022-36S4/92/2096-3542$03.00/0
not only on the composition of the dispersion medium”’ but also on the dispersion method itself.’2.13 Dioctadecyldimethyl( 1 ) Tanford, C. The Hydrophobic Effect; Wiley-Interscience: New York, 1982. (2) Israelachvili, J . N. Inrermoleculur and Surfuce Forces; Academic Press: London, 1985. (3) Bangham, A. D., Ed. Liposome Lerrers; Academic Press: London,
.,--. 19x7
(4) Kunitake, T.; Okahata, Y . ;Tamaki, K.; Kumamaru, F.; Takayanagi, M. Chem. Leu. 1977, 387. ( 5 ) Mortara, R . A.; Quina, F. H.; Chaimovich, H. Biochem. Biophys. Res.
Commun. 1978, 81. 1080.
0 1992 American Chemical Society
