Langmuir 1992,8, 744-746
744
Comments Comments on the Surface Energetics of Polyacetylene As mentioned elsewhere,l methods for the calculation of the surface free energy and its components using the contact angles of liquids on solids have been developed for about the last 40 years. Nevertheless, new hypotheses and disputes between various authors continue to appear in the literature. Among many other disputes, one was initiated by the paper by Guiseppi-Elie et aL2 These authors measured the contact angles, 8,of some liquids (L)on pristine polyacetylene, (CH),, and iodine-doped polyacetylene,(CHb.d,. ~ 8, , which they They tabulated the values of y ~y, ~and used to determine some characteristics of the above solids (S),such as,e.g., ysd (superscript d denotes the dispersion component of the surface free energy, y). From the plot corresponding to the Fowkes equation3 d
d 1/2
yL(l + COS 0) = 2(ys yL )
(1)
they obtained ysd = 58 and 90 mJ/m2 for (CH), and (CHIO.~),, respectively. However, reevaluation of their data led to the conclusion4that using eq 1was incorrect because it neglects polar interactions. I t was assumed that eq 1 can be employed in this case only for mercury on (CH),, which yielded ysd = 19.9 mJ/m2. This value was substituted in the equation
+
yL(i COS e) = 2(y,dy31/2
+ 2(yspyLp)1/2+ ;7
(2)
for the interaction of (CH), with water to yield ysp = 5.1 mJ/m2. Equation 2 was derived in a previous paper,5 in which the polar (Keesom), yp, and the hydrogen bond, yh, components of the surface free energy of water were also determined. The sum y s = ysd + ysp = 25 mJ/m2 for (CH), agrees with the ys value determined from the Girifalco and Good equation6 yL(l + COS e) = 2 4 ( ~ , ~ , ) ’ / ~
(3)
in which the empirical parameters for the interaction, $, were used for the interaction of (CH), with water and mercury. Similarly, eq 3 yielded ys = 58.5 mJ/m2 for (CHIO.~),.On the basis of the results obtained from eq 3, it was concluded that both the solid surfaces contain hydroxyl groups from atmospheric oxidation and/or hydration. Van Oss et al. suggested a new hypothesis. They7 combined dispersion (London), polar (Keesom), and induction (Debye) components of the surface free energy and called their sum apolar or Lifshitz-van der Waals component, yLw. Then8they split the acid-base (or polar) component, yAB,into the acid and base parameters, y+ and y-, respectively, and wrote the equation (1) Kloubek, J. Adu. Colloid Interface Sei., in press. (2) Guiseppi-Elie, A.; Wnek, G. E.; Wesson, S. P. Langmuir 1986, 2, 508. (3) Fowkes, F. M. Ind. Eng. Chem. 1964,56, 40. (4) Kloubek, J. Langmuir 1989,5, 1127. (5) Kloubek, J. Collect. Czech. Chem. Commun. 1987,52, 271. (6) Girifalco, L. A.; Good, R. J. J . Phys. Chem. 1967, 61, 904. (7) van Oss, C. J.; Good, R. J.; Chaudhury, M. K. J . Colloid Interface Sci. 1986, 1 1 1 , 378. (8) van Oss,C. J.;Chaudhury, M. K.; Good,R. J. Adu. Colloid Interface Sci. 1987, 28, 35.
0743-7463/92/2408-0744$03.00/0
YL(l+
cos e) = 2(y, LWyLLW11/2 + 2(ys+yL-)1/2+ 2(YiYL+)’/2
(4)
In their recent papers van Oss et al. agree that the ysd values reported by Guiseppi-Elie et ale2are much too high. However, they severely attacked my preceding reevalua t i ~ n .They ~ call attention to their eq 4 with comments that “Kloubek made an error”, “used an incorrect version”, “misread our papers”, “depicts a physical impossibility”, and “Kloubek’s paper seems to have an inappropriate formulation of the theory ...his comparison of the various theories ...is flawed.” They accuse me of “callingnitrobenzene apolar”. They find my ysd for (CH), “unrealistic”. The term yn used in my paper is “uncertain”. They consider eq 2 to be based “on a spurious additivity of yp and yh” and take yd from this equation to be “identical with ... yLw”because it “... comprises the Keesom and Debye components ...”. There are statements in the comments of van Oss et alms which are not true and should be rectified. Equation 4 was mentioned in my paper4 but it was not used for any calculations. The superscripts 1, a, and b were used there to indicate the component corresponding to the long-range forces and the acid and base elements instead of superscripts suggested by van Oss et aL7v8, LW, +, and -, respectively. Unfortunately, a misprint occurred in this + However, a few lines equation: 2(ys&I~~)’/~ below it, an explanation is given: “...one element interacts across the interface with the complementary element in contribthe second phase, and 2(y1&12b)1/2or 2(y1by2a)1/2 utes to the interaction” (between the monopolar and bipolar phases). Hence, no confusion could be caused by the above misprint. Moreover, the mentioned error was corrected in “Additions and Corrections” in this journallo before the critique of van Oss et al.s appeared. Furthermore, the calculation was carried out assuming the solids to be monopolar bases and the relationship corresponding to the hypothesis of van Oss et al.8
+ COS e) = 2(ysy,?1/2 + 2(y,byLa)1/2
yL(i
(5)
was correctly introduced in my paper4and solved by using liquids with a known yla/yzaratio. However, eq 5 did not seem to yield acceptable results. In my previous paper5 (also quoted in the comments of van Oss et al.9) nitrobenzene was shown to interact as a polar liquid. It was also used in the mentioned papel“l as a polar liquid in the above eq 5. On the other hand, Guiseppi-Elie et aL2considered nitrobenzene to be a nonpolar liquid with y~ = y ~ d .To show the procedure of the above authors to be incorrect, I also had to consider nitrobenzene as a nonpolar liquid, which I mentioned in my paper4 “Using the plot of cos 6 against ( T L ~ ) ’ / ’ / ~ L , Guiseppi-Elie et al. fitted their data ...Revaluating their data ...”. The same data were also used to show that even the frequently applied Owens and Wendt equation1’ yL(l
+ cos e) = 2 ( y , d y 3 ’ 2 + 2 ( y & 9 1 / 2
(6)
does not fit. (9) van Oss, C. J.; Giese, R. F., Jr.; Good, R. J. Langmuir 1990,6,1711. (10)Kloubek, J. Langmuir 1990, 6, 1034. (11) Owens, D. K.; Wendt, R. C. J . Appl. Polym. Sci. 1969,13,1741.
0 1992 American Chemical Society
Langmuir, Vol. 8, No. 2, 1992 745
Comments
The term of (ys"y~")l/~ substituted in eq 6 for (ys"/~~)l/~ represents the interaction of all nondispersion Components, Le., polar, induction, hydrogen bond, and acid-base components. The second term on the right-hand side of all equations of type 6 should generally represent an interaction of the sum of the nondispersion forces, regardless of whether the components are denoted yn,y p , or yh. Otherwise, an additional term is necessary as, e.g., in eq 2, where p then denotes only the Keesom and h the hydrogen bond forces. Superscript n was already used in my previous paper6 as well as in the papers of the other authors.l2J3 Unlike yd, the van Oss et ala7*value of yLwcomprises the London, Keesom, and Debye components of the surface free energy. It should be noted that the Debye component is negligibly small in pure condensed phases (containing one kind of molecule) while it can play an important role at interfaces between two phase^.^ On the other hand, the contribution of the Keesom forces to the surface free energy can be considerable in phases containing polar molecules with permanent dipoles while their contribution to the interaction at interfaces between a polar and a nonpolar phase is zero. Therefore, the van Oss et al. concept of the Lifshitz-van der Waalsterm, 2(y1Lwy2Lw)1/2, cannot be correct and neither their eq 4 nor 5 can hold in general. The interaction of the London components should be evaluated separately from the interaction of the Keesom and Debye components, as has been originally suggested by F ~ w k e s . ~ Doubts were raised by the authors,14 who frequently used eq 6, as to whether the components have a physical meaning. I t has been shown in many previous papers including the mentioned one4 that the terms 2(y1ny2n)1/2 or 2(71"/2~)l/~cannot be generally valid. Nevertheless, examples can be found where they yield good results.13J5 Equation 2, at least, can be considered to correspond to the respective system on an empirical basis because it suited a similar liquid-liquid system. In addition to for the London forces, Fowkes and Mostafal'j also suggested that the term 2(y1Py2P)1/2 is correct for the Keesom forces interaction. Introduction of the acid-base interaction8 expressed by the terms 2(y1+y2W2and 2(yl-y2+) certainly represented a contribution to the development of theories on the interactions at interfaces. However, in addition to the attraction between charges of the opposite sign, repulsion terms should also be ~0nsidered.l~ Therefore, the terms 2(y1&vz~)l/~ and 2(y1by2b)1/2 do not represent "a physical impossibility" and do not correspond to "forbidden and/ or nonsensical combinations" as van Oss et aL9suggested. These terms appeared in the mentioned paper4 as a misprint but they were actually used el~ewhere.'~J~ The statement of van Oss et aL9that ys for (cH10.2)~ in my paper4 was estimated "via an equation using a general combining rule" is also not true; ys was calculated by using eq 3. Van Oss et aL9 published their own estimates of ys, ysLW,ysm, YS+, and 7s- for (CHI, and (CHIo.dxusing eq (12) Bilihki, B.; W6jcik, W. Colloids Surf. 1989, 36, 77. (13) Jdczuk, B.; Bialopiotrowicz, T.;W6jcik, W. J.Colloid Interface Sci. 1989, 127, 59. (14) Jdczuk, B.; Bialopiotrowicz, T. J. Cplloid Interface Sci. 1989, 127, 189. (15)Kloubek, J. J . Adhes. 1974, 6, 293. (16) Fowkes, F. M.; Mostda, M. A. Ind. Eng. Chem. Prod. Res. Deu. 1978, 17, 3. (17) Hobza, P.; Zahradnik, R. Intermolecular Complexes; Academia:
Prague, 1988. (18) Kloubek, J. Collect. Czech. Chem. Commun. 1991, 56, 277. (19)Kloubek, J. Colloids Surf. 1991.55, 191.
4. They tabulated the y ~ and + y ~ values for the liquids employed (water, glycerol and formamide) and in the explanatory notes to their table they only mentioned that "the ratio yw+/yw- is not known with any certainty", "it has been assumed that, for water, yw+ = yw-", and "ally+ and y-values given" in their tables (i.e., also their ys+and 7s-) %re based on" this assumption. Unlike their assumption, the values ywa = 67.7 and ywb = 10.6 mJ/m2 were determined.18 Accordingly,these ys+ and ys- values cannot be correct. An equation was suggested18 which also includes the repulsion terms
1ll2 + 2(y;yLb)1/2 + b b 1/2 2(YsbYLa11/2 - 2(ysayLa)1/2 - 2(Y, YL 1 (7)
cos 0) = 2(Y,
Y,(l+
YL
It can be transcribed into a form suitable for evaluation by linear regression
+ COS e)/2(yLd)1/2= (ysdF2+ [(~sb)"~- (r;)1/21 [(TL")"~ -
yL(i
(8)
and the plot of y = y ~ ( 1 +cos 0 ) / 2 ( y ~ ~vs ) l x/ ~= [(yLa)1/2
- ( y ~ ~ ) ' / ~ l / ( yshould ~ ~ ) 'yield / ~ a straight line with (ysd)1/2 at x = 0 and with the slope (ysb)1/2- ( y ~ ~ ) lWhen / ~ . the solid is a nonpolar base (Le., ysd = ys, ysa = 0, and ysb > 01, the intersection yields ys1l2and the slope, ( y ~ ~ ) l / ~ . Equation 7 rewritten in the form d
d 112 +
yL(l+ COS e) = 2(ys yL 2[(y,b)1/2- (r;P2I [(rLa)1/2 - (yLb)1/21 (9)
shows that the acid-base interaction across the interface is the product of the difference between acidity and basicity of single phases. Equation 9 formally recalls eq 6 but the meaning of the terms in square brackets differs from yh (or 7"). The right-hand side of eq 9 depends on the type of adjoining phases. For solids with the surface free energy containing component ysab, the acid-base interaction with nonpolar (or monopolar) liquids having y~ = y~~and y~~ or y~~will be 2 [ ( y ~ ~ )-' /(~y ~ ~ ) l / ~ I ( or y ~2[(ysa)1/2 ~)'/~ ( y ~ ~ ) (' y/ ~ "l ) ' / ~respectively. , This interaction changes 1 2(ys&v~b)1/2for nonpolar to 2(ysW2[(y~b)1/2-( y ~ W 2 or solids with ys = ysd and ysa (i.e., monopolar acids) and - (yLb)l/2] or 2(ysby~a)'Pfor nonpoto 2(ysb)1/2[(y~a)1/2 lar solids with ys = ysd and ysb (i.e., monopolar bases), depending on the kind of the liquid. Therefore, the simple or 2 ( y ~ ~ y ~ in " ) equations l/~ of type 6 terms 2(yshy~h)1/2 cannot be generally valid and yh or yn are not constants with a physical meaning. Moreover, induction forces can be involved in the interaction, which introduces a further complication.18 Induced elements do not cause repulsion17and when, a a polar liquid induces ysibon a nonpolar solid e.g., y ~ of surface, eq 7 reduces to
+ COS e) = 2 ( y s y 3 1 / 2 + 2(y;byLa)1/2
yL(i
(io) This equation may seem identical with eq 5, but the d ysib differs from y ~ and l ysb, meaning of terms y ~ and respectively. Equation 10 also recalls eq 6 but the polar ) from the nondispersion comelements (ysib, y ~ a differ ponents (yhor 7"). It can be evaluated by linear regression in the form yL(i+ COS e)/2(y31/2= Y
ib 112
~ + (yS ~ /
d 1/2
~(yLa/yL
(11)
The contact angles given by Guiseppi-Elie et aL2 are evaluated anew here by using the YL, y ~Y L~~ ,and , Y L values for mercury, water, glycerol, formamide, and ni-
~
746 Langmuir, Vol. 8,No. 2, 1992
Comments
Table I. Values (in mJ/m2) Obtained by Linear Regression According to Equations 8 and 11 for Solids, (CH), and ( C H I & , Using Liquids
calculated from eq 8 solid
(CWx
liquidsa
M, W
ysd 26.4 63.9 53.7 77.3 86.6 83.2
[(ysb)'/* - (rsa)1/21
5.1 2.5 2.9 4.3 3.6 3.8
~ s (for b ysa
W, G,F, N M,W, G,F,N (CHbdx M,W W, G,F M, W, G,F Key: M, mercury; W, water; G, glycerol; F, formamide; N, nitrobenzene.
trobenzene from the previous paperla in eqs 8 and 11 yielding (ysd)l/2or ys1/2and (ysW2 - (ysa)1/2 or ( y s V 2 , respectively. The results are given in Table I. The straight line connecting the points for mercury and water in the plot according to eq 8 yields ysd for (CHI, which agrees with the value of ys = 26.4 mJ/m2from the preceding paper4 for the case in which eq 3 with q5 corresponding to an alcohol-like solid surface in contact with water was employed. However, other liquids used in eq 8 indicate a conspicuous deviation for mercury, which decreases the coefficient of determination in the set of liquids used. The ysd = 63.9 mJ/m2 value (determined from the plot after excluding the point for mercury) agrees with ys = 64.0 mJ/m2 obtained in the preceding paper4 from eq 3 for (CH), and water assuming a value of 4 corresponding to the aromatic hydrocarbon-like surfaces. If the surface of (CH), is similar to that of benzene (i.e. with a conjugated double bond system of CH groups), it would be a nonpolar base with ys = ysd = 63.9 and ysb = 6.2 mJ/m2. Table I shows that iodination of polyacetylene increased the ysd value according to eq 8 by more than 20 mJ/m2. is also higher than that of (CHI,, The ysbvalue of (CHIO.~), regardless of whether this substance is a nonpolar base or polar compound with ysab. The influence of the point obtained for mercury on the result calculated from the set of other liquids is not as significant as for (CH),. Nevertheless, the two-point extrapolation using water and mercury yields misleading values. Equation 11yields values of ysdsimilar to ysLWobtained
= 0)
25.7 6.2 8.2 18.8 13.0 14.2
coef of determ 0.963 0.628 0.925 0.927
calculated from eq 11 coef of YS YS'b determ 20.5 11.5 43.6 5.3 0.996 34.7 7.9 0.869 68.0 8.4 49.7 12.4 0.992 59.0 9.4 0.958
by van Oss et al.9 and of ysibsimilar to ysm. However, this similarity is only coincidental because of the difference between eqs 4 and 10as well as that between the elements used for the calculation. The values obtained in this paper differ from the original results2as well as from the first4and secondSreevaluations. Three new solutions of the surface free energy and its components have been described, based on the assumptions that the solids are (i) polar compounds, (ii) nonpolar bases with permanent ysb,and (iii) nonpolar compounds with induced ysib. Insufficient data are available to decide definitely which solution is correct and there is so far no independent method which would help to solve this question. However, it is believed that the methods and results presented here will help in the understanding of the surface energetics of polyacetylene, contribute to the development of the theories on interfacial interactions, and rebut the improper criticism of van Oss et al.9
Jan Kloubekt The J. HeyrovskS Institute of Physical Chemistry and Electrochemistry, Czechoslovak Academy of Sciences, 182 23 Prague 8, Czechoslovakia Received February 11,1991 In Final Form: May 15, 1991 Registry No.
(CH),, 25067-58-7. ~~
~
'Present address: TESSEK Ltd.,RadlickB.117/520,15801 Praha 5,Czechoslovakia.
