Interaction energetics on low energy surfaces

JOURNAL OF COLLOID AND INTERFACE SCIENCE 24, 372--378

(1967)

Interaction Energetics on Low Energy Surfaces Hydrocarbon ImmersionHeats for Teflon 6 J A M E S W. W H A L E N 1 AleD W I L L I A M It. W A D E 2 Received January 25, 1967 Immersion heats for Teflon 6 in a homologous series of hydrocarbons from hexane to hexadecane have been obtained in a joint calorimetric program. Thermodynamic interrelationships between energetic quantities derived from immersion heats of clear and film-covered surfaces and contact angle data have been examined. Temperature derivatives for spreading pressures and contact angles have been estimated for the systems studied. Further implications relative to interaction energetics on low energy surfaces are discussed. INTRODUCTION

that, with the use of Eqs. [1], a comparison of measured contact angles with adsorption Following Melrose (1) the adhesion energy data a n d / o r immersion heats obtained on ¢ between a liquid and a solid equilibrated analogous systems can resolve questions with vapor at saturation pressure p0 can related to the operational validity of conbe expressed in two alternative forms: tact angles (2) and serve as a useful test of the thermodynamic relationship (1). 0 [la] ~b = --h~(8~) = --hi(~) - ~r° + T ~d~r° ; Teflon 6 powder is an appropriate substrate for the indicated immersion heat and adsorption studies. Fox and Zisman (3) have reported contact angles for normal [lb] hydrocarbons on Teflon surfaces which d cos 0 increase gradually from 12 ° for hexane to T~L dt 46 ° for hexadecane. This hydrocarbon series In Eq. [la], h°(,~) is the heat of immersion is amenable to service as immersion fluids, of the solid equilibrated with vapor of the and identical contact angles have been immersion liquid at p0, h~(,) is the clean measured on the pressed powder and on solid immersion heat, and 0 is the Gibbs commercial extrusions (4). Some previous spreading pressure defined in terms of the adsorption studies (5) and a single immersion heat (6) have been reported. These measuresurface excess of adsorbate (P), ments are not sufficiently extensive to permit a treatment of the Eq. [1] relationships. ~r° = R T f0P°F dln p. [2] The requirement that an absolute surface area value for Teflon 6 be obtained is In Eq. [lb], 3'L is the surface tension of the implicit in Eq. [la], where unit area immerliquid and 0 the characteristic contact angle. sion heats and spreading pressures appear. Convenient units for ~b, h~ terms and ~r° In a previous publication (7) the B E T treatterms are ergs/cm?. I t has been suggested ment of argon and nitrogen data was dis1Mobil Oil Corporation, Field Research Lab- cussed, and surface area values so derived were compared with the geometric area oratory, Dallas, Texas 75224. Department of Chemistry, The University obtained from electron microscopy. The of Texas, Austin, Texas 78712. most acceptable surface area value appears 372

INTERACTION ENERGETICS ON LOW ENERGY SURFACES to be 15.4 m?/g. as compared to lower values reported by Graham (5) and Chessick et al (6). Both hio(.~) and ~0 refer to thermodynamic characteristics of the adsorbed film at saturation vapor pressure. In general there will be complications, as pointed out by Melrose (1), arising from the loss of film area due to condensation. The extent of film area loss has been discussed in detail by a number of authors (8, 9). The uncertainty which will be attached to h~(~) and ~0 depends greatly on the extent to which the condensed fluid contributes to film area loss and apparent film structure and on the degree to which satisfactory corrections can be accomplished. Direct measurements of the temperature derivative of contact angle have not been accomplished with certainty. The values are believed to be quite small, and there is not complete agreement as to the sign (10). Also, for direct measurement, d=°/dT will be the' very small difference between uncerrain 0 values at two temperatures and will be very difficult, if not impossible, to obrain with any degree of precision from isotherm data. In view of the uncertainties involved, it does not seem possible that questions of operational validity for the contact angle can be completely answered through the suggested correlation. It is, however, desirable to undertake the recommended studies with the intent of establishing sign and magnitude for the pertinent temperature derivatives. In establishing limiting values for these terms over a range of adhesion energies, a basis can be established for further study and discussion which may ultimately result in a thermodynamic verification. EXPERIMENTAL Immersion heat measurements were obtained independently using two microcalorimeters. Both of these instruments have been used extensively in previous studies and their mode of operation has been discussed (11, 12). They will be referred to as the JWW and WHW calorimeters. The JWW calorimeter uses a multijunction thermocouple detector whereas the WHW

373

calorimeter uses thermistor sensors. Both are best described as semiadiabatic, having Newtonian cooling time constants of approximtely 45 minutes. They have provision for electrical calibration and operate in oil baths the temperature of which can be regulated to ±O.O01°C. or better. The JWW measurements were made at 22.0°C. and the WI-IW measurements at 25.0°C. The J W W version accepts a duplicate set of sample bulbs whereas the WHW version can utilize up to a maximum of six duplicate sets of sample bulbs. In the J W W measurements the samples were contained in frangible thin-walled spherical bulbs. In the W H W measurements samples were inside thick-walled containers, only the thinwalled tips of which were broken. The Teflon 6 powder, which has been previously discussed (7), was used as received. Immersion heats of this powder were obtained in the following homologous series of aliphatic hydrocarbons: C~, Ca, C10, C1~, and C16. Research Grade liquids were used from C6 to C10 (99.9 mole % purity) and Pure Grade liquids from C12 to C1~ (99 mole % purity). The latter hydrocarbons were passed through silica gel columns prior to use. The materials so treated have been shown to exhibit identical contact angles (13) on Teflon to those repol~ed in the literature (3). Although trace I-I20 impurities should be of little consequence, the liquids were stored over activated 4A molecular sieves prior to use and maintained in a dry state during the immersion study by techniques previously described (14). In Table I the averaged values of the clean surface immersion heats for some 60 determinations are given together with pertinent literature data. The single clean surface value previously reported for heptane immersion (6), when expressed in terms of our surface area, lies (at 34 ergs/ era. 2) below the smoothed curve of our values. All immersion liquids were included in the individual programs at the two laboratories, and in no case does an experimental value represent less than 4 runs in each calorimeter. In view of the very small heat evolution on immersion of the Teflon

374

WHALEN AND WADE TABLE I

EXPERIMENTAL VALUES FOR CLEAN SURFACE IMMERSION HEATS. LITERATURE VALUES FOR CONTACT ANGLE AND LIQUID SURFACE ]~NERGY Immersion fluid

C1~ C~2

Clo Cs C6

-- hi(s) (ergs/cm3)

34.0 32.8 32.8 39.3 4.6.9

± ± 4± ±

1.6 1.6 1.6 4.0 6.3

Oo (degrees)

cos Oo

u L v (ergs/ cm3) (15)

46 42 35 26 12

.694 .743 .819 .899 .978

52.5 51.5 50.8 49.5 48.4

samples, all possible sources of error and correction terms were carefully re-examined, by direct experiment in many eases. These included reversibility in the adiabatic compression of the vapor space during the breaking operation, adiabatic expansion of the vapor space to aeeommodate the void volume associated with the sample, vaporization and break heats, sample compaction and electrical calibration at various levels of thermal input. In addition, the absence of inherent differences between the two calorimeters was demonstrated by measurements on the heats of dilution of sucrose solutions. Both systems substantiate the results obtained by Gucker, Piekard, and Planek (16). With the exception of the vaporization correetionl all error sources amount to less than 1% of the measured immersion heat. Break heats, never amounting to more than 0.03 joules, were applied to individual samples where applicable. Vaporization corrections were based on the void volume of the sample container and on heats of vaporization from Rossini (17). Although the total vaporization correction was greater than the measured immersion heat for hexane and quite significant for octane, the error in the correction term is less than 2 % of the corrected heat value. For the hydrocarbon studies, excepting octane and hexane, the indicated uncertainty in the average of the total data set is approximately equivalent to the precision attained in data sets from either laboratory. For hexane and octane somewhat poorer precision was obtained in separate sets. The reported uncertainty for hexane and octane average values is approximately 2.5 times the precision of the individual sets. There is

some indication that, for these two systems, the temperature derivative of the immersion heat is significant but neither calorimeter is adapted to the large operating temperature change required to substantiate the effect with adequate precision. In addition to the clean surface immersion heats, h¢(8~) values were obtained for those systems which, from contact angle considerations, appeared to exhibit reasonable interaction and therefore probably significant ~r° values. These data, obtained in the JWW calorimeter, are shown in Fig. I. For the purpose of the discussion which follows, only the quantity (hi(~)- hi(,~)) 0 and the general form of the curves are of significance. In order to eliminate confusion between the various data sets the ordinate is expressed in relative units. DISCUSSION At p0 and for multilayer adsorption (zero contact angle), Eqs. [i] reduce to the widely utilized form (18),

o

(d'~L~

--hi(,,) = 7L -- T \ ~ - ]

= uL,.

[3]

0 For finite contact angle systems --h~(~,) does not approach uL, as a limiting value but will, in general, reach some lower value. Several limiting cases are of wide interest. In cases (10) for which the contact angle temperature dependence is zero or negligible: -h~(,,) = uL, cos 0. [4a]

Also there is widespread consideration of the viewpoint expressed by Zisman (19) that, for poorly wetted or nomvet systems, 1.0 octane -

'~ 0.9

hexane

,

t-

0.8

I 0.2

I 0.4

I 06

I 0.8

I 1.0

P/Po

FIG. I. Immersion heats for hexane and octane films adsorbed on Teflon 6. Heats are expressed as fraction of clean surface (hi(s)) value.

INTERACTION ENERGETICS ON LOW ENERGY SURFACES

2,L and ZL, being the total solid-liquid and liquid-vapor interfacial areas involved in the condensation. Where ~L/2L~ = }

o = O, for which case: -h~)

= -h~(~) = u ~ cos 0 [4b]

d cos 0 --

T'),~

375

dT

"

Where both limiting assumptions hold: --h°(~) = -hi(~) = u~, cos O.

[4c]

The extent to which the 7r° = 0 approximation is valid can be established alternatively by the comparison of h°(~,) and h~(~) values or by direct spreading pressure calculations from adsorption isotherms. In either case condensation effects at particle contacts must introduce some uncertainty into measurements carried out at pressures approaching saturation. Qualitatively, however, it is obvious that such effects introduce a considerably greater and more difficultly treated error into ~r° calculations than into the h o¢(,~) determination. In cases of physical adsorption a practical precondition for multilayer adsorption is a condensed, but not necessarily close-packed, monolayer. Where multilayer adsorption is excluded, the absel~ee of a condensed film is implied and a submonolayer limit is placed on the relevant adsorption process. Since the interparticle condensed fluid can amount to the equivalent of several monolayers for particulate systems at high relative pressures, the contribution of condensation effects can override the adsorption process completely. Correction procedures (8, 9), being dependent upon regular and welldefined geometry, are not adequate for the resolution of the adsorbed quantity. On the other hand, for high porosity, low coordination number samples, the loss of film area due to condensation effects is never greater than a fraction of the original area and is largely replaced by the associated liquid-vapor interface. Where Z is the total sample area, the error ~H°(~) in the total measured immersion heat H°(=) is the difference between immersion heats at p0 of the real sample and a fictitious sample having no condensation but identical film structure. By such definition:

Since ~ values will always be greater than one, for wetted systems ( - h ~ o = u ~ ) 8H°(,~) will always be positive and immersion heats vs. film coverage will decline near pc to apparent values below uc~. Where -h°(,~) < uc~, the value of ~H~° may be positive, negative, or zero. Clearly, however, near-wet systems will approach wetted system behavior while the decreasing h°(,~) term will minimize the correction for poorly wet systems. The Fig. 1 results are compatible with this qualitative observation, hexane values indicating an abrupt drop in h~(,~) values above 0.77 pc while octane values remain constant to at least 0.92 p0. The apparent zero aH~(~> requirement for octane would be met for an ideal geometry system exhibiting a 26 ° contact angle (} = 1.55) if - h ~ ( ~ ) = 34.2 ergs/ era. =. A value of 36.2 (subject to the ± 4 ergs/cm. 2 uncertainty in h~(,)) is obtained .~ 0 . 9 ~ P 0 from - , ~ and the calculated value is seen to lie well within the assigned uncertainty. Two alternatives are available for the resolution of h°~.(,~) for the hexane data. Extrapolation of the Fig, J curve as indicated by the dotted segment, i.e., h °i ( ~ v ) -~- '~0.77po ~i(sv) , yields 41.2 ± 6.3 ergs/cm2 allowing for the uncertainty indicated by h~(,) data. Extrapolation of the h¢(~) data ~o p / p o = 1, yielding 0.8 h¢(,), and application of condensation corrections assuming spherical geometry gives 42.5 4- 7.6 ergs/cm2. The assumption of negligible film energy change in the interval 0.77 < P / P c = 1 appears preferable to the more uncertain extrapolation to p0 and associated condensation approximation. The general conclusions reached are not dependent upon the partieular choice in view of the relatively large uncertainty. Where, from Eq. [la], 7r° -- -~-~ T dTr°

=

--{hi(o

- -

h°c~)}

[6]

the right hand side has a value of 5.7 4- 2.7 ergs/cm3 for hexane and 3.2 ± 1 ergs/cm3

376

WHALEN AND WADE

for octane. The hexane value must be considered to represent the minimum adsorption-related contribution in that adsorption effects occurring at pressures greater than 0.77 p0 are neglected. However, the uncertainty associated with this approximation is less than that attached to the averaged h~(~) value to which it is applied. The condensation-related uncertainty attached to the octane data is negligible. Also, since in the latter case the total adsorption contribution is not greater than the averaged immersion heat uncertainty, adsorption-related terms for hydrocarbons above octane may be ignored in this correlation. Where v ° approximations are available, it is possible to obtain some further information regarding the sign and order of magnitude for the dz°/dT term. Graham has reported spreading pressures of several hydrocarbons on Teflon 6 as obtained from isotherms which had not been corrected for condensation effects. A value of z~ = 1.7 ergs/cm. 2 was obtained for octane by integrating up to a F value equivalent to one statistical monolayer (5). If hexane isotherms obtained in the course of this study (20) are treated in the same manner, we obtain 2.9 ergs/cm? for that system. Combining these data with the { h , ( , ) h ° (~)} data, dz°/dT values of --0.009±.007 erg cm. -~ deg. -1 for hexane and - 0 . 0 0 5 ± .003 erg cm. -: deg. -~ for octane were obtained. Although the accuracy is not high, negative d~r°/dT terms for these materials are confirmed, and the magnitude of the values establish that their direct determina-

NE

'X~

~

V

40

3(?

( ,I

C6

I

I

C8

Clo

I

I

CI2 CI4

I

CI6

Fro. 2. Energetic relationships for n-alkanes on Teflon 6 surfaces.

TABLE II EXPERIMENTAL VALUES FOR ADSORBED FILM IMMERSION I-IEATS. CALCULATED VALUES FOR CONTACT ANGLE TEMPERATURE DEPENDENCE

Immersion fluid C1~ C12 C~0 Cs C~

_hlO(sv) (ergs#m.:)

34.0 32.8 32.8 36.2 41.2

-4-4± ± ±

1.6 1.6 1.6 4.0 6.3

do

dT (deg. O/deg. K.)

0.037 0.061 0.125 0.185 0.282

± ± ± ± ±

.015 .017 .021 .084 .314

tion from adsorption data is impossible. The utilization of apparent ~r. values rather than true ~r° is not critical in view of the hi uncertainties although ~r~ > ~r° (20). No reasonable 7r° value, e.g., 1 erg/cm?, would raise --d~r°/dT above 0.015 erg cm. -~ deg. -1. Several curves of interest are shown in Fig. 2 including h~(~) and h°(~) as a function of hydrocarbon chain length. If Eq. [4a] holds, i.e., dO/dT = O, the h°(,~) and UL~ COS0 curves would coincide. And if Eq. [4c] applies, ~r° = 0 and dO/dT = O, the h~(,), h°(~), and UL~ COS 0 curves would coincide. In Fig. 2 an error of -4-2° has been assigned to the measured contact angle value for comparison with immersion heat errors. Those skilled in contact angle measurements achieve precision higher than -4-2° for contact angles above 10 ° (2, 13). Although it is not clear that absolute accuracy is higher than that value, in the calculations which follow, observed contact angles (00 Table I) are considered to be without error. From Eqs. [1] d cos 0

1

dT

T'yL

0~

{UL~ COS 0 + hi(8~)},

[7]

and values of dO/dT are shown in Table II. Although the uncertainty attached to the hexane value is quite large, the remaining data are significant. An increase in magnitude of dO/dT with decreasing chain length is indicated, and it is demonstrated that dO/dT can be safely ignored only for hexadeeane in the hydrocarbon series studied. The sign of dO/dT is negative for the systems studied. For systems exhibiting a contact angle

INTERACTION ENERGETICS ON LOW ENERGY SURFACES greater than 90 °, it is, however, a further consequence of Eqs. [1] that, as cos 0 becomes negative, dO/dT must become positive in order that exothermic immersion heats be observed. Such a system has been studied by Young et al. (21) and the exothermic immersion heats observed have been confirmed in both of these laboratories (22). Regarding the dO/dT variation as characteristic of the energetics of the wetting process, dO/dT m a y be considered a function of 0 and the generalized dO/dT variation given by the Fig. 3 sketch. Furthermore, as high contact angles may be associated with negligible ~r° terms, there is a region (a-b in Fig. 3) for which the contact angle cosine is closely approximated by Eq. [4@ A single clean surface immersion heat measurement m a y therefore suffice to describe the wetting character of m a n y systems not amenable to the limitations of contact angle measurement. Additional study with systems having contact angles greater than 90 ° will be required to define the pertinent region. Additional study is also indicated for the purpose of establishing the nature of dO/dT in the neighborhood of 0 ° and for very high contact angles. In this regard, however, immersion heat studies may be of little utility in view of the poor precision obtained with hexane systems in this study. Direct contact angle measurement over a reasonably narrow temperature

377

interval should establish whether dO/dT becomes increasingly large or approaches zero as 0 approaches zero. CONCLUSIONS For the homologous series of hydrocarbons hexane through hexadecane the adhesion energy is constant on Teflon at 32 42 ergs/cm? for hydrocarbons C10 and higher. Below C10 the adhesion energy increases sharply approaching uL~ as a limiting value. Temperature derivatives of the contact angle for the above series are negative, increasing in absolute value with increasing wetting character. Spreading pressure terms are negligible for paraffin hydrocarbons of ten or more carbon atoms. Where significant adsorption occurs, the temperature derivative of the spreading pressure is negative and of the order of 0.01 erg cm. -2 deg. -1. For m a n y interracial systems the contact angle can be obtained from a single clean surface immersion heat ACKNOWLEDGMENT One of the authors (JWW) wishes to acknowledge the permission granted by Mobil Oil Corporation to publish this work. The other (WHW) wishes to express appreciation to the Robert A. Welch Foundation for their continued interest and support. REFERENCES 1. MELROSE,J. C., J. Colloid Sci. 20, 801 (1965). 2. ADAMSON,A. W., A.NDLING, I., Advan. Chem. Ser. 43, 57 (1964). 3. FOX, H. W., AND ZISMA.N,W. A., J. Colloid

Sci. 5, 514 (1950). 4. ALLEN,A. J. G., ANDROBERTS,R., J. Polymer

i-

"O

J 180 (

Sci. 39, 1 (1959). 5. GRAH~'~,D. P., J. Phys. Chem. 69, 4387 (1965). 6. CItESSICK, J. J., HEALEY, F. H., AND ZETTLE~OYER, A. C., J. Phys. Chem, 60, 1345 (1956). 7. WHALEN, J. W., WADE, W. H., AND PORTER,

J. J., (Submitted this Journal). 8. WADE,W. tI., J. Phys. Chem. 68, 1029 (1964). 9. MELROSE, J. C., A.I.Ch.E.J. 12, 986 (1966). 10. PHILLIPS, M. C., AND RIDDIFORD, A. C.,

(+) FIG. 3. Schematic variation of the temperature dependence of contact angle from n-alkane data and wetting heat considerations.

Nature 203, 1006 (1965). 11. WHALEN,J. W., J. Phys. Chem. 65, 1676 (1961). 12. MAKRIDES, A. C., AND HAOKERMAN, N., J. Phys. Clam. 68, 594 (1959). 13. DUNLAP, P. M., Private communication. 14. WH•LEN, J. W., J. Phys. Chem. 66,511 (1962).

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WHALEN AND WADE

15. JASPER, J. J., AND Knmo, V. K., J. Phys. Chem. 59, 1019 (1955). 16. GUCKER,F. T., PICJKARD,H. B., AND PL:kNCK, R. W., J. Am. Chem. Soc. 61, 459 (1939). 17. ROSSINI, F. D., "Selected Values of Physical and Thermodynamic Properties of Hydrocarbons and Related Compounds." Carnegie Press, Pittsburg, Pa., 1953. 18. HAn~ms, W. D., "The Physical Chemistry

19. 20. 21.

22.

of Surface Films," p. 274. Reinhold, New York, 1952. ZISMAN,W. A., Advan. Chem. Ser. 43, 1 (1964). WHALEN, J. W., To be published. YOUNG, G. J., CHESSICK, J. ,}'., HEALEY, F. H., AND ZETTLEMOYER, A. C., J-. Phys. Chem. 58, 313 (1954). WADE, W. ~-~., AND WHALEN, J. W., To be published.