Contact Angle, Wettability, and Adhesion

5 Wettability by Heats of Immersion A . C. Z E T T L E M O Y E R AND J. J. CHESSICK

Downloaded by UNIV OF MINNESOTA on May 11, 2013 | http://pubs.acs.org Publication Date: January 1, 1964 | doi: 10.1021/ba-1964-0043.ch005

Surface Chemistry Laboratory Lehigh University, Bethlehem, Pa. Heats of immersion are effective in rating the average polarity or field strength of polar surfaces, and in assaying different batches of the same type of fine powders, whether pigment, catalyst, o r filler. The technique of following heats of immersion as a function of precoverage provides one of the simplest ways of ascertaining the heterogeneity of s u r faces; it is useful whether the heterogeneities take up a very minor or large fractions of the surface area. The wetting of surfactants for low energy surfaces can be readily rated by heats of immersion; the change with surface concentration as given by tracer measurements and the effect of added counterions have been followed. Differences between heats of immersion of Graphon into hexanoic acid and into the sodium salt yield directly estimates of heats of formation of the double layer at the solid-solution interface. This commentary on the current status of research on heats of i m mersion begins where our review written in 1958 concludes [6]. The classification of heats of immersion of solids into liquids as a function of precoverage is expanded to include two new types of curves. Several difficulties in heat of immersion research are discussed. Then, c u r rent applications of heats of immersion to determine the average polarity of solid surfaces, heterogeneities on solid surfaces, wetting by surfactants, hydrophilicity of solid surfaces, and thermodynamics of the specific interaction of molecules from solution onto solid surfaces are described. Heats of immersion can provide information on energies of interaction for systems in cases of spreading wetting or zero contact angles. The approach is particularly suitable for mapping the energetics of surfaces which, of course, are invariably somewhat heterogeneous. Even if the liquid has a very low vapor pressure, and consequently low equilibrium pressures to be employed in preconditioning to follow heats of immersion with coverage, ample time can be allowed to reach the required state; the bulbs containing the solid are sealed off and taken 88

In Contact Angle, Wettability, and Adhesion; Fowkes, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1964.


5.

ZETTLEMOYER

AND CHESSICK

Wettability

Heats of Immersion

Figure 1. Classification of heat of immersion as a function of preadsorbed wetting liquid

89

curves

a. Essentially homogeneous surface. Water on chrysotile asbestos [40] b. Heterogeneous surface. Water on anatase titanium dioxide. Classic case given by Harkins [11] c. Low-energy surface possessing isolated polar sites. Water on Graphon [31] or some polymers d. Swelling of stratified mineral. Polar adsorbate such as water absorbing at rather definite pressures. Water on and in Wyoming bentonite [39] e. Gradual filling of pores, leaving no appreciable area and so a negligible heat effect at high relative pressures. Methanol on charcoal [21]. Benzene on graphitized black [19]

to the calorimeter only after the absorbate has been distributed to the appropriate sites. Calorimeters can give precise results only for powders, except for the most sensitive types which can handle coarse materials measuring only a few hundredths of a square meter per gram. Classification The previous classification [6] is repeated in Figure 1. A sixth type of heat of immersion curve can be predicted from the isosteric heats determined by Graham [9] for carbon tetrafluoride and other gases on polytetrafluorethylene. The prediction can readily be made from the equation: = /

o

r

q

s t

dF

+

W

L


(1)

ADVANCES IN CHEMISTRY SERIES

90

V

Figure

2.

P/Po

New types of heat of immersion

curves

f. Weak adsorption such as C F on polytetrafluoroethylene g. Swelling of aggregates at high relative pressures 4

where h. and h refer to the heats liberated in ergs per square centimeter on immersion of the bare and film-covered solid, respec tively, into the liquid, Γ is the surface coverage in moles per square centimeter, q is the isosteric heat of adsorption (from two tempera ture isotherms), and A H is the heat of liquefaction per mole. Heter ogeneities plus low energies of adsorption yield a heat of immersion curve which starts above the heat of liquefaction but then falls below, as in Figure 2,f. Such a curve has not yet been directly measured. A seventh type as predicted from Kiselev's measurements of gassolid interactions [15] is depicted in Figure 2,g. Here, the commonly found decrease of heat of immersion with coverage is followed by a rising portion, which is believed to be due to the expansion of aggre gates and possibly consequent energy release.


( S L )

i ( S f L )

s t

L

Problems in Heat of Immersion

Technique

A number of difficulties in measurement and interpretation of heats of immersion have come to the fore in recent years. These problems are sometimes pertinent to the other techniques as well. They must be taken into account if valid results are to be obtained. Polar solids collect organic contaminants from the air merely on exposure in the laboratory. They may also become contaminated during manufacture, or by stopcock grease. It is sometimes exceedingly dif ficult to solve this problem. The organic impurities can be eliminated from titanium dioxide, for example, by heating in a vacuum at450°C. [13]. A liquid nitrogen trap must be inserted between the final stopcock and the sample being outgassed. If the solid is washed with an organic solvent or series of solvents to remove contaminants, the surface may be left even dirtier than at the start. This situation is particularly prevalent in the case of oxides. The removal of impurities by outgassing may include reduction of the surface. Thus, oxygen deficiencies are created in the surface of rutile T i 0 by outgassing at 450° in a vacuum; the reduction is indicated very sensitively by the gray color produced. When oxygen is returned to the surface by admitting it to the hot sample, the color returns to white. Oxygen is much less readily removed by outgassing if organic contaminants are kept from the surface. The heat of immersion of the deficient surface was about 50 ergs per square centimeter greater than for the stoichiometric surface in the case of one T i 0 sample studied [13]. Both problems of surface contamination and deficiencies must be attacked if proper wetting results are to be obtained. A prevalent problem is interpretation of the interaction occurring in the calorimeter as physical or chemical. Coupled with this problem 2

2



5.

ZETTLEMOYER

AND CHESSICK

Wettability

Heats of Immersion

91

is the omnipresent water which tenaciously holds to high-energy solids. Physical adsorption is not all that can occur during the wetting process. A particular difficulty arises if heats of immersion obtained after activa tion at several temperatures are compared. After low temperature activation, the interaction may be entirely physical, whereas after high temperature activation some of the interaction may be chemical. Such effects have been well explored in the case of silicas [13,30] interacting with water, and oxides such as titania [15] and alumina [25,26,27] i n teracting with water or with organic molecules such as alcohols. Yet, the complication of chemisorption as higher and higher activation tem peratures are employed is not always quantitatively considered [25, 26,27]. A plea should be made that other techniques should be brought to bear on the problem whenever any unknown system is investigated. A well established heat of immersion value in itself is not sufficient. A s indicated by the classification, heat of immersion followed as a function of precoverage reveals far more about the wetting characteristics of the system. But very different studies are also helpful. F o r example,butyl chloride was earlier believed to stand upright in close-packed array on rutile, as does the hydroxide or amine [35]. Recent adsorption isotherms, however, indicate that it lies flat [13]. Infrared spectroscopy is a power ful tool, when the powder is sufficiently fine, in determining the number per unit area and the nature of the hydroxyls on silicas [12, 16, 20] and alumina [17]. Water vapor isotherms locate the number of hydrophilic sites on an otherwise hydrophobic surface, such as presented by graphitized blacks [32], flame-hydrolyzed silicas [35], or silver halides [10,38], and this is a useful technique even for rather low area solids. In addition, as determined by Kiselev et al. [1], ground quartz agglom erated in such a fashion that the ground samples yielded much lower nitrogen areas than water areas as estimated from the isotherms. In this case, the heats of immersion per unit area (based on nitrogen) would be higher than if the total area available to the water molecules was used. The large decrease in heats of immersion of the same oxide in water or hexane as a function of increase in surface area (decrease in particle size), as reported by Wade and Hackerman [25, 26, 27], cannot be explained by Kiselev's finding. In fact, the trend i s opposite in the case of the silicas, aluminas, and titanias which they investigated. It appears, instead, that the less amorphous, coarser granules orient several layers of adsorbate. F o r silica-water systems, for example, the difference between low and high area silicas can be 500 ergs per square centimeter or more. Such a difference amounts to 7 kcal. per mole, if all of the extra energy is ascribed to the first adsorbed layer. Contrariwise, this additional interaction energy maybe distributed over several layers. The suggestion is made that an icelike structure i s developed over the crystalline faces of the coarse particles. N M R might reveal such structure; Pethica [18] has found a surprising amount of icelike structure around micelles using this tool. Application of Heats of Immersion The average polarity of solid surfaces can be estimated from heats of immersion into selected liquids, usually η-butyl derivatives possessing



92

a single functional group. F r o m the slope of the line of heat of i m m e r sion (or better, heats of adsorption obtained by substracting the enthalpy of the liquids, h ) vs. dipole moment, the surface force field, F, emanat ing from the solid can be estimated. The first such curve [4] obtained for a rutile is given in Figure 3. Additional values reported by Zettlemoyer, Chessick, and Hollabaugh [35], by Romo [22], and by D e a r , E l e y , and Johnson [8] are listed in Table I. L


800i

ι

ι

0

I

Figure 3.

ι

ι

2 jjl 3 DIPOLE MOMENT

.

ι

4

5

Determination of polarity of solid surface (rutile)

Chemisorption must be avoided. Close-packed, perpendicular array and same distance from dipole to surface assumed. Chloride lies flat on this substrate [13]

Table I. Electrostatic Field Strengths, F , and Dispersion Energies, E w

Powder Rutile (bare?) (bare) ( A l 0 - S i 0 coated) CaF S i 0 (Aerosil) 2

3

2

2

2

2

o

3

Graphon Teflon Iron blue (high strength) Chrome yellow (med.) Barium lithol (str. med. tone) Carbon black (short channel)

B E T Area, Sq. M . / G .

F, E.S.U./Sq.Cm. χ 10 " 5

Ergs/Sq.Cm.

Ref.

7.7 6.4 11.2 12.7 120 0.4 95 9.0

2.7 2.0 3.2 2.5 1.1 1.9 0 0

145 125 135 105 75 335 80 25

Î3] [22] [22] [35] [35] [8] [3] [5]

87

2.2

105

[33]

1.9

105

[33]

41

(1.7)

105

[33]

120

0.7

105

[33]

6.7


5.

ZETTLEMOYER

AND CHESSICK

Wettability

Heats of Immersion

93

These results are not discussed in detail here. The A e r o s i l silica was 75% hydrophobic, as indicated by 100 x η ο / ν î thus, surface regions of very different polarity are averaged in the value of 1.1 x 10 e.s.u. per sq. c m . The graphitic surface of Graphon displays no metallic character toward the dipoles employed; graphite had earlier been classified with metals as to adsorption characteristics [2]. The dispersion energy for the alumina on aluminum is usually large, as discussed by Eley [8]. Our values for rutile are probably less reliable than Romo's, because it has recently been concluded that a truly bare rutile surface is exceedingly difficult to prepare. A most important application of the F values will doubtless be to compare different samples of the same material and to relate them to other properties such as flocculation tendencies in organic media. It would also be interesting to compare F values for the different surface area samples studied by Wade and Hackerman. Finally, very little application and analysis of the E values have yet been made; they were c a l culated by subtracting 25 ergs per sq. cm. for the enthalpy of the hydrocarbon from the intercepts. ς

ς

2

2


s

w

Heterogeneities in Site Energies These heterogeneities can be assayed by exploring the variation in heat of immersion with coverage. F o r example, the acid sites on cracking catalysts can be explored by following the heat of immersion into liquid butylamine as a function of precoverage with butylamine from the vapor state. Rather, the reverse is done and the powder is first saturated with the vapor, so that a l l the active sites are covered after pumping at 25°C. Typically, about 50% of the surface retains butylamine. To prepare successive samples for heat of immersion determinations, increasing temperature are employed for outgassing. The derivative of the heat of immersion curve provides a differential heat curve, and the differential of the latter curve yields, when inverted, an approximate site energy distribution [34]:

d(AH ) d

Two such curves are plotted in Figure 4. The catalyst with the larger number of low energy sites was also the most active in cracking. The original heat of immersion curves, rather than these derived curves, reflect the site energy distributions. In an interesting sidelight, it was found that the differential heat curve for a kaolin catalyst [34] contained a maximum at 0.4 0. This maximum was suspected to be caused by lateral interactions, even though 0 = 1 for butylamine c o r responded to 50% coverage. To explore this possibility, sterically hindered diethylamine and pyridine were employed as adsorbates. Only about 40% coverage was retained in these cases, suggesting strongly that the acid sites were close neighbors and so occurred in patches. Wetting Ability of Surfactants The wetting ability of surfactants for low energy surfaces shows great promise and has only begun to be explored [20,24, 36,37 ]. Graphon


94


•

25%

Al 0

3

•

13%

Al 0

3

20

2

40 KCAL/MOLE


2

BuNH

2

Figure 4. Site energy distribution from heats of immersion into n-butylamine as a function of precoverage 25% A 1 0 2

3

catalyst showed greater activity

has been employed as the powder to be wetted, because it presents a large and uniform surface. Measurements were first made on a series of surfactants to examine the utility of the method. Then, sodium dodecyl sulfate (NaDS) and sodium dodecylbenzene sulfonate (NaDBS) were studied in detail; the amounts adsorbed were also monitored as a function of concentration. The NaDS adopted two preferred packings, the closer one occurring at higher concentrations or when salt was added. When the heats of immersion were put on a molar basis, the values in Table II were calculated. Heats of dissolution were negligible. The lower energy value obtained at the closer packing apparently r e flects repulsion between the head groups. Table II. Surfactant Heats of Adsorption on Graphon A r e a per Anion, Sq. A .

-AH , Kcal./Mole

NaDS

37 67

7.6 9.5

NaDBS

48

8.8

W

If calcium is added to the solution, an even closer packing results. Trace calcium had a large effect on either the heats or the packing. Indeed, the Graphon flocculated markedly even at a few parts per m i l lion. To avoid traces of polyvalent ions, an all-plastic calorimeter was developed, as depicted in Figure 5. Hydrophilic Nature of Surfaces The hydrophilic nature of surfaces deserves separate attention, if only because water is omnipresent. Except for silica surfaces, little is yet known about the precise nature of its interaction with solids. Figure l , a , is exemplified by the interaction of water with hydrophilic asbestos [40]; the heat of immersion falls in equal increments, indicating


5.

ZETTLEMOYER

AND CHESSICK

Wettability

Heats of Immersion

95


A

Β

Figure 5. Plastic calorimeter for measuring heats of immersion of low-energy surfaces in surfactant solutions Multivalent ions avoided Sensitivity. 0.01 c a l . ± 3%, with electronic galvanometer

a homogeneous surface (for water), to the value of 118.5 ergs p e r s q . c m . , the enthalpy of liquid water. Figure l , c , typifies an essentially hydro phobic surface possessing a few hydrophilic sites; in the case of Graphon, about 1/1500 of the nitrogen area accepts water and the coverage curve rises from about 32 to 37 ergs per sq. c m . when equilibrated at 0.95 relative pressure before immersion [32]. Water interacts, in this case, more energetically with the precoated surface; however, the water is adsorbed in isolated patches. Polar sites provided by surfactants and the like on polymer and essentially hydrophobic organic coatings will interact with water in a similar manner. This interaction may be the precursor to undesired attack of ambients on surfaces, or it may be desired, as with cloud nucleants such as silver iodide [16,32,38]. Curiously, this solid is largely hydrophobic (75%), in contrast to the hydrophilic character previously assigned by cloud physicists [10, 32,38]. The nature of the interactions of water with oxides or hydroxyl groups on oxides remains largely unsettled. In the case of titania, it is not yet certain whether hydroxyls exist on the surface ( T i - O H in solu tion is unknown). A n intriguing break in the curve of heat of immersion vs. activation temperature, recently found, is depicted in Figure 6. It occurs between 120° and 1 3 0 ° C , and its magnitude is about 10%. The strange finding is that alumina-coated titania behaves in the same way; and so do other oxide-coated titanias. Other techniques are needed to help to decide whether the 120° to 130° range under vacuum separates physically adsorbed molecules from the surface and leaves onlychemisorbed molecules behind. When such a finding is made for each mate r i a l , it suggests an experimental artifact; none has yet been found. F o r example, no appreciable change in heat of bulb breaking was discovered in the 120° to 130° range. Some attempt has been made recently to separate the total heat of immersion due to changes in the adsorbent and that due to changes in



96


130

140

TEMP. ° C

Figure 6. Heat of immersion of rutile as a function of activation temperature Break found for bare surface as well as oxide coated rutiles. Upper curve. Alumina-coated rutile

the adsorbate for the B a S 0 / H 0 system [7,29]. entropy changes in both were obtained. 4

2

The free energy and

Solution Adsorption Studies Solution adsorption studies also deserve to be set apart. A start was made in 1956 by following 1-butanol adsorption out of water onto Graphon by heat of immersion measurements [31]. A model of preferential adsorption of the butanol, plus the measured adsorption isotherm and measured heat effects due to wetting of the adsorbed film at various pertinent concentrations, allowed the heats of immersion to be calculated. Interaction between molecules in the adsorbed film were taken to be the same as in the bulk solution. The calculated values were in excellent agreement with the experimental heats of immersion, as illustrated in Figure 7. No further studies of this kind have been performed on other functional groups, different chain lengths, or other molecular structures. An incremental analysis of the contributions of various groups can be made by the heat of immersion technique. Heat of interaction of adsorbed layers with the equilibrium solutions can be directly determined by removing the equilibrated solid from the solution, drying it, and then re-immersing it into the equilibrium solution in the calorimeter. Lateral interactions can be determined from measurements at low and high coverages. Heats of immersion have been useful in elucidating the adsorption of heptyl derivatives out of paraffin o i l onto a polar silica [28]. It was established that heptyl chloride is not adsorbed from solution in this system, and that the other polar derivatives, such as the alcohol and amine, form close-packed monolayers. Heats of formation of double layers are being determined [14] by comparing the heats of immersion of Graphon in solutions of fatty acids at high (double layer forms) and low pH (un-ionized).



5.

ZETTLEMOYER

AND CHESSICK

Wettability

Heats of Immersion

97

INITIAL CONC.-GM./100ML.

Figure 7. Heats of immersion of Graphon into 1-butanol in water solutions •

Experimental points Calculated on basis of simple model

Rovinskaya and Koganovskiï [23] have recently made an incremental analysis of the effects of functional groups on the free energy of adsorption of benzene derivatives from water solutions on charcoal. This effort suffers from the use of an inhomogeneous and porous substrate. Furthermore, wetting of the adsorbed films and lateral interactions were not accounted for. By measuring the required heat effects at several temperatures, using a more satisfactory adsorbent, the entire thermodynamic picture could be unfolded. Summary Applications of the heat of immersion technique to determinations of polarity of surfaces, site heterogeneities, wetting of surfactants, hydrophilicity, and the interaction of specific groups from solution with solids are on the increase. The technique is certain to provide new and valuable information about the solid-liquid interface in the near future.

Literature (1)

(2) (3) (4) (5) (6) (7) (8)

Cited

Aleksandrova, G. I., Kiselev, V. F., Maiorova, M. P., Colloid J. (USSR) 24, 3 (1962); J. Phys. Chem. (USSR), No. 9, 2031 (1961); No. 10, 2234 (1961). de Boer, J. H., Advan. Catalysis 8, 102 (1956). Chessick, J. J., J. Phys. Chem. 66, 762 (1962). Chessick, J. J., Healey, F. H., Zettlemoyer, A . C . , Can. J. Chem. 33, 251 (1955). Chessick, J. J., Healey, F. H . , Zettlemoyer, A . C . , J. Phys. Chem. 60, 1345 (1956). Chessick, J. J., Zettlemoyer, A . C . , Advan. Catalysis 11, 263 (1959). Copeland, L. E., Yound, T. F., Advan. Chem. Ser., No. 33, 348 (1961). Dear, D . J. Α., E l e y , D . D . , Johnson, B . C . , T r a n s . Faraday Soc. 59, 713 (1963).



98


(9) (10) (11)

Graham, D., J. Phys. Chem. 66, 1815 (1962) Hall, P . G., Tompkins, F. C., T r a n s . Faraday Soc. 58, 1734 (1962). Harkins, W . D . , Jura, G . , " P h y s i c a l Chemistry of Surface F i l m s , " p. 256, Reinhold, New York, 1952. (12) Hockey, J. Α . , Pethica, Β. Α . , T r a n s . Faraday Soc. 57, 2247 (1961). (13) Hollabaugh, C . M . , Chessick, J. J., J. Phys. Chem. 65, 109 (1961). (14) Iyer, S. R., Zettlemoyer, A . C . , Narayan, K . S., National Colloid Symposium, Ottawa, Canada, June 1963; J. Phys. Chem. 67, 2112 (1963). (15) Kiselev, Α . V., private communication. (16) Kiselev, Α . V., Lygin, V . I., Kolloidn. Z h . 22, No. 4 (1960). (17) P e r r i , J. B., Hannan, R. B., J. Phys. Chem. 69, 1526 (1960). (18) Pethica, Β. Α . , private communication. (19) P i e r c e , W . C . , Mooi, J., H a r r i s , R. E., J. Phys. Chem. 62, 655 (1958). (20) "Proceedings of Second International Congress on Surface Activity," V o l . 2, p. 204, Academic P r e s s , New York, 1957. (21) Razouk, R. I., J. Phys. Chem. 45, 190 (1941). (22) Romo, L. Α . , J. Colloid Sci. 16, 139 (1961). (23) Rovinskaya, T . M., Koganovskiĭ, A . M., Kollidn. Z h . 24, 67-74(1962); 23, No. 5 (1961). (24) Skewis, J. D . , Zettlemoyer, A . C . , "Proceedings of 3rd International C o n gress on Surface Activity," V o l . II, p. 401, 1960. (25) Wade, W . H., Cole, H . D . , Meyer, D . E., Hackerman, N . , Advan. Chem. Ser., No. 33, 35 (1961). (26) Wade, W. H., Hackerman, N . , Ibid., No. 43, 222 (1963). (27) Wade, W . H . , Hackerman, N . , J. Phys. Chem. 66, 1823 (1962). (28) Wightman, J. P., Chessick, J. J., Ibid., 66, 1217 (1962). (29) Wu, Y. C . , Copeland, L. E., Advan. Chem. Ser., No. 33, 357 (1961). (30) Young, G . J., J. Colloid Sci. 13, 67 (1958). (31) Young, G.J., Chessick, J.J., Healey, F.H., J. Phys. Chem. 60, 394 (1956). (32) Young, G . J., Chessick, J. J., Healey, V . H . , Zettlemoyer, A . C . , Ibid., 58, 313 (1954). (33) Zettlemoyer, A . C., Offic. Digest Federation Paint Varnish P r o d . Clubs 28, 1238 (1957). (34) Zettlemoyer, A . C . , Chessick, J. J., J. Phys. Chem. 64, 1131 (1960). (35) Zettlemoyer, A . C . , Chessick, J. J., Hollabaugh, C. M . , Ibid., 62, 489 (1958). (36) Zettlemoyer, A . C . , Schneider, C . H . , Skewis, J. D . , "Proceedings of 2nd International Congress on Surface Activity," V o l . III, p. 472, Academic P r e s s , New York, 1957. (37) Zettlemoyer, A . C . , Skewis, J. D., Chessick, J. J., J. A m . O i l Chemists' Soc. 39, 280 (1962). (38) Zettlemoyer, A . C . , Tcheurekdjian, N . , Chessick, J. J., Nature 192, 653 (1961). (39) Zettlemoyer, A . C., Young, G . J., Chessick, J. J., J. Phys. Chem. 59, 962 (1955). (40) Zettlemoyer, A . C., Young, G . J., Chessick, J. J., Healey, F. H., Ibid.. 57, 649 (1953).

Received May 6, 1963.


Contact Angle, Wettability, and Adhesion

Recommend Documents