Contact angles on particles and plates

Colloids and Surfaces, 27 (1987) 57-64 Elsevier Science Publishers B.V., Amsterdam

57 -

Printed

in The Netherlands

Contact Angles on Particles and Plates RUSSELL

CRAWFORD

Department of Applied Chemistry, Swinburne Institute of Technology, P.O. Box 218, Hawthorn, Melbourne, Victoria 3122 (Australia) LUUK K. KOOPAL* and JOHN RALSTON** School of Chemical Technology, South Australian Institute of Technology, P.O. Box 1, Zngle Farm, Adelaide, South Australia 5098 (Australia) (Received

4 November

1986; accepted

13 March 1987)

ABSTRACT Contact angles on partially methylated quartz plates and particles of known surface coverage have been measured as a function of surface coverage and compared with predictions based on the Cassie equation. Advancing and receding water-contact angles on quartz plates have been determined by the captive-bubble and sessile-drop methods. Advancing water-contact angles have been measured on quartz particles using the Washburn equation. Both advancing and receding contact angles on methylated quartz plates are in good agreement with predictions based on the Cassie equation. Advancing water-contact angles measured on methylated quartz particles are in good agreement with Cassie equation predictions up to surface coverages of about 70% and angles of approximately 70”, above which the measured angles are less than anticipated.

INTRODUCTION

The wettability of surfaces is of great importance in fields as diverse as froth flotation, lithographic printing, detergency, cell adhesion and the thromboresistance of biomaterials [ 11. Wettability is commonly studied by contact-angle measurements on flat surfaces, less frequently on particulate solids [ 21. Both surface roughness and surface heterogeneity characterize real solid surfaces. Cassie [ 31 originally suggested that the equilibrium contact angle of a smooth microheterogeneous surface consisting of a “patchwork” arrangement of two homogeneous elements could be described by cos 8, =f1 cos 8, +f* cos 02

(1)

*Permanent address: Laboratory for Physical and Colloid Chemistry, Wageningen, The Netherlands. **To whom all correspondence should be addressed.

0166-6622/87/$03.50

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58

where fi is the area fraction of the surface having an intrinsic contact angle 8, and f2 is the area fraction of surface with an intrinsic contact angle 13,. In general, COS 8,

=

i

f,

COS 8i

LSl

(2)

for an n region surface. The Cassie equation has been confirmed experimentally and used in various situations [ 4,6,7]. Significantly, Oliver et al. [ 51 showed that spherical drops of mercury obeyed the Cassie equation on a composite, parallel grooved surface and also demonstrated in their scanning electron microscope study that the intrinsic or microscopic contact angle of mercury located on grooved surfaces was equivalent to that on smooth ungrooved surfaces [ 111. Huh and Mason [ 81 modified Wenzel’s original roughness equation [ 9,101 to account for the case of random surface roughness, relating the average apparent contact angle, 0,, to 8, (contact angle on a molecularly smooth surface), roughness r, and a surface texture factor @ cos 0,=cos

8, [r+ (r-l)@]

(3)

where rs 1. Two limiting cases of surface roughness and their effect on spreading were considered: spiral grooves producing a stick-jump contact line of movement and radial grooves on which contact-line movement was nearly reversible. Other forms of roughness (e.g., bead-blasted) showed behavior which varied between these limiting cases. For a Gaussian distribution of roughness height, Huh and Mason found that when the drop size was large compared with the roughness, $ approached zero and the original Wenzel equation was recovered on average. The determination of contact angles on particles is of key interest in the majority of practical situations. Compression of particles into a pellet and treatment of the latter’s surface in a similar fashion to flat surfaces present problems such as liquid absorption, porosity, and considerable surface roughness. The dynamic contact-angle technique is likely to be more reliable. Originally proposed by Washburn [ 121, it was modified by Studebaker and Snow [ 131, used by others (e.g., Ref. [ 141) , examined theoretically by Good [ 151, and tested stringently by Fisher and Lark [ 16,171. The driving force for penetration of liquid into a dry packed bed of particles is the capillary pressure across the liquid-vapour interface. The rate of liquid entry into a single capillary of circular cross-sectional radius r is given by the Washburn equation l2 -t-

r yL/v cos 8 2r/

(4)

59

where 1is the depth of the penetration in time t, q is the viscosity of the liquid, yLiv is the surface tension of the liquid and 0 is the equilibrium contact angle. In Eqn (4 1, gravitational effects have been neglected. For a packed bed of particles, the radius may be replaced by a factor Kwhich is a composite parameter composed of an effective radius for the bed together with a tortuosity factor which accounts for the complex pathway formed by the channels between the particles. Hence

For a particular, fixed packing of particles in a column, K will be a constant and a plot of l2 versus twill be linear [ 121. If a wetting liquid is used (for which cos 0 = 1) , a value of K will be obtained, enabling values of 0 to be determined for other liquids. For a liquid spreading over a solid surface which is not initially in equilibrium with the saturated vapour, the initial driving force for the liquid flow will be larger than that described by Eqn (5)) i.e., the “dry” solid will wet faster. Good [ 151 showed that the maximum rate of penetration is given by

where D, is the equilibrium spreading pressure and IY&=,is the spreading pressure when the solid surface is in equilibrium with whatever gases are present at zero time. To date there has been no stringent comparison of contact angles measured on plates and particles which have been chemically treated under the same conditions to yield surfaces of varying, known surface coverage with hydrophobic groups. We have recently developed a quantitative technique [ 71 whereby the surface of quartz particles and plates may be tailored to various known surface coverages with trimethylsilyl groups via a methylation reaction with trimethylchlorosilane. A specific quantity of strongly bound surface hydrophobic groups per unit mass of solid is obtained, i.e., the groups are not desorbable as in the case of conventional surfactants, circumventing the difficulties which plague dynamic contact-angle measurements. In the case of such surfactants, the equilibrium technique proposed by White [ 18,19] may be inherently more accurate. In this study we report the results of contact-angle measurements on quartz plates and particles which have been methylated under the same conditions and compare these results with predictions based on the Cassie equation.

60 EXPERIMENTAL

Analar-grade or equivalent chemicals and conductivity water (conductivity ~0.8xlO-” 52-l cm-‘, pH 5.620.1, y=72.8 mN m-’ at 20°C) were used throughout this investigation, unless otherwise stated. Experiments were performed at 25°C. Optical grade quartz ( G.Bottley Pty Ltd, London, U.K. ) was crushed, sized, and methylated to produce angular, rough particles of varying, known surface coverages using procedures previously described in detail [ 71. Quartz plates were polished so that there was no detectable surface roughness at 250x magnification and to better than one-quarter of a fringe as judged by interferometry. These plates were treated and methylated in an identical manner to the quartz particles. In fact they were methylated along with the particles in the same reaction vessel, but were kept isolated from the particles so as to avoid possible surface abrasion. Contact angles were measured on the quartz plates by both the captive-bubble and sessile-drop techniques using standard procedures [ 1,2,10,21]. Droplet and bubble diameters ranged from 2 to 4 mm. Measurements were performed inside a thermostatted, enclosed glass cell. The advancing and receding contact angles of water measured against air were recorded on photographic film; each individual angle was determined by direct measurement from the image with a precision of IT2 ‘. Each angle reported is the average of at least four separate measurements on at least two different plates produced in different methylation experiments. In order to determine contact angles on the quartz particles, the rate of liquid movement through a uniformly packed bed of particles held in a narrow size range in a cleaned glass column (diameter 0.5 cm) was measured relative to a calibrated background by visual observation of the wetting front [ 14,20,21]. Cyclohexane and toluene were used as wetting liquids. Viscosity and surface tension data were taken from the literature and checked experimentally. Particular care was taken to ensure reproducible packing of the quartz particles. The influence of the ( fl,- I&,,,) term in Eqn (6) was assessed by exposing the quartz particles to a saturated vapour of the relevant liquid for at least one hour, packing the particles in a column, and then storing the packed column in this saturated environment for a further four hours prior to the measurement being performed. Two particle size.fractions were examined, of average size 37 and 46 ,um. The advancing-water contact angles reported are the average of at least four separate determinations on at least two different columns containing particles produced in different methylation experiments. RESULTS AND DISCUSSION

The most reliable measurements of the contact angles of water measured against air [ 6,22,23] for fully methylated, nonheat-treated quartz give an

61

-Cawe

EcluatiOn

0 Advamng

AREA

FRACTION

SURFACE

OF TMS CROUPS

COVERAGE

CW

Fig. 1. Advancing and receding water-contact angles on smooth quartz plates, as a function of degree of surface methylation, compared with Cassie equation predictions.

advancing angle of 88’ and a receding angle of 72 ‘. The advancing contact angle of water for a paraffinic surface containing only exposed methyl groups is 110 -t 2 ’ [ 24,25 ] , whereas the receding angle is 88 2 10’ [ 24,25,27,28] . The contact angle for a pure cleaned quartz surface with predominantly exposed polar groups is zero [ 261. From Eqn (2) it may be shown that the maximum area fraction of the surface covered by trimethylsilyl groups is 0.72+0.10, a result which is also consistent with other data [ 71. The predicted dependence of contact angle on surface coverage of trimethylsilyl groups (100% corresponding to an area fraction of 0.72) for both advancing and receding watercontact angles is given by the full lines in Fig. 1. The abscissa is calibrated in terms of percentage surface coverage and area fraction for ease of reference. As noted previously [ 71, the percentage surface coverage of trimethylsilyl groups on the quartz surface is given by r (CH:r):@, X 100 [r

(CH.3

1

) :zS, plateau

is the adsorption density corresponding to the maxi[ r ( CH:I ) :)s, 1p~ateau mum surface coverage for a fully methylated surface (area fraction of 0.72) and I’ (CHZIjZIS, is any other exprimental adsorption density. Experimental advancing and receding water-contact angles are shown for quartz plates in Fig. 1. Within experimental error, there is a good agreement between the angles predicted from the Cassie equation and those determined experimentally. The measured advancing and receding contact angles for fully methylated quartz plates, i.e., 88” and 72’) respectively, agree with other comparable studies [ 6,22,23]. For quartz particles, plots of 1’ versus t [ refer to Eqn ( 6 j ] were linear, passing through or close to the origin, for cyclohexane, toluene, and water. Presaturation with vapour had no detectable effect on the rate of wetting; hence the where

62

I -Cawe

EquatlOn

100

t

qDry Powder

AREA

FRACTION

OF TMS

GROUPS 2 cl

SURFACE

COVERAGE

C%)

Fig. 2. Advancing water-contact angles on quartz particles, methylation, compared with Cassie equation predictions.

as a function

of degree of surface

(IT,- IT,,,) term is negligibly small. Measured advancing water-contact angles on quartz particles of varying, known degrees of methylation are shown in Fig. 2 where they are compared with values predicted from the Cassie equation. There was no effect, within experimental error, of variation in particle size on contact angle. Note that receding contact angles cannot be determined by this rate-of-wetting technique. For particles with surface coverages from zero to about 70% (area fraction 0.50)) there is good agreement between the experimental and predicted values. From observation under a scanning electron microscope, it is clear that the quartz particles posses rough surfaces and are angular in shape. The results indicate that when a moving wetting front whose rate of movement is described by the Washburn equation encounters a statistical assembly of such particles packed into a column, the measured contact angle agrees closely with that determined on a smooth surface. Such a macroscopic measurement may mask the microscopic events which take place on individual particles as the wetting front passes over them [ 5,8]. At surface coverages above 70%, the measured contact angle is less than the predicted value. For example, for fully methylated particles, the measured angle was 72’ against a predicted value of 88”. The value of 72 ’ was reproduced even when particles were methylated in a concentrated vapour of freshly distilled trimethylchlorosilane. These lower angles correspond to water-wetting rates which are greater than anticipated. The precise mechanism responsible for this effect is not clear but may be contributed to by capillary condensation ahead of the wetting front and/or packing and roughness effects enhancing movement of the contact line compared with a smooth surface [ 81. We will report the results of contact-angle measurements on similarly hydrophobized smooth and angular particles by both the rate-of-wetting and equilibrium techniques [ 18,191 at a future date. Such data will aid in the resolution of this issue. For both the quartz plates and particles, a contact angle of 0’ is reported for

63

surfaces which have not been methylated. There was no detectable bubble cling or particle “pickup”. Excellent wetting fringes were also observed on clean plates. It is recognized [ 291 that a distinction between an angle of 0’ and, say 2 ’ or 3’ is very difficult. The present techniques are certainly not accurate or precise enough to do this. Nevertheless, our observations do not enable us to assign other than a zero contact angle to such clean surfaces. SUMMARY AND CONCLUSIONS

Water-contact angles have been measured on quartz plates and particles which have been methylated under the same conditions. Advancing and receding water-contact angles determined on quartz plates as a function of degree of surface methylation are in good agreement with those predicted from the Cassie equation. Advancing water-contact angles determined on quartz particles as a function of a degree of surface methylation are also in good agreement with Cassie equation predictions up to surface coverages of about 70% (area fraction of trimethylsilyl groups equal to 0.50). At higher surface coverages, the measured contact angles are less than anticipated from the Cassie equation. For surface coverages between 0 and about 70% and angles between 0” and approximately 70”, advancing contact angles measured on quartz plates and particles methylated under the same conditions are in good agreement with each other. ACKNOWLEDGMENT

Financial support for this project Scheme is gratefully acknowledged.

from the Australian

Research

Grants

REFERENCES

8 9 10 11 12

J.F. Padday (Ed.), Wetting, Spreading and Adhesion, Academic Press, London, 1978. R.J. Good, Surf. Colloid Sci., 11 (1979) 1. A.B.D. Cassie, Discuss. Faraday. Sot., 3 (1948) 11. A.B.D. Cassie and S. Baxter, Trans. Faraday. Sot., 40 (1944) 456. J.F. Oliver, C. Huh and S.G. Mason, J. Adhes., 8 (1977) 273. R.M. Lamb and D.N. Furlong, J. Chem. Sot. Faraday. Trans. 1,8 (1977) 61. P. Blake and J. Ralston, Colloids Surfaces, 15 (1985) 101. C. Huh and S.G. Mason, J. Colloid Interface Sci., 60 (1977) 11. R.N. Wenzel, Ind. Eng. Chem., 28 (1936) 988. R.E. Johnson and R.H. Dettre, Surf. Colloid Sci., 2 (1969) 85. S.G. Mason, in J.F. Padday (Ed.), Wetting, Spreading and Adhesion, Academic Press, London, 1978, Ch. 14. E.W. Washburn, Phys. Rev., 17 (1921) 273,374.

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