water interface

Colloids and Surfaces A: Physicochemical and Engineering Aspects 92 (1994) 221-229

COLLOIDS AND SURFACES

A

Orientation and reversible (?) transitions at incipient collapse of a polymer resin monolayer at an air/water interface * D.J. Morantz

1

Pira International, Randalls Road, Leatherhead, Surrey KT22 7RU, UK

Received 5 January 1994; accepted 16 May 1994

Abstract A modified resin derived from tree rosin is found to share features with lung surfactant in the compression/expansion hysteresis of their monolayers on aqueous subphases. During re-expansion of a compressed resin monolayer, an interesting pressure reversal occurs, as also reported for lung surfactant. Moreover, monolayer collapse pressures, for resin mixtures and alveolar surfactants, may exceed slightly the surface tension value for water, the overpressure arising possibly from nucleation. The natural materials need to maintain aqueous interfaces without failure. The lithographic process benefits from such properties conferred by resin to printing inks. A model for these hysteresis features considers molecular rearrangements and energy gradients within the monolayer and at air and subphase interfaces. The effects of isopropanol addition to the subphase support the model. Keywords: Air/water interface; Orientation;

Polymer resin monolayer; Reversible transitions

1. Introduction

Ink varnish (unpigmented ink) monolayer collapse occurs at pressures above the value for water surface tension, after a rapid increase in film compliance. Appropriate ink interactions with water are crucial to the success of the lithographic process. The ink varnish incorporates a rosin-modified phenolic resin, a derivative of natural tree rosin. It is significant and of considerable interest that the varnish shares its hysteresis and high collapse pressure features with lung surfactant. The useful properties of the rosin, from which the commercial product is derived, appear to relate to the tree’s * Presented at the Polymers at Interfaces conference, held Bristol University, ’ Present address: 3AR, UK.

8-10 September 1993. 1 Chestnut Lane, Sevenoaks,

Kent,

at

TN13

0927-7757/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved .5X)10927-7757(94)02938-O

requirement to repair breaches in its sap transpiration system. Alveolar surfactant must, likewise, ensure fatigue-free channels for aqueous lung fluids during respiration. The resin component is a key to the monolayer behaviour of the varnish. This study explores the hysteresis and collapse behaviour of such monolayers spread over purified water in a Langmuir trough. Reference is made here to isopropanol added to the subphase, which is of help in clarifying the mechanisms; other, ionic, additives to the subphase will be reported elsewhere. The polymer monolayer collapse mechanisms are postulated to occur via transient, constrained intermediate states, in transformations between “solid” monolayer and collapsed bulk phase. The model addresses the scope for reversible monolayer regeneration.

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Surfaces A: Physicochem.

2. Experimental A Laude Filmwaage FW2 Langmuir trough applies pressure up to a preset maximum, via a movable barrier, on a prepared monolayer on a subphase, and was set to operate at 25 + 0.5”C. A sensor on a fixed barrier measures pressure which is recorded versus film area in square centimetres, given here as the “normalised area”. Trough components in contact with the film and subphase are of PTFE. Sample aliquots, in Analar toluene as spreading carrier, are deposited on the subphase. A calibrated 25 ~1 microsyringe delivers several drops and the solvent is evaporated. The resin sample (RL43) weights deposited were 35.2 mg. A comparable resin content, 34.1 mg for the varnish samples is an estimate (using 117.4 mg of varnish, of formulation 14% RL43 plus 15% RL54, the remainder being low molecular weight (M, ingredients). The RL43 resin, designed for use in typical ink formulations, was a rosin-modified phenolic resin (DSM BV; M, = 17 000); RL54 resin is similar, but of higher molecular weight (DSM BV; M, = 40 000). The deionised (Elgastat Spectrum Millipore) water subphase was freshly purified. In some experiments 0.9% Analar isopropanol (IPA) was premixed in the subphase. The Laude trough has preset operations for sweep time, area swept, and delay time prior to reverse sweep. The minimum sweep time of 3 min was used; 5 min sweeps

Eng. Aspects 92 (1994) 221-229

gave no significant differences. The samples (see Fig. 3 and 4 below) were deposited on an initial trough area of 314 cm’, representing a surface dilution of 2510 A” per molecule; the “normalised area” units, in square centimetres each represent 8.0 A’ per molecule. Compression is seen to begin at 283 area units, corresponding to a molecular area of 2264 A’ and a film thickness of 12 A (assuming a density of 1). Varnish experiments (Fig. 1) used an initial trough area of 524 cm2.

3. Results Fig. 1 for the ink varnish illustrates the hysteresis features to be discussed. Experiments showed that these could be produced by the rosin-modified phenolic resin constituents. (The features of the other resin ingredient of the varnish, namely a long oil alkyd resin, were qualitatively different. Monolayers of that alkyd showed much less hysteresis, no pressure reversal, and collapse at around 30 mN m- ‘, when there was a slight indication that the “solid” region had been reached.) The collapsing varnish monolayer survived up to around 75 mN m- ’ (Fig. 2, curve A), sustaining compressional pressure beyond the surface tension value of water (73 mN m-‘) before film collapse. Fig. 1 shows continuous transitions through the “gaseous”, “liquid” and into the “solid” phases.

80 ,

(expansion -10

I

/

100

Fig. 1. Hysteresis

150

200 Normalised

for ink varnish

( 1 250 Area

on water (pause,

b 300

0.1 min; sweep, 3 min).

3

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D. J. Morantz/CoNoids Surfaces A: Physicochem. Eng. Aspects 92 (1994) 221-229

7060$

50-

:

40-

? 7

30-

f

20-

10-

-10 0

Fig. 2. Collapse

50

of ink varnish

100

150

I 200

250

300

Normalised Area monolayer:

The “solid” compression shows a fall-off from linearity, just below compression maximum. At the commencement of expansion, the measured pressure drop is not vertical. During the preset pause, between compression and expansion, the instrument records to a midway pressure point. When expansion commences, the subsequent pressure drop profile depends on experimental conditions. The two cycles, for varnish (Fig. l), show irreversible area losses, larger after expansion than at the end of compression, confirmed by experiments of up to nine sequential cycles and was reproducible for fresh samples. It is possible that the lost area represents transformation to bulk phase at the end of compression. Resin (Fig. 3(a)), in the course of three cycles, is not significantly changed at the compression limit whereas initial areas do decrease progressively. This is due possibly to residual solvent loss, subphase-soluble ingredients and/or some structural changes. At a point near to 50 mN m-l (Fig. 1) an increase in compressibility begins in a “solid” region of the isotherm, suggesting a second-order process proceeding, as seen in Fig. 2 (curve A), to collapse at the preset compression sweep maximum of 75 mN m- ‘. The rate of change in compressibility accelerates, at around 70 mN m-l, to a value maintained until collapse sets in. During the increased compressibility regime the nature of the sample changes, as demonstrated by an experiment per-

curve A, monolayer;

curve B, recycled.

formed immediately afterwards (Fig. 2, curve B), when the initial area is reduced to 60% of the original value. This irreversible reduction in monolayer area, during high compliance compression, indicates conversion to bilayer/multilayer or other bulk phases. Layer thickness changes may be estimated from the cross-sectional area coordinate (assuming negligible material losses and no significant density changes). Thus, prior to collapse (Fig. 2, curve A) the linear “solid” region (between 30 and 70 mN m-‘) extrapolates, at zero pressure, to 160 area units, and within the region above 70 mN m-‘, the thickness is estimated to increase through doubling and tripling before collapse at 50 area units. During the expansion component of the hysteresis cycle, pressure falls to a minimum and then rises to a local maximum before continuing to fall. This overshoot by several units mN m-i depends, as does the overall hysteresis, on sample and stress history. Local pressure minima in the expansion cycle for the varnish and for the resin diminish monotonically with increasing delay between compression and expansion. This is illustrated in Fig. 3(b) for the resin and is seen to be the source of the effects in the varnishes. Note the large reduction, of around 120-130 area units, sustained by these monolayers compressed to 20 mN m-l. The effect of isopropanol as a subphase component is noted here. Used in the printing process, it

D. J. Morantz/Colloids Surfaces A: Physicochem. Eng. Aspects 92 (1994) 221-229

224

-5 1 140

I 160

180

(4

200 220 240 Normalised Area

260

280

3

260

280

300

I

pressure decreases with pause duration -10 140 (W

,

160

180

200 220 240 Normalised Area

Fig. 3. (a) Hysteresis for resin on water (pause, 0.1 min; sweep, 3 min; T=25”C. (b) Effect of pause on resin hysteresis (pause, 0.1 min, 1 min, 2 min).

uniquely improves performance, affecting ink interactions with water. A mole fraction of 0.05 reduces surface tension of water from 73 mN m-l to around 30 mN m-‘. Isopropanol in the subphase reduces the initial area of the resin molecule by more than 25% (cf. Figs. 3 and 4). 4. Discussion

4.1. Hysteresis processes Work on hysteresis effects appears to have been limited to alveolar surfactants and analogues since

1964, when Mendenhall and Mendenhall [l] drew attention to surface tension recoveries of more than 10 dyn cm-’ during expansion and compression branches of a cycle. Watkins [2] confirmed that interrupted compression or expansion led to pressure reversals, achieving surface pressures on compression close to 72 dyn cm- ‘, the surface tension approaching zero. Such behaviour corresponds closely to the present varnish and resin findings. Tabak et al. [3], examining lung surfactant dipalmitoyl phosphatidylcholine (DPL), obtained a 17 versus A curve in interrupted mode with reversals, whose upper envelope resembles

D. J. Morantz/CoNoids Surfaces A: Physicochem. Eng. Aspects 92 (1994) 221-229

-&-v-

r

140

Fig. 4. Resin hysteresis

160

(subphase

225

I

180 200 Normalised

220

240 m

!8‘0

Area

0.9% IPA): effects of pause time and recycling.

that in Fig. 2 curve A. The respreading of collapsed alveolar monolayer was considered by Snik et al. [4,5] who suggested reversible molecular rearrangements, pursuing a discussion raised by Watkins. Joos et al. [6] in 1992 invoked the Braun-Le Chatelier principle to account for the reversals. They also concluded that their evidence points to rearrangements in the surface in a twostep time-dependent mechanism and not to collapse phenomena. 4.2. Molecular rearrangements and monolayer collapse mechanisms

Ries and Kimball [7] gave microscopic evidence for a collapse mechanism, starting as a folding bulge which fractures to a trilayer nucleus. Mason and co-workers [S] studied molecular inversion times, and Sims and Zografi [9] considered molecular interactions, polymorphic transformations and expulsion of molecules from a collapsing monolayer. Neumann [lo] introduced the role of stress fields, associated with crystallisation. Gabrielli and co-workers [ 11,13,14] reported studies on the collapse of polymer monolayers: PVA collapse [ 111 involved expulsion of small segments. Simultaneously, Zatz [ 123 drew attention to the possible formation of a threedimensional “overfilm”. Cellulose acetate work [14] confirmed mechanisms of nucleation and

growth. Smith and Berg [ 151 clarified the role of nuclei. Where the monolayer surfactants become liquid in the bulk, the collapse was instantaneous and reversible; however, films of solid surfactants collapse by one of two mechanisms: nucleation and growth, or plastic compressional fracture, the latter under severe conditions. Commenting on initial area losses, they suggested that timedependent structural rearrangements may occur in the monolayer to relieve surface inhomogeneities. In contrast with other films studied, DPL fractured above 70 dyn cm-’ showing no collapse, the only area loss being due to initial rearrangement. Note analogously the initial area losses for resin and varnish whereas it seems likely that varnish films sustain fracture, showing progressive area reductions at the end of compression, when taken up to subcollapse pressures of 40 mN m-‘. On interruption of compression or expansion, the pressure relaxes towards equilibrium values, dependent on surface area and rate of pressure change, in hysteresis cycles for DPL, resin or fatty acid films. For ink varnish or its resin component the processes have time constants of the order of minutes. The overshoot phenomena, described in the literature, appear to need further attention, especially for the surface pressure recovery during expansion. Local molecular rearrangements of and interaction with the subphase need consideration. If the overshoot is described as an elastic effect,

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this involves bulk material, being sustained over minutes. Molecular processes may occur over a distance in the horizontal plane, and/or in the vertical plane. Note the extensive molecular compression sustained by varnish and resin, compelling increasing monolayer film thicknesses and distancing the air interface as it approaches zero tension from its underpinning to the subphase. Interactions of ink varnish with water must take account of bulk and multilayer reorganisation in turbulent environments under printing press conditions. Mechanisms for transfer between monolayers and multilayers thus take on technological and theoretical significance. Malcolm [ 161 demonstrated reorganisation of monolayer regions to bilayers during travel of the movable barrier of a Langmuir trough; polymers orientated parallel to the barrier and tension/extension encouraged orientation towards the direction of motion. A study by Peng and Barnes [ 171 showed that a polymer monolayer of poly(viny1 stearate) develops a pressure gradient during movement of the barrier. Relaxation of the gradient may take several hours. Such a gradient, if applicable to other polymer systems, could seriously affect conclusions concerning polymer monolayer transitions and may contribute to the anomalous effect observed in the present study.

rigidity and facilitates weakens inversions. “Persistence length” is the idealised unit square edge of a flat interface; it increases exponentially with rigidity. Considering long-range interactions for lamellar phases, including lecithin/water systems, the interfacial tensions of the layers are believed by De Gennes and Taupin to be zero, rendering fluctuations in structural order much more important. Long-range forces would thus destabilise inclusion of a water layer in a “wet bilayer”. These ideas may be adapted to the resin/water interface: co-surfactant (IPA) might attack the interface and so reduce the persistence length, cause “wrinkling”, and allow other structures to intrude. Such wrinkling may also arise during stress on compression of an interface in a Langmuir trough. Such a stressed interface may enhance the IPA effect. We postulate that the subphase induces a series of transient structural forms, as thermodynamic states, during compression of the monolayer. Indeed the compression-driven approach to zero surface tension, in the somewhat unique cases of DPL and the ink varnish, would set the stage for a proliferation of transient states.

4.3. The subphase in monolayer transformations

Nikomarov [ 211 developed Ries’s microscopic model for the slow collapse of a monolayer, providing a theoretical description in terms of bulk-phase nucleation and growth. A liquid bulk phase cannot be compressed above its spreading pressure, yet, depending on the pressure increase rate, a monolayer may be transformed to solid bulk phase before collapse, above the spreading pressure. Pronounced hysteresis will occur when solid phases are involved. Monolayer collapse derives from growth of bulk phase nuclei formed only during monolayer preparation. The non-uniformities do not anneal out, and result from solvent deposition of the monolayer precursor. Hysteresis derives from elastic shear stresses in such nuclei and their surrounding monolayer. Such nuclei must be able to grow, leading to the suggested nucleus structure. Nikomarov’s model for a low molecular weight surfactant is sketched in Fig. 5(a). This is adapted

Solute molecules diffusing to the air interface modify surface tension more slowly than diffusion predicts. Blair [IS] proposed a potential energy barrier to the creation of the holes needed to accommodate solute. Similarly, withdrawal of head groups from the interface, under compression, would also require energy. DPL and the resin function in coexistence with aqueous subphases. This role does not seem to have been adequately considered. Chen et al. [19] have discussed the effect of water of hydration, comparing monolayers with bilayers and noting molecular “packing” in multilayer structures. A review concerning oil/water interfaces for microemulsions presented by De Gennes and Taupin [20] highlights correlation time, to which fluidity is inversely related. Cosurfactant enhances fluidity,

4.4. Microscopic models for monolayer transformations

D. J. Morantz/Colloids Surfaces A: Physicochem. Eng. Aspects 92 (1994) 221-229

.

- -

_ :

221

collapse pressures. The “solid” resin is expected to be amorphous; plastic flow should be the mode of compression, probably accompanied by orientation. Note that the monolayer has thickened by a factor of 3 when the collapse regime is reached for the varnish (Fig. 2). This could accommodate trilayer folds.

- Dislocation region is analogous to - Micellar region

4.5. Hysteresis properties of polymer monolayer: preliminary

head group

(‘4 Fig. 5. Trilayer nucleus: (a) for short alkyl chain molecules; (b) for polymeric molecules.

for a polymeric nucleus in Fig. 5(b) showing a fractured monolayer folded to a trilayer fragment. Monolayer curvature approximating the reciprocal of molecular length would correspond to that of a micelle, small in the case of the short-chain fatty acid species but larger for polymeric surfactant. The model fracture nucleus, pinned at the headto-head bilayer of polar groups, may slip between adjacent tail-to-tail hydrophobic bilayer planes. Dislocations can thus occur through slippage at hydrophobic planes. Since a nucleus extracts material to form more than one layer, elastic stress and plastic deformation ensue, possibly leading to fissures. These can be removed by the water subphase. The measured spreading pressure can exceed the equilibrium value by several millinewtons per metre whilst overcoming shear stresses inside and outside the nucleus. It may, alternatively, be diminished by the stresses caused by evaporation of solvent (from deposited solution). The relevance of Nikomarov’s small molecular species model depends on the states of the resin monolayer and the bulk phase into which it transforms near

Proteins (e.g. DPL) in native forms in the bulk unfold to cover large monolayer areas; timedependent behaviour is determined by the relative rates of applied compression versus refolding relaxation. In DPL experiments, pressure changes were applied “suddenly”, taking 3 min for the present hysteresis sweeps. The theoretical analysis by Joos et al. seems appropriate to the high compliance features for ink varnish; collapse results (Fig. 2) imply conversion to bulk via bilayer/multilayer phases. Orientation of polymer monolayers and pressure gradients may occur, caused by the moving barrier of the trough, where, after initial compression some monolayer area is not recovered on expansion (Figs. 1 and 3). Since this is limited to a small conversion it may imply that heterogeneities are consolidated for collapse. However, the orientation process and a viscosity-driven retardation of compression may also cause chain entanglements or weak bonding, without actively participating thereafter. 4.5. Compression: compliance, interface changes, monolayer collapse

The commercial resin is of high molecular weight with a limited number of polar groups. The initial long compliant compression sweep, at pressures up to 20 mN m-’ (Fig. 3), suggests that the hydrocarbon chains lie parallel to the interface (Fig. 6(a)) in the condensed state monolayer. Within this phase the hydrocarbon chains fold progressively into vertical orientations until the solid phase is reached (Fig. 6(b)), where the compliance falls and compression continues against stronger repulsive forces characteristic of a solid. The resin cross-sectional area in contact with the

D. J. MorantzlColloids Surfaces A: Physicochem. Eng. Aspects 92 (1994) 221-229

228 air interface

(4

head group

(‘4

Fig. 6. (a) Monolayer at initial compression of 2 mNm_‘. (b) “Solid” phase under compression of 30 mNm_‘. (c) Collapse initiating with compression of 70 mNm-‘. The relative lateral head group spacings and relative monolayer thicknesses are estimated from data of Fig. 2 (curve A).

subphase reduces to two-thirds of its area under a 20 mN m- ’ compressional pressure. Plastic yield sets in, near 50 mN m-l, as compliance increases. The head groups will be forced out of the subphase, embedding into the thickened monomolecular layer of folded alkyl moieties. If the Ries-Nikomarov nucleation mechanism is applicable (Fig. 5(b)) the high curvature may place the polymer under sufficient tension so as to fracture, yielding small chain fragments. These subsequently reorganise to produce the initial compression losses. At 60 mN m ~ ’ pressure, the varnish/air interface stability is reduced with spreading pressure reduction to 13 mN m-i (i.e. 73 - 60). The compression also destabilises the monolayer/subphase interface. Thermal factors become more important to the high energy interfaces, reducing their “persistence lengths”. Thus the planar spatial confinement of the head groups weakens as compression increases, reducing repulsive forces between head groups at the interface and enabling further growth of com-

pliance. Concurrently, the displaced head groups are in an internally stressed hydrophobic environment with enfeebled surface tension. Cosurfactant, reducing the subphase interface tension, lowers the compression pressure at which enhanced “solid” compliance is initiated. For ink varnish on water, this sets in at 60 mN m-‘; the addition of IPA to the subphase reduces this transition to 43 mN m-‘. Incipient monolayer collapse develops with increasing compliance, head groups being forced (Fig. 6(c)) into a hydrophobic environment. This corresponds to a continuous series of states, such as those shown in Figs. 6(c) and 5(b), ranging in conformations and energies. Conformational excursions progressively increase, whilst the polymer/air interface reduces to zero. Recall that Nikomarov estimated nucleus shear stresses of several millinewtons per metre, accounting for an increase in equilibrium spreading pressure. We can explain a collapse pressure of 75 mN m- ‘, equating that to the internal stress value plus the water surface tension. The air/resin interfacial tension becoming zero (plus or minus internal stress) defines a condition for no containing force. The surface must disrupt and film collapse proceeds. 4.6. Expansion: pressure reversal in hysteresis cycle The foregoing concepts and mechanisms, as well as those which follow below, may be expressed in a model based on energetic and entropic considerations. Thus during compression of the “solid” monolayer, before reaching the lowered compressibility region, reversible time-dependent incursions occur at the interface; these increase with compression. The energy being supplied will increase both the energy and entropy content. During expansion, a more ordered interface structure is restored, and entropy decreases; this concept applies to the anomalous pressure reversal. Head groups are forced away from their hydrophilic interface into a hydrophobic environment; the compression rate for the process and the point at which it is interrupted must determine the transient degree of imposed disorder. This imposed disorder is in part determined by surviving potential energy forces,

D. J. MorantzJColloids

Surfaces

A: Physicochem.

whilst the diminishing compression force remains. This will relax, with time constants determined by the molecular dynamics of the resin in its given configuration. The pause period between compression and expansion will determine the extent of that relaxation. During expansion the degree of order will increase in the subphase/headgroup divide; head groups remaining within the hydrophobic environment will, increasingly, diffuse back under free energy gradient forces. The force gradients are determined by hydrophobic repulsion, interface attraction and a lowering of entropy at the interface. The experimental evidence indicates that such rearrangements occur over several minutes. A time-dependent chemical potential variation exists across the monolayer thickness dimension and along horizontal planes. This generates a timedependent shear force within the monolayer, and viscosity components along the monolayer and within its thickness. This recalls the observations of Peng and Barnes [ 171. Such mechanisms are consistent with complete structural reversibility, making the (tautological) proviso that no irreversible molecular processes occur. Such irreversible processes are met in the transfer of monolayer material into bulk phases, and their widest definition includes the subphase as well as the air interface.

5. Conclusions A model for the time-dependent pressure reversal in resin monolayer hysteresis cycles draws upon the pulmonary surfactant and microemulsion literature. There may be a wider range of interesting and useful polymer surfactants possessing exceptional collapse resistance and viscoelastic properties. The three-dimensional nature of such vicoelastic monolayers needs emphasis, and the behaviour of IPA cosurfactant in the subphase reinforces a focus on the monolayer as possessing a gradient of properties, influenced by and extending into the phases by which it is bound. The cooperative dynamics of such polymer resin monolayers need exploration in the context of transient

Eng. Aspects 92 (I 994) 221-229

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chemical potential gradients, driven by dynamic mechanical changes at the boundaries.

Acknowledgements I thank Dr. Spencer E. Taylor of BP Research Centre, Sunbury-on-Thames, Middlesex, UK, for facilities for this work, Dr. John H. Clint, now of the School of Chemistry, The University of Hull, UK, for introducing me to Langmuir trough studies, and Mr. John Birkenshaw of Pira International, Leatherhead, Surrey, UK, for supporting the research programme.

References Cl1 R.M.

Mendenhall and A.L. Mendenhall, Jr., Nature, 204 (1964) 747. c21 J.C. Watkins, Biochim. Biophys. Acta, 152 (1968) 293. c31 S.A. Tabak, R.H. Notter and J.S. Ultman, J. Colloid Interface Sci., 60 (1977) 117. c41 A.F.M. Snik, A.J. Kruger and P. Joos, J. Colloid Interface Sci., 66 (1978) 435. c51 A. Boonman, F.H.J. Machiels, A.F.M. Snik and J. Egberts, J. Colloid Interface Sci., 120 (1987) 456. C61P. Joos, M. van Uffelen and G. Serrien, J. Colloid Interface Sci., 152 (1992) 521. c71 H.E. Ries, Jr., and W.A. Kimball, Proc. 2nd Int. Congr. Surf. Act., Vol. 1, 1957, p. 75. R.F. Robertson and S.G. Mason, Can. C81 W. Rabinovitch, J. Chem., 38 (1960) 1881. c91 B. Sims and G. Zografi, Chem. Phys. Lipids, 6 (1971) 109. Cl01 R.D. Neumann, J. Colloid Interface Sci., 56 (1976) 505. Cl11 G. Gabrielli and M. Puggelli, J. Colloid Interface Sci., 37 (1971) 503. Cl21 J.L. Zatz, J. Colloid Interface Sci., 37 (1971) 505. Cl31 G. Gabrielli and G.G.T. Guarini, J. Colloid Interface Sci., 64 (1978) 185. Cl41 P. Baglioni, G. Gabrielli and G.G.T. Guarini, J. Colloid Interface Sci., 78 (1980) 347. Cl51 R.D. Smith and J.C. Berg, J. Colloid Interface Sci., 74 (1980) 273. Cl61 B.R. Malcolm, J. Colloid Interface Sci., 104 (1985) 520. Cl71 J.B. Peng and G.T. Barnes, Langmuir, 6 (1990) 578. Cl81 C.M. Blair, Jr., J. Chem. Phys., 16 (1948) 113. Langmuir, Cl91 Y.L. Chen, C.A. Helm and J.N. Israelachvili, 7 (1991) 2694. c201 P.G. De Gennes and C. Taupin, J. Phys. Chem., 86 (1982) 2294. Langmuir, 6 (1990) 410. c211 E.S. Nikomarov,