-&° Colloids and SurfacesELSEVIER

A: Physicochemical and Engineering Aspects 90 (1994) 63-78

Superspreading of water-silicone surfactant on

hydrophobic surfaces

S. Zhu a,W.G. Miller b , L.E. Scriven a, H.T. Davis a,*

Department ofChemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455, USAb Department of Chemistry, University ofMinnesota, Minneapolis, MN 55455, USA

Received 15 January 1994; accepted 25 April 1994

Abstract

We have studied the spreading of aqueous mixtures of six different silicone surfactants on Parafilm, a hydrophobicsurface . Two of the surfactants were soluble in water and did not superspread . The other surfactants were insolubleand their aqueous dispersions superspread . Thus we found that a dispersed surfactant-rich phase is needed forsuperspreading and that the initial rate of spreading increases with decreasing size of the particles of the dispersedphase. We also found that the radius of the spreading drop varies as the square root of time during the initialspreading. This time dependence is consistent with but not unique to Marangoni flow . The initial spreading ratepasses through a maximum with increasing concentration, but the equilibrium area, i .e. the area wetted by the dropafter spreading has ceased, is proportional to the surfactant concentration .

When the spreading experiment was conducted in a dry atmosphere the dispersions did not superspread. Thedispersions were superspreaders at 100% humidity and spreading rates were even greater in supersaturated air . Thuswe found that the presence of water vapor is necessary for superspreading . We speculate that the water vapor providesa thin high tension film at the leading edge of the spreading drop, and so spreading is driven by a Marangoni effect,but we do not know whether a pre-existing film is formed by the vapor or whether it is recruited during spreading . Awater film mechanically deposited near a spreading drop accelerates the spreading in the direction of the mechanicallydeposited film .

It was initially thought that a hammer-like geometry was responsible for superspreading . However, Hill et al . (R.M .Hill, M . He, H.T. Davis and L .E. Striven, Langmuir, (10 (1994) 1724) found that an analogous linear trisiloxanesurfactant is a superspreader. The natural next question was whether surfactant-rich dispersions of the hydrocarbonpolyoxyethylenes (C.E.) are superspreaders . In separate work we studied a series of dispersions of both hammer-likeand linear hydrocarbon polyoxyethylene surfactants and found that none were superspreaders . Thus, among thesurfactants compared, we found that a silicone hydrophobe is required for superspreading, but the molecular geometryof the trisiloxane surfactant does not appear to be a critical parameter .

Keywords : Low energy surfaces; Silicone surfactants ; Spreading; Wetting

-Corresponding author.

0927-7757194,/$07 .00 0 1994 Elsevier Science B .V . All rights reservedSSDI 0927-7757(94)02904-7

COLLOIDS

ASURFACES

6 4

S. Zhu et allColloids Surfaces A : Physicochent Eng. Aspects 90 (1994) 63-78

1 . Introduction

plants. Silicone surfactants are found to be usefuladditives to water-based paints applied to soiled

The polydimethylsiloxane chain consists of the

surfaces and may also be quite useful in decontami-repeat unit

nation operations.Silicone surfactants that induce water to wet

hydrophobic surfaces are called "superspreadingagents" or just "superspreaders." As a workingdefinition, we say a surfactant is a superspreaderif the addition of a small amount, say less than0.1%, to a small droplet of water enables it, whenplaced on a hydrophobic surface, to spread into athin, wetting film within tens of seconds .

The superspreader first to receive significantscientific scrutiny was a trisiloxane surfactantknown as Silwet L-77 or SS1 . Zabkiewicz andGaskin [1], Gaskin and Kirkwood [2], andKnoche et al . [3] showed that the silicone surfac-tant Silwet L-77 or SSl can be used as an effectivewetting agent for water-based insecticides onplant leaves .

The molecular structure of SSI is

CH3

-Si-O-ICH3

where Si, C, H and 0 are silicon, carbon, hydrogenand oxygen atoms . Many silicone oils are polymersof the monomer shown here. The siloxane chain ismore flexible than a hydrocarbon chain, has lowercohesive energy, a lower surface tension and yet itis strongly hydrophobic and is as insoluble inwater as the analogous hydrocarbon chain .Because of their flexibility and low cohesive energythe silicones are liquid at higher molecular weightsthan the corresponding hydrocarbons . Since theliquids are also good lubricants the silicones arethe oils and greases of choice as vacuum pumpoils and glassware sealants . The class of siliconesand the variety of their properties and uses can begreatly enlarged by replacing some of the methylgroups of the repeat units by other chemicalgroups .

Attachment of a hydrophilic group to a siliconechain results in a very effective surfactant. Forinstance, a small amount of trisiloxane surfactantswhen added to water can lower the surface tensionfrom 70 to about 20 dyn cm- '. Only a few perflu-orinated hydrocarbon surfactants are as effectiveat lowering the surface tension of water. Thesilicone surfactants are milder physiologically thanhydrocarbon surfactants, and so they find use inhygienic and cosmetic products such as deodor-ants, face creams and detergents . The largest tech-nological use of silicone surfactants is as foamingagents in the manufacture of polyurethane foams .A novel application, and the one that provided themotivation for the research reported here, is as anagent enabling water to wet hydrophobic surfaceswithout, incidentally, inhibiting the tendency ofwater to wet hydrophilic surfaces . Thus siliconesurfactants can greatly improve the effectiveness ofaqueous sprays in delivering nutrients, insecticidesand herbicides to the hydrophobic surfaces of

CH31

CH3-Si-CH3IO

ICH3O(CH2CH2O)-t , 5CH2CH2CH2-Si-CH3

IO

ICH3-St-CH,

ICH3

The number of OC 2H, groups in the surfactantas synthesized varies; what is recorded is the meannumber. SSI can also be represented by the nota-tion M(D'E9OMe)M where Me denotes the chemi-cal group CH3 , M the group Me3SiO, D' the groupSi(Me)(C3H,), and E„ a sequence of n OC 2H4groups terminated with an OH group. If thesequence E„ is terminated by something other thanthe OH group, for example the methoxy groupOMe or the acetate group OAc, the end group isdenoted explicitly. In the abbreviated notation themean value of n is rounded to the nearest integer .It is characteristic of silicone surfactant solutionsthat they reduce the surface tension of aqueous

solutions markedly (to about 21 dyncm- ') andthey adsorb readily on low energy surfaces [4,5] .

Recently Ananthapadmanabhan et al . [5] car-ried out a study of the wetting, or spreading,behavior of several surfactant-water mixtures .They compared the spreading rates of SS1 aqueousdispersions on certain low energy surfaces, e .g .Parafilm (a paraffin wax surface), with the rates ofother surfactant solutions . Their results, summa,rized in Table 1, show that after 5 min the radiusof the circular area covered by a droplet of theSS1 aqueous dispersion is much larger than thatcovered by any of the other surfactant solutionsstudied. The results in Table 1 also show that thereis no correlation between superspreading and thesurface tension of the surfactant solution .

At room temperature, the density, viscosity, sur-face tension, self-diffusion coefficient and solubilityin water of SS l are about 1 .007 g cm- t, 21 cP,24 dyn cm - ', 3.2 x 10-' emz s- ', and 0.007 wt .%respectively. Ananthapadmanabhan et al. [5]showed that at 0.007 wt.% SSi or above, thesurface tension levels off at about 20 .5 dyn cm - ',

Table ISpreading ability of various aqueous surfactant solutionsrelative to water on a Parafilm surface [5]

S. Zhu et al. /Colloids Surfaces A : Physicochem. Eng. Aspects 90 (1994) 63-78

Measurements were taken 5 min after depositing the drops.Spreading experiments were performed in the open air and atroom temperature .' Surfactant concentrations were 0.1 wt.%.b Covered radius by solution/covered radius by water .` From DuPont Corporation .° From Union Carbide Corporation .` Sodium dodecyl sulfate.

65

and the turbidity of fresh samples increases rapidlybecause the surfactant forms a two-phase disper-sion at that concentration . They also found thatthe area per SS1 molecule at the liquid-vaporinterface is about 68 A' (as estimated from theslope in the plot of the surface tension versus thelogarithm of the bulk SS1 concentration at lowconcentrations). A more recent estimate is 59 A2

[6] .Ananthapadmanabhan et al . [5] studied the

kinetics of the adsorption of SS1 together with twoother silicone surfactants, SS2 and SS3, onliquid-air and solid-liquid interfaces. The lattertwo surfactants are not superspreaders . They mea-sured the time it takes for the surface tension(liquid-air) to regain its original value, after asurface layer of the surfactant solution (0 .00125%)has been removed by suction. They found that ofthe three surfactants, SSI, when depleted fromthe liquid-air interface, is replenished mostrapidly. Transient solid-liquid adsorption studiesof SSl and SS2, a mixture of SS1 andM[D'(E, . 5OMe)]ZM (or SS2) on polyethylenepowder (6 µm diameter particles) revealed that thetime required for the SSl dispersion to wet thepowder completely is very short, e .g. about 12 minfor a 0.01% dispersion . They concluded that theSSI dispersion has a higher mobility and a highersolid-liquid adsorption rate than the SS2 solutionat comparable concentrations .Ananthapadmanabhan et al. [5] proposed a

mechanism of superspreading of SS1 dispersionson low energy surfaces: a primary fihn (precursorfilm) spreads (over the "dry" Parafilm surface)ahead of the main surfactant drop . Because of fastsurfactant adsorption at the solid-liquid interface,the primary film is presumably more dilute withrespect to the surfactant concentration, and hencehas a higher surface tension than the rest of thedrop. The concentration gradient induces a surfacetension gradient which drives liquid flow in theprimary film in the direction of increasing tension .They proposed that the superspreading is relatedto the fast SS1 adsorption at the solid-liquidinterface and speculated that this fast adsorptionis related to the "hammer-shaped" moleculargeometry of the surfactant . Their schematic dia-gram of this mechanism is shown in Fig . 1 . They

System' Spreading factorb Surface tension(dyn cm - I)

Water 1 72.8SS1 8 .6 20 .5SS2 2 .3 23 .5Fluorocarbonsurfactant, FSA° 2 16 .2Fluorocarbonsurfactant, FSB° 1 .8 16 .8Non-ionic surfactant,Tergitol NP-10° 1 .7 31 .1Fluorocarbonsurfactant, FSN° 1 .4 23 .4SDS' 1 .2 44 .3

66

SS1 Molecule

Hydrophobic FIGroup

Pob'etherGroup

S. Zhu et at /Colloids Surfaces A: Physicochem. Eng. Aspects 90 (1994) 63-78

Leading Film

Air

Water + 551

5'J75'J7AA

Water + 5S1

Polyethylene surface

Fig. 1 . A schematic depiction of transfer of surfactant moleculesfrom the liquid-air interface to the polyethylene surface . Theprogressive advance of SSI solution can be likened to"molecular zippering" of the polyethylene-liquid interface [5] .

did not explain explicitly how the primary film isformed, and they attributed the superspreadingability of SSl aqueous dispersions uniquely to themolecular geometry of SS1 . The actual spreadingrates of the water-surfactant systems were notreported in their study . Moreover, they attributedthe large final wetting coverage, an equilibriumproperty, to fast spreading, a dynamic property .The two properties may be correlated in a givensystem, but in general it is important to studyspreading rate and ultimate coverage separately .

In actual spreading experiments, the spreadingliquid may spread on a "wet" surface or on a "dry"surface . Spreading of a non-volatile liquid on a"dry" surface takes place by surface diffusion(molecular hopping) and is a slow process similarto molecular diffusion in liquids [7,8] . For a liquidto spread rapidly, either a liquid-like film mustpre-exist on the hydrophobic surface, or such afilm must be deposited from the vapor phaseduring the spreading .

The issue of a "dry" versus a "wet" surface wasnot addressed in the previous work. Moreover,spreading rates and ultimate coverage were notexamined systematically . Also, it seemed to us thatalthough the hypothesis that the uniqueness of themolecular structure of SSI is responsible for itssuperwetting properties is attractive, it has notbeen proved nor has a spreading force been iden-tified to support the hypothesis. In view of theseissues, we have undertaken further studies of SS1and related surfactants to try to add to what hasbeen learned in the previous studies [5] .

We have carried out a quantitative study ofspreading rates (i .e . spreading area versus time andconcentration) of the SS1 aqueous dispersions onParafilm and a qualitative study of spreading ratesof a few other surfactants on the same surfaces,and have examined the effects of surface roughness,air moisture level and surfactant dispersed stateon the spreading rate .

2. Experimental

2.1 . Materials

SSl and SS2 were gifts from the OrganosiliconesDivision of Union Carbide Corporation . Theywere synthesized using the procedure described byBailey and Snyder [9] . M(D'E5 )M, M(D'Es)M,M(D'E12)M, and M(D'E8OAc)M were gifts fromDow Corning. M(D'E 5)M was about 99% pure.The active components were about 95% for SS1,M(D'E8)M, M(D'E12)M, and M(D'E8OAc)M.The number of E units ascribed to each surfactantis an average value (Gaussian distribution) exceptfor M(D'E5)M in which E 5 is almost correct.Millipore water was used . Formamide waspurchased from Sigma . Parafilm (a paraffin waxsurface) is a product of American National Can.The polystyrene surface was of the brand Sterile-Falcon, a circular disk with a cover. The surfaceswere left in the air and no special cleaning treat-ment was carried out before use .

2.2. Apparatus

A Sony 8 mm CCD-F36 video recorder wasused to record the spreading process. The optical

S. Zhu et at lColloids Surfaces A : Physicochem . Eng. Aspects 90 (1994) 63-78

microscope used was equipped with a lens of 400 xmagnification and connected to a 9 in monitor anda U-Matic 3/4 in VCR recorder . Contact angles,viscosities, surface tensions and turbidities weremeasured by means of a telescope goniometer,Cannon capillary viscometers, a Wilhelmy platein a Langmuir-Blodgett balance (KSV 5000)and a UV-visible spectrophotometer (ShidmazuUV-160) respectively. The roughness of theParafilm surfaces was determined by use of ascanning electron microscope .

2.3 . Spreading set-up and measurement

The spreading experiments were performed withthe set-up shown in Fig . 2. Except when experi-ments were done to study the effect of air humidityon spreading, the desiccator contained a reservoirof water and so the air was saturated (i .e . at 100%humidity) . The substrates are Parafilm sheets cutinto 4 x 4 in 2 squares. A grid placed underneaththe Parafilm was used to measure the coveredspreading area. The desiccator was covered byanti-light-reflecting glass with a small hole at itscenter for depositing the solution . The system

Fig . 2. Spreading experiment set-up .

(inside the desiccator) was allowed to equilibratefor about 30 min before introducing a spreadingdrop. All spreading experiments were performedat ambient temperature (about 25°C). The spread-ing liquids were slowly pushed out of a micropipetand then deposited onto the Parafllm . The distancebetween the drop just releasing from the tip of themicropipet and the Parafllm was about 3 mm . Thespreading process was recorded using the videorecorder and was illuminated with either fiberoptics lamps or ordinary light bulbs . Because thecontrast between the solution and the Parafllmwas small, advantage was taken of the reflectionfrom the boundary between the spreading solutionand the Parafllm . The spreading areas as a functionof time were then obtained by playing back thevideo tape on a TV set.

The S51 dispersions studied were all turbid, i .e .all were above the solubility limit of 0.007 wt.% .The SS1 dispersions were formed by adding SSIto the solvent (in most cases water) and shakingwith the hands or sonicating when stated . Theywere all used immediately (about 5-10 min) aftermixing .

3 . Results

3 .1 . Spreading area versus time and concentrationof aqueous dispersions of SSl

In this study a 0.0078 g drop of a freshly pre-pared (by hand shaking) 5S1 aqueous dispersionof known concentration was deposited on aParafilm sheet. The results are shown in Fig . 3 . Atall concentrations, the initial spreading rateincreased monotonically with time but approacheda constant value at long time (about 10 min) .Curve a in Fig. 4 shows the spreading areas at2 min as a function of SS1 concentration . Thisfigure shows that the spreading area is proportionalto the concentration C when C is less than about0.16%. However, with increasing concentration, the2-min spreading area passes through a peak valueof 15.9 cm' at 0.161%. The viscosity of the corre-sponding dispersions is shown in curve b in Fig . 4.For time t < 2 min and C<0 .16%, the spreading

67

68 S. Zhu et al. /Colloids Surfaces A : Physicochem. Eng. Aspects 90 (1994) 63-78

0 2 4

area A can be expressed as

A=are=(47t)C

(1)

where r is the radius of the spreading drop Fig. 5shows the equilibrium spreading area, i.e . the areaachieved at the apparent termination of spreadingof a 0.0078 g drop of SS 1 dispersion at variousconcentrations. Even though the initial spreadingrates of drops ofconcentrations greater than 0 .16%are smaller than that at a concentration of 0 .16%,the final equilibrium spreading area is larger thehigher the concentration. In fact, the equilibriumspreading area is proportional to concentration .The estimated thickness of the equilibrium filmfor a drop (0.0078 g) of a 0 .3% solution (whichhas a covered area of about 48 cm') is about1 .6 µm if it is assumed that the drop does not

t (min)

Fig. 3 . Spreading of freshly prepared SSI dispersions (drop weight, 0 .0078 g on Parafilm) .

6 S 10

lose or gain mass during spreading . A droplet ofpure SS1 placed on Parafilm did not spread atall, and so between 0 .3% and 100% SS1 thespreading area would pass through a maximum .We did not determine the concentration of such amaximum.

The effect of the volume of the spreading dropon the equilibrium (limiting) area was alsoexplored. It was found that halving the spreadingdrop volume (from 0.0078 to 0.0039 g) halved theequilibrium area . This result is consistent with ahypothesis that at equilibrium, virtually all thesurfactant is at the spread drop surfaces and thesurface densities are independent of drop size .Under this hypothesis, at equilibrium the areaobeys the formula A = mdC/(a, + a2 ), where ,d isthe drop mass, C the SS1 concentration, and a,

S. Zhu et at/Colloids Surfaces A : Physicochem Eng. Aspects 90 (1994) 63-78

69

and az the surface density of SS1 at the air-liquidand air-solid interfaces respectively . The hypothe-sized situation is depicted in Fig. 6.

In the hand-shaken SS 1 dispersion, viewed underthe optical microscope at 400 x magnification,many spherical droplets (SS1-rich phase) were visi-ble; their sizes ranged from about 0 .5 mm (orperhaps smaller) to a few micrometers . We usedthe optical microscope to monitor the behavior ofthe droplets as the spreading proceeded . In thiscase, the polystyrene surface, which is much moretransparent than the Parafilm surface, was used asthe substrate, and a 0.4% SS1 dispersion (handshaken) was used as the spreading liquid . It wasobserved that, as the spreading proceeded, thedroplets marched towards the spreading front (i .e.toward the contract line region) and elongated intolong tubes and then disappeared at the contractline. After spreading was completed, the interior ofthe liquid thin film was free of visible surfactantdroplets .We also observed that the SS1 dispersion

superspread at temperatures as low as 5°C, butwe collected quantitative data only at roomtemperature .

C, wt. % SS1

Fig . 4. Curve a, spreading area at 2 min ; curve b, viscosity of 551 dispersions .

3.2 . Effect of size of droplets of SS1 dispersed inwater

From the above results it appears that dispersedSS I droplets play an important role in the spread-ing process . If this is true, then the dispersed state(e .g. dispersed droplet size) may affect the spreadingrate. In order to examine this possibility, two 0 .1%SS1 dispersions were freshly prepared, one bysonication and the other by hand shaking, and thespreading rates and extents of these two dispersionson Parafilm were compared in Fig . 7. The soni-cated dispersion spread faster . The equilibriumspreading area, however, was about the same inboth cases. As in the case of the hand-shakendispersion, the spreading area initially increaseslinearly in time and then approaches a plateau .The initial rate of spreading (calculated fromthe linear region) for the sonicated system was0.2 cm' s - t, and for the hand-shaken system was0.08 cm' s - r, i .e. spreading was 2 .5 fold fasterupon sonication.

The viscosity of the 0 .1% dispersions was essen-tially that of pure water and neither it nor thesurface tension was altered by sonication . The only

70

S. Zhu et al;/Colloids Surfaces A : Physicochem Eng. Aspects 90 (1994) 63-78

0

Initial State

0.115 0 .1 0 .15

0.2

SSI wt%

Free from SS Idroplets

0.25

Fig. 5. Equilibrium spreading area vs. SSI concentration .

Dispersed 5S1-richdroplet

0.3

Air saturated with watervapor

Final State

Fig . 6. Schematic drawing of spreading of a drop of SS1 dispersion on Parafilm. When the concentration is 0 .3 wt.%, h isabout 1 .6 µm .

difference between these two solutions was in the droplets, some as large as a few micrometers,dispersed droplet size. Studies using the optical originally present in the hand-shaken solutionmicroscope revealed that the large SS1 dispersed

almost completely disappeared after sonication, i .e.

0.35

TTTTTTTTTTTTTTTTTTTI T 1111111111111111111)

h

S. Zhu et al./Colloids Surfaces A : Physicochem. Eng. Aspects 90 (1994) 63-78

0 100 200 300

t (sec)400 500 600

71

Fig. 7 . Effect of sonication on spreading rate of a 0.104 wt ,% S51 dispersion on Parafilm (drop weight, 0.0078g; initial rates :0 .20 cm2 s" ' (sonicated), 0.080 cm2 s - ' (hand shaken)).

the dispersed droplets are too small to see in theoptical microscope at 400 x magnification . Thesize distribution of the dispersed phase was mea-sured by dynamic light scattering (DLS) . CONTINinversion [10,11] of the intensity correlation func-tion revealed that the average dispersed dropletradius was about 1370 A in a 0.16% hand-shakendispersion and about 260A in a 0.16% sonicateddispersion (Fig. 8) . DLS studies indicated that thedroplet size distributions of hand-shaken 0.1, 0 .12and 0.16% SS1 dispersions were similar, and thesame is true for sonicated dispersions . In Fig . 8, Ddenotes the diffusivity deduced from DLS and q isthe scattering vector. The average drop radius wascalculated from Stokes' law, R=kT/6nltD, where p

is the viscosity of water, k is Boltzmann's constantand Tis temperature .

3 .3 . Effect of adding formamide (a water-structurebreaker) on spreading area

In this experiment, 0 .17% hand-shaken mixturesconsisting of SS1, water and formamide were used .Formamide is known as a "water-structurebreaker" [12], and so the effect of adding theformamide is to increase the solubility of SSI andhence reduce the amount of dispersed phase . Theturbidity and equilibrium spreading area were mea-sured at constant SS1 concentration (0.17%) andvarious volume ratios of water to formamide . The

72

20

100

60

20

10 100

S. Zhu et al /Colloids Surfaces A : Physicochem. Eng. Aspects 90 (1994) 63-78

1000

D q2 (I/see)

260n

10000

100000

Fig. 8. Particle size distribution of SSI dispersions (0 .16 wt.% .hand shaken vs . sonicated) as studied by DLS at a 90°scattering angle.

turbidity (actually absorbance) of the SSl disper-sions was measured with a UV-visible spectrome-ter at wavelengths ranging from 420 to 600 ran .Fig. 9 shows the plot of the turbidity (measured at450 nm) and the equilibrium spreading area versusthe ratio of water to formamide .

The surface tension of a 0.17% SS1 solution wasfound to increase only slightly from 21 dyn cm -1(at 100% water) to 21 .3 dyn cm -' (at 80% water)and to 21.6 dyn cm -1 (at 60% water) . Thus thedifferent equilibrium spreading areas are definitelycontrolled by the dispersed phase and not by thesurface tension .

3 .4 . Qualitative observations ofspreading ofaqueous mixtures SS1, M(D'E 5)M, M(D'E8)M,M(D'E80Ac)M M(D'E 12)M and M(D'E,e)M onParafilm

The spreading on Parafilm of the six differentsilicone surfactants M(D'E5)M, SSl, M(D'E8)M,M(D'EBOAc)M and M(D'E12)M, which all havethe same siloxane back-bone and are hammer-shaped molecules, was compared qualitatively atroom temperature under saturated water vapor .

0

0.1

0.2

0.3

0 .4

0.6

0.7

0.8

1

W ster/(Water+rormemide) (voVval)

Fig. 9 . Effect of formamide on solution turbidity and spreading(concentration, 0.17g SSl in 100 ml solvent (solvent=water+formamide)) .

All except MD'(E 12)M form turbid dispersions atlow concentration (typically about 0.01%) .MD'(E12)M is soluble in all proportions in waterat room temperature . The spreading action of thesesurfactants was compared at about the sameconcentration, namely 0.17 wt.% surfactant(freshly prepared by hand shaking) . The equilib-rium area on Parafilm decreased in the followingorder: SS1 (26.9 cm2 ; 0.174%), M(D'E5)M andM(D'E8OAc)M (about 23 cm 2 ; 0.17%), M(D'E8)M(about l6 cm' ; 0,178%), M(D'E12)M (below10 cm'; 0.17%). The spreading rate also followedthe same order. The spreading rate of theM(D'E12)M solution was significantly lower thanthat of the dispersions of the other surfactants . TheM(D'E 18)M solution did not spread at all. Whenspreading was tested on Teflon, there was nosignificant spreading of any of these surfactantsystems.

3 .5 . Effect ofsurface roughness on spreading rate

Most solid surfaces are rough in nature . TheParafilm surface is no exception . Under the optical

25

20

9e

IS m9

IaN10

0

S. Zhu et aL/Colloids Surfaces A: Physicochem Eng. Aspects 90 (1994) 63-78

73

microscope at 400 x magnification, its roughnesscan be seen readily. Johnson et al. [13] reportedthat the Parafilm surface has steps or depressionson the order of 0 .4 gm deep . A scanning electronmicrograph of the Parafilm surface showed thatan unstretched Parafilm is rough but a stretchedone is even rougher (Fig. 10) . The additionalroughness results from exposing the interior of theParafilm, and may account for the contact anglehysteresis shown by the Parafilm surface . We foundthat with pure water the contact angle of a recentlyadvanced droplet meniscus was O a =106°, and thatof a recently receded meniscus is O r =96° . Thesevalues agree well with the literature values of Oa=107° and Or =96° [13] . When the spreading front

(a)

(b)

Fig. 10. Scanning electron micrograph of Parafilm surfaces :(a) unstretched; (b) stretched.

of the surfactant-water dispersion was observedunder the microscope, it was noted that it did notmove smoothly : in some regions the front movedfaster and sometimes the liquid moved laterallyand then backward to the missed regions . To thenaked eye the covered area on Parafilm was some-times oval in shape.

A roughness index of a Parafilm surface can beestimated from our spreading experiments. Thearea occupied per SS1 molecule is probably about59 A2 at both the liquid-air and liquid-solid inter-faces . If this is so, then since the molecular weightof SS1 is 624, a 0.0078 g drop with 0 .1 wt.% SS1would cover an area of 22 .1 cm 2 on a flat surface .Because the observed equilibrium area on Parafilmwas about 15 .9 cm2 , the roughness index a is 1 .8 .This estimate is obtained from the formula

XAobs + Aobs = 2Acatc

which assumes that the liquid-air surface is smoothand the liquid-solid interface is rough .

An experiment was performed in order to exam-ine the effect of surface roughness on the spreadingrate: a piece of Parafilm was stretched to a surfacearea about 4-5 times larger than an unstretchedfilm, and the spreading on the stretched Parafilmwas compared with that on an unstretched film .The advancing contact angle changed no morethan 1-2° after stretching . A drop (0 .0078 g) of anSS 1 dispersion (0.16 wt.%) spread on the stretchedParafilm surface much faster and apparentlyreached spreading equilibrium in about 1 min,compared with about 4 min on an unstretchedParafilm. However, the final equilibrium spreadingarea on the stretched surface was only about halfthat on the unstretched surface . The comparisonis shown schematically in Fig. 11(a) . The smallercovered area at equilibrium on a stretched Parafilmindicates that the stretched Parafilm is rougherthan an unstretched one .

We also observed that a water drop was harderto drag along a stretched Parafilm than along anunstretched one, and that an SS1 dispersionspreads faster along a thin scratch (Fig . 11(b)) .

3.6 . Effect ofsurface moisture level on the spreadingrate

A drop (about 0.01 g) of pure SS1 surfactant(viscosity, 21 cP; y=24 dyn cm- ') was placed on

74 S. Zhu et a1/Colloids Surfaces A : Physicochem . Eng. Aspects 90 (1994) 63-78

Spreading $51 Solution

UnmeshedParafilm

(a)

Spreading 551Solution

Parafilm

Scratch

N

Fig. 11 . Effect of roughness on the spreading of SS I dispersionson Parafilm in a container saturated with water vapor.Spreading is faster on the stretched Parafilm . (a) StretchedParafilm vs. unstretched Parafilm. (b) Thin scratch vs .unscratched region . Spreading is faster along the thin scratch .

the Parafilm in a desiccator with dry air and anexcess of pure SS1 underneath . Observation bynaked eye found that the drop spread slowly andformed a lens of small diameter, about 0 .5 cm .However, when a drop of the pure SS 1 was placedon the Parafilm in the presence of saturated watervapor, a thin liquid film with different refractiveindex from the Parafilm started to appear slowly .After about 12 h, the drop still looked almostunchanged but there was a thin film, readily visibleto the naked eye, that had spread over all theParafilm surface. Thus the water vapor appears toaid the formation of the thin spreading film .

In another experiment it was observed that astretched Parafilm surface became foggy morerapidly than did an unstretched one when exposedto saturated water vapor at room temperature . Inorder to test the effect of water vapor on thespreading rate, spreading experiments on a soni-cated 0-16% dispersion were performed under dryair (a low humidity was enforced by dryers present

inside the closed desiccator), room humidity (desic-cator open to the room air), 100% relative humidity(closed desiccator with an excess of water inside)and supersaturated water vapor (hot vapor blowninto the desiccator). Superspreading was notobserved in dry air, whereas in supersaturated airspreading was even faster than the superspreadingobserved in saturated air. In the saturated airexperiment, visual examination (by reflectance) ofthe spreading front revealed a very thin and short(1-1 .5 mm long) precursor film ahead of fat fingers(Fig. 12(a)) at spreading equilibrium (or almostequilibrium) . This phenomenon was more obviouswhen the spreading was observed on a stretchedParafilm in saturated water vapor . The equilibrium(or almost equilibrium) thickness profile of the

Finger tip -Imm

A

Finger tip-1 mm

(a)

Center region Precursor film(1-1.5 mm)

(b)

Fig. 12 . Schematic drawing of a spreading film profile showing(a) fingers and precursor film and (b) side view of (a) along lineAtoB .

spreading drop was qualitatively examined by eyeon a stretched Parafilm. There is a very thin(precursor) film at the edge of the drop . There is athick region consisting of finger-tips, about 1 mmlong and 1 mm wide, and just past the finger-tipregion there is a large thin region in the center ofthe covered area which was about 3 cm in radiusin the case of 0 .17% SSI (Fig . 12(b)). The fingersand the thicker ring persisted with little change upto 10-30 min before these features flattened into apart of the thin film .

In another experiment a drop of pure water wasdragged on a Parafilm (without scratching thesurface) ahead of a drop of SSI aqueous dispersionwhich was spreading on the same surface in openair. It was seen that the SS1 drop spread faster inthe direction toward the water drop being draggedahead of it (Fig. 13) .

3 .7. Report of other relevant observations

In the case of the silicone surfactants studiedhere, superspreading occurs only when a dispersedphase is present. A natural question is whether thepresence of a dispersed surfactant-rich phase issufficient to drive superspreading. We repeated our

S. Zhu et al /Colloids Surfaces A : Physicochenc Eng. Aspects 90 (1994) 63-78

75

Fig. 13 . Dragging a tiny water drop ahead of a spreading SS 1dispersion.

spreading experiment on Parafilm with a mixtureof water and C 12E, at a surfactant concentrationhigh enough to have a dispersed phase at roomtemperature . A droplet of this dispersion placedon Parafilm forms a lens within a few seconds anddoes not spread further. Thus it is not enough thatthe system is a dispersion. However, C 12E4 is alinear molecule, which suggests the following ques-tion: Would the hammer-shaped analogue of SS1drive superspreading? He et al . [ 14] have examineda series of hammer-shaped C ;E3 surfactants whosebalance of hydrophobicity and hydrophilicity isexpected to be similar to that of the silicones . Nosuperspreading was observed . It appears that thereis something special about the silicone surfactantscompared with their hydrocarbon analogs.Another question then is whether the siliconesurfactants must be hammer-shaped to drive super-spreading. The answer is no . Hill et al. [15] foundthat an aqueous dispersion of a linear siliconesurfactant, similar in molecular weight and phasebehavior to SS1, is a superspreader .

3 .8 . Spreading on a hydrophilic surface

I

Troian et al. [16], in their study of sodiumbis(2-ethylhexyl)sulfosuccinate (AOT ; a surfac-tant) spreading on a clean glass surface prewet bya water film, suggested that there was a "primary"surfactant-rich film (maybe a few millimeters long)which spreads out circularly ahead of the fingeringfront (the apparent contact line) . The flow wascaused by the Marangoni effect [17] : the surfacetension gradient between the pre-existing waterthin film (higher tension) and the drop containingthe AOT surface (lower tension). The Marangonieffect refers to flow produced by variations insurface tension . The surfactant concentration gra-dient, and hence the surface tension gradient, wereestablished in the thinned primary film region . Asurface tension gradient, here produced by a gradi-ent in the surfactant concentration, caused a shearstress at the air-water interface (in the direction ofincreasing surface tension) which induced motionin the interface and the adjoining liquid layers .The spreading rate was observed to be faster whenthe moisture level (the pre-existing water film) onthe glass was higher .

76

We studied the spreading of an SS1 dispersionon a freshly cleaved mica surface or on a hydrophi-lie silicon wafer in air at 100% humidity . In thesecases, the spreading proceeds with fingering fronts,and the rate of area covering by a visible thin film(which may contain fingers) is much faster thanthe spreading rate on Parafilm (a few secondscompared with about 1 min) . Unlike the spreadingon Parafilm, the majority of the drop volume couldnot keep up with the spreading of the thin film . Itwas observed that at the end of the fast growth ofthe thin film, most of the liquid drop was still atthe center . An aqueous dispersion of C 12E5 behavessimilarly to the SS1 dispersions on hydrophilicsurfaces.

3 .9 . Climbing of SS1 solution on a flat Parafilmsheet and on a flat polystyrene surface

Finally, a spreading experiment was performedthat was rather different from the drop spreadingexperiments. The edge of a vertically held piece ofParafilm was dipped in an SS1 dispersion (0 .16wt.% SS1) and held there . A film, visible to thenaked eye, climbed to about 4 .5 cm on Parafilmwithin a few hours (Fig . 14) and to about 6 cm

4

3e

0

0 20 40

S. Zhu et a1/Colloids Surfaces A : Physicochem. Eng. Aspects 90 (1994) 63-78

60

80t (mi .)

IN 120 140

Fig . 14. Climbing of SSI dispersions on Parafilm .

within 30 min on polystyrene (at 5 s the film hadalready climbed to about 1 cm) . Pure SSl in thepresence of its own vapor did not climbthe Parafilm surface .M(D'E8)M was found to behave in the same

way as SSl. A 0.17% M(D'E8)M dispersionclimbed up the polystyrene surface with an initialrate of 2 cm per 20 s. It was also found that SS Idispersions climbed much faster on a stretchedParafilm than on an unstretched film .

4. Discussion

From the experiments of Ananthapadmanabhanet al. [5] and those reported here it follows that asurfactant-rich dispersed phase is necessary forsuperspreading . However, an aqueous dispersionof C12E4 is not a superspreader, and so a surfactant-rich dispersed phase is not sufficient for super-spreading. Hammer-shaped C 1EI molecules, ana-logous to the silicone surfactants studied in thispaper, do not drive superspreading [14] . Thismeans that a hammer-like molecular geometry isnot sufficient to assure superspreading. Moreover,an aqueous dispersion of a linear silicone surfactantsimilar in molecular weight and phase behavior toSS1 is a superspreader, which establishes that ahammer-like molecular geometry is not necessaryfor superspreading . Thus, a silicone hydrophobicmoiety may be necessary for a surfactant to be asuperspreader . It is further required that the solu-bility limit be small so that the surfactant is presentmostly as a dispersed phase. Addition of formamideto solubilize SSI suppresses the superspreadingability of SSI .

An important question is how does the leadingedge of a spreading aqueous film move so fast ona hydrophobic surface . If the surface is dry at theleading edge and the film has to advance by surfacediffusion of the surfactant, the process would bemuch too slow to explain the observed spreadingrates. Tiberg and Cazabat [18] have made ellipso-metric studies of spreading rates of the super-spreader M(D'E$ )M on dry hydrophobic and dryhydrophilic surfaces. They observed diffusivespreading and found diffusion coefficients between10-6 and 10-7 am2 s- t, values typical of molecular

S. Zhu et al../Colloids Surfaces A : Physicachem. Eng. Aspects 90 (1994) 63-78

diffusion on a solid surface . The implication ofthese results is that diffusivity film spreading wouldtake days or weeks instead of the tens of secondsor minutes observed in our experiments . We foundthat in dry air a drop of pure SS1 does not visiblyspread on Parafilm, but in humid air a thin filmcan be seen forming around the drop in severalminutes. After 12 h the drop looks almostunchanged, but a thin film can be seen coveringthe entire surface of the Parafilm .

Comparison of the spreading of drops of aque-ous dispersions of SS1 on Parafilm under dry,saturated and supersaturated conditions provesthat the presence of water vapor is required forsuperspreading . If one assumes that water vapordeposits a thin fluid film of water, then super-spreading can be explained as a Marangoni effect,i .e. as being flow driven by a surface tensiongradient. Marangoni flow gives an initial spreadingpower law of the form roct1 f 2 , but this spreadinglaw is not unique to Marangoni flow. We are alsonot aware of any proof of the existence of anaqueous fluid film on hydrophobic surfaces . Also,if a pre-existing film is there, why does Marangoniflow not occur for all surfactant dispersions. Afterall, on a moist hydrophilic surface, we find thataqueous dispersions of SS1 and C 12E, spreadalmost as rapidly as the AOT solution shown byTroian et al. [16] to undergo Marangoni flow .

An alternative hypothesis to a pre-existing waterfilm on the hydrophobic surface is that waterdeposits from the vapor at the leading edge of thespreading film. The process would be driven bythe adsorption and solution chemistry in the air-fluid-solid contact region . The pertinent transportrate would then be diffusion of water vaporthrough air, a much faster process than surfacediffusion . Unfortunately this hypothesis still doesnot explain what is special about the siliconesurfactants and the need for a dispersed surfactant-rich phase to drive superspreading.

Although at this point, we can only speculateabout the mechanism of superspreading, our obser-vations do prove that water vapor must be presentfor superspreading on hydrophobic surfaces.Whatever mechanism is proposed must squarewith this fact and with the facts that a dispersed

77

phase is required and that silicone surfactants, butnot their molecular geometry, appear to be unique .

5. Summary

A study of superspreading of liquids containingsilicone surfactants on hydrophobic surfaces isreported. Spreading rate measurements were madeon unstretched Parafilm (paraffin wax) surfaces at100% humidity with aqueous dispersions of aspecial silicon surfactant SS1 which forms a turbiddispersion at or above 0.007% SS1 (prepared byhand shaking SS1 and water) . The initial spreadingarea A the (area in square centimeters covered bya 0.0078 g drop of SS1 dispersion) was observedto be proportional to the SSI concentration C andtime t (0.007% < C < 0 .16%; t < 2 min) : A=47Ct .Although the spreading area at 2 min peaks at0.16%, the final (or equilibrium) spreading area isproportional to the SS1 concentration . A finedispersion of SS1 (prepared by sonication) spreadsfaster than a coarse dispersion (prepared by handshaking), but the equilibrium spreading areas arethe same. When the solvent (water) is replacedgradually by formamide, both the turbidity andthe spreading ability decrease despite the fact thatthe solution surface tension remain relativelyunchanged. Spreading comparison was made withother silicone surfactants of similar chemical struc-tures. It was found that the surfactants which formturbid dispersions in water are better spreadingagents than those that formed clear micellar solu-tions. Surface roughness and surrounding humiditygreatly affect the spreading rate . Although thespreading rate on a stretched (rougher) Parafilmsurface is faster than that on an unstretched one,the equilibrium area on the stretched Parafilmsurface is smaller than that on the unstretched one .Spreading was found to be faster when the humid-ity was higher. On the basis of the experimentalresults, we conclude that superspreading on hydro-phobic surfaces requires the presence of watervapor and a dispersed surfactant-rich phase . Thesilicone hydrophobic moiety also appears to benecessary for superspreading, but the moleculargeometry of the surfactant is not a critical factor .

7 8

Acknowledgments

We would like to thank Dennis Murphy forattracting our interest to superspreading behaviorand K.P. (Anath) Ananthapadmanbhan for interes-ting discussions concerning his and his colleagues'experimental work . We would also thank the NSFCenter for Interfacial Engineering for financialsupport. We are grateful to Yaqiang Ming for theelectron micrographs and to Randal Hill for severalhelpful conversations .

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