Surfactant adsorption at superhydrophobic surfacesMichele Ferrari, Francesca Ravera, Silvia Rao, and Libero Liggieri

Citation: Applied Physics Letters 89, 053104 (2006); doi: 10.1063/1.2226771 View online: http://dx.doi.org/10.1063/1.2226771 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/89/5?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Coarse-graining MARTINI model for molecular-dynamics simulations of the wetting properties of graphiticsurfaces with non-ionic, long-chain, and T-shaped surfactants J. Chem. Phys. 137, 094904 (2012); 10.1063/1.4747827 From superhydrophobic to superhydrophilic surfaces tuned by surfactant solutions Appl. Phys. Lett. 91, 094108 (2007); 10.1063/1.2779092 Adsorption of Janus particles to curved interfaces J. Chem. Phys. 127, 054707 (2007); 10.1063/1.2756828 Effects of surfactant on droplet spreading Phys. Fluids 16, 3070 (2004); 10.1063/1.1764827 Surfactant induced Marangoni motion of a droplet into an external liquid medium J. Chem. Phys. 107, 630 (1997); 10.1063/1.474423

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APPLIED PHYSICS LETTERS 89, 053104 �2006�

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Surfactant adsorption at superhydrophobic surfacesMichele Ferrari,a� Francesca Ravera, Silvia Rao, and Libero LiggieriCNR-Institute for Energetics and Interphases, Via De Marini 6, I-16149 Genoa, Italy

�Received 10 March 2006; accepted 25 May 2006; published online 31 July 2006�

Here the wetting of ionic and nonionic surfactant solutions on a superhydrophobic interface,obtained by using a mixed inorganic-organic coating providing a fractal-like structure, has beeninvestigated to define the role of adsorption at these interfaces. The presence of the amphiphilicmolecules is effective in lowering the contact angle in a hydrophobic or still superhydrophobicrange. Influencing the adsorption properties by acting on the surfactant type and concentration, thespreading of a drop on the surface can be controlled by amphiphiles confining its volume in a

restricted space. © 2006 American Institute of Physics. �DOI: 10.1063/1.2226771�

Spreading control of liquids is an uprising field in mi-crofluidics, printing, and everywhere a solid substrate has tobe selective with different liquid phases. In the field ofmicrofluidics,1 for instance, small quantities of liquids mustbe addressed through microchannels to specific sites wherethey will be processed. To achieve this goal the capillaryproperties of these systems need to be carefully understood.Microfluidic applications involving surfaces with highchemical inertia are very critical. Indeed these surfaces arenormally associated with strong liquid repellence, whichmakes wetting and spreading very difficult to control. Asusual for capillarity-related applications, specific surface ac-tive amphiphiles can be efficiently added to the liquids totune the wetting and spreading on these interfaces. The de-scription of the properties of surfactant solutions on hydro-phobic or superhydrophobic solid surfaces, however, doesnot yet have a wide literature, providing only a partial over-view in the field.2,3

Conventionally, a surface shows superhydrophobic prop-erties when the contact angle �CA� with water is very high�generally �150°� with a low hysteresis. These features areobtained by a synergic effect of low surface energy and suit-able surface morphology and roughness.

The effect of surface roughness on wetting is accountedby the model developed by Wenzel,4 where it is assumed thatthe space between the protrusions on the surface is filled bythe liquid. The apparent ���� and thermodynamic ��� CAs arethen linked by

cos �� = r cos � , �1�

where r is the ratio between the true surface area and itshorizontal projection. This regime provides hydrophobic in-terfaces with contact angles below 120°, however, it cannotgive rise to superhydrophobicity.

Superhydrophobicity can be, instead, understood underthe Cassie-Baxter model, which suggested that the surfacetraps air in the hollow spaces of the rough surface,5,6 with thelink between the CAs given by

cos �� = f1 cos � − f2, �2�

where f1 is the fraction of liquid area in contact with thesolid and f2 is the fraction of liquid area in contact with thetrapped air.

a�Author to whom correspondence should be addressed; electronic mail:

m.ferrari@ge.ieni.cnr.it

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In this work the wetting behavior of surfactant solutionson a superhydrophobic interface have been investigated inorder to define the role of amphiphilic molecules in modify-ing, by adsorption, surfaces with particular microstructuregeometry such as those showing high water repellence.7 Thesuperhydrophobic surface has been prepared by means of theoriginal methodology described elsewhere8 based on thedeposition of a silica particle/fluorinated polymer coating.Contact angles of pure water on such interface are of theorder of 170°, with hysteresis �2°.

The wetting properties of different surfactant solutionson such interfaces have been studied as a function of theconcentration and of the presence of salts. Investigated sur-factants have a similar hydrophobic linear alkyl chain anddifferent head groups, ranging from nonionic C10E8�octaoxyethylene n-decylether�, semipolar C12DMPO�n-dodecyldimethylphosphine oxyde�, and ionic sodiumdodecylsulphate �SDS� and cetyl trimethylammonium bro-mide �CTAB�.9,10

In order to investigate the adsorption effect at the solidsurface starting from the same number of molecules in thebulk, for all these surfactants two concentrations have beenchosen: one diluted below the critical micellar concentration�cmc� �2.0�10−5M�, and one above the cmc �c=2cmc�. Theionic surfactants have been studied also in the presence of20 mM of NaCl. Table I reports the corresponding equilib-rium surface tensions measured by a pendant droptensiometer.11

Wetting was investigated by measuring the advancingand receding CAs by a similar drop shape tensiometer. Mea-surements were performed after achieving adsorption equi-librium at the liquid-air interface, the corresponding time be-ing evaluated via dynamic surface tension measurements. Astainless steel capillary with a diameter of 0.21 mm con-nected to a syringe pump is used to deposit a sessile drop of

TABLE I. Critical micellar concentration �cmc� and equilibrium surfacetension ��� of the investigated surfactant solutions.

cmc �M� � �2.0�10−5M� �mN/m� � �2cmc� �mN/m�

C10E8 1.0�10−3 58.5 34.0C12DMPO 1.0�10−3 60.0 33.0SDS 8.0�10−3 72.0 37.5SDS+NaCl 5.1�10−3 71.5 36.0CTAB 9.0�10−4 65.0 37.0CTAB+NaCl 5.0�10−4 60.0 37.0

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about 5 mm3 on the solid interface and to change its volume.Small flow rate is utilized in order to warrant mechanicalquasiequilibrium conditions.12 CA measurements have beenperformed for three cycles, in which the drop volume is ini-tially increased by 20% of its initial value and then decreasedby the same amount. In this way the advancing and recedingCAs are evaluated, evidencing also the history effect due tothe adsorption at the solid-liquid interface.13 Indeed, as afunction of the experiment time, at each cycle a progressivedecrease of both angles is observed. As an example, the cor-responding results for SDS solutions are reported in Fig. 1.Figures 2�a� and 2�b� report respectively the advancing CAand the CA hysteresis as a function of the equilibrium sur-face tension as evaluated at the first cycle. For all investi-gated systems the obvious effect of a decrease of hydropho-bicity with increasing surfactant concentration is coupledwith increase of the hysteresis effect.

Two regimes can be clearly evidenced.Submicellar concentrations. The solid surface keeps a

superhydrophobic character versus all investigated solutions,showing large CAs and a small hysteresis. Moreover, CAsshow a definite decreasing trend with decreasing interfacialtension. This calls for a secondary role of adsorption at solid-liquid interface, which is compatible with the Cassie-Baxtermodel, where most involved interface is liquid-gas. Indeed alimited solid-liquid interface is available for adsorption andspreading, which is at the origin of the CA hysteresis.

Supramicellar concentrations. The situation is more ar-ticulated: In spite of the very close values of the liquid-airsurface tension, the CAs and angle hysteresis are stronglydependent on the electrical characteristics of the surfactants.The superhydrophobic character of the solid interface is keptagainst solutions of both anionic and cationic surfactants inthe absence of salt. Instead, a “regular” hydrophobic behav-ior is observed both for nonionic and for ionic surfactantsolutions in the presence of salt, with contact angles close to120° and a large hysteresis.

In fact the solid surface is actually composed of pillar-like structures and empty spaces forming liquid bridges. Thissurface is initially uncharged and the experimental observa-tions can be explained by recalling the adsorption features of

FIG. 1. Advancing �filled symbols� and receding �empty symbols� contactangles for SDS aqueous solutions: c=2.0�10−5M �circles�, and c=2.0�10−5M +NaCl 20 mM �triangles�, c=1.6�10−2M �squares�, and c=1.6�10−2M +NaCl 20 mM �diamonds�.

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ionic surfactants. In the absence of salt, progressing the ionic

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surfactant adsorption, charge accumulates at the interface,inducing a distribution of the surfactant counterions and thenthe formation of a diffuse double layer �Stern layer�. Furtherprogress of adsorption is then hindered as well as the reduc-tion of the interfacial energy. In the presence of salt the ad-sorbed charge is screened, avoiding the formation of the dif-fuse double layer, resulting in a much larger value of theadsorption at the equilibrium. The consequent reduction ofinterfacial energy promotes the penetration of the liquid intothe grooves. Thus for concentrated solutions adsorption atthe solid-liquid interface seems to be the controlling param-eter for the wetting behavior. Moreover, the enhancement ofthe hysteresis shows that the liquid spreads in the surfacegrooves.

In conclusion, the presence of surfactants is effective inlowering the contact angle in a hydrophobic or still superhy-drophobic range. Moreover, increasing surfactant adsorptionresults in a switch between a Cassie-Baxter and a Wenzel

FIG. 2. Advancing contact angle �a� and contact angle hysteresis �b� vs.liquid-air equilibrium surface tension of the investigated surfactant solu-tions. Filling colours refer to the different surfactant types: ionic �black�,ionic+salt �white�; non ionic �grey�.

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regime for superhydrophobic surfaces. Due to the adsorption1 Apr 2014 21:57:18

053104-3 Ferrari et al. Appl. Phys. Lett. 89, 053104 �2006�

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at the solid surface and the increase of the ionic strength,more hydrophilic domains are created and the liquid fills thesurface grooves, enhancing the hysteresis. By coupling theadsorption properties and the repellence of the solid surface,drop spreading can be controlled, confining its volume in arestricted space.

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