Wetting of low free energy surfaces by aqueous surfactant solutions

Current Opinion in Colloid & Interface Science 16 (2011) 285–291

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Current Opinion in Colloid & Interface Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c o c i s

Wetting of low free energy surfaces by aqueous surfactant solutions N.A. Ivanova, V.M. Starov ⁎ Department of Chemical Engineering, Loughborough University, Loughborough, LE11 3TU, UK

a r t i c l e

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Article history: Received 12 January 2011 Received in revised form 13 June 2011 Accepted 13 June 2011 Available online 7 July 2011 Keywords: Wetting and spreading dynamics Surfactants Trisiloxanes Hydrophobic surfaces Adsorption/desorption of surfactants Free surface energy

a b s t r a c t Surfactant-enhanced spreading of water-based formulations over low-energy surfaces has attracted considerable interest in scientific and industrial communities because of its importance in agrichemical, pharmaceutical, coating and textile applications. Spreading of aqueous surfactant solutions is rather complex process than spreading of pure liquids due to a time-dependent adsorption/desorption of surfactant molecules at all three interfaces involved that results in changing the interfacial energy balance, producing interfacial tension gradients and, hence, Marangoni flows. The phase behavior and structures of surfactant aggregates in bulk solutions, structure and surface activity of surfactant molecule itself, physicochemical properties of substrates and a number of other parameters could strongly influence spreading dynamic of surfactant solutions on hydrophobic surfaces. Implication of all those factors on spreading behavior of solutions makes it hardly predictable from both theoretical and practical points of view. In this brief review we summarize different factors that determine spreading character of aqueous surfactant solutions on hydrophobic substrates such as polymers films and chemically modified solids. Focus is made on spreading and wetting behavior of nonionic hydrocarbon and organosilicone surfactants, which are widely used in commercial and analytical applications. © 2011 Elsevier Ltd. All rights reserved.

1. Introduction Most of natural (plant leaves [1,2]) and artificial surfaces (polymers [3]) are poorly wetted or non-wetted at all by water and aqueous solutions (see Fig. 1). Those surfaces are referred to as low energy surfaces or hydrophobic surfaces. However, various technological processes require aqueous solutions to spread out on originally hydrophobic surfaces [4–9]. Zisman and co-workers [10] introduced a critical surface tension, γс, as a parameter which characterizes a solid surface. This parameter shows whether liquid with a known liquid/vapor interfacial tension, γlv, wets a solid surface or not. Their simple method based on measuring of contact angles of a series of liquids with known γlv on the given solid surface. The critical surface tension is defined by a linear extrapolation of dependency cosθ vs. γlv to the intersection with the line cosθ = 1. According to Zisman's plot liquids with γlv b γс spread out over the solid substrate. The surface tension of pure water (γlv = 72.8 mN/m) is much higher than the critical surface tensions of hydrophobic materials (γс = 50 mN/m at 20 °C [3]). Hence, spreading of water does not occur on those materials. More recently Starov et al [11] proposed a model showing the ways liquids can wet hydrophobic solids spontaneously. According to this model the total excess free energy, Ф, of a system “substrates +

⁎ Corresponding author. Tel.: +44-1509 222508; fax: +44-1509 223923. E-mail address: [email protected] (V.M. Starov). 1359-0294/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.cocis.2011.06.008

sessile droplet”, which is the sum of all the excess free energies in the system: Φ = γlv S + PV + π a 2(γsl − γsv), where γlv, γsl and γsv are interfacial tensions of the liquid/vapor, the solid/liquid and the solid/vapor, respectively; a is the radius of the droplet base; S is the area of the liquid/vapor interface, P is the excess pressure, V is the liquid volume. The spontaneous spreading is the result of the decreasing of the total excess free energy of this system. Direct calculations of the excess free energy [11] results in the following conclusions concerning derivatives of the excess free energy with interfacial tensions γlv, γsl and γsv: ∂ Φ/∂ γlv N 0, ∂ Φ/∂ γsl N 0 and ∂ Φ/∂ γsv b 0 [11]. Those inequalities show that the excess free energy of the droplet decreases and droplet will spread out if either (i) γlv decreases (that is, as a result of adsorption of surfactants on the liquid/vapor interface), or (ii) γsldecreases (that is, if as a result of surfactants adsorption on the liquid/solid interface), or (iii)γsv increases (that is, surfactants adsorb in front of the moving three phase contact line on a bare hydrophobic solid/vapor interface). Hence, there are three ways to promote surface wetting and the liquid spreading. According to the previous consideration a direct way to promote spreading of aqueous droplets over hydrophobic surfaces is to reduce the interfacial tensions γlv and γsl or increase γsv through the addition of surfactants, which are amphiphilic surface active molecules capable of adsorption at all three interfaces involved [9,11–13]. Surfactants are widely used to facilitate wetting of surfaces and spreading water-based formulations in diverse industrial applications such as agrichemical, pharmaceutical, home care products, cosmetics, and coatings [4–9]. However, mechanism behind the spreading of

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Fig. 1. Rain droplets on a leaf of Agavaceae plant (from the left). Water droplets deposited from a spryer on a surface of a polypropylene film (from the right).

aqueous surfactant solutions over hydrophobic substrates is more complicated in comparison to the spreading of pure liquids and has not been completely understood yet. The reason is that the spreading dynamic of surfactant solutions is affected by (i) a time-dependent adsorption/desorption of surfactant molecules at all interfaces (i.e., liquid/vapor, solid/liquid and solid/vapor) involved, drastically change interfacial tensions and energy balance at the moving threephase contact line, (ii) resulting interfacial tension gradients and Marangoni flow as a consequence, (iii) disjoining/conjoining (Derjaguin's) pressure gradient. Very little is known on the latter pressure in the case of non-wetting. The adsorption processes as well as the changing interfacial tensions in their turn are influenced by concentration of surfactants in the bulk of the droplet that reflects on the spreading behavior (see for example, Fig. 2). Fig. 2 shows spreading kinetics of aqueous solutions of trisiloxane surfactant TEO8 (see below) on Teflon AF depending on concentration of TEO8. At lowest concentrations droplets spread essentially slow and a delay of spreading is detected (the upper curve in Fig. 2). Increasing concentration decreases an initial contact angle of droplets due to fast adsorption kinetics of molecules on a liquid/vapor interface and enhances the spreading allowing reaching a final quasi-equilibrium contact angle much faster. It is important to note that trisiloxane surfactants (organosilicone surfactants) on moderately hydrophobic surfaces exhibit “superspreading” that means a rapid spreading of droplet of those surfactant solutions to a final contact angle close to zero [18–20]. A mechanism responsible for superspreading is still debated, and a comprehensive

review concerning this problem is presented by Venzmer [14] in this issue. A character of spreading can be pre-determined by the surface free energy of solids and a pattern of surface chemical groups on the solid surface, which the deposition of surfactant solutions occurs onto. Let us consider as an example two hydrophobic surfaces: (i) Polyethylene (PE) having only relatively “weak” non-polar methylene \CH2 groups on its solid/vapor interface. PE exhibits the microscopic contact angles of pure water, θw, in the range of θw = 92° [15] to 103.5° [16] depending on a formation method; and (ii) Poly(methylmethacrylate) (PMMA) with “highly” non-polar methyl \CH3 groups (note that \CH3 groups are more non-polar as compared to \CH2 groups [10]). However, PMMA has on its surface comparative hydrophilic (polar) ester groups. Those ester groups despite the presence of highly non-polar \CH3 groups decrease the values of θw down to 80° [3] or even 68° [17]. Nature and structural composition of surfactant molecules influence the spreading behavior as well. Let us consider for example, the high performance wetting agents such as nonionic ethoxylated alcohols (CmEOn for short) with \CH2 hydrophobic tails and ethoxylated organosilicone surfactants (known as trisiloxane surfactants, TEOn) [18–20] with silicone-based hydrophobic tails screened by \CH3 groups. Both surfactants have identical hydrophilic polyoxyethylene (EO) chains, however, exhibit different character and power of spreading on identical hydrophobic surfaces [21,22]. Below we summarize the recent results reported on spreading behavior of aqueous mostly hydrocarbon and organosilicone surfactant solutions over low-energy substrates, with focus on an analysis of factors that determine spreading characteristics and on the consideration of the corresponding mechanisms. Capillary rise and imbibitions of surfactant solutions into hydrophobic capillaries are also overviewed. 2. Bimodal kinetics of spreading of surfactant solutions: the classical power law

Fig. 2. Evolution of advancing contact angle of droplets of aqueous trisiloxane solutions (TEO8) at various concentrations inside the droplet: C b CAC ( ), CAC b C b CWC ( ), and C N CWC ( ) on Teflon AF coated silicon wafers. We presented those plots at conference of “B&D Interfaces 2009”, Greece, September, 2009.

The analysis of the recent literature [13,15,23–31] shows that spreading of aqueous surfactant solutions on hydrophobic surfaces, as well as imbibitions of surfactant solutions into hydrophobic capillaries, shows mostly bimodal kinetic: two stages of spreading/ imbibition with different rates controlled by a surfactant adsorption mode. The latter in turn is predetermined by the concentration of surfactants, nature of both substrates and surfactants. In some cases even three stages of spreading/imbibitions were detected [26,28,31]. One of the widely used method to analyze the mechanism of spreading of surfactant solutions is matching of the experimental data by power law R ~ t n. This method is widely used to analyze the wetting by pure liquids. Briefly the relationship between the spreading/power exponent, n, and the corresponding mechanism is the following. In the case of small droplets and complete spreading, n = 0.1 (Tanner's

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law), showing the capillary-driven spreading regime [32], or n = 0.14 the same regime but based on molecular approach to the dissipation of energy [33]. Note the exponent n = 0.1 is in excellent agreement with experimental data in the case of complete wetting at spreading over both smooth solid substrates [34] and porous substrates saturated with the same liquid [35]. Marangoni forces (surfacetension gradients) are responsible for spreading with n = 0.25 [36,37]. For relatively large droplets the gravity-driven spreading dominates yielding n = 0.125. Von Bahr et al [29,30] detected two-stage's spreading dynamic of nonionic polyoxyethylene alcohols surfactant (CmEO6, where m is the length of hydrophobic tail, 6—is the length of hydrophilic head) solutions on hydrophobically modified glass and gold surfaces. The first stage was very fast (30 ms [30]) with the moving edge as n = 0.5 (referred to as a non-diffusive regime [29]), after this short period the spreading process slowed down relaxing towards the final contact angle. The first stage, according to the authors, is due to different factors such as inertia, capillarity, relaxation of interfacial tension balance [29,30], and lifetime of droplet [29], but the slow stage is determined by diffusion of surfactant monomers from bulk solution to the liquid/vapor interface to be adsorbed on that interface. The authors also noticed that adsorption at the solid/liquid interface leads to a decrease of the spreading rate during the second stage, especially at concentrations below CMC. Dutschk et al [23–25] found that the first stage (fast spreading) of CmEO5 surfactants on different polymers (Parafilm, polypropylene, etc) lasted less than 1 s with the radius evolving according to n = 0.5 but the second stage is much slower and characterized by spreading exponent n b b 0.1 that is in consistent with results obtained in [29,30]. To explain the extremely low spreading rate they used a theory proposed by Starov et al [12]. According to that theory the spreading at the slow stage is governed by slow adsorption of surfactant monomers in the front of the moving edge onto the solid/vapor interface making its hydrophilic (γsv increasing) [12]. This mechanism, firstly proposed by Churaev et al [27] and theoretically developed by Starov et al [12], has later been confirmed by direct experimental observation of adsorption of molecules in the front of the moving contact line by Garoff's group [38]. Note that adsorption of surfactant molecules in front of the moving three phase contact line results in a decrease of the total excess free energy of the system, however, a locally the solid/vapor interfacial tension increases [11]. The latter means that the mentioned process goes via a potential barrier, that is, is much slower as compared with other adsorption processes. Drelich et al [15] has shown that in the case of spreading of aqueous solutions of C12EOn over hydrophobic toner and polypropylene the first stage proceeds much longer, up to a few minutes after a deposition. During the first stage the contact angle significantly decreased, after that the slower relaxation was observed. That is, the first stage was found much longer as compared to those reported in [29,30,23–25], although type of surfactants and contact angle of water on solid substrates (close to 90°) used by all groups of researchers were quite similar. Probably, a reason of the difference could be due to the difference in chemical compositions of substrates used by different authors. In some cases polymer substrates were used with hydrophobic groups, while in another cases the gold surfaces coated by organosulfur monolayers with inclusions of OH polar groups that could affect the spreading rate. Svitova et al [31] reported two stages of complete spreading of C12EO3 surfactants and trisiloxanes TEO8 and TEO12 on graphite. For polyoxyethylene alcohols at C N CAC (CAC is the critical aggregation concentration; CWC N CAC) the 1st and 2nd stages were described by the power laws with n = 0.1 and n = 0.4, respectively. In the case of trisiloxanes at C N CWC (CWC is the critical wetting concentration) the authors found a higher spreading exponents: n = 0.2 and n = 0.5, respectively. The only one-stage partial wetting was observed for both types of surfactants at C b CMC/CWC.

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Ivanova et al [13] studied spreading kinetics of trisiloxane surfactants and polyoxyethylene alcohol surfactant solutions over Teflon AF 1600. Teflon AF 1600 has extremely low surface energy (15mN/m, according to Sigma Aldrich data), hence, only partial wetting took place and the term spreading itself in this case has to be understood as a microspreading. At concentrations above CAC/CWC two stages were observed: a short fast stage fitted using a power law with n ≤ 0.05, followed by a relatively longer stage, which looks like a slow motion of the three phase contact line towards the final position. At low concentrations only one slow stage of spreading occurs that is similar to results by Svitova et al [31] obtained on moderately hydrophobic graphite. Moreover, in some cases at low surfactant concentrations a delay of spreading up to a few seconds was detected [13]. The second stages were fitted according to Starov's theory [12] and reasonably well agreement was obtained. However, a mechanism behind the first stage was left unexplained in [13]. Recently Starov et al [11] suggested that the first stage is likely determined by the surface tension relaxation at the liquid/vapor interface immediately after deposition of a droplet. Some authors do not report the bimodal kinetics of spreading of surfactants studied, but they provide an evolution of the spreading exponents with concentration [39,40] that is useful for identification of driving forces responsible for spreading in a certain concentration range of surfactants. Zhang and Han [39] studied the spreading of complex surfactants, glucosamide-based trisiloxanes solutions, over polystyrene substrate. They considered the spreading kinetics over 2 s after a droplet was deposited on the substrate. It was found that for some surfactants the exponent n increases from roughly 0.1 to 0.25 showing that at low concentrations the spreading is driven by capillary forces, and at high concentrations the Marangoni forces dominate. Gemini-like glucosamide surfactants do not show noticeable spreading capability, the spreading exponent in this case hardly reaches n = 0.1 at highest concentrations [39]. Rafaï et al [40] showed that the power exponent for trisiloxane surfactant, TEO8, spreading over polyethylene terephthalate increases when the bulk surfactant concentration growth until a plateau value of n = 1 is reached for the highest concentrations. Marangoni forces (i.e. surface-tension gradient) were considered to be responsible for the spreading over the whole range of concentrations: the onset of radial surface-tension gradient corresponds to n = 0.25, but if the surface-tension gradient is rapidly established over the height of the droplet at very high concentrations then it yields n = 1. The radius of droplets of anionic surfactant solutions propagates due to the capillary forces, n = 0.1, at highest concentrations, but at low concentrations C b CMC (the critical micelle concentration) these solutions do not spread at all over polyethylene terephthalate [40]. Behavior of a spontaneous imbibition of aqueous surfactant solutions into hydrophobized capillaries was investigated by Churaev et al [27]. Solutions of nonionic surfactants penetrated into the thin capillary in two different regimes depending on the concentration: at C b CMC only one slow stage of penetration was detected. The mechanism was connected to the adsorption of surfactant molecules in front of the moving three-phase contact line and the rate of the penetration determined by the diffusion of surfactant molecules from the bulk. At C N CMC two stages of penetration were found: a fast first stage, which was followed by a slower stage similar to that at concentrations below the CMC. The fast first stage was explained by a disintegration of the first layer of micelles, which was a storage of surfactant molecules to be transferred onto the bare hydrophobic substrate in front of the moving three-phase contact line. Tiberg et al [28] identified two regimes of rising of nonionic surfactants solutions into hydrophobic capillaries: adsorption-controlled rise and diffusion-controlled rise. A third regime detected was the long time creeping of solution into the capillary which was caused according to the authors by a slow relaxation of adsorbed layers at the liquid/solid interface. Note that in the latter case authors did not take

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into account the adsorption of molecules onto the solid/vapor interface in the front on the moving meniscus as it was suggested in [27]. This brief overview demonstrates that the character of spreading is strongly affected by concentration of surfactants because different mechanisms of spreading observed at C b CAC/CMC and at C N CAC/CMC. A competition between the rate of adsorption of surfactant molecules and rate of depletion of molecules at the expanding interface due to their adsorption at the solid/liquid or due to the slow diffusion to the interface during the spreading reflects on the changing of spreading rate of droplets. Attempts to fit the data on spreading of surfactant solutions by power law showed that the fitting with only one exponent does not work. In some cases the power exponent is so small that the mechanism behind spreading can hardly be related to well-determined mechanisms corresponding to certain values of n. In many cases spreading occurs in a crossover mode and is controlled by competition between different mechanisms which drive the spreading.

3. Influence of surface energy of hydrophobic solid substrates Spreading and wetting behavior of surfactant solutions in respect to a relationship between the surface energy of substrate and the surface energy of the liquid/vapor interface of solutions has been studied by many authors [10,21–26,41–44]. In the case of pure liquids a number of works have been done by Zisman and co-workers [10]. Usually in the literature a solid surface is characterized by either the surface energy/critical surface tension or by the contact angle of the droplet of pure water on the surface. As typical surface free energies of polymers do not exceed 50 mN/m [10] then according to Zisman's rule aqueous solutions of surfactants can promote spreading if surfactants are capable to reduce surface tension of water below the critical surface tension. This conclusion is supported by Dutschk et al [23–25], who have shown that solutions of some ionic surfactants having the liquid/vapor surface tension at CMC close to 40 mN/m do not spread at all over polymers having the surface free energy below roughly 30 mN/m and characterized by θw N 90° (such as polypropylene, Parafilm, Teflon AF). At the same time nonionic surfactants lowering the surface tension water down to about 30 mN/m [18] exhibit partial wetting on those polymer substrates. The same trend was observed by Rafaï et al [40] for anionic surfactant solutions at high concentrations on polyethylene terephthalate. Radulovic et al [42] have undertaken a comparative analysis of spreading exponents for aqueous solutions of Silwet®L-77 (commercially available trisiloxane surfactant) at 0.1wt-% on hydrophobic substrates with different degrees of hydrophobicity, which was interpreted in terms of contact angles of pure water droplets, θw, on those substrates. The authors found nearly linear increasing in exponent n from roughly 0.1 to 0.8 with a decrease of pure water contact angle from 118° (Teflon AF coated Si wafers [13]) to 70° (polyethylene terephthalate) [40]. Detailed study has been done on the effect of the free energy of substrate for series of trisiloxane homologues by Wagner et al [43] and for trisiloxane surfactants, polyoxyethylene alcohols and ionic surfactants by Stoebe et al [21,22,44]. Wagner et al [43] have shown that spreading of trisiloxane homologous (γlv = 20 mN/m at 1 CAC) is favorable on non-polar surfaces and on slightly polar but still hydrophobic surfaces with the moderate surface energy 30–40mN/m. However, the lowest and highest surface energies of solid substrates suppress wetting capability all of those surfactants. A similar trend has been observed by Stoebe at al [21] for trisiloxanes on organosulfuric surfaces with different degrees of hydrophobicity varying from 57° to 112° in terms of θw. Polyoxyethylene alcohols and ionic surfactants solutions [22,44] regardless of surfactant concentration and the length of ethylene oxide (EO) chains exhibit sharp maximum in the spreading rate on slightly hydrophobic organosulfur monolayers with θw = 57°, containing mostly polar OH

groups. On substrates with only CH3 groups (θw = 112°) or small amount OH groups (θw = 93°) solutions do not spread at all. 4. A role of equilibrium and dynamic surface tension at liquid/ vapor interface It looks like it is possible to conclude from the above paragraph as well as from Young equation (cos θ = (γsv − γsl)/γlv) that the lower the liquid/vapor interfacial tension, γlv, the better spreading capability of a solution in terms of spreading rate/complete wetting, contact angle values and more hydrophobic surfaces can be wetted by that solution. However, despite this view it has been shown that the lowest equilibrium γlv values does not guarantee the fast spreading or “superspreading” [18,19] over hydrophobic surfaces. It is known that trisiloxane surfactants (having “hammer-like” structure of hydrophobic part of the molecule [18]) with different numbers of EO groups varying from 4 to 9 groups reduce the surface tension of water down to the equilibrium value about 20–22 mN/m [18,20,45,46]. Ivanova et al [45] have shown that in spite of that, those trisiloxanes demonstrate different capability to wet substrates with relatively high free surface energy (30–37 mN/m). A more hydrophobic trisiloxane with short EO chain (EO4) does not spread completely, but demonstrates a partial wetting and reaches a final contact angle on hydrophobic polystyrene (θw ≈ 89°) and polypropylene (θw = 97°), even at C N CWC [45]. Sieverding et al [47] compared spreading performance of some commercially available organosilicone surfactants used in agrochemical industry, such as Silwet® L-77, Break-Thru® S233 and BreakThru® S240 surfactants. All those surfactants have nearly the same equilibrium and dynamic surface tensions varying in the range of 24.9–23.6 mN/m, but demonstrate considerable difference in spreading capability: Silwet® L-77 and Break-Thru® S233 behave as real “superspreaders” on polypropylene surfaces, but Break-Thru® S240 does not promote spreading of water on this substrate. Another type of surfactants which are capable of reducing the liquid/vapor surface tension of water to 16–18mN/m is fluorocarbon surfactants [48,49]. Such low interfacial tension gives grounds for expectation of lowest contact angles and excellent wetting behavior of those surfactant solutions on hydrophobic surfaces. However, it has been shown in [49] and then in [50] that those solutions show much worse spreading performance on polypropylene and Parafilm surfaces [49] and on OTS monolayers [50] compared to trisiloxane surfactants. Anantha et al [49] pointed out that turbidity of solutions (presence of dispersed phase in solutions) at certain concentrations influences spreading, rather than the lowest equilibrium surface tension. Trying to explain lowest spreading exponent (n b 0.1), which was found in the case of glucosamide Gemini-like surfactants, Zhang and Han [39] suggested that the dynamic surface tension (DST) at the liquid/vapor interface reflects the spreading dynamics. Indeed, despite the fact that all surfactants studied reached the essentially equal equilibrium values of γlv at a concentration above CAC, the Geminilike surfactants exhibit a much slower decreasing of γlv down to γlv≈γс as compared with others surfactants, that, according to authors does not allowed to develop Marangoni forces over the surface of droplet enough to spread it over the substrate. According to Drelich et al [15] the kinetic of adsorption of surfactants on the liquid/vapor interface might have an effect on the contact angle relaxation at the beginning of spreading process. This is consistence with experiments by von Bahr et al [29] on influence of so-called lifetime of droplet, that is, the time after the droplet was formed on the syringe tip, on the spreading characteristics. Those results were also related to the liquid/vapor interfacial tension relaxation. The authors above found that the direct deposition (zero lifetime) of droplet results in a spreading delay and an initial contact angle close to the contact angle of pure water. Increasing the lifetime to tens of seconds leads to a rather low initial contact angle of droplet and a very fast reaching of the final contact angle compared to the zero lifetime

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case. The behavior mentioned above depends on concentration and the rate of diffusion/adsorption of a surfactant at the liquid/vapor interfaces. Starov et al [11] based on theoretical considerations have shown that adsorption at a liquid/vapor interface (or the relaxation of a liquid/vapor interfacial tension) does not favor the spreading at low concentrations, i.e. when an initial contact angle of droplet is above 90° [11], but it does at higher concentrations. In some cases the studies of DST of surfactant solutions can help to discover new interesting properties of those surfactants related to their spreading performance [46,51,52]. Kumar et al [51] measured DST of trisiloxane solutions to check whether the adsorption dynamics fulfilled the conditions necessary for high rate of spreading [8,18,20]. It appeared that Frumkin equation fitted well DST data only at relatively low concentrations, but not for the higher concentrations. Hence, the diffusion of monomers cannot provide the necessary amount of surfactants to maintain the spreading rate featured at high concentrates. Based on their DST data and analysis the authors [51] suggested that direct adsorption of bulk aggregates on interfaces is capable of maintaining the necessary flux of surfactants to the interface of the spreading droplet. More recently Ritacco et al [46] carried out detailed measurements of DST for those surfactants at the very short time (b1 s). It was discovered that in the case of trisiloxanes (with relatively long EOn chains, n = 6–9 possessing “superspreading” behavior) two inflection points were detected on DST curves [46]. Using Brewster Angle Microscopy Ritacco et al [46] directly observed appearance of aggregates on the liquid/vapor interface for those trisiloxanes in a range of concentrations (see Fig. 3). It allows suggesting that the surfactant molecules are present at the liquid/vapor interface in two states: as monomers and as surface aggregates [46,52]. The latter could act as reservoirs of surfactant monomers in the course of spreading that confirms the conclusions made in [51]. The above consideration shows that DST behavior plays an important role in the dynamic spreading and wetting processes. For more detailed information on the basic of DTS and its relation to diffusion and adsorption processes we could referred to some comprehensive reviews by Eastoe and Dalton [53] and Chang and Frances [54].

5. Spreading characteristics vs. the length of polyoxyethylene chains The role of the length of polyoxyethylene oxide chains of surfactant amphiphilic molecules on spreading behavior cannot be underestimated. On one hand this hydrophilic moiety provides a solubility of surfactants in water phase. However, on the other hand polyoxyethylene oxide chains influence an arrangement of surfactant molecules in adsorbed layers on air/liquid and solid/liquid interfaces and a shape of molecular aggregates formed in aqueous phase. As a

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consequence the length affects the dynamic and the equilibrium surface tension values of surfactant solutions. In the case of trisiloxanes with “hammer-like” shape [18] Ruckenstein [55] theoretically argued that molecules with moderately long EO7–8 chains are capable of stimulating high spreading capability of water on hydrophobic surfaces. While increasing of the EO length leads to lowering adsorbability of surfactants at interfaces because of strong attraction interaction with water and reduces spreading efficiency. Contrary to this Gentle and Snow [20] found experimentally that adsorption of trisiloxanes does not vary with the EOn length within the range of 4–16, but does decrease when n N 16. However, the theoretical arguments [55] are consistence with the results reported in [13,21,43], where it was found that trisiloxanes with the EO5– 8 show highest spreading rate on hydrophobic surfaces as compared with low-soluble short EO chains and highly-soluble long EO chains. It was also shown in [43] that on low hydrophobic (slightly polar) surfaces trisiloxanes with relatively longer EO chains show faster spreading. Authors [43] have shown also that concentration of surfactants is also important for the spreading behavior. The latter behavior was associated with the bulk phase conditions of trisiloxane solution, which in turn depends on the length of EO chains and temperature. In the case of ethoxylated alcohols Stoebe et al [22] detected decreasing spreading rate of homologues C12EO3–8 on hydrophobic surfaces with an increase of the length of EO chain, however, nothing similar was observed in a case of hydrophilic surfaces. Moreover, the maxima in spreading rate found in turbid (with bulk aggregates) solutions. It was suggested that the latter is a result of complex interplay between the adsorption capability of molecules at a solid and the aggregation in the bulk, which is determined by optimal hydrophilic/hydrophobic balance. However, AFM studies [56] of the relationship between surface aggregation on graphite and bulk aggregation of those C12EOn surfactants have shown that at highest surfactant concentrations with n = 5–10 EO chains formed hemicylindrical aggregates on graphite, while in the bulk solution spherical and rod-like micelles were present. It means there is no essential changes in the shapes of aggregates and consequently in the interplay mentioned above within selected range n = 5–8 of homologous. Nevertheless, the difference was found for very short EO [31,56] and very long EO [56] chains as compared to EO5–10 chains. Adsorption of bilayers at surface and formation of lamella in the bulk phase were observed for the EO3 surfactant, because the bending of those molecules is thermodynamically unfavorable; and nonstructural aggregation at the surface was exhibited by the EO23 surfactant. In [57] wetting capability of C10EOn surfactant solutions at concentrations ranging from 0.1 to 4 CMC on three polymeric surfaces: Teflon AF (117°), Parafilm (106°) and polypropylene (97°) was studied. It was shown that regardless of the hydrophobicity of surfaces the final quasi-equilibrium advancing contact angle increases as the number of EO units increases for all three substrates used. This

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Fig. 3. BAM images (inverted color) of surface aggregates for solutions TEO6 (a), TEO7 (b) and TEO8 (c) at concentration 0.002 mol/m3. These pictures were provided by RGRubio group [46].

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Fig. 4. Wetting performance of aged 0.1 wt.% Silwet® L-77 solutions after 1 (53°), 6 (72°), 8 (91°) and 10 (103°) days, respectively. Redrawn from [58]: droplet on Teflon AF surface.

implies that the common trend of the final contact angle behavior of C10EOn surfactants is maintained until the surfaces are characterized by θw N 90°. 6. Influence of others factors It is known that stability to hydrolysis is a weak point of organosilicone surfactants [18]. In spite of the high spreading performance these surfactants lose their efficiency very quickly after preparation [18,58,59]. Recently, a comparative study of aging processes of hydrocarbon and trisiloxane surfactants in water in terms of wetting power has been undertaken by Radulovic et al [58]. It was found that unlike conventional surfactants (like Triton X-100), commercial trisiloxane Silwet® L-77 gradually loses the wetting capability over days. Fig. 4 is an illustration of the aging process of Silwet L-77 solution [58]: a droplet increases its final contact angle day by day over 10 days due to hydrolysis, which caused degradation of surfactant molecules. However, concentration was found to play a crucial role to maintain the spreading capability: very dilute solutions lost their spreading ability over just 24 h, while very concentrated (0.1 wt.%) solutions are more viable and their spreading kinetics is slightly changed for a few days. As mentioned above the spreading behavior depends on the presence of dispersed phase, type and size of aggregates in the bulk solution [18,21,22,39,60,63] and the phase behavior of surfactant solutions [18,61,62]. Wagner et al [61] revealed that increasing spreading capability of trisiloxane solutions is observed when solutions were close to the transition region between the two-phase state and the lamellar phase state, which depend on temperature and concentrations. Zhu et al [60] showed that the size of aggregates in a bulk of solutions determines the spreading rate in the following way: the smaller aggregates the faster spreading. Zhang and Han [39] found that for glucosamide-based surfactants the existence of large bulk aggregates at low concentrations favored inducing Marangoni forces and increased the spreading area during the initial spreading stage. According to number of researches [18,21,22,63] presence of lamella or bilayer aggregates in trisiloxane solutions and its direct “unzipping” adsorption at the interfaces promote rapid spreading of those solutions. Humidity of the ambient atmosphere and pre-absorbed water layers on hydrophobic solid surfaces have been reported by some authors [60,63] as an important factor inducing “superspreading” behavior of trisiloxane surfactant solutions. However, a weak influence of humidity on the spreading dynamics of those solutions was shown in [40,64]. It is possible to state, despite those contradictory conclusions, that the evaporation rate of spreading droplets depends on the surrounding humidity and, hence, at least the contact angle values could be affected by a low humidity because the evaporation mostly occurs at the edge of the spreading droplets [65]. 7. Conclusions Spreading of aqueous droplets over low-energy surfaces can be facilitated by the addition of surfactants. However, the latter results in a complex mechanism of spreading and wetting, most of those mechanisms are different from those of spreading of pure liquids. Surfactants in aqueous phase tend to adsorb at all three interfaces involved and to form aggregates in the bulk solutions in order to

minimize the surface energy. The adsorption of surfactants at all interfaces drastically changes interfacial tensions and the energy balance at the three phase contact line. Hence, adsorption kinetics of surfactants and surface tension dynamics control the spreading behavior. The spreading rate is determined by the competition between the rate of adsorption of surfactants at the liquid/vapor interface and the rate of depletion of molecules due to expansion of this interface in the course of spreading. The rate of adsorption of surfactants in its turn depends on flux/diffusion of surfactant to the interface in a form of monomers or aggregates. The depletion of surfactants at the expanding liquid/vapor interface is caused by their adsorption at the solid/liquid or due to the slow diffusion/flux to the expanding interface. Additional factors, which strongly influence spreading dynamic of surfactant solutions on hydrophobic surfaces, are (i) phase behavior of surfactant aggregates in the bulk depending on temperature and surfactant concentration, (ii) structure and surface activity of surfactant monomers, (iii) physicochemical properties of substrates such as the energy and roughness (the latter was not discussed above); and (iv) humidity/evaporation. Acknowledgment This research was supported by Engineering and Physical Research Council, UK, Multiflow Project, EU, and PASTA project, European Space Agency. References [1] Otten A, Herminghaus S. How plants keep dry: a physicist's point of view. Langmuir 2004;20:2405–8. [2] Muller C, Reiderer M. Plant surface properties in chemical ecology. J Chem Ecol 2005;31:2621–51. [3] Wu S. Polymer interface and adhesion. New York: Dekker; 1982. p. 88. [4] Adams JW. In: Sharma MK, editor. Surface phenomena and additives in waterbased coatings and printing technology. New York: Plenum Press; 1991. p. 23. [5] Schramm LL. Emulsions, foam and suspensions. Fundamental and applications. Germany: Wiley-VCH: Verlag GmbH & Weinheim; 2005. p. 233–344. [6] Gecol H, Scamehorn JF, Christian SD, Grady BP, Riddell F. Use of surfactants to remove water based inks from plastic films. Colloids Surf A 2001;189:55–64. [7] Penner D, Burow R, Roggenbuck F. Use of organosilicone surfactants as agrichemical adjuvants. In: Hill RM, editor. Silicone surfactants. New York: Marcel Dekker; 1999. p. 241–58. [8] Hill RM. Silicone surfactants — new developments. Curr Opin Colloid Interface Sci 2002;7:255–61. [9] Tadros TF. Applied surfactants: principles and applications. Weinheim, Germany: Wiley-VCH; 2005. [10] Zisman W. Relation of the Equilibrium Contact Angle to Liquid and Solid Constitution. In: Gould RF, editor. Contact angle, Wettability, and Adhesion. Advances in Chemistry Series Vol. 43, Washington, DC: American Chemical Society; 1964. p. 1–51. [11] Starov V, Ivanova N, Rubio RG. Why do aqueous surfactant solutions spread over hydrophobic substrates? Adv Colloid Interface Sci 2010;161:153–62. [12] Starov VM, Kosvintsev SR, Velarde MG. Spreading of surfactant solutions over hydrophobic substrates. J Colloid Interface Sci 2000;227:185–90. [13] Ivanova N, Starov V, Johnson D, Hilal N, Rubio R. Spreading of aqueous solutions of trisiloxanes and conventional surfactants over PTFE AF coated silicone wafers. Langmuir 2009;25:3564–70. [14] Venzmer J. Superspreading — 20 years of physicochemical research. Curr Opin Colloid Interface. Sci in press, doi:10.1016/j.cocis.2010.11.006. [15] Drelich J, Zahn R, Miller JD, Borchardt JK. Contact angle relaxation for ethoxylated alcohol solution on hydrophobic surfaces. In: Mittal KL, editor. Contact angle, Wettability and adhesion, Vol. 2, 2002. p. 253–64. [16] Extrand CW. Water contact angles and hysteresis of polyamide surfaces. J Colloid Interface Sci 2002;248:136–42. [17] Ma Y, Cao X, Feng X, Ma Y, Zou H. Fabrication of super-hydrophobic film from PMMA with intrinsic water contact angle below 90°. Polymer 2007;48:7455–60.

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