Solvent effect on the aggregate of fluorinated gemini surfactant at silica surface

Journal of Colloid and Interface Science304(2006)

37–44

https://www.360docs.net/doc/603068211.html,/locate/jcis

Solvent effect on the aggregate of?uorinated gemini surfactant

at silica surface

Xiaoyan Song a,Peixun Li b,Yilin Wang a,?,Chuchuan Dong b,Robert K.Thomas b

a Key Laboratory of Colloid and Interface Science,Institute of Chemistry,Chinese Academy of Sciences,Beijing100080,People’s Republic of China

b Physical and Theoretical Chemistry Laboratory,Oxford University,South Parks Road,Oxford OXI3QZ,UK

Received20June2006;accepted22August2006

Available online22September2006

Abstract

The aggregate states of partially?uorinated gemini surfactant[(CF3)2CF(CF2)2(CH2)10N(CH3)2]2(CH2)6Br2(C F5C10–C6–C10C F5)on silica surface were investigated with atomic force microscopy(AFM)and water contact angle(CA)measurement by analyzing the effects of bulk concentration and adsorption time on stack state.On surfactant-adsorbed silica surfaces,there was a?at surface layer interspersed with some scattering surfactant aggregates.In the case of short adsorption times,the aggregates would be hemisphere.In the case of long adsorption times, the aggregates would be present in the form of bilayers.With the increase of bulk concentration,the adsorbed amount was enlarged and the surface layer became more compact.The formation of patchy bilayer aggregates indicated the saturation of the surface layer.Furthermore,organic solvent effects on the aggregate state of the surfactant on a silica surface were studied with four organic solvents,including n-hexane,dehydrated ethanol, 1,1,2-trichloro-1,2,2-tri?uoroethane,and toluene.With the treatment of different organic solvents,the hemisphere aggregates on the surface layer can rearrange into spherical bilayer,rodlike monolayer,and branched rodlike monolayer aggregates,respectively.The polarity of solvents and af?nity of organic solvents for surfactant molecules may have a great impact on the stack state of the?uorinated gemini surfactant molecules.?2006Elsevier Inc.All rights reserved.

Keywords:Fluorinated gemini surfactant;Surfactant adsorption;Silica surface;Organic solvent effect

1.Introduction

Fluorinated surfactants have attracted special interest be-cause of their unique characteristics[1–5].One of the character-istics of?uorocarbon chains is that they are stiffer than hydro-carbon chains,because of the bulky?uorine atoms[6].There-fore,?uorinated surfactants have a tendency to form structures with less surface curvature,which can explain the larger vol-ume of the aggregates compared to that of the corresponding hydrocarbon surfactants in aqueous solution[7–10].What is more,even single-chain?uorinated surfactants can form bilayer aggregates or vesicles,although this is usually unfavorable for single-chain hydrogenated surfactants[11,12].

Some attention has also been directed toward the adsorption of?uorinated surfactants onto solid surfaces[13,14].Rojas et

*Corresponding author.

E-mail address:yilinwang@https://www.360docs.net/doc/603068211.html,(Y.Wang).al.[15]investigated the adsorption of single-chain cationic?u-orosurfactants onto silica surfaces with respect to the adsorbed layer structure,long-range interactions,and adhesion forces. They found that an adsorbed monolayer was formed when the concentration was very low,while small loosely packed bi-layer aggregates formed readily as the surfactant concentration was increased.Further increase of the surfactant concentra-tion caused an increase in the surface density of the adsorbed bilayer https://www.360docs.net/doc/603068211.html,i et al.[16]used contact angle and fric-tion coef?cient measurements to evaluate the adsorbed?lms of sodium per?uorocarboxylate on aluminum oxide surface.It was shown that maximum adsorption decreased rapidly both with increasing hydrocarbon chain length and with increasing pH level for aqueous C n F2n+1COONa solutions on porous Al2O3 substrates.Contact angles and friction coef?cients for the ad-sorbed?lms were shown to be related to the structure of surfac-tant aggregates and bilayer completion.Davey and co-workers [12]determined the adsorbed layer structure of partially?u-orinated pyridinium surfactant HFePC by AFM imaging.The

0021-9797/$–see front matter?2006Elsevier Inc.All rights reserved. doi:10.1016/j.jcis.2006.08.045

38X.Song et al./Journal of Colloid and Interface Science304(2006)37–44

AFM image of HFePC showed very straight cylindrical aggre-gates,which might be due to the rigidity of the per?uoroalkyl chain.Li and Zu[2]investigated the adsorption of nonionic ?uorosurfactant onto gold and platinum electrodes,which was shown to signi?cantly retard the growth of the electrode oxide layers,compared with its hydrocarbon analogue Triton X-100.

Gemini surfactants consist of two conventional single-chain surfactant units connected at their head groups via a spacer[17]. In comparison to conventional single-chain surfactants,gemini surfactants have unusual aggregation properties,such as much lower critical micelle concentration(CMC),strong dependence on spacer structure,and special aggregate morphology[18].

A combination of gemini characteristics with stiff?uorocar-bon chains will lead to signi?cant changes for aggregate mor-phology.Oda’s group[6]observed very stable unilamellar and multilamellar vesicles for?uorinated gemini surfactants,which cannot be seen in their hydrocarbon analogs.Christenson et al.

[14]studied the adsorption of double-chained?uorinated sur-factant from solution onto mica surface.Monolayer was formed on surface within2h after the injection of the surfactant so-lution.After1day,the partial formation of a bilayer was ob-served.

As is well known,the formation of surfactant aggregates on surfaces can be attributed to the reduction of the contact area of nonpolar hydrophobic chains with surrounding polar water molecules[19].It is intriguing what will happen to the aggregates on silica surfaces when the solvent environment is altered to organic solvents.To the best of our knowledge,the effect of organic solvents on surfactant aggregates at silica sur-faces has been rarely reported.Kitaev et al.[20]mentioned that DTAB layers were stable in nonpolar solvents and not stable in polar organic solvents,according to the water contact angle (CA)results.Fluorocarbon chains exhibit limited miscibility with hydrocarbon amphiphiles or organic solvents.Therefore, the effect of organic solvents should lead more obvious changes in the aggregates of the?uorinated gemini surfactant on silica surface.

In the present work,we focus on the solvent effect of the partially?uorinated gemini quaternary ammonium surfac-tant[(CF3)2CF(CF2)2(CH2)10N(CH3)2]2(CH2)6Br2(C F5C10–C6–C10C F5)at a solid/liquid interface.To understand the re-arrangement of the surfactant under organic surroundings,?rst, we investigated the adsorption behavior of the partially?uori-nated gemini surfactant on a silica surface.Bulk concentration and adsorption time were found to have a great in?uence on the adsorption behavior of?uorinated gemini surfactant.The pos-sible packing state of the surfactant molecules on the surface was analyzed through?lm thickness,aggregate size,and wet-tability.Second,we explored the effect of organic solvents on the surface aggregate morphology with the aid of four differ-ent organic solvents,which are?uorocarbon or hydrocarbon polar or nonpolar organic solvents.Upon treatment with an organic solvent,the adsorbed surfactants underwent reorga-nization from hemisphere to bilayer,rodlike,and branched rodlike aggregates.A possible explanation was presented for the rearrangement of surfactant molecules from the perspective of interaction of the solvent surroundings with the surfactant molecules.

2.Experimental

2.1.Materials

The partially?uorinated gemini surfactant C F5C10–C6–C10C F5was synthesized according to the previous work[21]. The composition of the partially?uorinated gemini surfactant was con?rmed by NMR and MS.The purity of the compound was assessed from surface tension measurement,where no min-imum was observed.The surfactant solution was prepared using triply distilled water.

n-Hexane,toluene,1,1,2-trichloro-1,2,2-tri?uoroethane (F-113),and dehydrated ethanol were of analytical purity and obtained from Sinopharm Chemical Reagent Co.,Ltd., (SCRC).Before use,n-hexane and toluene were distilled over sodium,and F-113was distilled with P2O5.Dehydrated ethanol was used without further puri?cation.Single-crystal silicon wafers(100)polished on one side were obtained from the Gen-eral Research Institute for Nonferrous Metals.

2.2.Adsorption of surfactant onto silica surface

The polished silicon wafers were ultrasonicated in detergent solution and acetone for30min each.After being rinsed with water,the wafers were submerged in a freshly prepared mixture of H2SO4(98%)and H2O2(30%)at a volume ratio of7:3,and the solution was heated to80?C for1h to remove organic con-taminants.Then the wafers were rinsed thoroughly with triply distilled water and dried with a N2gas stream.After these treat-ments,a thin oxide layer was formed on the silicon surfaces. The substrates were immersed into C F

5

C10–C6–C10C F

5

solution, whose concentration was adjusted in the range from0.003to 1mM.Upon removal from the surfactant solution,the samples were blown dry with a nitrogen stream and stored in a desicca-tor.To further investigate the effect of organic solvents on the aggregate morphology of the partially?uorinated surfactant, the samples absorbed with C F5C10–C6–C10C F5were immersed into different organic solvents for24h.Then the samples were withdrawn and blown dry with a nitrogen stream.All the sam-ples were stored in a desiccator.

2.3.Surface characterization

The static water contact angles(CA)of the samples were measured with a2-μl water droplet at ambient temperature with an optical contact angle meter(Dataphysics Inc.,OCA20).The CA values reported are averages of?ve measurements made on different areas of surfaces.All measurements for all surfaces were within±2.0?of the averages.Morphology of the sam-ple surfaces was observed with atomic force microscopy(AFM, Digital Instruments,Nanoscope IIIa,tapping mode).The can-tilever used was a single-crystal silicon cantilever.The images were acquired with a scan rate of1.0Hz.

X.Song et al./Journal of Colloid and Interface Science304(2006)37–4439 One special method was used to carry out the?lm thick-

ness measurements.A ca.10-μm metallic tip was gently

placed against the surface.Several random and nearly parallel

scratches were made in the?lms.Moderate force was applied

to make sure that the silica surfaces were not damaged.The

?lms were then monitored by AFM.With the section analysis

of the morphology images,the thickness values,i.e.,the ver-

tical distances from the?lm plane to the bare substrate,were

obtained.The thickness values reported are averages of at least

?ve measurements made on different areas of each sample.

3.Results and discussion

3.1.Surfactant adsorption

The adsorption behavior of C F5C10–C6–C10C F5on the silica

surface was investigated by varying the concentration of bulk

solution from0.003to1mM.In the present work,the con-

centration was expressed as a ratio of CMC.The CMC of?u-

orinated gemini surfactant C F5C10–C6–C10C F5was0.060mM,

determined in our previous report[21].Therefore,the concen-

tration of the surfactant solution can be represented as rang-

ing from0.05to16.7CMC.Under different concentrations of

the surfactant solution,the adsorption morphologies of C F5C10–

C6–C10C F

5on silica surfaces for30min deposition are shown

in Fig.1.For30min deposition,some spherical aggregates formed on all of the silica surfaces.Even for the bulk surfactant concentration of0.05CMC,this kind of spherical aggregates are present.With increasing bulk concentration,the shape and amount of those aggregates change unobviously.The AFM im-ages in Fig.2show the morphology of the?lm adsorbed for 24h in C F5C10–C6–C10C F5solution and present spherical and disklike aggregates.For the bulk surfactant concentration from 0.05to1CMC,the morphology of adsorbed?lms is similar to that of the?lms deposited for30min.Some spherical ag-

gregates disperse randomly on the surface with a width of ca. 30nm.When the concentration is higher than2CMC,the dimension of the aggregates increases to ca.120nm.Those spherical aggregates turn out to be disklike.The surfactant ad-sorption for48h has also been https://www.360docs.net/doc/603068211.html,pared with the ?lms adsorbed for24h,AFM images of the?lms adsorbed for 48h show the same results,which are not presented here.

To further understand the adsorption behavior of C F

5

C10–C6–C10C F5on a silica surface,cross-section analysis of the adsorp-tion images was also performed.The height data of aggregates on the surface layer are shown in Table1.No matter what the bulk surfactant concentration is,higher or lower than the CMC, the spherical aggregates have apparent height ranging from1.8 to2.3nm in the case of30min deposition.This is consis-

tent with the length of a fully extended carbon chain C10C F

5 (2.1nm),which is calculated from L c=1.5+1.265n?,where n is the number of carbon atoms in the chain[22].Thus,it is possible that the aggregates for30min deposition should be monolayers or hemispheres owing to hydrophobic interaction among?uorinated chains.For the?lms adsorbed for24h,the height of spherical aggregates changes to ca.4nm,which is equal to the length of two extended surfactant molecules’hy-drophobic chains.This means that after a long deposition time, bilayer aggregates will form on the silica surface.

Water CA has been proven to be a sensitive method of in-vestigating the structure and chemical state of the outermost few angstroms of a surface[23].Therefore,water CA measure-ments can provide useful information on the arrange mode of the surfactant molecules on silica surface.The water CA results are shown in Fig.3.In Fig.3A,the water CAs of?lms adsorbed for30min are shown to increase with increasing bulk concen-tration.The surfactant?lm formed in0.05CMC solution after 30min shows a water CA of46.9?.For the surface

prepared Fig.1.AFM images of silicon surfaces self-assembled with gemini surfactant C F5C10–C6–C10C F5after30min deposition at concentrations of(A)0.05CMC, (B)0.1CMC,(C)0.5CMC,(D)1CMC,(E)2CMC,and(F)5CMC.

40X.Song et al./Journal of Colloid and Interface Science 304(2006)

37–44

Fig.2.AFM images of silicon surfaces self-assembled with gemini surfactant C F 5C 10–C 6–C 10C F 5after 24h deposition at concentrations of (A)0.05CMC,

(B)0.1CMC,(C)0.5CMC,(D)1CMC,(E)2CMC,and (F)5CMC.

Table 1

The ?lm thickness of height of aggregates and surface layer on silica surface Bulk concentration (CMC)Height of aggregates (nm)Film thickness (nm)30min 24h 30min 24h 0.051.94.00.40.50.12.23.70.40.40.52.14.30.50.411.94.10.40.521.83.80.60.55

2.3

3.9

0.6

0.

5

Fig.3.Water contact angles of silicon surfaces self-assembled with ?uorinated

gemini surfactant C F 5C 10–C 6–C 10C F 5(A)adsorbed after 30min,(B)adsorbed

after 24h.

in 16.7CMC solution,the value of water CA reaches 63.1?.The water CA results for the surfactant ?lms adsorbed for 24h are shown in Fig.3B.First,CA increases from 56.7?to 84.2?with an increase in bulk concentration from 0.05to 2CMC.The hydrophobicity of the adsorbed layer is shown to be improved with increasing bulk concentration.Then the CA drops to 71.4?at 5CMC and stays nearly constant afterward.

The relationship between the surface composition and water CA on the same solid surface is given by the Cassie equa-tion,which takes into account the ratio between two compo-nents [24].For a surface covered by two phases,the Cassie equation can be expressed ash cos θ=χA cos θA +χB cos θB .We de?ne θA as the fraction of the phase covered with closely packed surfactant molecules and θB as the fraction of the loosely covered phase.θA and θB are the CA on a surface composed purely of the corresponding phase.Because of the hydrophobicity of ?uorocarbon moiety and the closer packing,θA is de?nitely larger than θB .The hydrophobicity of com-plex surfaces will be improved when the fraction of the closely packed phase is increased.

From the variation in the values of water CA for 30min deposition shown in Fig.3A,the improvement of CA means that ?uorinated chains turn out to be packed more closely and the adsorbed amount is increased spontaneously.Owing to the weak interaction among ?uorocarbon chains,the increase of bulk concentration helps to promote the formation of closely packed ?lms.It should be pointed out that water CA still in-creases when the bulk concentration is as high as 16.7CMC,which implies that the monolayer adsorption does not reach saturation.For the surface adsorbed for 24h,which is shown in Fig.3B,CA reaches a maximum as the bulk concentration is in-creased to 2CMC.It may be expected that the silica surface has

been saturated with C F 5C 10–C 6–C 10C F 5.Further increase in the

bulk surfactant concentration results in a drop of water CA.The reduction of CA indicates that some more surfactant molecules may adsorb on the top of the surface layer with head groups fac-ing outside,because of the hydrophobic interaction between the ?rst-layer ?uorinated chains and the further adsorbed surfactant ?uorinated chains.

In addition,the water CAs of all the ?uorinated surfactant ?lms are at least 46.9?and larger than that of bare silica surface

X.Song et al./Journal of Colloid and Interface Science304(2006)37–44

41

Fig.4.Illustration of cross-section analysis on the surface layer thickness and the height of surfactant aggregates on silica surface.

(<5?),but the CAs are not higher than90?.Those scattering spherical and disklike aggregates cannot rationally explain the weak hydrophobicity.There might be a continuous?lm of dis-ordered molecules adsorbed in the areas between those aggre-gates.In order to con?rm this supposition,a scratch examina-tion was performed to study the?at regions surrounding those aggregates.AFM has the advantage of being capable of de-tecting surface variations at the subnanometer scale,which has been used to measure layer thickness[25,26].A clean metal-lic tip was used to make several scratches in the?lms to scrape away part of the adsorbed surface layer.With the cross-section analysis of the morphology images,the thickness data were ob-tained with a height difference between the?lm-occupied area and the bare substrate.The illustration of cross-section analy-sis of the surface layer thickness is shown in Fig.4.The?lm thickness data of the surface layer at different surfactant con-centrations are shown in Table1.The same thickness at differ-ent bulk concentrations indicates that the?lm thickness almost does not change with the concentration.The thickness of the surface layer is ca.0.5nm and is smaller than the length of

a fully extended hydrophobic chain C10C F

5.This suggests that

the surfactant molecules probably lie?at or nearly?at on the silica surface,which coincides with Rojas et al.’s report[15]. The surfactant molecules adsorb with their polar moiety in con-tact with the surface due to the electrostatic attraction between the surfactant head groups and the surface.The?uorinated hy-drocarbon moiety extends from the surface and results in the formation of a continuous?lm with disordered molecules.This kind of adsorption structure will improve the hydrophobicity of silica surface.Because of the loose packing of surfactant mole-cules and the existence of polar head groups,the water CA of the surfactant layer cannot exceed90?all the while.

The above AFM and water CA results provide information on the stack state of the surfactant molecules from different aspects.Bulk concentration and adsorption time both present great impacts on the adsorption behavior of?uorinated gemini surfactant C F5C10–C6–C10C F5on the silica surface.As we know, surfactant adsorption is determined by two main factors.The ?rst is the electrostatic interaction of surfactant molecules with the surface and the second is the hydrophobic interaction among hydrophobic chains of surfactant molecules[27].In the case of relatively short adsorption times,the surfactant head groups may bind with the silica surface through electrostatic interac-tion,and the?uorinated hydrophobic tails are assumed to lie ?at on the silica surface to minimize their contact area with water molecules and generate a weakly hydrophobic surface. The adsorbed molecules in turn will adsorb more surfactants cooperatively by hydrophobic interaction,resulting in the for-mation of hemisphere aggregates.With the increase of bulk concentration,the CA improvement means that more surfactant molecules are adsorbed and the unchanged small?lm thickness suggests that the adsorbed molecules in surface layer still lie ?at on the silica surface.

It is interesting to note that at the same bulk concentration, the?lms treated with a longer adsorption time are all more hy-drophobic than the?lms treated with a short adsorption time. This implies that the adsorbed amount can be improved after a long adsorption time.In the case of a relatively long adsorption time,the thickness of the surface layer remains approximately 0.5nm,suggesting a monolayer with surfactant molecules tak-ing a?at conformation.Apart from the bottom surface layer, some spherical or disklike aggregates exist on the AFM images, with a height of about4nm.The height of these aggregates formed for longer adsorption times is nearly twice the height of the aggregates formed for short adsorption times.This indicates that the surfactant molecules may adopt a bilayer conforma-tion for longer adsorption times.Bilayer formation requires a long equilibrium time,which can be partly ascribed to the rel-atively weak interaction among?uorinated chains.Our result is consistent with the time scale reported by Christenson et al.[14],who observed the partial formation of bilayer structures for a double-chained cationic?uorinated surfactant on mica af-ter1day,while Rojas et al.[15]found that bilayer structures could be formed1h after a single-chain?uorinated surfactant https://www.360docs.net/doc/603068211.html,ter on,the surface density of the bilayer structure was increased.Therefore,the divarication may be possibly in-duced by the slow adsorption dynamics of the gemini surfactant molecules.The molecule density in adsorbed surface layer is also improved with increasing the surfactant bulk concentra-tion.When the bulk concentration is increased up to2CMC, the adsorbed surface layer reaches saturation and water CA reaches the maximum.At the same time,disklike aggregates show their existence.Electrostatic repulsion between the head groups without contact with the surface favors the formation of small bilayer aggregates over complete bilayers.As a result, a further increase in surfactant concentration cannot cause the coalescence of patchy bilayer aggregates.

As mentioned above,the effect of equilibrium time and bulk concentration on the aggregate states of C F5C10–C6–C10C F5on a silica surface can be summarized.When the adsorption is per-formed in a short adsorption time,the surfactant molecules on the silica surface will form a?at surface layer with some hemi-sphere aggregates on.With the increase of bulk concentration, the adsorbed amount is raised and the surface layer turns out to be more compact.In the case of long adsorption time,the bi-layer aggregates are formed on top of the?at surface layer.With increasing bulk concentration,the molecular density of the sur-face layer is also improved.When the surface layer reaches saturation,patchy bilayer aggregates are formed.

Our group has studied the adsorption of hexyl- α,ω-bis-(dodecyldimethylammonium bromide)(C12C6C12Br2)gemini surfactant on silica surface[28].The experiment was performed on a silica surface for35min.The adsorbed?lm showed no ag-gregates at the concentration lower than0.5CMC.Then the surfactant molecules started to form circular bilayer islands at a concentration slightly above0.5CMC.With further increase

42X.Song et al./Journal of Colloid and Interface Science304(2006)37–44 of surfactant concentration,the aggregates merged with each

other to form semicontinuous bilayer islands and?nally formed

a two-bilayered structure.The adsorption of the present?uori-

nated gemini surfactant C F5C10–C6–C10C F5is totally different

from that of hydrocarbon surfactant C12C6C12Br2.In the case

of a short adsorption time,C F5C10–C6–C10C F5aggregates will

take the form of a monolayer at the whole concentration range.

It takes a much long equilibrium time for the?uorinated gemini

surfactant to form a bilayer structure.In contrast,the hydrocar-

bon gemini surfactant C12C6C12Br2tends to self-associate into

a bilayer structure.This difference might be due to the weak

interaction among the?uorinated chains.

3.2.Solvent effects

In addition to the chemical nature of the silica surface and

the molecular structure of the surfactant,another important fac-

tor is also known to govern the packing morphology of sur-

factants onto silica surface:the solvent environment[13].On

the basis of studying the adsorption of the?uorinated surfac-

tant C F

5C10–C6–C10C F

5

on silica surfaces,the investigation of

organic solvent effects on the aggregate morphology was car-ried out further.The samples adsorbed with the surfactant for 30min were immersed into different organic solvents for24h. The surfaces treated with n-hexane,dehydrated ethanol,F-113, and toluene are represented as surfaces A,B,C,and D,respec-tively.Fig.5shows a series of representative AFM images for the samples adsorbed at twice CMC and treated with the four organic solvents.Experiments at the concentrations0.05CMC, 0.1CMC,0.5CMC,1CMC,and5CMC were also performed. The aggregate size and shape are very similar and the AFM im-ages are not presented here.It is noteworthy that the surface morphologies in Fig.5signi?cantly differ from each other.Af-ter being treated with n-hexane or dehydrated ethanol,surfaces A and B present some randomly ranged spherical aggregates. When treated with F-113,surface C exhibits a few spherical ag-gregates and elongated rodlike aggregates.It is clear that these surfactant rods have a length ranging from500nm to1μm.Be-ing immersed into toluene leads to the coexistence of branched rodlike and spherical aggregates on surface D.The branched rodlike aggregates on surface D seem to be very?exible and can bend in any degree.These branched rodlike aggregates are as long as3μm,although they are not shown completely in Fig.5D.

The height data of aggregates above the surrounding low surface layer are listed in Table2.The spherical aggregates on surfaces A and B show a height of ca.4nm.The height of the aggregates is consistent with the length of two extended sur-factant hydrophobic chains.It indicates that the aggregates turn into a bilayer structure after being treated with n-hexane or de-hydrated ethanol.The rodlike aggregates on surfaces C and D show a height of slightly less than2nm,which is coincident with the length of one extended surfactant hydrophobic chain. This aggregate height suggests that the surfactant aggregates are Table2

The height data of aggregates on surface treated with organic solvents for24h after being immersed in surfactant solution for30min

Bulk concentration

(CMC)

n-Hexane Ethanol F-113Toluene

0.055.14.91.61.5

0.14.83.31.81.6

0.55.13.71.41.8 14.74.11.51.7 25.13.11.41.7 54.74.51.91.

4 Fig.5.The AFM images of silicon surfaces treated in organic solvents for24h after being self-assembled with gemini surfactant C F5C10–C6–C10C F5for30min deposition at2CMC.(A)n-Hexane,(B)dehydrated ethanol,(C)F-113,(D)toluene.The inserts show the possible stack states of?uorinated surfactants after solvent treatment.

X.Song et al./Journal of Colloid and Interface Science304(2006)37–44

43

Fig.6.Water contact angle of sample after being immersed in2CMC surfactant solution for30min and water contact angles of surfactant-adsorbed samples treated with organic solvents for24h.

present in the form of a monolayer structure after being treated with F-113or toluene.

Water CA measurements can give us much inspiration to understand the effect of organic solvents on the aggregate mor-phology of the?uorinated surfactant on silica surfaces.The water CA of the surfactant-adsorbed samples treated with the organic solvents for24h are listed in Fig.6.For the sake of comparing the wettability,the water CA of the samples ad-sorbed in the2CMC surfactant solution without organic solvent treatment is also provided here.Apparently,water CAs are all raised after being treated with n-hexane,toluene,and F-113, while water CA of the sample treated with dehydrated ethanol is lowered.These results demonstrate that n-hexane,toluene,and F-113can improve the hydrophobicity of surfactant-adsorbed surfaces to some extent.In contrast,dehydrated ethanol may impair the hydrophobicity.

From the discussion above,it can be concluded that the

adsorbed morphology of C F

5C10–C6–C10C F5on silica surfaces

after30min without solvent treatment is a monolayer with some hemisphere aggregates.By analyzing the change of mor-phology and wettability in concert,it might be expected that these hemisphere aggregates change their aggregate morphol-ogy upon treatment with organic solvents.The solubility of

C F 5C10–C6–C10C F

5

in organic solvents might have little effect on

the morphology alteration.The highly charged surfactant mole-cules have poor solubility in organic solvents and the molecules are bound on the silica surface through electrostatic interaction; thus the surfactant molecules cannot be dissolved into organic solvents from the silica surface to a great extent just by being immersed,and the impact of dissolving in organic solvent can be ignored.There is another important issue that needs to be addressed before the organic effect is analyzed.The unusual properties of?uorocarbons center on their simultaneous hy-drophobic and lipophobic nature[29].Accordingly,?uorinated amphiphiles have limited miscibility with hydrogenated am-phiphiles or organic solvents[12].In the solution mixed with ?uorinated and hydrogenated amphiphiles,two kinds of mi-celles coexist and each kind is mainly composed of one kind of surfactant[6].On the other hand,phase separation occurs in the mixed LB?lms of?uorinated and hydrogenated compo-nents[30,31].It is shown that the limited miscibility may push the?uorinated and hydrogenated amphiphiles to change their location.Some previous papers have also observed that differ-ent kinds of?uorinated amphiphile aggregates,such as micelles [32]and bilayer structures[33–35],are formed in different or-ganic solvent solutions.

The possible molecule stack states in the C F5C10–C6–C10C F5 aggregate above surface layer after being treated with organic solvents are shown in the inserts of Fig.5.n-Hexane is a kind of nonpolar organic solvent.The charged polar head groups are not likely to stay in the nonpolar atmosphere of n-hexane.For the sake of reducing the contact area of the head groups with n-hexane,a bilayer structure with the charged head groups in-side would be preferable.In the cases of F-113and toluene, to reduce the contact area of charged head groups with sol-vent molecules,the spherical aggregates on the silica surface tend to connect with each other,which would result in the formation of rodlike aggregates.In addition,the?uorinated hy-drophobic chains tend to extend from the surface and the polar head groups are converted inside.Moreover,compared with n-hexane,F-113and toluene are polarizable solvents;therefore, there will be some spherical aggregates left for these two sol-vents.With respect to F-113,a?uorinated solvent,there will be stronger af?nity of the solvent molecules with?uorinated chains,which may explain the difference in the morphology of these two kinds of rodlike aggregates.In all three cases, more hydrophobic moieties would be exposed to the outside, and the hydrophobicity would be expected to be improved.De-hydrated ethanol can create a kind of polar atmosphere around the surfactant molecules.The charged head groups tend to ex-pose themselves to the interface with ethanol.Thus,the sur-factant molecules may pile up with their hydrophobic chains inside,which will lead to the formation of a bilayer structure. This kind of rearrangement will lower the hydrophobicity of the surfactant-adsorbed surface.It is too complicated to analyze the molecular interaction between the?uorinated surfactant and organic solvent molecules quantitatively.Therefore,the expla-nation of solvent effects on aggregate morphology may just be a hypothesis.Further investigation needs to be done to provide more evidence for the assumption.

4.Conclusions

In this research,we explored the aggregates of?uorinated gemini surfactant C F5C10–C6–C10C F5on silica surfaces with AFM and CA measurement.The effects of bulk concentra-tion and adsorption time on the aggregate states of molecules were analyzed.In the case of short adsorption time,the surfac-tant molecules on solid surfaces exhibited a?at surface layer with some hemisphere aggregates.When the adsorption time was long enough,bilayer aggregates were formed on the?at surface layer.With increased bulk concentration,the surface layer turned out to be more closely packed.Furthermore,the organic solvent effect on the aggregate of the surfactant mole-cules was investigated.The polarity of solvents and af?nity of organic solvents with the surfactant molecules both in?u-enced the stack state of the molecules.Contacting with nonpo-

44X.Song et al./Journal of Colloid and Interface Science304(2006)37–44

lar n-hexane,hemisphere aggregates could be transformed into bilayer structures with polar head groups inside.When the sur-roundings were changed to be ethanol,bilayer structures would be in the form of polar head groups outside.When treated with F-113and toluene,the hemisphere aggregates tended to con-nect with each other and line up,behaving as?exible rodlike or branched rodlike aggregates.Different polarity and af?nity of organic solvents with surfactant molecules will push surfactant molecules to change their packing state. Acknowledgments

We are grateful for?nancial support from the National Natural Science Foundation of China(Grants20233010, 20473101).We greatly appreciate Professor Lei Jiang for valu-able discussions and providing the instrument for CA measure-ments.

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