Adsorption Behaviors of Cationic Surfactants and Wettability inPolytetrafluoroethylene−Solution−Air SystemsDan-Dan Liu,†,‡ Zhi-Cheng Xu,† Lei Zhang,*,† Lan Luo,† Lu Zhang,*,† Tian-Xin Wei,§ and Sui Zhao†

†Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China‡Graduate University of Chinese Academy of Sciences, Beijing 100049, P. R. China§Key Laboratory of Cluster Science of Ministry of Education, Beijing Institute of Technology, Beijing 100081, P. R. China

ABSTRACT: Measurements of the advancing contact angle(θ) and adsorption properties were carried out for aqueoussolutions of four cationic surfactants, hexadecanol glycidylether ammonium chloride (C16PC), Guerbet alcohol hexadecylglycidyl ether ammonium chloride (C16GPC), hexadecanolpolyoxyethylene(3) glycidyl ether ammonium chloride-(C16(EO)3PC), and Guerbet alcohol hexadecyl polyoxy-ethylene(3) glycidyl ether ammonium chloride (C16G(EO)3PC), on the polytetrafluoroethylene (PTFE) surface using thesessile drop analysis. The obtained results indicate that the contact angle decreases to a minimum with the increasingconcentration for all cationic surfactants. Surfactants with branched chain show lower θ values. Moreover, an increase ofadhensional tension on the PTFE−water interface has been observed for the four cationic surfactants, and the branched oneshave larger increases of adhensional tension. It is very interesting that the sharp decrease of θ appears mainly after critical micelleconcentration (cmc) for C16GPC, C16(EO)3PC, and C16G(EO)3PC, which is quite different from traditional cationic surfactantsreported in the literature. Especially for C16G(EO)3PC, there are two saturated adsorption stages on PTFE surface after cmc(which means the saturated adsorption film at air−solution interface has been formed). In the first saturated stage, theC16G(EO)3PC molecules are oriented parallel to the PTFE surface with saturated monolayer formed through hydrophobicinteraction and hydrogen bond. In the second saturated stage, the hemimicelle has been formed on the PTFE surface, which canbe supported by the QCM-D and SPR measurements.

1. INTRODUCTION

Many industrial processes depend on wetting. These processesinclude flotation, detergency, enhanced oil recovery, paintformulation, lubrication, coating, and deposition.1−8 Under-standing and characterizing the wettability of solid surfaces arethus highly essential. Proper wetting of hydrophobic solids withaqueous solutions becomes difficult due to low surface energyof these surfaces.9 Because water has a high surface tension(72.8 mN/m),10 it does not spontaneously spread over solidswith surface free energies smaller than 72.8 mN/m. Theaddition of a surface active agent to water to modify the surfacetension of the system is often necessary to enable water to wetthe surface of the solid.11 And it can also change solid−waterinterfacial free energy, which may cause contact angles todecrease or increase in solid−water drop−air system. There-fore, it is very difficult to establish the conditions of spreadingof aqueous solution of the surface active agents over the solidsurface.10

In recent years, the ability of surfactants to change wetting oflow-energy solids has been studied extensively.7,8,11−23 Zismanand co-workers24−26 characterized those conditions for lowenergetic solids by measuring contact angles, θ, for differentpure liquids and plotting cos θ vs γLV (the liquid surfacetension). Bernett and Zisman25,26 showed that there was alinear relationship between cos θ and the surface tension of notonly pure liquids but also aqueous surfactant solutions.

However, Bargeman and van Voorst Vader27 found, incontrast to Zisman and co-workers,24−26 that there is a linearrelationship for hydrophobic solids, but between the adhesionaltension (γLV cos θ) and the surface tension of aqueous solutionsof several types of surfactants, and that the slope of the lines forlow-energy hydrophobic solids such as paraffin and PTFE wasequal to −1. Our studies of PTFE wettability by aqueoussolution of the four cationic did not confirm the conclusionsdrawn by Bargeman and van Voorst Vader.27 Similar toBargeman and van Voorst Vader, Jan czuk11−20 and his groupshow that there is a linear relationship between adhesional andsurface tension in the case of CPyB, CTAB, C12(EDMAB),BDDAB, TX-165, TX-100, SDS, SHDS, SDDS, and surfactantswith short-chain alcohols at the PTFE surface. They also foundthe surfactant molecules at both interfaces in the saturatedmonolayer should also be the same.However, most of the reported wettabitily studies were

related to traditional surfactants; studies about the cationicsurfactants with branch-chain or polyoxyethylene units arequite rare. In order to systematically investigate surfactants withdifferent structure of the hydrophobic tail groups, we have donea series of wettability research study.

Received: July 22, 2012Revised: November 13, 2012Published: November 14, 2012

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In our previous work,28 we have discovered the four cationicsurfactants have interesting wetting behavior on quartz surface.Four cationic surfactants containing similar polar groups anddifferent nonpolar groups were used in the early and presentwork. The surfactants with branched chain have morehydrophobic group density on quartz surface than the oneswith straight chain due to the presence of branched chain in thehydrophobic group. And the presence of polyoxyethylene unitswill decrease the hydrophobic character of the surfactant: onone hand, the polyoxyethylene units itself has hydrophilicproperties; on the other hand, the bulky polyoxyethylene unitspresent in cationic surfactant molecules enlarge the surface areaoccupied by a molecule, which may result in more looserhydrophobic group density.In recent years, there are many reports about detecting the

adsorption of protein and surfactants on modified solid surfaceby surface plasmon resonance, SPR.29−40 SPR is an opticaltechnique that measures changes in the refractive index ofmedium near a metal surface.41,42 When detecting, first in thesensing chip surface is fixed with a layer of molecularrecognition membrane, and then the sample to be testedthrough the surface of the chip, when the molecules of thesample interacted with the surface biological molecularrecognition membrane, will cause the gold film surfacerefractive index changes, eventually leading to variation ofSPR angle; by detection of SPR angle changes, we can obtainthe concentration of sample, affinity, kinetic constants, andspecific information.The goal of this work was to investigate the four cationic

surfactants on their adsorption from water solutions atinterfaces with air and the PTFE surface. For this purposemeasurements of the contact angle in the PTFE drop ofcationic surfactants solution−air system and the adsorptionproperties through QCM-D and SPR measurements werecarried out.

2. EXPERIMENTAL SECTION2.1. Materials. The series of fatty alcohol hexadecyl polyoxy-

ethylene glycidyl ether ammonium chloride were two pairs of isomericcompounds, which were synthesized in our laboratory.43 Each isomerbears an identical polar group and an identical number of carbonatoms, differing only by the structure of the hydrophobic chain (seeScheme 1).Polytetrafluoroethylene (PTFE) plates used for contact angle

measurements were cut from a large sheet of PTFE. The surfacewas first washed with pure water and acetone, treated with freshlyprepared chromic acid and then in doubly distilled water, and washedin a water ultrasonic bath for 20 min. Next, these surfaces were heatedat 378 K for 2 h.2.2. Contact Angle Measurements. The measurements of

contact angles for the water and aqueous solutions of surfactants onPTFE plates were carried out via the sessile drop method with the

SCA20 from Dataphysics Instruments GmbH, Germany, at constanttemperature (303 ± 0.5 K). The contact angle measurements on bothsides of the drop of a given solution were carried out immediately afterdepositing the drop on the PTFE plate (within about 1−2 min afterdepositing the drop). The measurements were repeated several timesby settling other drops on the new parts of the same plate. Thestandard deviation of the contact angle values was less than 3°. Next, anew plate was placed in the chamber, and the above procedure wasrepeated.

2.3. Surface Free Energy (γSV) Calculation. From now on, thereis no direct method to determine the value solid−liquid interfacialtension (γSL). If we knew the values of the surface free energy (γSV) ofa given solid, the values of the surface tension of the liquid (γLV), andthe measured contact angle values (θ) of a liquid on a given surface (inthe case of PTFE surface), the PTFE−water interfacial tension can becalculated from the Young equation (eq 1):44,45

γ θ γ γ= −cosLV SV SL (1)

By using the method of OWRK,46 it is possible to determine thevalues of surface free energy (γSV) of PTFE. The measurements ofcontact angles for four kinds of liquids (deionized water, formamide,ethylene glycol, dimethylformamide (DMF)) on PTFE plates werecarried out via the sessile drop method with the SCA20 fromDataphysics Instruments GmbH, Germany, at constant temperature(303 ± 0.5 K). Input the test results (deionized water 115.2°,formamide 100.7°, ethylene glycol 90.1°, DMF 62.8°) to the surfacefree energy calculation software of SCA20 and choose the measure-ments method of OWRK; the software automatically calculated thevalues of γSV of the PTFE surface. The calculation result is 22.34 mN/m.

2.4. QCM Measurements. The quartz crystal microbalance(QCM) with Sweden Q-Sense E4 systems record the ac voltageelectrode vibration attenuation changes to determine of the electrodesurface quality changes and mechanical properties by using the inversepiezoelectric effect of quartz crystals. In this method, the chip consistsof a quartz crystal sandwiched between two electrodes to form asandwich structure. Adding an ac voltage at the electrodes at bothends, the resonant frequency of the sensor caused by a small shearvibration; when the ac voltage is turned off, the vibration decaysexponentially, recorded decay curve, resulting in resonant frequency( f) and dissipation factor (D) two parameters, after the appropriatesoftware processing quality changes and the change of mechanicalproperties on the electrode surface. In this study, we use standard goldchip, f 0 = 5 MHz, d = 14 mm.

For a rigid surface, the polarization of the chip before eachmeasurement, crystals were cleaned in a UVO-treatment for 10 min,then heat a 5:1:1 mixture of Milli-Q water, ammonia (25%), andhydrogen peroxide (30%) to 75 °C, immerse the sensors in thesolution using a cleaning holder (5 min), clean tweezers in the samebeaker, rinse in Milli-Q water, and dry with N2. After cleaning thecrystals, put the chip in 1H,1H,2H,2H-perfluorododecanethiol ethanolsolution (1.9 g/L). Then four cationic surfactants solution wascontinuously delivered to the measurement chamber by a multisyringepump (KD Scientific) at different concentrations, using a flow rate of20 μL/min. In order to switch between sample liquids, the flow was

Scheme 1. Formula and Abbreviations of the Surfactants

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interrupted for a few seconds without disturbing the measured signals.The working temperature was 23 °C.2.5. Surface Plasmon Resonance Measurements. The surface

plasmon resonance (SPR) substrates consist of a glass slide (LaSFN9)coated with a gold layer (50 nm in thickness). After deposition of gold,the glass slide was immediately placed in a 1H,1H,2H,2H-perfluorodecanethiol−ethanol solution (1 mmol/L) for 24 h. Afterthe completion of the reaction, pure ethanol solvent was used to rinserepeatedly the modified glass slide and dried under a stream ofnitrogen. Then place the glass slide to the SPR sensor chip system;after the optical path was debugged, we injected the Milli-Q water todo the angle scanning and the dynamic process scanning as thebaseline data. After the Milli-Q water was injected for 10 min, thesurfactants solution was injected to the flow cell and at the same timescanned the SPR dynamic process curve. When the surfactantssolution reached to adsorption equilibrium on the solid surface, theangle scanning was carried out again. Finally continue through the flowcell into water to wash. The surfactant concentrations were 1 × 10−5, 1× 10−4, 1 × 10−3, and 1 × 10−2 mol/L.

3. RESULTS AND DISCUSSION3.1. Wetting of PTFE Surface. The measured values of the

advancing contact angle (θ) for aqueous solutions of cationicsurfactants C16PC, C16GPC, C16(EO)3PC, and C16G(EO)3PCon the PTFE surface are presented in Figure 1. This figureshows the dependence of θ on the logarithm of the totalconcentration of surfactants in aqueous solution (C) for fourcationic surfactants.

From Figure 1 it is seen that the contact angle (θ) keepsconstant at a low concentration and then decreases with theincrease in surfactant bulk concentration, at last keepingconstant again after reaching a minimum at a givenconcentration. During the concentration range from 10−8 to10−5 mol/L, the θ values are slightly changed. However, if theconcentration of surfactant mixtures is close to or higher than10−4 mol/L, then a considerable decrease of θ as a function oflog C is observed. During the range of log C from −3.5 to −2the values of contact angles are almost constant, and they areminimal for a given surfactant.During the range of low concentrations, due to the few

adsorption of surfactants on PTFE surface, the contact angle(θ) changes slightly with the change of concentrations. Withthe increase in concentration, the adsorption of cationic

surfactants at low-energy PTFE surface can occur by weakreactions and hydrogen-bonding interactions, with hydrophilicionic head oriented to the liquid phase and causing a decreaseof γSL for PTFE−liquid interface and contact angle (θ). Thecontact angle (θ) reaches a plateau value when the adsorptionequilibrium is reached at solid surface.The contact angle values of C16PC and C16(EO)3PC are

similar to Jan czuk7,8,11−20 and other researchers’ conclusions;however, the cationic surfactants with branched chains have alower contact angle values when the system reached adsorptionequilibrium. When by the introduction of polyoxyethylene unitsbetween the hydrophilic and hydrophobic groups in thesurfactant molecule, the minimal contact angle (θ) valueschanged slightly.

3.2. Adsorption at the Water−Air and PTFE−WaterInterfaces. In our previous work,28 the surface properties ofcationic surfactants have been measured. The obtained valuesof the critical micelle concentration (cmc) are 4.88 × 10−4, 9.27× 10−5, 9.97 × 10−5, and 5.24 × 10−6 mol/L, and surfacetensions at the cmc (γcmc) are 37.0, 26.7, 26.8, and 30.1 mN/mfor C16PC, C16GPC, C16(EO)3PC, and C16G(EO)3PC,respectively.The solid−liquid interface tension, among other things, can

be determined by measuring the contact angle and using theYoung equation. This and Gibbs44,45 equations were taken intoaccount by Lucassen-Reynders47 to analyze the adsorption of asurfactant at solid−air, liquid−air, and solid−liquid interfaces.Many other researchers found that it is also possible to

establish the dependence between adhesional and surfacetension for both hydrophobic and hydrophilic solids:10,46,47

γ θ γ= +a bcosLV LV (2)

where a and b are constants. The value of a depends on thesolid surface property. From the above-mentioned equation, itis possible to determine not only the critical surface tension ofsolid wetting but also the relation between the adsorption ofsurfactant at the water−air and solid−water interfaces. Theyhave also found that the slope of the straight line is −1, whichimplies a similar adsorption density at solid−liquid and air−liquid interface in the case of hydrophobic solids.28

Figure 2 shows the relationship between the product of γLVcos θ and γLV for PTFE−cationic surfactant solutions systems.

Figure 1. Dependence of the measured values of the contact angle (θ)of surfactant solutions and log C.

Figure 2. Dependence of adhesional tensions as a function of surfacetensions of surfactant solutions.

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From this figure it results that for all investigated surfactants alinear relationship exists between the adhesional and surfacetension during a range of certain concentrations. Quite differentfrom Jan czuk11−20 and his group, the slope of the γLV cos θ−γLVcurve for the studied system is not equal to −1 on the PTFEsurface. In this system, for C16PC, as expected, a straight linearrelationship exists in the entire range of values, and the slope ofthe straight line is equal to about −0.75. For C16GPC,C16(EO)3PC, and C16G(EO)3PC, the slope of the straight lineis equal to about −0.47, −0.41, and −0.41, respectively.However, during the range of high C16GPC, C16(EO)3PC, andC16G(EO)3PC concentrations corresponding to values lowerthan 25.6, 25.4, and 28.2 mN/m, the curve is close to vertical.This vertical phenomenon is similar to FSN100 and FSO100,20

which are fluorocarbon surfactants at the PTFE−solutioninterface. It can be explained that the behavior of fluorocarbonsurfactants in the surface layer at the PTFE−solution interfaceis quite different from that of hydrocarbon surfactants, and insuch case, adsorption at this interface is not the same as at thewater−air interface as well as the structure of the surface.20 ForC16GPC, C16(EO)3PC, and C16G(EO)3PC, when the bulkconcentration arrives at its cmc, the surface tension keepsconstant. However, the adsorption of surfactants at PTFEsurface continues and leads to the steep drop of θ. This led tothe performance for the adhesional tension values are fromnegative to positive, and with the increasing of concentrationthe adhesional tension values increase significantly. This resultsin the vertical phenomenon.A direct method to investigate relative adsorption to

interfaces in wetting studies was developed by Lucassen-Reynders.48 By combining the Young (eq 1) and Gibbsequations, it was shown that

γ θγ

=Γ − Γ

Γd( cos )

dLV

LV

SV SL

LV (3)

where ΓSV, ΓSL, and ΓLV represent the surface excessconcentration of surfactants at solid−air, solid−water, andwater−air interfaces, respectively. Assuming that ΓSV ≈ 0, it ispossible to establish from eq 3 the ratio of ΓSL to ΓLV plottingγLV cos θ vs γLV; by combining eqs 2 and 3, eq 3 can be writtenin the form

ΓΓ

= −aSL

LV (4)

Table 1 shows the values of ΓSL/ΓLV; from the data we cansee the values equal 0.75, 0.47, 0.41, and 0.41 for C16PC,

C16GPC, C16(EO)3PC, and C16G(EO)3PC, respectively. Thisindicates that adsorption behaviors of these surfactants at thePTFE−water interface are quite different. It means that theamount of the surfactant adsorbed at the PTFE−water interfaceis a little smaller than at the water−air interface for C16PC.However, for C16GPC, C16(EO)3PC, and C16G(EO)3PC theamount of the surfactant adsorbed at the PTFE−waterinterfaces is about 2 times more smaller as compared to that

at the water−air interface. Moreover, the ΓSL/ΓLV values ofquartz systems are obviously higher than those of PTFEsystems, especially for C16GPC, C16(EO)3PC, and C16G-(EO)3PC. It is clear that the adsorption on PTFE interfacethrough hydrophobic interaction is weaker that the adsorptionon quartz interface through electrostatic attraction, and theintroduction of polyoxyethylene units or branched hydrophobicchain both make considerable reduction in ΓSL/ΓLV value dueto steric effect.

3.3. Work of Adhesion. The work of adhesion of liquid tosolid, WA, is defined by equation49

γ γ γ= + − WSL SV LV A (5)

Introducing eq 5 to the Young equation (eq 1), we obtain

γ θ= +W (cos 1)A LV (6)

Taking into account eq 6, the measured values of contact anglefor aqueous solution of surfactants on PTFE surface, and thedata of their surface tension,14 the values of the work ofadhesion of solution to PTFE surface were calculated.The obtained results are presented in Figure 3. From Figure

3, we can see that the adhesion work remains almost constant

over the low surfactant concentration range. With the increasein surfactant concentration of the solution, there is a decrease inthe adhesion work for all surfactants solutions. After WAreached its minimum value, the WA values of C16PC andC16(EO)3PC change slightly in the range of the followingconcentrations. For C16GPC and C16G(EO)3PC, there is amarked increase of WA. While for C16PC, the increase of thevalue is slight. It is interesting that there is a minimum of WA atconcentration close to their cmc. Because the work of adhesionis the sum of surface tension and adhesion tension, at theconcentration of cmc, surface tension reaches a minimum value.However, the surfactants keep adsorbing at the solid−liquidinterface, and the value of adhesion tension increases with theincreasing concentration after cmc. So the work of adhesion inthe concentration of cmc reached the lowest value.

3.4. Structural Dependence of Cationic Surfactants onWettability of PTFE Surface. In order to expound themechanism responsible for adsorption of cationic surfactants toPTFE surface, the dependences of surface tension, contact

Table 1. Values of ΓSL/ΓLV of Studied Cationic Surfactantson PTFE and Quartz Surface

cationic surfactant C16PC C16GPC C16(EO)3PC C16G(EO)3PC

PTFE surface 0.75 0.47 0.41 0.41quartz surface28 1.09 1.03 0.96 0.85

Figure 3. Dependence of the adhesion work (WA) of surfactantsolutions to PTFE surface and log C.

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angle, and adhesional tension on bulk concentration have beenreplotted for C16PC, C16GPC, C16(EO)3PC, and C16G-(EO)3PC, respectively.We can see clearly from Figure 4 that the variations in

contact angle, surface tension, and adhesional tension with

increasing bulk concentration can be divided into four stagesfor C16PC solutions. The possible schematic model of theadsorbed C16PCon the PTFE surface in the four regions ofconcentration is proposed in Figure 5.

In stage 1, the bulk concentration is too low to formadsorption films at both liquid−air and solid−liquid interfaces;therefore, the surface tension and γSL change slightly. Contactangle and adhesional tension all keep almost constant until 5 ×10−7 mol/L.In stage 2, the unsaturated adsorption films begin to be

formed by cationic surfactant molecules at both liquid−air andsolid−liquid interfaces; surface tension and the absolute valueof adhesion tension become lower with the increase ofconcentrations. As θ is higher than 90°, the decrease of surfacetension can lead to the increase of θ, and the absolute value ofadhesion tension can result in the decrease of θ. The result ofthe competition of the two factors at this stage is that thecontact angle decreases gradually.In stage 3, the surface tension at the liquid−air interface

keeps gradually decreasing, and there is a sharp decrease of theabsolute value of adhesional tension. Therefore, there is afurther increase of adsorption on PTFE surface. This leads tothe sharp decrease of θ.In stage 4, the saturated adsorption films have been formed

at both PTFE−liquid and liquid−air interfaces at the same bulkconcentration. The bulk concentration arrives at its cmc, andthere is no adsorption at PTFE−liquid interface anymore.Therefore, the surface tension, contact angle and adhesionaltension all keep constant.

In the case of C16GPC (Figure 6), there are also four stagesduring the whole adsorption process of C16GPC on PTFE

surface. The possible schematic model of the adsorbed C16GPCon the PTFE surface is proposed in Figure 7. It is clear that the

trend of surface tension and contact angle is inconsistent.However, the experimental results obtained by Jan czuk11,13,14and his group show that the trend of surface tension andcontact angle is consistent in the case of linear molecules suchas CTAB, TX-165, TX-100, C12(EDMAB), and BDDAB at thePTFE surface.In stage 1, the bulk concentration is too low to form

adsorption films at both liquid−air and solid−liquid interfaces;therefore, the surface tension and γSL change slightly. Contactangle and adhesional tension all keep almost constant until 1 ×10−7 mol/L.In stage 2, the unsaturated adsorption films have been

formed by cationic surfactant molecules at both liquid−air andsolid−liquid interfaces; surface tension and the absolute valueof adhesion tension become lower with the increase ofconcentrations. The contact angle decreases gradually withthe increase of concentrations.In stage 3, the bulk concentration arrives at its cmc, and

saturated adsorption film at liquid−air interface has beenformed. The surface tension keeps constant. However, theadsorption of surfactants at the PTFE surface is enhancedgreatly, which may be attributed to the formation of aggregatesby C16GPC molecules on the PTFE interface. The performancefor the adhesional tension values is from negative to positive,and with the increasing of concentration the adhesional tensionvalues increased significantly; this causes the steep drop of θ.In stage 4, the saturated adsorption film has been formed at

the PTFE−liquid interface. The adhesional tension does notchange any longer, and θ reaches the platform value.

Figure 4. Dependence of the adhesional data of C16PC solution toPTFE surface and log C (the lines are just for the eyes).

Figure 5. Schematic model of C16PC adsorption on the PTFE surface.

Figure 6. Dependence of the adhesional data of C16GPC solution toPTFE surface and log C (the lines are just for the eyes).

Figure 7. Schematic model of C16GPC adsorption on the PTFEsurface.

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It is very interesting to point out that the interaction betweensurfactant with branched hydrophobic tail is stronger thanlinear molecule. When saturated adsorption film has beenformed at liquid−air interface and micelles has been formed inbulk solution, the micelle-like aggregates may formed also at thePTFE−water interface, which will be discussed furtheraccording to data of adhesional tensions and QCM of foursurfactants.The dependences of surface tension, contact angle, and

adhesional tension on bulk concentration C16(EO)3PC areplotted in Figure 8.

We can see clearly from Figure 8 that the variations incontact angle, surface tension, and adhesional tension withincreasing bulk concentration can be divided into five stages forC16(EO)3PC solutions. The possible schematic model for thefive regions is proposed in Figure 9.

In stage 1, the bulk concentration is too low to formadsorption films at both liquid−air and solid−liquid interfaces;therefore, the surface tension and γSL change slightly. Contactangle and adhesional tension all keep almost constant until 1 ×10−7 mol/L.In stage 2, there is a low adsorption of C16(EO)3PC

molecules at the liquid−air interface, but the unsaturatedadsorption films have been formed at the solid−liquid interface,surface tension changes slightly, and the absolute value ofadhesion tension becomes lower with the increase ofconcentrations. The contact angle decreases gradually withthe increase of concentrations. The adsorption occurred at thesolid−liquid interface first may due to the hydrogen bondformed between polyoxyethylene units and fluorine atom in thePTFE surface.In stage 3, the unsaturated adsorption films have been

formed by cationic surfactant molecules at both liquid−air andsolid−liquid interfaces; surface tension and the absolute value

of adhesion tension become lower with the increase ofconcentrations. θ decrease gradually with the increase ofconcentrations.In stage 4, the bulk concentration arrives at its cmc, and the

saturated adsorption film at liquid−air interface has beenformed. The surface tension keeps constant. However, theadsorption of surfactants at PTFE surface continues and leadsto the steep drop of θ.In stage 5, the saturated adsorption film has also been formed

at the PTFE−liquid interface. The adhesional tension does notchange any longer, and θ reaches the platform value.The dependences of surface tension, contact angle, and

adhesional tension on bulk concentration C16G(EO)3PC areplotted in Figure 10. We can see clearly from Figure 10 that the

variations in contact angle, surface tension, and adhesionaltension with increasing bulk concentration can be also dividedinto five stages. The possible schematic model for the fiveregions is proposed in Figure 11.

In stage 1, the bulk concentration is too low to formadsorption films at both liquid−air and solid−liquid interfaces;therefore, the surface tension and γSL change slightly. Contactangle and adhesional tension all keep almost constant until 1 ×10−6 mol/L.In stage 2, the unsaturated adsorption films begin to be

formed by C16G(EO)3PC surfactant molecules at both liquid−air and solid−liquid interfaces gradually; surface tension andthe absolute value of adhesion tension become lower with theincrease of concentrations. The result of the competition of thetwo factors at this stage is that the contact angle decreasesgradually.In stage 3, the saturated adsorption film has been formed at

both PTFE−liquid and liquid−air interfaces at the same bulkconcentration; the surface tension, contact angle, and adhe-sional tension all keep almost constant.

Figure 8. Dependence of the adhesional data of C16(EO)3PC solutionto PTFE surface and log C (the lines are just for the eyes).

Figure 9. Schematic model of C16(EO)3PC adsorption on the PTFEsurface.

Figure 10. Dependence of the adhesional data of C16G(EO)3PCsolution to PTFE surface and log C (the lines are just for the eyes).

Figure 11. Schematic model of C16G (EO)3PC adsorption on thePTFE surface.

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In stage 4, the adsorption of surfactants at PTFE surface isenhanced greatly as the hemimicelle has been formed. Theperformance for the adhesional tension values is from negativeto positive, and with the increasing of concentration theadhesional tension values increase significantly, which results inthe steep drop of θ.In stage 5, the saturated adsorption layer has also been

formed at the PTFE−liquid interface. The adhesional tensiondoes not change any longer, and θ reaches the second platformvalue.The values of adhesional tensions can represent the

adsorption amount of surfactant molecules of solid surfaces.The adhesional tensions of four cationic surfactants atadsorption equilibrium on both PTFE and quartz surfaces arelisted in Table 2.

For quartz surface, the adsorption of surfactant moleculeswill lead to the increase of γSL and the decrease of adhesionaltension. The higher the hydrophobically modified degree is, thelower the adhesional tensions. However, in the case of low-energy PTFE surface, surfactant molecules adsorb onto solidthrough hydrophobic interaction and ionic head orients toward

aqueous bulk. Therefore, the adsorption of surfactant moleculeswill lead to the decrease of γSL and the increase of adhesionaltension. From Table 2 we can see that the values of adhesionaltensions on PTFE surface for C16PC and C16(EO)3PC aremuch lower than C16GPC and C16G(EO)3PC (the secondplatform). The possible reason is that the aggregates formed byC16GPC and C16G(EO)3PC molecules will enhance ionic headdensity and result in higher adhesional tension.The introduction of polyoxyethylene units and branched

chain has shown an interesting adsorption phenomenon. Thereare two adsorption equilibrium stages on PTFE surface. In thefirst adsorption equilibrium stage, the molecules of C16G-(EO)3PC may orient parallel to the PTFE surface due to thesteric of branched alkyl chain and the hydrogen bond betweenpolyoxyethylene units and PTFE. Therefore, the adhesionaltension is −8 mN/m, which is obviously lower than all othersurfactants. In the second equilibrium stage, the formation ofsemimicelle-like aggregates improves adhesional tensiondramatically.

3.5. Quartz Crystal Microbalance. In order to prove theinference of adsorption mechanisms for four cationicsurfactants on PTFE surface, we have addressed this questionby employing a quartz crystal microbalance with dissipationmonitoring ability, QCM-D. The layer QCM-D sensors wasspin-coated on gold, after using 1H,1H,2H,2H-perfluorodode-canethiol ethanol solution modified the sensor surface, the goldatom and thiol will form strong chemical bonds. Then thesurface properties are similar to PTFE surface which ishydrophobic. (The contact angle is about 105°, which issimilar to value on PTFE surface.) Figures 12 and 13 show the

Table 2. Adhesional Tension of Studied Cationic Surfactantsat the Time of Adsorption Equilibrium on PTFE Surface andthe Saturated Monolayers Formed on Quartz Surface

surfactant water C16PC C16GPC C16(EO)3PC C16G(EO)3PC

PTFE (mN/m) −27 1 15 2 −8/15quartz surface28

(mN/m)65 25 12 25 13

Figure 12. Resonant frequency ( f) and dissipation factor (D) change at 1.0 × 10−6 mol/L for all cationic surfactants on fluorine modified surface:(A) C16PC; (B) C16GPC; (C) C16(EO)3PC; (D) C16G(EO)3PC.

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experimental results of adsorption on the modified surface forfour cationic surfactants.The adsorption of surfactant on quartz crystal surface can

affect the resonance frequency and dissipation factor. Ifsurfactant molecules adsorbed on the surface of the quartzcrystal can be as an rigid spheres, and form monolayer with noobvious molecular interactions, then different resonancefrequency excitation conditions, resonance frequency, anddissipation factor changes are consistent. At the same time,along with the increase of the number of adsorbed molecules,resonant frequency will decrease and the dissipation factor willincrease. Therefore, the amplitude of the resonance frequencyand dissipation factor changes can be quantitative character-ization of surfactant absorption number.In addition, if there is surfactant molecular interaction in the

adsorbed layer on the quartz crystal surface, or is not amonolayer adsorption (formation of aggregates or bilayeradsorption), the data will appear different under differentresonant frequency. Moreover, the linear relationship may nolonger exist between the change in resonance frequency/dissipation factor and the mass of adsorption.From Figures 12 and 13 we can see clearly that the change of

flat 1.0 × 10−6 mol/L for all cationic surfactants increases withtime until the plateau value, while at the concentration of 1.0 ×10−2 mol/L the change of passes through a minimum forC16GPC and C16G(EO)3PC. These results allow us to concludethat the adsorbed mass of surfactants at the concentration of1.0 × 10−6 mol L−1 on PTFE surface is low, and the lowdissipation value indicates that the adsorbed layer is ratherflat.50 At even higher cationic surfactants concentrations, thedissipation value is much higher due to more adsorption to thesurface. For C16GPC and C16G(EO)3PC, the curve is different

from the others, which indicates the C16GPC and C16G-(EO)3PC molecules formed new structures on PTFE surface.This conclusion is qualitatively consistent with predictionsbased on the PTFE surface.

3.6. Test Results of SPR Measurements. The adsorptionproperties of surfactants on the modified metal surface werecharacterized by SPR spectroscopy. The gold surface wasmodified by 1H,1H,2H,2H-perfluorodecanethiol (1 mM), thesurface then just had the hydrophobic surface properties. Thecontact angle was 106° after modification. Detecting theresonance angle can calculate the adsorption capacity ofsurfactants. This angle variation is in the BIA core terminologyexpressed in resonance units (RUs), where 1 RU is equal to a0.0001° change in the intensity minimum. This variation, ΔRU,is directly proportional to the mass adsorbed, Δm, according toΔm = ΔRU × CSPR/β, where CSPR is a factor containing aninstrument constant, dn/dc is the variation of the refractiveindex with concentration for the adsorbent, and β is a factorcompensating for the decrease in the SPR signal with distancefrom the gold substrate.29 CSPR was calculated to 0.094 ± 0.008ng/cm2 using an average dn/dc for 18 different surfactants,29

and β was set to 1, which is the case for a plain gold surface.51

The adsorption isotherms for the four cationic surfactants areshown in Figure 14.We can see that the adsorbed capacity is increase with the

increase in surfactant bulk concentration for C16PC andC16(EO)3PC. However, for C16GPC at 10 mM and C16G-(EO)3PC at 1 and 10 mM, the adsorbed capacity is lower thanthe adsorbed capacity of low concentrations. The adsorbedcapacity for C16GPC and C16G(EO)3PC at high concentrationspresents strange regular when we set β to 1. The factor β willdiffer from 1 when the surface layer is thick.29 In this work,

Figure 13. Resonant frequency ( f) and dissipation factor (D) changes at 1.0 × 10−2 mol/L for all cationic surfactants on fluorine modified surface:(A) C16PC; (B) C16GPC; (C) C16(EO)3PC; (D) C16G(EO)3PC.

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when the concentration of surfactants is low, one can anticipatethe formation of a very thin layer and β will be close to 1. Whenat high concentrations especially for C16G(EO)3PC, it will formdifferent structure on solid surface, the surface layer is thick, atthis time, β is lower than 1. This conclusion is qualitativelyconsistent with predictions based on the PTFE surface.

4. CONCLUSIONSOn the basis of the obtained results of the contact anglemeasurements and the QCM-D and SPR measurements, wecan state the following:

(1) The contact angle (θ) decreases with the increase ofsurfactant concentration and then keeps constant afterreaching a minimum at a given concentration for allcationic surfactants. The cationic surfactants that havebranched chains have lower contact angle values.

(2) There is a linear relationship exists between theadhesional and surface tension in a range of certainconcentrations for all investigated surfactants. Theadsorption on the air−solution interface is higher thanthat on the PTFE−solution interface at low bulkconcentration.

(3) An increase of adhensional tension on the PTFE−waterinterface has been observed for the four cationicsurfactants. The branched cationic surfactants have anobviously increase of adhensional tension due to theincrease of adsorption densities.

(4) The sharp decrease of θ appears mainly after criticalmicelle concentration for C16GPC, C16(EO)3PC, andC16G(EO)3PC. The branched surfactants show lower θvalues due to the higher adsorption amounts on PTFEsurface resulting from the formation of aggregates.Especially for C16G(EO)3PC, there are two saturatedadsorption stages on the PTFE surface. In the firstsaturated stage, the C16G(EO)3PC molecules areoriented parallel to the PTFE surface with saturatedmonolayer formed through hydrophobic interaction andhydrogen bond. In the second saturated stage, thehemimicelle has been formed on the PTFE surface,which can be supported by the QCM-D and SPRmeasurements.

■ AUTHOR INFORMATION

Corresponding Author*E-mail zl2558@hotmail.com (Lei Zhang), luyiqiao@hotmail.com (Lu Zhang); Tel 86-10-82543587; Fax 86-10-62554670.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

The authors thank financial support from the National Science& Technology Major Project (2011ZX05011-004) of China.

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Figure 14. Mass adsorbed on modified gold surface determined bySPR for four surfactants with concentration dependence.

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