Relation between the Wetting Effect and the Adsorbed Amount of

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Relation between the Wetting Effect and the Adsorbed Amount of Water-Soluble Polymers or Proteins at Various Interfaces Lalit M. Pandey and Sudip K. Pattanayek* Department of Chemical Engineering, IIT Delhi, New Delhi, 110016, India S Supporting Information *

ABSTRACT: The contact angle of a solution is related to the adsorbed masses at contact points of three different interfaces through Lucassen-Reynders’s relation. The direct use of this relation is restricted by the difficulty of measurement of adsorbed mass at the solid−vapor and vapor−liquid interfaces. We have extended the LucassenReynders’s relation to show that adsorbed masses are related linearly with the wetting effect, which is defined as the difference of wetting tensions of solution and solvent. The wetting effects are determined from the measured contact angle of aqueous solution of poly(oxyethylene), poly[1-(2-oxopyrrolidin-1-yl)ethylene], poly(sodium 4-styrene sulfonate), poly(acrylic acid), albumin, and fibrinogen on various self-assembled surfaces and surface tension of solutions. The extended Lucassen-Reynders’s relation is verified using the experimentally determined wetting effect and adsorbed mass of polymers and proteins determined using a quartz crystal microbalance (QCM). The reasonable value of the determined adsorbed mass on liquid−vapor and solid−vapor interfaces indicates the applicability of our methodology.

1. INTRODUCTION Over the years self-assembled monolayers (SAMs) have been tested for their suitability as surface modifiers of biomaterials.1−6 The rule of thumb for checking for its suitability as a biomaterial, along with many other factors, is that (i) the water contact angle, θ, on the biomaterial4−6 should lie within the range 60° to 80° and (ii) the adsorbed amount of proteins should be less. These two experiments need to be done separately. Our thesis is that the contact angle of protein solution on the surfaces will give direct information about the adsorbing substrates, that is, its adsorption capacity. On a smooth surface, the contact angle of a solution of small molecular weight solute is related to interfacial tensions at the contact point of three phases through Young’s equation.7−9 The surface tensions are related to the adsorbed mass of small molecular organic solute at the interface through the Gibbs equation.10−12 Using the Gibbs equation in Young’s equation, Lucassen-Reynders13,14 derived the equation relating the adsorbed amount of the small molecular solute, contact angle, and surface tension of the solution. The above relations have been used for solution containing small molecular weight solutes like surfactants. However, the validity of the above equations is doubtful for solution containing a high molecular weight solute like proteins/polymer. As, for example, using the Gibbs equation, the adsorbed amount of various proteins from their solutions on liquid−vapor interface has been reported15,16 to be 60 times higher than the expected adsorbed mass that is typically 1 mg·m−2. On the other hand, the adsorbed mass of protein on solid surfaces, when a drop of protein solution is placed on a surface, has been calculated by using LucassenReynders’s equation assuming negligible adsorption of proteins at the solid−vapor interface. Cha et al.15 have studied the contact angle of solution of various proteins on four different © 2013 American Chemical Society

substrates: silica, SAMs of amine and methyl, and a polystyrene coated surface. They have determined the Gibbs excess adsorbed amount at the solid−liquid and liquid−vapor interfaces from the surface tension of solution and contact angle of the solution. The actual adsorbed masses of the proteins for all of these cases are not determined experimentally to verify the theory. Assuming a zero adsorbed amount at solid−vapor interface, Busscher et al.17 have tried to correlate the dynamic contact angle and time-dependent adsorbed mass of bovine serum albumin determined by using axisymmetric drop shape analysis by a profile and radio-labeling technique. Later on, a similar analysis of the experiments was done by the research group18 by using ellipsometry instead of a radiolabeling technique, to determine the adsorbed amount of protein at solid interfaces. But the verification of Gibbs equation and its subsequent application to determine the adsorbed amount at the solid−liquid interface are not done. The objective of present work is to quantify the adsorbed amount of water-soluble polymers on air−liquid and modified SAM−vapor interfaces using data of measured contact angle of their solution over the surfaces and determine the adsorbed mass using a quartz crystal microbalance (QCM).

2. EXPERIMENTAL SECTION The SAMs are used to obtain a physically smooth and chemically uniform surface. Physical smoothness is important on getting correct contact angle data. We have chosen watersoluble polymers of three different types: uncharged watersoluble polymer, poly(oxyethylene) (PEO), positively charged Received: August 4, 2013 Accepted: October 16, 2013 Published: October 31, 2013 3440

dx.doi.org/10.1021/je400712k | J. Chem. Eng. Data 2013, 58, 3440−3446

Journal of Chemical & Engineering Data

Article

Table 1. Chemicals Required for Formation of Various SAMs and Characteristics of the SAMs name SAMs/effective chemical groups on the surface mono

amine (NH2) chloro (Cl) mercapto (SH) methyl (CH3) octyl (CH3)

mixed

amine:methyl (1:1) (NH2 + CH3) amine:octyl (1:1) (NH2 + CH3)

γSV/mJ·m−2

θ/deg

chemical required (source and purity)

water

diiodomethane

polar γpSV

dispersive γdSV

total γSV

(3-aminopropyl) trimethoxysilane (APTMS) (Sigma Aldrich cat. no. 281778 with purity of 97 %) (3-chloropropyl) trimethoxysilane (CPTMS) (Sigma Aldrich cat. no. 440183 with purity of ≥ 97 %) (3-mercaptopropyl) trimethoxysilane (MPTMS) (Sigma Aldrich cat. no. 175617 with purity of 95 %) trimethoxymethylsilane (TMMS) (Sigma Aldrich cat. no. 246174 with purity of 98 %) octyltrimethoxysilane (OTMS) (Sigma Aldrich cat. no. 376221 with purity of 96 %) APTMS + TMMS

62 ± 1

38 ± 1

13.87

32.44

46.31

65 ± 1

34 ± 1

11.11

35.15

46.26

70 ± 1

35 ± 1

8.32

35.88

44.20

97 ± 1

57 ± 1

1.01

29.13

30.14

103 ± 1

60 ± 1

0.17

28.75

28.93

79 ± 1

44 ± 1

5.14

33.18

38.32

80 ± 1

46 ± 1

4.99

32.24

37.22

APTMS + OTMS

(iii) measuring its contact angle against two low molecular weight liquids, water and diiodomethane. We took the FTIR spectra of the modified surface of silicon wafer in the transmission mode on a Nicolet 6700 FTIR spectrophotometer. The processing of the spectra was done using the OMNIC 7.3 software. We have obtained images of the surface morphology by atomic force microscopy (make Digital Instruments Multimode Nanoscope IIIA) in contact mode using a silicon nitride tip (tip radius 50 nm). Average height (Ra) and root-mean-square average height (Rq) were determined using the Gwyddion software. The amplitude and extent of the surface features were used to determine the roughness factor (Rf) according to a previously described procedure.19 AFM images and details analysis are shown in the Supporting Information. The surface energies of the silica-modified surfaces are determined from its contact angle against deionized water and diiodomethane at 297 K using a semiempirical relation (equation 3), which is discussed later. The contact angles were measured by a contact angle goniometer (Kruss DSA-10). The drop images were stored by a video camera. Contact angles were calculated by fitting a circle on the obtained drop shape. The contact angle changed with time from the start of the placement of the drop on the surface. After a certain time, it reached an equilibrium value corresponding to a constant drop volume. We reported equilibrium angle as the static contact angle. About 12 readings were taken on each surface made from different preparations. 2.2. Characteristics of Proteins and Water-Soluble Polymer. We have used PEO of lot 03722CH, (Sigma Aldrich Catalog no. 372781), PVP of 01114 (Catalog no. CDH 024643), PAA of lot 00117BH (Sigma Aldrich Catalog no. 306215), NaPSS of lot 05920ME (Sigma Aldrich Catalog no. 434574), BSA of lot 110M7400 V and purity of ≥ 99 % (Sigma Aldrich Catalog no. A0281), and Fb of lot 029K7636 V and purity of 85 % (Sigma Aldrich Catalog no. F8630). The degree of sulfonation in the polystyrene is 83 % of the total styrene group present23 in the polymer. We have prepared their solution at 298 K using distilled water at the various concentrations as listed in Table 2. The surface tension of polymer solutions was measured using a Kruss Processor tensiometer K100 using the ring method. Each measurement was done in triplicate. 2.3. Contact Angles of the Polymer and Protein Solution. The contact angles of solutions of four different

polymer, poly[1-(2-oxopyrrolidin-1-yl)ethylene] (PVP), and negatively charged polymer, poly(sodium 4-styrene sulfonate) (NaPSS) and poly(acrylic acid) (PAA). We have chosen two model proteins, albumin from bovine serum (BSA) and fibrinogen from bovine plasma (Fb). All materials used here, unless it is mentioned, are purchased from Sigma-Aldrich and used without further purification. 2.1. Substrates and Its Characterization. We have modified silicon wafers by various monotype head groups of SAMs and mixed SAMs using organosilane modifiers as listed in Table 1. The table also lists the source of chemicals used for modifications and their purity as listed by manufacturer. The process of modifications in detail can be found in our earlier works.19−22 They are described briefly below. The silicon wafers (p-type, orientation 100) were cut into 10 mm square pieces. The cut pieces were sonicated consecutively with freshly prepared piranha solution for a period of one hour, ammonia solution for a period of half an hour, and hydrochloric acid (HCl) solution for a period of half an hour. After the sonication activated surfaces were washed consecutively with plenty of deionized water and acetone. Finally, the wafers were dried under vacuum and were ready for modification with SAMs. Cleaned wafers were immersed in solution (v = 0.01) of silane coupling agent in anhydrous toluene (Sigma Aldrich cat. no. 244511 with a purity of 99.8 %) for a period of 24 h at room temperature (300 K) under nitrogen atmosphere. Depending on the requirement of the headgroup, corresponding silane coupling agent is used (as listed in Table 1). Mixed SAMs were formed according to the procedure given in refs 11 and 6. Briefly, the cleaned wafers were dipped into a mixture of two silane coupling agents with the required volumetric ratio in anhydrous toluene for a period of 24 h at room temperature under nitrogen atmosphere; that is, we have synthesized mixed SAMs of amine−octyl (1:1) by mixing APTMS and OTMS in an equal volumetric ratio. The feed composition is indicated in parentheses. After the silanization, the wafers were sonicated consecutively in three different solvents: toluene, a binary mixture of toluene and methanol, and methanol for a period of 0.033 h in each step. Finally, wafers were dried under vacuum. The modified surfaces were characterized by following three methods: (i) the detection of newly formed functional groups on it by Fourier transform infrared spectroscopy (FTIR), (ii) viewing topography of the modified surfaces using AFM, and 3441



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γij = γi + γj − 2ϕ[(γidγjd)1/2 + (γi pγjp)1/2 ]

Table 2. Physical Characteristics of Various Polymers and Proteins

a

polymer

w/mass fraction

W/(g·mol−1), average molar massa

pH

PEO NaPSS BSA fibrinogen

0.001 0.001 0.00002 0.00002

106 106 6.6·104 3.4·105

6.8 7.4 6.9 7.0

where γpi and γdi are the polar and dispersive part of surface energy of pure phase i and ϕ is the interaction parameter. For like materials ϕ = 1, but it can have values from 0.5 to 1.15 for dissimilar molecules.8 However, researchers take ϕ = 1 for simplicity. We can determine the components of surface energy of an unknown surface from contact angle of two known liquids on the surface using eqs 1 to 3. When a drop of dilute solution of polymer−protein is placed on a substrate, the solute goes to the interface of the two phases according to the activity of solute. The interfacial tension between the two phases and amount of solute material at interfaces are related by the Gibbs adsorption equation10−12

The viscosity average molar mass for PEO and NaPSS.

polymers (PEO, NaPSS, PVP, and PAA) of concentration (w = 0.001) and two different proteins (BSA and Fb) of concentration (w = 0.00002) on the modified surfaces (listed in Table 1) were measured at room temperature 295 K using a goniometer (Kruss DSA10). The drop images were stored by a video camera. Contact angles were calculated by fitting a circle on the obtained drop shape. Contact angle may vary with time from the start of the placement of the drop on the surface. After a certain time, it reached an equilibrium value corresponding to a constant drop volume. We reported equilibrium angle as the static contact angle. We have measured at least 10 different data and report the average values. 2.4. Adsorbed Mass of Protein and Water-Soluble Polymers by Using QCM. Adsorption experiments of proteins (BSA and Fb) and polymer solutions (PEO and NaPSS) were carried out using QCM. Procedures of experiments were similar to that we have described in our previous work.19 Briefly, silica coated quartz crystal was modified with the required modifier in the QCM chamber. After surface modification, polymer/protein solutions were passed through QCM for adsorption and kept for a period of half an hour. We obtained changes in resonance frequency of different overtones, n, (ΔFn/n), and dissipation factor (ΔDn) of the quartz crystal in real time due to adsorption of the polymer/proteins. The adsorbed mass (Δm) is related to ΔFn by the Sauerbrey equation19,24 given as Δm = −C·ΔF/n. Here C is sensitivity constant of crystal given by 17.7·10−10 kg·m−2·Hz−1. This equation is valid for ΔDn ∼ 10−6. 2.5. Interrelation of Surface Tension, Adsorbed Amount, and Contact Angle of Solution on a Substrate. When a drop of pure liquid of small molecular weight is placed on a smooth solid surface, the contact angle (θ) depends on the interfacial tensions at the air−liquid, solid−liquid, and solid−air interfaces. The contact angle is related to interfacial tensions through Young’s equation7−9 γSV − γSL = γLV cos θ

Γj = −

dγj 1 · RT d ln a

(4)

where R is the universal gas constant; T is temperature; dγj is the change in interfacial tension at the jth interface with change in activity, a, of solute. The activity becomes concentration, c, for an ideal solution. We note that this relation is valid for small molecular solute. The interfacial tension at the three interfaces depends on the adsorbed amount and arrangement of solutes at the interfaces in comparison to that in the bulk solution. Applying eq 4 for three different interfaces, Lucassen-Reynders13 derived a relation relating adsorbed amount of solutes and the interfacial tension at the three different interfaces as d(γSV − γSL) ΓSV − ΓSL = ΓLV dγLV

(5)

where Γi (i = SV, SL, LV) are the adsorbed amounts of the solute at three different interfaces in contact. We note that Krishnan et al.15,26 have assumed the value of (ΓSV − ΓSL)/ΓLV is about unity for protein adsorption. The differential form of the right-hand side part of eq 5 is converted to difference from by taking difference between interfacial tensions of polymer−protein solution and water. Thus we can define, d(γSV − γSL) = (γSVP − γSVW ) − (γSLP − γSLW ) dγLV = ΔLV = γLVP − γLVW

and (6)

where subscripts p and w denote the interfacial tensions of polymer−protein solution and water, respectively. The difference of wetting tension27 of a solution and solvent is known as the wetting effect and is defined as

(1)

where γLV cos θ is known as wetting tension and γLV, γSL, and γSV are interfacial tensions between liquid−vapor (LV), solid− liquid (SL), and solid−vapor (SV) phases, respectively. This equation is further extended to determine the surface energy of the smooth solid surfaces25 by using following methodology. The surface energy of a solid surface or a pure liquid is assumed to have combination of two components: dispersion (γdi ) and the polar (γpi ). It is written in the general form as γi = γi p + γid

(3)

ΔγSL = γLVP cos θp − γLVW cos θw

(7)

The wetting effect indicates the effect of polymer−protein on solid surface tension in comparison to water on a substrate. Using eqs 3 to 5, we can rewrite the expression of adsorbed mass at the solid−liquid interface as

(2)

ΓSL = ΓSV − ΓLV

where i = L, S corresponding to pure liquid or solid surface, respectively. The interfacial tension between two pure phases (i and j) can be described by the following semiempirical relation25

ΔγSL ΔLV

(8)

We have determined the ΓSL, ΔγSL, and ΔLV experimentally and verified the modified Lucassen-Reynders equation. 3442



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Table 3. Surface Energy (γSV) at the cos θ = 0 Line for Different Solutes

3. RESULTS AND DISCUSSION 3.1. Characteristics of Substrates. In all of the FTIR spectra of the modified surfaces, peaks appeared around 2965 cm−1, 2920 cm−1, 2880 cm−1, and 2855 cm−1, which are due to CH3 asymmetric (νa-CH3), CH2 asymmetric (νa-CH2), CH3 symmetric (νs-CH3), and CH2 symmetric (νs-CH2) vibrations, respectively.19,28,29 These correspond to alkyl chain of silane modifier. The surfaces with SAM of amine headgroup show IR peaks at 3300 cm−1 to 3500 cm−1 and 1500 cm−1 to 1600 cm−1 corresponding to N−H stretching and bending, respectively. The details of FTIR spectra can be found in the Supporting Information (also see ref 19). Surface morphologies, characterized by AFM, result in nanoscale smooth surfaces (shown in the Supporting Information). The roughness factor is about unity for all the substrates. This indicates that the surface roughness of all the surfaces is too small to contribute to contact angle of liquids on the substrates. Using eqs 1 and 3, we calculate the surface energies from the contact angle data of water and diiodomethane on the surfaces and the known value of γpL and γdL of the two liquids.30 The calculated surface energies are listed Table 1. The characteristics of amine, octyl, 1:1 amine−octyl, and hybrid surfaces are similar to the reported data19 within the error limit of 10 %. 3.2. Contact Angle of Polymer−Protein Solutions and Qualitative Indication of Adsorption on Substrate. Figure 1 depicts the variation of cosine of contact angle of

sample no.

polymer−protein solution

γSV/mJ·m−2 at cos θ = 0

1 2 4 6 7

water PEO NaPSS BSA Fb

33.83 31.60 37.00 31.48 37.00

with ln c of PEO and NaPSS are shown in Figure 2. The figure also plots reported experimental data points of aqueous

Figure 2. Variation of surface tension (γLV) with bulk concentrations (ln c) of polymer/protein solutions; dark blue ×, PEO; dark blue ◇, NaPSS; red □, Fb;31 and red ○, BSA.16 Data points are the mean of three experiments with standard error of ± 0.3.

solution of BSA16 and Fb31 at different concentrations. γLV of both the polymers and proteins are varying linearly with ln c. The reported γLV of PEO and NaPSS are also very close to our results.23,32 We have determined surface tension solutions (w = 0.01) of PAA and PVP, which are 59.31 ± 0.04 mJ·m−2, 57.84 ± 0.04 mJ·m−2, respectively. These data are used to determine the wetting tension of solution of polymers and proteins on the substrates. The reported surface tension33 of PVP of average molar mass of 104 g·mol−1 at 298 K is 68 mJ·m−2 and PAA34 of average molar mass of 2.5·105 g·mol−1 at 303 K and pH 3.4 is 67.5 mJ·m−2. 3.4. Experimental Adsorbed Mass by QCM. Figure 3 shows the variation of ΔFn/n with time during adsorption of PEO and NaPSS from their solution on four representative surfaces. The curves obtained are similar in trend as reported earlier by us for adsorption of BSA and Fb on various modified surfaces.19,22 The ΔFn/n decreases with progress of time. Like the proteins, the polymer PEO and NaPSS has the highest affinity toward the octyl surface. On the amine surface, the rate of adsorption of PEO is the slowest, probably due to net charge repulsion among adsorbed PEO. The rate of adsorption of NaPSS is the slowest on the mixed surface. The polymer adsorption attains a plateau value after about 1000 s. The plateau value of −ΔFn/n for the adsorption of PEO, BSA, NaPSS, and Fb on various surfaces are determined and are listed in Table 4. These plateau values are converted to adsorbed mass using the Sauerbrey equation as discussed in section 2.3. In addition, we determine the wetting effect of solution on a surface from eq 7 using the surface tension of solution and contact angle of solution on the surface. Figure 4 shows the variation of adsorbed mass of different solutes with th ewetting effect. The adsorbed amount increases with the increase in the wetting effect for polymers. The adsorbed

Figure 1. Variation of contact angle (cos θ) of blue −, water; and solutions of red ○, BSA (w = 0.00002); black ×, PEO (w = 0.001); black +, PVP (w = 0.001); red □, Fb(w = 0.00002); dark blue ◇, NaPSS (w = 0.001); dark blue △, PAA (w = 0.001) with surface energy (γSV) at 295 K. Data points are the mean of 10 experiments with a standard error of ± 1.0.

various solutions on the various substrates with its surface energy. It also shows the contact angle of water on the substrates. At surface energy 31.5 mJ·m−2, the extrapolated value of contact angles of the solutions of PEO, PVP, and BSA is zero, while at surface energy 37 mJ·m−2 the extrapolated value of contact angles of the solutions of NaPSS, PAA, and Fb is zero (see Table 3). We denote these energies as critical surface energy of the solutions. The contact angle of the solutions on the surfaces with surface energy below this value is above 90°. We note that the “critical surface tension” is coined through Zisman plot,8 which is the variation of contact angle of homologous series of small molecular liquids on a solid surface with its surface tension. They have shown that the critical surface tension (surface tension at cos θ = 0) is characteristic of the solid. 3.3. Surface Tension, γLV, of Polymer−Protein Solutions. The variations of liquid−vapor interfacial tension γLV 3443



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Figure 4. Variation of adsorbed amount (Δm) and wetting effect (ΔγSL) with surface energy of substrates: dark blue ×, PEO; dark blue ◇, NaPSS; red ○, BSA; red □, Fb.

Table 5. Slope and Intercept of Lines Fitting of th eAdsorbed Amount of Polymers/Proteins on Various Substrates Using Equation 8 ΓLV/ΔLV

ΓSV

ΔLV

solute (w/mass fraction)

103 s2·m−2

mg·m−2

mJ·m−2

Fb (0.00002) BSA (0.00002) PEO (0.001) NaPSS (0.001)

0.138 0.066 0.067 0.048

4.115 2.091 2.164 2.059

19.18 20.18 11.41 0.96

be reasonable and varies from 0.5 mg·m−2 to 2 mg·m−2 depending on the solute. We note that reported7 ΓLV using eq 4 for proteins is 60 times higher than the expected values. Our semiempirical method is successful in predicting the reasonable adsorbed mass, ΓLV, probably due to presence of term containing ΔγSL, ΔLV, in the form of ratio in the linearized eq 8. Figure 5 shows the variation of (ΓSV − ΓSL)/ΓLV with −ΔγSL. (ΓSV − ΓSL)/ΓLV is calculated for any value of −ΔγSL using slopes and intercept of the curve obtained in Figure 4. From Figure 5, we can see that the absolute value of (ΓSV − ΓSL)/ΓLV is of the order of unity for BSA, FB, and PEO. However, this value is much higher of NaPSS (not shown in Figure 5 for clarity). This indicates that the assumption of Krishnan et al.7,16 is valid for certain solutes but not all solutes.

Figure 3. Variation of ΔFn/n with time during adsorption of polymers (a) PEO and (b) NaPSS (w = 0.001) on pink ■, amine; black ●, chloro; purple ★, amine−octyl (1:1) mixed; and green ▲, octyl SAMs. Adsorption data on remaining SAMs are omitted due to clarity.

amounts of BSA, PEO, NaPSS, and Fb were found to follow a similar trend. From eq 8, the slope and intercept of fitted straight lines through the adsorbed mass vs wetting effect for various proteins and polymers are found to be ΓLV/ΔLV and ΓSV. These are listed in Table 5. The intercept is the adsorbed mass of proteins/polymer, ΓSL, at ΔγSL = 0 and equal to ΓSV. We note that determination of ΓSV is not reported in the literature. Its estimation is based on assumptions as stated in the Introduction. ΓLV can be obtained by multiplying slope with the experimentally determined ΔLV. We have used ΔLV at the experimental concentration of proteins/polymer to determine the adsorbed mass at the experimental concentration of the solutes used. The obtained ΓLV through this method is found to

4. CONCLUSIONS We have studied the effect of hydrophobicity of the surface and adsorbing polymer−protein on the adsorption of protein− polymers at interfaces by measuring contact angle, surface tension, and adsorbed masses. We have quantified adsorbed mass on liquid−vapor and solid−vapor interfaces and

Table 4. Equilibrium Measured Value of −ΔFn/n by QCM on Various Substrates for a Given Concentration as Listed in Table 2 −ΔFn/n (Hz) substrate

PEO

BSA

NaPSS

Fb

amine mercapto chloro methyl octyl amine−methyl (1:1) amine−octyl (1:1)

7.3 ± 0.6 10.2 ± 1.1 10.1 ± 1.1 15.8 ± 1.5 16.9 ± 1.6 9.8 ± 1.0 11.0 ± 1.1

9.3 ± 1.0 14.6 ± 1.1 12.9 ± 1.1 15.7 ± 1.5 16.2 ± 1.6 10.2 ± 1.1 11.3 ± 1.1

8.4 ± 0.6 9.6 ± 1.0 9.0 ± 0.6 12.1 ± 1.1 12.4 ± 1.1 7.2 ± 0.6 7.6 ± 0.6

14.9 ± 1.5 39.4 ± 2.8 23.9 ± 2.2 26.4 ± 2.2 27.3 ± 2.2 9.0 ± 0.6 11.3 ± 1.1

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Figure 5. Variation of (ΓSV − ΓSL)/ΓLV with the wetting effect (ΔγSL). Long dashed line, Fb; short dashed line, PEO; red solid line, BSA.

determined (ΓSV − ΓSL)/ΓLV. The reasonable value of the determined adsorbed mass on liquid−vapor and solid−vapor interfaces indicates the applicability of our methodology. Also, we have shown that the absolute value of (ΓSV − ΓSL)/ΓLV is order of unity for some solute, but it cannot be taken as universal rule. The wetting effect can be a useful parameter to study the effect of surfaces on adsorption of solutes. We found that the critical surface tension of solute corresponding to cos θ = 0 on substrate can be defined.

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ASSOCIATED CONTENT

S Supporting Information *

FTIR spectra of modified surfaces, equations describing the kinetics of adsorption on surface, and AFM images and its analysis. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +91 11 26591018; fax: +91 11 26581120. E-mail address: [email protected]. Funding

The authors would like to thank department of biotechnology for their financial support (sanction no. BT/PR9683/MED/ 32/16/2007) for this work. Notes

The authors declare no competing financial interest.

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REFERENCES

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Relation between the Wetting Effect and the Adsorbed Amount of

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