Surface activity and thermodynamic of micellization and adsorption for isooctylphenol ethoxylates, phosphate esters and their mixtures with N-diethoxylated perfluorooctanamide

Colloids and Surfaces A: Physicochemical and Engineering Aspects 170 (2000) 127 – 136 www.elsevier.nl/locate/colsurfa

Surface activity and thermodynamic of micellization and adsorption for isooctylphenol ethoxylates, phosphate esters and their mixtures with N-diethoxylated perfluorooctanamide A.M. Al Sabagh a,*, N.Gh. Kandil b, A.M. Badawi a, H. El-Sharkawy a a

Department of Petroleum Applications, Egyptian Petroleum Research Institute Nasr City, Cairo 11727, Egypt b Faculty of Women, Ain Shams Uni6ersity, Roxy, Cairo, Egypt Received 23 June 1999; accepted 27 January 2000

Abstract Three nonionic surfactants; p-isooctylphenol ethoxylates p-[i-OPE10], p-[i-OPE15], and p-[i-OPE20], were phosphorylated to produce three anionic phosphate ester surfactants. In addition, N-diethoxylated perfluorooctanamide (N-DEFOA) was also prepared. The surface and thermodynamic properties of the three types of surfactants and mixtures of the fluorocarbon surfactant (FC) with the hydrocarbon surfactants (HC) have been investigated. Surface tension as a function of concentration of the surfactant in aqueous solution was measured at 30, 40, 50 and 60°C, using the spinning drop technique. From these measurements the critical micelle concentration (CMC), the surface tension at the CMC (gCMC), the maximum surface excess concentration (Gmax), the minimum area per molecule at the aqueous solution/air interface (Amin), and the effectiveness of surface tension reduction (pCMC), were calculated. The thermodynamic parameters of micellization (DGmic, DHmic, DSmic) and of adsorption (DGad, DHad, DSad) for these surfactants and their mixtures were also calculated. Structural effects on micellization, adsorption and effectiveness of surface tension reduction are discussed in terms of these parameters. The results show that the FC surfactant and its mixtures with HC surfactants enhance the efficiency in surface tension reduction and adsorption in the mixed monolayer at the aqueous solution/air interface, and also, reduce gCMC and the tendency towards micellization. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Surface properties; Thermodynamic; Fluorocarbon surfactants; Phosphate esters; Surfactant mixtures; Micellization; Adsorption

1. Introduction

* Corresponding author.

Fluorocarbon (FC) surfactants exhibit unique properties as a result of the high electronegativity of the fluorine atom. They are very stable and

0927-7757/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 ( 0 0 ) 0 0 4 7 5 - 1

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resistant to various chemicals, temperature and radiation. The low value of the free surface energy, 18 and 6 mNm − 1 for CF2CF2 and CF3 in comparison to 31 and 22 – 24 mNm − 1 for CH2CH2 and CH3, respectively [1], results in a high surface activity of the FC surfactants. Therefore, FC surfactants are more efficient than HC surfactants in lowering the surface tension of aqueous solutions [2]. Surfactants having

Table 1 Generalized formulae of the investigated surfactants a

Nonionic HC surfactants:

p-i-octyl phenol ethoxylates where, n= 10, 15, 20 p-[i-OPE10] p-[i-OPE15] p-[i-OPE20] b

Anionic HC surfactants:

fluorocarbon hydrophopes are used in extinguishing media, especially for quenching fires of petroleum and its products, in electroplating bathes to eliminate the mist formation over the electroplating vat, in lubricants and plastics surfaces and fabrics, and in glues to improve adhesion.[3–6]. The study of the surface and thermodynamic properties of FC surfactants mixed with HC surfactants are important for both basic research and industrial applications. Most reports in the literature on the mixing behavior of FC/HC surfactants have been mainly concerned with anionic–nonionic mixtures [7]. There have been some investigations of anionic–nonionic mixtures, such as sodium perfluorooctanoate or lithium perfluorooctane sulfonate mixed with polyoxyethylenated glycol alkyl esters [8,9], or polyoxyethylenated alkylphenols, having large number of ethylene oxide units (five to 20) [10]. For most of these anionic–nonionic mixed surfactant systems, the critical micelle concentration depends on the electrostatic stabilization due to interactions between the head groups for FC and HC surfactants in the mixture. The micellization process for mixed FC/HC surfactants was reported [7,11,12]. The aim of this work is to synthesize N-diethyoxylated perfluorooctanamide (FC), and alkylphenol ethoxylates (EO= 10, 15, 20) phosphate esters in order to compare their surface activity, thermodynamic of micellization and adsorption both individually and in mixtures.

2. Experimental

2.1. Preparation of phosphate ester surfactants (HC)

p-i-octyl phenol ethoxylates phosphate esters where, n= 10, 15, 20 p-[i-OPE10] PO4H2 p-[i-OPE15] PO4H2 p-[i-OPE20] PO4H2 c

Perfluorocarbon surfactants (FC): C7F15-CONHCH2CH2O CH2CH2OH N-diethoxylated perfluoroocatanamide

The p-isooctylphenol ethoxylates were selected, so that their degree ethoxylation was 10, 15 and 20. The three (HC) ethoxylated alkyl phenols were phosphorylated to give the corresponding phosphate esters [13]. The fluorocarbon surfactant (FC), N-diethoxylated perfluorooctanamide was prepared as reported [14]. A generalized formulae for these surfactants are shown in Table 1.

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Table 2 The CMC, gCMC and surface activity (−dg/d log C) for the individual investigated surfactants Surfactants

Ethylene oxide units (n)

HLB

Temp. (°C)

CMC (mol dm−3)

gCMC (mNm−1)

−dg/d log C, Pre-CMC (slope)

P,[i-OPE10] (HC1)

10

14.2

P,[i-OPE15] (HC2)

15

15.7

P,[i-OPE20] (HC3)

20

16.5

P,[i-OPE10] PO4H2 (HC4)

10

14.6

30 40 50 60 30 40 50 60 30 40 50 60 30

0.0139 0.0135 0.0125 0.0110 0.0149 0.0145 0.0139 0.0130 0.0168 0.0161 0.0152 0.0143 0.017

26.9 26.6 26.1 25.8 27.8 27.4 27.1 26.5 28.9 28.6 28.4 27.6 26.5

10.50 10.50 10.06 9.50 7.53 7.37 6.87 6.46 4.60 4.55 3.65 3.42 6.80

16.0

40 50 60 30

0.016 0.014 0.012 0.018

26.1 25.5 25.2 27.5

6.06 5.30 5.10 5.10

16.8

40 50 60 30

0.017 0.015 0.013 0.019

27.0 26.8 26.3 28.9

3.79 3.51 3.40 3.60

40 50 60 30 40 50 60

0.018 0.016 0.015 0.046 0.040 0.035 0.033

27.8 27.5 26.9 13.0 12.5 12.3 12.0

3.03 2.20 1.90 15.52 14.25 13.23 12.94

P,[i-OPE15] PO4H2 (HC5)

P,[i-OPE20]PO4H2 (HC6)

PFOAE2 (FC)

15

20

2

5.9

2.2. Surface tension measurements

3. Results and discussion

All the surface tension measurements were carried out by using deionized water. The surface tension as a function of concentration was measured at 30, 40, 50 and 60°C using the spinning drop technique (SDT Kruss model SITE 04). The critical micelle concentration (CMC) and the surface tension at CMC, (gCMC), were determined from the surface tension isotherms versus ln C (solute concentration) by least-square analysis of the data. The CMC and gCMC values are listed in Tables 2 and 3 for individual surfactants and the FC/HC mixtures.

The critical micelle concentration (CMC) of the investigated surfactants at 30, 40, 50 and 60°C were determined by plotting the surface tension (g) versus ln (solute concentration), C. The CMC values were determined from the abrupt change in the slope of g versus ln C plot. These values are listed in Table 2 for the individual surfactants and in Table 3 for the mixtures of FC/HC surfactants. Generally the increasing number of ethylene oxide units increases the CMC [15–18]. Increasing EO increases hydrophilicity, which increases solubility of the surfactant in water. Such improved solubil-

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ity lowers the tendency for surfactants to form micells in water and increases the CMC. In the present work the same behavior is observed for the individual surfactants (HC1 to HC6). Some investigators [19,20] found that increasing EO decreases CMC. However, these surfactants have very long hydrophobic tails (at least 16 carbons) and relatively low EO (about six to 19). It may be possible in these cases that the surfactants low water solubility (due to the long alkyl chain, not the long EO chain) causes this result over some EO ranges. The general trend in this work is a continuous increase in CMC from HC1 to HC3, and from HC4 to HC6, with the increase of the number of ethylenoxide units (n) as shown in Fig. 1, and hydrophilic – lipophilic balance (HLB) as shown in Fig. 2, for mixtures. This behavior is consistent with previous data [21,22]. It was observed that increasing the temperature from 30 to 60°C leads to a decrease in CMC for both nonionic (HC1 to HC3) and anionic (HC4 to HC6) HC

surfactants as well as the FC surfactants. The data presented in Table 2 indicate that (gCMC) also decrease with increasing (n), and continuously decrease with the rise of temperature. The surface activity (slope, −dg/d ln C) was plotted against the HLB and represented in Fig. 2. The surface activity (slope, − dg/d ln C) as listed in Table 2, indicates that the ethyoxylated alkylphenols (HC1 to HC3) have more surface activity than the phosphate esters (HC4 to HC6). On the other hand, the surface activity of the FC surfactant is more efficient than both the ethoxylated nonionic surfactants and their phosphorylated derivatives. The CMC of the FC surfactant has a higher value than those of the HC surfactants, but has the minimum gCMC value. As shown in Table 3 the mixtures of the FC surfactant and HC surfactants (HC1 to HC6) exhibit surface activity higher than either component, and gCMC close to that of the FC surfactant. However, the CMC of mixtures are higher than the CMC of the individual surfac-

Table 3 The CMC, gCMC and surface activity for mixed fluorocarbon and hydrocarbon surfactants Surfactant mixtures FC/HC (1:1)

HLB

Temp. (°C)

CMC (mol dm−3)

gCMC (mNm−1) −dg/d log C, Pre-CMC (slope)

FC/HC1

10.0

FC/HC2

10.8

FC/HC3

11.2

FC/HC4

10.2

FC/HC5

10.9

FC/HC6

11.3

30 40 50 60 30 40 50 60 30 40 50 60 30 40 50 60 30 40 50 60 30 40 50 60

0.073 0.070 0.067 0.057 0.081 0.073 0.070 0.065 0.089 0.085 0.080 0.072 0.072 0.062 0.056 0.048 0.073 0.064 0.057 0.051 0.074 0.066 0.058 0.057

17.94 17.13 17.12 16.28 17.8 17.1 16.8 16.0 18.95 18.27 18.00 17.77 16.8 17.7 15.46 14.12 16.3 15.5 15.1 14.4 17.07 15.73 13.98 12.76

18.47 17.95 17.24 15.53 16.7 16.1 15.6 14.7 15.64 15.10 14.52 13.35 17.7 16.8 15.46 14.12 17.4 17.1 16.6 16.1 18.08 16.66 15.40 15.08

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dration between the hydrophilic part of the FC molecules head group and the ether oxygen of the nonionic HC surfactants; (3) hydrogen bonding between the proton in the amide group in PFOAE2 and the phosphate group in anionic HC surfactants (HC4 to HC6). Maximum surface excess concentrations (Gmax) in mol dm − 2 were calculated from the relationship: Gmax =

1 − dg · RT d ln C

(1)

where −dg/d ln C is the surface activity (slope of g versus ln C plots at constant absolute temperature, T, and R=8.314 mol − 1 K − 1. The Gmax values are used to calculate the minimum area (Amin), in nm2 molecule − 1, at the aqueous/air interface using the relationship: Amin = ACMC =

1016 NGmax

(2)

Fig. 1. Relation between CMC and n for: (A) HC1, HC2 and HC3; (B) HC4, HC5 and HC6.

tants. In mixed surfactant systems, in which both the fluorocarbon component and the hydrocarbon component are anionic, the matual phobicity between the chains is the dominant interaction and the systems show strong positive deviation from ideal behavior and two types of mixed micells are formed [23,24]. Some investigators [7] reported that in mixtures of anionic FC/HC surfactants only one type of micelles is formed. In this respect the mixture of nonionic (PFOAE2) with (HC1 to HC3) and their phosphate esters (HC4 to HC6) show only one micelle. This result is observed in all mixtures and agree in harmony with the result of Wan Guo et al. [7]. This may be due to the possible interactions between the head groups in the mixed micelle which coming from (1) a reduction in the repulsive force between the amide– nonionic head due to the insertion of the nonionic surfactant molecules between the fluorocarbon surfactant molecules; (2) interactions through hy-

Fig. 2. Relation between CMC and HLB for: (A) FC1/HC1 to HC3; (B) FC/HC4 to HC6.

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Table 4 The surface properties for the HC, FC and mixed FC/HC surfactant systems at different temperatures Surfactant

Gmax 10−10 (mol/cm2)

pCMC (m N m−1)

Amin, (n m2)×10−2

Systems temp. (°C) 30 HC1 HC2 HC3 HC4 HC5 HC6 FC FC/HC1 FC/HC2 FC/HC3 FC/HC4 FC/HC5 FC/HC6

1.461 1.378 1.271 1.429 1.429 1.152 6.145 7.317 6.613 6.193 7.009 6.890 7.159

40 1.178 1.232 1.109 1.167 1.167 1.009 5.472 6.892 6.182 5.798 6.451 6.566 6.397

50 0.896 1.101 1.004 0.829 0.829 0.952 4.921 6.413 5.803 5.401 5.751 6.175 5.728

60 0.736 0.884 0.877 0.689 0.685 0.855 4.671 5.606 5.306 4.819 5.097 5.812 5.443

30 1.136 1.204 1.306 1.161 1.161 1.441 0.276 0.227 0.251 0.268 0.236 0.240 0.231

where N is Avogadro number (6.023× 1023). Values of Gmax and Amin are listed in Table 3. It is evident that Amin increases with temperature. This is probably due to the increased thermal motion [25,26]. The data presented in Table 4. indicate that Amin decreases with increase of (n) for each type of investigated HC surfactant. The same behavior was also observed in previous work [21] [22]. The Amin for the FC surfactants and for the FC/HC mixtures are much smaller than the Amin of the HC surfactants. This behaviour may be due to the much higher surface activity of the FC surfactants and the synergistic effect between the FC and the HC surfactants. On the other hand the Amin for mixtures of FC/HC1 to HC3 or FC/HC4 to HC6 increases with increase of ethylene oxide units as reported before [27]. The effectiveness of surface tension reduction was measured pCMC =g0 −gCMC, (where g0 is the surface tension of the water and gCMC, the surface tension of the solution at CMC). The pCMC continuously decreases with the rise of temprature and increases with increasing of the (n). This is consisted with reported for p-octylphenoxy poly (ethoxyethanol) [15]. This behavior is appeared for nonionic (HC1 to HC3) and the anionic phos-

40 1.409 1.347 1.497 1.422 1.422 1.645 0.303 0.240 0.268 0.286 0.257 0.252 0.259

50 1.853 1.507 1.653 2.002 2.002 1.744 0.337 0.258 0.286 0.307 0.288 0.286 0.289

60 2.255 1.878 1.893 2.409 2.409 1.941 0.355 0.296 0.312 0.344 0.325 0.285 0.305

30

40

50

60

44.6 42.6 42.6 42.6 44.0 44.5 58.5 55.1 53.7 53.3 54.7 55.2 54.4

44.4 42.6 41.4 45.0 42.6 43.2 58.5 53.8 52.9 52.7 53.3 54.5 58.2

43.2 40.9 39.6 43.8 41.2 42.5 57.7 52.8 51.2 51.1 55.8 52.9 56.0

42.2 40.5 39.4 41.8 40.7 41.1 56.0 50.7 51.0 49.2 52.5 52.6 53.2

phate esters (HC4 to HC6). The effectiveness for FC/HC mixtures shows a steady decrease with increase in the number of (n), this agrees with that reported for ethoxylated alcohols [28]. The effectiveness for the FC/HC1 and FC/HC2 mixtures is dominated by the FC surfactant. The thermodynamic parameters of micellization expressed by standard free energies DGmic, enthalpies DHmic and entropies DSmic, of micellzation for the investigated surfactants are calculated from the equations: DGmic = RT ln CMC

(3)

and − DSmic = (dDGmic/dT)

(4)

and by using the values of DGmic at 30, 40, 50 and 60°C. DHmic values are calculated as: DHmic = DGmic + TDSmic

(5)

From the results listed in Table 5, analyzing the thermodynamic parameters of micellization we may conclude that micellization process is spontaneous (DGmic B 0). Where DGmic was found to become less negative with the increase in oxyethylene content of the molecule, an effect attributed to steric inhibition of micellization [29]

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The thermodynamic parameters of adsorption DGad, DHad, and DSad are listed in Table 5. The DGad values are calculated by using the relationship [25].

for both the nonionic alkylphenol ethyoxylates (HC1 to HC3) and their phosphorylated derivatives (HC4 to HC6). The DGmic for FC surfactant is less negative than that of DGmic for (HC) surfactant. This behavior shows that the HC surfactants form micelles more easily than the FC surfactant. The values of DGmic for the mixtures of FC/HC indicate that they have even less tendency towards micellization. Table 4 covers the values obtained for DSmic at 30–60°C. The DSmic values are all positive, indicating increased randomness in the system upon transformation of the surfactant molecules into micelles. The slight increase in the positive DSmic value with increase in the number of (n) in the surfactant molecule has been previously reviewed [15]. The DHmic values in Table 5 are all positive, in contrast to those of Crook and co-workers [15], who report negative DHmic values at 25°C for p-ter-octylphenoxy poly (ethenoxyethanol) containing less than four oxyethylene unites. The values increase in (n) in the molecule, in accordance with the observations of others [15,30] indicating that a greater number of hydrogen bonds between polyoxyethylene chain oxygen and water molecules are broken in the micellization process as the (n) increases.

DGad = RT ln CMC− 0.6023·pCMC·ACMC

(6)

The DHad and DSad. Values listed in Table 6 are obtained from relationships corresponding to Eqs. (4) and (5). The DSad values are all positive and greater than the DSmic values for the same surfactant, reflecting the greater freedom of motion of the hydrocarbon chains at the planar air/aqueous solution interface compared to that in the relatively cramped interior beneath the convex surface of the micelle. The DSad of the FC (N-diethyoxylated perfluorooctanamide) is more than DSad for the individual HC surfactant probably due to the larger hydrophopicity character of the fluorocarbon chain than the hydrocarbon chain. The DHad values are all more positive than DHmic. indicating that the bonds between the (n) or phosphate group and water molecules are broken in the process of adsorption at air/aqueous solution interface than in micellization. The DGad values are all negative as listed in Table 6. The negative values of DGad are greater than the DGmic, showing that adsorption at the interface is associated with a decrease in free energy of the system.

Table 5 Thermodynamic parameters of micellization for the investigated surfactants at different temperatures Surfactant

DHmic (kJ mol−1)

DGmic (kJ mol−1)

DSmic (kJ mol−1 K−1)

Temp. (°C)

HC1 HC2 HC3 HC4 HC5 HC6 FC FC/HC1 FC/HC2 FC/HC3 FC/HC4 FC/HC5 FC/HC6

30

40

50

60

30

40

50

60

−10.77 −10.60 −10.30 −10.26 −10.12 −9.98 −7.76 −6.60 −6.33 −6.11 −6.62 −6.60 −6.56

−11.20 −11.00 −10.74 −10.76 −10.60 −10.45 −8.38 −6.92 −6.80 −6.42 −7.23 −7.20 −7.06

−11.76 −11.50 −11.20 −11.46 −11.28 −11.10 −9.00 −7.26 −7.14 −6.78 −7.74 −7.70 −7.64

−12.45 −12.00 −11.759 −12.25 −12.00 −11.62 −9.44 −7.93 −7.60 −7.28 −8.39 −8.23 −7.93

9.54 4.50 5.70 16.17 6.30 7.967 9.55 9.46 13.30 3.89 10.37 12.40 11.61

9.77 4.60 5.80 16.49 6.30 7.99 9.52 9.67 13.20 3.91 10.63 12.50 11.72

9.88 4.60 5.90 16.60 6.14 7.96 9.45 9.84 13.50 3.88 10.34 12.60 11.68

9.86 4.60 5.80 16.72 5.90 8.05 9.57 9.72 13.70 3.72 10.26 12.70 12.03

0.067 0.056 0.053 0.087 0.054 0.049 0.057 0.053 0.064 0.033 0.056 0.063 0.060

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Table 6 Thermodynamic parameters of adsorpation for the investigated surfactants at different temperatures Surfactant

DGad (kJ mol−1)

DHad (kJ mol−1)

DSad (kJ mol−1 K−1)

Temp.

HC1 HC2 HC3 HC4 HC5 HC6 FC FC/HC1 FC/HC2 FC/HC3 FC/HC4 FC/HC5 FC/HC6

30

40

50

60

30

40

50

60

−13.82 −13.89 −13.65 −15.42 −15.22 −16.11 −9.37 −7.85 −7.67 −7.53 −7.91 −7.92 −7.81

−14.96 −16.73 −16.93 −17.15 −16.65 −17.55 −10.15 −8.211 −8.217 −7.92 −8.59 −8.57 −8.56

−19.16 −17.66 −17.78 −20.22 −19.52 −18.51 −10.94 −8.62 −8.60 −8.34 −9.34 −9.11 −9.25

−21.96 −10.60 −18.64 −22.32 −21.80 −19.60 −11.42 −9.41 −9.19 −8.97 −10.09 −9.72 −9.55

26.17 16.71 11.19 30.43 21.44 12.37 17.29 21.54 14.75 8.52 16.93 11.47 8.24

26.35 14.88 9.27 30.73 21.22 11.87 17.39 22.15 14.95 8.66 17.07 11.46 8.02

22.87 15.26 8.70 29.19 19.56 11.85 17.00 22.71 15.30 8.77 17.14 11.56 7.86

21.99 14.03 8.66 28.62 18.49 11.70 17.88 22.89 15.45 9.12 17.21 11.59 8.09

0.132 0.101 0.082 0.153 0.121 0.094 0.088 0.097 0.074 0.053 0.082 0.064 0.053

Table 7 Structural effects on micellization and adsorption at different temperatures Surfactant

DGmic−DGad (kJ mol−1)

DHmic−DHad (kJ mol−1)

DSmic−DSad (kJ mol−1 K−1)

Temp. 30 HC1 HC2 HC3 HC4 HC5 HC6 FC FC/HC1 FC/HC2 FC/HC3 FC/HC4 FC/HC5 FC/HC6

3.05 3.29 3.35 5.16 5.10 6.13 1.61 1.25 1.34 1.42 1.29 1.32 1.25

40 3.76 5.73 6.19 6.39 6.05 7.10 1.77 1.29 1.41 1.50 1.36 1.37 1.50

50 8.00 6.16 6.54 8.76 8.24 7.41 1.94 1.36 1.46 1.56 1.60 1.41 1.61

60

30

40

50

60

9.51 7.60 6.88 10.07 9.80 7.98 1.98 1.48 1.59 1.69 1.70 1.49 1.62

16.63 12.21 5.49 14.76 15.14 4.403 7.74 12.08 1.45 4.63 6.56 −0.93 −3.37

16.58 10.28 3.47 14.24 14.92 3.88 7.87 12.48 1.75 4.75 6.44 −1.04 −3.70

12.99 10.66 2.80 12.59 13.42 3.89 7.55 12.87 1.80 4.89 6.80 −1.04 −3.82

12.13 9.43 2.86 11.90 12.59 3.65 8.31 13.17 1.75 5.40 6.95 −1.11 −3.94

3.1. Structural effect on adsorption and micellization From Eqs. (3) and (6) it follows that:

0.065 0.045 0.029 0.066 0.067 0.045 0.031 0.044 0.010 0.020 0.026 0.001 −0.007

pCMC·Amin pCMC·ACMC = DGmic − DGad

(7)

i.e. the pCMC. ACMC product expresses the work involved in transferring the surfactant molecule

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from a mono layer at zero surface pressure to the micelle. DGmic −DGad values represented in Table 7. The work of transfer, which explains the ease of adsorption to form a mono layer at 0 surface pressure relative to ease of micellization, shows observed change with temperature in the 30–60°C range. The enthalpy difference (higher on adsorption than on micellizatin) gives a negative contribution to the free energy difference between micellization and adsorption. The fact that the free energy difference is positive indicates that the entropy contribution is dominant over the enthalpy contribution. These observations indicate that steric factors inhibit micellization more than they do adsorption at the air/aqueous solution interface [31]. In the present case, the structural elements in the surfactant molecule that may cause steric inhibition of micellization are the alkyl hydrocarbon chain, alkyl perfluorocarbon chain, polyoxyethylene chain, amide and the phosphate ester group. The greater restriction controlling the motion of the hydrocarbon chain or perfluorocarbon chain in the relatively cramped interior of the micelle as compared to the planner air/aqueous solution interface may be due to the entropy (DSmic −DSad) contribution to the positive value of the work of transfer.

4. Conclusions The following conclusions are withdrawn from this article: the flourocarbon surfactant and its mixture with either nonionic or ionic hydrocarbon surfactants reduce the surface tension greater than the individual hydrocarbon surfactants; all mixtures of investigated FC/HC form only one type of micelle; all investigated surfactants (individual or mixtures) favor the adsorption than micellization process.

Acknowledgements The authors would like to express their appreciation to Dr B.M. Fung at the University of

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Oklahoma, USA, for helpful discussions and technical support.

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