poloxamer 407 mixtures

Accepted Manuscript Synergism and antagonism in mixed monolayers: Brij S20/poloxamer 407 and Triton X-100/poloxamer 407 mixtures Dejan Ćirin, Veljko Krstonošić, Darija Sazdanić PII:

S0378-3812(18)30245-0

DOI:

10.1016/j.fluid.2018.06.009

Reference:

FLUID 11864

To appear in:

Fluid Phase Equilibria

Received Date: 8 March 2018 Revised Date:

2 June 2018

Accepted Date: 19 June 2018

Please cite this article as: D. Ćirin, V. Krstonošić, D. Sazdanić, Synergism and antagonism in mixed monolayers: Brij S20/poloxamer 407 and Triton X-100/poloxamer 407 mixtures, Fluid Phase Equilibria (2018), doi: 10.1016/j.fluid.2018.06.009. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT

Synergism and antagonism in mixed monolayers: Brij S20/poloxamer 407 and Triton X-100/poloxamer 407 mixtures Dejan Ćirin*, Veljko Krstonošić, Darija Sazdanić

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University of Novi Sad, Faculty of Medicine, Department of Pharmacy, Hajduk Veljkova 3, 21000 Novi Sad, Serbia

*

Corresponding author. E-mail adress: [email protected]

ACCEPTED MANUSCRIPT Abstract Knowledge of nonideal behavior of surfactant mixtures of nonionic surfactants at air/aqueous solution interface is important in order to obtain surfactant mixtures with improved properties. In this work interfacial behavior of Brij S20/P407 and Triton X-100/P407 surfactant mixtures was investigated. The nonideal behavior was elucidated by extending Clint´s theory and by

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determination of the ideal maximum surface excess concentration values. Rosen´s model for mixed monolayers was used to investigate interactions between different surfactants at the interface. Synergism was noticed in Brij S20/poloxamer 407 mixtures. However, very

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different interfacial behavior was observed for Triton X-100/poloxamer 407 systems.

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Keywords: surfactants; interface; mixed monolayer; synergism; antagonism; Brij; Triton X100; poloxamer

ACCEPTED MANUSCRIPT Introduction Having in mind significance of surfactant mixtures in various industrial applications (detergency, mineral flotation, oil recovery, pharmaceutical formulations, food industry) it is important to know their fundamental properties. The knowledge of the nonideal behavior of

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surfactant mixtures at the air/aqueous solution interface, i.e., synergism or antagonism, is especially important because the nonideality often gives unique and improved properties to surfactant mixtures as compared to individual surfactants [1-16]. In case of strong synergism,

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a smaller amount of surfactant can be required for various applications, what is important from safety, ecological and economic perspective [17-19]. Therefore, it is important to

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investigate factors influencing the nonideal behavior not only for theoretical, but for practical reasons, also.

Nonionic surfactants have important role in various industrial applications. Owing to their nonionic nature they are regarded as biocompatible compounds, which gives them wide

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applicability in pharmaceutical, cosmetic and food industries. Brij surfactants are classic nonionic surfactants having hydrophilic head-hydrophobic tail architecture. In Brij S20 (B20) polar part is built of one hydrophilic, polyoxyethylene (PEO), group having 20 oxyethylene

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(OE) units on average, which is attached to a hydrophobic, stearyl, chain. Triton X-100 (T100) is also a nonionic surfactant with different structure of the hydrophobic part, however,

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as compared to the most classic nonionic surfactants. Namely, instead of aliphatic tail, T100 has an alkyl-aryl (octyl-phenyl) group, as its hydrophobic part. It is extensively used in biochemical research field and certain pharmaceutical formulations [20-22]. Poloxamers are nonionic surfactants, triblock copolymers, composed of ethylene oxide (EO) groups arranged in two hydrophilic polyoxyethylene side chains, which are attached to hydrophobic polyoxypropylene (PPO) base built of propylene oxide (PO) units. Apart from their usage as detergents, wetting agents and emulsifiers, they are also used as gelling agents because of 1

ACCEPTED MANUSCRIPT their polymeric structure [23]. Among the polymeric surfactants, poloxamer 407 (P407) is the most extensively investigated compound, especially in pharmaceutical field [24-28]. Surface properties of mixtures of ionic and nonionic surfactants are usually investigated in research studies, in which synergism is usually noticed [4-9]. In contrast, knowledge

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regarding the mixtures of nonionic surfactants is still scarce, although classic nonionic surfactants are generally recognized as less toxic and more biocompatible than ionic surfactants. To the best of our knowledge, there are also no reports about the influence of

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nonionic polymeric surfactants on the interfacial behavior of nonionic surfactant systems, even though mixtures of poloxamer 407 and classic nonionic surfactants are already used in

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certain industries, especially in pharmaceutical and cosmetics formulations. This is why in this study, binary mixtures of classic nonionic surfactants having different hydrophobic moieties and poloxamer 407 have been investigated.

The aim of this study is to investigate the nonideal behavior at the air/aqueous solution

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interface of Brij S20/poloxamer 407 and Triton X-100/poloxamer 407 mixtures. We have investigated the nonideality in surface tension reduction effectiveness and adsorption efficiency of surfactant mixtures. Also, interactions between different surfactants were

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elucidated. For the purpose of investigation of surface tension reduction effectiveness, we have extended Clint´s theory for mixed micelles to mixed monolayers. We have also obtained

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a relation for the calculation of the ideal maximum surface excess concentration ( Γ idmax ), which was used for the adsorption effectiveness assessment. Rosen´s model was used to elucidate the interactions between different surfactants at the air/aqueous solution interface.

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ACCEPTED MANUSCRIPT Materials and methods Materials Poloxamer 407 with an average composition of (EO)106-(PO)61-(EO)106 and a nominal molar mass of 12935 g/mol was donated by BASF Chemtrade GmbH, Germany.

Brij

S20

was

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purchased from Aldrich and Triton X-100 was obtained from J.T.Baker. All compounds were used as received, without further purification. Specifications of used surfactants, P407, B20 and T100 are presented in Table 1. The water used to prepare the aqueous solutions of

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individual surfactants and surfactant mixtures was double distilled. The specific conductivity

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of the double distilled water was <2 µS cm−1.

Table 1. Specifications of compounds used in this work Compound

poloxamer 407

Abbreviation

Source

CAS

Mass

Registry

fraction

Number

purity

P407

BASF

9003-11-6

B20

Aldrich

9005-00-9

T100

J.T.Baker

9002-93-1

Brij S20 (polyoxyethylene (20) stearyl ether) Triton X-100

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(poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol))

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(polyethylene glycol p-(1,1,3,3-tetramethylbutyl)-phenyl ether)

Methods (surface tension measurements) Surface tension (γ) measurements were carried out on a Krüss Easy Dyne tensiometer (Germany) using the du Noüy ring method. Based on this method, surface tension values of aqueous solutions of individual (pure) surfactants (poloxamer 407, Brij S20 and TritonX100), and surfactant mixtures (B20/P407 and T100/P407) were measured. The mole fraction of the poloxamer in the surfactant mixtures (α1) varied from 0.1 to 0.5. In order to obtain the 3

>0.980

ACCEPTED MANUSCRIPT surface properties, surface tension was monitored as a function of each surfactant or surfactant mixture concentration, i.e., natural logarithm of concentration (ln c). The surface excess concentration was determined from the constant slope of the curve (Figure 1). In order to investigate the nonideal behavior, the data in the slopes were fitted in the linear equations,

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from which the concentrations of a pure surfactant or a surfactant mixture required to produce given surface tension (45 mN m-1) were determined. The prepared aqueous solutions of individual surfactants and surfactant mixtures were allowed to stand for 10 minutes in a water

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bath at 298.15 ± 0.30 K, after the immersion of the platinum ring in the solutions, in order to allow equilibration of the solutions and to obtain a constant temperature. Temperature was

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kept constant during the surface tension measurements at 298.15 ± 0.30 K, as well. The measurements were repeated seven times. The relative standard uncertainty of the surface

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tension determination did not exceed 1.5%.

Figure 1. Dependence of the surface tension (γ) on logarithm of the concentration of the P407/T100 (α1=0.1) surfactant mixture. The arrow denotes the concentration of the surfactant mixture required to produce the given surface tension (c12).

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ACCEPTED MANUSCRIPT Theory The nonideal behavior at the interface based on pseudo-phase separation approach In Rosen´s model [1-3], Rubingh´s pseudo-phase separation approach for mixed micelles complemented with regular solution theory [29, 30], was extended to investigate nonideal

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behavior at the surface, i.e., in the mixed monolayer, of aqueous solutions. If the pseudophase separation theory is applied to adsorption of surfactants at the interface, in case of equilibrium between surfactants in monomeric state and in mixed monolayer following

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relations are obtained, according to Rosen [2]:

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α1c12 = x1 f1 c1

α 2 c12 = x2 f 2 c2

(1)

(2)

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where c1, c2 and c12 designate the solution concentrations of pure surfactants 1, 2 and their mixture, respectively, which can produce a given surface tension value; x1 and x2 are mole fractions of surfactant 1 and 2 in their mixed monolayer; f1 and f2 are the activity coefficients

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of surfactant 1 and 2, respectively, in the mixed monolayer.

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In Rosen´s model, based on the Eqs.(1) and (2), the relations for mole fractions of surfactants in mixed monolayer and interactions between different surfactants can be obtained. The mole fraction of surfactant 1 in mixed monolayer (x1) is calculated by following equation [1, 2]:

1=

(1 − x1 )

( x1 ) 2

2

ln (α1c12 c1 x1 )

(3)

ln ( (1 − α1 ) c12 c2 (1 − x1 ) )

where x1 is solved iteratively. 5

ACCEPTED MANUSCRIPT In binary mixtures, the mole fraction of surfactant 2 is calculated by the relation:

x2=1- x1

(4)

1

(1 − x1 )

2

ln

α1c12

(5)

c1 x1

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β12 =

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The value of x1 is used for the determination of the interaction parameter, β12 [1, 2]:

According to Rosen´s model, for synergism to exist, β12 must be negative and ǀln (c1/c2)ǀ must

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be < ǀβ12ǀ [18, 31].

However, if Clint´s theory for mixed micelles is extended to mixed monolayer, equations which enable additional insight into the nonideal behavior can be obtained. According to Clint´s theory, if surfactant mixing in the mixed micelles is assumed to be ideal, it is

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considered that the activity coefficients in mixed micelles are equal to unity [32]. In case of ideal mixing of different surfactants at the air/aqueous solution interface, it can be also regarded that the activity coefficients of surfactants in the pseudo-phase, i.e., in the mixed

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monolayer, are equal to unity, i.e., f1=f2=1. In this case, following relation can be obtained by

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combining Eqs. (1), (2) and (4):

1 α1 α 2 = + c id c1 c2

(6)

where cid designates the solution concentration of a surfactant mixture, required to produce a given surface tension value, in case of the ideal mixing in the mixed monolayer. For multicomponent surfactant system, cid can be shown as follows: 6

ACCEPTED MANUSCRIPT n αi 1 = ∑ id c i =1 ci

(7)

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where ci is a solution concentration of pure surfactant i, required to yield a given surface tension value, and n is a number of different surfactants in a surfactant mixture.

Therefore, cid, corresponding to certain γ value, is a hypothetical value which can be

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determined if concentrations of individual surfactants in their pure solutions, required to produce the given surface tension value, are known. If the experimentally obtained solution

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concentration of a surfactant mixture required to produce the given surface tension value, c12, is lower than the corresponding cid value, this means that synergism exists at the interface. In this case the solution concentration of the real system yields the same surface tension value as the ideal system does, but at higher concentration. This means that the surfactant mixture

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achieves more efficient reduction of the surface tension, relative to the ideal system. However, the higher c12 value as compared to cid value, for the same γ value, indicates existence of antagonism in the mixed monolayer. In order to quantify the nonideality, relative

c12 −1 cid

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c rc =

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change of c12 on cid can be determined:

(8)

where c12 and cid represent the determined solution concentration of a real and the ideal system, respectively, required to produce the same surface tension value. Therefore, the crc parameter represents the measure of the nonideality. The more negative the crc value is, the stronger synergism exists, i.e., while the more positive value indicates the stronger antagonism. In case of ideal mixing, the value of the crc parameter is zero. 7

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The nonideal behavior at the interface based on the maximum surface excess concentration determination The maximum surface excess concentration (Γmax) and the minimum area per molecule (Amin)

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can additionally elucidate the nonideal behavior. The Γmax value represents the maximum concentration certain surfactant or surfactant mixtures can have at the air/aqueous solution interface, and is regarded as the measure of the adsorption effectiveness [33]. The maximum

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surface excess, is usually calculated by surface tension measurements, based on the Gibbs

Γ max = −

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adsorption isotherm [4-16]:

1 dγ mRT d ln c

(9)

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where γ is surface tension of the aqueous solution, m is a constant, which shows the number of species constituting the surfactant; R is the universal gas constant (8.314 Nm/molK); T represents temperature in K; c is surfactant solution concentration, i.e., solution concentration

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of individual surfactant or surfactant mixture, in mol/l. The constant m has the value of one, in case of nonionic surfactants and nonionic surfactant mixtures. The factor (dγ/dlnc) is obtained

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from the slopes of the plots of surface tension vs. natural logarithm of pure surfactant or surfactant mixture concentration. Based on the Γmax minimum surface area per molecule can be calculated [4-16]:

Amin =

1 N A Γ max

(10)

where NA is Avogadro constant. 8

ACCEPTED MANUSCRIPT id The ideal minimum surface area per molecule for a binary surfactant mixture ( Amin ) is

determined with the help of Amin values for the pure surfactants constituting the mixture [4, 5]:

id Amin = x1 Amin,1 + x2 Amin,2

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(11)

where x1 and x2 represent the mole fraction of surfactant 1 and 2, respectively, in the mixed monolayer; Amin,1 and Amin,2 represent the Amin value for pure surfactants 1 and 2, respectively.

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In order to investigate the nonideality from the adsorption efficiency standpoint, the ideal

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maximum surface excess concentration for a surfactant mixture can be obtained based on the Eqs. (10) and (11) giving:

1

=

x1 Γ max,1

+

x2 Γ max,2

(12)

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Γ

id max

where Γmax,1, Γmax,2 represent the maximum surface excess for pure surfactants 1 and 2, respectively; x1 and x2 are mole fractions of components 1 and 2, respectively, in the mixed

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monolayer composed of surfactants 1 and 2.

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Thereby, for a surfactant mixture, composed of n number of different surfactants Γidmax can be described by following equation:

1 Γ

id max

n

=∑ i =1

xi

(13)

Γ max,i

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ACCEPTED MANUSCRIPT where xi represents the mole fraction of surfactant i in the mixed monolayer of a surfactant mixture composed of n number of different surfactants; Γmax,i designates the maximum surface excess concentration for pure surfactant i. If the experimentally obtained Γmax value for a surfactant mixture is higher than the Γ idmax

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value, i.e., if the adsorption efficiency of a surfactant system is higher as compared to the corresponding ideal value, it can be considered that a synergistic effect in improving the adsorption efficiency exists. However, a lower value of maximum surface excess for a

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surfactant mixture as compared to the ideal value indicates antagonism at the interface, i.e., lower adsorption efficiency of the real system than the corresponding ideal state.

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The nonideal behavior regarding the adsorption effectiveness can be quantified by determination of the relative change of the Γmax on Γ idmax :

Γ max −1 Γ idmax

(14)

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Γ rcmax =

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where Γmax and Γ idmax are the experimentaly obtained and the calculated ideal maximum surface excess concentrations for a surfactant mixture, respectively.

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The more positive value of Γ rcmax designates stronger synergism, while the more negative value of Γ rcmax indicates greater antagonism. In case of ideal mixing the value of parameter Γ rcmax is zero. The nonideal behavior regarding minimum surface area per molecule for a surfactant mixture can be quantified, by following relation:

rc Amin =

Amin −1 id Amin

(15)

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ACCEPTED MANUSCRIPT id where Amin and Amin are the experimentally obtained and calculated ideal minimum area per

molecule for a surfactant mixture. rc rc <0 is obtained, while Amin >0 is expected in case of antagonism. In In case of synergism Amin

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rc ideal mixtures Amin =0 is expected.

Results and discussion

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The c1, c2, and c12 values were obtained at the same surface tension value of 45 mN/m, from the plot of γ value vs. natural logarithm of surfactant or surfactant mixture concentration.

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These values were used for the cid and crc values calculation, as well as for the determination of the mole fractions of surfactants in the mixed monolyer and the interaction parameter values, according to Rosen´s model. The Γmax values for individual surfactants and surfactant mixtures were determined based on the Gibbs adsorption isotherm. The Amin values for pure surfactants and their mixtures were consequently determined based on the Γmax data. For the

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id calculation of Γ idmax and Amin data, the x1 and x2 values determined according to Rosen´s

model were used, because the model has been already validated for determination of the mole

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fractions of nonionic surfactants at the interface [1]. The Amin and Γmax values were used for id rc the determination of the ideal Amin and Γ idmax data and corresponding Amin and Γ rcmax values. All

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obtained values are presented in Table 2.

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ACCEPTED MANUSCRIPT Table 2. Experimental values of solution concentrations, surface and adsorption parameters of poloxamer 407, Brij S20 and Triton X-100 and their mixtures, poloxamer 407(1)+Brij S20(2) and poloxamer 407(1)+Triton X-100(2), at 298.15 K.a cid

α1

c

rc

ǀln(c1/c2)ǀ

x1

β12

x2

Γ idmax

Γ max

Γ

rc max

Amin

(µmol m-2)

(µM)

poloxamer 407(1)+Brij S20(2) 2.78

3.51

-0.208

0.3839

0.252

0.748

-1.462

3.798

0.2

2.67

3.36

-0.205

0.3839

0.341

0.659

-1.077

3.570

0.3

2.60

3.22

-0.192

0.3839

0.420

0.58

-0.884

3.420

0.4

2.55

3.09

-0.175

0.3839

0.496

0.504

-0.769

3.110

0.5

2.65

2.97

-0.108

0.3839

0.577

0.423

-0.475

2.941

rc Amin

2.742

0.385

43.72

60.49

-0.277

2.628

0.358

46.51

65.56

-0.291

2.563

0.334

48.55

70.06

-0.307

2.485

0.251

53.39

74.39

-0.282

2.407

0.222

56.46

79.01

-0.285

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0.1

id Amin

(10-20 m2)

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c12

poloxamer 407(1)+Triton X-100(2) 16.9

13.0

0.300

2.274

0.283

0.717

1.694

2.023

2.141

-0.0551

82.08

77.53

0.0587

0.2

16.3

8.86

0.840

2.274

0.299

0.701

2.997

1.730

2.099

-0.176

95.99

79.10

0.213

0.3

15.5

6.72

1.306

2.274

0.327

0.673

3.838

1.671

2.074

-0.194

99.38

80.06

0.241

0.4

14.2

5.41

1.625

2.274

0.363

0.637

4.520

1.620

2.042

-0.207

102.5

81.29

0.261

0.5

12.7

4.53

1.803

2.274

0.401

0.599

5.145

1.591

2.010

-0.208

104.4

82.60

0.264

α1 is mole fraction of P407 in surfactant mixtures;

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0.1

c1 is solution concentration of pure P407 required to produce a given surface tension (45 mN/m). c1 value for pure P407 is 2.50 µM.

c2 is solution concentration of pure B20 or T100 required to produce a given surface tension (45 mN/m). c2 values for pure B20 and T100 are 3.67 and 24.3 µM respectively. mN/m).

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c12 is solution concentration of an investigated P407/B20 or P407/T100 mixture required to produce a given surface tension (45

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cid is solution concentration of an investigated P407/B20 or P407/T100 mixture, in case of ideal mixing, required to produce a given surface tension (45 mN/m).

crc is the measure of the nonideality regarding surface tension reduction effectiveness of a surfactant mixture. ǀln (c1/c2)ǀ is the absolute value of the natural logarithm of the quotient of the solution concentration of pure 407 (c1) and pure B20 x1 is the mole fraction of P407 in mixed monolayer; x2 is the mole fraction of B20 or T100 in mixed monolayer; β12 is the interaction parameter of a surfactant mixture. or T100 (c2) required to produce a given surface tension (45 mN/m). Γmax is the maximum surface excess concentration for a surfactant mixture. Γmax values for pure P407, B20 and T100 are: 1.610 µmol m-2, 3.599 µmol m-2 and 2.411 µmol m-2 respectively.

Γ idmax is the maximum surface excess concentration for a surfactant mixture in case of ideal mixing.

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ACCEPTED MANUSCRIPT Γ rcmax is the measure of nonideality regarding the maximum surface excess for a surfactant mixture. Amin is the minimum area per molecule for a surfactant mixture. Amin values for pure P407, B20 and T100 are: 103.1x10-20 m2, 46.14x10-20 m2 and 68.87x10-20 m2 respectively. id is the minimum area per molecule for a surfactant mixture, in case of ideal mixing. Amin rc is the measure of nonideality regarding the minimum area per molecule for a surfactant mixture. Amin

Standard uncertainties u are u(c1)=±0.05 µM for P407, u(c2)= ±0.07 µM for B20, u(c2)= ±0.3 µM for T100, u(c12)= ±0.05 µM for

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B20/P407 mixtures, u(c12)= ±0.2 µM for T100/P407 mixtures; u( Γ max )=±0.030 µmol m-2 for P407, u( Γ max )=±0.052 µmol m-2 for B20, u( Γ max )=±0.039 µmol m-2 for T100, u( Γ max )=±0.047 µmol m-2 for B20/P407 mixtures, u( Γ max )=±0.035 µmol m-2 for T100/P407 mixtures; u( Amin )=±2.1x10-20 m2 for P407, u( Amin )=±1.29x10-20 m2 for B20, u( Amin )=±1.92x10-20 m2 for T100, u( Amin )=±1.08x10-20 m2 for B20/P407 mixtures, u( Amin )=±1.89x10-20 m2 for T100/P407 mixtures.

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rc Based on the values presented in Table 2, it can be noticed that the crc, Γ rcmax and Amin values

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for investigated mixtures are different from zero, what means that the nonideality exists in all investigated systems. Because hydrophilic domains of all investigated surfactants are composed of the same, oxyethylene, groups, it can be supposed that the observed nonideality arises because of the differences in hydrophobic parts. Based on the obtained results it can be

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noticed that the c12 values are lower than corresponding cid values, i.e., crc has negative values, for all investigated B20/P407 mixtures. This means that the synergism exists in the mixed monolayers of these mixtures, which leads to more efficient reduction of the surface tension,

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relative to corresponding ideal systems. All investigated B20/P407 mixtures have negative values of β12 interaction parameter. Also, it can be observed from Table 2 that ǀln (c1/c2)ǀ is <

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ǀβ12ǀ for the B20/P407 mixtures what means that synergism is confirmed by Rosen´s model, as well. The negative β12 values indicate existence of attractive interactions between B20 and P407 molecules at the air/aqueous solution interface. Most probably attractive dipole-induced dipole interactions between hydrophobic groups of P407 and proximal hydrophobic groups of B20 occur, i.e., between dipoles in PPO chains and induced dipoles in surrounding hydrocarbon groups of B20 molecules, phenomena already noticed in mixed micelles of P407 and classic nonionic surfactants [34]. It can be also supposed that P407 has elongated 13

ACCEPTED MANUSCRIPT conformation at the interface. Because of the presence of two hydrophilic, PEO, chains, poloxamer 407 has most probably inverted-U [35], or inverted-V conformation at the surface, as compared to nonionic surfactants having polar head-hydrophobic tail structure (Figure 2). The two hydrophilic chains present in the water phase are bending and thus elongating the

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PPO part at the interface. In this way, network of the propylene-oxide groups exists between B20 molecules at the surface, what enables one P407 molecule to interact with multiple B20 molecules. This increases the possibility for attractive interactions in mixed monolayers, i.e.,

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in this way synergism is enhanced.

Figure 2. Conformations of investigated surfactants, poloxamer 407 (A), Brij S20 (B) and

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Triton X-100 (C) at the air/water interface.

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According to results in Table 2, it can be noticed that a surfactant mixture having α1=0.1 has the Γ max value higher than pure poloxamer 407 and Brij S20 surfactant. This means that this mixture has higher adsorption efficiency as compared to individual surfactants constituting the system and is clear indication of synergism. It can be observed that the Γ rcmax values are positive for all α1 values, meaning that synergism in surface adsorption effectiveness exist in all investigated B20/P407 mixtures. The increase in the adsorption effectiveness can be attributed to the attractive interactions between Brij S20 and polymeric surfactant, which 14

ACCEPTED MANUSCRIPT increase the packing density at the surface. The tighter packing at the interface of surfactant mixtures is also reflected in the decrease of a minimum surface area occupied by surfactant id rc molecules, as compared to the hypothetical Amin values, what is observed from negative Amin

values for all investigated B20/P407 surfactant mixtures.

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For all T100/P407 mixtures, positive crc values are obtained, however, what indicates the existence of the antagonism in the mixed monolayers, which leads to less efficient reduction of the surface tension, than in case of ideal mixing. In all investigated T100/P407 mixtures

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positive values of the interaction parameter were noticed, meaning that antagonism exists according to the Rosen´s model, as well. Positive values of the β12 parameter indicate that the

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stronger repulsive interactions exist in the mixed monolayers than in corresponding ideal systems. This is probably the consequence of the electrostatic repulsions between the partially-negatively charged oxygen atoms of PO groups of P407 molecules and the πelectron cloud of phenyl groups of T100 molecules. Namely, in pure monolayer of Triton X-

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100 molecules repulsive interactions between aromatic groups, i.e., the π-electron clouds, of adjacent T100 molecules cannot be ruled out. However, the possibility of the repulsive interactions is higher if P407 molecules are present, because the polymeric surfactant has

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higher amount of partially negatively charged groups in the hydrophobic part, elongated

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conformation at the surface and less sterically rigid hydrophobic part as compared to the aromatic group of T100. This results in formation of a network of partially negatively charged propylene oxide groups between aromatic groups of T100 molecules, increasing the probability of repulsive interactions. It can be also noticed that the negative Γ rcmax values are obtained for all investigated T100/P407 mixtures, what means that the antagonism in adsorption effectiveness exists. The decrease in adsorption efficiency can be attributed to the looser packing of surfactants at the interface, as compared to the ideal mixtures. Lower

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ACCEPTED MANUSCRIPT rc packing density, observable from positive Amin values also, is most probably the consequence

of the repulsive interactions between the hydrophobic parts of T100 and P407 molecules at the surface.

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Conclusions

Based on the obtained values for surface and adsorption properties, synergism was noticed in mixed monolayers of all investigated B20/P407 mixtures. By extending the Clint´s theory to

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mixed monolayer synergism in surface tension reduction was noticed in B20/P407 mixtures. The attractive interactions exist between B20 and P407 molecules at the interface, most

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probably arising by dipole-induced dipole interactions between hydrophobic parts. By analyzing the experimentally obtained Γ max and the calculated Γ idmax values, the synergism was also noticed in the adsorption efficiency at the air/aqueous solution interface for these mixtures.

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However, the antagonism in interfacial behavior was determined in T100/P407 mixtures, resulting in less efficient reduction of the surface tension relative to the ideal systems. The

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antagonism is most probably the consequence of the repulsive interactions between PPO chains and phenyl groups. The antagonism was also noticed in the adsorption efficiency for

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all investigated T100/P407 mixtures.

Acknowledgments:

This study was financially supported by the Provincial secretariat for the Science and Technological Development, AP Vojvodina, Republic of Serbia, Grant No 114-451-360/2016. This work was also supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia (Grant no. 172021).

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