Surface and volume properties of dodecylethyldimethylammonium bromide and benzyldimethyldodecylammonium bromide

Surface and volume properties of dodecylethyldimethylammonium bromide and benzyldimethyldodecylammonium bromide

Journal of Colloid and Interface Science 331 (2009) 494–499 Contents lists available at ScienceDirect Journal of Colloid and Interface Science www.e...

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Journal of Colloid and Interface Science 331 (2009) 494–499

Contents lists available at ScienceDirect

Journal of Colloid and Interface Science www.elsevier.com/locate/jcis

Surface and volume properties of dodecylethyldimethylammonium bromide and benzyldimethyldodecylammonium bromide I. Surface properties of dodecylethyldimethylammonium bromide and benzyldimethyldodecylammonium bromide ∗ ´ Joanna Harkot, Bronisław Janczuk Department of Interfacial Phenomena of Chemistry, Faculty of Chemistry, Maria Curie-Skłodowska University, Maria Curie-Skłodowska Sq. 3, 20-031 Lublin, Poland

a r t i c l e

i n f o

a b s t r a c t

Article history: Received 19 September 2008 Accepted 27 November 2008 Available online 6 December 2008 Keywords: Surfactants Surface tension Adsorption Dodecylethyldimethylammonium bromide (C12 (EDMAB)) Benzyldimethyldodecylammonium bromide (BDDAB)

Surface tension measurements were carried out for aqueous solutions of two cationic surfactants: dodecylethyldimethylammonium bromide (C12 (EDMAB)) and benzyldimethyldodecylammonium bromide (BDDAB). Isotherms and thermodynamic adsorption parameters were determined from the surface tension data. Firstly, the surface excess concentration in the adsorbed monolayer and the total concentration of the surfactants were determined, then the standard free energy of adsorption was calculated by different methods. In the calculations, different orientations of the surfactants at the adsorbed monolayer were also taken into account. From the experimental and calculated data it results that the difference in the structure of the two cationic surfactants by changing the methyl group for aryl one in their heads causes an increase of the efficiency and a decrease of the effectiveness of adsorption at water–air interface, and that the standard free energy of adsorption can be predicted from the surface tension of the surfactants assuming the aryl group to be equivalent to 3.5 methylene groups. The experimentally obtained difference between the standard free energy of adsorption of the C12 (EDMAB) and BDDAB was in good agreement with that theoretically accounted, corresponding to the standard free energy of adsorption of the aryl group. However, the best correlation between the values was obtained when a parallel orientation of the surfactant molecules at the adsorbed monolayer was taken into account. © 2008 Elsevier Inc. All rights reserved.

1. Introduction The tendency of molecules to adsorb at interfaces in an oriented direction is one of the most interesting and important properties of surfactants. The interfacial chemistry created by adsorbed molecules of surfactants and the nature of the interfacial monolayers formed by these compounds determines the end-use properties of the materials in many different applications. Because of these reasons the adsorption has been studied largely [1–5] to determine the surfactant excess concentration at the interface (which is a measure of the extension of the interface covered by the surfactant), the orientation of the molecules at the interface, the efficiency and effectiveness of adsorption and energy changes in the system resulting from the adsorption. The most useful tools to study the surface phenomena in an aqueous system are changes of

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Corresponding author. Fax: +48 81 533 3348. ´ E-mail address: [email protected] (B. Janczuk).

0021-9797/$ – see front matter doi:10.1016/j.jcis.2008.11.064

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the physical properties of aqueous solutions of surfactants such as, for example, changes in the values of the surface tension of aqueous surfactant solutions. These changes can be used to determine the standard free energy of adsorption for surface-active agents at aqueous solution–air interfaces. The energy changes provide information on the type and mechanism of the interactions involving surfactants at the interface [6]. However, despite a large number of surface and interfacial tension data in the literature, there is a lack of correlation between the standard free energy of adsorption and surface tension of the tail and head of surfactant molecules. In addition, there are not many investigations using the calculations of the standard free energy of adsorption of surfactants where different orientations of the surfactants at the adsorbed monolayer were taken into account. Moreover, for the surfactants used in many branches of industry including, for example, cosmetic products and many others [7] it is difficult to find in the literature their adsorption behaviour at water–air interface in a wide concentrations range in the bulk aqueous phase.

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Fig. 1. The molecular structures of dodecylethyldimethylammonium bromide (C12 (EDMAB)) and benzyldimethyldodecylammonium bromide (BDDAB).

Cationic surfactants, being the subject of our investigations, have a wide applicability both from the biological and technical point of view [1,7–9]. Especially quaternary ammonium surfactants are widely applied in practical fields, as antimicrobial and antiseptic agents, as fabric softeners, as corrosion inhibitors, etc. [10]. The interest in cationic surfactants has also greatly increased because of the application of cationic surfactants as vectors in gene delivery [11,12]. However, the substitution of the methyl group in the cationic surfactant molecules bounded to the positive nitrogen by another (for example aryl group), the effect of the group on the adsorption tendency of the surfactants at the water–air interface and their aqueous solution thermodynamic properties have not been fully elucidated as yet. Despite the fact that we can find in the literature thermodynamic analysis of the influence of the methyl group addition to the tail of the dodecyltrimethylammonium bromide molecule or its substitution by the ethyl one in the surfactant polar head (gives rise to dodecylethyldimethylammonium bromide— C12 (EDMAB)) [10,13], it is still not possible to find full information about the influence of the methyl group substitution in the C12 (EDMAB) molecule by aryl one (being an equivalent to about three and half methylene groups) giving rise to benzyldimethyldodecylammonium bromide (BDDAB). It also seems to be useful (from the practical and theoretical point of view) to fully collect the received results of the influence of the substitution of the methyl group in the ethyl group of the head group by aryl one in the polar head of the dodecyltrimethylammonium bromide molecule. Because in the literature, there is also lack of correlation between the surface tension of the C12 (EDMAB) and (BDDAB) (having the same hydrophobic and different hydrophilic group) and the standard free energy of adsorption, the purpose of our paper was to determine the C12 (EDMAB) and (BDDAB) surface and adsorption properties at the aqueous solution–air system on the basis of surface tension measurements. Another aim of our paper was to find the correlation between the surface tension of aqueous solutions and these energies calculated on the basis of the surfactants surface tension. We also tried to explain the influence of the polar head of surfactant molecule structure on their tendency to adsorb at water–air interface. For these purposes measurements of the surface tension of aqueous solutions were carried out and some calculations were done.

2. Experimental 2.1. Materials The studied surfactants: dodecylethyldimethylammonium bromide (C12 (EDMAB)) of 98% purity or greater and benzyldimethyldodecylammonium bromide (BDDAB) of purity 99% or greater were supplied by Fluka (the structures of the molecules of these surfactants are shown in Fig. 1). The surfactant molecules have identical tails (12 carbon atoms) and different polar heads. C12 (EDMAB) was used as received without any further purification. BDDAB was purified as described in the literature [14,15]. For preparation of aqueous solutions of these surfactants doubly distilled and deionised water (Destamat Bi 18E) were used. The surface tension of water (γ w = 72.8 mN/m) was always controlled at 293 K with Krüss K9 tensiometer under atmospheric pressure by the ring method, which preceded the solution preparation. The surfactant concentration in solution was in the range from 1 × 10−7 M to 1 × 10−1 M (for C12 (EDMAB)) and from 1 × 10−7 M to 1 × 10−2 M (for BDDAB). 2.2. Surface tension measurements The surface tension measurements (γLV ) of C12 (EDMAB) and BDDAB aqueous solutions were made by the ring method using the Krüss K9 tensiometer, under atmospheric pressure. The platinum ring was cleaned and a flame dried before each measurement. Next it was dipped into the solution to measure its surface tension. Measurements of the surface tension of pure water at 293 K were performed to calibrate the tensiometer and to check the cleanliness of the ring and glassware. The measurements were conducted until constant surface tension values indicated that equilibrium had been reached. Standard deviation did not exceed ±0.2 mN/m. 3. Results and discussion 3.1. Surface properties of surfactants 3.1.1. Isotherms of surface tension The measured values of the surface tension of aqueous solutions of C12 (EDMAB) and BDDAB as a function of log C (C represents the concentration of the surfactants in solution) are presented in Fig. 2 (curve 1 and 2, respectively). The shape of the

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Fig. 2. The relationship between the values of the surface tension (γLV ) of aqueous solutions of C12 (EDMAB) (curve 1) and BDDAB (curve 2) and the concentration of the surfactants (log C ).

Fig. 3. The relationship between the values of the surface excess concentration (Γ ) of C12 (EDMAB) (curve 1) and BDDAB (curve 2) and the concentration of the surfactants (log C ).

γLV – log C curves is very similar for both surfactants, but the values of γLV at a given concentration depend on the type of the surfactant. It is clear that a liner dependence exists between γLV and

(curve 1 and 2, for C12 (EDMAB) and BDDAB, respectively). The maximal values of Γ for C12 (EDMAB) and BDDAB (Γmax ) were established from Eq. (3) on the basis of the linear relationships between the surface tension and log C . The calculated values of the Γmax are equal to 2.6 × 10−6 mol/m2 and 1.6 × 10−6 mol/m2 for C12 (EDMAB) and BDDAB, respectively. Of course, the amount of the surfactant adsorbed at air–water interface depends on the concentration of the surfactant in solution and increases in the range from 1 × 10−5 M for both surfactants to 3 × 10−3 M for C12 (EDMAB) and 3 × 10−4 M for BDDAB (the first points of C12 (EDMAB) and BDDAB saturation at air–water interface, respectively). The maximum surface excess concentration is the measure of the effectiveness of the surfactant adsorption at air–water interface [1]. Thus, on the basis of the obtained results we can state that the presence of the aryl group instead of methyl one in the molecule of the surfactant decreases its adsorption effectiveness. It is probable because of the influence of the substitution of the methyl group by the aryl one on a steric effect, resulting from the increase of the cross section area of the polar head of the BDDAB molecules [13]. The surface excess concentration in the adsorption monolayer determined on the basis of the Gibbs thermodynamic model of adsorption is practically equal to the total concentration of the surfactant in the range of its low concentrations in the bulk phase. However, at the concentration corresponding to the saturated monolayer there are some differences between the total surfactant concentration in the monolayer and its excess concentration. Knowing the length of the surfactants calculated by using the Mercury 1.4 programme (21.8 Å and 26.2 Å for C12 (EDMAB) and BDDAB, respectively) or their thickness (2.7 Å) [18], it is also possible to calculate the total concentration of the surfactant in the monolayer at water–air interface (C s ) [1]:

log C near the break point on the isotherms of the surface tension. On the other hand, it is known that the surface tension vs. the concentration curve breaks at a low surfactant concentration much below the CMC (critical micelle concentration) and that the breaking indicates the phase transition from gaseous to liquid expanded state in the adsorbed film [16,17]. It is interesting that the dependence between the surface tension and the surfactant concentration in the bulk phase in the range from 1 × 10−7 M to 3 × 10−3 M for C12 (EDMAB) and from 1 × 10−7 M to 3 × 10−4 M for BDDAB can be expressed by the second order of the exponential function:

γ = a1 exp(−b1 C ) + a2 exp(−b2 C ) + c ,

(1)

where a1 , a2 , b1 , b2 and c are the constants. It seems that this function is somewhat in contrast to the opinion that in the range of low concentrations of the surfactant the dependence between the surface tension of its aqueous solution and concentration is linear. However, it concerns the change of the surface tension as a function of C in the range from 0 to 3 mN/m [1]. Taking into account the error of the values of the surface tension (which depends on the surfactant concentration and it can be even equal to 0.4 mN/m) and the fact that some impurities can also influence the surface tension, it is difficult to state exactly which relationship, linear or exponential, is more real to present the γLV vs. C changes in the range of low surfactant concentrations. 3.1.2. Surfactants concentration in surface layer On the basis of the γLV vs. log C slopes the amount of surfactant adsorbed at air–water interface, Γ , can be determined using the Gibbs equation of adsorption [1,5]. For dilute solution (1 × 10−2 M or less) of 1:1 strong electrolyte type the Gibbs equation has the form [1,15]:

Γ =−

C



(2)

2R T dC

or

Γ =−

1



4.606R T d log C

(3)

. dγ

Introducing into Eq. (2) the values of dC determined from Eq. (1) the values of Γ were calculated which are presented in Fig. 3

Cs =

Γ h

+ Cb ,

(4)

where C b is the surfactant concentration in the bulk phase and h is the thickness of the monolayer of the surfactant at air–water interface. From the calculations of the surfactants concentration in the adsorption monolayer at water–air interface it results that at the same concentration of the surfactants in the bulk phase the concentration of BDDAB is higher than for C12 (EDMAB), however, at concentrations in the bulk phase corresponding to the break points on γLV – log C curves (Fig. 2) a reverse relation takes place (C s

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Fig. 4. The relationship between the standard free energy of adsorption (G 0ad ) of C12 (EDMAB) and the concentration of the surfactant in solution (log C ).

Fig. 5. The relationship between the standard free energy of adsorption (G 0ad ) of BDDAB and the concentration of the surfactant in solution (log C ).

is equal to 1.2 M and 0.6 M for C12 (EDMAB) and BDDAB, respectively). It suggests that the adsorption efficiency of BDDAB is higher than of C12 (EDMAB), but the effectiveness of C12 (EDMAB) is higher than of BDDAB. The efficiency of the adsorption can also be determined on the basis of the pC 20 (the negative logarithm of the bulk phase concentration of surfactant required to depress the surface tension of the solvent by 20 mN/m). The larger the value of the pC 20 , the more efficiently the surfactant is adsorbed at the interface and the more efficiently it reduces the surface or interfacial tension [1]. In the case of our studied surfactants there is a difference between the pC 20 values for C12 (EDMAB) and BDDAB. It equals 2.4 and 3.4 for C12 (EDMAB) and BDDAB, respectively. From these values it results that there is a better efficiency of BDDAB adsorption than in the case of the C12 (EDMAB) and that the smaller the bulk liquid phase concentration is required to attain saturation of the monolayer at the interface. From these facts it results that the efficiency at which the surfactant is adsorbed at an interface depends on the surfactant molecules structure. The efficiency of the surfactant adsorption can also be related to the standard free energy of the adsorption [1].

and

3.1.3. Standard free energy of adsorption Generally, the changes of the standard free energy of adsorption, G 0ad , tell us whether adsorption (at standard states) is spon-

taneous (when G 0ad is negative) or not. In the literature [1,19,20] different equations were found which may be used to evaluate the G 0ad for C12 (EDMAB) and BDDAB at air–water interface. According to the conception of Langmuir [1,19–23] and if mobile adsorption isotherms are considered, the area, A, per an adsorbed molecule will be related to the bulk concentration of the surfactant, and G 0ad will satisfy the equation introduced by de Boer [24]: A0 A − A0

exp

A0 A − A0

=

C

ω

 exp −

G 0ad



2R T

(5)

.

The G 0ad values calculated from Eq. (5) are presented in Figs. 4 and 5 for C12 (EDMAB) and BDDAB, respectively. The A 0 values used in Eq. (5) for calculations were determined from the following equations [1,5,25]:

 exp −

Π R T Γ0∞



 + exp −

Π 2R T Γ1∞

 ×

C a

=1

(6)

A0 =

1 N Γ1∞

(7)

,

where Γ0∞ , Γ1∞ are the maximum of the solvent and surfactant adsorption, respectively, Π is the surface pressure equal to the difference between the surface tension of the solvent and solution (γ0 − γ ), and a is the parameter expressed by the equation:

 a = exp

μS − μB 2R T

 × ω,

(8)

where μ S , μ B are the chemical potentials in the surface and bulk phase, respectively, under standard conditions. The Γ1∞ value calculated from Eq. (6) for C12 (EDMAB) is equal to 4.3 × 10−6 mol/m2 , and the A 0 value determined from Eq. (7) is equal to 38.4 Å2 . For BDDAB these values are equal to 2.9 × 10−6 mol/m2 and 74.5 Å2 , respectively. It is interesting that the values of “excluded area” both for BDDAB and C12 (EDMAB) calculated from Eq. (7) are close to those determined by using the Mercury 1.4 programme on the assumption that the cross section area of the hydrophilic groups of the surfactants can be expressed by the area of the rectangle (72 and 39 Å2 , respectively). It should also be mentioned that the value of A 0 for C12 (EDMAB) is almost two times and for BDDAB over three times higher than the cross section area for the alkyl group (21 Å2 ) [1,20,26]. Of course, for calculation of the standard free energy of adsorption, the A values were calculated from Eq. (7) using the data presented in Fig. 3. From Figs. 4 and 5 it results that for log C higher than −5.3 (for C12 (EDMAB)) and −6 (for BDDAB) there are attractive and repulsive interactions between the molecules of the surfactants studied. In the case of BDDAB and C12 (EDMAB) there are van der Waals attractive interactions and acid–base and electrostatic repulsive intermolecular interactions. According to the limitations of Eq. (5) application to determine the standard free energy of adsorption, the constant minimal values of G 0ad (Figs. 4 and 5 — curves 1) can be treated as reliable ones. If so, the values of −59 kJ/mol and −72.1 kJ/mol can be treated as the standard fee energy of C12 (EDMAB) and BDDAB, respectively. Thus, the difference between G 0ad values for C12 (EDMAB) and BDDAB is equal −13.1 kJ/mol and results from the presence of the aryl group in the BDDAB molecule.

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It is known that the standard free energy of adsorption is the sum of the standard free energy of adsorption of the structural groups present in the surfactant molecule and that the G 0ad of –CH2 – adsorption is in the range from −3 to −3.5 kJ/mol [1]. Thus, taking into account that the aryl group is equivalent to 3.5 methylene groups, the difference between the standard free energy of C12 (EDMAB) and BDDAB should be in the range from −10.5 kJ/mol to −12.25 kJ/mol. So, the difference between the standard surface free energy of adsorption of C12 (EDMAB) and BDDAB estimated in this way is close to that determined from the experimental data (−13.1 kJ/mol) on the basis of the Langmuir equation (Eq. (5)). As was mentioned above, the Langmuir equation can be applied only to surfactant solutions corresponding to unsaturated adsorption monolayer at water–air interface in which the molecules do not interact with each other. Rosen and Aronson [22] suggested a method for calculating the standard free energy of adsorption at water–air interface, using the surface tension data corresponding to saturated adsorption monolayer. The equation proposed by them for 1:1 electrolyte type of surfactant, on assumption that its activity coefficient is equal 1 has the form [1,24]:

G 0ad = 4.606R T log

C

ω

− 6.023Π A min .

(9)

The results of the G 0ad obtained from Eq. (9) are presented in Fig. 4 for C12 (EDMAB) and Fig. 5 (dotted lines) for BDDAB. The standard free energy of adsorption for C12 (EDMAB) and BDDAB was calculated using the A min values (minimal area per molecule) determined from Eq. (7) for Γ = Γmax . The values of the A min for C12 (EDMAB) and BDDAB are equal to 64.2 Å2 and 106.5 Å2 , respectively. It is seen from Figs. 4 and 5 that the values of the standard free energy of the adsorption of the surfactants studied obtained on the basis of the Rosen and Aronson method (Eq. (9)) are higher than those determined on the basis of the Langmuir theory (Eq. (5)), especially in the case of C12 (EDMAB). The difference between G 0ad values for C12 (EDMAB) and BDDAB is equal −16 kJ/mol and it is somewhat higher than that obtained for G 0ad values determined from Eq. (5) (−13 kJ/mol) and higher than the values established theoretically (from −10.5 kJ/mol to −12.25 kJ/mol). However, the G 0ad values for BDDAB obtained on the basis of Eq. (9) are very similar to those determined on the basis of Eq. (5) in the range of the surfactant concentration from 1 × 10−7 M to 3 × 10−6 M. Because there are considerable differences between the values of the standard free energy of adsorption for a given surfactant calculated from the Langmuir equation (Eq. (5)) and those from Rosen and Aronson’s equation (Eq. (9)), we calculated also the values of G 0ad on the basis of the surfactant concentration in the adsorbed monolayer at water–air interface and its concentration in the bulk phase from the following equation [1]: Cs Cb

  G 0ad . = exp − 2R T

(10)

For G 0ad calculations the values of C s obtained from Eq. (4) for h corresponding to parallel and vertical orientation of the surfactant molecule at water–air interface were used for a given surfactant concentration in the bulk phase. The G 0ad values calculated from Eq. (10) are presented in Figs. 4 and 5. From these figures it results that the shapes of G 0ad – log C curves obtained on the basis of Eqs. (10) and (5) are somewhat similar. It means that at log C higher than −5.3 (for C12 (EDMAB)) and −5.5 (for BDDAB) the mutual attractive and repulsive intermolecular interactions in adsorbed monolayer influence the values of G 0ad calculated from these equations.

It is interesting that the minimal values of G 0ad for C12 (EDMAB) obtained from Eq. (10) on the assumption of parallel orientation of surfactant molecules to water–air interface (Fig. 4, curve 3) are very similar to the G 0ad values determined on the basis of Rosen and Aronson’s method [22]. Moreover, it is also very interesting that the difference between G 0ad values determined on the basis of Eq. (10) on the assumption of parallel orientation of the surfactant molecule at water–air interface for C12 (EDMAB) is equal −10 kJ/mol. This value is very close to that determined theoretically on the assumption that the aryl group is equivalent to 3.5 methylene groups (from −10.5 to −12.25 kJ/mol). On the basis of the results presented above it is difficult to state which determination method of the standard free energy of adsorption gives more reliable values of this energy. It is possible that this problem can be somewhat clarified on the basis of correlation between the standard free energy of adsorption and the surface tension of the surfactants. 3.1.4. Standard free energy of adsorption and surfactant surface tension In the case of C12 (EDMAB) and BDDAB the standard free energy of adsorption can be broken into the standard free energy changes associated with transfer of the terminal methyl group, the –CH2 – groups of the hydrocarbon chain, and the hydrophilic group from the solution bulk phase to the interface [1]. It is known from the literature that the surface tension of the surfactant can be divided into the surface tension of the tail and head [18,26–29]. The surface tension of the hydrocarbon tail results from the Lifshitz–van der Waals intermolecular interactions, and that of the hydrophilic head from Lifshitz–van der Waals, Lewis acid–base and electrostatic interactions. If we assume that after the adsorption at water–air interface the hydrophobic tail of the surfactant is present at the air phase, and the hydrophilic head at the water phase, then the transfer of surfactant molecules from the bulk water phase to the interface is associated with changes in the interfacial tension of the water– tail (γWT ) to the surface tension of the tail (γ T ) and the interfacial tension of the water–head (γWH ) from γWH to γWH1 because of dehydration of the head during the adsorption process [18]. Thus, the standard free energy of adsorption at water–air interface should fulfill the condition [18]:

G 0ad = (γT − γWT ) × A T + (γWH1 − γWH ) × A H ,

(11)

where A T is the contactable area of the surfactant tail, and A H is the contactable area of the surfactant head. It is possible to calculate the standard free energy of adsorption from Eq. (11) if we assume that during the transfer of the surfactant molecule from water to water–air interface the surfactant head does not dehydrate. The free energy of adsorption of surfactant having a straight chain hydrocarbon tail (from hexyl to hexadecyl) was calculated earlier from Eq. (11) on the basis of the proper n-alkane–water interfacial tension and n-alkane surface tension [18]. From these calculations it results that the standard free energy of adsorption of surfactant having a dodecyl tail is equal to −51.04 kJ/mol [18]. This value is only about 2 kJ/mol higher than that calculated from Eq. (10) on the assumption of parallel orientation at water–air interface molecules of C12 (EDMAB) (Fig. 4, curve 3). It was mentioned above that the aryl group in the head of BDDAB molecule influences its standard free energy of adsorption like 3.5 methylene groups. If it is true, then the standard free energy of BDDAB adsorption should correspond to the changes of the average free energy during the transfer of the pentadecyl and hexadecyl groups. The average value of the free energy of the transfer of these groups is equal to −61.63 kJ/mol [18]. This value is very close to that calculated from Eq. (10) for BDDAB on the assump-

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tion of parallel orientation of its molecules at water–air interface (−62.8 kJ/mol) (Fig. 5, curve 3). Thus, it can be concluded that the standard free energy of surfactant adsorption can be determined on the basis of its surface tension and that the aryl group in the head of the BDDAB molecule positively contributes to its tendency to adsorb at water–air interface. Therefore, the efficiency of BDDAB adsorption, in contrast to its effectiveness, is higher than C12 (EDMAB). It can also be concluded that the values of the standard free energy of the surfactants adsorption evaluated for dilute aqueous solutions seem to be more real than those obtained for solutions corresponding to saturated adsorbed monolayer at water–air interface. However, it should be remembered that all equations used for calculations were derived for surfactants of 1:1 electrolyte type on the assumption that they are completely dissociated both in the water bulk phase and at water–air interface. This assumption cannot be always true, particularly in the adsorbed monolayer at water–air interface.

range of low concentrations if parallel orientation of C12 (EDMAB) and BDDAB molecules at water–air interface is assumed. In addition the standard free energy of adsorption of surfactant having a dodecyl tail is very close to that calculated for C12 (EDMAB). However, it must be noticed that the calculations and assumptions are true only in the region corresponding to the very dilute solution of the surfactant. These researches are very valuable, especially that there are only a few studies in literature on this region of the surface tension–concentration curves [1], since investigators of the effect of surfactants on the surface tension of solvents generally are interested in the region where surfactants show the maximum effect, rather than the region where they show little effect. In addition there is no in the literature systematic studies and calculations of the free energy of adsorption of the C12 (EDMAB) and BDDAB molecules at the water–air interface.

4. Conclusions

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Summarising, we can state that introduction of the aryl group into the head of the surfactant (BDDAB) instead of the methyl one (C12 (EDMAB)) causes the increase of the efficiency and decrease of the effectiveness of the BDDAB adsorption at water–air interface. A phenyl group that is located in a hydrophobic tail of surfactant has the effect of about three and one-half –CH2 – groups [1], however, in the literature, there is no studies on the effect of the phenyl group (that is a part of the polar head of the surfactant molecule) on the effectiveness and efficiency of adsorption of the studied surfactants at the water–air interface. Taking into account the fact that the cross-sectional area of the aliphatic chain oriented vertically to the interface is about 20 Å and that of benzene ring is about 25 Å [1], we can state that there should be no effect of the introduction of the phenyl group to the polar part of the surfactant on the effectiveness of the adsorption of the C12 (EDMAB) and BDDAB at water–air interface, if the vertical orientation of the molecules was assumed. However, on the basis of our studies we can state that the efficiency of BDDAB adsorption at water–air interface increases and the effectiveness of the surfactant decreases. It can suggest that the orientation of the BDDAB molecules at the solution–air interface is rather parallel than vertical in the range of a low surfactant concentration in solution. On the basis of our studies we can also state that it is possible to predict the standard free energy of adsorption of C12 (EDMAB) and BDDAB at water–air interface on the basis of their surface tension data if it is assumed that the aryl group is equivalent to 3.5 methylene groups. The determination of the value is especially important because it is a measure of a tendency of surfactant to adsorption at the interface. It tells us whether adsorption becomes spontaneously or not. Because the standard free energy of adsorption is a very valuable parameter both from the theoretical and practical point of view, it seemed to be very important to compare the obtained results of calculations carried out on the basis of different methods. The obtained values of the standard free energy of adsorption are close to those determined from the ratio of the concentration in the surface region and in the bulk solution in the

References