Introduction

New types of surfactants, known as gemini (or dimeric) surfactants, have been reported in the scientific literature in the past decade [1–9]. Such surfactants consist of two hydrophobic chains and two hydrophilic head groups covalently attached through a spacer. Their surface properties were first described by Okahara and his colleagues [10]. Recently, the study of gemini surfactants is being investigated very actively. A considerable number of investigations have reported their remarkable physicochemical properties, including their high surface activity [2, 4, 8, 11–13], unusual viscosity changes with an increase in surfactant concentration [14, 15], unusual micelle structure [3, 5, 16], aberrant aggregation behavior [17], and stronger interaction with oppositely charged surfactants [18]. The greater efficiency and effectiveness of geminis over comparable conventional surfactants [2–9] make them cost–effective as well as environmentally desirable.

In aqueous media, surfactants in pure and mixed states self-assemble to form micelles [1]. To improve the surface or interfacial properties of a surfactant, one of the best ways is to add to it another surfactant with which it can interact to produce synergy between them. Synergy [1] is defined here as the condition in which the properties of the mixture are better than those attainable with the individual components by themselves and can be predicted from molecular interactions between the two surfactants and relevant properties of the individual surfactants by themselves [1]. Recently, the molecular interactions in mixed systems involving conventional surfactants [18, 19] and gemini surfactants [20–23] have been of academic and industrial interest.

We have investigated the effect of alkanols on the micellization of gemini surfactant [24]. As the interactions between conventional cationic surfactants and alkylamines produce stable mixed micellar aggregates [25–28], the work has been extended to investigate the molecular interactions in the micellization of the cationic gemini surfactants pentanediyl-1, 5-bis(dimethylcetylammonium bromide) and hexanediyl-1, 6-bis(dimethylcetylammonium bromide) (referred to as 16-s-16 where s = 5, 6, see Scheme 1) and linear primary alkylamines (C n NH2, n = 4–8) at 303 K, using tensiometry. The mixtures are characterized by their CMC. The different surface properties of the gemini surfactants in aqueous solutions are evaluated using the surface tension (γ) versus logC t plots in absence as well as presence of additives (alkylamines). The standard Gibbs energies of adsorption (ΔG 0ads ) of the gemini surfactants are also evaluated. The work has relevance as regards the use of alkylamines as co-surfactants for microemulsion formulations with surfactant + oil systems [29, 30]. Further, microemulsions are considered to be good drug carriers and the presence of amines in such combinations may show their specific effects [31] as blood and saliva are known to contain various amines and polyamines [32].

Scheme 1
scheme 1

Molecular structure of gemini surfactants

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Experimental Section

The additives butylamine (C4NH2, >98%, Fluka, Switzerland), pentylamine (C5NH2, ≥98.5%, Fluka, Switzerland), hexylamine (C6NH2 >98%, Merck, Germany), heptylamine (C7NH2, ≥98%, Fluka, Switzerland) and octylamine (C8NH2, >98%, Fluka, Switzerland) were used as received. The gemini surfactants were synthesized by refluxing the corresponding α,ω-dibromoalkane (Br(CH2)sBr, s = 5, 6) with n,n-hexadecyl-N,N-dimethylamine in dry ethanol for 48 h. The solvent was removed under vacuum and the solids thus obtained were recrystallized three times from hexane/ethyl acetate mixtures to obtain pure surfactants. Both the geminis gave satisfactory 1H-NMR analysis which well matched the literature data [33–35]. Doubly distilled and deionized water (sp. conductivity = 1–2 × 10−6 S cm−1) was used throughout. Stock solutions of surfactants were prepared by dissolving the surfactant in aqueous + additive (alkylamine) solutions. The alkylamine concentrations which were used for the preparation of different mole fractions were 0.25, 0.7, 1.5, 4.0 mM.

The CMCs of gemini surfactants (with and without additives) in aqueous media were determined by measuring the surface tension of the pure gemini as well as of 16-s-16/additive (C4NH2–C8NH2) solutions of various mole fractions at 303 K. The surface tension values were measured by the ring detachment method using an S. D. Hardson tensiometer (Kolkata, India). For each set of experiments, the ring was cleaned by heating it in an alcohol flame. The CMC values were obtained by plotting surface tension (γ) versus logC t. The surface tension values decrease continuously and then become constant along a wide concentration range (Fig. 1). The break point, where the constancy of surface tension begins, was taken as the CMC of the system.

Fig. 1
figure 1figure 1

Plots of surface tension (γ) with logC t of pure gemini surfactants and alkylamine-16-s-16 mixtures [s = 5 (I), 6 (II)] at different mole fractions of alkylamines (C4NH2–C8NH2)

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Results and Discussion

Surface tension (γ) versus the logarithm of total surfactant concentration (logC t) profiles for different constant composition mixtures of 16-s-16 and alkylamines (C4NH2–C8NH2) in aqueous solutions are shown in Fig. 1. The break point concentrations corresponding to the CMCs are presented in Table 1. The CMC values decrease with increasing concentration as well as the alkyl chain length of the additives whereas an increase in spacer chain length of the gemini surfactant produces an opposite effect (Table 1). The trend is illustrated in Fig. 2 wherein we find the order to be: C8NH2 > C7NH2 > C6NH2 > C5NH2 > C4NH2 and 16-6-16 > 16-5-16 > 16-4-16 [36]. The CMC values of the gemini surfactants in water are in good agreement with the literature values [33–35] (Table 1).

Table 1 The effect of additive concentrations on the CMC (determined by surface tension measurements), C20, CMC/C20, Пcmc, A min, Гmax and ΔG 0ads values of cationic gemini surfactants in aqueous solutions at 303 K
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Fig. 2
figure 2

Values of CMC of the gemini surfactants [16-5-16 (I) and 16-6-16 (II)] at different mole fractions of alkylamines (C4NH2–C8NH2)

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Like our earlier findings with CTAB [26], which can be considered as the conventional counterpart of 16-s-16, we see, once again, that the amines are less effective than the corresponding alkanols in reducing the CMC values of all the gemini surfactants [36]. The amines are weak bases and, due to feeble hydrolysis \( ( - {\text{NH}}_{2} + {\text{H}}_{ 2} {\text{O}} \leftrightarrow {\text{NH}}_{3}^{ + } + {\text{OH}}^{ - } ), \) the protonated amine species get repelled by the cationic geminis. This specific interaction between the surfactant head groups and amines is responsible for the latter to be less effective.

The variation of C20 (the efficiency of the surfactant in reducing the surface tension of water is the surfactant concentration required to reduce the surface tension by 20 mN/m), the CMC/C20 ratio, and ПCMC (the surface pressure at the CMC), Гmax (the maximum surface excess), A min (the minimum surface area per molecule) and ΔG 0ads (the standard Gibbs energy of adsorption) values, obtained at different mole fractions of the added primary alkylamines in 16-s-16 solutions, are also collected in Table 1. In all cases, the C20 values decrease with additive concentration and follow similar trend for all the amines. The magnitude of the negative log of the C20 value is 2 or 3 orders smaller than those of comparable conventional cationic surfactants. This is in good agreement with previous work [18] showing that the presence of two hydrophobic groups in the gemini molecule results in greater surface activity. The C20 value increases with increasing the spacer chain length of the gemini surfactants. The CMC/C20 ratio is a measure of tendency of the surfactant to adsorb at the air/water interface, relative to its tendency to form micelles. The CMC/C20 ratio also measures how far the surface tension of water can be reduced by the presence of the surfactant. The CMC/C20 effectiveness is in the order: 16-6-16 > 16-5-16 > 16-4-16 [36], which supports the tendency of the 16-6-16 to adsorb at the air/water interface more than 16-5-16. In our previous study we found a similar trend of decreasing CMC with increasing amine concentrations [36].

Values of the ПCMC were obtained by using the equation

$$ \Uppi_{\text{CMC}} = \gamma_{0} -\gamma_{\text{CMC}} $$
(1)

where γ0 and γCMC are the surface tension of the solvent and the surface tension of the mixture at the CMC, respectively. On increasing the amine concentration, the values of ПCMC increase, indicating that the efficiency increases (Table 1). The values of ПCMC decrease with increasing the spacer length of the gemini surfactants.

Гmax of the gemini surfactant molecules at the air/water interface was calculated by using the Gibb’s equation

$$ \Upgamma_{\max } = - {\frac{1}{2.303nRT}}\left( {{\text{d}}\gamma /{\text{d}}\log C_{\text{t}} } \right)_{T} $$
(2)

where R and T are the universal gas constant (8.314 J mol−1 K−1) and temperature, respectively. The prefactor n is the number of species at the air/aqueous interface. For calculating Гmax of gemini surfactants, there is an ongoing dispute about the value of n. In this case, for calculating Гmax, we used a value of 2 for n. The slope of the tangent at the given concentration of the γ versus log C t plot was used to calculate Гmax. The value of Гmax increases with an increase in the additive (amine) concentrations (Table 1). This indicates that the gemini surfactant solutions in presence of amines have a greater tendency to be adsorbed at the air/water interface, compared to a pure gemini surfactant solution. The presence of alkylamines decreases the repulsion among head groups and more gemini surfactant molecules can be adsorbed at the interface.

Amin was evaluated by using the relation

$$ A_{\min } = 10^{20} /N_{A} \Upgamma_{\max } \, \left( {{\AA}^{2} } \right) $$
(3)

where N A is Avogadro’s number. The Amin area decrease with increasing additive concentration (Table 1) takes place due to progressive charge shielding and closer packing of the gemini surfactant ions at the surface. This result suggests that the orientation of the gemini surfactant molecules at the interface is almost perpendicular to the interface [37]. Whereas Гmax decreases with increasing the spacer length, both in the absence and presence of amines, the Amin followed a reverse order, i.e., 16-6-16 > 16-5-16 > 16-4-16 [36]. This may be due to intramolecular head group distances. In this case, the spacer chain could be in contact with water. With addition of amines, the values of Гmax increase and the values of Amin decrease and the trend is followed by all the systems.

To quantify the effect of alkylamines in the mixture on the micellization process, the standard Gibbs energy of micellization, ΔG 0m , and the standard Gibbs energy of adsorption, ΔG 0ads , were evaluated using Eqs. 4 and 5,

$$ \Updelta {\text{G}}_{m}^{0} = \left( {3 - 2{\text{g}}} \right)RT \, \ln C_{12}^{\text{m}} $$
(4)

(\( {\text{C}}_{ 1 2}^{\text{m}} \) is the CMC of the mixture of the two components at a given mole fraction and g is the degree of counterion dissociation to the micelles)

$$ \Updelta G^{0}_{\text{ads}} \, = \, \Updelta G^{0}_{\text{m}} -\Uppi \, _{\text{CMC}} /\Upgamma_{\max } . $$
(5)

The standard state for the adsorbed surfactant is a hypothetical monolayer at its minimum surface area per molecule, but at zero surface pressure. The last term in Eq. 5 expresses work involved in transferring the surfactant molecule from a monolayer at a zero surface pressure to the micelle. In the present case, the last term of Eq. 5 is very small compared to the ΔG 0m , which indicates that the work involved in transferring the surfactant molecule from a monolayer at zero surface pressure to the micelle is negligible. All the ΔG 0ads values are negative (Table 1), which implies that the adsorption of the surfactants at the air/mixture interface takes place spontaneously and are in the order: 16-4-16 [36] > 16-5-16 > 16-6-16. The average values of ΔG 0ads for amines follow the order: C8NH2 > C7NH2 > C6NH2 > C5NH2 > C4NH2 in the case of both gemini surfactants (16-5-16 and 16-6-16) (Table 1), which is in accordance with their hydrophobicity order. The hydrophobicity is the main cause of adsorption.

Molecular interactions between two compounds (amphiphiles) at an interface or in micelles are commonly measured by the so-called β parameters [1, 4, 11, 13], which are conveniently obtained from surface (or interfacial) tension or from CMC data by using well-known equations [11, 38]. By calculating the value of β (interaction) parameters, the nature and strength of the interaction between two components can be determined (βm is the interaction parameter for mixed micelle formation in an aqueous medium and βσ is the interaction parameter for mixed monolayer formation at an aqueous solution/air interface). The following alkylamine concentrations were used to calculate the values of β – for βm: C4NH2, C m1  = 8.28 × 10−3 M; C5NH2, C m1  = 4.17 × 10−3 M; C6NH2, C m1  = 3.98 × 10−3 M; C7NH2, C m1  = 3.48 × 10−3M; C8NH2, C m1  = 1.18 × 10−3M; for βσ: C4NH2, C σ1  = 3.02 × 10−3 M; C5NH2, C σ1  = 6.31 × 10−3 M; C6NH2, C σ1  = 7.58 × 10−4 M; C7NH2, C σ1  = 1.58 × 10−3M; C8NH2, C σ1  = 1.99 × 10−4M). The corresponding activity coefficients (f σ1 and f σ2 , f m1 and f m2 ) were calculated using the relevant Eqs. 6 and 7

$$ f_{ 1} \, = { \exp }\left\{ {\beta \, \left( { 1 -{\text{ X}}_{ 1} } \right)^{ 2} } \right\} $$
(6)
$$ f_{ 2} \, = { \exp }\left\{ {\beta \, \left( {{\text{X}}_{ 1} } \right)^ 2 } \right\} $$
(7)

β indicates the degree of interaction between the two components and also accounts for the deviation from ideality. For ideal mixing of two components, β assumes a value of zero. A positive β value means repulsive interaction among mixed species, whereas a negative β value implies an attractive interaction; the more negative its value, the greater the interaction. At all mole fractions of the mixed systems, the βm values are negative (Table 2), suggesting that the interaction is more attractive between the two components in the mixed micelle than the self-interaction of the two components before mixing. As the mole fraction of alkylamines increases, βm values become more negative. This indicates an increase in the attractive interaction with the increase in [amines] (also evident from the CMC values (Table 1), which decrease with increasing [amines]).

Table 2 Micellar compositions (X m1 , X σ1 ), interaction parameters (βm, βσ), and activity coefficients (f m1 , f m2 , f σ1 , f σ2 ) of binary mixtures of 16-s-16 and primary linear alkylamines at different mole fractions of alkylamines (αamine)
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The βσ trend is similar (Table 2), i.e., the mixtures of alkylamines/gemini surfactants show stronger attractive interaction at the solution/air interface. The βσ values are more negative than βm values which implies that the interactions at the solution/air interface are stronger than in mixed micelles. This is due to the steric factor which is more important in micelle formation than in monolayer formation at a planar interface. Increased bulkiness in the hydrophobic group causes greater difficulty for incorporation into the curved mixed micelle compared to that of accommodating at the planar interface. Table 2 data indicate that the attractive interactions of geminis/amines are more in the case of a smaller spacer chain length than that of the long spacer chain length of the geminis.

Synergism: In mixtures containing two amphiphiles, the existence of synergism has been shown to depend not only on the strength of interaction between them (measured by the values of the β parameter) but also on the relevant properties of the individual amphiphile components of a mixture [39]. The conditions for synergism in surface tension reduction efficiency (when the total concentration of mixed surfactant required to reduce the surface tension of the solvent to a given value is less than that of individual amphiphile) are the following:

  1. (a)

    βσ must be negative

  2. (b)

    |βσ| > |ln (C σ1 /C σ2 )|

where C σ1 and C σ2 are the molar concentrations of amphiphile 1 and 2, respectively, required to achieve that same surface tension value. All mixtures of the cationic gemini surfactant with alkylamines exhibit synergism in surface tension reduction efficiency. The data also show that there is very good synergism in surface tension reduction efficiency for the gemini/alkylamine mixtures (Table 3).

Table 3 Comparison table for synergism of the mixed systems of gemini surfactants and alkyl amines
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Synergism in the mixed micelle formation exists when the CMC of the mixture is less than that of either amphiphile of the mixture. The conditions for this to exist in a mixture of two surfactants are the following [39]:

  1. (a)

    βm must be negative

  2. (b)

    |βm| > |ln (C m1 /C m2 )|

  3. (c)

    |βσ − βm| > [|ln (C σ1 /C σ2 )| − |ln (C m1 /C m2 )|]

where C m1 and C m2 are the critical micelle concentrations of amphiphiles 1 and 2, respectively. Table 3 also shows that all the mixtures of cationic surfactants exhibit synergism in mixed micelle formation with the alkylamines.

Conclusions

The interaction of two cationic gemini surfactants [pentanediyl-1, 5-bis(dimethylcetylammonium bromide) and hexanediyl-1, 6-bis(dimethylcetylammonium bromide)] with primary linear alkylamines were investigated. The following conclusions were drawn:

  1. (a)

    The trend of the increase of Гmax and decrease of CMC and A min are due to formation of mixed micelles with the gemini surfactants.

  2. (b)

    Increasing the spacer chain length of the geminis increases the CMC, C 20, A min values and decreases the ПCMC, Гmax values.

  3. (c)

    The ΔG 0ads values indicate that the adsorption of the surfactant at the air/solution interface takes place spontaneously.

  4. (d)

    The β values (both βm and βσ) indicate the attractive interaction and the interaction is more in the case of a smaller spacer chain length because in the case of 16-5-16 we get more negative β values in comparison to 16-6-16 (Table 2).

  5. (e)

    The gemini surfactant/alkylamine systems show an increase in synergism with the increase in amine concentration.

  6. (f)

    From the values of interaction parameters, we can say that there is increased synergism in the mixed monolayer in comparison to the mixed micelle (as βσ > βm, Table 2).