Interfacial composition, thermodynamic properties and structural parameters of water-in-oil microemulsions stabilized by 1-pentanol and mixed anionic + polyoxyethylene type nonionic surfactants

Abstract

The interfacial composition $\left( {n_a^i} \right)$, thermodynamic properties and structural parameters of the stable water/(SDS + Brij-58 or Brij-78)/1-pentanol/heptane (or decane or isopropyl myristate) have been evaluated under various physicochemical environments by the dilution method. The results showed $n_a^i$ values increase with increasing water content (ω = [water]/[surfactant]) for all the systems, whereas reverse trend was observed for (SDS/Brij-58)/heptane-derived system. The spontaneity of the transfer process of 1-pentanol from bulk oil to the interface $\left[ { - \Delta G_t^0} \right]$ decreases with increase in ω for all the systems. The effective binding between 1-pentanol and surfactant(s) at the interface follows the order: SDS/Brij-78/IPM < SDS/Brij-58/IPM < SDS/Brij-78/Hp(or, Dc) < SDS/Brij-58/Hp(or, Dc), which corroborates well with the degree of spontaneity of the transfer process. The Gibbs free energy change $\left( {\Delta G_t^0} \right)$, standard enthalpy change $\left( {\Delta H_t^0} \right)$ and standard entropy change $\left( {\Delta S_t^0} \right)$ have been found to be dependent on ω, type of nonionic surfactant and its content (X_nonionic), oil and temperature, because of the interdependence of the partition equilibrium of Pn between bulk oil and the interface, and strong adsorption of both surfactants at the interface. Synergism in $\Delta G_t^0$ and $\left[ {{{\left( { - \Delta C_P^0} \right)}_t}} \right]$ (standard specific heat change) is evidenced at equimolar composition of SDS and Brij-58 in both oils at all temperatures and advocates more favorable applications for the synthesis of nanoparticles and the modulation of enzyme activity. The radius of water pool (R_w) was very sensitive to the increment of water content and tuned up by the addition of Brijs, which followed the order with decreasing size: IPM < Dc < Hp.

Appendix

Here, we describe the relationship used to evaluate the β parameter [the ratio of intercept (I) and slope (S)] as a function of K_d (distribution constant) or $\Delta G_t^0$ (free energy of transfer process of alkanol from interface to bulk oil) or $n_a^o$ (the moles of alkanol in the bulk oil) or temperature.

$$ \beta = {{I} \left/ {S} \right.} = \frac{{ \frac{{n_a^i}}{{{n_s}}} }}{{ \frac{{n_a^o}}{{{n_o}}} }} = \frac{{n_a^i \cdot {n_o}}}{{n_a^o \cdot {n_s}}} $$

(A1)

where, $n_a^i$ and $n_a^o$ are the number of moles of alkanol in the oil/water interface and bulk oil phase respectively, n_o and n_s are total number of moles of oil and surfactant.

$$ {K_d} = \frac{{I\left( {1 + S} \right)}}{{S\left( {1 + I} \right)}} $$

(A2)

where, K_d is distribution constant or the partition of alkanol between the continuous oil phase and the interface of the microemulsion droplet.

Now Eq. A1 can be rewritten as

$$ \beta = {K_d} \frac{{\left( {1 + I} \right)}}{{\left( {1 + S} \right)}} $$

(A3)

Rearranging Eq. A3, one finds

$$ \beta = {K_d}\frac{{\left( {\beta + \frac{1}{S} } \right)}}{{\left( {1 + {{1} \left/ {S} \right.}} \right)}} $$

(A4)

or,

$$ \beta = \frac{{{K_d}}}{{\left( {1 + S - {K_d}S} \right)}} $$

(A5)

or,

$$ \beta = \frac{{{K_d}}}{{\left( {1 + \frac{{n_a^o}}{{{n_o}}} - {K_d} \frac{{n_a^o}}{{{n_o}}} } \right)}} $$

(A6)

where, slope (S) is equal to ${{{n_a^o}} \left/ {{{n_o}}} \right.}$.

Now from Eq. 8, we have,

$$ {K_d} = e \frac{{ - \Delta G_t^0}}{{RT}} $$

(A7)

Now by using Eq. A7, one can find the relation between the β parameter with $\Delta G_t^0$ as well as temperature,

$$ \beta = \frac{{ e^{{ \frac{{ - \Delta G_t^0}}{{RT}} }} }}{{\left( {1 + \frac{{n_a^o}}{{{n_o}}} - \frac{{n_a^o}}{{{n_o}}} {e^{{ \frac{{ - \Delta G_t^0}}{{RT}} }}}} \right)}} $$

(A8)