Electronic Supplementary Material
Physicochemical Investigation of Biocompatible Mixed Surfactant Reverse Micelles: III.
Aqueous NaCl Solubilization, Thermodynamic Parameters of Desolubilization Process and
Conductometric Studies
Kaushik Kundu and Bidyut K. Paul*
Surface and Colloid Science Laboratory, Geological Studies Unit
Indian Statistical Institute, 203, B.T. Road, Kolkata-700 108 (India)
*author for correspondence <bidyut.isical@gmail.com>
Phone: (Off) (+91) (33)-2575-3164
Fax
: (+91) (33)-2577-3026
Isopropyl palmitate
Ethyl oleate (EO)
SMO20EO (HLB = 15)
Isopropyl myristate
Dioctyl sulfosuccinate sodium salt (DOSS)
STO20EO (HLB = 11)
Scheme. S1. Chemical structures of oils, ionic and nonionic surfactants
45
Solubilization capacity
40
35
30
25
20
15
10
0
0.1
0.2
0.3
10-3[NaCl] (mol/m3)
Fig. S1. Solubilization of aqueous NaCl in DOSS/STO20EO/EO mixed RMs at 303 K with
surfactant concentration of 0.10 × 103 mol/m3 at different XSTO20EO; where, XSTO20EO = 0.025
(●), XSTO20EO = 0.05 (■), XSTO20EO = 0.1 (▲) and XSTO20EO = 0.2 (*).
1. Solubilization of water in mixed RMs in polar lipophilic oils
DOSS RMs with total surfactant concentration, [ST] of 0.10 × 103 mol/m3 in IPM, IPP and EO
have been observed to solubilize a substantial amount of water, 0 (= [water]/[surfactant]) ~ 22,
14 and 17, respectively. It can be observed from Figs. 1A and 1B and Table 2 that the addition of
nonionic surfactant(s) (STO20EO and SMO20EO) to DOSS/IPM or, IPP or, EO (at a fixed [ST]
of 0.10 × 103 mol/m3) has been observed to enhance the solubilization capacity of water. The
solubilization capacities of DOSS/ STO20EO and SMO20EO systems are observed to increase
with increasing nonionic content (XSTO20EO
or SMO20EO)
in a mixture of DOSS/STO20EO or
SMO20EO, and then decrease after passing through maxima ( 0,max, indicates maximum amount
of water solubilized in RMs). The results are presented in Figs. 1A and 1B. It has been observed
that 0,max occurs at XSTO20EO or SMO20EO,max = 0.05 (where, XSTO20EO or SMO20EO,max indicates the
mole fraction of STO20EO or SMO20EO, at which maximum solubilization of water occurs) for
all three oils (Figs. 1A and 1B). It has also been observed that, 0,max values varies with the type
of oils (herein studied). Values of 0,max follows the order, IPP < EO < IPM.
The rationale behind using these nonionic surfactants is to underline their role of hydrophobic
configuration in water solubilization of mixed RMs. It can be observed from Table 2 that the
DOSS/STO20EO exhibits higher maximum solubilization capacity (0,max) (34.26, 32.41 and
27.78) compared to DOSS/SMO20EO (29.63, 25.93 and 22.22) at Xnonionic,max = 0.05 in oils
(IPM, EO and IPP, respectively). From the scrutiny of the chemical structures and HLB values of
the nonionic surfactants, it can be noted that STO20EO contains the same number EO units in
chain (i.e., 20) as that of SMO20EO, but STO20EO (HLB = 11) is more hydrophobic than
SMO20EO (HLB = 15) due to the presence of bulkier hydrophobic configuration. Hence, it can
be inferred that 0,max of these systems is influenced by the difference in chemical structures and
configurations of the nonionic surfactants and type of oils. Enhancement in water solubilization
capacity in mixed surfactant RMs was also reported earlier [1-7].
2. Effect of nonionic surfactant on solubilization of water in mixed RMs (DOSS/STO20EO or
SMO20EO) in polar lipophilic oils (EO, IPM and IPP)
In the present study, addition of nonionic surfactants with similar polar head group but differ in
hydrophobic configurations to DOSS/oil(s)/water RMs produces synergism in water
solubilization capacity. Solubilization of water in reverse micellar system depends upon many
factors, for example, type of surfactant (and cosurfactant or a second surfactant), oil,
temperature, and additives, etc. [8]. But the main driving force of such solubilization is the
spontaneous curvature and the elasticity (or rigidity) of the interfacial film formed by the
surfactant system [9]. If the interfacial curvature and the bending elasticity are fixed,
solubilization can be maximized by minimizing the interfacial bending stress of the rigid
interface and the attractive interdroplet interaction [10]. The critical packing parameter (CPP,
given by P = v/al where ‘v’ and ‘l’ are the volume and the length of hydrophobic chain,
respectively, and ‘a’ the area of polar head group of the surfactant) of the surfactant plays an
important role in water solubilization. For a mixed surfactant RM, the effective packing
parameter (Peff) follows the additive relation [11]:
Peff = [(xv/al)A + (xv/al)B]/(xA + xB)
(1)
where xA and xB are the mole fractions of surfactant A and B in the mixture, respectively.
Nonionic surfactants have larger head group area than DOSS [8, 12], and thus on mixing
STO20EO or SMO20EO with DOSS, Peff is decreased owing to the increase in ‘a’, which in turn
increases the natural radius of curvature (R). Thus, DOSS/nonionic(s)/oil RMs can accommodate
more water than the DOSS/oil RMs, and with increasing Xnonionic, solubilization capacity of the
mixed system increases. But with increasing the droplet size, interdroplet interaction also
increases, and at a certain droplet radius, Rc (the critical radius of curvature) interdroplet
interaction starts governing the solubilization behavior and the fluid interfaces can coalesce in
order to break the monophasic system into two distinct phases. Thus beyond Rc, addition of
nonionic surfactant decreased the solubilization capacity. For all these mixed systems, the
ascending curve in the solubilization capacity versus Xnonionic profile is governed by the curvature
effect due to the rigidity of the interface and the descending curve is governed by the interdroplet
interaction effect. Our observations are also supported by the argument of Nazario et al. [13].
They reported that the main driving force for water solubilization in mixed surfactant,
DOSS/CiEj RMs (where ‘i’ is the number of carbon atoms in the hydrophobic part and ‘j’ is the
number of ethylene oxide units in the hydrophilic part of the nonionic surfactant) is the presence
of the hydrophilic group of the nonionic surfactant.
In the present study, we also performed dye solubilization experiment [8, 14] to estimate the
phase
separation
phenomenon
in
single
DOSS/oil(s)/water
as
well
as
mixed
DOSS/nonionics/oil(s)/water with Xnonionic < Xnonionic,max and Xnonionic ≥ Xnonionic,max. The results
have been depicted in our previous report [15].
Growth of microemulsion droplets during the solubilization process and consequently the phase
separation in such systems is limited by two opposing factors, namely, the radius of spontaneous
curvature (Ro) as a result of curvature effect and the critical radius of droplets (Rc) due to the
result of attractive interaction among the droplets. A maximum in solubilization occur when both
of these effects are minimized [9].
3. Solubilization efficiency parameter (SP*water and SP*NaCl)
Maximization of solubilization capacity of water in mixed RMs (0,max) have been found to
depend on the content, hydrophobic moiety, polar head group of the added nonionic surfactant
and oil type vis-à-vis its chemical structure. An attempt has been made to introduce a new
parameter, “solubilization efficiency parameter (SP*)” to underline the efficacy of oils (of
different lipophilicities) on synergistic behavior in solubilization capacity in mixed systems.
Solubilization efficiency parameter (SP*water or SP*NaCl) can be defined as a relative increase in
solubilization capacity of water or aqueous NaCl in presence of surfactant mixture as a unique
number for a particular oil at constant composition in microemulsion systems. Mathematically,
SP*water or SP*NaCl are represented as follows:
SP*water = (0,max - 0)/0
SP*NaCl = (NaCl,max - [NaCl]=0(M))/[NaCl]=0(M)
(2)
(3)
Mathematical symbols are used with their usual significance (mentioned earlier). The calculated
values SP*water or SP*NaCl are presented in Tables 2-4. A representative diagram of solubilization
efficiency parameter (SP*NaCl) as a function of XSTO20EO for all oils, has been presented (Fig. S2).
It has been observed that SP*water and SP*NaCl follows the order: IPM < EO < IPP, which is in the
reverse order of both 0,max and NaCl,max. Order of SP*water has also been found to be of reverse
order proposed by Shah et al. (which states that solubilization of water decreases with increase in
molecular volume of oil) [9], with exchange in position of EO with respect to IPP. Further, it is
evident from Fig. S2, SP*NaCl increases with increase in XSTO20EO for studied oils, which
corroborate well with results presented in Sec. 3.2.1. The result might be explained in the
following way: this type of fatty acid ester oils have a tendency to penetrate in the surfactant
palisade layer, depending on the alkyl chain (short) associated with ester moiety and make
oil/water interface rigid. It has been reported by Shah et al. [9], that the solubilization of a
microemulsion with a rigid interface can be increased by increasing fluidity of the interface. So,
variation in penetration ability of these oils due to their different chemical structures exhibit such
type of order of SP*water and SP*NaCl (as mentioned above). Owing to the presence of cis-double
bond in EO (i.e., adjacent hydrogen atoms are on the same side of the double bond), it has a
natural tendency for bending [16]. Thus, the conformational freedom of the fatty acid chain of
EO restricts due to the bending configuration and as a result, the oil/water interface becomes less
flexible compared to saturated chain containing IPP system. Hence, SP*water and SP*NaCl values
of EO is lower than IPP. All these phenomena are attributed to the fact that the solubilization
depends not only on the geometric parameters but also on branching, saturation or unsaturation
in the oil molecule etc. In the present case, it might be the mutual miscibility between the
hydrophobic chains of the surfactant(s) and oil that affects the degree of oil penetration into the
amphiphilic film as well as the spontaneous curvature [17, 18]. Hence, it can be inferred that the
efficacy of the solubilization of the dispersed phase(s) in mixed RMs depends on both the fatty
acid chain length (as indicated by increases with increase in molar volume or fatty acid chain
length) and chemical structures of the oils.
1
IPM
SP*NaCl
EO
IPP
0.5
0
0.05
0.1
0.2
XTween-85
Fig. S2. Solubilization efficiency parameter (SP*NaCl) as a function XSTO20EO in
DOSS/STO20EO/oil/water mixed reverse micellar system.
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