Characterizing the Oil-like and Surfactant-like Behavior of Polar Oils

Abstract

In this work, a bifunctional model was developed to fit and predict the phase inversion point (PIP) of microemulsions containing polar oils. This model incorporated the hydrophilic−lipophilic difference (HLD) equations, where HLD = 0 at the PIP. The model uses a Langmuir isotherm to account for the interfacial segregation of polar oils as a function of their concentration in the bulk oil phase. The segregated polar oil was treated as being surfactant-like, having a characteristic curvature (Cc). The polar oil in the bulk oil phase was characterized via an equivalent alkane carbon number (EACN). The Cc value was obtained considering deviations in the PIP at low polar oil concentrations. The EACN was determined considering PIP deviations at high polar oil concentrations. Naphthenic acid and dodecanol were used as model polar oils mixed with ionic and nonionic surfactants and nonpolar oils. The EACN of the polar oil was shown to be independent of the EACN of the nonpolar oil and likely independent of the surfactant. The Cc for dodecanol was likely independent of the surfactant used. For naphthenic acid, the Cc was independent of the nonpolar oil, and within a certain surfactant type (ionic, nonionic, or extended ionic), it was likely independent of the surfactant. For the naphthenic acid systems, the segregation predicted via the bifunctional model was consistent with experimental measurements of this segregation. Given that the bifunctional model only involves phase inversion experiments, it is a convenient method to determine the oil-like and surfactant-like nature of polar oils.

Effect of surfactant concentration on the hydrophobicity of polydisperse alkyl ethoxylates

“…For partitioning polar oils, added at relatively low concentrations (often less than 20 mol%) in relation to the surfactant, previous work has found that values of q max ~0.7 are appropriate for the segregation-bifunctional model (Ghayour & Acosta, 2019). However, in our preliminary simulations, we determined that if the ratio C j /C np was larger than 0.7, then using q max = 0.7 produced an overestimation of the partition of these species.…”

“…To represent the polar oil segregation of the partitioning surfactant species in the surfactant pseudophase using a Langmuir isotherm, one needs two parameters, q max,j and Km j , such that q j = q max,j ÁKm j ÁCe j /(1 + Km j ÁCe j ). This segregation approach was previously validated for naphthenic acid and dodecanol, used as an example of polar oils, with ionic and nonionic surfactants (Ghayour & Acosta, 2019, 2020. The segregation-bifunctional model has also been used to predict the phase behavior of cosmetic formulations for skin cleaning, including cholesterol and oleic acid as polar oils (Acosta, 2020); and to evaluate the effect of polar oils (oleic acid) in oily substrates removed with a detergent formulation based on alkyl ethoxylates (Natali & Acosta, 2019).…”

“…Equation (A.7), Figure 3, presents another important simplification in the algorithm: all partitioning species have the same EACN polar oil = À20. Values of EACN of fatty alcohols and fatty acids range from À9 to À80, often with large uncertainties that overlap with the value of EACN polar oil = À20 (Acosta, 2020;Ghayour & Acosta, 2019, 2020. Given the large uncertainty in the EACN of polar oils, the model uses the most common value of À20, but this is an aspect that is open to re-examination.…”

“…It should be clarified that although Figure 5 shows pictures where middle phases are identified (or at least the closest salinity to a middle phase system) as the points of inversion (S*), the reality is that one can obtain several middle phase systems, depending on the step used in the salinity scan. In the absence of middle phases, a more representative value of S* was obtained by fitting the entire salinity scan using the NAC model, as described in the literature (Ghayour & Acosta, 2019;Zarate-Muñoz et al, 2016).…”

Design of industrial wastewater demulsifier by HLD-NAC model

“…F(S) is the function of salinity, being expressed as Ln(S) and b(S) for ionic and nonionic surfactants, respectively. Equivalent Alkane Carbon Number ( EACN ) represents oil hydrophobicity 10 and f(A) is the function of alcohol, which usually acts as co-surfactant or co-solvent in the system 11 . It is expressed as m a C a in the HLD formulation, where m a is a constant value depending on the alcohol type and C a represents its concentration 12 .…”