Introduction

Fatty acids, neutralized by sodium or potassium hydroxides to prepare soaps, have been widely used, especially in household and personal cleaning products. The earliest recorded evidence of the production of soap-like materials dates back to around 2800 BC in ancient Babylon. The original composition of soaps was analyzed by Leeds in 1883 [1], and the characterization of soaps had been studied by Divine [2], Hillyer [3] and other scholars. Liquid soap was an innovation in the twentieth century, with the launch of “Palmolive” brand soap (prepared with palm and olive oils), and properties such as concentration, surface tension, and irritation had been studied by White [4], Hoyt [5] and other scholars. Due to its cleansing and low skin irritation, liquid soap became popular in hand and face cleansing and bathing.

It was said that the solubility of fatty acid salts, the main components of soaps, was a challenge in the twentieth century [6–10]. With the increase in alkyl chain length of fatty acid salts, their solubility decreases, limiting their applications [11–16]. A lot of measurements have analyzed the properties of soaps to improve their solubility, such as polarized microscopy, X-ray diffraction (XRD) [17–19], differential scanning calorimetry (DSC), to examine the effect of temperature and stabilizers on the microstructure of fatty acids [20–26]. There was not any information on the phase behavior of fatty acid soaps with mixtures of different fatty acids at varying ratios, and the instability of fatty acid soap was still a factor in the limitation of fatty acid soaps in personal and home care applications.

The objective of the present work was to characterize the phase behavior of soaps prepared with single, multiple, and ternary kinds of fatty acids. In particular, the influence of glycerin on the phase formation and stabilization of soaps are discussed. All the phase formations were analyzed by pH, viscosity, polarized light microscopy, X-ray diffraction and differential scanning calorimetry analysis. The investigation aimed at studying the phase behavior of soaps prepared with ternary fatty acids and glycerin, finding an appropriate proportion of fatty acids to improve the stabilization of soaps.

Experimental

Materials Used

Lauric acid (Reyland L A-1299, with purity ≥99%, molar mass 200.32 g mol−1, melting point 43.8 °C, acid value 278-282 mg/g, Wilmar, China), myristic acid (Reyland MA-1499, with purity ≥99%, molar mass 228.38 g mol−1, melting point 54.4 °C, acid value 244–248 mg/g, Wilmar, China), palmitic acid (Reyland PA-1699, purity ≥99%, molar mass 256.43 g mol−1, melting point 62.9 °C, acid value 218-220 mg/g, Wilmar, China) and glycerin (purity ≥99.7%, Longqi, with a viscosity 1.25 g/cm3) were all purchased from Yihai Kerry. Stearic acid (Analytical Reagent, with a melting point from 69 to 70 °C, molar mass 284.48 g mol−1, acid value 193–199 mg/g, Sinopharm Chemical Reagent Co., Ltd.) and potassium hydroxide (Analytical Reagent, purity 88%, where the pH of 0.1 mol/L water solution was 13.5, Sinopharm Chemical Reagent Co., Ltd.) were purchased from China Pharmaceutical Group Shanghai Chemical Reagent Company. Deionized water was used throughout the experiments.

Preparation of Soaps

Soaps were prepared by a neutralization or saponification reaction process. The mixture of fatty acids, with addition of glycerin, acted as phase A. Potassium hydroxide at a stoichiometric concentration according to the fatty acid’s acid value was dissolved in deionized water, acting as phase B. Phase A was added to phase B continuously at 80–85 °C while stirring at 400–500 rpm for 40 min. Samples were then cooled to the desired temperature while stirring, being careful that there were no bubbles generated in the process.

The mass fraction of potassium hydroxide used was calculated by the formula (1):

$$w({\text{KOH}}) = \frac{{W \times I_{A} }}{1000 \times 88\% } \times 100\% ,$$
(1)

where w(KOH) is the mass fraction of potassium hydroxide; W, is the mass fraction of fatty acid, %; I A is the acid value of fatty acid, mg/g.

pH Measurements

The pH of the soap samples was measured (Benchtop pH/mV meter 210, Bante Instruments, China) as a function of neutralization degree by potassium hydroxide. The temperature was kept at 25 ± 2 °C.

Viscosity Measurements

The viscosity of the soap samples was measured by means of a viscometer (DV-II + Pro, Brookfield, USA) at 20 rpm and 1 min, the temperature was set at 25 ± 2 °C.

Polarization Microscopy

An Olympus polarized optical microscope with a digital camera (Digital Eyepiece DCM300-USB2.0, Hangzhou Huaxin IC Technology Inc.) was used for microscopic texture observation. The sample was placed on an optical slide, and then with a cover slip lightly covered, keeping the sample homogeneous.

X-ray Diffraction (XRD) Measurements

Structure characterization of soaps by X-ray diffraction was carried out on a Bruker ADVANCE D8 (Germany) diffractometer using Cu Kα radiation (λ = 0.154 nm) from an X-ray generator operating at 40 kV and 200 mA. Scans were run between 2θ values of 0.7° and 10° with a step size of 0.004°.

X-ray diffraction results were displayed as intensity (I) versus scattering vector (2θ), based on Bragg’s law,

$$q = \frac{4\pi }{\lambda }\sin \left( {\frac{\theta }{2}} \right).$$
(2)

Bragg distance (d) was obtained from the first diffraction peak through the relation:

$$d = \frac{2\pi }{q}.$$
(3)

Differential Scanning Calorimetry (DSC) Measurements

DSC Q2000 apparatus (TA Instruments, USA) was used to measure thermodynamic properties of samples. Samples (∼2–5 mg) were sealed in standard aluminum pans, with an empty pan used as a reference. The measurement range was from 0 °C up to 150 °C at the rate of 10 °C min−1, and then cooled from 150 °C down to 0 °C.

Binary Phase Diagrams

Binary phase diagrams describe the phase behaviors of soaps with different concentration of fatty acids at different temperatures. A colorimetric tube with 10 ml of sample was stored for 1 h in a water bath at a series of temperature, namely at 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85 °C. The clarity, turbidity and viscosity were observed at every test temperature, and the optical properties of samples at every test temperature were obtained by a polarizing microscope equipped with a temperature controlled heating plate. The binary phase diagram was drawn with a comprehensive data set including turbidity, viscosity, isotropic/anisotropic structure and polarized light texture.

The Ternary Phase Diagram

The ternary phase diagram describes the phase behavior of soaps at different ratios of three different fatty acids. Samples were stored 24 h in a constant temperature environment at 25 ± 2 °C. Based on the properties of the samples such as turbidity, viscosity, isotropic/anisotropic structure, and polarized light texture, the ternary phase diagrams were drawn.

Results and Discussion

Phase Behaviors of Soaps Prepared with Myristic Acid

Phase behaviors of soaps prepared with myristic acid were studied, the mass fraction of myristic acid used in the experiments was from 0 to 28%, and the mass fraction of potassium hydroxide used was calculated based on the formula (Eq. 1). The phase behaviors of soaps at different temperature were observed after the myristic acid was neutralized completely, based on the analysis of appearance, viscosity, polarizing character and X-ray diffraction measurements. The result was displayed in Fig. 1, and the polarizing characters of liquid + hexagonal + lamellar phase and liquid + lamellar phase were displayed in Fig. 2, the X-ray diffraction curves of soap samples were displayed in Fig. 3.

Fig. 1
figure 1

Phase behaviors diagram of soap prepared with myristic acid. (Liquid phase, a liquid micelle phase; Liq + Hex + Lam, liquid micelle and hexagonal and lamellar phase; Liq + Lam, a liquid and lamellar phase; Liq + Crystal, a liquid micelle and crystal phase; Ice + Crystal, an ice and crystal phase; other phase, not been studied phase region)

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Fig. 2
figure 2

Polarized optical microscopy graphs for liquid + hexagonal + lamellar (a) and liquid + lamellar (b) (×40)

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Fig. 3
figure 3

X-ray diffraction patterns for soaps prepared with different percentages of myristic acid (a) and soaps prepared with 26% percentage of myristic acid stored for different times (b)

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Phase behavior of soaps prepared with different concentrations of myristic acid at different temperature was complicated (Fig. 1). Liquid crystalline phase behavior was observed where the mass fraction of myristic acid was 22–28%. A single liquid phase was observed with temperatures above 70 °C; then a transformation into Liq + Hex + Lam and Liq + Lam phases occurred when the temperature decreased, then another transformation into Liq + Crystal phase when the temperature was below 10 °C. A simple phase behavior of soaps was observed when the mass fraction of myristic acid was less than 22%, The liquid phase directly transformed into a Liq + Crystal phase when the temperature decreased to 5 °C. All samples of soaps transformed into Ice + Crystal phase at temperatures lower than 0 °C.

The liquid phase is isotropic, no characteristics of liquid crystalline phase behavior was observed by polarized optical microscopy; all samples of Liquid phase were clean and transparent, suggesting that the solutions were micellar. The Liq + Hex + Lam and Liq + Lam phases are both anisotropic phases, and their polarized optical properties are displayed in Fig. 2; these separated with the anisotropic phase at the bottom and the isotropic micelle phase on the top. The Liq + Crystal phase separated into with the precipitate at the bottom and the isotropic micelle phase on the top in one week. Furthermore, the pH range of the soap samples was 10.7–12.10, demonstrating that all of the myristic acid molecules had been ionized [27]. As the mass fraction of myristic acid increased, the monolayer of soap expanded because of the repulsion between the similarly charged molecules, leading to the weakening and instability of the film and ultimately to its separation into multiphase [28–30].

Detailed structural information was obtained through X-ray diffraction analysis. When the mass fraction of myristic acid was 18%, an absence of peaks were revealed in Fig. 3a, indicating that the sample was in a Liquid phase. By contrast, two obvious peaks were observed when the soap was prepared with 27% myristic acid (Fig. 3a), in which every peak reflects the layer spacing, and the d value between group intensity peaks was 1:0.6456:0.4990 based on Bragg’s law, in line with the d value ratio of lamellar phase which is 1/2:1/3:1/4 [28]. The X-ray diffraction of soaps prepared with 26% myristic acid for different storage times (12, 24, 36, and 48 h) were also analyzed (Fig. 3b). No obvious difference was observed in the 2-theta of every peak, except the intensity, and the calculated d values of every group peak under storage time are shown in Table 1. The values of ratio d1:d2:d3 were maintained at 1:0.65:0.50, which is consistent with the 1/2:1/3:1/4 [28], illustrating that the soap system was in the lamellar phase.

Table 1 d value ratios of group peaks with different storage times
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The thermal behaviors of the samples of Liq + Lam phase soap and the pure potassium myristate were also investigated by DSC measurements (Fig. 4a, b). The DSC curve of Liq + Lam phase sample revealed a double peak and a single peak at 75 °C and 115 °C respectively; these peaks are possibly caused by heat absorbed by water in the system, The Liq + Lam phase may have thus transitioned to become a sole lamellar phase. The small, negative peak at 120 °C is possibly the phase transition from the lamellar phase to the isotropic phase. Moreover, no peak was observed when the temperature decreased from 150 to 0 °C, demonstrating that the phase transition was irreversible and the structure of the phase was disrupted during heat absorption, indicating the instability of the Liq + Lam phase. The DSC curves of the pure potassium myristate were different from that of sample with Liq + Lam phase (Fig. 4b). A single negative peak on the curve resulting from endothermic melting was centered at 60 °C, indicating pure potassium myristate absorbed heat and underwent an endothermic phase transition from solid to liquid, then the phase transferred into a third state, an anisotropic liquid crystalline phase [31]. A slight change in energy during phase transition from liquid crystalline into an isotropic liquid at 134 °C were indicated by small negative peaks (Fig. 4b). Two positive peaks at 122 and 38 °C as the temperature decreased from 150 to 0 °C correspond to the negative peaks, except the temperature hysteresis of phase transformation due to the slightly faster cooling rate. The difference in the DSC curves of the liquid and pure potassium myristate indicates that the Liq + Lam phase was a separation phase.

Fig. 4
figure 4

DSC curves of Liq + Lam phase with different sample handling. a Liq + Lam phase sample of soap; b pure potassium myristate

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Effect of Binary Fatty Acids Ratio on the Phase Behaviors of Soaps

Aggregation properties and phase behaviors of soaps prepared with two kinds of fatty acids, such as the mixture of myristic with lauric/palmitic/stearic acids, were investigated by polarized microscopy, viscosity and X-ray diffraction analysis. The total fatty acid percentage of every formulation was kept at 30%, with any proportion of myristic acid and lauric/palmitic/stearic acid ratios, soaps were prepared by complete neutralization with potassium hydroxide. The phase behaviors of soaps prepared with binary fatty acids are shown in Fig. 5a–c separately.

Fig. 5
figure 5

Phase behavior diagrams of soaps prepared with binary fatty acids compounds. a Lauric-myristic acids; b myristic-palmitic acids; c myristic-stearic acids. (Cub + Hex, a cubic and hexagonal phase)

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Figure 5a shows the phase behaviors of the soaps prepared with lauric and myristic acids, with different ratios of myristic acid to lauric acid, with the total fatty acid content of 30% held constant. A liquid phase was observed when 0.1–5% myristic acid was mixed with lauric acid at all tested temperatures. When soap samples were prepared with 5–12% myristic acid, the Liquid phase transformed into the Liq + Hex + Lam phase at approximately 50 °C, and a portion of the phase transformed into the Cub + Hex phase at 0–10 °C. When the percentage of myristic acid increased to 12–27%, the Liquid phase transformed into the Liq + Lam phase at approximately 70 °C, a portion of the phase transformed into the Liq + Hex + Lam phase at approximately 50 °C, then transformed into the Cub + Hex phase at approximately 30 °C, while some parts of the phase also directly transformed into the Cub + Hex phase at approximately 30 °C. In addition, the Liq + Crystal phase of soaps was observed at the testing temperature when the percentage of myristic acid was increased to 27% (lauric acid decreased to 3%). All of the samples displayed Ice + Crystal phase at temperatures below 0 °C. Phase behaviors of soaps prepared using myristic acid compounded with palmitic acid or stearic acid were relatively simple (Fig. 5b, c). The prepared soaps displayed a Liquid phase at approximately 80 °C, transformed into the Liq + Lam phase at approximately 70 °C, then the phase of soaps prepared with myristic and palmitic acids transformed into the Cub + Hex phase when the temperature fell (Fig. 5b). The phase of soaps prepared using 2–10% stearic acid (28–20% myristic acid) displayed the Cub + Hex phase, which transformed into the Liq + Lam and Liq + Crystal phases as the temperature decreased.

There was a correspondingly large viscosity variation from Cub + Hex, Liq + Lam, and Liq + Hex + Lam phases to the Liquid phase, Cub + Hex phase exhibited a high viscosity and transparent appearance, while the viscosity of the Liquid phase was lower than 100 mPa·s at 20 rpm, whereas the values of viscosities of the other phases were between them. The phase behaviors varied when different kinds of fatty acids were used, possibly owing to the different pKa of ionized fatty acids when they were completely neutralized in a pH ≫ pKa solvent [32–35]. The molecular packing between the surfactant molecules in the bulk solution tightened because of the Van der Waals interaction, results in the phase behaviors changed. In addition, K+ was proposed to screen the electrostatic repulsion between the deprotonated fatty acid molecules and thus reduce the distance between the polar head groups. In terms of the packing parameter, p = v/a 0 l, the average head group area was lower, and the packing parameter p increased, resulting in the formation of bilayer as the temperature decreased, and the homogeneous lamellar phase separated in the Liq + Lam phase after 3–5 h.

Derjaguin and Landau, Verwey, and Overbeek (DLVO) theory proposes that the stability of a solution depends on the balance between the repulsion and attraction energies [36, 37], solutions remain stable when the repulsion energy is larger than the attraction energy; solutions are unstable with aggregates deposited when the attraction energy is larger than the repulsion energy. When the soap system remained in the Liq + Hex + Lam, Liq + Lam, or Liq + Crystal phase, aggregates formed by fatty acid salt molecules deposited because of the attraction energy being larger than the repulsion energy, leading to multiphase formation [38]. By contrast, aggregates of the surfactant molecules in the Liquid and Cub + Hex phases repelled each other, stabilizing the solution over time.

Detailed structure information of the Cub + Hex phase was characterized by X-ray diffraction analysis (Fig. 6a, b, c), the d value ratio of group peaks displayed in Table 2. When soap was prepared with 6% lauric acid and 24% myristic acid, the d value of group peaks was 1:0.8645:0.5852 (Table 2a), approximately equal to \(1:\sqrt {{3 \mathord{\left/ {\vphantom {3 4}} \right. \kern-0pt} 4}} :\sqrt {{1 \mathord{\left/ {\vphantom {1 3}} \right. \kern-0pt} 3}}\), indicating that cubic and hexagonal phases were present. As soap was prepared with 28% myristic acid and 2% palmitic acid, or 25% myristic acid and 5% stearic acid, the d value of group peaks were both approximately equal to \(1:\sqrt {{3 \mathord{\left/ {\vphantom {3 4}} \right. \kern-0pt} 4}} :\sqrt {{3 \mathord{\left/ {\vphantom {3 8}} \right. \kern-0pt} 8}} :\sqrt {{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-0pt} 2}}\) (Table 2b, c), also illustrated the presence of cubic and hexagonal phases with the appearance of a transparent gel.

Fig. 6
figure 6

X-ray diffraction patterns for Cub + Hex soaps prepared with binary fatty acid compounds. a Myristic–lauric acids; b myristic–palmitic acids; c myristic–stearic acids

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Table 2 X-ray diffraction group peaks d values of soaps prepared with binary fatty acids compounds
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Effect of Ternary Fatty Acids Ratio on Phase Behaviors of Soaps

It had been found that some properties of soaps such as detergency, foaming, appearance and stability could be improved by mixing different kinds of fatty acids, and among plots of Krafft point with different composition of ternary soap mixtures, there exists a minimum Krafft temperature, at or close to the minimum Krafft temperature, the mixture have a higher solubility than that of single components [29, 34, 39]. In our investigations, we selected three kinds of fatty acids from myristic acid, lauric acid, palmitic acid and stearic acid, on condition that the percentage of three total fatty acid was kept at 30% as well, soap systems were prepared with various proportion of three fatty acids, phase behaviors of test systems are shown in Fig. 7.

Fig. 7
figure 7

Phase behavior diagrams of soaps with ternary fatty acid soaps compounds. a Lauric–myristic-palmitic acids; b Lauric–myristic–stearic acids; c Myristic–palmitic–stearic acids

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The phase behaviors of soaps prepared with ternary fatty acid mixtures were complicated at a temperature of 25 ± 2 °C. When soaps prepared with the mixture of lauric, myristic, and palmitic acids by neutralization (Fig. 7a), Liq + Lam phase was displayed at most of compositions of ternary fatty acids mixtures, which displayed a regular texture revealed by polarized microscopy (Fig. 8b). An isotropic Liquid phase formed, when the proportion of lauric acid was higher than myristic and palmitic acids, and Cub + Hex phase was formed with the proportion of myristic acid increased, Liq + Crystal phase was formed with the proportion of palmitic acid increased, whose polarized texture was shown in Fig. 8c. The Liq + Hex + Lam phase, whose polarized texture is shown in Fig. 8a, was intermediate between the Liq + Lam and the Liquid phases. The liquid phase region was larger in soaps prepared using lauric, myristic, and stearic acids mixtures (Fig. 7b). Soaps displayed Liq + Lam and Cub + Hex phases at high proportions of myristic acid, and the polarized texture of Liq + Lam phase is shown in Fig. 9a. By contrast, the Liq + Crystal phase, whose polarized texture is shown in Fig. 9b, formed when more than 15% stearic acid was used. No Liquid phase formed in the phase behavior of soaps prepared using myristic, palmitic, and stearic acids mixtures; in addition, the Liq + Crystal phase covered a large region (Fig. 7c), which is possibly caused by the limited solubility of potassium palmitate and potassium stearate in water [32]. Moreover, small regions of the Cub + Lam and Liq + Lam phases formed when 15–28% myristic acid was used.

Fig. 8
figure 8

Polarization microscopy of soaps with lauric-myristic-palmitic acids ×40 (a Liq + Hex + Lam; b Liq + Lam; c Liq + Crystal)

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Fig. 9
figure 9

Polarization microscopy of soaps with lauric–myristic–stearic acids ×40 (a Liq + Lam; b Liq + Crystal)

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Owing to the alkyl chains from C12 and C14 to C16, the ternary soap mixtures maintained a large Liq + Lam phase region wherein micelles aggregated and formed large stacks of double-layered sheets. By contrast, the micelle formed by C12 and C14 alkyl chain ions had solubility properties on the stearate molecule, preventing some crystallization. Thus, the soap displayed a large Liquid phase region. When the ternary soap was prepared with lauric, palmitic, and stearic acids mixtures, the Krafft point of the mixture decreased. The results indicated that the application of palmitic and stearic acids in soaps can be improved by myristic and lauric acids.

Effect of Glycerin on the Phase Behaviors of Ternary Fatty Acids Soaps

Glycerin is a common material in personal care and cleansing products, improving moisturizing properties, and preventing skin dryness caused by soap [40]. In order to investigate the effect that glycerin has on the phase behavior of fatty acid soaps, a concentration of total ternary fatty acids mixtures kept 30% percentage was a rigid condition, 20% percentage glycerin was added into the fatty acids mixtures, the soaps were prepared by the procedure of neutralization as before. According to the comprehensive result of appearance, polarized microscopy and viscosity, influence of glycerin on the phase behaviors of soap was investigated.

A significant change was observed in the phase behavior of the soaps prepared using ternary fatty acid mixtures and glycerin compared with those of the soaps without glycerin. The phase behaviors of the soaps prepared using lauric, myristic, palmitic acid mixture and glycerin (Fig. 10) were characterized as follows: the Cub + Lam phase disappeared, the Liq + Lam and Liq + Hex + Lam phase regions reduced, Liq + Crystal phase was obtained when stearic acid was present in the greatest proportion. Soaps prepared using ternary lauric, myristic, and stearic acid mixtures and glycerin, displayed different phase behaviors with different fatty acid proportions (Fig. 10b), in which existed expanded Liquid phase region; the Liq + Crystal phase region decreased but still formed when stearic acid constituted the greatest proportion in the mixture. The influence of glycerin on the phase behavior of soaps prepared with ternary myristic, palmitic, and stearic acid mixtures was not significant except enhancing its stability, the Liq + Crystal phase region decreased slightly and its stability improved in two weeks; in addition, the original Liq + Lam phase changed into a Cub + Hex phase, whereas the original Liq + Hex + Lam phase changed into a new Liq + Lam phase.

Fig. 10
figure 10

Phase behavior of soaps prepared with ternary fatty acids and 20% glycerin. a Lauric–myristic–palmitic acids; b lauric–myristic–stearic acids; c myristic–palmitic–stearic acids

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Glycerin added into the fatty acid soap and aqueous solutions disturbed the original bonding between the ions of the system, namely the hydrogen bonding between aqueous and alkyl chain ions, the Van der Waals force bonding between alkyl chains, thus giving more chance to hydrogen bonding between alkyl chain ions and solvents, and decreased the interaction between the polar head groups of soaps and potassium ions, increasing the solubility of fatty acid soap and made most of anhydrate soaps converted to a Liquid phase.

Conclusion

This paper described the phase behavior of fatty acid soap prepared with single myristic acid, binary or ternary fatty acids, the effect of glycerin on the phase behavior of ternary fatty acids soaps was also discussed. The phase structure and behaviors were characterized by pH, viscosity, polarization microscopy, X-ray scattering and differential scanning calorimetry (DSC) analysis. According to the phase behavior of soap systems, the composition of fatty acid mixture played an important role in the preparation of soaps, including the kind of fatty acid and the proportion of fatty acid mixtures. The soap samples in the Liquid phase and the Cub + Hex phase were stable, while samples in the Liq + Crystal phase, the Liq + Lam phase and the Liq + Hex + Lam phase were unstable with phase separation during storage time. Glycerin had a significant impact on the phase behavior of soaps, expanding the stable Liquid phase by giving more chance for hydrogen and other bonding, and improved the stability of the Liq + Lam and Liq + Hex + Lam phases for two weeks. It was expected that additional investigation of phase behavior of soaps, enabling the stability of Liq + Lam, Liq + Hex + Lam, and Liq + Crystal phase to be increased by a long time in our latter study, giving more liquid crystal phases the chance of being used in applications.