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Multi-responsive emulsion of stearic acid soap aqueous solution
Accepted Manuscript Title: Multi-responsive emulsion of stearic acid soap aqueous solution Author: Yue Hong Wenlong Xu Yuanyuan Hu Guihua Li Mengjun Chen Jingcheng Hao Shuli Dong PII: DOI: Reference:
S0927-7757(17)30313-8 http://dx.doi.org/doi:10.1016/j.colsurfa.2017.03.053 COLSUA 21501
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
23-9-2016 24-1-2017 26-3-2017
Please cite this article as: Y. Hong, W. Xu, Y. Hu, G. Li, M. Chen, J. Hao, S. Dong, Multi-responsive emulsion of stearic acid soap aqueous solution, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2017), http://dx.doi.org/10.1016/j.colsurfa.2017.03.053 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Multi-responsive emulsion of stearic acid soap aqueous solution Yue Hong, Wenlong Xu, Yuanyuan Hu, Guihua Li, Mengjun Chen,
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Jingcheng Hao, and Shuli Dong*
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Key Laboratory of Colloid and Interface Chemistry & Key Laboratory of Special
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Aggregated Materials (Shandong University), Ministry of Education, Jinan 250100, P.
an
R. China
* Corresponding author. Tel.: +86-531-88363768, Fax: +86-531-88564750
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E-mail: [email protected] Research Highlights:
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1. The phase behavior of stearic acid and alkali in water was observed.
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2. Multi-responses of the emulsion stabilized by bilayers were achieved.
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3. The ability of stabilizing the emulsion by the bilayers and micelles was compared.
ABSTRACT:
Stearic acid (SA) mixed with alkali in water can form different fascinating aggregates in solution. The phase behavior of SA and alkali in water was observed in this work, and the apparent viscosity was measured by rheological measurements. Fatty acid self-assembled into bilayers at pH ≈ pKa and the structure was determined by cryogenic transmission microscopy (Cryo-TEM) observations. The ability of stabilizing the emulsion by the bilayers and micelles was compared. The
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homogeneous emulsion formed by bilayers was gained, indicating that the bilayers have a better emulsion stability. By adjusting the conditions and external stimuli,
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emulsification and demulsification can be achieved. Based on this background, we carried out the experiments of multi-responses of the emulsion stabilized by bilayers,
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including pH, CO2, light, and temperature.
Keywords: Multi-responsibility; Emulsion; Stearic acid soap; Phase behavior;
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Bilayer
1. Introduction
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Stimuli-responsive materials are on the leading edge of materials research and have been recognized as one of the most important issues this century [1,2]. It was
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reported that surfactants were widely used to change self-assembled structures over
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macrocosm to microcosmic scales. They can self-assemble into micelles [3,4], tubes [5], nanodiscs [6], vesicles [7,8] and other various morphological structures in
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different systems. Switchable surfactants can change their self-assembled structures in response to a trigger (like varying pH [9,10], bubbling CO2 [11,12] changing
temperature [13,14], introducing light [15,16], etc. which can in turn affect various macroscopic properties such as viscosity, foaming and emulsion stability [17]. Fatty acid is a kind of biocompatibility anionic surfactant which extensively exists
in nature and a cheap resource from oils of animals, marine and plants, which has successfully appealed to researchers committing to the study of fatty acid for many years [18-23]. From a chemical structural viewpoint, fatty acids are amphiphilic molecules with a hydrophobic aliphatic chain which can be crystalline or dynamic and a hydrophilic polar headgroup which could be protonated or deprotonated [24].
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Diversity of the self-assembled structures is attributed to the molecule structure itself. Since Gebicki reported the discovery of fatty acid vesicles in 1973 [25], bilayers formed by fatty acid soaps have become a hot field of research. In our previous work
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[26], we systematically reported the phase behavior of fatty acid/soap mixtures in the aqueous solutions. We studied the interaction between lauric acid (LA) and various
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counterions including inorganic cations and organic cations, and discussed the vesicle stability in theory [27]. In general, when the pH is around the pKa of the fatty acid,
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aggregated structures are commonly bilayers [28]. When the pH of the solution is
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considerably higher than the pKa, micelles are obtained [29]. It was reported that fatty acid bilayers have better emulsification than micelles [17]. However, to our
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knowledge, the study in regard to the emulsion based on the fatty acid is relatively few. As is well known, emulsion has important applications in our daily life and
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industrial manufacture. Under many circumstances, we hope the emulsion could keep
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stable in a certain period of time and then could be demulsified when we need, for example, in the process of oil transportation [30]. So, it is of great significance to
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investigate the emulsification and demulsification and a lot of related works focused on the smart emulsions have been conducted [30-34]. However, little coverage of deep study about emulsions based on the fatty acids has been performed up to now. Herein, because fatty acids are sensitive to pH and temperature, we studied the
emulsion stabilized by bilayers and its multi-responsive performances. We discussed the aggregation behavior of stearic acid (SA) and (CH3)4N+OH- and delineated the
phase diagram. Typical samples were analyzed by a series of characterizations such as the microstructure of bilayers was observed by cryo-TEM. It was proved that the bilayers of SA have better emulsification than micelles. We also investigated the multi-responsive properties of emulsion from following aspects: pH, CO2, light, as
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well as temperature.
2. Materials and Methods 2.1. Materials
＞98 wt %), (CH ) N OH 3 4
+
-
(25 wt % aqueous solution ) and
diphenyliodonium nitrate (97 %) were purchased from J
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Stearic acid (SA,
＆K Scientific Co. Ltd
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(China). Hydrochloric acid (HCl, 36 – 38 %) was purchased from Kant Chemical Co.
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Ltd in Lai Yang (China). Paraffin oil (Trade Name: Marcol 52) with viscosity of 12.0 mPa·s and density of 0.833 g·cm-3 at 20 °C was purchased from Exxon Mobil and
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used as the oil phase. All chemicals were used without further purification. Ultrapure water with a resistivity of 18.25 MΩ·cm was obtained using a UPH-IV ultrapure
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water purifier (China). 2.2. Phase behavior study
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The phase diagram of SA/(CH3)4N+OH- was delineated at 25.0 °C. The samples
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were prepared in glass tubes. Different amounts of SA were weighted into glass tubes
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and then different volumes of a 400 mmol·L-1 (CH3)4N+OH- aqueous solution were added into these tubes. The total volume of each sample was 5 mL eventually by supplying ultrapure water. The samples were dissolved with the help of ultrasonication at 50.0 °C and then cooled to room temperature. All the samples were equilibrated in an incubator at 25.0 °C for further study. The phase diagram was drawn by visual inspection utilizing crossed polarizers. 2.3. Conductivity and pH measurements The conductivity measurements were measured on a DDSJ-308A (China) conductivity meter with a DJS-1C glass electrode at 25.0 °C. The values of pH were obtained on a GB/T11165 pH meter at 25.0 °C. 2.4. Cryogenic transmission electron microscopy (cryo-TEM) observations
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A drop of sample solution was dropped on a microgrid with carbon support film in a high humidity environment
（＞ 80 %） to minimize water loss. Two pieces of
filter paper were used to blot up the excess sample, leaving a thin film sprawled on
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the microgrid. The microgrid was then quickly plunged into liquid ethane at -165 °C which was cooled by liquid nitrogen in advance. The vitrified sample was transferred
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into a sample holder (Gatan 626) and inserted into a TEM (JEOL JEM-1400 TEM
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operated at 120 kV). The images were recorded on a Gatan multiscan CCD and processed with a Digital Micrograph. The samples were kept in the liquid nitrogen in
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the whole process in order to protect the frozen structure. 2.5. Rheological characterization
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The rheological experiments were carried out on a HAAKE Rheo Stress 6000 rheometer with a coaxial cylinder sensor system (Z41 Ti) for samples with low
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viscosity with the constant water reflux of 25.0 °C.
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2.6. Differential scanning calorimrtry (DSC) measurements The phase transition temperature was obtained on a DSC-Q10 (TA Instruments,
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New Castle, PA, USA). The measuring range of temperature was from 25 to 60 °C at a rate of 5 °C/min.
2.7. Surface tension measurements Surface tension measurements were performed on a Tension meter K100 (Krüss
Company, Germany) using the plate method. A Haake K10 (Germany) superconstant temperature was used to control the temperature at 25.0 °C. Stock solution was added to 50 mL water quantificationally with pipette to measure surface tension of different concentrations of SA and the solution was stirred with magneton for 5 min and standing for 5 min. The plate was cleaned with ultrapure water and heated vertically until red with alcohol lamp before each measurement.
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2.8. Multi-responsive emulsion Emulsions could be prepared with the oil/water ratios (rO/W) was 1:1. The oil phase was paraffin oil. SA was dissolved in the (CH3)4N+OH- first with the volume of 1 mL,
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and then it was mixed with 1 mL paraffin oil. After a vortex oscillation, the samples with different amount of SA presented different phenomena. Then, multi-responsive
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abilities were found between stabilization and destabilization of emulsions.
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3. Results and Discussion 3.1. Phase behavior
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The phase behavior was investigated for the SA/ (CH3)4N+OH-/H2O system at 25.0 °C. The phase diagram is shown in Fig. 1. SA, as a kind of long chain fatty acid,
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does not dissolve in water at room temperature. According to the literature [35], SA was found to be dissolved with the addition of amines. When the content of
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(CH3)4N+OH- was low, only part of SA dissolved (P phase). With the addition of
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(CH3)4N+OH- gradually, the microstructure of the system changed from bilayer (Lα
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phase) to bilayer/micelle (Lα/L1 phase) to micelle (L1 phase) eventually.
Figure 1
The appearance of typical sample solutions of SA mixed with (CH3)4N+OH- is
shown in Fig. 2. The concentration of (CH3)4N+OH- was changed at constant
concentration of SA. The concentration of SA was fixed at 40 mmol·L-1 and the concentration of (CH3)4N+OH- ranged from 10 to 60 mmol·L-1. When the
concentration of (CH3)4N+OH- was below 13 mmol·L-1, SA was partially dissolved and formed a mixture of precipitation (P phase). When the concentration of (CH3)4N+OH- was between 13 and 37 mmol·L-1, a homogeneous bluish solution (Lα phase) with birefringence under crossed polarizers was observed. When the
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concentration of (CH3)4N+OH- was between 37 and 41 mmol·L-1, the bluish solution with birefringence under crossed polarizers at the top and the colorless transparent solution at the bottom (Lα/L1 phase) was found. When the concentration of
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(CH3)4N+OH- was higher than 41 mmol·L-1, transparent micelle phase (L1 phase) was
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obtained.
3.2. Microstructures and formation mechanism
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Figure 2
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Cryo-TEM observations were sued to determine the self-assembled structures. A sample from Lα phase was chosen to characterize the microstructures by Cryo-TEM.
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Bilayer vesicles were observed in Fig. 3, showing a good dispersion with an average
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diameter of about 74.94 nm and the thickness of the vesicle is approximately 6.18 nm.
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Figure 3
,
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In general, hydrophobic interaction hydrogen bonding and electrostatic interaction are the main driving forces for the formation of bilayers. Especially, hydrogen bonding is the most important factor in the self-assembly process and attributes to the formation of bilayers. When the pH of the system is around the pKa of the SA, the amount of protonated and deprotonated SA molecules are approximately equal and then form bilayers by hydrogen bonds. Based on above observations, we hypothesized the formation mechanism of bilayers: when the pH is around the pKa of the fatty acid, the protonated and deprotonated fatty acid molecules can form hydrogen bonds and then form bilayers. With the increase of pH, more and more fatty acid molecules become deprotonated which leads to a higher ratio of ionized to the protonated molecules. As a result,
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micelles are formed as a result of the electrostatic repulsion forces between the deprotonated fatty acid molecules (Scheme 1).
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Scheme 1
3.3. Conductivity and pH measurements
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At a fixed fatty acid concentration of cSA = 40 mmol·L-1, we systematically
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measured the pH and conductivity of typical samples. As shown in Fig. 4, with the
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increase of the concentration of alkali, the pH of the system is on the rise generally due to the neutralization reaction. The pH of the Lα phase was near 10 in accordance
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with the pKa values of SA, i.e., pKa = 10[36]. The conductivity increases sharply in the Lα/L1 phase and L1 phase while it shows a decrease or a slow increase in the Lα
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phase because the counterions were bound to bilayers. Figure 4
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3.4. Rheological measurements
The change of self-assembled microscopic structures in solution can be reflected
by rheological measurements. The apparent viscosity data (η, Pa·s) of the typical
samples were obtained by rheological measurements and shown in Fig. 5. The η
values of the Lα phase samples is larger than the L1 phase samples. Because of the
destruction of the bilayers structure with the increase of shear rate, η values of the Lα phase samples show shear thinning behavior in the whole shear rate range which is the property of non-Newtonian fluid property. However, L1 phase samples show the
character of Newtonian fluid that is independent of shear rate at high shear rate. The experimental results are in conformity with our cognition.
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Figure 5 3.5. Surface tension (CH3)4N+OH-, as an organic amine, can be miscible with water in any proportion.
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The surface tension of aqueous solutions has no obvious changes with the increase of the (CH3)4N+OH- concentration, which indicates that the (CH3)4N+OH- itself does not
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have surface activity.
The stock solution of SA/(CH3)4N+OH- (20 mmol·L-1, 800 mmol·L-1) was prepared
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in order to investigate the surface activity of the system. The surface tension of SA
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could not obtained directly by water solution because of the poor solubility. As can be seen from Fig. 6, the surface tension was decreased with the increasing of the
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concentration of fatty acid, which proved that SA has surface activity when it is deprotonated in amines.
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Figure 6
3.6. Multi-responsive emulsion
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It was established the ability of stabilizing emulsion of bilayers solution is better
than that of micelles solution. We chose both samples of cSA = 40 mmol·L-1 with
varying 20 (bilayers) and 50 mmol·L-1 (micelles) of (CH3)4N+OH- and added
equivalent volume paraffin oil. After a vortex oscillation, both samples formed homogeneous emulsion. However, the phase separation of the emulsion which was stabilized by micelles was found after several minutes. In contrast, the sample stabilized by bilayers can keep stable for several months (Fig. 7). These observations were in good agreement with the literature that bilayers possess the better emulsifying properties [17]. Reports on successful attempts of controlling the process of emulsification and demulsification of the emulsion are relatively scarce. The
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responses are the result of modifications in the affinity between emulsifier and water or oil and can be induced by more or less instructive methods. Therefore, emulsions stabilized by surfactant could be damaged by applicable triggers. In order to apply to
response performance of the emulsion stabilized by the bilayers.
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3.6.1. pH response
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Figure 7
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the industry on the emulsification and demulsification better, we studied various
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Surfactants which are sensitive to pH tend to increase interest because of their wide range of potential in novel applications, where pH variations can be used to
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control aggregation. SA, as a common and conventional surfactant, is equipped with pH-sensitive property and can behave itself like a binary mixture consisting of the
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protonated and deprotonated forms. As a result, the bilayers have pH dependence, as
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well as the emulsion stabilized by bilayers.
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When 2 µL HCl was added into the emulsion and then the sample was vortex oscillated, a clear oil phase separation from the upper layer was observed after holding for several minutes in Fig. 8a. Originally, the emulsion is stabilized by bilayers. When the acid is added into the emulsion, deprotonated fatty acid molecules are transformed into protonated molecules, leading to losing of surface activity and destroying of bilayers subsequently. Finally demulsification occurs. 3.6.2. CO2 response Another method to vary pH is realized by bringing in CO2. Bubbling CO2 has an advantage of avoiding contamination of the medium solution comparing with adding acid or base to change the pH. After introducing CO2 into the emulsion, we
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also observed phase separation in line with expectations (Fig. 8b).
Figure 8
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3.6.3. Photoresponse Light is a trigger that is non-invasive and can avoid the direct contact with the
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specimen. In general, there are two methods to produce light-response: one is to
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modify a responsive group to a component and the other is to bring in materials which have responsive groups. Fatty acid itself does not have light response, so we introduce
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a photoacid generator (PAG) [16] in our work. PAG can be photolyzed by UV light and then produces acid causing the decrease of the pH and the change of
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self-assemblies based on fatty acid molecules.
To investigate the photoresponsive performance, 6.86 mg PAG was added to the
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emulsion. The separation was found after UV irradiation for 2 hours. Then holding for
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some time, we can obtain a sample similar to the emulsion added acid (Fig. 8c). This
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kind of light-response is another pH-response from the source. 3.6.4. Temperature response
DSC measurement was performed to investigate the temperature effect (Fig. 9).
We can see that 40 mmol·L-1 SA/20 mmol·L-1 (CH3)4N+OH- (bilayers) sample has a
broad peak at 50.02 °C which indicates the phase transition of the solution. However, micelles do not have a phase transition temperature which is a basis for next temperature response. Comparing with other stimuli, the temperature could change the system without adding chemical materials. The sample was placed in a water bath pot in order to investigate the temperature effect. When the temperature was rose to 50.0 °C, the
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demulsification was obtained (Fig. 9). The phenomenon is in good agreement with the result of the DSC measurement. Because of the existence of the phase transition temperature, temperature will affect the stability of the emulsion.
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Figure 9
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4. Conclusions
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In summary, we studied the phase behaviors of the mixture of SA and (CH3)4N+OH-. The conductivity and pH value were measured. The phase-transition temperature was
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obtained by DSC measurements and the apparent viscosity of typical samples was measured by rheological characterizations. Cryo-TEM demonstrated the bilayers. We
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focused on the emulsion stability of bilayers and micelles and found that bilayers have better emulsifying properties than micelles. Furthermore, the multiple responses of
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emulsion are the key research as an application and we expect the study is meaningful
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for industrial emulsification, demulsification and simplifying chemical routes for
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accessing more commercially viable systems.
Acknowledgements
This work is financially supported by the NSFC (grant nos. 21273136 and 21420102006).
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Captions of Figures and Tables in Manuscript Text:
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60
30
3 4
L α/L1
d
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L1
te
40
-
c(CH ) N OH / mmol⋅L
-1
50
Lα
20 10
0 10
P 20
30
cSA / mmol⋅L
-1
40
50
Fig. 1. Phase behavior of the SA/(CH3)4N+OH-/H2O system.
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Fig. 2. Photographs of typical samples at 25.0 °C without (top) and with (bottom)
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polarizers. cSA = 40 mmol·L-1 with varying (CH3)4N+OH- concentration gradually. From left to right: 10, 20, 30, 40, 50 and 60 mmol·L-1. The solution volume was fixed
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d
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at 5 mL.
Fig. 3. Cryo-TEM images of a sample in Lα phase region.
Scheme 1. Microstructures of the fatty acid soap system at different pH.
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13
3000
pH κ
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Lα/L1
10
Lα
10
20
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9 8
1000
L1
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P
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pH
2000 11
-1
2500
κ / µS•cm
12
30
40
50
60
500 0
-1
c(CH ) N OH / mmol⋅L +
-
M
3 4
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Fig. 4. Conductivity and pH value with the increase of concentration of (CH3)4N+OH-
1
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10
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at cSA = 40 mmol·L-1 at 25.0 °C.
η / Pa· s
10
40:20 40:50
0
10
-1
10
-2
10
-3
10
-2
10
-1
•
10
0
γ/s
-1
10
1
10
2
Fig. 5. Shear rheogram of the samples with cSA = 40 mmol·L-1 and c(CH3)4N+OH- = 20, 50 mmol·L-1, respectively, at 25.0 °C.
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80
γ / mN • m
-1
70 40
γ / mN•m
-1
60
60
50
20
100
-1
c(CH ) N OH / µmol⋅L 3 4
+
1000
-
30 20 10
10
100 -1
cr
cSA / µmol⋅L
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40
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Fig. 6. Surface tensions (γ, mN·m-1) vs. log c of SA or (CH3)4N+OH- (interior
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illustration) in solution.
(b)
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(a)
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Fig. 7. Emulsification by vortex oscillation holding for a period of time. (a) cSA = 40 mmol·L-1, c(CH3)4N+OH- = 20 mmol·L-1, (b) cSA = 40 mmol·L-1, c(CH3)4N+OH- = 50
Ac ce p
mmol·L-1.
Fig. 8. Multiple response of the emulsion stabilized by the bilayers. (a) pH response, (b) CO2 response, (c) light response, (d) temperature response.
Page 20 of 21
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40 : 20
o
50.02 C
40
50
T /° C
60
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30
cr
Heat Flow / a.u.
40 : 50
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Fig. 9. Phase transition temperature of the samples with cSA = 40 mmol·L-1 and
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c(CH3)4N+OH- = 20 and 50 mmol·L-1, respectively measured by DSC.
Page 21 of 21
S0927-7757(17)30313-8 http://dx.doi.org/doi:10.1016/j.colsurfa.2017.03.053 COLSUA 21501
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
23-9-2016 24-1-2017 26-3-2017
Please cite this article as: Y. Hong, W. Xu, Y. Hu, G. Li, M. Chen, J. Hao, S. Dong, Multi-responsive emulsion of stearic acid soap aqueous solution, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2017), http://dx.doi.org/10.1016/j.colsurfa.2017.03.053 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Multi-responsive emulsion of stearic acid soap aqueous solution Yue Hong, Wenlong Xu, Yuanyuan Hu, Guihua Li, Mengjun Chen,
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Jingcheng Hao, and Shuli Dong*
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Key Laboratory of Colloid and Interface Chemistry & Key Laboratory of Special
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Aggregated Materials (Shandong University), Ministry of Education, Jinan 250100, P.
an
R. China
* Corresponding author. Tel.: +86-531-88363768, Fax: +86-531-88564750
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E-mail: [email protected] Research Highlights:
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1. The phase behavior of stearic acid and alkali in water was observed.
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2. Multi-responses of the emulsion stabilized by bilayers were achieved.
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3. The ability of stabilizing the emulsion by the bilayers and micelles was compared.
ABSTRACT:
Stearic acid (SA) mixed with alkali in water can form different fascinating aggregates in solution. The phase behavior of SA and alkali in water was observed in this work, and the apparent viscosity was measured by rheological measurements. Fatty acid self-assembled into bilayers at pH ≈ pKa and the structure was determined by cryogenic transmission microscopy (Cryo-TEM) observations. The ability of stabilizing the emulsion by the bilayers and micelles was compared. The
Page 1 of 21
homogeneous emulsion formed by bilayers was gained, indicating that the bilayers have a better emulsion stability. By adjusting the conditions and external stimuli,
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emulsification and demulsification can be achieved. Based on this background, we carried out the experiments of multi-responses of the emulsion stabilized by bilayers,
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including pH, CO2, light, and temperature.
Keywords: Multi-responsibility; Emulsion; Stearic acid soap; Phase behavior;
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Bilayer
1. Introduction
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Stimuli-responsive materials are on the leading edge of materials research and have been recognized as one of the most important issues this century [1,2]. It was
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reported that surfactants were widely used to change self-assembled structures over
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macrocosm to microcosmic scales. They can self-assemble into micelles [3,4], tubes [5], nanodiscs [6], vesicles [7,8] and other various morphological structures in
Ac ce p
different systems. Switchable surfactants can change their self-assembled structures in response to a trigger (like varying pH [9,10], bubbling CO2 [11,12] changing
temperature [13,14], introducing light [15,16], etc. which can in turn affect various macroscopic properties such as viscosity, foaming and emulsion stability [17]. Fatty acid is a kind of biocompatibility anionic surfactant which extensively exists
in nature and a cheap resource from oils of animals, marine and plants, which has successfully appealed to researchers committing to the study of fatty acid for many years [18-23]. From a chemical structural viewpoint, fatty acids are amphiphilic molecules with a hydrophobic aliphatic chain which can be crystalline or dynamic and a hydrophilic polar headgroup which could be protonated or deprotonated [24].
Page 2 of 21
Diversity of the self-assembled structures is attributed to the molecule structure itself. Since Gebicki reported the discovery of fatty acid vesicles in 1973 [25], bilayers formed by fatty acid soaps have become a hot field of research. In our previous work
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[26], we systematically reported the phase behavior of fatty acid/soap mixtures in the aqueous solutions. We studied the interaction between lauric acid (LA) and various
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counterions including inorganic cations and organic cations, and discussed the vesicle stability in theory [27]. In general, when the pH is around the pKa of the fatty acid,
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aggregated structures are commonly bilayers [28]. When the pH of the solution is
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considerably higher than the pKa, micelles are obtained [29]. It was reported that fatty acid bilayers have better emulsification than micelles [17]. However, to our
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knowledge, the study in regard to the emulsion based on the fatty acid is relatively few. As is well known, emulsion has important applications in our daily life and
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industrial manufacture. Under many circumstances, we hope the emulsion could keep
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stable in a certain period of time and then could be demulsified when we need, for example, in the process of oil transportation [30]. So, it is of great significance to
Ac ce p
investigate the emulsification and demulsification and a lot of related works focused on the smart emulsions have been conducted [30-34]. However, little coverage of deep study about emulsions based on the fatty acids has been performed up to now. Herein, because fatty acids are sensitive to pH and temperature, we studied the
emulsion stabilized by bilayers and its multi-responsive performances. We discussed the aggregation behavior of stearic acid (SA) and (CH3)4N+OH- and delineated the
phase diagram. Typical samples were analyzed by a series of characterizations such as the microstructure of bilayers was observed by cryo-TEM. It was proved that the bilayers of SA have better emulsification than micelles. We also investigated the multi-responsive properties of emulsion from following aspects: pH, CO2, light, as
Page 3 of 21
well as temperature.
2. Materials and Methods 2.1. Materials
＞98 wt %), (CH ) N OH 3 4
+
-
(25 wt % aqueous solution ) and
diphenyliodonium nitrate (97 %) were purchased from J
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Stearic acid (SA,
＆K Scientific Co. Ltd
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(China). Hydrochloric acid (HCl, 36 – 38 %) was purchased from Kant Chemical Co.
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Ltd in Lai Yang (China). Paraffin oil (Trade Name: Marcol 52) with viscosity of 12.0 mPa·s and density of 0.833 g·cm-3 at 20 °C was purchased from Exxon Mobil and
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used as the oil phase. All chemicals were used without further purification. Ultrapure water with a resistivity of 18.25 MΩ·cm was obtained using a UPH-IV ultrapure
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water purifier (China). 2.2. Phase behavior study
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The phase diagram of SA/(CH3)4N+OH- was delineated at 25.0 °C. The samples
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were prepared in glass tubes. Different amounts of SA were weighted into glass tubes
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and then different volumes of a 400 mmol·L-1 (CH3)4N+OH- aqueous solution were added into these tubes. The total volume of each sample was 5 mL eventually by supplying ultrapure water. The samples were dissolved with the help of ultrasonication at 50.0 °C and then cooled to room temperature. All the samples were equilibrated in an incubator at 25.0 °C for further study. The phase diagram was drawn by visual inspection utilizing crossed polarizers. 2.3. Conductivity and pH measurements The conductivity measurements were measured on a DDSJ-308A (China) conductivity meter with a DJS-1C glass electrode at 25.0 °C. The values of pH were obtained on a GB/T11165 pH meter at 25.0 °C. 2.4. Cryogenic transmission electron microscopy (cryo-TEM) observations
Page 4 of 21
A drop of sample solution was dropped on a microgrid with carbon support film in a high humidity environment
（＞ 80 %） to minimize water loss. Two pieces of
filter paper were used to blot up the excess sample, leaving a thin film sprawled on
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the microgrid. The microgrid was then quickly plunged into liquid ethane at -165 °C which was cooled by liquid nitrogen in advance. The vitrified sample was transferred
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into a sample holder (Gatan 626) and inserted into a TEM (JEOL JEM-1400 TEM
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operated at 120 kV). The images were recorded on a Gatan multiscan CCD and processed with a Digital Micrograph. The samples were kept in the liquid nitrogen in
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the whole process in order to protect the frozen structure. 2.5. Rheological characterization
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The rheological experiments were carried out on a HAAKE Rheo Stress 6000 rheometer with a coaxial cylinder sensor system (Z41 Ti) for samples with low
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viscosity with the constant water reflux of 25.0 °C.
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2.6. Differential scanning calorimrtry (DSC) measurements The phase transition temperature was obtained on a DSC-Q10 (TA Instruments,
Ac ce p
New Castle, PA, USA). The measuring range of temperature was from 25 to 60 °C at a rate of 5 °C/min.
2.7. Surface tension measurements Surface tension measurements were performed on a Tension meter K100 (Krüss
Company, Germany) using the plate method. A Haake K10 (Germany) superconstant temperature was used to control the temperature at 25.0 °C. Stock solution was added to 50 mL water quantificationally with pipette to measure surface tension of different concentrations of SA and the solution was stirred with magneton for 5 min and standing for 5 min. The plate was cleaned with ultrapure water and heated vertically until red with alcohol lamp before each measurement.
Page 5 of 21
2.8. Multi-responsive emulsion Emulsions could be prepared with the oil/water ratios (rO/W) was 1:1. The oil phase was paraffin oil. SA was dissolved in the (CH3)4N+OH- first with the volume of 1 mL,
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and then it was mixed with 1 mL paraffin oil. After a vortex oscillation, the samples with different amount of SA presented different phenomena. Then, multi-responsive
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abilities were found between stabilization and destabilization of emulsions.
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3. Results and Discussion 3.1. Phase behavior
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The phase behavior was investigated for the SA/ (CH3)4N+OH-/H2O system at 25.0 °C. The phase diagram is shown in Fig. 1. SA, as a kind of long chain fatty acid,
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does not dissolve in water at room temperature. According to the literature [35], SA was found to be dissolved with the addition of amines. When the content of
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(CH3)4N+OH- was low, only part of SA dissolved (P phase). With the addition of
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(CH3)4N+OH- gradually, the microstructure of the system changed from bilayer (Lα
Ac ce p
phase) to bilayer/micelle (Lα/L1 phase) to micelle (L1 phase) eventually.
Figure 1
The appearance of typical sample solutions of SA mixed with (CH3)4N+OH- is
shown in Fig. 2. The concentration of (CH3)4N+OH- was changed at constant
concentration of SA. The concentration of SA was fixed at 40 mmol·L-1 and the concentration of (CH3)4N+OH- ranged from 10 to 60 mmol·L-1. When the
concentration of (CH3)4N+OH- was below 13 mmol·L-1, SA was partially dissolved and formed a mixture of precipitation (P phase). When the concentration of (CH3)4N+OH- was between 13 and 37 mmol·L-1, a homogeneous bluish solution (Lα phase) with birefringence under crossed polarizers was observed. When the
Page 6 of 21
concentration of (CH3)4N+OH- was between 37 and 41 mmol·L-1, the bluish solution with birefringence under crossed polarizers at the top and the colorless transparent solution at the bottom (Lα/L1 phase) was found. When the concentration of
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(CH3)4N+OH- was higher than 41 mmol·L-1, transparent micelle phase (L1 phase) was
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obtained.
3.2. Microstructures and formation mechanism
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Figure 2
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Cryo-TEM observations were sued to determine the self-assembled structures. A sample from Lα phase was chosen to characterize the microstructures by Cryo-TEM.
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Bilayer vesicles were observed in Fig. 3, showing a good dispersion with an average
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diameter of about 74.94 nm and the thickness of the vesicle is approximately 6.18 nm.
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Figure 3
,
Ac ce p
In general, hydrophobic interaction hydrogen bonding and electrostatic interaction are the main driving forces for the formation of bilayers. Especially, hydrogen bonding is the most important factor in the self-assembly process and attributes to the formation of bilayers. When the pH of the system is around the pKa of the SA, the amount of protonated and deprotonated SA molecules are approximately equal and then form bilayers by hydrogen bonds. Based on above observations, we hypothesized the formation mechanism of bilayers: when the pH is around the pKa of the fatty acid, the protonated and deprotonated fatty acid molecules can form hydrogen bonds and then form bilayers. With the increase of pH, more and more fatty acid molecules become deprotonated which leads to a higher ratio of ionized to the protonated molecules. As a result,
Page 7 of 21
micelles are formed as a result of the electrostatic repulsion forces between the deprotonated fatty acid molecules (Scheme 1).
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Scheme 1
3.3. Conductivity and pH measurements
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At a fixed fatty acid concentration of cSA = 40 mmol·L-1, we systematically
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measured the pH and conductivity of typical samples. As shown in Fig. 4, with the
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increase of the concentration of alkali, the pH of the system is on the rise generally due to the neutralization reaction. The pH of the Lα phase was near 10 in accordance
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with the pKa values of SA, i.e., pKa = 10[36]. The conductivity increases sharply in the Lα/L1 phase and L1 phase while it shows a decrease or a slow increase in the Lα
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phase because the counterions were bound to bilayers. Figure 4
Ac ce p
3.4. Rheological measurements
The change of self-assembled microscopic structures in solution can be reflected
by rheological measurements. The apparent viscosity data (η, Pa·s) of the typical
samples were obtained by rheological measurements and shown in Fig. 5. The η
values of the Lα phase samples is larger than the L1 phase samples. Because of the
destruction of the bilayers structure with the increase of shear rate, η values of the Lα phase samples show shear thinning behavior in the whole shear rate range which is the property of non-Newtonian fluid property. However, L1 phase samples show the
character of Newtonian fluid that is independent of shear rate at high shear rate. The experimental results are in conformity with our cognition.
Page 8 of 21
Figure 5 3.5. Surface tension (CH3)4N+OH-, as an organic amine, can be miscible with water in any proportion.
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The surface tension of aqueous solutions has no obvious changes with the increase of the (CH3)4N+OH- concentration, which indicates that the (CH3)4N+OH- itself does not
cr
have surface activity.
The stock solution of SA/(CH3)4N+OH- (20 mmol·L-1, 800 mmol·L-1) was prepared
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in order to investigate the surface activity of the system. The surface tension of SA
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could not obtained directly by water solution because of the poor solubility. As can be seen from Fig. 6, the surface tension was decreased with the increasing of the
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concentration of fatty acid, which proved that SA has surface activity when it is deprotonated in amines.
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Figure 6
3.6. Multi-responsive emulsion
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It was established the ability of stabilizing emulsion of bilayers solution is better
than that of micelles solution. We chose both samples of cSA = 40 mmol·L-1 with
varying 20 (bilayers) and 50 mmol·L-1 (micelles) of (CH3)4N+OH- and added
equivalent volume paraffin oil. After a vortex oscillation, both samples formed homogeneous emulsion. However, the phase separation of the emulsion which was stabilized by micelles was found after several minutes. In contrast, the sample stabilized by bilayers can keep stable for several months (Fig. 7). These observations were in good agreement with the literature that bilayers possess the better emulsifying properties [17]. Reports on successful attempts of controlling the process of emulsification and demulsification of the emulsion are relatively scarce. The
Page 9 of 21
responses are the result of modifications in the affinity between emulsifier and water or oil and can be induced by more or less instructive methods. Therefore, emulsions stabilized by surfactant could be damaged by applicable triggers. In order to apply to
response performance of the emulsion stabilized by the bilayers.
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3.6.1. pH response
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Figure 7
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the industry on the emulsification and demulsification better, we studied various
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Surfactants which are sensitive to pH tend to increase interest because of their wide range of potential in novel applications, where pH variations can be used to
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control aggregation. SA, as a common and conventional surfactant, is equipped with pH-sensitive property and can behave itself like a binary mixture consisting of the
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protonated and deprotonated forms. As a result, the bilayers have pH dependence, as
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well as the emulsion stabilized by bilayers.
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When 2 µL HCl was added into the emulsion and then the sample was vortex oscillated, a clear oil phase separation from the upper layer was observed after holding for several minutes in Fig. 8a. Originally, the emulsion is stabilized by bilayers. When the acid is added into the emulsion, deprotonated fatty acid molecules are transformed into protonated molecules, leading to losing of surface activity and destroying of bilayers subsequently. Finally demulsification occurs. 3.6.2. CO2 response Another method to vary pH is realized by bringing in CO2. Bubbling CO2 has an advantage of avoiding contamination of the medium solution comparing with adding acid or base to change the pH. After introducing CO2 into the emulsion, we
Page 10 of 21
also observed phase separation in line with expectations (Fig. 8b).
Figure 8
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3.6.3. Photoresponse Light is a trigger that is non-invasive and can avoid the direct contact with the
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specimen. In general, there are two methods to produce light-response: one is to
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modify a responsive group to a component and the other is to bring in materials which have responsive groups. Fatty acid itself does not have light response, so we introduce
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a photoacid generator (PAG) [16] in our work. PAG can be photolyzed by UV light and then produces acid causing the decrease of the pH and the change of
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self-assemblies based on fatty acid molecules.
To investigate the photoresponsive performance, 6.86 mg PAG was added to the
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emulsion. The separation was found after UV irradiation for 2 hours. Then holding for
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some time, we can obtain a sample similar to the emulsion added acid (Fig. 8c). This
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kind of light-response is another pH-response from the source. 3.6.4. Temperature response
DSC measurement was performed to investigate the temperature effect (Fig. 9).
We can see that 40 mmol·L-1 SA/20 mmol·L-1 (CH3)4N+OH- (bilayers) sample has a
broad peak at 50.02 °C which indicates the phase transition of the solution. However, micelles do not have a phase transition temperature which is a basis for next temperature response. Comparing with other stimuli, the temperature could change the system without adding chemical materials. The sample was placed in a water bath pot in order to investigate the temperature effect. When the temperature was rose to 50.0 °C, the
Page 11 of 21
demulsification was obtained (Fig. 9). The phenomenon is in good agreement with the result of the DSC measurement. Because of the existence of the phase transition temperature, temperature will affect the stability of the emulsion.
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Figure 9
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4. Conclusions
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In summary, we studied the phase behaviors of the mixture of SA and (CH3)4N+OH-. The conductivity and pH value were measured. The phase-transition temperature was
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obtained by DSC measurements and the apparent viscosity of typical samples was measured by rheological characterizations. Cryo-TEM demonstrated the bilayers. We
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focused on the emulsion stability of bilayers and micelles and found that bilayers have better emulsifying properties than micelles. Furthermore, the multiple responses of
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emulsion are the key research as an application and we expect the study is meaningful
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for industrial emulsification, demulsification and simplifying chemical routes for
Ac ce p
accessing more commercially viable systems.
Acknowledgements
This work is financially supported by the NSFC (grant nos. 21273136 and 21420102006).
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Captions of Figures and Tables in Manuscript Text:
M
60
30
3 4
L α/L1
d
Ac ce p +
L1
te
40
-
c(CH ) N OH / mmol⋅L
-1
50
Lα
20 10
0 10
P 20
30
cSA / mmol⋅L
-1
40
50
Fig. 1. Phase behavior of the SA/(CH3)4N+OH-/H2O system.
Page 17 of 21
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Fig. 2. Photographs of typical samples at 25.0 °C without (top) and with (bottom)
an
polarizers. cSA = 40 mmol·L-1 with varying (CH3)4N+OH- concentration gradually. From left to right: 10, 20, 30, 40, 50 and 60 mmol·L-1. The solution volume was fixed
Ac ce p
te
d
M
at 5 mL.
Fig. 3. Cryo-TEM images of a sample in Lα phase region.
Scheme 1. Microstructures of the fatty acid soap system at different pH.
Page 18 of 21
13
3000
pH κ
ip t 1500
Lα/L1
10
Lα
10
20
an
9 8
1000
L1
us
P
cr
pH
2000 11
-1
2500
κ / µS•cm
12
30
40
50
60
500 0
-1
c(CH ) N OH / mmol⋅L +
-
M
3 4
d
Fig. 4. Conductivity and pH value with the increase of concentration of (CH3)4N+OH-
1
Ac ce p
10
te
at cSA = 40 mmol·L-1 at 25.0 °C.
η / Pa· s
10
40:20 40:50
0
10
-1
10
-2
10
-3
10
-2
10
-1
•
10
0
γ/s
-1
10
1
10
2
Fig. 5. Shear rheogram of the samples with cSA = 40 mmol·L-1 and c(CH3)4N+OH- = 20, 50 mmol·L-1, respectively, at 25.0 °C.
Page 19 of 21
80
γ / mN • m
-1
70 40
γ / mN•m
-1
60
60
50
20
100
-1
c(CH ) N OH / µmol⋅L 3 4
+
1000
-
30 20 10
10
100 -1
cr
cSA / µmol⋅L
ip t
40
us
Fig. 6. Surface tensions (γ, mN·m-1) vs. log c of SA or (CH3)4N+OH- (interior
M
an
illustration) in solution.
(b)
d
(a)
te
Fig. 7. Emulsification by vortex oscillation holding for a period of time. (a) cSA = 40 mmol·L-1, c(CH3)4N+OH- = 20 mmol·L-1, (b) cSA = 40 mmol·L-1, c(CH3)4N+OH- = 50
Ac ce p
mmol·L-1.
Fig. 8. Multiple response of the emulsion stabilized by the bilayers. (a) pH response, (b) CO2 response, (c) light response, (d) temperature response.
Page 20 of 21
ip t
40 : 20
o
50.02 C
40
50
T /° C
60
us
30
cr
Heat Flow / a.u.
40 : 50
an
Fig. 9. Phase transition temperature of the samples with cSA = 40 mmol·L-1 and
Ac ce p
te
d
M
c(CH3)4N+OH- = 20 and 50 mmol·L-1, respectively measured by DSC.
Page 21 of 21