Hydration-Induced Phase Transitions in Surfactant and Lipid Films

Article pubs.acs.org/Langmuir

Hydration-Induced Phase Transitions in Surfactant and Lipid Films Sebastian Björklund* and Vitaly Kocherbitov Department of Biomedical Science, Faculty of Health and Society and Biofilms Research Center for Biointerfaces, Malmö University, Malmö, Sweden S Supporting Information *

ABSTRACT: For several surfactant and lipid systems, it is crucial to understand how hydration influences the physical and chemical properties. When humidity changes, it affects the degree of hydration by adding or removing water molecules. In many cases, this process induces transitions between liquid crystalline phases. This phenomenon is of general interest for numerous applications simply because of the fact that humidity variations are ubiquitous. Of particular interest are hydration-induced phase transitions in amphiphilic films, which in many cases appear as the frontier toward a vapor phase with changing humidity. Considering this, it is important to characterize the film thickness needed for the formation of 3D liquid crystalline phases and the lyotropic phase behavior of this kind of film. In this work, we study this issue by employing a recently developed method based on the humidity scanning quartz crystal microbalance with dissipation monitoring (HS QCM-D), which enables continuous scanning of the film hydration. We investigate five surfactants films (DDAO, DTAC, CTAC, SDS, and n-octylβ-D-glucoside) and one lipid film (monoolein) and show that HS QCM-D enables the fast characterization of hydration-induced phase transitions with small samples. Film thicknesses range from tens to hundreds of nanometers, and clear phase transitions are observed in all cases. It is shown that phase transitions in films occur at the same water activities as for corresponding bulk samples. This allows us to conclude that surfactant and lipid films, with a thickness of as low as 50 nm, are in fact assembled as 3D-structured liquid crystalline phases. Furthermore, liquid crystalline phases of surfactant films show liquidlike behavior, which decreases the accuracy of the absorbed water mass measurement. On the other hand, the monoolein lipid forms more rigid liquid crystalline films, allowing for an accurate determination of the water sorption isotherm, which is also true for the sorption isotherms corresponding to the solid surfactant phases.

■

INTRODUCTION When water molecules are added to a system, two general processes may occur: (i) water uptake by a single phase where the phase gradually swells as a result of the incorporation of water molecules and (ii) water uptake involving a phase change where the addition of water molecules causes a transition from one phase to another. During both types of hydration processes, several properties of the system may change, such as structural parameters, molecular mobility, viscosity, density, reactivity, permeability, and so forth.1 In general, these changes can be induced by hydration from the surrounding air due to variations in the relative humidity (RH). Therefore, the effect of hydration is relevant for many situations where the system is in contact with air of varying humidity. For surfactant- and lipid-based systems, used in, for example, pharmaceutical, technical, and industrial applications, it is mandatory to characterize hydration-induced phase transitions to enable control of the phase structure required for successful application. The reason is that the physicochemical properties of the system may change dramatically from one phase structure to another; for example, the diffusion properties in a liquid crystalline lamellar phase are very different as compared to those in cubic phases.2 Therefore, hydration-induced phase © XXXX American Chemical Society

transitions are of particular importance to consider for the development of carrier systems for the delivery of drugs or food nutrients where structural changes can drastically affect the drug or nutrient release profile.3 In these situations, it is important to characterize the hydration process when the formulation enters the aqueous interior of the human body to enable successful release profiles. This well-known aspect is taken advantage of in some cases. For example, the hydrationinduced formation of cubic liquid crystalline phases has been recognized for several decades as potential drug-delivery matrixes for controlled and sustained release due to the complex diffusion pathway for the loaded and entrapped drug molecules.4 Another related aspect is that many pharmaceutically active substances are amphiphilic and may in fact form liquid crystalline structures during initial hydration before they are dispersed in the aqueous human body, which is important to consider in the drug development process.5 Considering these features, it is important to enable good characterization of how water sorption or desorption from the Received: February 5, 2016 Revised: April 7, 2016

A

DOI: 10.1021/acs.langmuir.6b00452 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

and can, in addition, be modified to allow measurements of water desorption.10 In general, this type of experimental approach, where the hydration of a small sample is performed continuously, is difficult to implement in many of the methods usually employed for investigating the phase behavior of surfactants. This kind of approach has been employed in some cases. For example, the structural properties of multilayered lamellar phospholipid films, with approximate thicknesses ranging from tens to hundreds of nanometers, have been investigated at partial and full hydration.11,12 In addition, a cubic micellar (I2) liquid crystalline phase has been successfully formed in films with thickness ranging from 0.5 to 6 μm as concluded from small-angle X-ray scattering measurements in excess water. However, to our knowledge there is no study showing lyotropic phase transitions in surfactant or lipid films of such small thicknesses as in the present work because of continuous film hydration. Moreover, the minimum film thickness sufficient for the formation of 3D liquid crystalline phases with complicated geometries remains unknown. In the present study, we employ a novel method based on the humidity scanning quartz crystal microbalance with dissipation monitoring (HS QCM-D) that enables continuous scanning of the hydration of surfactant and lipid films cast on a quartz resonator.13 We show that HS QCM-D is a fast and easy technique that is advantageously used to characterize phase transitions induced by hydration and that requires very little substance. This is achieved by investigating the hydration effect of five surfactants (DDAO, DTAC, CTAC, SDS, and n-octyl β-D-glucoside) and one lipid (monoolein) with known phase behavior. The amphiphilic molecules are deposited on QCM-D sensors by spin-coating, giving films with thicknesses ranging from tens to hundreds of nanometers. In all cases, clear phase transitions are observed, and we show that the transitions occur at the same levels of water activity as for bulk samples investigated by sorption calorimetry. This allows us to conclude that surfactant and lipid films, with an average thickness of as low as 50 nm, are in fact assembled as 3D-structured liquid crystalline phases. Furthermore, the liquid crystalline phases of surfactant films show liquid-like behavior, which decreases the accuracy of the absorbed water mass measurement. On the other hand, the lipid monoolein forms more rigid liquid crystalline films, allowing for an accurate determination of the water sorption isotherm, which is also true for the sorption isotherms corresponding to the solid surfactant phases.

ambient air or the aqueous environment of the body influences the physical properties of the system by performing lyotropic phase studies. However, accurate phase studies of surfactants are time- and sample-consuming because large amounts of samples are required and hence a long time is needed to achieve an equilibrium distribution of water between liquid crystalline phases. Commonly, the effect of hydration on amphiphilic molecules is investigated by equilibrating the sample at a specific RH and registering the water content before investigating the physical properties with an appropriate technique such as X-ray scattering, NMR, FTIR, or microscope methods.6 Alternatively, the sample is simply mixed with water in different ratios before measurement at several temperatures to construct a temperature− composition phase diagram. As mentioned, some drawbacks related to this kind of experimental design are that it can be timeconsuming as a result of the slow equilibration of large samples and that one sample merely gives one data point for each RH (or water content). An alternative approach is to use small amounts of sample, which strongly decrease the time needed to obtain an equilibrium distribution of water. Working with small amounts, on the other hand, involves another complication: the transfer of samples into a cell where the actual measurement that is performed inevitably changes the water content of the sample. A solution to this problem is to prepare the required concentrations in situ in the equipment that performs the measurements of physical properties. For binary surfactant−water phase diagrams, the best method is to scan the water content from a dry surfactant to a wet system and to measure changes occurring during this process. This type of humidity scanning method, where the sample is subjected to continuous hydration during the measurement, results in multiple data points at different hydration levels.7 However, this methodology requires that the RH can be regulated in a controlled manner in situ during the measurement, which can be practically challenging. Sorption calorimetry is one example of a scanning technique that involves a double twin isothermal calorimeter.8,9 On the sample side of the twin calorimeter, one chamber contains the initially dry sample, and the other cell is empty from the start and later filled with pure water by injection. The two chambers are connected by a tube to allow water vapor to diffuse from the liquid water to the sample. From the thermal power of vaporization measured in the cell containing water, one can obtain the sorption isotherm in the form of water content as a function of RH (or water activity). In addition, by combining the thermal powers measured in both cells one can calculate the partial molar enthalpy of mixing of water as a function of water content. This method therefore provides a complete thermodynamic characterization of the water sorption process

■

EXPERIMENTAL SECTION

Materials. The chemical structures of the studied amphiphiles are shown in Figure 1. Dimethyldodecylamine-N-oxide (DDAO, 99% grade,

Figure 1. Chemical structures of nonionic surfactants DDAO and OG, lipid MO, and ionic surfactants DTAC, CTAC, and SDS. B


Article

Langmuir

the film thickness was primarily determined by the concentration and not the number of solution applications (Figure S1). Subsequently, the coated sensor was dried in vacuum for a minimum of 15 min and then put back in the module for measurement under a flow of N2 gas until a stable baseline of frequency was obtained. Next, the humidity scanning experiment was initiated according to the procedure described in detail elsewhere.13 In brief, HS QCM-D is based on the controlled dilution of LiCl solution to adjust the water activity aw continuously to regulate the RH (aw = RH/100%) in the QCM-D chamber. The LiCl solution is pumped through the QCM-D humidity module where water vapor can pass across the Gore membrane and thereby set the RH of the gas phase above the coated sensor in a continuous manner. Control experiments were performed to confirm that the kinetics of the hydration process was independent of the water activity scan rate by changing the flow rate of the solution that dilutes the LiCl solution. In this manner, the time period for the hydration process was varied from hours to days. From these experiments, it was concluded that the hydration of the amphiphilic films was kinetically independent of the water activity scan rate, within the limits investigated. Water Activity Measurements. The aw of aqueous surfactant solutions was measured with a NovaSina LabMaster-aw apparatus at 25 °C. The instrument was calibrated with saturated salt solutions (standards provided with the instrument) at suitable RHs before measurements.

CAS no. 1643-20-5), dodecyltrimethylammonium chloride (DTAC or C12TACl, 99% grade, CAS no. 112-00-5), cetyltrimethylammonium chloride (CTAC or C16TACl, 99% grade, CAS no. 112-02-7), and sodium dodecyl sulfate (SDS, 99% grade, CAS no. 151-21-3) were purchased from Sigma-Aldrich, n-octyl β-D-glucoside (OG, 98% grade, CAS no. 29836-26-8) was obtained from Boehringer Mannheim, and monoolein (MO, 98% grade, CAS no. 111-03-5) was purchased from Danisco. The amphiphiles were used without further purification. Close-to-saturated LiCl solution was prepared from anhydrous LiCl (p.a. quality, Sigma-Aldrich) by mixing excess amounts of LiCl in water for several days. The saturated solution was then filtered two times to remove excess LiCl salt. All water used in this work was ultrapure grade. Humidity Scanning (HS) QCM-D. QCM is an ultrasensitive method for the mass determination of materials adsorbed on a piezoelectric quartz sensor according to the methodology described by Sauerbrey.14 In particular, QCM-D has been widely employed because of its capacity to provide additional information on the viscoelastic properties of the adsorbed material.15 The QCM-D technique monitors the frequency of the oscillating shear motion of a quartz crystal, which is stimulated by an applied potential. The oscillating motion generates an acoustic wave, and the resonance condition occurs when the wavelength is an odd integer of the thickness of the (un)coated quartz sensor. The resonance frequency gives information on the adsorbed mass on the quartz crystal sensor. If the material mass is small relative to the mass of the quartz crystal and the material forms a homogeneous rigid film on the sensor, then the Sauerbrey equation can be used to calculate the film mass:14 −

2f 2 mf Δf = 0 n Zq

■

RESULTS AND DISCUSSION HS QCM-D for Studying Hydration-Induced Phase Transitions of 3D Surfactant Films. We investigated hydration-induced (lyotropic) phase transitions in films of six different amphiphiles (five surfactants and one lipid) with well-known phase behavior when mixed with water. The type of phase induced by the hydration process was determined on the basis of previously published studies on these molecules.1,16−23 Below, we discuss the phase transitions of one of the surfactants (DDAO) in detail and then describe the results obtained for the other surfactants and the lipid. Phase Behavior of DDAO (Dimethyldodecylamine-Noxide). The HS QCM-D method was thoroughly tested by performing multiple experiments on DDAO films with thickness varying from approximately 50 to 700 nm (Figure S1). Representative data is shown in Figure 2, where frequency Δf/n and dissipation ΔD/n data for a 49-nm-thick DDAO film are presented. The measurement is performed as a function of time during continuous hydration (Figure 2A,B). Next, the water activity of the LiCl solution is calculated according to the procedure previously described,13 after which the data is reevaluated as a function of water activity (Figure 2C,D). In other words, the results in Figure 2 show how Δf/n and ΔD/n are affected when the initially dry DDAO film is exposed to continuously increased RH, which leads to water uptake and lyotropic phase transitions. In this work, the data are consistently presented with the dry film as a reference state, which means that the frequency shifts Δf/n for all overtones n are zero in the dry state. The prominent and abrupt changes in resonance frequency Δf/n and dissipation ΔD/n observed in Figure 2 are related to hydration-induced transitions between different lyotropic phases of the binary DDAO−water system (indicated by arrows). The results are consistent with the established phase sequence of the DDAO−water system that forms at the following water activities:1,19 • solid phases, aw ≈ 0−0.2 • lamellar liquid crystalline phase, aw ≈ 0.2−0.7 • cubic phase, aw ≈ 0.7−0.8 • hexagonal phase, aw ≈ 0.8−1 • liquid micellar phase, aw ≈ 1

(1)

The Sauerbrey equation (eq 1) describes the proportionality between the negative frequency change Δf (Hz), normalized per overtone n, and the areal film mass mf (kg m−2). In eq 1, Zq is the acoustic impedance of quartz (8.8 × 106 kg m−2 s−1) and f 0 is the fundamental resonance frequency of the quartz sensor (∼5 MHz). In the present work, we describe the films by their estimated thicknesses in addition to their corresponding areal masses, which are obtained from the QCM-D experiments. The film thickness d of the dry film can be calculated from the areal mass of the dry film according to d = mf/ρ, assuming that the density ρ of the dry sample is known. For this estimation, we used a density equal to 1.0 g cm−3 for all dry surfactant films, and we stress that this is only a rough estimation of the film thickness. In addition to the frequency change, the QCM-D technique also monitors the dissipation D, which is related to the decay time of the oscillating resonator when the alternating potential is turned off. The decay time is related to the energy dissipation of the quartz resonator and greatly depends on the viscoelastic properties of the film that coats the sensor. Thus, the dissipation gives information on the rheological properties of the film and provides complementary data during the hydration process. A q-sense QCM-D E4 with humidity module QHM 401 and AT-cut SiO2 (QSX 303, 5 MHz) or Au (QSX 301, 5 MHz) sensors from Biolin Scientific AB were used in this work. New sensors were washed with water and ethanol before use. Reused sensors were cleaned by the procedures described in the q-sense guidelines manual (cleaning protocols B and A-I for QSX 303 and 301, respectively). No difference was observed between measurements with new or reused sensors. The humidity module is equipped with a Gore membrane that separates the flowing solution from the sensor. The membrane is permeable only to water vapor, which diffuses from the solution into the gas phase above the sensor and thus regulates the RH above the film coated on the sensor. The general procedure of an experiment was started by measuring the uncoated sensor in a dry N2 atmosphere at 25 °C. Next, the sensor was coated with a sample film by spin-coating where 20 μL of solution was applied 1−15 times in total (in one case, 50 times was used; see Figure S1). All surfactants were dissolved in water, and ethanol was used as a solvent for MO. The concentration of the solutions used for spin-coating the sensor varied between approximately 1 and 20 wt % to obtain different film thicknesses. It can be noted that C


Article

Langmuir

Figure 2. Frequency shift Δf/n and dissipation ΔD/n as a function of time (A and B) and as a function of water activity aw (C and D) for a 49-nm-thick film of DDAO. Arrows indicate time points or water activity levels where abrupt changes in Δf/n and ΔD/n occur as a result of hydration-induced phase transitions. Abbreviations: s, solid; lam, lamellar; cub, cubic; hex, hexagonal. All data are presented with the dry film as the reference state.

sample roughness and domain structure of liquid crystalline phases may have an effect on Δf/n and ΔD/n. In any case, in most experiments we see a general trend of how Δf/n and ΔD/n change with increasing water activity and water content. One can expect that first-order phase transitions (due to the formation of two-phase regions and the gradual accumulation of the second phase) lead to continuous smooth steplike changes in frequencies and dissipations. However, a closer inspection of how Δf/n and ΔD/n changes at phase transitions and in two-phase regions reveals more complicated behavior. For example, the lamellar-cubic and cubic-hexagonal phase transitions are in general associated with peak/troughlike changes in Δf/n and ΔD/n, which are probably caused by complex interactions between domains of two different phases in the two-phase region. This feature may lead to the uncoupling of domains of different phases and also weaker coupling between the surfactant film and the sensor, which can explain the peak/troughlike changes in Δf/n and ΔD/n in the twophase regions. Furthermore, in some cases Δf/n and ΔD/n increase dramatically at high water activities (examples in Figure 4), which indicates that hydration-induced changes to more fluidlike structures may cause an uncoupling of surfactant structures upon addition of water. The results in Figure 2 demonstrate that it is possible to gain detailed information on hydration-induced phase transitions of surfactant systems from HS QCM-D in a single experiment performed with very small amount of sample and in short time (i.e., hours). For example, the areal mass of the dry DDAO film used in the experiment in Figure 2 was 4.9 ± 0.1 μg/cm2. The area of the quartz sensor is 1.54 cm2. Thus, the deposited mass of the DDAO film was merely (4.9 × 1.54) = 7.5 μg. To investigate the reproducibility of the changes of Δf/n and ΔD/n at specific values of aw and to increase the understanding of the underlying phase transitions of these changes, we performed multiple experiments on the DDAO−water system. The results from these experiments are compiled in Table 1

The dry solid crystals do not take up any water until the water activity level reaches about 0.2, where a phase transition from the solid-crystalline to liquid crystalline state is clearly seen. In the lamellar liquid crystalline state, the Δf/n curves start to diverge. The deviation of Δf/n follows a clear dependence where Δf/n is more negative the lower the value of n. This behavior is in contrast to that observed for viscoelastic biopolymers in QCM-D experiments,13,24 where Δf/n was more negative the higher the value of n. On the other hand, similar behavior, with Δf/n being more negative the lower the value of n, is expected according to the semi-infinite liquid model where −Δf/n ∝ n−1/2.25 In other words, the results show that the surfactant film is characterized by liquidlike properties after the hydration-induced transition from the solid to the lamellar liquid crystalline phase. The dissipation data support this interpretation, showing that the dry film is completely rigid with ΔD/n close to zero, whereas the transition from the solid to the lamellar phase results in a sharp increase in ΔD/n around aw ≈ 0.2. The lamellar phase has previously been demonstrated to be a plastic material characterized with extensive energy dissipation as a result of the parallel slip of the lamellae,26 in agreement with the sharp increase in ΔD/n seen in Figure 2 at the transition into this phase. In general, our data show that in the water activity range where liquid crystalline phases exist, Δf/n and ΔD/n change rather continuously. In other words, water addition leads to the fluidization of the liquid crystalline phases in a continuous manner, and despite the expected differences in the rheological properties of different liquid crystalline phases, the Δf/n and ΔD/n values change relatively little upon transition from one phase to another. This can be rationalized by the fact that Δf/n and ΔD/n are proportional to the square root of the viscosity according to the semi-infinite liquid model,25 implying relatively small changes in Δf/n and ΔD/n due to changes in the rheological properties of liquidlike films. Moreover, the D


Article

Langmuir

of the monohydrate, two clear steps of Δf/n and ΔD/n are observed (Figure 3B,D). However, the ΔD/n data go back to values close to zero after the first transition (solid monohydrate), which is in agreement with the solid characteristic of the monohydrate state. This is in contrast to the direct transition from a solid to lamellar phase where Δf/n and ΔD/n change in a single step (Figure 3A,C). Furthermore, the fact that ΔD/n increases significantly from values close to zero in Figure 3A,C implies that the solid film is transformed into a liquidlike structure in agreement with the liquid crystalline lamellar phase. In general, the HS QCM-D data in Table 1 are in good agreement with the corresponding sorption calorimetry data. In other words, the water activity levels where hydration-induced phase transitions take place for the DDAO−water system is similar for both bulk samples (sorption calorimetry) and thin films. However, some minor discrepancies are observed, which can be related to the kinetics of phase transitions. For example, the transition from the lamellar to the cubic phase was seen at slightly higher water activities in HS QCM-D experiments compared to sorption calorimetry. This is explained by the fact that humidity scanning in HS QCM-D is faster and the transition to the cubic phase is kinetically hindered.19 On the other hand, from a point of view of water diffusion kinetics, the HS QCM-D method can be even more accurate than sorption calorimetry and other methods that use powder samples because of much smaller film thicknesses in QCM-D (tens or hundreds of nanometers) compared to powder particle sizes. Moreover, when powders get hydrated, they turn into continuous gel-like samples with characteristic sizes in the several millimeters range, where diffusion paths for water molecules become very long (3−6 orders of magnitude longer than in QCM-D). Thus, despite faster scan rates in HS QCM-D experiments, as a result of small sample thicknesses they can provide more homogeneous water distributions as compared to methods that use powder samples. Taken together, the results compiled in

Table 1. Comparison between Literature Data and HS QCM-D Data (n = 14) of Water Activity Values of Phase Transitions in the DDAO−Water System aw phase transition

literature data

solid−monohydrate monohydrate−lamellar solid−lamellar lamellar−cubic cubic−hexagonal hexagonal−micellar

0.15 0.27 0.19 0.65 0.79 0.980b

a

HS QCM-D 0.168 0.268 0.178 0.674 0.794 0.985

± ± ± ± ± ±

0.007 0.004 0.005 0.013 0.007 0.000

(n (n (n (n (n (n

= = = = = =

3) 3) 11) 14) 14) 2)

a

Data from ref 19. bWater activity of a concentrated DDAO micellar solution (68.0 wt % water content) measured with a NovaSina LabMaster-aw apparatus at 25 °C.

together with previously published results from sorption calorimetry for comparison.19 Interestingly, two groups of data were obtained where the initial water uptake of the solid DDAO film resulted in either monohydrate formation or a direct transition into the lamellar liquid crystalline phase. For clarity, Figure 3 shows results from two experiments, with and without monohydrate formation. From previous sorption calorimetry experiments, monohydrate formation has been established as the equilibrium path, while the direct change from the dry surfactant to the liquid crystalline lamellar phase is a metastable transition.19 This kind of solid-state polymorphism is a typical feature of surfactants, is regularly observed by other methods,19 and is likely related to the sample history and preparation procedure. We performed 14 experiments with DDAO samples of different thicknesses but could not find any clear dependence of monohydrate formation on the sample thickness (further commented on in Figure S1). The data in Figure 3 clearly illustrate the benefits of combining the frequency and dissipation data to obtain more information on the phase transition. For example, upon the formation

Figure 3. Frequency shift Δf/n and dissipation ΔD/n as a function of water activity aw for two experiments with DDAO films without (A and C) and with (B and D) the formation of monohydrate. Film thicknesses: (A, C) 49 nm and (B, D) 168 nm. Abbreviations: s, solid; smh, solid monohydrate; and lam, lamellar. All data are presented with the dry film as a reference state. E


Article

Langmuir

Figure 4. Δf/n (left panel) and ΔD/n (right panel) as a function of aw for 3D films of amphiphilic molecules. Arrows indicate water activity levels where abrupt changes in Δf/n and ΔD/n occur as a result of hydration-induced phase transitions. Film thicknesses: (A, B) OG, 190 nm; (C, D) MO, 153 nm; (E, F) DTAC, 158 nm; (G, H) CTAC, 293 nm; and (I, J) SDS, 231 nm. Abbreviations: s, solid; sxh, solid unknown hydrate; smh, solid monohydrate; lam, lamellar; cub, cubic; int, intermediate; hex, hexagonal; lc, liquid crystalline or gel phase; and mic, micellar. All data are presented with the dry film as the reference state.

At higher water activities, aw ≈ 0.92, there is another transition from the lamellar to the cubic phase.20 A more detailed investigation of Δf/n at higher water activities shows a clear change in Δf/n around aw ≈ 0.96, which may be explained by a transition from the cubic phase to an isotropic micellar phase. (See Figure S3 for a close-up of the high water activity range.) The structure of the micellar phase, close to the phase boundary, has been suggested to resemble a melted cubic phase where the micelles form a connected network.21 In other words, close to the phase boundary the micellar phase is similar to the cubic phase, while further hydration of the micellar phase may result in additional changes in the rheological properties that can explain the changes in Δf/n observed above aw ≈ 0.96 (cf. Figure S3). Also, it can be noted that at lower temperature (below approximately 22 °C) the cubic phase is followed by a hexagonal phase, before the isotropic micellar phase.21 In other words, it is not likely that these additional changes in Δf/n observed above aw ≈ 0.96 are related to the formation of a hexagonal phase. Finally, we note that the Δf/n data in Figure 4

Table 1 illustrate that HS QCM-D is a reliable technique for identifying the water activity levels where lyotropic phase transitions occur. Phase Behavior of Different Surfactant and Lipid Films. For a more complete demonstration of how HS QCM-D can be used to determine lyotropic phase changes of 3D surfactant and lipid films, we investigated six different amphiphiles in total (Figure 1). Representative results are shown in Figure 4, where Δf/n (left panel) and ΔD/n (right panel) are plotted as functions of aw for the various surfactant or lipid molecules. The corresponding plots of Δf/n and ΔD/n as functions of time are presented in Figure S2. From the results in Figure 4, we again conclude that hydration-induced (lyotropic) phase transitions of surfactant 3D films result in clear changes in Δf/n and ΔD/n. OG (n-Octyl β-D-glucoside). The results in Figure 4A,B show that the hydration of alkylglucoside OG is associated with a clear phase change at aw ≈ 0.6, which is due to a transition from a solid state to a liquid crystalline lamellar phase.20 F


Article

Langmuir

Figure 5. Demonstration of how hydration-induced phase transitions from solid to liquidlike phases change the order of overtones from Sauerbrey conditions (−Δf/n ∝ n0) to liquidlike behavior (i.e., negative dependence similar to −Δf/n ∝ n−1/2). Film thicknesses: (A) DDAO, 204 nm; (B) DTAC, 54 nm; and (C) SDS, 231 nm. Abbreviations: s, solid; sxh, solid unknown hydrate; smh, solid monohydrate; lam, lamellar; cub, cubic; and mic, micellar. All data are presented with the dry film as the reference state.

A show a positive change for the solid film at around aw ≈ 0.2, which is most likely not due to any loss of adsorbed mass. Instead, this can be explained by the fact that the dry OG film is in an amorphous state initially and then converts into a more ordered and rigid solid state upon addition of a small number of water molecules, allowing for structural relaxation (further commented on in Figure S4 in the SI). MO (Monoolein). The hydration process MO (Figure 4C,D) is associated with two clear changes in Δf/n and ΔD/n at around aw ≈ 0.7 and 0.98, which can be explained by formation of the liquid crystalline lamellar phase and a transition from the lamellar to the cubic phase, respectively.22,23 The phase state of MO in the dry film is discussed below in the section on water sorption isotherms. DTAC (Dodecyltrimethylammonium Chloride). The Δf/n and ΔD/n data corresponding to the DTAC film (Figure 4E,F) show clear changes at aw ≈ 0.21, 0.26, 0.34, 0.53, 0.58, 0.95, and 0.98. From the established phase diagram of water−DTAC, we expect the following phase transitions at 25 °C: solid−lamellar, lamellar−cubic (Ia3d), cubic−hexagonal, hexagonal−cubic (Pm3n), and cubic−micellar.16 In addition, an intermediate phase has been identified between the cubic (Ia3d) and the hexagonal phases on the basis of 14N and 2H NMR experiments at 24 °C.17 Starting with the first transition at aw ≈ 0.21, we note that the ΔD/n values remain close to zero, which implies that the change in Δf/n is due to water uptake and the formation of a solid hydrate. The water content at this aw, calculated by the Sauerbrey equation, is between 0.5 and 1 water molecule per DTAC molecule, suggesting that this transition corresponds to the formation of either a hemi- or monohydrate. Next, the transition at aw ≈ 0.26 is due to the formation of a liquid crystalline lamellar phase that is changed into a cubic phase at aw ≈ 0.34. The transitions at aw ≈ 0.53 and 0.58 are consistently observed in the HS QCM-D experiments and interpreted as transitions from the cubic to the intermediate and from the intermediate to the hexagonal phases, respectively. From the aforementioned NMR experiments, the intermediate phase was established between approximately 18 to 20 wt % water content.17 This range of water contents correspond to aw ≈ 0.525−0.573 based on sorption calorimetry isotherms (data not shown), which is in good agreement with the HS QCM-D data. Finally, the changes in Δf/n and ΔD/n at aw ≈ 0.95 and 0.98 are attributed to transitions from the hexagonal to the cubic and from the cubic to the micellar phases, respectively. We determined the water activity of a concentrated

micellar solution (61.8 wt % water) to aw = 0.973, which supports our interpretation that the transition at aw ≈ 0.98 corresponds to the formation of the micellar phase. CTAC (Cetyltrimethylammonium Chloride). CTAC has four more carbons in the surfactant tail as compared to DTAC, which clearly influences the QCM-D results as seen in Figure 4G,H. At low humidity (aw < 0.5), there are clear changes in Δf/n at aw ≈ 0.3 and 0.4, which can be related to formation of hydrates of different stoichiometries. The fact that ΔD/n remains close to zero in this region (i.e., aw < 0.5) supports the interpretation that these changes are related to transitions between different solid states. The water content (calculated by the Sauerbrey equation) in the water activity range around aw ≈ 0.3 is 0.2−0.6 mole per mole of CTAC, which implies the formation of a hemihydrate. Similarly, the water content at aw ≈ 0.4−0.5 is approximately constant at 1 molecule of water per CTAC molecule, suggesting the formation of a monohydrate. Taken together, it can be concluded that two different solid hydrates form,; however, the precise stoichiometries of these hydrates remain to be determined. At higher water activities, the most prominent changes in Δf/n and ΔD/n occur at aw ≈ 0.55 and 0.81. The equilibrium phases before and after these transitions remain unclear; however, the change at aw ≈ 0.55 is likely from solid monohydrate to a liquid crystalline or gel phase, whereas the change at aw ≈ 0.81 can be explained by a transition from a liquid crystalline or gel phase to a hexagonal structure. A more detailed investigation of the phase behavior of the CTAC−water system is currently being performed in our laboratory (unpublished data, Kocherbitov, V.; Engblom J.). SDS (Sodium Dodecyl Sulfate). Finally, the results corresponding to the SDS film in Figure 4I,J show that Δf/n and ΔD/n remain unchanged until aw ≈ 0.98, at which point a drastic change is observed. This change is likely due to water uptake leading to a phase transition from the solid state to the two-phase region between the solid and micellar phases.18 To investigate this further, we determined the water activity at 25 °C of a two-phase SDS−water mixture with a gross concentration of 37.0 wt % SDS.18 In this measurement, the water activity was determined to be aw = 0.978, which coincides with the level of aw where the onset of the abrupt change in Δf/n occurs (cf. Figure 5C). At higher water activities, additional changes in Δf/n and ΔD/n are observed, which is likely related to the transition from the two-phase region to the micellar phase. (See the time-dependent hydration process of SDS in Figure S2I,J). G


Article

Langmuir

Figure 6. Sorption isotherms of (A) DDAO, (B) OG, and (C) MO calculated according to the Sauerbrey equation from HS QCM-D data (blue curves) or from sorption calorimetry (red curves, data from refs 19, 20, and 23). Each trace represents an independent experiment. In (A), dashed lines show the direct transition from solid to lamellar, whereas solid lines show monohydrate formation. The inset in (C) shows a clear kink in the HS QCM-D sorption isotherm at aw ≈ 0.98, which correspond to a transition from a lamellar to a cubic phase (not detected in the sorption calorimetry experiment, red curve). Abbreviations: s, solid; smh, solid monohydrate; lam, lamellar; and cub, cubic.

dependence expected according to the viscoelastic film in air model (i.e., − Δf/n ∝ n2), which indicates that liquid crystalline films exhibit more viscous liquidlike than viscoelastic properties, whereas in the dry state they behave like solids. Water Sorption Isotherms of Surfactant and Lipid Films. As illustrated, the clear changes in Δf/n can easily be used to locate the water activities where hydration-induced phase transitions occur. This information can advantageously be complemented with the corresponding composition of the surfactant−water system to generate a water sorption isotherm where the water content is plotted as a function of the water activity. The absorbed water content can be calculated from the Δf/n as previously shown for different types of biopolymers.13,24 However, this requires that the overtones follow either the Sauerbrey equation or some other model (e.g., the viscoelastic film in air model25). Unfortunately, the overtone dependence in the case of QCM-D data from surfactant films under continuous hydration is very complex, with several changes due to multiple phase transitions (cf. Figure 4), which make it difficult to calculate the water uptake. However, this is not always the case; for example, in solid surfactant films (i.e., dry solids and crystal hydrates) Δf/n is independent of the overtone number; therefore, it is possible to calculate the water uptake by the Sauerbrey equation. In some cases, this is possible not only for surfactant films in the solid state but also for more rigid liquid crystalline phases when the film thickness is relatively small. To illustrate this, we focus on the hydration of DDAO, OG, and MO films, which have previously been investigated by sorption calorimetry.19,20,23 Figure 6A shows that the QCM-D results (blue curves) from DDAO are in good agreement with the reference sorption calorimetry data (red curves) in the initial part of the isotherm where the DDAO films are solid (i.e., formed by dry crystals or crystal hydrates). As already mentioned, the initial water uptake of the solid DDAO film results in either monohydrate formation (solid curves) or a direct transition into the lamellar phase (dashed curves), and these different hydration traces are observed both in the case of sorption calorimetry and HS QCM-D. It can be pointed out that once the DDAO film is changed from a solid state to the liquid crystalline lamellar phase the Sauerbrey equation does not hold because of diverging overtones (e.g., Figure 2A,C), which results in an erroneous calculation of the water mass and thus deviating results. The QCM-D data on OG hydration (blue curves in Figure 6B) are similar to the corresponding sorption calorimetry data

Overtone Order Is Dependent on the (Liquid) Crystalline State of 3D Surfactant Films. Under conditions where the Sauerbrey eq (eq 1) is valid, it is expected that the normalized frequency shifts, from each overtone, are equal. However, in most cases the films on the quartz sensor are not rigid, which leads to deviations from the Sauerbrey condition. In this situation, alternative methods for data analysis are required. As demonstrated in previous work, QCM-D data can be analyzed according to the single viscoelastic film in air model.25 This approach has been successful for investigating hydration and water uptake by various samples, such as pig gastric mucin and lysozyme films.13,24 An important feature of this model is that −Δf/n ∝ n2, which implies that Δf/n becomes more negative for increasing overtone number n. This is in contrast as compared to the Sauerbrey model where − Δf/n ∝ n0 (i.e., the normalized frequency shifts are independent of overtone number). In addition, if the adsorbed material is liquid-like, then the QCM-D data can be treated according to the aforementioned semi-infinite liquid model, where it is expected that the normalized frequency shifts are inversely proportional to the square root of the overtone number (i.e., −Δf/n ∝ n−1/2).25 Taken together, the dependence of −Δf/n on n is related to the rheological properties of the film, which can change drastically as a result of hydration-induced phase transitions. Thus, changes in the rheological properties of the film can be studied during the hydration process by comparing the observed overtone dependence with the limiting cases described by the mentioned theoretical models. This is illustrated in Figure 5 for (A) DDAO, (B) DTAC, and (C) SDS. The results show that when the surfactant films are solid, the QCM-D data obey the Sauerbrey equation. However, upon hydration-induced transformations, from solid states to liquid crystalline phases, the overtone dependence becomes negative, which is similar to the overtone dependence expected from the semi-infinite liquid model (i.e., − Δf/n ∝ n−1/2). The overtone dependence with respect to the water activity was also evaluated in more detail for DDAO, where the exponent of n was determined from a generalized equation using multidimensional unconstrained nonlinear minimization (Supporting Text and Figure S5), which supports our interpretation of the results in Figure 5. Similarly, both lysozyme and mucin films have been shown to undergo hydration-induced rheological transformations, which resulted in −Δf/n changing from viscoelastic (i.e., −Δf/n ∝ n2) to liquidlike behavior (i.e., − Δf/n ∝ n−1/2).13,24 However, the amphiphilic films studied here did not show the overtone H


Article

Langmuir

Furthermore, the fact that surfactant and lipid films with thicknesses of as low as tens of nanometers form liquid crystalline phases is promising for the development of new materials. For example, it has been shown that lyotropic liquid crystalline mixtures can be used as a template for the electrochemical deposition of metallic mesoporous platinum films, which may serve as electrode materials in batteries, fuel cells, and sensors.32 Potentially, amphiphilic films with 3D liquid crystalline phases can be used in a similar manner as templates for mesoporous materials without the need for electrochemical deposition.

(red curves). In both cases, the results show an abrupt increase in water content at the transition from the solid state to the lamellar liquid crystalline phase at around aw ≈ 0.6, followed by swelling of the lamellar phase at higher water activities. The sorption isotherms from calorimetry on MO bulk samples show no water uptake for the solid phase, whereas the transition from the solid to the liquid crystalline lamellar phase is associated with the clear water uptake of one water molecule per MO molecule, followed by continuous swelling. The corresponding sorption isotherm from HS QCM-D shows a continuous small absorption between aw ≈ 0.1 and 0.7 and a clear kink at aw ≈ 0.7 due to the transition to the liquid crystalline lamellar phase. After the calorimetric transition into the lamellar phase (aw ≈ 0.8) and above, there is excellent agreement between the results from the two techniques. The difference between the results, obtained by the two techniques at low water activities, is most likely due to the fact that the initial states of the samples differed. In sorption calorimetry, the experiment started from solid crystalline powder that does not absorb water until an abrupt phase transition to the lamellar phase. In the case of HS QCM-D, the lipid was first dissolved in ethanol, which destroyed the crystalline structure, and then was deposited on the sensor by spin-coating. The solvent evaporation in spin-coating is a fast process, and the sample did not have enough time for crystallization. It was probably the fluid isotropic phase that was formed upon solvent evaporation because it exists in the dry state of MO above the melting temperature of the solid crystals (phase diagram in ref 22). A closer analysis of the isotherms in Figure 6C reveals that it is only the HS QCM-D method that detects the phase transition from the lamellar to the cubic phase at aw ≈ 0.98 (inset in Figure 6C). In sorption calorimetric experiments, this transition was not seen, although it was observed when the calorimetric method was used in desorption mode.23 Taken together, the sorption isotherms of MO are in good agreement at high water activities where MO is in a liquid crystalline lamellar state, which is in contrast to the other surfactants. Because the sample thicknesses were of the same order of magnitude, the reason for the difference in their behavior is probably due to the rheological properties of MO versus those of the other surfactant films. One should note that MO is more hydrophobic as compared to the surfactants studied in this work and unlike them forms reverse, not direct, cubic phases. Furthermore, the amount of water taken up by liquid crystalline phases of MO is typically smaller than that taken up by the surfactants at the same water activities. These effects lead to more solidlike behavior of MO liquid crystalline films as compared to that of films formed by other surfactants. This opens up an opportunity for a more accurate determination of water sorption isotherms and phase diagrams of lipids using the HS QCM-D method, which will be tested in further studies. Outlook. This work demonstrates that HS QCM-D can be employed to investigate how the external humidity influences the phase behavior of amphiphilic films in a precise and systematic manner. This methodology will be useful for future investigations of relevant biological, pharmaceutical, and technical applications exposed to environments with varying humidity where hydration-induced phase transitions may occur. Examples of systems like these can be the lipid tear film formed on eyes,27 cuticle wax films on plants,28 the protein−lipid composite of the skin barrier (referred to as the stratum corneum),29 lipid-based pharmaceutical formulations for topical treatment/drug delivery,30 and complex coatings of polymers and surfactants such as complex salts.31

■

CONCLUSIONS In this work, we used the recently developed humidity scanning (HS) QCM-D method for a detailed investigation of the phase behavior of six different surfactant/lipid−water systems. Surfactant films deposited on quartz sensors show the same phase transitions as bulk surfactant samples, even though the film thickness can be as thin as a few tens of nanometers. The hydration-induced phase transitions result in clear changes in Δf/n and ΔD/n for the phase-transition water activities, and the phase transitions are proven to be very reproducible. The overtone dependence of Δf/n can be analyzed to obtain qualitative information on the rheological properties of the surfactant films where transitions into liquidlike structures typically result in a negative overtone dependence. The sorption isotherms of the solidlike films are in good agreement with the corresponding isotherms from sorption calorimetry. Furthermore, it is shown that HS QCM-D in some cases provides information complementary to calorimetry from the hydration-induced changes in both Δf/n and ΔD/n, such as the intermediate phase of DTAC and the lamellar−cubic phase transition of MO at high water activity. Finally, the HS QCM-D method requires small amounts of material and is very fast, which makes it a powerful technique for investigating the phase behavior of both exotic/expensive materials and standard materials when time is a limiting factor.

■

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b00452. Mean thickness of DDAO films as a function of DDAO concentration used for spin-coating, number of 20 μL applications for different DDAO concentrations used for spin-coating, Δf/n and ΔD/n as a function of time for 3D surfactant films under the influence of continuous hydration, Δf/n and ΔD/n as functions of aw for an OG film, exponent β as a function of water activity for a DDAO film, and dependence of exponent n with respect to water content (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Biofilms - Research Center for Biointerfaces at Malmö University and the Knowledge Foundation are acknowledged for financial support. I


Article

Langmuir

■

phase transition in the monoolein-distearoyl phosphatidyl glycerolwater system. Biochim. Biophys. Acta, Biomembr. 2004, 1665 (1−2), 156−166. (24) Znamenskaya, Y.; Sotres, J.; Gavryushov, S.; Engblom, J.; Arnebrant, T.; Kocherbitov, V. Water Sorption and Glass Transition of Pig Gastric Mucin Studied by QCM-D. J. Phys. Chem. B 2013, 117 (8), 2554−2563. (25) Johannsmann, D. Viscoelastic analysis of organic thin films on quartz resonators. Macromol. Chem. Phys. 1999, 200 (3), 501−516. (26) Mezzenga, R.; Meyer, C.; Servais, C.; Romoscanu, A. I.; Sagalowicz, L.; Hayward, R. C. Shear rheology of lyotropic liquid crystals: A case study. Langmuir 2005, 21 (8), 3322−3333. (27) Leiske, D. L.; Miller, C. E.; Rosenfeld, L.; Cerretani, C.; Ayzner, A.; Lin, B.; Meron, M.; Senchyna, M.; Ketelson, H. A.; Meadows, D.; Srinivasan, S.; Jones, L.; Radke, C. J.; Toney, M. F.; Fuller, G. G. Molecular structure of interfacial human meibum films. Langmuir 2012, 28, 11858. (28) Bargel, H.; Koch, K.; Cerman, Z.; Neinhuis, C. Structurefunction relationships of the plant cuticle and cuticular waxes - A smart material? Functional Plant Biology 2006, 33 (10), 893−910. (29) Björklund, S.; Nowacka, A.; Bouwstra, J. A.; Sparr, E.; Topgaard, D. Characterization of stratum corneum molecular dynamics by natural-abundance 13C solid-state NMR. PLoS One 2013, 8 (4), e61889. (30) Barauskas, J.; Johnsson, M.; Johnson, F.; Tiberg, F. Cubic phase nanoparticles (Cubosome): Principles for controlling size, structure, and stability. Langmuir 2005, 21 (6), 2569−2577. (31) Gustavsson, C.; Li, J.; Edler, K. J.; Piculell, L. Water-Responsive Internally Structured Polymer Surfactant Films on Solid Surfaces. Langmuir 2014, 30 (42), 12525−12531. (32) Attard, G. S.; Bartlett, P. N.; Coleman, N. R. B.; Elliott, J. M.; Owen, J. R.; Wang, J. H. Mesoporous platinum films from lyotropic liquid crystalline phases. Science 1997, 278 (5339), 838−840.

REFERENCES

(1) Laughlin, R. G. The Structures and Properties of Surfactant Phases. The Aqueous Phase Behavior of Surfactants; Academic Press: London, 1996; Chapter 8, pp 181−237. (2) Åberg, C.; Pairin, C. Responding double-porous lipid membrane: lyotropic phases in a polymer scaffold. Biochim. Biophys. Acta, Biomembr. 2008, 1778 (2), 549−558. (3) Shah, J. C.; Sadhale, Y.; Chilukuri, D. M. Cubic phase gels as drug delivery systems. Adv. Drug Delivery Rev. 2001, 47 (2−3), 229−250. (4) Drummond, C. J.; Fong, C. Surfactant self-assembly objects as novel drug delivery vehicles. Curr. Opin. Colloid Interface Sci. 1999, 4 (6), 449−456. (5) Schreier, S.; Malheiros, S. V. P.; de Paula, E. Surface active drugs: self-association and interaction with membranes and surfactants. Physicochemical and biological aspects. Biochim. Biophys. Acta, Biomembr. 2000, 1508 (1−2), 210−234. (6) Evans, D. F.; Wennerström, H. Bilayer Systems. The Colloidal Domain: Where Physics, Chemistry, Biology and Technology Meet, 2nd ed; Wiley-VCH: New York, 1999; Chapter 6, pp 295−350. (7) Kocherbitov, V. Application of scanning methods to distinguish between entropy and enthalpy driven phase transitions. Curr. Opin. Colloid Interface Sci. 2013, 18 (6), 510−516. (8) Wadsö, I.; Wadsö, L. A new method for determination of vapour sorption isotherms using a twin double microcalorimeter. Thermochim. Acta 1996, 271, 179−187. (9) Wadsö, L.; Markova, N. A double twin isothermal microcalorimeter. Thermochim. Acta 2000, 360 (2), 101−107. (10) Kocherbitov, V.; Wadsö, L. A desorption calorimetric method for use at high water activities. Thermochim. Acta 2004, 411 (1), 31− 36. (11) Mennicke, U.; Salditt, T. Preparation of solid-supported lipid bilayers by spin-coating. Langmuir 2002, 18 (21), 8172−8177. (12) Simonsen, A. C.; Bagatolli, L. A. Structure of spin-coated lipid films and domain formation in supported membranes formed by hydration. Langmuir 2004, 20 (22), 9720−9728. (13) Björklund, S.; Kocherbitov, V. Humidity scanning quartz crystal microbalance with dissipation monitoring setup for determination of sorption-desorption isotherms and rheological changes. Rev. Sci. Instrum. 2015, 86 (5), 055105. (14) Sauerbrey, G. Verwendung Von Schwingquarzen Zur Wägung Dünner Schichten Und Zur Mikrowägung. Eur. Phys. J. A 1959, 155 (2), 206−222. (15) Rodahl, M.; Höök, F.; Krozer, A.; Brzezinski, P.; Kasemo, B. Quartz-crystal microbalance setup for frequency and q-factor measurements in gaseous and liquid environments. Rev. Sci. Instrum. 1995, 66 (7), 3924−3930. (16) Balmbra, R. R.; Clunie, J. S.; Goodman, J. F. Cubic Mesomorphic Phases. Nature 1969, 222 (5199), 1159−1160. (17) Kang, C.; Söderman, O.; Eriksson, P. O.; Vonholstein, I. S. Intermediate phases in the dodecyltrimethylammonium chloride water-system. Liq. Cryst. 1992, 12 (1), 71−81. (18) Kekicheff, P.; Grabiellemadelmont, C.; Ollivon, M. Phasediagram of sodium dodecyl-sulfate water-system 0.1. A calorimetric study. J. Colloid Interface Sci. 1989, 131 (1), 112−132. (19) Kocherbitov, V.; Söderman, O. Hydration of dimethyldodecylamine-N-oxide: Enthalpy and entropy driven processes. J. Phys. Chem. B 2006, 110 (27), 13649−13655. (20) Kocherbitov, V.; Söderman, O.; Wadsö, L. Phase diagram and thermodynamics of the n-octyl beta-D-glucoside/water system. J. Phys. Chem. B 2002, 106 (11), 2910−2917. (21) Nilsson, F.; Söderman, O.; Johansson, I. Physical-chemical properties of the n-octyl beta-d-glucoside/water system. A phase diagram, self-diffusion NMR, and SAXS study. Langmuir 1996, 12 (4), 902−908. (22) Qiu, H.; Caffrey, M. The phase diagram of the monoolein/water system: metastability and equilibrium aspects. Biomaterials 2000, 21 (3), 223−234. (23) Sparr, E.; Wadsten, P.; Kocherbitov, V.; Engström, S. The effect of bacteriorhodopsin, detergent and hydration on the cubic-to-lamellar J


Hydration-Induced Phase Transitions in Surfactant and Lipid Films

Recommend Documents