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101.
Katarzyna Kaa Wojciech Pajk Katarzyna Sukowska-Ziaja Agata Krakowska Jan Lazur Maciej Fidurski Krystian Marzec Piotr Ziba Agata Fijakowska Agnieszka Szewczyk Boena Muszyska 《Molecules (Basel, Switzerland)》2022,27(24)
Hypsizygus marmoreus is an edible medicinal mushroom species with a high dietary value. In this study, the fruiting bodies of commercial and self-cultivated crops and mycelium from in vitro H. marmoreus cultures (both white and brown varieties) were evaluated. This study aimed to analyze the presence of indole compounds and other biologically active substances and determine the effect that the addition of zinc and magnesium ions to the culture medium has on the content of the tested compounds in mycelial cultures. The content of indole compounds and other organic compounds was determined using high-performance liquid chromatography, the content of bioelements was determined using flame atomic absorption spectrometry, the glucan content was determined spectrophotometrically, and the antioxidant activity of extracts was estimated using DPPH, FRAP, and ABTS methods. The results showed that H. marmoreus mycelium from in vitro cultures is a good source of indole compounds, bioelements, glucans, and lovastatin. Mycelia from in vitro cultures showed the most diverse composition of indole compounds (L-tryptophan, 5-hydroxy-L-tryptophan, tryptamine, 5-methyltryptamine, and melatonin). Additionally, in vitro cultures of H. marmoreus enriched with Zn and Mg salts increased the contents of Na, Ca, Zn, 5-methyltryptamine, melatonin, protocatechuic acid, sterols, and total glucans. Only in the case of the white variety of mycelial enriched cultures, ergothioneine and Mg levels also increased. To summarize, the content of the active compounds differed depending on the H. marmoreus variety and the tested material. 相似文献
102.
Stefan Mommer Kamil Sokoowski Magdalena Olesiska Zehuan Huang Oren A. Scherman 《Chemical science》2022,13(30):8791
Extended polymeric structures based on redox-active species are of great interest in emerging technologies related to energy conversion and storage. However, redox-active monomers tend to inhibit radical polymerisation processes and hence, increase polydispersity and reduce the average molecular weight of the resultant polymers. Here, we demonstrate that styrenic viologens, which do not undergo radical polymerisation effectively on their own, can be readily copolymerised in the presence of cucurbit[n]uril (CB[n]) macrocycles. The presented strategy relies on pre-encapsulation of the viologen monomers within the molecular cavities of the CB[n] macrocycle. Upon polymerisation, the molecular weight of the resultant polymer was found to be an order of magnitude higher and the polydispersity reduced 5-fold. The mechanism responsible for this enhancement was unveiled through comprehensive spectroscopic and electrochemical studies. A combination of solubilisation/stabilisation of reduced viologen species as well as protection of the parent viologens against reduction gives rise to the higher molar masses and reduced polydispersities. The presented study highlights the potential of CB[n]-based host–guest chemistry to control both the redox behavior of monomers as well as the kinetics of their radical polymerisation, which will open up new opportunities across myriad fields.Extended polymeric structures based on redox-active species are of great interest in emerging technologies related to energy conversion and storage.Polyviologens are redox-active polymers based on N-substituted bipyridinium derivatives which have emerged as promising materials for energy conversion and storage.1–5 Their physicochemical properties can be adjusted through copolymerisation of the redox-active viologen monomers.6–8 The resultant materials are stable, water soluble and exhibit fast electron transfer kinetics. Polyviologens have been commonly fabricated through step-growth polymerisation in linear and dendritic architectures,9–13 as supramolecular polymers,14–16 networks,6,17,18 and covalent organic frameworks.19,20 Alternatively, anionic/cationic or metathesis-based polymerisations are used to avoid interference of radical-stabilising monomers with the radical initiators, however, these techniques are highly water- and/or oxygen-sensitive.21,22 When free-radical polymerisation (FRP) is conducted in the presence of viologen species, its reduction can cause a depletion of active radicals and thus disruption of the polymerisation process. Despite varying solvents, comonomers and initiator loadings, the direct FRP of viologen-containing monomers remains therefore limited to molar masses of 30 kDa.23–25 Accessing higher molar masses has been possible via post-polymerisation modification,26–28 which has impacted the electrochemical properties of the resultant materials.29,30 Alternative strategies to access higher molar masses of redox-active polymers and control their polymerisation are highly desirable.Incorporation of cucurbit[n]uril (CB[n]) macrocycles have lead to a variety of functional materials through host–guest chemistry.31–34 Moreover, the redox chemistry of viologens can be modulated through complexation with CB[n].35–38 Specifically, CB[n] (n = 7, 8) can tune the redox potential of pristine viologens and efficiently sequester monoreduced viologen radical cations, avoiding precipitation in aqueous environments. Further to this, we recently demonstrated that the viologen radical cation is stabilised by −20 kcal mol−1 when encapsulated in CB[7].39Consequently, we envisioned that incorporating CB[n]s as additives prior to polymerisation could (i) overcome current limits in accessible molar masses, (ii) increase control over FRP of viologen-based monomers through encapsulation and (iii) enable separation of radical species avoiding aggregation.Here, we demonstrate a new approach to control FRP of redox-active monomers leading to high molar masses and decreased dispersity of the resultant polymers. In absence of CB[n], co-polymerisation of the N-styryl-N′-phenyl viologen monomer 12+ and N,N-dimethylacrylamide (DMAAm) only occurs at high initiator loadings (>0.5 mol%, Fig. 1a), leading to low molecular weights and high polydispersity. Using our synthetic approach, 12+ is efficiently copolymerised with DMAAm in the presence of CB[n] (n = 7, 8) macrocycles resulting in control of the polymer molar mass across a broad range, 4–500 kDa (Fig. 1b). Finally, CB[n] are successfully removed from the polymer via competitive host–guest binding and dialysis. Spectroscopic and electrochemical studies revealed that solubilisation/stabilisation of the reduced species and/or shielding of the redox-active monomers from electron transfer processes was responsible for this enhancement.Open in a separate windowFig. 1Schematic representation of the investigated polymerisation. (a) Conventional free radical polymerisation either completely fails to copolymerise redox-active monomers (low initiator loading) or delivers copolymers with limited molar masses and high dispersities (high initiator loading). (b) CB[n]-mediated protection suppresses interference of viologen monomers with radicals formed through the initiation process facilitating copolymerisation. The molar mass of the resulting copolymers is readily tunable via the amount of present CB[n] macrocycles and the CB[n] is post-synthetically removed via competitive binding to yield the final copolymer with desired molar mass. Cl− counter-ions are omitted for clarity.Recent studies on symmetric aryl viologens demonstrated 2 : 2 binding modes with CB[8] and high binding constants (up to Ka ∼ 1011 M−2).40,41 Incorporation of polymerisable vinyl moieties, in combination with the relatively static structure of their CB[n] host–guest complexes, was postulated to allow polymerisation without unfavorable side reactions. The asymmetric N-styryl-N′-phenyl viologen monomer 12+ prepared for this study (Fig. S1a and S2–S13†) displays a linear geometry and was predicted to bind CB[n] (n = 7, 8) in a 2 : 1 and 2 : 2 binding fashion (Fig. S1b†).40,42 Binding modes between CB[n] (n = 7, 8) and 12+ were investigated through titration experiments (1H NMR and ITC) which confirmed the formation of 1·(CB[7])2 and (1)2·(CB[8])2 (see Fig. S25 and S26†). 1H NMR titration of CB[7] with 12+ demonstrates encapsulation of both aryl moieties (including the vinyl group) through upfield chemical shifts of the respective signals (Fig. 2a). Similar upfield shifts were observed for CB[8] (Fig. 2c). Different para-aryl substituents (vinyl vs. hydrogen) resulted in either head-to-tail or head-to-head (1)2·(CB[8])2 dimers (Fig. S1b and S26†), a previously reported phenomenon.43 Nonetheless, the reversible nature of the complex renders the vinyl group temporarily available for copolymerisation. In the presence of CB[8], 12+ yields polymer molar masses of up to 500 kDa as its complexation is more robust. ITC data confirmed binding stoichiometry, with binding constants of Ka = 2.64 × 106 M−1 for 1·(CB[7])2 and Ka = 9.02 × 1010 M−2 for (1)2·(CB[8])2 (Table S2, Fig. S29a and b†).Open in a separate windowFig. 2Supramolecular complexation of 12+ and CB[n]. 1H NMR spectra of 12+ at (a) χCB[7] = 2, (b) χCB[n] = 0 and (c) χCB[8] = 1 in D2O. Cl− counter-ions are omitted for clarity.The free radical copolymerisation of 12+ and DMAAm ([M] = 2 M), in the absence of CB[n], was based on optimised DMAAm homopolymerisations (Fig. S14 and S15†) and full conversion was confirmed by 1H NMR spectroscopy (Table S1 and Fig. S16†). 12+ was maintained at 1 mol% relative to DMAAm and by varying the radical initiator concentration molar masses of up to 30 kDa with broad dispersities (Đ = 11.4) were obtained (Fig. S17†). Lower initiator concentrations (<0.25 mol%) limited polymerisation (Mn = 3.7 kDa) and size exclusion chromatography elution peaks exhibited extensive tailing, suggesting that 12+ engages in radical transfer processes.To verify our hypothesis that CB[n] macrocycles can modulate the redox behavior of 12+, FRP of 12+ and DMAAm was conducted with varying amounts of CB[n] (n = 7, 8) (Fig. 3, S18 and S20†). Full conversion of all monomers including their successful incorporation into the polymer was verified via1H NMR spectroscopy and SEC (Fig. S18 and S21–S23†). Using CB[7], the molar mass of the copolymers was tunable between Mn = 3.7–160 kDa (Fig. 3b and S21a†). Importantly, in the presence of CB[8], a broad range of molar masses Mn = 3.7–500 kDa were accessible for 0 < χCB[8] < 1.2 (Fig. S20 and S21b†). Increasing the CB[n] (n = 7, 8) concentration caused dispersity values to converge to Đ = 2.2 (χCB[8] = 1.2, χ is the ratio of CB[n] to the redox-active monomer, Fig. S20†). The copolymers were purified by addition of adamantylamine (competitive binder) prior to dialysis to deliver CB[n]-free redox-active copolymers (Fig. S23†).Open in a separate windowFig. 3(a) In situ copolymerisation of DMAAm with 12+ and CB[7]. (b) Molar mass and dispersity vs. amount of CB[7] in the system. Fitted curve is drawn to guide the eye. Cl− counter-ions are omitted for clarity.The range of molar masses obtainable through addition of CB[n] (n = 7, 8) correlated with the measured Ka (Fig. 3b and S20†). Binding of 12+ to CB[8] was stronger and therefore lower concentrations of CB[8] were required to shift the binding equilibrium and mitigate disruption of the polymerisation. Dispersity values reached a maximum at χCB[7] = 0.6 or χCB[8] = 0.3, suggesting 1+˙ is only partially encapsulated. Consequently, higher CB[n] concentrations can enable FRP with lower initiator concentrations (0.10 mol%, Fig. S19†), which demonstrates the major role of complexation to modulate electron accepting properties of 12+.The redox-active monomer 12+ can engage with propagating primary radicals (Pm˙) to either be incorporated into the growing polymer chain (Pm–12+˙) or to abstract an electron deactivating it (Pm). This deactivation likely occurs through oxidative termination producing 1+˙ (energetic sink), inactive oligo- and/or polymer chains (Pm) and a proton H+, causing retardation of the overall polymerisation. Oxidative terminations have been previously observed in aqueous polymerisations of methyl methacrylate, styrenes and acrylonitriles that make use of redox initiator systems.44–47 Another example by Das et al. investigated the use of methylene blue as a retarder, with the primary radical being transferred to a methylene blue electron acceptor via oxidative termination, altogether supporting the outlined mechanism of our system (extended discussion see ESI, Section 1.4†).48The process of retardation can, however, be successfully suppressed, when monomer 12+ is encapsulated within CB[n] macrocycles. Herein the formation of 1·(CB[7])2 or (1)2·(CB[8])2 results in shielding of the redox-active component of 12+ from other radicals within the system, hampering other electron transfer reactions. This inhibits termination and results in extended polymerisation processes leading to higher molar mass polymers through mitigation of radical transfer reactions. Moreover, suppressing the formation of 1+˙ through supramolecular encapsulation minimises both π and σ dimerisation of the emerging viologen radical species,39 preventing any further reactions that could impact the molar mass or polydispersity of the resulting polymers.Cyclic voltammetry (CV) and UV-Vis titration experiments were conducted to provide insight into the impact of CB[n] on the redox behavior and control over FRP of 12+. Excess of CB[n] (n = 7, 8) towards 12+ resulted in a complete suppression of electron transfer processes (Fig. S31 and S32†). Initially, 12+ shows a quasi-reversible reduction wave at −0.44 V forming 1+˙ (Fig. 4a). Increasing χCB[7], this reduction peak decreases and shifts towards more negative potentials (−0.51 V, χCB[7] = 1) accompanied by the formation of 12+·(CB[7])1. A second cathodic peak emerges at −0.75 V due to the increased formation of 12+·(CB[7])2. At χCB[7] = 2, this peak shifts to −0.80 V, where it reaches maximum intensity, once 12+·(CB[7])2 is the dominating species in solution. When 2 < χCB[7] < 4, the intensity of the reduction peak decreases and the complexation equilibrium is shifted towards the bound state, complete suppression of the reduction peak occurs at χCB[7] = 4. Similarly, the oxidation wave intensity is reduced by 95% at χCB[7] = 4 causing suppression of potential oxidative radical transfer processes (Fig. 4c).Open in a separate windowFig. 4Mechanism of the CB[n]-mediated (n = 7, 8) strategy for the controlled copolymerisation of redox-active monomer 12+. (a) Cyclic voltammogram with varying amounts of CB[7]. (b) UV-Vis titration of 12+ with varying amounts of CB[7]. (c) Intensity decay of the oxidation peak at −0.27 V and change in absorption maximum of 1+˙ at 590 nm vs. χCB[7]. (d) Electron transfer processes of 12+ to generate 1+˙ and 10. (e) Reduction of 12+ resulting in precipitation of 10. (f) Stabilisation of 1+˙ through encapsulation with CB[7]. (g) Protection of 12+ from redox processes through CB[7]-mediated encapsulation.The concentration of 1+˙ can be monitored using UV-Vis (Fig. 4b and S34†).49 Absorbance at 590 nm (λmax) vs. χCB[7] was plotted and the concentration of 1+˙ increases, reaching a maximum at χCB[7] = 4 (Fig. 4c). When χCB[7] > 4, a decrease in concentration of 1+˙ was observed. We postulate the following mechanism: at χCB[7] = 0, 12+ is reduced to produce high concentrations of 1+˙ that partially disproportionates to form 10, which precipitates (Fig. 4e and S34†). When 0 < χCB[7] < 4, increasing amounts of green 1+˙ are stabilised through encapsulation within CB[7] suppressing disproportionation (Fig. 4c (cuvette pictures), Fig. 4f). For χCB[7] > 4, 12+ is protected from reduction through encapsulation (Fig. 4g).To further demonstrate applicability of this strategy, we chose another viologen-based monomer 22+ for copolymerisation (Fig. 5a). As opposed to 12+, CB binds predominantly to the styryl moiety of 22+ (Fig. S27 and S28†).50 ITC data showed that 22+ binds CB[7] in a 1 : 1 fashion with a binding affinity of Ka = 2.32 × 106 M−1 (Fig. S30 and Table S2†). Monomer 22+ was also analysed via CV and showed three reversible reduction waves at −0.91 V, −0.61 V (viologen) and 0.40 V (styrene). Similar to 12+, excess CB[7] selectively protects the molecule from redox processes, while the vinyl moiety remains accessible (Fig. 5a, S33c and d†). For CB[8], only partial suppression of electron transfer processes was observed (Fig. S33e and f†). We therefore chose CB[7] as an additive to increase control over FRP of 22+ (Fig. 5b). Copolymerisation of 22+ (1 mol%) and DMAAm ([M] = 2 M) at χCB[7] = 0 resulted in Mn = 28 kDa. When χCB[7] = 0.1, 0.2 or 0.3, Mn increased gradually from 124 to 230 and 313 kDa, respectively, demonstrating the potential of this strategy for FRP of redox-active monomers. Higher percentages of CB[7] led to copolymers with presumably higher molar masses causing a drastic decrease in solubility that prevented further analysis. Investigations on a broader spectrum of such copolymers, including those with higher contents of 22+ are currently ongoing.Open in a separate windowFig. 5(a) Cyclic voltammogram of viologen-containing monomer 22+ and its complexation with CB[n] (n = 7, 8) at a concentration of 1 mM using a scan rate of 10 mV s−1 in 0.1 mM NaCl solution. (b) Molar mass and dispersity of 22+-containing copolymers vs. equivalents of CB[7]. Cl− counter-ions are omitted for clarity.In conclusion, we report a supramolecular strategy to induce control over the free radical polymerisation of redox-active building blocks, unlocking high molar masses and reducing polydispersity of the resulting polymers. Through the use of CB[n] macrocycles (n = 7, 8) for the copolymerisation of styrenic viologen 12+, a broad range of molar masses between 3.7–500 kDa becomes accessible. Our mechanistic investigations elucidated that the redox behavior of monomer 12+ is dominated by either CB[n]-mediated stabilisation of monoradical cationic species or protection of the encapsulated pyridinium species from reduction. In the stabilisation regime (χCB[7] < 4), 12+ is reduced to form the radical cation 1+˙, which is subsequently stabilised through CB[7] encapsulation. Upon reaching a critical concentration of CB[7] (χCB[7] > 4), the system enters a protection-dominated regime, where reduction of 12+ is suppressed and the concentration of 1+˙ diminishes. The resulting copolymers can be purified by use of a competitive binder to remove CB[n] macrocycles from the product. This strategy was successfully translated to a structurally different redox-active monomer that suffered similar limitations. We believe that the reported strategy of copolymerisation of redox-active monomers will open new avenues in the synthesis of functional materials for energy conversion and storage as well as for applications in electrochromic devices and (nano)electronics. 相似文献
103.
Trichothecene mycotoxins are sesquiterpenoid compounds primarily produced by fungi in taxonomical genera such as Fusarium, Myrothecium, Stachybotrys, Trichothecium, and others, under specific climatic conditions on a worldwide basis. Fusarium mold is a major plant pathogen and produces a number of trichothecene mycotoxins including deoxynivalenol (or vomitoxin), nivalenol, diacetoxyscirpenol, and T-2 toxin, HT-2 toxin. Monogastrics are sensitive to vomitoxin, while poultry and ruminants appear to be less sensitive to some trichothecenes through microbial metabolism of trichothecenes in the gastrointestinal tract. Trichothecene mycotoxins occur worldwide however both total concentrations and the particular mix of toxins present vary with environmental conditions. Proper agricultural practices such as avoiding late harvests, removing overwintered stubble from fields, and avoiding a corn/wheat rotation that favors Fusarium growth in residue can reduce trichothecene contamination of grains. Due to the vague nature of toxic effects attributed to low concentrations of trichothecenes, a solid link between low level exposure and a specific trichothecene is difficult to establish. Multiple factors, such as nutrition, management, and environmental conditions impact animal health and need to be evaluated with the knowledge of the mycotoxin and concentrations known to cause adverse health effects. Future research evaluating the impact of low-level exposure on livestock may clarify the potential impact on immunity. Trichothecenes are rapidly excreted from animals, and residues in edible tissues, milk, or eggs are likely negligible. In chronic exposures to trichothecenes, once the contaminated feed is removed and exposure stopped, animals generally have an excellent prognosis for recovery. This review shows the occurrence of trichothecenes in food and feed in 2011–2020 and their toxic effects and provides a summary of the discussions on the potential public health concerns specifically related to trichothecenes residues in foods associated with the exposure of farm animals to mycotoxin-contaminated feeds and impact to human health. Moreover, the article discusses the methods of their detection. 相似文献
104.
Agnieszka Lewiska Marta Doma-Kdzia Kinga Kierul Micha Bochynek Dominika Pannert Piotr Nowaczyk Marcin ukaszewicz 《Molecules (Basel, Switzerland)》2021,26(4)
The skin is constantly exposed to external and internal factors that disturb its function. In this work, two nanosystems-levan nanoparticles and a surfactin-stabilized nanoemulsion were preserved (tested for microbial growth) and characterized (size, polydispersity, Zeta potential, and stability). The nanosystems were introduced in the model formulations-cream, tonic, and gel, and confirmed by TEM. The analysis showed that nanoemulsion has a spherical morphology and size 220–300 nm, while levan nanoparticles had irregular shapes independently of the use of matrix and with particle size (130–260 nm). Additionally, we examined the antiradical effect of levan nanoparticles and nanoemulsion in the prototype of formulations by scavenging DPPH (2,2-diphenyl-1-picrylhydrazyl; EPR spectroscopy). The model cream with both nanosystems and the whole range of products with nanosystems were evaluated in vivo for hydration, elasticity, smoothness, wrinkles and vascular lesions, discoloration, respectively. The cream improved skin condition in all tested parameters in at least 50% of volunteers. The use of more comprehensive care, additionally consisting of a tonic and gel, reduced the previously existing skin discoloration to 10.42 ± 0.58%. The presented prototype formulations are promising in improving skin conditions. 相似文献
105.
Szymon Szufa Piotr Piersa ukasz Adrian Justyna Czerwiska Artur Lewandowski Wiktoria Lewandowska Jan Sielski Maria Dziku Marek Wrbel Marcin Jewiarz Adrian Knapczyk 《Molecules (Basel, Switzerland)》2021,26(4)
Miscanthus is resistant to dry, frosty winters in Poland and most European Union countries. Miscanthus gives higher yields compared to native species. Farmers can produce Miscanthus pellets after drying it for their own heating purposes. From the third year, the most efficient plant development begins, resulting in a yield of 25–30 tons of dry matter from an area of 1 hectare. Laboratory scale tests were carried out on the processes of drying, compacting, and torrefaction of this biomass type. The analysis of the drying process was conducted at three temperature levels of the drying agent (60, 100, and 140 °C). Compaction on a hydraulic press was carried out in the pressure range characteristic of a pressure agglomeration (130.8–457.8 MPa) at different moisture contents of the raw material (0.5% and 10%). The main interest in this part was to assess the influence of drying temperature, moisture content, and compaction pressure on the specific densities (DE) and the mechanical durability of the pellets (DU). In the next step, laboratory analyses of the torrefaction process were carried out, initially using the Thermogravimetric Analysis TGA and Differential Scaning Calorimeter DSC techniques (to assess activation energy (EA)), followed by a flow reactor operating at five temperature levels (225, 250, 275, 300, and 525 °C). A SEM analysis of Miscanthus after torrefaction processes at three different temperatures was performed. Both the parameters of biochar (proximate and ultimate analysis) and the quality of the torgas (volatile organic content (VOC)) were analyzed. The results show that both drying temperature and moisture level will affect the quality of the pellets. Analysis of the torrefaction process shows clearly that the optimum process temperature would be around 300–340 °C from a mass loss ratio and economical perspective. 相似文献
106.
Chromatographia - The relationship between Hashimoto's thyroiditis and vitamin D concentration was already presented in many studies. The aim of this study was to analyze the differences in the... 相似文献
107.
Dr. Joice Thomas Dr. Liliana Dobrzańska Prof. Luc Van Meervelt Prof. Mario Alfredo Quevedo Prof. Krzysztof Woźniak Marcin Stachowicz Prof. Mario Smet Prof. Wouter Maes Prof. Wim Dehaen 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(3):979-987
A synthetic route towards homodiselenacalix[4]arene macrocycles is presented, based on the dynamic covalent chemistry of diselenides. The calixarene inner rim is decorated with either alkoxy or tert‐butyl ester groups. Single‐crystal X‐ray analysis of two THF solvates with methoxy and ethoxy substituents reveals the high similarity of their molecular structures and alterations on the supramolecular level. In both crystal structures, solvent channels are present and differ in both shape and capacity. Furthermore, the methoxy‐substituted macrocycle undergoes a single‐crystal‐to‐single‐crystal transformation during which the molecular structure changes its conformation from 1,3‐alternate (loaded with THF/water) to 1,2‐alternate (apohost form). Molecular modelling techniques were applied to explore the conformational and energetic behaviour of the macrocycles. 相似文献
108.
109.
110.
Abstract Experimental results referring to the transformation of smectic phases, mainly smectic A, into nematic and reentrant nematic phases are reviewed. A new explanation of some experimental results is proposed. Factors which are responsible for the depression of smectic phases in mixtures of polar mesogens are discussed and the possibility of forming mixtures with a broad temperature range of nematic phase from smectic compounds, which can be useful for liquid crystal displays (LCDs), is shown. A nematic gap observed in some cases between monolayer (SA1) or monolayer and partially bilayer (SAd) smectics results from the differences in the organization of the molecules in the smectic layers. It is concluded that polar phase from smectic A1 phases can be divided into two groups: (a) the first one is characteristic for compounds with the -NCS, -F, -CI, -I or ?COC m H2m + 1 terminal group. The spacing of the smectic layer slowly expands with the increase in alkyl chain length and the structure of the smectic A1 phase slowly changes to be more like the smectic Ad phase (d/1 > 1). It is proposed that such a smectic is called an enhanced monolayer smectic (SA1e (b) the second one is typical for compounds with the -CN terminal group. This kind of smectic A1 phase is rapidly transformed into the smectic Ad phase with increasing alkyl chain length. These latter monolayer mesogens easily form the reentrant nematic phase when they are mixed with other polar smectic mesogens. 相似文献