Phase inversion sequences

Lee et al. described a promising way to prepare multiple emulsion that is derived from the emulsion inversion path (Lee et al., 2002). An oil-in-water microemulsion based on 2-butoxyethanol/n-decane/water system was mixed with oleic acid, and they observed a phase inversion sequence that includes microemulsion-in-oil to oil-in-microemulsion. 

Figure 2 illustrates the following sequence of steps. During the reactions, forty ml samples were removed from the reactor by pipeting the solution into bottles, followed by quenching in an ice bath. Samples were removed every 10 minutes early in the reaction and every 5 minutes close to the phase inversion point. 

A pulse scheme recovering the zero-quantum Hamiltonian was proposed by Baldus and Meier.142 It is weakly dependent on spectral parameters and a faithful measure of internuclear distances. This sequence is based on the former rotor-synchronized R/L-driven polarization transfer experiments.143,144 It uses the LG or FS-LG, which is used to decouple the high-7 spins, and combined MAS and RF irradiation of low-7 spins to decouple the hetero-nuclear dipolar interactions. With phase-inversion and amplitude attenuation in the rotating frame and refocusing pulses in the laboratory frame part of the pulse sequence, a zero-quantum average Hamiltonian can be obtained with optimum chemical-shift/offset independence. 

The polymerization sequence can be plotted using a phase diagram (Figure 14.1) [11]. This diagram is representative for nearly all polymer (rubber-polymer (glassy polymer)-common solvent systems. Point a is the composition of the feed. The critical point (phase separation) is reached at point b. Further polymerization generates more glassy polymer (SAN) and phase inversion occurs at point c (rubber continuous —> discontinuous). Point d is reached when all monomer is converted. 

Symmetrically shifted pulses have been proposed as a means of solvent suppression. Symmetrically shifted pulses are symmetrically shifted laminar pulses that contain equal numbers of rectangular pulse components of the same phase at an offset frequency. The basis of the symmetrically shifted pulse family is the SS pulse which is conceptually equivalent to applying simultaneous ir/2 rectangular pulses with two separate, but in-phase, transmitters at offset frequency from the water. On a practical basis an SS pulse is obtained by a complete Itt cosine modulation of a single transmitter (see Fig. 15). An S pulse is half of an SS pulse (i.e. a half-cycle tt pulse) which results in a narrower null and a 180° phase inversion at the transmitter frequency. They are also the soft, continuous equivalent of binomial sequences. The SS and S pulses have broader excitation maxima than the sinusoidal profile of the JR sequence. The method has maximal excitation at an offeet frequency of second-order U-shaped water suppression. The exdtation profile is related to the maximum amplitude modulation and can be determined by numerical evaluation of the Bloch equations. Hence a new pulse shape must be used for each excitation window. The SS pulses give better water suppression than the JR sequence, but at the expense of poorer excitation of resonances closer to the water. Also, there is no phase inversion at zero frequency. The S pulse gives better excitation near the water frequency but with less water suppression. 

The binomial sequences aim to improve the zero excitation profile and provide schemes that are less sensitive to spectrometer imperfections. The series may be written 1-1, 1-2-1, 1-3-3-1. .. and so on, where the numbers indieate the relative pulse widths, each separated by a delay x, and the overbar indicates phase inversion of the pulse. For off-resonance spins the pulse elements are additive at the exeitation maximum so for example, should one require 90° off-resonance excitation, 1-1 corresponds to the sequence 45x-t-45 x. Of this binomial series, it turns out that the 1-3-3-1 sequence [66] has good performance and is most tolerant of pulse imperfections by virtue of its symmetry [67]. The trajectory of spins with frequency offset l/2x from the transmitter for a net 90° pulse (1 = 11.25°) is shown in Fig. 9.26. During each... 

Whilst maximum excitation occurs at l/2t Hz from the transmitter offset, further nulls occur at offsets of n/x (n = 1, 2, 3,. .. corresponding to complete revolutions during each x) so a judicious choice of x is required to provide excitation over the desired bandwidth. The excitation profiles of the 1-1 and 1-3-1 sequences are shown in Fig. 9.25b and c. Clearly the excitation is non-uniform, so places limits on quantitative measurements, and once again there exists a phase inversion either side of the solvent. Both provide an effective null at the transmitter offset and suppression ratios in excess of 1000-fold can be achieved. 

As apparent from the previous section, a binomial sequence has a suitably tailored profile for the element S, and the series 3a-9a-19a-19a-9a-3a (Fig. 9.28a, with 26a =180° and a delay t between pulses, here termed W3 [71]) has a desirable off-resonance inversion profile for this purpose. The WATERGATE excitation profile for this is shown in Fig. 9.29a. Once again characteristic nulls also occur at offsets of n/x Hz, but between these the excitation is quite uniform and does not suffer the phase inversion of the unaccompanied 90° binomials. More recently, extended binomial sequences have been shown to provide a narrower notch at the transmit-... 

The moral of all this is that considerable care must be taken when composite pulses are introduced into pulse sequences. It is wise to test the compensated pulse sequences vs the original uncompensated sequences on a known sample to see if improvements are achieved in reality. This offers a check on whether the composite elements have been correctly introduced and on whether the spectrometer is capable of correctly executing the desired sequences, many of which demand accurate control of rf amplitudes and phases for extended periods. The experimental performance of the composite pulse itself is best tested with a simple experiment, such as an inversion sequence (180 t, y -90 r-FIDA ) or a spin-echo sequence (90 180 c,y, c, y—FID with... 

The handedness inversion behaviors upon UV irradiation were observed in both wedge cells and homeotropic cells with the cholesteric (N )—nematic (N)— cholesteric (N ) phase transition sequence as confirmed by the Cano s lines and fingerprint texmres (Fig. 5.12). The mechanism of the helix inversion is proposed... 

The electronic structure of the group-5 anionic complexes [MCls], [MOCl4], [M(OH)2Cl4] , and [MOCls] was calculated with the DS DVM code (Pershina et al. 1994). By applying Born s theory of phase transition between the aqueous phase and the organic phase (mixed HCl/HF solutions and TiOA, see (Kratz et al. 1989), the extraction sequence Pa > Db > Nb was predicted, which is the inverse sequence compared to that observed experimentally (Kratz et al. 1989). Due to the complicated situation in mixed HCl/HF solutions with possibilities to form mixed chloride/fluoride- or fluoride complexes, it was recommended to repeat the experiments in the pure HCl system (Pershina et al. 1994). While these new experiments were prepared (Paulus 1997), Pershina (1998a, b) considered hydrolysis and complex formation of Nb, Ta, Pb, and Pa in HCl solutions as competing processes in equilibria such as... 

Devia et explored the effects of stirring on phase inversion in a series of castor oil polyester/polystyrene SINs (see Section 5.5.2.2). For materials subjected to a somewhat different polymerization sequence, Devia showed that quite dissimilar morphologies may ensue. The... 

The competition between different inter-molecular interactions results in a complicated thermal behavior. Mostly, the cubic phase is found in a sequence with the SmC phase, in which the latter one is the low temperature form. But the hydrazine derivatives exhibit the inverse sequence. In some cases (compounds la-1, la-2, and 8a-2-8a-7), the cubic phase is observed between the SmC and SmA phase. In polycatenar substances the cubic phase is found together... 

If the amphiphile concentration is larger than about 50 wt%, the amphiphile aggregates may revert to inverse structures (inverse hexagonal or inverse cubic structures). In these cases, the solvent becomes the minority phase. The sequence in increasing amphiphile concentrations is typically the following ... 

---